Abstract
Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. However, the intrinsic optical properties of monolayer boron nitride remain largely unexplored. In particular, the theoretically expected crossover to a direct-gap in the limit of the single monolayer is presently not confirmed experimentally. Here, in contrast to the technique of exfoliating few-layer 2D hexagonal boron nitride, we exploit the scalable approach of high-temperature molecular beam epitaxy to grow high-quality monolayer boron nitride on graphite substrates. We combine deep-ultraviolet photoluminescence and reflectance spectroscopy with atomic force microscopy to reveal the presence of a direct gap of energy 6.1 eV in the single atomic layers, thus confirming a crossover to direct gap in the monolayer limit.
Highlights
Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides
Angleresolved photoemission spectroscopy (ARPES) shows that the Hexagonal boron nitride (hBN) layers are epitaxially aligned with graphite, with a welldefined energy band structure reflecting the high quality of our hBN films[33]
We emphasise that our demonstration of the direct bandgap crossover in the monolayer limit did not follow the standard methods documented in the literature
Summary
Hexagonal boron nitride is a large band-gap insulating material which complements the electronic and optical properties of graphene and the transition metal dichalcogenides. The investigation of the directgap properties of mBN is an important issue, from a fundamental point of view and for applications in DUV optoelectronics, with the exciting prospect of it becoming an active layer with highly efficient light–matter coupling in the DUV. In this context, reliance on exfoliated few-layer samples of hBN may prove to be of limited use. Cathodoluminescence measurements on hBN could only resolve the emission spectrum down to six monolayers[20], leaving unanswered the question of luminescence in mBN, either because of potential intrinsic limitations of cathodoluminescence for atomically thin layers of BN, or because of coupling with the substrate
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