The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe2 has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe2 hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe2, but the difficulty of precisely controlling solid-state TMD precursors (WO3, MoO3, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe2 by utilizing methane (CH4 ) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe2 without a gas-phase promoter, the catalytic growth of the monolayered WSe2 with a CH4 promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe2 up to 100 µm. The significant improvement in the optical properties of the resulting WSe2 indicates that CH4 is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.
4. Conclusions 4. Conclusions In summary, we developed a CH4-assisted vapor transport growth method to obtain high-quality monolayer WSe2 crystals with large domain sizes. Unlike other promoter s or growth promoters quality monolayer WSe2 crystals with large domain4sizes. Unlike other promoter s o2r growth promoters previously reported (polymer, halide, and metal), CH4 only2acts as a promoter for W5Se2 growth2witho\u2212u2 t producing any residue. M4oreover, the nucleation density of WSe2 was tuned (from of1.6the\u00d7 1synthesized05 to 1.5 \u00d7 10monolayer2 mm\u22122) by WSeusin2g bythe Raman,gas-phaPL,se CHand4KPFMpromoterconfirmedwith precthatise CHflow 4 iscoantrolsuitab. The le characterization of the synthesized monolayer WSe2 by Raman, PL, and K2PFM confirmed th4 at CH4 is a suitable candidate as a promoter for the growth of high-quality monolayer WSe2. Finally, our CH4-assisted growth approach may be applicable for the controlled growth of high-quality single crystals of other TMDs. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/9/11/1642/s1: Figures S1\u2013S4\u2014The SEM results at each synthesis condition for statistical analysis, Figure S5\u2014Distribution of grain sizes in WSe2 according to carrier gas ratios, Figure S6\u2014Raman spectrum of synthesized monolayer WSe2, Figure S7\u2014The work function of Pt-coated AFM tip and reference HOPG, and Figure S8\u2014XPS wide-range spectra according to carrie2r gas ratios, Figure S6\u2014Raman spectrum of synthesized monolayer WSe2, Figure S7\u2014The work function of Pt-coated AFM tip and reference HOPG, and Figure S8\u2014XPS wide-range spectra of Author Contributions: H.-S.J. and J.-Y.L. contributed equally to this work. H.-S.J. and S.-G.K. synthesized the synthesized WSe2. samples. J.-Y.L. and S.-K.S. analyzed and performed the scanning electron microscopy, aAFM, KPFM, and XPS. Author Contributions: H.-S.J. and J.-Y.L. contributed equally to this work. H.-S.J. and S.-G.K. synthesized the analyzed the results. H.-S.J., J.-Y.L., S.S., J.-H.L., and D.W. wrote the paper. All authors participated in reviewing the manuscript. All authors approved the final version of the manuscript. H.-S.J. and S.-H.H. measured the optical properties (Raman and PL). J.-H.L. and D.W. guided the project and analyzed the results. H.-S.J., J.-Y.L., S.S., J.-H.L., and D.W. wrote the paper. All authors participated in reviewing Facilities and Equipment Center (NFEC) grant funded by the Korean government (Ministry of Education) the manuscript. All authors approved the final version of the manuscript. (No. 2019R1A6C1010031) and the National Research Foundation of Korea (NRF) grant funded by the Korean Funding: This work was supported by the Korea Basic Science Institute (KBSI) National Research Facilities and Equipment Center (NFEC) grant funded by the Korean government (Ministry of Education) (No. 2019R1A6C1010031) and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2019R1F1A1062330) and the Ajou University research fund.
