GB2548628A

GB2548628A - Process

Info

Publication number: GB2548628A
Application number: GB1605121.1A
Authority: GB
Inventors: Babenko Vitaliy; Grobert Nicole
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2016-03-24
Filing date: 2016-03-24
Publication date: 2017-09-27
Also published as: GB201605121D0

Abstract

A process for producing a two-dimensional metal chalcogenide material by chemical vapour deposition (CVD), the process comprising contacting a substrate in a reaction chamber with a first flow comprising a metal precursor, a second flow comprising a chalcogen precursor and a third flow comprising an oxygen-containing species. The oxygen-containing species may be oxygen, water vapour or a chalcogen oxide, preferably oxygen. Chalcogenides include sulfides, selenides and tellurides, preferably WS2, TiS2 or MoS2. Mixed-metal chalcogenides and metal di- chalcogenides are also encompassed. The material is preferably applied on an Al2O3 substrate in monolayer form. The metal precursor may be a metal halide, oxyhalide or metal carbonyl, or an organometallic compound, preferably a metal oxychloride, such as WOCl4 or MoOCl4. The chalcogen precursor may be sulfur, selenium, tellurium, hydrogen chalcogenide, oxygen chalcogenide, organic chalcogenide or chalcogen halide, the plain chemical elements being preferred. The third flow may further comprise hydrogen and all flows preferably comprise a carrier gas, such as argon, helium and nitrogen. The second and third flows, i.e. the chalcogen precursor and the oxygen-containing compound, are preferably mixed and introduced into the reaction chamber simultaneously. Use of an oxygen-containing compound as an etchant in the aforementioned process is also claimed.

Description

PROCESS

Field of the Invention

This invention relates to a process for producing two-dimensional metal chalcogenide materials by chemical vapour deposition (CVD), and to two-dimensional metal chalcogenide materials which are obtainable by said process.

Background to the Invention

Before the experimental discovery of graphene in 2004, the existence of two-dimensional crystals was not known and such crystals were considered impossible to produce. Recently, however, many types of two-dimensional materials have been identified, including two-dimensional metal chalcogenide materials.

Metal chalcogenides are a group of inorganic compounds comprising one or more atoms of a metal and one or more atoms of a chalcogenide. Metals can be chosen from transition metals, post-transition metals, and lanthanides; and chalcogenides can be chosen from sulfur, selenium, and tellurium. Metal di-chalcogenides are the largest subgroup of metal chalcogenide materials. Many metal di-chalcogenides exist as a result of different metal/chalcogenide combinations. Many metal di-chalcogenides have been found to have desirable properties. Some are insulators (HfS2), some are semiconductors (M0S2, WS2), some are metals (NbS2, VSe2) and some are superconductors (NbS2, NbSe2, TaS2, TaSe2).

Metal chalcogenide materials can have layered structures comprising one or more layers, and can be obtained in two-dimensional form. In particular, monolayer metal chalcogenide materials, consisting of a single metal chalcogenide material layer, have desirable properties {e.g. they can be semiconductors, conductors or insulators) and typically have significantly different properties to bulk or multi-layer structures. For example, M0S2 is a direct bandgap semiconductor in monolayer form, but changes to an indirect bandgap semiconductor when two or more layers are present. Monolayer semiconducting metal chalcogenide materials are promising materials for transistors. LEDs, photodetectors and photovoltaics. Conducting or insulating monolayer metal chalcogenide materials are promising materials for the next generation of nano-electronic heterostructure devices. For electronic, optoelectronic and other applications, materials have to possess sufficiently large defect-free areas in order to exhibit technologically useful properties. Some few layer metal chalcogenide materials, e.g. TiS2, also exhibit desirable properties. Such materials are advantageous in a wide variety of nanodevices and applications due to their high flexibility, low weight and high transparency.

Due to their enhanced properties and their potential use In numerous devices, processes to produce monolayer and few layer metal chalcogenide materials are highly sought after. However, the scalable production of such materials has not been realised in the art to date.

Only a small number of methods have previously been used for the production of monolayer metal chalcogenide materials, namely mechanical exfoliation, liquid exfoliation in solution, and chemical vapour deposition (CVD) methods. Of these methods, CVD is the only scalable production method where the thickness and the crystallinity of the material can be controlled. A small number of CVD methods to produce two-dimensional metal chalcogenide materials are currently available. In one method, layers of metal or a metal oxide are deposited on a target substrate by physical vapour deposition followed by a CVD process where the layer Is converted to a metal chalcogenide material (see Lin et al, Nanoscale, 2012,4, 6637-6641). However, this method produces materials of poor quality as the thickness of the metallic layer deposition and the crystallinity of the deposits cannot be precisely controlled. Another method Is a “close proximity” CVD process where metal precursors are evaporated at high temperature close to a target substrate In a chalcogenide-rich atmosphere (as described in C’Brien et al, Scientific Reports, 2014, 4:7374). However, due to very low vapour pressure of metal oxides, the product deposits on the substrate are highly non-uniform, ranging from bulk deposits to empty regions. Various geometric configurations of precursor positioning over the substrate have been investigated but have not resulted in industrially-applicable production. A further CVD process uses a volatile metal precursor and a chalcogenide for the production of nanoparticles and nanotubes (as described by Margolin et al, Nanotechnology, 19, 2008, 095601). However, large-area, single-crystal two-dimensional metal chalcogenide materials have not been produced with this method. The volatile metal precursors that are used contain elements such as chlorine that disturb the chemical reaction needed for the formation of metal chalcogenide materials, making the process inefficient and introducing residual impurities. Additionally, there is no mechanism by which to control the thickness of crystallinity of the produced material, leading to small, thick, low quality materials.

There remains a need in the art for improved processes for the production of two-dimensional metal chalcogenide materials, in particular monolayer and few layer metal chalcogenide materials.

Summary of the Invention

According to a first aspect of the invention there is provided a process for producing a two-dimensional metal chalcogenide material by chemical vapour deposition (CVD), the process comprising contacting a substrate in a CVD reaction chamber with a first flow comprising a metal precursor, a second flow comprising a chalcogen precursor, and a third flow comprising an oxygen-containing species, wherein the contacting takes place under conditions such that the two-dimensional metal chalcogenide material is formed on a surface of the substrate.

The process of the present invention may be advantageous in a number of respects. In particular, the present process may allow the removal of unstable defects from the lattice of the two-dimensional metal chalcogenide material. Further, the present process may allow multiple layer deposits to be etched, thereby allowing the production of two-dimensional, in particular monolayer, materials. Accordingly, in further aspects, the present invention provides two-dimensional metal chalcogenide materials obtainable by the present process, as well as devices and other products comprising such materials.

Brief Description of the Drawings

Figure 1 shows a list of exemplary metal chalcogenide materials, with known stoichiometries highlighted in bold.

Figure 2 shows a schematic diagram of an exemplary CVD system useful for the production of metal chalcogenide materials according to the present process.

Figure 3 shows various images and spectra characterising WS2 materials. Specifically, Figure 3 shows: (a) an SEM image of a WS2 domain produced without the addition of oxygen; (b) a Raman spectrum of the same material measured with a 533 nm laser wavelength (the ratio of peaks’ intensities of wavenumbers 416 cm'^ and 349 cm"' respectively is 0.3, confirming few-layer WS2 deposits); (c) weak photoluminescence intensity at 618 nm (normalized to the sapphire substrate peak at 694 nm); (d) a comparative SEM image for WS2 produced with the addition of oxygen at a flow rate of 15 seem; (e) a comparative Raman spectrum of bulk WS2 deposits produced with the addition of oxygen at a flow rate of 15 seem; (f) a corresponding photoluminescence spectrum; (g) a comparative SEM image for WS2 produced with the addition of oxygen (the triangular shape indicates good crystallinity and the crystals are much larger (>20 pm) than those produced without oxygen); (h) a comparative Raman spectrum for WS2 produced with the addition of oxygen (the ratio of peaks’ intensities at wavenumbers 414 cm''’ and 351 cm"’' respectively is 0.19 confirming monolayer WS2 deposits); and (i) high photoluminescence intensity (normalized to the sapphire substrate peak at 694 nm).

Figure 4 shows SEM images and spectroscopic characterisation of TiS2 deposits produced by the present process. Specifically, Figure 4 shows: (a) an SEM image of a thick TiS2 nanoflower produced when the flow rate of the oxygen-containing flow was 15 seem; (b), (c) corresponding Raman and photoluminescence spectra; (d) a comparative SEM image of a thin TiS2 domain; and (e), (f) corresponding Raman and photoluminescence spectra respectively, confirming significant differences in the electronic properties between the 2D and bulk forms.

Description of Various Embodiments

According to the present invention, there is provided a process for producing a two-dimensional metal chalcogenide material. The metal chalcogenide materials comprise at least one metal atom and at least one chalcogen atom. The term “chalcogen” as used herein refers to sulfur (S), selenium (Se) or tellurium (Te).

Metal chalcogenide materials produced by the process of the present Invention may be defined by the formula: MOxChy wherein Me is a metal, Ch is a chalcogen, x is from 1 to 5 and y is from 1 to 8.

Each Me may be independently selected from scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Nl), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), aluminium (Al), gallium (Ga), germanium (Ge), indium (In), tin (Sn), antimony (Sb), thallium (TI), lead (Pb), bismuth (Bi), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu): and each Ch may be selected from sulfur (S), selenium (Se), tellurium (Te) and combinations thereof.

The definitions of x and y encompass fractional variations as well as integers. Thus, for example, when x is 1 this may include a mixed-metal chalcogenide compound such as MezMe(z-i)Chy, wherein z + (z-1) = 1 and each Me is a different metal. An example of a mixed-metal chalcogenide material Is a material where the first metal dominates the stoichiometry and the second metal Is a dopant, such as in Mo0.9W0.1S2. Alternatively, the metals can be of similar fractions, for example, Mo0.4W0.eS2. In an alternative embodiment, the metal chalcogenide material produced by the process of the present Invention Is a metal mixed-chalcogenide. Metal mixed-chalcogenide materials are materials that contain two or more different chalcogens in the structure, for example MoSeTe or WSi.8Seo.2·

Preferably the two-dimensional metal chalcogenide material produced by the present process is one of the metal chalcogenide materials listed in Figure 1. In an embodiment, the two-dimensional metal chalcogenide is one of the metal chalcogenide materials listed in bold in Figure 1. In another embodiment, the two-dimensional metal chalcogenide is one of the metal chalcogenide materials not listed in bold in Figure 1.

In an embodiment, the metal chalcogenide material has a fractional stoichiometry and comprises a combination of two metals or a combination of two chalcogenide {e.g. Mo0.3W0.7S2 or MoSi.sSoo.s)·

In a preferred embodiment the two-dimensional metal chalcogenide material produced by the present process comprises a sulfide.

In a preferred embodiment, x and y are integers. Preferably the metal chalcogenide material produced by process of the present invention is a metal chalcogenide material wherein only a single type of metal and a single type of chalcogenide is present, for example ZnS or M0S2.

Preferably, the metal chalcogenide material is a metal di-chalcogenide. Metal di-chalcogenides comprise one atom of a metal and two atoms of a chalcogen, having the formula MeiCh2. Metal di-chalcogenides typically comprise only one type of chalcogen. Preferably the metal di-chalcogenide produced by the process of the invention is selected from WS2, TiS2and M0S2.

Two-dimensional metal chalcogenide materials generally comprise one or more atomically thin crystalline repeating units referred to as layers. The make-up of a layer of a metal chalcogenide material will depend on the composition of the material. For example, a layer of a metal chalcogenide material with the formula MeiChi, for example ZnS, is generally considered to be one atom thick. A layer of a metal di-chalcogenide, for example M0S2 and WSe2, is formed of a sheet of metal atoms sandwiched between two sheets of chalcogen atoms. The bonds between sheets in a layer are typically covalent or ionic bonds. Where more than one layer is present, the layers may be weakly bonded by van der Waals interactions. The crystallographic space groups of layered metal chalcogenide materials produced by the present process are, e.g. Pes/mmc, P3m1, P3m1, C2/m, Pnm2i, Pf, Pbca and P2i/m. A two-dimensional metal chalcogenide material may comprise from 1 to about 20 layers, e.g. from 1 to about 10 layers, e.g. from 1 to about 5 layers. Preferably the metal chalcogenide material produced by the process of the invention is at least partially in monolayer form, more preferably substantially in monolayer form. In a preferred embodiment the metal chalcogenide material is entirely in monolayer form. Monolayer metal chalcogenide materials typically have significantly different properties to bulk or multi-layer structures due to quantum confinement. It has been found that the process of the present invention allows metal chalcogenide materials to be obtained in monolayer form. The number of layers can be determined by techniques known in the art. Atomic force microscopy, high resolution transmission electron microscopy, Raman spectroscopy, photoluminescence and selected area electron diffraction can be used to determine the number of layers present in the metal chalcogenide material. Raman mapping and high resolution TEM can be used to determine much of the material is in monolayer form.

The two-dimensional metal chalcogenide material may have a thickness ranging from about 0.1 nm to about 10 nm, e.g. from about 0.3 nm to about 1 nm. The dimensions of the two-dimensional metal chalcogenide material and the number of layers present may be determined using techniques known in the art. The thickness of the structure in one dimension, out of the three, is generally sufficiently small for the quantum confinement to be substantial to give the materials additional properties distinctive from the bulk material. For example, the Raman spectroscopy or photoluminescence signal of a given material may vary depending on the thickness of the material and thus can be used for characterisation.

Monolayers of two-dimensional metal chalcogenide materials may be polycrystalline, in which case the lattice may have line defects or crystal boundaries. Alternatively, a monolayer of a metal chalcogenide material may be a single crystal, in which case the lattice does not have line defects, but may include occasional point-defects. Preferably, the monolayers produced herein are crystalline. Single-crystal or polycrystalline, monolayer, bilayer or few-layer two-dimensional metal chalcogenide materials or their combinations may be produced by the present processes. Stacked monolayers layers need not be of the same size, morphology or coverage.

The process of the present invention comprises contacting a substrate in a CVD reaction chamber with a first flow comprising a metal precursor, a second flow comprising a chalcogen precursor, and a third flow comprising an oxygen-containing species, wherein the contacting takes place under conditions such that the two-dimensional metal chalcogenide material is formed on a surface of the substrate.

The metal chalcogenide material may be formed on the surface of the substrate by the following reaction: metal precursor (gas) + chalcogen precursor (gas) + oxygen (gas) metal chalcogenide material (solid) + by-productS(gas)

The first, second and third flows are gaseous flows and may enter the CVD chamber as separate flows such that contacting and mixing of the flows occurs in the CVD chamber. Alternatively, two of the flows, e.g. the second and third flows, may be combined before entering the CVD chamber, whereupon they are combined with the other flow. In this case, the flows which are combined before entering the chamber should be combined at a temperature such that little or no reaction takes place until all three flows are intermixed.

In a preferred embodiment, the first, second and third flows enter the CVD chamber as separate flows such that contacting and mixing of the flows occurs in the CVD chamber.

The first flow comprises a metal precursor. Preferably the metal precursor comprises a metal selected from scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), aluminium (Al), gallium (Ga), germanium (Ge), indium (In), tin (Sn), antimony (Sb), thallium (Tl), lead (Pb), bismuth (Bi), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and combinations thereof. Preferably, the metal precursor is selected from: metal halides (comprising a metal with one of F, Cl, Br or I); metal oxy-halides (comprising a metal with oxygen and one of F, Cl, Br or I); metal carbonyls (comprising a metal and one or more CO groups); and organometallic compounds (comprising a metal and combinations of C, 0, H, N). Most preferably the metal precursor is a metal oxy-halide, e.g. a metal oxy-chloride.

The first flow may consist essentially of the metal precursor. Alternatively, the first flow may comprise the metal precursor and a carrier gas. Preferably the carrier gas is selected from noble gases and nitrogen. The metal precursor may be formed by heating the metal precursor to a temperature of from -10 °C to 600 °C, preferably from 40 °C to 300 °C, wherein the heating may take place at a pressure of from 0.01 mTorr to 760 Torr, preferably from 1 Torr to 760 Torr.

In an embodiment, the process produces a mixed-metal chalcogenide material. In this embodiment a further flow comprising a further metal precursor is contacted with the first, second and third flows, wherein the first flow and the further flow comprise different metals. The first flow and the further flow may be added sequentially or simultaneously to the CVD chamber. Where the production of a material comprising more than two different metals is desired, still more flows containing metal precursors may be added in the same way as the further flow comprising a metal precursor described herein.

The second flow comprises a chalcogen precursor. The chalcogen precursor can be selected from: chalcogens; hydrogen-chalcogens; oxygen-chalcogens; organic chalcogens; and chalcogen-halides. Preferably the chalcogen precursor comprises sulfur, selenium or tellurium.

The second flow may consist essentially of the chalcogen precursor. Alternatively, the second flow may comprise the chalcogen precursor and a carrier gas. Preferably the carrier gas is selected from argon, helium and nitrogen. The chalcogen precursor may be formed as a vapour by heating the chalcogen precursor to a temperature of from -10 °C to 600 °C, preferably from 40 °C to 300 °C, wherein the heating may take place at a pressure of from 0.01 mTorr to 760 Torr, preferably from 1 Torr to 760 Torn

In an embodiment, the process produces a metal mixed-chalcogenide. In this embodiment a further flow comprising a further chalcogen precursor is contacted with the first, second and third flows on the substrate, wherein the second flow and the further flow comprise different chalcogens. The second flow and the further flow may be added sequentially or simultaneously to the CVD chamber. Where the production of a material comprising more than two different chalcogens is desired, still more flows may be added.

The third flow comprises an oxygen-containing species. Preferably, the oxygen-containing species is oxygen (O2), water or a chalcogen-oxide. In a more preferred embodiment the oxygen-containing species is oxygen (O2). The third flow may further comprise one or more gases in addition to the oxygen-containing species. In an embodiment a carrier gas is present in the third flow, wherein the carrier gas may be an inert gas. Preferably, the inert gas is argon. In an embodiment the third flow is substantially free, more preferably free, of any metal-containing species.

The third flow may also comprise hydrogen. In some embodiments, the third flow comprises hydrogen and the metal precursor that is present in the first flow is a metal oxychloride or a metal chloride. When the metal precursor is a metal oxychloride or metal chloride, the presence of hydrogen in the third flow may be useful for improving the efficiency of the reaction, as it may remove chlorine from the metal and favour the chalcogen reaction with the metal.

In an embodiment, the third flow Is obtained by mixing a flow comprising Ar, a flow comprising O2 in Ar and a flow comprising H2 in Ar. In an embodiment, the third flow comprises a mixture of argon, oxygen and nitrogen.

Previously, oxygen-containing species have been regarded as adverse contaminants and are rigorously avoided in CVD. However, it has surprisingly been found that oxygen-containing species are advantageous in the production of two-dimensional metal chalcogenide materials. The presence of an oxygen-containing species may improve reaction efficiency as well as the purity of the metal chalcogenide material by facilitating the removal of secondary molecules {e.g. organic groups) from the metal and/or chalcogen precursors. Further, the oxygen atoms from the third flow may act as an etchant. This surprising etchant property allows the oxygen to break energetically unstable defects and defects which are energetically less stable than the desired metal chalcogenide materials. The occurrence of these defects in the metal chalcogenide material products is limited, or even non-existent, due to the presence of oxygen and its etchant properties. Further, the etching process provided by oxygen atoms may allow monolayer metal chalcogenide materials to be formed. The etching process provided by oxygen suppresses the formation of multiple layers and therefore allows thin metal chalcogenide materials to be formed reproducibly and uniformly down to monolayer size. A generalised chemical reaction for the formation of two-dimensional metal chalcogenide materials by a process of the invention is as follows: metal precursor (gas) + chalcogen precursor (gas) + oxygen (gas) metal chalcogenide material (solid) + by-productS(gas)

Without wishing to be bound by theory, the below reaction (which is based on experimental observations) shows the oxygen-mediated reaction in the formation of crystalline metal chalcogenide materials: crystalline metal chalcogenide material(soiid) + defective metal chalcogenide material(soiid) —► crystalline metal chalcogenide material (solid) + by-product(gas)

The production of two-dimensional metal chalcogenide materials may be based on the competition between metal, chalcogen and oxygen species to form metal-chalcogenide bonds, metal-oxygen bonds and chalcogen-oxygen bonds. By selecting suitable synthesis parameters it is possible to form a metal-chalcogenide structure, with occasional metal-oxygen and chalcogen-oxygen compound formation only from the less stable defect sites. These metal-oxygen and chalcogen-oxygen compounds are removed from the two-dimensional material lattice.

Depending on the nature of defect formed and whether they are caused by additional metal or chalcogen atoms in the lattice, or missing metal or chalcogen atoms, byproducts may form or may not form respectively.

In an embodiment, the quantity of oxygen in the CVD chamber is much smaller than the quantity of metal precursor. In an embodiment, the molar ratio of oxygen atoms from the third flow to metal atoms from all metal precursors combined is from 0.001:1 to 10:1, more preferably from 0.1:1 to 1:1.

The amount of oxygen-containing species and chalcogen precursor in the present process may be determined relative to the molar flow of the metal precursor.

The ratio of metal atoms from the metal precursor, or precursors, to chalcogen atoms from the chalcogen precursor is sufficient to achieve a suitable stoichiometry between the metal atom and the chalcogen atom. The amount of chalcogen used in the process is typically higher than is necessary to compensate for any reaction of the chalcogen precursor with oxygen. Preferably, the molar ratio of metal atoms from the metal precursor to chalcogen atoms from the chalcogen precursor is from 1:0.5 to 1:10000, more preferably from 1:1 to 1:100.

If hydrogen is used, the molar ratio of the metal atoms in the metal precursor to hydrogen is preferably sufficient to achieve removal of secondary molecules from the metal precursor. Preferably, the molar ratio of metal atoms from the metal precursor to hydrogen atoms is from 1:0.01 to 1:50, more preferably, from 1:0.1 to 1:10.

The molar ratios used for the production of mixed-metal chalcogenide materials or metal mixed-chalcogenide materials can be determined by scaling from the ratios used in pristine metal chalcogenide materials.

The substrate employed in the process of the present invention can be selected from AI2O3 (sapphire), metal oxides, Si, Si02, SiN, boron nitride, gold, noble metal substrates, graphene and hBN thin film. Preferably the substrate is AI2O3. In preferred embodiments the substrate is conditioned before use in order to clean the substrate surface. The substrate is conditioned by annealing in a reducing atmosphere (for metallic and non-oxide substrates) or an oxidising atmosphere (for metal oxide substrates). The annealing may take place under dilute H2 or O2 gases in Ar at a temperature of from 100 °C to 1000 °C.

In the process of the present invention the flows are brought into contact with each other on the surface of the substrate in a CVD reaction chamber. The precursors are contacted with the substrate for an appropriate period of time to enable one or more layers of the two-dimensional metal chalcogenide material to form on a surface of the substrate. In an embodiment of the invention, the flows are contacted with the substrate for a time period of from 1 second to 2 days, preferably from 5 minutes to 480 minutes. Longer synthesis times may result in larger or thicker coverage.

The flows are introduced into the CVD device at any flow rates suitable for the production of two-dimensional metal chalcogenide materials, e.g. from 0.001 standard cubic centimetres per minute (seem) to 10 standard cubic litres per minute (sIm). The flow rates can be controlled in a number of ways. In some embodiments the metal and chalcogen precursors are contained in pressurised vessels, e.g. a gas cylinder or a heated container with a solid or liquid. In such vessels the flow rates can be controlled with mass flow controllers, rotameters, needle valves or similar devices.

Facilitated CVD processes, aimed at reducing the synthesis temperatures or other improvements, such as increasing the growth rates, namely plasma-assisted CVD, UV-assisted CVD, pulsed-CVD or cold-wall reactor configurations may be employed in the present processes.

In an embodiment the CVD apparatus comprises vaporisation chambers which are used to heat metal and/or chalcogen precursors which are in solid form. In this way the solid precursors are heated separately from the substrates. The system can operate in atmospheric and low-pressure configurations.

In an embodiment, the two-dimensional metal chalcogenide material is produced by atmospheric-pressure CVD (APCVD). That is, the CVD process is performed at a pressure that is substantially equal to (for example equal to) atmospheric pressure. In an embodiment, the two-dimensional metal chalcogenide material is grown under a pressure of about 760 Torn

In an embodiment, the two-dimensional material is produced by controlled chamber pressure chemical vapour deposition (CPCVD; for example high vacuum CVD [HVCVD]), ultra-high vacuum CVD (UHVCVD), high pressure CVD (HPCVD) or generally any low pressure CVD (LPCVD). In a preferred embodiment, the CVD process may be performed at a pressure below atmospheric pressure, typically usually in the 0.01 mTorr to hundreds of Torr pressure range. In an embodiment, the flows and the substrate are contacted in the reaction chamber at defined pressure. The substrate may be conditioned before synthesis of the two-dimensional metal chalcogenide material in said controlled pressure chamber. In a preferred embodiment of the invention the flows are contacted with the substrate at a pressure of from 0.01 mTorr to 760 Torre.g. from 1 Torr to 760 Torr.

The temperature in the reaction chamber may be any temperature suitable for formation of the two-dimensional metal chalcogenide material. The temperature may vary depending on the nature of the precursors that are used. For instance, the temperature in the reaction chamber may range from about 100 °C to about 1700 °C, for example from about 50 °C to 2000 °C. In a preferred embodiment, the temperature in the reaction chamber is from about 300 °C to 800 °C. The temperature may be maintained throughout the course of the process or it may be varied. Preferably the temperature is sufficiently high as to overcome the activation energy of the metal chalcogenide material formation reaction and is such that it favours the thermodynamically favourable products to be metal chalcogenide material or their derivatives.

The substrate may be heated before the flows are contacted with it. Preferably the substrate is heated to a temperature of from 50 °C to 2000 °C, e.g. from 300 °C to 800 °C.

The process may comprise a further step of cooling the substrate once the metal chalcogenide material has been formed thereon. For example, the substrate may be cooled to room temperature, e.g. to a temperature of about 25 °C. A fast cooling rate can be used to quench the growth of the two-dimensional metal chalcogenide material. The substrate may be cooled by any suitable means.

The process may also comprise a step of separating the two-dimensional metal chalcogenide material from the substrate. Methods are known to separate these materials. Any suitable separation method may be used.

The process may also include forming products comprising the two-dimensional metal chalcogenide materials produced. Any suitable product that may be formed using the two-dimensional metal chalcogenide materials may is encompassed by this step.

Two-dimensional metal chalcogenide materials produced by the present process may be used in the manufacture of devices and other products. By way of illustration, the device may be an electronic device, a semiconductor, a transistor, a photodetector, a photovoltaic device, a memory device, a field effect transistor (FET), a light emitting diode (LED), a catalyst e.g. a catalyst in a hydrogen evolution reaction or in a Li-ion battery, a nanoelectronics device, a remote control, a controllable band-gap device, a two-dimensional spintronics device, or a radiation-resistant logic circuit.

The following non-limiting Examples illustrate the present invention.

Example 1: Oxygen-mediated tungsten disulfide (WS?) production from tungsten (VI) oxychloride precursor, sulfur, hydrogen gas and oxygen gas

Substrate preparation

Polished sapphire (AI2O3) substrates with 430 pm thickness, 10 mm x 10 mm, with (0001) C-plane orientation were cleaned in a standard RCA clean process. The first step consisted of organic cleaning and particle removal in an ultrasonic bath at 70 °C for 20 minutes with a solution composed of H2O, NH4OH and H2O2 in volume ratio of 5:1:1. This was followed by an ionic cleaning step in an ultrasonic bath at 70 °C for 20 minutes with a solution composed of H2O, HCI and H2O2 in volume ratio of 5:1:1. The substrates were then rinsed in deionised water and dried with a nitrogen gun. CVD system configuration A schematic of the main components of the CVD system is shown in Figure 2. Briefly, the system comprises of: gas cylinders (Ar, O2, H2) connected to a stainless steel gas mixing manifold; vaporisation chambers connected to a stainless steel injection manifold; and a reaction chamber, which consisted of a fused silica tube (32 mm outer diameter, 2 mm thick) inside a tube furnace (38 mm inner diameter, maximum continuous operation temperature of 1150 °C). The fused silica tube was connected to the stainless steel manifolds with stainless steel compression flanges, and the outlet flange was also connected to an exhaust manifold, consisting of a pump and an outlet at atmospheric pressure. The flows of gases from the cylinders and the flows of carrier gases through the vaporisation chambers were controlled with digital mass flow controllers. The gases utilised were Ar (99.995 % purity), 0.1 % O2 in Ar (99.995 % purity) and 2.5 % H2 in Ar (99.995 % purity). A pressure valve (0.3 psi) was utilised to give priority to flows through the vaporisation chambers, thus the by-pass flow was the difference between the total flow introduced through the gas mixing manifold and the flows through the vaporisation chambers. Electrically-heated jackets connected to PID controllers were used to heat the vaporisation chambers. Additionally, the stainless steel manifolds were heated with heating tapes connected to PID controllers to prevent precursors vapour condensation on the tubing. It is beneficial to cool the substrates quickly to quench the material growth. For this reason, the furnace was mounted on a rail platform and was shifted along the fused silica tube at the end of the synthesis. Additionally, fast cooling with a movable furnace allowed the throughput of the material production to be improved.

Two external vaporisation chambers or equivalent configurations were utilised for the metal precursor and chalcogen precursor respectively. It is advantageous to control the precursors in separate chambers, in comparison to the “two-stage” or “three-stage furnace” apparatus typically used for the synthesis of bulk structures or films. Advantages of controlling the precursors in separate chambers include: (i) the flow rate in the chamber is independent of the flow rate over the substrate, and thus the precursor evaporation rate inside the chambers is independent of the gas flow over the substrates; (ii) the main furnace typically operates at a much high temperature than that needed for evaporating the precursors, thus in the “two-stage” or “three-stage furnace” configurations, infrared heating from the mains furnace leads to overheating of the precursors and therefore poor control and poor reproducibility; and (ill) when precursors are evaporated from ceramic boats in a “two-stage” or “three-stage furnace” apparatus, a steady state convective backflow can occur that leads to ongoing contamination of the system and even blockage, causing safety concerns, this is avoided by using separate chambers.

Synthesis

Tungsten (VI) oxychloride, WOCU (98 % purity) and sulfur, S (99.5 % purity) were used as the metal and chalcogen precursors respectively. The substrate was loaded into the reaction tube and the precursors were loaded into the vaporisation chambers. The system was purged with argon. 100 mg of WOCU and 500 mg of S were utilised in each experiment. Both masses were in excess of what was needed for the synthesis of WS2 in order to avoid precursor exhaustion over the course of the synthesis. The stainless steel injection manifold was heated to 200 °C to prevent vapour condensation. The heating jacket for the metal precursor and the heating jacket for the sulfur precursor were pre-heated away from the vaporisation chambers to 135 °C and 270 °C respectively. The furnace was pre-heated away from the substrate to 800 °C.

The synthesis procedure comprised of an annealing stage, a growth stage and a cooling stage. For the annealing stage the furnace was shifted over the substrate and the total gas flow was changed to a mixture of 180 seem Ar, 5 seem of the 2.5 % H2 in Ar and 15 seem of the 0.1 % O2 in Ar. Such an atmosphere is useful to remove carbonaceous or organic contamination typically present on the substrates. The annealing stage lasted 20 minutes to clean the substrate and also to allow the furnace temperature to stabilize. Immediately thereafter, the heating jackets were connected to the precursor vaporisation chambers to start the precursor flows. A 30 seem of Ar was set through the vaporisation chamber of the WOCU precursor and a total of 170 seem of a mixture of Ar, Ar/H2 and Ar/02 through the vaporisation chamber of the S precursor. The gas composition, Ar, Ar/H2 and Ar/02 was changed as is shown in Table 1, but the total flow was kept at 200 seem. The synthesis stage lasted 20 minutes. Immediately thereafter, the heating jackets were removed from the precursors to stop their vapour flow, the gas atmosphere was changed to 200 seem Ar through the bypass line, and the furnace was shifted away from the substrate to quench WS2 growth. The initial cooling rate of the substrates was estimated to be around 50 °C per second, allowing the chemical reaction inside the reaction tube to be sufficiently halted within tens of seconds. When the substrate temperature reached room temperature it was removed from the CVD system.

During the growth stage, sulfur and tungsten deposits accumulated on the fused silica tube after the furnace. Beneficially, the utilization of a moving furnace allowed these deposits to shift to a cold region on the tube by moving the furnace over them under Ar flow in vacuum. The region was chosen sufficiently far from the substrates and allowed the unwanted deposits to be permanently captured. This procedure allowed the material production throughput to be improved, such that hundreds of experiments could be performed without the need to change the tube.

The gas composition of Ar, Ar/H2 and MO2 used in each experiment is shown in Table 1 below. These three flows together make up the third flow. The total flow was kept at 200 seem.

Table 1

Results and Discussion

In Experiment Aw, it was seen that hydrogen does not facilitate the formation of two-dimensional material, and produces large quantities of defective and thick WS2. The role of hydrogen is tentatively identified as a way to bind chlorine molecules from the metal precursors and to allow sulfur to react easily with the metal. Experimental observations confirm this hypothesis: when H2 gas flow was added in some experiments the fused silica tube was covered with an optically visible deposit, whereas without H2 gas there was no deposit. The following chemical reaction has been identified: WOCI4 + 3 S + 3 H2 :5 0.99 WS2 + 4 HCI + 0.7 H2O + 0.3 S2 +0.2 H2S + 0.1 SO2 (etc.)

Most notably, the thermodynamic calculations suggest nearly 100 % efficiency of the metal oxychloride precursor conversion to tungsten disulfide. A reference experiment was conducted with WOCU, sulfur and H2 precursors only, where the flow rates for the growth stage were set to 195 seem Ar and 5 seem of 2.5 % H2 in Ar (Experiment Aw, Table 1). A scanning electron micrograph of the resulting material is shown in Figure 3a. The WS2 deposits are very small (<1 pm). The overall shape is circular with irregular edges, indicating that the deposits are not single crystals, but are polycrystalline with many defects and domain boundaries that degrade the properties of the material. Figure 3b shows a Raman spectrum of the WS2 material produced in this experiment, measured with a 533 nm laser wavelength. The ratio of peaks’ intensities of wavenumbers 416 cm'^ and 349 cm'^ respectively is 0.3, confirming few-layer WS2 deposits. The photoluminescence intensity in Figure 3c at 618 nm is very weak (normalized to the sapphire substrate peak at 694 nm), confirming that the material produced without the addition of oxygen has poor optoelectronic properties.

The oxygen etching mechanism was confirmed experimentally. The growth was performed with 180 seem Ar, 5 seem of 2.5 % H2 in Ar and 15 seem of 0.1 % O2 in Ar (Experiment Bw, Table 1), no deposits were observed on the substrate.

Systematic experiments with varying oxygen concentrations were performed, as shown in Table 1 and Figure 3. In all experiments where oxygen gas was added and WS2 deposits were formed on the substrate, the deposits had triangular shape with very sharp edges, indicating good crystallinity. Additionally, large triangles could be grown. The molar fraction of oxygen had significant influence on the thickness of the WS2 material produced. For example, an experiment with 165 seem Ar, 20 seem of 2.5 % H2 in Ar and 15 seem of 0.1 % O2 in Ar (Experiment D) resulted in bulk triangular deposits that had a ratio of the main peaks of around 1 and no photoluminescence peak (Figure 3d-f). However, when the molar fraction of oxygen was increased approximately 6 times, with 100 seem Ar, 20 seem of 2.5 % H2 in Ar and 80 seem of 0.1 % 02 in Ar (Experiment F), large monolayer triangles appeared. A scanning electron micrograph of the sample is shown in Figure 3g. The triangular shape indicates good crystallinity and the crystals are much larger (>20 pm) than those produced without oxygen. From Raman spectroscopy (Figure 3h) monolayer WS2 deposits were confirmed: the ratio of peaks’ intensities at wavenumbers 414 cm'^ and 351 cm'^ respectively was 0.19. Photoluminescence intensity of the material was high, as shown in Figure 3i (normalized to the sapphire substrate peak at 694 nm) confirming good crystallinity and monolayer thickness of the WS2 material synthesised with the addition of oxygen.

Example 2: Oxvaen-mediated molybdenum disulfide (MoS?) production from molybdenum (VI) oxychloride precursor, sulfur, hydrogen gas and oxygen gas

Following similar procedures to those described in Example 1, two-dimensional M0S2 was grown by utilising a molybdenum (VI) oxychloride precursor (MoOCU) instead of the tungsten (VI) oxychloride precursor (WOCU). The vapour pressure of MoOCU is slightly higher than that of WOCI4 for the 100-150 °C temperature range. Therefore, the temperature of the heating jacket for the MoOCI4 precursor was set to 120 °C to achieve the synthesis of crystalline, monolayer two-dimensional M0S2 material.

Example 3: Oxygen-mediated mixed-metal chalcoqenide materials production, WvMoi-vS? production with tungsten (Vh oxychloride, molybdenum (Vh oxychloride, sulfur, hydrogen gas and oxygen gas

Following similar procedures, a two-dimensional mixed-metal chalcogenide material WxMoi-xS2 was produced by heating a mixture of WOCU and MoOCU precursors to 135 °C. The vapour pressure of MoOCU precursor is approximately double that of WOCU precursor at this temperature. Therefore, to achieve the stoichiometry of W0.5Mo0.5S2, the molar ratio of WOCU to MoOCU should be 2:1. By adjusting the ratio by the molar mass of 341.65 g/mol for WOCI4 and 253.77 g/mol respectively, the mass ratio is therefore around 2.7:1. For example, 73 mg of WOCU and 27 mg of MoOCU can be used.

Example 4: Oxvaen-mediated titanium disulfide (TiS?) production from titanium (IV) chloride, sulfur, hydrogen oas and oxygen gas

Following similar procedures, a two-dimensional mixed-metal chalcogenide material TiS2 was produced by maintaining the temperature of titanium tetrachloride (TiCU) to 25 °C while sulfur was heated to 280 °C. Due to a different reaction kinetics and thermodynamics, the furnace was heated to 600 °C. During the synthesis, the flow through the TiCU precursor was set to 6 seem, while the flow through the sulfur precursor was set to 94 seem. The synthesis was performed for 10 minutes resulting in TiS2 domains of tens of micrometres. When a flow comprising 64 seem Ar, 20 seem Ar with 2.5 % H2 and 10 seem Ar with 0.1 % 02was used, bulk nanoflowers were formed, as shown in the SEM micrograph in Figure 4a. Raman and photoluminescence measurements in Figure 4b-c confirm thick, bulk TiS2 deposits. With a flow comprising 54 seem Ar, 20 seem Ar with 2.5 % H2 and 20 seem Ar with 0.1 % O2, thinner deposits were obtained, as shown in Figure 4d. The corresponding Raman (Figure 4e) and photoluminescence (normalized to the sapphire substrate peak at 694 nm) spectra show that the deposit is very thin and that the optoelectronic properties of two-dimensional and bulk TiS2 deposits are very different.

It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. Each feature disclosed in the description, and where appropriate the claims and drawings may be provided independently or in any appropriate combination.

Claims

1. A process for producing a two-dimensional metal chalcogenide material by chemical vapour deposition (CVD), the process comprising contacting a substrate in a CVD reaction chamber with a first flow comprising a metal precursor, a second flow comprising a chalcogen precursor, and a third flow comprising an oxygen-containing species, wherein the contacting takes place under conditions such that the two-dimensional metal chalcogenide material is formed on a surface of the substrate.

2. The process according to Claim 1, wherein the metal chalcogenide material is of the formula MexChy, wherein: each Me is independently selected from scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), aluminium (Al), gallium (Ga), germanium (Ge), indium (In), tin (Sn), antimony (Sb), thallium (TI), lead (Pb), bismuth (Bi), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu) and combinations thereof; Ch is selected from sulfur, selenium and tellurium; X is from 1 to 5; and y is from 1 to 8.

3. The process according to Claim 1 or Claim 2, wherein the metal chalcogenide material is selected from the list in Figure 1.

4. The process according to any preceding claim, wherein the metal chalcogenide material is M0S2, WS2, MoSe2, WSe2, HfS2, HfSe2, ZrS2, ZrSe2, TiS2, TiSe2, VS2, VSe2, CoSe2, NbS2, NbSe2, TaS2, TaSe2, lnS2, InSe, SbS2, SbSe, GaS2 or GaSe2, preferably WS2, TiS2 or M0S2.

5. The process according to any preceding claim, wherein the metal chalcogenide material is a metal di-chalcogenide.

6. The process according to Claim 1 or Claim 2, wherein the metal chalcogenide material comprises at least two different metals.

7. The process according to Claim 6, said process comprising contacting a further flow comprising a further metal precursor with the first, second and third flows, wherein the first flow and the further flow comprise different metals.

8. The process according to any preceding claim, wherein the metal chalcogenide material comprises at least two different chalcogens.

9. The process according to Claim 8, said process comprising contacting a further flow comprising a further chalcogen precursor with the first, second and third flows on the substrate, wherein the further flow and the second flow comprise different chalcogens.

10. The process according to Claim 2, wherein the metal chalcogenide material is of the formula MeiChi.

11. The process according to any preceding claim, wherein the metal chalcogenide material is at least partially in monolayer form.

12. The process according to any preceding claim, wherein the metal chalcogenide material is substantially in monolayer form.

13. The process according to any preceding claim, wherein the metal precursor is a metal halide, a metal oxy-halide, a metal carbonyl, or an organometallic compound.

14. The process according to Claim 13, wherein the metal precursor is a metal oxy-halide.

15. The process according to Claim 14, wherein the metal precursor is a metal oxychloride.

16. The process according to any preceding claim, wherein the chalcogen precursor is selected from sulfur, selenium, tellurium, a hydrogen-chalcogenide, an oxygen-chalcogenide, an organic chalcogenide and a chalcogen-halide.

17. The process according to Claim 16, wherein the chalcogen precursor is sulfur, selenium or tellurium.

18. The process according to any preceding claim, wherein the oxygen-containing species is oxygen (O2), water or a chalcogen-oxide.

19. The process according to Claim 18, wherein the oxygen-containing species is oxygen.

20. The process according to any preceding claim, wherein the third flow further comprises an inert gas selected from noble gases and nitrogen, e.g. argon.

21. The process according to any preceding claim, wherein the third flow further comprises hydrogen.

22. The process according to any preceding claim, wherein the substrate is selected from AI2O3, metal oxides, Si, Si02, SiN, boron nitride, gold, noble metal substrates, graphene and hBN thin film.

23. The process according to Claim 22, wherein the substrate is AI2O3.

24. The process according to any preceding claim, wherein the molar ratio of oxygen atoms from the third flow to the combined number of metal atoms from all metal precursors is from 0.001:1 to 10:1, preferably from 0.1:1 to 1:1.

25. The process according to any preceding claim, wherein the flows are contacted with the substrate for a period of time from 1 second to 2 days, preferably from 5 minutes to 480 minutes.

26. The process according to any preceding claim, wherein the substrate is heated to a temperature of from 50 °C to 2000 °C.

27. The process according to Claim 26, wherein the substrate is heated to a temperature of from 300 °C to 800 °C.

28. The process according to any preceding claim, wherein the flows are contacted with the substrate at a pressure of from 0.01 mTorr to 760 Torn

29. The process according to Claim 28, wherein the flows are contacted with the substrate at a pressure of from 1 Torrto 760 Torn

30. The process according to any preceding claim, wherein the first flow further comprises a carrier gas.

31. The process according to Claim 30, wherein the carrier gas is selected from argon, helium and nitrogen.

32. The process according to any preceding claim, wherein the second flow comprises a carrier gas.

33. The process according to Claim 32, wherein the carrier gas is selected from argon, helium and nitrogen.

34. The process according to any preceding claim, wherein the process further comprises a step of cooling the substrate and the two-dimensional metal chalcogenide material formed thereon to room temperature.

35. The process according to any preceding claim, wherein the process further comprises forming a product comprising the two-dimensional metal chalcogenide material.

36. A two-dimensional metal chalcogenide material obtainable by the process of any of the preceding claims.

37. The material according to Claim 36, wherein the two-dimensional metal chalcogenide material is in at least partially monolayer form, e.g. substantially in monolayer form.

38. The material according to Claim 36 or Claim 37, wherein the metal chalcogenide is M0S2, WS2, MoSe2, WSe2, HfS2, HfSe2, ZrS2, ZrSe2, TiS2, TiSe2, VS2, VSe2, CoSe2, NbS2, NbSe2, TaS2, TaSe2, lnS2, InSe, SbS2, SbSe, GaS2 or GaSe2, preferably WS2, TiS2 or M0S2.

39. The material according to any of Claims 36 to 38, wherein the metal chalcogenide is a metal di-chalcogenide.

40. A product comprising a two-dimensional metal chalcogenide material according to any of claims 36 to 39.

41. A product according to Claim 40, wherein the product is an electronic device, a semiconductor, a transistor, a photodetector, a photovoltaic device, a memory device, a field effect transistor (FET), a light emitting diode (LED), a catalyst (e.g. a catalyst In a hydrogen evolution reaction or in a Li-ion battery), a nanoelectronics device, a remote control, a controllable band-gap device, a two-dimensional spintronics device, or a radiation-resistant logic circuit.

42. Use of an oxygen-containing species as an etchant in a process for forming a two-dimensional metal chalcogenide material by chemical vapour deposition (CVD).

43. A process or product substantially as described herein with reference to the accompanying figures.