Silicene Sheet Research Articles

Study on the influence of material properties in femtosecond laser ablation of 6H-SiC in water and SERS-based applications

We report on the formation of SiC nanoparticles coated with graphene oxide layers by femtosecond (fs) laser ablation of 6H-SiC (n-type and semi-insulating V-doped) in deionized water. Distinct structural features were observed in the surrounding matrix of colloidal spherical nanoparticles. This corresponds to the formation of micro-nanostructures of silicene sheets in the colloidal suspension. Further, the formation of subwavelength (∼λ/8) high spatial frequency laser-induced periodic surface structures (HSFL) is noticed on the ablated SiC surface. The variation in the size and periodicity of LIPSS is noted among two SiC targets with different resistivities. The study continued to investigate the morphology of furnace annealed (FA) and rapid thermal annealed (RTA) Au-deposited LIPSS towards sensing applications based on the surface-enhanced Raman scattering (SERS) technique. The formation of fine spherical Au nanoparticles is observed in the case of RTA samples, which exhibited predominant SERS enhancement in trace detection of the explosive analyte, Tetryl. This study on SiC using fs laser pulses unveils the uniqueness of SiC material in the ablation process, especially in producing the LIPSS and silicon-carbon-based nanoparticles/nanostructures that can be tailored for diverse applications.

Transport through a monolayer-tube junction: Sheet-to-tube spin current

We develop a method to calculate the electron flow between an arbitrary atomic monolayer sheet and an arbitrary tube by expressing the corresponding sheet-tube tunneling matrix elements with those between sheets. We use this method to calculate the spin current from a monolayer silicene sheet with sublattice-staggered current-induced spin polarization to a silicene tube. The calculated sheet-to-tube spin current exhibits an oscillation as a function of the tube circumferential length because the Fermi points in the tube cross the Fermi circle in the sheet. Furthermore, the spin current with spin in the out-of-plane direction, which is absent in the sheet-sheet junction (including twisted sheets) with C3 rotational symmetry, appears in an oscillating form in the tube-sheet junction due to the broken C3 rotational symmetry. This is an example of the symmetry manipulation which realizes switching a particular component of the spin current.

Exploring the electrocatalytic performance of silicene and single atom doped silicene for HER, OER and ORR activity using density functional theory

The transition towards hydrogen energy is paramount due to its potential to mitigate environmental pollution caused by fossil fuels. The greener and cleaner way of producing and utilizing hydrogen is through water electrolysis and hydrogen fuel cell where the sluggish reaction kinetics bottlenecks the widescale implementation. Hence, in this study, density functional theory (DFT) is utilized to study the HER, OER and ORR performance of pristine and doped silicene sheets where the dopants (B, C, N, Al and P) are introduced as single atom. The thermodynamical stability for pristine and doped sheets are confirmed with ADMP study and the cohesive energy for all the sheets are found to be around -4.2 eV per atom which validates their stability. The hydrogen evolution reaction (HER) performance of all the sheets is studied and the N doped silicene exhibits better catalytic property with a lowest overpotential of 70 mV and the C doped silicene shown good HER performance with an overpotential of 180 mV. In case of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the pristine silicene is found to behave better in the ORR performance while the C doped silicene has the lowest overpotential compared with the other sheets. The variation in the catalytic performance is studied with the reactivity parameters which implies that the nucleophilic nature of the material enhances the HER performance. Furthermore, the results suggest that C doped silicene can be utilized as a bifunctional catalyst for the HER and OER performance. This study suggests that the silicene in the pure form as well as with dopant can be a good electrocatalyst for the water splitting and fuel cell application.

Enhancing the Glucose Sensing Properties of Silicene Through Transition Metal Doping: a DFT Investigation

AbstractThe attractive physiochemical properties of two dimensional materials demonstrate them to be an emerging class of potential candidates for use in sensing devices. Studies have shown that silicene can act as an excellent chemical sensor; however, no work has been studied about silicene as a glucose sensor. In this paper, using density functional theory, the interaction of pristine silicene and transition metal‐doped (Ag, Au, Cu, Ni, and Pt) silicene is studied with glucose molecules. The calculated adsorption energy, energy gap, and charge transfer value confirmed that the nickel‐doped silicene sheet can be a good glucose sensor with high sensitivity, moderate recovery time, and high conductivity. Further, the electron localization function and the reduced density gradient confirm the physisorption between the glucose molecule and the metal‐doped silicene sheets.

A CrO₂ and Out-of-Plane Silicene based Sub-10nm MTJ with perfect spin filtering efficiency and high tunnel magnetoresistance

In this paper, properties of spin transport for MTJs (magnetic tunnel junction) consisting of CrO2 Half Metallic Ferromagnetic (HMF) electrodes with out of plane silicene sheet as scattering region are analyzed. The transport phenomena in the proposed structure are purely tunneling due to the absence of transmission states near the Fermi level. This is contrary to the in plane structures where transmission states exist near the Fermi level. The proposed device shows the highest value (100%) of magneto-resistance. Spin-filtering efficiency reported in out-of-plane silicene-based MTJ is also increased because of silicene acting as a perfect barrier, results in feeble spin down current in both parallel and anti-parallel configurations. The proposed device has dimensions below 10-nm. Due to the use of silicene sheet as a scattering region, it is also likely to integrate well with the already existing silicon technology. It can be used for high-performance memory applications similar to MRAMs.

Silicene growth mechanisms on Au(111) and Au(110) substrates

Despite the remarkable theoretical applications of silicene, its synthesis remains a complex task, with epitaxial growth being one of the main routes involving depositing evaporated Si atoms onto a suitable substrate. Additionally, the requirement for a substrate to maintain the silicene stability poses several difficulties in accurately determining the growth mechanisms and the resulting structures, leading to conflicting results in the literature. In this study, large-scale molecular dynamics simulations are performed to uncover the growth mechanisms and characteristics of epitaxially grown silicene sheets on Au(111) and Au(110) substrates, considering different temperatures and Si deposition rates. The growth process has been found to initiate with the nucleation of several independent islands homogeneously distributed on the substrate surface, which gradually merge to form a complete silicene sheet. The results consistently demonstrate the presence of a buckled silicene structure, although this characteristic is notably reduced when using an Au(111) substrate. Furthermore, the analysis also focuses on the quality and growth mode of the silicene sheets, considering the influence of temperature and deposition rate. The findings reveal a prevalence of the Frank–van der Merwe growth mode, along with diverse forms of defects throughout the sheets.

Adsorption of Hydrogen Molecules in Nickel Decorated Silicene

First-principles simulations based on density functional theory (DFT) have been used to study the structural, electronic and magnetic properties of pristine and Ni decorated silicene sheets. Generalized Gradient Approximation (GGA) based exchange correlation functionals are used under software package Quantum ESPRESSO (QE), 6.5 versions. We have reconstructed the optimized unit cell of silicene, which has a face centered cubic (fcc) structure with two silicon atoms having lattice parameters a = b = 3.8 Å. The distance between two nearest silicene monolayers is found to be 20.5 Å which is large enough to neglect the interlayer interactions between 4×4 supercells of silicene monolayers. The atoms in the prepared supercell are fully relaxed under Bloyden-Fletcher-Goldfarb-Shanno (BFGS) scheme prior to the self-consistent, band structure and density of state (DoS) calculations. The pristine silicene is semi-metallic in nature possessing a Dirac-cone as in graphene. The h-site adsorption is found to be the most stable adsorption site of nickel in silicene with the binding energy of 4.69 eV. The addition of nickel atom completely distorts the hexagonal structure of silicene destroying the Dirac cone and the system becomes slightly insulating from its semi-metallic nature. We then construct a 4×4 nickel dimer silicene which further destroys the hexagonal silicene structure with further opening of the band gap. The charge transfer analysis in the Ni decorated systems shows the charge transfers of 0.163e and 0.294e in Ni adatom silicene and Ni dimer silicene respectively showing that the nickel atoms are adsorbed by weak van der Waals forces in both of the systems. We then proceed to hydrogen molecule adsorption in these prepared 4×4 silicene systems: pristine, Ni adatom and Ni dimer silicene systems. The adsorption energy of hydrogen in the Ni adatom silicene is found to be the largest making it the most effective system for hydrogen storage.

Silicene-2DEG heterostructures: Collective excitations investigations

This paper investigates collective excitations in a heterostructure consisting of a silicene sheet and a parallel GaAs quantum well by employing the random-phase approximation within exchange-correlation effects. We find that two plasmonic modes exist in the systems, corresponding to in-phase (optical) and out-of-phase (acoustical) oscillations of charged particles in two layers. Calculations show that interactions between electrons in two layers decrease the energy of both two plasmon modes, completely differing from other double-layer structures. The increase in inter-layer separation significantly decreases both optical and acoustical modes’ frequencies. We observe that taking into account local field correction and increasing quantum well width pronouncedly decrease acoustical mode frequency. However, the increase in the bandgap, caused by an external electric field, decreases only the frequency of the optical collective mode. Finally, as carrier density in quantum well decreases, the acoustical mode energy noticeably declines while that of the optical one seems extremely stable.

First principle modeling of a silicene-aluminum composite anode for lithium ion batteries

The creation of new environmentally friendly and portable energy sources requires the development of technology used to produce lithium-ion batteries, especially their anodes. The aim of this study was to show the possibilities of the state-of-the-art ab initio approach for modeling battery anode materials. A quantum-mechanical study of the limiting filling of a silicene/aluminum anode with lithium has been performed. In this case, the changes in the structure, energy, and electronic properties of silicene that occur upon filling the anode have been studied. Lithium atoms are deposited both inside and on top of the channel formed by two parallel silicene sheets. The ratio of lithium to silicon varies in the range of 0.1–1.5. The gap in the silicene channel increases as the channel is filled with lithium. The filling of the silicene channel with lithium is associated with an increase in the Si–Si bond length in both silicene sheets without destruction of the channel. The infiltration of lithium into the space between the aluminum substrate and silicene sheet has been determined. Our calculation of the band structure has shown that, regardless of filling with lithium, the silicene-aluminum system exhibits metallic conductivity. The storage capacity of the combined anode (729 mAh g−1) significantly exceeds the capacity of graphite anodes. It was found that the top sheet of silicene acquires a negative electric charge upon significant lithium filling of the anode, exceeding the absolute value of the negative charge of the bottom sheet in contact with the aluminum substrate. Therefore, our model study has elucidated that the new silicene-aluminum anode is a promising material for the creation of a new generation of lithium-ion batteries.

Electronic and work function-based glucose sensors on graphene, silicene, and germanene sheets – DFT studies

Diabetes is a chronic health condition caused due to a metabolic disorder that results in high blood sugar levels. Diabetes would be the seventh-prominent reason for mortality according to recent studies. There is a need for the improvement of selective and sensitive glucose detection in medicine, clinical diagnostics, and the food industry. We have studied a density functional theory (DFT) method of the adsorption of glucose on pure graphene, silicene, and germanene sheets. For the above-mentioned structures the optimized geometries adsorption energies have been calculated to analyze the suitability and efficiency for glucose sensing. The electronic properties such as charge transfer analysis, band gap, fermi energy, work function, and density of states have been studied into attention. Our result reveals that the interaction between glucose with pristine graphene sheet is strong chemisorption when compared to the case of pristine silicene and pure germanene which exhibits weak chemisorption suitable for sensors. However pristine germanene shows high bandgap variation when compared to silicene indicating that it can act as an electronic sensor. Tomodify the adsorption properties by improving its chemical reactivity and electronic structure, we suggest either doping with different atoms, preparing sheets with defects or chemical functionalization have to be applied.

Theoretical Analysis of High-Harmonic Generation in Silicon Nanotubes

Silicene hexagonal nanotube (Si h-NT) is a one-dimensional periodic system consisting of the rolling silicene layer, a monolayer of silicon atoms. In this research, we explore high-order harmonic generation (HHG) with a strong mid-infrared (IR) field on the single-walled nanotube. Moreover, the electronic intraband and interband dynamics are significantly and especially investigated in order to study the HHG mechanisms thoroughly. We then show and discuss the numerical results of the HHG characteristics from Si h-NT compared with single-walled carbon nanotubes and silicene sheets.

Role of interface and valley-mixing scattering in a ferromagnetic silicene/superconductor junction

We theoretically investigate the subgap transport in a ferromagnetic silicene/superconductor junction and highlight the role of the interface. It is demonstrated that the subgap conductance spectra for the continuous and discontinuous interface models are significantly different. In particular, the intravalley Andreev reflection (AR) may occur at the discontinuous interface, and then the subgap conductance will be remarkably enhanced when the silicene sheet is highly valley polarized. This novel AR resulted from the valley-mixing scattering, which is absent in the continuous interface model. The influence of the interface coupling strength on the subgap conductance is also computed. Our results can be directly tested experimentally using scanning tunnel microscope measurements and/or point-contact spectroscopy.

Molecular dynamic study of the applicability of silicene lithium ion battery anodes: A review

Lithium-ion batteries (LIBs) are the main energy storage devices that have found wide application in the electrical, electronics, automotive and even aerospace industries. In practical applications, silicene has been put forward as an active anode material for LIBs. This is facilitated by its high theoretical capacitance, strength, and small volume change during lithiation. Thin-film materials containing two-layer silicene and intended for use in the LIB anode have been studied by the method of classical molecular dynamics. Among the important characteristics obtained is the fillability of the silicene anode (under the influence of an electric field), which was determined depending on the type of vacancy defects in silicene and the type of substrate used. Both metallic (Ag, Ni, Cu, Al) and non-metallic (graphite, silicon carbide) substrates are considered. The behavior of the self-diffusion coefficient of intercalated lithium atoms in a silicene channel as it is filled has been studied. Based on the construction of Voronoi polyhedra, the packing of lithium atoms and the state of the walls in the channel has been studied in detail. The change in the shape of silicene sheets, as well as the stresses in them caused by lithium intercalation, are analyzed. It has been established that two-layer silicene with monovacancies on a nickel substrate is the most optimum variant of the anode material. The results of this work may be useful in the development of new anode materials for new generation LIBs.

Analog and Digital Performance of Graphene-Germanene-Graphene Heterojunction Based Field Effect Transistor

In this study, by taking the advantage of electrical properties of graphene, silicene and germanene sheets we propose and analyze a heterojunction based field effect transistor. It is comprised of germanene sheet in the channel and gapless graphene in the source/drain regions. We investigate and compare the performances of proposed device from two points of view comprised of analog and digital applications. A comparative study is carried out with three other simulated devices including gapless graphene, germanene and graphene-silicene-graphene heterojunction based field effect transistors. Our theoretical analysis show that for digital applications, Ion/Ioff ratio in the proposed graphene-germanene-graphene (G-Ge-G) heterojunction based field effect transistor shows a significant better value than other counterpart simulated devices and reaches to 106. However for the analog applications, the output characteristic curves show different behavior for the simulated devices under study. The output characteristic of (G-Ge-G) heterojunction based field effect transistor displays a very good saturation which owes to improved pinch-off in the channel.

Numerical simulation of functioning a silicene anode of a lithium-ion battery

The development of next generation lithium-ion batteries (LIB) requires stable anodes with high capacity and long service life. In this work, the molecular dynamics method is used to study the functioning of a combined anode constructed from a nickel plate and two layers of silicene. Such anodes demonstrate high efficiency as opposed to similar anodes with supporting plates made of other metals. The creation of monovacancies in silicene increases the capacity of the anode. The ultimate stresses and strains of a nickel plate and a silicene sheet under uniaxial tension are calculated. When lithium is intercalated into the silicene channel, significant fluctuations in the value of its self-diffusion coefficient are observed. The most significant effect of polyvacancies in the channel walls on the lithium packing in it is manifested at small values of the number of nearest neighbors and small angles of their mutual arrangement. Four- and five-membered rings are most often found in packings of lithium atoms in a channel. The strength of silicene with monovacancies and its adhesion to the nickel plate are sufficient to ensure long and trouble-free operation of the anode in LIB.

Van der Waals Heteroepitaxy of Air-Stable Quasi-Free-Standing Silicene Layers on CVD Epitaxial Graphene/6H-SiC.

Graphene, consisting of an inert, thermally stable material with an atomically flat, dangling-bond-free surface, is by essence an ideal template layer for van der Waals heteroepitaxy of two-dimensional materials such as silicene. However, depending on the synthesis method and growth parameters, graphene (Gr) substrates could exhibit, on a single sample, various surface structures, thicknesses, defects, and step heights. These structures noticeably affect the growth mode of epitaxial layers, e.g., turning the layer-by-layer growth into the Volmer-Weber growth promoted by defect-assisted nucleation. In this work, the growth of silicon on chemical vapor deposited epitaxial Gr (1 ML Gr/1 ML Gr buffer) on a 6H-SiC(0001) substrate is investigated by a combination of atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Raman spectroscopy measurements. It is shown that the perfect control of full-scale almost defect-free 1 ML Gr with a single surface structure and the ultraclean conditions for molecular beam epitaxy deposition of silicon represent key prerequisites for ensuring the growth of extended silicene sheets on epitaxial graphene. At low coverages, the deposition of Si produces large silicene sheets (some hundreds of nanometers large) attested by both AFM and SEM observations and the onset of a Raman peak at 560 cm-1, very close to the theoretical value of 570 cm-1 calculated for free-standing silicene. This vibrational mode at 560 cm-1 represents the highest ever experimentally measured value and is representative of quasi-free-standing silicene with almost no interaction with inert nonmetal substrates. From a coverage rate of 1 ML, the silicene sheets disappear at the expense of 3D Si dendritic islands whose density, size, and thickness increase with the deposited thickness. From this coverage, the Raman mode assigned to quasi-free-standing silicene totally vanishes, and the 2D flakes of silicene are no longer observed by AFM. The experimental results are in very good agreement with the results of kinetic Monte Carlo simulations that rationalize the initial flake growth in solid-state dewetting conditions, followed by the growth of ridges surrounding and eventually covering the 2D flakes. A full description of the growth mechanism is given. This study, which covers a wide range of growth parameters, challenges recent results stating the impossibility to grow silicene on a carbon inert surface and is very promising for large-scale silicene growth. It shows that silicene growth can be achieved using perfectly controlled and ultraclean deposition conditions and an almost defect-free Gr substrate.

Temperature-Dependent Growth and Evolution of Silicene on Au Ultrathin Films-LEEM and LEED Studies.

The formation and evolution of silicene on ultrathin Au films have been investigated with low energy electron microscopy and diffraction. Careful control of the annealing rate and temperature of Au films epitaxially grown on the Si(111) surface allows for the preparation of a large scale, of the order of cm2, silicene sheets. Depending on the final temperature, three stages of silicene evolution can be distinguished: (i) the growth of the low buckled phase, (ii) the formation of a layered heterostructure of the low buckled and planar phases of silicene and (iii) the gradual destruction of the silicene. Each stage is characterized by its unique surface morphology and characteristic diffraction patterns. The present study gives an overview of structures formed on the surface of ultrathin Au films and morphology changes between room temperature and the temperature at which the formation of Au droplets on the Si(111) surface occurs.

Ti atom doped single vacancy silicene for hydrogen energy storage: DFT study

AbstractHydrogen adsorption on titanium (Ti) atom doped single vacancy silicene (SV‐SL) is investigated through first principles density functional theory (DFT) study. Strong hybridization of d‐orbitals of Ti atom to p‐orbitals of Si atoms results in a tight bond to the silicene sheet with energy of −6.48 eV and keeps away from metal clustering. Maximum 8 H2 molecules firmly bind to Ti atom doped SV‐SL sheet with an adsorption energy ranging from −0.481 to −0.201 eV per H2 molecule and hydrogen storage capacity (HSC) of 6.3 wt%. Double‐side H2 adsorptions on hollow sites of Ti atom doped SV‐SL sheet are verified by structural and electronic properties. The partial density of states (PDOS) analysis shows the kubas interaction mainly caused the molecular H2 adsorption. Further, the absence of spin‐up and spin‐down channels in electronic band structures of nH2 molecule adsorption to Ti atom doped SV‐SL systems indicates its nonmagnetic nature. Conclusively, this study reveals that the Ti atom doped SV‐SL can be a promising candidate for hydrogen storage applications.

An Ab Initio Study of Lithization of Two-Dimensional Silicon-Carbon Anode Material for Lithium-Ion Batteries.

This work is devoted to a first-principles study of changes in the structural, energetic, and electronic properties of silicene anodes during their lithium filling. Anodes were presented by silicene on carbon substrate and free-standing silicene. The ratio of the amount of lithium to silicon varied in the range from 0.06 to 1.125 for silicene on bilayer graphene and from 0.06 to 2.375 for free-standing silicene. It is shown that the carbon substrate reduces the stability of the silicene sheet. Silicene begins to degrade when the ratio of lithium to silicon (NLi/NSi) exceeds ~0.87, and at NLi/NSi = 0.938, lithium penetrates into the space between the silicene sheet and the carbon substrate. At certain values of the Li/Si ratio in the silicene sheet, five- and seven-membered rings of Si atoms can be formed on the carbon substrate. The presence of two-layer graphene imparts conductive properties to the anode. These properties can periodically disappear during the adsorption of lithium in the absence of a carbon substrate. Free-standing silicene adsorbed by lithium loses its stability at NLi/NSi = 1.375.

Adatom-induced dislocation annihilation in epitaxial silicene

The transformation of the stripe domain structure of spontaneously-formed epitaxial silicene on ZrB2 thin films into a single-domain driven by the adsorption of a fraction of a monolayer of silicon was used to investigate how dislocations react and eventually annihilate in a two-dimensional honeycomb structure. The in-situ real time scanning tunneling microscopy monitoring of the evolution of the domain structure after Si deposition revealed the mechanisms leading to the nucleation of a single-domain island into a domain structure through a stepwise reaction of partial dislocations. After its nucleation, the single-domain island extends by the propagation of edge dislocations at its frontiers. The identification of this particular nucleation-propagation formation of dislocation-free silicene sheet provides insights into how crystallographic defects can be healed in two-dimensional materials.