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.

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• Electronic structure
• Excitons
• Flat bands
• Nanophotonics
• Optoelectronics
• 2-dimensional systems
• Boron nitride
• Wide band gap systems
• Photoabsorption
• Photoluminescence
• Raman spectroscopy