Indirect Band Nature of Atomically Thin Hexagonal Boron Nitride Identified by Resonant Excitation in the Deep Ultraviolet Regime

作者：Lei Fu, Yuqing Hu, Ning Tang, Junxi Duan, Xionghui Jia, Huaiyuan Yang, Zhuoxian Li, Xiangyan Han, Guoping Li, Jianming Lü, Lun Dai, Weikun Ge, Yugui Yao, Bo Shen · 发表于：Physical Review Letters · 年份：2025 · DOI：10.1103/rt4w-v9r8 · 被引用次数：6 · 研究领域：Graphene research and applications、Boron and Carbon Nanomaterials Research、Diamond and Carbon-based Materials Research

Atomically thin hexagonal boron nitride (h-BN), especially monolayer, has emerged as a pivotal quantum material due to its intriguing optical and light-matter-interaction properties. Nevertheless, fundamental ambiguities persist regarding its intrinsic band structure and deep-UV optical responses. Here, a multispectroscopic approach-combining near-resonance deep-UV photoluminescence, Raman spectroscopy, and reflectance contrast measurements-is employed to systematically resolve the layer-dependent optoelectronic evolution of h-BN. It is revealed that the absence of band-edge luminescence in 1-3 layers h-BN is indicative of their indirect band gap nature, thereby rectifying longstanding misinterpretations of monolayer BN as a direct band gap semiconductor. Strikingly, band-edge luminescence signals and indirect band gap 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 band gap transition, even in the 4-layer case, and furthermore indicating the formation process of flat bands at K/M valleys as the periodicity along z direction increases. Moreover, the prominent resonance Raman signals in atomically thin h-BN reveals exceptionally strong electron-phonon coupling, a critical parameter for quantum optoelectronic applications. Our findings provide definitive experimental benchmarks for the long-debated monolayer BN's ba...