Raman imaging of strained bubbles and their effects on charge doping in monolayer WS2 encapsulated with hexagonal boron nitride

Abstract

Interfacial defects significantly affect the optical and electronic properties of two-dimensional (2D) materials. Particularly, bubbles inevitably formed during the layer-by-layer fabrication of 2D heterostructures can cause spatially inhomogeneous distributions in charge density and strain, leading to modifications in emission efficiency and electronic structures that are markedly different depending on which interface the bubbles form. Here, we report spatially resolved Raman results of a hBN/WS2/hBN heterostructure over a large area in which microbubbles are present. Spatial variations in the optical phonon characteristics of both WS2 and hBN reveal that the bubbles are formed at the interface between the top hBN layer and the underlying WS2 monolayer. The presence of the hBN bubbles results in a relatively higher electron density of the underlying WS2 than that of the WS2 in the bubble-free surrounding flat region, possibly due to the flexoelectric effect of the bent hBN layer. In addition, the Grüneisen parameter of hBN is obtained using the relationship between E2g phonon frequency shifts and corresponding strain profiles of the hBN bubble layer. Our work inspires a more comprehensive understanding of charge and strain distributions under the effect of interfacial defects.