Argonne National Laboratory

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First Glimpse of a New Type of Two-Dimensional Crystal

NST Colloquium
Gong Gu, University of Tennessee, Knoxville
May 16, 2018 11:00AM to 12:00PM
Building 440, Room A105-106

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.