First Principles study on structural and electronic properties of silicene on √3x√3 Si (111) - Ag (Au) monolayer

Abstract

Graphene, two-dimensional honeycomb lattice with C atoms, has attracted much attentions since it was synthesized in 2004 because of its peculiar sructural and electronic properties. Since Si atom also has same valence electrons with C atom, theoretical studies calculated the properties of two-dimensional honeycomb lattice with Si atoms and recent experiment has succeeded in synthesis of silicene on Ag (111) surface. Many researches have investigated in silicene that various superlattice structures on Ag (111) surface and the other metal substrates are found. Though silicene is expected to be promising nanoelectronics with experimental success, insulating substrate for silicene is still not reported yet. Furthermore, recent studies insist that the linear dispersion band measured form angle-resolved photo emission spectroscopy (ARPES) in above experiment is not originated from Si pz orbital, but from the hybridization between Si and Ag atoms. In this thesis, we analyse stuructural and electronic properties of freestanding silicene, silicene on metal substrate usgin abinito calculation and suggest the structure with silicene-insulating substrate. First, we show that bandgap of freestanding silicene can be opened through various processes because of its peculiar structure, light-buckling hexagonal lattice: (1) changing buckling pattern can open bandgap since it breaks sublattice inversion symmetry. (2) applying electric field perpendicular to silicene plane also can open bandgap. Si atoms located on different heights feel different potential and hence have asymmetry wavefunction. (3) hybridization between silicene and substrate also can open bandgap. Practically, all the three process play a part in opening bandgap. Analysing the amount of electrons in each atoms when silicene is formed on Ag (111) surface shows that the electrons from Ag substrate to Si atoms are 0.15e on average. Therefore, we can predict that silicene band in total electronic band structure shift down compared with freestanding silicene band. Projection of electronic band structure to Si pz orbital reveals that π band shift down 1.2 eV from Fermi level and the linear dispersion band in previous experiment, which extends from -0.3 eV to -3 eV, is not originated from silicene. Silicene π band also shifts down when silicene is put on Au (111) surface and hybridization between Si and Au is stronger than that between Si and Ag. We calculate energy barrier of Si atom on substrate, because it play an important role in diusion. The results are that Ag and ZrB2 have similar values while Au has much less value. We can suggest from above results the magnitude of energy barrier for substrate in usual usual deposition conditions. Finally,We propose insulating substrate consists of insulator and metal thin film. Ag thin film is used to make deposited Si atoms to form silicene as Ag (111) surface does. Since metal thin lm interacts with underneath insulating substrate, hybridization between silicene and metal atoms also can be weaken that Dirac-electron character of silicene is not destroyed. Among a couple of candidates, we choose and investigate the structural and electronic properties of √3x√3 Si (111) - Ag (Au) monolayer. Silicene on both substrate has bangap with ~ 120 meV originated from the three processes as mentioned above and hence off-current state can be realized with silicene. In sequence, We analyse the first conduction band by projecting it on orbital basis in order to see the contribution of Si pz orbital, which can act as conduction channel. The Si pz orbital character is dominant between K and Γ point at 0.2 eV. On-current state can be realized around this energy level. We show the possibility of silicene on substrate as nanoelectronics in this study.