Correlative spectroscopic investigations of the mechanisms of inhomogeneity in CVD-grown monolayer WS2

Chemical vapor deposition (CVD) has been proved to be the most useful method to produce two-dimensional (2D) materials, including tungsten disulfide (WS2). However, the existence of inhomogeneity of strain, doping, and defects in the CVD-grown WS2 monolayers may significantly influence the optical and electronic properties of the materials, thus affecting their device applications. In this work, we systematically characterized the inhomogeneity of strain, doping, and nonradiative defect centers in mesoscopicsize, triangular-shape monolayer WS2 grown by CVD on sapphire substrate by using spatially resolved micro-Raman and photoluminescence (PL) spectroscopy. We performed correlative analyses on the variations of the pertinent spectral parameters (i.e., peak position, intensity, and full width at half maximum) of Raman and PL signatures in two physical scales: (1) the complete-data-set level, including the data of the whole sample, and (2) the sub-data-set level for individual special regions (e.g., apexes, edges, center) that exhibit distinctly different strain, doping, and defect states. This study reveals and explains the inhomogeneous strain, doping, and defects across the WS2 monolayer. Additionally, we find the inhomogeneity substantially diminishes when a mesoscopic-size triangle structure expands into a continuous film. Our work demonstrates that the correlative analyses, supported with physics insights, can offer comprehensive understanding on the underlying mechanisms of the inhomogeneity and guidance for optimizing the growth process and device processing of 2D materials.

Automated summary

This study systematically characterized the inhomogeneity of strain, doping, and defects in CVD-grown WS2 monolayers and revealed the effects of different regions on the properties. It was found that the inhomogeneity substantially diminished in continuous films.