Correlated Structural and Optical Characterization of Hexagonal Boron Nitride

Abstract: Hexagonal boron nitride (hBN) plays a central role in nanoelectronics and nanophotonics. Moreover, hBN hosts room-temperature quantum emitters and optically addressable spins, making it promising for quantum sensing and quantum photonics. Despite many investigations of their optical properties, however, the emitters' chemical structure remains unclear, as does the role of contamination at surfaces and interfaces in forming the emitters or modifying their properties. We prepare hBN samples that are compatible with confocal photoluminescence (PL) microscopy, transmission electron microscopy (TEM), and atomic-force microscopy (AFM), and we use those techniques to quantitatively investigate correlations between fluorescent emission, flake morphology, and surface residue. We find that the microscopy techniques themselves induce changes in hBN's optical activity and residue morphology: PL measurements induce photobleaching, whereas TEM measurements alter surface residue and emission characteristics. We also study the effects of common treatments $\unicode{x2014}$ annealing and oxygen plasma cleaning $\unicode{x2014}$ on the structure and optical activity of hBN. The methods can be broadly applied to study two-dimensional materials, and the results illustrate the importance of correlative studies to elucidate structural factors that influence hBN's functionality as a host for quantum emitters and spin defects.