Bogaert Kevin, Liu Song, Liu Tao, Guo Na, Zhang Chun, Gradečak Silvija, Garaj Slaven
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA.
Centre for Advanced 2D Materials, National University of Singapore, 6 Science Drive 2, 117546, Singapore, Singapore.
Sci Rep. 2018 Aug 27;8(1):12889. doi: 10.1038/s41598-018-31220-z.
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 MoWS alloy by investigating the effect of continuously-varying composition on photoluminescence intensity. We developed a new process for growth of two-dimensional MoWS 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 WS 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 MoWS 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.
二维（2D）过渡金属二硫属化物可以通过在金属原子位点进行取代来形成合金，这对晶格应变的影响可忽略不计，但对光学和电学性质有显著影响。在这项工作中，我们通过研究连续变化的成分对光致发光强度的影响，建立了MoWS合金成分与光学性质之间的关系。我们开发了一种生长二维MoWS合金的新工艺，该合金沿单晶几乎跨越了整个成分范围，从而避免了任何与样品相关的不均匀性。梯度合金晶体是使用基于扩散的化学气相沉积（CVD）方法生长的，该方法首先合成具有梯度点缺陷分布的WS晶体，然后在第二阶段进行Mo合金化。我们表明，点缺陷促进了扩散和合金化，拉曼光谱和光致发光测量、反应路径的密度泛函理论计算以及观察到在缺陷密度低的CVD处理的剥离晶体中不发生合金化均证实了这一点。我们观察到光学量子产率强烈依赖于合金成分，在等成分的MoWS合金中达到最大强度。此外，我们绘制了合金化晶体中生长诱导的应变分布，以解耦成分和应变对光学性质的影响：在相同成分下，我们观察到量子产率随着诱导应变而显著降低。我们的方法通常适用于其他二维材料以及单晶中其他成分依赖性质的优化。
