Raman Signature and Phonon Dispersion of Atomically Thin Boron Nitride

TL;DR

This study combines experimental Raman spectroscopy and first-principles simulations to characterize the intrinsic phonon and Raman signatures of atomically thin boron nitride, revealing substrate effects and layer independence.

Contribution

It provides the first comprehensive analysis of Raman signatures and phonon dispersion in atomically thin BN using both experiments and advanced DFT calculations.

Findings

Atomically thin BN's G band is similar to bulk BN without substrate interaction.

Substrate-induced strain causes significant Raman shifts in BN.

The intrinsic E2g phonon mode is largely independent of layer number.

Abstract

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.