Density Functional Theory Investigation of the Physical Properties of Hydrogenated Borophene Nanosheets

Two-dimensional materials such as graphene, borophene, hydrogen boride, and hexagonal boron nitride exhibit extraordinary chemical stability and structural flexibility required for high-speed modern technologies ^[1-2] in many different research fields viz. spintronics, thermoelectronics, magnetism, hydrogen storage, lithium batteries, sensors, and detectors ^[3-4]. In this study, we focus on hydrogenated borophenes (sometimes referred to as hydrogen borides (HBs)), which can be readily synthesized by cation-exchange methods ^[5-6]. We have performed density functional theory to investigate the physical chemistry of some free standing 6,6 and 5,7 HB nanosheets with the empirical formula B_xH_x.<br/>We used Atomic Simulation Environment (ASE) tool to create the 6,6 and 5,7 HB geometries and the VASP code for DFT calculations at the PBE level of theory. The results suggest that the orthorhombic <i>Cmmm </i>geometry (B₄H₄) is energetically favorable over the triclinic <i>Pm</i> geometry (B₂H₂) of the 6,6 HB (energy per atom formula unit: -4.8 eV vs. -4.3 eV), and are dynamically stable. The XAS spectrum (simulated using supercell core hole method) gave B-K edge peaks approximately at 181, 185, 192, 194 and 198 eV for 6,6 HB, in which, orthorhombic crystal lattice was used. The origin of the first peak is due to the transition from B’s 1s orbital to the π* antibonding orbital, and the remaining peaks are due to transitions from B’s 1s to the σ* antibonding orbitals. A blue shift about 4 eV for each peak position was observed while changing and lattice symmetry to triclinic.<br/>For 5,7 HB nanosheets, we considered two different hydrogenation patterns, comprising 5,7-1 and 5,7-2 phases. As found for 6,6 HB, our calculations suggested that the orthorhombic <i>Pbam</i> geometry of 5,7 HB (5,7-2) is nearly isoenergetic to the monoclinic <i>P_21/c</i> geometry (5,7-1) (energy per atom formula unit: -4.92 eV vs. -4.90 eV), which are dynamically stable. The computed XAS spectrum of 5,7-1 HB sheet gave B-K edge peaks approximately at 180, 185, 187, 188, 190,193 and 195 eV. These results demonstrate that there are indeed some small changes to peak positions compared to that observed for 6,6 HB, which can be understood as a result of electronic structure changes. A detail of the geometric, electronic, vibrational, charge density topology, optical and XAS spectral aspects of the studied HB nanosheets will be discussed in this presentation.<br/><br/><br/><b>References:</b><br/>[1] Kaneti YV <i>et al</i>., Two-dimensional Boron Monolayer: Synthesis, Properties, and Potential Applications, <i>Chem. Rev</i>., 122, 1, 1000–1051, (2022).<br/>[2] Castro Neto AH, <i>et al</i>., The electronic properties of graphene, <i>Rev. Mod. Phys.</i> 81,109, (2009).<br/>[3] Chaves A<i> et al</i>., Bandgap engineering of two-dimensional semiconductor materials, <i>npj 2D Materials and Applications</i>, 4, 29, (2020).<br/>[4] Ma J-J <i>et al</i>., First-principles calculations of thermal transport properties in MoS₂/MoSe₂ bilayer heterostructure, <i>Phys. Chem. Chem. Phys</i>., 21, 10442, (2019).<br/>[5] Nishino H, <i>et al.</i>, Formation and Characterization of Hydrogen Boride Sheets Derived from MgB2 by Cation Exchange, <i>J. Am. Chem. Soc.</i>, 139, 39, 13761–13769, (2017).<br/>[6] Zhang X, <i>et al</i>., Accelerated Synthesis of Borophane (HB) Sheets through HCl-Assisted Ion-Exchange Reaction with YCrB4, <i>Molecules</i>, 28, 7, 2985, (2023).<br/>[7] Tateishi I, <i>et al</i>., Electronic Structures of Polymorphic Layers of Borophane, Molecules, 27,6, 1808, (2022).<br/><br/><b>Acknowledgements:</b><br/>AV and MK thanks Institute of Molecular Science, Okazaki, Japan for supercomputing facilities received for all calculations (Project: 23-IMS-C137), and all authors thank CREST project for generous funding (JPMJCR21O4).