  Raman spectroscopy has become an essential technique to characterize and
investigate graphene and many other two-dimensional materials. However, there
still lacks consensus on the Raman signature and phonon dispersion of
atomically thin boron nitride (BN), which has many unique properties distinct
from graphene. Such a knowledge gap greatly affects the understanding of basic
physical and chemical properties of atomically thin BN as well as the use of
Raman spectroscopy to study these nanomaterials. Here, we use both experiment
and simulation to reveal the intrinsic Raman signature of monolayer and
few-layer BN. We find experimentally that atomically thin BN without
interaction with substrate has a G band frequency similar to that of bulk
hexagonal BN, but strain induced by substrate can cause pronounced Raman
shifts. This is in excellent agreement with our first-principles density
functional theory (DFT) calculations at two levels of theory, including van der
Waals dispersion forces (opt-vdW) and a fractional of the exact exchange from
Hartree-Fock (HF) theory through hybrid HSE06 functional. Both calculations
demonstrate that the intrinsic E2g mode of BN does not depend sensibly on the
number of layers. Our simulations also suggest the importance of the exact
exchange mixing parameter in calculating the vibrational modes in BN, as it
determines the fraction of HF exchange included in the DFT calculations.
