Mathias Dolci

Delphine Toulemon

Zaineb Chaffar

Jean-Luc Bubendorff

Frederik Tielens

Monica Calatayud

Spyridon Zafeiratos

Sylvie Begin-Colin

Benoit P Pichon

Nanoparticle Assembling through Click Chemistry Directed by Mixed SAMs for Magnetic Applications

Keywords: click chemistry, defect mediated replacement, structure, collective properties, thiol molecule, phase segregation, theoretical modeling

Click" chemistry, used to promote nanoparticle assemblies, is a powerful strategy which has emerged very recently to control the spatial arrangement of nanoparticles onto surfaces. Such a strategy may be of high interest for applications such as magnetic recording media or magnetic sensors which are based on the fine control of the collective properties of nanoparticles. Nevertheless, self-assembly driven by clickable functional groups still remains to be understood. Mixed self-assembled monolayers (SAMs) of alkane-thiol molecules were used to control the spatial arrangement of nanoparticles onto gold substrates. This approach was combined with click chemistry in order to control the immobilization of nanoparticles on selective areas through specific copper catalyzed alkyne-azide cycloaddition (CuAAC) reaction. Mixed SAMs consist of co-adsorbed 11-(undec-1-ynyl)thiol (S-CC) and 12-(dodecane)thiol (S-CH 3 ) molecules. The variation of the molar ratio between both molecules resulted in significant modulation of the structure of nanoparticle assemblies. The spatial arrangement of nanoparticles revealed the very complex structure of alkyne/methylene terminated mixed SAMs. Alkyne terminal groups could not be only studied by the usual characterization surface techniques such as PM-IRRAS and XPS. Therefore, azido-terminated nanoparticles acted as probing agents to determine the spatial distribution of alkyne groups at the surface of mixed SAMs. This approach was combined with scanning tunneling microscopy (STM) and DFT calculations to get a deeper insight into the structure of mixed SAMs of S-CC and S-CH 3 molecules. Gold substrate topography, chemical affinity of molecules, intermolecular interactions and length of alkyl chains were found to be critical parameters that rule the SAM structure.

■ INTRODUCTION

Nanoparticles are very attractive thanks to their tunable physical properties arising from collective behavior. [START_REF] Nie | Properties and Emerging Applications of Self-Assembled Structures Made from Inorganic Nanoparticles[END_REF] Addressing the spatial arrangement of nanoparticles immobilized onto a surface allows accessing to an infinite and precise modulation of their physical properties, which are of high interest for applications such as magnetic recording media or magnetic sensors. [START_REF] Nie | Properties and Emerging Applications of Self-Assembled Structures Made from Inorganic Nanoparticles[END_REF] It is particularly true for magnetic nanoparticles, the properties of which are directly dependent on magnetostatic coupling resulting from dipolar interactions. [START_REF] Kinge | Self-Assembling Nanoparticles at Surfaces and Interfaces[END_REF][START_REF] Singamaneni | Magnetic Nanoparticles: Recent Advances in Synthesis, Self-Assembly and Applications[END_REF][START_REF] Neouze | Nanoparticle Assemblies: Main Synthesis Pathways and Brief Overview on Some Important Applications[END_REF] Among many assembly techniques, self-assembly driven by chemical interactions onto surfaces is very appealing to address the structure of nanoparticle assemblies. [START_REF] Bao | Self-Assembly of Superparamagnetic Nanoparticles[END_REF][START_REF] Smith | Patterning Self-Assembled Monolayers[END_REF] In this case, the assembly is mediated on selective areas by specific interactions between functional groups localized at the surface of both substrate and nanoparticles.

Self-assembled monolayers (SAMs) of n-alkanethiolate present the advantage of being an easy way to prepare chemically functionalized gold substrates in ambient conditions. [START_REF] Bao | Self-Assembly of Superparamagnetic Nanoparticles[END_REF][START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF] Therefore, many different chemical groups have been used to bring specific functionalities at SAM surfaces. It has resulted in a wide panel of interfacial properties such as hydrophobicity, chemical reactivity, cell adhesion, and molecular recognition. [START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF][START_REF] Vericat | Self-Assembled Monolayers of Thiols and Dithiols on Gold: New Challenges for a Well-Known System[END_REF] SAMs are also particularly efficient to easily address nanoparticle assembly through multiple Hbonds, [START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF] chelates, [START_REF] Van Den Brom | Selective Immobilization of Nanoparticles on Surfaces by Molecular Recognition Using Simple Multiple H-Bonding Functionalities[END_REF][START_REF] Sakuragi | A Self-Assembled Monolayers Assisted Solid-State Conversion of Boehmite Particles to Aluminum Oxide Film[END_REF][START_REF] Pichon | Iron Oxide Magnetic Nanoparticles Used as Probing Agents to Study the Nanostructure of Mixed Self-Assembled Monolayers[END_REF][START_REF] Pichon | Magnetic Properties of Mono-and Multilayer Assemblies of Iron Oxide Nanoparticles Promoted by SAMs[END_REF] electrostatic interactions, or covalent bonds. [START_REF] Pichon | Tunable Magnetic Properties of Nanoparticles 2D Assemblies Addressed by Mixed SAMs[END_REF] Recently, a very attractive approach emerged as a new and promising way to prepare complex nanostructures onto surfaces. Click chemistry was initially reported for organic chemistry reactions between molecules [START_REF] Bae | Selectively Assembled Co Nanoparticle Stripes Prepared by Covalent Linkage and Microcontact Printing[END_REF] or surface functionalization. [START_REF] Kolb | Click Chemistry: Diverse Chemical Function from a Few Good Reactions[END_REF][START_REF] White | Toward the Syntheses of Universal Ligands for Metal Oxide Surfaces: Controlling Surface Functionality through Click Chemistry[END_REF][START_REF] Rengifo | Click-Functional" Block Copolymers Provide Precise Surface Functionality via Spin Coating[END_REF][START_REF] Prakash | Modification of Silica Surfaces and Glass Microfluidic Channels[END_REF] Its high reliability, no byproduct formation, mild conditions, and high yields made it very popular. Therefore, it recently became a powerful tool to easily prepare highly stable nanoparticle assemblies onto surfaces through specific chemical reaction. Some of these click assembling strategies allowed nanoparticle assembling to be addressed with a variety of structures such as isolated nanoparticles, [START_REF] Toulemon | 2D Assembly of Non-Interacting Magnetic Iron Oxide Nanoparticles via "Click[END_REF] monolayers, [START_REF] Toulemon | 2D Assembly of Non-Interacting Magnetic Iron Oxide Nanoparticles via "Click[END_REF][START_REF] Toulemon | Fast Assembling of Magnetic Iron Oxide Nanoparticles by Microwave-Assisted Copper-(I) Catalyzed Alkyne-Azide Cycloaddition (CuAAC)[END_REF][START_REF] Williams | Building High-Coverage Monolayers of Covalently Bound Magnetic Nanoparticles[END_REF] multilayers, [START_REF] Liu | Nanoparticle Layer Deposition for Highly Controlled Multilayer Formation Based on High-Coverage Monolayers of Nanoparticles[END_REF][START_REF] Rydzek | Selective Nanotrench Filling by One-Pot Electroclick Self-Constructed Nanoparticle Films[END_REF] or chains. [START_REF] Kinge | Magnetic Nanoparticle Assembly on Surfaces Using Click Chemistry[END_REF][START_REF] Toulemon | Enhanced Collective Magnetic Properties Induced by the Controlled Assembly of Iron Oxide Nanoparticles in Chains[END_REF] Nevertheless, the control of assemblies at the nanoparticle scale remains critical to fully address the collective properties. In this aim, mixed SAMs which consist of different thiol molecules were developed to modify the surface reactivity at the nanoscale. [START_REF] Smith | Patterning Self-Assembled Monolayers[END_REF][START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF] Mixed SAMs represent a very easy way to introduce different functional groups with tunable spatial arrangement onto substrates. Indeed, the arrangement of adsorbed molecules depends on their respective chemical affinities which allows controlling phase segregation vs homogeneous mixing. [START_REF] Smith | Patterning Self-Assembled Monolayers[END_REF][START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF] Nevertheless, mixed SAMs supported onto gold thin films consist of dynamic systems with complex phase behaviors. Therefore, their structure is ruled by many factors. For instance, structural defect sites such as adsorbate vacancies in the molecular lattice and domain boundaries are critical sites which favor the exchange of molecules from the gold surface with other thiol derivatives in solution. [START_REF] Bumm | Directed Self-Assembly to Create Molecular Terraces with Molecularly Sharp Boundaries in Organic Monolayers[END_REF][START_REF] Chen | Controlled Chemical and Structural Properties of Mixed Self-Assembled Monolayers of Alkanethiols on Au(111)[END_REF] This dynamic feature of SAM structure is particularly interesting with regard to surface functionalization since host molecules can be replaced by guest molecules. [START_REF] Choi | Phase Separation of a Mixed Self-Assembled Monolayer Prepared via a Stepwise Method[END_REF][START_REF] Zhang | Probing Carboxylic Acid Groups in Replaced and Mixed Self-Assembled Monolayers by Individual Ionized Dendrimer Molecules: An Atomic Force Microscopy Study[END_REF] Mixed SAMs have been poorly investigated to control the structure of nanoparticle assemblies. [START_REF] Goldmann | Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces[END_REF] We reported on phase segregation in mixed SAMs of 12-dodecanethiol and 11mercaptoundecanoic acid to prepare sizable domains of iron oxide nanoparticle by controlling chelating interactions. [START_REF] Pichon | Tunable Magnetic Properties of Nanoparticles 2D Assemblies Addressed by Mixed SAMs[END_REF] We also used iron oxide nanoparticles as probing agents to study the mechanisms pathway of replacement of 12-dodecanethiol by 11-mercaptoundecanoic at the gold surface. [START_REF] Pichon | Iron Oxide Magnetic Nanoparticles Used as Probing Agents to Study the Nanostructure of Mixed Self-Assembled Monolayers[END_REF] Finally, we also investigated theoretically the structure of mixed SAMS at the periodic DFT level [START_REF] Tielens | Characterization of ω-Functionalized Undecanethiol Mixed Self-Assembled Monolayers on Au(111): A Combined Polarization Modulation Infrared Reflection-Absorption Spectroscopy/X-Ray Photoelectron Spectroscopy/Periodic Density Functional Theory Study[END_REF] and studied its dynamics, very recently. [START_REF] Meena | Nanophase Segregation of Self-Assembled Monolayers on Gold Nanoparticles[END_REF] Herein, we report on the structuring of nanoparticle assembly by adapting the click chemistry approach to mixed SAMs supported on gold thin films which were used to direct nanoparticle assemblies (Figure 1). In contrast to our previous studies which were conducted on single components alkyneterminated SAMs, we took advantage of the ability of molecules to segregate in mixed SAMs in order to modulate the structure of nanoparticle assembly. The surface of mixed SAMs was decorated by alkyne and methylene terminal groups. The former allows immobilizing azido-terminated nanoparticles onto the surface, while the latter acts as a dilution agent and directs the spatial distribution of alkyne groups. We show that the molar ratio between 11-(undec-1-ynyl)thiol (S-CC) and 1-dodecanethiol (S-CH 3 ) molecules strongly modulate the structure of nanoparticle assembly. The structure of mixed SAMs is investigated indirectly by the spatial arrangement of nanoparticles. In addition, the spatial arrangement of molecules was studied directly by scanning tunneling microscopy (STM) and density functional (DFT) calculations. Finally, the collective magnetic properties were investigated as a function of the structure of nanoparticle assemblies.

■ MATERIALS AND METHODS

Chemicals. 11-(Undec-1-ynyl)thiol (S-CC) and 1-dodecanethiol (S-CH 3 ) were purchased from Aldrich. Tetrahydrofuran (THF) and ethanol were purchased from Carlo Erba and used as received. Triethylamine was obtained from Fluka. Iron stearate was purchased from Strem Chemicals. Oleic acid was purchased from Alfa Aesar.

Synthesis of Azide-Terminated Iron Oxide Nanoparticles (NP@N3). The procedure is fully described in our previous article. [START_REF] Toulemon | 2D Assembly of Non-Interacting Magnetic Iron Oxide Nanoparticles via "Click[END_REF] Iron stearate (Fe(stearate) 2 ; 1.38 g, 2.2 × 10 -3 mol) was dissolved in octyl ether (20 mL) with oleic acid (1.24 g, 3.3 × 10 -3 mol). The mixture was kept at 110 °C for 30 min and was brought up to reflux at 5 °C•min -1 without stirring for 120 min under air. After cooling to room temperature, the black suspension was washed three times with a mixture of chloroform and ethanol (v:v, 1:3) and centrifuged (14000 rpm, 10 min). Nanoparticles were easily suspended in tetrahydrofuran (THF). A solution of (12-azidododecyl)phosphonic acid (AP12N3; 15 mg) in THF (10 mL) was added to 10 mL of the nanoparticle solution and stirred for 48 h at room temperature. Free molecules were removed by ultrafiltration (using a 30 kDa membrane, Millipore) by washing three times with 20 mL of THF.

Preparation of Mixed SAMs. Ion sputtered gold substrates were cleaned under O 2 /H 2 plasma for 2 min and were soaked in a 10 mM ethanolic solution of thiol derivatives at room temperature for 20 h. Each SAM was then rinsed with copious amounts of pure ethanol and dried under nitrogen stream.

Assembling of Iron Oxide Nanoparticles onto Mixed SAMs. Mixed SAMs were immersed directly after preparation in the suspension of azido-terminated nanoparticles in THF/triethylamine mixture (v:v, 10:1). After addition of CuBr(PPh 3 ) 3 (6.5 mg, 6.7.10 -3 mmol), the reaction was performed under reflux for 48 h. Substrates were then extensively rinsed with THF. They were placed in an ultrasonic bath in THF for 10 s to remove any physisorbed nanoparticles and finally dried under a nitrogen stream.

■ CHARACTERIZATION TECHNIQUES

Transmission electronic microscopy (TEM) was performed by using a TOPCON microscope (model 002B) operating at 200 kV with a point-to-point resolution of 0.18 nm. Nanoparticle size distribution was calculated from the size measurements of more than 300 nanoparticles from TEM micrographs by using the ImageJ software. Dynamic light scattering (DLS) measurements were performed on the suspension of nanoparticles in tetrahydrofuran, using a nanosize MALVERN (nano ZS) apparatus.

Polarization modulation infrared reflection-absorption spectroscopy (PMIRRAS) was performed on gold substrates after being immersed in thiol solutions, using a IF66S Bruker spectrometer with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. Typical data were derived from 2000 scans at a resolution of 1 cm -1 , 85°beam angle of incidence, and 74 kHz modulation frequency.

Scanning electronic microscopy (SEM) was performed by using a JEOL 6700 microscope equipped with a field emission gun (SEM-FEG) operating at an accelerating voltage of 3 kV.

Atomic force microscopy (AFM) was performed by using an ICON atomic force microscope from Bruker operating with a Nanoscope V controller. All experiments were conducted in air and atmospheric pressures. The images were acquired in standard tapping mode with probe amplitude oscillations around 15 nm. We used probes with a normal stiffness of the order of 10 N/m (first normal-resonant mode of about 260 kHz). All AFM tips had apex radii around 5 nm with an upper conical shape of an angle of 25°. These geometrical parameters kept to a minimum the tip convolution effects which can then be precisely evaluated. Collected data were analyzed with Nanotec WSXM software. [START_REF] Horcas | A Software for Scanning Probe Microscopy and a Tool for Nanotechnology[END_REF] Ellipsometry was performed by using a SENpro ellipsometer in a spectral range from 350 to 1050 nm using a tungsten halogen lamp and spot size of 2 nm.

X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultrahigh vacuum setup (base pressure ∼ 5 × 10 -9 mbar) described elsewhere. [START_REF] Luo | Tuning Morphology and Redox Properties of Cobalt Particles Supported on Oxides by an in between Graphene Layer[END_REF] A monochromated Al Kα X-ray source was used as incident radiation (energy resolution better than 0.4 eV). Prior to individual elemental scans, a survey scan was taken for all the samples to detect all of the elements present. The CASA XPS program with a Gaussian-Lorentzian mix function and Shirley background subtraction was employed to deconvolute the XP spectra. All binding energies were calibrated to the Au 4f 7/2 peak at 83.8 eV. The sulfur, carbon and gold contents were calculated by S 2p, C 1s, and Au 4f core level peaks, properly normalized to the photoemission cross-section and assuming a homogeneous distribution arrangement model. The full width at halfmaximum intensity (fwhm) of the S 2p 3/2 peak was better than 1 eV as measured by a reference sample under the same spectrometer settings.

Scanning tunneling microscopy (STM) was performed in air and at ambient temperature by using a commercial STM head mounted on a commercial atomic force microscope (Multimode basis with a Nanoscope V electronic from Bruker). A hand-cut Pt-Ir wire (diameter = 0.25 mm) is used as a tip. A preamplificator has allowed measurements at tunnel currents around 1 pA for an applied tip-sample difference of potential around 0.2-1 V, experimental conditions necessary to image the end of the thiol molecules and not the gold substrate. The potential is applied to the sample, and the tip is ground, so the electrons flow from the tip to the sample. STM analysis has been done using gold on mica samples. Gold substrates were prepared by vacuum evaporation of 100 nm thick gold films onto freshly cleaved mica substrate at a pressure of 10 -8 Torr and a substrate temperature of 600 K. Each sample was then flame annealed at 800 K for a few seconds and quenched in ultrapure water, leading to (111) textured gold films with terraces 100-200 nm wide as verified by STM. Afterward the gold substrate follow the mixed SAM deposition procedure.

Magnetic curves were recorded at 300 and 5 K by applying a magnetic field in the plane of the substrate by using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS SQUID-VSM dc magnetometer). Magnetization was recorded at 300 and 5 K by applying a magnetic field from -7 to +7 T. Magnetization curves were recorded against temperature from 5 to 300 K under a magnetic field of 75 G. M(T) curves were normalized to the maximum values of the magnetization of ZFC curve. The magnetic field was applied in the plane of substrates.

■ THEORETICAL MODEL Computational Details. Calculations were performed in the frame of periodic DFT by means of the Vienna ab Initio Simulation Package (VASP 5.2.11). 35,[START_REF] Kresse | Norm-Conserving and Ultrasoft Pseudopotentials for First-Row and Transition Elements[END_REF] The electron-ion interactions were described by the projector augmented wave (PAW) method, [START_REF] Blochl | Projector Augmented-Wave Method[END_REF][START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] representing the core electrons, as provided in the code libraries. The convergence of the planewave expansion was obtained with a cutoff of 500 eV. The generalized gradient approximation (GGA) was used with the functional of Perdew-Burke-Ernzerhof (PBE). [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] The sampling in the Brillouin zone was performed on a 3 × 3 × 1 kpoint mesh.

Since our system involves organic molecules which interact through weak forces, dispersion-corrected DFT was used in all the calculations through the Grimme D3 method [START_REF] Grimme | A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu[END_REF] as implemented in VASP 5.4. In this approach, DFT-D3 describes the dispersion interactions between a particle and its neighbors in a given radius, via a simple pairwise force field. This force field is optimized for several popular DFT functionals, especially to the one used here, i.e., PBE. Finally, the dispersion energy ΔE D3 is summed to the pure DFT energy ΔE DFT :

Δ = Δ + Δ E E E ads DFT D3 (1)
The adsorption (physisorption) or binding (chemisorption) energy is calculated as follows (the terms physi-and chemisorption indicate the presence of an S-H group or not, differently from the usual terminology indicating a weak or a strong adsorption):

Δ = - - E E E E ads/bind thiol/slab slab thiol (2)
From ΔE bind one can obtain an approximation for the interchain interaction energy ΔE int chain as follows:

Δ = Δ -Δ E E E int chain ads bind (3)
Models. The thiol molecules studied are H-S-(CH 2 ) 11 -CH 3 and H-S-(CH 2 ) 9 -CC-H. The adsorption processes of both thiols on the Au(111) surface are modeled using slabs containing four Au layers. [START_REF] Luque | Effect of Coverage and Defects on the Adsorption of Propanethiol on Au(111) Surface: A Theoretical Study[END_REF] The vacuum between subsequent slabs in the infinite three-dimensional representation was fixed to 25 Å. The unit cell employed to study the SAM structures is (2√3 × 2√3)R30°, which is equivalent to the c (4 × 2) unit cell, with 48 Au atoms. The unit cell parameter (4.177 Å, compared with 4.078 Å experimentally) of Au is obtained after optimization of the bulk at the same level of calculation as the thiol SAM. It should be noted that only a perfect defect free Au(111) surface has been considered.

■ NANOPARTICLE SYNTHESIS AND ASSEMBLY

The assembly of iron oxide nanoparticles was achieved onto gold substrates by performing the copper(I) catalyzed alkyneazide cycloaddition (CuAAC) "'click'" reaction. [START_REF] Toulemon | 2D Assembly of Non-Interacting Magnetic Iron Oxide Nanoparticles via "Click[END_REF] The specific reaction between azide-terminated nanoparticles (NP@N 3 ) and alkyne-terminated gold substrate (SAM-CC) resulted in the irreversible formation of multicovalent triazol bonds. This strategy contributes to the preparation of highly stable nanoparticle assemblies onto substrates. Iron oxide nanoparticles were prepared by the thermal decomposition method and were postfunctionalized by 12-azidododecylphosphonic acid (AP12N3) as we have reported earlier. [START_REF] Toulemon | 2D Assembly of Non-Interacting Magnetic Iron Oxide Nanoparticles via "Click[END_REF] This strategy allows the preparation a highly stable suspension in THF by avoiding any aggregates (Supporting Information), which is highly critical to address precisely the structure of nanoparticle assemblies in the frame of the bottom-up approach. Azidoterminated nanoparticles (NP@N 3 ) were featured by narrow size distribution centered at 11.5 ± 1.1 nm and homogeneous spheric shape. Gold substrates were functionalized by a mixture of 11-(undec-1-ynyl)thiol (S-CC) and 12-dodecanethiol (S-CH 3 ) molecules. The density and the spatial arrangement of alkyne and methylene terminal groups at SAM surfaces were modulated by preparing 10 mM ethanolic solutions of S-CC/S-CH 3 with molar ratios of 0, 0.2, 0.5, 0.8, and 1. Gold substrates were dipped in these solutions for 20 h at room temperature to allow stabilization. Molecules selfassembled onto the gold surface through van der Waals interactions which favor the specific localization of alkyne and methylene terminal groups at the SAM surface. The alkyne groups acted as active groups toward nanoparticle assembly by click chemistry, while the methylene groups are inactive and control indirectly the spatial arrangement of nanoparticles. The resulting functionalized substrates were named according to S-CC/S-CH 3 molar ratio: SAM-0, SAM-0.2, SAM-0.5, SAM-0.8 and SAM-1.

SEM micrographs show each SAM after performing CuAAC reaction, e.g., after dipping in THF suspension of NP@N 3 (0.5 mg/mL) for 48 h at 60 °C (Figure 2). SAM-0 does not show any nanoparticles which agree with the presence of terminal CH 3 groups at the SAM surface. The SEM micrograph shows the gold grains underneath the self-assembled monolayer of organic molecules. In contrast, the observation of nanoparticles corresponds to the presence of alkyne groups at the SAM surface. The nanoparticle density (3820 ± 20 NPs/μm 2 ) is the highest on SAM-1 and is very close to the maximum theoretical value (4770 NPs/μm 2 ). This result agrees with the fact that SAM-1 only consists of S-CC molecules of which alkyne groups fully cover the surface. Both samples clearly show the high stability of SAMs upon the assembly process. Indeed, the desorption of S-CH 3 would have led to the presence of nanoparticles through the interaction of azide groups with the gold thin film. S-CC molecules also remain adsorbed onto a gold thin film as we reported previously in similar conditions. [START_REF] Toulemon | Fast Assembling of Magnetic Iron Oxide Nanoparticles by Microwave-Assisted Copper-(I) Catalyzed Alkyne-Azide Cycloaddition (CuAAC)[END_REF] Furthermore, the nanoparticle density is intermediate at the surface of SAM-0.5 (210 ± 50 NPs/μm 2 ) and SAM-0.8 (2620 ± 30 NPs/μm 2 ), which is in agreement with lower S-CC/S-CH 3 molar ratios used for solutions. It is worth noting that the nanoparticle density does not increase linearly with S-CC/S-CH 3 solution ratios and that the spatial arrangement of nanoparticles is not homogeneous. SAM-0.2 displays very few isolated nanoparticles and looks similar to SAM-0. SAM-0.5 displays a higher number of nanoparticles, which tend to assemble in lines on areas mostly corresponding to gold grain junctions and from which 2D assemblies seem to grow. SAM-0.8 shows larger 2D assemblies of nanoparticles (up to 100 nm) which coexist with small uncovered areas. Finally, as reported recently, [START_REF] Dolci | Robust Clicked Assembly Based on Iron Oxide Nanoparticles for a New Type of SPR Biosensor[END_REF] nanoparticles assembled by click chemistry are all expected to contain azide groups at their surface, which may be suitable for postfunctionalization by biomolecules for instance.

■ SAM CHARACTERIZATION

SAMs were deeply characterized by several characterization techniques. Ellipsometry was used first for preliminary measurements in order to demonstrate that gold substrates were covered by an organic layer with a thickness below 2 nm, whatever the composition of SAMs. AFM height images of the gold substrate, profile cross-sections, and the measured roughness of 1.6 nm showed it consists of a typical grain structure resulting in terraces separated by junctions.

The chemical compositions of SAM-0 and SAM-1 were also investigated by XPS (Figure 3). The survey scan of SAM-1 is dominated by the photoelectron peaks of Au, O, and C (see the Supporting Information (SI)). High-resolution spectra showed that the S 2p core level spectrum could be deconvoluted in two doublets, according to the literature. [START_REF] Castner | X-Ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces[END_REF] The doublet at 161.9 eV (magenta peaks) with the highest intensity is attributed to the S-Au bond. The second doublet at higher BEs (163.9 eV) is much weaker and is related to S-H or S-C groups corresponding to some unbound thiol derivatives at the SAM surface which represent 30%. [START_REF] Pasquali | XPS, and NEXAFS Study of Self-Assembly of Standing 1,4-Benzenedimethanethiol SAMs on Gold[END_REF] One may also observe a very noisy peak at 169 eV in the SAM-1 spectrum which may be attributed to a very low amount of oxidized thiol. [START_REF] Joseph | Aging of 1,ω-Alkyldithiol Interlinked Au Nanoparticle Networks[END_REF] The main contribution in the C 1s peak at 284.5 eV corresponds to single bonded carbons (C-C) of alkylene chains. [START_REF] Heister | Adsorption of Alkanethiols and Biphenylthiols on Au and Ag Substrates: A High-Resolution X-Ray Photoelectron Spectroscopy Study[END_REF] The two components at higher binding energies (286.0 and 288.1 eV) are induced by carbons bonded to oxygen (mainly C-O, CO, and O-CO species) as it is also confirmed from the O 1s peak (not shown). [START_REF] Luo | CHAPTER 5:XPS Characterization of Metal-Free Functionalized Carbons[END_REF] The presence of these species might be provoked by CO 2 adsorption over the SAM surface; [START_REF] Luo | CHAPTER 5:XPS Characterization of Metal-Free Functionalized Carbons[END_REF] however, one cannot exclude the possibility that the high binding energy peak is partially induced by the oxidation of S-CH 3 and S-CC molecules. Unfortunately, the contribution of alkyne groups in the C 1s spectra cannot be distinguished since these species, with binding energies around 285 eV, [START_REF] Teschner | Alkyne Hydrogenation over Pd Catalysts: A New Paradigm[END_REF] overlap with the main C 1s peaks due to C-C bonds. The calculated S/C ratio was found to be 0.08 ± 0.01 and was the same for both samples, in close agreement with the stoichiometries of S-CC and S-CH 3 molecules. In addition, the similar S/Au ratio of 0.09 ± 0.01 for both SAM-0 and SAM-1 is indicative of the high density and tight packing of both thiol molecules onto the gold substrates. [START_REF] Hamoudi | Chain-Length-Dependent Branching of Irradiation-Induced Processes in Alkanethiolate Self-Assembled Monolayers[END_REF][START_REF] Lee | The Influence of Packing Densities and Surface Order on the Frictional Properties of Alkanethiol Self-Assembled Monolayers (SAMs) on Gold: A Comparison of SAMs Derived from Normal and Spiroalkanedithiols[END_REF] PM-IRRAS spectra recorded for SAM-0 and SAM-1 (Figure 4) showed typical signals of alkylene chains. ν as (CH 2 ) and ν s (CH 2 ) bands were centered at 2918 and 2849 cm -1 for SAM-0 and at 2925 and 2854 cm -1 for SAM-1. These positions suggest ordered and close-packed alkylene chains into a primarily all-trans configuration for S-CH3 molecules, while S-CC molecules are slightly disordered. [START_REF] Nuzzo | Fundamental Studies of the Chemisorption of Organosulfur Compounds on Gold(111). Implications for Molecular Self-Assembly on Gold Surfaces[END_REF] Bands at 2963 cm -1 (νas) and 2878 cm -1 (νs) corresponding to CH 3 stretching are only observed for SAM-0 and correspond to the terminal methylene groups. The broad band at about 3300 cm -1 observed for SAM-1 corresponds to the ν(CC-H) vibration. [START_REF] Paoprasert | Grafting of Poly(3-Hexylthiophene) Brushes on Oxides Using Click Chemistry[END_REF] It is confirmed by δ(CC-H) vibration at 680 cm -1 . This band is usually difficult to observe because the absorption coefficient is very low due to the vanishing small transition dipole moment of the CC triple bond. [START_REF] Toulemon | Fast Assembling of Magnetic Iron Oxide Nanoparticles by Microwave-Assisted Copper-(I) Catalyzed Alkyne-Azide Cycloaddition (CuAAC)[END_REF] The structure of alkyne/methylene terminated mixed SAMs was investigated directly by performing STM in order to get additional information (Figure 5). As shown by the 500 nm × 500 nm sized images noted A, D and G, STM confirms that gold substrates were covered by an organic layer. In fact, Au (111) terraces and also grains boundaries present "holes" domains randomly distributed over the entire surface. This result is typical for SAMs on gold surfaces as discussed in the literature. [START_REF] Smith | Patterning Self-Assembled Monolayers[END_REF] As shown on the zoomed image B, and the crosssections C1 and C2, these holes are substrate vacancies consisting of one atomic layer deep of gold. Monoatomic step edges (C2), where one atomic layer of gold separates the gold terraces from each other with a height difference of ∼0.235 nm, are present. These vacancies have been formed during the adsorption process. The involved mechanism is based on the fact that the Au-S bond is strong enough to weaken proximal Au-Au bonds in the lattice beneath the assembly. Gold desorbed in the solution and the vacancies coalesce into domains. The process occurs for both molecules since it is related to the thiol end group.

Second, since we do not observe significant height changes other than the presence of gold vacancies domains as a function of the SAMs solution composition, we can conclude that our mixed SAMs do not segregate in air at room temperature. That is coherent with previous studies [START_REF] Smith | Patterning Self-Assembled Monolayers[END_REF] showing that molecules of similar enough composition do not phase separate if formed at room temperature from the same solution. The likelihood of preferential exchange is decreased, as the energies of salvation and exchange will be similar between the two adsorbates. In fact, Monte Carlo simulations [START_REF] Mizutani | Monte Carlo Simulation of Phase-Separated Self-Assembled Films[END_REF] predict phase segregation when two components have a chain length difference of seven or eight carbon atoms and experimentally [START_REF] Chen | Controlled Chemical and Structural Properties of Mixed Self-Assembled Monolayers of Alkanethiols on Au(111)[END_REF] phase segregation was observed at room temperature when the chain length difference is greater than four carbons, which is not our case. If the chain length difference is only one or two carbon atoms, it is hard to detect the difference in a mixed SAMs monolayer with a low-current STM. [START_REF] Takami | Recognition of Individual Tail Groups in Self-Assembled Monolayers[END_REF] It is why, on the cross-sections C1, C2, F, and I, we observe a rather "homogeneous" height distribution around the height changes of 0.235 nm. The small height fluctuations are due to the fact that the molecules stand up at our high surface coverage with the hydrocarbon tails tilting approximately 30°f rom the surface normal and nominally in the all-trans configuration (confirmed by PM-IRRAS) so as to maximize van der Waals interactions (see simulation below). Since the tunneling current just reflects the surface Fermi level, STM cannot distinguish the true height difference between two chains separated by one carbon atom. In our case, it is highly probable that small angle fluctuations conduct to small height changes when the tip scans over these tightly packed structures.

Third, STM measurements show that S-CH 3 molecules desorbed much more easily from the gold surface than S-CC molecules. With the flooding procedure of the freely available WSxM software, [START_REF] Horcas | A Software for Scanning Probe Microscopy and a Tool for Nanotechnology[END_REF] we determine the percentage of the surface covered by holes domains using images B, E, and H as input (not shown). It decreases from around 30% for the SAM-0 samples to 20% for SAM-1 with an intermediate value of 22-23% for SAM-0.5. Since the holes are related to the desorption ability of molecules connected to a gold atom, we conclude that S-CH 3 molecules desorbed easier than S-CC molecules, which is coherent with the computational results.

■ COMPUTATIONAL RESULTS

In order to study the stability of the S-CC/S-CH 3 mixture in SAMs, a series of models representing each different S-CC/S-CH 3 molar ratio of mixed SAMs were considered. The mean adsorption energy per thiol chain (ΔE ads ) was calculated for perfect mixtures of S-CC and S-CH 3 as follows: Adsorption energy (Δ ads ) values including the dispersion interaction energy are much larger than without correction (Table 1). This result confirms the importance of dispersion interactions in long alkyl chain thiol derivatives. Indeed, the dispersion interactions represent 60% (eq 3) of the total adsorption energy, and 30% and 40% of the intramolecular interaction energy for the S-CH 3 and 68% for the S-CC thiol SAM, respectively. The latter has been calculated using the Au-S binding energy of 2.60 eV of alkyl chain thiols. [START_REF] Nassoko | Rationalizing the Formation of Binary Mixed Thiol Self-Assembled Monolayers[END_REF] Adsorption energy (ΔE ads ) values also clearly show that interactions between S-CC chains are more favorable than between S-CH 3 chains. Although the alkyl chain is shorter in S-CC than in S-CH 3 (9 CH 2 against 11 CH 2 ), van der Waals interactions are compensated by dipolar interactions between alkyne head groups. It is confirmed by ΔE ads -corrected and uncorrected values, which show that the dispersion interaction energy predominates between chains of S-CH 3 molecules.

Δ = [ × - - °- - - °- ] E E E nE n E 1 4 ( 2
Mean values of adsorption energy (ΔE ads,mean ) were also calculated for segregated mixtures by considering pure phases. For instance, ΔE ads,mean = (ΔE ads,4alkyl + ΔE ads,4alkyne )/4 = -3.38 eV for SAM-0.5. This value is further used as a reference value in order to quantify the stabilization of the SAM mixture. [START_REF] Goldmann | Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces[END_REF][START_REF] Nassoko | Rationalizing the Formation of Binary Mixed Thiol Self-Assembled Monolayers[END_REF][START_REF] Guesmi | Does the S-H Bond Always Break after Adsorption of an Alkylthiol on Au(111)?[END_REF] ΔE ads is the lowest for all segregated mixtures whatever the S-CC/S-CH 3 molar ratio (Figure 6). It shows that intermolecular interactions are thermodynamically favored in segregated mixtures. Nevertheless, small variations being observed (below 1 eV), we expect both segregated and perfect mixtures to coexist thermodynamically. These results agree with studies which usually report on perfectly mixed SAMs (no segregation) prepared from mixtures of n-alkyl thiols presenting small differences between alkyl chain length and terminal groups. [START_REF] Bao | Self-Assembly of Superparamagnetic Nanoparticles[END_REF] This finding does not include entropic considerations to stabilization/formation, which are expected to be of similar magnitude for all SAMs because of the similar physicochemical properties of both thiol derivatives. [START_REF] Goldmann | Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces[END_REF][START_REF] Meena | Nanophase Segregation of Self-Assembled Monolayers on Gold Nanoparticles[END_REF][START_REF] Nassoko | Rationalizing the Formation of Binary Mixed Thiol Self-Assembled Monolayers[END_REF] Furthermore, the binding energy (2.6 eV) was calculated to be constant throughout the mixture series. This result agrees with our earlier findings, stating that the Au-S binding energy is expected to be related to the sulfur hybridization. [START_REF] Goldmann | Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces[END_REF][START_REF] Meena | Nanophase Segregation of Self-Assembled Monolayers on Gold Nanoparticles[END_REF][START_REF] Nassoko | Rationalizing the Formation of Binary Mixed Thiol Self-Assembled Monolayers[END_REF] Concerning the effect of the thiol adsorption on the Au(111) surface, a precursor state for the formation of an adatom can be seen (Figure 7). STM measurements and theoretical calculations showed that the S-CC assembly is more stable than S-CH 3 assembly. Although van der Waals interactions between alkyl chains are stronger for the former, interactions between alkyne head groups in the latter predominate.

■ MAGNETIC MEASUREMENTS

The magnetic properties of nanoparticle assemblies have been studied as a function of their structure controlled by SAM-0.5, SAM-0.8, and SAM-1 (Figure 8). Magnetization curves recorded against a magnetic field are representative of the superparamagnetic behavior at 300 K and ferrimagnetic behavior at 5 K as we already observed for Fe 3-δ O 4 nanoparticles with similar size. [START_REF] Baaziz | Magnetic Iron Oxide Nanoparticles: Reproducible Tuning of the Size and Nanosized-Dependent Composition, Defects, and Spin Canting[END_REF] These results agree with nanoparticles which consist of a single magnetic domain which is blocked at low temperature thanks to magnetic anisotropy energy. In contrast, by increasing temperature, thermal energy becomes the highest, which results in the oscillation of magnetic moments and corresponds to superparamagnetic behavior. Hysteresis cycles at 5 K display similar coercive fields (300-400 Oe) for all samples, which agree with our previous studies. [START_REF] Toulemon | 2D Assembly of Non-Interacting Magnetic Iron Oxide Nanoparticles via "Click[END_REF][START_REF] Pauly | Size Dependent Dipolar Interactions in Iron Oxide Nanoparticle Monolayer and Multilayer Langmuir-Blodgett Films[END_REF] Temperature dependent magnetization curves were also very similar. Maxima recorded for ZFC curves can be assimilated to the blocking temperature (T B ) at which the transition between ferrimagnetic and superparamagnetic behaviors happens. T B being highly sensitive to dipolar interactions, it shows that the increase in nanoparticle density does not influence the magnetic anisotropy energy. Although nanoparticle domains became larger, they reached a critical size onto SAM-0.5. Nanoparticles being tightly packed in all samples, their environment is very similar, so that dipolar interactions occur. These results are confirmed by the plateau observed below T B in FC curves, which correspond to strong dipolar interactions between nanoparticles, whatever the SAM structure.

■ DISCUSSION Investigation on the Structure of Alkyne-Terminated Mixed SAMs. Alkyne groups are difficult to characterize at the SAM surface because no signal could be observed by XPS and PM-IRRAS due to low sensitivity. [START_REF] Paoprasert | Grafting of Poly(3-Hexylthiophene) Brushes on Oxides Using Click Chemistry[END_REF] Therefore, these techniques are not suitable for the characterization of alkyne terminal groups at the surface of mixed SAMs. It has been reported that this issue could be circumvented by using probing agents such as an azide-functional molecule containing an infrared active label to detect indirectly alkyne groups. [START_REF] Maidenberg | Mixed Silane Monolayers for Controlling the Surface Areal Density of Click-Reactive Alkyne Groups: A Method to Assess Preferential Surface Adsorption on Flat Substrates and a Method to Verify Compositional Homogeneity on Nanoparticles[END_REF] In this study, we took advantage of azide-terminated nanoparticles as probing agents.

Since nanoparticles can only be immobilized onto areas terminated by alkyne groups, they offer the possibility to map indirectly their spatial arrangement at the surface of mixed SAMs. Therefore, the spatial arrangement of nanoparticles clearly shows that the S-CC/S-CH 3 molar ratio in solution modulated the composition of mixed SAMs. SEM micrographs gave first insights on the amount and distribution of alkyne groups at the SAM surface, which were completed by STM measurements and theoretical calculations.

First of all, the nanoparticle density onto mixed SAMs does not systematically reflect the molar ratio of S-CC and S-CH 3 in solution. The absence of nanoparticles on SAM-0.2 is unexpected and may correspond to the absence of alkyne groups at the SAM surface. However, our computational model showed that S-CC and S-CH 3 molecules are well mixed in SAM-0.2. Nevertheless, the shorter alkyl chain of S-CC (9 carbons) molecules than S-CH 3 molecules (11 carbons) may render difficult the accessibility of terminal alkyne groups to react with azide groups at nanoparticle surface. These results agree with the non-observation of nanoparticles on SAM-0.2 and large uncovered areas on SAM-0.5. Therefore, uncovered areas at the SAM-0.5 surface may also consist of CH 3 groups perfectly mixed with alkyne groups, which may be in rather large amount but are not accessible.

Second, n-alkyl thiol monolayers are a highly dynamic system which is featured by exchange of thiol derivatives from the gold substrate with molecules in solution. STM measurements have shown that S-CH 3 molecules desorbed much more easily from the gold surface than S-CC molecules. This result agrees with theoretical modeling which demonstrated that the most stable system was SAM-1. It is commonly admitted that longer alkyl chains favor stronger intermolecular interactions (van der Waals, hydrophobic, and so on) which improve the stability of SAMs and slow the desorption of thiol derivatives. [START_REF] Love | Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology[END_REF] In this study, S-CC molecules display the shortest alkyl chain, but interactions between alkyne groups induce significant dipolar moments at the SAM surface which contribute to enhance the stability of adsorbed S-CC molecules.

Third, the spatial arrangement of S-CC and S-CH 3 molecules also depends on the topography of gold substrates which consist OF gold grain, i.e., terraces separated by junctions. [START_REF] Pichon | Tunable Magnetic Properties of Nanoparticles 2D Assemblies Addressed by Mixed SAMs[END_REF] Indeed, grain junctions favor topologically the interaction between SAM and nanoparticles. However, they can be assimilated as defect sites which favor rapid desorption/ adsorption process of molecules during equilibration times (20 h in our case). Therefore, according to our computational calculations (higher stability of S-CC assembly) and STM measurements (greater desorption of S-CH 3 ), S-CC molecules are expected to be preferentially localized at the grain junction.

Fourth, such a defect mediated exchange of S-CH 3 by S-CC molecules was confirmed by immersing SAM-0 (pure SAM of S-CH 3 ) in a S-CC ethanolic solution (10 mM) for 24 h at room temperature (Figure 9a). The new SAM was fully covered by nanoparticles after performing click reaction, which demonstrates that S-CH 3 molecules were replaced by S-CC molecules. The opposite experiment which consisted of dipping a SAM-1 in a S-CH 3 solution before performing click reaction also showed the full coverage of SAM by nanoparticles (Figure 9b). These results confirmed the better stability of the S-CC assembly than the S-CH 3 one deduced from STM and theoretical data.

Nevertheless, the absence of nanoparticles at the surface of SAM-0.2 shows that both stronger interactions between S-CC molecules and topology of gold substrate are not sufficient to produce large enough areas of CC and to counterbalance shorter alkyl chains and homogeneous mixing with S-CH 3 molecules. A critical S-CC/S-CH 3 molar ratio has to be reached to allow nanoparticles immobilization onto mixed SAMs. Indeed, the critical number of alkyne groups (i.e., S-CC molecules) per surface unit which is necessary to immobilize each nanoparticle at the SAM surface remains an open question. Given the size of iron oxide nanoparticles (11.5 nm), we estimated that the surface contact between each nanoparticle and the SAM supported by the gold substrate is about 8 nm 2 . Therefore, the surface occupied by a S-CC molecule being about 20 Å 2 , we expect a maximum of 40 molecules which may interact with nanoparticles in the case of SAM-1.

Gold Topography and Chemical Reactivity Direct Nanoparticle Assembling. Nanoparticles being able to assemble only on areas containing alkyne groups, the SAM structure may direct the assembling process of nanoparticles. Indeed, nanoparticles assembled preferentially at gold grain junctions where the topography favors interactions with alkyne groups and azido-terminated nanoparticles. It results in rather unidirectional assemblies (chains) from which the nanoparticles assemble in 2D domains. This assembly process may result from interparticle interactions triggered by larger dipolar moments as long as the nanoparticle assembly grows. [START_REF] Klokkenburg | Direct Imaging of Zero-Field Dipolar Structures in Colloidal Dispersions of Synthetic Magnetite[END_REF][START_REF] Butter | Direct Observation of Dipolar Chains in Iron Ferrofluids by Cryogenic Electron Microscopy[END_REF][START_REF] Toulemon | Enhanced Collective Magnetic Properties in 2D Monolayers of Iron Oxide Nanoparticles Favored by Local Order and 1D Shape Anisotropy[END_REF] Furthermore, nanoparticle domains also grow on areas corresponding to terraces of gold grains. In contrast to grain junctions, the desorption of S-CH 3 at the surface of terraces is longer because of tight packing of self-assembled molecules. [START_REF] Pichon | Iron Oxide Magnetic Nanoparticles Used as Probing Agents to Study the Nanostructure of Mixed Self-Assembled Monolayers[END_REF] Therefore, nanoparticles can only assemble on terraces for the largest amount of S-CC molecules (>50%). The presence of nanoparticle domains in the center of terraces in SAM-0.5 is indicative of the replacement of S-CH 3 by S-CC molecules. As shown by our computational model, some pure areas of S-CC become large enough for nanoparticle immobilization on terraces than in junctions; it may be triggered by the lower energy of pure S-CC SAM structure.

A larger amount of S-CC in solution contributes to extend nanoparticle assemblies at the surface of SAM-0.8. Gold grain junctions seem to be much more covered and nanoparticle domains are much larger. This result shows that alkyne groups are localized onto both gold grain junctions and gold terraces. The cohabitation with uncovered areas, which represent about 20% of the SAM surface, agrees with areas containing S-CH 3 molecules. Although it is difficult to tell about the presence of S-CC in these uncovered areas, it may count for half according to theoretical calculations.

The distribution of alkyne terminal groups at the surface of mixed SAMs significantly influenced the spatial arrangement of nanoparticles. Domains of relatively tight packed nanoparticles were formed and grew from gold grain junctions to terraces.

■ CONCLUSION

The assembly of magnetic iron oxide nanoparticles was controlled through specific chemical interactions between azide and alkyne groups localized at nanoparticle and SAM surfaces, respectively. The spatial arrangement of nanoparticles was controlled by the structure of mixed SAMs of S-CC and S-CH 3 molecules. While the characterization of alkyne groups was not possible by usual surface techniques such as XPS and PM-IRRAS, nanoparticles allowed determining spatially the presence of alkyne groups at the monolayer surface. Although a dense monolayer of nanoparticles was obtained onto a SAM-CC, intermediate nanoparticle densities were obtained by modulating the S-CC/S-CH 3 molar ratio used to prepare mixed SAMs. In addition to SEM, the combination of STM measurements and theoretical calculations offered a deeper understanding of the structure of mixed SAMs. First, S-CC SAM is much more stable than S-CH 3 SAM, which agrees with faster desorption of S-CH 3 molecules. Second, homogeneous mixing of S-CC and S-CH 3 is favored against segregation, especially in mixed SAMs with high S-CC content. Therefore, alkyne terminal groups are less accessible than methylene terminal groups because the S-CC alkyl chain is shorter than the S-CH 3 one (9 vs 11 carbons). Binding with azideterminated nanoparticles was disfavored and resulted in lower nanoparticle density. Nevertheless, the topography of gold substrates favored significant defect mediated exchange of S-CH 3 by S-CC molecules. Therefore, above a critical content in S-CC, the formation of large enough alkyne areas favored the assembly of nanoparticles at grain junctions which further extended to terraces. Such a mechanism was triggered by the lower energy of S-CC assemblies than S-CH 3 , ones as shown by theoretical calculations. Finally, the magnetic properties of nanoparticle assemblies were studied. Although nanoparticle density increases with alkyne content in mixed SAM, assemblies consisting of relatively large 2D domains which favored rather strong dipolar interactions. Therefore, the collective magnetic properties of nanoparticles are rather similar whatever the domain size, which means that, above a critical size, dipolar interactions predominate over the size effect of 2D assemblies. Finally, our approach is versatile and can be used for the assembly of a variety of nanoparticles. The fine control of phase segregation in mixed SAMs will pave the way to the patterning of nanoparticles, which is of high interest for advanced applications such as bit patterned recording media or magnetic sensors.

■ ASSOCIATED CONTENT

* S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b02152.

TEM micrograph, size distribution, and granulometry measurements for azido-terminated nanoparticles; atomic force micrographs of gold thin film and SAM-0 and SAM-1; survey XPS scan recorded for SAM-0 and SAM-1 (PDF)

■

Notes

The authors declare no competing financial interest. (Grants 2016-[x2016082022] and 2017-x2017082131) and the CCRE of UniversitéPierre et Marie Curie were provided for computational calculations. Dr. X. Cattoen from Institut Neél (Grenoble, France) is acknowledge for providing phosphonic acid derivative for nanoparticle functionalization prior to the preparation of azido-terminated nanoparticles.

Figure 1 .

 1 Figure 1. Schematic illustration of the assembly of iron oxide nanoparticles directed by mixed SAMs.

Figure 2 .

 2 Figure 2. SEM micrographs of mixed SAMs after performing CuAAC "click" reaction in the presence of NP@N 3 : (a) SAM-0, (b) SAM-0.2, (c) SAM-0.5, (d) SAM-0.8, and (e) SAM-1.

Figure

  Figure XPS spectra of SAM-0 and SAM-1: (a) S 2p signal and (b) C 1s signal.

Figure 4 .

 4 Figure 4. PM-IRRAS spectra of SAM-0 and SAM-1.

Figure 5 .

 5 Figure 5. STM images of mixed SAMs on a Au(111) surface. SAM-0: (a) 500 × 500 nm image; (b) 100 × 80 nm image; (c1 and c2) sections along the white lines. SAM-0.5: (d) 500 × 500 nm image; (e) 80 × 70 nm image; (f) section along the white line. SAM-1: (g) 500 × 500 nm image; (h) 80 × 70 nm image; (i) section along the white line. All images have been taken at a tunneling current of 2 pA and a tip bias of 1 V.

  n = 0, 1, 2, 3, or 4 corresponding to the number of S-CC molecules per unit cell, E(2 × 2 system) the energy of the SAM adsorbed on the Au(111) surface, E(surface slab) the energy of the bare Au(111) slab, and E(CC-(°S-CC) and E(°S-CH 3 ) the energies of an isolated alkyne and methylene thiol radicals, respectively.

Figure 6 .

 6 Figure 6. Adsorption energy calculated for mixed SAMs consisting of different mixtures of S-CC and S-CH 3 molecules (mixed), compared with the mean adsorption energy calculated from the pure faces (segregated).

Figure 7 .

 7 Figure 7. Optimized geometry of the SAM-0.75 consisting of alternative S-CC and S-CH 3 molecules. The top two Au(111) layers (only shown here) of four used in the unit cell present a reconstruction/reorganization.

Figure 8 .

 8 Figure 8. Magnetic measurements of mixed SAMs containing nanoparticles: (a, b) M(H) curves of SAM-1 recorded at 300 K (both curves overlap) and 5 K; (b) same curves in panel a at a lower magnetic field; (c) M(T) zero field cooled (ZFC) and field cooled (FC) curves recorded for SAM-0.5, SAM-0.8, and SAM-1.

Figure 9 .

 9 Figure 9. Click chemistry reaction performed after (a) immersion of a SAM-0 in an ethanolic solution of S-CC and (b) immersion of a SAM-1 in an ethanolic solution of S-CH 3 .

■

  ACKNOWLEDGMENTS Funding was provided by Direction geńeŕale de l'armement (DGA) and Reǵion Alsace for D.T.'s Ph.D. This work was supported by the French Agence Nationale de la Recherche (ANR) under Reference ANR-11-LABX-0058-NIE within the Investissement d'Avenir program ANR-10-IDEX-0002-02. HPC resources from GENCI-[CCRT/CINES/IDRIS]

Table 1 .

 1 Adsorption Energy (ΔE ) per Thiol Chain Calculated for Each S-CC/S-CH 3 Molar Ratio by Taking into Account the Dispersion Interaction Energy (PBE-D3)[START_REF] Grimme | A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu[END_REF] or Not (PBE) According to Equation1

	S-CC/S-CH 3 molar ratio (%)	PBE-D3 (eV) (ΔE DFT + ΔE D3 )	PBE (eV) (ΔE DFT )
	100/0	-3.60	-1.13
	75/25	-3.47	-1.30
	50/50	-3.31	-1.20
	25/75	-3.24	-1.29
	0/100	-3.17	-1.27

  E-mail: benoit.pichon@ipcms.unistra.fr.

	AUTHOR INFORMATION
	Corresponding Author
	ORCID
	Frederik Tielens: 0000-0002-6760-6176
	Spyridon Zafeiratos: 0000-0001-8165-2585
	Sylvie Begin-Colin: 0000-0002-2293-2226
	Benoit P. Pichon: 0000-0003-3708-6461

*


G Ounoughene

O Le Bihan

∥ C Chivas-Joly

C Motzkus

⊥ C Longuet

B Debray

∥ A Joubert

L Le Coq

J-M Lopez-Cuesta

Behavior and Fate of Halloysite Nanotubes (HNTs) When Incinerating PA6/HNTs Nanocomposite

Nanoclay-based nanocomposites have been widely studied and produced since the late 1990s, and frequently end up in waste disposal plants. This work investigates the behavior of PA6/HNTs nanocomposites (nylon-6 incorporating halloysite nanotubes) during incineration. Incineration tests were performed at lab-scale using a specific tubular furnace modified in order to control the key incineration parameters within both the combustion and postcombustion zones. The combustion residues and combustion aerosol (particulate matter and gas phase) collected downstream of the incinerator furnace were characterized using various aerosol analysis techniques. Time tracking of the gas and particle-number concentrations revealed two-step char formation during combustion. HNTs transformed into other mineral structures which were found in both the aerosol and the residues. During combustion of the polymer, it appears that HNTs contribute to the formation of a cohesive char layer that protects the residual material. 49 (1-10 wt %) of silicate nanofiller can remarkably enhance the 50 properties of the material. 2,13 Nanofillers based on montmor-51 illonite layered silicates have been the subject of many studies. 52 More recently, halloysite nanotubes (HNTs) have also been 53 studied, and have been reported to improve many properties, 54 especially thermal properties, when incorporated in a PA6 55 polymer matrix. 14-17 HNTs are alumino-silicates. Their 56 chemical composition is Al 2 Si 2 O 5 (OH) 4 ,2H 2 O. They are 57 extracted from natural deposits and are chemically similar to 58 kaolinite. Their nanotubular structure gives them very interesting 59 properties that attract the attention of researchers for many 60 applications. 17-19 61 Many articles and book chapters have been published about 62 the thermal degradation of nanocomposites, 13,16,20-25 but none 63 of them relate to tests involving the combustion parameters of 64 incineration (temperature around 850 °C, highly ventilated 65 combustion, at least 2 s residence time for the combustion gas in 66 a postcombustion chamber at 850 °C, and high oxygen/fuel 67 contact) or time tracking of the gas and particle concentrations 68 in the combustion aerosol. In the present work, we used a Environmental science and technology Number Concentration. The graphs showing the evolution of 235 concentrations during the incineration of the nanocomposite 236 PA6/5HNTs are given in Figure 3a. The two verticals delimit 237 the time range with more than 5% O 2 consumption from the 238 baseline. This area of interest is denoted "AOI". The averages of 239 three runs are presented with the associated standard deviation. 240 When the neat matrix is compared to the nanocomposite, weight 241 standardization was performed in order to compare the same 242 mass of polymer. 243 Before the AOI, from 0 to 48 s, O 2 consumption is low, little 244 gases are produced, but particles are strongly emitted with a 245 domination of PN 0.1 . Ignition is observed during this period. 246 During the AOI, which extends over a period from 48 to 100 s, 247 oxygen consumption is maximal. From 48 to 80 s, peaks of CO 2 , 248 CO and NOx are noticed while PN x-y decreases. From 80 to 249 100 s, a second peak of CO begins along with an increase in 250 PN 0.1 . After the AOI, from 100 s to the end (250 s) a shoulder in 251 NOx concentration appears with a second peak of CO, particles 252 are less and less emitted, and O 2 reaches its baseline. 253 High concentration levels are noticed here. Then, considering 254 such high concentrations, coagulation/agglomeration phenom-255 ena must occur in the furnace. Even if the pilot does respect a 256 residence time of two seconds, it would be difficult to say if these 257 phenomena occur in a real scale industrial furnace. This is rightly 258 a limit of an incinerator lab-scale pilot.

INTRODUCTION

In recent decades, polymer nanocomposites have drawn much attention due to their enhanced performance. [1][2][3][4] The physicochemical properties of polymer matrices (tensile strength, stiffness, heat resistance, barrier properties, etc.) are enhanced by adding small amounts of nano-objects. This is the case with polyamide 6 (PA6) incorporating silicate nanoclays. These nanocomposites have been extensively studied and developed by both universities and industry for many applications in various industrial areas such as transportation, electronic and electrical equipment, and packaging. Nevertheless, many of these state-ofthe-art materials are expected to end up in waste treatment facilities such as incineration plants due to the lack of specific recovery procedures. [START_REF]Concerns about Incineration of Waste Containing 452 Nanomaterials. A nanomaterial can be defined as any 453 intentionally manufactured material, containing particles[END_REF] Hence, the potential environmental risk arising from the incineration of waste containing nanomaterials is a new field which deserves further attention. Some recent studies have begun to focus on this topic, but the data are incomplete. [START_REF]Concerns about Incineration of Waste Containing 452 Nanomaterials. A nanomaterial can be defined as any 453 intentionally manufactured material, containing particles[END_REF][START_REF] Walser | Persistence of engineered nanoparticles in a municipal solid-waste incineration plant[END_REF][START_REF] Mueller | Modeling the flows of engineered nanomaterials during waste handling[END_REF][START_REF] Holder | Nanomaterial disposal by incineration[END_REF][START_REF] Vejerano | Emissions of Polycyclic Aromatic Hydrocarbons, Polychlorinated Dibenzo-p-Dioxins, and Dibenzofurans from Incineration of Nanomaterials[END_REF][START_REF] Vejerano | Characterization of particle emissions and fate of nanomaterials during incineration[END_REF][START_REF] Massari | Behavior of TiO2 nanoparticles during incineration of solid paint waste: A labscale test[END_REF][START_REF] Stahlmecke | Investigations on CNT Release from Composite Materials During End of Life[END_REF] The present study therefore gives some insight into the fate and behavior of nano-objects (incorporated in a polymer matrix) during the incineration of nanocomposites. The work focuses on PA6/HNTs nanocomposites (nylon-6 incorporating halloysite nanotubes).

Polyamide 6 (also called Nylon-6) is a semicrystalline polyamide thermoplastic used in a wide range of fiber, film and engineering applications. It has been shown that a small amount 69 laboratory scale furnace modified to control the key incineration 70 parameters. The combustion aerosol was sampled downstream 71 of the postcombustion chamber (i.e., analogous to upstream of 72 filtration in a real-scale incineration plant), and was characterized 73 using various aerosol analysis techniques. This was a first step in 74 the evaluation of the efficiency of flue-gas cleaning systems. After 75 that, the combustion residues were analyzed. The aim of this 76 work is to study the behavior and the fate of HNTs during 77 incineration of PA6/HNTs nanocomposites while investigating 78 the influence of HNTs on combustion mechanisms (aerosol 79 release and decomposition) and the release of nano-objects 80 during incineration. It is relevant to ask whether the nano-objects 81 initially incorporated in the polymer are destroyed or undergo 82 changes during their stay in the incinerator and their fate 83 thereafter. S1) were prepared by melt extrusion and injection 87 molding. The halloysite (shown in Figure 1) was provided by 88 Imerys Ceramics from New Zealand. PA6 (polyamide 6) 89 Technyl C206 was supplied by Rhodia Plastics Engineering 90 SA. Neat matrix and two nanocomposites at two loadings (1 and 91 5%wt) of HNTs were prepared using the same extrusion and 92 injection process to ensure the same thermo-chemical history for all samples. A twin-screw extruder (Clextral BC21, 1200 mm length) was used (rotational speed of 250 rpm, and processing temperature of 260 °C). The pelletized extruded material obtained was finally injection molded (Krauss Maffei KM50/ 180CX) into standard specimens (100 × 100 × 4 mm 3 ). 250 mg samples from these specimens were cut and then tested in the incinerator furnace.

MATERIALS AND METHODS

The pristine HNTs and the prepared nanocomposite samples were characterized prior to the incineration tests. Imagery on pristine HNTs and on the dispersion of HNTs in the nanocomposite (Figure 1) was carried out using transmission electronic microscope (Philips CM12 TEM 200 kV). In addition, elemental analysis of the polymer material, laser diffraction and X-ray fluorescence analysis of pristine Halloysite were performed (see SI, Tables S2 andS3 and Figure S2). TEM micrographs show the nanotubular structure of the pristine halloysite (Figure 1).

In agreement with literature, [START_REF] Guo | Structure and Performance of Polyamide 6/Halloysite Nanotubes Nanocomposites[END_REF][START_REF] Hedicke-Hochstoẗter | Novel polyamide nanocomposites based on silicate nanotubes of the mineral halloysite[END_REF][START_REF] Marney | The suitability of halloysite nanotubes as a fire retardant for nylon 6[END_REF][START_REF] Liu | Recent advance in research on halloysite nanotubes-polymer nanocomposite[END_REF][START_REF] Du | Newly emerging applications of halloysite nanotubes: A review[END_REF] the observed halloysite tubes are nanosized with a thickness lower than 100 nm.

Thermogravimetric analysis (TGA) was performed using a Pyris-1 Perkin-Elmer apparatus. Samples of 10 (±2) mg were heated under a nitrogen flow at a 10 °C/min heating rate from ambient temperature to 850 °C. Figure 2 the "combustion chamber" is maintained at 850 °C (at least) and where air cross flows penetrate and mix with the combustibles. Then, the "post-combustion chamber" is the furnace zone where the combustion aerosol stays 2 s at 850 °C (at least).

Finally the combustion released gases and the aerosol particles were conveyed to the measurement line as described in the following paragraph.

In order to superimpose the curves of time tracking, it is important to know the response time of each measurement line to synchronize all events detected. The reaction time of each unit of the measurement line is determined by stopwatch. An uncertainty of two seconds is taken into account.

The combustion aerosol was sampled appropriately as explained below and analyzed in three different ways: time tracking for the gas and particles from the aerosol and two off-line analyses for the particulate matter. Further analyses were conducted on the combustion residues.

3.2.1. Time Tracking for the Gas Phase. The sampling of the gas phase from the combustion aerosol followed the methods used for sampling direct emissions from stationary combustion sources. The flue gas leaving the furnace was dried and filtered using a line with a hot filter and a condenser. A multigas analyzer (portable gas analyzer PG-250 Horiba) was used. It indicates the evolution of the combustion gases concentrations: the consumption of O 2 (% vol .) and the production of NOx (ppm vol .), CO 2 (% vol .) and CO (ppm vol .). They are respectively detected by paramagnetism, chemiluminescence and an infrared technique with 1 s time resolution.

Time Tracking and off-Line Analysis for Particulate

Matter. The combustion aerosol sampled downstream of the postcombustion chamber has to be representative of the aerosol sampled upstream of the flue-gas cleaning systems. The target temperature is 170 (±10) °C. In fact, under industrial conditions, the aerosol leaves the postcombustion chamber and goes through an upstream filtration processes at a temperature around 150 °C in accordance with BREF Waste Incineration (2006). [START_REF]BREF Waste Incineration[END_REF] Different samplers and impactors were used to conduct the analyses with respect to the temperature constraint mentioned above.

Regarding the time tracking of the particles from the combustion aerosol, an electrical low pressure impactor, Dekati (ELPI) was used downstream of a fine particle sampler dilutor (FPS, Dekati). The objective of the sampling was to avoid cold spots below 170 °C before and during the first dilution. FPS dilutor is well adapted to measurements related to combustion research. This system allows sampling from hot flue gas and provides controlled temperature decrease with minimal losses. [START_REF] Niemeläv | A Novel method for particle sampling and size-classified electrical charge measurement at power plant environment[END_REF][START_REF] Andersson | Particle Measurement Programme (PMP) Heavy-Duty Inter-Laboratory Correlation Exercise (ILCE_HD) Final Report[END_REF][START_REF] Motzkus | Aerosols emitted by the combustion of polymers containing nanoparticles[END_REF] The FPS dilutor performs two successive dilutions: the first heated dilution at 170 °C and the second at room temperature. The primary dilution air was heated to 170 (±10) °C. Thus, the sampled combustion aerosol underwent two dilutions with a total factor of 1:30. The dilution ratio was given in real time by the FPS software. However, in order to check the displayed FPS value, the dilution ratio was recalculated continuously during the test using a gas analyzer connected to the output the FPS which indicates the concentration of CO and CO 2 after the dilution. [START_REF] Tissari | A novel field measurement method for determining fine particle and gas emissions from residential wood combustion[END_REF] The ELPI provided a real-time measurement of particle number concentration through 12 channels from 17 nm to 5 μm. [START_REF] Keskinen | Electrical low pressure impactor[END_REF] The PN x-y are defined as the number concentration of particles counted by the ELPI with mean geometric diameter (Di) between x and y μm. HNTs* aggregates represent a minor fraction. Figure 4b shows the particles collected on a TEM grid during ten seconds preceding AOI. Submicron HNTs* aggregates are also minor components among the soot particles.

Combustion

Residues. TEM and SEM (coupled with X-ray microanalysis) performed on combustion residues reveal that the residues are entirely inorganic and seem to consist of mineral aggregates and individual mineral particles (Figure 4c). No residues remain for the neat matrix case since neat PA6 decomposes completely. The incorporation of HNTs in PA6 reduces the PCFC peak (463 W/g for the pristine PA6 and 459 W/g for the composite). Besides, the shift toward high temperatures is almost unnoticeable because it is correlated with a mixing law (Figure 5). Nevertheless, PCFC peaks coincide roughly with the derivates of TGA curves (Figure 5, Figure 6). For both techniques, the thermal degradation of PA6 occurs at lower temperatures in the presence of HNTs.

XRD on Pristine HNTs and Residues.

For pristine HNTs, X-ray fluorescence analysis (SI Table S3) and X-ray diffraction pattern (Figure 7) highlight that quartz (cristobalite) is present (nodular shape) as impurity of halloysite. The presence of quartz leads to an excess of silicon over aluminum, since in 304 pure halloysite, their ratio is 1:1.

305

XRD spectra performed on residues show that they essentially 306 contain cristobalite and quartz. A bump can be noticed for the 307 residue from PA6/5HNTs (between 2θ =20 to 30°) which can 308 be ascribed to an amorphous alumino-silicate structure. Only a 309 peak at 44° was not identified. A temperature of at least 850 °C 310 is attained in the furnace. Consequently, no halloysite remains 311 in the residue even if SEM micrographs show pseudotubular 312 structures, also for individual particles.

DISCUSSION

Considering the results about time tracking and imagery, 314 interpretations can be proposed concerning the behavior and 315 fate of HNTs during the incineration of PA6/HNTs nano-316 composites. 317 5.1. HNTs Behavior: A Two Step Mechanism. First of all, 318 a clear difference exists between the neat matrix and the 319 nanocomposite. A two-step mechanism can be considered for 320 PA6/HNTs nanocomposite. The evidence for this is the second 321 CO peak, the NO x shoulder, and the second phase in the 322 emission of particles. Thus, the presence of HNTs is clearly 323 responsible for this two step mechanism. It takes around 15 s to introduce the sample in the furnace. 326 This step involves particle emissions due to aerodynamic 327 disturbance from the set up and due to the first thermal effects 328 on the sample. Then, from 15 to 50 s, the thermal decomposition 329 of the sample is indicated by the release of gases and particles, and 330 by the presence of a flame for a few seconds. Effectively, as shown 331 by TGA (Figure 5), HNTs lose water, so a hydrolysis of PA6 332 occurs and there is an increase in the release of volatile 333 combustibles. [START_REF] Davis | Processing degradation of polyamide 6/montmorillonite clay nanocomposites and clay organic modifier[END_REF] Furthermore, during this step, according to the 334 literature, it is assumed that due to the thermal ablation of the 335 polymer and the dehydration of HNTs, decohesion occurs 336 between the PA6 and the HNTs. [START_REF] Lewin | Nanocomposites at elevated temperatures: Migration and structural changes[END_REF][START_REF] Vaia | Polymer layered silicate nanocomposites as high performance ablative materials[END_REF] Through these processes, 337 the HNTs concentration at the surface of the sample increases 338 and a protective layer gradually forms. TEM observation of 339 particles collected during this phase reveals the presence of 340 HNTs (in small quantities). The high concentration at the 341 surface would promote the release of HNTs and make them free 342 from the matrix and able to be carried away by the aerosol flow.

343

As reported in the literature, polyamide 6 could be considered 344 as a charring polymer and the nanoclays as char promoters. 24,38 345 In fact, during a thermal decomposition, some materials can 346 develop on their surface a protective layer which limits mass 347 transfer (volatile combustible and oxygen transfer) as well as 348 thermal transfer. Actually, CO time tracking with a double peak is 349 a good indicator of the protective layer evolution. It was 350 established that the decomposition of charring polymer involve 351 two phases of gas release: the first one is associated with the 352 primary char formation and the second one with the 353 decomposition of the nonstable primary char. [START_REF] Bahramian | High temperature ablation of kaolinite layered silicate/phenolic resin/ asbestos cloth nanocomposite[END_REF][START_REF] Lyon | Pyrolysis kinetics of char forming polymers[END_REF] Hence in 354 the present case, the protective layer can be made of a 355 carbonaceous char reinforced by HNTs. This char/HNTs layer 356 forms progressively, since under-ventilated combustion occurs 357 with a significant increase in CO emissions (Figure 3c). 358 Simultaneously, the particle emission decreases from 45 s up to 359 70 s (Figure 3a) which is in agreement with the formation of a 360 cohesive char layer. After 65s which corresponds to a CO peak, 361 CO emission decreases and reaches a local minimum while 362 oxygen consumption rises dramatically. This corresponds to the 363 oxidation of the primary char. [START_REF] Bahramian | High temperature ablation of kaolinite layered silicate/phenolic resin/ asbestos cloth nanocomposite[END_REF][START_REF] Lyon | Pyrolysis kinetics of char forming polymers[END_REF] Moreover, this oxidation is 364 accompanied by an increase in particle emissions, which is 365 irregular and suggests a partial cracking of the protective layer. So 366 the second peak of PN 0.1 reflects the presence of a second 367 combustion event. Furthermore, the NO x shoulder from 100 s 368 reflects the oxidation of the residual material which is no longer 369 protected by the cracked char/HNTs layer (Figure 3d). 370 At the end of the combustion test, the residues from PA6/ 371 5HNTs incineration consist only of mineral compounds derived 372 from the initial HNTs objects. However, some studies report 373 carbon/clay residues after pyrolysis tests on polymer/clay 374 nanocomposites [START_REF] Vaia | Polymer layered silicate nanocomposites as high performance ablative materials[END_REF] and especially for PA6/HNTs nanocompo-375 sites. [START_REF] Marney | The suitability of halloysite nanotubes as a fire retardant for nylon 6[END_REF] This difference should come from the combustion 376 conditions implemented in the furnace. In fact, in our case the 377 sample is well oxygenated at high temperature (850 °C or more) 378 and for a high rate of increase in temperature (from 25 to 850 °C 379 in 10 s). Therefore, under our incineration conditions, combus-380 tion is complete and no organic residues remain in the sample 381 holder.

382

As with other nanoclays (montmorillonite for example), 383 HNTs are known to have fire-retardant properties when in-384 corporated in a polymer matrix. [START_REF] Marney | The suitability of halloysite nanotubes as a fire retardant for nylon 6[END_REF][START_REF] Liu | Recent advance in research on halloysite nanotubes-polymer nanocomposite[END_REF] During exposure to a heat 385 source, these refractory materials can protect the matrix in which 386 they are incorporated through the formation of a protective clay 387 barrier which acts as a screen toward mass and heat transfers. The fire retardancy conferred by HNTs is confirmed by both the reduction in the peak heat release rate (pHRR) observed with the PCFC and the two-step mechanism discussed above. But this retardant character is not effective enough since it has been established that more than 15%wt is necessary to meet fire retardancy standards for industrial applications. [START_REF] Marney | The suitability of halloysite nanotubes as a fire retardant for nylon 6[END_REF] The comparison between neat PA6 matrix and PA6/5HNTs nanocomposite shows that CO emission is stronger for the nanocomposite with a first peak at lower time. Besides, this tendency is confirmed for PA6/1HNTs (Figure 3c) with its CO curve just inserted between the two other CO curves. This suggests better char formation in the presence of HNTs. This easier char formation could be related to the lower thermal stability for the composite shown by TGA and PCFC, which indicate that the release of water from the clays leads to hydrolysis of PA6 polymer chains. Since the emission of particles is significantly lower for neat PA6, it shows that the char is more unstable with the unfilled polymer, and burns more easily.

We can thus surmise that the HNTs modify the thermal behavior of the PA6/HNTs nanocomposite during its incineration and that the more HNTs incorporated in PA6, the more the phenomenon is accentuated. In fact, these nanoclays enhance PA6's ability to form a protective char layer, and reinforce it via a cohesive char/HNTs layer. To sum up, through a two-step mechanism as suggested by analysis of the time tracking, this char/HNTs layer gradually forms, acts as a barrier, cracks, oxidizes and ends up entirely inorganic.

Fate of HNTs: Aggregates in Aerosol and Residues.

The imagery results reveal the presence of mineral nano-objects in both the combustion aerosol and the combustion residues as aggregates and as prismatic (pseudotubular) structures which could result from partial distortion of the tubular structure. Thus, HNTs are not destroyed during the incineration of PA6/HNTs nanocomposites, but seem to be transformed into other mineral structures (named HNTs*). Only qualitative considerations can be discussed here.

At high temperatures (beyond 1000 °C), the tubular morphology tends to distort and then halloysite transforms into new crystalline or amorphous structures such as gammaalumina, cristobalite, and mullite. [START_REF] Yuan | Changes in structure, morphology, porosity, and surface activity 646 of mesoporous halloysite nanotubes under heating[END_REF][42][43] Actually, it is established that thermal decomposition of HNTs occurs in three steps: dehydration (loss of H 2 O from ∼200 °C), dehydroxylation (loss of OH from ∼500 to 900 °C) and a mullite-like phase formation (from 1200 to 1400 °C). The distortion of the tubular structure begins from 1000 °C and fusion up to 1400 °C. [START_REF] Yuan | Changes in structure, morphology, porosity, and surface activity 646 of mesoporous halloysite nanotubes under heating[END_REF][42][43] Considering the high temperatures reached in the furnace (at least 850 °C and more in the presence of a flame), the HNTs undergo the aforementioned thermal transitions during the incineration of PA6/HNTs. Thus, the presence of mineral aggregates identified as pseudo-HNTs with transformed mineral structures (HNTs*) could be explained by the thermal process occurred during the HNTs' stay in the furnace, and the aggregated state could also be explained by a sintering process occurring during the stay above 850 °C. Nevertheless, the temperature is not high enough to ensure the crystallization of mullite.

Finally, regarding the behavior of multilayered silicates, it can be suggested that for the incineration of other PA6/silicate nanocomposites, mineral nano-objects could be found both in the aerosol downstream of the postcombustion chamber and in residues, but possibly transformed into other mineral structures, taking into account the temperature of the incinerator and the residence time. This should be confirmed by prospective additional incineration tests on other nanocomposites.

  84 3.1. Sample Preparation and Characterization. PA6/ 85 HNTs nanocomposites (see Supporting Information (SI), 86 Table

  PCFC (pyrolysis combustion flow calorimeter) tests were carried out using a fire testing technology apparatus (FTT, ASTM D7309). A sample of 2 (±1) mg was heated under nitrogen flow to 750 °C at a heating rate equal to 1 K/s. Gases were extracted and sent to a combustion chamber in the presence of a N 2 /O 2 flow (80/20). The combustor ensured complete oxidation of pyrolysis products at a combustion temperature fixed at 900 °C.

26

 26

3. 2 .

 2 Figure 1. Pristine HNTs dispersed in PA6/HNTs nanocomposite.

Figure 2 .

 2 Figure 2. Basic diagram of the modified tubular furnaceLab-scale incinerator and the measurement lines.

200

  Regarding off-line analyses, there were two types of sampling: 201 in the first type, combustion aerosol particles were collected 202 throughout the test; in the second type, combustion aerosol 203 particles were collected over a targeted time range. 204 In the first case, the aerosol combustion underwent just one 205 hot dilution at 170 °C with a porous tube dilutor from VTT 206 laboratory (Valtion Teknillinen Tutkimuskeskus Technical 207 research center of Finland). 33 This diluted hot sample was 208 routed to a gravimetric impactor (DGI, Dekati Gravimetric 209 Impactor) which was heated to the same temperature (170 °C ± 210 10 °C). This PM 2.5 four-stage impactor resolves particles into 211 four size fractions with 50% collection efficiency for aerodynamic 212 (dp50) cut-points of 2.5, 1.0, 0.5, 0.2 μm plus a back-up filter 213 (after the impaction stages) which collects all particles typically 214 smaller than 0.2 μm. 34 In this way, the particulate matter of 215 the combustion aerosol was collected on PTFE impaction 216 substrates with controlled temperature during sampling. Then, 217 these particles were analyzed by a scanning electronic micro-218 scope (JEOL 7600F High Resolution Analytical SEM energy 219 dispersive spectrometer SDD BRUKER (EDS) X-ray detector). 220 This gravimetric impactor provided a qualitative analysis 221 (imagery and chemistry) of particles collected during the 222 whole combustion test. 223 In the second case, particles from the combustion aerosol were 224 collected on a TEM grid for above 10 s downstream of the tubular 225 furnace with a MPS (Mini-Particle-Sampler, Ecomesure 35 ). A 226 Philips CM12 TEM 200 kV was used for the imagery.

  Figure 3b-d show the comparisons between neat PA6 and

Figure 3 .

 3 Figure 3. (a) Time tracking for PA6/5HNTs incineration: particle number concentration and gas concentration, (b) Comparison between PA6 and PA6/5HNTs for PN 0.1 concentration, (c) Comparison between PA6, PA6/5HNTs and PA6/1HNTs for CO concentration, (d) Comparison between PA6 and PA6/5HNTs for NOx concentration (log scale).

  4.2. Imagery. The results from imagery provide only 268 qualitative considerations. 269 HNTs* are defined as pseudo-HNTs with transformed 270 mineral structures from HNTs. 4.2.1. Combustion Aerosol Particles. Imagery on particles collected on PTFE impaction substrates reveals the presence of HNTs* among the soot particles as shown in Figure 4a. These particles are from the fourth stage with dp50 = 1 μm. Submicron

Figure 4 .

 4 Figure 4. (a) Combustion aerosol particles collected on PTFE impaction substrate: soot and HNTs*, (b) Combustion aerosol particles collected on TEM grid: soot and HNTs* aggregates, (c) Combustion residues: HNTs* aggregates and individual HNTs*.

  4.2.3. TGA and PCFC on Nanocomposite.

  Figure 5 presents TGA curves of neat matrix, pristine HNTs and PA6/5HNTs. The mass loss of HNTs (14%wt) corresponds to water release and occurs mainly in the 350-600 °C range. The mass loss range of PA6 occurs between 350 and 500 °C. It can be noticed that the presence of HNTs reduces the degradation onset temperature and the thermal stability up to 450 °C.

  313

Figure 5 .

 5 Figure 5. TGA under N 2 .

Figure 6 .

 6 Figure 6. Heat Rate Release at PCFC of neat matrix and PA6/5HNTs nanocomposite.

Figure 7 .

 7 Figure 7. XRD of pristine HNTs and residues of nanocomposites.