Unambiguous identification of the indirect band nature of atomically thin hexagonal boron nitride

TL;DR

This study clarifies that atomically thin hexagonal boron nitride has an indirect band gap, with optical properties and interlayer interactions influencing its electronic structure, challenging previous assumptions of a direct band gap in monolayer BN.

Contribution

The paper provides experimental evidence that monolayer h-BN has an indirect band gap, correcting prior beliefs, and explores how layer number affects its optical and electronic properties.

Findings

Monolayer h-BN exhibits no luminescence, indicating an indirect band gap.

Band-edge luminescence appears in 4-layer and thicker h-BN.

Interlayer interactions enhance phonon-assisted indirect transitions.

Abstract

Atomically thin hexagonal boron nitride (h-BN), especially monolayer, has garnered increasing attention due to its intriguing optical and light-matter-interaction properties. However, its intrinsic optical properties and electronic band structure, have long remained elusive. In this study, near-resonance excited deep-UV photoluminescence/Raman spectroscopy and deep-UV reflectance contrast spectroscopy are utilized to experimentally investigate the optical properties of atomically thin h-BN across various layer numbers. It is revealed that the absence of luminescence in 1-3 layers h-BN is indicative of their indirect band gap nature, rectifying previously adopted identification of a direct band gap in monolayer BN. Notably, band-edge luminescence signals and indirect bandgap absorption start to appear in 4-layer, and the luminescence intensity increases with the number of layers, suggesting that interlayer interactions and periodicity along the z-axis enhance phonon-assisted indirect bandgap transition, even in the 4-layer case, and furthermore indicating the formation process of flat bands at the K and M valleys as the periodicity along the z direction increases. Additionally, the prominent resonance Raman signals in atomically thin h-BN underscore strong electron-phonon coupling in this material.