Growth of Wafer‐Scale Single‐Crystal 2D Semiconducting Transition Metal Dichalcogenide Monolayers

Abstract Due to extraordinary electronic and optoelectronic properties, large‐scale single‐crystal two‐dimensional (2D) semiconducting transition metal dichalcogenide (TMD) monolayers have gained significant interest in the development of profit‐making cutting‐edge nano and atomic‐scale devices. To explore the remarkable properties of single‐crystal 2D monolayers, many strategies are proposed to achieve ultra‐thin functional devices. Despite substantial attempts, the controllable growth of high‐quality single‐crystal 2D monolayer still needs to be improved. The quality of the 2D monolayer strongly depends on the underlying substrates primarily responsible for the formation of grain boundaries during the growth process. To restrain the grain boundaries, the epitaxial growth process plays a crucial role and becomes ideal if an appropriate single crystal substrate is selected. Therefore, this perspective focuses on the latest advances in the growth of large‐scale single‐crystal 2D TMD monolayers in the light of enhancing their industrial applicability. In the end, recent progress and challenges of 2D TMD materials for various potential applications are highlighted.


Introduction
The growth of large-scale ultrathin twodimensional (2D) monolayers of semiconducting transition metal dichalcogenides (TMDs) has emerged as an important topic and generated significant interest in fundamental research and industrial applications.3][4][5][6][7][8][9][10] However, the controllable growth of high-quality single-crystal 2D monolayers still needs to be applied in potential applications. [11,12]he performance of TMD-based devices might be substantially influenced by the grain boundaries and crystal defects, which commonly act as scattering/trapping centers for charge carriers and hinder device carrier mobility. [13][16] In 2020, highly oriented epitaxy of wafer-scale monolayers of MoS 2 on sapphire has first been demonstrated on 4-inch wafers to exhibit carrier mobility of ≈70 cm 2 V•s −1 and on/off ratio of ≈10 9 . [15]Recently, 4-inch flexible wafer scale monolayer MoS 2 field-effect transistors (FETs) have been demonstrated by employing polyethylene terephthalate (PET) substrates and atomic layer deposition (ALD) deposited HfO 2 as dielectric layers, showing average mobility of ≈70 cm 2 V•s −1 , on/off ratio of 5 × 10 7 , and subthreshold swing of 83 mV•dec −1 for over 500 randomly picked devices. [16]Bulk crystalline TMD, such as MoS 2 and WS 2 , can be obtained from molybdenite and tungstenite minerals.Due to the weak van der Waals coupling between layers, monolayer or few-layer TMD can be prepared through the exfoliation process using scotch tape or liquid-phase exfoliation. [17,18]In bulk, most TMD semiconducting materials reveal indirect bandgap, whereas monolayers exhibit direct bandgap due to the quantum confinement effect. [19]This unique layer-dependent characteristic of 2D TMDs opens a new window for various possible applications. [20]For example, it is reported that the bandgap of 2D TMDs can be varied effortlessly through strain engineering. [21,22]he enormously increased Coulomb interaction in the direct bandgap TMD monolayer makes it an ideal model for many-body-associated quasiparticles' fundamental properties in www.advancedscience.comcondensed matter physics. [23]26][27] Top-down exfoliation techniques of TMDs result in microscale flakes along with an arbitrary dispersion of film thickness, which can only be applicable in fundamental research. [28,29]Over the years, various bottom-up strategies have been devoted to producing large-scale TMD films and monolayers.[32] As-grown TMD films using the above techniques exhibit polycrystalline structures with various randomly oriented domains and boundaries.For semiconductors and dielectrics, boundary density can significantly affect the device's performance.To restrain the TMD domain boundaries, the epitaxial growth process plays a crucial role.It could become an ideal method if appropriate single-crystal metal substrates like Au or Cu are selected.
Sapphire substrates have been broadly utilized to grow 2D monolayers due to their exclusive lattice constant and insulating surfaces. [33]Numerous reports have embraced c-plane sapphire as the substrate for developing 2D monolayers. [34]However, it has been realized that the c-plane sapphire surface was unsuccessful in terminating the generation of antiparallel domains and twin boundaries, resulting in the synthesis of polycrystalline films.Recently, the epitaxial growth of centimeter-scale single-crystal MoS 2 monolayer on the Au (111) thin film has been reported. [35]espite this achievement, a significant drawback is the high cost of single-crystal Au (111) and its limited size (≈1 cm) although it has the advantage of commercial availability.Further, it has been investigated that using single-crystal Cu thin films is favorable to grow graphene film with no apparent grain boundaries and high carrier mobility. [36]Meantime, the growth of a wafer-scale single-crystal hexagonal boron nitride (h-BN) monolayer has been demonstrated on a Cu (111) thin film. [37]Besides, it has also been noticed that graphene could also be developed along with a favorable orientation on Cu (111) with seamless stitching domains. [38,39]Therefore, the single-crystal metal thin film becomes a favorable substrate for the growth of singlecrystal 2D monolayers. [37,39,40]It will not be easy to grow a 2D TMD monolayer on Cu (111) thin film due to the catalytic nature of Cu.It tends to form CuS, which leads to the degradation of Cu.Therefore, only single-crystal Au (111) thin film offers an excellent opportunity to grow 2D TMD monolayers with suppressed grain boundaries for various applications. [41,42]Eliminating the twin/grain boundaries of metal structures is also beneficial for electronic and optoelectronic devices.There are few works reported on the growth of 2D TMDs monolayer on a single crystal Au (111) thin films deposited on c-plane sapphire by using the CVD technique. [34,43][46][47] In this perspective, we first highlight the growth of single-crystal metal using a single nucleation approach.Then, multi-nucleation growth has been reviewed for growing large domain-size single-crystal 2D TMDs monolayers.Besides, we also outline the re-utilization of the developed metal thin film after detaching successfully grown single-crystal 2D TMD monolayers.In the end, progress and challenges are highlighted.

Growth Approaches
Two major approaches, single-nucleation and multi-nucleation methods, have been reported for growing large domain-size single-crystal 2D monolayers (Figure 1).[50][51] The best-in-class techniques have broadened the size of segregated single-crystal TMD monolayers from 100 μm to >1 mm range.However, the significant drawback of such a vapor transport-mediated growth method is that presenting only one single nucleus is extremely difficult.The subsequent domain size is still too small to be viable with the development of exceptionally integrated devices.On the other hand, the multi-nucleation approach depends on a lattice-matched substrate, which permits the epitaxial TMD domains to grow in a similar direction and later mix into a monocrystalline film. [52]It allows the synthesis of numerous nucleation sites on the substrate, and a TMD single-crystal film can thus be adaptable to the ideal size.Table 1 shows the electrical performance of 2D TMD FETs using various CVD methods, some of the representative works will be discussed in detail in the following sections.
In this manner, the fundamental issue is how to initiate the uniform orientation of each domain.Examinations have been directed at developing enormous area TMD films on epitaxial substrates such as mica and sapphire. [53]However, the sixfold symmetry of such substrates has been discovered to be incongruent with the threefold symmetry of TMD materials, typically prompting the development of antiparallel domains and unavoidable twin boundaries.Those boundaries are generally developed during the growth process and tend to work as conducting channels, which hinder the electrical and optical properties of related devices. [54]Thus, controlling the domain orientation and decreasing the formation of twin boundaries are crucial in the growth of large-scale TMD single-crystal films.It has been studied that the underlying substrate plays an essential role in the growth of TMD films by suppressing twin boundaries.In this article, we will review both the growth approaches and their advantages and challenges.

Nuclei Etching
Nuclei etching has drawn intensive attention to grow large domain-size 2D TMD monolayers.Previously, Chen et al. have successfully reported the growth of large single-crystal and highquality 2D MoS 2 monolayer on the c-plane sapphire substrate through nuclei etching using the oxygen-assisted (OA) CVD technique. [48]The schematic diagram of the OA-CVD technique is shown in Figure 2a.During the growth process, a mixture of Ar and a small amount of O 2 was introduced as a carrier gas, and the pressure was kept at 0.5 Torr.It was observed that this small amount of O 2 strongly reduced the MoS 2 nucleation density, preventing poisoning of the MoO 3 precursor for the large domain size growth of MoS 2 .Besides, it also led to eliminating defects that usually form during the growth process.In this work, the authors observed that the largest size of triangular shape single crystal domains were formed with side lengths ≈350 μm and very high room temperature mobility of 90 cm 2 V-s −1 on SiO 2 .
The optical images of the MoS 2 monolayers in Figure 2b-d reveal the growth of large triangular shape domains of 350 μm size at 2 sccm O 2 flow rate (Figure 2c).In this work, authors have claimed that the observed domain size and mobility were higher than ever observed.The atomic force microscopy (AFM) results (insets of Figure 1b-d) of their work confirmed that a small amount of O 2 could etch off unsteady nuclei and reduce the growth of nanoparticles/nanotubes. [63,64] The structural and optical properties of the MoS 2 film before/after oxygen flow were observed by measuring Raman and photoluminescence (PL) spectra (Figure 2e,f), respectively, revealing the monolayer characteristics of MoS 2 . [65,66]They have also demonstrated the effect of oxygen flow during the growth process and investigated the growth of MoS 2 with an O 2 flow rate at 2 sccm with various growth durations.The average side lengths of grown domains as a function of growth durations are shown in Figure 2g.It was observed that the maximum domain size and the nucleation density of the MoS 2 film were gradually suppressed with further increase in the O 2 flow rate.The relationship between the growth rates/etching of MoS 2 domains as a function of growth durations was estimated and plotted in Figure 2h.The initial growth rate remains constant (<15 min) and drops gradually after the critical growth time.On the other hand, the etching rate was kept constant by maintaining the flow rate of O 2 .The MoS 2 growth can be facilitated when the growth rate dominants over the etching rate.By further increasing the growth time (>30 min), the etching rate overcomes the growth rate, and the MoS 2 domains shrink substantially.Hence, it was concluded that high quality MoS 2 domains are highly associated with the synergistic effect of growth and etching. [67,68]urthermore, the above studies have evaluated the electrical properties of MoS 2 monolayers by fabricating FETs.The schematic of the fabricated device is shown in Figure 2i, and its output characteristics are presented in Figure 2j.The device exhibits n-type behavior with linear I-V characteristics (ohmic behavior) at low bias region.The estimated electron mobility of 90 cm 2 V-s −1 is twice the value of the reported mobility in the case of exfoliated and CVD-grown MoS 2 films.The high mobility can be mainly attributed to fewer sulfur vacancies, leading to less carrier  [48] Copyright 2015 American Chemical Society.
trapping sites.Thus, nuclei etching using OA-CVD shows much better quality on the growth of single-crystal MoS 2 monolayers.Despite these achievements, the disadvantage is that introducing only a single nucleus is extremely difficult, which limits the potential for large-scale production.

Catalytic Substrates
It has been noticed that catalytic substrates for promoting the rapid growth of 2D materials.Copyright 2017 Johns Wiley and Sons.
[71] For example, ultrafast growth of single-crystal graphene on Cu substrate with a remarkable growth rate of 26 μm −1 s in the presence of oxygen has been reported to achieve graphene domains with lateral size ≈0.3 mm in 5 s. [72][74] However, these substrates exhibit lattice mismatch and poor catalytic nature, resulting in various problems in terms of growth rate, domain orientation, uniformity, and controllability.Therefore, metal catalytic substrates such as Au and Cu have been admitted as ideal substrates for the ultrafast growth of high quality 2D materials. [65]The ultrafast growth of highquality uniform tungsten diselenide (WSe 2 ) monolayers on catalytic Au foil has been reported using ambient pressure CVD. [49]he schematic of the CVD with a mounted precursor is shown in Figure 3a.This work demonstrates the growth rate of ≈26 μm −1 s, which is 2-3 order of magnitude faster than those previously reported for the growth of 2D TMDs.Also, this growth rate is comparable to the highest growth rate of CVD-grown graphene, which permits the growth of a large-area single-crystal WSe 2 domain and a continuous monolayer film in 30 s (Figure 3b) and 65 s, respectively.The AFM image in Figure 3c reveals that the WSe 2 domains exhibit a uniform thickness of ≈0.73 nm with an average surface roughness of ≈0.3 nm.The Raman spectra (Figure 3d) exhibit peaks ≈250 cm −1 (E 1 2g ) and 261 (A 1g ) of WSe 2 further confirm the monolayer characteristic and high thickness uniformity.The PL spectrum (Figure 3e) reveals a single fine peak at ≈752 nm with a half-width at half maximum (FWHM) of ≈29 nm, which corresponds to the feature of a direct bandgap semiconductor. [61,75,58]o evaluate the electrical properties of the ultrafast-grown WSe 2 domain, the as-grown WSe 2 was transferred from Au onto the SiO 2 /Si substrate using the electrochemical bubbling transfer method to fabricate back gate field-effect transistors (FETs).The transfer characteristics (Figure 3f) represent the p-type nature of the monolayer WSe 2 semiconductor.The carrier mobility and corresponding ON/OFF ratio at applied V ds = -1 V were ≈129 cm 2 V-s −1 and 9.2 × 10 6 , respectively.[78][79] Despite the significant achievements, the drawback is a more significant trap density at the interface. [80]Still, more efforts are required to suppress the interface trap density to enhance the performance of ultrafast-grown WSe 2 monolayer FETs devices.

Molten Substrates
Previously, the growth of the polycrystalline graphene film on a glass substrate was reported using the molten glass substrate method. [81]The growth of graphene on the molten metallic substrate was also demonstrated to permit the formation of hexagonal crystallites to be gathered in an aligned orientation. [62]ecently, a millimeter-size high-quality MoSe 2 monolayer was demonstrated to be grown successfully on a molten glass  [50] Copyright 2017 American Chemical Society.
substrate. [50]In the study, it was suggested that the melting/regeneration of glass creates a smooth surface, thus enabling low-density nucleation for significant crystal domain growth.The study claimed that triangular shape monolayer MoSe 2 domains with a width in millimeters can be grown within 5 min.Using this approach, the growth of large-area single-crystal MoS 2 monolayer was also achieved, indicating the universal suitability of molten glass substrate.
The growth of high-quality MoSe 2 monolayer on molten glass was performed at room temperature using CVD, as shown in Figure 4a.Glass substrates were inserted in a quartz tube placed inside the tube furnace to grow MoSe 2 .Glass with a Mo foil on the surface was first placed on a SiO 2 /Si with MoO 3 precursor.During the heating process (up to 1050 °C), the solid glass was melted to form a clean and atomically flat surface.The melting of glass significantly suppressed the high-energy interface trap sites, kinks, and asperities to further promote large-area high-quality MoSe 2 crystal growth. [62]Millimeter-size MoSe 2 triangular crystals on molten glass can easily be visually observed, as shown in Figure 4b,c.The results show large-size domain formation with a uniform structure in the growth temperature range from 700 to 1050 °C.Upon increasing the temperature, the MoSe 2 nucleation density reduced while domain size increased.At 1050 °C, the nucleation density is ≈20 nuclei cm −2 .In addition, the growth can be further controlled by the H 2 carrier gas.By increasing the H 2 gas, the etching of MoSe 2 crystals was facilitated; by reducing the H 2 gas, the growth of hexagonally shaped MoSe 2 crystals was enhanced.As-grown MoSe 2 crystals were transferred from the molten glass onto SiO 2 /Si substrate for AFM characterization (Figure 4d).The step height of ≈0.97 nm is consistent with the height of exfoliated MoSe 2 monolayers. [82]urthermore, the authors investigated the microstructural properties of MoSe 2 crystals using TEM (Figure 4e,f) and X-ray photoelectron spectroscopy (XPS) (Figure 4g,h).The SAED of MoSe 2 crystals reveal identical orientation of diffraction patterns for various spots on the crystal, confirming the single crystal nature of MoSe 2 .The results were also confirmed with low-energy electron diffraction (LEED) and scanning TEM.The elemental compositions and bonding among Mo and Se were investigated by recording the XPS spectra of MoSe 2 .The XPS spectra contain the binding energy peaks of Mo 3d 3/2 (231.5 eV), Mo 3d 5/2 (228.3 eV), Se 3d 5/2 (55 eV), and Se 3d 3/2 (55.9 eV) core levels.The MoO 3 peaks were not observed, indicating low defect density at the surface.The integrated peaks area reveals the desired stoichiometry (1:2) of Mo and Se in MoSe 2 crystals.The crystal quality and optical properties of MoSe 2 crystals were further investigated by Raman, PL, and absorption spectra shown in Figure 4i,j. [83,84]Both Raman and PL results show evidence of single crystal characteristics of as-grown MoSe 2 on molten glass with high thickness uniformity.
To evaluate the potential of the method in other 2D TMDs, it was also applied to grow MoS 2 crystals.The approach and parameters were similar to MoSe 2 growth, aside from the fact that S (instead of Se) was utilized as a source.The Raman and PL spectra of the as-grown MoS 2 crystal are shown in Figure 4k.The Raman bands observed ≈400 cm −1 and 382 cm −1 correspond to the A 1g and E 2g modes of MoS 2, respectively.The PL characteristic emission peak at ≈673 nm indicates the direct bandgap excitonic transition of MoS 2 monolayer.In this work, the size of the as-grown MoS 2 domain can reach up to 2.6 mm, which is relatively higher than domain size of 2D TMDs grown using different growth approaches. [63,73]he electrical performance of the MoSe 2 crystal was investigated by fabricating FETs on the SiO 2 /Si substrate (Figure 4l).The I-V characteristic of the FET with a channel length (L) of ≈10 μm and width (W) of 2 μm is shown in Figure 4m. Figure 4n shows the transfer characteristics of the device as a function of V ds , revealing a gradual increase in the I ds with increasing the V gs bias voltage (n-type behavior).Further, the ON/OFF ratio and threshold voltage were found to be ≈10 7 and ≈-20 V (Figure 4n), respectively.The carrier mobilities by and ON/OFF ratios from 19 devices have also been extrated and found to be in the range of 5-95 cm 2 V-s −1 and ≈10 4 -10 7 , respectively.These values are comparable to the previously reported ones in the case of mechanically cleaved MoSe 2 , suggesting the high crystal quality of MoSe 2 crystals grown by the molten glass approach. [85,86]

Growth Promotors
One of the major challenges in the production of large-scale and high-quality 2D TMDs is the high melting point of metal and metal oxide precursors.Therefore, molten-salt-assisted methods have been employed over the past several years to promote the growth of 2D TMD monolayers at relatively low temperatures. [87,60]It has been discovered that the molten-saltassisted CVD can be widely utilized to synthesize ≈47 com-pounds including 32 binary compounds, 13 alloys, and a couple of 2D heterostructures. [51]In the study, salt has been demonstrated to reduce the melting point of the reactants, promote the development of intermediate products, and enhance the overall reaction rate.
The mass flux decides the quantity of metal precursors associated with the nucleation process, while the growth rate affects the grain size of the as-grown film.High mass flux and low growth rate lead to polycrystalline films formed with small grain size.However, high mass flux and high growth rate lead to the formation of a continuous film with large grains (≈millimeters). [88]onversely, low mass flux and low growth rate tend to form small flakes.It was suggested that tiny nuclei were frequently found in the middle of flakes, which require extra adatoms or atom clusters to be reliably bounded to an existing nucleus during the growth process. [89]Further, low mass flux and high growth rate assist in forming an individual large-area single-crystal 2D TMD domain. [90]It was also observed that some TMDs, such as those made of Nb, Pt, and Ni, were difficult to synthesize.The high melting points and low vapor pressures of these metal or metal oxide precursors result in low mass flux to limit the reaction process.Therefore, molten salt was used to increase the mass flux by producing oxychlorides through the reaction with certain metal oxides, reducing the melting point of metal precursors, and enhancing the reaction rate.The growth mechanism of the moltensalt-assisted CVD was demonstrated using salt (as a growth promotor) that could decrease the melting point of metal precursors and therefore make the reaction possible.For instance, a correlation of Nb nucleus with and without salt was added, demonstrates a high mass flux of metal precursors facilitated by the salt, notwithstanding the downfall in the melting point.The existence of metal oxychloride has also been investigated by analyzing the intermediate products through XPS and density function theory (DFT) calculations.It was found that the sulfurization of metal oxychlorides is extremely energetically preferable than the sulfurization of metal oxides.Therefore, salt-assisted CVD can be used as a universal approach for the growth of various 2D TMDs, which holds much potential for industrial applicability.

Multi-Nucleation Methods
To obtain the controllable growth of wafer-scale single crystal 2D TMDs with high crystallinity, the lattice-matched approach was demonstrated to become a promising method for large-scale production. [43,88]Recently, epitaxial growth of 100 cm 2 singlecrystal hexagonal boron nitride (h-BN) monolayer has been reported on a single-crystal vicinal Cu (100) surface. [35]It was observed that a unidirectional alignment of the h-BN appeared through the coupling of h-BN zigzag edge with the Cu (211) step edge.Subsequently, epitaxial growth of single-crystal h-BN monolayer was introduced on Cu (111) thin film at a temperature ≈1050 °C using the ammonia borane precursor in the presence of H 2 gas.The step edge of single-crystal Cu (111) was introduced to induce the mono-orientation growth of h-BN monolayers.It was also theoretically realized that the equivalence 0 to 60°domains can be broken if the surface of the substrate is vicinal. [91]A similar study has been reported on the growth of identically oriented WS 2 domains on a single crystalline h-BN layer on a melted gold surface. [92]This work provided the  [43] Copyright 2020 American Chemical Society.
possibility of growing large-area single-crystal 2D TMD monolayer on single crystal metallic substrates.Quite recently, singlecrystal 2D MoS 2 monolayers have been successfully grown on Au (111) thin film using CVD. [43]Before the CVD process, a vicinal Au (111) thin film was first grown on a tungsten (W) substrate through the melting and re-solidifying process of commercially available Au foils (Figure 5).However, the re-solidified Au on the W substrate formed a polycrystalline film.After the post thermal annealing process, single-crystal Au (111) film can be obtained.Generally, Au (111) forms twin grains isolated through twin boundaries during growth kinetic process.Therefore, optimized temperature (1040-1080 °C) was proposed in the presence of Ar/H 2 (300/50 sccm) gas, revealing key factors for eliminating the twin grains.
To grow Au (111) film on W (etched in 30 wt% H 2 O 2 ), a piece of gold foil was first mounted on the etched W foil.This structure was heated up to 1050 °C from room temperature within 55 mins and maintained for 10 min at 1050 °C in the presence of Ar/H 2 (300/50 sccm) mixed gas.Eventually, the molten gold liquid spreads on the surface of the W foil.After cooled down, the liquid gold solidified and formed steps on the Au (111) surface.Later, single-crystal MoS 2 was grown on the face-down Au (111)/W substrate using the CVD method.To confirm the formation of the Au (111) plane, X-ray diffraction (XRD) and high resolution (HR)-XRD patterns of the Au/W film have been obtained and shown in Figure 6a,b, respectively.The XRD pattern of the Au/W film comprises four diffraction peaks which associate with (111) and (222) planes of Au and (200) and (211) planes of W. This demonstrates the growth of a single-crystal Au (111) film on the polycrystalline W foil. Also, the HR-XRD pattern reveals only three peaks with an interval of 120°in the ϕ scan.In the case of face-centered cubic (FCC) metals, (111) exhibits minimum surface energy among all the planes. [93]The melting of gold into a liquid state could generally reduce the thermal pressure from its interfacial contact with bare W foils.During the cooling process, Au (111) tends to form for achieving surface energy reduction.Further, the formation of single-crystal Au (111) film has also been confirmed by measuring its topography using AFM, as shown in Figure 6c.The AFM image shows the atomic step is aligned in three directions, including a crossing angle of 60°.
To demonstrate the single-crystal nature of MoS 2 grown on the Au (111)/W substrate, low-energy electron diffraction (LEED) patterns were recorded at various positions (Figure 6d).The results reveal identical lattice orientations, indicating the singlecrystal nature of the MoS 2 film.The SEM (Figure 6e) image shows the formation of triangular and well-oriented aligned domains on the Au (111) substrate (synthesized at ≈720 °C for 30 min).It was also found that the proportion of aligned domains was as high as ≈98%.The unidirectional domains merged (Figure 6f) upon increasing the growth time and eventually converted into a continuous MoS 2 film (Figure 6g).The Raman measurement (Figure 6h) was performed at five different positions on MoS 2 and revealed similar peak positions ≈385 and 403 cm −1 , indicating monolayer characteristics and high thickness uniformity.To understand the electrical properties of the as-grown single-crystal MoS 2 film on Au (111)/W, the MoS 2 film was transferred to the SiO 2 /Si substrate to fabricate a FET (Figure 6i).It was noted that the Au/W template is chemically inert and may be re-utilized after the transfer process.The current-voltage characteristics of FETs (channel width and length of 15 and 5 μm) were measured and shown in Figure 6j, which reveals the n-type behavior.Figure 6k shows the I DS -V GS curve at V DS = 0.5 V.The ON-OFF ratio exceeds 10 5 has been observed.Figure 6l shows the carrier mobility (≈11.2 cm 2 V-s −1 ) over 110 devices.Copyright 2020 American Chemical Society.

Conclusion and Outlook
This perspective makes a significant step toward understanding main approaches on single and multiple nucleations for largescale 2D TMD preparation.Investigating the underlying phenomenon of the uniform growth of 2D TMD monolayers on single-crystal film is still a fundamental issue in related fields.In this perspective, we have first reviewed the approach of single nucleation growth through which the nucleation density down to a single nucleus is possible with the help of four major methods: (i) nuclei etching, (ii) utilizing substrates with high catalytic capacity, (iii) choosing molten substrates as growth assistant, and (iv) growth promotors.Despite the major success being achieved in single nucleation for designing millimeter-size high quality single crystal FET devices with high mobility and large ON/OFF ratio, it seems to be extremely difficult to go beyond this scale.Thus, the multi-nucleation method should be the mainstream for developing the industrially applicable route for achieving wafer-scale 2D film production in the next several years.However, multi-nucleation strategies in the past few years require single-crystal metallic substrate such as Au(111) prior to the growth procedure, which could largely increase the cost of the production process.In addition, the fabrication of devices such as FETs requires the 2D film to be further transferred to other insulating substrates, which may possibly affect the uniformity, integrity, and internal strain of the film.Nonetheless, the strategy indeed opens a promising new path for the possibility of large-scale high-quality 2D film production.The recently proposed wafer-scale epitaxial growth of monolayer TMD on vicinal a-plane or miscut c-plane sapphires indeed opens a new avenue in developing appropriate substrates to efficiently inhibit antiparallel growth as well as twin boundaries to achieve large-scale uniformity.By tuning the miscut angle, vicinal step density can be further controlled.Although the development of such strategy is still under progress, we believe that similar design will dominate the growth of wafer-scale single crystal monolayer TMD by evaluating various insulating substrate materials, miscut (off-cut) angles, growth/substrate pre-annealing temperatures, and TMD materials.It can be expected that in the near future, similar techniques combining the lattice-matched dielectric substrate, effective CVD (plasma-enhanced CVD or metal-organic CVD), clean and strain-free transfer methods, and appropriate catalysts will be on the way for the development of commerically available highquality single-crystal TMD films.

Figure 1 .
Figure 1.The approaches for growth of large domain-sized single-crystal 2D TMD monolayers.

Figure 2 .
Figure 2. a) Schematic diagram of OA-CVD.b-d) Optical and AFM images of MoS2 monolayers grown under various oxygen flow rates (0-2 sccm).e) Raman and f) photoluminescence (PL) spectra of MoS 2 films grown with/without O 2 .g) Evolution of MoS 2 domain size and variation of etching with the growth duration.h) Dependence of growth and the etching rates of domains as a function of the growth duration.i) Schematic of the fabricated monolayer MoS 2 FET device on SiO 2 /Si substrate.j) Output characteristics of the FET device.Reproduced with permission.[48]Copyright 2015 American Chemical Society.

Figure 3 .
Figure 3. a) Growth-time dependent edge length of single-crystal monolayer WSe 2 domains.b) Optical image of single-crystal WSe 2 domain on Au foil at t = 30 s. c) AFM image of the WSe 2 domain, indicating a thickness of 0.73 nm.d) Raman and e) PL spectra of the WSe 2 domain.f) Transfer characteristic of the WSe 2 FET device.Reproduced with permission.[49]Copyright 2017 Johns Wiley and Sons.

Figure 4 .
Figure 4. a) The growth mechanism of MoSe 2 crystals on molten glass substrate using CVD.b) Photograph of MoSe 2 crystals on a molten glass substrate.c) Optical image of MoSe 2 crystals on a molten glass substrate (scale bar = 500 μm).d) AFM image of the MoSe 2 crystal (scale bar = 1 μm).e) High-resolution TEM image of the MoSe 2 crystal (scale bar = 20 nm).f) SAED pattern of the MoSe 2 crystal.g) XPS spectra of Mo 3d core levels.h) XPS spectra of Se 3d core levels.i) Raman and PL (inset) spectra of MoSe 2 crystals.j) Absorption and transient absorption (inset) spectra of MoSe 2 crystals.k) Raman and PL (inset) spectra of MoS 2 crystals.l) Schematic of the fabricated MoSe 2 FET.m) I ds -V ds curves of the FET.n) Transfer characteristics of the device as a function of V ds .Reproduced with permission.[50]Copyright 2017 American Chemical Society.

Figure 5 .
Figure5.The growth process of single-crystal metal Au (111) on W, MoS 2 monolayer on Au (111)/W, the schematic of the CVD quartz tube, and the schematic illustration of continuous MoS 2 film growth during CVD.Reproduced with permission.[43]Copyright 2020 American Chemical Society.

Figure 6 .
Figure 6.a) XRD pattern of the Au/W foil.b) HR-XRD azimuthal ϕ scans of the off-axis ⟨111⟩ reflection of the Au (111) film.c) AFM image of the re-solidified single-crystal Au (111) substrate.d) LEED pattern of the MoS 2 continuous film collected at various positions.e) SEM image of identically oriented small MoS 2 domains at the growth time of 3 min.f) SEM image of identically oriented extended MoS 2 domains at the growth time of 5 min.g) SEM image of the continuous MoS 2 film at the growth time of 8 min.h) Raman spectra of MoS 2 film on Au (111)/W film at various positions.i) Schematic of the single-crystal MoS 2 FET device.j) I DS -V DS characteristics of the single-crystal MoS 2 FET device.k) I DS -V GS curves of the single-crystal MoS 2 FET device.l) The distribution of carrier mobilities over 110 FET devices.Reproduced with permission.[43]Copyright 2020 American Chemical Society.

Table 1 .
Electrical performance of 2D TMDs FET using various CVD methods.