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

Lei Fu; Yuqing Hu; Ning Tang; Junxi Duan; Xionghui Jia; Huaiyuan Yang; Zhuoxian Li; Xiangyan Han; Guoping Li; Jianming Lu; Lun Dai; Weikun Ge; Bo Shen

arXiv · 2410.12545

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

Lei Fu·Yuqing Hu·Ning Tang·Junxi Duan·Xionghui Jia·Huaiyuan Yang·Zhuoxian Li·Xiangyan Han·Guoping Li·Jianming Lu·Lun Dai·Weikun Ge·Bo Shen

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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.

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Lei Fu, Yuqing Hu, Ning Tang, Junxi Duan, Xionghui Jia, Huaiyuan Yang, Zhuoxian Li, Xiangyan Han, Guoping Li, Jianming Lu, Lun Dai, Weikun Ge, Bo Shen. 2024-10-16. Unambiguous identification of the indirect band nature of atomically thin hexagonal boron nitride. https://arxiv.org/abs/2410.12545

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