  The hexagonal boron nitride (hBN) encapsulation has been widely used in the
electronics applications of 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 MoS$_2$
encapsulated in hBN flakes by using a combination of theoretical and
experimental Raman spectroscopy. We found that after the encapsulation the
out-of-plane A$_{1g}$ mode is upshifted, while 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 MoS$_2$, which can be attributed to tensile
strains in bilayer and trilayer MoS$_2$ 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 MoS$_2$, so greatly modifies 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.
