Bilayer Silicene Research Articles

Superconductivity in Ca-intercalated bilayer silicene

Superconductivity in Ca-intercalated bilayer silicene

Magnetic interactions in chromium adsorption on silicene

Silicon is a material with well established technological applications. Thus, obtaining this material in nanostructured form increases its possibility of integration in the current technology. Silicene, a two-dimensional allotrope of silicon, has a natural compatibility with current silicon-based technology. In this work we use density-functional theory to identify the electronic and magnetic properties of chromium atoms on monolayer and bilayer silicene. We investigate several properties of chromium in silicene, such as dopant solubility limits, site preference for adsorption and charge transfer. We find that magnetization depends on key parameters such as buckling, interlayer distance and adsorption site. Chromium intercalated in silicene bilayers showed a very interesting relation between the buckling of silicene and the total magnetic moment. The larger is the buckling the smaller is the magnetic moment. Our results demonstrate that it is possible to manipulate the magnetic properties of silicene by changing chromium concentration, adsorption site and structural parameters of the host material.

The effect of carbon chain doping at different positions on the electrical properties of bilayer silicene nanoribbons

The effect of carbon chain doping at different positions on the electrical properties of bilayer silicene nanoribbons

Feasibility study of dative bond formation for bilayer silicon growth under excessive strain

Energy bandgap opening has been found in low-buckle bilayer silicene under tensile in-plane strain. Practically such substantial in-plane strain could be provided from the substrates. The intense interfacial covalent bonds ensure an in-plane lattice-matching expitaxial growth, but at the same time impose a challenge in forming low-buckle two-dimensional films. We performed a theoretical study using density function theory to investigate the feasibility of growing bilayer silicon under excessive in-plane strain on various substrates. By the insertion of an air gap, dative bonds have been found at the interface on the substrates with the preferred polarized surface. The interactions of the transferred electrons from the surface-terminating metallic atoms of the substrates and the electron sea in the bilayer silicon was observed. The strength of the dative bond is reduced to about ∼0.05% of the Ga–Si covalent bond in the absence of the air gap. Preservation of substantial in-plane strain has been obtained in the bilayer silicon, resulting in a low-buckle bilayer silicon with opened energy bandgap up to ∼75 meV.

Structural, electronic and optical properties of fluorinated bilayer silicene

Silicon (Si) is undoubtedly an excellent material for electronic applications. However, Si is not a suitable material for optoelectronic applications due to its indirect bandgap. Based on the first principles approach, we have studied different stacking (AA and AB) patterns in bilayer silicene. The phonon band structure of AA and AB-stacked “bilayer silicene” reveals that only AB-stacked silicene is stable. Different fluorinated configurations (1F, 2F, 3F and 4F) of AB-stacked bilayer silicene are analyzed in terms of structural parameters and the phonon band structure. Of the four fluorinated configurations, 2F-AB-stacked bilayer silicene has a direct bandgap of 1.23 eV. The direct bandgap allows the carriers to transit from the “valence band” to the “conduction band” with a low probability of phonon generation. Meanwhile, the other configurations either lack stability or have a zero bandgap. The proposed fluorinated bilayer silicene, in addition to having an expected integration with silicon technology, could also be suitable for visible and near-infrared light detection applications.

Half-metallic antiferromagnets induced by non-magnetic adatoms on bilayer silicene.

Transition metal-free magnetism and two-dimensional p-state half-metals have been a fascinating subject of research due to their potential applications in nanoelectronics and spintronics. By applying density functional theory calculations, we predict that bilayer silicene can be an interlayer antiferromagnetic ground state. Interestingly, the half-metallicity is realized by adsorbing non-magnetic atoms on the antiferromagnetic bilayer silicene in the absence of transition magnetic atoms, nanoribbons, ferromagnetic substrates and magnetic field. Then, on the basis of first principles calculations and theoretical analysis, we show that the realization of half-metallicity is induced by the split of antiferromagnetic degeneracy due to the localization of transfer charge from the adatom to silicene. Our findings may open a new avenue to silicene-based electronic and spintronic devices.

Spin-valley polarized transport in a field-controllable bilayer silicene superlattice

We have theoretically investigated the spin-valley asymmetric transport of massive Dirac fermions in the field-controllable bilayer silicene superlattices. The spin-valley dependent ballistic transmission, conductance, and polarization have been systematically calculated by formulating the scattering matrix method for the completed four-band low-energy effective Hamiltonian. Our results uncover that for a single valley transport, near-perfect spin polarization and its perfect switching could be efficiently modulated by the gate field engineering. Under the one-dimensional periodic field modulation, two types of flat bands with less dispersion and, importantly, the perfect contrast in the spin-dependent subbands are observed for the bilayer silicene superlattice. Together with its larger spin-orbit coupling and better stability, these spin-valley asymmetric characteristics engineered by the gate field indicate that the field-controllable bilayer silicene could be a potential component candidate to achieve a fully spin-valley polarized beam for quantum logic applications.

Feasibility study of Mg storage in a bilayer silicene anode via application of an external electric field.

With the goal of developing a Si-based anode for Mg-ion batteries (MIBs) that is both efficient and compatible with the current semiconductor industry, the current research utilized classical Molecular Dynamics (MD) simulation in investigating the intercalation of a Mg2+ ion under an external electric field (E-field) in a 2D bilayer silicene anode (BSA). First principles density functional theory calculations were used to validate the implemented EDIP potentials. Our simulation shows that there exists an optimum E-field value in the range of 0.2–0.4 V Å−1 for Mg2+ intercalation in BSA. To study the effect of the E-field on Mg2+ ions, an exhaustive spread of investigations was carried out under different boundary conditions, including calculations of mean square displacement (MSD), interaction energy, radial distribution function (RDF), and trajectory of ions. Our results show that the Mg2+ ions form a stable bond with Si in BSA. The effects of E-field direction and operating temperature were also investigated. In the X–Y plane in the 0°–45° range, 15° from the X-direction was found to be the optimum direction for intercalation. The results of this work also suggest that BSA does not undergo drastic structural changes during the charging cycles with the highest operating temperature being ∼300 K.

Zigzag nanoribbon of gated bilayer hexagonal crystals with spontaneous edge magnetism

Zigzag nanoribbons of monolayer graphene-like two-dimensional materials host spontaneous edge magnetism at the zigzag terminations, whose configuration controls the band gap. In this article, the edge magnetism of zigzag nanoribbons of bilayer hexagonal crystals are studied. The specific models of bilayer graphene and bilayer silicene are both studied. As the gated voltage increases, the total energy level, magnetic structure and band structures of the ground state and the first quasi-stable excited state are tuned. For certain region of the gated voltage, the band gaps of spin up and down are opened and closed, respectively, so that the systems are in the spin-polarized metallic phase. The spin-polarized conducting edge band of the bilayer nanoribbons could be applied for gate tunable spintronic devices.

Electronic transport through a silicene bilayer barrier

We investigate the electronic transport properties of a finite-size silicene bilayer barrier. We got the boundary conditions at the interface of monolayer and bilayer silicene using the tight-binding model. By matching the wavefunctions, we got the transmission probability as a function of incident angle for several lengths of bilayer barrier and incident energys. We found that a clear resonant tunneling appears because of evanescent modes in bilayer silicene. We also calculate the conductance in a overlap configuration which displays an oscillatory behavior as increase of either incident energy or length of bilayer barrier.

First principle modeling of a silicene anode for lithium ion batteries

This work is devoted to a first principle study of changes in the structural, energy, and electronic properties of a silicene anode, represented by a bilayer silicene, as it is filled with lithium. The parallel folded silicene sheets form a flat channel with an initial gap of 0.75 nm. Lithium atoms were deposited both inside and outside the channel. The ratio of the amount of lithium to silicon varied in the range from 0.1 to 2.3. The maximum number of lithium atoms in the channel is revealed, which does not lead to defect formation in the silicene walls. The types of clusters in the formed packing of lithium atoms are defined and their stability is investigated. The tensile limit of silicon bonds in silicene sheets is found. The cohesive energy between the surfaces formed by lithium and the walls of the silicene channel has been established at various ratios of lithium to silicon. The change in the volume of the silicene channel is calculated depending on the amount of lithium deposited on it. The gravimetric capacities for different degrees of filling of the silicene anode with lithium were obtained. The voltage profile of the simulated anode was determined. The conductor-semiconductor and semiconductor-conductor transitions were found when the silicene channel was filled with lithium.

Mechanism of monolayer to bilayer silicene transformation in CaSi2 due to fluorine diffusion.

Calcium disilicide (CaSi2) possesses a layered structure composed of alternating monolayers of silicene (MLSi) and calcium. Here the mechanism by which fluorine (F) diffusion into CaSi2 leads to a phase transformation from MLSi to bilayer silicene (BLSi) was investigated. Disorder in intra-layer atomic arrangements and F aggregation were observed using HAADF-STEM in areas of low F concentration. Transformation of MLSi to BLSi in CaSi2Fx was predicted to occur at x = 0.63 based on cluster expansion (CE) and density functional theory (DFT) analyses, and these results agreed well with HAADF-STEM observations. The occurrence of F aggregation at low concentrations was also confirmed by Monte Carlo simulations using the interaction parameters obtained in CE analysis. Bader charge analysis, DFT calculations of charged states, and ab initio molecular dynamics simulations indicated that the aggregated F atoms withdrew electrons from MLSi, destabilizing the buckled honeycomb structure of MLSi in CaSi2. This charge imbalance caused the transformation of MLSi to the covalent-like BLSi.

Nanoscale synthesis of ionic analogues of bilayer silicene with high carrier mobility

High carrier mobility of both electrons and holes is found in nanofilms of layered SrAl2Si2 integrated with silicon. The salient feature of its atomic structure is anionic bilayers [Al2Si2]2−, isostructural and isoelectronic to bilayer silicene.

Subfemtosecond charge dynamics in vertically stacked bilayer silicene

AbstractUltrafast charge dynamics in two‐dimensional materials have attracted great interest from both fundamental and applied perspectives due to its significance in the next‐generation battery and solar cells. In this work, using the realistic quantum mechanical real‐time propagation time‐dependent density functional theory (rt‐TDDFT) simulations, we disclose the ultrafast charge dynamics in vertically stacked bilayer silicene subjected to an external bias. Among other insights, our study indicates that even modest changes in interlayer distance produce significant modulations in the ultrafast charge dynamics at the subfemtosecond time scale. The findings in this work may open new pathways for the use of bilayer silicene in novel optoelectronic and photovoltaic devices.

Stacking-configuration-enriched essential properties of bilayer graphenes and silicenes.

First-principles calculations show that the geometric and electronic properties of silicene-related systems have diversified phenomena. Critical factors of group-IV monoelements, like buckled/planar structures, stacking configurations, layer numbers, and van der Waals interactions of bilayer composites, are considered simultaneously. The theoretical framework developed provides a concise physical and chemical picture. Delicate evaluations and analyses have been made on the optimal lattices, energy bands, and orbital-projected van Hove singularities. They provide decisive mechanisms, such as buckled/planar honeycomb lattices, multi-/single-orbital hybridizations, and significant/negligible spin-orbital couplings. We investigate the stacking-configuration-induced dramatic transformations of essential properties by relative shift in bilayer graphenes and silicenes. The lattice constant, interlayer distance, buckling height, and total energy essentially depend on the magnitude and direction of the relative shift: AA → AB → AA' → AA. Apparently, sliding bilayer systems are quite different between silicene and graphene in terms of geometric structures, electronic properties, orbital hybridizations, interlayer hopping integrals, and spin interactions.

Computational investigation of silicene/nickel anode for lithium-ion battery

Silicon-based anodes are distinguished by an exceptionally high energy capacity, which is an order of magnitude superior to that of a graphite anode. The thin-film design of the anode creates the conditions for increasing the specific energy, energy density and power density of LIB. Bilayer silicene on a nickel substrate is the anode material that can increase LIB performance. In the present work, using the molecular dynamic method, a detailed analysis of the packings of lithium atoms in silicene channels with different types of walls is performed. The occupancy of the channels with lithium depends on the type of defects present in its walls. Lithiation and delithiation are carried out in the presence of a constant electric field. The type of substrate affects the packing of lithium atoms in the channel and consequently on battery capacity and performance. The maximum filling of the lithium channel is achieved when monovacancies are present in silicene sheets. It is shown that the most preferable location of Li atoms in a channel is their location over hexagonal Si cells. The most significant stresses in silicene σzz are maximal in the presence of trivacancies in silicene sheets.

Moderate bandgap and high carrier mobility simultaneously realized in bilayer silicene by oxidation

Semiconductors simultaneously possessing high carrier mobility, moderate bandgap, and ambient environment stability are so important to modern industry, and Si-based semiconducting materials can match well with the previous silicon-based electronic components. Thus, searching for such Si-based semiconductors has been one hot project due to the lack of them nowadays. Here, with the help of density functional theory, we found that the full-oxidized bilayer silicene exhibits high carrier mobility with a moderate direct bandgap of 1.02 eV. The high carrier mobility is derived from the remaining of big π bond, and the moderate bandgap is opened by the saturation of dangling Si 3p bonds. Originated from the formation of strong Si-O and Si-Si bonds, the sample exhibits strong thermodynamic and dynamical stabilities. Our work indicates that the full-oxidized bilayer silicene has many potential applications in modern electronic fields.

Electronic Properties of Silicene Films Subjected to Neutron Transmutation Doping

The radiation doping of single-crystal silicon with phosphorus retains the structure of the sample, reduces internal stresses, and increases the lifetime of minority charge carriers. The study is concerned with the effect of phosphorus additives on the electronic properties of silicene. The electron density-of-states spectra of a phosphorus-doped single layer and 2 × 2 bilayer silicene on a graphite substrate are calculated by the quantum-mechanical method. The carbon substrate imparts semiconductor properties to silicene due to p–p hybridization. Doping with phosphorus can retain or modify the metal properties gained by silicene. The position of phosphorus dopant atoms in silicene influences the semiconductor–conductor transition. The theoretical specific capacity of a phosphorus-doped silicene electrode decreases, and the electrode becomes less efficient for application in lithium-ion batteries. However, the increase in the conductivity is favorable for use of this material in solar cells.

Computer Study of Silicene Applicability in Electrochemical Devices

Novel anode materials made of silicene on metal substrates are studied by molecular dynamics. Ni(111) and Cu(111) substrates are most preferable in terms of higher anode’s occupancy and mechanical strength. The highest crystallinity degree is exhibited by the lithium packing in the silicene channel on the Ag(111) substrate. The lowest local normal stresses appear in the walls of the channel on the Al(111) substrate. The voltage profile is built as a function of concentration of Li adsorbed on free-standing bilayer silicene.

Growth and fluorination of CaSi2 thin film

We fabricated bilayer silicene (BLSi) from an epitaxial CaSi2 thin film grown on a Si (111) substrate followed by treatment with BF−4-based ionic liquid. First, a 6R-CaSi2 thin film was grown from simultaneously supplied Ca and Si sources. After reacting the CaSi2 thin film with BF−4-based ionic liquid, the BLSi was formed. The reaction proceeded laterally along the CaSi2 layer. The rearrangement of the Si and Ca atomic layers in the CaSi2 thin film was investigated by high-angle annular dark-field scanning transmission electron microscope images.