The use of elemental substances(Mo,W)as precursors has a simple chemical reaction process,so it has been used to prepare 1T'-MoTe₂and 1T'-WTe₂earlier,and the preparation method is usually to Te Mo,W films(Fig.2a,B )^[30,34,35]。 In 2015,Lee et al.Deposited a 50 nm amorphous Mo film on a 300 nm SiO₂/Si substrate by electron beam evaporation or sputtering^[34]。 Centimeter-scale multilayer 1T'-MoTe₂films were obtained by annealing the Te-treated Mo films at low Te vapor pressure(Figure 2A).In the same year,Dresselhaus et al.Reduced the thickness of the Mo film to 1 nm and inverted it on a boat containing Te powder,thus obtaining a thinner and more uniform 1T'-MoTe₂film^[30]。 In 2017,Cha et al.Used a similar method to prepare centimeter-scale 1T'-WTe₂（(Fig.2b )^[35]。 Different from before,Cha et al.Studied the effect of W film thickness on the product thickness,and they prepared W films with different thicknesses(1–30 nm)by sputtering method,and the results showed that the 1T'-WTe₂films of 10,31 and 54 nm could be obtained from W films of 2.3,4.2 and 7.1 nm thickness,respectively.From the statistical results,the thickness of 1T'-WTe₂film is about 6–7 times of that of W film.In addition,they analyzed two chemical reactions in the growth process:

Te is reduced by H₂to hydrogen telluride(H₂Te)as an intermediate product,which then reacts with the W film to produce 1T'-WTe₂（Fig.2 C).Atomic force microscope(AFM)characterization showed that the product was rough(Fig.2C),on the one hand,due to the uneven deposition of particles during the sputtering process of W film;On the other hand,because the step(1)is a reversible reaction and the H₂Te is unstable,Te is not sufficient 。

Large area products with the same size as the substrate can be obtained by using elemental 1T'-MTe₂,but the thickness of the products is often more than 10 nm due to the thickness of Mo and W films,and the thickness is not uniform.It is worth noting that the reaction temperature of CVD system is mostly not more than 1100℃,while the melting point of Mo is 2620℃,and the melting point of W is as high as 3400℃.Therefore,the direct use of elemental Mo and W powders as precursors has the problems of difficult volatilization,low concentration and instability of gaseous precursors,and it is difficult to achieve the controllable synthesis of high-quality,large-area 1T'-MoTe₂and 1T'-WTe₂^[36]。

In order to overcome the problem of high melting point of single substance,researchers began to consider using oxides with relatively low melting point as precursors to improve the concentration and controllability of gaseous precursor supply.The specific strategies include:(1)directly using MO_x（x=2,3）as the precursor(the melting point of（MoO₂is about 1100°C,that of MoO₃is about 800°C,and that of WO₃is about 1470°C);(2)on the basis of the MO_x,salt and other substances are introduced as a fluxing agent or a sleeve method is used,the fluxing agent can further improve the gaseous concentration of the precursor,and the sleeve method can prevent two gaseous precursors from reacting in the carrier gas transportation process,thereby ensuring the continuous and stable supply of the precursor at the downstream substrate position^{[36][29,37⇓⇓⇓⇓~42][43,44]}; And(3)adding molecular sieves and reagents(OIs)for controlling the volatilization of oxides into the MO_x,so that the supply of the precursor is more stable 。

In 2016,Dresselhaus et al.Found that centimeter-scale,uniform-thickness,multilayer 1T'-MoTe₂single crystals could be obtained by oxidizing 1 nm Mo films into MoO₃films in air and then Te-oxidizing them in Te-poor atmosphere at 700°C(Figure 2D )^[45]。 The melting point of MoO₃is only 800℃,which can obtain more sufficient gas concentration in the temperature range of CVD equipment.Moreover,by comparing the AFM characterization results,it can be found that the 1T'-MoTe₂（prepared with MoO₃(Fig.2e)is smoother and denser than that prepared with elemental Mo(Fig.2F).The researchers tested the electrical properties of the 1T'-MoTe₂film,and the resistance value was about 1120Ω·sq^-1,which was much smaller than that of the 1T'-MoTe₂grown with Mo.The improvement of film quality greatly promotes the electrical properties of materials,and also makes MoO_xbecome the mainstream precursor for the preparation of 1T'-MoTe₂by CVD 。

The concentration of the gaseous precursor can be further increased by adding salts and other substances to the oxide as a flux.Liu et al.added Te powder and MoCl₅(WCl₆)powder into MoO₃(WO₃)powder(Fig.3A),on the one hand,Te powder can form a low melting point eutectic with MoO₃(WO₃))powder,which can reduce the melting point of the precursor to below 450°C,thereby increasing the precursor concentration^[29]; On the other hand,MoCl₅(WCl₆)itself has a low melting point,with a MoCl₅of about 190°C and a WCl₆of about 270°C,which is easy to evaporate and contributes to rapid nucleation^[36]。 Liu et al.Placed the boat containing the mixed powder in the center of the quartz tube,another boat containing Te powder in the upstream,and the SiO₂/Si substrate in the downstream,and reacted at 820°C for 5 min to obtain monolayer 1T'-MoTe₂and 1T'-WTe₂single crystals with sizes of∼350 and∼150μm,respectively.Few-layer 1T'-MoTe₂and 1T'-WTe₂can also be obtained by prolonging the growth time or changing the Te source concentration.In the Scanning transmission electron microscopy(STEM)images of few-layer Scanning transmission electron microscopy and 1T'-WTe₂(Fig.3 B,C),no obvious vacancy defects were observed,indicating that the crystallinity of Scanning transmission electron microscopy and 1T'-WTe₂is good.The researchers further studied the electrical properties of the two materials.Interestingly,at low temperature and zero magnetic field,the bilayer 1T'-WTe₂showed insulating behavior,while the 1T'-MoTe₂showed superconducting properties.It is worth noting that although this method can prepare 1T'-MoTe₂and 1T'-WTe₂with large area,controllable number of layers and uniform thickness,it has not been widely used in subsequent studies because of the high reaction speed of MoCl₅(WCl₆),which is dangerous.In addition to the addition of Te powder and MoCl₅(WCl₆)to the oxide,the addition of alkali metal salts can also improve the volatility of the precursor.On the one hand,alkali metal salts(NaCl,KCl,KI,etc.)Can also reduce the melting point of metal oxides and react with metal oxides to form volatile intermediates;On the other hand,alkali metals(Na,K,Li,etc.)can reduce the reaction barrier and thus increase the reaction rate^[46]。 KI can react with MoO₂to produce volatile intermediates such as MoO₂I₂,which can greatly increase the concentration of Mo in the gaseous state.Fu et al.Placed a mixture of MoO₂powder and KI powder in a boat,and inverted the substrate on the boat,which could increase the Mo source concentration on the surface of the substrate^[37]。 The mixture was heated to 700°C for 80 min,and high coverage 1T'-MoTe₂single crystal flakes with thickness of 1.0–5.1 nm were obtained.In addition,KI can volatilize at high temperature,thus reducing impurity residues.The relationship between the conductivity of 1T'-MoTe₂and temperature is analyzed,and the results show that 1T'-MoTe₂has a smaller relative localization length than MoSe₂^[47]。 Intermediates such as gaseous MoO₂Cl₂and MoOCl₄can also be produced by the reaction of NaCl and MoO₃,thus increasing the concentration of precursors.Jo et al.Heated the mixture of MoO₃powder and NaCl powder to 710°C for 30 min,and obtained 1T'-MoTe₂flakes with thickness varying from 3.5 nm(5 layers)to 7.7 nm(11 layers)on the surface of SiO₂/Si substrate^[38]。 Subsequently,they utilized the same method and fabricated few-layer 1T'-MoTe₂flakes on various substrates(sapphire,silicon nitride,and mica).The amount of NaCl can greatly affect the morphology of the product(Fig.3D).Appropriate use can increase the nucleation density,while excessive use can lead to the formation of thick nuclei.In the preparation of 1T'-WTe₂,water can also be used as a growth adjuvant to form a volatile WO₂(OH)₂with WO₃,thereby increasing the concentration of W source and optimizing the quality of the product.Hao et al.Oxidized W foil into a WO₃film,vertically stacked with c-plane sapphire,and placed in the center of a quartz tube^[42]; Add 0.05 mL of water to the Te powder and place it upstream of the quartz tube(Fig.3e).The stacking of the WO₃film and the substrate realizes the spatial confinement,which can improve the stability of the Te process.Based on this method,Hao et al.Prepared 1T'-WTe₂flakes with a thickness of 1.8 nm at 720°C.By changing the flow rate of H₂,the morphology of the product changed from rectangular(10~25 sccm)to triangular(30~40 sccm)to irregular(45 sccm).Compared with the alkali metal salt auxiliary agent,the water auxiliary agent better avoids the problem of doping.It should be noted that excessive water will lead to the etching of 1T'-WTe₂,which will inhibit the nucleation and growth of 1T'-WTe₂.Previous studies have shown that the substrate symmetry can modulate the activation energy barrier of different crystal phases of the domain,and then affect the morphology of the domain^[48,49]。 Hao et al.Counted the growth orientation of 1T'-WTe₂on different substrates.On the c-plane sapphire substrate with threefold symmetry,1T'-WTe₂showed three growth orientations with an angular spacing of 60°(Fig.3F);While on the a-plane sapphire substrate with dual symmetry,1T'-WTe₂exhibits a single orientation(Figure 3G),and this result well confirms the influence of substrate symmetry on the product morphology 。

In the CVD growth process,the unstable supply of gaseous precursors can also affect the uniformity and size of the product,which can be avoided to some extent by the addition of molecular sieves and reagents(OIs)to control the volatilization of oxides.Feng et al.Placed a mixture of MoO₃and NaCl in a small boat and completely covered the powder with a molecular sieve with a pore size of about 44Å^[40]。 Molecular sieves can not only act as a physical barrier to prevent the transfer of MoO₃powder to the substrate surface(contributing to the cleanliness of the substrate);It can also be used as a microcontainer to control the stable release of Mo source precursor,thus realizing the large-scale and uniform nucleation of MoO₃on the SiO₂/Si substrate,and the domain size of single crystal 1T'-MoTe₂can reach almost 100μm.This study also found that temperature had a significant effect on the product morphology(Figure 3H,I).The calculation results show that the formation energy of the B(100)orientation of MoTe₂increases greatly with the increase of the growth temperature,which leads to the faster growth rate of the a(010)orientation than that of the B(100)orientation(Fig.3j),and finally the ribbon morphology is obtained.This achievement further explains the growth mechanism of 1T'-MoTe₂.Cheng et al.Used OIs to cover the MoO₃to optimize the release of Mo vapor in the CVD reaction,and finally obtained a monolayer 1T'-MoTe₂sheet on the SiO₂/Si substrate^[50]。 They found that higher Mo∶Te favored the in-plane fast growth of MoTe₂monolayer,while lower Mo∶X favored the out-of-plane fast growth,resulting in the formation of multilayer structure 。

Ammonium heptamolybdate((NH₄)₆Mo₇O₂₄,AHM)and ammonium metatungstate((NH₄)₁₀W₁₂O₄₁)can decompose to produce gaseous oxides of Mo and W at about 300 and 400°C,respectively.The decomposition temperature of these two compounds is much lower than the melting point of simple substances and oxides,so they can significantly increase the concentration of gaseous precursors^[51,52]。 This summary will be further summarized according to the supply mode of ammonium salt precursor,including:(1)heating ammonium salt powder;(2)depositing droplets of ammonium salt aqueous solution;(3)spin coat ammonium salt aqueous solution.Compared with the ammonium salt powder,the concentration of the metal precursor can be controlled more finely by using the ammonium salt aqueous solution,and the uniformity of the precursor on the surface of the substrate can be improved by using the spin coating method,so that the preparation of a large-area thin film is realized.In addition,the substrate is pretreated by plasma or acidic solution before spin coating to enhance the hydrophilicity of the substrate,which helps the precursor solution to be distributed more uniformly。

Xu et al.Dissolved(NH₄)₆Mo₇O₂₄or(NH₄)₁₀W₁₂O₄₁powder and KCl in deionized water(DI)and obtained a uniform mixture of(NH₄)₆Mo₇O₂₄and KCl((NH₄)₁₀W₁₂O₄₁and KCl)after drying^[53]。 The mixture was placed in the middle of the quartz tube as the Mo(W)source,and the Te powder was placed in the upstream as the Te source.High-quality and large-area single-layer and multilayer 1T'-MoTe₂and 1T'-WTe₂single crystals(Fig.4 B,C)were grown by using both the inversion method and the sleeve method(Fig.4A),and the maximum size of the products could reach 1 mm and 350µm,respectively.Since the MoO₃produced by precursor decomposition is more active than WO₃,the growth size of MoTe₂is larger than that of WTe₂.It is found that K(Na)Cl is the trigger factor for the growth of 1T'-MoTe₂and 1T'-WTe₂in the CVD process,and the flow rate of H₂and Ar can greatly affect the size of crystal domains.The researchers found some crystal cracks in the Raman scanning images,which are caused by the difference between the thermal expansion coefficients of the 1T'-MoTe₂and the 1T'-WTe₂and the substrate during the cooling process.Overcoming these cracks can further improve the quality of large-size crystal domains.The electrical property test results of 1T'-MoTe₂and 1T'-WTe₂flakes show that the thermal activation energy(Schottky barrier)of the bilayer 1T'-MoTe₂device is about 35 meV,and that of the trilayer 1T'-WTe₂device is about 5 meV,which is close to the value of the mechanically exfoliated sample in the literature,further demonstrating the high quality of the sample^[54,55]。 Although the direct use of ammonium salt powder as metal precursor is simple,it is easy to form a large concentration gradient on the substrate surface during the mass transfer process of gaseous precursor,which leads to uneven nucleation and limited size.This problem can be optimized to some extent by using an aqueous solution of an ammonium salt as the precursor。

When(NH₄)₆Mo₇O₂₄((NH₄)₁₀W₁₂O₄₁))aqueous solution is used as the Mo(W)source precursor,the precursor can be applied by depositing droplets or spin-coating the solution on the substrate surface.In 2016,Johnson et al.First spin-coated a layer of sodium cholate solution on the surface of a SiO₂/Si substrate at a speed of 4000 R/min as a growth promoter,and then deposited(NH₄)₆Mo₇O₂₄saturated aqueous solution droplets on the substrate as a Mo source to prepare 1T'-MoTe₂flakes with different morphologies^[56]。 Due to the lack of uniform spin coating,there is a difference in Mo flux in different regions of the substrate during the growth process,resulting in a significant nucleation density gradient(Figure 4D).In the region with high Mo flux,dendritic polycrystalline 1T'-MoTe₂flakes will grow compactly,while in the region with relatively low Mo flux,the nucleation density is small,and rectangular monocrystalline 1T'-MoTe₂flakes will grow.In the STEM images,the atomic structures of 1T'-MoTe₂with different layer numbers show good agreement with the ball-and-stick model,indicating the high quality of 1T'-MoTe₂,which is also the first report on the atomic resolution images of 1T'-MoTe₂monolayer.Shortly thereafter,Johnson et al.Prepared 1T'-WTe₂flakes by a similar method^[57]。 It is worth mentioning that this study is the first to report the decay of monolayer 1T'-WTe₂in air,and graphene was used to encapsulate 1T'-WTe₂sheets to delay its degradation in air.The results show that the stability of 1T'-WTe₂encapsulated by graphene can be improved from tens of minutes to several days,which is of historic significance for the related applications of 1T'-WTe₂.In the electrical tests of 1T'-MoTe₂and 1T'-WTe₂,the resistance of 1T'-MoTe₂showed a clear temperature dependence,while 1T'-WTe₂showed a clear metallic behavior.Monolayer 1T'-MoTe₂and 1T'-WTe₂both have antilocalization(WAL)cusps in low-temperature magnetoconductance measurements,which is crucial for the study of their potential topological insulator properties and their applications in new switching and sensing devices 。

Spinning(NH₄)₆Mo₇O₂₄and(NH₄)₁₀W₁₂O₄₁solution on the substrate surface can significantly reduce the effect of gas concentration gradient on the morphology of the product compared with the deposition of droplets^[58]。 Lee et al.Mixed AHM,NaOH,OPTI(density gradient medium)aqueous solution at a ratio of 0.3:3:3.5 and spin-coated them uniformly on a SiO₂/Si substrate,and obtained a rectangular 1T'-MoTe₂sheet with uniform thickness and consistent morphology in the whole domain after the reaction^[59]。 By increasing the relative content of(NH₄)₆Mo₇O₂₄aqueous solution in the mixed solution,the size of the crystalline domain can be adjusted,and when the ratio of AHM to NaOH reaches 1:3,a large-area monolayer 1T'-MoTe₂film covering the whole substrate is obtained(Fig.4E).In the optical test,the absorption of 1T'-MoTe₂to light shows obvious anisotropy in the visible range,which provides a good idea for the visualization of the boundary 。

In order to further improve the uniformity of Mo source precursor distribution on the substrate surface,Coletti et al.Used oxygen plasma to pretreat the surface of SiO₂/Si substrate,which effectively increased the free energy of the substrate surface,thus realizing the uniform distribution of reactants on the substrate surface^[60]。 Coletti et al.Mixed AHM,NaOH,and OPTI solutions at a ratio of 1∶1∶5.5,and uniformly spin-coated them on the pretreated substrate at a rotation speed of 2900 R/min.After growth at 730°C,single-layer 1T'-MoTe₂single crystal flakes with an average size of 250μm×30μm(maximum size of 480μm×65μm)were obtained.Comparing the growth results before and after substrate pretreatment(Fig.4F,G),the advantages of oxygen plasma treatment can be clearly reflected.Encapsulation of 1T'-MoTe₂with hexagonal boron nitride(hBN)can extend the lifetime of monolayer 1T'-MoTe₂to more than one month without compromising performance.The researchers believe that this packaging method can be widely applied to other two-dimensional materials that are easy to decompose.Zou et al.used 15 mL of 30%hydrogen peroxide and 35 mL of concentrated sulfuric acid to prepare an etching solvent to etch the substrate,which not only cleaned the substrate surface,but also improved the hydrophilicity of the substrate,so as to promote the uniform distribution of the precursor solution on the substrate surface^[61]。 Subsequently,Zou et al.Spin-coated a 1:1 mixed solution of sodium molybdate(Na₂MoO₄)and NaOH on the pretreated substrate at a rotation speed of 8000 R/min,and obtained a centimeter-scale 1T'-MoTe₂monolayer at 700°C.The electrical measurement results show that the average resistance of the whole monolayer is 3409Ωand the average conductivity is 1.5×10^-5S/m,which indicates the good uniformity of the monolayer 1T'-MoTe₂.The temperature dependence test also reveals the typical semiconductor characteristics of the monolayer 1T'-MoTe₂ 。

During the transformation of Mo,MoO₃to MoTe₂,severe lattice distortion will occur,resulting in a large strain in the lattice structure^{[30,32,39,64,65]}。 Theoretical and experimental results show that 1T'-MoTe₂is more stable than 2H-MoTe₂under strain conditions.Based on this principle,researchers can control the 1T'-MoTe₂by changing the precursor type and adjusting the Te concentration^{[30,65][34,43,45,66]}。

Dresselhaus et al.Found that during the transformation to MoTe₂,the lattice distortion of Mo is more serious than that of MoO₃(about 3 times),and more strain will be generated in the lattice,which will promote the generation of 1T'-MoTe₂^[30]。 Thus,Dresselhaus et al.Prepared centimeter-scale 1T'-MoTe₂thin films using Mo thin films.In 2022,Hu et al.Used this principle to deposit a 5-nm-thick elemental Mo film on the surface of a SiO₂/Si substrate,and then used photolithography and oxygen plasma treatment to form a Mo/MoO₃assembly,which was Te-treated to obtain a polycrystalline 1T'/2H-MoTe₂heterojunction(Figure 5A )^[65]。

The gaseous partial pressure of Te will affect the concentration of Te vacancies in the product MoTe₂,and then change the lattice strain of MoTe₂.Density functional theory(DFT)calculation shows that when the vacancy concentration of Te exceeds 2%,1T'is more stable than 2H phase^[67]。 Johns et al.used Mo as a metal precursor,placed it in the middle of the quartz tube,placed Te powder in the upstream as a Te source,and adjusted the temperature of the Te source by adjusting the position of the Te powder,thereby changing the gaseous partial pressure of Te(Fig.5B)^[66]。 The results show that the content of 1T'-MoTe₂in the product increases with the decrease of Te vapor pressure,but when the Te powder moves to site D,the complete structure of 1T'-MoTe₂can not be formed due to the serious lack of Te atoms.Based on the same principle,Dresselhaus et al.Prepared 1T'-MoTe₂thin films by Te-impregnation of MoO₃thin films in Te-poor atmosphere^[45]。 It should be noted that this defect-induced phase transformation will greatly reduce the relevant properties of 1T'-MoTe₂and is not conducive to the practical application of 1T'-MoTe₂.Therefore,researchers often use the sleeve method or put Te powder in a small neck container to ensure the stable supply of Te source and avoid the phase transformation caused by the lack of Te^{[43,44,53][41]}。 When the concentration of Te source is sufficient,the gaseous partial pressure of Te is proportional to the rate of Te formation.The higher Te-doping rate will make the strain in the lattice unable to release in time,which is more conducive to the growth of 1T'-MoTe₂.Xu et al.Used a two-zone furnace to prepare 1T'-MoTe₂,and adjusted the partial pressure of Te by adjusting the temperature of Te source,thereby changing the rate of Te addition^[43]。 With the increase of Te source temperature,the product changed from 2H-MoTe₂to 1T'-MoTe₂(Figure 5C),and finally the 1T'-MoTe₂single crystal flake was obtained at about 700°C.Chang et al.Used the flow rate of carrier gas to regulate the rate of Te formation,and the faster the flow rate of carrier gas,the faster the rate of Te formation^[68]。 High-purity 1T'-MoTe₂films can be obtained at 750°C when the N₂flow rate reaches 100 sccm 。

at room temperature,2H phase is a thermodynamically stable phase and 1T'phase is a metastable phase.In 2016,Bartels et al.Found through calculation that 1T'is more stable than 2H phase At high temperature^[69]。 the results of this study provide theoretical guidance for The subsequent phase control by temperature。

In 2017,Jo et al.Used MoO₃and Te powder as precursors to controllably synthesize high-purity 1T'-MoTe₂on a variety of substrates by precisely controlling the growth temperature^[38]。 The researchers found that when the growth temperature was higher than 710℃,the uniform 1T'-MoTe₂sheet could be obtained.With the decrease of the growth temperature,the product began to transform to the 2 H phase,and when it was reduced to 670℃,it was completely transformed into 2H-MoTe₂（(Fig.5d).Using this research result,Jo et al.Prepared 1T'/2H-MoTe₂hetero-identical junctions,and the interface between the two phases was atomically sharp and seamless.It is found that the contact barrier is only 25 meV,which directly leads to the advantage of coplanar contact FET over the traditional top contact FET in terms of current and gate tunability.Yan et al.Also controllably synthesized 1T'-MoTe₂sheets and films by precisely controlling the growth temperature^[39]。 In addition,they calculated the Gibbs free energy of each component in the reaction mixture at different temperatures to deduce the thermodynamic equilibrium product of the final state of the reaction.This study is helpful to deeply understand the growth mechanism of 1T'-MoTe₂from the perspective of thermodynamics 。

Cooling temperature and speed are also the key factors affecting the phase state of the product.In the process of slow cooling,the energy of 2H phase is gradually lower than that of 1T'phase,which makes the 1T'-MoTe₂grown at high temperature relax and gradually transform into 2H phase,and finally the stable 1T'-MoTe₂can not be obtained at room temperature^[69]。 To solve this problem,Bartels et al.Prepared a 1–1.5 nm thick Mo film on a Si/SiO₂substrate by magnetron sputtering,and then Te-treated the Mo film at 620°C,and the Mo film was completely tellurized after 5 min.At this time,due to the short Te-treatment time and the lack of Te source,the product was a 1T'-MoTe₂film(Figure 5G )^[69]。 After that,with the extension of the Te annealing time,the Te source was continuously supplied,and the MoTe₂began to transform from the 1 T′phase to the 2 H phase(Fig.5 H),and it was completely transformed into the 2H-MoTe₂film after 180 min of Te annealing.On this basis,a dense 30-nm-thick Atomic layer deposition layer was deposited on the wafer surface by Y(Atomic layer deposition,ALD),and a small hole was introduced in the seed region by a tungsten probe as the only channel for the supply of Te atoms,so as to avoid the spontaneous nucleation of randomly oriented 2H-MoTe₂and make the phase transition extend outward from the seed region,which is essential for the growth of wafer-scale single-crystal thin films 。

In a word,in order to prepare high-quality 1T'-MTe₂,researchers have successively used elemental substances(Mo,W),oxides（MoO_X,WO₃）,ammonium salts[((NH₄)₆Mo₇O₂₄,(NH₄)₁₀W₁₂O₄₁)]as precursors.The growth process and product structure of 1T'-MTe₂were studied.In order to further optimize the growth process and improve the product quality,the researchers also developed optimized growth measures such as salt-assisted method,sleeve method,space confinement method and substrate engineering,and obtained large-area,uniform,single-layer and few-layer 1T'-MTe₂sheets and films.For the controllable preparation of 1T'-MoTe₂,it is necessary to precisely control the phase state of the product by strain and temperature.The phase state of MoTe₂is affected by thermodynamics and kinetics.From the thermodynamic point of view,the phase state with the lowest energy under the corresponding conditions can be obtained by adjusting the lattice strain or growth temperature.For example,the lower the gaseous partial pressure of Te and the higher the growth temperature,the more favorable it is to grow 1T'-MoTe₂.From the kinetic point of view,the mutual transformation between different phase States needs to cross the energy barrier.The room temperature metastable product can be maintained during cooling to room temperature on the basis of the thermodynamically advantageous 1T'-MoTe₂product prepared at high temperature by shortening the relaxation time through a rapid cooling strategy(Fig.5e,f )^[43,69]。

The acting force between an insulating substrate such as SiO₂/Si,sapphire,mica and the like and a surface material is relatively weak,the prepared product is easy to transfer,and the price of the substrate is relatively low,so that the insulating substrate has significant advantages in device manufacturing,and is widely applied to large-area preparation of two-dimensional materials.In recent years,in addition to controlling the morphology of the product by changing the basic growth parameters,such as temperature and substrate type,researchers have chosen to improve the experimental device and introduce salt auxiliaries to improve the surface material quality and structural inhomogeneity of the insulating substrate 。

Zhai et al.Selected ReO₃as the rhenium-containing precursor to synthesize a uniform double-layer polycrystalline 1T'-ReS₂film on the surface of a c-plane sapphire substrate(Fig.7 a),and a large-area thick-layer 1T'-ReS₂product with a hexagonal grain shape could be obtained on the surface of a SiO₂/Si substrate under the same growth conditions^[80]。 Meanwhile,as shown in Fig.7 B,Zhai et al.Synthesized a hexagonal thick layer 1T'-ReSe₂product on the surface of the SiO₂/Si substrate after the reduction of ReO₃by Se powder,with a lateral size of about 5μm^[81]。 Lee et al.Used the same growth method to prepare triangular and rhombic 1T'-ReSe₂grains with thickness varying from 1.5 nm to 30 nm on the surface of SiO₂/Si substrate^[82]。 Xiu et al.Synthesized a thick-layer 1T'-ReSe₂single crystal product on the surface of a SiO₂/Si substrate by an inversion method,which showed excellent electrical properties at room temperature,with an on/off ratio exceeding that of a 10^7[83]。 Wang et al.Obtained monolayer 1T'-ReS₂with different morphologies on the surface of sapphire substrate by adjusting the growth temperature.With the increase of growth temperature,the products gradually changed from circular(700℃)to triangular(800℃),and the growth mechanism was revealed by diffusion-limited mechanism^[84]。 Wang et al.Found that the thickness of 1T'-ReS₂also has a great dependence on the growth temperature,and they compared the HER performance of 1T'-ReS₂with different thicknesses,revealing various stacking structures in 1T'-ReS₂from the atomic level 。

In order to further improve the controllability of the crystal quality,domain size and morphology of the product,Xu et al.Ingeniously stacked two identical mica substrates together to form a narrow space,and limited the precursor concentration in the reaction process by space confinement(Fig.7C).Ffectively reduces the nucleation density and the growth rate of the 1T'-ReX₂,can realize large-area epitaxial growth,obtains a single-layer 1T'-ReX₂product,and has excellent anisotropic physical and chemical properties^[85,86]。 In addition,the vapor pressure of the rhenium source can be controlled by adjusting the valence state of the rhenium source during the growth process by introducing an appropriate amount of H₂into the 1T'-ReSe₂preparation process^[86]。 Zhang et al.Achieved the regulation of the vapor pressure of the rhenium-containing precursor in the CVD system by changing the position of the ReO₃precursor^[87]。 When the ReO₃is located upstream,the precursor is transported to the reaction zone along the carrier gas,at which time the rhenium source concentration is high,and a sunflower-shaped product is obtained at a reaction temperature of 650℃(Fig.7d);However,when the ReO₃is located downstream of the reaction zone,the vapor pressure of the rhenium source in the system is small,and the compact truncated triangular products are easy to grow on the surface of the SiO₂/Si substrate(Fig.7e).The sunflower-shaped product has more marginal active sites,lower Tafel slope(~76 mV/dec),and higher exchange current density（10.5μA/cm²） 。

In the above study,the CVD growth apparatus was improved and the concentration of rhenium-containing precursor in the preparation process was controlled to achieve the preparation of 1T'-ReX₂with controllable morphology.As shown in Fig.7 f,Zhang et al.Selected the method of sodium salt assisted CVD growth to obtain hexagonal(650°C),triangular(700°C)and zigzag（750℃）1T'-ReSe₂sheets on the surface of sapphire substrate by controlling the growth temperature,with the domain size of∼106,∼86 and∼97μm,respectively,and the 1T'-ReSe₂film can be obtained by extending the growth time(from 10 min to 20 min )^[88]。 Among them,the introduction of NaCl can form volatile intermediate products,inhibit nucleation and reduce the nucleation density.Wang et al.Spin-coated an aqueous solution of an alkali metal salt on the surface of a SiO₂/Si substrate and then performed 1T'-ReS₂growth,so that the original acidic substrate was modified to be alkaline,and the matching between the alkaline substrate and the acidic ReO₃was realized,thereby increasing the amount of precursor adsorbed on the substrate surface and promoting the lateral growth of the product to obtain a monolayer product with a larger size^[89]。 the results of STEM characterization show that The product prepared by this method has rich grain boundary structure and is polycrystalline。

Interestingly,1T'-ReX₂with low symmetry atomic structure are more likely to produce macroscopic products with high symmetry shapes,such as hexagons,triangles and so on.Xu et al.Studied the structure of polycrystalline 1T'-ReS₂at atomic scale and proposed a nano-assembly growth model^[25]。 The in-plane orientation angle between the daughter domains of the polycrystal is 60°,and the angle in the vertical direction is 0°and 180°.During the growth process,the Re4 chain is randomly deflected to form seven grain boundaries with different structures(Fig.7G),and finally the polycrystal product with high macroscopic shape symmetry is formed.Among them,GB with different structures have different effects on the electrical transport properties.By controlling the preparation temperature of 1T'-ReS₂,the grain boundary structure and density are controlled from the thermodynamic point of view,and the coherent twin superlattice structure with carrier mobility of 3.4 cm²V^-1·s^-1and switching ratio of 10⁶is constructed.The grain boundary structure can affect the physical and chemical properties of materials,such as electrical and optical properties,but it is difficult to achieve rapid retrieval of grain boundary and high-resolution imaging of structure at the same time in the atomic-level study of GB.Wang et al.Used the high chemical activity of GB to selectively adsorb platinum(Pt)nanoparticles,selected ReO₃and S powder as precursors,and prepared high-quality monolayer 1T'-ReS₂on the surface of mica substrate^[90]。 As shown in Fig.7h,overlapping grain boundaries formed between grains parallel to each other in 1T'-ReS₂due to the presence of horizontal displacement were observed.Then,Wang et al.Used electron beam irradiation in STEM to study the fracture mechanics properties at the grain boundary of monolayer 1T'-ReS₂.When the orientation of Re chain is parallel to the grain boundary direction,the grain boundary is fragile and easy to break^[91]。 On the contrary,the strength of GB increases,and the crack can be blocked.Therefore,as shown in fig.7i,in the complex polycrystalline region with different structures,the direction of crack propagation can be observed to change continuously。

At present,most of the 1T'-ReX₂prepared by CVD are polycrystalline products.The diversity of grain boundary structure makes it difficult to control the mechanical,electrical and optical properties of the products,and the existence of different oriented daughter domains reduces the anisotropy of the properties of 1T'-ReX₂,so the controllable preparation of single crystal products is an important link to realize the large-scale device application of 1T'-ReX₂.In 2018,Zhang et al.Chose Au substrate,ReO₃and Se powder,which have strong coupling effect with 1T'-ReSe₂,as precursors to synthesize products(Fig.8A )^[92]。 The Au foil substrate has a catalytic effect in the CVD preparation process,which catalyzes the decomposition of S and Se polymers in the system into a single active chalcogen atom,and the Re atom has a low solubility on the surface of the Au foil substrate,so that the uniform growth of a monolayer 1T'-ReX₂can be realized^[26][93][94]。 Trapezoidal and banded Selected area electron diffraction sheets can be obtained after 15–30 min of growth,and completely coincident diffraction spots can be obtained by randomly selecting four different areas for Selected area electron diffraction(SAED),which proves that the product is a single crystal.The intensity of Raman active vibration mode changes with the rotation angle of the sample and the change period is 180°,which shows the polarization dependence of Raman,and further verifies that the synthesized 1T'-ReSe₂product is a single crystal.As shown in Fig.8B,Zhang et al.prepared a strip-shaped single crystal Scanning tunneling microscope on an Au substrate by the same growth method,and identified four different Se atoms(Fig.8C)and defect structures such as grain boundaries(Fig.8d,e)in the unit cell of the anisotropic monolayer Scanning tunneling microscope with the help of in-situ Scanning tunneling microscope(STM)and Scan tunnel spectroscopy(STS )^[95,96]。

In addition to suppressing the deflection of Re4 chains due to the strong coupling interaction between Au substrate and 1T'-ReX₂,it is found that the atomic arrangement of different crystal planes of Au substrate has an important influence on the growth of materials.Xu et al.Annealed the gold foil to obtain Au substrates with different low-index crystal faces,and then selected ReO₃and S powder for CVD growth,and grew crystal domains with regular shapes on the surface of the Au(111)substrate with six-fold symmetry(Fig.8f),and the grain boundaries induced by Re4 chain reconstruction could be clearly observed^[97]。 The orientation control of 1T'-ReS₂preparation was realized on the surface of the four-fold symmetric Au(001)substrate and the two-fold symmetric Au(101)substrate,and single crystal grains with two orientations and a single orientation were obtained,respectively(Fig.8g,H),and the orientation distribution decreased with the decrease of substrate symmetry 。

In order to better meet the actual application scenarios of 1T'-ReX₂,Lee et al.Directly prepared two-dimensional 1T'-ReS₂on the surface of flexible glass substrate by reducing ReO₃with H₂S.By controlling the concentration of H₂S gas flow,films from 2 to 8 layers could be obtained(Figure 8 I).Randomly selecting multiple places for testing,multiple results were obtained with the same position and intensity,which proved that the obtained films were uniform everywhere^[98]。 The growth of 1T'-ReS₂directly on the surface of a flexible exfoliated substrate not only avoids damage or pollution in the transfer process,but also lays a foundation for its application in transparent electronic and flexible optoelectronic devices.Liu et al.Selected ReO₃and S powder as precursors to synthesize ultra-high density 1T'-ReSe₂sheets on the surface of porous carbon cloth(PCC),with a thickness of about 2.5–4 nm^[99]。 X-ray photoelectron spectroscopy(ray photo electron spectroscopy,XPS)characterization showed that the characteristic peak of C-Se bond appeared at 286.4 eV,and the interfacial interaction between the product and the substrate promoted the electron transport between the material and the electrode,which enhanced the HER performance(overpotential of 140 mV,Tafel slope of 64 mV/dec)。