Introduction:

Structure of the manuscript:

This thesis covers different colloidal syntheses, properties, and applications of transition metal dichalcogenides (TMDCs). The emphasis is on group VI TMDCs, in which the semiconducting MoS2, MoSe2, WS2, and WSe2 exist. The important effects that control the growth and dispersion are investigated and well-dispersed nanomonolayers with different mean sizes are synthesized.

In the first chapter, a broad introduction to TMDCs is discussed. In the second chapter, a comprehensive review of the colloidal synthesis of different members of group IV-VI of TMDCs is reported. As group V of TMDCs is out of the scope of this thesis, the details for this group are not deeply clarified. In the third chapter, the first part of this thesis, which is about developing a protocol to synthesize well-dispersed size-controlled nanomonolayers of WS2, is explained. After developing a controlled synthesis for 1T'-WS2, inducing phase change is reported in the fourth chapter. After developing a colloidal protocol to synthesize well dispersed NMLs of 1T'-WS2, the protocol is also extended to the other TMDCs and alloys. This part is explained in chapter five. In the last chapter, a new colloidal protocol is presented to synthesize heterostructures of 1T-WS2/2H-WS2, WS2/WSSe and WS2/WSe2.

An introduction to TMDCs:

TMDCs are a family of van der Waals solids, with a strong covalent in-plane bonding and weak van der Waals inter-layer one, consisting of over 63 members with different properties and applications started to attract attention since graphene isolation in 2004. 1,2 In these lamellar materials, each layer is consisted of a row of transition metal sandwiched between two layers of chalcogens (S, Se, Te) with a thickness of about 4-7 Å. 3 As a consequence of different bonding energies (strong inplane and weak inter-layer bonding), it is possible to isolate the nanosheets by direct or indirect methods, which cause high surface area and 1D quantum confinement. 4 Regarding this specific structure, they can possess multiple fascinating physical, electronic, and optical properties, as well as numerous applications. 47 members of this family are synthesized and characterized by Liu et al. including 32 binary compounds, 13 alloys, and 2 heterostructures 5 . As this family of materials consists of a vast number of members, our focus in this thesis is on the group VI TMDCs, in which the transition metals are Mo and W and the chalcogens are S, Se, and Te in the form of MX2. These materials are semiconductor in the stable form and beside the tunable bandgap, high carrier mobility, and large surface area, which provide different applications in electronics, 6 optoelectronics, 7 and catalysis, 8 as they can possess metastable crystal structures (octahedral and distorted octahedral), they also have other considerable applications such as hydrogen evolution reaction (HER) 9,10 and electrode materials for sodium ion batteries. 11 These different structures will be presented in detail in this chapter. Figure 1-1 represents different applications of TMDCs.

Different synthesis methods are available to produce TMDCs from multi to single layers. These fabrication techniques are categorized into two groups; top-down and bottom-up. The top-down methods consist of various mechanical and chemical exfoliation methods and bottom-up methods include vapor deposition methods and solution-based syntheses. 12 Recently, colloidal synthesis methods gained more attention due to their potential to produce large batches of materials with acceptable properties such as high crystallinity, size dispersion, and the shape of nanosheets. [13][14][15] Different synthesis techniques are presented in the next part and the colloidal method is also explained in detail in the next chapter.

One of the most attractive properties of TMDCs is the effect of quantum confinement and lack of inversion symmetry on the monolayers of these materials which cause the transition from an indirect bandgap to a direct one during the reduction from bulk to the monolayers. 16,17 There are other methods to manipulate the bandgap in these materials such as alloying and forming heterostructures. 15,18 Because of the strain due to the difference in size of atoms and the position of CBM and VBM in each TMDC, it is possible to tailor the bandgap size and position by alloying and by heterostructures formation. 15 The diverse applications of TMDCs in the form of bulk, single layers, alloys, and heterostructure are explained at the end of this chapter.

Synthesis methods:

Depending on the chemical composition, crystal structure, and the number of layers in nanosheets, TMDCs materials exhibit different properties. Due to the strong in-plane covalent bonding and weak inter-layer van der Waals bonding, it is possible to isolate single monolayers of TMDCs. 19 As it is mentioned before, there are two main categories of fabrication techniques and for each technique, different challenges exist that limit their applications. Although it is possible to produce large amounts of nanosheets by exfoliation methods, there is no control on the size, phase, and shape of the edges. 20 In contrast, substrate growth methods such as CVD, ALD, and PLD, lead to a good control on the shape, size, and crystallinity of the formed monolayers but the amount of produced monolayers limits the applications. 21 The last synthesis method is colloidal solution synthesis, which has the potential to produce a large quantity of well-controlled monolayers. 13 Concerning the applications and the required properties, the synthesis method would be different. Figure 1-2 displays a schematic of the two different techniques.

Top-down methods:

In the top-down approach, a sheer force is needed to overcome the inter-layer van der Waals forces, break the crystal lattice, and produce thinner materials. This process could be repeated until reaching isolated few to single layers of nanosheets. This sheer force could be induced by different methods such as micromechanical exfoliation, 22 liquid mediated exfoliation, 23 and chemical Figure 123A schematic presentation of exfoliation of a van der Waals crystal. 26 Figure 1-4: The procedure of micromechanical exfoliation is presented: (1) a layer of gold is deposited on the silicon substrate, (2) a layer of PVP and thermal release tape (TRT) is coated, (3) three layers of gold/PVP/TRT is exfoliated, (4) it is then pressed on to p of a van der Waals crystal, (5) a monolayer is peeled off, (6) after transferring to a new substrate the TRT is removed by heating, (7) PVP is dissolved in water, (8) gold layer is dissolved in an etchant solution, (9) a nano-monolayer is formed. 26 exfoliation. 24 Figure 1-3 shows the schematic presentation of a van der Waals structure in the form of a bulk crystal and exfoliated.

Historically, the first method has been the micromechanical exfoliation or scotch tape method, by which it is possible to isolate high-quality single layer and few layers of TMDC nanosheets. 25 In this method, which has a very poor yield, by making a bonding stronger than inter-layer van der Waals bonds of the TMDC layers, in such a way that the upper layers would exfoliate. 26 The next approach is also a mechanical exfoliation but in a liquid environment. The forces that are applied to the nanosheets can be categorized into two distinct forms: the vibrational and cavitation forces. 27 In a liquid mediated exfoliation, a proper solvent is necessary, as it has three roles: 1) the sonication force transferring media, 2) minimize the enthalpy of mixing of the exfoliated sheets and the solvent, 3) stabilize the exfoliated sheets and prevent the aggregation. 20 Coleman et al. did different studies to find the key factors for the liquid mediated exfoliation. 23 They also showed that not only the sheer force from a sonication bath or a sonication probe tip causes the exfoliation, but also the sheer force from a kitchen blender can induce exfoliation. 28 Although the production rate is quite high, the result is a mixture of single and few layers of TMDCs with different sizes. 28 SEM images of liquid-mediated exfoliated nanosheets from the study of Coleman et al. are presented in Figure 12345.

One of the most studied approaches to prepare TMDCs nanosheets is chemical exfoliation/intercalation methods. The reaction between small alkali metal especially lithium, which can diffuse into the inter-layer space, reacts with water and forms LiOH and H2, thus the formation of hydrogen gas between the nanosheets produces enough force to break the van der Waals bonding and separate the nanosheets. 29 The reaction of lithium salts that helps to intercalate the sheets is described as an ion-electron transfer reaction in which the electrons from lithium transfer to the lowest unoccupied energy levels of the TMDCs. 30 This excessive charge changes the metal coordination geometry and causes a distortion by which the structure of TMDCs change from 2H to 1T (different structures are explained in the next part). 31 Cheon et al. used tandem intercalation by different length alkylamines instead of the typical alkali salts exfoliation methods. 32 Figure 1-6 displays schematically the chemical exfoliation. Although the production rate of these approaches is quite high, some limits such as production of alloys or heterostructures exist.

Figure 1-5: SEM images of exfoliated nanosheets of four different TMDCs. 23

Bottom-up methods:

A proper reaction, in a solid, liquid, or gaseous state, between M (the transition metal) and X (the chalcogen) is required to form the desired crystal lattice of MX2. Diverse studies have been done on these three different reaction mediums to find out the best reaction parameters. 33 The bottom-up approaches eliminated the two main limits of the top-down methods, which are alloying and making heterostructures. 34,35 The greatest advantage of bottom-up methods is the controllability of the size, shape, and composition of the nanosheets by changing reaction parameters such as the reaction temperature, reaction time, nature of the reactants, and the concentration of the reactants. [START_REF] Han | Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures[END_REF] In the bottom-up approach, the most studied approach is the gas phase synthesis. As it is possible to produce large monolayers with controlled shape and edges and high crystallinity, 13 different studies have been completed on this synthesis technique not only to optimize the parameters but also to better know these materials and their applications. 37 Based on the methods of vapor generation, the gas phase method could be divided into two main categories: physical vapor deposition (PVD) 38 and chemical vapor deposition (CVD). 39 To understand better the gas phase method, it is necessary to know the TMDC formation mechanism. After preparation of the gaseous precursor (depending on the method, chemically or physically) the nucleus forms on a proper substrate. The last step is the growing step in the preferred direction. There are important factors to control the final morphology, thickness, and sizes of the nanosheets: the type of growing substrate, 40 reaction chamber pressure, 19 and reaction time. 41 In PVD methods by subliming the metal and chalcogen part of the desired TMDC under reduced pressure, the reactive gaseous phase would form, while different heat sources can be used such as a heating element or laser. 42 While, in CVD methods, the reactive vapor, which is made by the volatilization of precursors that contain the elements, reacts to form the TMDC nanosheets. Figure 1234567represents the schematic and SEM images of triangular MoS2 formed by CVD.

Although colloidal synthesis has the potential for the preparation of a large batch with high crystallinity of well-dispersed nano-size monolayers, it is the least studied method among all. 13 In the Figure 1-6: Schematic presentation of MoS2 exfoliation by lithium ions. 30 colloidal synthesis, the reaction between precursors happens after the decomposition in the solution either at high or low temperatures. As it has been mentioned, the greatest advantage of bottom-up methods is the size, shape, and composition controllability. There are three important factors that control the properties of a colloidal synthesis: the difference between the surface energy of nanosheets and the surfactants, the kinetics of the reactions, and the nature of bonding between ligands and the nanosheets. [START_REF] Han | Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures[END_REF] Colloidal syntheses are categorized into three different methods: hot injection method, one-pot synthesis, and hydrothermal method. It should be noted that, the proper layered TMDCs are not achievable unless the suitable precursors, solvents, and ligands are chosen. 33 As hot injection method is a classic nucleation and growth process, the monomers for the nucleation step forms within a reaction between the precursor and the reaction medium. Then, the nuclei reach the critical size and the growth step starts. However, in the one-pot synthesis and hydrothermal method, the nuclei formation and growth happen together and they are not separable. 43 Because of the difference between in-plane covalent bonding energy and inter-layer van der Waals bonding energy, the anisotropic growth happens in the 2D lamellar nanocrystals. Mahler et al. reported that the change in the solvents can control the crystal structure. 13 While, Cheon et al. described in two different reports that, the effect of the chalcogen sources and the capping ligand on the thicknesscontrol. 44,45 Schimpf et al. proclaimed the role of ligands as well as the reactivity of precursor on the size and phase of nanocrystals. 46 As it has been already mentioned, it is possible to produce alloys and heterostructures by colloidal synthesis. 15,34 This method will be explained in more details in the next 41 chapter. Figure 12345678shows the size of nanosheets can be controlled by changing the reaction parameters.

The last bottom-up method is a solid phase (solventless) reaction. To date, only a few studies (mostly MoS2) have been done on this subject. In a sulfur-rich environment (H2S), a proper salt of the transition metal decomposes and the TMDC nanosheets form. 47 TMDCs properties:

As it has been described, TMDCs are lamellar materials with strong in-plane covalent bonding and weak inter-layer van der Waals bonding, which help to isolate monolayers. These materials can possess different structures such as metallic, semiconducting, and superconducting. The very iconic properties of these materials are the change of bandgap from an indirect to a direct one by decreasing the size to a monolayer of the semiconductor. 48 Crystal structure:

Regarding the atomic coordination and oxidation state of the metal atoms, TMDCs possess metallic, semiconducting, or superconducting properties. 49 Depending on the arrangement of the atoms, TMDCs possess two different structures: H-phase (trigonal prismatic) and T-phase (octahedral). The stacking order of chalcogens and transition metals along the z-axis defines the different polymorphs. Considering B denotes the transition metal while A and C represent the chalcogen atoms. The stacking sequence of ABA demonstrates the H-phase and ABC shows the T-phase. 50 Because the stable polymorph of the majority of the group 6 TMDCs is the H-phase, here the first explained polymorph is this one. 1H, 2H, and 3R (H stands for hexagonal and R stands for rhombohedral) are the different polytypes of the H-phase, which has trigonal prismatic coordination of transition metals with hexagonal symmetry (the D3h group, space group P6 ̅ m2). Depending on the number of stacking sequences of each layer needed to represent the structure, these different polytypes form. [START_REF] Mattevi | Direct synthesis of metastable phases of 2D transition metal dichalcogenides[END_REF] DFT calculations represent that the 2H-phase is the stable phase for MoS2, MoSe2, MoTe2, WS2, and WSe2, which have semiconducting properties. 50 The M-M bond's length depends on the size of transition metal, chalcogen, and the structure. 3 The metastable polymorph of group 6 TMDCs except for WTe2 (the stable polymorph of WTe2 is 1T phase), is the T-phase, which forms by an interlayer shift of one of the chalcogen planes. [START_REF] Mattevi | Direct synthesis of metastable phases of 2D transition metal dichalcogenides[END_REF] The coordination of transition metals in the T-phase is octahedral with tetragonal symmetry (the D3d group, space group P3 ̅ m1). [START_REF] Voiry | Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering[END_REF] In comparison with H-phase, there are no different polytypes of the T-phase, and due to the large energy difference between the 2H (stable phase) and 1T (unstable phase) phase, to reduce the internal energy, 1T monolayers would distort to the 1T' crystal structure. [START_REF] Zhao | Metastable MoS2: Crystal Structure, Electronic Band Structure, Synthetic Approach and Intriguing Physical Properties[END_REF] In contrast with the 1T phase, which is unstable for group 6 TMDCs (except WTe2), the 1T' phase is metastable and obtainable under specific conditions. [START_REF] Mattevi | Direct synthesis of metastable phases of 2D transition metal dichalcogenides[END_REF] Albeit the 2H-phase TMDCs have semiconducting properties, the bandgap of the 1T-and 1T'-phases is almost zero and they have semi-metallic properties. 50 It should be noted that depending on the synthesis method, it is possible to produce either the stable or metastable phases, or transform them into each other. [START_REF] Eda | Supporting Information for "Photoluminescence from chemically exfoliated MoS2[END_REF] A sufficient amount of charges is needed to transform the 2H phase to 1T'. Due to the Jahn-Teller effect, to reduce the energy of the system, the symmetry would break and the hexagonal symmetry changes to a zigzag structure. 3 Different techniques are used to induce the 2H to 1T' phase change such as charge accumulation methods, defect induced methods, and mechanical strain. [START_REF] Mattevi | Direct synthesis of metastable phases of 2D transition metal dichalcogenides[END_REF] The 1T'-phase as the metastable phase is 13 In the next chapter, the fundamentals, history, and the evolution of the colloidal synthesis will be explained comprehensively.

Vibrational properties:

As TMDCs have different symmetry groups, distinct lattice vibrations would form. Zhang et al. reported that there are 9 vibrational modes for a monolayer, which is composed of 3 acoustic and 6 optical modes, while for a bulk there are 18 vibrational modes. [START_REF] Paquin | Multi-phase semicrystalline microstructures drive exciton dissociation in neat plastic semiconductors[END_REF] There are two kinds of Raman peaks, in-plane (E 1 2g and E1u) and out-of-plane (A1g) modes, which change with variation in the number of layers. The A1g and E 1 2g of bulk MoS2 are 408 and 383 cm -1 (λ=532nm), respectively, [START_REF] Zhang | Shear and Layer Breathing Modes in Multilayer MoS2[END_REF] and for WS2 these numbers are 421 and 356 cm -1 (λ=473nm), respectively (these numbers are reported for 2H-phases). [START_REF] Paquin | Multi-phase semicrystalline microstructures drive exciton dissociation in neat plastic semiconductors[END_REF] Electronic and band structure:

The electronic band structure of TMDCs is not only dependent on their crystal structure, but also depends on the number of layers. As it has been already mentioned, bulk semiconducting TMDCs (MoS2, MoSe2, WS2, and WSe2) possess indirect bandgap, however, their single layers have direct bandgap. [START_REF] Zhao | Metastable MoS2: Crystal Structure, Electronic Band Structure, Synthetic Approach and Intriguing Physical Properties[END_REF] In the bulk (until bilayers) TMDCs, the conduction band minimum is located at Γ point and the valance band maximum is placed at K point. [START_REF] Rivera | Interlayer valley excitons in heterobilayers of transition metal dichalcogenides[END_REF] By reducing the size of sheets to a single monolayer, due to the quantum confinement and lack of inversion symmetry, both valance band maximum and conduction band minimum are located at K point. 16 Eda et al. reported that the emission from a highquality monolayer is 100 to 1000 times stronger than the bulk in WS2 and WSe2, due to the indirect to direct transformation of the bandgap. [START_REF] Zhao | Evolution of Electronic Structure in Atomically Thin Sheets of WS 2 and WSe 2[END_REF] As different valleys are present in the Brillouin zone of a semiconductor, charge carriers could populate the valleys to different ranges. This charge accumulation leads to valley polarization and affect the optical properties of the semiconductor. [START_REF] Wilson | The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties[END_REF] Moreover in TMDCs, reducing the size to the monolayer, due to the lack of symmetry inversion, generates a strong coupling between the valley and spin polarizations at the K point, which provides the ability of controlling the spin polarization. [START_REF] Peng | Two-dimensional dichalcogenides for light-harvesting applications[END_REF] [START_REF] Zhang | Shear and Layer Breathing Modes in Multilayer MoS2[END_REF] In comparison with other semiconductors, TMDC semiconductors possess very high exciton binding energy. While for typical semiconductors, the exciton binding energy is in the order of tens of meV, the exciton binding energy of a single monolayer of group 6 TMDCs is reported to be in the range of 0.2-0.9 eV. [START_REF] Cao | Two-dimensional transition-metal dichalcogenide materials: Toward an age of atomic-scale photonics[END_REF][START_REF] Mak | Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides[END_REF] Yu et al. reported that due to this very high exciton binding energy, MoS2 and WS2 monolayers can absorb more than 5% of light with energy higher than 2eV. [START_REF] Yu | Equally efficient interlayer exciton relaxation and improved absorption in epitaxial and nonepitaxial MoS2/WS2 Heterostructures[END_REF]

Electrical properties:

Single layers of TMDCs are a combination of quantum-confined semiconductors with direct bandgap and strong excitonic binding energy in the 2H-phase. [START_REF] Manzeli | 2D transition metal dichalcogenides[END_REF] As it has been already mentioned, regarding the crystal structure, group 6 TMDCs exhibit either semiconducting (2H-phase) or metallic (1T-or 1T'-phase) properties, so in metallic phase conduction increased in comparison with the semiconducting one. [START_REF] Wang | Electronics and optoelectronics of two-dimensional transition metal dichalcogenides[END_REF] Due to the different bonding energies for in and out of plane, the electrical properties of these materials are anisotropic. [START_REF] Wilson | The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties[END_REF] DFT calculations demonstrate that in the group 6 TMDCs, the selenides are naturally p-type semiconductors while sulfides are n-type semiconductors. [START_REF] Fang | High-performance single layered WSe 2 p-FETs with chemically doped contacts[END_REF][START_REF] Radisavljevic | Single-layer MoS2 transistors[END_REF] Defects and impurities can also change the electrical properties of TMDCs. induce n-type doping. They also reported in a large sheet local n-type and p-type semiconductor could exist at the same time. [START_REF] Voiry | Phase engineering of transition metal dichalcogenides[END_REF] Nayeri et al. proved by doping V, Nb, and Ta atoms in WS2, the system would become a p-type semiconductor while Re and Tc cause an n-type semiconductor transformation. [START_REF] Nayeri | The Influence of Impurity on the Electronic Properties of WS2 Single Layer[END_REF] In another study, Zhao et al. substituted sulfur atoms with atoms from group V (N, P, and As) and VII (F, Cl, and I). They reported that the nitrogen-doped system has the lowest formation energy which leads to p-type doping. [START_REF] Zhao | Effective p-type N-doped WS2 monolayer[END_REF] Javey et al. studied the effect of chalcogen vacancies on the Fermi level. Unlike selenides, vacancies cause an increase in the Fermi level of transition metal disulfides, which can be eliminated by bis(trifluoromethane) sulfonamide (TFSI) treatment. This Lewis acid treatment decreases the number of electron on the surface by transferring the charges. [START_REF] Lien | Electrical suppression of all nonradiative recombination pathways in monolayer semiconductors[END_REF] To make p-n junction diodes, it is possible to make heterostructures of these TMDCs. [START_REF] Lee | Atomically thin p-n junctions with van der Waals heterointerfaces[END_REF] One of the most interesting electrical properties of TMDCs is their charge carrier mobility. In theory, their high charge carrier mobility is very promising but in practice, different scattering sources can affect the charge carrier mobility such as crystal defects and optical phonons. A theoretical simulation confirms that the charge carrier mobility of electrons in 2H-MoS2 at room temperature is about 10-1000 cm 2 V -1 S -1 and this number can exceed 10 5 cm 2 V -1 S -1 at low temperature, which depends on the impurities and environment. It should be noted, as the thin layer materials are more sensitive to the environment, that reducing the size from bulk to monolayer, the charge mobility would reduce. [START_REF] Manzeli | 2D transition metal dichalcogenides[END_REF] Figure 1-13: Schematic illustration of p-n junction of TMDCs. [START_REF] Lee | Atomically thin p-n junctions with van der Waals heterointerfaces[END_REF] TMDCs can also exhibit superconducting behavior at low temperatures. Manzeli et al. reported that all bulk TMDCs possess superconductivity behavior in a specific condition (specific temperature, pressure, or doping induces superconductivity). [START_REF] Manzeli | 2D transition metal dichalcogenides[END_REF]

Chhowalla et al. reported that transition metal vacancies induce p-type doping and sulfur vacancies

Optical properties:

As it has been already explained, the band structure and morphology of TMDCs influence the optical properties by controlling the phase. [START_REF] Wang | Electronics and optoelectronics of two-dimensional transition metal dichalcogenides[END_REF] The other important parameters that affect the optical properties are the number of layers, the band nesting effect, and defects. 73 In contrast with other twodimensional materials such as graphene with an almost zero bandgap and h-BN with a very large bandgap, semiconductor TMDCs have a tunable bandgap, which changes to a direct one by reducing the size to monolayer. [START_REF] Mak | Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides[END_REF] To understand better the optical properties of TMDCs, getting to know their exciton dynamics is decisive. The exciton formation, separation, transfer, and relaxation in these materials are different from other materials as the spin-valley coupling is very strong in TMDCs. [START_REF] Cao | Two-dimensional transition-metal dichalcogenide materials: Toward an age of atomic-scale photonics[END_REF] Theoretical calculations estimate the exciton Bohr radius of monolayer TMDCs is in the order of 1 nm and it increases by increasing the number of layers. [START_REF] Cheiwchanchamnangij | Quasiparticle band structure calculation of monolayer, bilayer, and bulk MoS 2[END_REF] Figure 1-14: The changes in absorption coefficient vs energy for a) bulk WS 2, b) bilayer WS2, and c) monolayer WS2. [START_REF] Roy | Electronic and optical properties of ultra-thin 2D tungsten disulfide for photovoltaic applications[END_REF] The presentation of optical properties of semiconducting TMDCs will be separated in two categories: absorption and emission. The group 6 TMDCs have bandgaps in the range of 1-2 eV with absorbance and photoluminescence near the infrared region. [START_REF] Xia | Two-dimensional material nanophotonics[END_REF] Absorbance of TMDCs depends on the number of layers. Roy et al. measured the absorption coefficient of the bulk, bilayers, and monolayers of WS2 in the range of 0-4 eV and they reported that the absorption coefficient of WS2 increase by decreasing the number of layers to a monolayer. [START_REF] Roy | Electronic and optical properties of ultra-thin 2D tungsten disulfide for photovoltaic applications[END_REF] Figure 1-14 shows the changes in the absorption coefficient of 2H-WS2 when reducing the size from bulk to monolayer.

Semiconducting TMDCs possess a bandgap in the range of 1.5 eV to 2 eV, which make them good choices for luminescence as well. As reducing the size to monolayer semiconductors induces bandgap transformation, they have also the potential to be used as light-emitting devices. The change from indirect to direct bandgap increases the photoluminescence quantum yield. 21 Because of the high quantum confinement and exciton bonding energy, defects have a great influence on the photoluminescence of TMDCs and significantly reduce their quantum yield. Lattice defects such as impurities and vacancies can change the bandgap size. [START_REF] Zhou | How defects influence the photoluminescence of TMDCs[END_REF] While Javey et al. reported an increase in the quantum yield of sulfur-based TMDCs by super-acid treatment, Yao et al. announced that Na2S can also improve the photoluminescence of WS2 due to a n-type doping and oxide reduction. [START_REF] Wallace | Recombination Kinetics and Effects of Superacid Treatment in Sulfur-and Selenium-Based Transition Metal Dichalcogenides[END_REF][START_REF] Yao | Significant photoluminescence enhancement in WS2 monolayers through Na2S treatment[END_REF] The maximum quantum yield of WS2, MoS2, WSe2, and MoSe2 before and after TFSI treatment. [START_REF] Wallace | Recombination Kinetics and Effects of Superacid Treatment in Sulfur-and Selenium-Based Transition Metal Dichalcogenides[END_REF] The other applicable property of semiconducting TMDCs comes from bandgap split at K point. Cui et al. reported that due to the lack of inversion symmetry in the monolayers of MoS2, orbital magnetic moment and circular dichroism would form, which increases the diversity of applications of these materials. [START_REF] Zeng | Valley polarization in MoS 2 monolayers by optical pumping[END_REF] Multiple excitons are identified in these materials. The first two excitons (A and B) come from the transition between the split valance band maximum at point K and the conduction band maximum. The strong spin-orbit coupling splits the valence band of MoX2 by almost 160 meV and this number for WX2 is almost 400 meV. [START_REF] Peng | Two-dimensional dichalcogenides for light-harvesting applications[END_REF] Moreover the last exciton (C) comes from the band nesting of the VBM and CBM between K and Γ points. [START_REF] Cao | Two-dimensional transition-metal dichalcogenide materials: Toward an age of atomic-scale photonics[END_REF] The cleavage of the valence band maximum and the band nesting, which causes the formation of three excitons are demonstrated in Figure 1234567891011121314151617. Due to the valley splitting, the K' valley absorbs and emits left -handed photons and the K valley absorbs and emits the right -handed photons. [START_REF] Hong | Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures[END_REF]

Alloys:

A brief explanation on alloys is reported here, of course there are some nice review papers about this subject. There are different approaches to modify the TMDCs bandgap such as surface and defect engineering, heterostructures fabrication, and alloying. 15 As the photoluminescence of these materials is sensitive, especially to the defects, the best methods to modify their bandgap are alloying and heterostructures fabrication. A very comprehensive review on bandgap engineering of TMDCs is reported by Chaves et al.. [START_REF] Chaves | Bandgap engineering of two-dimensional semiconductor materials[END_REF] They explained the impacts of alloying on the bandgap size and band structure in TMDCs. The synthesis, properties, and applications of heterostructures will be shortly presented in the next part. Different studies have been carried out on alloying and the properties of the alloys, as this class of materials are still not well known. Xie et al. did both experiments and DFT calculations to find out the effect of alloying on the bandgap of Mo1-xWxS2 monolayers, and they reported a large bowing effect, which comes from the difference in the bandgap compositions. [START_REF] Chen | Tunable band gap photoluminescence from atomically thin transition-metal dichalcogenide alloys[END_REF] Li et al. reported the selenization of CVD-grown MoS2 monolayers (MoS2(1-x)Se2x) and the influence of the selenization temperature on the optical bandgap. [START_REF] Su | Band gap-tunable molybdenum sulfide selenide monolayer alloy[END_REF] Terrones et al. managed to fabricate Mo1-xWxS2 triangular nanocrystals by annealing the MoS2 and WO3 in a sulfur-rich atmosphere. [START_REF] Lin | Facile synthesis of MoS2and MoxW1-xS2triangular monolayers[END_REF] In another study from Xie et al., monolayer alloys of MoS2(1-x)Se2x with controlled morphology were grown by PVD on a SiO2/Si substrate. In comparison with the Mo1-xWxS2 alloys, no bowing effect is reported for MoS2(1x)Se2x. 38 Sun et al. synthesized the whole range of Mo1-xWxSe2 to WS2(1-x)Se2x multilayers by colloidal synthesis and depicted the presence of bowing effect for Mo1-xWxSe2 and not for WS2(1-x)Se2x. 15 We developed a new colloidal synthesis to produce the 1T' phase of alloyed WS2(1-x)Se2x. Consequently, we annealed the 1T' alloys and transferred the structure to 2H. 34 TMDC alloys have the potential to be used in a diverse range of applications. Figure 1-18 shows changes in the bandgap size due to alloying. [START_REF] Hong | Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures[END_REF] Wang et al. reported a study on the spin-orbit engineering of MoWSe2 by changing the composition. They examined the effect of alloying on the valance and conduction band splitting through the bright and dark exciton population. [START_REF] Wang | Spin-orbit engineering in transition metal dichalcogenide alloy monolayers[END_REF]

Heterostructures:

Although no colloidal protocol to synthesize heterostructures of TMDCs is developed yet, it is one of the most interesting techniques to tailor the bandgap of TMDCs. As it has been already mentioned, the first tuning method (alloying) has been previously described. The second one, which is studied more, is heterostructure manufacturing. Hu et al. reported a review computational properties of heterostructures of TMDCs. [START_REF] Hu | Two-dimensional van der Waals heterojunctions for functional materials and devices[END_REF] They represented the different type of bandgaps and band alignments, which forms by formation of heterostructures. The changes in the physical properties of 15 the materials depend on the interaction between two layered materials. Regarding the electronic properties of the materials, such as insulators, semiconductors, and semimetals, different types of heterostructure could form. [START_REF] Li | Heterostructures based on two-dimensional layered materials and their potential applications[END_REF] First, it should be noted that there are two types of heterostructures for these 2D materials: In-plane (lateral) and out-of-plane (lamellar) heterostructures. [START_REF] Amin | Heterostructures of transition metal dichalcogenides[END_REF] Although out-ofplane TMDCs heterostructures possess indirect bandgaps, in-plane heterostructures have direct bandgaps and exhibit significant changes in the electronic properties. [START_REF] Wang | First-principles study of the structural and electronic properties of MoS2-WS2 and MoS2-MoTe2 monolayer heterostructures[END_REF] In a simulation study, Amin et al. reported that out-of-plane heterostructures have less formation energy than the in-plane ones, which leads to indirect bandgaps with type-II band alignment. In addition, they reported that the in-plane heterostructures possess direct bandgaps [START_REF] Amin | Heterostructures of transition metal dichalcogenides[END_REF] . In contrast, Kosmider et al. announced that the bandgap of an out-of-plane heterostructure would remain direct, as there is no inversion symmetry. [START_REF] Koåmider | Electronic properties of the MoS2-WS2 heterojunction[END_REF] Only two different methods of heterostructure manufacturing are developed up to now, sticking of exfoliated nanosheets and vapor deposition methods. Moreover, by the first method, only out-of-plane heterostructure could be fabricated. [START_REF] Li | Heterostructures based on two-dimensional layered materials and their potential applications[END_REF] 2D heterostructures have a broad range of applications, as that will be discussed in the next section.

Characterization techniques:

Different methods are available to determine different phases. As the crystal structure, the bonding energy, and the optical properties of different phases are different, we can separate them by different techniques. X-ray Photoelectron Spectroscopy (XPS) is a powerful technique to determine the chemical states of atoms by measuring the kinetic energy and the number of ejected electrons after X-ray radiation. XPS is a precise qualitative and quantitative method that can detect all the elements except hydrogen and helium with the precision of part per thousand (ppt) to part per million (ppm) depending on the signal acquisition time. In this thesis, XPS has been used to determine the phase of nanosheets before and after phase change, as well as define the exact content of each phase after annealing. Figure 1-21 shows the 1T'-WS2 sample that was analyzing by XPS.

X-ray diffraction analysis (XRD) is another method to define the phases. XRD or in this case, powder-XRD (PXRD) is a great tool to define the atomic structure of crystalline materials. When the Xray radiations collide with the crystal, in the diffraction, they can have constructive and destructive behavior. By using the Brag's law, it is possible to measure the distance between the planes, and as the result, the crystal structure would be defined. The lattice parameters, the crystal orientation, the crystal size and shape, and the residual stress in the crystals are measurable by XRD. UV-Vis spectroscopy measures the absorbance or reflectance of a chemical substance in the part of ultraviolet and visible spectrum. A quantitative technique that measures the intensity of the light before passing through the sample (I0) and the intensity of the light passed through the sample (I). The logarithm of I0 divided by I represents the absorbance (A) of the sample. Beer-Lambert's law helps us to measure the absorption coefficient of chemical compounds if their absorbance is specified by this equation:

A = εlc
Where ε is the absorption coefficient, l is the length of the light path, and c stands for the concentration. In this thesis, UV-Vis spectroscopy was used as the first spectroscopy method to define the structure, as it is the most applicable technique. In contrast with semiconducting TMDCs that have a recognizable excitonic peak related to their bandgap size, metallic TMDCs have a significant absorption at all wavelengths. Because of these features, UV-Vis spectroscopy was used as a preliminary technique to recognize the phase of TMDCs.

Scanning Transmission Electron Microscopy (STEM) is a powerful technique to observe the structure of the materials. STEM is a kind of transmission electron microscopy, in which the fine beam is scanned to form the image. Because the images form by scanning the beam that passes through the sample, to have a high-quality atomic resolution image, reducing the fluctuation of the electron beam source, temperature changes, and any vibration is vital. Two different imaging modes are available for STEM, which depends on the position of the detector: annular Dark-field (ADF) and annular Bright-field (ABF). By High-Angle Annular Dark-Field (HAADF-STEM), forming atomic resolution images is possible where the contrast depends on the atomic mass of the elements. ABF images are used as complementary images for ADF to show the light elements. In this study, low magnitude HAADF-STEM is used to check the dispersion of nanosheets and high magnitude HAADF-STEM is used to investigate both the phase change (different atomic structure) and the heterostructure formation (different signal intensity due to the difference in elements mass).

Raman spectroscopy is a powerful technique to observe the different vibrational modes of molecules. In Raman spectroscopy the scattering of a known source of light (X-ray, UV, visible, or infrared) and the matter is measured, which can have three kinds of interaction: Rayleigh, Stokes and Anti-Stokes scattering. Raman spectroscopy relies on the inelastic scatterings (Stokes and Anti-Stokes) while the Rayleigh scattering is an elastic one and it is not useful for Raman spectroscopy. By Raman spectroscopy, it is possible to define the compound, crystal structure, and the number of layers in TMDCs. In this study, Raman spectroscopy is used to determine the phase change for annealed WS2.

(PL) spectroscopy is a useful technique to determine the optical properties. PL is the emitted light after absorption of exciting light. The emitted light from a semiconductor depends on the bandgap and various relaxation mechanisms happens in the semiconductor. It should be noted that, the PL is different from Photo luminescent excitation (PLE). In PLE the luminescence is observed at a specific wavelength while the frequency of the exciting light varies, while in PL the emitted light in full spectrum is monitored. In this study, UV-Vis spectroscopy is used to determine the structure of TMDCs and PL spectroscopy is used to investigate the properties of semiconducting TMDCs.

Energy Dispersive X-ray analysis (EDX) is a powerful analytical technique to do elemental analysis. EDX is a qualitative and quantitative method, which measures the x-ray induced by the sample after an interaction with an electron or x-ray beam. It can characterize materials due to this fundamental principle that each element has a unique atomic structure allowing a unique set of peaks to form on its electromagnetic emission spectrum. In this study, EDX has been used to determine the exact composition of each alloy. STEM, and Electron energy loss spectroscopy (EELS) could be used to determine the formation of heterostructures. The mean size of the nanosheets before and after heterostructure manufacturing has been observed by TEM and the growth in the mean size proves the formation of heterostructures. In HAADF-STEM, the contrast from different elements with different mass is significantly different, by which it is possible to observe the formation of heterostructures. Both of these methods can be applied to characterize the lateral heterostructures. EELS is a powerful technique, which could be used to evaluate the formation of either lamellar or lateral heterostructures. EELS is an analytical technique, which relies on the inelastic interaction of radiated electron to the sample. Therefore, the amount of energy loss could be measured to determine the source of the loss. In this study, all of these three techniques have been used to evaluate the formation of heterostructures.

Applications:

To clarify the applications of TMDCs, a piece of general knowledge about their properties is needed. To summarize, group 6 TMDCs are lamellar materials with strong in-plane covalent bonding and weak out-of-plane van der Waals one. These materials can possess two different structures, which affect their electronic properties. By reducing the size to monolayers, the indirect bandgap would change to a direct one. The vast range of properties as well as large surface area, make this group of materials very promising not only for fundamental studies but also for different applications. Here the main applications areas of TMDCs are discussed.

Electrocatalysts:

TMDCs in the form of single layers possess a large surface area. However, the atoms on the surface do not have any dangling bond, edges are considered active sites for catalytic reactions and the catalytic active sites are located on the edges due to the presence of unsaturated atoms. So, the most important part of the study on electrocatalytic properties of TMDCs is to manipulate the morphology to have higher amount of edges. Platinum electrodes can be substituted by this class of materials in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). 92 To be more precise, metallic phase TMDCs are exceptional replacements for precious metal electrocatalysts. [START_REF] Shi | Synthesis, stabilization and applications of 2-dimensional 1T metallic MoS2[END_REF] Huang et al. reported that by doping WSe2 with cobalt, the electrocatalytic activity for HER improved, even though mixing CoSe2 and WSe2 weakens their electrocatalytic performance, if the mixing happens physically. Thermal treatment of nanosheets, as a rule, decrease the electrocatalytic activity due to improving the crystallinity, which annihilates the active sites, but in a study by Schenato et al. the HER electrocatalytic activity of annealed WS2 was improved. 92 Some studies have shown that the 1T-phase of TMDCs have better electrocatalytic activity in comparison with the 2H phase. Voiry et al. and Lukowski et al. reported that the electrocatalytic activity of MoS2 is significantly improved when the 1T-phase is used, due to the higher charge mobility and less resistivity of the metallic phase. Several studies have been conducted on this subject, but, as it has been explained, the best morphology for electrocatalysis is a small metallic nanosheet, which is still not synthesized. Figure 1-23 represents the schematic of an electrocatalysis device used for water splitting. From these studies, smaller nanosheets with 1T-phase structure have a better electrocatalytic performance. 92

Photodetectors:

Electron-hole pairs form by shining light with energy higher than the semiconductor's bandgap. If it is possible to separate and collect the excitons, it is possible to sense the light. As the conductivity of TMDCs changes by the incident of light and also they have a relatively high absorption coefficient, they are a good candidate for photo-detection applications.95 Depending on the incident light, the resistance of 2H-phase TMDCs would change, which happens in 50 ms and makes these materials suitable as they have high photo-responsivity. The other property that makes TMDCs a very interesting choice for photodetection is their high broadband absorption. Composites of TMDCS and graphene are powerful composites for photodetection as they can separate the electron-hole pair at the interface.96 Figure 1-24 shows the changes in the current density of a phototransistor under light and dark. TMDCs can also be used as gas sensors since their photocurrent changes in the presence of chemicals. Different studies have been done on this subject and MoS2 and WS2 are used to sense chemicals such as ethanol and ammonia. 92

Photocatalysts:

TMDCs attracted great attention due to their potential application as photocatalysts. Different studies have been done on the photovoltaic applications of TMDCs as they have relatively high absorption coefficient and it is possible to produce them as n-type or p-type conveniently. [START_REF] Li | Heterostructures based on two-dimensional layered materials and their potential applications[END_REF] Hence, a heterostructure with different n and p type of TMDCs could lead to a very efficient solar cell. Because of their bandgap size and bandgap position, TMDCs are good candidates for hydrogen evolution reactions. A computational study shows different TMDCs such as WS2, PtS2, and PtSe2 are suitable for spontaneous water splitting in an acidic medium when they are thinned to monolayers. In addition, monolayer semiconductors of TMDCs possess a relatively long electron diffusion pass due to the high exciton mobility, which simplifies the transfer and recombination of photo-induced electron and hole pair into the redox sites. [START_REF] Peng | Two-dimensional dichalcogenides for light-harvesting applications[END_REF] The other photocatalytic application of TMDCs is the photocatalytic oxidation or photo-degeneration of organic molecules. Sang et al. proposed WS2 as a good candidate for photo-degeneration application due to the capability to work in the full solar spectrum. 92 The 1T phase TMDCs are good choices not only in electrocatalytic applications but also they are better than the 2H phase for photocatalytic activities due to the higher conductivity of the metallic phase. Urmimala et al. reported a huge increase in the production of hydrogen when they used 1T-MoS2 as a cocatalyst in a hydrogen evolution system with Eosin Y as the sensitizer and triethanolamine as the sacrificial agent. [START_REF] Shi | Synthesis, stabilization and applications of 2-dimensional 1T metallic MoS2[END_REF] Mahler et al. reported that the nanocomposite of TiO2 with metallic WS2 considerably improves the hydrogen produced by photocatalytic HER. In this study, P25-TiO2 was used as the synthesizer and methanol was used as the hole scavenger. 13

Energy storage:

TMDCs are very important not only to produce energy as a semiconducting material but also as a candidate for energy storage applications. Acceptable mechanical properties, good conductivity, and tunable bandgap make this class of materials a promising candidate for energy storage applications. TMDCs were investigated as the cathode in Li-ion batteries. Graphite is the most frequently used anode in lithium-ion batteries but it has a low theoretical capacity, which reduces the yield of these batteries. MoS2 is a good alternative for graphite because its theoretical capacity is almost two times higher than graphite. [START_REF] Shi | Synthesis, stabilization and applications of 2-dimensional 1T metallic MoS2[END_REF] Different alloys of TMDCs have been studied for energy storage applications. Zhang et al. studied a strategy to tune the structure of MoSSe for lithium-ion batteries, where the metastable MoSSe possesses a high charge capacity. [START_REF] Wang | Recent advances in ternary two-dimensional materials: Synthesis, properties and applications[END_REF] WS2 and WSe2 were also studied by Qian et al. and Chen et al. respectively. Qian et al. deposited WS2 nanosheets vertically and horizontally on the substrate and reported that vertically deposited nanosheets improve the battery performance more than horizontally deposited film. Chen et al. synthesized a mesoporous anode. In the electrochemical application for batteries, the important factor is the solid-state diffusion, which is fast for layered materials like TMDCs. A composite of TMDCs with a thermodynamically suitable material for Na or Li intercalation can improve the ionic-battery applications. 92 The voltage vs current Some studies on the application of TMDCs in energy storage support that the 1T-phase TMDCs improve the cycling performance and Li storage ability of the anode. Moreover, the 1T-phase composites are more sustainable than the 2H-phase and the capacity does not decrease. [START_REF] Shi | Synthesis, stabilization and applications of 2-dimensional 1T metallic MoS2[END_REF]

Conclusion:

Although TMDCs attracted attentions due to their promising properties, lack of a proper technique to produce separated single layers with reasonable price limits their applications. Due to their unique crystal structure, optical, electronic, and electrical properties this group of materials could be used in different areas. Up to now, different methods are developed to synthesize single layer TMDCs, but none of them leads to formation of big batches of size-controlled and well-dispersed nanomonolayers. In this thesis, we developed a colloidal protocol to produce well-dispersed and sizecontrolled nano-monolayers 1T'-WS2 and then we induced phase transformation by annealing. The protocol was extended to the other members of TMDCs and by reinjection heterstructures of different TMDCs have been synthesized. In the first chapter, the importance and the role of transition metal dichalcogenides (TMDCs) has been stressed out. We understood that due to the special optical, electronic, electrical, and structural properties, they are promising materials for new generation technologies. Three main techniques to synthesize monolayers, their advantages and weaknesses were also discussed. It was assumed that by changing the experimental parameters, the shape, size, and dispersion of the final products could be modified, but regarding Lin et al. no protocol is developed yet to produce welldispersed and size-controlled nanosheets of TMDCs. 1 As the main part of this thesis is related to the development of colloidal syntheses, this chapter will explain how the path of TMDCs colloidal syntheses was and how this study can affect it.

In this chapter, the general idea of colloidal synthesis, the history of TMDCs colloidal synthesis, and the results will be discussed. The discussion will start with an explanation of the principles of colloidal synthesis. Nucleation and different growth mechanisms will be exposed comprehensively. After the description of nucleation and growth, crucial parameters of colloidal synthesis will be discussed. Different studies on the colloidal synthesis of TMDCs are pointed out generally to clarify the evolution of TMDCs colloidal syntheses. In the end, a pathway for the evolution of TMDCs colloidal synthesis is presented.

The principles of colloidal synthesis:

Colloidal synthesis or solution-based synthesis of nanomaterials is a bottom-up method to produce inorganic nanoparticles. In this technique, there are different parameters, which control the properties of the final product. The basics of colloidal synthesis will be clarified, and their effect on the final product becomes more understandable to better understand them. There are three components in every colloidal synthesis: precursors or reactants, solvent, and surfactant. It should be noted that in some cases the solvent and the surfactant are the same.

Nucleation:

In the reaction medium, the reactants will turn into active atoms or molecules at a specific temperature. The next step is the formation of the nanoparticles, which consists of two phases: nucleation and growth. Nucleus forms when the number of atoms or molecules reaches a minimum amount. The minimum number of atoms that form a nucleus is called the critical radius and the proper equation to calculate the size of this cluster is:

ΔGT = ΔGV + ΔGS ΔGV = 4 3 πr 3 Δgv, ΔGS = 4πr 2 γ ΔGT = 4 3 πr 3 Δgv + 4πr 2 γ  𝑑ΔGT 𝑑𝑟 = -4πrc 2 Δgv + 8πrcγ =0 rc = -2𝛾 ΔGv = 2𝛾 𝐾𝑇𝑙𝑛𝑆 Δgv = 𝛥𝜇 𝑉𝑎 = - 𝐾𝑇 𝑉𝑎 ln ( 𝐶0 𝐶𝑒𝑞 )
Here, ΔGT is the Gibbs free energy, ΔGV is the volume energy, and ΔGS is the surface energy. Δgv is the Gibbs free energy per volume and γ is the surface tension. C0 is the solution concentration and Ceq is the equilibrium concentration. T represents the temperature. K stands for the gas constant. A cluster with the size of the critical radius is thermodynamically stable and ready to absorb more atoms or molecules to reduce the free energy by growing. 2 The nucleation process could be classified into two categories, homogeneous nucleation and heterogeneous nucleation. In homogenous nucleation, atoms or molecules attach and when the size of the cluster of atoms reaches the critical size, stable nucleus would form. In heterogeneous nucleation, as there are some solid particles and the nucleus forms at the seed surface, the number of atoms required to form the nucleus is lower. Thus, the probability of heterogeneous nucleation increases by adding seeds to the solution. 4 Regarding the attachment of the atoms to the seeds, the difference in their surface energy, which defines the contact angle, rules the reduction in the Gibbs free energy due to the heterogeneous nucleation. This relation is explained in the next equations: 2 4

ΔG het = ϕΔG hom ϕ= (2𝑐𝑜𝑠Ө)(1-𝑐𝑜𝑠Ө)
Ө is the contact angle between the seed and the nucleating phase. The nucleation and growth process is predictable by knowing the contact angle. If Ө is equal to zero, there is no energy barrier anymore, and atoms would join the surface for the growth process. If Ө is equal to π, the change in the Gibbs free energy is equal to the homogeneous one, which means that there is no heterogeneous nucleation and if 0 < Ө < π, then 0 < ΔG het < ΔG hom .

The other important factor is the concentration of reactants in the solution. S is the supersaturation. This variable means that the formation of nuclei needs a certain concentration of reactants. The nucleation rate is also predictable by an Arrhenius equation:

𝑑𝑁 𝑑𝑡 = 𝐴 exp[ -𝛥𝐺 𝐾𝑇 ]  𝑑𝑁 𝑑𝑡 = 𝐴 exp[ 16𝜋𝛾 3 𝑣 2 3𝐾 3 𝑇 3 (𝑙𝑛𝑆) 2 ]
Three variables control the nucleation rate: supersaturation, reaction temperature, and surface tension. The most important parameter in the nucleation rate is the super saturation. 3 Figure 234shows that the nucleation rate increases exponentially due to the supersaturation. The nuclei are not crystalline and in the growth step, atoms should rearrange to form crystalline material. The mobility of the atoms in the solid phase is shown by diffusivity, which works as a function of temperature. Thus, the reaction temperature is an important factor to produce crystalline materials.

Growth:

After nucleus formation, excess atoms would join the surface and the size of the cluster increases. Depending on the concentration, surface energy, and temperature, there is always a competition between growth and new nucleation. In this part, three different theories about nucleation and growth are detailed.

LaMer mechanism:

In the LaMer mechanism, nucleation and growth take place in two separate steps. This process, which is debatable about nanoparticles formation, could be divided into three stages: (1) An increase in the number of atoms, (2) the burst-nucleation stage, which induces a depletion in the number of atoms and after that no nucleation happens, (3) the growth stage, which depends on the diffusion rate of atoms in the solution. 2 The extended LaMer mechanism can help to reach a monodispersed and size-controlled nanoparticles synthesis. In this mechanism, the reactants that produce the atoms should be continuously added to the system. 5

Ostwald ripening and digestive ripening:

The most well-known phenomenon in the nucleation and growth process is Ostwald ripening. Regarding the reduction in the Gibbs free energy of the system, reducing the amount of surface to volume will reduce the total energy of the system. Therefore, smaller particles would shrink and larger particles would grow. 6 In contrast to the Ostwald ripening, digestive ripening takes place, where the larger particles dissolve and smaller particles grow. It happens due to the low surface tension of particles in the solution. 2 Both the Ostwald ripening and the digestive ripening lead to production of monodispersed particles. 7

Finke-Watzky two-step mechanism:

In this process, both nucleation and growth happen simultaneously and depending on the temperature, surface energy, and concentration, the rate of nucleation or growth is higher. This process is considered the main mechanism in the formation of transition metal nanoparticles. In the first step, nuclei form from the precursor and in the second step, particles grow by autocatalytic growth and nuclei formation. 8 Although there are other mechanisms for nucleation and growth, as they are so specific for a certain system and happen under specific conditions, we will not explain them here. Mahiddine et al. reported a comprehensive review on different growth mechanisms.

Different components of a colloidal synthesis:

As already explained, different mechanisms occur during a colloidal synthesis. Now, it is time to explain the different parts of a colloidal synthesis. A colloidal synthesis includes different parts such as precursors or reactants, solvent or reaction medium, and surface ligands or surfactants.

Normally in a colloidal synthesis, the precursor transforms into active atoms or molecules that react, and depending on the situation, the nucleation and growth process would follow one of the explained mechanisms. The ligands also control the shape and size of the final product. Surfactants attach to the surface of the nanoparticles with different energy and this energy controls the shape, size, and crystallinity of the final products. 9

Precursors:

In a colloidal synthesis, a chemical reaction or series of chemical reactions leads to the production of the nanoparticles. This is achievable by mixing suitable reactants that are constituent of the elements with appropriate proportions. Thermal decomposition of the precursors at elevated temperatures or salt dissociation causes a burst in the concentration of the elements in the reaction medium, which leads to nucleation and growth of nanoparticles. There are three different techniques to inject the precursor into the solvent. In the first method, the precursor is injected into the solution very fast to limit the nucleation phase in a very short and well-defined period. Whether the decomposition/dissociation happens at room or high temperature (in this case, it is called hot injection), this technique usually leads to uniform-sized nanoparticles. In the second method, the precursor is injected dropwise and in a specific period to reduce the nucleation time. The last method is the one-pot synthesis method, in which the precursor, the solvent, and the capping ligands are mixed and heated up to react. 10

Solvent:

Wet chemistry or colloidal synthesis is a highly efficient method to produce different types of nanoparticles. One of the most important parts of colloidal synthesis is the solvent. Because normally colloidal synthesis works at relatively high temperatures (200-400°C), a proper organic solvent should work at this range (it should be noted that there are low-temperature colloidal syntheses also). Some of the solvents do not act only as a solvent and they work as the reducing agent and/or surface ligands. Diethylene glycol (DEG) and dimethylformamide (DMF) are two organic solvents that could be used as the solvent in reactions to produce nanoparticles by reducing the salts. In a study by Wang et al. octadecylamine (ODA) has been used as a high boiling point solvent, reducing agent, and surfactant to produce noble metal and metal oxide nanoparticles. 11 Oleic acid (OA) is another high boiling point solvent, which is an oxidative agent. Oleyl alcohol (OAl) is a high boiling point solvent with a slight reduction ability. Oleylamine (OAm) is another high boiling solvent that has a high reducing ability. OA, OAl, and OAm are also used as surface ligands. Cheon et al. reported a study on 2H-WSe2 synthesis with three different solvents, OA, OAl, and OAm. They discovered the role of the solvent in the properties of the final products, as by changing the solvents, the mean size and the number of layers of the final products changed. 12 In the next table, some of the most used high boiling point solvents are introduced. Regarding the experiments of this thesis, OAm is the most used solvent, so it is explained here in detail. A long chain alkyl-amine with 18 carbons. Oleylamine is used as the solvent in different kinds of colloidal synthesis of nanoparticles such as magnetic nanoparticles, non-plasmonic and plasmonic nanomaterials, and semiconductor or metallic nanomaterials. OAm is usually used as the solvent to synthesize chalcogenide semiconductors. CdS nanorods, CdSe nanoparticles, CuS2 nanoplatelets, PbS nanocrystals, and PbTe are just some examples of chalcogenide semiconductors in the synthesis of which, OAm is used as the solvent. OAm is broadly used as the solvent in high boiling point colloidal syntheses as it has many advantages such as being a reducing agent and surfactant with a liquid state at room temperature that has a high boiling point, and low cost. Moreover, other reagents could be added to OAm to improve the controllability of the reaction. 11

Solvent

Surface ligands:

Surface ligands or capping ligands are not only responsible for controlling the size and shape of nanoparticles by controlling the rate of nucleation and growth but also they improve the dispersion of the nanoparticles in the solution. Depending on the functional group, polar or apolar tail, and the shape of the capping ligand, the shape, size, and dispersion of the nanoparticles would change. 10 Regarding the type of bonding between ligands and the surface, four types of ligands could be considered. X-type: with ion pair bonding, L-type: they are natural donors, and Z-type: they are natural acceptor. Depending on the type of bonding, they can be replaced by each other. 13 Every colloidal synthesis follows the same basics. However, different 2D colloidal syntheses should be categorized to better understand the reactions and the situation in the TMDCs colloidal synthesis. 13 2D semiconducting materials gained huge attention due to their promising properties and different protocols are developed to synthesize these materials in wet chemistry.

The evolution of TMDCs colloidal synthesis:

Transition metal dichalcogenides have different ranges of properties that raise the attention to these materials. Colloidal synthesis of TMDCs is a promising strategy to produce a scalable amount 13 of nanosheets. Now to find out the importance of colloidal synthesis of TMDCs, the evolution of wet chemistry of these materials is studied. There are nice comprehensive reviews from Schaak et al. and Cheon et al., which discussed the subject very well. Lin et al. wrote a comprehensive review in 2016 and they announced that a protocol to synthesize well-dispersed nanosheets with control on size and morphology is not developed yet. 1 In this part, recent developments in colloidal syntheses are explained. During the next discussion, it will become clear that there is a lack of common points in the syntheses protocols. It worth to point that, due to the separation, some of the studies have been led to the same results, which is completely irrational.

Mansouri et al. conducted a study on the colloidal synthesis protocols that can control the shape and size of NbS2 to clarify the effect of solvent and ligand on the final products. Carbon disulfide and niobium pentachloride were used as the sulfur and niobium precursor, respectively. OAm, OA, and ODE were used as the high boiling point solvents and results showed that depending on the solvents ratio, the products could be formed as nanosheets, nanoplatelets, nanohexagons, nanorods, and nanospheres. 14

Group IV dichalcogenides:

The first group of transition metals that can form van der Waals solids is group IV, which starts with Ti and follows by Zr, Hf, and Rf. Generally, sulfides are synthesized by a hot-injection technique with carbon disulfide, elemental sulfur, or thiols as the sulfur precursor, and selenides are synthesized by heating up technique with the elemental selenium or selenide compounds as the selenium precursor. The stable phase of this group of TMDCs is the metallic one.

TiS2:

Here syntheses of TiS2, the lightest member of 2D TMDCs are described. In most protocols, titanium tetrachloride was used as the source of titanium and elemental sulfur and carbon disulfide Figure 2-12: Schematic illustration of different NbS 2 protocols and the effect of different solvents on the final product. 14 were utilized as the source of sulfur. It is worth to be noted that a protocol to produce well-dispersed size-controlled nanosheets has not been developed yet.

In 2007, Let et al. reported a method to synthesize nanoclusters of TiS2 with asymmetrical shapes and sizes between 2 and 5 nm. For more details on the protocol, titanium tetrachloride is dissolved in tridodecylmethylammonium iodide and dispersed in a mixture of hexanol and octane. Then dihydrogen sulfide bubbled steadily through the solution at room temperature for 5 minutes. In the next step, the mixture was stirred for 2 hours to complete the reaction. They reported that a large blue shift happened in the absorption spectrum of nanoclusters in comparison to the bulk TiS2. They also reported that the TiS2 clusters are photoluminescent. 15 In 2008, Park et al. reported another protocol to synthesize unstable monolayers of TiS2. They dissolved titanium tetrachloride and elemental sulfur in some ultra-dry OAm. Then the reaction mixture was heated up to 215°C for 12 hours. Circular shape disks formed with a distance of 1.9±0.3 nm, which is related to the ligands on the surface. The mean size of nanodisks changed by changing the sulfur to titanium tetrachloride ratio and the heating rate. They reported that the synthesized TiS2 nanosheets, which were produced by this protocol are unstable and oxidize under ambient condition. 16 Figure 2-14 illustrates the TEM images and size distributions of nanodisks synthesized with different reactant ratios. As it is shown, they managed to control the mean size, but the nanosheets are not well-dispersed.

In 2011, Jeong et al. reported a transformative synthesis protocol to produce doughnut-like Cu2S with TiS2 as the template. In this protocol, they produced nanodisks of TiS2 by adding carbon disulfide into the mixture of titanium tetrachloride and OAm at 300°C. They succeeded to produce multilayers nanodisks with 150 nm diameter. Then, CuCl2 was added to the mixture of TiS2 in OAm and heated up to 200°C for 30 minutes to form toroids of Cu2S. In this study, the main goal was the production of other materials (CuS2) and they synthesized TiS2, as an intermediate. They reported that Figure 2-13: TEM image of 2-5 nm nanoclusters of TiS2. 15 it is possible to produce toroids of other substances such as Ag2S, MnS, and CdS. 17 In 2012, Plashnitsa et al. reported another synthesis protocol in which elemental sulfur has been used as the sulfur source and titanium tetrachloride was the source of titanium. In a one-pot synthesis, they dissolved precursors in OAm and heated them to 300°C for 3 hours. They produced large crystalline nanosheets of TiS2 with colloidal synthesis. They also reported that the DFT calculations support that TiS2 undergoes a semimetal to semiconducting transformation by reducing 17 the thickness to a monolayer. 18 In another study in 2015, Qian et al. reported a dropwise protocol to produce nanosheets of TiS2. They added titanium tetrachloride into the mixture of OAm and ODE and heated the solution to 300°C. In the next step, 5 ml of dissolved sulfur in OAm was injected into the mixture in 10 minutes. The solution was stirred for 1 hour at 300°C and then cooled down and washed. As they wanted to use the nanosheets for tumor imaging and treatment, they functionalized the surface of the TiS2 nanosheets by PEG grafted poly(maleic anhydride-alt-1-octadecene). The results confirm the improvement in the photoacoustic imaging and photothermal cancer treatment, but the nanosheets are aggregated and no control on the mean size is reported. 19 XRD pattern, low magnification and high magnification TEM images, and EDS spectrum are displayed in Figure 2-17.

To conclude, up to now no protocol is developed to produce well-dispersed and sizecontrolled nanosheets of TiS2. Although Park et al. developed a protocol to produce size controlled nanosheets, the nanosheets were not well-dispersed and stable in an ambient atmosphere.

TiSe2:

The next member of this group that will be discussed is TiSe2, which is mostly used as the cathode in ion batteries. As TiSe2 nanosheets are mostly produced by exfoliation methods, here, I will explain just two synthesis protocols. Similar to the protocols developed to produce TiS2, titanium tetrachloride is used as the source of titanium. In this case, a protocol to produce well-dispersed nanosheets of TiSe2 is not developed yet.

In 2017, Yang et al. reported a colloidal synthesis protocol to produce heterostructures of S doped TiSe2/Fe3O4. They used TOPSe as the selenium source, which was produced by stirring elemental selenium in TOP. OAm was used as the solvent and elemental selenium was also added to the flask. Then, the flask was heated up to 120°C and remained for 30 minutes to remove moisture. After that, they reduced the temperature to room temperature and added titanium tetrachloride, and heated the solution to 300°C. The solution was stirred at 300°C for 90 minutes and then cooled down and washed to get sulfur-doped TiSe2. 20 In 2020, Ge et al. reported a TOP-free synthesis protocol of TiSe2 platelets. They dispersed elemental selenium in a mixture of OAm and ODE and after degassing the solution at 120°C for 30 minutes, titanium tetrachloride was injected into the solution. Then, the mixture was heated up to 260°C for different timespans to synthesize TiSe2 with different mean sizes. XRD patterns and SEM images support that, by increasing the time of reaction both crystallinity and mean size are increased. 21 XRD diffractogram and SEM images are demonstrated in Figure 2-19. At it has been illustrated, they managed to control the mean size of the platelets, but the multilayer platelets are not well-dispersed. 20 To conclud, different protocols to synthesize TiSe2 lead to formation of thick platelets constituted from multilayers sheets of TiSe2.

ZrS2 and HfS2:

In 2014, Yoo et al. reported a colloidal synthesis protocol, which presents the effect of precursors on final products. They used CS2 or dodecanethiol as the source of sulfur and due to the different decomposition rates, the morphology of the final products changed. They dissolved ZrCl4 (for ZrS2 synthesis), TiCl4 (for TiCl4 synthesis), or HfCl4 (for HfCl4 synthesis) in OAm. Carbon disulfide or dodecanethiol was also dissolved in the reaction mixture and after degassing, it was heated up to 245°C and stirred for 10 hours. The results show that, when CS2 is used as the sulfur source, as the decomposition rate is quite high, H2S forms explosively, which leads to the formation of multilayers of small nanosheets. In contrast, dodecanethiol decomposes slowly and as they named it "diluted chalcogen continuous influx", leads to the formation of large monolayers. 22

Group V dichalcogenides:

Since this group of dichalcogenides is out of the scope of this thesis, they will not be explained in detail. So just one protocol is explained for each member. This group of TMDCs possesses superconductor properties at low temperatures.

VS2, VSe2, TaS2, and TaSe2:

In 2012, Jeong et al. reported a standard protocol to synthesize different 2D TMDCs such as, TiS2, ZrS2, HfS2, VS2, NbS2, TiSe2, VSe2, NbSe2, and TaSe2. They investigated the effect of different sources on the final size, structure, and dispersion of the product. The protocol of VS2 and TaS2 is a hot injection synthesis, in which they dissolved vanadium tetrachloride or tantalum tetrachloride in OAm and heated it to 330°C. Then, carbon disulfide was injected into the reaction mixture. The reaction was stirred for 6 hours at 330°C. VSe2 was synthesized by a one-pot synthesis. They dissolved vanadium tetrachloride or tantalum tetrachloride and elemental selenium in OAm and heated the reaction mixture to 300°C. After 1 hour of stirring, they obtained the nanosheets. They reported that transition metal chlorides are good choices for the colloidal synthesis of TMDCs. 23

NbS2:

In 2018, Mansouri et al. reported a comprehensive colloidal synthesis protocol for NbS2 with different morphology. They showed that any changes in the solvent (ligands) induce diversity in the shape of the product. As this protocol has been already explained to show the importance of the solvent, here the results are discussed merely. They explained that changing the solvent (ligand) could change the growth, shape, and size of the nanoparticles. In fact, any changes in the solvents affect the decomposition of the precursor (carbon disulfide), which leads to a change in the sulfur content of reaction mixture. 14 23 In conclusion, this group of dichalcogenides did not attract so much attention as they do not possess extraordinary properties, in comparison with group 6, which has semiconducting behavior. The study from Jeong et al. covers most of the members of this family, but there is still a challenge to control the mean size and dispersion of the nanosheets.

Group VI dichalcogenides:

Semiconducting materials, except WTe2, with direct bandgap as monolayer and high absorption in the visible region. With promising electrical, optical, and spintronic properties, this group of materials attracted attention. Here, some colloidal synthesis protocols for MoS2, MoSe2, WS2, and WSe2 will be described. Different sources of Mo and W such as molybdenum hexacarbonyl, molybdenum oxide, molybdenum (II) acetate dimer, and molybdenum pentachloride, tungsten hexachloride, and tungsten hexacarbonyl are used for colloidal synthesis and depending on the precursors, solvents, reaction temperatures, and synthesis methods, factors such as dispersion, mean size, and shape of the final products are completely different. Recently, the colloidal synthesis of this group of materials attracts much more attention.

MoS2:

MoS2 is the lightest member of this group with a bandgap size of 1.3 eV as bulk and 1.8 eV as a monolayer. 24 In 2016, Alam Khan et al. reported a one-pot synthesis to produce MoS2 nanosheets intended for the anode of ion batteries. They dissolved molybdenum hexacarbonyl in octylamine and after degassing at 110°C, they heated up the mixture to 270°C. A mixture of elemental sulfur and octylamine, which was already prepared, was injected swiftly into the mixture. The mixture was stirred at 270°C for 1 hour. TEM images represent the formation of aggregated nanosheets and the PXRD diffraction pattern shows the MoS2 nanosheets are formed (Figure 2-23). 25 Figure 2-22: The structure of the NbS 2 nanoparticles depends on the composition of the solvent and the amount of CS 2 in the reaction 14 .

In 2016, another study has been done by Zhou et al. to produce single and multilayers of MoS2 nanosheets. They succeeded to control the number of layers by changing the concentration of the precursor. Molybdenum(II) acetate dimer was dissolved in a mixture of stearic acid, OAm, and TOPO. As the mixture reached 330°C, the mixture of thioacetamide and OAm with different concentrations was injected rapidly into the solution. They kept the mixture at 320°C for 5 minutes and then cooled it down. To control the number of layers, they repeated the injection after cooling down the system to 170°C and then they heated the mixture to 320°C. They managed to change the mean size of the nanosheets from 8 to 25 nm, by changing the concentration of the precursors. In addition, by repeating the injections, they increased the number of layers from monolayer to 3, 5, and 7 layers. 26 Here, I presented some of the studies that have been done on colloidal protocol of MoS2. All the studies have led to formation of nanosheets with no control on the size. Moreover, nanosheets are not colloidally stable.

MoSe2:

The second member of group VI TMDCs with bandgap size of 1.1 eV as bulk and 1.55 eV as monolayer. Although one of the first synthesis that developed for MoSe2 leaded to production of monolayer nanosheets, other studies produced nanoflwers. 29 In 2015, Jung et al. reported a new colloidal protocol to produce MSe2 (M= Mo, W). This is one of the earliest studies on the colloidal synthesis of MoSe2 nanosheets. Molybdenum hexacarbonyl was dissolved in a mixture of tetradecylphosphonic acid and OA. The mixture was heated to 350°C, then 28 the dissolved diphenyl diselenide in OA was injected rapidly into the reaction mixture and stirred for 10 hours. The results show that the functional group of the surface ligand can control the growth procedure to form single monolayers or multilayers. 30 TEM images, SAED-pattern, AFM image, and the Raman spectroscopy of MoSe2 nanosheets are demonstrated in Figure 2-26. Although they managed to synthesize single nanosheets, there is no control on the mean size. Additionally, all our attempts to reproduce this synthesis failed.

In 2015, Sun et al. reported a colloidal synthesis protocol to produce size-controlled MoSe2 nanoflowers. The crystalline 2H-MoSe2 nanoflowers are composed of a few layers of nanosheets, which are stuck out from the central core. They dissolved Na2MoO4 in a mixture of OA and octylamine. In parallel, they prepared ODE-Se by adding elemental selenium to ODE and mixing for 1 hour at 200°C. When the molybdenum solution reached 240°C, ODE-Se was injected dropwise into the solution. As the injection finished, the mixture was annealed at 300°C for 30 min. The size of nanoflowers is controlled by changing the concentration of the precursors. 31 31 In 2018, a one-pot colloidal synthesis was developed by Xu et al. to produce MoSe2-Pt nanoflowers. Molybdenum diacetylacetonate dioxide and dibenzyl diselenide were dissolved in OAm and degassed at 140°C for 30 minutes. Then, the temperature was raised to 250°C and the solution was stirred at this temperature for 20 minutes. In a glass bottle, Platinum (II) acetylacetonate was dispersed in OAm and when the first reaction was cooled down to 200°C, the platinum solution was injected swiftly into the nanoflowers solution and remained for 30 minutes. Low magnification TEM images represent the MoSe2 nanoflowers with 100-200 nm size and high magnification TEM images show that platinum nanoparticles are attached to the MoSe2 nanoflowers. They also proclaimed that the MoSe2 hybrid improved the HER performance. 33 35 Although Jung et al. developed a protocol to produce well-dispersed nanomonolayers of MoSe2, other protocols are also developed as this protocol is not quite reproducible. In contrast, other protocols have resulted in nanoflowers and changes in the solvent composition and reaction time control the size of nanoflowers.

WS2:

The next member of group VI TMDCs is WS2, which is studied the most in colloidal synthesis. The bandgap size of WS2 is 1.5 eV as bulk and 1.9 eV as a monolayer. It is almost one decade that the first colloidal synthesis was developed for 1T and 2H-WS2, but up to now, no colloidal protocol is developed to synthesize well-dispersed and size controlled WS2 nanosheets. 34 Although the colloidal synthesis of WS2 started by Cheon et al. 37 in 2007, the real evolution in the group VI dichalcogenides happened in 2014 when Mahler et al. developed a new continuous injection protocol to produce 1T and 2H-WS2. They dissolved tungsten hexachloride and carbon disulfide into a mixture of OA and OAm to prepare the injection mixture. The injection mixture was injected into the hot OAm (320°C) to have 1T-WS2 and injected into the hot mixture of OAm and hexamethyldisilazane (330°C) to have 2H-WS2. TEM images prove that the 1T-WS2 formed as nanosheets and 2H-WS2 formed as nanoflowers. The synthesized nanoparticles were used for photocatalytic hydrogen evolution reaction by loading them on TiO2 nanoparticles. The results support that the 1T-WS2 improved the hydrogen production due to the metallic behavior, while 2H-WS2, the semiconducting phase, reduced it. 35 As it has already been explained, they also developed a new protocol to synthesize MoS2 nanosheets with controlled number of layers. They dissolved tungsten hexacarbonyl in a mixture of stearic acid, octadecanamine, and trioctylphosphine oxide. After degassing, the mixture was heated to 330°C and then, an already prepared mixture of thioacetamide and OAm was injected swiftly and stirred for 1 more hour. To synthesize controlled multilayers of WS2, the reaction temperature was increased to 350°C and 30 minutes after the first injection, for the second time thioacetamide and OAm were injected. The results show that the absorption peak energies of WS2 nanosheets increase as a function of the number of layers. [START_REF] Han | Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures[END_REF] 35 In 2018, Liu et al. reported a new 1T'-WS2 colloidal synthesis, which could be used to produce electrocatalysis anodes. In a one-pot synthesis, they dissolved ammonium tungstate and thiourea in OAm and after degassing, the reaction mixture was kept at 280°C for 90 minutes. TEM and STEM images represent the aggregated nanosheets with a 162 nm mean size. They reported an acid treatment protocol that can improve the HER performance of the WS2 anodes due to the reduction in the amount of the 2H phase. The HER performance results confirm that the cheap 1T'-WS2 anodes are comparable with the expensive platinum anodes. 37 [START_REF] Han | Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures[END_REF] In 2019, Zhou et al. reported a new hot injection colloidal protocol to synthesize 2H-WS2 nanosheets. Tungsten hexacarbonyl was dissolved and degassed in a mixture of OAm, OA, trioctylphosphine oxide, and squalene. After degassing, the mixture was heated up to 330°C. The sulfur precursor was made by dissolving elemental sulfur in OAm and injected rapidly into the solution. Then the reaction mixture was kept at 330°C for 1 hour. They developed this protocol to measure the carrier dynamics in multilayers of WS2, which shows fast electron trapping, followed by an Auger recombination phenomenon. 38 37 In 2019, Mastria et al. reported a new colloidal synthesis protocol to produce WS2 nanoflakes. This is a two steps protocol to produce WS2 nanoflakes, in which a sulfidation process is performed on the colloidally synthesized W18O49 nanorods. They dissolved tungsten tetrachloride into a mixture of ODE, OAm, and OA and heated it under an inert atmosphere to 300°C. The produced W18O49 nanorods were dispersed in OAm, subsequently, the mixture was heated up to 250°C, and dissolved carbon disulfide in OAm was injected rapidly into the solution. The sulfidation process took 2 hours at 250°C. TEM and STEM images and PXRD diffraction patterns support the as-synthesized nanoflakes are 1T'-WS2, which was transformed partially to 2H-WS2 by annealing at 270°C (Figure 2-35). 39 HRSTEM images show the nanosheets are severely defected and TEM images confirm the nanosheets are not welldispersed. 39 In comparison with other members of group VI TMDCs, colloidal synthesis of WS2 is studied more, however, a protocol to produce well-dispersed nanomonolayers with control on the mean size has not been developed yet.

WSe2:

The last member of group VI TMDCs with bandgap size of 1.2 eV as bulk and 1.7 eV as monolayer. Different protocols are developed to synthesize single layer WSe2. Although Jung et al. developed a protocol to synthesize single nanosheets, other studies leaded to formation of either nanoflowers or multilayer nanosheets. 40 The first colloidal synthesis of WSe2 was developed in 2015 by Jung et al. to produce MSe2 (M= W, Mo). The first part of the study, which was devoted to MoSe2 was already explained and now the part for WSe2 will be explained. As it has been explained, in this study they checked different solvents to control the number of layers of the nanosheets. Tungsten hexacarbonyl and diphenyl diselenide were dissolved in three kinds of solvents: OAm, OA, and OAl. They kept the mixtures at 330°C for 12 hours to complete the reactions. They managed to develop a protocol to produce WSe2 nanosheets with a different number of layers. 30 45 In 2021, in a study by Zhou et al., a new colloidal synthesis protocol was developed to produce different polymorphs of WSe2 by changing the high boiling point solvent. Nanoflowers were synthesized by dissolving tungsten hexacarbonyl in OA and heating the reaction mixture to 330°C after degassing. In parallel, elemental selenium was dispersed in OA and injected rapidly into the reaction mixture. The nanosheets protocol is the same, but the high boiling point and the injection mixture solvent are OAm. They reported that by changing the high boiling point solvent, the tungsten hexacarbonyl decomposition rate would change. They believed tungsten hexacarbonyl decomposes faster in OA than OAm, as it makes a new complex with OAm. 41 In conclusion, different synthesis protocols have been described, which lead to products with different properties. As the most important feature of group VI TMDCs is their direct bandgap as a monolayer, a proper colloidal synthesis protocol to produce well-dispersed nanosheets is still missing. Some reports claimed that it is possible to control the mean size of the products, but they developed a protocol that leads to the formation of nanoflowers. Therefore, a colloidal synthesis to produce sizecontrolled nanosheets (not nanoflowers) is missing as well.

Alloys:

Colloidal synthesis is an interesting method to produce TMDCs nanosheets, as it is possible to produce different structures, large batches of nanosheets, and alloys. However, synthesizing alloys with colloidal synthesis is not well-studied yet. It is worth to be noted that most of the studies on alloys, either in colloidal synthesis or other synthesis methods, are done on group VI of TMDCs because they have semiconducting properties and alloying is an attainable technique to manipulate the bandgap.

In 2017, Sun et al. reported a colloidal synthesis of MoWSe2 and WSSe. They synthesized both the alloys with 2H structure and verified the change in the optical properties of TMDCs due to the alloying. MoWSe2 was synthesized by dissolving diphenyl diselenide into OAm. After the solution is degassed, bis(trimethylsilyl)amine was injected into the mixture and it was heated up to 300°C. The injection mixture is composed of molybdenum pentachloride, tungsten hexachloride, or a mixture of them in OAm. The injection mixture was injected dropwise into the reaction mixture. WSSe was synthesized and carbon disulfide was used as the sulfur source and diphenyl diselenide as the source of selenium. They were also dissolved in a mixture of OAm and bis(trimethylsilyl)amine and degassed. Then, the reaction mixture was heated up to 300°C and dissolved tungsten hexachloride into OAm was 42 injected dropwise into the reaction mixture. The results confirm that alloying can tune the optical properties of TMDCs. Although bowing effect is observed in WSSe alloys, the bowing effect of MoWSe2 is larger, which is associated with the larger lattice mismatch. 43 In Figure 2 In another study by Sun et al. in 2017, a colloidal synthesis protocol for multilayer MoWTe2 was reported. They dispersed elemental tellurium powder in a mixture of OAm and TOP and after degassing, heated up the mixture to 260°C to dissolve tellurium powders completely. Then, the reaction mixture was cooled down to 100°C and after injection of bis(trimethylsilyl) amine, it was heated up to 300°C. Concurrently, molybdenum pentachloride, tungsten hexachloride, or a mixture of them was dissolved in OA to form the injection mixture. The injection mixture was injected dropwise 44 into the reaction mixture and MoWTe2 nanosheets were formed. TEM and STEM images show the aggregated multilayers formed as nanostars with diameters from 30 to 150 nm. 44

Heterostructures:

Only few studies on colloidal protocol of heterostructure formation have been investigated, which led to formation of aggregated nanoflowers. Hwang et al. developed a colloidal protocol to synthesize MoSe2-WSe2 heterostructure. They used multiple injection method to form the heterostructure. They used Mo(CO)6 and W(CO)6 as the sources of transition metals and Ph2Se2 was used as the source of selenium. Tetradecylphosphonic acid and transition metal hexacarbonyl were dissolved in OA. After degassing, the reaction mixture heated to 350°C for MoSe2 and 330°C for WSe2. Then the precursor was injected rapidly. The reactions were kept at the reaction temperature for 2 hours. Figure 2-43 shows the TEM images of MoSe2, WSe2, and MoSe2-WSe2 heterostructure. The change in the size of the nanoflowers indicates the formation of heterostructures. 50

Conclusion:

Here, the evolution of the colloidal synthesis of TMDCs was reported. To conclude, colloidal synthesis is a strong technique to produce low-cost TMDCs in large batches and tune their properties.

As it has been explained, different protocols to synthesize nanosheets of TMDCs and alloys are developed. Indeed, different protocols to synthesize stable and metastable phases are also developed. However, a protocol to produce well-dispersed nanosheets of group VI chalcogenides is still needed. Except for nanoflower structure TMDCs, the mean size is not controllable yet. XRD and spectroscopy techniques have been used to characterize the structure of the products and different microscopy methods have been used to define the dispersion and size of the products.

Different reactants, different solvents, different reaction temperatures, and different injection methods have been used to develop a colloidal protocol to produce well-dispersed nanosheets. Regarding the protocols the most important factor is the reaction temperature. Increasing the reaction temperature leads to formation of 2H phase, which is more stable. However, increasing the temperature induces more aggregation. Transition metal dichalcogenides (TMDCs) are lamellar materials with strong covalent in-plane bonding and weak van der Waals inter-layer interaction, classifying them as van der Waals solids. 1,2 Tunable bandgap, high carrier mobility, and large surface area make these materials relevant for different applications in electronics, 3 optoelectronics, 4 and catalysis. 5 In addition, because they can possess meta-stable crystal structures (octahedral or distorted octahedral), they are candidates for a considerable number of applications, such as hydrogen evolution reaction (HER) and electrode materials for sodium ion batteries. [6][7][8] As a consequence of different bonding energies (strong in-plane and weak inter-layer bonding), isolation of single monolayers is achievable, which creates a high surface area, induces quantum confinement and bandgap transformation. 9 This alluring feature results from a lack of symmetry due to the reduction in the size to a single monolayer. 10,11 Due to these unique properties and numerous potential applications of TMDCs monolayers, the preparation of single layer materials and control of their size have attracted much attention. 12,13 Different thin layer preparation techniques have been developed and applied over the years, such as exfoliation techniques, 14,15 substrate growth techniques, 16,17 and colloidal synthesis. 18 But so far, no studies have succeeded in synthesizing size-controlled and well-dispersed nano-monolayers (NML). Colloidal synthesis of TMDCs could be a promising strategy to produce a scalable amount of nanosheets by modifying the experimental parameters. 19 It has been almost one decade since the first colloidal synthesis of WS2 nanosheets was developed, yet this field suffers from the lack of a direct colloidal protocols to produce both well-dispersed and size controlled WS2 nanosheets. Previous work from our group reported a protocol to produce 1T-WS2 nanosheets, and by a modification, 2H-WS2 nanoflowers were also synthesized. 20 However, these 1T-WS2 nanosheets were severely defected. Zhou et al. developed another hot injection protocol to synthesize WS2 nanosheets. Although they managed to control the number of layers by reinjection of the precursors, the nanosheets were not well-dispersed in solution. 21 Yin et al. also developed a colloidal protocol to produce 1T'-WS2 nanosheets by one-pot synthesis. However, TEM and STEM images support the formation of 1T'-WS2 nanochains with 162 nm diameter assembled nanosheets. 22 Son et al. developed a protocol to produce colloidal quantum dots of WS2, where they sonicated WS2 powders in an appropriate solvent, then separated the WS2 quantum dots. 23 More recently, Liu et al. developed a comprehensive colloidal protocol to produce metallic phase nanosheets of MoS2, MoSe2, WS2, and WSe2. In the last step of the synthesis, they induced phase change by annealing the nanosheets under argon atmosphere at 500°C for 1 hour. TEM images show that the nanosheets are neither monodisperse nor colloidally stable. 24 This last study does not yet fulfill the aforementioned demand for colloidally stable and size-controlled nanosheets of WS2.

This study presents a novel colloidal protocol to produce well-dispersed and size-controlled NMLs of 1T'-WS2. We developed a colloidal synthesis protocol in which 1-octadecanethiol (ODT) is used as the tungsten coordinating agent, reducing its reactivity and allowing for a better synthetic control. We use different characterization techniques such as UV-Vis spectroscopy, powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and small angle X-ray scattering (SAXS) to prove that the products of the synthesis are colloidally stable 1T'-WS2 monolayers. Additionally, Fourier transform infrared spectroscopy (FTIR), ligand exchange, and thermogravimetric analysis (TGA) are used to probe the nature of the NMLs surface ligands. Finally, we demonstrate control over the size of 1T'-WS2 NMLs by changing the amount of ODT as well as the reaction time, producing monodisperse NMLs with mean sizes ranging from 10 to 60 nm diameter. This high-yield reaction leads to the formation of the highest quality nano-monolayers that have been synthesized to date. We believe this new colloidal synthesis can spur the widespread study and use of colloidal TMDC nano-monolayers.

Results and discussion

The previous WS2 synthetic protocol developed by our group consisted of a slow precursor injection at high temperature, where oleylamine (OAm) was the solvent and a mixture of oleic acid (OA) and OAm were used for the precursors injection. 20 In this case, OAm is crucial for the reaction as it reacts with the sulfur precursor, CS2, during the injection mixture preparation and produces oleylammonium dithiocarbamate, which in turn decomposes at high temperature to oleylammonium sulfide. OA coordinates partially with tungsten hexachloride and regulates the reactivity of this tungsten source. But at high temperatures (depending on the environment, between 200°C to 350°C) OA can decompose or react with oleylamine, yielding water or hydroxides which can induce oxidation of the nanosheets. 25 Hence, to eliminate this partial oxidation of tungsten disulfide, OA should be suppressed from the reaction. In a preliminary experiment, OA is totally suppressed from the protocol and WCl6 is dissolved directly in OAm in the injection mixture. The other synthetic parameters remained the same, with slow injection (30 min) of the precursor mixture at high temperature in OAm.

After synthesis, UV-Vis absorption spectroscopy of a diluted sample (Figure 3-1: a) shows an excitonic feature at 612 nm, which is characteristic of the 2H structure and a strong deep red and infrared absorption coming from the presence of 1T' domains. 26 This first characterization already indicates the presence of both phases in the product: 2H and 1T' WS2.

PXRD measurement (Figure 3-1: b) performed on a drop-casted sample on a low background silicon sample holder confirms the existence of the two phases. Although oscillations in the low angles can be due to a variety of reasons, they can be a characteristic of stacking of lamellar materials. However, in this case, as the distances between the peaks are not the same, they are not only simple stacks of monolayers. The peak at 14° corresponds to the 2H (002) plane reflection, and confirms the formation of 2H-WS2 multilayers. In the inset, the area between 30° and 40° is magnified to better illustrate the presence of the 1T' and 2H phases. The peak at 31.8° corresponds to the reflection from (2-20) of 1T'-WS2 and the peak at 32.8° corresponds to the reflection from (100) of 2H-WS2. This confirms that the WS2 nanosheets synthesized using this pure OAm protocol are a mixture of 1T' and 2H-WS2. 20 In contrast with the previous study, where the 2H-WS2 formed as aggregated nanoflowers, TEM observation shows that this polymorphic WS2 is formed as nanosheet multilayers (Figure 3-1: c,d), with a mean diameter of (82 ± 12) nm. The diversity of contrast between different nanosheets confirms that nanosheets are of different thicknesses, partially aggregated or formed of multilayers. Furthermore, the size dispersion of the produced nanosheets is large (see Figure 3-3: a).

In the original protocol, OA has been used to form a tungsten oleate complex that regulate the reactivity of the tungsten precursor. This preliminary experiment highlights the strong dependence of this WS2 colloidal synthesis on the precursor activity. Strong activity leads to aggregated 2H-WS2 whereas lower precursors activity can lead to 1T'-WS2 monolayers or polymorphic samples if the activity is not low enough. As a conclusion, an oxygen-free coordinating agent is needed to coordinate further with tungsten hexachloride and regulate the reactivity of the tungsten precursor to improve the dispersion and produce pure 1T' monolayers.

Thiols or thiolates can play several different roles in the 1T'-WS2 NML synthesis. They can either act as strong complexing agents for tungsten, thereby tuning its reactivity, or they can be an additional sulfur source, as previously discussed in the literature. 27 Additionally, they can act as the main ligands stabilizing the NMLs in a solution. In the injection mixture, oleylamine reacts with carbon disulfide at room temperature and produces oleylammonium dithiocarbamate, which decomposes at high temperature to oleylammonium sulfide (when injected into the reaction mixture). Tungsten hexachloride reacts with thiols in the reaction mixture and forms tungsten thiolate. The tungsten thiolate reacts in turn with oleylammonium sulfides to ultimately generate 1T'-WS2. 20 At this point, tungsten thiolate can also thermally decompose into WS2. These hypotheses are tested in the following where a new protocol is developed using alkanethiols as the coordinating agents (detailed under Methods). The UV-Vis spectra of WS2 NMLs synthesized using different alkanethiols (dodecanethiol DDT, hexadecanethiol HDT, and octadecanethiol ODT) (Figure 3-2: a) are characteristic of the pure metallic 1T' phase, as there is high absorption in the entire spectrum with no excitonic features. Comparing the absorption spectra with other studies supports the formation of pure 1T' phase. 20,28 X-ray diffractograms (Figure 3-3) further confirm that all the syntheses using alkanethiols yield phase-pure 1T' WS2 nanosheets (evidenced by the absence of 2H peaks, and especially at 32.8°). The homogenous contrast of different nanosheets supports that they are well-dispersed NMLs. However, Figure 34: a shows that regions exhibiting minor aggregation exist. Thiols with longer chains, specifically hexadecanethiol (HDT) and octadecanethiol (ODT), are also explored to investigate the effect of thiols on the nanosheets synthesis and dispersion. TEM images (Figures 3-2: c, d) demonstrate that increasing the chain length of the alkanethiol decreases the mean-size of the NMLs and improves the dispersion. However, Figures 34c show that the nanosheets synthesized by HDT are partially aggregated, whereas the use of ODT as the coordinating agent leads to the formation of well-dispersed nanomonolayers. Using thiols as the coordinating Since these thiols-synthesized NMLs are well-dispersed, we initially hypothesized that thiolates are attached to the surface of NMLs. 29 Due to steric hindrance imparted by the long alkyl chain, the dispersion may then be improved. This is consistent with the colloidal stability enhancement induced by increasing the length of the thiol. However, in the following sections, we will show that the surface is not passivated by thiols, and in fact OAm acts as the sole surface ligand present. PXRD can be a good method to measure the distance between layers in NMLs when they orient flat on the substrate during sample deposition and drying. The series of low-angle oscillations are characteristic of the interlayer distance of stacks of 2D materials and are observable for all three of the 1T'-WS2 NMLs synthesized with thiols, with remarkable coincidence in the oscillation peaks. The position of the smallest measured angle for all of the NMLs is at 7°, which corresponds to the reflection from the second oscillation, as the spacing between peaks is 3.5° (Figure 3-6: a). The position of the peaks implies a distance between NMLs of 2.7 nm, which is in good agreement with the distance between nanosheets previously reported, which was measured by STEM imaging. 20 Small angle x-ray scattering (SAXS) was also performed on the well-dispersed WS2 NMLs obtained with the ODT protocol, confirming both the absence of aggregation in toluene, and illustrating their high colloidal stability. For comparison purposes, aggregation has also been induced by controlled addition of ethanol at different volumetric fractions. 30 On slightly aggregated and aggregated NMLs (Figure 3-6: b), SAXS measurements confirm that the thickness of 1T'-WS2 NMLs with an organic ligand layer is about 2.7 nm. It should be stressed that this value is independent of the thiol used during the synthesis (Figure 34567), suggesting that thiols are not the main stabilizing ligands here. Considering the thickness of the crystalline portion of a single monolayer which is reported as 0.6 nm, it can be seen that the position of the first peak in the SAXS pattern is at 0.21 Å -1 , and the corresponding distance between two sheets is thus 2.38 nm. 31 Furthermore, this is much smaller than twice an OAm molecule, which shows that there is a strong interdigitating of the alkyl chains. Figure 3-8 displays a photograph of colloidal dispersions of WS2 NMLs synthesized using the three thiols (DDT, HDT, and ODT) one month after synthesis. It confirms that the WS2 NMLs synthesized using the procedure with thiols are colloidally stable for an extended period of time.

Although these thiols have different chain lengths (from 12 carbons in the backbone for DDT to 18 carbons for ODT), the inter-layer distance stays the same. These results are in stark contrast with the assumption that the

DDT HDT ODT

Figure 3-8: The photograph of colloidal dispersions of WS 2 NMLs synthesized using the three thiols (DDT, HDT, and ODT) one month after synthesis improvement in the dispersion is caused by the presence of thiols on the surface. From this observation, two possible conclusions can be drawn: either the surface is exclusively passivated by OAm, or there is a mixture of OAm and thiols. In both cases, the inter-layer distance would be dictated by the presence of OAm, and thus the following characterizations have been performed to exclude the presence of any residual thiols.

To check the presence of such residual thiols on the surface of the nanosheets, we first performed FTIR on dried nanosheets to eliminate solvent effects. The FTIR spectra of different compounds is shown in Figure 3456789. Increasing the chain length in the thiols induces a shift in the S-H bonding, which causes the S-H bonding peak for ODT to occur below 600 cm -1 . 32 It is no longer measurable using our setup. Furthermore, WS2 NMLs absorb a high amount of FTIR signals, and less than 20% of the infrared light is collected after interaction with the sample. Consequently, the signal-to-noise ratio is low and the results are difficult to interpret. For these reasons, FTIR results were inconclusive for analyzing the ligands composition on the WS2 NMLs synthesized with thiols, and the results can neither prove nor disprove the presence of thiols on the NMLs surface.

Thermogravimetric analysis has been performed on completely dried NMLs in an attempt to determine both the ligand identity and the ligand density on the WS2 surface. This is made possible by the unique decomposition temperature for each different thiol and for OAm (Figures 3-10 and 3-11). After synthesis, the final product is a mixture of WS2 nanosheets with ligands on their surface. In good accordance with PXRD results, TGA shows that WS2 synthesized using different thiols and from pure OAm have the same ligands decomposition temperature onset. The first derivative of the TGA curves determines the temperature at which the greatest rate of change in the weight loss happens. as the coordination agent, and for WS2 nanosheets synthesized using the pure OAm synthesis. This confirms that both samples have the same ligands on the surface, as both peaks are located at the same temperature (312°C).

Furthermore, removing the ligands by thermal decomposition allows for determination of the net weight of the nanosheets. Heating the nanosheets above 400°C leads to complete decomposition of the ligands on the surface, with the experiment being performed under nitrogen flow to avoid oxidation of the NMLs themselves. The reaction stoichiometry indicates a large excess of sulfur, making tungsten the limiting reagent. Therefore, by measuring the mass of the NMLs before and after TGA analysis and comparing it with the completed reaction calculated based on the amount of reactants, both the yield of reaction and the number of ligands on the surface are quantifiable. Calculations indicate that thiols not only improve the dispersion, but also improve the yield of the reaction, as indicated by the increase in yield from 73% for pure oleylamine synthesis to 84% for 1T'-WS2 with DDT, 86 % for 1T'-WS2 with HDT, and 90% for 1T'-WS2 with ODT.

The onset of decomposition temperatures of tungsten thiolates can also be indicative of their stability in solution during synthesis, with a lower onset temperature correlating to lower stability. This has been investigated as well using TGA, showing an onset of decomposition temperature for W(DDTate)6 at 223°C, W(HDTate)6 at 249°C, and W(ODTate)6 at 268°C (Figure 345678910). The increase in the decomposition temperature occurs with increasing chain length of the thiol. An attractive hypothesis is than that the in-situ formation of tungsten thiolate leads to the creation of well-dispersed 1T'-WS2 NMLs by regulating the reactivity of the tungsten precursor. Reducing the reaction rate by conjugating the reactants produces the metastable 1T' phase. This phase generally occurs due to the presence of excess charges in the NMLs and then in the reaction system. Thus, due to the presence of charges on the nanosheets, oleylammonium ligands are bound to the surface and are responsible for the colloidal stability. Another protocol was developed to clarify the role of thiols in the reaction. As has been already discussed, the reaction of WCl6 with thiols forms tungsten hexathiolate and HCl. This reaction happens at room temperature, so instead of adding ODT into the reaction mixture, WCl6, ODT, OAm, and CS2 were mixed to prepare the injection mixture. As explained in detail in the methods section, the injection mixture was added into the OAm at 320°C over a period of 30 minutes. The TEM image and the absorption spectrum are presented in Figure 3-13. The TEM image illustrates that aggregated nanosheets are produced by this protocol and the absorption spectrum confirms the formation of the 1T'-WS2 phase. Although the amount of ODT in both protocols is the same, the dispersion is here completely different. One can hypothesize that when ODT is present in the reaction medium, at every moment during the injection, tungsten precursors are at low concentrations and in presence of a large excess of ODT, which can efficiently bind the tungsten and regulate the reactivity. On the contrary, when ODT is added into the injection mixture, the amount of ODT relative to the reactive tungsten species during injection increases over time in the injection mixture and is too low at the early stages of the synthesis to properly regulate the NMLs growth.

Since a direct measurement through FTIR was inconclusive, an indirect proof was required to investigate if the ligands on the surface are exclusively OAm. As such, ligand exchange has been performed on the NMLs synthesized by ODT as the coordinating agent to verify if the mean size changes. As described in further detail under Methods, octylamine has been used for the ligand exchange. Figure 3-14 illustrates the PXRD diffractograms of 1T'-WS2 NMLs before and after ligand exchange. The shift in the low angle oscillations supports that the interplanar distance has decreased. Calculating the distance of NMLs after ligand exchange shows that the spacing has decreased from 2.7 nm to 2.07 nm. It should be noted that octylamine with 8 carbons in the ) ) Clarifying the role of ODT in the synthesis of colloidally stable NMLs paves the way to develop protocols to synthesize size-controlled NMLs. There are different key factors in wet chemistry synthesis that, that can modify the final nanocrystal size and morphology. Because the results from 1T'-WS2 synthesized by ODT are colloidally stable nanosheets, different parameters in this protocol are modified to produce size-controlled and well-dispersed NMLs with diameters ranging from 10 to 60 nm. If the formation mechanism of WS2 NMLs occurs through nucleation and growth, regulating the injection rate should alter the rate of nucleation, and the mean size and the size-dispersion of the nanosheets should also change. Thus, the first parameter used to control the mean size of the final products is the injection rate. Reducing injection time should cause an increase in the nucleation rate, and consequently, the mean size should reduce. On the other hand, prolonging the injection time reduces the concentration of precursors in the reaction mixture, which decreases the nucleation rate, and thus the mean size should increase. Although the jellification of the injection mixture (through oleylammonium dithiocarbamate formation) limits the injection time, diluting the injection mixture with excess OAm (10 mL instead of 5 mL) makes it possible to prolong the injection time.

The other modified factor is the amount of ODT in the reaction mixture. Although it is now quite clear that the dispersion does not improve due to the presence of ODT on the surface of the nanosheets and that it only adjusts the reactivity of the tungsten precursor, changing the amount of ODT in the reaction mixture shifts the mean size of the products. New colloidal protocols are then developed to synthesize colloidally stable and size-controlled 1T'-WS2 NMLs by controlling both the tungsten precursor reactivity with the amount of ODT and the nucleation rate by varying the injection time. Decreasing the amount of ODT and increasing the injection time causes the NMLs to become larger. Figure 3-16 demonstrates the absorption spectra of 1T'-WS2 NMLs with different sizes, which confirms the different sized NMLs are semi-metallic polymorphs. A reduction in the amount of ODT increases the decomposition rate, which means a decrease in the stability of the precursor. It was already known that the increase in the stability of the precursor decreases the size of products and vice versa. As the formation mechanism is nucleation and growth, increasing the injection time means dilution of the reaction mixture, which decreases the nucleation rate and boosts the growth. Depending on the desired size of NMLs, the injection rate and amount of ODT can be varied.

Conclusions:

In conclusion, a new protocol has been developed to produce colloidally stable, monodisperse and sizecontrolled 1T'-WS2 NMLs with nearly 90% chemical reaction yield. Three different alkanethiols were used as the tungsten coordinating agents, improving the colloidal stability and yield of the reaction by regulating the reactivity of the tungsten precursor. UV-Vis absorption spectroscopy and PXRD confirm that the NMLs produced are 1T'-WS2, and TEM images and SAXS support that the NMLs are well dispersed. A combination of PXRD, TGA, and ligand exchange supports that the surface chemistry is not changed, regardless of the thiol used, and implies that the NMLs are systematically stabilized with oleylamine/oleylammonium ligands. 1T'-WS2 NMLs with different mean sizes (10-60 nm) are then synthesized by changing the amount of thiol and reaction time to regulate the nucleation rate and the tungsten precursor reactivity. This new protocol yields the most stable WS2 monolayers that have been colloidally synthesized to date. We strongly believe this new colloidal protocol that we have developed paves the way for other studies into the applications of 1T and 2H-WS2, such as electrocatalysis, sensors, and optoelectronic devices. This protocol could also potentially be extended to other group VI TMDCs to produce well-dispersed and size-controlled NMLs of all the members of this family.

Methods:

Materials:

1-octadecanthiol (ODT) (97%, Sigma-Aldrich), hexadecanethiol (HDT) (95%, Sigma-Aldrich), dodecanethiol (DDT) (98%, Sigma-Aldrich), oleylamine (OAm) (80%-90%, Acros), tungsten hexachloride (WCl6) (99.9%, Acros), carbon disulfide (CS2) (99%, Sigma-Aldrich), hexane (95%, Carlo erba), anhydrous ethanol (99.9%, Carlo erba), toluene (98%, VWR-BDH chemicals), acetonitrile (99.9% Sigma-Aldrich), and 1-octylamine (99% Alfa Aesar) were used as received with no additional purification.

Synthesis of WS2 nanosheets without thiols:

In a typical synthesis, 15 mL (46.6 mmol) OAm was added into a 50 mL three-neck-flask and degassed under vacuum at 80°C for 1 hour. Then the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 5 mL (15.5 mmol) OAm and just before the injection, to avoid jellification, 240 µL CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 30 minutes. After injection, the reaction mixture was cooled down to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Synthesis of WS2 nano-monolayers:

In a typical synthesis, 35 mmol thiol (10 gr ODT, 9 gr HDT, or 7 gr DDT) was dissolved in 15 mL (46.6 mmol) OAm. The mixture was added into a 50 mL three-neck-flask and degassed under vacuum at 80°C for 1 hour. Then, the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 5 ml (15.5 mmol) OAm and just before the injection, to avoid jellification, 240 µL CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 30 minutes. After injection, the reaction mixture was cooled to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Synthesis of tungsten thiolates:

In a 20 mL vial under a fume hood, 50 mg (0.126 mmol) WCl6 was mixed in 0.756 mmol thiol (216 mg ODT, 195 mg HDT, 151 mg DDT), 5 mL absolute ethanol was added to the mixture to insure thorough mixing. As the reactants were mixed, bubbles formed and the color of the mixture turned dark red. When the mixture stopped bubbling, the product was centrifuged and washed 3 times with a mixture of hexane and acetonitrile. The resulting products were silvery-white solid particles.

Synthesis of different size WS2 nano-monolayers:

Dmean= 14 nm:

In a typical synthesis, 10 gr ODT (35 mmol) was dissolved in 15 mL (46.6 mmol) OAm. The mixture was added into a 50 mL three-neck-flask flask and degassed under vacuum at 80°C for 1 hour. Then, the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 5 mL (15.5 mmol) OAm and just before the injection, to avoid jellification, 240 µl CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 15 minutes. After injection, the reaction mixture was cooled to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Dmean= 38 nm:

In a typical synthesis, 5 gr ODT (17.5 mmol) was dissolved in 15 mL (46.6 mmol) OAm. The mixture was added into a 50 mL three-neck-flask and degassed under vacuum at 80°C for 1 hour. Then, the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 10 mL (31 mmol) OAm and just before the injection, to avoid jellification, 240 µL CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 60 minutes. After injection, the reaction mixture was cooled to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Dmean= 45 nm:

In a typical synthesis, 5 gr ODT (17.5 mmol) was dissolved in 15 mL (46.6 mmol) OAm. The mixture was added into a 50 mL three-neck-flask and degassed under vacuum at 80°C for 1 hour. Then, the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 10 mL (31 mmol) OAm and just before the injection, to avoid jellification, 240 µL CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 120 minutes. After injection, the reaction mixture was cooled to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Dmean= 59 nm:

In a typical synthesis, 5 gr ODT (17.5 mmol) was dissolved in 15 mL (46.6 mmol) OAm. The mixture was added into a 50 mL three-neck-flask and degassed under vacuum at 80°C for 1 hour. Then, the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 10 mL (31 mmol) OAm and just before the injection, to avoid jellification, 240 µL CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 180 minutes. After injection, the reaction mixture was cooled to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

WS2 nanosheets synthesis with injection of thiols:

15 mL (46.6 mmol) OAm was degassed at 80°C for 1 hour in a three-neck-flask and then heated to 320°C. 50 mg (0.126 mmol) WCl6 was dispersed in a mixture of 5 mL (15.5 mmol) OAm and 5 gr (17.5 mmol) ODT. Just before the injection, 240 µL CS2 was added to the mixture and the injection mixture was injected dropwise over the course of 1 hour. After injection, the reaction mixture was cooled to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Ligand exchange:

200 µL (2 mmol) octylamine was added to 500 µL synthesized WS2 dispersed in 500 µL toluene in a closed vial. The mixture was kept in the oven at 65°C. After 24h, the samples were centrifuged and washed 3 times with ethanol. Nanosheets were dispersed in 10 µL ethanol for PXRD characterization.

Characterization methods: TEM:

Transmission electron microscopy (TEM) images were acquired on a JEOL JEM-2100 TEM equipped with a LaB6 electron source, operating at 200 kV. Samples were prepared by drop-casting a diluted sample on a 400mesh copper grid.

UV-Vis spectroscopy:

The UV-Visible absorption spectra of 15 µL of WS2 nanosheets diluted in 3 mL toluene were recorded using an AVANTES Avaspec ULs2048L spectrometer.

PXRD:

Powder X-Ray Diffraction (PXRD) diffractograms were recorded on a PANalytical PIXcel1D Medipix3 detector with a copper tube operating at 30 kV. All the samples were prepared by drying few a drops of the WS2 dispersed in ethanol on a low background silicon sample holder.

TGA:

Thermogravimetric analysis (TGA) measurements were performed on a Netzsch STA 409 PC Luxx thermogravimetric analyzer. Nanomonolayer samples were dried to a powder and portions of (10 ± 2) mg were loaded in an Al₂O₃ crucible. Tungsten thiolates were dried similarly before being diluted to an approximate concentration of 10% in a matrix of powdered alumina. Samples were heated from 25 ᵒC to 550 ᵒC at a rate of 10.0 ᵒC/min under flow of nitrogen gas.

FTIR:

Fourier-transform infrared spectroscopy (FTIR) measurements were performed on an IR Affinity -1 Shimizu FII spectrophotometer with MIRacle natural diamond ATR -PIKE. Different samples were drop-casted directly onto the crystal optic. Concentrated solutions of WS2 nanomonolayers were dried on top of the crystal optic. The range of measurement was 600-4000 wavenumber (cm -1 ) and the acquisition time for each sample was 300 s.

SAXS:

Small-angle X-ray scattering (SAXS) measurements were carried out on a XENOCS Xeuss 3.0 instrument using a fixed tension of 50 kV, a current of 0.6 mA, and a sample-to-detector distance of 0.28 m. The typical accessible range of scattering vector modulus q was 0.01 to 1 Å -1 , for q=(4π/λ) sin(θ), where 2θ is the scattering angle and λ = 1.54 Å is the wavelength. The intensity was radially averaged to yield intensity versus q data. Typical exposure times were 300 seconds, repeated for 5 scans before being averaged and background-corrected with the corresponding solvent blank. For aggregated samples, colloidal dispersions were destabilized using varying amounts of ethanol (10%v/v, 20%v/v or 50%v/v), and loaded into glass capillaries.

ABBREVIATIONS

NML, nano-monolayer; OAm, oleylamine; OA, oleic acid; ODT, octadecanethiol; HDT, hexadecanethiol; DDT, dodecanethiol; TEM, transmission electron microscopy; PXRD, powder X-Ray Diffraction; TGA, thermogravimetric analysis; FTIR, fourier-transform infrared; SAXS, small angle X-ray scattering.

Chapter 4: Production of quantum confined semiconducting WS 2 nanomonolayers using size dependent 1T' to 2H WS 2 phase transition in solution

Introduction:

Group VI transition metal dichalcogenides are van der Waals solids with semiconducting properties (with the exception of WTe2) in their most stable phase. Depending on the atomic coordination and oxidation state of the metal atoms, they can be synthesized in two different polymorphs (2H and 1T) with promising properties and applications. 1,23 In addition to their tunable bandgap, high carrier mobility, and large surface area that provide different applications in electronics, 4 optoelectronics, 5 and catalysis, 6 they can also possess metastable crystal structures with metallic properties, giving rise to other considerable applications such as catalysts for hydrogen evolution reaction (HER) 7,8 and electrode materials for ion batteries. 9 Depending on the arrangement of the atoms, group VI TMDCs possess different crystal structures: 2H with hexagonal symmetry which has semiconducting properties, and 1T with tetragonal symmetry and metallic properties. 10 The large energy difference between the phases (the stable 2H phase and the unstable 1T phase) induces a further lattice distortion, which makes the 1T' polymorph. 11 Depending on the synthesis method, it is possible to produce or transform the stable and metastable phases between one-another. 12 A sufficient amount of charges are needed, due to the Jahn-Teller effect, to transform the 2H phase to 1T' and the hexagonal symmetry would break, changing to a zigzag pattern to reduce the energy of the system. 13 Thus, the 1T'-phase is obtainable through a number of methods such as chemical intercalation with lithium salts, charging through contact with electrodes, defects production by laser or plasma irradiation, mechanical strain on the 2H-phase, or directly by colloidal synthesis. 10,[14][15][16] The 1T' phase can also be transformed to the 2H under certain conditions such as charge suppression and annealing. 17 Due to the fact that the stable phase of group VI TMDCs is 2H, most of the phase engineering studies have been performed on 2H to 1T' transformation, 10 as lithium intercalation is also an efficient way to exfoliate these van der Waals materials and produce monolayers. On the contrary, only a few studies have been devoted to the 1T' to 2H phase transformation. 18 DFT calculations in conjunction with experimental measurements show that while the 2H phase is more stable, a large energetic barrier must be overcome to transform the 1T' phase to the 2H phase. 7 Starting with well-dispersed and size controlled 1T'-WS2 nano-monolayers (NMLs), we study here the relationship between size, defect density, and the 1T' to 2H phase transformation. Evaluation of high-resolution transmission electron microscopy (HRSTEM) images reveals that the density of defects increases with an increase in size. Moreover, a careful analysis of the nano-monolayer phase change under HRSTEM imaging conditions highlights that defects hinder the phase transformation. Differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) confirm that defects improve the 1T' stability as the phase change temperature for larger nanosheets is higher, and it can be seen that annealing different sized nanosheets under the same conditions leads to less phase transformation for larger nanosheets. This work is the first fundamental study on 1T' to 2H phase transformation of colloidal WS2 nanosheets. We strongly believe believe that this study clarifies the metallic to semiconducting phase transformation of WS2, and can be extended as a model to other TMDCs to better understand phase transformations in these materials.

Results and discussion:

1T'-WS2 NMLs with different sizes have been synthesized using 1-octadecanethiol (ODT) as the complexing agent. Dispersion and size of the nanosheets were verified through TEM. The contrast homogeneity of the TEM images of well-dispersed nanosheets support that the synthesis of NMLs with different sizes is reproducible (Figure 4-1). Size dispersion histograms were constructed from the TEM images by averaging the High resolution HAADF-STEM, has been used to observe the crystal structure of the NMLs. Figure 4-3 demonstrates low and high magnification images of three different sizes of well-dispersed NMLs (Dmean= 12 nm, Dmean= 22 nm, Dmean= 60 nm). From the low-magnitude STEM images, well-dispersed NMLs are evident as the contrast of nanosheets is homogeneous, and a higher but uniform contrast is exhibited when the edges of two NMLs are overlapping. The HAADF-HRSTEM images illustrate the structure and vacancies of the NMLs, due to the high resolution of this imaging technique. In comparison with the 2H structure which has a hexagonal pattern, the 1T' phase possesses a characteristic zigzag pattern. As explained previously, the hexagonal to zigzag deformation comes from a broken symmetry due to the presence of excess charges in the system. We use the high resolution images to extract an approximate evaluation of the density of defects that increases simultaneously with the increase in the mean size of the NMLs.

When the nanosheets are observed under HRSTEM, the high energy of the electron beam induces a phase change from 1T' to 2H. Figures 4-4: a -j exhibit the 1T' to 2H phase change of a single 1T'-WS2 NML due to the incident electron beam, evident as the zigzag pattern is progressively changing to a hexagonal pattern. Figures 4-4: a' -j' show color maps of the two crystal phases throughout each step of the phase change. The distance of each atom with the six nearest neighbors has been computed to plot these maps, with the yellow and red colors representing the 1T'-WS2 and 2H-WS2 crystal structures, respectively. Superimposing the STEM images with the maps shows that tungsten vacancies hinder the phase change and act as barriers against the propagation of the 2H phase, as the phase transformation propagates by reorganizing neighboring atoms. We can assume that the atomic coordination transformation from distorted octahedral to trigonal prismatic is hindered by vacancies due to the imbalance of charge. Therefore, the phase change passes higher barrier energy for nanosheets with a higher density of vacancies. According to the mapping in Figure 4-4, Table 1 represents the phase change propagation as a function of the time. According to this first set of observations, two hypotheses can be made about the 1T' to 2H phase transition in WS2 NMLs. First, tungsten vacancies slow the phase change, and a highly defected sample should be more stable in the 1T' structure. Second, the 1T' to 2H phase transition originates from an edge and propagates through the whole layer. Small layers with more edges would then be easier to transition than large ones.

In a subsequent stage, DSC has been performed to determine the size effect on the onset temperature of the 1T' to 2H phase transition. Generally, by reducing the size of a particle, as the free energy increases, 19 the particle becomes more unstable and more prone to phase change. But unexpectedly and in contrast with the other reports which have been performed on large 1T' sheets, the phase change temperature observed here is higher. The phase change temperature for large 1T'-WS2 monolayers obtained via lithium exfoliation has been reported to be around 207°C, 20 while from the measurements performed in this study, the phase change The final image was taken after 631 s. temperature for nanosheets is higher, and depending on the size of nanosheets, ranges between 300°C to 340°C. As the density of vacancies increases with the growth of the nanosheets, the phase change temperature increases simultaneously until a plateau is reached. Figure 4-5 illustrates the phase transformation temperature of nanosheets. For each size of nanosheets, two types of DSC measurements have been performed: with and without the ligands on the surface. Surface ligands (oleylammonium ions) are connected to the WS2 NMLs by electrostatic charge interaction as the nanosheets are negatively charged. They can improve the stability of the 1T' phase as the phase change temperature for the as-synthesized nanosheets is higher than the bare nanosheets without surface ligands. Now, as the phase change temperature has been determined, controlled phase transformation from 1T'-WS2 to 2H-WS2 should be achievable by annealing in a suitable solution. Due to the higher phase transition onset temperature of nanosheets with ligands on the surface, as they still have charges on the surface, the phase transformation happens in a small portion of nanosheets. Figure 4-6 shows the absorption spectrum of solutionannealed WS2 NMLs containing surface ligands, and this spectrum is devoid of any clear excitonic transition. Thus, to be able to induce a complete phase transformation, the ligands and charges must first be removed by an oxidizing agent such as iodine. Following this, the bare nanosheets can be annealed in a high boiling point solvent to induce phase transformation. Iodine treatment clears the charges from the surface of the nanosheets by reducing iodine to I3 -. 21 Charge removal led to the elimination of ligands on the surface of the well-dispersed NMLs. Hence, after iodine treatment, as there are no ligands on the surface, the well-dispersed NMLs (Figure 4-7a) aggregate (Figure 4-7b). However, after annealing the aggregated nanosheets in OAm at 340°C for several hours, both the phase changed and the dispersion was improved (Figure 4-7c). Aliquots were taken at different moments throughout the annealing process, and after 19 hours, phase change was observed.

Metallic (1T') and semiconducting (2H) WS2 possess different physical properties, which makes them easy to distinguish. Dilution of the product in toluene shows that 1T'-WS2 has a dark grayish color, whereas 2H-WS2 is dark brown (Figure 45678). 15 UV-Vis spectroscopy is used to investigate the structure of the well-dispersed NMLs before and after annealing. The absorption spectra confirm the as-synthesized NMLs with different sizes are all 1T'-WS2 (Figure 4-9a), and that 1 hour of iodine treatment followed by subsequent annealing for 19 hours induces the phase transformation from 1T' to 2H (Figure 4 22 this corresponds to a hypsochromic shift for nanosheets smaller than 12 nm due to quantum confinement, and to a bathochromic shift for nanosheets larger than 45 nm due to aggregation. edges, dangling bonds present here, which have different binding energy with the 2H bonding. Regarding the XPS results, annealing could induce three different scenarios. First, the nanosheet undergoes a complete phase transformation and becomes exclusively a 2H-WS2 nanosheet. Secondly, the phase change happens partially in the nanosheet and it becomes a mixture of 1T' and 2H-WS2. Lastly, the nanosheet remains entirely 1T'-WS2. Only in the first scenario could the nanosheets be luminescent even if they aggregate. In the other two possibilities, the nanosheets would not be luminescent, as in the second situation presence of metallic 1T'-WS2 domains would quench the luminescence of the 2H part, as electron hole pairs would recombine in the 1T' zone, acting as a nonradiative recombination site. Photoluminescence spectroscopy (PL) with excitation wavelength at 365 nm has been performed on the annealed nanosheets dispersed in a 1:1 mixture of toluene and isopropyl alcohol. Figure S7 represents the PL spectra of different sized WS2 nanosheets which have been annealed. The multiple emissions at energies higher than the bandgap energy are considered to be caused by large spin-valley coupling and presence of new defect levels.24 Moreover, as the larger nanosheets possess more residual 1T' structure, they are less luminescent.

Conclusions:

In conclusion, this work investigates the effect of size on the phase transformation of WS2 nanomonolayers. To investigate this, an in-solution annealing method was developed to induce 1T' to 2H-WS2 phase change. Different characterizations have been utilized to explore the phase change in WS2 nanosheets. HAADF-HRSTEM observations elucidated that the increase in the size of colloidally synthesized nanosheets increases the defect density. DSC measurements determined the smaller NMLs have higher phase transformation temperature than the larger 1T'-WS2 sheets. Although all the 1T'-WS2 NMLs have been annealed under the same conditions, XPS spectra indicated the larger nanosheets exhibited less complete phase transformation. Absorption spectra of annealed nanosheets represent a blue-shift in the position of the first excitonic peak with a reduction in the size, highlighting lateral quantum confinement effects for the smaller sizes around 10nm diameter. Due to the defect levels and large spin-valley coupling, annealed nanosheets emit white light.

)

)

) ) This study opens new doors for future applications of 1T' and 2H-WS2 as the phase transformation and phase stability is now better understand. Furthermore, it opens a new synthetic route for the synthesis of 2H-WS2 NMLs exhibiting lateral quantum confinement. This phase transformation technique should be also extendable to other TMDCs synthesized in the 1T' crystal structure.

Methods:

Materials:

1-octadecanthiol (ODT) (97%, Sigma-Aldrich), hexadecanethiol (HDT) (95%, Sigma-Aldrich), dodecanethiol (DDT) (98%, Sigma-Aldrich), oleylamine (OAm) (80%-90%, Acros), tungsten hexachloride (WCl6) (99.9%, Acros), carbon disulfide (CS2) (99%, Sigma-Aldrich), hexane (95%, Carlo erba), anhydrous ethanol (99.9%, Carlo erba), toluene (98%, VWR-BDH chemicals), acetonitrile (99.9% Sigma-Aldrich), and 1-octylamine (99% Alfa Aesar) were used as received with no additional purification.

Synthesis of WS2 nanosheets without thiols:

In a typical synthesis, 15 mL (46.6 mmol) OAm was added into a 50 mL three-neck-flask and degassed under vacuum at 80°C for 1 hour. Then the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 5 mL (15.5 mmol) OAm and just before the injection, to avoid jellification, 240 µL CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 30 minutes. After injection, the reaction mixture was cooled down to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Synthesis of WS2 nano-monolayers:

In a typical synthesis, 35 mmol thiol (10 gr ODT, 9 gr HDT, or 7 gr DDT) was dissolved in 15 mL (46.6 mmol) OAm. The mixture was added into a 50 mL three-neck-flask and degassed under vacuum at 80°C for 1 hour. Then, the reaction mixture was heated to 320°C under an argon atmosphere. 50 mg (0.126 mmol) WCl6 was dispersed in 5 ml (15.5 mmol) OAm and just before the injection, to avoid jellification, 240 µL CS2 was added to the mixture. The injection mixture was added dropwise to the reaction mixture over the course of 30 minutes. After injection, the reaction mixture was cooled to room temperature and the product was centrifuged and washed 3 times with a mixture of hexane and ethanol. The product was dispersed in 10 mL toluene for storage.

Synthesis of tungsten thiolates:

In a typical synthesis, 5 gr ODT (17.5 mmol) was dissolved in 15 mL OAm (45.6 mmol) in a 50 mL threeneck-flask. The mixture was degassed under vacuum at 80°C for 1 hour and then heated to 320°C under argon atmosphere. 50 mg of WCl6 (0.126 mmol) was dispersed in 5 mL of OAm, and 240 µL of CS2 (3.97 mmol) was added to the mixture. This precursor mixture was injected dropwise into the mixture of ODT and OAm over a period of 30 minutes by a syringe pump. By the end of the injection, the mixture was cooled to room temperature and the product was centrifuged and washed 3 times with hexane and ethanol. The product was dispersed in 10 mL of toluene for storage.

Iodine treatment:

1269 mg iodine (5 mmol) was dispersed in 10 mL toluene and stirred for 15 minutes, resulting in a 0.5 M solution. The products of one synthesis were dispersed in 10 mL toluene and then added to 10 ml I2 (0.5 M) and stirred for 1 hour. Nanosheets were washed six times with hexane and ethanol to remove iodine from the solution and then dispersed in 10 mL toluene.

Annealing protocol:

To anneal the NMLs in solution, first the surface ligands were removed by iodine treatment. Following this, in a 50 mL three-neck-flask, the bare nanosheets in toluene were dispersed in 20 mL of OAm. This solution was degassed under vacuum at 80°C for 1 hour and then heated to 340°C under argon. After 19 hours, the mixture was cooled to room temperature. The final product was a thick black oil which was centrifuged and washed 3 times with hexane and ethanol. The nanosheets were dispersed in a mixture of toluene and isopropyl alcohol (1:1).

Characterization methods:

TEM: Transmission electron microscopy (TEM) images were acquired on a JEOL JEM-2100 TEM equipped with a LaB6 electron source, operating at 200 kV. Samples were prepared by dropcasting a diluted sample on a 400 mesh copper grid.

UV-Vis spectroscopy:

The UV-Visible absorption spectra of 15 µL of WS2 nanosheets diluted in 3 mL toluene were recorded using an AVANTES Avaspec ULs2048L spectrometer.

PXRD: Powder X-Ray Diffraction (PXRD) diffractograms were recorded on a PANalytical PIXcel1D Medipix3 detector with a copper tube operating at 30 kV. All the samples were prepared by drying few a drops of the WS2 dispersed in ethanol on a low background silicon sample holder. DSC: Differential Scanning Calorimetry (DSC) measurements were performed on a Perkin Elmer DSC 8500 under the flow of argon gas (20 mL/min). Sample amounts of 5 mg were heated from 30.00°C to 500.00°C at a rate of 5.00°C/min. The onset temperatures of thermal events were obtained from tangent lines determined from calculated first derivatives. XPS: X-ray Photoelectron Spectroscopy (XPS) was performed in a homemade system with an ultra-high vacuum chamber using an Al anode. The spectra were calibrated by using the C1s level of carbon species as standard references with a nominal value of 285.0 eV.

PL Spectroscopy:

The photoluminescent spectra of 5 µL of annealed WS2 dispersed in 3 mL mixture of toluene and isopropyl alcohol (1:1) were recorded using a spectrofluorometer FS5 Edinburgh instrument equipped with a double spectrometer and a Xenon white-light lamp. The excitation wavelength was 350 nm with 1 s dwell time and 1 nm step size. The spectra were normalized regarding their absorbance.

Chapter 5: Extention of the protocol to other TMDCs and alloys

Introduction:

After developing a protocol to synthesize well-dispersed NMLs of WS2, as there is not a good protocol to produce well-dispersed NMLs of other TMDC members, I extended the protocol to the other ones as well. Different parameters such as precursors, the reaction temperature, the solvent, and the ligand should be optimized to be able to produce well-dispersed NMLs. In this study, I developed protocols to produce TiS2, TiSe2, WSe2, MoS2, MoSe2, and WSSe. Although well-dispersed NMLs of TiS2, TiSe2, and WSSe are synthesized, WSe2, MoS2, and MoSe2 need more study. In all the protocols, chloride salt of the transition metal is used as the source of transition metal and OAm is used as a high boiling solvent.

Introduction on the alloys and other TMDCs:

Semiconductor nanocrystals are nanoparticles whose optical properties can easily be tuned by tailoring the bandgap thanks to alloying. This spectral tunability alongside their bright luminescence are certainly responsible for the large interest they have generated over the past three decades.

TMDCs became a central building block for 2D nanophotonics and optoelectronics. Methods such as mechanical exfoliation and Chemical Vapor Deposition have led to the highest quality samples. However, they are difficult to scale up for applications requiring a large amount of materials as is the case for light-conversion, energy storage or photocatalysis. Therefore, chemical growth appears as a more viable sample production path. Liquid phase exfoliations, in which small cations such as Li + coupled to a solvent such as N-methyl-2-pyrrolidone (NMP), have been proven as a viable strategy to obtain TMDC nanosheets. However, the obtained materials are often aggregated and the control of the thickness at the single monolayer scale is still challenging. Meanwhile, in the field of nanocrystals, great progresses has been achieved in the control of the nanoparticles shape, in particular, with synthesis of 2D nanoplatelets made of II-VI and IV-VI semiconductors.

TiS 2 :

Titanium disulfide (TiS2) possesses semi-metallic crystal structure and has promising electrical properties to be used as new electrode materials. 1 Some theoretical studies showed the number of layers influences the physical properties such as bandgap. 2,3 Fang et al. reported the single layer TiS2 disks have semiconducting properties. 3 Up to now, there is no colloidal protocol to produce welldispersed and stable single layers and different synthesis protocols lead to multilayers or unstable nanosheets. 4,5 Here, I reported a new hot injection colloidal synthesis to produce stable single layers of TiS2. As it is reported in the experimental section, titanium tetrachloride (TiCl4) has been used as the source of titanium. It should be noted, as TiCl4 is very reactive, solvents, in this case oleylamine, should be completely degassed, otherwise TiO2 would form.

Experimental section:

In a typical synthesis, in a three-neck-flask 15 mL oleylamine (OAm) was degassed at 80°C for 1 hour. Then, the degassed OAm was heated to 280°C under argon. In parallel, 24 mg TiCl4 was dispersed in 5 mL high purity OAm in the glovebox and just before the injection, CS2 was added to the mixture. The injection mixture was injected into the reaction mixture over the course of 30 minutes. As the injection was done, the reaction mixture was cooled down to room temperature and centrifuged 3 times and washed by a mixture of ethanol and hexane. In the end, nanosheets were dispersed in 10 mL toluene for storage.

Results and discussion:

TEM and UV-Vis spectroscopy have been used to characterize the products. The UV-Vis spectrum of diluted solution is depicted in Figure 5-1. TEM image shows that the TiS2 nanosheets are single layers with 140 nm mean diameter (Figure 5-2). Small nanowires beside TiS2 nanosheets are TiO2 anatase, which should be formed due to the presence of trace of water and oxygen in the glove box. The same as TiS2, titanium diselenide (TiSe2) is a promising material to be used as new electrode materials for conversion and storage of energy. 6 Different synthesis protocols are developed to produce TiSe2 colloidally, but none leaded to formation of single layers. [6][7][8] Here, a new colloidal synthesis protocol is developed to produce stable multilayers of TiSe2.

Experimental section:

In a typical synthesis, in a three-neck-flask 15 mL oleylamine (OAm) was degassed at 80°C for 1 hour. Then, the degassed OAm was heated to 320°C under argon. In parallel, 24 mg TiCl4 and 298 mg selenium were dispersed in 5 mL super clean OAm in the glovebox. The injection mixture was injected into the reaction mixture in 30 minutes. As the injection was done, the reaction mixture was cooled down to room temperature and centrifuged 3 times and washed by a mixture of ethanol and hexane. In the end, nanosheets were dispersed in 10 mL toluene for storage. There is no characteristic feature in the absorption spectrum, which might be due to the large size of the nanosheets.

nm

nm

WSSe:

In the previous reference carboxylic acids have been used but it introduced oxide defects in the structure. To tackle this, here we used thiols. When present in large excess, they also complex the tungsten precursor to form thiolates. This bulky form of tungsten also slows down the reaction enabling a better control of the reactivity. In addition, it has been recently demonstrated that the addition of long ligands has a beneficial effect on the trap passivation which leads to a photoluminescence enhancement in the case of the 2H phase. This observation suggests that long ligands can be incorporated within the TMDC crystalline structure. Here we use a combination of long chain amines and thiols.

Experimental section:

Tungsten chloride is first dissolved in a mixture of oleylamine and is then mixed with carbon sulfide and trioctylphosphine selenide (respectively used as sulfur and selenium precursor). This mixture is then injected dropwise into a hot solution of oleylamine and octadecanethiol (from 280 to 320 °C depending on Se content). At the end of the injection, the flakes are separated from the reaction medium thanks to a series of precipitations. Finally, the particles are stored in a non-polar solvent such as toluene.

Results and discussion:

Transmission electron microscopy imaging (Figure 5-5: a-b) reveals the formation of small flakes with typical planar extension around 20 nm. High resolution image (Figure 5-5b) clearly exhibits the formation of atomic lines, characteristic of the 1T' distorted phase (see atomic organization on Figure 5-5b). Flakes appear to be defective with the presence of atomic vacancies and even holes.

The formation of the 1T' phase is also clearly highlighted by the absorption spectrum, see Figure 1c. The latter does not present any feature in the 600-700 nm range where the exciton of the hexagonal wide band gap phase would have been expected. However, by annealing the flakes at high temperature (380°C, see method), it is possible to convert the distorted 1T' phase into the hexagonal phase. This can be observed in absorption with the apparition of a clear feature associated with the A exciton of the hexagonal phase of WSSe. As the Se content is changed, we observe a shift in the energy of this exciton (inset of Figure 5-5c), which confirms the introduction of the selenium into the TMDC lattice. This incorporation can also be observed from the X-ray diffraction pattern, see Figure 1d and its inset. A clear shift of the peak associated with the (110) plans is observed with the increase in the Se ratio.

Other characterization and the DFT calculation part have also been performed on this material, which is available in the paper that is available in the end of this chapter. As that part of the study has not done by me, I did not bring it here.

WSe 2 :

After we developed a colloidal protocol to synthesize well-dispersed and size-controlled 1T'-WS2 nano-monolayers, we extended the protocol to the other TMDCs. Semiconducting WSe2 has a bandgap size of 1.2 eV as bulk and 1.7 eV as monolayer. Different protocols are developed to synthesize single layer WSe2. Although Jung et al. developed a protocol to synthesize single nanosheets, other studies leaded to formation of either nanoflowers or multilayer nanosheets. 9 Here, different experimental parameters have been investigated to develop a colloidal protocol to synthesize single nanosheets of WSe2.

Experimental section:

Different sources of selenium have been used and different morphologies have been synthesized. Elemental selenium, trioctylphosphine selenide (TOPSe), and octadecane selenol (ODSe) have been used as the selenium source.

TOPSe:

TOPSe 2.24 M was synthesized by dissolving 3.54 gr elemental selenium into 20 mL of trioctylphosphine (TOP) in a 50 mL three-neck-flask. After degassing the mixture for 1 hour at 80°C, it was stirred for 72 hours at room temperature under argon until a colorless solution was obtained.

ODSe:

ODSe was synthesized by flushing a three-neck flask with argon, which maintains the inert gas. 4.6 gr elemental selenium and 4.5 gr borohydride were added to the flask. 21 mL of anhydrous ethanol was added to them slowly and after about 20 minutes of stirring a white greyish mixture would form. 100 mL anhydrous DMF was added to the reaction mixture, which induced a shift in the solution color first to a brownish red solution and after a while, it became colorless. 4.6 mL formic acid was added dropwise to the reaction mixture. After 20 minutes of stirring, 16.67 gr 1-bromooctadecane was added to the reaction mixture and stirred for 6 hours. Then, it was hydrolyzed with 100 mL HCl (10 wt%). The organics were extracted by 100 mL diethyl ether 3 times.

WSe2:

It should be noted, in different protocols, the only variable that changes is the source of selenium (in all protocols the quantity of the selenium is 30 times more than tungsten in the injection mixture). 50 mg tungsten hexachloride and 300 mg selenium or 1.68 mL TOPSe or 420 mg ODSe were dissolved in 5 mL OAm. The injection mixture was injected dropwise into 15 mL OAm at 280°C. In the end, the cooled down mixture was centrifuged at 9000 rpm and washed by a mixture of hexane and ethanol. Final product was dispersed in 10 mL toluene.

Results and discussions:

The first investigated source of selenium is elemental selenium. As it does not have any kind of binding, the reactivity of elemental selenium is the highest. The next source of selenium that has been investigated was TOPSe. Absorption spectrum of WSe2 nanoparticles synthesized by TOPSe is presented in Figure 567. In comparison with the results from WSSe synthesis, we can understand the product is pure WSe2. TEM image of aggregates small nanosheets of WSe2 is illustrated in Figure 5678. Small aggregated nanosheets of WSe2 were formed as tiny clusters with 20 nm diameter. Although by changing the source of the selenium, the reactivity decreased, but still, it is not enough. So, another source of selenium should be investigated.

Conclusion:

To be summarized, we extended the colloidal protocol, which we had developed for 1T'-WS2, to TiS2, TiSe2, WSSe alloys, and WSe2. The UV-Vis absorption spectroscopy and TEM have been taken advantage to characterize the products. In the case of TiS2, TiSe2, and WSSe, the reaction temperature was the only variable that we changed to reach good results, but for WSe2 finding the source of selenium was the main challenge. In the end, new protocols to produce nanosheets of these TMDCs have been developed.

Introduction:

Different protocols are developed to produce well-dispersed nanomonolayers (NMLs) of 1T'tungsten dichalcogenides and their alloys. As it has already been mentioned, a new protocol is developed to produce size-controlled 1T'-WS2 NMLs, which leads to colloidally stable nanosheets. Although colloidal protocols to produce well-dispersed NMLs of tungsten dichalcogenides and the alloys are developed, a colloidal protocol to produce homostructure and heterostructures of them is still missing. In this chapter, new protocols, which are developed to produce homostructures of WS2 with different crystal structures and heterostructures of WS2-WSe2, will be explained. A double injection method is developed to produce homostructures and heterostructures. UV-Vis absorption spectroscopy, STEM, and EELS spectroscopy confirm the formation of heterostructures.

Introduction on heterostructures of TMDCs:

In a study from Amin et al. different properties of group VI TMDCs heterostructures have been investigated. They reported that, out-of-plane heterostructures leads to formation of indirect bandgap with type two alignment. While, in-plane heterostructures formed direct bandgaps without type two alignment. 1 In another study from Hong et al., a CVD method was developed to produce out-of-plane heterostructure of MoS2/WS2. They measured the photoluminescence, transient absorption measurement, which confirm the band alignment, is type two, and the photoluminescence decreased after the heterostructure formation. 2 In the study from Soukoulis et al., the structure, electronic, and optical properties of out-of-plane heterostructures of group VI TMDCs have been investigated. They reported that, depending on the size and position of bandgaps and different stacking of the layers, three kinds of band alignment can form. Interlayer coupling induces changes in the band structure of heterostructures with different stackings. In addition, the direct bandgap at K valley would change to an indirect one. 3 Kobayashi et al. reported a CVD method to grow hetroepitaxially WS2 and MoS2 by changing the salt. STM spectroscopy shows the interface between WS2 and MoS2 is very sharp. 4 Chen et al. developed a protocol to produce heterostructures and regarding the stacking sequence and sulfurization steps, it is possible to produce heterostructure of single monolayers of WS2/MoS2 or alloys of them. Photoluminescence spectroscopy shows alloying enlarges the emission peak. 5 In a very recent study, Jin et al. reported that formation of moiré superlattice induce new exciton states in an out-ofplane heterostructures of WS2/WSe2. Depending on the twist-angle between TMDCs, reflection contrast and photoluminescence would change. Photoluminescence spectroscopy supports that the interlayer exciton photoluminescence is stronger than the reflection photoluminescence as excitation happens at all moiré exciton states. 6 In another simulation study by Kang et al., electronic properties of different TMDCs were calculated. They reported that, by increasing the atomic number in TMDCs, CBM and VBM increase. They also showed that all MoX2-WX2 heterostructures would have type two band alignment. 7 In another computational study from Komsa et al., regarding the electronic structure of heterostructures of group VI TMDCs, they reported that the interlayer interaction is very severe for TMDCs and it changes the position of VBM. 8 Wang et al. compared the heterostructures of MoS2-WS2 and MoS2-MoTe2. They reported that, while MoS2-WS2 heterostructure forms type-II band alignment, which is about 1.58 eV, MoS2-MoTe2 heterostructures show semi-metallic behavior. It should be noted that this study was a simulation also. 9 Zhu et al. developed a strategy to produce large size lateral or vertical heterostructures of WS2-MoS2. Analyzes show that the interface is very sharp and alloying did not happen in the lateral heterostructure. Photoluminescence spectroscopies confirm that lateral heterostructure does not affect the photoemission but vertical heterostructure reduces the intensity of the emitted light. 10 Gong et al. developed another protocol to produce WS2-MoS2 vertical and lateral heterostructures. Zhu et al. used hydroxide-assisted nucleation, while Gong et al. used oxides as precursors. The sharp interface in lateral heterostructures was made with this method also. Micro-Raman spectroscopy and STEM images support the formation of heterostructures. 11 Meng et al. reported a layer-by-layer transfer technique to form vertical heterostructure of MoSe2-WS2. They reported that, depending on the gate voltage, quenching or enhancement of photoluminescence happens, which causes by the change in the position and size of VBM and CBM of MoSe2 and WS2. 12 In other studies, heterostructures of TMDCs with graphene, h-BN, TiO2, and perovskite are investigated. The changes in the bandgap size and electronic structure, improvements in the photoluminescence, and photocatalytic behavior of these heterostructures have been investigated. [13][14][15][16] From all the studies, making heterostructures is a robust method to manipulate the properties of TMDCs. As it has been explained, vapor deposition methods and transferring methods have been used to make heterostructures. Moreover, up to now no colloidal protocol to synthesize heterostructures of NMLs of TMDCs is developed. Here, the first colloidal protocol to produce heterostructures of TMDCs is explained.

Developed protocol:

As it has been already mentioned, the formation mechanism of TMDCs is nucleation and growth. Regarding the formation mechanism and depending on the energy of the system, if enough available sites for growth are available, lateral growth will happen instead of formation of new nuclei. According to this, reducing the size of seeds is in the favor of increasing the available sites for growth. Hence, in all of the protocols that were developed to synthesize heterostructures, 1T'-WS2 with 17 nm was used as the seed, as it was the smallest nanosheet with very good colloidal stability. The 1T'-WS2 seeds were dispersed in the reaction medium and the crown was injected slowly to decrease the concentration of second phase in the reaction medium to prevent secondary nucleation.

In this study different protocols were developed to produce different types of TMDCs; 1T'-WS2, 2H-WS2, 1T'-WSSe, and 1T'-WSe2. Results from synthesis of size-controlled NMLs of 1T'-WS2 confirm the formation mechanism is nucleation and growth. This finding ignites the idea of using well-dispersed NMLs as seeds to form heterostructures. Although annealing induces the phase change, as the 2H-WS2 possess triangular edges, it would be so beneficial to conjugate it with well-dispersed NMLs. Other heterostructures that have been investigated are WS2-WSSe and WS2-WSe2.

Homostructure of 1T'-WS 2 /2H-WS 2 :

As the NMLs are in the nanometer range (D= 10-60 nm), edges have great effects on the different properties such as luminescence and defected edges quench the luminescence, as they are non-emitting recombination sites. Formation of triangular edges on well-dispersed NMLs can reduce the non-emitting recombinations and improve the photoluminescence. As both the seeds and the edges are the same material (WS2) with different structures (1T' and 2H), we named this structure as homostructure. Maximum amount of growth without secondary nucleation would form when the edge growth needs less energy than formation of new nuclei. As the edge growth is more energetically favourable than the nucleation, by increasing the portion of the edges, the nucleation could be stopped. Reducing the mean size of seeds increases the portion of edges in a certain amount of nanosheets. However, the most important factor is the seeds dispersion. So, smallest NMLs that are colloidaly stable should be used as the seeds. In the next part, the protocol used to produce heterostructures is explained.

Synthesis protocol:

In a typical synthesis, 10 gr ODT was dissolved in 15 mL OAm in a 50 ml three-neck-flask. The mixture was heated to 320°C under Argon, after 1 hour degassing at 80°C. 50 mg WCl6 was dispersed in 5 mL OAm and 240 µl CS2 was added to the mixture. The injection mixture was injected to the reaction mixture dropwise in 15 minutes. The product was centrifuged and washed 3 times by a mixture of ethanol and hexane. In the end, the product was dispersed in 20 mL toluene.

In a 100 mL three-neck-flask, all the products from a synthesis were dispersed in a mixture of 30 mL OAl and 10 mL OA. After degassing the mixture at 80°C for 1 hour, it was heated to 330°C under Argon. 50 mg WCl6 was dissolved in a mixture of 10 mL OAm and 300 µL OA. 240 µl CS2 was added to the injection mixture, which was injected in the reaction medium in 3 hours. The product was centrifuged and washed 3 times with a mixture of ethanol and hexane. The final product was dispersed in 10 mL of a mixture of 1:1 of toluene and Isopropyl alcohol.

Results and discussion:

As it has been already explained, this protocol is developed to reduce the defect density in the edges of NMLs. Formation of triangular 2H-WS2 on the defected edges of 1T'-WS2 nanomonolayers can passivate the edges to reduce the non-emitting exciton recombination. The first and most accessible spectroscopy method to characterize the results from a colloidal synthesis is UV-Vis spectroscopy. UV-Vis absorption spectroscopy is a robust technique to determine different structures. The difference in the color of diluted dispersions of WS2 before and after forming the heterojunction supports the second injection induced the formation of another structure. As 1T'-WS2 possesses semi-metallic properties, a high amount of absorption is expected in the entire UV-Vis spectrum. Moreover, 2H-WS2 has semiconducting properties, so a first excitonic peak that defines the bandgap is anticipated. Figure 6-1 shows the absorption spectra of the core and the homostructure of 1T'-WS2/2H-WS2. The change in the absorption spectrum from metallic to semiconducting implies the second injection was led to formation of a new crystal structure.

Although the formation of a new phase was proved by checking the absorption spectra, a direct proof is required to show the second phase is formed on the edge of the first crystal. TEM images ( Figure 6-2) of the first injection (core) and second one (homostructure) illustrate that the mean size and shape of well-dispersed nano-monolayers is changed. As we already knew that the products from the second injection were formed as nanoflowers, which might happen due to the nucleation as multilayers and very fast growth. 13 However, here, instead of nanoflowers, nanosheets with triangular edges were formed. It should be noted, STEM observation can be useful to determine the formation of core and crown structure, as the core is 1T'-WS2, which has zigzag structure and crown is 2H-WS2 with hexagonal symmetry.

Conclusion:

A new protocol is developed to produce homostructures of 1T'-WS2/2H-WS2 by reinjection of the second phase. Absorption spectra of nanosheets before and after the second injection depict the formation of semiconducting phase after the second injection. TEM images support the formation of heterostructures, as the shape and size of the nanosheets are changed.

Heterostructure WS 2 -WSSe/ WS 2 -WSe 2 :

Formation of Nano-heterostructures induces huge changes in the electric properties of both the core and crown. 17 Regarding the bandgap size and position of different group VI TMDCs, they always form type-II band alignment. 1 Although, in large sheets, the interface and edges are not sensible and important, in the Nano-heterostructures, as the size of core and crown is comparable with the exciton radius, the interface and edges become important. 18 Investigating this consequence has not yet been studied, since no protocol is capable of producing Nano-heterostructures, these materials could be studied only by simulation. Colloidal synthesis is a powerful method to produce nanoparticles in different shapes and sizes and it has the potential for the preparation of large batches with high crystallinity. As the formation mechanism is nucleation and growth, it is possible to produce heterostructures if all the conditions of heterostructure formation are passed. One of the most important conditions to form a heterostructure is the lattice mismatch. As the lattice mismatch between WS2 and WSe2 is about 3.4%, 19,20 the strain in the interface is low enough to induce defect formation.

As it has been explained for the homostructure of WS2-WS2 with different crystal structures, increasing the portion of edges reduces the probability of secondary nucleation. Therefore, the synthesis protocol of core is the same as the core for homostructure formation, the smallest NMLs with the best dispersion. Furthermore, the time of the second injection was increased to reduce the concentration of the precursors in the reaction medium to prevent secondary nucleation.

Synthesis protocol:

As it has been mentioned, the synthesis protocol of core of the heterostructures is the same as the core of the homostructures. Two different protocols were used to produce the crown; the first one was WSSe with almost 52% selenium and then WSe2. At first, a protocol to produce WS2-WSSe was used to be sure that it is possible to produce heterostructures by colloidal synthesis.

To synthesize WS2-WSSe heterostructures, in a three-neck-flask, 20 mL 1T'-WS2 was dispersed in a mixture of 15 mL OAm and 5 gr ODT. The reaction mixture was heated to 280°C after 1 hour degassing at 80°C. 50 mg WCl6 was dispersed in 10 mL OAm and 1.68 mL TOPSe (2.24M), then the injection mixture was injected in the reaction medium over the course of 5 hours. After cooling the reaction mixture to room temperature, the product was centrifuged and washed 3 times with a mixture of ethanol and hexane, and the washed product was dispersed in a mixture of 1:1 of toluene and Isopropyl alcohol.

To synthesize WS2-WSe2 heterostructures, in a three-neck-flask, 20 mL 1T'-WS2 was dispersed in 40 mL OAm and the mixture was degassed at 80°C. The reaction mixture was heated to 280°C after 1 hour degassing. 50 mg WCl6 was dispersed in 10 mL OAm and just before injection 300 mg octadecane selenol dissolved in the solution and it was injected over the course of 5 hours. After cooling the reaction mixture to room temperature, the product was centrifuged and washed 3 times with a mixture of ethanol and hexane, and the washed product was dispersed in a mixture of 1:1 of toluene and Isopropyl alcohol.

Results and discussion:

The most effective property of the seeds is their good dispersion, so every time the dispersion was verified before making the heterostructure. Figure 5a and Figure 10a represent the well-dispersed nanosheets with the mean size of 24 nm and 22 nm, respectively. The homogeneity of the contrast in TEM images supports that the nanosheets are well-dispersed monolayers. Absorption spectra of diluted solutions of well-dispersed nanosheets are shown in Figure 6-3 and 6-7 with the red curve. The results show that the structure of nanosheets, which have been used as the seeds in the both heterostructures is 1T'-WS2. As there is high absorbance everywhere without any first excitonic peak, they have metallic behavior.

The certainty of production of well-dispersed seeds led us to run the second injections as they have been explained in the synthesis protocol. Changes in the absorption spectra determine the formation of another crystal structure. heterostructures at 340°C for 19 hours. The presence of the first excitonic peak at around 670 nm supports the formation of WSSe with 52% selenium. 21 TEM images of the nanosheets before (Figure 6-5a) and after (Figure 6-5b) the second injection show the size difference between the seeds (24nm) and the products after the second injection (40 nm) support the formation of heterostructures. The increase in the size of nanosheets indicates the second phase (WSSe) is formed on the edges, and a core-crown heterostructure is formed.

As the contrast in TEM depends on the atomic mass, HRSTEM illustrates better the formation of heterostructures. As selenium is more than two times heavier than sulfur, in HRSTEM images the contrast of WSSe will be higher than WS2. Figure 6-6 shows the HRSTEM image of a single monolayer

) b )

of heterostructure of WS2-WSSe. Due to the difference in the mass of selenium and sulfur, their contrast is greater and as selenium is heavier, the contrast form selenium is higher.

As mentioned previously, up to now, all the lateral TMDCs heterostructures have been produced by vapor deposition methods. As the formation of heterostructures of WS2 and an alloy has been demonstrated by different characterization techniques, our ambition led us to the development of a new heterostructure, WS2-WSe2. The reactivity of the precursors of the second injection can induce either formation of new nuclei or growth on the edges. In accordance with the synthesis protocol, octadecane selenol was used as the source of selenium. The difference in the absorbance spectra of the diluted products, before and after the second injection confirms the formation of a new crystal structure (Figure 67). Comparing the absorption spectra of WS2-WSSe and WS2-WSe2 with WS2xSe2(1-x) with different compositions shows that the crown formed in the WS2-WSe2 heterostructure second phase has been proved, a direct characterization method is needed to determine if the second phase is formed on the edges. TEM images of nanosheets before and after the second injection are illustrated in Figure 6789. The size enlargement of nanosheets from 22 nm to 47 nm confirms the second phase is formed on the edges.

As previously explained, due to the difference in the contrast of atoms with different masses, STEM is a robust method to define the formation of heterostructures. This technique has been used with EELS method to better define the quality of the interface. HAADF-HRSTEM and EELS images of two different heterostructures are presented in Figures 6-10 and 6-11. It is clear from both nanosheets that the interface is quite sharp and alloying has been not occurred. The main goal is to understand the behavior of the nanosheets so we can control the in-plane and out-of-plane heterostructure. Regarding this aim, different protocols to synthesize nanoheterostructures have been developed. WS2-WSSe/ WS2-WSe2 heterostructures have been synthesized and characterized by TEM, STEM, and UV-Vis absorption spectroscopy. TEM images prove the growth of the nanosheets after the second injection. As, signals in electron microscopy depends on the atomic mass, HAADF-HRSTEM images support the formation of nano-heterostructures by the difference in the contrast of the core and the crown.

Conclusions:

An urgent demand for colloidal synthesis protocols that are able to produce well-dispersed nano-monolayers with different transition metal dichalcogenides, alloys, and heterostructures promoted us perform this study. Since this group had already developed a colloidal synthesis for 1T-WS2 nanosheets, in the first step we changed some experimental parameters of that protocol to produce well-dispersed and size-controlled nano-monolayers. As we managed to develop a protocol to produce well-dispersed and size-controlled nano-monolayers, we extended the protocol to the other members of TMDCs. The success in the development of colloidal protocols to produce WSSe alloys and WSe2 nanosheets, as well as the realization of formation mechanism of TMDC nanosheets paves the way to produce heterostructures.

Although TMDCs have promising properties, they are not yet well used, as a method to produce big batches of TMDCs was not available. Substituting oleic acid with a non-oxidative coordinating agent (octadecane thiol) improved the dispersion and crystallinity of the nanomonolayers of 1T'-WS2 by regulating the reactivity of the source of tungsten. UV-Vis absorption spectroscopy and PXRD measurements confirm the structure is pure 1T' and TEM images illustrate that the WS2 nanosheets are colloidally stable. Low angles oscillations support the nanosheets are single layers and position of the first peak in the PXRD diffractogram defines the inter-planar distance between nanosheets is 2.7 nm. TGA and FTIR have been done to check the ligands on the surface. As the thermal decomposition onset temperature and first derivation position of WS2 nanosheets synthesized with ODT as the coordinating agent and without any coordinating agent is the same, we concluded the ligands on both nanosheets should be the same, which is oleylamine. However, FTIR was not a useful technique as C-S bonding for ODT is out of range of FTIR measurement and WS2 reflects about 20% of the signals, then the signal to noise ratio is very low. Ligand exchange has been used as an indirect proof. After the ligand exchange with octylamine, which is shorter than oleylamine, the inter-planar distance reduced, so it confirms that oleylamine was the only kind of ligand on the surface.

The first step, producing well-dispersed nano-monolayers, was done. Other experimental parameters have been changed also to control the size of nanosheets. The amount of ODT, reaction temperature, and injection time were the most effective parameters that have been changed to control the mean size. Reducing the amount of ODT in the reaction mixture decreases the stability of the precursors, hence increasing the size of nanosheets. Increasing the reaction temperature reduces the amount of super saturation needed to form nuclei, so nucleation increases and the size of nanosheets decreases. The last parameter is the injection time. Increasing the injection time reduces the solution concentration, which reduces the rate of nucleation and increases the size of nanosheets.

Although 1T'-WS2 nano-monolayers with semi-metallic properties have different applications such as hydrogen evolution reaction (HER) and electrode materials for sodium ion batteries, 2H-WS2 with semiconducting properties has other promising application such as electronics, optoelectronics, and catalysis. As the 1T' crystal structure is a metastable phase, annealing induces phase change to the 2H one. DSC analysis represented that the phase change temperature in the NMLs synthesized by using ODT as the coordinating agent, is much higher than the reported temperature in other studies. STEM images showed the propagation of phase change under electron beam. However, defects hinder the phase change. Due to the presence of defects in these 1T'-WS2 nanosheets, here, the phase change temperature is higher. STEM images illustrated the density of defects increases with the size enlargement. XPS spectra of annealed nanosheets confirm that defects stop the phase change and as the larger nanosheets possess more defects, annealing at the same temperature for a certain time induces less phase transformation in the larger nanosheets.

A new colloidal synthesis protocol was developed for 1T'-WS2 nano-monolayers, after that we decided to extend this protocol to the other members of group VI TMDCs. WSe2 is the most similar member of this group to WS2. So, at first new protocols are developed to produce WSSe alloys and then WSe2 nanosheets. CS2 as the source of sulfur was substituted by TOPSe as the source of selenium. UV-Vis absorption spectroscopy confirmed that the as-synthesized samples were 1T'-WSSe and annealing them on the thin film induced phase change to 2H. HRSTEM images supported that the crystal structure was 1T', due to the presence of zigzag patterns. PXRD patterns and UV-Vis spectrum of alloys with different amount of selenium showed that by increasing the amount of selenium in the structure, the peak positions changed.

As the colloidal protocols to synthesize single-layer WS2, WSSe, and WSe2 were developed, we developed another protocol to produce lateral heterostructures. 1T'-WS2/2H-WS2, WS2/WSSe, and WS2/WSe2 were synthesized by reinjection of the second part in the well-dispersed 1T'-WS2. UV-Vis spectroscopy has been used to validate the formation of the second phase. However, the formation of the second phase does not mean the heterostructures are formed. So, TEM and STEM images are needed to clarify the formation of heterostructures. TEM images before and after the second injection confirm the mean size of the nanosheets is increased, which means the heterostructures are formed. Due to the difference in the contrast of atoms with different mass, HAADF-HRSTEM images support the WS2/WSSe and WS2/WSe2 are formed as the contrast of the edges are higher. EELS measurement illustrates the interface is quite sharp. Different colloidal protocols were developed by which big batches of high quality welldispersed NMLs of 1T'-WS2, 1T'-WSSe, and heterostructure of WS2/WSSe and WS2/WSe2 can be synthesized. As the 1T' structure is transformable to 2H one by annealing in the proper situation, as a thin film or in the solution, this study opens new doors into the future of application of TMDCs. It is also possible to extend these protocols to other members of TMDCs. In this study, we had a chemist point of view, whom developed a protocol to synthesize well-dispersed and size-controlled nanomonolayers. As material scientists, we characterize the materials we synthesized. Now it is time for physicists and engineers to check for the properties and application, respectively.

Perspectives:

In this study, we managed to develop a protocol to synthesize well-dispersed and sizecontrolled 1T'-WS2. Then we transform the phase to 2H by annealing. We extended the protocol to other members and colloidal protocols of WSSe alloys and WSe2 nanosheets were also developed. Then, by reinjection, different heterostructures such as 1T'-WS2/2H-WS2, WS2/WSSe, and WS2/WSe2 nanosheets have been synthesized. Although, we synthesized size-controlled nano-monolayers, there is no control on the shape of nanosheets. Due to the different application of WS2 nanosheets, shapecontrol is a very interesting subject, as their properties will be different.

Les métaux de transition dichalcogenides du groupe VI (TMDCs) sont des matériaux semiconducteurs en 2 dimensions possédant de fortes liaisons covalentes dans le plan mais des faibles liaisons de Van der Walls en dehors du plan. Ce groupe de matériaux peut aussi bien avoir des propriétés semi-conductrices impliquant une structure cristalline stable, mais aussi des propriétés semi-métalliques impliquant une structure cristalline métastable. En réduisant la taille de ces semiconducteurs à une monocouche, il est possible d'induire des transitions directes ou indirectes dans la bande interdite. Ces propriétés prometteuses montre le potentiel important de ces matériaux pour certaines applications en électronique, optoélectronique, catalyse ou encore les électrodes des batteries ioniques. De plus, grâce à la taille et la position de la bande interdite, ces matériaux peuvent être utilisés pour le water splitting et l'électrolyse de l'eau.

Cette thèse est dans la continuité de la recherche faite sur le développement de protocoles colloïdaux afin de synthétiser des batchs importants et qualitatifs de différents TMDCs. A ce propos, la première étape a été de développer des protocoles pour produire des nano feuillets de WS2 avec des tailles et des dispersions bien contrôlés. Cet objectif a été rempli grâce à la modification de vieux protocoles d'échange de l'acide oléique par de l'octadécanethiol comme agent complexant. Nous avons étudié la structure de ces nano feuillets par spectroscopie UV-Visible et par diffraction de rayons X ce qui a permis de mettre en lumière la structure cristalline en 1T'-WS2. Les techniques de microscopie électronique ont également été utilisées pour vérifier la dispersion et la taille des nano feuillets. L'analyse thermogravimétrique couplée à la spectroscopie infrarouge à transformée de Fourrier ont été réalisées pour analyser les ligands présents sur la surface. Après que le premier objectif ait été rempli, la transformation de phase de 1T' à 2H a ensuite été étudiée, le but étant essentiellement d'examiner le changement de phase. Nous avons alors réussi à obtenir des nano feuillets 2H-WS2 après recuit dans l'oleylamine à 340°C pendant 19 heures. La spectroscopie UV-Visible, l'analyse par diffraction de rayons X et par spectrométrie photoélectronique X ont permis de caractériser ce changement de phase. De plus, grâce aux différentes structures cristallines présentes, la microscopie électronique à transmission a été un outil indispensable pour montrer cette transformation. La calorimétrie différentielle à balayage nous a aidé à définir la température exacte au moment du changement de phase. Une extension du protocole de base des 1T'-WS2 a permis de développer de nouvelles procédures afin de produire des alliages de nano feuillets à une monocouche comme des TiS2, TiSe2, WSe2 ou encore WSSe. Finalement, nous avons développé un protocole pour synthétiser des hétérostructures de WS2-WSSe, WS2-WSe2 et de 1T'-WS2/2H-WS2 en réinjectant la deuxième phase. Les changements en absorbance et en taille des nano feuillets ont été analysés par spectroscopie UV-Visible et par les méthodes de microscopie électroniques.

Figure 1 - 1 :

 11 Figure 1-1: Some of the applications of TMDCs are represented here. As these materials have extraordinary properties, they can be used in different application aspects.

Figure 1 - 2 :

 12 Figure 1-2: Two different methods are available to produce isolated TMDCs layers. In top-down methods, a mechanical force is needed to break the inter -layer bonding energy. In bottom-up methods, layers of TMDCs form by a chemical reaction between reactants.

  Figure 1-4 illustrates an improved procedure of the micromechanical exfoliation technique.

Figure 1 - 7 :

 17 Figure 1-7: Low-pressure CVD growth of MoS2 on mica. (a) The schematic presentation of the formation of nuclei and their growth, (b) -(d) the SEM images of MoS2 sheets after different reaction times, and (e) Statistical relation of surface nucleation density with flake size.41

Figure 1 - 8 :

 18 Figure 1-8: The size of the nanosheets can be controlled by changing the amount of reactants or injection time.

  Figure 1-9 illustrates the different structures of TMDCs.

Figure 1 - 9 :

 19 Figure 1-9: Three different structures of TMDCs. For most group VI TMDCs, 2H -phase is the stable phase.

Figure 1 - 10 :

 110 Figure 1-10: Different techniques to induce 2H to 1T' phase change. The two main categories are phase change by extra charges and mechanical s train.[START_REF] Mattevi | Direct synthesis of metastable phases of 2D transition metal dichalcogenides[END_REF]

Figure 1 -

 1 12 illustrates different events happen to semiconducting TMDCs.

Figure 1 - 11 :

 111 Figure 1-11 : Raman spectra for a) 2H-WS2 with different number of layers, b) 2H-MoS2 with different number of layers.Increasing the number of layers decrease the A1g but increase the E 1 2g in both WS2 and MoS2.[START_REF] Zhang | Shear and Layer Breathing Modes in Multilayer MoS2[END_REF]

Figure 1 -

 1 Figure 1-12: a) Schematic illustration of hexagonal structure of 2H -TMDCs. b) The Brillouin zone of 2H-TMDCs. c) DFT calculated the Brillouin zone for MoS2 from bulk to monolayer.[START_REF] Cao | Two-dimensional transition-metal dichalcogenide materials: Toward an age of atomic-scale photonics[END_REF]

Figure 1 -

 1 13 illustrates the schematic of a p-n junction diode made of n-MoS2 and p-WSe2.

  Figure 1-15 represents the changes in the quantum yields of four members of group VI TMDCs after super acid treatment.

Figure 1 -

 1 Figure 1-15:The maximum quantum yield of WS2, MoS2, WSe2, and MoSe2 before and after TFSI treatment.[START_REF] Wallace | Recombination Kinetics and Effects of Superacid Treatment in Sulfur-and Selenium-Based Transition Metal Dichalcogenides[END_REF]

Figure 1 -

 1 16 illustrates the schematic of valley polarization in single layer TMDCs.

Figure 1 - 16 :

 116 Figure 1-16: The schematic illustration of polarization in single layer TMDCs. Due to the valley splitting, the K' valley absorbs and emits left -handed

Figure 1 - 17 :

 117 Figure 1-17: Schematic illustration of a) density of states of 2H -TMDCs, b) valley splitting and band nesting in 2H-TMDCs, c) absorption and photoluminescence spectra of MoS2, MoSe2, WS2, and WSe2.[START_REF] Hong | Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures[END_REF]

Figure 1 - 18 :

 118 Figure 1-18: Tailoring the optical properties of the TMDC alloys. a) The first excitonic peak positions and b) correspond ing bandgap energy transition of Mo 1- x WxSe2 alloys. c) The first excitonic peak positions and d) corresponding bandgap energy transition of WS2(1-x)Se2x alloys.15

  Figure 1-19 schematically shows the in-plane and out-of-plane heterostructures. Depending on the bandgap size and bandgap position of each semiconductor, three types of band alignments could form: type-I band alignment or straddling gap, type-II band alignment or staggered gap, and type-III band alignment or broken gap. 87 Due to the bandgap size and bandgap position of TMDCs, by the heterostructure manufacturing, type-II bandgap alignment would form, which causes a reduction in bandgap size. Figure 1-20 illustrates schematically different types of band alignment in semiconductors.

Figure 1 - 19 :

 119 Figure 1-19: Schematic illustration of in -plane and out-of-plane heterostructure.

Figure 1 - 20 :

 120 Figure 1-20: Three types of band alignments in heterostructures. In TMDC heterostructures, usually band alignment type II happens.[START_REF] Hu | Two-dimensional van der Waals heterojunctions for functional materials and devices[END_REF]

  Figure 1-22 shows the PXRD diffractograms of 1T' and 2H-WS2.

Figure 1 - 21 :

 121 Figure 1-21: 1T'-WS2 thin film in the XPS chamber.

Figure 1 -

 1 Figure 1-22: Powder X-ray diffractograms of a) calculated bulk diffraction peaks for a hypothetical distorted 1T'-WS2 b) 1T'-WS2 nanosheets, c) 2H-WS2 nanostructures, and d) bulk diffraction peaks of 2H-WS2.

Figure 1 - 23 :

 123 Figure 1-23: The schematic of an electrocatalysis device used for water splitting

  Bandgap size and bandgap position of different TMDC semiconductors are shown in Figure 1-25.

Figure 1 - 24 :

 124 Figure 1-24: Current stability check of a single layer MoS 2 phototransistor with and without emission of light. 6

Figure 1 - 25 :

 125 Figure 1-25: Bandgap size and bandgap position of different TMDC semiconductors.[START_REF] Li | Heterostructures based on two-dimensional layered materials and their potential applications[END_REF]

Figure 1 -Chapter 2 :

 12 Figure 1-25: voltage vs current hysteresis loop diagrams of WS 2 and WSe2. 92

Figure 2 -

 2 1 represents a schematic of the formation of a nucleus.A cluster is stable after reaching the critical size.

Figure 2 -

 2 2 shows the change in the system energy as the function of the size of the nucleus. As it has been shown, energy of a system would reduce when the nuclei grow.

Figure 2 - 1 :

 21 Figure 2-1 : Schematic illustration of nucleation formation.3

Figure 2 -

 2 3 displays the heterogeneous formation of the nucleus.

Figure 2 - 1 :

 21 Figure 2-1: Description of the classic nucleation and growth process.3

Figure 2 - 2 :

 22 Figure2-2: Growth of embryo on the nucleating particle. The most important factor that controls the growth rate is the contact angle.3

Figure 2 - 3 :

 23 Figure 2-3: Supersaturation causes an exponentially rise in the nucleation rate.3

Figure 2 -

 2 5 depicts the diagram of solution concentration versus time for the LaMer mechanism.

Figure 2 - 4 :

 24 Figure 2-4: Schematic diagram of LaMer mechanism. In the stage (I) the rapid increase in the concentration happens, (II) burst of nucleation, (III) growth of nuclei happens by the reduction in the concentration.2

Figure 2 -

 2 6 and 2-7 schematically illustrate the Ostwald ripening and digestive ripening, respectively.

Figure 2 - 5 :

 25 Figure 2-5: Schematic illustration of Ostwald rip ening.7

Figure 2 -

 2 8 displays the two steps mechanism of Finke-Watzky.

8

 8

Figure 2 - 6 :

 26 Figure 2-6: Schematic illustration of digestive ripening. Step1: polydisperse nanoparticles are formed. Step2: larger nanoparticles dissolve and smaller ones grow. Step3: monodisperse nanoparticles form. 7

Figure 2 - 7 :

 27 Figure 2-7: Schematic presentation of Finke -Watzky mechanism. On the right the first graph shows the change in the concentration of cyclohexane by time, and the second graph shows the size distribution in the iridium nanoparticle synthesis.8

Figure 2 -

 2 9 represents the different parts of a colloidal synthesis.

Figure 2 - 8 :

 28 Figure 2-8: Different parts of a solution-based synthesis are demonstrated. Part 1 is the reaction mixture and part 2 is the injection mixture.10

Figure 2 -

 2 10 shows the schematic presentation of ligands on the surface.

Figure 2 - 9 :

 29 Figure 2-9: Schematic illustration of ligands on the surface of a nanoparticle. 10

Figure 2 - 10 :

 210 Figure 2-10: Classification of different 2D semiconducting nanomaterials colloidal synthesis.13

Figure 2 -

 2 12 represents different structures produced by changing the solvent. One of the most important advantages of colloidal synthesis for CVD and PVD is that the procedure happens at a lower temperature.

Figure 2 -

 2 13 displays a TEM image of 2-5 nm nanoclusters of TiS2.

  TEM images of TiS2 nanodisks and CuS2 toroids are presented Figure 2-15.

Figure 2 -

 2 Figure 2-14: TEM images and size distribution diagrams for di fferent sizes of nanodisks.16

Figure 2 -

 2 Figure 2-16: (a) Low-magnification TEM image, (b) high-magnification TEM image, (c) side-view TEM image, and (d) atomically resolved HRTEM image of TiS 2 nanosheets. (e) Low-magnification TEM image, (f) high-magnification TEM image, (g) side-view TEM image, and (h) atomically resolved HRTEM image of CuS2 toroids.17

Figure 2 -

 2 16 shows low-resolution and high-resolution TEM images of multilayer TiS2.

Figure 2 -

 2 Figure 2-15: TEM images of multilayer TiS2 with (a) low magnification, (b) high magnification, (c) SAED pattern of multilayers, (d) edge of a single layer, and (e) SAED pattern of a single layer.18

Figure 2 -

 2 Figure 2-17: (a) XRD pattern, (b), (c) low and high resolution TEM images, and (d) EDS spectrum of TiS2.19

Figure 2 -

 2 18 represents SEM and TEM images of TiSe2 thick platelets.

Figure 2 -

 2 Figure 2-18: (a) SEM image of TiSe2 platelets. (b), (d) Low-magnification TEM image of TiSe2 platelets. (c) High-magnification TEM image of TiSe 2 platelets.20

Figure 2 -

 2 20 illustrates the ZrS2 products when the source of sulfur is CS2 and DDT.

Figure 2 -

 2 Figure 2-19: XRD pattern and SEM images of TiSe 2 platelets with different mean size 21 .

Figure 2 -

 2 21 shows the schematic and TEM images of VS2, VSe2, TaS2, and TaSe2.

Figure 2 - 20 :

 220 Figure 2-20: The schematic presentation of ZrS 2 nanosheets formation with different precursors. (a) DDT is used as the source of sulfur, (i) TEM image of nanosheets, (ii) single and double layers are defined using the difference in contrast, (iii) HRTEM and FFT pattern of ZrS2 nanosheets, (iv) SAED pattern of ZrS2 nanosheets, and (v) AFM image approves there is a monolayer. (b) CS2 is used as the source of sulfur, (i) TEM image of nanodisks of ZrS 2, and (ii) TEM image of the side view of the nanodisks of ZrS 2. 22

Figure 2 -

 2 22 illustrates the changes in the final product by changing the composition of the reaction solvent. They managed to produce some well-dispersed nanosheets but without any control on the mean size.

Figure 2 - 21 :

 221 Figure 2-21: Schematic representation, low -resolution TEM image, high resolution TEM image, and side view TEM image of (a) VS 2, (b) VSe2,( c) TaS2, and (d) TaSe2.23

  Figure2-24 shows the SEM and TEM images of MoS2 nanosheets. Although they alleged this protocol leads to formation of nanosheets with control on number of layers, the mean size and dispersion is not controlled.

Figure 2 -

 2 Figure 2-23: (a) TEM image of aggregated nanosheets of MoS 2. (b) XRD patterns of MoS2 produced by molybdenum hexacarbonyl (red) and molybdenum oxide (black).25

Figure 2 -

 2 Figure 2-24: (a) SEM image of MoS2 nanosheets. (b) Low magnification image o f MoS2 nanosheets. (c) High magnification TEM image of MoS 2 nanosheet. (d) sideview of stacked nanosheets.27

Figure 2 -

 2 Figure 2-25: (a) XRD diffractogram of as-synthesized MoS2 nanosheets. (b) The absorption spectrum of MoS2 nanosheets dispersed in toluene. (c), (d) HAADF -STEM image of MoS2 nanosheets support the interdistance between two layers is 0.65 nm and the structure is hexagonal.28

Figure 2 -

 2 27 illustrates the TEM images of the MoSe2 nanoflowers with different mean sizes. They managed to control the size of nanoflowers, but the nanoflowers are severely aggregated and polydisperse. Another protocol to synthesize nanoflowers was developed in 2017 by Guo et al. for electrocatalytic hydrogen evolution activity. They made a solution of ODE-Se by adding elemental selenium into ODE and stirring for 1 hour at 200°C. In a dropwise synthesis, they dissolved molybdenum hexacarbonyl into OAm, heated up to 200°C, and then added slowly ODE-Se into the

Figure 2 -

 2 Figure 2-26: (a) Low-resolution TEM image of single -layer MoSe2 nanosheets. (b) Mono and bilayers are defined due to the different contrast. (c) High -resolution TEM image of MoSe2 nanosheet. (d) The SAED-pattern of MoSe2 nanosheets. (e) AFM image of a single layer nanosheet. (f) Raman spectra of the bulk and single la yer of MoSe2.30

Figure 2 -

 2 Figure 2-27: TEM images of MoSe2 nanoflowers after (a), (b) 15 min, (c), (d) 20 min, (e), (f) 30 min, (g),and (h) 40 min.31

Figure 2 -

 2 29 shows TEM images of the MoSe2-Pt nanoflowers. Platinum nanoparticles are attached to the MoSe2 sheets.

Figure 2 -

 2 Figure 2-28: TEM images of MoSe2 nanoflowers with different ratios of OAm and OA as the reaction solvent.32

  Figure 2-30 shows the absorption spectrum and TEM image of MoS2 and MoSe2 nanosheets.35

Figure 2 - 29 :

 229 Figure 2-29: Low and high magnification TEM images of MoSe 2-P t nanoflowers.33

Figure 2 -

 2 Figure 2-30: a, c) Absorption spectra of MoS 2 and MoSe2. b, d) TEM Images of MoS2 and MoSe2.35

Figure 2 -

 2 31 represents the TEM images and hydrogen production of 1T and 2H-WS2. STEM images illustrate the nanosheets are severely defected. In 2016, Zhou et al. reported a new colloidal protocol to synthesize size and layer numbers controlled WS2 nanosheets.

Figure 2 -

 2 32 illustrates TEM images of multilayers of WS2. TEM images show that the products are not well-dispersed nanosheets and there is no control on their size.

Figure 2 -

 2 Figure 2-31: (a) low-magnification TEM and (b) HAADF-HRSTEM images of 1T-WS2. (c) low-magnification TEM and (d) HAADF-HRSTEM images of 2H-WS2. (e) Hydrogen production rate of TiO2, 1TWS2-TiO2, and 2HWS2-TiO2.35

Figure 2 -

 2 33 represents TEM and STEM images of WS2 multilayers. Although they developed a protocol to produce monodisperse nanosheets, the multilayer nanosheets are completely aggregated.

Figure 2 -

 2 Figure 2-32: (a) low-magnification and (b) high -magnification TEM images of 5 layered WS2 nanosheets.[START_REF] Han | Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures[END_REF]

Figure 2 -

 2 34 shows the TEM image, AFM image, Raman spectroscopy, and absorption spectrum of 2H-WS2 multilayers.

Figure 2 -

 2 Figure 2-33: (a), (f) Low-magnification TEM image of WS2 nanoparticles and nanochains, respectively. (b), (d) High-magnification TEM image of WS2 nanoparticles. (g) HAADF-STEM of WS2 nanochains.37

Figure 2 -

 2 Figure 2-34: (a) High-resolution TEM image of WS2 multilayer, top right the FFT pattern and bottom right the filtered HRTEM image. (b) AFM image of WS 2 multilayers spin coated on a silicon substrate. (c) Raman spectra of bulk and synthesized WS 2. (d) The absorption spectrum of WS2 multilayers dispersed in dichlorobenzene.38

Figure 2 -

 2 Figure 2-35: (a) Low-resolution TEM image, (b) ADF-STEM image, (c) HAADF-HRSETM image of WS2 nanoflakes. Atomically resolved HAADF -STEM d) 1T'-WS2, (e) 2H-WS2. (f), (g), and (h) simulated and experimental XRD patterns of 1T' and 2H-WS2.39

Figure 2 -

 2 37 shows the schematic presentation of a single WSe2, TEM images, XRD patterns, and absorption spectra of WSe2 nanosheets with a different number of layers.

Figure 2 -

 2 Figure 2-36: TEM and HRTEM of single and multilayers of WS 2 nanosheets.42

Figure 2 -

 2 Figure 2-37: (a) Schematic illustration of single layer WSe 2. (b) The chemical equation is presented. (c) TEM images of WSe2 nanosheets synthesized with: (i), (i') OAm, (ii), (ii') OAl, and (iii), (iii') OA. d) XRD diffractogram of WSe 2 nanosheets synthesized with OAm (top), OAl (middle), and OA (bottom). (e) Absorption spectra of WSe 2 nanosheets synthesized with OAm (purple), OAl (orange), and OA (red).30

Figure 2 -

 2 Figure 2-38: TEM image of multilayers of WSe 2 nanosheets.45

Figure 2 -

 2 Figure 2-39: (a) Low-resolution TEM image, (b) High-resolution TEM image, c) High-resolution TEM image defined in the image b, of WSe 2 nanoflower. (d) The absorption spectrum of WSe 2 nanoflower. (e), (f) XPS spectra of W 4f and Se3d of WSe2 nanoflower.41

Figure 2 -

 2 Figure 2-40: TEM images of a) MoS2 nanosheets, b) MoSe2 nanosheets, c) WS2 nanosheets, and d) WSe2 nanosheets.42

  -41, STEM images of two alloys of MoWSe2 and WSSe are represented. The nonhomogeneity of contrast in STEM images shows that the products are formed as multilayer nanosheets.

Figure 2 -

 2 Figure 2-41: a) ADF-HRSTEM image of Mo0.35W0.65Se2 multilayers, b) defects are specified in the atomically resolved ADF-HRSTEM image of Mo0.35W0.65Se2. c) ADF-HRSTEM image of WS0.88Se1.12 multilayers, d) atomically resolved ADF -HRSTEM image of WS0.88Se1.12 multilayers.43

Figure 2 -

 2 Figure 2-42: a) HAADF-STEM and b) ADF-HRSTEM images of Mo0.39W0.61Te2 nanostructures.44

Figure 2 -

 2 42 shows HAADF and ADF-HRSTEM images of Mo0.39W0.61Te2.

Figure 2 - 50 Chapter

 250 Figure 2-43: TEM image of a) MoSe2, b) WSe2, c) MoSe2-WSe2 heterostructures.50

Figure 3 -

 3 Figure 3-12: a) UV-Vis spectrum of WS2 nanosheets synthesized by pure oleylamine protocol. b) PXRD diffractogram of WS2 nanosheets synthesized by pure oleylamine protocol, with inset showing the magnified region between 30°-40°. c, d) TEM images of aggregated WS 2 nanosheets synthesized by the pure oleylamine protocol.

Figure 3 - 2 :

 32 Figure 3-2: a) absorption spectra of 1T'-WS2 synthesized with different thiols as coordinating agents. b-d) TEM images of well-dispersed NMLs of 1T'-WS2 synthesized with, respectively, dodecanethiol (DDT), hexadecanethiol (HDT), and octadecanethiol (ODT) as the coordinating agent. formation.

Figure 3 - 3 :

 33 Figure 3-3: PXRD patterns of 1T'-WS2 synthesized by DDT (green line), HDT (red line), and ODT blue line as the coordinating agent. The area between 30° to 38° is zoomed in to define the crystal struc-ture. Lack of a peak at 32.8° confirms the crystal structure is

Figure 3 - 2 :

 32 Figure 3-2: b displays TEM images of the well-dispersed WS2 NMLs obtained using dodecanethiol (DDT).The homogenous contrast of different nanosheets supports that they are well-dispersed NMLs. However, Figure3-4: a shows that regions exhibiting minor aggregation exist. Thiols with longer chains, specifically hexadecanethiol (HDT) and octadecanethiol (ODT), are also explored to investigate the effect of thiols on the nanosheets synthesis and dispersion. TEM images (Figures3-2: c, d) demonstrate that increasing the chain length of the alkanethiol decreases the mean-size of the NMLs and improves the dispersion. However, Figures3-4: b, cshow that the nanosheets synthesized by HDT are partially aggregated, whereas the use of ODT as the coordinating agent leads to the formation of well-dispersed nanomonolayers. Using thiols as the coordinating

Figure 3 - 4 :

 34 Figure 3-4: TEM images of 1T'-WS2 nanosheets synthesized by a) DDT, b) HDT, and c) ODT as coordinating agent. Black circles define the aggregations.

Figure 3 - 5 :

 35 Figure 3-5: Size dispersion of WS2 nanosheets synthesized with different alkylthiols and with oleylamine only.

Figure 3 - 7 :

 37 Figure 3-7: SAXS diffractograms of aggregated WS2 nanosheets synthesized with different thiols as the coordinating agent. Ethanol was added to the well-dispersed NMLs to induce aggregation.

Figure 3 -

 3 Figure 3-9: FTIR spectra of ODT (gray), OAm (red), WS 2 synthesized by OAm (black), WS2 synthesized by ODT (blue), and tungsten thiolate (violate).

Figure 3 -

 3 Figure 3-10: TGA spectra of different tungsten thiolates and WS 2 nanosheets synthesized by DDT, HDT, ODT, and OAm protocols. It should be noted the W(DDthiolate) 6 is mixed with TiO2, as the thermal decomposition of thiolates produces ashes, which damages the device .

Figure 3 -

 3 Figure 3-11: a) Thermogravimetric analysis of WS2 synthesized by ODT as the coordinating agent (red) and WS2 synthesized by pure OAm protocol (black). b) First derivation of TGA of WS 2 synthesized by ODT as the coordinating agent (red) and WS2 synthesized by pure OAm protocol (black).

33

 33

Figure 3 -

 3 Figure 3-12: TGA measurements of 1T'-WS2 synthesized with different coordinating agents.

Figure 3 -

 3 Figure 3-13: a) TEM image of WS2 nanosheets synthesized by ODT added into the injection mixture. b) UV-Vis absorption spectrum of WS2 nanosheets synthesized by ODT added into the injection mixture.

Figure 3 -

 3 Figure 3-14: PXRD diffractograms of WS2 nanosheets synthesized by ODT as the coordinating agent, as synthesized (black line) and after ligand exchange with octylamine (red line).

Figure 3 -

 3 Figure 3-15: TEM image of well-dispersed 1T'-WS2 NMLs with different mean sizes. Decreasing the amount of coordinating agent (ODT) as well as prolonging the injection time increases the mean size.

Figure 3 - 16 :

 316 Figure 3-16: Absorption spectra of 1T'-WS2 NMLs synthesized with different amount of ODT and different injection time.

Figure 4 -

 4 Figure 4-13: TEM images of well-dispersed NMLs when a) Dmean = 10 nm, b) Dmean = 12 nm, c) Dmean = 17 nm, d) Dmean = 22 nm, e) Dmean = 45 nm, f) Dmean= 60 nm.

Figure 4 - 14 :

 414 Figure 4-14: Size dispersion histograms resulting from different synthesis protocols.

Figure 4 -

 4 Figure 4-15: HAADF-STEM images of well-dispersed NMLs when a, b) Dmean = 10 nm, c, d) Dmean = 22 nm, e, f) Dmean = 60 nm. The zigzag patterns in the HAADF -HRSTEM images confirm the structure is 1T'.

Figure 4 - 4 :

 44 Figure 4-4: HAADF-HRSTEM images (a to j) of a nanosheet that undergoes phase change induced by the electron beam. The zigzag pattern represents the 1T' phase and the hexagonal pattern demonstrates the 2H. Mapping of the phase change progression (a' to j') corresponds to the STEM images, in which we calculate the number of neighbors for each atom, which defines the structure of the nanosheet. The yellow color represents the 1T' phase and the red color shows the 2H phase.

Figure 4 - 5 :

 45 Figure 4-5: The phase change temperature s resulting from DSC measurements for samples with different mean sizes. The dashed line represents the reported temperature for the 1T' to 2H phase change 207°C.

Figure 4 - 6 :

 46 Figure 4-6: Absorption spectrum of annealed 1T'-WS2 nanosheets in oleylamine for 19 hours without iodine treatment.

  -9b) clearly visible through the appearance of the characteristic excitonic features. Absorption spectra of annealed WS2 show that the different sized 2H-WS2 nanosheets have different first excitonic peaks. The bandgaps of different sized nanosheets were defined by determination of the first excitonic peak positions as a function of the mean diameter (Figure 4-10). The changes in the first excitonic peak represents the change in the bandgap. In relation to the bandgap size reported by Qixing Wang et al. (2.02 eV),

Figure 4 - 7 :

 47 Figure 4-7: TEM images of a) well-dispersed NMLs of WS2, b) WS2 nanosheets after iodine treatment, c) WS2 nanosheets after being annealed at 340°C for 19 hours in OAm.

Figure 16 - 8 :

 168 Figure 16-8: Photographs of dispersed nanosheets of 1T' -WS2 in toluene on the left and 2H-WS2 in a mixture of toluene and isopropyl alcohol on the right.

Figure 4 - 9 :

 49 Figure 4-9: UV-Vis spectra of a) as-synthesized nanosheets b) annealed nanosheets (19 hours at 340°C).

Figure 4 - 10 :Figure 4 -

 4104 Figure 4-10: Bandgap energy of annealed nanosheets with different mean sizes.

Figure 4 -

 4 Figure 4-12: XPS analysis of annealed nanosheets when D me a n = 10 nm. In the fitting, the red curves represent the peaks for the 1T' phase and the green curves show the 2H phase.

Figure 4 -

 4 Figure 4-13: XPS spectra of annealed nanosheets with differen t sizes: a) Dmean = 12 nm, b) Dmean = 22 nm, c) Dmean = 45 nm, d) Dmean= 60 nm.

Figure 5 - 17 :

 517 Figure 5-17: Absorption spectrum of diluted TiS2 solution.

Figure 5 -

 5 Figure 5-18: TEM image of TiS2 nanosheets with Dmean=140 nm.

Figure 5 - 4 :

 54 Figure 5-4: Absorption spectrum of diluted TiSe 2 solution.

Figure 5 - 3 :

 53 Figure 5-3: TEM image of hexagonal TiSe 2 multilayers.

Figure 5 -

 5 Figure 5-19: a) TEM image of WSSe nanosheets. b) HAADF-HRSTEM image of separated nanosheet of WSSe. c) Absorption spectra of WS2xSe2(1-x) before and after annealing on a thin film. d) XRD pattern of WS2xSe2(1-x) with different amount of selenium.

Figure 5 -

 5 6 shows the TEM image of WSe2 nanoflowers synthesized with elemental selenium. Due to the high reactivity of this source of selenium, the product forms as nanoflowers. The reactivity of selenium would reduce by compounding it. It should be noted, due to the formation of big aggregation of nanoflowers, UV-Vis spectroscopy was not useful.

Figure 5 -

 5 Figure 5-20: TEM image of nanoflowers synthesized by elemental selenium.

Figure 5 - 21 :

 521 Figure 5-21: Absorption spectrum of WSe 2 nanosheets synthesized by TOPSe.

Figure 5 - 8 :

 58 Figure 5-8: TEM image of small aggregated WSe2 nanosheets synthesized by TOPSe.

Figure 5 - 9 :

 59 Figure 5-9: Absorption spectrum of WSe2 nanosheets synthesized by ODSe.

Figure 5 -

 5 Figure5-10 shows the TEM image of WSe2 nanosheets synthesized by ODSe. Although the dispersion is not very good, the product formed can be identified as nanosheets.

5

 5

Figure 5 -

 5 Figure 5-22: TEM image of WSe2 nanhseets synthesized by ODSe.

Figure 6 - 24 :Figure 6 -

 6246 Figure 6-24: Absorption spectra of 1T'-WS2/2H-WS2 homostructure before (black curve) and after (red curve) the second injection.

  Figure 6-3 shows the absorption spectra of the seeds and the products after the second injection, which induced the formation of WSSe. The change in the absorbance supports the formation of another crystal structure.Although the change in the absorption of the as-synthesized heterostructure shows the formation of a new crystal structure, the exact crystal structure of the crown is not clear. Annealing the nanosheets at 340°C for 19 hours in OAm induces phase change, which could be used to determine the composition of the crown.

  Figure 6-4 represents the absorption spectrum of annealed WS2-WSSe

Figure 6 - 25 :

 625 Figure 6-25: Absorption spectra of WS2 core (red curve) and WS2/WSSe heterostructure (black curve) when 1.68 mL TOPSe is used as the source of selenium in the second injection .

Figure 6 - 26 :

 626 Figure 6-26: Absorption spectrum of annealed WS2-WSSe heterostructures when 1.68 mL TOPSe is used as the source of selenium in the second injection.

Figure 6 -

 6 Figure 6-27: TEM images of a) well-dispersed 1T'-WS2 NMLs used as the seeds. b) WS2-WSSe heterostructures. The scale bars show 100 nm.

Figure 6 -

 6 Figure 6-28: a) HAADF-STEM image of a single layer of WS2-WSSe heterostructure. The scale bar shows 20 nm. b) The difference in the contrast of core and crown is plotted.

Figure 6 - 9 :

 69 Figure 6-9: TEM images of a) WS2 (seeds) b) WS2/WSe2 heterostructure. The increase in the mean size confirms the formation of heterostructures.

ABSTRACT:

The difference between in-plane and out-of-plane bonding energy of transition metal dichalcogenides has provided the possibility of isolating single layers. A one step synthesis protocol to produce size-controlled single layers has always been challenging. Here we developed a new colloidal synthesis to produce monodisperse sizecontrolled 1T'-WS2 nano-monolayers with outstanding colloidal stability by using 1-octadecanethiol as the coordinating agent. Changes in the reaction time and amount of coordinating agent regulate the mean size of the nanosheets. We investigated the effect of octadecanethiol and injection rate on the dispersion and mean-size, using UV-Vis spectroscopy, X-ray diffraction techniques, and transmission electron microscopy. Furthermore, thermogravimetric analysis and Fourier transform infrared spectroscopy allow for ligands detection and analysis at the surface of the nanosheets. These results open a new pathway to synthesize, control and explore the properties of nanoscale transition metal dichalcogenides.

PXRD has been performed on a drop-casted sample on a low background silicon sample holder.

Abstract:

Phase engineering of group VI transition metal dichalcogenides (TMDCs) has drawn attention due to the versatile electronic and optoelectronic properties that are accessible through different crystal structures. These properties can be manipulated by controlling the phase of the material. Although for most TMDCs the semiconducting phase is more stable, the metastable crystal structure with semi-metallic properties can also be directly synthesized. Different studies have been performed on 2H to 1T' phase transformation, but there is still no fundamental examination of 1T' to 2H phase transformation. Here, we study the 1T' to 2H phase transformation of well-dispersed 1T'-WS2 nanomonolayers prepared through colloidal synthesis. The phase transformation is induced through annealing, and is confirmed through UV-Vis spectroscopy, X-ray photoemission spectroscopy, and X-ray diffraction. Differential scanning calorimetry reveals two surprising facts: that the phase transformation temperature is higher than that previously reported, and that the presence of surface ligands stabilizes the phase and further increases the phase transformation temperature. Transmission electron microscopy proves the phase transformation launches from the edges and is hindered by structural defects. Such defects prevent the phase transformation, and thus larger nanosheets possessing more defects show a greater resistance to the phase change. These results provide insights into the phase engineering of WS2, and help to unlock new properties in the promising library of TMDC materials.

Chapter 6: Heterostructures is pure. 21 Annealing the heterostructures can clarify the atomic composition of the crown also. Figure 8 represents the absorption spectrum of annealed WS2-WSe2 heterostructures.

Appearance of a peak at 780 nm indicates the formation of pure WSe2, as the bandgap is 1.6 eV the second injection induced pure WSe2 nanosheets. Although the formation of pure WSe2 as the

Chapter 7: Conclusion and perspectives

Colloidally stable nano-monolayers are developed, but to be able to use them, a single layer thin film or a column of nanosheets is needed. The auto assembly formation of a single layer of nanomonolayers on top of an anti-solvent or induction of lamellar aggregation by using a bad solvent is another interesting subject that could be studied in future.

The formation of lateral heterostructures depends on the energy of the system. There is a competition between edge growth and secondary nucleation on top of the seed. Choosing the right experimental parameters helps us to control the growth and secondary nucleation. Both lateral and lamellar heterostructures have unique properties. We managed to synthesize lateral heterostructures only, more study is needed to choose the best reaction parameters to have lamellar heterostructures also.

It is the first time that nano-heterostructures of TMDCs are synthesized and in comparison with the heterostructures that are in the range of micrometer, they have different properties. Due to the bandgap size and position, we already knew that band alignment type 2 happens. However, the exciton binding energy in TMDCs is an order of magnitude larger than other semiconductors. Then, the interface of heterostructures could form either a direct or indirect bandgap. STM and STS measurements on the interface of the heterostructures can illustrate the nature of the bandgap. Some studies have been done on the effect of WS2 nanosheets as electrodes for hydrogen evolution reaction (HER). They used lithium intercalation to make the metallic phase. Indeed, exfoliation methods lead to formation of large sheets with different number of layers. As the active site for HER is the edge of WS2 sheets, reducing the size increases the portion of edges. As it has already mentioned, due to the presence of defects, these 1T'-WS2 nanosheets are quite stable. So, they are good candidate to use as electrodes for HER.

French summary