Silicene Nanosheets Research Articles

Resolving the Controversial Existence of Silicene and Germanene Nanosheets Grown on Graphite.

The highly oriented pyrolytic graphite (HOPG) surface, consisting of a dangling bond-free lattice, is regarded as a potential substrate for van der Waals heteroepitaxy of two-dimensional layered materials. In this work, the growth of silicon and germanium on HOPG is investigated with scanning tunneling microscopy by using typical synthesis conditions for silicene and germanene on metal surfaces. At low coverages, the deposition of Si and Ge gives rise to tiny and sparse clusters that are surrounded by a honeycomb superstructure. From the detailed analysis of the superstructure, its comparison with the one encountered on the bare and clean HOPG surface, and simulations of the electron density, we conclude that the superstructure is caused by charge density modulations in the HOPG surface. At high coverages, we find the formation of clusters, assembled in filamentary patterns, which indicates a Volmer-Weber growth mode instead of a layer-by-layer growth mode. This coverage-dependent study sets the stage for revisiting recent results alleging the synthesis of silicene and germanene on the HOPG surface.

Nonlinear absorption properties of silicene nanosheets

As the cousins of graphene, i.e. same group IVA element, the nonlinear absorption (NLA) properties of silicene nanosheets were rarely studied. In this paper, we successfully exfoliated the two-dimensional silicene nanosheets from bulk silicon crystal using liquid phase exfoliation method. The NLA properties of silicene nanosheets were systemically investigated for the first time, as we have known. Silicene performed exciting saturable absorption and two photon absorption (2PA) behavior. The lower saturable intensity and larger 2PA coefficient at 532 nm excitation indicates that silicene has potential application in ultrafast lasers and optical limiting devices, especially in visible waveband.

Lithiation/Delithiation Synthesis of Few Layer Silicene Nanosheets for Rechargeable Li-O2 Batteries.

Silicene has recently received increasing interest due to its unique properties. However, the synthesis of silicene remains challenging, which limits its wide applications. In this work, a top-down lithiation and delithiation process is developed to prepare few layer silicene-like nanosheets from ball-milled silicon nanopowders. It is found that delithiation solvent plays a critical role in the structure evolution of the final products. The use of isopropyl alcohol renders 2D silicene-like products 30-100 nm in length and ≈2.4 nm in thickness. The electrochemical characterization analysis suggests that the product shows high performance for rechargeable Li-O2 batteries with 73% energy efficiency and high stability. The top-down synthesis strategy proposed in this work not only provides a new solution to the challenging preparation issue of few layer silicene but also demonstrates the feasibility of producing 2D materials from nonlayered starting structures.

Tuning the mechanical properties of silicene nanosheet by auxiliary cracks: a molecular dynamics study.

Silicene has become a topic of interest nowadays due to its potential application in various electro-mechanical nanodevices. In our previous work on silicene, fracture stresses of single crystal and polycrystalline silicene have been investigated. Existence of defects in the form of cracks reduces the fracture strength of silicene nanosheets to a great extent. In this study, an engineering way has been proposed for improving the fracture stress of silicene nanosheets with a pre-existing crack by incorporating auxiliary cracks symmetrically in a direction perpendicular to the main crack. We call this mechanism the “Failure shielding mechanism”. An extensive molecular dynamics simulation based analysis has been performed to capture the atomic level auxiliary crack-main crack interactions. It is found that the main crack tip stress distribution is significantly changed with the presence of auxiliary cracks for loading along both armchair and zigzag directions. The effects of temperature and the crack propagation speed of silicene have also been studied. Interestingly, in the case of loading along the zigzag direction, SW defect formation is observed at the tip of main crack. This leads to a reduction of the tip stress resulting in a more prominent failure shielding in case of zigzag loading than in armchair loading. Moreover, the position and length of the cracks as well as the loading directions have significant impacts on the tip stress distribution. Finally, this study opens the possibilities of strain engineering for silicene by proposing an engineering way to tailor the fracture strength of silicene.

Spin- and valley-dependent electrical conductivity of ferromagnetic group-IV 2D sheets in the topological insulator phase

In this work, based on the Kubo-Greenwood formalism and the k.p Hamiltonian model, the impact of Rashba spin-orbit coupling on electronic band structure and electrical conductivity of spin-up and spin-down subbands in counterparts of graphene, including silicene, stanene, and germanene nanosheets has been studied. When Rashba coupling is considered, the effective mass of Dirac fermions decreases significantly and no significant change is caused by this coupling for the subband gaps. All these nanosheets are found to be in topological insulator quantum phase at low staggered on-site potentials due to the applied perpendicular external electric field. We point out that the electrical conductivity of germanene increases gradually with Rashab coupling, while silicene and stanene have some fluctuations due to their smaller Fermi velocity. Furthermore, some critical temperatures with the same electrical conductivity values for jumping to the higher energy levels are observed at various Rashba coupling strengths. For all structures, a broad peak appears at low temperatures in electrical conductivity curves corresponding to the large entropy of systems when the thermal energy reaches to the difference between the energy states. Finally, we have reported that silicene has the larger has the larger electrical conductivity than two others.

Graphene and its elemental analogue: A molecular dynamics view of fracture phenomenon

Graphene and some graphene like two dimensional materials; hexagonal boron nitride (hBN) and silicene have unique mechanical properties which severely limit the suitability of conventional theories used for common brittle and ductile materials to predict the fracture response of these materials. This study revealed the fracture response of graphene, hBN and silicene nanosheets under different tiny crack lengths by molecular dynamics (MD) simulations using LAMMPS. The useful strength of these two dimensional materials are determined by their fracture toughness. Our study shows a comparative analysis of mechanical properties among the elemental analogues of graphene and suggested that hBN can be a good substitute for graphene in terms of mechanical properties. We have also found that the pre-cracked sheets fail in brittle manner and their failure is governed by the strength of the atomic bonds at the crack tip. The MD prediction of fracture toughness shows significant difference with the fracture toughness determined by Griffth's theory of brittle failure which restricts the applicability of Griffith's criterion for these materials in case of nano-cracks. Moreover, the strengths measured in armchair and zigzag directions of nanosheets of these materials implied that the bonds in armchair direction have the stronger capability to resist crack propagation compared to zigzag direction.

Fracture behavior of hydrogen-functionalized silicene nanosheets by molecular dynamics simulations

Molecular dynamics simulations are used to investigate the mechanical properties of the hydrogen-functionalized silicene nanosheets (silicane). The nanosheet elastic modulus, fracture stress and strain are computed. It is shown that the mechanical properties of the silicene nanosheets degrade by functionalizing with hydrogen atoms. Comparing the mechanical properties of armchair and zigzag silicene and silicane nanosheets, it is shown that the armchair nanosheets have higher Young's modulus, fracture stress and fracture strains than the zigzag silicene and silicane nanosheets with the same dimensions. Following up the fracture pattern of the silicane nanosheets, a brittle behavior is observed for both of the armchair and zigzag nanosheets. It is also shown that the mechanical properties of the armchair and zigzag silicane has an inverse relation to the temperatures.

Thermal Rectification of Silicene Nanosheets With Triangular Cavities by Molecular Dynamics Simulations

Silicene, the silicon-based two-dimensional structure with honeycomb lattice, has been discovered and expected to have tremendous application potential in fundamental industries. However, its thermal transport mechanism and thermal properties of silicene have not been fully explained. We report a possible way to control the thermal transport and thermal rectification in silicene nanosheets by distributing triangular cavities, which are arranged in a staggered way. The nonequilibrium molecular dynamics (NEMD) simulation method is used. The influences of the size, number, and distribution of cavities are investigated. The simulation results show that reflections of phonon at the vertex and the base of the triangular cavities are quite different. The heat flux is higher when heat flow is from the vertex to the base of cavities, resulting in thermal rectification effect. The thermal rectification effect is strengthened with increasing cavity size and number. A maximum of thermal rectification with varying distance between columns of cavities is observed.

Formation of Silicene Nanosheets on Graphite.

The extraordinary properties of graphene have spurred huge interest in the experimental realization of a two-dimensional honeycomb lattice of silicon, namely, silicene. However, its synthesis on supporting substrates remains a challenging issue. Recently, strong doubts against the possibility of synthesizing silicene on metallic substrates have been brought forward because of the non-negligible interaction between silicon and metal atoms. To solve the growth problems, we directly deposited silicon on a chemically inert graphite substrate at room temperature. Based on atomic force microscopy, scanning tunneling microscopy, and ab initio molecular dynamics simulations, we reveal the growth of silicon nanosheets where the substrate-silicon interaction is minimized. Scanning tunneling microscopy measurements clearly display the atomically resolved unit cell and the small buckling of the silicene honeycomb structure. Similar to the carbon atoms in graphene, each of the silicon atoms has three nearest and six second nearest neighbors, thus demonstrating its dominant sp2 configuration. Our scanning tunneling spectroscopy investigations confirm the metallic character of the deposited silicene, in excellent agreement with our band structure calculations that also exhibit the presence of a Dirac cone.

Progressively Doping Graphene with S i: from Graphene to Silicene, a Numerical Study

For three different sizes of graphene nanosheets, we computed the Density of states when these nanosheets are progressively doped with an increasing percentage of S i atoms. The pure graphene nanosheets are semi conducting or not depending on their size. The pure silicene nanosheets are conducting with a conduction due to π (pi) electrons. <br />The S i doped graphene nanosheets are also semi conducting or not depending on their size: for small sizes, there are semi conducting and they become conducting for larger sizes and larger percentages of S idoping. We computed also the total electronic energy which is linked to the mechanical stability of all our nanosheets. This mechanical stability decreases regularly as a function of the S i percentage of doping , but for the pure silicene nanosheets, the mechanical stability decreases more abruptly.

Unusual structural and electronic properties of porous silicene and germanene: insights from first-principles calculations.

Using first-principles calculations, we investigate the geometric structures and electronic properties of porous silicene and germanene nanosheets, which are the Si and Ge analogues of α−graphyne (referred to as silicyne and germanyne). It is found that the elemental silicyne and germanyne sheets are energetically unfavourable. However, after the C-substitution, the hybrid graphyne-like sheets (c-silicyne/c-germanyne) possess robust energetic and dynamical stabilities. Different from silicene and germanene, c-silicyne is a flat sheet, and c-germanyne is buckled with a distinct half-hilled conformation. Such asymmetric buckling structure causes the semiconducting behaviour into c-germanyne. While in c-silicyne, the semimetallic Dirac-like property is kept at the nonmagnetic state, but a spontaneous antiferromagnetism produces the massive Dirac fermions and opens a sizeable gap between Dirac cones. A tensile strain can further enhance the antiferromagnetism, which also linearly modulates the gap value without altering the direct-bandgap feature. Through strain engineering, c-silicyne can form a type-II band alignment with the MoS 2 sheet. The combined c-silicyne/MoS 2 nanostructure has a high power conversion efficiency beyond 20% for photovoltaic solar cells, enabling a fascinating utilization in the fields of solar energy and nano-devices.

Adsorption of choline benzoate ionic liquid on graphene, silicene, germanene and boron-nitride nanosheets: a DFT perspective

The adsorption of choline benzoate ([CH][BE]) ionic liquid (IL) on the surface of different hexagonal nanosheets has been studied using Density Functional Theory (DFT) methods. For this, the interaction mechanism, binding energies and electronic structure of [CH][BE] ionic liquid on four types of nanosheets, i.e., graphene, silicene, germanene and boron-nitride, were estimated and compared. The adsorption of [CH][BE] ionic liquid on different nanosheets is mainly featured by van der Waals forces, leading to strong benzoate ion-surface π-stacking. Likewise, there is also an important charge transfer from the anion to the sheet. The electronic structure analysis shows that Si- and Ge-based sheets lead to the largest changes in the HOMO and LUMO levels of choline benzoate. This paper provides new insights into the capability of DFT methods to provide useful information about the adsorption of ionic liquids on nanosheets and how ionic liquid features could be tuned through the adsorption on the suitable nanosheet.

Engineering the electronic properties of silicene by tuning the composition of MoX2 and GaX (X = S,Se,Te) chalchogenide templates

By using first-principles simulations, we investigate the interaction of a 2D silicon layer with two classes of chalcogenide-layered compounds, namely MoX2 and GaX (X = S, Se, Te). A rather weak (van der Waals) interaction between the silicene layers and the chalcogenide layers is predicted. We found that the buckling of the silicene layer is correlated to the lattice mismatch between the silicene layer and the MoX2 or GaX template. The electronic properties of silicene on these different templates largely depend on the buckling of the silicene layer: highly buckled silicene on MoS2 is predicted to be metallic, while low buckled silicene on GaS and GaSe is predicted to be semi-metallic, with preserved Dirac cones at the K points. These results indicate new routes for artificially engineering silicene nanosheets, providing tailored electronic properties of this 2D layer on non-metallic substrates. These non-metallic templates also open the way to the possible integration of silicene in future nanoelectronic devices.

Thermal conductivity of silicene nanosheets and the effect of isotopic doping

This paper investigates the thermal conductivity of silicene nanosheets (SiNSs) via molecular dynamic simulation. It shows that the thermal conductivity of pristine SiNSs is about 25–30 W mK−1 and exhibits anisotropic behaviour. Moreover, it is found that isotopic doping is efficient in reducing the thermal conductivity of SiNSs. When SiNSs are randomly doped with 30Si at the doping percentage of 50%, a maximum reduction of about 20% is obtained. This reduction can be increased when the dopants are arranged into a superlattice pattern. The thermal conductivity of these superlattice-structured SiNSs changes non-monotonically as the thickness of their lattice layers decreases. Detailed analysis of the phonon spectra demonstrates that the competing mechanism between the interface scattering and confinement effects of the phonon modes is responsible for this non-monotonical behaviour.

Preference for a propellane motif in pure silicon nanosheets.

Free standing silicene nanosheets remain elusive presumably due to the instability associated with sp(2) hybridized silicon atoms. Here we show that silicon prefers nanosheets based on the non-classical Si5 unit with a [1.1.1]-propellane motif that has two inverted tetrahedral atoms bridged by three tetrahedral atoms. DFT calculations show that nanosheets constructed exclusively from propellane building blocks are consistently more stable than those with sp(2) silicon atoms or their hybrids. These nanosheets also exhibit a narrow but definite band gap, unlike those reported earlier.

Interactions between stacked layers of phenyl-modified silicene

We use density functional theory (DFT) calculations to determine the interaction between phenyl-modified silicene nanosheets. The adhesion energy curves between the nanosheets are compared for different van der Waals density functional (vdW-DF) functionals and the DFT-D2 Grimme method. Our results show that there is a weak attraction between the sheets at close separations, that is stronger when vdWs forces are included. Without including vdWs forces the interaction is negligible and occurs at a much larger separation, highlighting the need to include such forces when modelling these nanosheets. Of the vdWs methods, the optB88 functional gives the strongest interaction energy while the Grimme gives the weakest, with the separation at which the nanosheets adhere more strongly varying between 10.04 and 11.24 Å, as measured by the distance between the silicene layers. As the modified nanosheets are brought closer together at separations as close at ∼ 8 Å, the phenyl groups on the bottom of one nanosheet can fit in between the phenyl groups on the top of the adjacent nanosheet allowing some π − π interaction between the phenyl groups. We showed that the band gap can be modified by compressing the nanosheets together while retaining a small attraction between them. There is also a change from a direct to an indirect band gap. Such a property may be exploited for the application of these nanomaterials in optoelectronic devices.

Elastic properties and large deformation of two-dimensional silicene nanosheets using molecular dynamics

This paper aims to compute the elastic properties and large deformation of two-dimensional silicene, a low buckled honeycomb structure of silicon, under uniaxial and biaxial tension by implementing molecular dynamics simulations in canonical ensemble (NVT). The results demonstrate that Young’s and bulk moduli and ultimate stress of silicene nanosheet are lower than those of graphene. Ultimate strain is found to be higher than that of graphene for armchair silicene, unlike the zigzag one. Moreover, Poisson’s ratio of silicene is found to be greater than that of its carbon counterpart due to longer Si–Si bond length and its low buckled honeycomb structure. Further, it is observed that bulk modulus is strongly size-dependent and it decreases by increasing the length of nanosheet. Finally, the silicene behavior under large deformation and fracture pattern are investigated and the formation of topological defects and silicon chains are observed. It is further revealed that the silicene is noticeably weaker than graphene in zigzag direction.

Novel electronic and magnetic properties in N or B doped silicene nanoribbons

Motivated by experimental developments on silicene nanosheets, we performed first-principles calculations to study the geometric, electronic and magnetic properties of pristine, N or B doped, as well as N and B co-doped silicene nanoribbons (SiNRs). It is shown that the substitution of N or B for Si is preferentially at the ribbon edge sites. A singly substituted N or B atom at the edges results in a semiconductor–metal transition in armchair silicene nanoribbons (ASiNRs) because of the appearance of half-filled impurity band near the Fermi level. When the N/N or B/B atoms are doped into ASiNRs at two opposite edges, they preserve the metallic character due to a negligible impurity–impurity interaction, independent of the ribbon widths. However, the co-doped systems with N and B atoms exhibit semiconducting behavior with band gaps smaller than the corresponding pristine forms, due to effective charge compensation between N or B atoms. When Si is substituted by an N or B atom in zigzag silicene nanoribbons (ZSiNRs), the systems show ferromagnetic (FM) character, which is attributed to the perturbation of π and π* states localized at the doped edges. More importantly, the marvelous half-metal and spin gapless semiconductor with 100% spin polarized currents around the Fermi level has been found in N-doped ZSiNRs. Providing that the doping with two N/N or B/B atoms is made in ZSiNRs, the spin-polarization at both Si edges is found to be compressed, and thus they exhibit nonmagnetic behavior. However, when the N and B atoms are co-doped into ZSiNRs at the most stable edge positions, a transition from metallic to semiconducting state will appear. These predicted properties may lead to a new route for energy band engineering of SiNRs and benefit the design of silicene-based electronic devices in nanoelectronics.

Strain-induced self-doping in silicene and germanene from first-principles

By first-principles calculations, the variations of band structures by strains are investigated for silicene and germanene nanosheets, i.e. the Si and Ge analogs of graphene. It is found that both systems exhibit a strain-induced self-doping phenomenon, which is closely related with their buckled structures and cannot arise in graphene. Under the compressive strain, the Dirac point is moved below the Fermi level, making the nanosheets behave as n-type doped. While under the tensile strain, the p-type doping is rendered by shifting the Dirac point above the Fermi level. Our studies demonstrate that the n-type/zero-band-gap/p-type semiconducting features can be switched for silicene and germanene by applying strains.