Planar hexagonal B36 as a potential basis for extended single-atom layer boron sheets

Abstract

Boron is carbon's neighbour in the periodic table and has similar valence orbitals. However, boron cannot form graphene-like structures with a honeycomb hexagonal framework because of its electron deficiency. Computational studies suggest that extended boron sheets with partially filled hexagonal holes are stable; however, there has been no experimental evidence for such atom-thin boron nanostructures. Here, we show experimentally and theoretically that B36 is a highly stable quasiplanar boron cluster with a central hexagonal hole, providing the first experimental evidence that single-atom layer boron sheets with hexagonal vacancies are potentially viable. Photoelectron spectroscopy of B36(-) reveals a relatively simple spectrum, suggesting a symmetric cluster. Global minimum searches for B36(-) lead to a quasiplanar structure with a central hexagonal hole. Neutral B36 is the smallest boron cluster to have sixfold symmetry and a perfect hexagonal vacancy, and it can be viewed as a potential basis for extended two-dimensional boron sheets.

SmB6– Cluster Anion: Covalency Involving f Orbitals

“…This is seen particularly well in the lowest energy orbitals: 1a 1 , 1b 2 , 2a 1 , and 2b 2 form a sigma bonding and antibonding system, and 3a 1 , 4a 1 , 3b 2 , and 4b 2 from a peripheral sigma bonding and antibonding system. There is also clearly a π-antiaromatic system composed of 1b 1 and 1a 2 . By employing a natural population analysis (NPA), we see that the B 6 portion of SmB 6 − caries a −1.796 charge, meaning it is close in electronic structure to the B 6 2− cluster.…”

Elasticity, Flexibility, and Ideal Strength of Borophenes

“…Since the advent of graphene, 2 twodimensional (2D) materials that are one or several atoms thick reign the current field of materials research. As boron has demonstrated striking similarity to carbon, forming planar clusters 1,[3][4][5][6][7][8] , cage-like fullerences [9][10][11][12][13][14][15] and 1D nanotubes 7,[16][17][18][19][20][21] , extensive theoretical efforts have been devoted to exploring graphene analogues of boron-borophenes [22][23][24][25][26] . Unlike graphene or hexagonal boron nitride (h-BN) that have exclusively stable honeycomb lattice, the borophene is predicted to be polymorphic 27 with numerous states near the ground-state energy line, due to a highly variable network of hollow hexagons (HHs) in a reference triangular lattice.…”

“…The brittle fracture can be understood from two aspects: i) Due to excessive electrons, the triangular sheet has out-of-plane buckling, which is decreased by applied tensile strain; the decreased buckling diminishes the mixing of in-plane and out-ofplane orbitals and thereby drives more of the excessive electrons to occupy the in-plane antibonding states, ending up with severely weakened B-B bonds. ii) The dangling bonds along the cleaved edges are self-passivated due to the variability of B coordination; the self-passivation is particularly efficient at the cleaved flat B edge (insets in Figure 5a), which naturally exists in many planar B molecules 1,4,17 .…”

Perfectly planar CoB 18 – as a motif for metallo-borophenes

“…Small boron clusters have the propensity for planarity with delocalized  and  electrons over the whole molecular plane, giving rise to the concept of multiple aromaticity. Particularly, the discovery of the planar B 36 with a central hexagonal vacancy [8] has been a major landmark in the study of boron clusters, because it can be viewed as the basis for the formation of monolayer two-dimensional (2D) boron materials and provided the first indirect evidence for the viability of 2D borons with hexagonal holes [9,10]. This momentous finding prompted the proposal of a new name "borophene" for the new 2D boron [8], in analogy with graphene, by Jun Li and Lai-Sheng Wang.…”

“…Particularly, the discovery of the planar B 36 with a central hexagonal vacancy [8] has been a major landmark in the study of boron clusters, because it can be viewed as the basis for the formation of monolayer two-dimensional (2D) boron materials and provided the first indirect evidence for the viability of 2D borons with hexagonal holes [9,10]. This momentous finding prompted the proposal of a new name "borophene" for the new 2D boron [8], in analogy with graphene, by Jun Li and Lai-Sheng Wang.The reason that the name borophene was coined, rather than borene, was because the later had already been used in chemistry to represent R-B=B-R types of organoboron molecules. Very recently, borophenes have been successfully synthesized by atomic deposition on a silver surface [11,12].…”