Robust synthesis of two-dimensional metal dichalcogenides and 1 their alloys by active chalcogen monomer supply 2


 Two-dimensional (2D) transition metal dichalcogenides (TMDs), with their atomic thicknesses, high carrier mobility, fast charge transfer, and intrinsic spin-valley couplings, have been demonstrated one of the most appealing candidates for next-generation electronic and optoelectronic devices. The synthesis of TMDs with well-controlled crystallinity, quality and composition is essential to fully realize their promising applications. Similar to that in III-V semiconductor synthesis, the precise precursor supply is a precondition for controllable growth of TMDs. Although great efforts have been devoted to modulate the transition metal supply, few effective methods of chalcogen feeding control were developed. Herein we report a strategy of using active chalcogen monomer supply to grow TMDs and their alloys in a robust and controllable manner. It is found that at a high temperature, the active chalcogen monomers (such as S, Se, Te atoms or their mixtures) can be controllably released from metal chalcogenides and, thus, enable the synthesis of TMDs (MX2, M = Mo, W; X = S, Se, Te) with very high quality, e.g., MoS2 monolayers exhibit photoluminescent circular helicity of ~92%, comparable to the best exfoliated single-crystal flakes and close to the theoretical limit of unity. More intriguingly, a uniform quaternary TMD alloy with three different anions, i.e., MoS2(1-x-y)Se2xTe2y, was accomplished for the first time. Our mechanism study revealed that the active chalcogen monomers can bind and diffuse freely on a TMD surface, which enables the effective nucleation and reaction, quick chalcogen vacancy healing, and alloy formation during the growth. The chalcogen monomer supply strategy offers more degrees of freedom for the controllable synthesis of 2D compounds and their alloys, which will greatly benefit the development of high-end devices with desired 2D materials.

generation semiconductor, silicon, can have the extremely low impurity of ~10 -11 and is nearly 48 threading dislocation free, but the synthesized third-generation semiconductor, GaN, generally 49 have a much higher impurity, ~10 -4 , and a threading dislocation density of ~10 4 -10 5 cm -2 (ref 22 ). 50 Analogously for the growth of 2D materials, the as-grown graphene already has excellent 51 properties, which is comparable to the samples exfoliated from natural crystals, and the measured 52 carrier mobilities are close to the theoretical limit 23 , while the as-grown 2D compounds, such as 53 TMDs, typically have lower quality than the natural crystals or the theoretical expectations 24 . 54 The main difficulty in controllable compounds' growth lies in the complicated feeding of several 55 elements simultaneously during the growth process. Therefore, in the semiconductor industry, 56 advanced and expensive techniques such as molecular beam epitaxy (MBE) and metal-organic 57 chemical vapour deposition (MOCVD) have been developed to realize the precise control of 58 multi-element supplies for the compound film growth. 59 The synthesis of high-quality TMD materials requires the precise feeding control of both the 60 transition metal and chalcogen precursors as well. In the past decade, intensive efforts have been 61 devoted to optimizing the feeding of metal precursors by thermal evaporation or molten-salt-62 assisted evaporation of metal oxide 14 , decomposition of metal-organic precursor 15 , direct 63 deposition of metal layers, and others [16][17][18] . Although some methods for controllable chalcogen 64 feeding, such as using either elemental chalcogen or chalcogen compounds (i.e., heating sulfur 65 powder, using H2S gas and ammonium sulphide 19-21 ), have also been developed, it turns out that 66 the chalcogen feeding control is much less effective than metal feeding control, as indicated by 67 the most challenging problem in TMD quality control that the most synthesized TMDs are rich 68 with chalcogen vacancies 25 . Therefore, developing more effective chalcogen supply methods to 69 enable the growth of high-quality TMDs is of critical importance. In this work, we propose to use a chalcogen monomer feeding method in controllable TMD 71 growth because of the following advantages: (i) the chalcogen monomers or atoms are generally 72 very active than corresponding dimers or bulk and thus they can quickly react with metal 73 precursors to form TMDs, (ii) the active chalcogen monomers can bind and quickly diffuse on a 74 TMD surface to scavenge the vacancy defects effectively, which will greatly improve the quality 75 of the TMDs and (iii) an active chalcogen monomer can react with a TMD and easily substitute 76 a chalcogen atom and, thus, allow the synthesis of uniform TMD chalcogen alloys. However, as 77 chalcogen of monomer state only exists at very high temperature (>2500 K) under normal 78 circumstance 26 , the most used methods can't produce enough chalcogen monomers at the typical 79 TMD growth temperature, which is generally less than 1273 K. Herein, we developed an effective 80 route to provide chalcogen monomer by heating a metal chalcogenide. The success of this 81 approach lies in that the dangling bonds on the surface of metal chalcogenides are unstable and 82 can be easily broken to release chalcogen atoms (monomers) under a relatively low temperature 27, 83 28 , and the slowly released chalcogen atoms have a very low probability to react with each other 84 to form dimers.

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In our design, metal chalcogenide plate, ZnS, which served as the source for S monomers, 86 was directly placed above the growth substrates of sapphire or SiO2/Si (see Methods for details).

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The substrates were precoated with a thin layer of Na2MoO4 precursor to provide Mo source for 88 MoS2 growth (Fig. 1a, left panel). A confined space with a narrow gap of ~20 μm was naturally 89 formed between the ZnS and the growth substrates. At an elevated growth temperature (750-90 950 °C), S monomers slowly released from the ZnS surface can quickly reach the substrate 91 surface and the possibility of their combination into dimers or polymers in the gas phase is very 92 low (Fig. 1a, middle panel). These S monomers will react with melted Na2MoO4 pellets on the 93 substrate to form monolayer MoS2 (Fig. 1a, right panel). Indeed, the release of S monomers from 94 the metal sulphide surface was observed long time ago 29 , and was unambiguously revealed by 95 the in-situ mass spectroscopy in our experiment (Fig. 1b). It is important to note that the S both Mo and S atoms (Fig. 1d). The low-temperature photoluminescence (PL) spectra of the S-105 monomer-feeding-grown MoS2 (Fig. 1e, orange curve) has a characteristic neutral exciton (X 0 ) 106 emission peak accompanied with a trion (X T ) peak (believed to be caused by the n-type doping 107 from substrate 31 ), but the X D peak (believed to be caused by S vacancy 32 and was obvious in S-108 powder-feeding-grown samples, Fig. 1e, dark yellow curve) is nearly invisible, which clearly 109 proves the high quality of the S-monomer-feeding-grown MoS2 samples.

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In addition, its high quality can be further confirmed by the measured optical circular helicity, 111 which was detected to be as high as 92% (Fig. 1f) and comparable with the best exfoliated flakes 112 from high-quality natural crystals 33 . The circular helicity is directly related to the scattering 113 between K and K' valleys in the Brillouin zone of MoS2 whilst the defects will greatly enhance 114 the inter-valley scattering and decrease circular helicity value. Thus, the near-unity circular 115 helicity strongly proves the high quality of the as-grown MoS2 samples. Here, we would like to 116 note that intrinsic quality information of MoS2 accessed by the optical helicity measurement is 117 more reliable than that via electronic device measurements, where the device fabrication process, 118 contact quality and device configuration will all bring great uncertainties in quality assessment 34 .

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Our strategy on MoS2 growth by monomer feeding has also been proved to be applicable for Extended Data Fig. 2). It's worth noting that the formation of transition metal tellurides, e.g.,

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MoTe2 and WTe2, are usually less favourable when chalcogen bulks applied 35 , due to their higher 127 formation energy compared to the corresponding sulphide and selenide (Fig. 2b). Thanks to the 128 introduction of active Te monomers, the synthesis of WTe2 and MoTe2 turns to be much easier 129 because of the greatly reduced formation energy.

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The chalcogen monomer feeding method has a unique advantage in the growth of TMD 131 chalcogen alloys. Since the evaporation temperatures, saturated vapour pressures and reaction 132 energies of S, Se and Te are significantly different, it is nearly impossible to form high-quality 133 TMD alloys with more than two anion elements by traditional approaches 36 . Till now, there is no 134 report on the successful growth of MoS2(1-x-y)Se2xTe2y alloy. In our experiment, we applied a 135 compressed plate mixed with different metal chalcogenide powders, i.e., ZnS, ZnSe, and ZnTe, 136 to supply three kinds of chalcogen monomers (S, Se, and Te) simultaneously (Fig. 3a). The as-137 grown alloy of MoS2(1-x-y)Se2xTe2y has a triangular domain similar to 2H phase TMDs (Fig. 3b).

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The X-ray photoelectron spectroscopy (XPS) unambiguously revealed the coexistence of S, Se, 139 and Te atoms in the synthesized TMD alloy (Extended Data Fig. 3), and the energy dispersive X-      WTe2, respectively). During the growth process, the system pressure was kept at ~120 Pa and the 296 growth duration was set as 10-60 minutes. After growth, the system was naturally cooled down 297 to room temperature. Similar growth conditions were applied to the TMD alloy growth, wherein 298 the major difference lies in the use of a chalcogenide mixture plate.

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To evaluate the distribution of Se and Te atoms in our quaternary alloy, statistical analysis was 318 conducted in a 32 × 32 nm STEM image (Extended Data Fig. 5a). The atoms in the image were 319 sorted into metal atom (M) sites and chalcogen atom (X2) sites. The intensity histogram of all X2 320 sites shows three peaks, which are assigned to be "S site", "Se site" and "Te site" regions, 321 respectively, according to the Z-contrast nature of STEM image (Extended Data Fig. 5b). Note 322 that every X2 site in STEM image is actually a projection of two overlapped X atoms along the 323 electron beam direction, "S site" denotes both X atoms are S, "Se site" means two possible 324 configurations: Se-S or Se-Se, while "Te site" stands for Te-S, Te-Se, or Te-Te. According to the 325 overall statistical result, P(S), the probability for an X2 site to be "S site", is 326 14 / 30 9122/(9122+2417+771) = 0.741. Similarly, the probability of "Se site" and "Te site" denoting as 327 P(Se) and P(Te), are 0.196 and 0.063, respectively.

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Assume that Se and Te are randomly distributed in the X2 sites, the overall probability of 329 P(Se) and P(Te) will also be valid to every X2 site. Thus, the probability of Se distribution could Extended Data Fig. 5d), which match well with the purple dotted lines calculated by the binomial 337 distribution, suggesting the random distribution of Te and Se atoms in our quaternary alloy.

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In addition, there is a relationship between the probability P and the atom concentration c:

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(2) wave basis set with a cutoff energy of 450 eV was adopted. All the structures were fully relaxed, 352 and the convergence criteria for energy and force were set at 10 -5 eV and 10 -2 eV/Å, respectively.

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The Brillion zone is sampled by 1×1×1 grid meshes. A vacuum spacing larger than 15 Å was set 354 to avoid the interaction between neighboring images along the non-periodic direction. The energy

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Based on the MD simulations, the sulfidation processes of Na2MoO4 are analyzed at atomic-level.

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Here we consider the sulfur source in the form of S monomers and S2 dimers respectively. As