Theoretical Study On Boron Monolayers And Clusters

Due to the electron deficiency,boron nano-structures show rich structural diversity.Photoelectron spectroscopy combined with first-principles theoretical studies have revealed that,boron clusters possess planar and quasi-planar structures up to relatively large sizes,due to the complicated multicenter bonds between boron atoms.Recently,two independent experiments reported the synthesis of boron monolayers on the Ag（111）surfaces,which have provoked a rush of attentions in the community of physical chemistry and material research.Herein,based on the first-principles high-throughput calculation and bonding analysis,we have systematically investigated the structural stability and electronic properties of the boron clusters and monolayers.With the congruence check,we have investigated the stability and electronic properties of the quasi-planar and tubular B_n（n=30-85）clusters based on the first-principles high-throughput calculations.Based on the triangular lattice fragments with convex profiles for the given number of atoms,we have predicted the stable planar clusters B_41,46 with one hexagonal vacancy and a double-hexagon vacancy planar cluster B₄₉.As the size increased,the competition between 2D planar clusters and 3D tubular clusters indicated the tubular clusters are the potential ground-state boron clusters,and we have found two stable five layers tubular clusters of B_70,76.Combined our new method of distance matrix and eigen-subspace projection function to describe the atomic structure under the periodic boundary conditions with the first-principles high-throughput calculations,we have confirmed the ground-state boron monolayers at the given hexagon-vacancy concentration,a new stable phaseβ_arm was confirmed,and we have revealed that boron atoms’coordination number（CN）of the stable candidates is not smaller than 4.After high-throughput screening the electronic properties of the boron monolayers at the fixed vacancy concentration range,we have first found a family of boron monolayers with the novel semiconducting property.We demonstrate that the connected network of hexagonal vacancies dominates the gap opening for both the in-plane s+p_x,y,y and out-of-plane p_z orbitals,with which various band gaps semiconducting boron monolayers are designed by control the shape and arrangement of the heptagons in the unit cell.In order to expand the application of boron nano-materials,we have thought about the potential of metal-boron two-dimensional alloy.Combined the USPEX search with the DFT calculation,we have confirmed that the structures of the stable 2D iron borides FeB_x are multilayer structures.By using HSE06 functional,we carried out the band structure calculation and found the stable FeB_{（4,6,8,10）}are 2D wide-band-gap semiconductors.In addition,the bandgap and optical properties of FeB₆ sandwich can be effectively adjusted by applying a biaxial strain.We have theoretically investigated the formation of boron monolayers on the Ag（111）surface,and a penetration mechanism of initial nucleation is especially proposed.Our calculations indicate that B_1–3–3 clusters deposited on the Ag surface would penetrate the first layer and further expel Ag atoms for extra B atoms.As the number of B atoms increases,the bigger size clusters evolve into the nanoribbons,and the wide nanoribbons grow into a stripe pattern boron monolayer of 1/6 vacancy.In addition,the corresponding STM image is in good agreement with the experimental observation.As the metal substrates can alter the stability of the boron monolayers,the semiconducting boron monolayers we predicted may be synthesized on the proper substrates in the future.Based on the first-principles high-throughput calculations,we have proposed a8-electron model based on 2c-2e and 3c-2e bond to understand the complicated bonding inboron nanostructures,which are consistent with the charge distribution from DFT calculations,and the 8-electron model provides a practical criterion for structure searching.Furthermore,an average electron compensation（AEC）model is proposed to describe the electron deficiency in boron monolayers quantitatively,providing an excellent explanation of the stability evolutions for boron monolayers with/without substrates.Our findings show a valuable insight to help us understand the intriguing electronic properties of boron monolayers,and provide theoretical support for synthesizing various boron monolayers for practical applications in the future.