Effects of Hexagonal Boron Nitride Encapsulation on the Electronic Structure of Few-Layer MoS2

Hexagonal boron nitride (hBN) encapsulation has been widely used in the electronic applications of two-dimensional (2D) materials to improve device performance by protecting 2D materials against contamination and degradation. It is often assumed that hBN layers as a dielectric would not affect the electronic structure of encapsulated 2D materials. Here, we studied few-layer MoS2 encapsulated in hBN flakes using a combination of theoretical and experimental Raman spectroscopy. We found that after the encapsulation the out-of-plane, A(1g), mode is upshifted, whereas the in-plane, E-2g(1), mode is downshifted. The measured downshift of the E-2g(1) mode does not decrease with increasing the thickness of MoS2, which can be attributed to tensile strains in bilayer and trilayer MoS2 caused by the typical experimental process of the hBN encapsulation. We estimated the strain magnitude and found that the induced strain may cause the K-Q crossover in the conduction band of few-layer MoS2, so greatly modifying its electronic properties as an n-type semiconductor. Our study suggests that the hBN encapsulation should be used with caution, as it may affect the electronic properties of encapsulated few-layer 2D materials.