Abstract
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.
Highlights
The discovery of graphene and its unique properties has triggered the development of various types of layered materials [1]
Among the numerous transition metal dichalcogenides (TMDs) materials, WSe2 has been extensively studied because its electrical transport properties can be adjusted from p-type to bipolar behavior depending on the type of contact metal [5,6,7]
Domain size of MoS2 increased up to 60 μm through vaporized aromatic-molecule catalysts such as perylene-3,4,9,10- tetracarboxylic acid tetrapotassium salt (PTAS) and F16CuPc. They reported that uniform monolayer MoS2 can be synthesized on the entire area of the SiO2/Si substrate; the use of such an organic catalyst leaves a residue on the growth substrate that acts as a defect of the synthesized TMD
Summary
The discovery of graphene and its unique properties has triggered the development of various types of layered materials [1]. The second is a noncatalytic growth method, in which a transition metal source and sulfur (or selenium) are heat-treated in a growth tube and flowed in a gaseous state to synthesize the TMD layer on a target substrate [19,20] These CVD approaches have not been successful in uniform, high-quality TMD synthesis because it is difficult to control the thickness and nucleation density of TMDs [21]. Liu et al demonstrated a Cu-assisted self-limited growth (CASLG) method that allowed the synthesis of a high-quality, uniform WSe2 monolayer while maintaining a balance between high crystallinity and fast growth rates They explained that Cu atoms, which occupy the hexagonal sites positioned at the center of the six-membered rings of the WSe2 surface, induce self-limited growth of WSe2 and prevent unwanted reactions [15]. This approach had disadvantages, for example, the synthesized WSe2 had small grain sizes with multilayered regions and the vapor pressure of the solid catalyst could not be precisely controlled
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