WO2018231153A1

WO2018231153A1 - Synthesis of atomically-thin metal dichalcogenides

Info

Publication number: WO2018231153A1
Application number: PCT/SG2018/050296
Authority: WO
Inventors: Zheng Liu; Jiadong Zhou
Original assignee: Nanyang Technological University
Priority date: 2017-06-16
Filing date: 2018-06-14
Publication date: 2018-12-20
Also published as: TW201905231A

Abstract

The present invention relates to the manufacture of a wide range of two-dimensional transition metal dichalcogenide on a substrate using a salt-assisted chemical vapour deposition method, wherein the metal dichalcogenide is a binary, tertiary, quaternary, or quinary compound. The transition metal in the dichalcogenide compound is preferably titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, niobium, molybdynum, cadmium, hafnium, tantalum, tungsten, or rhenium, and the salt used in the method is an alkali metal halide, preferably sodium chloride and potassium iodide.

Description

Synthesis of Atomically-thin Metal Dichalcogenides

Field of Invention This invention relates to a method to synthesise a wide range of two-dimensional transition metal dichalcogenides.

Background The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Two-dimensional transition-metal dichalcogenides (TMDs) have demonstrated interesting physical phenomena, including the quantum spin Hall effect,^1,2 valley polarization^3,4 and two- dimensional superconductivity,⁵ suggesting their potential applications for functional devices,^6"10 universal band gap engineering and heterogeneous catalysis.^11"14 Although 2D TMDs are by far the largest family in the field of 2D crystals, only a limited number of them have been produced, commonly via chemical vapour deposition (CVD).

In the past few years, most of the synthetic work on TMDs has been focused on Group VI MX₂ (M: Mo, W; X: S, Se) compounds and more than forty TMDs are left to be explored. Many of them were predicted in theory to possess diverse and novel properties. For example, Group IV TMDs (Ti, Zr and Hf) are predicted to have properties of high mobility and phase transition, while Group V TMDs are predicted to possess ferromagnetic, superconducting and charge density wave effects. In addition, Group VI tellurides are predicted to behave like type-ll Weyl semimetals and topological insulators.

The Mo- and W-based TMDs have been synthesised, typically via sulfurisation^15"19, selenisation^20"21 and tellurisation²² of metals and metal compounds. Many other TMDs remain inaccessible due to the high melting points and low vapour pressure of the metal precursors (such as oxides of Ti, Zr, Hf, Nb, Ta, W, P and Pd).

Given the potentially useful properties and applications of TMDs, there remains a need to develop an effective and general synthetic method to produce a wide range of 2D TMDs. This will open up opportunities for studying the properties and potential application of a wide variety of 2D TMDs. Other than gaining access to the desired TMDs, there are inherent requirements on what an ideal synthesis method needs to demonstrate. In the first instance, it is important that the method is applicable to a wide range of TMDs and should allow the user to have the flexibility to control the properties of the materials accordingly. The method should also allow the production of high quality (with little or no defects), large size TMDs crystals which are highly desired for practical use.

Summary of Invention It has been surprisingly discovered that the use of salts in combination with a transition metal or metal oxide enables the facile formation of two-dimensional transition metal dichalcogenides. Thus, in a first aspect of the invention, there is provided a process of forming a two-dimensional transition metal dichalcogenide of formula I: MX₂ I

where:

M is selected from one or more of the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, Hf, Ta, W, and Re: and

X is selected from one or more of the group consisting of S, Se, and Te, the process comprising the step of depositing the transition metal dichalcogenide of formula I on a substrate by chemical vapour deposition, wherein the vapour deposition is accomplished by: passing a carrier gas through a fluid pathway in a furnace, where the fluid pathway comprises a first temperature zone housing one or more of elemental sulfur, selenium or tellurium and a second temperature zone that houses a substrate above a mixture of one or more metallic precursors and an alkali metal halide, where the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 1 :2 to 15:1 , where:

the temperature of the first temperature zone is from 150 to 500°C;

the temperature of the second temperature zone is from 350 to 950°C;

the substrate is from 0.01 to 2.0 cm above the mixture of one or more metallic precursors and the alkali metal halide; and

the one or more metallic precursors are selected from an elemental metal, a metal oxide or a metal halide, where each respective metal is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, Hf, Ta, W, and Re, wherein the growth rate of the two-dimensional transition metal dichalcogenide of formula I on the substrate is from 0.01 pm/s to 10 pm/s. Embodiments of the first aspect of the invention that may be generally applicable include those in which:

(a) the carrier gas may comprise an inert gas having a flow rate of from 30 to 200 seem and may optionally further comprise hydrogen, the hydrogen having a flow rate of from 1 to 20 seem, optionally the inert gas may be argon and/or nitrogen;

(b) the alkali metal halide may be selected from one or more of the group consisting of NaCI, KCI, LiCI, NaBr, KBr, LiBr, Nal, Kl, and Lil;

(c) the thickness of the deposited transition metal dichalcogenide may be from 1 nm to less than 1 μιη (e.g. from 1 nm to 500 nm);

(d) the process may further comprise laying one or more further layers of a two- dimensional transition metal dichalcogenide of formula I onto a substrate that has already undergone the deposition process described above, optionally wherein the two-dimensional transition metal dichalcogenide of formula I deposited in each further layer is different to the layer on the surface of the substrate;

(e) the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be from 1.5:1 to 15:1 , such as from 1.5:1 to 5:1 ;

(f) the flow rate of the carrier gas is varied during the process to provide an in- plane heterostructure. In embodiments of the first aspect of the invention, when the transition metal dichalcogenide of formula I is WS₂ the temperature of the second temperature zone is from 600 to 670°C, or when the transition metal dichalcogenide of formula I is WSe₂ the temperature of the second temperature zone is from 650 to 670°C. In certain embodiments of the invention, the transition metal dichalcogenide of formula I that is formed may be MoS₂ or MoSe₂. When the process relates to the formation of MoS₂, the temperature of the second temperature zone may be from 350 to 800°C (e.g. from 350 to 600°C) and the w wt ratio of the one or more metallic precursors to the alkali metal halide may be from 5:1 to 10:1. When the process relates to the formation of MoSe₂, the temperature of the second temperature zone may be from 550 to 900°C (e.g. from 550 to 650°C, such as from 550 to 600°C) and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be from 5:1 to 10:1.

In embodiments where the transition metal dichalcogenide of formula I that is formed is MoS₂ or MoSe₂:

(a) the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be 6:1 ; and/or (b) the metallic precursor may be Mo0₃; and/or

(c) the alkali metal halide may be NaCI.

In certain embodiments of the invention, the transition metal dichalcogenide of formula I that is formed may be one selected from the group consisting of PdTe_2> PdSe₂, PdS₂, PtTe₂, PtSe₂, and PtS₂₎ where the temperature of the second temperature zone in the process may be from 750 to 860°C and the w wt ratio of the one or more metallic precursors to the alkali metal halide may be from 8:1 to 12:1 (e.g. 10:1). In these embodiments, the alkali metal halide may be NaCI. In embodiments where the transition metal dichalcogenide of formula I contains Pd, the metallic precursor may be PdCI₂. In embodiments where the transition metal dichalcogenide of formula I contains Pt, the metallic precursor may be PtCI₂.

In certain embodiments of the invention, the transition metal dichalcogenide of formula I that is formed may be one selected from the group consisting of HfTe₂, HfSe₂, HfS₂, VTe₂, VSe₂, VS₂₎ TiTe₂, TiSe₂, TiS₂, NbTe₂, NbSe_2l NbS₂, ZrTe₂, ZrSe₂, ZrS₂, TaTe₂, TaSe₂, TaS₂, MoTe₂, and WTe₂, where the temperature of the second temperature zone in the process may be from 600 to 860°C and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be from 1.6:1 to 4:1. In these embodiments, the alkali metal halide may be NaCI. In embodiments where the transition metal dichalcogenide of formula I contains Hf, the metallic precursor may be Hf. In embodiments where the transition metal dichalcogenide of formula I is selected from V, Ti, Nb, Zr, Ta, Mo, W, the metallic precursor may be a metal oxide of the respective metal.

In certain embodiments of the invention, the transition metal dichalcogenide of formula I that is formed may be one selected from the group consisting of FeS₂, FeSe₂ or FeTe₂, the temperature of the second temperature zone in the process may be from 500 to 850°C (e.g. from 500 to 600°C) and the wtwt ratio of the one or more metallic precursors to the alkali metal halide may be from 4:1 to 6:1 , such as 5:1. In these embodiments, the alkali metal halide may be NaCI and/or LiCI. In embodiments where the transition metal dichalcogenide of formula I contains Fe, the metallic precursor may be a metal oxide (e.g. Fe₂0₃) or a metal chloride (e.g. FeCI₂).

In certain embodiments of the invention, the transition metal dichalcogenide of formula I that is formed may be an alloy that has the formula II:

AaBbCcDdEeFfGgShSeJe_j II

where: each of A to G is a different metal selected from one of the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, Hf, Ta, W, and Re;

each of a to g is independently 0 to 0.99 and the sum of a+b+c+d+e+f+g is 1 ; and each of h to j is 0 to 2 and the sum of h+i+j is 2,

provided that at least two of a to g is greater than 0 and/or at least two of h to j is greater than 0. In certain embodiments, the formation of the alloy may be accomplished using a process where the temperature of the second temperature zone is from 600 to 850°C and/or the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 5:1 to 7.5:1 , such as 5:1. Suitable transition metal dichalcogenides of formula II that may be formed from the process described above may be selected from the group consisting of MoSe2xTe₂(i-x), Mo_1-xRe_xS₂, MoS_2xTe2(i-x), Moi__xNb_xSe₂, Moi_-xNb_xS₂, Μο_χ ^. _xS_2ySe₂₍₁._y), WS_2xTe₂₍₁._x), WSe_2xTe₂₍₁._x), Mo_1-xW_xTe₂, NbS_2xSe₂₍₁._x), W_1-xNb_xS₂, W₁._xNbxSe₂, and V_xW_yMO_(1-x._y)S_2zSe₂₍₁._z). It will be appreciated that in each case x, y and z are greater than zero and less than 1.

In a second aspect of the invention, there is provided a transition metal dichalcogenide alloy of formula II:

A_aB_bC_cD_dEeF_fG_gS_hSeiTe_j II

where:

each of A to G is a different metal selected from one of the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, Hf, Ta, W, and Re;

provided that at least two of a to g is greater than 0 and/or at least two of h to j is greater than 0.

In an embodiment of the second aspect of the invention, the transition metal dichalcogenide of formula II may be selected from the group consisting of MoSe_2xTe₂₍ ._x), Mo_1-xRe_xS₂, MoS_2xTe2_(1-x), Moi_-xNb_xSe₂, Mo_1-xNb_xS₂, Mo_xNbi_-xS_2ySe₂₍₁._y), WS_2xTe₂₍i-_X), WSe_2xTe₂(i-_x)) Mo_1-xW_xTe₂, NbS_2xSe₂₍₁._x)> W_1-xNb_xS₂, W₁._xNbxSe₂, and V_xW_yMo₍₁._x-y)S_2zSe₂₍₁._z). It will be appreciated that in each case x, y and z are greater than zero and less than 1.

Brief Description of Drawings

Fig. 1 Depicts the flow chart of the general growth process for the production of TMDs by the chemical vapour deposition method. The growth of 2D TMDs can be classified into four routes based on different mass flux of metal precursor and growth rate. High mass flux of metal precursor offers the opportunity to synthesize large-scale continuous monolayer polycrystalline films with small (route I) or large (route II) domains depending on the growth rate. On the other hand, low mass flux of metal precursor results in discrete single-crystalline monolayers with different sizes. Low growth rate leads to small crystal size with atom clusters decorated in the center and edge of the monocrystal (route III), while high growth rate gives rise to large monocrystals (route IV).

Fig. 2 Depicts: (a) the chemical vapour deposition (CVD) setup for the growth of TMD monolayers; and (b) the expanded view of the aluminium oxide boat 15 and the Si/Si0₂ wafer 20.

Fig. 3 Depicts the optical images of 47 TMDs synthesized using the method in this application. These include 32 binary 2D crystals, 11 ternary alloys, a quaternary alloy, a quinary alloy, a 1 T MoTe₂-2 H MoTe₂ in-plane and MoS₂-NbSe₂ vertically stacked heterostructures.

Fig. 4 Depicts the optical images, Raman and photoluminescence characterisations of MoX₂ (X: S, Se and Te): (a, d and g) show the optical image of MoS₂, MoSe₂ and MoTe₂ respectively; (b, e and h) show their respective Raman spectra; and (c and f) show the photoluminescence spectra of MoS₂ and MoSe₂.

Fig. 5 Depicts the optical images, Raman and photoluminescence characterisations of WX₂ (X: S, Se and Te): (a, d and g) show the optical image of WS₂, WSe₂ and WTe₂ respectively; (b, e and h) show their respective Raman spectra; and (c and f) show the photoluminescence spectra of WS₂ and WSe₂.

Fig. 6 Depicts the optical images and Raman characterisations of TiX₂ (X: S, Se and Te): (a, c and e) show the optical image of TiS₂, TiSe₂ and TiTe₂ respectively; and (b, d and f) show their respective Raman spectra.

Fig. 7 Depicts the optical images and Raman characterisations of ZrX₂ (X: S, Se and Te): (a, c and e) show the optical image of ZrS₂, ZrSe₂ and ZrTe₂ respectively; and (b, d and f) show their respective Raman spectra.

Fig. 8 Depicts: (a, b and c) the optical images of MoS_2xTe₂₍₁._X), MoSe_2xTe₂₍i_-X) and WS_2xTe₂₍i. _X) respectively; and (d, e and f) the respective Raman spectra. Fig. 9 Depicts: (a and c) the optical images of quaternary alloy M0xNb_1-xS_2ySe₂₍i-y₎ and the quinary alloy V_xW_yMo₁._x._yS_2ZSe₂₍i-_Z) respectively; and (b and d) the respective Raman spectra. Fig. 10 Depicts the optical images and Raman characterisations of heterostructures: (a and b) optical image and Raman spectrum of 1 ΜοΤβ₂-2Η MoTe₂ in-plane heterostructure; (c and d) optical image and Raman spectrum of MoS₂-NbSe₂ vertically stacked heterostructure.

Fig. 11 Depicts the STEM images and the corresponding fast Fourier transform patterns and atomic structural models of (a) MoS₂ in the 1 H phase; (b) PtSe₂ in the 1 T phase; (c) WTe₂ in the 1 T' phase; and (d) ReSe₂ in the 1 T" phase.

Fig. 12 Depicts: (a) the EDS spectrum of MoTe₂; (b and c) atomic resolution STEM image and EELS spectrum of 2H MoTe₂ respectively; and (d and e) the STEM image and EELS spectrum of 1T MoTe₂.

Fig. 13 Depicts: (a) the atomic resolution STEM image of a few-layer WTe₂; (b) the EDS spectrum; and (c) the EELS spectrum of WTe₂. Fig. 14 Depicts: (a) the atomic resolution STEM image; (b) the EDS spectrum; and (c) the EELS spectrum of TiS₂ monolayer.

Fig. 15 Depicts: (a) the atomic resolution STEM image of a few-layer ZrS₂; (b) the EDS spectrum; and (c) the EELS spectrum of ZrS₂.

Fig. 16 Depicts: (a) STEM image (left) of the quinary monolayer alloy V_xW_yMoi-_x-_yS_2zSe₂₍₁-_z), with the corresponding atomic model from atom-by-atom intensity mapping (right); (b) EDS spectra of the alloyed monolayer, confirming its chemical composition; and (c) line intensity profiles along dashed lines A (representing cation) and B (representing anion) in (a), indicating the different intensity of each chemical species.

Fig. 17 Depicts the intensity histogram of the V_xW_yMo_1-x-yS_2zSe₂₍₁._z) quinary alloy monolayer and EELS of the V atoms inside the lattice: (a) STEM image of Fig. 16a with a larger field of view; (b) respective intensity fitting of the cation and anion sites using the Gaussian models. The position of the peak intensity is highlighted with markers; (c and d) intensity histogram of the cation (c) and anion (d) sites. The image intensity of different chemical species maintains a Gaussian distribution that is well separated, giving the ground to the correct assignment of the chemical composition of each atomic columns. The local concentration of each chemical species based on atom counting in this image: cation - V 3.7%, Mo 95.6%, W 0.7%; anion - S 93.6%, Se 6.4%; and (e) EELS of a single atom in the cation site that has the lower intensity, showing the characteristic V L2,3 ionization edge which confirms the presence of V.

Fig. 18 Transport measurements of monolayer NbSe₂ and MoTe₂ 2D crystals: (a and c) temperature dependence of longitudinal resistance R_xx in a zero magnetic field from 300 K to 0.26 K of the monolayer NbSe₂ and MoTe₂. Inset shows an expanded view of the low- temperature data; (b) temperature dependence of the longitudinal resistance R_xx in different magnetic fields applied perpendicular to the NbSe₂ crystal plane; and (d) longitudinal resistance R_xx as a function of magnetic field B at different temperatures for MoTe₂.

Fig. 19 Depicts two-dimensional superconductivity in MoTe₂: (a) magnetic field dependence of resistance under different tilt angles; and (b) angular dependence of upper critical fields H_c2(d). The inset shows the arrangement of the experimental configuration.

Fig. 20 Depicts temperature dependence of the upper critical field H_c2 of (a) NbSe₂; and (b) MoTe₂. The solid line is the linear fit to H_c2. Fig. 21 Depicts transport measurements of monolayer MoS₂ and ReS₂: (a and b) show the respective l_d-V_d and l_COnductivity-Vg plots of monolayer MoS₂; and (c and d) show the respective l_d-V_d and l_d-V_g plots of monolayer ReS₂.

Fig. 22 Depicts the TG-DSC curves of salts mixed with the metal sources: (a) shows that the melting points of the systems after adding salt are all within the highlighted windows from 600 °C to 850 °C; and (b) shows the thermogravimetry versus time curves.

Fig. 23 Depicts the XPS spectra which reveal the existence of CI bonds to other elements, resulting from the intermediate products during the synthesis of Nb-, Mo- and W-based 2D crystals. XPS spectra of Nb 3d, Mo 3d and W 4 f are shown in (a), (b) and (c), respectively. Nb 3d³'² and 3d⁵'², Mo 3d³'² and 3d⁵'² and W 4f⁵¹² and 4f^m are the core level energy states of Nb, Mo and W, respectively, all of which are well fitted with Gaussian peaks at energies indicating the bonding to CI. Fig. 24 Depicts the comparison of nucleation of TMD with and without salt: (a, c, and e) show the SEM images of the nuclei during synthesis of ReX₂, TiX₂, and WX₂ with salt promoter; and (b, d, and f) show the SEM images of the nuclei during synthesis of ReX₂, TiX₂, and WX₂ without salt promoter.

Fig. 25 Depicts the optical images of MoS₂ and growth rate without NaCI. The inset shows the size evolution of MoS₂ single crystal monolayers and the main panel is the fitting results of single crystal size over distance.

Fig. 26 Depicts the optical image of MoS₂ and growth rate with NaCI. The inset shows the size evolution of MoS₂ single crystal monolayers and the main panel shows the fitting results of single crystal size over distance.

Fig. 27 Depicts the optical images of MoS₂ of various morphologies obtained by controlling the nucleation and growth rate. The corresponding weight of MoCVNaCI sources used are: (a) 2mg/0.2mg; (b) 3mg/0.3mg; and (c) 10mg/0.8mg. An increasing weight ratio of salt over M0O₃ increases the nucleation density for the growth of MoS₂, thus the distribution of mono- layered MoS₂ transits from (a) low-density single crystal to (c) a continuous film.

Fig. 28 Depicts the optical images of MoS₂ of various thickness obtained by controlling the nucleation and growth rate. Nuclei of different sizes can be obtained by varying distance between the substrate and the MoCVNaCI sources: (a) 1.2 cm; (b) 0.8 cm; and (c) 0.6 cm.

Fig. 29 Depicts (a and b) optical images of MoS₂ and MoSe₂ grown at 750 °C, and (c) MoTe₂ grown at 700 °C; (d, e, and f) optical images of WS₂, WSe_2l and WTe₂ grown at 800 °C, 810 °C and 820 °C respectively; and (g, h, and i) optical images of NbS₂, NbSe₂ and NbTe₂ grown at 780 °C, 790 °C and 800 °C respectively.

Fig. 30 Depicts optical images of some 2D TMDs which show that most of the flakes are monolayer: (a) TiS₂ grown at 800 °C; (b) NbS₂ grown at 780 °C; (c) NbSe₂ grown at 790 °C; (d) PtSe₂ grown at 810 °C; (e) WTe₂ grown at 810 °C; and (f) MoTe₂ grown at 700 °C.

Fig. 31 Depicts the optical images of PtSe₂ grown at (a) 815 °C and at (b) 800 °C; and VTe₂ grown at (c) 750 °C and at (d) 700 °C.

Fig. 32 Depicts the linear sweep voltammograms of Vo.o3Wo.2Moo.77Si.₆Se₀.4 in comparison with MoS₂, carried out using a typical three-electrode cell consisting of a working electrode, a graphite carbon counter electrode and a saturated calomel reference electrode (SCE), with 0.5 M H₂S0₄ as the electrolyte. Fig. 33 Depicts the temperature dependence of square resistance of various samples of MoSe_xTe₂-_x containing different ratios of Se to Te. This shows a transition from semiconductor to metal with a decreasing ratio of Se to Te.

Description

As noted above, the inventors have surprisingly found a generic method to manufacture a wide range of two-dimensional transition metal dichalcogenides (TMDs) in a rapid manner and a low temperature. This process can be used to deposit a monolayer or a few layers of TMD onto a substrate by controlling the weight of the source materials. This process enables TMD crystals having a thickness of from 1 nm to less than 1 pm to be formed expeditiously. Thus, there is provided a process of forming a two-dimensional transition metal dichalcogenide of formula I:

MX₂ I

where:

X is selected from one or more of the group consisting of S, Se, and Te, the process comprising the step of depositing the transition metal dichalcogenide of formula I on a substrate by chemical vapour deposition, wherein the vapour deposition is accomplished by: passing a carrier gas through a fluid pathway in a furnace, where the fluid pathway comprises a first temperature zone housing one or more of elemental sulfur, selenium or tellurium and a second temperature zone that houses a substrate above a mixture of one or more metallic precursors and an alkali metal halide, where the wt.wt ratio of the one or more metallic precursors to the alkali metal halide is from 1 :2 to 15:1 , where:

the temperature of the first temperature zone is from 150 to 500°C;

the temperature of the second temperature zone is from 350 to 950°C;

the one or more metallic precursors are selected from an elemental metal, a metal oxide or a metal halide, where each respective metal is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, Hf, Ta, W, and Re, wherein the growth rate of the two-dimensional transition metal dichalcogenide of formula I on the substrate is from 0.01 μηΊ/s to 10 μιτι/s.

When used herein, the word "comprising" may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word "comprising" may also relate to the situation where only the components/features listed are intended to be present (e.g. the word "comprising" may be replaced by the phrases "consists of or "consists essentially of). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word "comprising" and synonyms thereof may be replaced by the phrase "consisting of or the phrase "consists essentially of or synonyms thereof and vice versa.

It will be appreciated that the TMDs formed using this process may relate to materials containing a single metal and a single chalcogenide (e.g. TaS₂), but also enables the formation of any suitable alloy (e.g. TaSo.₈Se₀.2, Hfo.3Tao.7S2, Fe₀.3Vo.3Pto.4So.5Seo.2Teo.₃)_> where the alloy may contain up to six metals and multiple chalcogenides (i.e. S, Se, Te).

The term "houses" when used herein may refer to material(s) deposited directly onto a particular region of the fluid pathway, or it may refer to the use of a suitable container/vessel that contains the desired material(s) situated within a particular region of the fluid pathway.

As noted above, the process relies on the use of a furnace having a fluid path through it. This fluid path may be created by any suitable means, such as the use of a quartz/glass tube of a suitable diameter to enable the positioning of the substrate, a first boat (containing the chalcogenide(s) may be placed upstream of the substrate and a second boat may be placed beneath the substrate and containing the metal source(s) and alkali metal halide. The furnace is also adapted to provide two separate temperature zones. The first temperature zone is upstream of the second temperature zone and houses the first boat, which contains the chalcogenide(s), the second temperature zone is downstream from the first temperature zone and houses the substrate and second boat, which boat contains the metal precursor and alkali metal halide. The first temperature zone is heated up to and maintained at a temperature capable of vaporising the chalcogenide(s), such a temperature may be from 150 to 500°C. The second temperature zone is heated up to and maintained at a temperature of from 350 to 950°C in order to vaporise the metal source. In operation, a carrier gas is passed through the fluid path and first comes into contact with the first boat and vaporised chalcogenide(s). The vaporised chalcogenide(s) is then carried to the second temperature zone by the carrier gas where it mixes with the vaporised metal and forms a deposited TMD on the surface of the substrate. The carrier gas exits through an outlet and it may still contain traces of the chalcogenide(s), metal source/metal and alkali metal salts. The carrier gas may be any suitable gas for chemical vapour deposition. In certain embodiments, there may be a single carrier gas and in others there may be more than one. When there is a single carrier gas, the gas is suitably an inert gas, such as nitrogen or argon. When there is more than one carrier gas the gas may be made from a mixture of an inert gas (e.g. nitrogen and/or argon) and hydrogen gas. The flow rate of the inert gas, whether used alone or in combination is from 30 to 200 standard cubic centimetres per minute (seem), such as from 60 to 120 seem. The flow rate of the hydrogen gas is independently from 1 to 20 seem (e.g. from 5 to 20 seem, such as 10 to 15 seem). As will be appreciated, it may be advantageous to remove oxygen from the fluid pathway before the process is conducted. This may be achieved purging the system with a stream of the carrier gas (or at least the inert gas) for a period of time (e.g. 5 minutes to 1 hour). Alternatively the fluid pathway may be purged by applying a vacuum to the fluid pathway to remove the gas present and then allowing the carrier gas (e.g. an inert gas) to flow into the pathway, which steps may be repeated a suitable number of times (e.g. 3 to 5 times). As noted herein, it is possible to obtain an in-plane heterostructure (i.e. a single layer of material on the substrate made of a single transition metal dichalcogenide in two or more phases (e.g. from 2 to 5, such as 2 to 4, such as 2 to 3 phases). In order to obtain such heterostructures, the flow rate of the carrier gas(es) may be varied during the deposition of the transition metal dichalcogenide in order to provide different phases of material. As an example of such a process, an in-plane heterostructure of MoTe₂ was obtained by first using a flow rate of 80 seem Ar and 20 seem H₂ to deposit MoTe₂ in the 1T phase, followed by changing the flow rate in the same deposition run to 20 seem Ar and 4 seem H₂ to deposit MoTe₂ in the 2H phase. The substrate may be any suitable substrate. A typical example of a substrate (but not limited thereto) is a silicon substrate. The substrate is placed above the boat containing the metal source and alkali metal salt. In order to ensure that there is a flow path for the carrier gas between the substrate and the second boat (i.e. second mixture) a gap of from 0.01 to 2.0 cm is used (e.g. 0.1 to 2.0 cm, such as 0.5 cm to 1.9 cm, such as 0.7 to 1.5 cm). When used herein, the term "boat" refers to any suitable vessel for holding the chalcogenide(s), metal source/metal and alkali metal salts that is capable of surviving the temperatures used in the process. The resulting thickness of the deposited TMD may be from 1 nm to 1 pm (e.g. 1 nm to 500 nm) on the surface of the substrate. That is, the layer of deposited TMD material may be a monolayer (e.g. 1 nm thick) or may be formed from several layers of the same TMD stacked on top of one another. The thickness of the layer deposited may be controlled by the reaction time and the amount of material provided. As will be appreciated, it is possible to repeat the process outlined above using the newly-coated substrate as the substrate, in which case a second TMD may be deposited on the first TMD deposition, which can be repeated any number of times. This second (and subsequent) deposition may deposit the same TMD as already on the surface of the substrate (e.g. if there is a need for a particularly thick coating of said TMD), but may particularly facilitate the formation of a heterostructure, where each TMD layer deposited is in contact with a TMD that is different to it (e.g. all TMD layers are distinct or a second TMD layer may be sandwiched between a first and third TMD layer, where the first and third TMD layers are the same TMD, while the second TMD layer is a different material (e.g. substrate/WS₂/HfSe₂ WS₂)). As noted above, the processing conditions outlined above enable the formation of the TMDs at a fast rate on the surface of the substrate once the deposition has been initiated. That is, the growth rate may be from 0.01 pm/s to 10 pm/s (e.g. from 0.05 pm/s to 10 pm/s, such as, more particularly, from 1 pm/s to 10 pm/s, such as from 2.5 to 7.5 pm/s), which allows TMDs having a size of from 5 pm to 1 ,000 pm to be grown quickly on the surface of the substrate. Indeed, growth time needed for many of the TMDs may be less than 30 seconds. Examples of TMDs produced under the conditions above that may be produced in less than 30 seconds include, but are not limited to, MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, WTe₂, VS₂, VSe₂, ReS₂, ReSe₂, TiS₂, TiSe₂, TaS₂, TaSe₂, NbS₂ and NbSe₂. The one or more metallic precursors are selected from an elemental metal, a metal oxide or a metal halide. The oxides and chlorides mentioned herein may be non-stoichiometric or, more particularly, stoichiometric in nature. Particular non-stoichiometric compounds that may be mentioned herein are iron and tungsten oxides. An advantage of the currently disclosed process is that the temperatures needed to heat the metal source may be significantly below those conventionally used. As such, in certain embodiments of the invention, when the desired transition metal dichalcogenide is WS₂ the temperature of the second temperature zone may be from 600 to 670°C. Alternatively, when the transition metal dichalcogenide is WSe₂ the temperature of the second temperature zone may be from 650 to 670°C. Particular embodiments for other TMDs and their associated alloys are provided below.

It will be appreciated that any suitable alkali metal salt (or mixture thereof) may be used in the process disclosed here. Examples of suitable alkali metal salts include, but are not limited to, NaCI, KCI, LiCI, Nal, KBr, LiBr, NaBr, Kl, Lil and mixtures thereof. In the process, the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be from 1 :2 to 15:1, such as from 1.5:1 to 15:1 , from 1.6:1 to 13:1 , from 4:1 to 10:1 , from 5:1 to 8:1, such as 6:1. Other suitable ratios are presented in the experimental section and below.

When the process above is used to make the transition metal dichalcogenide MoS₂, the temperature of the second temperature zone may be from 350 to 800°C (e.g. from 350 to 600°C, such as from 350 to 500°C) and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be from 5:1 to 10:1 (e.g. 6:1). When the process above is used to make the transition metal dichalcogenide MoSe₂, the temperature of the second temperature zone may be from 550 to 900°C (e.g. from 550 to 650°C, such as from 550 to 600°C) and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be from 5:1 to 10:1 (e.g. 6:1 ).

When the process above is used to make the transition metal dichalcogenides MoS₂ or MoSe₂, the metallic precursor may be a molybdenum oxide (e.g. Mo0₃). In addition, while any suitable alkali metal salt may be used, a particular alkali metal salt that may be mentioned in these processes is NaCI.

When the process above is used to make the transition metal dichalcogenides PdTe₂, PdSe₂, PdS₂, PtTe₂, PtSe₂, and PtS₂, the temperature of the second temperature zone may be from 750 to 860°C and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 8:1 to 12:1. In these processes, the wt.wt ratio of the one or more metallic precursors to the alkali metal halide may be 10:1 and/or the alkali metal halide may be any suitable salt, such as NaCI. In addition, a suitable metallic precursor for Pd may be PdCI₂ and a suitable a suitable metallic precursor for Pd may be PtCI₂. The process described herein may be used to make a transition metal dichalcogenide that is selected from the group consisting of HfTe₂, HfSe₂, HfS₂, VTe₂, VSe₂, VS₂, TiTe₂, TiSe₂, TiS₂, NbTe₂, NbSe₂, NbS₂, ZrTe₂, ZrSe₂, ZrS₂₎ TaTe₂, TaSe₂, TaS₂, MoTe₂, and WTe₂, where the temperature of the second temperature zone may be from 600 to 860°C and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide (e.g. NaCI) may be from 1.6:1 to 4:1. In particular embodiments of these processes, the metal Hf may be used as the precursor for TMDs containing Hf, while for each of V, Ti, Nb, Zr, Ta, Mo and W the metallic precursor may be a metal oxide.

The process described herein may be used to make a transition metal dichalcogenide that is FeS₂, FeSe₂ or FeTe₂, where the temperature of the second temperature zone is from 500 to 850°C (e.g. from 500 to 600°C) and the w wt ratio of the one or more metallic precursors to the alkali metal halide (e.g. NaCI and/or LiCI) is from 4:1 to 6:1 , such as 5:1. The metallic precursor for Fe may be a metal oxide (e.g. Fe₂0₃ or a non-stoichiometric oxide) or a metal chloride (e.g. FeCI₂).

As discussed above, the process disclosed herein may be used to manufacture a transition metal dichalcogenide that is an alloy of formula II:

AaB_bCcD_dEeF_fG_gS_hSe e_j II

where:

provided that at least two of a to g is greater than 0 and/or at least two of h to j is greater than 0. In embodiments relating to alloy formation, the wt:wt ratio of the one or more metallic precursors to the alkali metal halide may be from 5:1 to 7.5:1 , such as 5:1 and/or the temperature of the second temperature zone may be from 600 to 850°C. As will be noted, there may be up to six metals and three chalcogenides in the alloy.

Particular alloys that may be made using the above process include, but are not limited to alloys selected from the group consisting of MoSe_2xTe₂₍₁-_X), Mo_1-xRe_xS₂, MoS_2xTe₂₍₁._x), Mo_1-xNb_xSe₂, Mo_1-xNb_xS₂, Mo_xNb_l-xS_2ySe₂₍₁._y), WS_2xTe₂₍₁._x), WSe_2xTe₂₍i-_x), Mo_1-xW_xTe₂, NbS_2xSe_2(1-x), W_1-xNb_xS₂, W_1-xNb_xSe₂, and V_xW_yMo_(1-x._y)S₂₂Se_2(1-z). It will be appreciated that x, y and z are greater than 0 and less than 1. Also disclosed herein are transition metal dichalcogenide alloy of formula II per se, which are as defined above. These alloys may have particularly good catalytic properties compared to other materials. The invention will now be further described by reference to the following non-limiting examples. Examples

Fig. 1 shows a flow chart of the general growth process for the production of TMDs by the chemical vapour deposition method, based on the competition between mass flux of metal precursors and reaction rate of the domains.

The mass flux determines the amount of metal precursors involved in the reaction for the formation of nucleus and the growth of domains, while the growth rate dominates the grain size of the as-grown films. At high mass flux, low growth rate results in monolayer polycrystalline film (Route I) with small grain, and high growth rate tends to form continuous monolayer films with large grain (up to millimeter size, Route II) (Dumcenco, D. et al. ACS Nano, 2015, 9, 4611-4620). On the other hand, at low mass flux, low growth rate leads to form small flakes. Tiny nucleus are often observed at the center of the flakes (Li, B. et al. Angew. Chem. Int. Edit, 2016, 55, 10656-10661 ), suggesting that the extra adatoms or atom clusters will consistently attach to an existing nucleus or the edge during the growth (Route III), while high reaction rate prefers to produce individual large single crystalline 2D monolayer (Route IV) (Gong, Y. J. er a/. Adv. Func Mater., 2016, 26, 2009-2015).

Unfortunately, for many TMDs, such as Nb, Pt and Ti based ones, it is very difficult to produce them because their metal or metal oxides precursors have high melting points and low vapor pressure, which leads to very low mass flux and therefore limits the occurrence of the reaction. It has been surprisingly found that the use of a molten salt mixture can increase the mass flux by reducing the melting point of the metal precursors and forming oxyhalides (e.g. oxychlorides) via reaction with some of the metal/metal oxide of the precursor, which helps to facilitate the desired reactions.

Materials and Methods

General procedure for synthesis of the 2D TMDs The setup of the reaction is shown in Fig. 2a with an expanded view of the middle section shown in Fig. 2b. The 2D compounds and heterostructures were synthesised in a quartz tube 10 having a 1 inch (2.54 cm) diameter and a length of approximately 70 cm. The length of the furnace was about 36 cm. A carrier gas 35 (e.g. mixture of H₂/Ar gas) was used, with the direction of gas flow as shown in Figs. 2a and b. An aluminium oxide boat 15 with dimensions of about 8 cm x 1 .1 cm x 1.2 cm, containing a mixture of precursor powder 25 and salt 45, was placed in the center of the tube 10. A Si substrate 20 with a 285 nm top layer of Si0₂ was placed on boat 15 with the polished surface down; the distance between the sources and substrate was set at a distance of from 0.2 cm to 1.2 cm. Another aluminium boat (of similar dimension) containing S or Se or Te powder 30 was placed on the upstream (upwind) of the tube furnace at 200 °C, 300 °C and 450 °C, respectively. The distance between the S or Se or Te boat 30 and the precursor boat was about 18 cm, 16 cm and 15 cm, respectively. The heating rate of all reactions was 50 °C min^"1. All the reactions were carried out at atmospheric pressure. The temperature was cooled down to room temperature naturally. All reaction materials were bought from Alfa Aesar with a purity of more than 99%. The synthesis conditions and parameters for each compound made are summarised in Tables 1-

2.

The weight ratio between salt and metal precursor(s), or the weight ratio between the different metal precursors or chalcogenides (for alloys) can be tuned in accordance with the needs of the starting materials and/or desired properties of the final TMDs. Taking Mo^Re^ and WS_xTe₂-_x as examples, by tuning the ratio between Mo and Re, and between S and Te, it is possible to control the value of x. Optical imaging, Raman and photoluminescence spectroscopy

Raman measurements were performed on a WITEC alpha 200R Confocal Raman system, with an excitation laser of 532 nm and with a laser power of less than 1 mW. Prior to Raman characterisation of the samples, the system was calibrated against a silicon standard with a Raman peak centered at 520 cm^"1. The laser powers are less than 1 mW to avoid overheating of the samples. Photoluminescence measurements were also performed on the same system. The optical images of the as-synthesised samples were obtained using a light microscope. Fig. 3 shows the optical images of 47 TMDs synthesised using the general method above from Ti to Pt, in which various morphologies including triangles, hexagons, ribbons and films can be observed. The TMDs synthesised include 32 binary 2D crystals, 11 ternary alloys, a quaternary alloy, a quinary alloy and a 1 T MoTe₂-2 H MoTe₂ in-plane and MoS₂- NbSe₂ vertically stacked heterostructures.

All of the materials display a purple color because they were directly grown on 285 nm Si0₂. The size of the 2D TMDs can reach as large as one millimeter in the case of MoS₂, WS₂ and WSe₂. It is notable that ΊΠ-, Nb-, V-, Fe-, Zr-, Pd-, Hf-, Ta- and Pt-based TMDs have rarely been synthesised before.

Scanning Transmission Electron Microscopy (STEM), Energy-Dispersive X-ray Spectroscopy (EDS) and Electron Energy Loss Spectroscopy (EELS).

STEM samples were prepared with a poly(methyl methacrylate) (PMMA)-assisted method or PMMA-free method with the assistance of an iso-propyl alcohol droplet. For some water- sensitive materials, we used a non-aqueous transfer method. STEM imaging and EELS analysis were performed on a JEOL 2100 F with a cold field-emission gun and an aberration corrector (the DELTA corrector) operating at 60 kV. A low-voltage modified Gatan GIF Quantum spectrometer was used for recording the EELS spectra. The inner and outer collection angles for the STEM image (β1 and β2) were 62 mrad and 129-140 mrad, respectively, with a convergence semi-angle of 35 mrad. The beam current was about 15 pA for the annular dark-field (ADF) imaging and the EELS chemical analyses. EDS analyses were also carried out using the same instrument.

The atomic structures and chemical compositions of the as-synthesised 2D crystals and compounds can be further revealed by these techniques. Most 2D crystals in the metal chalcogenide family maintain a chemical stoichiometry of MX₂ (M=metal, X = chalcogen) with an X-M-X structure, where the layer of the metal atoms is sandwiched by the other two chalcogen layers.

The atomic structures of most 2D crystals can be classified into four types: (1 ) the trigonal prismatic 1 H phase; (2) the undistorted 1 T phase with the metal atom located at the centre of an octahedral unit; (3) the one-dimensional distorted 1 T phase (called the 1 T' phase), in which pairs of metal atoms move closer to each other perpendicularly, resulting in a quasi- one-dimensional chain-like structure consisting of distorted octahedral units; and (4) and the two-dimensional distorted 1 T phase (called the 1 T" phase), in which four nearby metal atoms move closer to each other to form a new unit cell, producing repeatable diamond-like patterns. As the image intensity in Z-contrast STEM imaging is directly related to the atomic number of the imaged species, the structural phase of each synthesised material can be determined by the Z-contrast STEM image. A summary of different phases for the as-synthesised 2D materials that have been examined is shown in Table 3.

Notably, the 2D TMDs for group IV and X metals were observed to be highly sensitive to water and oxygen under ambient conditions, thus, they were easily oxidised during the transfer process before they were characterised structurally. As such, there are some missing data points due to the difficulties in characterising the TMDs, including the degraded samples.

X-ray Photoelectron Spectroscopy (XPS)

XPS measurements were performed using a monochromated Al Ka source (hv = 1486.6 eV) and a 128-channel mode detection Physical Electronics Inc. original detector. XPS spectra were acquired at a pass energy of 140 eV and a take-off angle of 45°.

Atomic Force Microscopy (AFM) AFM measurements on the thickness of the TMDs were carried out using the Asylum Research, Cypher S system. During the measurement, some dots were often found on the surface on the AFM images due to the oxidation of some of the TMDs. Given this, the thickness morphology of some of the TMDs was also confirmed using STEM. Reference to the methodology involving the AFM step heights and identifying layer thicknesses can be found in Shearer, C. J., et al., Nanotechnology, 2016, 27, 125704.

The AFM images and the corresponding thicknesses of the 2D TMDs were measured. All the sulphides except PdS₂ were found to be monolayer. In comparison to sulfides, only some selenides such as MoSe₂, WSe₂, NbSe₂, VSe₂, PtSe₂ and ReS₂ were confirmed as monolayer crystals. For the tellurides, monolayer WTe₂ and MoTe₂ were obtained easily, which were confirmed by the AFM images and STEM results. As a comparison, other few- layer tellurides such as Ti-, V-, Nb-, Zr-, and TaTe₂ were obtained.

Some TMDs like TiSe₂, HfSe₂, ZrSe₂, and TaSe₂ were easily oxidised as observed from the AFM images which often showed dots found on the surface of the materials. Given this, the AFM images cannot accurately reveal the information on the precise thickness. Fortunately, this can be distinguished from the STEM results which showed that the monolayer zone was often found.

A summary of the sample thickness for the as-synthesised 2D materials that have been examined is shown in Table 4.

Synthesis Conditions and summaries of phases/thicknesses of resulting products

Table 1a depicts the synthesis conditions and parameters for the 32 binary compounds (based on transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe) obtained from initial studies, while Table 1b depicts the synthesis conditions and parameters obtained after repeated studies.

Table 2a depict the synthesis conditions and parameters for the 13 alloys (including 11 ternary, one quaternary and one quinary) obtained from initial studies, while Table 2b depicts the synthesis conditions and parameters obtained after repeated studies.

The phases of the 2D TMD samples examined by high-resolution STEM is summarised in Table 3, while the thickness of the samples examined by AFM is summarised in Table 4.

Table 1a. Synthesis conditions and parameters for 32 binary TMD obtained from initial studies

Table 1b. Synthesis conditions and parameters for 32 binary TMD obtained after repeated studies

Table 2a. Synthesis conditions and parameters for the13 alloys obtained from initial studies.

Table 3. Summary of the phases of the 2D TMD samples examined by high-resolution STEM

Table 4. Summary of the thickness of the 2D TMD samples examined by AFM

Example 1. Synthesis and characterisation of MoS₂

MoS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. A large MoS₂ single crystal with a size up to 1.2 mm was synthesised with corresponding photoluminescence (PL) peak at 675 nm (Fig. 4a and c). The Raman spectrum of the monolayer MoS₂ crystal shows the distance between the two peaks to be about 19 nm, indicating the monolayer nature of the MoS₂ crystal (Fig. 4b). Fig. 11a shows the STEM image of monolayer MoS₂ in 1 H phase, with the corresponding atomic structural model. The patterns obtained by fast Fourier transform further indicate that the 1 H phase maintains a hexagonal unit cell. In addition, the thickness of the monolayer was measured to be 0.7 nm by AFM (Table 4). Example 2. Synthesis and characterisation of MoSe₂

MoSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. A MoSe₂ single crystal with a size up to 1.0 mm was synthesised (Fig. 4d) and the PL spectrum is shown in Fig. 4f. The Raman spectrum of MoSe₂ shows a A_1g peak at 249 cm^"1, which is in agreement with previous report (Fig. 4e) (Gong, Y. J. et al., Adv. Fund. Mater., 2016, 26, 2009-2015).

The as-synthesised MoSe₂ was determined to be in 1 H phase by STEM and the thickness was measured to be 0.7 nm by AFM (Tables 3 and 4 respectively).

Example 3. Synthesis and characterisation of MoTe₂

MoTe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. In comparison with MoS₂ and MoSe₂, MoTe₂ has two phases which are 2H and 1T\ The synthesis of the few-layer 2H- and 1T- MoTe₂ films have been reported by tellurisation of the Mo or Mo0₃ film (Zhou, L. et al., J. Am. Chem. Soc, 2015, 137, 11892-11895; Park, J. C. er a/., ACS Nano, 2015, 9, 6548-6554). The monolayer 1T MoTe₂ crystal with a size smaller than 10 pm was also obtained by a similar method (Naylor, C. H. ef al., Nano Lett., 2016, 16, 4297-4304). However, using the method as disclosed in this application, the synthesis of a large-size MoTe₂ monolayer film of up to 150 μιτι was achieved and was characterised by Raman spectroscopy (Fig. 4g and h). More importantly, the growth of 2H- and 1T- MoTe₂ can be controlled by varying the amount of Te sources (this is described in detail in Example 36, which relates to the synthesis of 1T MoTe₂ - 2H MoTe₂ in-plane heterostructures).

The as-synthesised MoTe₂ thin film has 1T and 2H phases, which should show semiconducting and semi-metallic behaviour, respectively. Fig. 12a shows the EDS spectrum of MoTe₂ thin film where the signals due to Te and Mo elements were observed strongly, therefore confirming the high purity of MoTe₂.The Au signals appear to be from the Au TEM grid bars and signals for Cu may come from the contaminations during the samples transfer. This confirms the high purity of the as-synthesised MoTe₂.

The high-resolution Z-contrast STEM images of few-layer 2H and 1 MoTe₂ are shown in Fig. 12b and d, respectively. A hexagonal crystal structure of the 2H phase MoTe₂ was observed which confirms the AA' structure (Fig. 12b). It is evident that the atomic structure of few-layer MoTe₂ in 1 phase is different from the 2H phase (Fig. 12d).

The EELS spectrum of 2H phase MoTe₂ is different from that of 1T phase MoTe₂, in which an additional small pre-peak at around 30 eV was observed for the 1Γ phase (Fig. 12c and e). This may be caused by different Mo-Te chemical bonding in the 2H and 1T phases, which can be used as a "fingerprint" for differentiating these two phases. The thickness of the monolayer was measured to be 0.8 nm by AFM (Table 4).

Example 4. Synthesis and characterisation of WS₂

WS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. The synthesis of WS₂ monolayer has been reported by CVD method. However, large size WS₂ was usually obtained with long-time growth at high temperature of 900 to 1100 °C (Rong, Y. M. et a/. Nanoscale, 2014, 6, 12096-12103). Using the method disclosed in this application, WS₂ monolayer as large as 0.5 mm was achieved in a short time of 3 min and at a reduced temperature (Fig. 5a). The Raman spectrum and PL peak (at 635 nm) of the monolayer are shown in Fig. 5b and c.

The as-synthesised WS₂ was determined to be in 1 H phase by STEM and the thickness was measured to be 0.8 nm by AFM (Tables 3 and 4 respectively).

Example 5. Synthesis and characterisation of WSe₂

WSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The synthesis of a large-size monolayer WSe₂ crystal was achieved as shown in Fig. 5d and the corresponding Raman spectrum of the monolayer crystal (Fig. 5e) is consistent with values in previous report (Huang, J. K. er a/., ACS Nano, 2014, 8, 923-930). The peak of the PL spectrum at -760 nm suggests that the WSe₂ is a monolayer (Fig. 5f).

The as-synthesised WSe₂ was also characterised by STEM and determined to be in 1 H phase, while the thickness was measured to be 0.9 nm by AFM (Tables 3 and 4 respectively). Example 6. Synthesis and characterisation of WTe₂

WTe₂ was synthesised in accordance to the above general procedure with the growth conditions and parameters in Tables 1a and 1 b. Monolayer and bilayer of WTe₂ with size up to 600 μιη was synthesised (Fig. 5g), and the Raman spectra of monolayer shown in Fig. 5h is in agreement with previous report. WTe_2l a semimetal, is only stable in 1T phase (usually called 1Td phase in bulk due to the small misalignment in stacking) in nature. Tungsten chains are formed within the dichalcogenide layers along the a axis of the unit cell, making the compound structurally one dimensional. Fig. 11 c shows the STEM image of monolayer WTe₂ in 1T phase with the corresponding atomic structural model. The patterns obtained by fast Fourier transform further indicate that the lT phase forms a rectangular unit cell owing to one-dimensional metal-pair distortion.

In addition, Fig. 13a shows the STEM image of the as-grown WTe₂ in the 1T phase, where the chain-like structure can still be observed. The EDS and EELS spectra in Fig 13b and c respectively further confirm the chemical composition and purity of the WTe₂ sample. The thickness of the sample was measured to be 0.7 nm by AFM (Table 4).

Example 7. Synthesis and characterisation of TiS₂

TiS₂ was synthesised in accordance to the above general procedure with the growth conditions and parameters in Tables 1a and 1 b. The synthesis of a monolayer TiS₂ with a size of up to 50 μιτη was achieved (Fig. 6a). The Raman peaks located at 230 cm^"1 and 332 cm^"1 are the same as the reported result, confirming that the flakes are TiS₂ (Fig. 6b).

TiS₂ is a semimetal with a small band gap opening when thinned down to few layers, and it maintains the 1T structure which belongs to the space group P3m1 with a crystal lattice of a= 3.4071 A, c=5.6953 A. Fig. 14a shows the STEM image of the TiS₂ layers, revealing the 1T phase with the hexagonal crystal structure. The EDS and EELS spectra are shown in Fig. 14b and c respectively, which further confirm the chemical composition of the as-synthesised TiS₂. The thickness of the sample was measured to be 0.9 nm by AFM (Table 4).

Example 8. Synthesis and characterisation of TiSe₂ TiSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. A few-layer TiSe₂ with a size up to 30 pm was synthesised, with Raman peaks located at 190 cm^"1 and 250 cm^"1 corresponding to the A_1g mode and E _g mode of TiSe₂ respectively (Fig. 6c and d). The thickness was measured to be 2.0 nm by AFM (Table 4).

Example 9. Synthesis and characterisation of TiTe₂ TiTe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. A TiTe₂ ultra-thin film was synthesised (Fig. 6e). The Raman peaks locating at ~90 cm^"1, 120 cm^"1, and 140 cm^"1 are in agreement with the reported result, in which the latter two peaks correspond to the vibration of A_1g and Ei_g respectively (Fig. 6f). The thickness was measured to be 5.0 nm by AFM (Table 4).

Example 10. Synthesis and characterisation of ZrS₂ ZrS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. A monolayer ZrS₂ of size up to 30 pm was synthesised (Fig. 7a). The Raman spectrum of ZrS₂ is shown in Fig. 7b, with a Raman peak locating at 305 cm^"1 corresponding to A_2u vibration. ZrS₂ crystal is a semiconductor with crystal constants a = b = 3.66 A and c = 5.82 A. The atomic structure of a monolayer ZrS₂ consists of S-Zr-S sandwich structure in the octahedral 1T phase. Fig. 15a shows a STEM image of a few-layer ZrS₂ crystal. Hexagonal arrangement of atoms was observed in the projected view, an evidence of the presence of 1T phase. It is noted that Zr-based compounds are highly sensitive to the ambient conditions. The image shown here is a few-layer ZrS₂ film covered by substantial amount of oxidised residual. The corresponding EDS and EELS spectra (Fig. 15b and c respectively) show the Zr and S signal, which confirms the chemical composition of the as-synthesised ZrS₂ flakes. In addition, the thickness of the monolayer was measured to be 1.0 nm by AFM (Table 4). Example 11. Synthesis and characterisation of ZrSe₂

ZrSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. A ZrSe₂ monolayer was synthesised as shown in Fig. 7c. The corresponding Raman intensity of the monolayer ZrSe₂ shown in Fig. 7d is weaker than that in bulk ZrSe₂. In addition, the thickness of the monolayer was measured to be 2.0 nm by AFM (Table 4).

Example 12. Synthesis and characterisation of ZrTe₂

ZrTe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of ZrTe₂ is shown in Fig. 7e and the Raman spectrum confirms that the flake is ZrTe₂ crystal (Fig. 7f). In addition, the thickness of the monolayer was measured to be 2.0 nm by AF (Table 4).

Example 13. Synthesis and characterisation of HfS₂

HfS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of HfS₂ is shown in Fig. 3. The as- synthesised HfS₂ was also characterised by Raman spectroscopy in which the A_1g vibration mode is in agreement with the reported result.

The as-synthesised HfS₂ was characterised by STEM and was determined to be a semiconductor in octahedral 1T phase with van der Waals interaction between the layers. This is similar to the structure of ZrS₂ since both Zr and Hf are group IVB elements. HfS₂ is not stable in air, which therefore restricts the structural characterisation of the HfS₂ atomic layers. The STEM images of the as-synthesised HfS₂ flake showed that it was heavily oxidised and this made the identification of the exact atomic structure challenging. However, the EDS spectrum showed strong Hf and S signals. Together with the corresponding EELS spectrum, which also showed the Hf and S EELS fingerprints spectra, it was confirmed that the flakes should be the HfS₂ crystal, before they were oxidised.

In addition, the thickness of the monolayer was measured to be 1.0 nm by AFM (Table 4).

Example 14. Synthesis and characterisation of HfSe₂ HfSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. A HfSe₂ monolayer of around 15 m was synthesised and the optical image is shown in Fig. 3. It was also characterised by Raman spectroscopy which showed the A_1g mode in the spectrum. In addition, the thickness of the monolayer was measured to be 1.0 nm by AFM (Table 4).

Example 15. Synthesis and characterisation of HfTe₂

HfTe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. A few-layer HfTe₂ was synthesised and the optical image is as shown in Fig. 3. The flake was characterised by Raman spectroscopy in which the spectrum confirmed that the flake is HfTe₂ crystal. Example 16. Synthesis and characterisation of VS₂

VS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of VS₂ is shown in Fig. 3. The as- synthesised VS₂ was characterised by Raman spectroscopy and the spectrum showed the 2-photonon and A _g vibrations which are the same as the values reported.

The VS₂ was characterised by STEM and it was determined to be in 1T phase whereby it is composed of layers of VS₆ octahedral separated by a van der Waals gap and it belongs to space group P3m1. The hexagonal atomic arrangement observed at the edges confirmed the 1T structure of VS₂. The EDS spectrum showed the presence of only S and V atoms in this region and the EELS spectrum collected along the edge further confirmed the high quality of the as-synthesised VS₂. In addition, the thickness of the as-synthesised sample was measured to be 0.7 nm by AFM (Table 4).

Example 17. Synthesis and characterisation of VSe₂ VSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. A monolayer of VSe₂ of 50 μηι was synthesised and the optical image is shown in Fig. 3. The sample was also characterised by Raman spectroscopy. The STEM image of few-layer VSe₂ show the hexagonal arrangement of atomic columns in alternate bright and dark pattern results from the stacking of the few layers in 1T structure. The EDS and EELS spectra collected in the same region further confirmed the high purity of the as-synthesised VSe₂. The thickness of the as-synthesised sample was measured to be 0.9 nm by AFM (Table 4).

Example 18. Synthesis and characterisation of VTe₂

VTe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. A monolayer of VTe₂ was synthesised and the optical image is shown in Fig. 3. The as-synthesised sample was also characterised by Raman spectroscopy and the Raman spectrum showed peaks at 1 17 cm^"1 and 137 cm^"1 which were slightly shifted as compared to reported values. The thickness of the sample was measured to be 3.0 nm by AFM (Table 4).

Example 19. Synthesis and characterisation of NbS₂

NbS₂ was synthesised in accordance to the above general procedures with growth conditions and parameters in Tables 1a and 1b. The synthesis of a monolayer of NbS₂ of a size up to 80 m was achieved and the optical image is shown in Fig. 3. The sample was also characterised by Raman spectrum which confirmed that the as-synthesised flake is an NbS₂ crystal.

The NbS₂ crystal generally exists in 2H phase (or 1 H in the case of a single layer). The STEM image of a monolayer NbS₂ showed that the 2H phase of NbS₂ can be directly identified by the hexagons composed by Nb and S₂ columns with different contrast profile - this is similar to the well-known atomic structure of MoS₂. The EDS and EELS spectra showed the presence of Nb and S, therefore confirming the chemical composition of NbS₂. The thickness of the sample was measured to be 0.7 nm by AFM (Table 4).

Example 20. Synthesis and characterisation of NbSe₂

NbSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of monolayer NbSe₂ with a size up to 50 pm is shown in Fig. 3 and the Raman spectrum confirmed that the as- synthesised sample is NbSe₂.

NbSe₂ is generally in the 2H phase, where the Nb and Se₂ atomic columns are arranged in a hexagonal manner. The STEM image of NbSe₂ showed the presence of both monolayer and bilayer regions. The hexagonal atomic structure in the monolayer, similar to those observed in NbS₂, confirmed that the monolayer is in 1H phase. The uniform intensity of each atomic site in the bilayer, on the other hand, revealed the as-synthesised NbSe₂ is in the 2H stacking phase. The EDS and EELS spectra collected in the same region confirmed the chemical composition of NbSe₂. The Cu signals were from the background signals of the Cu TEM grid used. The thickness of the sample was measured to be 0.8 nm by AFM (Table 4). Example 21. Synthesis and characterisation of NbTe₂ NbSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. NbTe₂ is a layered-structure compound and was reported to demonstrate charge-density wave (CDW) effect and superconducting behaviour (Nagata, S., et al., J. Phys. Chem. Solids, 1993, 54, 895-899. However, the synthesis of NbTe₂ flakes has not been reported by CVD method. Here, large-size NbTe₂ obtained using CVD method was achieved and the optical image is shown in Fig. 3. The sample was characterised by Raman spectroscopy which revealed that the flake is NbTe₂ crystal. Like other telluride-based materials, the bulk crystal structure of NbTe₂ is monoclinic, belonging to space group C2/m. NbTe₂ is known to demonstrate charge density wave (CDW) transition when the temperature drops to the transition point. The STEM image of a NbTe₂ monolayer showed hexagonal patterns of atom arrangement instead of the MoTe₂- like 1T phase, therefore suggesting that the NbTe₂ monolayer is presumably in 1T phase. The corresponding EDS and EELS spectra further confirmed that the flakes were only composed of Nb and Te elements. The thickness of the sample was measured to be 4.0 nm by AFM (Table 4).

Example 22. Synthesis and characterisation of TaS₂

TaS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of TaS₂ film with a size up to 70 μιη is shown in Fig. 3. The sample was also characterised by Raman spectroscopy. TaS₂ monolayer can exist stably in 1 H and 1T phase. A high resolution Z-contrast STEM image of a monolayer region and bilayer region of TaS₂ showed the hexagonal patterns which indicated that the Ta (bright spots) and S₂ were arranged similarly to that of MoS₂. This therefore confirmed the 1 H phase of the as-synthesized TaS₂. In addition, the bilayer region showed a 3R stacking order, similar to that of CVD-grown MoS₂. The EDS and EELS spectra were collected in the same region where the image was collected. This confirmed the chemical composition of the sample which consists of Ta and S without any other obvious impurities. The thickness of the sample was measured to be 0.8 nm by AFM (Table 4)· Example 23. Synthesis and characterisation of TaSe₂ TaSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. The optical image of TaSe₂ is shown in Fig. 3 and the sample was also characterised by Raman spectroscopy. The thickness of the sample was measured to be 2.0 nm by AFM (Table 4).

Example 24. Synthesis and characterisation of TaTe₂

TaTe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1b. TaSe₂ with a size up to 20 pm is shown in Fig. 3. The sample was also characterised by Raman spectroscopy and the thickness of the sample was measured to be 5.0 nm by AFM (Table 4).

Example 25. Synthesis and characterisation of ReS₂ ReS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of monolayer ReS₂ with a size up to 50 pm is shown in Fig. 3. The sample was also characterised by Raman spectroscopy.

ReS₂ is a semiconductor with a distorted 1T structure which is defined as 1T" phase. A high resolution Z-contrast STEM image of a ReS₂ monolayer showed a diamond shape pattern of the Re atoms due to the two-dimensional distortion from the 1T phase. Typically, the Re atoms are observed clearly as bright spots, while the S atoms are generally not observable due to the smaller size as compared to Re atom. The EDS and EELS spectra showed Re and S characteristic peaks, without any other obvious impurities, therefore confirming the composition of the material. The thickness of the sample was measured to be 0.9 nm by AFM (Table 4).

Example 26. Synthesis and characterisation of ReSe₂ ReSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of ReSe₂ is shown in Fig. 3. The sample was also characterised by Raman spectroscopy.

Fig.11d shows the STEM image of monolayer ReSe₂ in 1T" phase, with the corresponding atomic structural model. The patterns obtained by fast Fourier transform further indicate that the 1T" phase changes to a much larger hexagonal cell owing to the aggregation of four metal atoms into a new unit cell. The EDS spectrum collected from a larger region confirms the presence of Re and Se. The Mo signals were likely to be from the TEM Mo grid bar, while the Cu may be from clusters that were attached to the sample during the transfer. The EELS spectrum further demonstrated that only Re and S characteristic peaks were found, which confirmed the chemical composition of the film. In addition, the thickness of the sample was measured to be 0.8 nm by AFM (Table 4).

Example 27. Synthesis and characterisation of FeSe

FeSe was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of the monolayer flake is shown in Fig. 3 and the Raman spectrum of the sample confirmed that the as-synthesised flake is FeSe crystal.

The two different phases of FeSe are the tetragonal phase α-FeSe with PbO-structure, and the NiAs-type β-phase with a wide range of homogeneity showing a transformation from hexagonal to monoclinic symmetry. Specifically, the single crystal α-FeSe can undergo a clear tetragonal to orthorhombic phase transition at temperature T = 91 K and at T_c= 9.3 K. In addition, FeSe is also known to demonstrate superconductivity property. In this case, a hexagonal phase of monolayer FeSe was obtained as shown by the high- resolution Z-contrast STEM image of a monolayer region collected in the as-grown FeSe sample. Interestingly, this hexagonal phase, viewed along [001] zone axis, maintained a layered structure, which is similar to the 1 H phase observed in MoS₂. However, this phase has not been reported for FeSe. The chemical composition was first confirmed by the EDS spectrum of a large flake which only showed the presence of Fe and Se. The EELS spectrum showed that only Fe and Se characteristic core-loss peaks were present. This further confirmed the high purity of the FeSe sample and a new phase of FeSe may have been obtained. The Mo signals were from the Mo TEM grid. The thickness of the sample was measured to be 1.2 nm by AFM (Table 4).

Example 28. Synthesis and characterisation of PtS₂

PtS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of PtS₂ is shown in Fig. 3 and the sample was also characterised by Raman spectroscopy. The thickness of the sample was measured to be 1.0 nm by AFM (Table 4). Example 29. Synthesis and characterisation of PtSe₂

PtSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of PtSe₂ is shown in Fig. 3 and the Raman spectrum of PtSe₂ showed the A_1g and E_g vibration modes.

PtSe₂ crystal is an emerging new layered two-dimensional metal dichalcogenide material with group X metal cation. It has a structure similar to Cdl₂, which belongs to the space group P3m1 with the lattice constant of a = 3.724 A, c = 5.062 A. A PtSe₂ monolayer has the octahedral 1T phase and is able to perform like a semiconductor. On the other hand, the few-layer counterpart demonstrates metallic property due to the strong interlayer interaction.

Fig.11 b shows the STEM image of monolayer PtSe₂ in 1T phase, with the corresponding atomic structural model. The patterns obtained by fast Fourier transform further indicated that the 1T phase maintain a hexagonal unit cell. Some holes observed on the samples during STEM characterisation (due to irradiation damage from the electron beam) further confirmed the presence of a single monolayer. In addition, both the EDS and EELS spectra indicated the characteristic features of Pt and Se, therefore further confirming the chemical composition of the as-synthesised PtSe₂ film. . The thickness of the sample was measured to be 0.9 nm by AFM (Table 4).

Example 30. Synthesis and characterisation of PdS₂

PdS₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical image of PdS₂ with hexagonal shapes is shown in Fig. 3. The sample was also characterised by Raman spectroscopy. The thickness of the sample was measured to be 1.5 nm by AFM (Table 4).

Example 31. Synthesis and characterisation of PdSe₂

PdSe₂ was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 1a and 1 b. The optical images of PdSe₂ with hexagonal shapes is shown in Fig. 3. The sample was also characterised by Raman spectroscopy. The thickness of the sample was measured to be 3.0 nm by AFM (Table 4). Example 32. Synthesis and characterisation of MoS₂xTe₂₍i-x₎, MoSe_2xTe₂₍i__X), WS₂xTe₂₍i-x₎, WSe2xTe₂₍i-_X) and NbS₂ Se₂₍₁-._X)

MoS_2xTe₂(i-_X), MoSe_2xTe₂(i-_X), WS_2xTe_2{1-x ), WSe₂xTe₂₍₁-.x )and NbS₂xSe₂(i-_X) Were synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 2a and 2b.

Specifically, MoSe₀.₃Tei.₇ was obtained as follows:

Mixed powder of NaCI and Mo0₃ with weight of 2 mg and 10 mg in the alumina boat was placed in the center of the tube. Another alumina boat containing mixed powder of Se and Te in a weight ratio of 3:7 (Se:Te) was placed in the upstream. The furnace was heated with a ramp rate of 50 °C/min to the growth temperature 700-800 °C and held at the temperature for 10-20 mins before cooled down to room temperature naturally. The mixed Ar/H₂ with a flow rate of 100/5 seem was used as the carrier gas.

The optical images of the as-synthesised ternary samples are shown in Fig. 3 and the samples were also characterised by Raman spectroscopy. For example, some other representative images of MoS₂xTe₂(i-x), MoSe₂xTe₂₍₁-x), WS₂xTe₂₍₁-x ₎ are shown in Fig. 8a-c, along with the respective Raman spectra in Fig. 8d-f.

Example 33. Synthesis and characterisation of Moi__xNb_xSe₂, Mo₁-xRe_xS₂, W₁-_xNb_xS₂, W₁-_xNb_xSe₂, Mo_xNbi__xS₂ and Mo_xW^_xTe₂

Mo₁-xNb_xSe₂, Mo₁-xRe_xS₂, W^xNbxSa, W₁-_xNb_xSe₂, Mo_xNb₁-xS₂ and Mo_xW₁-xTe₂₎ were synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 2a and 2b. For the synthesis of this group of ternary alloys, the aluminium oxide boat placed in the center of the quartz tube will contain a mixture of two metal precursors in a weight ratio from 0.01 :1 to 1 :0.01. The optical images of the as- synthesised ternary samples are shown in Fig. 3 and the samples were also characterised by Raman spectroscopy.

Example 34. Synthesis and characterisation of Mo_xNbi-xS_2ySe₂₍i-_y)

The quaternary TMD, Mo_xNb₁-xS2ySe₂(i-y), was synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 2a and 2b. Specifically, the aluminium oxide boat placed in the center of the quartz tube will contain a mixture of Nb₂0₅:Mo0₃ in a weight ratio from 0.01 :1 to 1 :0.01 , while another aluminium oxide boat containing S and Se powder was placed in the upstream.

The optical image of the as-synthesised sample is shown in Fig. 9a and the corresponding Raman spectrum is shown in Fig. 9b.

Example 35. Synthesis and characterisation of

The quinary TMD,

was also synthesised in accordance to the above general procedure with growth conditions and parameters in Tables 2a and 2b.

Specifically, Vo.03W02 oo.77SL6Seo.4 can be obtained as follows:

A powder mixture of 2 mg NaCI and 10 mg of V₂0₅:Mo0₃:W0₃ = 1 :5:3 in the aluminium oxide boat was placed in the centre of the quartz tube. Another aluminium oxide boat containing S and Se powder of weight ratio from 1 :10 to 10:1 was placed in the upstream. The furnace was heated with a ramp rate of 50 °C min^"1 to the growth temperature (760-840 °C) and held at this temperature for 10-20 min before cooling down to room temperature naturally. Ar/H₂ with a flow rate of 100/5 seem was used as the carrier gas. The optical image of the as-synthesised quinary sample is shown in Fig. 9c and the corresponding Raman spectrum is shown in Fig. 9d. A STEM image of a quinary V_xWyMoi-_x-_yS₂zSe2₍i-z₎ monolayer alloy is shown in Fig. 16a, where the chemical composition was verified by the EDS spectrum (Fig. 16b). Different chemical species give rise to the distinct atomic contrast in the image. Combined with the intensity histogram analysis of the cation and anion sites (Fig. 17), each atomic column can be directly associated with their chemical identities using the image contrast, as shown by the representative line intensity profile in Fig. 16c. The atom-by-atom mapping further confirms the successful synthesis of a quinary alloyed monolayer. Example 36. Synthesis and characterisation of 1 T ΜοΤβ₂-2 Η oTe₂ in-plane heterostructure

The 1 T MoTe₂-2 H MoTe₂ in-plane heterostructure was synthesised as follows:

A mixed powder of 4 mg NaCI and 14 mg M0O₃ in the aluminium oxide boat was placed in the centre of the quartz tube. Another aluminium oxide boat containing Te powder was placed in the upstream. The furnace was heated to a growth temperature of 720 °C with a ramp rate of 50 °C min^"1 and held for 3 min, and then quickly cooled to a growth temperature of 650 °C and held for 5 min and then cooled down to room temperature naturally. 1 T MoTe₂ was first synthesised using an Ar/H₂ flow rate of 80/20 seem. This was then followed by the growth of 2H MoTe₂ on the as-synthesised 1 T' MoTe₂ using an Ar/H₂ flow rate of 20/4 seem to obtain the 1 T MoTe₂-2 H MoTe₂ in-plane heterostructure.

Fig. 10a shows the optical image of 1T MoTe₂-2H MoTe₂ in-plane heterostructure. The shape of 2H MoTe₂ is hexagonal while the shape of 1T MoTe₂ is rectangular. The Raman spectra of corresponding 1T' MoTe₂ and 2H MoTe₂ in the heterostructure confirmed the formation of in-plane heterojunction (Fig. 10b)

Example 37. Synthesis and characterisation of MoS₂-NbSe₂ vertically stacked heterostructure

MoS₂ was synthesised first. A mixed powder of 0.5 mg NaCI and 3 mg Mo0₃ in the aluminium oxide boat was placed in the centre of the tube. Another aluminium oxide boat containing S powder was placed in the upstream. The furnace was heated to the growing temperature (600-800 °C) with a ramp rate of 50 °C min^-1. The growth time was 3 min. Ar (or Ar/H₂) with a flow rate of 80 seem (or 80/5 seem) was used as the carrier gas. The as- obtained MoS₂ was quickly transferred to another furnace for heterostructure growth. For the NbSe₂ growth, a mixed powder of 2 mg NaCI and 10 mg Nb₂0₅ and in the aluminium oxide boat was placed in the centre of the quartz tube. Another aluminium oxide boat containing Se powder was placed in the upstream. The furnace was heated with a ramp rate of 50 °C min^"1 to the growth temperature of 700 °C and held at this temperature for 10 min before cooling down to room temperature naturally. Ar/H₂ with a flow rate of 60/4 seem was used as carrier gas.

The optical image of MoS₂-NbSe₂ vertically stacked heterostructure is shown in Fig. 10c. The Raman spectra in Fig. 10d confirmed the formation of vertically stacked heterojunction. Example 38. Comparison of the as-synthesised 2D TMD with previous reports

In this experiment, the synthesis of more than 40 different styles of 2D TMDs was demonstrated. As a comparison, many previous reported works on VS₂, VSe₂ and NbS₂ showed that thick flakes of these materials were obtained using normal CVD method (Table 5). The work on the synthesis of WS₂ and WSe₂ using salt was also reported (Table 5). In that work, only WS₂ and WSe₂ were obtained with no growth mechanism discussed. Table 5. Comparison of the as-synthesised 2D TMD with previous reports

Ref. 1 - Li, S. et al. Applied Materials Today, 2015, 1, 60-66.

Ref. 2 - Boscher, N. D., et al., Appl. Surf. Sci., 2007, 253, 6041-6046.

Ref. 3 - Yuan, J. T. et al. Adv. Mater., 2015, 27, 5605-5609.

Ref. 4 - Ge, W. Y., et al., Nanoscale, 2013, 5, 5773-5778.

In addition, the growth conditions and the size of MoX₂, WX₂ and NbX₂ obtained were compared with the reported values (Table 6). Table 6. Comparison of the as-synthesised 2D TMD with previous reports

References:

1 ) Gong, Y. J. er a/., Adv. Fund Mater., 2016, 26, 2009-2015.

2) Rong, Y. M. et al., Nanoscale, 2014, 6, 12096-12103.

3) van der Zande, A. M. er a/., Nat. Mater., 2013, 12, 554-561.

4) Zhao, S. H. et al., 2D Mater., 2016, 3, 025027. Example 39. Superconductivity of as-synthesised MoTe₂ and NbSe₂ monolayers

Method

The transport experiments were carried out in a top-loading Helium-3 cryostat equipped with a 15-T superconducting magnet. Standard low-frequency lock-in technique was employed to measure the longitudinal resistance Rxx. An ac probe current

nA at 30.9 Hz was applied from the source to the drain. Then a lock-in amplifier was used to monitor the longitudinal Rxx through two additional electrical contacts. Discussion

Superconductivity has been demonstrated to exist in high-quality exfoliated NbSe₂ monolayers and in bulk TaS₂ and MoTe₂ (Xi, X. X. er a/., Nat. Phys., 2016, 12, 139-143; Navarro-Moratalla, E. er a/., Nat. Commun. 2016, 7, 11043; Qi, Y. P. er a/., Nat. Commun., 2016, 7, 11038).

To examine the quality of the as-grown 2D crystals, Hall-bar devices were fabricated with their transport properties measured. The transport results for the as-synthesised monolayer NbSe₂ and MoTe₂ are shown in Fig. 18a and c respectively, and for bilayer MoTe₂ in Fig. 19. Interestingly, these two materials exhibited similar transport behaviors. At high temperatures, they exhibited metallic behavior, with dR/dT > 0. When the temperature was reduced to 1.5 K, the superconducting transition began to emerge; zero resistance was finally obtained at 7^"co = 0.4 K and 0.5 K for the monolayer NbSe₂ and MoTe₂, respectively. From the temperature and field dependence of the longitudinal resistance R_xx shown in Fig. 18b and d, the phase diagrams of the upper critical field H_c2(T) as a function of temperature were obtained (Fig. 20a and b). To the best of our knowledge, this represents the first observation of superconductivity in monolayer MoTe₂. Similar superconducting behavior has also been observed in NbSe₂ and MoTe₂ layered samples with different thicknesses.

Fig. 20a and b show the upper critical field H_cZ-T_c phase diagrams, where the superconducting transition temperature T_c under different magnetic fields is defined as the temperature at which the resistance drops to 10% of the normal state resistance R . A linear relationship between H_c2 and T_c (closer to T_c) was observed, which is regarded as a characteristic property of 2D superconductors. The observation of superconductivity in the as-synthesised monolayer NbSe₂ and MoTe₂ represents the realisation of superconductivity in non-ultrahigh-vacuum-grown monolayer materials. Combined with the high mobility of monolayer MoS₂ and ReS₂ (in Example 40), these results indicate the high quality of the as-prepared 2D TMDs.

Example 40. Transport measurements of monolayer oS₂ and ReS₂

The transport measurements of monolayer MoS₂ and ReS₂ were carried out. Fig. 21a and b show the l_d-V_d and l_Conduct ty- g of monolayer MoS₂. The mobility of MoS₂ which is about 30 cm² V V¹ can be calculated from Fig. 21b. The on/off ratio is as high as 10⁸, which is similar to the reported result of monolayer MoS₂ using CVD method (Najmaei, S. et al., Nat. Mater., 2013, 12, 754-759).

Fig. 21c and d show the l_d-V_d and l_d-V_g of monolayer ReS₂. The mobility of ReS₂ was about 6.5 cmW¹ can be calculated from Fig. 21c and this is similar to the reported result of monolayer ReS₂ using CVD method (Keyshar, K. et al., Adv. Mater, 2015, 27, 4640-4648). The mobility was calculated using the equation μ = dl_ds/dV_bg * L_ch (W_chC_ox _ds), where μ is the mobility, W is the channel width, L is the channel length, and C_ox (C_ox = 1.26 ^χ 10^"4 F/m²) is the capacitance per unit area of the Si0₂ layer.

Example 41. Melting points of salt and metal precursors mixtures, determined using thermogravimetry and different scanning calorimetry (TG-DSC)

Method

Thermogravimetry and differential scanning calorimetry (TG-DSC) measurements were performed using a Netzsch STA 449 C thermal analyser. Approximately 10 mg of the sample were loaded into an aluminium oxide crucible and heated at 10 K min^"1 from 20 °C to 920 °C. The 95 vol% Ar/5 vol% H₂ with a flow rate of 40 ml_ min^"1 was used as the carrier gas.

Discussion

The melting points of the precursors for all binary 2D systems were obtained using TG-DSC measurements. They all fell within the temperature window from 600 °C to 850 °C, as shown in Fig. 22a, which matches the temperature range in which the resulting materials grow. This is further supported by the thermogravimetry versus time curves in Fig. 22b. During the growth, coarsening forms a stable nucleus (Fig. 24), then adatoms and atom clusters of chalcogen and metal attach to the edges of as-grown 2D monolayers and grow quickly owing to their high mobility. This helps to produce millimetre-sized single-crystal 2D TMDs, such as the W-, Nb- and Mo-based TMDs (Fig. 3). Example 42. Determining the existence of intermediate products, metal oxychlorides, using X-ray photoelectron spectroscopy (XPS) and X-ray Powder Diffraction (XRD)

Method

XRD was carried out using Bruker D8 Advance XRD with a Cu-Ka radiation at 40kV and 40 mA.

Discussion

To confirm the existence of metal oxychloride, the intermediate products were collected and analysed by X-ray photoelectron spectroscopy (XPS) during the synthesis of monolayer NbX_2> MoX₂ and WX₂ (X = S, Se, Te). The signals from M-CI and M-O (M = W, Nb, Mo) bonds in Nb 3d, Mo 3d and W 4f confirmed the existence of the oxychloride compounds NbO_xCl_y , MoO_xCl_yand WO_xCl_y (Fig. 23a-c). These values are in agreement with reported values (Wu, H.-M., et al., Synth. Met., 1987, 20, 169-183; Alov, N. V„ Phys. Stat. Solidi C, 2015, 12, 263-266; McGuire, G. E., et al., Inorg. Chem., 1973, 12, 2450-2453).The XRD spectrum of the materials obtained during the synthesis of NbX₂ (X: S, Se, Te) indicated the formation of the intermediate metal oxychloride (NbOCI₃). The (101 ) and (111 ) peaks located at 23.6° and 25.5° are in agreement with previous report (Z. Anorg. Allg. Chem., 2002, 628, 488-491 ). Example 43. Nucleation of TMD with and without salt

Nucleation is the first and a key step for the growth of TMD layers. The nucleation density will dominate the geometries of TMD layers. In the reaction between metal oxides and NaCI, metal oxychlorides like MoCl_xO_y, WCI_xO_y, NbCl_xO_y will be formed. The high volatility nature of metal oxychlorides can result in a higher nucleation rate as compared to those without the use of salts. This was observed experimentally where the use of salt during the synthesis process of ReX₂, TiX₂, and WX₂ (X: S, Se, Te) gave large number of nucleation (Fig. 24a, c and e). When no salt was used, almost no nucleus were formed on the substrate (Fig. 24b, d, and f)

Example 44. Comparing the growth rate of MoS₂ monolayer single crystal with and without NaCI

It was observed that the growth time of MoS₂ with NaCI was as short as 3 min and the growth rate was up to 8 μηη s^"1(Fig. 26), owing to the high chemical activities of oxychloride during the reaction. The growth rates can be determined from the ratio of the sample size over the growth time. Fig. 25 shows the optical image of MoS₂ and growth rate without NaCI while Fig. 26 shows the optical image of molten-salt-assisted MoS₂ and the relationship between the growth rate and the distance from the center of substrate to the edge of the substrate. For the formation MoCI₂0₂, it was observed that the growth time was ~3 min.

Example 45. Morphologies of 2D TMD controlled by the nucleation and growth rate

By controlling the nucleation and growth rate, TMD 2D materials with various morphologies can be realised. For example, the parameters can be controlled to form large-size MoS₂ monolayer, continual MoS₂ film and MoS₂ flake with different layers, accordingly.

To synthesise continuous MoS₂ monolayer, high carrier gas flow rate of 100 seem was used to increase the concentration of S vapour, which can react with metal oxide (or oxychloride) nuclei (Fig. 27). The distance between the substrate and MoCyNaCI sources was 1.2 cm with growth temperature at 750 °C and growth time of 3 min. The corresponding weight of MoCyNaCI sources are: (a) 2mg/0.2mg, (b) 3mg/0.3mg and (c) 10mg/0.8mg. An increasing weight ratio of salt over Mo0₃ increases the nucleation density for the growth of MoS₂, thus the distribution of monolayered MoS₂ transits from low-density single crystal (Fig 27a), to mediate-density single crystal, to high-density single crystal (high coverage) and finally to a continual film (Fig. 27c).

Controlling the thickness of MoS?

The layer-controlled growth of MoS₂ needs large size nuclei to provide enough source for the layer-by-layer growth, so the distance between the substrate and the MoCVNaCI sources was shortened to 0.6 cm and the carrier gas flow rate was decreased to 60 seem to increase the size of the nucleus. Decreasing the flow rate of carrier gas can make the nuclei surplus so that the nuclei will grow larger. The growth time was increased to 15 min to ensure sufficient time for layer-by-layer growth, considering that the in-plane growth was generally faster than out-of-plane growth. The growth temperature was set at 750 °C. The corresponding optical images are shown in Fig. 28a-c.

The optical images of different 2D TMDs under different growing conditions are as shown in Fig. 29 to 31 , which indicate the uniformity of the as-synthesised 2D TMDs. Fig. 29a-c show the optical images of MoS₂ and MoSe₂ grown at 750 °C, and MoTe₂ grown at 700 °C. Fig. 29d-f show the optical images of WS₂, WSe₂ and WTe₂ grown at 800 °C, 810 °C and 820 °C respectively, and Fig. 29g-i show the optical images of NbS₂, NbSe₂ and NbTe₂ grown at 780 °C, 790 °C and 800 °C respectively. Due to the rate constant k_s>k_Se>k_Te, TMD 2D films with an increasing thickness can be obtained. Generally, few-layered selenide and telluride compounds can be obtained easily.

The optical images of various TMDs grown in various growing temperatures are as shown in Fig. 30a-f at 800 °C, 780 °C, 790 °C, 810 °C, 810 °C and 700 °C, respectively.

In addition, the thickness of the TMDs (from monolayer to few layers) can be controlled by varying the temperature. It was observed that monolayer PtSe₂ can be obtained with a higher growing temperature of 815 °C (in Fig. 31a), as compared to 800 °C which gave few- layer PtSe₂ (Fig. 31 b). Similar observation was made for VTe₂ (in Fig. 31 c) in which a monolayer was obtained at a higher temperature of 750 °C, as compared to 700 °C (Fig. 31d).

Example 46. Comparison of the properties of Vo.o3Wo.2Moo.77S_{1 6}Seo.4 with MoS₂ for catalysing hydrogen evolution reaction (HER)

Electrochemical measurements of Vo.o3Wo.₂Moo.77Si.₆Seo.4 were carried out using a typical three-electrode cell consisting of a working electrode, a graphite carbon counter electrode and a saturated calomel reference electrode (SCE), with 0.5 M H₂S0₄ as the electrolyte. The electrochemical cell was connected to an electrochemical workstation (CHI760) coupled with a rotating disk electrode (RDE) system (AFMSRCE3529, Pine Research Instrumentation, USA). A glassy carbon electrode (GCE) covered with catalyst samples was used as the working electrode. The potential, measured against a SCE electrode, was converted to the potential versus the reversible hydrogen electrode (RHE) according to E_RHE = E_SCE + E°_SCE (0.2412) + (0.059 x pH). Linear Sweep Voltammetry (LSV) was conducted in 0.5 M H₂S0₄ with a scan rate of 2 mVs^"1 under 1500 rpm. The current density vs potential data plots were corrected for 90% ohmic compensation throughout the system.

TMDs have been proposed as promising candidates for catalysing HER. In comparison to MoS₂, the quniary alloy Vo.o3Wo.₂Moo.77S_1.6Seo_.4 gave an overpotential of -65 mV which was significantly lower than that of MoS₂ (Fig. 32). This shows that the alloy has better catalytic activity than MoS₂ as the alloy has a smaller Gibbs free energy according to the following reaction formula: H⁺+e ^H*- 1/2H₂

Example 47. Properties of MoSe_xTe₂._x containing different ratios of Se to Te The properties of MoSe_xTe₂-_x can be adjusted by varying the ratio of Se:Te. Fig. 33 shows the temperature dependence of square resistance of pristine MoTe₂ in comparison with different MoSe_xTe_2-x containing different ratios of Se to Te.

The square resistances of the samples at room temperature generally varied from 63 Ω to 3120 Ω and it was observed that a larger resistance was obtained with an increase in Se doping. This demonstrates that MoSe_xTe_2-x can transit from semiconductor to metal with decreasing ratio of Se to Te. At a lower temperature, samples of MoSe_xTe_2-x with x ranging from 1.1 to 1.3 showed a semiconductor behaviour, while MoSe_xTe_2-x with x ranging from 0 to 1.0 gave metallic behaviour which finally became superconducting at extremely low temperature. However, phase transition at 150K to 250K from 1Γ to Td were not observed.

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Claims

1. A process of forming a two-dimensional transition metal dichalcogenide of formula I:

MX₂ I

where:

the temperature of the first temperature zone is from 150 to 500°C;

the temperature of the second temperature zone is from 350 to 950°C;

the one or more metallic precursors are selected from an elemental metal, a metal oxide or a metal halide, where each respective metal is selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Cd, Hf, Ta, W, and Re, wherein the growth rate of the two-dimensional transition metal dichalcogenide of formula I on the substrate is from 0.01 pm/s to 10 pm/s.

2. The process according to Claim 1 , wherein the carrier gas comprises an inert gas having a flow rate of from 30 to 200 seem, optionally where the inert gas is argon and/or nitrogen.

3. The process according to Claim 2, wherein the carrier gas further comprises hydrogen, the hydrogen having a flow rate of from 1 to 20 seem.

4. The process according to any one of the preceding claims, wherein the alkali metal halide is selected from one or more of the group consisting of NaCI, KCI, LiCI, NaBr, KBr, LiBr, Nal, Kl, and Lil.

5. The process according to any one of the preceding claims, wherein the thickness of the deposited transition metal dichalcogenide is from 1 nm to less than 1 pm (e.g. from 1 nm to 500 nm).

6. The process according to any one of the preceding claims, wherein the process further comprises laying one or more further layers of a two-dimensional transition metal dichalcogenide of formula I onto a substrate that has already undergone the deposition process described in the preceding claims, optionally wherein the two-dimensional transition metal dichalcogenide of formula I deposited in each further layer is different to the layer on the surface of the substrate.

7. The process according to any one of the preceding claims, wherein the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 1.5:1 to 5:1.

8. The process according to any one of the preceding claims, wherein the flow rate of the carrier gas is varied during the process to provide an in-plane heterostructure.

9. The process according to any one of Claims 1 to 6, wherein when the transition metal dichalcogenide of formula I is MoS₂, the temperature of the second temperature zone is from 350 to 800°C (e.g. from 350 to 600°C) and the wtwt ratio of the one or more metallic precursors to the alkali metal halide is from 5:1 to 10: .

10. The process according to any one of Claims 1 to 6, wherein when the transition metal dichalcogenide of formula I is MoSe₂, the temperature of the second temperature zone is from 550 to 900°C (e.g. from 550 to 650°C, such as from 550 to 600°C) and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 5:1 to 10:1.

11. The process according to Claim 9 or Claim 10, wherein:

(a) the wtwt ratio of the one or more metallic precursors to the alkali metal halide is 6:1 ; and/or

(b) the metallic precursor is Mo0₃; and/or

(c) the alkali metal halide is NaCI.

12. The process according to any one of Claims 1 to 6, wherein when the transition metal dichalcogenide of formula I is selected from the group consisting of PdTe₂, PdSe₂, PdS_2l PtTe₂, PtSe₂, and PtS₂, the temperature of the second temperature zone is from 750 to 860°C and the wtwt ratio of the one or more metallic precursors to the alkali metal halide is from 8:1 to 12:1.

13. The process according to Claim 12, wherein:

(a) the wtwt ratio of the one or more metallic precursors to the alkali metal halide is 10:1 ; and/or

(b) the metallic precursor for Pd is PdCI₂ and the metallic precursor for Pt is PtCI₂; and/or

(c) the alkali metal halide is NaCI.

14. The process according to any one of Claims 1 to 7, wherein when the transition metal dichalcogenide of formula I is selected from the group consisting of HfTe_2> HfSe₂, HfS₂, VTe₂, VSe₂, VS₂, TiTe₂, TiSe₂, TiS₂, NbTe₂, NbSe₂, NbS₂, ZrTe₂, ZrSe₂, ZrS_2l TaTe₂, TaSe₂, TaS₂, MoTe₂, and WTe_2> the temperature of the second temperature zone is from 600 to 860°C and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 1.6:1 to 4:1.

15. The process according to Claim 14, wherein:

(a) the metallic precursor for Hf is Hf and the metallic precursor for each of V, Ti, Nb, Zr, Ta, Mo and W is a metal oxide of each respective metal; and/or

(b) the alkali metal halide is NaCI.

16. The process according to any one of Claims 1 to 6, wherein when the transition metal dichalcogenide of formula I is FeS₂, FeSe₂ or FeTe₂, the temperature of the second temperature zone is from 500 to 850°C (e.g. from 500 to 600°C) and the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 4:1 to 6:1 , such as 5:1.

17. The process according to Claim 16, wherein:

(a) the metallic precursor for Fe is a metal oxide (e.g. Fe₂0₃) or a metal chloride (e.g. FeCI₂); and/or

(b) the alkali metal halide is NaCI and/or LiCI.

18. The process according to any one of Claims 1 to 6, wherein the transition metal dichalcogenide of formula I has the formula II:

AaBbCcDdEeFfGgS_hSeiTe_j II

19. The process according to Claim 18, wherein:

(a) the wt:wt ratio of the one or more metallic precursors to the alkali metal halide is from 5:1 to 7.5:1 , such as 5:1 ; and/or

(b) the temperature of the second temperature zone is from 600 to 850°C.

20. The process according to Claim 18 or Claim 19, wherein the transition metal dichalcogenide of formula II is selected from the group consisting of MoSe₂xTe₂₍i._x), Mo_1-xRe_xS₂, MoS₂xTe₂₍i-_x), Mo_1-xNb_xSe₂, Md.xNbxSz, Mo_xNb_1-xS_2ySe_2(1-y), WS_2xTe₂₍₁._x), WSe_2xTe₂₍₁._x), Mo _xW_xTes, NbS_2xSe₂₍₁._x), \N . b£₂, W^Nb^, and V_xvV_yMo₍₁._x._y)S_2zSe₂(₁._z).

21. A transition metal dichalcogenide alloy of formula II:

A_aB_bC_cD_dE_eF_fG_gS_hSeiTe_j II

where:

22. The alloy according to Claim 21 , wherein the transition metal dichalcogenide of formula II is selected from the group consisting of MoSe_2xTe₂₍i._x), Mo_1-xRe_xS₂, MoS_2xTe₂₍i._x), Moi_-xNb_xSe₂, Mo_1-xNb_xS₂, Mo_xNb₁._xS_2ySe₂₍i._y), WS_2xTe_2(1-x), WSe_2xTe₂₍₁._x), Moi_-xW_xTe₂, NbS_2xSe₂₍₁._x), W_1-xNb_xS₂, W_1-xNb_xSe₂, and V_xW_yMo_(1-x._y)S_2zSe₂₍₁.₂₎.