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

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 band structure.