Abstract: Traditional and new mainstream semiconductors are all sp3-coordinated crystals, including III-V, IV-IV, and, to a lesser extent, II-VI compounds, which are collectively referred to as octet compounds. Among octet compounds that exist in sp3-coordinated polymorphs, only boron nitride (BN) is known to exist also in sp2-coordinated forms, the most common of which is hexagonal BN (h-BN). Given the tremendous interest in two-dimensional (2-D) crystals, a natural question is whether h-BN-like polymorphs can exist for at least some octet compounds other than BN.

A theoretical study, based on an energetic consideration, predicted that each cation-anion bilayer in a wurtzite {0001} film would collapse into a planar, h-BN-like structure if and only if the film thickness is below a certain threshold. This transformation to the nonpolar structure is deemed a new stabilization mechanism for the otherwise polar crystals to avert the polar field in the ultrathin limit; a multitude of known mechanisms counter the would-be catastrophic divergence of potential because of the polar field for bulk crystals. While the h-BN-like ultrathin films were hailed as ​“precursors to wurtzite films,” experimental evidence has been illusive despite efforts to grow ultrathin films.

This talk presents the discovery of h-BeO, the h-BN-like form of BeO, made in a serendipitous experiment at the Center for Nanoscale Materials. Nanocrystals of BeO formed in graphene-sealed liquid cells were identified by HRTEM and EELS. Since h-BeO and the usual wurtzite BeO (w-BeO) have nearly identical basal plane lattice constants, we resorted to the ​“fine structure” of EELS, or energy loss near edge structure (ELNES), to show the sp2 electron configuration. Furthermore, we measured h-BeO thicknesses significantly larger than the thermodynamic threshold above which w-BeO is more stable. I will explain why this can be achieved, as well as why previous attempts did not lead to h-BN-like films. Our theoretical work further shows that the h-BN-like thin films of octet compounds with wurtzite bulks are not so much like h-BN. They constitute a new type of 2-D materials, of which we just had a first glimpse.