Ultrafast growth of submillimeter-scale single-crystal MoSe₂ by pre-alloying CVD

Abstract

The synthesis of large-scale monolayer single-crystal MX₂ (M = Mo, W; X = S, Se), a typical transition metal dichalcogenide (TMD), is the premise for their future applications. Compared with insulating substrates such as SiO₂ and sapphire, Au is more favourable for the fast growth of TMDs by chemical vapor deposition (CVD). Recently, large-scale single-crystal WX₂ was successfully grown and transferred on Au. In sharp contrast, the growth and transfer for monolayer MoX₂ is still very challenging, because Au has a higher solubility of Mo and stronger interaction with MoX₂ than WX₂. Compared with the most studied MoS₂, MoSe₂ is superior in many aspects because of the narrower band gap and tunable excitonic charging effects. However, the synthesis of large-scale single-crystal MoSe₂ on Au has not been reported so far. Here, a pre-alloying CVD method was developed to solve the problems for the growth and non-destructive transfer of MoX₂. It has realized the ultrafast growth (30 s) of submillimeter-scale (560 μm) single-crystal MoSe₂ for the first time. As-grown samples are strictly monolayers with good optical and electrical properties, which can be easily transferred without sacrificing Au foils by the electrochemical bubbling method. It was found that pre-alloying not only passivates the energetically active sites on Au but also weakens the interaction between Au and MoSe₂, which is responsible for the ultrafast growth and easy transfer of MoSe₂. This method is also universal for the fast growth and non-destructive transfer of other 2D TMDs.

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New concepts

Ultrafast growth of 2D materials is one of the hot research topics, which is not only beneficial for the synthesis of large-scale 2D materials but also can greatly reduce the production cost. Among 2D materials, monolayer transition metal dichalcogenides (TMDs) are highly desired for electronic and optoelectronic applications. Although numerous efforts have been devoted to such materials, the ultrafast growth and transfer of large-scale monolayer single-crystal TMDs is still challenging. Here, a new pre-alloying CVD method was developed to solve the problems for the growth and transfer of MoX₂. It has realized the ultrafast growth and non-destructive transfer of submillimeter-scale monolayer single-crystal MoSe₂ for the first time. We also discussed the growth mechanism and confirmed it is universal for other 2D TMDs. Pre-alloying CVD offers important insights for the synthesis of large-scale high-quality TMDs and other 2D materials, which paves the way for their electronic and optoelectronic applications.

Introduction

Due to the direct band gap structure, monolayer two-dimensional (2D) MX₂ (M = Mo, W; X = S, Se) transition metal dichalcogenides (TMDs) have attracted much attention for their unique properties and promising electronic and optoelectronic applications.^1–5 However, defects, grain boundaries and the uniformity of layer-number intensively influence their various properties.^6–9 On the other hand, ultrafast growth of 2D materials is one research focus. It is not only beneficial for the synthesis of large-scale 2D materials, but also can greatly reduce the production cost by shortening the growth time during the energy-consuming chemical vapor deposition (CVD) process.^10–13 Therefore, the ultrafast growth of large-scale, high-quality and uniform monolayer MX₂ single crystals is essential for their future applications. Recently, numerous efforts have been devoted to growing such materials on various substrates. Insulating substrates (SiO₂/Si, sapphire, mica and so on) have poor catalytic activity, resulting in the low growth rate and limited grain size of monolayer MX₂.^14–17 Moreover, many defects (vacancies, adatoms etc.) and uncontrollable multilayers were inevitably formed during TMD growth on these substrates. By contrast, Au is a good candidate for the fast growth of large-scale high-quality uniform monolayer MX₂ because of its catalytic effects.^10,18–21

The very low solubility of W in Au (∼0.08 at% at 1050 °C)²² makes WX₂ on Au follow the self-limited catalytic surface growth mechanism. In our previous reports, we have demonstrated the synthesis of millimeter-sized single-crystal WS₂ and WSe₂ by using Au foils.^10,19 WX₂ can be easily transferred from Au foils because of the weak interaction between WX₂ and Au. In sharp contrast, Au has a higher solubility of Mo (∼1.25 at% at 1050 °C)²³ than W. Although MoS₂ has been achieved on Au, the growth rate was very low (∼0.03 μm s⁻¹).^24–26 Furthermore, Au substrates had to be etched away when transferring MoS₂ for subsequent applications because of the strong interaction between MoX₂ and Au. Compared with the most studied MoS₂, MoSe₂ is superior in many aspects because of the narrower band gap (1.5 eV for MoSe₂ and 1.9 eV for MoS₂) and tunable excitonic charging effects.^27–30 However, the synthesis of large-scale single-crystal monolayer MoSe₂ on Au has not been realized so far.

Here, we develop a pre-alloying CVD method to solve the problems for the growth and transfer of MoX₂ on Au and realize the ultrafast growth of submillimeter-scale single-crystal MoSe₂ for the first time. 560 μm-large MoSe₂ triangles were synthesized in 30 s with an ultrahigh growth rate of ∼18.7 μm s⁻¹. As-grown samples are strictly monolayer with good optical and electrical properties, which were easily transferred without sacrificing Au foils by the electrochemical bubbling method. We also revealed that pre-alloying can not only passivate the energetically active sites on Au but also weaken the interaction between MoSe₂ and Au, which is responsible for the ultrafast growth and easy transfer of MoSe₂. Finally, we demonstrated that pre-alloying CVD is applicable for the fast growth and non-destructive transfer of other 2D TMDs. The growth rate of as-grown 450 μm-large single-crystal MoS₂ was increased by over 100 times compared to those reported works.^24–26

Experimental

Preparation of pre-alloyed Au

First, Au foils (99.99 wt%, 100 μm-thick, 1 cm × 1 cm, Beijing Kairui Material Technology Co. Ltd) were ultrasonically treated with acetone, isopropyl alcohol and ethanol, respectively, for 20 minutes, and then annealed at 1050 °C for more than 12 hours. Pre-alloyed Au foils were obtained by atmospheric pressure CVD (APCVD) in a 1 inch tube furnace. A piece of Au foil was placed in the center of the tube furnace and 80 mg molybdenum oxide (MoO₃) powder (99.999 wt%, Adamas) was located at 1 cm upstream of the Au foil as the Mo source. The tube furnace was heated from room temperature to 900 °C within 30 min and then to 1050 °C in 15 min under an argon (Ar) flow of 60 standard cubic centimeter per minute (sccm). At the same time, 25 sccm hydrogen (H₂) was introduced to reduce MoO₃ to form pre-alloyed Au for 6 min in the single time of pre-alloying (Fig. S1, ESI†). Pre-alloyed Au foils with different Mo content were obtained by repeating the above process for different times (from 1 to 25 times). Finally, Au foils were rapidly moved away from the high temperature zone.

Ultrafast growth of MoSe₂ on pre-alloyed Au

Pre-alloyed Au foil was placed at the center of the tube furnace, and 23 mg MoO₃ powder (99.999 wt%, Adamas) was located at 3 cm upstream of the pre-alloyed Au foil as the Mo source. 100 mg selenium (Se) powder (99.99 wt%, Aladdin) was situated in the upstream of the pre-alloyed Au foil outside the high temperature zone by heating it separately. The distance between the Se powder and pre-alloyed Au foils was ∼25 cm. The tube furnace was heated from room temperature to 750 °C within 20 min and then to 900 °C in 10 min under an Ar flow of 100 sccm. When the tube furnace and Se powder were heated to 900 °C and 350 °C, 3 sccm H₂ was introduced. We defined this moment as the beginning of MoSe₂ growth (t = 0).

Electrochemical bubbling transfer of MoSe₂

First, a protective layer of polymethyl methacrylate (PMMA: ∼1.25 dL g⁻¹, 4 wt% in ethyl lactate) was spin-coated on the surface of MoSe₂/Au at 500 rpm for 17 s and then it was heated at 170 °C for 100 min. Then, PMMA/MoSe₂/Au, platinum foil and 2-M NaOH solution were used as the cathode, anode and electrolyte, respectively. With a continuous current of 60 mA, H₂ produced at the cathode will separate the PMMA/MoSe₂ films from the Au foil. The PMMA/MoSe₂ film can be transferred to target substrates: Si substrates with a 295 nm-thick SiO₂ layer or a TEM grid (Quantifoil Au grid). Finally, PMMA was removed in hot acetone.

Characterization

The morphology of MoSe₂ was characterized by optical microscopy (Olympus BX51 microscope) and scanning electron microscopy (SEM, QYANTA FEG 250). An atomic force microscope (AFM, Bruker Dimension Icon) system was used to determine the thickness of MoSe₂. X-Ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to characterize the chemical composition of MoSe₂ and determine the variation trend of Mo content in pre-alloyed Au. The atomic structure of monolayer MoSe₂ was carried out by TEM on an FEI Titan Themis Cube with an X-FEG electron gun and a DCOR aberration corrector operating at 60 kV. Raman and PL spectra were obtained with a Raman spectrometer (JY HR-800 LabRAM) by using a 488 nm semiconductor laser (output power: 50 μW, spot size: 1 μm) as the excitation source.

FET fabrication and electrical property measurements

Monolayer single-crystal MoSe₂ domains were transferred onto highly doped Si substrates with a 295 nm-thick SiO₂ layer, and then a 5 nm Ti/50 nm Au layer was deposited by electron beam evaporation as the source and drain electrodes. The channel width and length of the FET device were 1 and 20 μm, respectively. Electrical properties of FET were measured under ambient conditions at room temperature using an Agilent semiconductor parameter analyser (4155C Semiconductor Parameter Analyser). The carrier mobility was determined by the following equation:
Where L and W represent channel width and length, respectively, and C_ox is the capacitance of the dielectric layer.

Results and discussion

Ultrafast growth of MoSe₂ by pre-alloying CVD

Different from normal CVD, there exists a pre-treatment process before CVD growth by our method (Fig. 1a): Au foils were annealed with MoO₃ powder under a mixture of Ar and H₂. The single pre-treatment lasts for 6 min to ensure sufficient MoO₃ supply (see Experimental, Fig. S1, ESI†). Note that the above process was implemented at a high temperature (1050 °C) to speed up the pre-treatment of Au foils. XPS was used to detect the elemental variation of the Au foils. The formation of Au–Mo alloys in Fig. S2a (ESI†) illustrates that the pre-treatment is actually a pre-alloying process of Au foils with Mo. MoSe₂ single crystals and films were then synthesized on pre-alloyed Au by APCVD at a lower temperature (900 °C) using MoO₃ and Se powder as precursors (Fig. 1b). During the MoSe₂ growth, we define the growth time t = 0 s as the moment when H₂ was introduced. On reaching the preset growth time t, we rapidly moved the precursors and samples to the low temperature zone in order to accurately control t. It is surprising to see that ∼560 μm-large single-crystal MoSe₂ was formed within 30 s by optimizing the growth conditions (Fig. 1c). The determined growth rate is ∼18.7 μm s⁻¹, which is much higher than most reported TMDs (Fig. 1e and Tables S1, S2, ESI†). As shown in Fig S2b and c, (ESI†) Mo 3d_5/2 and 3d_3/2 peaks are located at ∼228.3 and 231.3 eV, while Se 3d_5/2 and 3d_3/2 peaks are located at ∼54.2 and 54.9 eV, respectively. These results prove the formation of MoSe₂ on pre-alloyed Au foil. Interestingly, we found that pre-alloying plays a dominant role for the synthesis of large-scale MoSe₂. The grain size of single-crystal MoSe₂ grown on pristine Au foil without pre-alloying is only limited to 80 μm, which will be discussed in detail in the following sections.

Fig. 1 Ultrafast growth of monolayer MoSe₂ single crystals and their films by pre-alloying CVD. Schematic diagram of (a) pre-alloying of Au foils and (b) ultrafast growth of submillimeter-scale monolayer MoSe₂ single crystals on pre-alloyed Au foils. Optical images of (c) a 560 μm single-crystal MoSe₂ and (d) a continuously uniform film. Before the MoSe₂ growth, Au foil was pre-alloyed at 1050 °C for 9 times. (e) Comparison of growth rate for MX₂ reported in the literature and by our method (marked by the red star). The squares and triangles in (e) represent that the substrates used for MX₂ growth are insulating and Au substrates, respectively. The references numbered by 1–31 in (e) correspond to those in Table S1 (ESI†). Optical images of (f) a MoSe₂ single crystal and (g) a continuous film transferred onto the SiO₂/Si substrates. (h) AFM height topography of a single-crystal MoSe₂ domain. The inset in (h) is the height profile measured along the black dotted line.

The SEM images (Fig. S3, ESI†) show that MoSe₂ single crystals can expand across the grain boundaries of polycrystalline Au foils. The morphology of MoSe₂ on the different areas of the Au foils is very similar and MoSe₂ domains distribute on the whole Au surface uniformly (Fig. S4, ESI†). With the growth time increasing, individual MoSe₂ triangles join together to form a continuous monolayer film within 60 s (Fig. 1d). Some reports have indicated that Au foils need to be etched away for the subsequent transfer of MoS₂.^24–26 By contrast, as-grown MoSe₂ monolayers can be successfully transferred onto SiO₂/Si substrates by the electrochemical bubbling method³¹ (Fig. 1f, g and Fig S5, ESI†), making the pre-alloyed Au foils reusable (Fig. S6, ESI†). The thickness of the obtained MoSe₂ measured by AFM (Fig. 1h) is ∼0.9 nm, illustrating its monolayer characteristic.

Structural characterization and properties of MoSe₂

As shown in Fig. 2, the atomic structure of monolayer single-crystal MoSe₂ was characterized by selected-area electron diffraction (SAED) and aberration-corrected scanning transmission electron microscopy (STEM). Fig. 2a shows an optical image of a single-crystal MoSe₂ transferred onto a carbon supported TEM grid. The perfect coincidence of all SAED patterns (Fig. 2b) confirms that the monolayer MoSe₂ is single-crystal. From the TEM image in Fig. 2c, we can see that the MoSe₂ domain was clearly transferred. The energy-dispersive X-ray spectroscopy (EDS) mappings (Fig. 2d and e) were acquired at the same area in Fig. 2c, which indicates the homogeneous distribution of Mo and Se atoms in the MoSe₂ flake. The stacking of monolayer MoSe₂ is further observed by using atomic resolution high-angle annular dark field (HAADF) STEM. Fig. 2f is a representative HAADF-STEM image of our MoSe₂, which shows a perfect hexagonal lattice structure. The distance of adjacent Se₂ (Fig. 2g) and interplanar crystal spacing is 5.64 and 2.82 Å, respectively, which is consistent with the theoretical value. The atomic arrangement at the triangle edge indicates that the monolayer MoSe₂ single crystal has a zigzag edge terminated with Mo atoms (Fig. 2h). These results demonstrate the good crystal quality of as-grown monolayer MoSe₂ single crystals.

Fig. 2 Atomic structure of monolayer MoSe₂ characterized by TEM. (a) Optical image of a monolayer MoSe₂ flake. (b) The 8 SAED patterns arbitrarily taken from the different regions in the same flake in (a). All scale bars are 50 nm⁻¹. The yellow and green arrows represent lattice plane (100) and (010), respectively. (c) Low-resolution (HAADF-STEM) image of an area selected at the edge of the MoSe₂ flake in (a). EDS mappings of (d) Mo and (e) Se element taken from the same area in (c). (f) Wiener filtered HAADF-STEM image of the inner area of the MoSe₂ flake in (a). Due to the different atomic numbers between Se₂ and Mo, the two stacked Se atoms exhibit a brighter spot than the Mo atom. (g) Element intensity profile acquired along the blue line in (f). The distance between the adjacent Se₂ is 5.64 Å. (h) HAADF-STEM image of the triangle edge of the MoSe₂ flake in (a). The red and green balls in (f and g) represent Se₂ atoms and Mo atoms, respectively.

Raman and photoluminescence (PL) spectroscopy are effective methods to characterize the crystal quality and optical properties of as-grown MoSe₂. Fig. 3a–c show the optical image of transferred MoSe₂ and the corresponding Raman and PL spectra. There exist two main peaks in the Raman spectra: one is a very sharp A_1g mode at low wavenumber of ∼240 cm⁻¹, representing the out-of-plane vibration. The other one is a broad E¹_2g mode at higher wavenumber of ∼288 cm⁻¹, representing the in-plane vibration. The A_1g mode is much stronger than the E¹_2g mode, which is a typical feature of monolayer MoSe₂. The PL spectrum shows a sharp peak at ∼790 nm (A exciton) and a narrow full-width at half-maximum of ∼30 nm, corresponding to a band gap of 1.57 eV and 41 meV, respectively, which indicates that MoSe₂ grown on pre-alloyed Au is a monolayer with a direct band gap. The peak position and relative intensity of the spectra in Fig. 3b and c are almost identical, suggesting that our MoSe₂ domain is homogeneous. The corresponding intensity mappings of Raman and PL spectra in Fig. 3d–f further illustrate the above results. The uniformity of the large-area continuous MoSe₂ film was also observed in Fig. S7 (ESI†). In addition, the electronic transport properties of MoSe₂ were evaluated by fabricating a back-gate field effect transistor (FET) device (Fig. S8, ESI†). The estimated carrier mobility (11.6 cm² V⁻¹ s⁻¹) is comparable to many reported TMDs (Table S1, ESI†), demonstrating the potential applications of our MoSe₂ in transistors.

Fig. 3 Raman and PL characterization of single-crystal MoSe₂. (a) Optical image of a monolayer MoSe₂ domain transferred on the SiO₂/Si substrate. (b) Raman and (c) PL spectra taken from the 5 positions marked with different colours in (a), showing the characteristic peaks of MoSe₂. The negligible peak in (c) at ∼705 nm is attributed to the B excitonic transition, in agreement with the reported results.^27,30 The intensity mappings of (d) A_1g (240 cm⁻¹), (e) E¹_2g (288 cm⁻¹) and (f) PL peak (790 nm) of MoSe₂ in (a).

Growth mechanism of MoSe₂ by pre-alloying CVD

The grain size of MoSe₂ grown on original Au foils without pre-alloying was only limited to 80 μm by changing the growth temperature or precursor supply (Fig. S9, ESI†). Combining with the results in Fig. 1, we can see that pre-alloying of Au is a prerequisite to grow large-scale single-crystal MoSe₂. To further evaluate the effects of pre-alloying on MoSe₂ growth, we have prepared pre-alloyed Au foils with different Mo contents. Interestingly, we found that (Fig. 4a and b): with increasing the pre-alloying times, the Mo content in the pre-alloyed Au gradually increases and gets saturated after 18 times, while the grain size and nucleation density of the as-grown MoSe₂ show a non-monotonic variation. To intuitively observe the relationship between the morphology evolution of MoSe₂ and Mo content in pre-alloyed Au, we estimated the Mo content, grain size and nucleation density of MoSe₂ with increasing pre-alloying times (Fig. 4c and d). Obviously, with the Mo content increasing, the grain size increases at first (from O to M Point), then gradually decreases (from M to S Point) and tends to be stable finally (after S Point). On the contrary, nucleation density decreases initially (from O to M Point) and then increases (from M to S Point) until it remains constant (after S Point).

Fig. 4 Relationship between the morphology evolution of single-crystal MoSe₂ and Mo content in pre-alloyed Au. (a) A series of optical images of MoSe₂ single crystals grown on the Au foils with different Mo content by pre-alloying for 0–25 times. Every pre-alloying lasts 6 min and all the MoSe₂ samples were obtained in 30 s with the same CVD parameters. Scale bar in all images is 100 μm. (b) XPS spectra showing the different Mo content in pre-alloyed Au with increasing the pre-alloying times. The Mo 3d_5/2 and Mo 3d_3/2 peaks (238.0 eV and 231.1 eV) verify the formation of Au–Mo alloys as discussed in Fig. S2 (ESI†). (c) The variation of Mo content in Au with pre-alloying times increasing. As Mo 3d_5/2 was much stronger than Mo 3d_3/2, the Mo content was calculated as the peak intensity of Mo 3d_5/2 in (b).³⁶ (d) The change of nucleation density and grain size of MoSe₂ in (a) with Mo content increasing. We defined the O, M and S Point as when the original Au was not pre-alloyed, the largest grain size of MoSe₂ appears and the grain size keeps still, respectively. The grain counts and the side length of a triangle in (a) represent the nucleation density and grain size of MoSe₂, respectively.

Apparently, the Mo content in pre-alloyed Au strongly effects the MoSe₂ growth. (1) Active imperfections on the metal surface, such as void sites and other defects, are favourable for nucleation of 2D materials. The passivation of these sites can effectively restrict nucleation.^32–34 At the O Point, many active sites (vacancies and grain boundaries) on the original Au surface (Fig. 5a) induced the maximum nucleation density and minimum grain size of MoSe₂ domains. From the O Point to M Point, the increased Mo content will gradually passivate the active sites on the Au surface, and thus the nucleation density decreases and the grain size of MoSe₂ increases continuously. (2) It is worth noting that the largest grain size and smallest nucleation density appeared at the M Point (Fig. 5b), but not the S Point. Because the Mo solubility in Au decreases with the temperature decreasing according to the Au–Mo phase diagram (Fig. S10, ESI†), although the Mo content at the M point is not saturated at 1050 °C (pre-alloying temperature, ∼1.25 at%), it is saturated at the temperature for MoSe₂ growth (900 °C, ∼1.11 at%). (3) After the M Point, redundant Mo atoms precipitate onto the Au surface more and more (Fig. S11, ESI†). Although the pre-deposited Mo can act as a Mo source for MoSe₂ growth, it has a slight effect on promoting the grain size (Fig. S12, ESI†). The excess Mo atoms will provide active sites and accelerate the MoSe₂ nucleation (Fig. 5c).³⁵ As a result, the grain size of MoSe₂ decreased from the M Point to the S Point.

Fig. 5 Schematic of MoSe₂ growth on pre-alloyed Au with different Mo content. (a) The original polycrystalline Au surface with many defects, including vacancies and grain boundaries (O Point in Fig. 4). Au–Mo alloys appeared (Process 2) and MoSe₂ formed (Process 1) simultaneously with the Mo source supply introduced by a carrier gas, which makes a strong interaction between MoSe₂ and Au. (b) Mo content got saturated at 900 °C. Actively defective sites on the Au surface were totally passivated by Mo atoms (M Point in Fig. 4), which induced the minimum nucleation density of MoSe₂ and a weak interaction between MoSe₂ and Au. (c) Mo was oversaturated at 900 °C (S Point in Fig. 4). Excess Mo atoms diffused onto the Au surface, which provide the new nucleation sites and increase the nucleation density of MoSe₂ (Process 3). Mo atoms coming from pre-alloyed Au are coloured by light blue, while those introduced during MoSe₂ growth are coloured by dark blue. The schematic of Au was built based on the cross section of pre-alloyed Au foils (Fig. S13, ESI†).

In addition, we found that pre-alloying not only suppressed the nucleation density by passivating the active sites on the Au surface but also induced a weak interaction between MoSe₂ and Au, which makes the following transfer easy. When MoSe₂ was synthesized on original Au without pre-alloying, Mo atoms occupy the void sites of the Au surface and react with Se simultaneously to form a Se–Mo–Au structure.³⁷ This phenomenon will greatly enhance the interaction between Au and MoSe₂. By contrast, void sites and other defects of the Au surface can be passivated in advance by pre-alloying CVD (M Point in Fig. 5). In this way, the main Mo source for MoSe₂ growth comes from Mo introduced by carrier gas but not pre-deposited Mo in Au (Fig. S12, ESI†). We think that such a growth process is similar to the reported surface catalytic growth of WS₂ or WSe₂ on Au,^10,19 which may induce a weak interaction between MoSe₂ and Au (Fig. S14, ESI†). As a result, the MoSe₂ domains obtained at the O Point were seriously damaged, while those grown on pre-alloyed Au (M and S Point) were completely transferred (Fig. S15, ESI†).

Except for the low nucleation density, high growth rate also gets involved in the formation of submillimeter-scale single-crystal MoSe₂. As reported, MoS₂ can be synthesized in pure Ar,³³ but MoSe₂ domains only appeared when H₂ was introduced in our experiments. H₂ acted as both a reduction promoter for the adequate selenylation of MoO₃ and an etching agent of as-grown MoSe₂. The growth rate was intensively influenced by the H₂ concentration. Introducing a moderate amount of H₂ (3 sccm) will speed up the growth of uniform monolayer MoSe₂ without any multilayer (Fig. S16, ESI†).

Fast growth of large-scale single-crystal MoS₂

We have also confirmed that pre-alloying CVD is also applicable for the fast growth and non-destructive transfer of other 2D TMDs (Fig. S17 and S18, ESI†). As reported, the growth rate of MoS₂ on Au is very low (∼0.03 μm s⁻¹) and Au foils need to be etched away for transferring MoS₂.^24–26 By contrast, ∼450 μm-sized triangular MoS₂ was formed within 2 min by pre-alloying CVD. Although the growth rate of MoS₂ (∼3.75 μm s⁻¹) is much lower than MoSe₂ (details discussed in Fig. S17, ESI†), it is still over 100 times higher than reported results, as shown in Table S2 (ESI†). Furthermore, Au foils were reusable and MoS₂ can be completely transferred.

Conclusions

In conclusion, the ultrafast growth of submillimeter-scale monolayer single-crystal MoSe₂ was realized by pre-alloying CVD. The uniform high-quality MoSe₂ monolayers can be successfully transferred without sacrificing Au foils by using the electrochemical bubbling method. We further revealed the significant role of pre-alloying in the ultrafast growth of MoSe₂. It can not only suppress the nucleation density by passivating the active sites on the Au surface, but also induce a weak interaction between MoSe₂ and Au, which makes the following transfer easy. This method is also universal for the fast growth and non-destructive transfer of other 2D TMDs. Pre-alloying CVD offers important insights for the synthesis of large-scale high-quality TMDs and other 2D materials, which paves the way for their electronic and optoelectronic applications.

Author contributions

Conceptualization: X. Xin, H. Xu and W. Ren; Investigation: X. Xin, J. Chen, Y. Zhan, Y. Bao and M.-L. Chen; Funding acquisition: X. Xin, H. Xu, W. Liu, Y. Liu and W. Ren. Writing: X. Xin, J. Chen, H. Xu and W. Ren.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Fund for Distinguished Young Scholars (No. 52025022), the Program of National Natural Science Foundation of China (No. 52002056, 11874104, 12074060, 51872043, U19A2091 and 51732003), the Chinese Academy of Sciences (No. XDB30000000 and ZDBS-LY-JSC027), the National Key Research and Development Program of China (No. 2021YFA0716400 and 2019YFB2205100), the Fundamental Research Funds for the Central Universities (No. 2412020QD014, 2412021ZD006, 2412019BJ006 and 2412021ZD012), the fund from Jilin Province (No. YDZJ202101ZYTS046, YDZJ202101ZYTS133 and JJKH20211273KJ) and LiaoNing Revitalization Talents Program (No. XLYC1808013).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nh00105e

‡ Equal contribution.

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