Computational Study On Silicon-based Atomically Thin Structures

The experimental realization of two-dimensional materials such as graphene,silicene and germanene has attracted incredible interest ranging from understanding their physical properties to device applications.Silicene,an atomically thin layer of Si atoms in a honeycomb lattice structure,is in the spotlight of ongoing research due to its unique properties.It is theorized that the low-buckled silicene is even more advantageous than planar graphene.For instance,the prominent structural characteristic of buckling for silicene facilitates the tuning of the electronic structure by vertical electric fields.In addition,silicene is clearly much more compatible with existing well-established Si technologies.However,the intrinsic zero-bandgap of silicene is a great barrier in the realization of silicene-based devices.The aim of this thesis is to tune the structural and electronic properties of Si-based atomically thin structures by means of vacancies,oxidation and nanostructuring in the framework of density functional theory.Following states the key results of this work:(1)The formation energies,electronic and magnetic properties of silicene with varying the vacancy concentration are systematically investigated and compared to those of graphene with vacancies.It is found that the magnetic moment of silicene with vacancies decreases with the increase in the concentration of vacancies.Moreover,low-buckled silicene with vacancies may possess remarkable band gaps,in contrast to planar graphene with vacancies.With the formation of vacancies silicene demonstrates a transition from semimetal to semiconductor,while graphene turns to be metallic.(2)We study the formation,structural and electronic properties of silicene oxides(SOs)that result from the oxidation of silicene on Ag(111)surface.It is found that the honeycomb lattice of silicene on the Ag(111)surface changes after the oxidation.SOs are strongly hybridized with the Ag(111)surface so.that they possess metallic band structures.Charge accumulation between SOs and the Ag(111)surface indicates strong chemical bonding,which dramatically affects the electronic properties of SOs.When SOs are peeled off the Ag(111)surface,however,they may become semiconductors.(3)The structural and electronic properties of hydrogen passivated black phosphorene quantum dots(BPQDs)/silicene hybrid nanostructures are investigated by means of changing the BPQD size on silicene.The adsorption energies and bandgaps of BPQD/silicene hybrid nanostructures increase with the increase of QD size.The adsorption of P76H22-QD on silicene generates a bandgap of 102 meV for silicene.Charge transfer analysis has revealed that the charge transfers from silicene to BPQDs and the magnitude of charge transfer depends on the surface coverage of BPQD on silicene.(4)We investigate the electronic properties of atomically thin superlattice of Si6-XCX/C formed by the incorporation of Si atoms into graphene.The superlattices demonstrate direct bandgaps at Dirac point and the charge effective masses vary from 0.016 to 0.646me.It is found that the bandgap of superlattices greatly depends on the numbers and sites of Si in the hexagonal ring of graphene.The maximum bandgap of 0.3 eV is calculated for ?-Si3C3/C superlattice.Since ?-Si3C3/C superlattice possesses the largest bandgap among all the superlattices,we have further investigated the carrier mobility and thermoelectric properties of ?-Si3C3/C superlattice only.The Boltzmann transport method within relaxation time approximation has shown the carrier mobility as high as 1.2854 x 105 cm2V-1s-1 at room temperature.The incorporation of Si into graphene reduces the phonon thermal conductivity to 15.48 Wm-1 for ??Si3C3/C superlattice.The thermoelectric figure of merit for ?-Si3C3/C can be optimized to a prominent value of 1.95 by tuning of the carrier concentration.