Theory of sigma bond resonance in flat boron materials

An Extended Rudolph Diagram: B3H5 and B3H6+ Relate 3D-, 2D-, 1D-, and 0D-Boron Allotropes

The structural chemistry of boron goes beyond the sp, sp2, and sp3 hybridization paradigms of carbon chemistry. We relate the apparently unconnected polyhedral boranes and 3D allotropes on the one hand and 2D clusters, borophenes, and multilayer borophenes on the other hand, through an extended Rudolph diagram. All-boron equivalents of cyclopropenium cation viz the flat B3H5 and the nonplanar B3H6+ constitute the missing links. The nonplanar B3H6+ (C3v) is the starting point for construction of polyhedral boranes; e.g., fusion of two of them leads to octahedral B6H62-. On the other hand, planar B3H6+ and B3H5 relate to borophenes with hexagonal holes. These borophene sheets can be further stacked with diverse interlayer BB bonds, ranging from bilayers to infinite layers. The tendency to achieve electron sufficiency as in the parent C3H3+ dictates the preference for hexagonal holes in the constituent layers and the interlayer bonds between them in multilayer borophenes. The design principles and theoretical validations for the formation of multilayer borophenes are also presented, indicating the variety and complexities involved.

Quantitative Analogue Simulation of Planar Molecules

Synthetic quantum systems provide a pathway for exploring the physics of complex quantum matter in a programmable fashion. This approach becomes particularly advantageous when it comes to systems that are thermodynamically unfavorable. By sculpting the potential landscape of Cu(111) surfaces with carbon monoxide quantum corrals in a cryogenic scanning tunneling microscope, we created analogue simulators of planar organic molecules, including antiaromatic and non-Kekulé species that are generally reactive or unstable. Spectroscopic imaging of such synthetic molecules reveals close replications of molecular orbitals obtained from ab initio calculations of the organic molecules. We further illustrate the quantitative nature of such analogue simulators by faithful extraction of bond orders and global aromaticity indices, which are otherwise technically daunting using real molecules. Our approach therefore sets the stage for new research frontiers pertaining to the quantum physics and chemistry of designer nanostructures.

Superhard and Superconducting Bilayer Borophene

Two-dimensional superconductors, especially the covalent metals such as borophene, have received significant attention due to their new fundamental physics, as well as potential applications. Furthermore, the bilayer borophene has recently ignited interest due to its high stability and versatile properties. Here, the mechanical and superconducting properties of bilayer-δ6 borophene are explored by means of first-principles computations and anisotropic Migdal-Eliashberg analytics. We find that the coexistence of strong covalent bonds and delocalized metallic bonds endows this structure with remarkable mechanical properties (maximum 2D-Young's modulus of ~570 N/m) and superconductivity with a critical temperature of ~20 K. Moreover, the superconducting critical temperature of this structure can be further boosted to ~46 K by applied strain, which is the highest value known among all borophenes or two-dimensional elemental materials.

B63: The Most Stable Bilayer Structure with Dual Aromaticity

The emergence of a bilayer B48 cluster, which has been both theoretically predicted and experimentally observed, as well as the recent experimental synthesis of bilayer borophene sheets on Ag and Cu surfaces, has generated tremendous curiosity in the bilayer structures of boron clusters. However, the connection between bilayer boron cluster and bilayer borophene remains unknown. By combining a genetic algorithm and density functional theory calculations, a global search for the low-energy structures of the B63 cluster was conducted, revealing that the Cs bilayer structure with three interlayer B-B bonds is the most stable bilayer structure. This structure was further examined in terms of its structural stability, chemical bonding, and aromaticity. Interestingly, the interlayer bonds induce strong electronegativity and robust aromaticity. Furthermore, the dual aromaticity stems from diatropic currents originating from virtual translational transitions for both σ and π electrons. This unprecedent bilayer boron cluster is anticipated to enrich the concept of dual aromaticity and serve as a potential precursor for bilayer borophene.

Self-Termination of Borophene Edges

Due to boron's unique bonding nature, planar boron materials, including borophenes, boron nanoclusters, and nanoribbons, show very puzzling features, especially the superior stability of the free-standing planar boron edges. Here, we present a systematic investigation of the bonding configurations of various edges of borophene. Because of the flexibility of forming either three-center two-electron (3c-2e) or two-center two-electron bonds (2c-2e), an edge of borophene tends to be self-terminated by adopting a different bonding configuration at the edge from that in bulk. Among various borophene edge types, the double-chain-terminated flat edge is found to be significantly stable. As a consequence, we found that the double- and triple-chain borophene nanoribbons with a triangular lattice and wider ribbons with hexagonal holes in the central area are more stable than the quadruple-chain borophene nanoribbon. This study greatly deepens our understanding of the bonding configurations, electronic properties, and stabilities of planar boron nanostructures and paves the way for the rational design and synthesis of various boron materials.