Abstract
Two-dimensional (2D) transition metal dichalcogenides can be alloyed by substitution at the metal atom site with negligible effect on lattice strain, but with significant influence on optical and electrical properties. In this work, we establish the relationship between composition and optical properties of the MoxW1−xS2 alloy by investigating the effect of continuously-varying composition on photoluminescence intensity. We developed a new process for growth of two-dimensional MoxW1−xS2 alloys that span nearly the full composition range along a single crystal, thus avoiding any sample-related heterogeneities. The graded alloy crystals were grown using a diffusion-based chemical vapor deposition (CVD) method that starts by synthesizing a WS2 crystal with a graded point defect distribution, followed by Mo alloying in the second stage. We show that point defects promote the diffusion and alloying, as confirmed by Raman and photoluminescence measurements, density functional theory calculations of the reaction path, and observation that no alloying occurs in CVD-treated exfoliated crystals with low defect density. We observe a significant dependence of the optical quantum yield as a function of the alloy composition reaching the maximum intensity for the equicompositional Mo0.5W0.5S2 alloy. Furthermore, we map the growth-induced strain distribution within the alloyed crystals to decouple composition and strain effects on optical properties: at the same composition, we observe significant decrease in quantum yield with induced strain. Our approach is generally applicable to other 2D materials as well as the optimization of other composition-dependent properties within a single crystal.
Highlights
In analogy to conventional semiconductors, substituting various transition metal (M = Mo, W, etc.) and chalcogen (X = S, Se, Te) atoms into the TMD lattice while maintaining an MX2 stoichiometry would enable property modulation for specific applications
We study optical properties of 2D TMDs with compositional variations within single crystals that were produced using a novel 2-step chemical vapor deposition (CVD) process based on diffusion-mediated growth method
WS2 crystals are grown via CVD, which are exposed to MoS2 precursors in the second step
Summary
Details of the synthesis are described in the Methods section. In short, to produce compositionally-graded MoxW1−xS2 2D single crystals, WS2 crystals were first grown at 825 °C using WO3/NaCl precursor, followed by the deposition of MoS2 at 680 °C. An additional example of a crystal demonstrating this behavior is presented in Supporting Figure S2 Taken together, these results indicate that the crystal has a three-fold symmetry with pure MoS2 in the core and gradually-shifted composition toward the crystal edges that are composed of almost pure WS2. These results indicate that the crystal has a three-fold symmetry with pure MoS2 in the core and gradually-shifted composition toward the crystal edges that are composed of almost pure WS2 The graded LA(M)/A intensity ratio present in NaCl-assisted WS2 crystals is indicative of a graded point defect distribution, with a greater density of defects near the crystal core compared to the exterior This is likely due to a W-rich atmosphere during early stages of the crystal growth leading to non-stoichiometric crystal growth and formation of sulfur vacancies.
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