Abstract:
We report on first-principles density functional theory (DFT) calculations of interactions between extrinsic defects and intrinsic structural defects in silicene. Specifically, we investigate the stability, structural, magnetic and electronic properties of a monolayer silicene containing vanadium (V), hydrogen (H) and oxygen (O) atoms. Vanadium is a magnetic transition-metal and its incorporation in silicene lattice introduces magnetization. Thus, we have considered various configurations of vanadium either as interstitial or substitutional atoms, and their interactions with silicene vacancies. Hydrogen and oxygen are ubiquitous elements which are inadvertently introduced during material synthesis. Therefore, for practical purposes, it is important to investigate how their presence impact on the host material which in this case, are silicene or silicene containing vanadium impurities.
We show that a monovacancy introduces a magnetic moment of 2.02 μB in an otherwise non-magnetic monolayer silicene. Nonetheless, the vacancy possesses a significant formation energy of 3.52 eV, which suggest that it may only be produced through external perturbation such as electron irradiation. Also, we show that a divacancy is more stable than a single vacancy, but unlike a single vacancy, it has a zero magnetic moment. Also, divacancies at different separation in silicene lattice have a similar formation energy irrespective of their separation. Furthermore, when a silicene atom is substituted by a vanadium atom, the latter makes the monolayer silicene metallic while introducing a magnetic moment of 2.61 μB. The presence of a vacancy at a different atomic separation from vanadium shows that the nearest-neighbour vanadium-vacancy defect complex, that is, vanadium in a divacancy has the highest stability, however, all the substitutional vanadium-vacancy configurations are stable and both types of defects can co-exist in a monolayer silicene.
Regarding small vanadium clusters consisting of a pair of vanadium at varying separations, we found that the relative stability of the V-V pair is sublattice dependent, which oscillates between ferromagnetic (FM) and antiferromagnetic (AFM) configuration as the substitutional lattice sites of the V-V pair varies. When the V-V dimer are on a similar sublattice type, they prefer to couple together antiferromagnetically. However, when they are on a different sublattice type, the V-V dimer prefer to be in ferromagnetic configuration.
Comparison between the binding energy of substitutional V-vacancy pair and V-V pair shows that vanadium clustering is more probable without the vacancy than with vacancy. Consideration of interstitial hole V-V pair, that is, V-V pair at the centre of silicene hexagons affirms that indeed, small V-V pair are stable without vacancies. The presence of V atoms, however, induces finite magnetic moment in monolayer silicene, while annihilating the Dirac point and opening a narrow band gap of under 0.1 eV in the monolayer silicene electronic band structures.
We found that the V atom attracts the O and H either in atomic or molecular form, and when adsorbed they impact on the magnetization of V-doped monolayer silicene by reducing or annihilating its magnetic moment. Furthermore, a V-doped silicene having adsorbed atomic H and O behaves like a ferromagnetic semiconductor. On the other hand, molecular H2 and O2 adsorbed on a V-doped silicene do not result in a ferromagnetic semiconductor, although the resulting structures are metallic with a finite magnetization. We also found that the impact of H and O on the electronic and magnetic properties of V-doped silicene depends on their respective lattice locations, that is, whether these adsorbates are on the V atom or on the silicene atom near to the V dopant.
