James C. McGroddy Prize for New Materials Winner: Optical properties of Far-UV luminous hexagonal boron nitride and its applications

Since the discovery of graphene, two-dimensional (2D) van der Waals atomic-layer materials have been extensively studied for the last decade and a half in terms of the 2D physics of quantum correlation, superconductivity, photonics, topology and so on. From the early stage of research on graphene, hexagonal boron nitride (h-BN) single crystals grown by high-pressure, high-temperature (HPHT) synthesis have been used as the best substrate dielectric for studying 2D physics of graphene. A single crystal of h-BN has a layered structure, in which each layer is composed of atomically flat with sp² bonding between boron and nitrogen atoms, and the interlayers are coupled weakly via van der Waals interaction. The surface of the cleaved layer is thus almost free of dangling bonds and charge traps, which would scatter charged carriers and cause inhomogeneous fluctuations in chemical potential. The use of h-BN dielectric is also applicable for other 2D materials, and it is indispensable for the study of physics in 2D materials.

Originating from this anisotropic 2D crystal structure, this material is also known to have very interesting optical properties, such as high luminous efficiency of exciton in the far-UV region, three-dimensionally confined hyperbolic polaritons, symmetrically controlled polarisation characteristic, and single photon emitting point defect centers. Although h-BN is an indirect band gap material, the luminous efficiency is comparable to that of typical direct transition materials such as ZnO. The feature of confined hyperbolic polaritons is known as "natural metamaterial." The polarization switching is promising for atomically thin nonvolatile memory application, and the single photon source is attracting attention as the future quantum devices. We will review these new aspects of h-BN especially the peculiar and outstanding optical characteristics.