Silicene Sheet Research Articles

Band structure of monolayer of graphene, silicene and silicon-carbide including a lattice of empty or filled holes

We have developed a -orbital tight-binding Hamiltonian model taking into account the nearest neighbors to study the effect of antidot lattices (two dimensional honeycomb lattice of atoms including holes) on the band structure of silicene and silicon carbide (SiC) sheets. We obtained that the band structure of the silicene antidot superlattice strongly depends on the size of embedded holes, and the band gap of the silicene antidot lattice increases by increasing of holes diameter. The band gap of SiC antidot lattice, except for the lattice of the small unit cell, is independent of the holes diameter and also depends on the distance between holes. We obtained that, the band gap of the SiC antidot lattice is the same as the band gap of the corresponding sheet without hole. Also, the electronic properties of the SiC antidot superlattice occupied either by carbon or by silicon atoms are investigated, numerically. Furthermore, we study the effect of occupation of graphene antidot by Si atoms and vice versa. Also, we have calculated the band structure of graphene and silicene antidot lattice filled by Si + C atoms. Finally, we compute the band structure of the SiC antidot lattice including the holes which are filled by C or by Si atoms. Really, in this paper we have generalized the method of paper[] about graphene antidot with empty holes to the cases of filled holes by different atoms and also to the case of silicene and silicon carbide antidot lattices.

Density Functional Theory Study on the Mechanical Properties and Interlayer Interactions of Multi-layer Graphene: Carbonic, Silicon-Carbide and Silicene Graphene-like Structures

The main purpose of this study is to calculate the Young’s modulus of carbonic (c-graphene), silicon-carbide (SiC) and silicene graphene-like structures using density functional theory (DFT). Our results show that an increase in the number of layers did not noticeably change the Young’s modulus of carbonic and silicon-carbide graphene, while the Young’s modulus of silicene sheets decreased. Moreover, we found that carbonic graphene had the highest Young’s modulus among the above-mentioned graphene sheets due to having the shortest distance between its elements. In contrast, silicene graphenes had the lowest mechanical properties and highest equilibrium Si-Si distance. We also investigated the existing van der Waals interfacial interaction between the layers of the multilayer graphene structure using the Lennard-Jones potential. We used the Lennard-Jones parameters (e and s) to model the van der Waals interaction as a classical linear spring. Finally, the densities of states (DOS) were calculated to better understand the electronic properties of these systems.

Vacancy-defect scattering in Ag(111)-supported silicene interface

The role of vacancy-defect scattering in electron mobility near the two-dimensional (2D) Ag(111)-supported silicene sheet interface was investigated in this paper. The interface structures of Ag(111)-supported silicene and silicene-sheet charge density were considered. Mobility limited by the vacancy-defect scattering was calculated in the order of 102–105 cm2/V⋅s, which was comparable with first-principle calculations and experimental results for free-standing and substrate-supported silicene. Results further showed the decay relationship between the electron mobility and vacancy density of silicene sheet. Furthermore, the influence of the Ag(111)-supported silicene sheet structure on electron mobility was confirmed through calculations. This study can contribute to the prediction of the electron mobility of other 2D graphene-like honeycomb-structure systems.

Novel method to detect the lung cancer biomarker volatiles using hydrogen vacant silicane nanosheets: A DFT investigation

Abstract Using first-principles calculation, the electronic properties including geometrical stability of bare (BSi) and hydrogenated silicene (HSi) and hydrogen vacant silicane sheets (HvSi) are investigated. The geometrical stability of BSi, HSi and HvSi nanosheets are ensured in accordance with the formation energy and phonon band structure. The prime emphasis of the proposed work is to detect the exhaled human breath biomarkers and to detect the earlier stage of lung cancer disease using HvSi base material. Moreover, the adsorption of various breath biomarkers namely, 2,3,4-trimethyl hexane, 4-methyl octane, ethyl benzene and undecane molecules on HvSi material is explored with essential parameters namely Bader charge transfer, energy gap, adsorption energy and average energy gap variation. Moreover, the interaction of biomarkers with HvSi base material changes the adsorption energy of the base material, which is noticed to be in the range of −0.778 to −1.274 eV. The novel feature of the present study is to detect the lung cancer disease at the earlier stage upon adsorption of breath biomarkers on HvSi nanosheets.

Mode-I stress intensity factors of silicene, AlN, and SiC hexagonal sheets

The crack-tip displacement field and molecular dynamics finite element method with Tersoff potentials were used to find the mode-I stress intensity factors (SIF) of silicene, aluminum nitride (AlN), and silicon carbide (SiC) hexagonal sheets. Fracture properties of graphene and boronitrene are also included for comparison. It is found that KIct (KIc is mode-I critical SIF and t is the sheet’s thickness) of silicene, AlN, and SiC sheets are approximately 80, 66, and 47%; and 73, 64, and 45% smaller values of those of graphene for crack along the armchair and zigzag directions, respectively. The estimated fracture toughness of silicene is close to the experimental data of single-crystal silicon.

Optical properties of silicene, Si/Ag(111), and Si/Ag(110)

We present a state-of-the-art study of the optical properties of free-standing silicene and of single-layer Si 1D and 2D nanostructures supported on Ag(110) and Ag(111) substrates. Ab initio simulations of reflectance anisotropy spectroscopy (RAS) and surface differential reflectivity spectroscopy (SDRS) applied to the clean Ag surface and Si/Ag interfaces are compared with new measurements. For Si/Ag(110) we confirm a pentagonal nanoribbon geometry, strongly bonded to the substrate, and rule out competing zigzag chain and silicenelike models. For Si/Ag(111) we reproduce the main experimental features and isolate the optical signal of the epitaxial silicene overlayer. The absorption spectrum of a silicene sheet computed including excitonic and local field effects is found to be quite similar to that calculated within an independent particle approximation, and shows strong modifications when adsorbed on a Ag substrate. Important details of the computational approach are examined and the origins of the RAS and SDRS signals are explained in terms of the interface and substrate response functions. Our study does not find any evidence for Si adlayers that retain the properties of freestanding silicene.

Few-Layer Silicene Nanosheets with Superior Lithium-Storage Properties.

Silicene, a 2D silicon allotrope with unique low-buckled structure, has attracted increasing attention in recent years due to its many superior properties. So far, epitaxial growth is one of the very limited ways to obtain high-quality silicene, which severely impedes the research and application of silicene. Therefore, large-scale synthesis of silicene is a great challenge, yet urgently desired. Herein, the first scalable preparation of free-standing high-quality silicene nanosheets via liquid oxidation and exfoliation of CaSi2 is reported. This new synthesis strategy successfully induces mild oxidation of the (Si2n )2n- layers in CaSi2 into neutral Si2n layers without damage of pristine silicene structure and promotes the exfoliation of stacked silicene layers. The obtained silicene sheets are dispersible and ultrathin ones with monolayer or few-layer thickness and exhibit excellent crystallinity. As a unique 2D layered silicon allotrope, the silicene nanosheets are further explored as new anodes for lithium-ion batteries and exhibit a nearly theoretical capacity of 721 mAh g-1 at 0.1 A g-1 and an extraordinary cycling stability with no capacity decay after 1800 cycles in contrast to previous most silicon anodes showing rapid capacity decay, thus holding great promise for energy storage and beyond.

Tuning Optoelectronic Properties of the Graphene-Based Quantum Dots C16- xSi xH10 Family.

The electronic and optical properties of graphene-based quantum dots (QDs) are investigated using DFT and many-body perturbation theory. Formation energy, hardeness and electrophilicity show that all structures, from pyrene to silicene QD passing through 15 CSi QD configurations, are energetically and chemically stable. It is also found that they are reactive which implies their favorable character for the possible electronic transport and conductivity. The electronic and optical properties are very sensitive to the number and position of the substituted silicon atoms as well as the directions of the light polarization. Moreover, quantum confinement effects make the exciton binding energy of CSi quantum dots larger than those of their higher dimensional allotropes such as silicene, graphene, and SiC sheet and nanotube. It is also higher those of other shapes of quantum dots like hexagonal graphene QDs and can be tailored from the ultraviolet region to the visible one. The values of the singlet-triplet splitting determined for the X- and Y-light polarized indicate that all configurations have a high fluorescence quantum yield compared to the yield of typical semiconductors, which makes them very promising for various applications such as the light-emitting diode material and nanomedicine.

Low-temperature, ultrahigh-vacuum tip-enhanced Raman spectroscopy combined with molecular beam epitaxy for in situ two-dimensional materials' studies.

Tip-enhanced Raman spectroscopy (TERS), which combines scanning probe microscopy with the Raman spectroscopy, is capable to access the local structure and chemical information simultaneously. However, the application of ambient TERS is limited by the unstable and poorly controllable experimental conditions. Here, we designed a high performance TERS system based on a low-temperature ultrahigh-vacuum scanning tunneling microscope (LT-UHV-STM) and combined with a molecular beam epitaxy (MBE) system. It can be used for growing two-dimensional (2D) materials and for in situ STM and TERS characterization. Using a 2D silicene sheet on the Ag(111) surface as a model system, we achieved an unprecedented 109 Raman single enhancement factor in combination with a TERS spatial resolution down to 0.5 nm. The results show that TERS combined with a MBE system can be a powerful tool to study low dimensional materials and surface science.

Nanoindentation study of mechanical behavior and response of a single layer pristine silicene sheet using molecular dynamics simulations

In the present study, molecular dynamics simulations were carried out to study the mechanical behavior and response of a single layer, rectangular, planar and pristine silicene sheet during nanoindentation process. The nanoindentation simulation on the silicene sheet was done using a spherical diamond indenter and the different force vs depth of indentation curves at different conditions of temperature, indenter radius and depth of indentation were obtained. The variation of mechanical properties like maximum force, hardness and Young’s modulus were studied with respect to temperature, indenter radius and depth of indentation by varying one parameter while keeping the other two parameters constant. The deformation behavior of the silicene sheet up to failure was thoroughly studied and the mechanical properties like maximum force bearing capacity, maximum hardness and Young’s modulus of the silicene sheet were calculated. The maximum force and hardness of the silicene sheet at its maximum indentation depth of 39.5 Å are found to be 286.3 nN and 461.2 GPa respectively. The Young’s modulus of the silicene sheet is found to be 176.9 GPa which is in close agreement with the existing literature reported value of 178 GPa.

Comparison of mechanical properties of silicene estimated using different testing procedures: A molecular dynamics study

Silicene, a two-dimensional allotrope and silicon counterpart of graphene, has recently attracted scientists all over the world due to its superior material properties and thus can be a potential applicant as a reinforcing agent. The mechanical properties of silicene have been studied using several testings (tensile, bending, oscillation, and equilibrium) through the molecular dynamics (MD) simulation technique. Plastic flow occurs, and 46% elongation is observed in a silicene sheet with dimensions of (200 × 700) Å for room temperature (298 K) tensile testing. The yield strength, ultimate tensile strength, Young's modulus (E), cohesive energy, and bulk modulus are found to be 18.28 GPa, 23.96 GPa, 5.25 TPa, 3.72 eV atom−1, and 3.62 TPa, respectively. For the same sample, a Poisson ratio of 0.75 is observed. An ultrahigh mechanical strength of silicene, even higher than the previously predicted value of 0.178 TPa, is observed in this study.

Silicene-terminated surface of calcium and strontium disilicides: properties and comparison with bulk structures by computational methods

The bulk and surface structures of calcium and strontium disilicides are investigated by computational methods using density functional theory. The investigated structures are R6, R3 and P1-CaSi2 and P1-SrSi2. The investigated properties are the cleavage energy at the silicene sheet, buckling of the bulk and surface silicene layers, charge transfer from calcium to silicon, band structure of bulk and surface-terminated structures and adsorption energies on H atoms and H2 molecules on the silicene-terminated surface of the R3 phase. The cleavage energy at the silicene surface is low in all cases. Structures P1-CaSi2 and R3-CaSi2 contain silicene sheets with different coordination to Ca, while R6-CaSi2 contains both types of the sheets. It is shown that the properties of the two types of silicene-like sheets in R6-CaSi2 are similar to those of the corresponding sheets in P1-CaSi2 and R3-CaSi2, and the thermodynamically stable R6 phase is a good candidate for experimental investigation of silicene-terminated surface in calcium disilicide.

Spin filtering in silicene by edges and chemically or electrically induced interfaces

Silicene is a graphene-like material with relatively strong spin-orbit coupling exhibiting gapless topologically protected edge states. In addition, it has a buckled structure, and hence, it stands as a feasible candidate for spintronic applications, where spin-polarized channels could be controlled with external electric fields realized with voltage gates attached to a Silicene sheet. Breaking the periodicity in 2D-materials with spin-orbit coupling produces one-dimensional edge and interface nanostructures that may give rise to an intrinsic locking of spin-polarization to electron momentum. We consider field induced and chemical ways to create interfaces to give way to spin polarized states for both zigzag and armchair alignments. While the spin polarization of a field induced interface channel can be feasibly tuned, a chemical interface is less flexibly tunable. However, controlling Fermi-level, e.g. with a gate voltage, might serve as a spin valve along the interface.

Computer Study of Atomic Mechanisms of Intercalation/ Deintercalation of Li Ions in a Silicene Anode on an Ag (111) Substrate

The ability of silicon to hold a large amount of lithium puts silicene in a series of the most promising materials for the anode of lithium-ion batteries. An increase in the rate of movement of lithium ions through silicene can be achieved with silicene having vacancy-type defects. The effect of vacancy-type defects on the fill ability with lithium of the channel formed by silicene sheets on the Ag (111) substrate, as well as on the structural and kinetic properties of lithium, has been studied by the molecular dynamics method. The limit number of intercalated lithium atoms and their self-diffusion coefficient increase with the transition from the perfect silicene channel to the channel containing mono- and bivacancies. The lithium structure in the channel was studied using the method of statistical geometry. The packing of the lithium atoms in the channel turns out to be partially ordered due to the regular placement of some of the atoms at their fixation opposite the centers of hexagonal Si-cells. The σzz stress in the sheets of silicene decreases during intercalation of lithium and increases at the final stage of deintercalation.

Reduction of thermal conductivity in silicene nanomesh: insights from coherent and incoherent phonon transport.

Silicene nanomesh (SNM), a silicene sheet with periodically arranged nanoholes, has gained increasing interest due to its unique geometry and novel properties. In this paper, we have conducted molecular dynamics simulations to study the phonon transport properties of SNMs. The results demonstrate that the thermal conductivity of SNM, which is shown to be much lower than that of silicene, is little affected by temperature but can be effectively tuned by varying the porosity. To elucidate the underlying mechanisms for decreased thermal conductivity, we have investigated both coherent and incoherent phonon transport in SNMs. It is found that the phonon backscattering at the nanopore edges leads to extra thermal resistances. Additionally, the introduction of nanopores induces phonon localization and consequently hinders phonon transport in SNMs. The phonons of SNM exhibit coherent resonant behavior, which is believed to reduce the phonon group velocities and thus leads to a further reduction in thermal conductivity of SNMs. Our findings could be useful in the design of thermal properties of silicene for applications in thermoelectrics, thermal insulation and thermal protection.

Guided modes in silicene-based waveguides

Silicene is a new Dirac-type electron system similar to graphene. A monolayer silicene sheet forms a quantum well induced by an electrostatic potential, which acts as an electron waveguide. The guided modes in the silicene waveguide have been investigated. Electron waves can propagate in the silicene-based waveguide in the cases of Klein tunneling and classical motion. The behavior of the wave function depends on the spin and valley indices. The amplitude of the electron wave function in the silicene waveguide can be controlled by the external electric field. These phenomena may be helpful for the potential applications of silicene-based electronic devices.

Vibrational Properties of a Monolayer Silicene Sheet Studied by Tip-Enhanced Raman Spectroscopy.

Combining ultrahigh sensitivity, spatial resolution, and the capability to resolve chemical information, tip-enhanced Raman spectroscopy (TERS) is a powerful tool to study molecules or nanoscale objects. Here we show that TERS can also be a powerful tool in studying two-dimensional materials. We have achieved a 10^{9} Raman signal enhancement and a 0.5nm spatial resolution using monolayer silicene on Ag(111) as a prototypical 2D material system. Because of the selective enhancement on Raman modes with vertical vibrational components in TERS, our experiment provides direct evidence of the origination of Raman modes in silicene. Furthermore, the ultrahigh sensitivity of TERS allows us to identify different vibrational properties of silicene phases, which differ only in the bucking direction of the Si-Si bonds. Local vibrational features from defects and domain boundaries in silicene can also be identified.

Adsorption characteristics of DNA nucleobases, aromatic amino acids and heterocyclic molecules on silicene and germanene monolayers

Binding of DNA/RNA nucleobases, aromatic amino acids and heterocyclic molecules on two-dimensional silicene and germanene sheets have been investigated for the application of sensing of biomolecules using first principle density functional theory calculations. Binding energy range for nucleobases, amino acids and heterocyclic molecules with both the sheets have been found to be (0.43–1.16 eV), (0.70–1.58 eV) and (0.22–0.96 eV) respectively, which along with the binding distances show that these molecules bind to both sheets by physisorption and chemisorption process. The exchange of electric charges between the monolayers and the incident molecules has been examined by means of Bader charge analysis. It has been observed that the introduction of DNA/RNA nucleobases, aromatic amino acids and heterocyclic molecules alters the electronic properties of both silicene and germanene nano sheets as studied by plotting the total (TDOS) and partial (PDOS) density of states. The DOS plots reveal the variation in the band gaps of both silicene and germanene caused by the introduction of studied molecules. Based on the obtained results we suggest that both silicene and germanene monolayers in their pristine form could be useful for sensing of biomolecules.

Electrical and optical conductivities of bilayer silicene: Tight-binding calculations

The electronic structures, densities of state and electrical and optical conductivities of monolayer and bilayer silicene sheets are investigated using tight-binding approximation and Green’s function method. We found that applying the electric field on doped bilayer silicene leads to band structure modification. For AA-stacked bilayer silicene, applying and increasing the bias [Formula: see text] does not create the energy gap but for AB-stacked bilayer, a small bias [Formula: see text] is enough to create an energy gap. It is shown that the electrical conductivity depends on temperature, doping and electric field. In pristine and doped AA- and AB-stacked bilayers, electrical conductivity increases (decreases) linearly with [Formula: see text] at low (high) temperature and decreases by increasing the doping onsite energy [Formula: see text] at all [Formula: see text] ranges. The effects of temperature on the conductivity increase with the increase of electric field. It is found that the electrical conductivity depends on the amount of interlayer hopping integral [Formula: see text]. For the case of AA-stacking, there are three steps in optical conductivity at energies [Formula: see text] and [Formula: see text] for [Formula: see text] and [Formula: see text]. For the case of AB-stacking the optical conductivity has a step at [Formula: see text] similar to monolayer sheet and some steps at energies [Formula: see text].

Effect of a velocity barrier on the spin- and valley-dependent transport in ferromagnetic silicene

We investigate the transport properties in a ferromagnetic silicene sheet in the presence of a velocity barrier, defining the region in which the Fermi velocity vF is tunable compared to other regions. It is shown that the valley and spin transports could be modulated by a pure velocity barrier. To achieve the high spin and valley polarization more efficiently, a local gate voltage is considered in the velocity barrier region, to induce a on-site potential difference between two sublattices of the silicene sheet. For the Fermi velocity inside the barrier smaller than the other regions, it is found that a velocity barrier induces a strong oscillation in the conductance and the valley and spin polarizations are strongly enhanced. In the opposite case, the valley and spin polarizations are suppressed by the velocity barrier. In particular, there exists a sharp transition of the polarizations by varying the Fermi velocity in this case, which has a potential application in the spin- and valley-polarized devices.