Browsing by Subject "Silicene"

Now showing 1 - 4 of 4

Open Access
Frictional properties of quasi-two-dimensional materials from the Prandtl-Tomlinson model
(2016-09) Adeel, Shaharyar
Tribology, the study of friction, is both an old theoretical problem in physics and an area of great practical importance. The invention of experimental instruments such as Atomic Force Microscope (AFM) has lead to the emergence of the eld of nanotribology, the exploration of friction phenomenon at the nanoscale. While more complete descriptions of friction make use of density functional theory (DFT) and molecular dynamics (MD) simulations, many essential features of frictional phenomena are accurately modeled by so called "reduced order models" such as the Prandtl-Tomlinson (PT) Model. We illustrate the PT model in both one-dimensional and two-dimensional forms via application to various crystal lattice surfaces (cubic, planar hexagonal) and reproduce important results from the literature by solving the resulting Langevin equation within the PT model. We also discuss the parameter dependence in this model via relevant simulations. We then generalize the PT model to a three-dimensional case and analyse quasi-two-dimensional systems. These systems thus exhibit a small amount of "buckling" - i.e. with out-of-plane basis atoms. The equations of motion of the Prandtl-Tomlinson model are solved numerically and the resulting friction force curves, tip path and lattice are analysed comparatively. The results agree with underlying theory and make testable predictions. We conclude that our generalized, three-dimensional PT model is a good approximation to the frictional dynamics at this scale for these systems and has the advantage of being computationally less intensive than full scale MD or DFT calculations.
Open Access
Magnetization of silicene via coverage with gadolinium: effects of thickness, symmetry, strain, and coverage
(American Physical Society, 2021-12-14) Demirci, S.; Gorkan, T.; Çallioğlu, Şafak; Yüksel, Y.; Akıncı, Ü.; Aktürk, E.; Çıracı, Salim
When covered by gadolinium (Gd) atoms, silicene, a freestanding monolayer of Si atoms in a honeycomb network, remains stable above the room temperature and becomes a two-dimensional (2D) ferromagnetic semiconductor, despite the antiferromagnetic ground state of three-dimensional bulk GdSi2 crystal. In thin GdSi2 multilayers, even if magnetic moments are ordered parallel in the same Gd atomic planes, they are antiparallel between nearest Gd planes; hence they exhibit a ferrimagnetic behavior. In contrast, a freestanding Gd2Si2 monolayer constructed by covering silicene from both sides by Gd atoms is a stable antiferromagnetic metal due to the mirror symmetry. While multilayers covered by Gd from both sides having an odd number of Gd planes have a ferrimagneticlike ground state, even-numbered ones have antiferromagnetic ground state, but none of them is ferromagnetic. Silicon atoms intervening between Gd planes are responsible for these intriguing magnetic orders conforming with the recent experiments performed on Si(111) surface. Additionally, the magnetic states of these 2D gadolinium disilicide monolayers can be monitored by applied tensile strain and by the coverage/decoration of Gd. These predictions obtained by using first-principles, spin-polarized, density functional theory calculations combined with Monte Carlo simulations herald that C, B, Si, Ge, Sn, and their compounds functionalized by rare-earth atoms can lead to novel nanostructures in 2D spintronics.
Open Access
Novel honeycomb nanostructures for energy storage and nanoscale device design
(2015-06) Özçelik, Veli Ongun
This thesis presents a variety of new two dimensional honeycomb-like structures and heterostructures; the main objective being to determine their fundamental electronic, magnetic, mechanical and optical properties for new device and material design. Utilization of existing two dimensional materials for nanoscale device design, understanding the fundamental properties of their composite structures, explaining the existing data on known two dimensional materials and using computational simulations to discover new materials are the main concerns of this thesis. We begin by assessing the validity of density functional theory on monolayer composites of graphene and boron nitride. We show that it is possible to grow vertical graphene / boron nitride heterostructures on top of each other and reveal the growth mechanisms at the atomistic level. We then utilize this vertical heterostructure for a nanoscale capacitor design by applying an external electric eld. We test and show how rst principles methods can be used to investigate the properties of materials under electric eld. After explaining the reliable methods, capacitance values are calculated for the model for various thicknesses, which show quantum mechanical size e ects at small separations that recede as the separations get larger; as the later is con rmed by experimental observations. The next part of the thesis, investigates the electronic properties of lateral graphene / boron nitride heterostructures, and show how these composites act di erently depending on the concentrations of graphene and boron nitride in the composite system. Namely, di erent behaviors of alloys, -doping and line compounds are revealed. Following this, these lateral heterostructures are utilized as nanoscale planar capacitors for atomically thin circuitry. As a nal remark on carbon and boron nitride nanocomposites, the next chapter of this thesis describes the growth mechanisms of one dimensional carbon/ boron nitride short atomic chains and show their stabilities at elevated temperatures. The electronic and magnetic properties of these chains exhibit even/odd disparity depending on the number of atoms in the chain. These chains also construct another two dimensional allotrope of graphene, namely graphyne, when connected to each other on the same plane. The properties of graphyne and its boron nitride analogue described in the following chapter introduces a new monolayer allotrope of carbon and boron nitride. The following chapter turns to silicon and germanium analogue of graphene, silicene and germanene. Dumbbell type reconstructions of silicene and germanene are introduced, which lead to layered silicene and germanene. Dumbbell units introduced here form the fundamental building blocks of experimentally observed layered silicene and germanene. The last chapter of the thesis looks at new material design and prediction studies based on computational simulations. Oxygenated silicene leads to a new monolayer piezoelectric material called silicatene. Finally, the monolayer structures of Group V elements nitrogen and antimony are also shown to be stable by phonon calculations and high temperature molecular dynamics simulations.
Open Access
Silicene dynamic optical response in the presence of external electric and exchange fields
(Institute of Physics Publishing Ltd., 2022-01-04) Mirzaei, M.; Vazifehshenas, T.; Salavati-fard, T.; Tanatar, Bilal
We investigate the dynamic optical transition of monolayer silicene in the presence of external electric and exchange fields within the low-energy tight-binding model. Applying external electric and exchange fields breaks the silicene band structure spin and valley degeneracies. Three phases of silicene corresponding to different strengths of perpendicular electric field with respect to the spin–orbit coupling (Δz < Δso, Δz = Δso and Δz > Δso) are considered. We obtain the spin-valley-dependent optical responses to the incoming circularly polarized light using the Kubo formula. We show and discuss how the magnitude and direction of the transverse and longitudinal optical responses of such a system change with the electric and exchange fields. Our calculations suggest that the intraband part of the longitudinal optical response as well as the initial point of the interband part have strong dependencies on the exchange field. Furthermore, we show that one of the spin subbands plays a dominant role in the response to polarized light. Depending on the type of incident light polarization, the dominant subband may change. Our results shed light on the relation between silicene dynamic optical responses and externally applied fields.