Peter Rosenzweig

Patrick Schlee

Jakob Hübner

Keywords: symmetric, bend -bending, rock -rocking, tors -torsion, strstretching, twist -twisting

The research chapters of this thesis (chapter 3, 4 and 5) are written in form of self-contained research articles. Consequently, the terms "abstract", and "supporting information" are used.

To avoid repetitions, the presented thesis does not contain a section on methodology and instrumentation, since the experimental details are given in the research chapters. Although the research chapters consist of individual articles, this thesis should give a coherent picture of structure investigations of organic nano-composites via near-and far-field Raman spectroscopy methods.

Introduction

The kinetics of chemical and physical reactions depend not only on the used chemicals but also on their sizes, morphologies, surface compositions and molecular arrangements. This relationship can be observed, for example, in the adsorption of gases on particle surfaces or when chemicals are dissolved in a solvent. The influence of size, morphology, surface composition and arrangement becomes particularly clear in solid state reactions, e.g. combustions, of particulate precursors. [1][2] Therefore, a detailed knowledge of these parameters is essential in order to be able to describe their reactions in detail and thus produce tailor-made reactants for several applications. This task is relatively easy for reactants consisting of one compound only. For this purpose, it is normally sufficient to examine the particles using standard analytical methods. The chemical composition can be determined by nuclear magnetic resonance spectroscopy, infrared or Raman spectroscopy, for example. Particle morphologies and sizes, in turn, can be determined by microscopic methods such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning tunnel microscopy (STM) or atomic force microscopy (AFM). However, it becomes considerably more difficult if the reactant particles are formed from different chemical compounds. Especially, if the particles sizes are on the sub-micro or nanoscale. The influence of particle size and arrangement on the reaction behavior as well as their structural investigation will be briefly explained using the example of two different aluminum-based iron oxide/-hydroxide nanothermites (i) and (ii) which were both synthesized via a wet-chemical process. 1 The difference in the synthesis routine of both thermites lies only in the temperature, i.e. (i) is synthesized at 20°C and (ii) at 50°C. Nevertheless, (i) shows a significantly higher flame propagation velocity (472 ms -1 ) than (ii) (323 ms -1 ). This difference cannot only be explained by the chemical composition because differential scanning calorimetry analyses show that both thermites consist of almost the same composition. Hence, TEM was performed to obtain information about the structural arrangements of the two nanothermites. However, the TEM images shown in Figure 1 do not present any major structural differences between the two samples. Therefore, a further measuring method had to be found which allows simultaneous imaging of microscopic and spectroscopic results. For this purpose, energy dispersive X-ray (EDX) spectroscopy coupled with TEM was applied. This technique measures emitted element-specific X-rays and images Chapter 1 them microscopically in nanometric resolution. Only through the application of EDX-TEM, the difference between the two nanothermites becomes apparent. Accordingly, (i) is formed from aluminum nanoparticles embedded in an iron oxide/-hydroxide matrix, whereas (ii) are built up by aluminum/iron oxide core/shell particles also embedded in an iron oxide/-hydroxide matrix (Figure 1. 1, right). This structural difference finally allows to draw conclusions about the different reaction behaviors of the nanothermites (i) and (ii).

Is the combination of TEM and EDX therefore also the method of choice for structure investigations of organic nanocomposites? Unfortunately, it is not, and there are several reasons for this. On the one hand, electron microscopy methods like SEM and TEM provide a high energy input to the investigated sample. This high energy input can lead to dissociation of the molecules of organic nanocomposites. Especially organic energetic and pharmaceutical substances tend to do so. In addition, both SEM and TEM are carried out under high vacuum (10 -5 -10 -7 mbar). 3 The applied high vacuum can cause sublimation of organic substances and thus falsify the measurement results. Moreover, EDX provides only information about the elemental composition of the sample. Organic substances (especially energetic substances), Chapter 1 however, are formed mainly from carbon, hydrogen, nitrogen and oxygen. Hence, it is not possible with EDX-TEM to map the arrangement of the different molecules within a composite nanoparticle. Consequently, another analysis method must be found fulfilling the following requirements. Firstly, the method must be relatively gentle in terms of energy input to the sample and measurement environment. Secondly, the method must be able to distinguish between different molecules. And thirdly, the method must offer a high spatial resolution that allows individual particles to be examined.

All these requirements for the investigation of nanoscale composites are fulfilled by tip enhanced Raman spectroscopy (TERS). If the nanoscale composites aggregate to sub-micro composites, surface enhanced Raman spectroscopy (SERS) can be performed on a confocal Raman microscope to investigate these superstructures. As the name suggests, both methods are based on Raman spectroscopy. However, Raman spectroscopy requires a relatively large amount of sample molecules because the probability of a photon becoming Raman scattered is very low, which makes it impossible to study individual particles. The difference between SERS and conventional Raman spectroscopy is that in SERS the sample to be analyzed is first deposited on or near a nanostructured metal substrates (mainly gold or silver). If the sample is now irradiated with a laser, the valence electrons of the metal particles are excited to a collective oscillation. The collective oscillation of the valence electrons causes an enhancement of the electric field near the metal particle surfaces (<10 nm), especially, if they oscillate in resonance with the incident laser light (localized surface plasmon resonance). [4][5] If a sample is placed within this enhanced field, the probability that a photon becomes Raman scattered is drastically increased. This makes it possible to study individual nanoparticles. In special experimental setups, it is even possible to investigate the vibrations of single molecules with SERS. [6][7] Due to its ability to study small amounts of sample molecules, SERS is used in a variety of applications. Thus, SERS is used in bio-medical approaches like the detection of biomarkers, the detection of circulating tumor cells or detection of pathogens. [8][9][10][11][12][13][14][15][16][17][18] Moreover, SERS is used for the detection and the quantification of environmental toxins and chemicals. 7,[19][20][21][22][23] In addition, SERS can be used for analyzing drinking water and food. [24][25] Tip enhanced Raman spectroscopy is an advanced technology based on SERS. TERS combines the outstanding properties of SERS with a scanning probe microscopy (SPM) method, such as STM or AFM. In contrast to SERS, where the sample is applied to metallic nanoparticle surfaces, the tip of a SPM is equipped with a metallic nanoparticle in TERS. This allows a sample to be microscopically scanned and the metal nanoparticle at the tip to be moved to any Chapter 1 position on the sample. Accordingly, it is possible to bring the field-enhancing nanoparticle onto positions of interests of the sample surface and not the sample onto the metal nanoparticle.

This and the fact that the electric field amplification is generated by a single nanoparticle makes TERS much more selective than SERS. In addition, TERS offers a lateral resolution of less than 1 nm making TERS a promising tool for structure investigations of organic nanoscale composites. 5,[26][27][28] TERS is used inter alia for analyzing organic molecules, self-assembled monolayers, chemical reactions, biological samples and for material characterization. [29][30][31][32][33][34][35][36][37][38][39] Within this thesis, various organic and energetic organic nano and submicron composites are characterized via conventional Raman spectroscopy (far-field Raman), TERS and SERS (nearfield Raman). The experimental details, the results and the conclusions which could be drawn from these investigations as well as a detailed description of the theoretical background of Raman scattering, SERS and TERS are presented in the following.

Chapter 2 2 Theoretical Background Within this chapter the theoretical background of Raman spectroscopy in general, surface enhanced Raman spectroscopy (SERS) and tip enhanced Raman spectroscopy (TERS) is presented. Especially, intrinsic features of Raman scattering, SERS and TERS are examined in more detail since they form the basis for the presented spectra analyses.

Basics of Light-Molecule Interactions

To develop a deeper understanding about the mechanism of Raman scattering, it is useful to discuss shortly the basic principles of the interactions between light and matter. Basically, there exist various mechanisms of light-matter interactions. These mechanisms depend strongly on the energy of the incident light. Since Raman spectroscopy is exclusively undertaken with near ultra-violet (NUV), visible (VIS) or near infrared (NIR) light, only interactions between matter and light with wavelengths between 200 nm and 50000 nm (50000 cm -1 -200 cm -1 ) are presented. Furthermore, the here investigated composite systems consist solely of organic submicron and nanoparticles, thus the focus of the following introduction is on light-molecule interactions in the previously mentioned spectral range.

Molecular Energy Levels

Interactions between light and matter are typically connected with energy transfers. Therefore, it is of fundamental importance for the description of light-matter interactions to know how energy is distributed in molecules. Basically, the total molecular energy comprises of the summation of its electronic, vibrational, rotational, nuclear and translational energy: 1 𝐸 𝑡𝑜𝑡 = 𝐸 𝑒𝑙 + 𝐸 𝑣𝑖𝑏 + 𝐸 𝑟𝑜𝑡 + 𝐸 𝑛𝑢𝑐 + 𝐸 𝑡𝑟𝑎𝑛𝑠 .

Eq. 2.1 Chapter 2

Due to quantum mechanics, molecules cannot absorb all energy values. Like atoms, molecules have specific energy levels (often energy states) which can solely adopt discrete values.

Excepted from this quantization is only the translational energy 𝐸 𝑡𝑟𝑎𝑛𝑠 . Moreover, the occupation of the energy levels of the atom nuclei and thus 𝐸 𝑛𝑢𝑐 stays normally constant during interactions with light. Thus, it is not necessary to include 𝐸 𝑡𝑟𝑎𝑛𝑠 and 𝐸 𝑛𝑢𝑐 into consideration for the description of molecular energy levels. This simplification is, in addition, permissible, since spectroscopic investigations deal only with the energy differences between certain energy levels and not with total energy level values.

Anyway, the main tool to describe quantum mechanically the behavior of atoms and molecules is the wave function 𝜓. This wave function describes distinctly the state of a quantum system although it has no physical meaning. To ascertain all possible energy levels a quantum mechanically system can adopt, the Hamiltonian 𝐻 ̂ is applied on the wave function 𝜓 resulting in the Schrödinger equation: 2 𝐻 ̂𝜓 = 𝐸𝜓.

Eq. 2.2

All wave functions solving the Schrödinger equation are eigenfunctions of the Hamiltonian 𝐻 ̂ and describe the adoptable quantum states. The energy eigenvalues 𝐸 corresponding to the eigenfunctions represent the energy values of the quantum state, respectively energy level. The Hamiltonian 𝐻 ̂ itself is built up by the sum of the operators of the kinetic energy 𝑇 ̂ and potential energy 𝑉 ̂: 2 𝐻 ̂= 𝑇 ̂+ 𝑉 ̂. Eq. 2.3

Therefore, the energy levels of atoms and further molecules can be calculated by the set-up of an appropriate Hamiltonian and the solution of the Schrödinger equation. If one applies this scheme to describe quantum mechanically the energy levels for one of the simplest real existing quantum mechanical systems the hydrogen atom, it turns out that some energy eigenvalues Chapter 2 correspond to different eigenfunctions. These quantum states have equivalent energy levels and are called degenerated quantum states. To distinguish these quantum states, it is necessary to introduce four quantum numbers. These are the principal quantum number 𝑛 describing the energy level of an electron, the azimuthal quantum number 𝑙 giving the shape of the orbital, the magnetic quantum number 𝑚 𝑙 subdividing the orbital shape referring to its spatial orientation towards a freely chosen coordinate system axis. The spin quantum number 𝑚 𝑠 describes the electron spin within the orbital. The squares of the eigenfunctions solving the Schrödinger equation for a hydrogen atom give the probability density of the electron. The projection of the real part of the spherical harmonic of the wave functions onto a Cartesian coordinate system depict graphically the shapes of the orbitals. Table 2.1 summarizes the possible values of the quantum numbers. The calculations are not shown since the mathematical background is not of relevance for a better understanding of the later presented spectroscopic experiments and results. Interested readers are referred here to comprehensive standard works about quantum mechanical and molecular physics (e.g. references 1-3).

𝑚 𝑠 -½ , + ½ ↓, ↑
The quantum numbers allow to describe each atomic electronic state which can be occupied within an atom and thus the electronic energy 𝐸 𝑒𝑙 . This changes if several atoms form chemical bonds and arrange each other to molecules with molecule orbitals. Since molecules consists from nature of at least two atoms, molecular total energy 𝐸 𝑡𝑜𝑡 contains additionally vibrational energy 𝐸 𝑣𝑖𝑏 and rotational energy 𝐸 𝑟𝑜𝑡 proportions (compare Eq. 2.1). The vibrational energy 𝐸 𝑣𝑖𝑏 and rotational energy 𝐸 𝑟𝑜𝑡 proportions arise from the higher amount of degrees of freedom which is provided by molecules compared to atoms. The total number of degrees of freedom 𝑓 𝑡𝑜𝑡 of atoms or molecules is given as

𝑓 𝑡𝑜𝑡 = 3𝑛 , Eq. 2.4
respectively as 𝑓 𝑡𝑜𝑡 = 𝑓 𝑡𝑟𝑎𝑛𝑠 + 𝑓 𝑣𝑖𝑏 + 𝑓 𝑟𝑜𝑡 , Eq. 2.5

where 𝑛 represents the number of atoms, 𝑓 trans the translation degrees of freedom, 𝑓 vib the vibration degrees of freedom and 𝑓 𝑟𝑜𝑡 the rotation degrees of freedom. 1 The amount of vibrational degrees of freedom 𝑓 vib and rotational degrees of freedom 𝑓 𝑟𝑜𝑡 is next to the number of atoms related to the molecular arrangement. Thus, linear molecules provide more vibration degrees of freedom but less rotation degrees of freedom than nonlinear molecules with the same number of atoms. These differences are summed up in Table 2.2. vibrations can be quantum mechanically approximated as harmonic oscillator. In accordance to classical mechanics, the quantum mechanical harmonic oscillator describes a particle, e.g. a molecule, in a potential of the general form

𝑉(𝑥) = 1 2 𝑘𝑥 2 = 1 2 𝑚𝜔 2 𝑥 2 , Eq. 2.6
with 𝑘 is the force constant, 𝑥 is the particle position, 𝑚 the mass of the particle and 𝜔 is the angular frequency. 3 Since molecule vibrations are in focus of this thesis, the particle position 𝑥 is given as the normal coordinate 𝑞 of a molecule vibration in the following. The normal coordinate 𝑞 gives the relative position of all molecule atoms during the molecule vibration relative to their equilibrium positions (Figure 2.1, left). The vibration energy eigenstates of a quantum mechanical oscillator can be calculated by finding an appropriate Hamiltonian (Eq.

2.

3) and solving the Schrödinger equation (Eq. 2.2). The kinetic energy operator of a particle is given as: ) . 2 Eq. 2.11

The harmonic oscillator eigenfunctions, especially their symmetries, are important for estimating if a vibration is Raman active or not and is discussed in more detail later within the semi quantum mechanical description of Raman scattering. The energy levels of the corresponding molecule vibration states are equally distributed with an energy difference of ℏω between two successive states. Furthermore, the zero-point energy of the vibrational ground state 𝑣 0 is ℏω/2 and thus in accordance with the Heisenberg uncertainly principle (Figure 2.1, right). Anyway, the model of the quantum mechanical harmonic oscillator is not sufficient to describe the energy distribution of molecules in general. The harmonic oscillator provides an infinite parabolic potential resulting in the occurrence of vibrational energy states characterized by infinitely high energy levels. This is in contradiction to the natural behavior of molecules.

Chapter 2

Higher vibration energy levels are related with higher minimal and maximal deflections of the normal coordinate 𝑞. The shorter the minimal deviation the stronger are the repulsive forces between the atomic nuclei. Accordingly, the atomic nuclei involved in the molecule vibration cannot arbitrarily near each other. On the other hand, the wider the maximal deviation the weaker are the molecular bond resulting in the dissociation of the molecule. 1 A more accurate quantum mechanical description of molecule vibrations is the anharmonic oscillator. The quantum mechanical anharmonic oscillator defines a maximal vibration state related with a maximal energy level, the dissociation energy (Figure 2.2). Furthermore, the energy differences between two successive vibrational states are not constant like they are in case of the harmonic oscillator; with increasing 𝑣 the energy differences become smaller. However, this effect gains only increasing importance for higher vibrational states. Since only fundamental vibrational frequencies (𝑣 0 to 𝑣 1 ) are investigated in the later presented Raman spectroscopy experiments, the quantum mechanical harmonic oscillator remains a sufficient tool to describe the results.

By "combing" the electronic energy states 𝑛 and the vibrational energy states 𝑣 an impression can be created how energy is distributed in molecules. Thus, every electronic state has In conclusion, quantum mechanical considerations allow to describe the energy distribution within a molecule. Accordingly, solely discrete energy states can be occupied. The energy states are built up by electronic states subdivided by vibration states which are further subdivided by the rotation states. This approximated quantum mechanical model enable to describe transitions between the diverse energy states. An atom or a molecule undergoes such transitions e.g. by absorbing or emitting a photon. Next to the required photon energy, further conditions must be satisfied so that a transition is "allowed". A simplified quantum mechanical explanation about photon absorption and emission is presented within the next sub-chapter. As a consequence, the molecule dissociates.

Chapter 2

Photon Absorption, Emission and Non-Radiative Relaxation

When a molecule interacts with electromagnetic radiation e.g. by absorbing or emitting a photon it can undergo an energy transition. Depending on the wavelength of the incident light, the transition can be electronic, vibrational or both. In case of wavelengths in the IR range (2500 nm -50000 nm, 4000 cm -1 -200 cm -1 ), molecules typically experience vibration state transitions. If the wavelengths of the electromagnetic radiation are localized between the UV and the VIS range (200 nm -800 nm, 50000 cm -1 -12500 cm -1 ), molecules may undergo an electronic state transition. This electronic state transitions can be connected with an additional vibration state transition. In order to induce a molecular energy transition, the energy of the absorbed photon must coincide with the energy differences between the initial state and the excited state. However, this is only half the story. Not every molecular transition, independently if it is of electronic or vibrational nature, is allowed. In case of molecular electronic transitions, the molecule orbital symmetry and the spin multiplicity influence if a transition is allowed or forbidden. The spin multiplicity (2S+1) gives the number of degenerated eigenstates related to the total spin angular momentum (simplified: number of unpaired electrons). In most of all cases, molecules have no unpaired electrons resulting in a spin multiplicity of one of the ground state. This configuration is called singlet S. However, some molecules have unpaired electrons (e.g. O2). Accordingly, the spin multiplicity is unequal to one. If the spin multiplicity is two, it is called doublet; if it is three, the configuration is called triplet T. The change in spin multiplicity must be zero so that a transition is spin-allowed. In case of vibrational transitions, the symmetry of the molecule vibration in the ground and in the excited state gives information whether the transition is allowed or forbidden. A molecular energy state transition is quantum mechanically expressed by the transition moment integral

µ = ∫ 𝜓 2 * 𝑜 ̂𝜓1 +∞ -∞
Eq. 2.12 with 𝜓 2 * represents the complex-conjugate wave function of the final state, 𝑜 ̂ the transition operator and 𝜓 1 the wave function of the initial state. In general, the wave functions are formed from the product of the electronic wave function and a wave function describing the positions Chapter 2

of the atom nuclei within the molecule. However, electronic transitions normally do not appear in Raman spectroscopy (except for resonance Raman spectroscopy) and thus the electronic wave functions stay constant during vibrational transition. Therefore, a detailed consideration of the behavior of this proportion of the wave functions is not shown. Anyway, if the transition moment integral turns zero, the transition is called forbidden. However, solving the transition moment integral is not a trivial matter and can be only solved numerical for complex quantum mechanical system like molecules, especially if electronic and vibrational transitions occur simultaneously. Fortunately, the concrete solution of this integral is not required since symmetry determinations of the wave functions and the transition operators are already expedient. Thus, the integral turns zero if the symmetry of the transition function 𝜓 1 * 𝑜 ̂𝜓2 is odd.

Contrary, if the transition function is even, the transition function is unequal to zero and consequently the transition is allowed. The transition moment integral and the Raman transition selection rules are discussed in detail within the semi quantum mechanically description of Raman scattering.

In order to visualize the changes of the occupations of the energy states in a molecule while interacting with electromagnetic radiation, the Jablonski diagram has proven its worth.

Accordingly, the Jablonski diagram illustrates in a simple manner all possible energy transitions a molecule can undergo as presented in Figure 2.3. If a molecule absorbs a photon satisfying the necessary requirements, the molecule undergoes an energy transition from its ground state to an excited state: 𝑆 0 𝑣 0 → 𝑆 𝑥 𝑣 𝑦 . In case of IR absorption (IR A, Figure 2.3), the molecule resists in its electronic ground state but are excited into an excited vibrational state: 𝑆 0 𝑣 0 → 𝑆 0 𝑣 𝑦 . The transition 𝑆 0 𝑣 0 → 𝑆 0 𝑣 1 is the fundamental transition which occurs most likely.

Vibrational frequencies found in typical IR or Raman spectra correspond in general to the energy difference between 𝑆 0 𝑣 0 → 𝑆 0 𝑣 1 . If a molecule is excited from its vibrational ground state into a vibrational state >1, the transition is called overtone (e.g. 𝑆 0 𝑣 0 → 𝑆 0 𝑣 2 ). A transition from an already excited vibrational state into a higher vibrational state is called hot transition (e.g. 𝑆 0 𝑣 1 → 𝑆 0 𝑣 2 ). The relaxation from an excited vibrational state to the ground state (e.g.

𝑆 0 𝑣 1 → 𝑆 0 𝑣 0 ) appears non-radiative, that means without an emission of a photon. The nonradiative relaxation are accomplished by the release of thermal energy. 1 If a molecule absorbs a photon from the UV-VIS range (UV-VIS A, Figure 2.3), an electronic state transition may take place which is often accompanied by an additional vibrational transition (e.g. 𝑆 0 𝑣 0 → 𝑆 1 𝑣 3 , Figure 2.3). The relaxation from such an excited state occurs first over a non-radiative Chapter 2 relaxation to the vibrational ground state of the excited electronic state (𝑆 1 𝑣 3 → 𝑆 1 𝑣 0 ). The further relaxation from the excited electronic state to the electronic ground state depends strongly of the molecule characteristics. Three different relaxation mechanisms exist. The first is driven by the internal conversion (IC), a non-radiative transition. This transition takes place between an excited electronic state and the excited vibrational state of the electronic ground state (e.g. 𝑆 1 𝑣 0 → 𝑆 0 𝑣 7 , Figure 2.3). Hereby, the energy levels of both, the initial and the final state, must be equivalent, or in other words they must be degenerated eigenstates. 1 The final energy relaxation of this mechanism occur non-radiatively in the vibrational ground state of the electronic ground state (e.g. 𝑆 0 𝑣 7 → 𝑆 0 𝑣 0 , Figure 2.3). The second relaxation mechanism starts with an intersystem crossing (ISC), another non-radiative transition between the excited electronic singlet state and an excited vibrational state of a lower energy electronic triplet state (e.g. 𝑆 1 𝑣 0 → 𝑇 1 𝑣 2 , Figure 2.3). The ISC is followed by a non-radiative vibrational relaxation from the excited vibrational state to vibrational ground state of the electronic triplet state Chapter 2 (e.g. 𝑇 1 𝑣 2 → 𝑇 1 𝑣 0 ). From here the molecule can undergo another ISC to an excited vibrational state of the electronic ground state (e.g. 𝑇 1 𝑣 0 → 𝑆 0 𝑣 4 ) followed by non-radiative relaxation (e.g.

𝑆 1 𝑣 4 → 𝑆 0 𝑣 0 ). However, the molecule can relax from the excited electronic triplet state to the ground state (e.g. 𝑇 1 𝑣 0 → 𝑆 0 𝑣 0 ) also by emitting a photon. This radiative relaxation mechanism is called phosphorescence (P, Figure 2.3). The third relaxation mechanism from the excited electronic state to the electronic ground state (e.g. 𝑆 1 𝑣 0 → 𝑆 0 𝑣 0 ) is the fluorescence (F, Figure 2.3). The relaxation appears here by emitting a photon with an energy corresponding to the energy difference between the excited and the ground state. Since the energy difference of the absorbed and the emitted photon is close to the energy differences between two vibrational states, fluorescence bands can occur in Raman spectra. These fluorescence bands appear sometimes extreme broad and intense so that an accurate spectral analysis becomes impossible.

Chapter 2

Raman Scattering

The Raman effect is an inelastic scattering process of light at matter whereby the energy (respectively the wavelength or the angular frequency) of the scattered light is either decreased or increased (Raman scattering). The Raman effect was already predicted in 1923 by Adolf Smekal and quantum mechanically described by Kramer, Heisenberg and Dirac before it could be experimentally verified. [4][5][6][7] It took five more years after its prediction until Raman and Krishnan could observe and describe the Raman effect by investigations of organic liquids illuminated with sunlight. [8][9][10][11][12][13] Within the same year, Landsberg and Mandelstam observed the Raman effect in inorganic crystals independently from Raman and Krishnan. 14 Though the Raman effect was predicted, described and observed by various researchers, it is named after the Indian physicist Chandrasekhara Venkata Raman.

If a photon with an energy ℎ𝜈 0 which coincides not with the energy difference between two molecular energy states interacts with a molecule (or matter in general), the molecule can be excited in a virtual energy state 𝑉. The photon energy is hereby typically smaller than required for a molecular electronic transition (except for resonance Raman spectroscopy). Virtual states are short-lived, not stationary states of a molecule because they are not eigenfunctions of any operator. 15 Virtual states are an interesting research topic per se which do not have to be discussed in more detail here since a closer description is not necessary for Raman spectra analyses. Anyway, if a molecule is excited from its electronic and vibrational ground state to a virtual state by absorbing a photon (𝑆 0 𝑣 0 → 𝑉), it immediately relaxes by emitting a photon. In most of the cases, the molecule relaxes back to its electronic and vibrational ground state (𝑉 → 𝑆 0 𝑣 0 ). Accordingly, the photon energy of the absorbed and the emitted photon is equal (Figure 2.4). This elastic scattering process is called Rayleigh scattering. Rayleigh scattering typically occurs with a probability of 10 -4 -10 -3 . However, with a possibility of 10 -8 -10 -5 the molecule can also relax from the virtual state to the first excited vibrational state of the electronic ground state (𝑉 → 𝑆 0 𝑣 1 ). This inelastic scattering process is the Stokes-scattering. The energy difference between the absorbed and the emitted photon coincides hereby with the energy difference between the vibrational ground state and the first vibrational state (Figure

Classical Description of Raman scattering

In the following, a classical description of Raman scattering is developed since it is in most of the cases sufficient to interpret spectroscopic data. 7 Accordingly, the classical description of Raman scattering gives if a molecular vibration is Raman active or inactive. Furthermore, the vectorial consideration allows to draw conclusion about the intensity ratios of different molecular vibration depending on their orientation towards the electric field vector of the incident light. The following presented mathematical description is based on reference 7. Chapter 2

In classical theory, a dipole moment is induced in a molecule while interacting with electromagnetic radiation. The induced dipole moment oscillates with the same frequency as the incident light. The electromagnetic radiation created by the induced dipole spreads over the whole solid angle. The electric field vector E ⃗ ⃗ (t) of the incident light is

𝐸 ⃗ (𝑡) = 𝐸 ⃗ 0 𝑐𝑜𝑠 𝜔 0 𝑡 , Eq. 2.
13

with E ⃗ ⃗ 0 is the directed amplitude of the electromagnetic wave and ω 0 is the angular frequency.

The induced dipole moment µ ⃗ (t) is then given by vector relation

𝜇 (𝑡) = 𝛼 ̃𝐸 ⃗ (𝑡) .
Eq. 2.14

The proportionality factor α ̃ is the polarizability of the molecule. The polarizability α ̃ is a second rank tensor assigning E ⃗ ⃗ (t) to µ ⃗ (t) thus it can be described as a 3x3 matrix (Figure 2.5, left). Generally, the polarizability is a measure for the deformability of the molecule orbitals in an external electric field. Since the polarizability is a second rank tensor, it can be illustrated as ellipsoids. Anyway, in order to simplify the mathematical description, the electric field vector

E ⃗ ⃗ (t) is reduced on its z -component E z (t).
Additionally, only the z -component of the induced dipole moment is discussed µ z (t). By applying both simplifications the polarizability tensor is reduced to α zz and thus Eq. 2.14 becomes a scalar relation resulting in 𝜇 𝑧 (𝑡) = 𝛼 𝑧𝑧 𝐸 𝑧 (𝑡) .

Eq. 2.15

The simplification can be realized experimentally by using z -polarized light and a z -analyzer placed in front of the Raman spectrometer (

Eq. 2.16

Inserting of Eq. 2.13 results in 𝜇(𝑡) = 𝛼𝐸 0 𝑐𝑜𝑠(𝜔 0 𝑡) .

Eq. 2.17 Chapter 2

According to classical electrodynamics, a time-varying dipole moment emits electromagnetic radiation with the frequency of its own oscillation (Hertz'scher dipole). If the polarizability 𝛼 is constant by meaning 𝛼 is not a function of time, the induced dipole will re-emit an electromagnetic wave with the same angular frequency 𝜔 0 than the incident light. Since the frequency and thus the wavelength of the incident and the emitted light are identic, Eq. 2.17 𝑞(𝑡) of a molecular vibration can be described as a harmonic oscillator

𝑞(𝑡) = 𝑞 0 𝑐𝑜𝑠(𝜔 𝑞 𝑡) Eq. 2.18
with q 0 is the maximal deviation from the equilibrium position (amplitude) and ω q is the angular frequency of the molecular vibration. Since the polarizability depends on the molecule atom positions, it follows this oscillation and become a function of the normal coordinate 𝛼(𝑞).

Since the concrete form of the polarizability function is unknown, a Taylor series is applied to approximate the polarizability around the equilibrium position (𝑞 = 0). The atom position deviation during a molecular vibration are relatively small. Accordingly, only terms of first order must be considered resulting in

𝛼(𝑞) = 𝛼 0 + (
𝜇(𝑡) = 𝛼 0 𝐸 0 cos(𝜔 0 𝑡) + 1 2 ( 𝜕𝛼 𝜕𝑞 ) 0 𝑞 0 𝐸 0 cos[(𝜔 0 -𝜔 𝑞 )𝑡] + 1 2 ( 𝜕𝛼 𝜕𝑞 ) 0 𝑞 0 𝐸 0 cos[(𝜔 0 + 𝜔 𝑞 )𝑡] .
Eq. 2.23

The influences of the diverse scattering mechanisms can directly be seen in the single addends of the rearranged time-dependent dipole moment µ(t). The first addend represents the Rayleigh scattering, the second addend gives the Stokes-scattering and the third describes the Anti-stokes

𝛼 𝑣 0 →𝑣 1 𝑡𝑟𝑎𝑛𝑠 = ∫ 𝜙 1 * 𝛼 ̂𝜙0 . +∞ -∞
Eq. 2.24

The vibrational wave functions ϕ 0 and ϕ 1 arise from the Schrödinger equation of the harmonic oscillator (Eq. 2.10) as presented within chapter 2.1.1:

𝜙 0 (𝑞) = ( 𝑚𝜔 ℏ𝜋 ) 1 4 𝑒 (- 𝑚𝜔 2ℏ 𝑞 2 )
Eq. 2.25

𝜙 1 (𝑞) = 2 √2 𝜋 1 4 ( 𝑚𝜔 ℏ ) 3 4 𝑞𝑒 (- 𝑚𝜔 2ℏ 𝑞 2 ) .
Eq. 2.26

From Eq. 2.24 -Eq. 2.26 it becomes obvious that a concrete solution of the transition momentum integral is a difficult task, especially since the normal coordinate 𝑞 can be arbitrarily complicated for large molecules. Moreover, the concrete character of the polarizability operator 𝛼 ̂ is unknown. However, symmetry investigations of the vibrational wave functions and the operator are already sufficient to predict if a molecular vibration is Raman-active or -inactive.

A molecular vibration is Raman-active if 𝛼 𝑣 0 →𝑣 1 𝑡𝑟𝑎𝑛𝑠 is unequal to zero. Accordingly, the symmetry of the polarizability transition momentum function must be even so that 𝛼 𝑣 0 →𝑣 1 𝑡𝑟𝑎𝑛𝑠 ≠ 0 (compare 2.1.2). In case of 1 -dimensional wave functions, the symmetry of the transition momentum integral could be easily determined by multiplying the parities (+1 for even functions, -1 for odd functions) of the single wave functions and the operator resulting in the parity of transition momentum integral function. If the parity of the transition momentum integral function is -1, the function symmetry is odd (-𝑓(-𝑥) = 𝑓(𝑥)) and the integral turns zero. On the other hand, if the total parity is +1, the function symmetry is even (-𝑓(𝑥) = 𝑓(𝑥)) resulting in a 𝛼 𝑣 0 →𝑣 1 𝑡𝑟𝑎𝑛𝑠 ≠ 0. However, the variable of vibrational wave functions is the normal coordinate 𝑞. 1 The symmetry of the vibrational wave functions are related to the occurrence of the normal coordinate within the wave function. The wave function of the vibrational ground state ϕ 0 contains the normal coordinate as q 2 only (Eq. 2.25).

Consequently, ϕ 0 transforms always like the total symmetric irreducible representation of the point group. On the other hand, ϕ 1 contains additional to q 2 a linear factor 𝑞. Since q 2 results in a total symmetric transformation, the vibrational wave function ϕ 1 transforms always like 𝑞. 1 Finally, the polarizability operator 𝛼 ̂ transforms like the basic functions x 2 , y 2 , z

𝑫 ∞𝒉 𝑬 𝟐𝑪 ∞ 𝝓 … ∞𝝈 𝒗 𝒊 𝟐𝑺 ∞ 𝝓 … ∞𝑪 𝟐 𝛤 𝜙 0 𝛼 ̂𝜙1 𝒂 𝟏𝒈 1 1 … 1 1 1 … 1 𝒂 𝟏𝒈 1 1 … 1 1 1 … 1 𝒂 𝟏𝒈 1 1 … 1 1 1 … 1 Product 1 1 … 1 1 1 … 1 𝒂 𝟏𝒈
𝑫 ∞𝒉 𝑬 𝟐𝑪 ∞ 𝝓 … ∞𝝈 𝒗 𝒊 𝟐𝑺 ∞ 𝝓 … ∞𝑪 𝟐 𝛤 𝜙 0 𝛼 ̂𝜙1 𝒂 𝟏𝒈 1 1 … 1 1 1 … 1 𝒆 𝟏𝒈 2 2 cos ϕ … 0 2 -2 cos ϕ … 0 𝒂 𝟏𝒈 1 1 … 1 1 1 … 1 Product 2 2 cos ϕ … 0 2 -2 cos ϕ … 0 𝒆 𝟏𝒈
𝑫 ∞𝒉 𝑬 𝟐𝑪 ∞ 𝝓 … ∞𝝈 𝒗 𝒊 𝟐𝑺 ∞ 𝝓 … ∞𝑪 𝟐 𝛤 𝜙 0 𝛼 ̂𝜙1 𝒂 𝟏𝒈 1 1 … 1 1 1 … 1 𝒆 𝟐𝒈 2 2 cos 2ϕ … 0 2 2 cos 2ϕ … 0 𝒂 𝟏𝒈 1 1 … 1 1 1 … 1 Product 2 2 cos 2ϕ … 0 2 2 cos 2ϕ … 0 𝒆 𝟐𝒈
Finally, the product of the irreducible representations Γ ϕ 0 α ̂ϕ1 for the CO2 symmetric stretching vibration can be given as

𝛤 𝜙 0 𝛼 ̂𝜙1 = 𝑎 1𝑔 ( 𝑎 1𝑔 𝑒 1𝑔 𝑒 2𝑔 ) 𝑎 1𝑔 = ( 𝒂 𝟏𝒈 𝒆 𝟏𝒈 𝒆 𝟐𝒈 ) Eq. 2.29
The product contains the total symmetric irreducible representation 𝑎 1𝑔 of the 𝐷 ∞ℎ symmetry point group. Therefore, the polarizability transition moment integral 𝛼 𝑣 0 →𝑣 1 𝑡𝑟𝑎𝑛𝑠 is unequal to zero and thus the CO2 symmetric vibration is Raman-active. In case of the CO2 asymmetric stretching vibration, the product of the irreducible representations Γ ϕ 0 α ̂ϕ1 is

𝛤 𝜙 0 𝛼 ̂𝜙1 = 𝑎 1𝑔 ( 𝑎 1𝑔 𝑒 1𝑔 𝑒 2𝑔 ) 𝑎 1𝑢 = ( 𝒂 𝟏𝒖 𝒆 𝟏𝒖 𝒆 𝟐𝒖 ) Eq. 2.30
𝛤 𝜙 0 𝛼 ̂𝜙1 do not contain the total symmetric representation 𝑎 1𝑔 . Thus, the CO2 asymmetric stretching vibration is Raman-inactive.

Both, the classical as well as the quantum mechanical treatment of the

Surface Enhanced Raman Spectroscopy

As already mentioned within the previous section, the possibility that a photon is Raman scattered at a molecule is 10 -8 -10 -5 and thus very low. Furthermore, the scattered photon carries only the information of a single vibrational mode and it must find the way to the detector. For

Raman spectra analyses, however, it is necessary that the spectra contain all molecular vibrations with sufficient intensities. To fulfill these requirements, a relatively large amount of substance is needed to guarantee that enough Raman scattered photons reach the detector.

Electromagnetic Enhancement

The electromagnetic theory of SERS is based on an electric field enhancement provided by the metal nanostructures. If a metal surface is placed within the electric field of incident light, the valence electrons start to oscillate. In general, the collective oscillation of the valence electrons relative to the atom nuclei can be mathematically described as a quasi-particle called plasmon.

In case of metal nanoparticles, the collective oscillation of the valence electrons is called localized surface plasmon (LSP). If a metal nanoparticle interacts with light, the valence electrons are forced towards the particle edges in opposite direction to the electric field vector of the incident light 𝐸 ⃗ 𝑖𝑛 . Due to the displacement of the valence electrons with respect to the atomic nuclei in the crystal lattice, a charge separation between positively charged atom nuclei and a negatively charged electrons occurs (Figure 2.8, left). Consequently, a restoring force 𝐹 𝐶 arises from Coulomb attraction. 19 This force induces an oscillation of the valence electrons.

Because of the collective oscillation of the valence electrons, an electric field is created at the nanoparticle surface parallel towards the electric field vector 𝐸 ⃗ 𝑖𝑛 of the incident light. The intensity of this electric field is strongly enhanced compared to the intensity of the incident light. The strongest field enhancement occurs if the LSP of the metal nanoparticle oscillates in resonance with the incident light (localized surface plasmon resonance, LSPR). The LSPR frequency depends on material properties like the electron density, the effective electron mass and the size and shape of the charge distribution. 19 Besides material properties, also the size and shape of the nanoparticles influences the LSPR frequency. 19 The metal nanoparticle size, however, must be in the order of the penetration depth of the incident light and thus significantly smaller than its wavelength. 20 The LSPR frequencies can be experimentally determined via extinction (absorption) spectroscopy. Generally, the LSPR occurs as broad peak within such spectra. Thus, the LSPR of nanoparticles formed from given materials with given shapes and sizes occurs maximal at wavelengths related to the strongest optical extinctions in the extinction Chapter 2 spectra. According to Mie-theory, the extinction spectrum E(λ) and thus the range of LSPR frequencies of spherical nanoparticles can be calculated by

𝐸(𝜆) = 24𝜋 2 𝑁𝑎 3 𝜀 𝑜𝑢𝑡 3 2
𝜆 𝑙𝑛 (10) [ 𝜀 𝑖 (𝜆) (𝜀 𝑟 (𝜆) + 𝜒𝜀 𝑜𝑢𝑡 ) 2 + 𝜀 𝑖 (𝜆) 2 ] ,

Eq. 2.31 with 𝜆 is the wavelength of the incident light, N is the number of nanoparticles, a is the nanoparticle radius, ε 𝑜𝑢𝑡 is the dielectric constant of the environment, ε r (λ) and ε i (λ) are the real and imaginary components of the metal dielectric function ε 𝑖𝑛 (λ) and χ is a shape factor which is 2 for spherical nanoparticles. [21][22] Both experimental results and theoretical calculation show that the LSPR wavelengths for silver and gold nanoparticles are in the VIS range. Due to this fact and their relative stability against oxidation, silver and gold nanostructures are typically used for SERS experiments. 22 Anyway, the electric field E 𝑜𝑢𝑡 at and close to the surface of a spherical nanoparticle excited by z -polarized light E 0 can be calculated using a quasi-statistic approximation allowing to replace Maxwell's equations of electrodynamics by the Laplace equation of electrostatics. 19,22 Following the previously presented approximations, theoretical enhancement factors for spherical silver nanoparticles can be calculated appearing within a range between 10 4 -10 5 . 22 Higher-ordered structures can even reach theoretical enhancement factors of 10 8 and higher. 22,25 The EFs can further be increased by placing molecules between metal nanoparticle aggregates reaching EFs of 10 10 -10 14 which allow single molecule investigations. [25][26][27][28][29] However, theoretical approximations deal normally with perfectly smooth nanostructure surfaces without any sub-structures. If sub-structures formed from single metal atoms are drawn under consideration, the field enhancement is significantly higher leading to theoretical maximal Raman intensity 𝐸 4 enhancements of 10 12 directly at the sub-structure of silver nanoparticle surfaces as shown by Trautmann et al. 30 The strong field enhancement at the atomic substructures is explained as a consequence of an atomic lightning rod effect. [31][32] The lightning rod effect appears related to the LSPR at ellipsoid nanoparticles resulting in a stronger field enhancement at the ellipsoid tips. 33 Anyway, as already mentioned, the enhanced field is highly

Chapter 2 localized at the nanoparticle surface and decays rapidly with increasing distance. 34 The range of field enhancement was found to be less than 8 nm and 90 -95 % of the maximum signal enhancement decreases within 10 nm. [34][35][36][37][38][39][40] The extreme field enhancement found at atomic substructures decays even faster. Thus, the magnitude of the field enhancement and Raman intensity enhancements can be found within a distance less than 1 nm from the atomic feature. 30 The field decay plays an important role for SERS investigations of sub-micro or nano composites. Since the magnitude of the enhanced field can be found within a range smaller than Raman spectra. This is firstly related to the conformations, surface molecules may adopt, leading to an increase or decrease of the polarizability and thus to more or less intense peaks in the Raman spectra. Further, the orientation of the surface molecules of nanoparticles towards the field vector of the enhanced field influences the Raman intensity. In contrast to far-field Raman spectroscopy where a lot of possible alignments between scattering molecules and the electric field vector are summed up in one spectrum, SERS spectra contain less alignments.

Due to these facts, SERS spectra can differ -sometimes strongly -from far-field Raman spectra. Anyway, the strong field enhancement and the resulting Raman intensity enhancement provided by the LSPR of metallic nanoparticles allow already Raman investigations of organic nanoparticles.

Chemical Enhancement

Although the electromagnetic enhancement gives with Raman intensity enhancements of 10 6 -10 10 the lion's share in SERS, the theory of chemical enhancement should be shortly discussed. The chemical enhancement is with enhancement factors between 10 2 and 10 4 less effective than the electromagnetic enhancement, however, it provides intrinsic features recognizable in band position shifts and intensity changes in Raman spectra. 41 The theory of Chapter 2 chemical enhancement can be subdivided into a non-resonant chemical effect and a resonant charge transfer chemical effect. [41][42] The non-resonant chemical effect is related to the molecule conformation which changes if a molecule is absorbed on a nanoparticle's surface. Like surface molecules of organic crystals as described in chapter 2.3.1, the conformation alteration of absorbed molecules can lead to slight shifts of band positions in Raman spectra. Moreover, the conformation change can magnify the polarizability of certain vibrational modes and thus increase the Raman intensities. The resonant charge transfer chemical effect is related to the resonance Raman effect. In contrast to the "normal" non-resonant Raman effect, the wavelength of the incident light coincides with the energy difference between the electronic ground to the first (or even higher) excited electronic state of the molecule (Figure 2.9, left). Since an electronic transition occurs more probable from the electronic ground state to an excited electronic state instead to a virtual state, the Raman intensities are enhanced. 15 ). 41 Chapter 2

during the electronic excitation are enhanced. 43 In case of the resonant charge transfer chemical effect, molecules are chemisorbed at the nanoparticle surface. If the Fermi level of the nanoparticle is close to the energy level of the frontier orbitals of the molecule, they can form a metal-molecule (or molecule-metal) electronic charge transfer (CT) state (Figure 2.9, right). 41 If the wavelength of the incident light is in coincidence with the energy difference between the molecular ground state and the CT state, the Raman intensities of certain molecular vibrations can be enhanced as described above. Since the chemical enhancement is not inevitably related to a LSPR, it can appear also on non-metallic nanostructures. 44 Anyway, if metal nanoparticles like silver or gold nanoparticles are used for SERS, both the electromagnetic and the chemical enhancement can occur simultaneously. 25,41 Chapter 2

Tip Enhanced Raman Spectroscopy

Although SERS allows Raman investigations of small amounts of samples like single nanoparticle, however, it does not provide information about size and morphology of the particles. To overcome this last limitation, SERS must be combined with a microscopic technique providing a spatial resolution allowing to depict single nanoparticles. Scanning probe microscopy (SPM) methods like atomic force microscopy (AFM) or scanning tunneling microscopy (STM) are perfectly suited for this purpose. 34,[45][46][47] The combination of both methods is called tip enhanced Raman spectroscopy (TERS) which was firstly performed by Stöckle et al. in 2000. 48 Since the experiments presented in chapter 3 and in chapter 4 were undertaken using AFM-TERS, only AFM-TERS measurement methods and setups are presented within this chapter. Anyway, to combine SERS with a SPM technique, the used probe tip must provide a plasmonic feature like a single silver or gold nanoparticle mounted at the tip apex or the sharp tip must consist completely of these materials to promote a lightning rod effect. Furthermore, the tip apex must be illuminated by a laser with a wavelength matching the LSPR of the plasmonic feature. Typically, silver tips are used for excitation wavelengths between 488 nm and 568 nm, whereas gold tips are used for wavelengths between 633 nm and 785 nm. 34 Moreover, the geometry of the TERS setup by meaning of the alignment between the illumination optics and the tip must be chosen so that the sample is placed within the enhanced electric field area at the silver or gold nanoparticle surface. Also, the SPM techniques have certain measurement modii providing specific advantages and disadvantages. The different TERS setup geometries, diverse TERS tip production procedures and TERS applications are presented in the following.

TERS Illumination Setups

Three different illumination geometries are typically used for AFM-TERS experiments. These are the bottom-illumination, the side-illumination and the top-illumination setups. Each illumination setup has its specific advantages and disadvantages regarding the field enhancement at the tip apex, the laser focus, the background signal and the character of the sample being examined. Anyway, the first TERS experiments were undertaken using the bottom-illumination setup. 34,[48][49][50][51] In this illumination mode, an inverse optical microscope equipped with an oil-immersion objective with a high numerical aperture (NA, typically 1.4 -1.6) illuminates the sample from the bottom (Figure 2.10, left). Using high NA immersion-oil objectives involves two advantages. Firstly, these objectives generate small laser foci providing high laser intensities on the one hand and reducing far-field Raman signals to a minimum on the other hand. 34,52 Secondly, high NA oil-objectives refract the incident light under a relatively large angle (compared to small NA) resulting in a total reflection of the outer parts of the laser beam at the glass substrate, creating an evanescent filed polarized parallel toward the tip axis. Due to the evanescent field and the strongly refracted laser beams, the enhanced electric field at the tip apex is mainly polarized along the tip axis and thus the excitation of Raman scattering of the sample beneath the tip is maximized. The field enhancement can be further increased in bottom-illumination setups by using radially polarized laser light. 34,[53][54] Raman scattered light is collected in back-reflection mode by the same immersion-oil objective as used for sample illumination. Since the excitation light and the Raman scattered light must travel through the sample, only transparent samples are accessible for TERS in bottom-illumination setups. Furthermore, the samples may not exceed a certain thickness because the evanescent field created at the glass substrate surface contributes mainly to the LSPR polarized along the tip axis. 55 Since evanescent fields do spread in the direction of the electric field vector, they decay strongly with respect to the distance from the surface. Thus, TERS samples should not be thicker than 100 -150 nm (depending on the wavelength of the incident light). Although this limitation, the bottom-illumination is the most efficient setup for TERS investigations. 34 Due to its high efficiency, the bottom-illumination setup was used for structure investigations of organic nanoscale composites as presented within chapter 3 and 4. Chapter 2

In contrast to the bottom-illumination, side-illumination allows TERS investigations of opaque and thicker samples. 34,[56][57][58][59][60][START_REF] Marquestaut | Imaging of single GaN nanowires by tip-enhanced Raman spectroscopy[END_REF] As already indicated by the name, the incident light illuminates the TERS tip from the side and above the sample within an angle between 45° -80° (Figure 2.10, middle). 34,52,[START_REF] Stadler | Developments in and practical guidelines for tipenhanced Raman spectroscopy[END_REF] Long working distance objectives with smaller NAs (0.28 -0.7) have to be used in side-illumination since the objective cannot brought close enough to the tip to use short working distance objectives with higher NAs. Long working distance objectives provide broader focused laser spots. Furthermore, the angle between sample and illumination optics deforms the laser spot to an ellipse at the sample. Both, the broader laser focus and its elliptical character increase the laser spot size resulting in a higher contribution of far-field signals in the TERS spectra. 52 Furthermore, the increased spot size provided by long working distance objectives compared to short working distance objectives requires higher laser powers to receive equivalent results as in bottom-illumination setups. In contrast to bottom-illumination geometries, strongest field enhancements at the tip apex are obtained for using linear polarized light along the tip axis. 34,54 Raman scattered light can be collected via the same objectives in back-reflection mode for all samples or in bottom-collection mode for transparent samples. 52 As like as in side-illumination setups, also top-illumination setups allow TERS measurements on opaque samples. The illumination of the metal tip apex and the sample take place from above perpendicular to the sample surface (Figure 2.10, right). 52 Also here, long working distance objectives with small NAs (compared to bottom-illumination mode) are used. [START_REF] Berweger | Signal limitations in tip-enhanced Raman scattering: the challenge to become a routine analytical technique[END_REF][START_REF] Stadler | Nanoscale chemical imaging using top-illumination tip-enhanced Raman spectroscopy[END_REF] This results in broader laser spots and thus in a higher contribution of far-field signals in TERS spectra. The far-field contribution is, however, smaller than for side-illumination setups since the incident light strikes the sample perpendicular forming a circular laser spot instead of angled resulting in an elliptical spot. Anyway, the components of the incident light being polarized perpendicular to sample (parallel to the tip axis) are less pronounced in bottom-illumination setups compared to bottom-and side illumination geometries. Hence, also the enhanced field at the tip apex is less polarized perpendicular to the sample leading to weaker TERS signals.

Furthermore, special AFM-probes providing an angle between tip and tip holder should be used to reduce shadowing effects and thus further TERS signal losses. 34 Anyway, top-illumination setups can be easier integrated into standard AFM microscope systems compared to the other illumination geometries. 34 In sake of completeness, it should be mentioned that a fourth, less commonly used setup exists, the parabolic mirror illumination. Here, the sample is illuminated from the bottom but in contrast to common bottom-illumination, a parabolic mirror focuses additionally the incident laser light onto the sample and the tip apex. 52, 65-67

TERS Tip Preparation Methods

One of the most important (if not the most important) element in TERS is the tip. As already mentioned above, it must provide a plasmonic feature like a nanoparticle localized at the tip apex which fulfill the requirements so that a LSPR can be induced by the incident light.

However, the sharp tip can also consist completely of a plasmonic material like silver and gold.

In this case, the field enhancement at the tip apex is generated mainly by the lighting rod effect. 33,45 Anyway, one of the most common methods to produce TERS active tips is undertaken by vacuum deposition of thin metal layers (< 50 nm) onto commercially available silicon or silicon-nitride AFM tips using thermal evaporation or sputtering techniques. 34,[48][49][50][START_REF] Bailo | Tip-Enhanced Raman Spectroscopy of Single RNA Strands: Towards a Novel Direct-Sequencing Method[END_REF][START_REF] Hayazawa | Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy[END_REF][START_REF] Deckert-Gaudig | A Modified Transmission Tip-Enhanced Raman Scattering (TERS) Setup Provides Access to Opaque Samples[END_REF][START_REF] Yeo | Enhancement of Raman signals with silver-coated tips[END_REF][START_REF] Hayazawa | Highly reproducible tip-enhanced Raman scattering using an oxidized and metallized silicon cantilever tip as a tool for everyone[END_REF][START_REF] Yang | The controlled fabrication of "Tip-On-Tip" TERS probes[END_REF][START_REF] Asghari-Khiavi | Exploring the origin of tip-enhanced Raman scattering; preparation of efficient TERS probes with high yield[END_REF] Vacuum deposition of silver or gold results in a metal coating onto the AFM tips characterized by the formation of metal islands. Due to the geometry of sharp AFM tips, a single silver respectively gold nanoparticle is formed at the tip apex. The formation of single silver or gold nanoparticles at the tip apex can be supported by a subsequent annealing of the metallized tips. [START_REF] Hübner | Surface Characterization of Nanoscale Co-Crystals Enabled through Tip Enhanced Raman Spectroscopy[END_REF][START_REF] Hübner | Formation Mechanism of Anisotropic RDX-TNT Core-Shell Nanoparticles and their Influence onto Nanodiamond Detonation Syntheses[END_REF] However, silver and especially gold nanoparticles do not adhere strongly to the silicon surface of the tips. Hence, if the metalized tips are exposed to mechanical stress, the nanoparticle positioned at the tip apex may get lost rendering the tip TERS inactive. This problem can be handled by coating the tips previously with an adhesive layer consisting of chromium or titanium. 52,[START_REF] Hübner | Surface Characterization of Nanoscale Co-Crystals Enabled through Tip Enhanced Raman Spectroscopy[END_REF][START_REF] Hübner | Formation Mechanism of Anisotropic RDX-TNT Core-Shell Nanoparticles and their Influence onto Nanodiamond Detonation Syntheses[END_REF][START_REF] Park | Tip-Enhanced Raman Scattering with a Nanoparticle-Functionalized Probe[END_REF][78][START_REF] Weber-Bargioni | Hyperspectral nanoscale imaging on dielectric substrates with coaxial optical antenna scan probes[END_REF] Next to vapor deposition techniques, AFM tip apexes can be equipped chemically with a single metal nanoparticle. Different chemical procedures are reported in literature including inter alia electrochemical deposition, electroless chemical deposition, silver mirror reactions and photoinduced reactions. 34,52,[START_REF] Yang | Rational Fabrication of a Gold-Coated AFM TERS Tip by Pulsed Electrodeposition[END_REF][START_REF] Huang | Tip-Enhanced Raman Spectroscopy: Tip-Related Issues[END_REF][START_REF] Wang | Tip-enhanced Raman spectroscopy with silver-coated optical fiber probe in reflection mode for investigating multiwall carbon nanotubes[END_REF][START_REF] Barsegova | Controlled fabrication of silver or gold nanoparticle near-field optical atomic force probes: Enhancement of second-harmonic generation[END_REF][START_REF] Coluccio | Plasmonic 3D-structures based on silver decorated nanotips for biological sensing[END_REF][START_REF] Wang | Controllable method for the preparation of metalized probes for efficient scanning near-field optical Raman microscopy[END_REF][START_REF] Wang | Controllable method for the preparation of metalized probes for efficient scanning near-field optical Raman microscopy[END_REF][START_REF] Umakoshi | Fabrication of Near-Field Plasmonic Tip by Photoreduction for Strong Enhancement in Tip-Enhanced Raman Spectroscopy[END_REF] Furthermore, TERS active tips can be prepared by picking up gold nanoparticles previously coated with a glue from a flat substrate. 34,52,[START_REF] Park | Tip-Enhanced Raman Scattering with a Nanoparticle-Functionalized Probe[END_REF][START_REF] Kalkbrenner | A single gold particle as a probe for apertureless scanning near-field optical microscopy[END_REF] Moreover, etching and focus ion beam milling procedures for TERS active tip preparation are also reported in literature. [START_REF] Hartschuh | Nanoscale optical imaging of single-walled carbon nanotubes[END_REF][START_REF] Neacsu | Scanning-probe Raman spectroscopy with single-molecule sensitivity[END_REF][START_REF] Paulite | Full Spectroscopic Tip-Enhanced Raman Imaging of Single Nanotapes Formed from β-Amyloid(1-40) Peptide Fragments[END_REF][START_REF] Zhang | Single molecule tip-enhanced Raman spectroscopy with silver tips[END_REF][START_REF] Wang | Tip-enhanced Raman spectroscopy for investigating adsorbed species on a single-crystal surface using electrochemically prepared Au tips[END_REF][START_REF] Ren | Preparation of gold tips suitable for tip-enhanced Raman spectroscopy and light emission by electrochemical etching[END_REF][START_REF] Chen | Tipenhanced Raman scattering of p-thiocresol molecules on individual gold nanoparticles[END_REF][START_REF] Roy | Single crystal gold tips for tip-enhanced Raman spectroscopy[END_REF][START_REF] Kharintsev | Near-field optical taper antennas fabricated with a highly replicable ac electrochemical etching method[END_REF][START_REF] Xu | Constant current etching of gold tips suitable for tip-enhanced Raman spectroscopy[END_REF][START_REF] Kharintsev | Electrochemical design of plasmonic nanoantennas for tip-enhanced optical spectroscopy and imaging performance[END_REF][100][101][102] Within this work, TERS active tips were produced by sputtering a thin layer of silver onto a commercially available AFM probe.

Before silver sputtering, the AFM tips were previously coated with a titanium adhesion layer.

Afterwards, TERS tips were annealed to promote the formation of a single silver nanoparticle at the tip apex. This procedure is described in more detail within chapter 3 and 4.

Special Features of TERS

Since TERS is in principle the combination of SERS and AFM, the same special features are expected to occur in TERS spectra like in SERS spectra. In contrast to SERS, however, where a various number of metal nanoparticles are localized within the laser spot and thus recorded spectra contain information of all molecules close or in direct contact to these nanoparticles' surfaces, only a single nanoparticle providing the necessary field enhancement is placed in the laser focus in TERS. Accordingly, the intrinsic features are more prominent in TERS than in SERS. Especially the orientation between the investigated molecules and the direction of the enhanced field which is mainly aligned along the tip axis in bottom-illumination setups influences strongly TERS spectra. Thus, molecular vibrational modes aligned parallel to the tip axis are maximally enhanced. 34 Vice versa, vibrational modes aligned perpendicular to the polarization of the enhanced field are less or even not enhanced and may miss in TERS spectra.

This phenomenon is observable at in-plane modes of molecules lying flat on the substrate. 34,38,[103][104][105][106] The orientation of vibrational modes towards the polarization of the enhanced field is for example of pivotal importance in TERS investigations of nanoscale co-crystals as shown and described in more detail within chapter 3. Although, presented TERS experiments are exclusively performed in non-contact AFM mode and thus the TERS tip is normally not in direct contact to the sample surface, tip-sample interactions may further influence TERS spectra. Hence, temporal fluctuations in Raman band intensities and positions are reported assigned to molecular diffusions and orientation changes during the measurements. 34,[START_REF] Zhang | Single molecule tip-enhanced Raman spectroscopy with silver tips[END_REF][107][108][109] If the tip should "accidently" touch the sample, the sample surface can be deformed and chemical interactions between the tip and sample molecules may arise. Both, the surface deformation and chemical interactions can cause further changes in molecular orientation and conformation leading to modified selection rules and alternating Raman spectra. 34,[110][111][112] Since the enhanced field arises of a single metal nanoparticle (or of the apex of a sharp metal tip) and due to the rapid field decay, TERS provides a high spatial resolution (< 1 nm). 30,[113][114] Hence, only vibrational modes of a few surface molecules contribute to TERS spectra of organic nanoparticles. As already described (see chapter 2.2.2 and 2.3.1), molecular conformations of surface molecules are supposed to be different than that in the bulk. Thus, TERS spectra of nanoscale organic crystals can differ strongly from conventional far-field Raman spectra as presented in the following chapters.

Chapter 3

3 Surface Characterization of Nanoscale Co-Crystals Enabled through Tip Enhanced

Raman Spectroscopy

Before the first TERS structure investigations of nanoscale composites could be performed, an appropriate measurement mode had to be found. During the first TERS measurements on single compound nanoparticles, it turned out already that contact AFM measurement methods are not suited for this purpose. Due the direct contact between tip and sample, nanoparticles or particle fragments were often picked up contaminating the TERS tip. Hence, Raman signals were recorded at positions were no nanoparticle could be found in AFM scans. Accordingly, the dynamic non-contact mode was chosen for TERS measurements. Furthermore, a reproducible tip production procedure had to be found. Both, the measurement mode and the tip production procedure are presented in more detail in the following. After these "teething problems" could be eliminated successfully, the first nanoscale composite, a co-crystal consisting of CL-20 and HMX, could be investigated. Since this co-crystal was the first co-crystal ever characterized by TERS and thus no reference data existed, it figured out a challenging task. Anyway, TERS investigations gave new insights into the surface composition of these nano co-crystals allowing to draw conclusions about their reaction behavior. Moreover, a simplified 3D approximation of vibrational normal coordinates could be introduced enabling predictions about the intensity ratio of normal modes in TERS spectra.

(This chapter is published at: Hübner, J.; Deckert-Gaudig, T.; Glorian, J.; Deckert, V. K.;

Introduction

Co-crystals are crystal systems built up from two or more chemical compounds. 1 Due to different intermolecular interactions like hydrogen bonding, van der Waals forces or π-π stacking chemical compounds can align in a specific manner during the crystallization process which is energetically more favored than single compound crystals. These co-crystals often differs in their physical properties e.g. solubility, melting point and mechanical properties from their origin single compounds. 2 Hence co-crystals have found their usage in chemical industry like the pharmaceutic sector. Here, for example, adding specific co-formers can improve the rate of dissolution of active pharmaceutical ingredient and thus accelerate the pharmaceutic effects. [2][3] Next to pharmaceutic industry co-crystallization has been also started to be applied for engineering of energetic materials. Especially in this field it is necessary to combine two properties which often behave diametrically opposed. These properties are safety characteristics like impact or friction sensitivity and a high energy release. One of these compounds showing a very pronounced diametrical behavior between energetic performance and stability is

.5.0.0 3,11 .0 5,9 ]dodecane (CL-20). CL-20 is one of the most energy-rich known organic explosives which was firstly synthesized by Nielsen in 1987. [4][5] Unfortunately, CL-20 is not only characterized by its high detonation velocity but also by its relatively high sensitivity against impact and friction. Next to diverse methods like adding of binders, nano-structuring, or micro-capsulation to or of CL-20 crystals, co-crystallization of CL-20 with diverse inert or even energetic co-formers could successfully decrease sensitivites. [6][7][8][9][10][11][12] One of the most promising of these compounds is a co-crystal between CL-20 and the energetic cyclic nitroamine 1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX) in a molar ratio of 2 : 1 (CL-20/HMX) which was firstly synthesized by Bolton et al. in 2012. 13 CL-20/HMX demonstrates perfectly the synergy between its single components; thus the co-crystal shows a higher detonation velocity than pure HMX which is close to CL-20 and nearly the sensitivity values of HMX. [13][14][15] Additional downsizing onto sub-micron or nano-scale of CL-20/HMX co-crystal improves further the sensitivity. Here, An et al. could show that nano CL-20/HMX particles are even less sensitive than pristine HMX. 16 However, up to now it is not completely understood how the co-crystal structure effects on performance and sensitivity.

A dispersion-corrected density functional theory (DFT-D) study undertaken by Liu et al.

reveals that the CL-20/HMX co-crystal is more compressible than its pristine compounds. 17 Though the increased compressibility can explain the improved sensitivity for structure defectfree single crystals it does not include the influences of defect sites in micro or nano-structured CL-20/HMX powders leading to the formation of hot-spots. Since crystal surfaces are defect sites per se, crystal morphology and surface condition participate strongly to the sensitivities of energetic materials especially if they are nano-structured. [18][19][20] Therefore, a detailed study of these parameters might give a deeper insight into the impact sensitivity of nano CL-20/HMX co-crystals. Next to its extraordinary pyrotechnical characteristics CL-20/HMX was chosen additionally as the co-crystal sample system of this study since its structure properties are already well investigated.

The method of choice for this purpose is Tip-Enhanced Raman Spectroscopy (TERS) based on Atomic Force Microscopy (AFM; combined: AFM-TERS). AFM-TERS combines the lateral resolution of AFM with the advantages of Surface Enhanced Raman Spectroscopy. Hereby the apex of an AFM tip is equipped with a silver or gold nanoparticle. This modified tip is then brought close to the sample surface and becomes irradiated by a laser. The irradiation leads to a strong electric field enhancement close to the particle surface which rapidly decays within a few nanometers. [21][22][23][24] This strong field enhancement allows to perform Raman spectroscopy on small amounts of substances like single nanoparticles, molecular monolayer or even single molecules. [25][26][27][28][29][30] Furthermore, the fast decay of the field enhancement makes TERS to an extremely surface sensitive measurement method. AFM-TERS enables a simultaneous recording of the sample height and the corresponding Raman spectrum at single measurement points. These point measurements can be assembled to a topographic and spectrographic map depicting particle morphology and composition. Hereby lateral resolutions < 1 nm can be reached. 21 A further reason making TERS a perfectly suited tool for surface or structure investigations of nano energetic materials is the relatively low energy input compared to common techniques like electron microscopy which is necessary since organic molecules especially energetic organic molecules tend to dissociate under the electron beam. The suitability of AFM-TERS for investigations of energetic nano-composites could be already demonstrated by structure determination of RDX/TNT core/shell and patchy nanoparticles. [31][32] In this work investigated CL-20/HMX nano co-crystals were synthesized by the Spray Flash Evaporation process. This process was invented at the NS3E lab and enables the continuous production of organic nanoparticles which are used in pharmaceutic and energetic sector. [33][34] During the process a pressurized solution of one or more components is sprayed through a heated hollow cone nozzle into an evacuated reaction chamber. The strong pressure drop forces Chapter 3

an immediate evaporation of the solvent leading to the release of the desired nanoparticles.

These particles are collected in a filter system. Due to the composition of the precursor solution a wide variety of nano-structured products can be obtained. If the precursor solution consists only of a solvent and a single solved compound, the SFE reactor acts as a recrystallizer providing single compound submicron-or nanoparticles. 15,[35][36] If two or more compounds are solved in the precursor solution the obtained products depend strongly on the intermolecular interactions. Nano-structured products can occur e.g. as uncoupled but physically perfectly mixed single compounds, core/shell nanoparticles or nano co-crystals. 15,31,[37][38][39] One of these nano co-crystals systems which can be produced by SFE is the already introduced CL-20/HMX co-crystal. Synthesis details, X-Ray powder diffraction-(XRPD), AFM, Raman-and AFM-TERS investigations of this nano co-crystal system are presented in the following sections.

Preparation of CL-20 γ -Polymorph

The γ -polymorph of CL-20 (γ-CL-20) was fabricated by forcing a thermal induced phase transition from the є -phase into the γ -phase. Therefore ~ 10 mg of dried є-CL-20 were evenly spread onto a microscope glass slide. Afterwards the prepared slide was heated in an oven for 3h by 160 °C. A comparable route is described by Thiboutot et al. 40

Preparation of CL-20 β -Polymorph

The CL-20 β -polymorph (β-CL-20) was synthesized using the Spray Flash Evaporation (SFE) process which is described in more detail elsewhere. [33][34] during the process. After the complete β-CL-20 acetone solution was sprayed, the final product was collected from the walls of the filter system as a white-yellowish powder.

TERS Tip Preparation

AppNano ACCESS-NC AFM-probes (Mountain View, USA) were used for TERS-active tip production. Since the tips of the used probes are mounted onto the cantilever at an angle of 120° (Figure 3.2.a), the probes were fixed onto a 30° prism template to generate a perpendicular arrangement between the tip and the sputtering target. This template was installed in the vacuum chamber of an HHV Auto 306 (Bangalore, India) argon plasma sputtering system at a distance of 10 cm from the sputtering target. In a first step, 3 nm of titanium were deposited onto the tip at a deposition rate of 0.5 Å/s. Titanium is used as an adhesive promoter between the native oxide layer of the Si -tip and silver. Afterwards the Si/Ti -tips were coated with a 25 nm layer of silver (sputtering rate 0.5 Å/s). Finally, the Si/Ti/Ag -tips were annealed on a heating plate for 1 min at 300 °C under an argon atmosphere. The SEM image in Figure 3.2.b) shows the Chapter 3 successful formation of a single Ag nanoparticle on the tip apex after the manufacturing procedure. Atomic force microscopy (AFM) was carried out on an AIST-NT CombiScope TM -1000SPM atomic force microscope (Novato, USA) in non-contact mode. All samples were applied onto glass cover slides, by depositing a spatula's tip between two 2 cm x 2 cm cover glasses. The

Analysis Methods

X-ray powder diffraction (XRPD) analysis in

Simulation of Molecular Normal Modes

Molecular normal modes were computed through computational chemical calculations using Gaussian 16. 41 The use of density functional theory (DFT) with a B3LYP-6-31G(d,p) basis set was chosen since it is suitable for quite large systems such as CL-20. DFT B3LYP-6-31G(d,p) relevancy for energetic materials was demonstrated by Osmont et al. 42 For each studied molecules, the most stable geometry was obtained from the minimization of the ground state energy. All vibrational frequencies were checked to be positive.

Results and Discussion

XRPD and AFM

After SFE synthesis the final product was collected as a dry, ultra-fine, white-yellowish powder Moreover, the measured particle morphology resembles strongly to the simulated shapes, though particle dimensions in the substrate plane are three to four times bigger. These differences hint towards crystal structure defects. A possible explanation for these defect sites is a non-classical growth mechanism of the nano-sized CL-20/HMX plates via oriented aggregation. In this case a classical formation of primary nanoparticles with the simulated dimensions is the first step. Afterwards these primary particles accumulate along similar crystal faces and aggregate to the measured secondary n-CL-20/HMX particles. A comparable formation of hierarchal superstructured ZnO submicron particles has been described in detail by Klaumünzer et al. 43 Since HR-TEM investigations were not possible due to dissociation of CL-20/HMX triggered by the relatively high energy input through the electron beam, a direct evidence of the suggested building mechanism cannot be given here. However, particle heights can be aligned to the crystallographic [300] -direction regarding to results of AFM and XRPD. Contrary, Ghosh et al. assign the crystal height of platelet macro CL-20/HMX crystals to crystallographic [111] -direction because of weakest short contacts between the molecules in this crystallographic direction. 14 These weak short contacts lead to the slowest growing rate in this direction and hence to a domination of the crystal morphology by (111) -planes. On the other hand, Ghosh et al. used a slow evaporation rate of 1 mL/min to grow CL-20/HMX crystal with a size around 200 µm, this slow evaporation method is hardly comparable to the ultra-fast SFE process. Therefore, other crystallographic planes can determine n-CL-20/HMX nano-plate morphology due to massive differences in thermodynamic and kinetic circumstances during the co-crystal growth process. Furthermore, the height of the simulated nano-crystal is extended in [300] -direction and measured AFM heights and (1598 cm -1 , literature: 1600 cm -1 ) and n-CL-20/HMX (1602 cm -1 ) spectra. 14,45 Moreover, in the region between 1580 cm -1 and 1620 cm -1 no vibrational mode is found in pristine β-HMX far-field Raman spectrum. Thus, the broad peak at 1600 cm -1 in n-CL-20/HMX spectrum can be clearly assigned to NO2 asymmetric stretching vibrations of the diverse CL-20 conformations. The NO2 asymmetric stretching vibrations of the two CL-20 conformations broaden in the n-CL-20/HMX Raman spectrum compared to the same vibration in the spectra of pristine CL-20 polymorphs because of a superposition of single NO2 asymmetric stretching vibrations of both conformations. Furthermore, no signals appear in the region between 1400 cm -1 and 1500 cm -1 in β-CL-20 and γ-CL-20 spectra. As a consequence, the CH2 out of plane wagging vibration of β-HMX localized at 1416 cm -1 (literature: 1418 cm -1 -1422 cm -1 )

was selected as the HMX marker band. [46][47][48] This vibrational mode is detected slightly blue-shifted at 1417 cm Chapter 3 vibrational modes appear evenly distributed in TERS spectra over all particles, although they appear slightly shifted and differ in their intensities (Figure 3.10). The shifts may appear because of the missing molecular layer at the particle surface leading to a change of intermolecular interactions such as Van-der-Waals forces or hydrogen -bonds. The lack of these interactions leads to changes in molecular arrangements to the energetically most favorable conformation. 30 Consequently different arrangements influence the polarizabilities and the energy levels of the vibrational states implying a shift of the detected normal modes.

Also interactions between the Ag nanoparticle located at the tip apex and the surface molecules have been shown to influence the band positions. 25 However, the detectability of all vibrational modes clarifies that TERS can demonstrate the co-crystallinity of nanoparticles due to the even distribution of Raman signals over all measured particles. Moreover, all TERS spectra contain CL-20 and HMX marker bands (1602 cm -1 ; 1416 cm -1 ). If no co-crystal formation but the formation of other hierarchical arranged nano-particle would occur (e.g. core/shell particles), superimposed CL-20 and HMX far-field Raman spectra could be misinterpreted as CL-20/HMX co-crystal signature due to the less surface sensitivity of far-field Raman spectroscopy. Contrary to far-field Raman TERS is able to identify these particle arrangements due to its surface sensitivity demonstrated recently on RDX/TNT core/shell nano-particles. 31 This renders TERS a powerful tool for the investigation of co-crystalline systems, especially if it is not possible to grow these new co-crystals to a necessary size for single crystal X-ray diffraction. By using SFE process which is designed for recrystallization on the sub-micro and nano-scale, this necessary size will not be reached. On the other hand, it is supposed that this rapid crystallization process may force two (or more) compounds into specific co-crystal arrangements which they would not adopt in slow crystallization processes. Though, such a nano co-crystal growth may occur, integral methods like XRPD and far-field Raman spectroscopy may lead to wrong assumptions because signal superpositions or the arising of new signals can be misinterpreted as new co-crystal structures. However, as already mentioned TERS spectra differ from far-field Raman spectra not only in slight band positions shifts but also in intensity changes. One particular reason for this effect is the fixed orientation of the investigated molecules towards the electric field vector of the enhanced electric field at the Ag Chapter 3 appear more intense in TERS spectra. 22,27,[50][51] If one compares the intensity ratio between the CL-20 ONO asymmetric stretching vibration and the HMX CH2 wagging vibration, an inversion of the intensity ratio from ~ 2.5 : 1 to ~ 1 : 1.5 (CL-20 : HMX) on average is recognizable within TERS spectra compared to far-field Raman-spectra. This inversion is illustrated in the TERS maps in Figure 3. appear green-yellowish to green instead of red-orange in TERS maps. This is indeed interesting and unexpected, since none of the investigated n-CL-20/HMX agglomerates show such an intensity ratio inversion in far-field Raman maps. As described above it is assumed that n-CL-20/ HMX nano-plates aggregate semi-orientated parallel towards their crystallographic bc -planes. Thus, it is highly likely that one of this agglomerates will be arranged in a manner that the bc -planes of single n-CL-20/HMX nano-plates are aligned parallel towards the substrate plane, respectively the xy -plane of the scattering system. Thus, the measured intensity ratio inversion should not arise due to a specific orientation.

Approximated Normal Coordinate Analysis

To verify this presumption, an approximated qualitative investigation of the arrangement between the n-CL-20/HMX nano-plates and the present TERS scattering system is necessary at this point. For this purpose, approximations of the normal coordinates of the investigated normal modes of β-CL-20, γ-CL-20 and β-HMX have to be found in a first step. This approximation is formed by the following three assumptions. Firstly, the norms of the vectors play only a subordinate role here, since only their orientations towards the electric field of the incident light are of interest. Thus, they are normalized. Secondly, only atoms showing a relatively strong derivation from their equilibrium position are to be considered in this approximation. Thirdly, all x -, y -and z -components of the 3N dimensional normal coordinates, where N represents the number of the investigated oscillating atoms, can be summed up -resulting in a three-dimensional vector -since the assumption is not focused on the emitted electromagnetic field. Therefore, the detected Raman intensities of the investigated Normalization was done to ensure a comparability between CL-20 and HMX normal modes.

The summation of 𝑞 0;𝛽-𝐶𝐿-20 and 𝑞 0;𝛾-𝐶𝐿-20 is necessary due to the molar ratio of 2:1 between CL-20 and HMX in the CL-20/HMX co-crystal system. Regarding these facts, the approximation of the normalized 𝑞 0; 𝛽-𝐻𝑀𝑋 can be expressed as

𝑞 0;𝛽-𝐻𝑀𝑋 = 1 2 ( ∑ 𝑞 ⃗ 𝑚;𝛽-𝐻𝑀𝑋-1 4 𝑚=1 𝑛 𝛽-𝐻𝑀𝑋-1 + ∑ 𝑞 ⃗ 𝑚;𝛽-𝐻𝑀𝑋-2 4 𝑚=1
𝑛 𝛽-𝐻𝑀𝑋-2

) . Eq. 3.4

The approximated vectors 𝑞 0; 𝐶𝐿-20 and 𝑞 0; 𝛽-𝐻𝑀𝑋 are shown in larger than the tip-surface distance. 13 Regarding this fact, the surface layer is always located in a field region showing a stronger Raman enhancement than the second molecular layer (Figure

Proposed Mechanism for Ignition by Impact

The HMX surface finishing may contribute to explain why the impact sensitivity of CL-20/HMX is much closer to HMX than to CL-20 (Table 3.1). [13][14][15] Up to now it is assumed that the relative low impact sensitivity of CL-20/HMX arises inter alia due to an enhanced compressibility of the co-crystal system compared to its pure compounds. This increased compressibility was shown by a DFT-D study undertaken by Liu et al. 17 Therefore, input impact energy are able to dissipate easier over the crystal lattice and thus more energy is needed to overcome the bond-dissociation energies of CL-20 and HMX. Moreover, a profusely electron the compressibility of the CL-20/HMX co-crystal system is higher than those of its pristine compounds, a higher amount of impact energy should be necessary to induce the dissociation of CL-20 and HMX in the co-crystal as in its pure compounds. However, impact sensitivity experiments were not able to validate this assumption. [13][14][15] Therefore, it should be considered that at the measured impact energies an ignition of CL-20/HMX does not take place in the bulk material. Thus, the following mechanism is proposed. In a first step input impact energy which The adapted conformations will lead to a surface energy minimization but should be energetically less favored than that of pure β-HMX. Furthermore, energy transfers between compressed air and HMX surfaces may contribute to the impact ignition mechanism. For the sake of completeness, it should be mentioned that An et al. show a less sensitivity against impact of nano sub-structured CL-20/HMX micro particles. 16 This improved sensitivity may be caused by the underlying 3D-networks and a possible interaction between these networks and included air. Anyway, the proposed mechanism would explain the nearly similar impact sensitivities of CL-20/HMX and HMX without contradiction to previously found theoretical and experimental data.

Chapter 3

Conclusion

The

Raman Active Vibrational Frequencies and Assignments

In the following Raman spectra and tables of Raman active vibrational frequencies of β-CL-20, γ-CL-20, β-HMX, CL-20/HMX as well as their assignments are presented.

β-CL-20

Raman frequencies of β-CL-20

Experiment / cm -1 Literature / cm γ-CL-20

Raman frequencies of γ-CL-20

Experiment / cm -1 Literature / cm The approximated normal coordinate of both HMX conformations can be expressed as the average of 𝑞 0;𝐻𝑀𝑋-1 and 𝑞 0;𝐻𝑀𝑋-1 : Finally, a correlation between the TNT shell thickness, the symmetry of the anisotropic hexolite precursor nanoparticles, and the resulting nanodiamond sizes is discussed in detail.

Raman frequencies of β-HMX

𝑞 0;𝛽-𝐻𝑀𝑋 = 1 2 ( ∑ 𝑞 ⃗ 𝑚;𝛽-𝐻𝑀𝑋-1 4 𝑚=1 𝑛 𝛽-𝐻𝑀𝑋-1 + ∑ 𝑞 ⃗ 𝑚;𝛽-𝐻𝑀𝑋-2 4 𝑚=1 𝑛 𝛽-𝐻𝑀𝑋-2 ) = (

Introduction

Since the first detonation synthesis of nanodiamonds in the USSR back in the 1960s, nanodiamonds have become a focal point of interest in various fields of science and industries. 1- 2 Functionalized nanodiamonds found their use inter alia in biomedical approaches as non-toxic compounds for biomedical imaging or as drug and gene delivery agents. [3][4][5][6][7][8] In addition to the medical approaches, nanodiamonds are used in electronic, chemical, material and energetic applications. [9][10][11][12][13] Probably due to the broad field of application and the constantly growing demand for nanodiamonds, research increasingly aims at gaining insights into the mechanism of nanodiamond formation in detonation syntheses. [14][15][16][17][18][19][20][21][22] A detailed knowledge of the building mechanism is crucial for the adaption of nanodiamond sizes and size distribution associated with their application. Furthermore, especially the structure of the precursor material influences strongly the resulting nanodiamonds. Pichot et al. demonstrated that nanoscale RDX and TNT (hexolite) precursor particles result in the formation of nanodiamonds characterized by smaller mean sizes and narrower size distribution as obtained by employing micron sized precursor materials. [23][24] Using nanostructured RDX and TNT precursor particles, they obtained nanodiamonds defined by particle mean sizes between 2.8 nm -4.2 nm depending on the precursor compositions. Since the precursor structure affects strongly the resulting nanodiamonds different nanoscale hexolite mixtures of various RDX and TNT mass ratios are investigated in this study. All presented nano hexolites were produced by Spray Flash Evaporation (SFE). This technique allows the continuous production of organic submicron-and nanoparticles. [25][26] During the SFE process a preheated and pressurized solution containing one (or more) different components is sprayed through a hollow cone nozzle into a permanently evacuated reaction chamber. The strong pressure and temperature difference in front and behind the nozzle implement an immediate evaporation of the dissolved micro droplets resulting in the release of submicron or nanoparticles. Depending on the composition of the dissolved precursors a huge variety of submicron or nanoscale particles can be obtained. [23][24][26][27][28][29][30][31][32] However, the characterization of SFE-produced hexolite composites turned out to be quite difficult using common measurement methods for several reasons. On the one hand, electron microscopy and energy dispersive X-ray spectroscopy failed to determine the structure and composition of single hexolite particles although these techniques allow structure analysis of other energetic materials. 33 Due to the thermal instability of RDX and TNT, hexolite, nanoparticles tend to dissociate under the electron beam. Other standard methods like infrared and far-field Raman spectroscopy or X-ray diffraction may provide molecular and crystallographic information but due to the lack of an adequate sensitivity and resolution, the characterization of single nanoparticles remain impossible. Furthermore, X-ray photoelectron spectroscopy (XPS) failed to investigate structures and chemical compositions of SFEproduced hexolite mixtures since nanostructured TNT sublimates rapidly under the high vacuum conditions of XPS. Thus, AFM-based tip-enhanced Raman spectroscopy (TERS) was chosen for hexolite nanoparticle characterization since it provides ultrahigh sensitivity and spatial resolution at a low energetic load on the sample (compared to electron microscopy methods). In AFM-TERS a common cantilever tip is coated with silver (or gold) nanoparticle and irradiated with a laser wavelength that matches the plasmon resonance of the nanoparticles.

The incident laser light causes a collective oscillation of the conductive electrons within the silver nanoparticle. This so called localized surface plasmon resonance (LSPR) leads to an enhancement of the electrical field close to the silver particle surface, which rapidly decays within a few nanometers. [34][35][36][37] The strong field enhancement enables spectroscopic investigations of nanoparticles, molecular monolayers and even single molecules. [32][33][38][39][40][41][42][43][44] Furthermore, Deckert-Gaudig et al. successfully analyzed hexolite RDX/TNT core/shell nanoparticles with TERS. 44 In this contribution, the synthesis and characterization of diverse SFE produced anisotropic RDX/TNT core/shell nanoparticles and inhomogeneous RDX/TNT nanoparticles are presented. Furthermore, a building mechanism of these nanoparticles including the spectroscopic data and the dynamics of the SFE process is proposed. Finally, the influence of the precursor structure onto the resulting nanodiamonds is discussed in detail.

Experimental Section

Production of Hexolite Mixtures

Diverse hexolite mixtures were produced via Spray Flash Evaporation (SFE) which is described in detail elsewhere. 25,29 A flow chart of the SFE setup as well as a photography of the used SFE reactor are depicted in Figure 4.1. For hexolite mixture production 2 wt.% solutions of various RDX and TNT mass fractions in 500 mL acetone were prepared. Thus, the mass of RDX plus TNT amounted 8.2 g in each solution. The amounts of RDX and TNT for each hexolite mixture solution are summed up in Table 4.1. Each solution was given into the solution storage tank of the vertical SFE -reactor. A pressure of 40 bar was applied onto the solution storage tank. Afterwards the RDX-TNT-acetone solutions were sprayed through a preheated ruby hollow cone nozzle (160°C) with a diameter of 200 μm into the permanently evacuated atomization chamber. During the spraying, the pressure inside the reaction chamber amounted 2 -5 mbar. Final hexolite products were collected as white-yellowish powders whereby the yellowish shade increases slightly with the amount of TNT. In Table 4.1 the hexolite mixtures are named by the mass ratios of RDX and TNT and the first number gives the percentage of RDX and the second one the percentage of TNT (e.g. 80/20).

Production of Physically Mixed Hexolite Reference Samples

A reference sample using a 60/40 hexolite mixture was prepared through physical mixing.

60 mg of RDX nanoparticles and 40 mg of TNT nanoparticles were transferred into a porcelain mortar and mixed carefully and pressureless with a porcelain pestle. Pure RDX and TNT nanoparticles were also prepared via SFE. In a first step 8.2 g (36.7 mmol) RDX, respectively 8.2 g (35.9 mmol) TNT were solved in acetone in order to obtain 4 wt.% solutions.

Recrystallization of pure RDX and pure TNT were performed using the same SFE set-up and settings as described above.

Nanodiamond Detonation Syntheses

Each SFE-produced hexolite mixture was pressed to cylinders with lengths and diameters of 15.8 mm at room temperature for 10 minutes. These cylinders were cemented afterwards with pyrotechnic glue. The cylindrical explosive charges were transferred into a water-filled pocket before firing in a detonation tank. Obtained nanodiamonds were dried via rotation evaporation.

Analysis Methods

Confocal far-field Raman spectroscopy investigations were performed with a HORIBA (Kyoto, Japan) LabRam HR evolution confocal Raman microscope. In the experiment a spatula's tip of the diverse hexolite mixtures was applied to a glass slide. Single point spectra were obtained by sample excitation with a linear polarized illumination light from a 532 nm diode laser with an adjusted output power of 13.5 mW at an acquisition time of 3 s per spectrum.

Raman mappings across the substrate were recorded with the same laser and output power. The acquisition time was set to 0.5 s per spectrum with a step size of 250 nm for these mappings.

The laser light was focused onto the sample through a 100x, 0.9 NA objective. Raman scattered photons were collected with the same objective in back reflection geometry. Scattered light passed an edge filter, a confocal aperture with a diameter adjusted to 100 nm and a diffraction grating with 300 lines/mm before entering a deep cooled CCD camera (-60°C). Confocal

Raman maps were calculated and depicted using LabSpec Spectroscopy Suite 6.

Spectroscopy

In a first step, far-field Raman spectra of pristine RDX and TNT were recorded to obtain information about the structure composition of hexolite mixtures. Marker bands of each compound were chosen from these spectra. These marker band were detected in wavenumber regions were the other compound did not show a signal. The distinguishability of RDX and TNT molecules was already demonstrated in an earlier work and the same RDX and TNT marker bands were chosen. 44 The RDX marker band at 882 cm -1 can be assigned to the ring breathing vibration. [44][45][46][47] The TNT marker band between 1358-1371 cm -1 can be assigned to three NO2 symmetric stretching vibrations. 44,[48][49] studies. 32,44 TERS surface maps each containing 100 TERS spectra (10 x 10 spectra) with step sizes between 2 nm -5 nm (depending on the nanoparticle size) were recorded. 15 nanoparticles of each hexolite mixture were investigated. Thus, differences between far-field Raman spectra and TERS spectra of hexolite nanoparticles allowed to conclude on the distinct arrangement of RDX and TNT molecules in the particles. The RDX marker band (882 cm -1 ; ring breathing) is highlighted in green. The TNT marker band (1358 cm -1 -1371 cm -1 ; three NO2 symmetric stretching vibrations) is marked in red. interactions. 32 The different molecular arrangements result in slight changes of the molecular energy levels and thus, molecular vibrations can be detected with slightly shifted band positions in TERS spectra. 50 This phenomenon is comparable with Raman frequency shifts in Raman spectra of different organic crystal polymorphs. [51][52][53] Furthermore, interactions between surface molecules and the silver coated tip may contribute to a band position shift as described above. 38,54 In contrast, the far-field Raman spectra of the bulk material contain averaged information from all molecule orientations. Discrepancies of Raman intensities of same vibrational modes between far-and near-field Raman spectra can be explained by the orientation between illuminated molecules towards the electric field vector of the enhanced electric field. In general normal modes appear more intense if the vibrational direction and the electric field vector are Chapter 4 aligned parallel. 35,40,[55][56] However, TERS spectra of p-60/40 nanoparticles contain all relevant peaks of RDX or TNT and can be consequently easily assigned to the specific compound (Figure 4.4). The RDX marker band is detected between 870-890 cm -1 in TERS spectra of RDX nanoparticles. Also, the TNT marker band is well identifiable in a wavenumber range between 1350 cm -1 and 1375 cm -1 . The shape of this broad peak can differ in TERS spectra compared to far-field Raman spectra since it contains several symmetric NO2 stretching vibrations which can be enhanced differently due to specific orientations as described above. In contrast to nano hexolite mixtures produced by SFE only six particles p-60/40 were investigated by TERS, since a molecular intermixture of RDX and TNT could be already excluded by confocal Raman microscopy. Thus, a statistical more reliable investigation was not necessary at this point.

However, three RDX and three TNT nanoparticles could be clearly identified by TERS. 4.2). Since some NO2 modes of RDX and TNT overlap in the TERS spectra, only clearly assignable vibrational modes are used for detailed analysis. In RDX two N-NO2 stretching vibrations are localized at 1270 cm -1 and 1309 cm -1 , respectively, and one NO2 is localized at 1592 cm -1 in SFE-80/20 far field spectra. The RDX N-NO2 stretching vibrational modes appear slightly shifted within a wavenumber range between 1256 cm -1 and 1283 cm -1 respectively 1296 cm -1 and 1322 cm -1 in SFE-80/20 TERS spectra. The RDX NO2 asymmetric vibration can be found between 1585 cm -1 and 1622 cm -1 . The TNT marker band containing three clearly identifiable Raman active NO2 symmetric stretching vibrations (between 1358 cm -1 and 1371 cm -1 in far-field

Raman spectra) appears shifted between 1354 cm -1 and 1399 cm -1 in TERS spectra. The TNT NO2 asymmetric stretching vibration localized at 1617 cm -1 in far-field Raman spectra is shifted between 1617 cm -1 and 1653 cm -1 in near-field TERS spectra. Generally, RDX N-NO2 vibrations appear with a shift of ± 14 cm -1 less shifted than the RDX NO2 asymmetric stretching vibration, which are detected within + 30 cm -1 . The TNT symmetric and asymmetric stretching vibrations show a wavenumber shift of up to 31 cm -1 and 36 cm -1 , respectively. Described shifts are supposed to appear due to intermolecular interactions between the NO2 functional groups of RDX on the one hand and NO2 functional groups of TNT on the other hand. Since the RDX N-NO2 vibrations seem to be less affected by these interactions than the RDX NO2 asymmetric stretching vibration, it is supposed that negatively charged oxygen atoms of the RDX NO2 functional groups interact electrostatically with positively charged nitrogen atoms of the TNT NO2 functional groups. If the electrostatic interaction would appear between the positively charged nitrogen atoms of the RDX NO2 functional group and the negatively charged oxygen atoms of the TNT NO2 functional groups, shifts of RDX N-NO2 stretching vibrations are expected stronger than shifts of TNT and RDX NO2 stretching vibrations. The described intermolecular electrostatic interactions force RDX respectively TNT molecules in specific molecular arrangements which differ from their arrangements in the bulk material. Thus, vibrational modes are slightly shifted in the TERS spectra compared to far-field Raman spectra. 44 The authors demonstrated that the surfaces of 5 out of 8 SFE-60/40 nanoparticles were dominated by a TNT contribution. RDX and TNT marker bands were found in 3 of 8 SFE-60/40 TERS maps. By comparing far-field Raman and TERS spectra it was concluded that RDX cores were encapsulated by TNT shells. In the case of SFE-60/40 TERS maps showing RDX and TNT marker bands it is supposed that the thickness of the TNT shells is only a few nanometers.

Thus, vibrations of TNT surface molecules and RDX molecules close to particle surfaces are identified in the TERS spectra. TERS spectra containing both RDX and TNT marker bands show similar band position shifts as already discussed in the SFE-80/20 section above and point building mechanism of such anisotropic core/shell nanoparticles will be discussed in detail in the next section. Directly after the injection of the hexolite solution droplets into the evacuated reaction chamber, the concentrations of RDX and TNT molecules within these microscale droplets are still the same as it is in the previously prepared acetone solutions. The continuous evaporation of acetone during SFE process and the related droplet shrinking results in a consistent increase of the RDX and TNT concentrations within the microscale droplets until the solution is supersaturated with RDX molecules. The supersaturation level of RDX molecules within the hexolite solution droplets is reached first since RDX exhibits with 6.8 g in 100 g acetone a sixtime less solubility than TNT with 42.4 g per 100 g acetone at room temperature. 44 Following the classical theory of nucleation and crystallization, RDX molecules organizes to nuclei while the concentrations of RDX and TNT molecules further increase (Figure 4.9.c). 57 of RDX within the hexolite precursor solutions allowing a complete TNT surface coverage.

While the TNT concentration increases within the droplets due to the related droplet size shrinking, RDX cores are coated with a TNT shell following the previously described mechanism. Additionally, further RDX core growths are quenched by the TNT encapsulations.

Up to now, the presented reaction mechanism allows to explain the occurrence of different particle structure and sizes. However, the building mechanism does not include an explanation 34 Therefore the contribution of RDX Raman active frequencies to diverse TERS spectra must be related to the position of the RDX cores within the hexolite RDX/TNT core/shell nanoparticles. As mentioned before, the previous building mechanism describes the hexolite nano-composite formation without considering the translational and angular accelerations of the droplets in the SFE atomization chamber. Without these accelerations it can be assumed that RDX primary nanoparticles would be located randomly within the bulk of the droplets without stronger movements until the complete evaporation of acetone is finished resulting in the formation of isotropic RDX/TNT core/shell nano-spheres with a centered RDX core. However, during SFE process droplets are accelerated due to the release of the pressurized precursor solution into the evacuated atomization chamber through a hollow cone nozzle, the interactions of acetone droplet surface molecules with the nozzle surface while leaving the nozzle, turbulences in the atomization chamber and the flow generated by the vacuum pump.

The first steps of the hexolite core/shell nanoparticle building mechanism by means of RDX superstation and nucleation within the micron sized droplets take place as already described for the non-accelerated case. After the phase change of solved RDX into solid RDX nuclei and the aggregation of these RDX nuclei into primary RDX nanoparticles, the acceleration of the droplets starts to influence the formation of the hierarchical order hexolite nanoparticles. Due to the translational accelerations (by nozzle and pump, Figure 4.9.c) of the shrinking dynamic micro reactor droplets, a fictitious force affects the RDX primary nanoparticles pushing them back towards the edge of the droplets. This fictitious force is qualitatively the same fictitious force which presses a passenger into the seat of a car while the car is accelerated. Additionally, the inhomogeneous acceleration of the droplet while leaving the nozzle and turbulences within the SFE crystallization chamber induce a spin of the acetone micro droplets. This droplet spin is described by an angular acceleration of the droplet around itself. Consequently, a centrifugal force affects the RDX primary nanoparticles within the droplet pushing them towards the edge of the acetone droplet (Figure 4.9.c). The combination of both effects forces RDX primary particles to looping motions starting in the droplet bulk and ending at the droplet surface.

Similar looping motions and particle migrations are described by Chung for the motion of coal particles inside accelerated n-hexane respectively methanol micro droplets. 59 has to be slightly reconsidered.

For this purpose, an ideal case which is defined by monodispersed isotropic RDX/TNT core/shell nanoparticles forming a hexagonal close-packed precursor material after pressing is discussed first (Figure 4.13). After ignition and detonation, RDX reaction products are accelerated from the particle centers outwards whereas TNT reaction products are accelerated both towards the particle center and away from the particle center. The migration of RDX and TNT reaction products is driven by their specific detonation pressures. Depending on the RDX and TNT mass fractions which define the RDX core diameter and the TNT shell thickness, the competing migration effects can be dominated either by the RDX detonation pressure or by the TNT detonation pressure. Generally, RDX providing a higher detonation pressure than TNT.

In case of RDX detonation pressure domination, TNT reaction products are forced into the center of a tetrahedron while RDX reaction products form a spherical diffusion barrier around it. If TNT detonation pressure dominates the competing migration effects due to a relatively low RDX mass fraction, RDX reaction products are forced to spherical diffusion barriers around the center of a tetrahedron. The tetrahedrons arise from the hexagonal close-packing of the material and represents the smallest repetitive area. The sites of the tetrahedron are defined by the connection lines between the centers of two neighboring isotropic hexolite particles. Thus, four times a fourth of the TNT reaction products of one single hexolite core/shell particles contributes to one diffusion zone. Anyway, depending on whether the RDX or TNT detonation pressure dominates, different thermodynamic circumstances regarding the pressure and the temperature prevail within the diffusion zones. Since in case of the RDX detonation pressure dominance the TNT reaction products are forced towards the tetrahedron centers, pressure and temperature are higher within these resulting diffusion zones than for diffusion zones formed in a TNT dominated system. Higher pressures and temperatures promote a more complete conversion of carbon atoms into nanodiamonds leading consequently to larger nanodiamond diameters. Contrary, smaller pressure and temperature values result in a less amount of carbon Due to the size distribution of the hexolite precursor particles a perfect hexagonal close-packing is not possible.

The random arrangement of the anisotropic core/shell particles leads to the formation of more ellipsoid diffusion zones of slightly differing sizes. Consequently, diffusion zones contain different amounts of carbon atoms and nucleation sites. Therefore, synthesized nanodiamonds provide slightly different sizes leading to a nanodiamonds size distribution. Thus, nanodiamond mean sizes and size distribution depend on the TNT shell thickness, the hexolite precursor particle size, the hexolite particle size standard deviation and on the level of anisotropy within the precursor structure.

Chapter 4 atoms forming nanodiamonds and thus in smaller nanodiamond mean sizes. However, due to the high symmetry of ideal packed and perfectly monodispersed isotropic hexolite core/shell particles, the size of the diffusion zones and the thermodynamic circumstances within one diffusion zone is constant for all diffusion zones by a given RDX respectively TNT mass fraction. Consequently, the number of nucleation sites within the diffusions zones appears also constant and finally the number and sizes of obtained nanodiamonds within one diffusion zone (Figure 4.13). Therefore, the nanodiamond size depend only on the RDX mass fraction and the TNT shell thickness in the ideal case. Following the suggested mechanism for the ideal case it becomes obvious that the correlation between precursor particle size and nanodiamond size proposed by Pichot et al. remains untouched since it is not germane for estimating the nanodiamond size if the TNT carbon atoms which form the nanodiamonds arises from a single TNT nanoparticle or of four fourth RDX/TNT core/shell nanoparticles. However, under real conditions the hexolite precursor material consist of anisotropic core/shell particles exhibiting a size distribution. Due to the size distribution, a perfect symmetrical hexagonal close-packing is not accomplishable even if a dense packing of the hexolite precursor material is undertaken before detonation synthesis (see Experimental section). Moreover, the anisotropy of the core/shell precursor particles (by meaning of the RDX core is not centered in the middle of the particle) causes a further decrease of the symmetry within the precursor structure. The symmetry decrease leads to formation of diffusion zones respectively diffusion barriers with more ellipsoid character and different dimensions (Figure 4.13). As a consequence, the thermodynamic and kinetic circumstances as well as the amount of TNT carbon atoms within the single diffusion zones differ leading to a varying number of nucleation sites. This results in slightly different nanodiamond sizes and thus in a nanodiamond size distribution.

Consequently, the nanodiamond mean size 𝑑 𝑛𝑑 of the obtained nanodiamonds depends not only on the RDX mass fraction 𝜔 𝑅𝐷𝑋 and the TNT shell thickness 𝑑 𝑇𝑁𝑇 but also on the parameters defining the symmetry of the precursor material. These parameters are the hexolite precursor particle mean size 𝑑 ℎ𝑒𝑥 , the hexolite precursor size standard deviation 𝑠 ℎ𝑒𝑥 and the level of anisotropy 𝑎 ℎ𝑒𝑥 within the precursor structure. Thus 𝑑 𝑛𝑑 can be assumed by the following correlation: thickness 𝑑 𝑇𝑁𝑇 within an anisotropic hexolite core/shell nanoparticle should be as small as possible in order to receive the smallest nanodiamonds (Eq. 4.1) but both parameters behave inversely proportional to each other (due to the varying RDX mass fractions), SFE-40/60 seems to provide the best mixture between its TNT shell thickness 𝑑 𝑇𝑁𝑇 and its level of anisotropy 𝑎 ℎ𝑒𝑥 .

𝑑 𝑛𝑑 ~

Conclusion

Different hexolite mixtures with various RDX/TNT mass ratios were produced via Spray Flash The influence of the hexolite precursor structure on the size and the distribution of the resulting nanodiamonds was discussed in detail. It is proposed that the TNT shell thickness, the symmetry reduction by meaning of the level of anisotropy (position of the RDX core center related to the hexolite particle center) and the precursor size distribution determine the resulting nanodiamond mean size. Further studies are necessary to verify the correlation between these parameters and the nanodiamond mean size. For this purpose, more RDX/TNT mass ratios have to be investigated to find the perfect relationship between TNT shell thickness and the anisotropy level within the RDX/TNT core/shell nanoparticles. This way, the size distribution of nanodiamonds produced from the detonation of hexolite mixtures could be further reduced.

2-Methyl-1,3,5-trinitrobenzene (TNT)

In the following measured Raman active frequencies of pristine 2-Methyl-1,3,5-trinitrobenzene (TNT) are summed up, assigned and compared with literature. Particle size distributions of all SFE produced hexolite nanocomposites were estimated by Atomic Force Microscopy. For this purpose, the heights of 200 nanoparticles of each hexolite mixture were measured. The results, the fitted size distributions and the particle mean sizes (dhex) of each hexolite sample are presented in Figure 4.17. Figure 4.17 of single nanoparticles could not be proven, the particle was considered within the statistics.

An AFM topographic map of one of these typical agglomerates (SFE-20/80) is depicted in

Measurement of Droplet Size and Velocity in the SFE Process

In order to obtain information about the acetone droplets size evolution and their axial and radial velocities as functions of the distance from the nozzle during the Spray Flash Evaporation (SFE) process, Phase Doppler particle analysis (PDPA) measurements were performed on a pure acetone spray using the same SFE conditions as presented in the experimental section of the main text (nozzle diameter: 80 μm; nozzle temperature: 160 °C; pressure: 40 bar). The used PDPA device consists of a FlowExplorer 2D (Dantec Dynamics, Danemark), a HiDense PDA Chapter 4

For a given RDX/TNT mass fraction ratio, the RDX and TNT masses of a hexolite core/shell nanoparticle behave like:

𝑚 𝑅𝐷𝑋 𝑚 𝑇𝑁𝑇 = 𝜔 𝑅𝐷𝑋 𝜔 𝑇𝑁𝑇 . Eq. 4.6
Furthermore, the densities of RDX and TNT behave like:

𝜌 𝑅𝐷𝑋 𝜌 𝑇𝑁𝑇 = 𝑚 𝑅𝐷𝑋 𝑉 𝑅𝐷𝑋 𝑚 𝑇𝑁𝑇 𝑉 𝑇𝑁𝑇 = 𝑚 𝑅𝐷𝑋 𝑉 𝑅𝐷𝑋 • 𝑉 𝑇𝑁𝑇 𝑚 𝑇𝑁𝑇 . Eq. 4.7
The Volume of a typical spherical hexolite core/shell nanoparticle can be given as the sum of RDX core volume and TNT shell volume:

𝑉 ℎ𝑒𝑥 = 𝑉 𝑅𝐷𝑋 + 𝑉 𝑇𝑁𝑇 . Eq. 4.8
In order to calculate the mean radius 𝑟 𝑅𝐷𝑋 of the RDX core, Eq. 4.6 is rearranged to 𝑚 𝑇𝑁𝑇 and Eq. 4.7 is rearranged to 𝑉 𝑇𝑁𝑇 :

𝑚 𝑇𝑁𝑇 = 𝜔 𝑇𝑁𝑇 • 𝑚 𝑅𝐷𝑋 𝜔 𝑅𝐷𝑋 .
Eq. 4.9

𝑉 𝑇𝑁𝑇 = 𝑚 𝑇𝑁𝑇 • 𝑉 𝑅𝐷𝑋 • 𝜌 𝑅𝐷𝑋 𝑚 𝑅𝐷𝑋 • 𝜌 𝑇𝑁𝑇 . Eq. 4.10
Inserting Eq. 4.9 in Eq. 4.10 results in:

𝑉 𝑇𝑁𝑇 = 𝜔 𝑇𝑁𝑇 • 𝑉 𝑅𝐷𝑋 • 𝜌 𝑅𝐷𝑋 𝜔 𝑅𝐷𝑋 • 𝜌 𝑇𝑁𝑇 . Eq. 4.11
Inserting Eq. 4.11 in Eq. 4.8 and rearranging towards 𝑉 𝑅𝐷𝑋 result in:

𝑉 𝑅𝐷𝑋 = 𝑉 ℎ𝑒𝑥 1+ 𝜔 𝑇𝑁𝑇 𝜔 𝑅𝐷𝑋 • 𝜌 𝑅𝐷𝑋 𝜌 𝑇𝑁𝑇
. Eq. 4.12 By using the sphere volume (𝑉 = 4 3 𝑟 3 𝜋), 𝑟 𝑅𝐷𝑋 can be finally given as:

𝑟 𝑅𝐷𝑋 = 𝑟 ℎ𝑒𝑥 √1+ 𝜔 𝑇𝑁𝑇 𝜔 𝑅𝐷𝑋 • 𝜌 𝑅𝐷𝑋 𝜌 𝑇𝑁𝑇 3 .
Eq. 4.13

Finally, the mean diameter of the TNT shell thickness is:

𝑑 𝑇𝑁𝑇 = 𝑟 ℎ𝑒𝑥 -𝑟 𝑇𝑁𝑇 .
Eq. 4.14

Calculated RDX core radii and diameters for the diverse assumed isotropic hexolite RDX/TNT core/shell nanoparticles are summed up in Table 4.6. The density of RDX amounts 1.82 g/cm 3 , the density of TNT is given with 1.65 g/cm 3 . The previously presented studies confirm that TERS respectively near-field Raman investigations of nanoscale organic composites provide new insights into their structure, arrangement and surface composition. These data allow to derive formation mechanisms and on the one hand and on the other hand they provide additional information which help to describe their specific chemical and physical behaviors. As described within chapter 2.4.1, bottom-illuminated TERS setups are limited to thin and opaque samples only. Moreover, TERS experiments are extremely time and material consuming in order to obtain statistically reliable results. So how to analyze in an appropriate time larger particles which exceed the maximal thickness accessible to TERS? Hence, a different way had to be found to enable near-field Raman structure investigations on thicker samples like sub-micron composite particles.

Unfortunately, SERS experiments performed on typical SERS substrates cannot be applied for this purpose since incident light has to travel through the whole sample before it can couple to the LSPs. Accordingly, far-field signals containing information about the whole composite particle would dominate the Raman spectra and no conclusion about the surface composition could be drawn. Therefore, sub-micron composite particles consisting of Furosemide and PVP were directly coated with a thin layer of silver by sputtering. The thickness of the silver layer (40 nm) was chosen to correspond to silver nanoparticles providing a LSPR while interacting with laser light with a wavelength of 533 nm. In addition, a relatively high sputtering rate was adjusted to promote the formation of silver islands instead of a continuous silver layer. Strong

Raman intensities attest that an enhanced field is formed at the surfaces of the silver island.

Introduction

Within the last decades, sub-micron and nanoparticles have found their use in a huge variety of medical and pharmaceutic applications. Among others composite particles are commonly used as drug delivery agents, as drug carriers, as contrast agents in diagnostic techniques like magnetic resonance imaging and in cancer therapies. [1][2][3][4][5] In pharmaceutic research and industry, the downsizing of active pharmaceutical ingredients (API) on the sub-micron or nanoscale is paramount topic attracting more and more interests. In particular, downsizing of pharmaceutic particles would allow increasing the aqueous solubility and the dissolution velocity of API molecules due to the increased surface-to-volume ratio. [6][7][8] Clinical tests confirm that downsizing of APIs on the sub-micron or nanoscale improves their solubility, their dissolution velocity and thus their bioavailability within the human body. [8][9] The formulation of nanoscale APIs is often undertaken by the use of biocompatible polymers working as host materials confining the drug within the polymeric network or as nanocapsules embedding nanocrystalline or amorphous APIs. [10][11][12][13] The development and the design of improved amorphous solid API/polymer composites, arranged in a high surface area matrix, requires the parallel development of advanced spectroscopies and microscopies capable of chemical and molecular imaging on such fragile organic sub-micron particles. [14][15][16] This information would allow determining the very crucial structure-to-function relations for solubility, dissolution velocity and bioavailability. Conventional analytical techniques struggle to face this challenge, because of the need of combining high sensitivity, high chemical and molecular selectivity versus possibly similar organic molecules, imaging capability with high lateral resolution (below 100 nm) and a sufficiently low energy input in order to preserve APIs and prevent their dissociation, oxidation or internal recombination.

Not surprisingly, it has been recently reported that the lack of methods allowing for the precise characterization of molecular nanocomposite at the sub-micron scale is hampering the development of such systems in the pharmaceutical area. [17][18] The first reason is that the poor knowledge about particles surface composition hinders the understanding of their in vivo behavior which depends on the interaction between biological elements encountered in the body and the particle surface. 17,19 Secondly, for such increasingly complex molecular systems, the batch-to-batch consistency of needs to be validated with the highest accuracy, which is unfortunately not possible today with conventional methods. Therefore, the development of viable advanced characterization methods and protocols is very crucial for organic applied to investigate surface and structural properties of nanoscale organic composites. [20][21][22] However, up to now, SERS experiments were typically carried out by depositing the analyte onto special SERS substrates. [23][24][25] Typical SERS substrates consist of (precious) metal nanostructures (e.g. nanoparticles) providing a collective oscillation of their conductive band electrons in resonance with the incident light (localized surface plasmon resonances, LSPR) leading to a strong enhancement of the electric field close to the metal nanostructure surfaces. 23- 26 A multitude of SERS substrates and manufacturing procedures have been reported in literature providing for nanostructures with different sizes, shapes, materials and arrangements. 23,[27][28][29][30] However, the deposition of sub-micron composite particles on SERS substrates may not lead to the desired accuracy in measuring the particle structure and arrangement since SECoRM maps are recorded in reflection-mode. Accordingly, the fieldenhancing metal nanoparticles would be located below the sub-micron analyte particles from the view of the microscope objective. Thus, the incident light would have to travel through the whole analyte particle before interacting with the conductive band electrons of the metal nanoparticles in this configuration. Consequently, the far-field signals would dominate the Raman spectra. To overcome this limitation, in this study organic sub-micron particles are directly coated with silver nanoparticles deposited by sputtering. The so formed silver nano grains, better referred as silver spasers (surface plasmon amplification by stimulated emission of radiation), are directly formed on top of the analyte particle surface. 31 As a consequence of the strong local field enhancement the intensity of the incident laser light can be drastically reduced so that Raman spectra will be dominated by near-field signals. As a model system, results obtained on Furosemide/PVP sub-micron composite particles produced using the spray flash evaporation (SFE) process are presented. Furosemide is a poorly water-soluble loop diuretic and polyvinylpyrrolidone is a common biocompatible polymer. The technique has been previously described elsewhere and it has the advantage of allowing for the continuous production of sub-micron and nanoscale organic compounds and composites from the vacuum spraying of a pressurized and heated solution. 22,[32][33][34][35][36][37][38] 5.4 Results and Discussion Chapter 5 coherence lengths along the main crystallographic directions reveal that furosemide crystals dimensions are below 50 nm, i.e. about ten times smaller than the average particles size estimated from SEM analysis. From these results, we conclude that SEM particles correspond to large aggregates, however, no information is available on the arrangement of furosemide and PVP molecules within single sub-micron composite particles. To fill in this gap, far-field and SERS Raman spectroscopy is subsequently applied to investigate Furosemide/PVP sub-micron particles. and CL-20 and HMX layers in CL-20/HMX nanoscale co-crystals. [21][22] For Furosemide, the chosen marker band is the in-plane ring bending vibration at 686 cm -1 . 39 For PVP, the selected marker band is the most intense CH2 asymmetric stretching vibrations of the saturated PVP chains at 2924 cm -1 . 40 The far-field Raman spectra and the marker bands of pure Furosemide Since the silver spasers are in direct contact with the sample surfaces also chemical enhancements may contribute to the total enhancement. This may be evident in the CH2 asymmetric stretching vibration of PVP (marker band) occurring at the Stoke shift of 2924 cm -1 , equivalent to 630 nm, which seems to be additionally enhanced by a chemical interaction with the silver surfaces. Light of this wavelength does not (or only barely) induce a resonant oscillation of the localized surface plasmons of silver nanoparticles with a mean diameter of few tens of nm. 41 Therefore, Stoke shifted light emitted in this range is expected to be less enhanced as compared to resonant wavelengths (E 2 instead of E 4 enhancement). 26 Despite of this, the intensity ratio between the PVP marker band intensity and the next most intense band only decreases from about 5:1 (far-field spectra) to ~2:1 (in SERS spectra). Thus, it is supposed that a charge transfer between the silver surface and the absorbed PVP molecules may take place resulting in an amplification of the molecule polarizability. Interestingly, both ) due to the chemical interaction of PVP carbonyl functional group from with the silver surfaces. 40 In our case the broad features at 1600 cm -1 , and 1350 cm -1 appearing in both silver-coated Furosemide and PVP SERS spectra, are rather ascribed to the D and G bands of the sp 2 -hybridized carbon atoms from amorphous carbon species formed at the surface of the silver spasers during the exposure to the incident laser beam. More in details, the collective oscillation of the conductive band electrons induces the heating up of silver nanoparticles leading to the partial graphitization of organic molecules in the proximity of silver particles. In order to exclude that the organic material degradation occurs during the silver sputtering process, pure Furosemide and PVP were coated with a 5 nm thick titanium layer. Raman spectra of titanium-coated Furosemide and PVP show the absence of broad bands at 1600 cm -1 and 1350 cm -1 (Figure 5.5). Raman spectra from titanium-coated Furosemide and PVP samples differ from far-field spectra only by the presence of fluorescence signals arising from the titanium nano species formed at the organic particle surfaces (Figure 5.5). Anyway, since the origin of these broad peaks is related to amorphous carbon contamination, these bands can be removed by background subtraction for SERS spectra analysis.

Since a small amount of surface molecules contribute to most of the signal in SERS, band position shifts and the modification of relative intensities may occur due to electrostatic and chemical interactions with the metal nanoparticle surface inducing slight changes in molecular orientations and arrangement. [42][43][44][45] Despite of this, Furosemide and PVP marker bands identified in far-field spectra can still be applied in SERS to identify each compound in Furosemide/PVP composite particles. Chapter 5 must be sensibly smaller than what predicted from an ideal core/shell structure. In the SFE process, the mixed solution is sprayed through a preheated hollow cone nozzle into the constantly-pumped vacuum reactor. The micron sized aerosol droplets undergo a fast solvent evaporation when travelling from the injection nozzle to the particles filter. When the saturation point is reached, the less soluble compound will be the first to crystallize. Since PVP has a 100× higher solubility in the used ethanol/DCM solvent mixture (about 1000 g/L) respect to Furosemide (10 g/L), Furosemide is very likely to crystallize before PVP transform into its problem might be solved by protecting the sample surface with few nm of a poor thermal conductive insulation layer before silver coating. We believe that many composites could be tested by this combination of far-and near field Raman approach as a standard method for chemical surface characterizations and molecular arrangement investigations of sub-micron organic composite particles.

Supporting Information

The molecular fraction of PVP and Furosemide in the nanocomposite is investigated by X-Ray This ratio is then used to simply derive the Furosemide contribution within the total nitrogen content in the Furosemide/PVP composite (%𝑁 𝐹+𝑃𝑉𝑃 ) knowing that %𝑁 𝐹+𝑃𝑉𝑃 = %𝑁 𝐹 + %𝑁 𝑃𝑉𝑃 = (%𝐶𝑙 × 1.6) + %𝑁 𝑃𝑉𝑃 , Eq. 5.1

where %𝑁 𝑃𝑉𝑃 and %𝑁 𝐹 are nitrogen contributions from PVP and Furosemide surface molecules. This simple formula allows counting Furosemide and PVP molecules at the surface and gives a PVP surface content between 60% if we use R(th) and 65%, if we use R(exp). Both analyses point out that the PVP surface fraction is slightly higher than Furosemide while the average molecular composition in the SFE solution is 53% Furosemide and 47% PVP.

Overall, the XPS analysis confirms the tendency of PVP molecules organize on the top of nanoscale Furosemide domains; the average PVP layer thickness is extremely small (reasonably less than 3 nm) so that the Furosemide signal intensity is only slightly screened by PVP molecules.

Chapter 6

However, near-field Raman spectroscopy and microscopy methods do not only have advantages. TERS is an extremely time and material consuming measurement technique. As described within chapter 3 and 4, a lot of TERS spectra hast to be recorded and analyzed to obtain robust results. For example, all recorded hexolite TERS maps contain together 6600 individual TERS spectra. Furthermore, the tips can be easily damaged or contaminated while scanning making them TERS-inactive. Moreover, TERS tips must be produced before TERS measurements can be performed. Due to the manufacturing processes, not every tip produced is TERS active. Although, the manufacturing procedure presented provide a success rate of about 90 %, not each TERS tip show the same field enhancement. Thus, while it is possible to record single-point spectra and long exposed surface maps, TERS maps of larger areas require a large field enhancement factor, since the individual measuring points can be only exposed for a short time to ensure that the tip remains in the focus of the laser. Accordingly, further experiments have to be carried out to find a time-saving and reproducible manufacturing process that delivers strong field-enhancing TERS tips allowing TERS maps of several particles. For economic reasons, it is therefore advisable to use TERS for structure investigations of organic nano-composites only if other standard methods cannot provide the information sought. However, the results then found can provide insights into the molecular composition of organic nanocomposites, allowing new conclusions to be drawn about their specific reaction behaviors or their formation mechanisms.

On the other hand, SECoRM is a time-saving measurement method since it allows the investigation of a high number of particles in a single map. However, it does not provide the lateral resolution of TERS by far and is therefore only suitable for sub-micro or micro (composite) particles. Since the sample is directly coated with a thin metal layer, it is, moreover, a destructive measuring method. Despite these disadvantages, SECoRM can also provide information that is not accessible by other methods. However, the strong D and G bands still interfere with the spectrum evaluation. Possibly, the Raman intensity of these bands can be minimized by applying an insulating layer before the silver coating. Various rapidly oxidizing metals could be suitable for this purpose.

In conclusion, TERS and SERS are suitable methods for structure investigations of organic nanocomposites despite the described disadvantages. However, once these are eliminated, TERS and SERS themselves could become standard methods for structure investigations of organic nanoscale composites. des cas, la molécule se relaxe pour revenir à son état électronique et vibratoire de base (𝑉 → 𝑆 0 𝑣 0 ). En conséquence, les énergies du photon absorbé et du photon émis sont égales (Figure 0.1). Ce processus de diffusion élastique est appelé diffusion de Rayleigh. La diffusion de Rayleigh se produit typiquement avec une probabilité de 10 -4 -10 -3 . Toutefois, avec une probabilité de 10 -8 -10 -5 , la molécule peut également se relaxer de l'état virtuel au premier état vibratoire excité de l'état électronique de base (𝑉 → 𝑆 0 𝑣 1 Résumé rapport à l'intensité de la lumière incidente. L'augmentation de champ la plus forte se produit si le LSP de la nanoparticule métallique oscille en résonance avec la lumière incidente (résonance plasmonique de surface localisée, LSPR). La fréquence de la LSPR dépend des propriétés du matériau comme la densité électronique, la masse électronique effective, la taille et la forme de la distribution de la charge. 4 Quoi qu'il en soit, l'augmentation du champ diminue rapidement par rapport à la surface des nanoparticules métalliques. Ainsi, après quelques nanomètres, l'augmentation du champ est à peine perceptible. [5][6][7][8][9][10][11][12] En fonction des nanoparticules utilisées, on peut atteindre des améliorations de l'intensité Raman de 10 4 -10 14 . [13][14][15][16][17][18] Outre l'exaltation électromagnétique, les interactions chimiques entre les molécules et les nanoparticules métalliques peuvent également participer à l'exaltation globale de l'intensité

Section expérimentale

Le co-cristal CL-20/HMX a été produit par le procédé d'évaporation flash de spray. [30][31] La Les pointes TERS ont été préparées comme décrit dans la section précédente.

Résultats et discussion

Dans un premier temps, des spectres Raman en champ lointain de RDX et de TNT purs ont été pris et des bandes de référence ont été choisies pour chaque composé. La bande de référence de RDX à 882 cm -1 peut être attribuée à la vibration du cycle. [36][37][38][39] La bande de référence TNT comprise entre 1358 et 1371 cm -1 peut être attribuée à trois vibrations d'étirements symétriques NO2. 37,[40][41]

Conclusion générale

Dans le cadre de cette thèse, plusieurs composites nanostructurés produits par évaporation flash de spray ont été étudiés en utilisant la spectroscopie et la microscopie Raman, TERS et SERS.

Figure 1 . 1 .

 11 Figure 1.1. Left: TEM images of two aluminum-based iron oxide/-hydroxide nanothermites (i) and (ii). The TEM images present no significant structural difference. Right: EDX-TEM images of the same nanothermites . The EDX-TEM images reveal, that (i) is formed from aluminum particles (red) embedded in an iron oxide/-hydroxide matrix (iron: green, oxide: blue), whereas (ii) is formed by hierarchically structured aluminum/Iron oxide/hydroxide core/shell particles embedded in an iron oxide/-hydroxide matrix.

Figure 2 . 1 .

 21 Figure 2.1. Left: The normal coordinate q defines the relative position of atoms during a molecular vibrations related to their equilibrium positions. Blue spheres represent any atoms. Right: Parabolic potential of the harmonic oscillator. Blue lines depict certain vibrational energy states 𝑣. The energy difference between two vibrational states is ℏ𝜔. The vibrational ground state 𝑣 0 energy is ℏ𝜔/2.

Figure 2 . 2

 22 Figure 2.2 Simplified schematic representation of energy distribution within a molecule. The electron states n are subdivided by the vibration states 𝑣. The dissociation energy D gives the energy level where the maximum deviation of the normal coordinate 𝑞 is too large so that the molecule cannot relax into its equilibrium position.

Figure 2 . 3 .

 23 Figure 2.3. The Jablonski diagram illustrates the molecular energy states and the possible transitions. 𝑆 0 : singlet ground state; S1: first excited singlet state; 𝑇 1 : lowest triplet state; 𝑣 0 : vibrational ground state; 𝑣 𝑥 : excited vibrational states; IR A: infrared absorption; UV-VIS: ultra-violet or visible light absorption; F: fluorescence; P: phosphorescence; IC: internal conversion; ISC: inter system crossing; arrows represent photonic processes; horizontal wave arrows depict non-radiative transitions; vertical wave arrows diagram non-radiative relaxation. 1

Figure 2 . 4 .

 24 Figure 2.4. Schematic representation of the Raman effect. 𝜈 0 and 𝜈 1 represent the vibrational ground state and the first excited vibrational state of a molecule. Black arrows depict the energy ℎ𝜈 0 and ℎ𝜈 𝑟 of an incident photon as well as the energy of a Rayleigh scattered photon. The red arrow represents the energy ℎ𝜈 𝑠 of a Stokes scattered photon, the green arrow symbolizes the energy of an Anti-Stokes scattered photon ℎ𝜈 𝑎𝑠 .

Figure 2 . 5 , 2 a

 252 right). The investigated molecule, here Chapter CO2 molecule, is centered at the origin of the Raman scattering system aligned along the z -axis of the scattering system. The z -analyzer and the Raman spectrometer are localized along the x -axis of the Raman scattering system. The indices describing the z -components in Eq. 2.15 are dropped within the following to guarantee comprehensibility 𝜇(𝑡) = 𝛼𝐸(𝑡) .

Figure 2 . 5 .

 25 Figure 2.5. Left: Matrix representation of the polarizability tensor α ̃ and vectorial description of the induced dipole moment µ ⃗ . Coloured polarizability components participate at µ ⃗ if incident light is z -polarized. Right: Setup of the investigated theoretical scattering system. A CO2 molecule is located in the origin of the scattering system and aligned parallel to the z-axis. The incident light is z -polarized; 𝐸 ⃗ → 𝐸 𝑧 . Only z -components of scattered light can pass the z -analyser and reach the detector; 𝜇 → 𝜇 𝑧 . Accordingly, the polarizability tensor is reduced to 𝛼 𝑧𝑧 and thus 𝜇 𝑧 (𝑡) = 𝛼 𝑧𝑧 𝐸 𝑧 (𝑡) is a scalar relation.

  already describes the elastic Rayleigh scattering. Anyway, if a molecule (or each other scattering system) oscillates, the polarizability 𝛼 changes depending on the relative positions of the molecule atoms to their equilibrium position. As already introduced in chapter 2.1.1, the relative molecule atom positions can be expressed with the normal coordinate 𝑞. Since molecular vibrations are innately periodic oscillations, the time-dependent normal coordinate

  , it is obvious that the difference of the modified angular frequencies 𝜔 0 ± 𝜔 𝑞 correspond to the energy of the Stokes or Anti-stokes scattered photons (𝐸 𝑝 = ℏ𝜔 = ℎ𝑣; 𝐸 𝑣 0 →𝑣 1 = ℏ𝜔 𝑞 ; compare Figure 2.4). However, the Stokes respectively the Anti-Stokes term provides additionally information whether a molecular vibration is Raman active or not. Therefore, the Stokes and Anti-Stokes terms disappear if the polarizability alteration while the molecule oscillates through its equilibrium position (𝜕𝛼 𝜕𝑞 ⁄ ) 0 is zero. Consequently, the molecular vibration would not appear in the Raman spectrum. In contrast, if (𝜕𝛼 𝜕𝑞 ⁄ ) 0 is unequal to zero, the Stokes and the Anti-Stokes terms participate to the time-dependent induced dipole moment 𝜇(𝑡) and thus the vibration can be find within the Raman spectrum. As an example, the CO2 symmetric and asymmetric stretching vibrations are investigated (Figure 2.6). In case of the CO2 symmetric stretching vibration, the polarizability alteration is unequal to zero while the molecule oscillates through its equilibrium position as visualized by the different polarizability ellipsoid shapes of -𝑞 and +𝑞 in Figure 2.6. Accordingly, the CO2 symmetric stretching vibration is Raman-active. On the other hand, the CO2 asymmetric stretching vibration provides no polarizability alteration while oscillating through the equilibrium position (same ellipsoid shapes of -𝑞 and +𝑞). Therefore, the CO2 asymmetric stretching vibration is Raman-inactive. The intensity of Stokes or Anti-Stokes scattered photons in the Raman

Figure 2 . 6 .

 26 Figure 2.6. Left: Schematic representation of the CO2 symmetric stretching vibration. This molecular vibration provides an alteration of the polarizability when oscillating through the equilibrium position. Accordingly, the vibration is Raman-active and occurs in the Raman spectrum. Right: Schematic representation of the CO2 asymmetric stretching vibration. The polarizability does not alternate while oscillating through the equilibrium position. Thus, this vibration is Raman-inactive and do not appear within the Raman spectrum.

Figure 2 . 8 .

 28 Figure 2.8. Schematic representation of the localized surface plasmon resonance (LSPR). If a metal nanoparticle interacts with electromagnetic radiation, the valence electrons are displaced relatively to the atomic nuclei towards the particle surface in opposite direction to the electric field vector 𝐸 ⃗ 𝑖𝑛 . Because of the related charge separation, a restoring force due to Coulomb attractions occurs leading to a collective oscillation of the valence electrons. If the frequency of the electromagnetic radiation is in resonance with the oscillation frequency of the valence electrons (LSPR), an electric field is created at the particle surface parallel to 𝐸 ⃗ 𝑖𝑛 .

  However, only certain molecular vibrations are enhanced by the resonance Raman effect. Since electronic transitions accompany with structural changes of the molecule orbitals and of the electron density, only the Raman intensities of vibrations which are related with a change in bond length

Figure 2 . 9 .

 29 Figure 2.9. Left: Schematic representation of the resonance Raman effect. In contrast to the non-resonant Raman effect, the wavelength of the incident light coincides with the energy difference between the electric ground state and the first excited electronic state. Raman resonant vibrations appear more intense in Raman spectra. Right: Schematic representation of the resonant charge transfer chemical effect. A chemisorbed molecule forms a charge transfer (CT) state with the nanoparticle surface. If the wavelength of the incident light coincides with the energy difference between the electric ground state and the electronic CT state, the Raman intensities of certain molecular vibrations are enhanced (10 2 -10 4 ).41

Figure 2 . 10 .

 210 Figure 2.10. Schematic representation of different TERS illumination geometries. Green arrows depict the incident laser light; the red arrows depict Raman scattered light; the grey triangles and spheres represent the metal tips; green ellipses illustrate the enhanced field provided by LSPRs and the lightning rod effect; the blue circle segments represent optical lenses, the different lens sizes represent diverse numerical apertures (NA). Left: Bottom-illumination setup. Middle: Side-illumination setup. Right: Top-illumination setup.

1 (

 1 Spitzer, D., Surface Characterization of Nanoscale Co-Crystals Enabled through Tip Enhanced Raman Spectroscopy. Nanoscale 2020, 18, 10306-10319, DOI: 10.1039/D0NR00397B)3.1 AbstractAtomic Force Microscopy coupled with Tip Enhanced Raman Spectroscopy (AFM-TERS) was applied to obtain information about structure and surface composition of single nano co-crystals. For this purpose, a co-crystalline system consisting of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazatetracyclo-[5.5.0.03,11 .05,9 ]-dodecane (CL-20) and 1,3,5,7-Tetranitro-1,3,5,7-tetrazocane (HMX) in a molar ratio of 2:1 (CL-20/HMX) were chosen. CL-20/HMX nano-plates were prepared by Spray Flash Evaporation. To ensure co-crystallinity and nanostructure, powder X-Ray Diffraction and AFM investigations were performed. Results demonstrate that coherence lengths and particle dimensions are on a similar level though coherence lengths appear shorter than measured particle dimension. According to this fact, defects inside the nano co-crystals are minimized. The co-crystallinity was additionally proven by confocal Raman spectroscopy. Here, marker bands for pristine CL-20 and HMX were chosen which appear in the CL-20/HMX spectrum in an intensity ratio of ~ 2.5:CL-20:HMX). Afterwards surface investigations of single CL-20/HMX nano-plates were performed by AFM-TERS. Due to the surface sensitivity of TERS, these experiments reveal that the ratio of the Raman intensities between CL-20 and HMX inverts at CL-20/HMX nano-plate surfaces. Therefore, it is concluded that nano co-crystal surfaces consist of molecular layers of HMX. A theoretical approximation of the normal coordinates of the investigated marker vibrations supports this conclusion, since it can exclude the occurrence of the intensity ratio inversion because of the given orientation between CL-20/HMX nano-plates and the Raman scattering system. Based on this finding, an impact ignition mechanism is proposed enabling to explain the close impact sensitivity values of β-HMX and CL-20/HMX.

  10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane crystallized in є phase (є-CL-20) and the β -polymorph of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (β-HMX) were purchased by EURENCO (Massy, France). є-CL-20 and β-HMX were dried for 6h at 50 °C in a vacuum drying oven before further use. Acetone (CHROMASOLV®) was sourced by Sigma-Aldrich (Munich, Germany) and used without further purification.

Figure 3 .

 3 1 depicts a photography of the lab version of a SFE reactor as well as a flow chart of the set-up. For β-CL-20 synthesis, 3.14 g (7.2 mM) of dry є-CL-20 were dissolved in 200 mL acetone. This solution was given into the solution storage tank of the SFE device. For the spraying process an 80 µm hollow cone nozzle was used, which was heated up to 160 °C. The pressure in front of the nozzle was set to 40 bar. The pressure behind the nozzle inside the reaction chamber amounted 8 -15 mbar

3. 3 . 4

 34 Preparation of CL-20/HMX (2:1) Co-Crystal CL-20/HMX co-crystals were synthesized using SFE process as well. For performed synthesis, a 2 wt.% solution of 2.4 g (5.4 mM) є-CL-20 and 0.80 (2.7 mM) β-HMX in 200 ml acetone were prepared and transferred into the SFE solution tank. During the synthesis procedure same nozzle size, temperature and pressure settings were used as like as for β-CL-20 synthesis.

Figure 3 . 1 .

 31 Figure 3.1. a) Photography of a lab version SFE -reactor. b) Flow-chart of SFE setup. B1, B2: solvent and solution tank; R: reaction chamber; D: hollow cone nozzle, F: filter, P: vacuum pump; PI: pressure sensor; TI: temperature sensor.

  Bragg-Brentano geometry was performed on a Bruker AXS Advance D8 (Karlsruhe; Germany) X-ray powder diffractometer using Cu-Kα radiation (λ= 1.54 Å). The acceleration voltage amounts 40 kV and the operating current is 40 mA. The step size for all scans was set to 0.0148° (2 θ). The loose powder samples were measured on a rotated sample holder. Coherence length L hkl was calculated from the full width at half-maximum (FWHM) of single diffraction peaks by using the well-known Scherrer equation assuming Gaussian shaped peaks that were fitted with OriginPro 2015 (version b9.2.214). A crystal shape model was simulated with VESTA (version 3.4.4) by using obtained coherence lengths.

Figure 3 .Figure 3 . 3

 333 Figure 3.2. a) Untreated AFM probe (AppNano Access-NC). The tip is mounted onto the cantilever at an angle of 120°. b) Annealed Ti/Ag coated Si -tip.

Figure 3 . 3 .

 33 Figure 3.3. Schematic representation of the experimental setup.

  (n-CL-20/HMX). XRPD patterns were recorded to investigate the crystal structure of n-CL-20/HMX. Obtained XRPD pattern shows same Bragg reflex positions as simulated XRPD diffractogram from single crystal X-ray diffraction results of CL-20/HMX monoclinic co-crystals (molar ratio CL-20 : HMX 2 : 1) undertaken by Bolton et al.Figure 3.4 compares XRPD patterns of synthesized n-CL-20/HMX and literature data in the relevant data range between 10° and 40° (2θ). 13 In this crystal system layers of HMX alternate with bilayers of CL-20 in [300] -direction. The conformation of HMX corresponds to the chair conformation which is also found in the β -polymorph of HMX. CL-20 molecules are present in pairs, whereby one takes on the conformation seen in the β -polymorph of pure CL-20 while the other adopt the conformation which is present in the γ -polymorph. 13 No additional signals appear in the n-CL-20/HMX diffractogram indicating a complete conversion of pristine CL-20 and HMX into CL-20/HMX. Additionally, the crystal morphology of single nano-crystals was simulated by calculated coherence lengths (table in Figure 3.4). The plate shape of this simulated CL-20/HMX nano co-crystal is shown in Figure 3.5d. AFM investigations were aimed to gain insight into size and structure of n-CL-20/HMX particles. n-CL-20/HMX forms angular plates with heights of ~29 nm. Their lengths and widths range between 100 nm and 400 nm.Figure 3.5.a) depicts an agglomerate of two of these nano-crystals. These particles are well representing the general appearance of n-CL-20/HMX, since similar particles are found in all AFM measurements (further AFM maps can be found in the supporting information). The detected heights of 29 nm correspond with the calculated coherence length in [300] -direction.

Figure 3 . 4 .

 34 Figure 3.4. Upper left: Relevant section of n-CL-20/HMX XRPD pattern between 10° and 40°. Bottom left: Simulated XRP diffractogram of single crystal X-Ray diffraction results of CL-20/HMX taken from ref. 1. Bragg reflexes of n-CL-20/HMX appear at same positions as reference CL-20/HMX reflexes. Upper right: Gaussian fitted (111) -reflex. Bottom right: Calculated coherence length of crystal faces determining the crystal morphology.

Figure 3 . 5 .

 35 Figure 3.5. a) AFM topographic maps of n-CL-20/HMX co-crystals. b) Height profile of particles shown in a) along y -direcFigure 3.6tion. c) 3D image of a). d) Crystal morphology simulated by using crystal faces and corresponding coherence lengths from XPRD data. Calculated and measured shapes resemble strongly one another. Moreover, particle heights fit to coherence length in [300] -direction. Particles sizes in crystallographic bc -plane are larger than corresponding coherence length. This may be caused by crystal structure defects occurring due to oriented attachment of primary nanoparticles to regarding n-CL-20/HMX plates.

  -1 and broadened in the n-CL-20/HMX Raman spectrum. Shift and broadening are explainable by the different chemical environment in the CL-20/HMX co-crystal system compared to the β-HMX polymorph leading to various interactions between H atoms of HMX and -NO2 functional groups of CL-20. Marker bands appear in the CL-20/HMX spectrum with an intensity ratio of ~ 2.5:1 (CL-20/HMX). Confocal far-field Raman maps were recorded to investigate if CL-20 and HMX marker bands are equally

Figure 3 . 7 . 3 Figure 3 . 8

 37338 Figure 3.7. Far-field Raman spectra of β-CL-20, γ-CL-20, n-CL-20/HMX and β-HMX between 1700 cm -1 and 600 cm -1 . Marker bands of CL-20 (β-CL-20: 1595 cm -1 , γ-CL-20: 1598 cm -1 , O-N-O asymmetric stretching vibration, red) and HMX (1417 cm -1 , CH2 wagging vibration, green) were chosen to indicate the contribution of pristine compounds in n-CL-20/HMX TERS maps. Raman intensities are normalized to improve comparability.

Figure 3 . 9 .

 39 Figure 3.9. Oriented agglomerates of n-CL-20/HMX nano-plates. a) Crystallographic bc -plane lies in xy -plane of the Raman scattering system. b) Here the crystallographic bc -plane is aligned perpendicular to the xy -plane of the scattering system. Incident laser light in z-direction is linear polarized thus the orientation of the electric field vector 𝐸 ⃗ towards the crystallographic axes depends only on the orientation of n-CL-20/HMX agglomerates.

  nanoparticle surface. Contrary to far-field Raman spectroscopy where signals are averaged over various molecular orientations, TERS investigations are by nature on single n-CL-20/HMX nano-plates. Thus, TERS experiments are rather comparable to single crystal far-field Raman Chapter 3or single molecule TERS investigations with arbitrary but fixed orientations.49 Due to the experimental setup the longitudinal field (along z -axis, Figure3.11. c) and d)) provides the main contribution with respect to the localized surface plasmon resonance and hence normal modes with stronger vibrational components perpendicular to the substrate plane (xy -plane)

Figure 3 . 10 .

 310 Figure 3.10. AFM topography images, TERS maps and corresponding spectra of n-CL-20/HMX nano-plates. TERS spectra represent the average over all TERS spectra on the particle surfaces. At all measured surface areas an intensity inversion between CL-20 and HMX marker bands occur. TERS maps and TERS spectra are normalized for better comparability. AFM topography images were recorded after TERS mapping. Step sizes: particle_1 20 nm, particle_2 and particle_3 50 nm.

  10. In relation to the far-field Raman maps shown above, intensities of the CL-20 marker band are depicted in red in these TERS maps while the intensities of the HMX marker band are depicted in green. Due to the intensity ratio inversion from CL-20 marker band towards HMX marker band which can be recognized by comparing intensities of far-field and TERS spectra (Figure 3.10), surfaces of n-CL-20/HMX nano-plates

Chapter 3 normal 2

 32 modes depend on the orientation of this simplified normal coordinate vector towards the electric field vector of the linear polarized incident light as shown later. The oscillation directions of the molecular fragments involved in the investigated vibrations were computed by a DFT calculation. In case of the ONO asymmetric stretching vibrations of the CL-20 polymorphs, the N atoms oscillate oppositely to the O atoms of the NO2 functional groups. This oscillation takes place in a plane formed by the involved N and O atoms. Additionally, the atomic deviations during the oscillation are equal in all NO2 groups. Therefore, a vector 𝑞 𝑚;𝛽-𝐶𝐿-20 , respectively 𝑞 𝑚;𝛾-𝐶𝐿-20 , starting at one O atom and pointing to the other O atom of the same functional group is able to describe qualitatively the oscillation of this NO2 group. Hence, the approximated normal coordinates 𝑞 0;𝛽-𝐶𝐿-20 and 𝑞 0;𝛾-𝐶𝐿-20 arise from 𝑞 0;𝛽-𝐶𝐿-20 = ∑ 𝑞 𝑚;𝛽-𝐶𝐿-The directions of the single vectors 𝑞 𝑛;𝛽-𝐶𝐿-20;𝑥𝑦 and 𝑞 𝑛;𝛾-𝐶𝐿-20;𝑥𝑦 are represented as yellow arrows in Figure 3.11. a). It should be mentioned that only the four marked NO2 functional groups contribute to the investigated CL-20 normal modes. Resulting CL-20 normal coordinates were normalized and added: 𝑞 0;𝐶𝐿-20 = 1 𝑛 𝛽-𝐶𝐿-20 𝑞 0;𝛽-𝐶𝐿-20 + 1 𝑛 𝛾-𝐶𝐿-20 𝑞 0;𝛾-𝐶𝐿-20 , Eq. 3.3 where 𝑛 𝛽-𝐶𝐿-20 and 𝑛 𝛾-𝐶𝐿-20 represent the normalization factors of 𝑞 0;𝛽-𝐶𝐿-20 and 𝑞 0;𝛾-𝐶𝐿-20 .

Figure 3 .

 3 Figure 3.11. a) Orientations and conformations of β-CL-20, γ-CL-20 and β-HMX mirrored conformations in the CL-20/HMX co-crystal lattice. Yellow arrows represent the approximated single vector components of the approximated normal coordinates. b) Approximated normal coordinates; 𝑞 0;𝐶𝐿-20 is depicted in red, 𝑞 0;𝐻𝑀𝑋 is depicted in green. Since the crystallographic bc -plane is aligned parallel towards the xy -plane, all possible orientations between the normal coordinates and the electric field vector 𝐸 ⃗ 𝑛𝑓 can be illustrated as cones around the z -axis. Therefore, the present semi-orientation does not affect the Raman intensity ratio in TERS, since the angles included by the normal coordinates and the z -axis (𝐸 ⃗ 𝑛𝑓 ) remains unchanged in all possible alignments. c) and d) two possible arrangements of the semi-orientated scattering system. The crystallographic bc -plane is always aligned parallel to the xy -plane of the scattering system. The electric field vector 𝐸 ⃗ 𝑛𝑓 of the near-field located one the Ag nanoparticle at the tip apex is approximated perpendicular towards the xy -plane.

Figure 3 . 3 3. 4 . 5

 3345 11.b). The detailed calculations of these vectors are provided within the supporting information. Due to the semi-orientated scattering system the crystallographic ac -plane can be aligned in every possible direction inside the xy -plane of the scattering system. Therefore all possible orientations of 𝑞 0; 𝐶𝐿-20 and 𝑞 0; 𝛽-𝐻𝑀𝑋 in the scattering system can be depicted as circles around the z -axis (Figure3.11.b). The cones shown in Figure 3.11.b) represent furthermore the changes of the approximated normal coordinates over time. As already mentioned above, the z -component of the electric field vector 𝐸 ⃗ 𝑛𝑓 (nf = near field) is the main source of 𝐸 ⃗ 𝑛𝑓 and hence 𝐸 ⃗ 𝑛𝑓 can be approximated to 𝐸 ⃗ 𝑛𝑓 = ( Stokes term of the classical interpretation of the Raman scattering ⃗ 0 cos (𝜔 0 -𝜔 𝑅 )𝑡 Eq. 3.6 the measured Raman intensity is proportional to the absolute value of the scalar product between the normal coordinate 𝑞 0; 𝐶𝐿-20 , respectively 𝑞 0; 𝛽-𝐻𝑀𝑋 , and the electric field vector 𝐸 ⃗ 𝑛𝑓 : 𝐼 𝐶𝐿-20 ~ |𝐸 ⃗ 𝑛𝑓 • 𝑞 0;𝐶𝐿-20 | = |𝐸 𝑧;𝑛𝑓 • 𝑞 0;𝑧;𝐶𝐿-20 | Eq. 3.7 𝐼 𝐻𝑀𝑋 ~ |𝐸 ⃗ 𝑛𝑓 • 𝑞 0;𝐻𝑀𝑋 | = |𝐸 𝑧;𝑛𝑓 • 𝑞 0;𝑧;𝐻𝑀𝑋 | . Eq. 3.8 From this correlations it is apparent that only the z -components 𝑞 0;𝑧;𝐶𝐿-20 and 𝑞 0;𝑧;𝐻𝑀𝑋 of the approximated normal coordinates contributes to the scalar products. These z -components are constant for each possible parallel orientation of the crystallographic bc -plane inside the xy -plane of the scattering system. Therefore, the theoretical approximation of the intensity ratios can be finally given as: 𝐼 𝐶𝐿-20 𝐼 𝐻𝑀𝑋 = |𝐸 𝑧;𝑛𝑓 • 𝑞 0;𝑧;𝐶𝐿-20 | |𝐸 𝑧;𝑛𝑓 • 𝑞 0;𝑧;𝐻𝑀𝑋 | = |𝑞 0;𝑧;𝐶𝐿-20 | |𝑞 0;𝑧;𝐻𝑀𝑋 | already assumed from far-field Raman data, the intensity ratio inversion of the marker bands within the TERS spectra of n-CL-20/HMX nano-plates is very unlikely to occur because of the (semi-)orientated scattering system. Chapter Surface Composition of CL-20/HMX Nano-Plates Next to the molecular arrangement towards the TERS scattering system, the distance between the Ag nanoparticle at the tip apex and the sample surface influences strongly the Raman intensities. This dependency arises from the rapid decrease of the localized enhanced field at the tip apex as a function of the distance. Experimental studies and simulations have shown that the decay of the majority of the field enhancement occurs at a distance less than 8 nm -10 nm from the surface of the Ag nanoparticle. 21-24 Furthermore, TERS experiments presented here were recorded in non-contact mode (see experimental section). In this dynamic AFM mode, the tip oscillates with a certain frequency and amplitude over the sample surface. Thus, the distance between the tip apex and the n-CL-20/HMX nano-plates surfaces varies constantly during the measurement. Regarding this fact, measured signal intensities represent a weighted average over all particle-surface distances during the acquisition time. Trautmann et al. could show via electrodynamic simulations that the average Raman enhancement strongly depends on the closest distance between Ag nanoparticle and sample surface at dynamic AFM modes. Further they could demonstrate that the main field enhancement is localized within a distance less than 2 nm from the particle surface (green spheres in Figure 3.12). 21 In case of single n-CL-20/HMX nano-plates surfaces parallel to the xy -plane of the scattering system always consists of pure HMX or CL-20 layers (see XRPD and AFM section). Accordingly, the tip -surface distance represents always the distance between the tip apex and a specific molecular layer. Therefore, the distance between the next molecular layer and the tip apex is always around a third of the height of the crystallographic unit cell ℎ 𝑢𝑛𝑖𝑡 ℎ 63 𝑛𝑚 • cos 9.23°= 0.54 𝑛𝑚 Eq. 3.10

3. 12 )

 12 . This difference leads to a significantly stronger enhancement of the actual surface layer and hence to stronger Raman intensities of this layer. Thus, the Raman intensity inversions between the CL-20 and HMX marker bands from ~2.5:1 to ~1.5:1 (CL-20 : HMX) in the TERS spectra can be explained by HMX as the surface finishing layer. Since the intensity inversion arises at each measured n-CL-20/HMX nano-plate surface, it is assumed that an HMX surface finishing is energetically favored. This assumption is explicable by the molecular structure of HMX. Contrary to the CL-20 cage structure, HMX is a cyclic nitroamine and hence it is easier to deform. Regarding this HMX should be able to adopt an energetically more favorable conformation than CL-20. Therefore, surface energy of CL-20/HMX nano-plates is minimized through an HMX surface finishing.

Figure 3 . 12 .

 312 Figure 3.12. Schematic representation of a dynamic non-contact TERS measurement of n-CL-20/HMX. The grey sphere represents the TERS active Ag nanoparticle. The green circle symbolizes the localized enhanced field at the tip apex. a) Closest distance between TERS active Ag -tip and particle surface. b) TERS active Ag -tip is moving upwards. a) and b) During the whole TERS measurements the HMX surface layer is always located in an electric field region showing a stronger Raman enhancement. Thus, the HMX marker band appears more intense in TERS spectra.

Table 3 . 1 . 14 1

 3114 Impact sensitivities of CL-20/HMX, є-CL-20 and β-HMX measured by various groups Bolton et al. 13 h50 = 43 cm -48 cm h50 = 25 cm -28 cm h50 = 45 cm -50 cm Gosh et al. assumption deals solely with perfect defect free crystals and does not include defect sides or crystal surfaces. Due to the nanostructure of n-CL-20/HMX, the nano-plates exhibit only a few crystal defects but a relatively high surface area because of the high surface to volume ratio (see AFM and XPRD measurements). These surfaces consist approximately to 85% of HMX (see XRPD and AFM section). Next to the (300) -faces also the other crystal lattice planes defining the particle surfaces (or internal defect sides) are very likely finished by an HMX molecular layer because of the surface energy minimization due to the more easily deformable HMX molecule. However, TERS investigations of these inclined planes including a theoretical approximation are significantly more difficult, since also the xy -components of the enhanced electric field at the Ag nanoparticle surface must be considered. Anyway, since

Chapter 3 still

 3 can be absorbed by compression of the CL-20/HMX co-crystal but sufficient to ignite pure β-HMX and thus β-CL-20 and γ-CL-20 compresses n-CL-20/HMX. During the relaxation of the compressed crystal input impact energy is converted into thermal energy dissipating over the crystal lattice. If the amount of the dissipating thermal energy while reaching the HMX determined particles surface (or structural defect) is still high enough to overcome the dissociation energy of HMX, HMX molecules will dissociate, since a further dissipation of the thermal energy is hindered. The released heat of reaction induces the dissociation of neighboring HMX and CL-20 leading to a chain-reaction and finally to the combustion of the CL-20/HMX co-crystal. Thus, the ignition by impact is spread from the surfaces respectively structure defects towards the inside of the bulk material by starting with dissociation of HMX surface molecules. However, the measured sensitivities against impact values are subjacent to this of β-HMX. This fact can be explained by the conformation -or better conformations, since different surface faces should lead to different HMX conformations -adapted by surfaces HMX molecules.

  paper shows the successful synthesis of CL-20/HMX nano-plates via the Spray Flash Evaporation process. XRPD prove impeccably the co-crystallinity of n-CL-20/HMX and AFM investigations demonstrates the nanostructure of n-CL-20/HMX nano-plates. Furthermore far-field Raman maps support the XPRD results and demonstrate besides a complete conversion of pristine CL-20 and HMX into n-CL-20/HMX. Marker bands for HMX and CL-20 were chosen from far-field Raman spectroscopy data with the premise that no signal appears at these wavenumbers at the other pristine compound Raman spectrum. In far-field spectra the CL-20 marker band (NO 2 asymmetric stretching vibrations localized at 1600 cm -1 ) appears with an around 2.5 times higher intensity as the HMX marker band (CH2 out of plane wagging vibration, localized at 1416 cm -1 ). The intensity ratio of these marker bands inverts to a ratio of around 1.5 : 1 (HMX to CL-20) in TERS spectra which strongly hints to an HMX determined surface of n-CL-20/HMX. To ensure this assumption, an approximated normal coordinate analysis was undertaken, since the intensity ratio inversion could occur due to a specific arrangement of n-CL-20/HMX nano-plates towards the Raman scattering system. Therefore, the movements of the atoms involved in the investigated normal modes were simulated with a DFT method. Only atoms with strong derivations from their equilibrium position were drawn under consideration by expressing their movements as 3D dimensional vectors leading to simplified 3N dimensional normal coordinates. Since only the interactions between the incident light and the normal modes and not the emitted field are of interest for this purpose, the x-, y-and z -components of these 3N dimensional normal coordinates were summed up and normalized resulting in 3D normalized vectors. This simplification demonstrates that under the given orientation the intensity of CL-20 marker vibration should triple the intensity of HMX marker vibration which is close to the measured ratio. Thus, the intensity ratio inversion arises most likely from an HMX surface finishing, since HMX surface molecules are always located in a higher enhanced field region during the dynamic AFM-TERS measurements. In consideration of this finding an impact ignition mechanism is proposed enabling to explain the close sensitivity values of HMX and CL-20/HMX in spite of a higher compressibility of CL-20/HMX.In conclusion, this study demonstrates again the extraordinary possibilities that lie behind TERS. It was possible to figure out co-crystallinity on single nanoscale co-crystals. This is of analytical interest, for example, if synthesized nano co-crystals are not present in sufficient amounts to perform more common analytical methods like XRPD. Moreover, we could show that TERS enables surface investigations of nano (co-) crystalline particles which may help to receive a better understanding of co-crystal formation and co-crystal design for specific Topographic Mappings of n-CL-20/HMX Nano-PlatesIn the following further AFM topographic maps of n-CL-20/HMX nano-plates recorded in non-contact mode are presented. n-CL-20/HMX nano-plates occur as single particles as well as agglomerates along their crystallographic bc-axis. Figure3.13 shows one of these agglomerates. The substructure of single nano-plates can be identified easily from the AFM topographic image. n-CL-20/HMX agglomerates can be randomly orientated towards the scattering system. Therefore, only single CL-20/HMX nano-plates were investigated via AFM-TERS in this study. Figure3.14 depict a selection of different single and agglomerated CL-20/HMX nano-plates to demonstrate that these structures appear in the whole sample.

Figure 3 . 13 .

 313 Figure 3.13. Left: AFM topographic mappings of several n-CL-20/HMX nano-plates. Single nano-plates show a height of ~30 nm. n-CL-20/HMX nano-plates built up agglomerates along their crystallographic bc-planes. Right: Single n-CL-20/HMX nano-plates and a CL-20/HMX nano-plate agglomerate.

  it could be shown that TERS provides new insights into the structure and the surface composition of nanoscale composites, different composite nanoparticles consisting of RDX and TNT with varying mass ratios were analyzed. These so called hexolite mixtures are used as precursor for nanodiamond detonation syntheses. TERS investigations were performed aimed to elucidate the structural composition of the hexolite particles. It could be figured out that the hexolite composite are formed by anisotropic patchy or core/shell nanoparticles with an RDX core and a TNT shell. From these findings, a formation mechanism could be derived allowing a deeper understanding of the kinetics within in the SFE process. More interestingly, since TERS structure determination allowed to link the hexolite precursor structures and arrangements with the resulting nanodiamond sizes, a nanodiamond formation mechanism could be derived. (This chapter is published at: Hübner, J.; Pichot, V.; Lobry, E.; Deckert-Gaudig, T.; Deckert, V.; Spitzer, D., Formation Mechanism of Anisotropic RDX-TNT Core-Shell Nanoparticles and their Influence onto Nanodiamond Detonation Syntheses, chemrxiv.org, 2020, DOI: 10.26434/chemrxiv.12005625.v1) 4.1 Abstract Mixtures of RDX and TNT or hexolite mixtures are well known precursors for nanodiamond detonation syntheses. In this study, diverse nanoscale hexolite mixtures varying in mass ratios of RDX and TNT are synthesized via Spray Flash Evaporation. The hexolite mixtures are characterized by confocal Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) to obtain information about their molecular structure composition. The marker bands of pristine RDX and TNT enable the identification and distinction of both compounds in the Raman spectra. Confocal Raman spectroscopy indicates an intermixture of RDX and TNT molecules on the nanoscale since both marker bands are detected in all spectra. TERS investigations of single hexolite particles reveal that the particle surfaces are mainly composed of TNT. The comparison of confocal Raman and TERS results suggests that (depending on the mass ratio) hexolite particles are either inhomogeneous patchy RDX/TNT nanoparticles or anisotropic RDX/TNT core/shell nanoparticles. A building mechanism to explain the formation/growth of those nanoparticles is derived from the spectroscopic data and the dynamics of the SFE process.

4. 3 . 1 Chemicals 1 , 3 , 5 -

 31135 Trinitro-1,3,5-triazine (RDX; M = 222.12 g/mol) and 2-Methyl-1,3,5-trinitrobenzene (TNT; M = 227.13 g/mol) were purchased by EURENCO (Massy, France). Both explosives were dried in a vacuum drying oven for 6h at 50 °C before further use. Acetone (for HPLC, ≥99.9%) was sourced by Sigma-Aldrich (Munich, Germany) and used without further purification.

Figure 4 . 1 .

 41 Figure 4.1. a) Photography of a medium sized vertical SFE -reactor. b) Flow-chart of SFE setup. Final product is collected with axial cyclones (marked in grey). B1, B2: solvent and solution tank; R: reaction chamber; D: hollow cone nozzle, P: vacuum pump; PI: pressure sensor; TIC: temperature sensor.

Figure 4 . 2 .

 42 Figure 4.2. a) Pristine AppNano Access-NC AFM probe. The tip is mounted onto the cantilever at an angle of 120°. b) Si -tip sputtered with a 5 nm layer of Ti and a 25 nm layer of Ag. After the sputtering process the tips were annealed in a muffle furnace.

  4.4. (HORIBA; Kyoto, Japan). Atomic Force Microscopy (AFM) size distribution measurements of all SFE-produced hexolite mixtures and nanodiamonds were performed with an AIST-NT CombiScope TM -1000SPM Atomic Force Microscope (Novato, USA) in non-contact mode. The amplitude of the cantilever vibration was set to 10 nm. All measurements were performed with AppNano ACCESS-NC AFM-probes (Mountain View, USA). The size of all AFM topographic images was 5 μm x 5 μm with a resolution of 256 x 256 measurements points and a scanning rate of 0.6 Hz. 200 nanoparticles of each SFE-produced hexolite mixture and 200 nanodiamonds of each hexolite precursor were measured. Samples were prepared by depositing a spatula's tip of the diverse hexolite mixtures or nanodiamonds between two 2 cm x 2 cm cover slips. The sample was spread on the glass surface through pressureless rubbing of the two glass slides against each other. Tip Enhanced Raman Spectroscopy (TERS) surface maps were recorded with a combination of the AFM microscope and the confocal Raman microscope described above. The confocal Raman microscope acts in this combination as light source and spectrometer. All measurements were performed in non-contact AFM mode with an amplitude of 10 nm and samples were illuminated from the below (transmission mode). Incident linear polarized laser light (532 nm) was focused onto the tip with by a 100x, 1.4 NA oil-immersion objective. The precise positioning of the AFM-tip in the focused laser spot was achieved by objective scans perpendicular and parallel towards the AFM-TERS tip. A typical square-shaped TERS surface map consists of 100 (10 x 10) single TERS spectra. The acquisition time was 3 s per measurement point at a laser power on the sample of 54 μW. Step sizes between the measurement points depend on the size and surface area of the investigated nanoparticle and varied between 2-5 nm. The same samples were used for TERS investigations as for AFM size distribution measurements. High Resolution Transmission Electron Microscopy (HR-TEM) images of obtained nanodiamonds were recorded on a JEOL ARM2000F (Tokio, Japan) microscope with a nominal point resolution of 0.8 Å at Scherzer defocus. The microscope was operated at 200 kV acceleration voltage.4.4 Results and Discussion 4.4.1 Distinction of RDX and TNT via Far-and Near-Field Raman

Figure 4 .

 4 3 shows the Raman spectra of RDX and TNT with marker bands highlighted in green respectively red. The complete Raman spectra and the assignment are given in the Supporting Information. In confocal far-field Raman experiments maps were recorded of each hexolite mixture to obtain a first insight into the structure compositions. TERS was performed to investigate morphology, size and structure composition of the hexolite mixtures on the nanoscale. Due to its surface sensitivity, TERS is perfectly suited for surface characterization of composite nano materials as already demonstrated on 60/40 hexolite nanoparticles and on CL-20/HMX nano co-crystals in previous

Figure 4 . 3 .

 43 Figure 4.3. Far-field Raman spectra of RDX and TNT nanoparticles in the range between 1700 cm -1 and 600 cm -1 .

4. 4 . 2

 42 Physically Mixed 60/40 Hexolite Physically mixed 60/40 hexolite (p-60/40) was received as white-yellowish powder. Confocal Raman maps of this powder were recorded to provide insight into the intermixture of RDX and TNT nanoparticles.

Figure 4 .

 4 4 shows a microscope image and the corresponding confocal Raman map of p-60/40. These confocal Raman maps were recorded pixel-wise. The Raman intensity of the RDX marker band in the single spectra is shown in green and the Raman intensity of TNT is shown in red. If both marker bands were detected simultaneously, the band intensities are superimposed and appear as light green over yellow to dark orange pixels. Evidently, the confocal Raman maps demonstrate that RDX and TNT nanoparticles consist of microscale agglomerates without a further intermixture on the nanoscale. In order to visualize this finding Figure 4.4.a) und Figure 4.4.b) present only the intensity of RDX respectively TNT marker band. Even though confocal far-field Raman maps already exclude a nanoscale intermixture of RDX and TNT nanoparticles, TERS measurements were performed on single nanoparticles to verify this result and to receive reference data for TERS investigation of SFE produced nano hexolite mixtures. As expected, only pure RDX and pure TNT nanoparticles could be identified in the TERS spectra. Generally, TERS spectra differ from far-field Raman spectra by several features. Thus, slight shifts of band positions and differences in Raman intensities of same vibrational modes are evident in TERS spectra (Figure 4.4). are evident in TERS spectra (Figure 4.4). Since TERS is a surface sensitive detection technique, the orientation of molecules on the probed particles plays an important role. The surface molecules are supposed to have different molecular environments due to the crystal symmetry break at the particle's surface. This environmental change is associated with a change of intermolecular

Figure 4 . 4 .

 44 Figure 4.4. Left: Far-field Raman and TERS spectra of pristine RDX and TNT. Right: Confocal Raman microscopy maps of physically mixed 60/40 hexolite particles. A better comparability of the Raman intensities was achieved by normalization. a) Microscope image of p-60/40 nano hexolite particles. b) Corresponding confocal Raman map of superimposed RDX (882 cm -1 ; ring breathing, NNO2 stretching) and TNT (1358 cm -1 -1371 cm -1 ; three NO2 symmetric stretching vibrations) marker band Raman intensities. c) Confocal Raman map of RDX marker band Raman intensities. d) Confocal Raman map of TNT marker band Raman intensities.

Figure 4 . 5 .

 45 Figure 4.5. Left: Far-field Raman and TERS spectra of 80/20 hexolite powder produced by SFE. Typical TERS spectra show either both RDX (882 cm -1 ; ring breathing) and TNT (1358 cm -1 -1371 cm -1 ; three NO2 symmetric stretching vibrations) marker bands or only the TNT marker band. Δ indicate the N-NO2 stretching vibration and the NO2 asymmetric stretching vibration of RDX; * indicate the investigated NO2 stretching vibrations of TNT. Right: Confocal Raman microscopy maps of SFE-80/20. Raman intensities are normalized for a better comparability. a) Microscope image of physically SFE-80/20 nano hexolite. b) Corresponding confocal Raman map of superimposed RDX and TNT marker band Raman intensities. c) Confocal Raman map of RDX marker band Raman intensities. d) Confocal Raman map of TNT marker band Raman intensities.

Figure 4 . 6 .

 46 Figure 4.6. Left: Far-field Raman and TERS spectra of 60/40 hexolite powder produced by SFE. Typical TERS spectra show either both RDX (882 cm -1 ; ring breathing) and TNT (1358 cm -1 -1371 cm -1 ; three NO2 symmetric stretching vibrations) marker bands or only the TNT marker band. Δ indicate the N-NO2 stretching vibration and the NO2 asymmetric stretching vibration of RDX; * indicate the investigated NO2 stretching vibrations of TNT. Right: Confocal Raman microscopy maps of SFE-60/40. Raman intensities are normalized for a better comparability. a) Microscope image of physically SFE-60/40 nano hexolite. b) Corresponding confocal Raman map of superimposed RDX and TNT marker band Raman intensities. c) Confocal Raman map of RDX marker band Raman intensities. d) Confocal Raman map of TNT marker band Raman intensities.

Figure 4 . 7 .

 47 Figure 4.7. Left: Far-field Raman and TERS spectra of 40/60 hexolite powder produced by SFE. Typical TERS spectra show either both RDX (882 cm -1 ; ring breathing) and TNT (1358 cm -1 -1371 cm -1 ; three NO2 symmetric stretching vibrations) marker bands or only the TNT marker band. Δ indicate the N-NO2 stretching vibration and the NO2 asymmetric stretching vibration of RDX; * indicate the investigated NO2 stretching vibrations of TNT. Right: Confocal Raman microscopy maps of SFE-40/60. Raman intensities are normalized for a better comparability. a) Microscope image of physically SFE-40/60 nano hexolite. b) Corresponding confocal Raman map of superimposed RDX and TNT marker band Raman intensities. c) Confocal Raman map of RDX marker band Raman intensities. d) Confocal Raman map of TNT marker band Raman intensities.

Figure 4 . 8 .

 48 Figure 4.8. Left: Far-field Raman and TERS spectra of 20/80 hexolite powder produced by SFE. Typical TERS spectra show either both RDX (882 cm -1 ; ring breathing) and TNT (1358 cm -1 -1371 cm -1 ; three NO2 symmetric stretching vibrations) marker bands or only the TNT marker band. Δ indicate the N-NO2 stretching vibration and the NO2 asymmetric stretching vibration of RDX; * indicate the investigated NO2 stretching vibrations of TNT. Right: Confocal Raman microscopy maps of SFE-20/80. Raman intensities are normalized for a better comparability. a) Microscope image of physically SFE-20/80 nano hexolite. b) Corresponding confocal Raman map of superimposed RDX and TNT marker band Raman intensities. c) Confocal Raman map of RDX marker band Raman intensities. d) Confocal Raman map of TNT marker band Raman intensities.

  These RDX nuclei grow to RDX nanoparticles. The growth of RDX nuclei to RDX nanoparticles appear due to the attaching of solved RDX molecules onto RDX nuclei, the aggregation of various RDX nuclei and nanoparticles and further due to Ostwald ripening. The driving force behind the nuclei growth and aggregation to nanoparticles is the minimization of surface energy which is realized by decreasing the surface-area-to-volume ratio. Anyway, nucleation and crystallization of RDX occurs first in all produced hexolite nano-composites independent of the used RDX and TNT mass fractions within the precursor solution. Due to further shrinking of Chapter 4 the droplets and the related increase of RDX and TNT concentrations within the droplets the point of supersaturation of TNT in the solution is reached in the next step. Consequently, TNT molecules start to nucleate on top of RDX nanoparticle surfaces acting as nucleation seeds. As discussed within the SFE-80/20 section, the physisorption of at least the first layer of TNT molecules onto RDX nanoparticles surfaces is supposed to be initiated by electrostatic interactions between positively charged nitrogen atoms and negatively charged oxygen atoms of the NO2 functional groups of both RDX and TNT molecules. Further TNT molecules grow onto this first TNT spots during the continued SFE process leading to the formation of the

Figure 4 . 9 .

 49 Figure 4.9. a) Mean sizes of anisotropic hexolite core/shell nanoparticles (SFE-80/20, SFE-60/40, SFE-40/60) and patchy hexolite sub-micron particles (SFE-20/80) as a function of the RDX mass fraction inside the precursor solutions. Green spheres represent RDX molecules; red spheres symbolize TNT molecules. Anisotropic hexolite core/shell nanoparticles are depicted as cross sections aimed to visualize its core/shell structure. b) Numbers of anisotropic hexolite nanoparticles and patchy nano particles providing RDX only, TNT only and RDX/TNT mixed surfaces. c) Flowchart of the supposed anisotropic RDX/TNT core/shell nanoparticle building mechanism.

  During the RDX nanoparticle migration the droplet sizes decrease continuously leading to TNT supersaturation and TNT nucleation on top of the RDX nanoparticles as already described for the nonaccelerated case. Since the RDX core particles are localized close to or at the edges of the acetone droplets during the growth of the TNT shell, the formation of hexolite core/shell nano-spheres processes anisotropic (Figure4.9.c). Consequently, RDX cores are not centered in the middle of hexolite core/shell nanoparticles but are localized near or at the particle edges.Due to the varying RDX mass fractions within the different hexolite precursor solutions, kinetics of RDX nucleation and RDX core growths differ. RDX nucleation and RDX nanoparticle growth occur earlier the higher the RDX mass fraction. Accordingly, RDX primary nanoparticles are less affected by the fictitious forces the less the RDX mass fraction. Thus, a complete migration of RDX nanoparticles towards the droplets edges might not takes place for smaller RDX mass fraction hexolite systems. Therefore, it is expected that the RDX cores of SFE-20/80 and SFE-40/60 appear more centered inside hexolite nanoparticles. On the other hand, SFE-60/40 nanoparticles provide RDX cores which are located closer to the particle edges than to the particle centers. However, all of these arrangements cause nanoparticles providing partially thicker and thinner TNT shell areas. Thus, particle surface areas formed by RDX only or by RDX covered with a thin TNT layer occur. These RDX or RDX/TNT determined surface areas result in the presence of RDX Raman active frequencies within some TERS spectra of SFE-60/40, SFE-40/60 and SFE-20/80 TERS surface maps. Due to the anisotropic RDX/TNT core/shell nanoparticle morphology the number of TERS spectra containing RDX signals decreases with the increase of the TNT mass fraction within the hexolite mixtures. An increase of the TNT mass fraction leads to smaller RDX cores and thicker TNT shells. Therefore, the

Figure 4 . 10 .

 410 Figure 4.10. Different alignments of SFE-80/20 RDX/TNT core/shell particles towards the silver particle mounted at the AFM tip apex. Axis units are given in nm. a) Isotropic SFE-80/20 RDX/TNT core/shell nanoparticle. Due to the closed TNT shell only TNT Raman active frequencies would occur in TERS spectra. b) Anisotropic SFE-80/20 RDX/TNT core/shell nanoparticle. The RDX core is aligned towards the silver nanoparticle localized at the AFM tip apex. RDX or RDX and TNT Raman active frequencies appear in TERS spectra. c) Anisotropic SFE-80/20 RDX/TNT core/shell nanoparticle. The RDX core is not localized in the enhanced electric field of the silver nanoparticle. Only TNT Raman active frequencies appear in TERS spectra .

Figure 4 .

 4 11 (right) depicts a sample HR-TEM image of nanodiamonds synthesized from SFE-40/60 (ND-40/60). TERS spectra of nanodiamonds contain the broad graphitic G -and D -bands localized at ~1600 cm -1

Figure 4 . 11 .

 411 Figure 4.11. Left: TERS spectra of certain nanodiamonds synthesized from SFE-40/60 contain graphitic G -and D -bands (~1600 cm -1 , 1400 cm -1 ) as well as the diamond peak (1325 cm -1 ). The shift of the diamond peak from 1333 cm -1 to 1325 cm -1 hints onto a nanodiamond size < 10 nm. The AFM topographic map and the corresponding TERS map depict an agglomerate of nanodiamonds synthesized from SFE-40/60. Right: HR-TEM image of nanodiamonds synthesized from SFE-40/60.

Figure 4 . 12 .

 412 Figure 4.12. Evolution of hexolite RDX/TNT core/shell nanoparticles and evolution of detonation synthesized nanodiamond mean sizes as a function of the RDX mass fraction within the hexolite precursor nano-composite.

Figure 4 . 13 .

 413 Figure 4.13. Top: Supposed building mechanism of nanodiamonds under ideal conditions presented as 2D crosssections. The hexolite precursor are built up by hexagonal close-packed isotropic RDX/TNT core/shell nanoparticles. Due to the higher detonation pressure of RDX compared to TNT and the high symmetry of the precursor, RDX reactions products form spherical diffusion barriers after detonation. Inside the diffusion zone nanodiamonds are formed from TNT carbon atoms. In the ideal case, all diffusion zones have the identic size and contain the same amount carbon atoms and nucleation sites. Thus, the same amount of monodispersed nanodiamonds are formed within the diffusion zones. Consequently, the nanodiamond size depends only on the TNT shell thickness in the ideal case. Bottom: Supposed building mechanism of nanodiamonds under real conditions presented as 2D cross sections. SFE-produced anisotropic core/shell nanoparticles are packed dense.

  Evaporation and investigated with AFM, confocal far-field Raman spectroscopy and TERS. The combination of these techniques gave insight into the sizes of the produced hexolite particles and into the arrangements of RDX and TNT molecules within single particles on the nanoscale. It was demonstrated that SFE-80(RDX)/20(TNT) is built up by patchy RDX/TNT nanoparticles while SFE-60/40, SFE-40/60 and SFE-20/80 form anisotropic RDX/TNT core/shell nanoparticles. The anisotropy of the particles, which depends on the RDX/TNT mass ratio, expresses a non-centered RDX core appearing closer to the hexolite nanoparticle edges. A building mechanism for the different SFE produced hexolite mixtures based on the experimental results and the dynamics of the SFE process is proposed. Due to the higher solubility, RDX nanoparticles are formed first during SFE process. Driven by fictitious forces, RDX nanoparticles migrate towards acetone droplet edges while TNT grows onto RDX nanoparticle surfaces. Since RDX nanoparticles are not centered in the droplet, hexolite nanoparticle formation occur anisotropic.

Figure 4 . 15 .

 415 Figure 4.15. Raman spectrum of RDX in a spectral range between 3200 cm -1 and 300 cm -1 .

  .a) (and b) depicts an example AFM topographic map of SFE-40/60 nanoparticles which is typical for hexolite size distribution AFM measurements. Particles with relatively large heights were measured individually by using smaller scan areas and less scan rates to ensure if they consist of single nanoparticles or if they are single submicron particles. If these individual AFM scans could clearly identify agglomerates these particles were not counted. Contrary, if an agglomeration

Figure 4 .

 4 Figure 4.17.c) (and d). The height profile shows unambiguously that the scanned agglomerate is formed by several single nanoparticles.

Figure 4 . 16 .

 416 Figure 4.16. Size distribution of SFE-produced hexolite nanoparticles.

Figure 4 .

 4 Figure 4.17. a) AFM topographic map of SFE-40/60. b) 3D representation of a). c) AFM topographic map of a SFE-40/60 agglomerate. d) 3D representation of c).

Figure 4 .

 4 Figure 4.18 c) depicts the droplet mean size distribution in the spray as a function of the scanned area. The origin of the coordinate system was set to the nozzle position.Figure 4.19 gives the

Figure 4 .

 4 19 gives the evolution of the droplet mean size, the axial velocity and the radial velocity as functions of the distance from the nozzle. Thus, the droplet mean size was found as 2.6 µm ± 0.8 µm behind the nozzle. The shrinking of the droplet size follows approximately a linear decay. The partially negative velocity and the relatively large error bars of the radial velocity V (Figure4.19, right) can be interpreted as turbulences inside the spray.

Figure 4 .

 4 Figure 4.18. a) Schematic representation of the spray and the measurement zone. The measurement zone depends on x-, y-and z-positions set by the 3D axes table. The origin of the coordinate system is centered at the nozzle outlet position. Shown dimensions are (given in mm). b) Mean axial velocity U distribution in the first quarter of the spray (given in ms -1 ). c) Mean radial velocity V distribution in the first quarter of the spray (given in ms -1 ). d) Mean droplet size distribution in the first quarter of the spray (given in µm).

4. 6 . 4

 64 Estimation of RDX Core Diameters and TNT Shell Thicknesses for Isotropic Hexolite Core/Shell Nanoparticles The theoretical diameters of the RDX cores and the TNT shell thicknesses of the diverse isotropic hexolite core/shell nanoparticles were calculated from the RDX and TNT mass fractions (𝜔 𝑅𝐷𝑋 , 𝜔 𝑇𝑁𝑇 ), the measured particle means radius (𝑟 ℎ𝑒𝑥 ) and the densities of RDX and TNT (𝜌 𝑅𝐷𝑋 , 𝜌 𝑇𝑁𝑇 ). A typical isotropic core/shell particle is presented in Figure 4.20.

Figure 4 . 19 .

 419 Figure 4.19. Left: Evolution of the droplet mean size as a function of the distance from the nozzle. Middle: Axial velocity U as a function as a function of the distance from the nozzle. Right: Axial velocity U as a function as a function of the distance from the nozzle.

Figure 4 . 20 .

 420 Figure 4.20. Schematic representation of an isotropic hexolite RDX/TNT core/shell nanoparticle; 𝑟 ℎ𝑒𝑥 gives the particle radius, 𝑟 𝑅𝐷𝑋 represents the radius of the RDX core and 𝑑 𝑇𝑁𝑇 the TNT shell thickness.

  photoelectron spectroscopy. (This chapter is published at: Hübner, J.; Coty J.-B.; Busby, Y.; Spitzer, D., Formation Mechanisms of Sub-Micron Pharmaceutical Composite Particles Derived from Far-and Near Field Raman Microscopy, chemRxiv.org, 2020, DOI: 10.26434/chemrxiv.12098478.v1)

Chapter 5 nanocomposites.

 5 In this work, surface-enhanced confocal Raman microscopy (SECoRM) and far-field confocal Raman microscopy are applied to investigate sub-micron pharmaceutic nanocomposite particles. The SECoRM technique ensures the high surface sensitivity of surface-enhanced Raman spectroscopy (SERS) and the high resolution of confocal Raman microscopy. Far-field Raman microscopy provides for additional information from a more volume-sensitive technique. Near-field (tip-enhanced) Raman spectroscopy was recently

5. 4 . 1

 41 Structure and Morphology Analysis of Furosemide/PVP Particles A first insight on the size, morphology and crystallinity of Furosemide/PVP particles is provided by SEM and XRPD analyses (Figure 5.2) showing the presence of spherical submicron particles with a rough cauliflower-like surface. Furosemide/PVP particles feature a mean diameter size of 0.38 µm (± 0.24 µm). The Furosemide/PVP sub-micron particles crystal structure was investigated by XRPD. XRPD pattern shows typical reflections from Furosemide crystals superimposed a broad amorphous reflection between 16° and 25° ascribed to PVP. The

Figure 5 . 2 .

 52 Figure 5.2. Left: XRPD pattern of Furosemide and PVP precursors, Furosemide/PVP sub-micron particles and a reference XRD pattern of Furosemide. The coherence lengths derived from the XRPD pattern of Furosemide/PVP indicates the formation of nano-structured Furosemide crystals. Right: SEM image of Furosemide/PVP particles characterized by a rough cauliflower-like surface. Furosemide/PVP particles provide a mean size of 0.38 µm (± 0.24µm).

5. 4 . 2

 42 Far-Field Raman and SERS Investigations of Pure Furosemide and PVP In order to facilitate the interpretation of Raman spectra from Furosemide/PVP composites, far-field Raman spectra and maps of pure, commercial Furosemide and PVP particles are characterized first. Afterwards, specific marker bands allowing to clearly distinguish between Furosemide and PVP signals in the composite particles spectra and images are identified from these datasets. Ideally, marker bands should be selected in wavenumber regions where no (or nearly no) signal superposition occurs. This procedure was already successfully applied in a TERS study aimed to identify RDX and TNT molecules in RDX/TNT composite nanoparticles

(

  red) and PVP (green) compounds are shown in Figure 5.3.a. Specific molecular orientation is known to affect the relative peak intensities in surface-enhanced Raman spectra. Because of this, marker bands selected in far-field spectra may not be useful in the molecular identification in surface-enhanced spectra from silver-coated pure Furosemide and PVP particles. The eventual modification of relative peak intensities is useful to demonstrate that the silver spasers are effective in enhancing the local field at the particles surface. For this, pure compounds particles are sputtered with silver at the relatively high sputtering rate of 4.0 Å/s to favor the formation of silver islands spasers instead of a continuous metal layer. As expected, the obtained SECoRM maps of silver-coated Furosemide and PVP sensibly differ from the corresponding far-field Raman maps by slight peak position shifts and by the relative Raman

Figure 5 . 3 .

 53 Figure 5.3.a) Optical microscopy images and the corresponding far-field Raman maps and spectra for pure Furosemide and PVP. The in-plane ring bending vibration localized at 686 cm -1 (red) is chosen as marker band for Furosemide and the CH2 asymmetric stretching vibrations of PVP at 2924 cm -1 (green) is selected for PVP identification. b) SECoRM maps and SERS spectra of silver coated Furosemide and PVP showing the presence of the chosen marker bands.

Furosemide

  and PVP SERS spectra clearly display two very broad background signals (with point-to-point variable intensity) roughly centered at 1600 cm -1 and 1350 cm -1 . Mao et al. found similar signals in Raman spectra of PVP functionalized with one-dimensional silver nanowires and ascribed the signal at 1605 cm -1 to the shifted C=O stretching vibration (from 1665 to 1605 cm -1

Figure 5 . 4 .

 54 Figure 5.4. Schematic geometrical illustration of the coupling between the incident radiation field (𝐸 ⃗ 𝑖𝑛 ) with localized surface plasmons generating a strong electric field parallel to 𝐸 ⃗ 𝑖𝑛 at the silver nanoparticle surface. Left: The illuminated Ag NP lies on a flat surface thus no (or nearly no) molecules are localized in the enhanced field area. Consequently, no (or nearly no) photons are Raman scattered and contribute the SERS spectrum. Right: The illuminated Ag NP is attached on a perpendicular surface (towards 𝐸 ⃗ 𝑖𝑛 ). Many molecules are localized within the enhanced field leading to frequent Raman scattering.

Figure 5 . 5 .

 55 Figure 5.5. Left: SERS spectrum of PVP before and after background correction. Intense D and G bands from sp 2 -hybridized amorphous carbon species are seen at between 1100 cm -1 and 1700 cm -1 . These signals are ascribed to carbon formed on silver nano grains during laser irradiation. A perfectly similar feature is observed for silvercoated Furosemide. Right: Far-field Raman spectrum of Furosemide coated with 5 nm Titanium. No signals from amorphous carbon species are found. The fluorescence band (between 3300 and 2000 cm -1 ) is ascribed to nanoscale titanium particles.

Figure 5 . 6 .

 56 Figure 5.6.a) Far-field confocal Raman microscopy map of Furosemide/PVP sub-micro composite particles. The yellow pixels indicate the presence of both Furosemide and PVP marker bands in each single point Raman spectrum as shown in the background-corrected spectrum. b) SECoRM map of Furosemide/PVP sub-micro composite particles. The green colored pixels demonstrate that PVP is dominantly found at the Furosemide/PVP particle surface. Background-corrected SERS spectra showing only PVP markers (majority) and PVP-Furosemide markers are presented.

Figure 5 . 7 .

 57 Figure 5.7. Top: Proposed building mechanism of Furosemide/PVP sub-micron composite particles. Bottom left: Schematic representation of a SERS experiment on an estimated classical Furosemide/PVP core/shell particle. In this arrangement no Furosemide molecules are located within the enhanced field area. Thus Stokes-scattering occur only on PVP molecules resulting in the absence of Furosemide vibrational modes within the SERS spectrum.Bottom right: Schematic representation of a SERS experiment on a real Furosemide/PVP sub-micron composite particle. Since the PVP shell appear much smaller in the real case, surface-near Furosemide molecules can be located in the enhanced field area. Consequently, Furosemide vibrational modes appear in some SERS spectra.

Figure 5 . 8 .

 58 Figure 5.8. Results from XPS analysis on Furosemide (a) and Furosemide-PVP composite (b). The peak fitting of the C1s spectra allows comparing the characteristic functional groups of Furosemide (R-COOH) and PVP (R-CNOH). The normalized fraction of PVP (0.55) and Furosemide (0.45) caAn be derived. Tables report the atomic percentage compositions derived from the quantification on high-resolution spectra (Exp) and the values calculated from the chemical formula of Furosemide (Th). The atomic percentage can be used to derive PVP (0.60) and Furosemide (0.40) molecular fractions at the surface.

  photoelectron spectroscopy (Escalab 250 Xi, Thermo Scientific, Waltham, Massachusetts, USA) by acquiring survey and high-resolution scans of C1s, N1s, Cl2p, O1s, S2p with a monochromatic Al k-a source.Two independent procedures are applied to determine the molecular fraction in the composite; the first is by peak fitting the C1s spectrum and comparing the peak area of the Furosemide characteristic signal from R-COOH carboxylic groups (centered at a binding energy of 288.8 eV) and the well-separated characteristic component from R-CNOH groups from PVP (at 287.6 eV). This method allows directly estimating the PVP (55%) and Furosemide (45%) molecular fractions at the surface (Figure8.b) with a sensitivity of few percent mainly limited by the peak fitting accuracy. Alternatively, from the XPS analysis of the surface composition of the pure Furosemide compound formed by SFE (Figure8.a) we can determine the experimental N/Cl ratio R(exp)=1.6. This ratio is lower than the theoretical one based on the chemical compositions R(th)=2 probably because of the use of a chlorinated solvent (DCM).

3 Figure 0 . 1 .

 301 Figure 0.1. Représentation schématique de l'effet Raman. 𝜈 0 et 𝜈 1 représentent l'état vibratoire fondamental et le premier état vibratoire excité d'une molécule. Les flèches noires représentent l'énergie ℎ𝜈 0 et ℎ𝜈 𝑟 d'un photon incident ainsi que l'énergie d'un photon diffusé de Rayleigh. La flèche rouge représente l'énergie ℎ𝜈 𝑠 d'un photon diffusé de Stokes, la flèche verte symbolise l'énergie d'un photon diffusé Anti-Stokes ℎ𝜈 𝑎𝑠 .

Figure 0 . 2 .

 02 Figure 0.2. Haut : Représentation schématique de la résonance plasmonique de surface localisée (LSPR). En bas à gauche : Représentation schématique de l'effet Raman de résonance. En bas à droite : Représentation schématique de l'effet chimique de transfert de charge par résonance.

Figure 0 . 3 .

 03 Figure 0.3. Représentation schématique des différentes géométries d'illumination du TERS. Les flèches vertes représentent la lumière laser incidente ; les flèches rouges représentent la lumière diffusée Raman ; les triangles et sphères gris représentent les pointes métalliques ; les ellipses vertes illustrent le champ amélioré fourni par les LSPR et l'effet paratonnerre ; les segments de cercle bleus représentent les lentilles optiques. A gauche : Configuration de l'éclairage du fond. Au milieu : Configuration de l'éclairage latéral. A droite : Configuration de l'éclairage par le haut.

Figure 0 . 4

 04 Figure 0.4 montre une photographie de la version laboratoire d'un réacteur SFE ainsi qu'un schéma de l'installation. Une solution à 2 % en masse de 2,4 g (5,4 mM) є-CL-20 et de 0,80 g (2,7 mM) β-HMX dans 200 ml d'acétone est préparée. Cette solution est introduite dans le réservoir de stockage de solution du dispositif SFE. Pour le processus de pulvérisation, une buse à cône creux de 80 µm a été utilisée. Elle a été chauffée à 160 °C. La pression en amont de la buse a été réglée à 40 bar. La pression après la buse à l'intérieur de la chambre d'atomisation s'élève à 8 -15 mbar pendant le processus. Après la pulvérisation de la solution complète d'acétone CL-20/HMX, le produit final est recueilli sur les parois du système de filtration sous forme de poudre blanc-jaunâtre (n-CL-20/HMX). Les pointes TERS ont été préparées par une procédure de pulvérisation. Les pointes AFM AppNano ACCESS-NC (Mountain View, USA) ont été fixées sur un gabarit à prisme de 30° pour générer un arrangement perpendiculaire entre la pointe et la cible de pulvérisation. Ce gabarit a été installé dans la chambre à vide d'un système de pulvérisation à plasma d'argon HHV Auto 306 (Bangalore, Inde) à une distance de 10 cm de la cible de pulvérisation. Dans une première étape, 3 nm de titane ont été déposés sur la pointe à une vitesse de dépôt de 0,5 Å/s. Le titane est utilisé comme promoteur d'adhésion entre la couche d'oxyde native de la pointe en Si et l'argent. Ensuite, les pointes Si/Ti ont été recouvertes d'une couche d'argent de 25 nm (vitesse de pulvérisation de 0,5 Å/s). Enfin, les pointes Si/Ti/Ag ont été recuites sur une plaque chauffante pendant 1 minute à 300 °C.

Figure 0 . 4 .

 04 Figure 0.4. a) Photographie d'une version de laboratoire SFE -réacteur. b) Schéma du SFE. B1, B2 : réservoir de solvant et de solution ; R : chambre de réaction ; D : buse à cône creux, F : filtre, P : pompe à vide ; PI : capteur de pression ; TI : capteur de température.

Figure 0 . 6 .

 06 Figure 0.6. À gauche : Spectres Raman en champ lointain de β-CL-20, γ-CL-20, n-CL-20/HMX et β-HMX entre 1700 cm 1 et 600 cm-1. Bandes de référence du CL-20 (β-CL-20 : 1595 cm -1 , γ-CL-20 : 1598 cm -1 , O-N-O vibration d'étirement asymétrique, rouge) et HMX (1417 cm -1 , CH2 vibration d'étirement, vert) ont été choisies pour indiquer la contribution des composés vierges dans n cartes TERS CL-20/HMX. Les intensités Raman sont normalisées pour améliorer la comparaison. A droite : a) Images microscopiques optiques des agglomérats de n-CL-20/HMX. b) -d) Cartes Raman confocale des agglomérats de n-CL-20/HMX présentés en a). La surface balayée est de 15 μm x 15 μm avec un pas de mesure de 250 nm. Les intensités Raman sont normalisées dans toutes les cartes indiquées. b) Les bandes de référence de CL-20 (vibration d'étirement asymétrique O-N-O ; 1602 cm -1 ; rouge) et HMX (vibration d'agitation CH2; 1416 cm -1 ; vert), sont représentées en superposition. En général, si les deux signaux présentent la même intensité, les superpositions sont illustrées en jaune. Comme l'intensité de la bande de référence CL-20 double l'intensité de la bande de marqueurs HMX dans le spectre Raman CL-20/HMX, le CL-20 domine, ce qui donne un aspect plus rougeâtre. c) Cartes Raman confocal de la bande de référence CL-20 (1602 cm -1 ) colorées en rouge. d) Cartes Raman confocal de la bande de référence HMX (1416 cm -1 ) colorées en vert.

Figure 0 . 8 .

 08 Figure 0.8. En haut : Coordonnées normales approximatives ; 𝑞 0;𝐶𝐿-20 est représenté en rouge, 𝑞 0;𝐻𝑀𝑋 est représenté en vert. Comme le plan cristallographique bc -est aligné parallèlement au plan xy -, toutes les orientations possibles entre les coordonnées normales et le vecteur de champ électrique 𝐸 ⃗ 𝑛𝑓 peuvent être illustrées sous forme de cônes autour de l'axe z -. Par conséquent, la semi-orientation actuelle n'affecte pas le rapport d'intensité Raman dans le TERS, puisque les angles inclus par les coordonnées normales et l'axe z (𝐸 ⃗ 𝑛𝑓 ) restent inchangés dans tous les alignements possibles. En bas : Représentation schématique d'une mesure TERS dynamique sans contact de n CL-20/HMX. La sphère grise représente la nanoparticule d'Ag active du TERS. Le cercle vert symbolise le champ renforcé localisé au sommet de la pointe. a) Distance la plus proche entre la pointe de la nanoparticule TERS active Ag et la surface de la particule. b) La pointe de la nanoparticule TERS active Ag se déplace vers le haut. a) et b) Pendant toute la durée des mesures TERS, la couche de surface HMX est toujours située dans une région de champ électrique présentant un renforcement Raman plus important. Ainsi, la bande de référence HMX apparaît plus intense dans les spectres TERS.

  Ensuite, des cartes de Raman confocal en champ lointain ont été enregistrées pour chaque mélange d'hexolite afin d'obtenir un premier aperçu de l'interpénétration des molécules de RDX et de TNT dans les divers mélanges d'hexolites. Le TERS a été réalisé pour étudier la morphologie, la taille et la composition structurale des mélanges d'hexolites à l'échelle nanométrique. Des cartes de surface TERS contenant chacune 100 spectres TERS (10 x 10 spectres) avec des pas de 2 nm -5 nm (selon la taille des nanoparticules) ont été enregistrées. 15 nanoparticules de chaque mélange d'hexolite ont été étudiées. Ainsi, les différences entre les spectres Raman en champ lointain et les spectres TERS des nanoparticules d'hexolite ont permis de conclure sur la disposition distincte des molécules de RDX et de TNT dans les particules. Les cartes Raman confocal en champ lointain révèlent un mélange de molécules RDX et TNT à l'échelle nanométrique dans tous les mélanges d'hexolite, puisque tous les spectres enregistrés contiennent à la fois les bandes de référence RDX et TNT, comme le montre la Figure 0.9 sur l'exemple du SFE-40/60. Contrairement au Raman en champ lointain, les cartes TERS sensibles à la surface contiennent soit la bande de référence TNT uniquement, soit les bandes de référence RDX et TNT. Pas une seule surface de nanoparticule d'hexolite n'est formée uniquement de RDX. Le nombre de surfaces terminées uniquement par le TNT augmente avec la diminution de la fraction massique de RDX dans les mélanges d'hexolites (voir l'histogramme de la ligne inférieure de la Figure 0.9). En outre, de légers décalages de position des groupes fonctionnels N-NO2 et NO2 du RDX et du TNT sont présents dans les spectres TERS, ce qui indique une interaction électrostatique entre les molécules de surface du RDX et du TNT. La différence entre la spectroscopie Raman en champ lointain sensible au volume et le TERS sensible à la surface peut être comprise en prenant l'hypothèse que les mélanges d'hexolite sont constitués de nanoparticules composites structurées hiérarchiquement comme les particules de type patch ou les particules de type coeur/coquille (Figure 0.9). Un mécanisme de formation des diverses nanoparticules d'hexolite est dérivé des données spectroscopiques, de la dynamique du processus SFE et des différentes solubilités du RDX (6,8 g dans 100 g d'acétone) et du TNT(42,4 g pour 100 g) dans l'acétone.37 Dans le procédé SFE, une solution de précurseur sous

Figure 0 . 9 .Figure 0 . 10 .

 09010 Figure 0.9. Top: Cartes de microscopie confocale Raman du SFE 40/60. Les intensités Raman sont normalisées pour une meilleure comparaison. a) Image microscopique de la nano hexolite SFE-40/60. b) Carte Raman confocal correspondante des intensités Raman superposées des bandes de référence RDX et TNT. c) Carte Raman confocal des intensités Raman des bandes de référence RDX. d) Carte Raman confocal des intensités Raman des bandes de référence TNT. α) Spectres Raman et TERS en champ lointain de la poudre d'hexolite 40/60 produite par SFE. Les spectres TERS typiques montrent soit les deux bandes de référence RDX (882 cm -1 ; vibration de cycle) et TNT (1358 cm -1 -1371 cm -1 ; trois vibrations d'étirement symétriques de NO2), soit seulement la bande de marqueurs TNT. Δ indique la vibration d'étirement N-NO2 et la vibration d'étirement asymétrique de NO2 du RDX ; * indique les vibrations d'étirement NO2 du TNT étudiées. En bas à gauche : Tailles moyennes des nanoparticules anisotropes à coeur/coquille d'hexolite (SFE 80/20, SFE 60/40, SFE 40/60) et des particules d'hexolite de taille inférieure au micron (SFE 20/80) en fonction de la fraction massique du RDX dans les solutions précurseurs. Les sphères vertes représentent les molécules de RDX ; les sphères rouges symbolisent les molécules de TNT. En bas à droite : Nombre de nanoparticules d'hexolite anisotropes et de nanoparticules de type patch donnant des surfaces de RDX uniquement, de TNT uniquement et de RDX/TNT mélangés.

Figure 0 . 11 . 2 3 Résumé 1 (

 011231 Figure 0.11. En haut : Mécanisme supposé de formation des nanodiamants dans des conditions idéales présentésous forme de sections transversales en 2D. Le précurseur de l'hexolite est constitué de nanoparticules isotropes hexagonales à coeur/coquille RDX/TNT très serrées. En raison de la pression de détonation plus élevée du RDX par rapport à celle du TNT et de la grande symétrie du précurseur, les produits de réaction du RDX forment des barrières de diffusion sphériques après la détonation. À l'intérieur de la zone de diffusion, des nanodiamants sont formés à partir des atomes de carbone du TNT. Dans le cas idéal, toutes les zones de diffusion ont la même taille et contiennent la même quantité d'atomes de carbone et de sites de nucléation. Ainsi, la même quantité de nanodiamants monodisperses en taille se forme dans les zones de diffusion. Par conséquent, la taille des nanodiamants ne dépend que de l'épaisseur de l'enveloppe du TNT et de la fraction massique du RDX dans le cas idéal. En bas : Mécanisme supposé de construction de nanodiamants dans des conditions réelles, présenté sous forme de sections transversales en 2D. Les nanoparticules anisotropes à coeur/coquille produites par SFE sont très denses. En raison de la distribution de taille des particules précurseurs d'hexolite, une compression hexagonale parfait n'est pas possible. La disposition aléatoire des particules anisotropes coeur/coquille conduit à la formation de zones de diffusion plus ellipsoïdes de tailles légèrement différentes. Par conséquent, les zones de diffusion contiennent des quantités différentes d'atomes de carbone et de sites de nucléation. Ainsi, les nanodiamants synthétisés présentent des tailles légèrement différentes, ce qui conduit à une distribution de la taille des nanodiamants. Par conséquent, la taille moyenne et la distribution de taille des nanodiamants dépendent de l'épaisseur de la coquille du TNT, de la fraction massique du RDX, de la taille moyenne des particules du précurseur d'hexolite, de la distribution de taille du précurseur d'hexolite et du niveau d'anisotropie au sein de la structure du précurseur.

  Abbreviations

Table 2 .1. Quantum numbers

 2

	Quantum numbers Values	Atom terms
	𝑛	1, 2, 3, 4, 5, …	1, 2, 3, 4, 5, …
	𝑙	0	s
		1	p
		2, 3, 4, …	d, f, g, …
		n -1	after f in alphabetic order
	𝑚 𝑙	-l, …, -l, 0, +1, …, +l	e.g. px, py, pz

Table 2 . 2

 22 Differences in degrees of freedom between linear and nonlinear molecules 𝐸 𝑒𝑙 , molecular vibrations and rotations can only absorb (and emit) certain energy values. Thus, also 𝐸 𝑣𝑖𝑏 and 𝐸 𝑟𝑜𝑡 are quantized. Molecular rotations are quantum mechanically treated as linear ridged rotators. However, since later presented Raman experiments are performed on solids only, molecular rotational energy levels are not further discussed. Anyway, molecular

	degrees of freedom	linear molecules	nonlinear molecules
	𝑓 trans	3	3
	𝑓 vib	3n-5	3n-6
	𝑓 𝑟𝑜𝑡	2	3
	𝑓 𝑡𝑜𝑡	3n	3n

Each vibrational degree of freedom and rotational degree of freedom represents a single molecular vibration and rotation which can absorb (and emit) energy. Like the electronic energy Chapter 2

  Chapter 2 spectrum is directly proportional to the polarizability alteration. Since the polarizability is a measure of deformability of molecular orbitals and the shapes of molecule orbitals depend on the molecule conformations, changes of the molecule conformation can influence strongly the measured Raman intensity of a molecular vibration. Furthermore, the classical description of Raman scattering presents that also the orientations of the electric field vector of the incident light 𝐸 ⃗ and the normal coordinate 𝑞 affect the Raman intensities. Thus, the highest Raman intensity is expected for a parallel alignment of 𝐸 ⃗ and 𝑞. On the other hand, if 𝐸 ⃗ and 𝑞 are aligned perpendicular the Raman intensity will tend towards zero.

	transition momentum integral
	µ = ∫ 𝜓 2 * 𝑜 ̂𝜓1

2.2.2 Semi-Quantum Mechanical Description

Principally, the scattering system and the electromagnetic field can be described quantum mechanically. However, a closer quantum mechanical description of the incident and emitted light is not necessary since only the changes of the scattering molecules are of interest. As already introduced in chapter 2.1.2, a molecule can solely undergo an energy transition, if the +∞ -∞ Eq. 2.12 is unequal to zero. This certainly also applies to Raman scattering. In case of Raman scattering, the transition operator 𝑜 ̂ turns to the polarizability operator 𝛼 ̂ and the transition moment integral is given as 𝛼 𝑣 0 →𝑣 1

𝑡𝑟𝑎𝑛𝑠

. Since only fundamental vibrational transition are investigated and thus only the vibrational proportion of the wave functions are of interest, the wave function of the vibrational ground state is called 𝜙 0 and the wave function describing the first excited vibrational state is given as 𝜙 1 in the following to avoid confusions with the total wave function ψ (compare chapter 2.1.1). Accordingly, the Raman transition momentum integral can be given as

  dimensional vector, where N represents the number of molecule atoms. Consequently, the vibrational wave functions are also 3N -dimensional making a symmetry investigation significantly more difficult. However, the normal coordinate 𝑞 is related to the atom positions within the molecule and thus to the symmetry of the molecule. Therefore, the normal coordinate is a basis of an irreducible representation in the symmetry point group of the molecule. The product of the irreducible representations Γ ϕ 0 α ̂ϕ1 of 𝜙 0 , 𝛼 ̂ and 𝜙 1 𝛤 𝜙 0 𝛼 ̂𝜙1 = 𝛤 𝜙 0 𝛤 𝛼 ̂ 𝛤 𝜙 1Eq. 2.27thus provides information about the symmetry of the polarizability transition momentum integral function. The polarizability transition momentum integral 𝛼 𝑣 0 →𝑣 1

The normal coordinate 𝑞 is formed from the x -, y -and z -coordinates of all molecule atoms resulting in Chapter 2 a 3N -𝑡𝑟𝑎𝑛𝑠 is solely unequal to zero if the product Γ ϕ 0 α ̂ϕ1 contains the total symmetric irreducible representation of the symmetry point group of the molecule.

  combinations transform like 𝑎 1𝑔 , 𝑒 1𝑔 and 𝑒 2𝑔 of the 𝐷 ∞ℎ symmetry point group. Therefore, the product of the irreducible representations Γ ϕ 0 α ̂ϕ1 for the CO 2 symmetric

	Chapter 2
	yz respectively their stretching vibration can be given as
	𝑎 1𝑔
	𝛤 𝜙 0 𝛼 ̂𝜙1 = 𝑎 1𝑔 (	𝑒 1𝑔 𝑒 2𝑔	) 𝑎 1𝑔	Eq. 2.28

2

, xy, xz, yz of the underlying symmetric point group. The correlation between 𝛼 𝑣 0 →𝑣 1

𝑡𝑟𝑎𝑛𝑠

and Γ ϕ 0 α ̂ϕ1 is presented in the following by the examples of the CO2 symmetric and asymmetric stretching vibrations.

The CO 2 molecule provides a 𝐷 ∞ℎ symmetry. The CO 2 symmetric stretching vibration respectively its normal coordinate 𝑞 transforms like the irreducible representation 𝑎 1𝑔 of the 𝐷 ∞ℎ point group, the CO2 asymmetric stretching vibration transforms like the irreducible representation 𝑎 1𝑢 . 1 The character table of the 𝐷 ∞ℎ symmetry point group is presented in Figure 2.7. The symmetric behavior of a molecular vibration can also be derived from the molecule symmetry but is not shown here. A detailed introduction to this topic is given in reference 1. Anyway, the wave function of the vibrational ground state ϕ 0 transforms always like the total symmetric representation of the underlying point group. In case of the 𝐷 ∞ℎ symmetry Chapter 2 point group, the total symmetric irreducible representation is 𝑎 1𝑢 (compare Figure 2.7). The wave function of the first excited vibrational state ϕ 1 transforms like the normal coordinate of the vibration. Consequently, the wave function of the first excited vibrational state ϕ 1 of the CO2 symmetric stretching vibration transforms like 𝑎 1𝑔 and the wave function of the first excited vibrational state ϕ 1 of the CO2 asymmetric stretching vibration like 𝑎 1𝑢 . The components of the polarizability operator 𝛼 ̂ by meaning of the basic functions x 2 , y 2 , z 2 , xy, xz, Figure 2.7. Top: Schematic representation of the CO 2 symmetric and asymmetric stretching vibrations. The normal coordinate 𝑞 of the CO2 symmetric stretching vibration transforms like 𝑎 1𝑔 of the 𝐷 ∞ℎ symmetry point group, the normal coordinate 𝑞 of the CO2 asymmetric stretching vibration transforms like 𝑎 1𝑢 . Bottom: Character table of the 𝐷 ∞ℎ symmetry point group. Γ ϕ 0 α ̂ϕ1 can be solved by multiplying the values of each symmetry operation of the irreducible representation with the corresponding values of the other irreducible representations. This product results in a further irreducible presentation as presented for the CO2 symmetric stretching vibration in Table 2.3 -Table 2.5. Table 2.3. Calculation of the first component of 𝛤 𝜙 0 𝛼 ̂𝜙1 of CO2 symmetric stretching vibration

Table 2 . 4 .

 24 Calculation of the second component of 𝛤 𝜙 0 𝛼 ̂𝜙1 of CO 2 symmetric stretching vibration

Table 2 . 5 .

 25 Calculation of the third component of 𝛤 𝜙 0 𝛼 ̂𝜙1 of CO2 symmetric stretching vibration

Table 3 . 2 .

 32 Raman frequencies and their assignments of β-CL-20.

-1 [ 45 ] Assignments [ 45 ]

  Raman spectrum of β-CL-20 in a spectral range between 3200 cm -1 and 300 cm -1 .

								Chapter 3
		vw			456 vw		N-NO2 bend
		vw			511 vw		NNO bend
		vw			560 vw		cage def
		vw			583 vw		cage def
		w				621 w		cage def
		vw			673 vw		NO bend
		vw			718 vw		NO bend, NNO2 bend
		vw			744 vw		ONO bend
		vw			762 vw		ONO bend
		w w	4000		791w b -CL-20 803 w		ONO bend ONO bend
	-	vs				832 s		ring CH wag
	/	w				912 w		ring str, ring CH wag
	intensity Raman	1000 2000 3000 vw w vw w m vw w m m vs vw		932, 944 w 982, 999 w 1052 vw 1071 vw 1093 w 1154 vw 1229 w 1276 m 1295 w 1328 s 1377 vw		ring str, ring CH wag NN str NN str NN str NN str CH bend CH bend, NO str sym CH bend, NO str sym CH bend, NO str sym CH bend CH bend
		vw			1390 vw		CH bend
		vw			1496 vw		CH bend
		vw			1557 vw		ONO str asym
		vw w w	0	3000	2500 1570 vw 1585 w 1593 w	2000	1500 ONO str asym 1000 ONO str asym ONO str asym	500
		m m			1631 w wavenumber / cm -1 ONO str asym 3027 w CH str
		m			3036 m		CH str
		s				3045 s		CH str
		m			3056 m		CH str
	320 m				319 m		cage def, CNN bend
	356 m				353, 369 w		cage def, N-NO2 bend
	400 vw			398 vw		N-NO2 bend
	436 vw			435 vw		N-NO2 bend

Figure 3.15.

β-CL-20 marker vibration (1595 cm -1 ; ONO asymmetric stretching)) is marked in red. Notes: vs -very strong, s -strong, m -medium, w -weak, vw -very weak; asym -asymmetric, sym -symmetric, bend -bending, defdeformation, str -stretching, wag -wagging Chapter 3

Table 3 .3. Raman

 3

frequencies and their assignments of γ-CL-20.

-1 [ 45 ] Assignments [ 45 ]

	Chapter 3			Chapter 3
	vw β-HMX		718 w	ONO bend
		w			750 vw	ONO bend
		vw m		770 vw g -CL-20 795, 807 w	ONO bend ONO bend
	-	w vs	4000	827 vw 846 m	ONO bend ring CH wag
	/	m			858 m	ring CH wag
	intensity Raman	1000 2000 3000 vw vw w m m w w w vw w m	877 w 909 935 vw 956 w 978 w 1055 w 1105 w 1145 m 1179 w 1220 vw 1238 w	ring CH wag ring CH wag ring str, ring CH wag ring str, ring CH wag ring str, ring CH wag ring str, NN str NN str CH bend, ring str CH bend, ring str CH bend CH bend
		s			1254 m	CH bend, NO str sym
		s			1273 m	CH bend, NO str sym
		s			1310	CH bend, NO str sym
		vs	0	1324 s	CH bend, NO str sym
		m w	3000	2500 1336 w 1389 w	2000	1500 CH bend, NO str sym 1000 500 CH bend
		vw vw		1498 vw wavenumber / cm -1 ONO str asym 1554 vw ONO str asym
		vw		-	ONO str asym
		m		1600 m	ONO str asym
		vw		-	ONO str asym
		m		3034 s	CH str
		vs		3043, 3050 s	CH str
		s			3061 m	CH str
	312 m			318 m	cage def
	334 w			340 vw	cage def
	370 w			366 w	cage def
	406 w			406 w	N-NO2 bend
	464 m			470, 479 vw	N-NO2 bend
	508 vw		512 w	N-NO2 bend
	557 vw		560 vw	N-NO2 bend
	585 w			592 w	cage def
	614 vw		613, 619 w	cage def
	635 vw		639 vw	cage def
	684 w			686 vw	ONO bend

Figure 3.16. Raman spectrum of γ-CL-20 in a spectral range between 3200 cm -1 and 300 cm -1 .

γ-CL-20 marker vibration (1598 cm -1 ; ONO asymmetric stretching)) is marked in red. Notes: vs -very strong, s -strong, m -medium, w -weak, vw -very weak; asym -asymmetric, sym -symmetric, bend -bending, defdeformation, str -stretching, wag -wagging

Table 3 .4. Raman

 3

frequencies and their assignments of β-HMX.

Experiment / cm -1 Literature / cm -1 [ 46-47 ] Assignments [ 47 ]

	Chapter 3
	885 s CL-20/HMX (2:1) co-crystal 881		NNC2 str sym
	954 s	950		NN str, CH2 rock
	5000 1082 vw 967 w	965 b -HMX 1090		CNN str asym, CH2 rock NNC2 str asym
	1168 m	1168		CNN str asym, CH2 rock
	/ -1188 m	1190		NC2 str asym
	3000 4000 1267 m 1248 s 1313 s 1352 m 1365 w intensity 1417 m	1248 1268 1312 1350 1368 1418		NC2 str asym NO2 str sym CH2 twist CH2 twist CH2 wag CH2 wag
	2000 1570 m 1436 w 1455 w 1526 w Raman 2995 vs 1000 3027 s	1438 1460 1532 1558 2992 3028		HCH bend HCH bend NO2 str asym NO2 str asym CH2 str sym CH2 str asym
	3038 s	3037		CH2 str asym
	0
	3000	2500	2000	1500	1000	500
		wavenumber / cm -1
	364 m	342		CN str, NN str, NCN bend
	414 m	412		NNO bend, NNC bend
	436 m	432		CNC bend, NNC bend
	593 w	597		NNO bend
	635 w	638		NNO bend, NN str
	664 w	662		NNO bend, NN str
	720 vw	721		ONO bend, CN str
	761 v	759		ONO wag
	837 vs	834		NC2 str sym

Figure 3.17. Raman spectrum of β-HMX in a spectral range between 3200 cm -1 and 300 cm -1 .

β-HMX marker vibration (1417 cm -1 ; CH2 wagging) is marked in green. Notes: vs -very strong, s -strong, m -medium, w -weak, vw -very weak; asym -asymmetric, sym -symmetric, bend -bending, rock -rocking, str -stretching, twist -twisting, wag -wagging

Table 3 . 5 .

 35 Raman frequencies and their assignments of CL-20/HMX. The summation of 𝑞 𝑚;𝛽-𝐶𝐿-20 and 𝑞 𝑚;𝛾-𝐶𝐿-20 leads to the approximated normal coordinates 𝑞 0;𝛽-𝐶𝐿-20 and 𝑞 0;𝛾-𝐶𝐿-20 of the ONO asymmetric stretching vibrations of the CL-20 polymorphs.Since the Cartesian coordinates of crystallographic data do not correspond with that of the Raman scattering system, they had to be transformed. Calculations and number values of 𝑞 0;𝛽-𝐶𝐿-20 and 𝑞 0;𝛾-𝐶𝐿-20 are summed up in Table3.6 respectively Table3.7.

			CL-20/HMX (2:1)
		4000
	-
	/
	intensity	3000
	Raman	1000 2000
		0
		3000	2500	2000	1500	1000	500
			wavenumber / cm -1
		Raman frequencies of CL-20/HMX (2:1)
	Experiment / cm -1	Literature / cm -1 (experimental) [ 14 ]		Assignments [ 14 ]
	356 m		360.4 w			HMX: ring def, CNC bend; CL-20: cage def
	393 w		395.3 w			CL-20: CNC bend, N-NO2 bend
	414 w		415.1 w			HMX: NCN bend, N-NO2 bend
	443 w		441.1 m			CL-20: CNC bend, N-NO2 bend

Figure 3.18. Raman spectrum of CL-20/HMX co-crystal (2:1) in a spectral range between 3200 cm -1 and 300 cm-1 .

Table 3 . 6 .

 36 Calculation and number values of 𝑞 0;𝛽-𝐶𝐿-20

	β-CL-20

Table 3 . 7 .

 37 Calculation and number values of 𝑞 0;𝛾-𝐶𝐿-20In case of the HMX CH2 wagging vibration the path which is covered by the H is a circular segment around the central C atom. This movement can be described mathematically by a partially rotation of a plane defined by C and H atoms of a CH2 functional group around the C atom. Thus the direction of this movement can be expressed approximately by the normal vector 𝑞 𝑚;𝐻𝑀𝑋 of this plane at the equilibrium position (Figure3.19, bottom right). The direction of inside the CL-20/HMX co-crystal 𝑞 0;𝐻𝑀𝑋 has to be split into 𝑞 0;𝐻𝑀𝑋-1 and 𝑞 0;𝐻𝑀𝑋-2 . The calculation and the number values of 𝑞 0;𝐻𝑀𝑋-1 and 𝑞 0;𝐻𝑀𝑋-2 can be found in Table3.8 and Table3.9. The approximated normal coordinate of both HMX conformations can be expressed as the average of 𝑞 0;𝐻𝑀𝑋-1 and 𝑞 0;𝐻𝑀𝑋-1 :

	γ-CL-20

these vectors and thus their algebraic signs are depicted in Figure

3

.19 (bottom left). Here again the summation of 𝑞 𝑚;𝐻𝑀𝑋 gives the approximated normal coordinate 𝑞 0;𝐻𝑀𝑋 of the HMX CH2 wagging vibration. Since HMX molecules in β conformation appear as image and mirror image Chapter 3

Table 3 . 8 .

 38 Calculation and number values of 𝑞 0;𝐻𝑀𝑋-1

	𝒒 ⃗ ⃗ 𝟏;𝑯𝑴𝑿-𝟏		𝒒 ⃗ ⃗ 𝟐;𝑯𝑴𝑿-𝟏
	x	-0.293	x	0.324
	y	-0.531	y	0.444
	z	0.795	z	0.835
	𝒒 ⃗ ⃗ 𝟑;𝑯𝑴𝑿-𝟏		𝒒 ⃗ ⃗ 𝟒;𝑯𝑴𝑿-𝟏
	x	0.324	x	-0.293
	y	0.444	y	-0.531
	z	0.835	z	0.795
	𝒒 ⃗ ⃗ 𝟎;𝑯𝑴𝑿-𝟏		crystal axis	projection
	x	1.234	a	z → x
	y	1.950	b	x → y
	z	0.080	c	y → z
	𝒒 ⃗ ⃗ 𝟎;𝑯𝑴𝑿-𝟏 (in Raman scattering system)
	x		0,080
	y		1,234
	z		1,950

β-HMX-1 (along b-axis)

Table 3 . 9 .

 39 Calculation and number values of 𝑞 0;𝐻𝑀𝑋-1

	β-HMX-2 (along b-axis)
	𝒒 ⃗ ⃗ 𝟏;𝑯𝑴𝑿-𝟐		𝒒 ⃗ ⃗ 𝟐;𝑯𝑴𝑿-𝟐
	x	0.293	x	-0.324
	y	0.531	y	-0.444
	z	-0.795	z	-0.835
	𝒒 ⃗ ⃗ 𝟑;𝑯𝑴𝑿-𝟐		𝒒 ⃗ ⃗ 𝟒;𝑯𝑴𝑿-𝟐
	x	-0.324	x	0.293
	y	-0.444	y	0.531
	z	-0.835	z	-0.795
	𝒒 ⃗ ⃗ 𝟎;𝑯𝑴𝑿-𝟐		crystal axis	projection
	x	1.234	a	z → x
	y	1.950	b	x → y
	z	0.080	c
	x		1.950
	y		0.080
	z		1.234

y → z 𝒒 ⃗ ⃗ 𝟎;𝑯𝑴𝑿-𝟐 (in Raman scattering system)

Table 3 .

 3 10. Standardization of approximated normal mode vectors

			Standardization of 𝒒 ⃗ ⃗ 𝟎;𝜷-𝑯𝑴𝑿
		𝒒 ⃗ ⃗ 𝟎;𝜷-𝑪𝑳-𝟐𝟎		standardized 𝒒 ⃗ ⃗ 𝟎;𝜷-𝑪𝑳-𝟐𝟎
	x	1.397	factor:	x	0.44
	y	-1.313	3.182	y	-0.41
	z	2.539		z	0.80
			Standardization of 𝒒 ⃗ ⃗ 𝟎;𝜷-𝑯𝑴𝑿
		𝒒 ⃗ ⃗ 𝟎;𝜸-𝑪𝑳-𝟐𝟎		standardized 𝒒 ⃗ ⃗ 𝟎;𝜸-𝑪𝑳-𝟐𝟎
	x	1.323	factor:	x	0.42
	y	-1.384	3.175	y	-0.44
	z	2.533		z	0.80
			Standardization of 𝒒 ⃗ ⃗ 𝟎;𝜷-𝑯𝑴𝑿
		𝒒 ⃗ ⃗ 𝟎;𝜷-𝑯𝑴𝑿		standardized 𝒒 ⃗ ⃗ 𝟎;𝜷-𝑯𝑴𝑿
	x	1.950	factor:	x	0.84
	y	0.080	2.309	y	0.03
	z	1.234		z	0.53

Table 4 . 1 .

 41 Mass and amount of substance of RDX and TNT for each hexolite mixture

	hexolite mass	RDX		TNT
	fraction		amount of		amount of
	(RDX/TNT)	mass / g	substance /	mass / g	substance /
			mmol		mmol
	80/20	6.5	29.3	1.7	7.3
	60/40	4.9	22.1	3.3	14.3
	40/60	3.3	14.7	4.9	21.6
	20/80	1.7	7.5	6.5	28.6

Table 4 . 2 .

 42 Band positions of NO 2 vibrational modes of RDX and TNT detected in the far-field

	Raman and TERS spectra
	vibration [ref. 44-46, 48-49 ]	far-field Raman wavenumber / cm -1	TERS wavenumber / cm -1
	RDX: N-NO2 str	1270	1256 -1283
	RDX: N-NO2 str	1309	1296 -1322
	RDX: NO2 asym str	1541	1527 -1546
	RDX: NO2 asym str	1573	1565 -1584
	RDX: NO2 asym str	1592	1585 -1622
	TNT: 3x NO2 sym str	1358 -1371	1354 -1399
	TNT: NO2 asym str	1534	1540 -1546
	TNT: NO2 asym str	1553	1559 -1572
	TNT: NO2 asym str	1617	1617 -1653
	Thus, in SFE RDX nanoparticles are formed followed by the attachment of TNT molecules. 44
	4.4.4 60/40 SFE Hexolite Mixture
	60/40 hexolite (SFE-60/40) nanoparticles produced in SFE were collected as a yellowish
	powder. SFE-60/40 are composed of spherical nanoparticles with a mean size of 32.2 nm ±
	12.3 nm. Confocal Raman microscopy of SFE-60/40 points to a similar RDX/TNT mixture
	found for SFE-80/20. The TERS experiments show that none of the 15 investigated SFE-60/40

asym: asymmetric; sym: symmetric; str: stretching; investigated RDX vibrations are marked in green; investigated TNT vibrations are marked in red Since only RDX/TNT mixed surfaces and pure TNT surfaces could be identified within the TERS spectra, it is concluded that TNT nano-patches grow on RDX particle surfaces in the SFE process. This assumption is supported by the lower solubility of RDX in acetone than TNT. nanoparticles surfaces is built up by RDX only. In contrast to SFE-80/20, the TERS surface investigations of SFE-60/40 particles reveal that 10 of 15 nanoparticle surfaces consist of TNT only whereas 5 of 15 TERS maps of SFE-60/40 surfaces contain both RDX and TNT marker bands. Similar results were earlier reported by Deckert-Gaudig et al. using a different TERS setup.

  why TERS surface maps of RDX/TNT core/shell nanoparticles still include RDX signals even if the mass fraction of TNT amounts 80 % yet. Estimations of the RDX core diameters and the TNT shell thicknesses reveal that a typical SFE-60/40 core/shell nanoparticle provides a RDX core with a diameter of 26.7 nm and a TNT shell thickness of 2.7 nm. SFE-40/60 is formed by an RDX core with a diameter of 29.4 nm and a 4.1 nm thick TNT shell. The diameter of the RDX core of a typical SFE-20/80 nanoparticle amounts 59.7 nm and the TNT shell thickness amounts 12.9 nm. The estimations are based on the particle mean sizes of the diverse hexolite nano-composites and the densities of crystalline RDX and TNT. The results are summarized in Table4.3. The detailed calculations are given within the Supporting Information.

	Table 4.3. Estimated RDX core diameters and TNT shell thicknesses for isotropic hexolite
	core/shell nanoparticles
	mass fraction ratio (RDX/TNT)	particle mean size	RDX core diameter TNT shell thickness
	60/40	32.2 nm	26.7 nm	2.7 nm
	40/60	29.4 nm	21.2 nm	4.1 nm
	20/80	59.7 nm	34.0 nm	12.9 nm
	Due to the rapid field decay of the enhanced electric field generated by the LSPR of the silver
	nanoparticle localized at the AFM tip apex, the appearance of RDX signals within TERS spectra
	is at least very unlikely for SFE-60/40 and SFE-40/60 de facto impossible for SFE-20/80
	(Figure 4.10.a).

  Chapter 4 decrease of nucleation sites and thus in smaller nanodiamonds. However, TERS investigations could show that SFE produced hexolite mixtures appear as anisotropic RDX/TNT core/shell particles and not as uncoupled RDX and TNT nanoparticles as presumed. Furthermore, here presented nanodiamonds do not follow this trend completely. Nanodiamonds resulting from the detonation of SFE-20/80 show a larger particle mean size than nanodiamonds produced from SFE-60/40 although SFE-60/40 core/shell nanoparticles provide a smaller particle mean size than SFE-20/80 nanoparticles. Consequently, the building mechanism proposed byPichot et al.

  𝜔 𝑅𝐷𝑋 𝑑 𝑇𝑁𝑇 + 𝑑 𝑇𝑁𝑇 (𝑎 ℎ𝑒𝑥 + On the other hand, ND-20/80 exhibit a mean size of 3.7 nm ± 0.08 nm though the SFE-20/80 precursor shows a larger core/shell nanoparticle means size and a larger TNT shell thickness 𝑑 𝑇𝑁𝑇 . Since this hexolite mixture includes the lowest RDX mass fraction SFE-20/80 particles should provide the smallest level of anisotropy of all presented hexolite mixtures. The lowest RDX mass fraction results further in the lowest temperature and pressure values inside the diffusion zones of all hexolite materials. Consequently, a less amount of carbon atoms converts into nanodiamonds leading to smaller nanodiamond mean sizes. Due to the smaller mean size of ND-20/80 compared to ND-60/40, the symmetry parameters, especially the level of anisotropy 𝑎 ℎ𝑒𝑥 , seem to affect the nanodiamond mean size at least as strong as the TNT shell thickness 𝑑 𝑇𝑁𝑇 and the RDX mass fraction 𝜔 𝑅𝐷𝑋 . Anyway, the smallest nanodiamond mean size including the narrowest size distribution could be found at ND-40/60 with 3.6 ± 0.04 nm. The SFE-40/60 hexolite precursor presents a thicker TNT shell but a less level of anisotropy 𝑎 ℎ𝑒𝑥 than SFE-60/40. Contrary, SFE-40/60 is defined by a thinner TNT shell but a higher level of anisotropy than SFE-20/80. Since the level of anisotropy 𝑎 ℎ𝑒𝑥 and the TNT shell

			Chapter 4
	above.
	|𝑠 ℎ𝑒𝑥 | 𝑑 ℎ𝑒𝑥	)	Eq. 4.1

Table 4 . 4 .

 44 Raman frequencies and their assignments of RDX

		Raman frequencies of RDX
	Experiment / cm -1	Literature / cm -1 [ 47 ]	Assignments [ 44, 46-47 ]
	345 m	347	ring twist
	410 w	415	ring bend
	461 m	464	ring bend, N-N str
	489 vw	488	ring twist, NO2 sci
	589 w	588	ring twist
	603 w	607	ring rock
	667 vw	670	ring bend
	736 vw	739	N-NO2 umb
	757 vw	757	ring bend, NO2 sci
	785 vw	788	C-N str, NO2 sci
	847 m	848	N-N str, NO2 sci
	882 vs	885	ring breath
	943 w	945	N-N str
	1024 vw	1023	CH2 rock
	1031 vw	1031	N-C str
	1218 m	1215	N-C str
	1270 w	1273	N-NO2 str sym
	1309 s	1309	CH2 twist, N-NO2 str
	1346 vw	1346	CH2 wag
	1377 w	1377	CH2 twist
	1387 w	1388	CH2 wag
	1432 w	1436	CH2 sci
	1458 vw	1460	CH2 sci
	1509 vw	1508	CH2 sci
	1541 vw	1542	NO2 str asym
	1573 vw	1573	NO2 str asym
	1592 w	1595	NO 2 str asym
	2907 w	2906	C-H str
	2949 vs	2949	C-H str
	3003 s	3003	C-H str
	3072 m	3067	C-H str
	3077 m	3075	C-H str
	RDX marker band vibration (882 cm-1; ring breathing) is marked in green. Notes: vs -very

strong, s -strong, m -medium, w -weak, vw -very weak; asym -asymmetric, bend -bending, breath -breathing, rock -rocking, sci -scissoring, sym -symmetric, , str -stretching, twisttwisting, umb -umbrella, wag -wagging Chapter 4

Table 4 . 5 .

 45 Raman frequencies and their assignments of TNT

		Raman frequencies of TNT
	Experiment / cm -1	Literature / cm -1 [ 49 ]	Assignments [ 44, 49 ]
	323 w	326	C-N tors, ring bend
	352 vw	355	ring bend
	367 w	366	CH3 def, ring bend
	439 vw	442	C-CH3 wag

Table 4 . 6 .

 46 Estimated RDX core radii and diameters and TNT shell thicknesses for isotropic

	hexolite core/shell nanoparticles
	mass fraction ratio (RDX/TNT)	particle mean size	𝒓 𝒉𝒆𝒙	𝒓 𝑹𝑫𝑿	RDX core diameter	TNT shell thickness
	60/40	32.2 nm	16.1 nm	13.4 nm	26.7 nm	2.7 nm
	40/60	29.4 nm	14.7 nm	10.6 nm	21.2 nm	4.1 nm
	20/80	59.7 nm	29.9 nm	17.0 nm	34.0 nm	12.9 nm

  Dans le cadre de cette thèse, divers composites organiques et énergétiques nano et submicroniques sont caractérisés par la spectroscopie Raman conventionnelle (Raman en champ lointain), la spectroscopie Raman exaltée au voisinage d'une pointe AFM (TERS) et la spectroscopie Raman exaltée en surface (SERS) (Raman en champ proche). Les détails expérimentaux, les résultats et les conclusions qui ont pu être tirés de ces recherches ainsi qu'une description détaillée du contexte théorique de la diffusion Raman, du SERS et du TERS sont présentés dans ce qui suit. Si un photon ayant une énergie ℎ𝜈 0 qui ne coïncide pas avec la différence d'énergie entre deux états d'énergie moléculaire interagit avec une molécule (ou la matière en général), la molécule peut être excitée dans un état d'énergie virtuel 𝑉. L'énergie des photons est donc

	Résumé
	Investigation structurale de nanocomposites Contexte théorique
	La diffusion Raman
	énergétiques Le TERS et le SERS sont basés sur l'effet Raman. L'effet Raman est un processus de diffusion et pharmaceutiques par
	inélastique de la lumière par la matière, par lequel l'énergie (respectivement la longueur d'onde AFM-TERS et SERS ou la fréquence angulaire) de la lumière diffusée est soit diminuée soit augmentée (diffusion
	Raman).
	197

Introduction

La cinétique des réactions chimiques et physiques dépend non seulement des produits chimiques utilisés, mais aussi de leur taille, de leur morphologie, de la composition de leur surface et de la disposition de leurs molécules. Cette relation peut être observée, par exemple, dans l'adsorption de gaz sur la surface des particules ou lorsque des produits chimiques sont dissous dans un solvant. L'influence de la taille, de la morphologie, de la composition et de la disposition des surfaces devient particulièrement évidente dans les réactions à l'état solide, par exemple les combustions des précurseurs de particules. 1-2 L'analyse de ces paramètres est une tâche difficile, en particulier pour les nanoparticules organiques et les nanoparticules submicroniques nanostructurées. Les molécules organiques métastables, en particulier les composés énergétiques et pharmaceutiques, ont tendance à se dissocier sous l'apport énergétique de diverses méthodes de mesure telles que la microscopie électronique. Il faut donc utiliser une technique d'analyse douce qui offre des résolutions latérale et spectroscopique suffisantes. généralement inférieure à celle requise pour une transition électronique moléculaire (sauf pour la spectroscopie Raman par résonance). Les états virtuels sont des états de courte durée et non stationnaires d'une molécule car ils ne sont pas des fonctions propres d'un opérateur.

  ). Ce processus de diffusion inélastique est la diffusion de Stokes. La différence d'énergie entre le photon absorbé et le photon émis coïncide ainsi avec la différence d'énergie entre l'état vibratoire fondamental et le premier état vibratoire (Figure 0.1). Si la molécule est déclarée dans le premier état vibratoire excité de l'état électronique de base avant d'absorber un photon (𝑆 0 𝑣 1 → 𝑉), la molécule peut se relaxer à ses états vibratoires et électroniques de base (𝑆 0 𝑣 0 → 𝑉). Par conséquent, l'énergie du photon émis est augmentée par la différence d'énergie entre l'état vibratoire fondamental et le premier état vibratoire excité. Ce processus de diffusion inélastique est appelé diffusion anti-Stokes. Du fait que beaucoup moins de molécules se trouvent dans des états vibratoires excités à température ambiante, la diffusion Anti-Stokes se produit moins probablement que la diffusion Stokes, ce qui se traduit par des intensités de fréquence moindres dans les spectres Raman. appelée plasmon de surface localisé (LSP). Si une nanoparticule métallique interagit avec la lumière, les électrons de valence sont forcés vers les bords de la particule dans la direction opposée au vecteur de champ électrique de la lumière incidente 𝐸 ⃗ 𝑖𝑛 . En raison du déplacement des électrons de valence par rapport aux noyaux atomiques dans le réseau cristallin, il se produit une séparation de charge entre les noyaux atomiques chargés positivement et les électrons chargés négativement (Figure 0.2). Par conséquent, une force de restauration 𝐹 𝐶 résulte de l'attraction de Coulomb. 4 Cette force induit une oscillation des électrons de valence. En raison de l'oscillation collective des électrons de valence, un champ électrique est créé à la surface des nanoparticules parallèlement au vecteur de champ électrique

	Raman se trouvent considérablement augmentées lorsque les molécules sont situées à proximité
	ou sur des surfaces de métaux (précieux) nanostructurés. Cet effet est devenu célèbre sous le
	nom de diffusion Raman exaltée de surface (SERS). Le mécanisme exact du SERS est toujours
	en cours de discussion, mais deux théories ont été avancées. D'une part, une amplification du
	champ électrique fournie par les nanostructures métalliques participe principalement à
	l'amélioration globale de l'intensité Raman (amplification électromagnétique). Si une surface
	métallique est placée dans le champ électrique de la lumière incidente, les électrons de valence

Spectroscopie Raman amplifiée de surface (SERS)

Comme nous l'avons déjà mentionné précédemment, la probabilité qu'un photon soit diffusé par effet Raman au niveau d'une molécule est de 10 -8 -10 -5 ce qui est donc très faible. Par conséquent, un très grand nombre de molécules est nécessaire, ce qui rend impossible les recherches Raman sur des nanoparticules isolées. On peut cependant montrer que les intensités commencent à osciller. En général, l'oscillation collective des électrons de valence par rapport aux noyaux des atomes peut être mathématiquement décrite comme une quasi-particule appelée plasmon. Dans le cas des nanoparticules métalliques, l'oscillation collective des électrons de Résumé valence est 𝐸 ⃗ 𝑖𝑛 de la lumière incidente. L'intensité de ce champ électrique est fortement augmentée par

  comprises entre 488 nm et 568 nm, tandis que les pointes en or sont utilisées pour des longueurs d'onde comprises entre 633 nm et 785 nm.5 La géométrie du dispositif TERS, c'est-à-dire l'alignement entre l'optique d'éclairage et la pointe, doit être choisie de manière à ce que l'échantillon soit placé dans la zone de champ électrique renforcé à la surface des nanoparticules d'argent ou d'or. En fonction de l'échantillon, la pointe du TERS peut être éclairée par le bas, par le côté ou par le haut (Figure 0.3). Comme le champ exalté provient d'une seule nanoparticule métallique dans le TERS et en raison de la rapide décroissance du champ, le TERS offre une haute résolution spatiale (< 1 nm).

	Spectroscopie Raman à exaltation de pointe (TERS)
	Fondamentalement, le TERS est la combinaison du SERS et de la microscopie à sonde à
	balayage (SPM) comme la microscopie à force atomique (AFM). La première expérience TERS
	a été réalisée par Stöckle et al. en 2000. 20 Afin de combiner le SERS avec l'AFM, la pointe de
	la sonde utilisée doit présenter une caractéristique plasmonique comme une simple

Raman. L'exaltation chimique est fortement liée à l'effet Raman de résonance. Ici, la longueur d'onde de la lumière incidente coïncide avec la différence d'énergie entre niveau électronique fondamental et le premier état électronique excité (ou même supérieur) de la molécule (Figure

0

.2, en bas à gauche). Comme il est plus probable qu'une transition électronique se produise de l'état électronique de base à un état électronique excité plutôt qu'à un état virtuel, les intensités Raman sont augmentées.

3

Dans le cas de la théorie de l'exaltation chimique, les molécules sont chimisorbées à la surface des nanoparticules. Si le niveau de Fermi de la nanoparticule est proche du niveau d'énergie des orbitales de la molécule, elles peuvent former un état de transfert électronique de charge (TC) métal-molécule (ou molécule-métal) (Figure

0

.2, en bas à droite).

19

Si la longueur d'onde de la lumière incidente coïncide avec la différence d'énergie entre l'état moléculaire fondamental et l'état TC, les intensités Raman de certaines vibrations moléculaires peuvent être augmentées comme décrit ci-dessus. Le produit chimique participe avec des facteurs compris entre 10 2 et 10 4 à l'exaltation totale de l'intensité Raman. nanoparticule d'argent ou d'or montée au sommet de la pointe. L'apex de la pointe doit être éclairé par un laser dont la longueur d'onde correspond au LSPR de la caractéristique plasmonique. En général, les pointes en argent sont utilisées pour des longueurs d'onde d'excitation

  12,[21][22] Les spectres de TERS et de SERS peuvent être différents des spectres Raman en champ lointain car le TERS est très sensible à la surface. Les molécules de surface s'adaptent à des conformations différentes de

celles des molécules de masse, ce qui entraîne de légers changements de position de bande, des changements d'intensité et l'apparition et la disparition de modes vibratoires uniques.

Tableau 1 .

 1 Masse et quantité de substance du RDX et du TNT pour chaque mélange d'hexolite

	fraction		RDX		TNT
	massique de		quantité de		quantité de
	l'hexolite	masse / g	substance /	masse / g	substance /
	(RDX/TNT)		mmol		mmol
	80/20	6,5	29,3	1,7	7,3
	60/40	4,9	22,1	3,3	14,3
	40/60	3,3	14,7	4,9	21,6
	20/80	1,7	7,5	6,5	28,6

RDX/TNT de type coeur/coquille formant un matériau précurseur hexagonal compact est d'abord examiné (Figure4.13). Après l'allumage et la détonation, les produits de réaction du RDX sont accélérés en s'éloignant des centres des particules, tandis que les produits de réaction du TNT sont accélérés à la fois vers et à l'opposé des centres des particules. La migration des produits de réaction du RDX et du TNT est déterminée par leurs pressions de détonation spécifiques. En fonction du rapport massique RDX/TNT, les effets de migration concurrents peuvent être dominés soit par la pression de détonation du RDX, soit par celle du TNT. En général, le RDX fournit une pression de détonation plus élevée que le TNT. Dans le cas où la pression de détonation du RDX domine, les produits de réaction du TNT sont poussés au centre du tétraèdre composé de particules d'hexolite adjacentes très serrées, tandis que les produits de réaction du RDX forment une barrière de diffusion sphérique autour de celui-ci. Si la pression de détonation du TNT domine les effets de migration, les produits de réaction du RDX forment des barrières de diffusion sphériques autour du centre d'un tétraèdre. Selon que la pression de détonation du RDX ou du TNT domine, différentes conditions thermodynamiques de pression et de température prévalent dans les zones de diffusion. Comme dans le cas de la pression de détonation du RDX, les produits de la réaction du TNT sont poussés vers les centres du tétraèdre, la pression et la température sont plus élevées dans ces zones de diffusion résultantes que dans les zones de diffusion formées dans un système dominé par le TNT. Des pressions et des températures plus élevées favorisent une conversion plus complète des atomes de carbone en nanodiamants, ce qui conduit à des particules de nanodiamants plus grosses. Par conséquent, une pression et une température plus basses entraînent une diminution du nombre d'atomes de carbone formant des nanodiamants et donc des nanoparticules plus petites. En raison de la

Acknowledgment

At this point I would like to express my gratitude to all who have assisted and motivated me during the period of this doctoral thesis.

At first, I would like to thank the French-German-Research Institute of Saint-Louis and the Région Grand Est for the financial support. ONO str asym 1576 w 1575.8 m HMX: ONO str asym 1595 m 1597.ONO str asym 1602 m 1604.ONO str asym 1608 m 1615.ONO str asym 3006 s 3006.5 s HMX: CH str 3016 s 3016.4 m HMX: CH str 3032 s 3033.

Chapter 5 acceleration voltage was 40 kV and the operating current 40 mA. The step size for all scans was set to 0.0148° (2 θ). The loose powder samples were measured on a rotating sample holder.

The coherence length Lhkl was calculated from the full width at half maximum (FWHM) of single diffraction peaks by using the well-known Scherrer equation assuming Gaussian-shaped peaks.

Calculation of Approximated 3D Normal Coordinates

In a first step x-, y-and z-coordinates from all atoms which are involved mainly at the investigated molecular vibration were extracted from crystallographic data published by Bolton et al. 13 These atoms are depicted and numbered in Figure 3.19. As already described within the paper, the N atoms of the NO2 functional groups of CL-20 oscillate oppositely to the O atoms. Furthermore, the atomic derivations from their equilibrium position are the same for all involved NO2 groups. Thus, the vectors 𝑞 𝑚;𝛽-𝐶𝐿-20 , respectively 𝑞 𝑚;𝛾-𝐶𝐿-20 , describe approximately the oscillation of the NO2 group. These vectors are presented as yellow arrows in Figure 3. 19. The directions of these vectors correspond to the direction of movement of the

Chapter 4

The level of anisotropy is defined as the distance between the hexolite particle center and the center of the corresponding RDX core 𝑑 ℎ𝑒𝑥→𝑅𝐷𝑋 divided by the hexolite particle mean radius 𝑟 ℎ𝑒𝑥 :

𝑑 ℎ𝑒𝑥→𝑅𝐷𝑋 𝑟 ℎ𝑒𝑥 Eq. 4.2

The described correlation presented in Eq. 4.1 allows an interpretation of the nanodiamond mean sizes and size distributions of the presented detonation synthesized nanodiamonds.

ND-80/20 provides with 6.1 nm ± 0.2 nm the largest particle means size and the broadest size distribution. Due to the formation of patchy or Janus particles, the level of anisotropy 𝑎 ℎ𝑒𝑥 of SFE-80/20 nanoparticles has the highest value of all investigated hexolite nano-mixtures because 𝑑 ℎ𝑒𝑥→𝑅𝐷𝑋 is nearly equal to 𝑟 ℎ𝑒𝑥 (compare Figure 4.9.a). Furthermore 𝑑 𝑇𝑁𝑇 -which is in case of SFE-80/20 more a TNT patch diameter than a shell thickness -is supposed to be relatively large since TNT solely form patches at single spots at the RDX surface. Additionally, the formation of patchy particles causes a reduction of the symmetry within the precursor charge compared to core/shell particles. Moreover, SFE-80/20 provides the broadest size distribution and thus the highest particle size standard deviation reducing further the symmetry of the precursor charge. Chapter 5 A solution of 3.0 g Furosemide and 1.0 g PVP in a solvent mixture of 240 mL ethanol and 160 mL dichloromethane was prepared first. This solution was given into the solution tank of a vertical SFE crystallizer pressurized at 40 bar (N2). The Furosemide/PVP solution was sprayed through a 120 °C preheated hollow cone nozzle with a diameter of 100 µm into a vacuum reactor chamber with a base pressure below 0.1 mbar. The pressure inside the SFE atomization chamber varied between 5 and 10 mbar during the spraying process. After the fast solvent evaporation Furosemide/PVP submicron particles are collected with a steel filter as a white-yellowish ultra-fine powder.

Silver Coating of Furosemide, PVP and Furosemide/PVP Sub-Micron Particles

For surface-enhanced Raman spectroscopy, Furosemide, PVP and Furosemide/PVP submicron particles were deposited on glass substrates and uniformly distributed by a gentle robbing of two glasses. Afterwards, the cover glasses were sputtered with silver in an HHV Auto 306 (Bangalore, India) sputtering device at a distance of 15 cm from the silver target. All prepared samples were directly coated with 40 nm nominal thickness of silver at a deposition rate of

Analysis Methods

Scanning Electron Microscopy (SEM) images were recorded on a FEI Nova NanoSEM 450 (Hillsboro, Oregon, USA) at an acceleration voltage of 10 kV using an in-lens secondary electron detector. To prevent charge accumulation, Furosemide/PVP particles were coated with a 10 nm thick layer of gold by sputtering before the analysis. The size distribution and particle mean size of Furosemide/PVP sub-micron particles were determined by measuring 260 particles using ImageJ software. The particle size distribution was fitted with a Gaussian line shape using OriginPro 2019b (version 9.6.5.169).

X-ray powder diffraction (XRPD) was performed on a Bruker AXS Advance D8 (Karlsruhe;

Germany) diffractometer using Cu-Kα radiation (λ= 1.54 Å) in Bragg-Brentano geometry. The Alternatively, from the N/Cl ratio in pure Furosemide formulated by SFE, we derive the Furosemide contribution to the total nitrogen content in the PVP/Furosemide surface which allows counting the ratio between Furosemide and PVP molecules at the composite surface.

This ratio is compared with the Furosemide:PVP molar ratio in the precursor solution to evaluate the tendency of each compound to form a core/shell arrangement.