Real-space light-reflection mapping of atomically thin WSe₂ flakes revealing the gradient local strain

Abstract

The spatially continuous control of the physical properties in semiconductor materials is an important strategy in increasing electron-capturing or light-harvesting efficiencies, which is highly desirable for the application of optoelectronic devices including photodetectors, solar cells and biosensors. Unlike the multi-layer growth of chemical composition modulation, local strain offers a convenient way to continuously tune the physical properties of a single semiconductor layer, and open up new possibility for band engineering within the 2D plane. Here, we demonstrate that the gradient refractive index and bandgap can be generated in atomically thin transition metal dichalcogenide flakes due to the effect of thermal strain difference. A highly resolved confocal scanning optical microscopy is used to perform a real-space light-reflection mapping of suspended atomically thin WSe 2 flakes at the low temperature of 4.2 K, in which the parabolic light-reflection profiles have been observed on suspended monolayer and bilayer WSe 2 flakes. This finding is corroborated by our theoretical model which includes the effect of strain on both the refractive index and bandgap of nanostructures. The inhomogeneous local strain observed here will allow new device functionalities to be integrated within 2D layered materials, such as in-plane photodetectors and photovoltaic devices.

Suspended 2D Materials: A Short Review

“…The findings indicate that the thermal expansion efficiency of few- Suspended 2D materials offer a unique opportunity to study their mechanical properties, which possess an exceptionally large surface area and an extremely thin thickness, leading to remarkable mechanical characteristics such as stiffness and elastic modulus. One article by Guo et al employed real-space light-reflection mapping to investigate the spatially varying strain distribution in atomically thin suspended WSe 2 flakes (Figure 4) [118]. The suspending configuration allows for more accurate and higher-resolution measurements of strain compared with supported ones, where the substrate-induced strain obscures local strain measurements.…”