Investigation of new polymorphs of borophene and their functionalization

Abstract

The realization of buckled monolayer sheets of boron (i.e., borophene) and its other polymorphs has attracted signi cant interest in the eld of two-dimensional systems. Motivated by their chameleonic behavior we analyzed di erent polymorphs of borophene and discovered two new phases with unprecedented crystal structures namely symmetric washboard and asymmetric washboard using ab initio methods based on density functional theory. While symmetric washboard borophene is a metal with high electronic density of states in the vicinity of Fermi level asymmetric washboard borophene is a narrow band gap semiconductor. Asymmetric washboard structure is actually a 2 1 reconstructed form of symmetric structure with in plane and out of plane Peierl's distortion along the chains of boron atoms which is the key reason for the contrasting electronic behavior of these phases. Phonons dispersion calculations based on density functional perturbation theory reveal that both structures are stable at 0 K however ab ini- tio molecular dynamics simulations showed that symmetric washboard structure is stable only at temperatures close to absolute zero and at nite temperatures this structure gets deformed transforming into asymmetric washboard structure. Moreover we discovered that asymmetric washboard structure has a positive Poissons's ratio however symmetric one has a negative Poisson's ratio. In the next work, motivated by buckled borophene's tendency to donate electrons, we analyzed the interaction of single halogen atoms (F, Cl, Br, I) with borophene. The possible adsorption sites are tested and the top of the boron atom is found as the ground state con guration. The nature of bonding and strong chemical interaction is revealed by using projected density of states and charge di erence analysis. The migration of single halogen atoms on the surface of borophene is analyzed and high di usion barriers that decrease with atomic size are obtained. The metallicity of borophene is preserved upon adsorption but anisotropy in electrical conductivity is altered. The variation of adsorption and formation energy, interatomic distance, charge transfer, di usion barriers, and bonding character with the type of halogen atom are explored and trends are revealed. Lastly, the adsorption of halogen molecules (F2, Cl2, Br2, I2), including the possibility of dissociation, is studied. The obtained results are substantial for fundamental understanding and possible device implementations of borophenes and their halogenated derivatives.