Single-layer Silicene Research Articles

Computer development of silicene anodes for lithium-ion batteries: A review

Lithium-ion batteries (LIB) have many advantages, the main ones being high energy density, long service life, small size, and low environmental pollution. This review is devoted to further development of LIBs based on quantum mechanical calculations in order to use them for energy storage in the future. Energetically favorite places occupied by lithium atoms on silicene are found. Lithium filling of free-standing two-layer silicene and single-layer silicene on graphene was studied. The geometric, energy, charging characteristics, as well as the open circuit voltage are determined. The effect of metallic (Al, Cu, Ni, Ag and Au) and non-metallic (C, SiC and BN) substrates on the geometric, energy and electronic properties of silicene has been studied. The effect of an intermediate nickel layer on the characteristics of the "silicene on a multilayer copper substrate" system has been studied. The effect of nuclear transmutation doping (NTD) of the silicene/graphite system with phosphorus on the density of electronic states of one- and two-layer silicene has been determined. Promising applications for silicene and the advantages of its use as an anode in a lithium-ion battery are discussed.

Plasmon modes in N-layer silicene structures

We investigate the plasmon properties in N-layer silicene systems consisting of N, up to 6, parallel single-layer silicene (SLS) under the application of an out-of-plane electric field, taking into account the spin–orbit coupling within the random-phase approximation. Numerical calculations demonstrate that N undamped plasmon modes, including one in-phase optical (Op) and (N − 1) out-of-phase acoustic (Ac) modes, continue mainly outside the single-particle excitation area of the system. As the number of layers increases, the frequencies of plasmonic collective excitations increase and can become much larger than that in SLS, more significant for high-frequency modes. The Op (Ac) plasmon mode(s) noticeably (slightly) decreases with the increase in the bandgap and weakly depends on the number of layers. We observe that the phase transition of the system weakly affects the plasmon properties, and as the bandgap caused by the spin–orbit coupling equal that caused by the external electric field, the plasmonic collective excitations and their broadening function in multilayer silicene behave similarly to those in multilayer gapless graphene structures. Our investigations show that plasmon curves in the system move toward that in SLS as the separation increases, and the impacts of this factor can be raised by a large number of layers in the system. Finally, we find that the imbalanced carrier density between silicene layers significantly decreases plasmon frequencies, depending on the number of layers.

Silicene Anodes for Lithium-Ion Batteries on Metal Substrates

This article discusses heterogeneous materials containing silicene, which can be a promising anode for lithium-ion batteries. In addition to the current collector on an ultrathin insulator, the anode includes sheets of silicene spaced 0.75 nm apart. One of these sheets is on a metal substrate. Using the molecular dynamics method, we study new anode materials obtained from silicene on various metal substrates. In terms of the degree of filling of the anode and its mechanical strength, preference is given to Ni (111) and Cu (111) substrates. The highest degree of crystallinity of the packing is realized in a silicene channel on an Ag (111) substrate. The smallest local normal stresses appear in the channel walls on the Al (111) substrate. The voltage profile is defined as a function of the concentration of Li adsorbed on a two-layer silicene. The charge capacity of a two-layer freestanding silicene was estimated based on the study of its local destruction. Each of the considered metal substrates has a significant effect on the electronic properties of single-layer silicene, which leads to its metallization. The calculated partial densities of the electronic state allow us to establish the causes of the occurrence of metallic conductivity in silicene.

RKKY interaction in biased single-layer silicene

RKKY interaction in biased single-layer silicene

Influence of specimen size and strain rate on tensile deformation and fracture behavior of single-layer Silicene

Abstract Silicene is potential material for electronic industry and therefore an understanding of the deformation features of silicene is significant for the better design of electronic components. In this Molecular Dynamics (MD) simulation work, deformation behavior of silicene and its underlying mechanism based on structural evolution during deformation has been investigated by applying tensile load parallel to the arm chair direction. To understand the effect of specimen size on deformation behaviour four specimens containing 600, 20000, 60000 and 100000 atoms are considered. The MD simulations are performed at 50 K and for different strain rates such as 109 sec-1, 1010 sec-1 and 1011 sec-1using EDIP potential. Evaluated Young modulus is found to be independent of strain rate for the specimen having 100000 atoms. Fracture strain is found to be dependent on the specimen size and the strain rate. It is observed that the UTS, fracture stress and fracture strain of the silicene nano sheet improves with the increasing strain rate. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i-2017).

Spin–orbit effects on the spin and pseudospin polarization in ac-driven silicene

We study the pseudospin and spin dynamical effects in single-layer silicene due to a perpendicular electric field periodically driven and its interplay with the intrinsic and extrinsic (Rashba) spin–orbit interaction. We find that the spin nonconserving processes of the real spin of the quasiparticles in silicene, induced by the rather weak spin–orbit mechanisms, manifest themselves as shifts of the resonances of its quasienergy spectrum in the low coupling regime to the driving field. We show that there is an interesting cooperative effect among the, in principle, competing Rashba and intrinsic spin–orbit contributions. This is explicitly illustrated by exact and approximated analytical solutions of the dynamical equations. In addition, we show that a finite Rashba spin–orbit interaction is indeed necessary in order to achieve a nonvanishing spin polarization. As additional feature, trivial and nontrivial topological phases might be distinguished from each other as fast or slow dynamical fluctuations of the spin polarization. We mention the possible experimental detection schemes of our theoretical results and their relevance in new practical implementation of periodically driven interactions in silicene physics and related two-dimensional systems.

Can an element form a two-dimensional nanosheet of type 15 pentagons?

Type 15 pentagon was recently discovered three decades since the 14th type of pentagon was reported in 1985. In contrast to such a long time span, the list of two-dimensional (2D) materials keeps growing longer on a monthly (if not daily) basis according to emerging experimental and theoretical reports. Combining these two seemingly irrelevant topics, we apply density functional theory (DFT) calculations to examine the possibility of forming a 2D nanosheet with the vertices of type 15 pentagons occupied by boron, carbon, silicon, phosphorous, sulfur, gallium, germanium, or tin atoms. We find that none of the optimized eight nanosheets remain the same initial structure filled with type 15 pentagons after the geometry optimizations by a DFT calculator. Instead, we obtain different rearranged structures for each nanosheet, most of which have not yet been reported before. For example, gallium atoms form a 2D nanosheet structure with a triangular pattern similar to single-layer boron sheet called borophene. These new 2D materials exhibit formation energies comparable to that of single-layer silicene, implying the feasibility of being grown on a substrate. The electronic structure shows that all the eight nanosheets are metallic (or semi-metallic for the carbon nanosheet), compensating for the dearth of metallic systems in existing 2D materials that are mostly semiconducting. Our work shows that linking a pentagonal geometry with DFT calculations yields both educational and scientific merits.

Assessment of bilayer silicene to probe as quantum spin and valley Hall effect

Silicene takes precedence over graphene due to its buckling type structure and strong spin orbit coupling. Motivated by these properties, we study the silicene bilayer in the presence of applied perpendicular electric field and intrinsic spin orbit coupling to probe as quantum spin/valley Hall effect. Using analytical approach, we calculate the spin Chern-number of bilayer silicene and then compare it with monolayer silicene. We reveal that bilayer silicene hosts double spin Chern-number as compared to single layer silicene and therefore accordingly has twice as many edge states in contrast to single layer silicene. In addition, we investigate the combined effect of intrinsic spin orbit coupling and the external electric field, we find that bilayer silicene, likewise single layer silicene, goes through a phase transitions from a quantum spin Hall state to a quantum valley Hall state when the strength of the applied electric field exceeds the intrinsic spin orbit coupling strength. We believe that the results and outcomes obtained for bilayer silicene are experimentally more accessible as compared to bilayer graphene, because of strong SO coupling in bilayer silicene.

Adsorption of Li on single-layer silicene for anodes of Li-ion batteries.

We report an investigation of the interactions of Li with silicene. We find silicene to be a promising anode material for high energy density lithium-ion batteries. Based on first-principles calculations, we study the interaction between Li and silicene and explore the microscopic mechanism of Li storage on silicene. We find that Li ion is adsorbed on hollow sites at a very low concentration of less than 1.56%. At a high concentration of Li, the Li chains with up-down pairs on top sites become popular. With the formation of the Li chains, the local structure of silicene is modulated. In addition, the electronic states near the Fermi level are found to be renormalized. There is no obvious charge transfer from Li to Si, distinct from the C based materials, although the local charge distributions are polarized under the trapping of Li, implying a greater chemical interaction than that of the weak interaction with graphene. It is predicted that the capacity of silicene can reach 1196 mA h g-1 due to Li intercalation without the breaking of the Si-Si bonds.

DFT studies on the Al, B, and P doping of silicene

The search for efficient adsorbents of atoms and molecules has motivated the study of systems in the presence of defects. For this reason, we have investigated theoretically the creation of mono- and di-vacancies on single layer silicene, as well as the Al, B, and P doping of silicene. Using the first-principles method with the generalized gradient approximation in the parameterization of Perdew-Burke-Ernzerhof, we have found that Al, B, and P interact strongly with Si atoms. Besides, when the vacancies are generated, the dangling bonds are saturated in pairs to form new bonds. Optimal geometries, binding energies, density of states (DOS) and charge density are reported. The results suggest that new chemical modifications can be used to modify the electronic properties of single-layer silicene.

Synthesis of Multilayer Silicene on Si(111)√3 × √3-Ag

The compelling experimental evidence of multilayer silicene synthesis on one monolayer of silver deposited on a silicon substrate, Si(111)√3 × √3-Ag, used as a template is reported. Auger electron spectroscopy, low-energy electron diffraction, scanning tunneling microscopy/spectroscopy, in situ Raman spectroscopy, and energy-dispersive in-plane X-ray diffraction were applied to achieve the fingerprints of √3 × √3 multilayer silicene. Density functional theory calculations provided the structural models of single, bilayer, and three-layer silicene on the Si(111)√3 × √3-Ag surface. Single-layer silicene retains its free-standing form. The structural models, which are consistent with the experimental findings, feature atomic structure substantially different from that of Si(111).

Strain-modulated anisotropy of quantum transport properties in single-layer silicene: Spin and valley filtering

Strained two-dimensional crystals often offer novel physical properties that are usable to improve their electronic performance. Here we show by the theory of elasticity combined with the tight-binding approximation that local strains in silicene can open up new prospects for generating fully polarized spin and valley currents. The trajectory of electrons flowing through locally strained regions obeys the same behavior as light waves propagating in uniaxial anisotropic materials. The refraction angle of electrons at local strain boundaries exhibits a strong dependence on the valley degree of freedom, allowing for valley filtering based on the strain direction. The ability to control the spin polarization direction additionally requires a perpendicular electric field to be involved in combination with the local strain. Further similarities of the problem with optics of anisotropic materials are elucidated and possible applications in spin- and valleytronic nanodevices are discussed.

α-Silicene as oxidation-resistant ultra-thin coating material.

By performing density functional theory (DFT)-based calculations, the performance of α-silicene as oxidation-resistant coating on Ag(111) surface is investigated. First of all, it is shown that the Ag(111) surface is quite reactive against O atoms and O2 molecules. It is known that when single-layer silicene is formed on the Ag(111) surface, the 3 × 3-reconstructed phase, α-silicene, is the ground state. Our investigation reveals that as a coating layer, α-silicene (i) strongly absorbs single O atoms and (ii) absorbs O2 molecules by breaking the strong O–O bond. (iii) Even the hollow sites, which are found to be most favorable penetration path for oxygens, serves as high-energy oxidation barrier, and (iv) α-silicene becomes more protective and less permeable in the presence of absorbed O atom. It appears that single-layer silicene is a quite promising material for ultra-thin oxidation-protective coating applications.

Optimized interatomic potential for silicon and its application to thermal stability of silicene

An optimized interatomic potential has been constructed for silicon using a modified Tersoff model. The potential reproduces a wide range of properties of Si and improves over existing potentials with respect to point defect structures and energies, surface energies and reconstructions, thermal expansion, melting temperature and other properties. The proposed potential is compared with three other potentials from the literature. The potentials demonstrate reasonable agreement with first-principles binding energies of small Si clusters as well as single-layer and bilayer silicenes. The four potentials are used to evaluate the thermal stability of free-standing silicenes in the form of nano-ribbons, nano-flakes and nano-tubes. While single-layer silicene is mechanically stable at zero Kelvin, it is predicted to become unstable and collapse at room temperature. By contrast, the bilayer silicene demonstrates a larger bending rigidity and remains stable at and even above room temperature. The results suggest that bilayer silicene might exist in a free-standing form at ambient conditions.

Silicon Nanosheets: Crossover between Multilayer Silicene and Diamond-like Growth Regime.

The structural and electronic properties of nanoscale Si epitaxially grown on Ag(111) can be tuned from a multilayer silicene phase, where the constitutive layers incorporate a mixed sp2/sp3 bonding, to other ordinary Si phases, such as amorphous and diamond-like Si. Based on comparative scanning tunneling microscopy and Raman spectroscopy investigations, a key role in determining the nanoscale Si phase is played by the growth temperature of the epitaxial deposition on Ag(111) substrate and the presence or absence of a single-layer silicene as a seed for the successive growth. Furthermore, when integrated into a field-effect transistor device, multilayer silicene exhibits a characteristic ambipolar charge carrier transport behavior that makes it strikingly different from other conventional Si channels and suggestive of a Dirac-like character of the electronic bands of the crystal. These findings spotlight the interest in multilayer silicene as a different nanoscale Si phase for advanced nanotechnology applications such as ultrascaled nanoelectronics and nanomembranes, as well as for fundamental exploration of quantum properties.

First-principles investigation of mechanical properties of silicene, germanene and stanene

Two-dimensional allotropes of group-IV substrates including silicene, germanene and stanene have recently attracted considerable attention in nanodevice fabrication industry. These materials involving the buckled structure have been experimentally fabricated lately. In this study, first-principles density functional theory calculations were utilized to investigate the mechanical properties of single-layer and free-standing silicene, germanene and stanene. Uniaxial tensile and compressive simulations were carried out to probe and compare stress-strain properties; such as the Young’s modulus, Poisson’s ratio and ultimate strength. We evaluated the chirality effect on the mechanical response and bond structure of the 2D substrates. Our first-principles simulations suggest that in all studied samples application of uniaxial loading can alter the electronic nature of the buckled structures into the metallic character. Our investigation provides a general but also useful viewpoint with respect to the mechanical properties of silicene, germanene and stanene.

(Invited) Is the Silicene a 2D Dirac Material?

Silicene, a 2D honeycomb layer of Si, has attracted much attention, since a Dirac fermion character like graphene was predicted. Furthermore the silicene has a 2D topological insulator due to the 1000 times stronger spin-orbit coupling than graphene. The hypothetical silicene is a free standing layer, but in practice, experimentally fabricated silicenes so far are 2D layers of silicon epitaxially grown on substrates. We will discuss, on the basis of the structure determination on an atomic scale by using quantitative LEED analysis, the nature of the epitaxial silicenes on Ag(111) surface; the influence of the silicene-substrate interaction for the single layer silicene and if the multi-layer silicene fabricated on the substrate has the features expected for the free-standing silicene.

(Invited) Silicene As a 2D Material Candidate

Silicene, a new Si-based 2D material with a graphene-like honeycomb structure, has attracted considerable interest, because its topology confers to it the same remarkable electronic properties as those of graphene [1]. Contrary to graphene, silicene does not exist in nature and had to be synthesized on a substrate material, such as Ag(111) [2]. In comparison to graphene, such silicene layers might have a potential advantage of being easily integrated in current Si-based nano/micro-electronics offering novel technological applications. Only very recently the first silicene transistor was reported, based on silicene layers grown on a Ag(111) substrate [3]. In this talk the recent development in epitaxial formation of single layer silicene will be reviewed, including structural and electronic properties for different silicene and 2D-Si layers. Furthermore, the formation of silicene double-layer and multi-layer structures will be discussed [4]. Different structure models will be compared to experimental results in order to unveil the nature of these layered materials. Such a detailed understanding of single/multi-layer silicene structures and their formation is crucial in order to optimize the performance of future devices which are based on such layers. We will also discuss the stability of single/multi-layer silicene upon heating and oxidation and look a possible modifications of silicene by adsorbates. It is expected that silicene can be functionalized this way, which could open a new way for the application of Si in electronic devices. 1) G. G. Guzmán-Verri and L. C. Lew Yan Voon, Phys. Rev. B 76, 075131 (2007); S. Lebègue and O. Eriksson, Phys. Rev. B, 79 115409 (2009); S. Cahangirov et al., Phys. Rev. Lett. 102, 236804 (2009). 2) P. Vogt et al., Phys. Rev. Lett. 108, 55501 (2012); C.-L. Lin et al., Appl. Phys. Exp. 5, 045802 (2012); B. Feng et al., Nano Lett. 11, 3507 (2012). 3) Li Tao et al., Nature Nanotechnology (2015). 4) P. Vogt et al., Appl. Phys. Lett. 104, 021602(2014), P. De Padova et al., Appl. Phys. Lett. 102, 163106 (2013)

(Invited) Is the Silicene a 2D Dirac Material?

Silicene, a 2D honeycomb layer of Si, has attracted much attention, since a Dirac fermion character like graphene was predicted. Furthermore the silicene has a 2D topological insulator due to the 1000 times stronger spin-orbit coupling than graphene. The hypothetical silicene is a free standing layer, but in practice, experimentally fabricated silicenes so far are 2D layers of silicon epitaxially grown on substrates. We will discuss, on the basis of the structure determination on an atomic scale by using quantitative LEED analysis, the nature of the epitaxial silicenes on Ag(111) surface; the influence of the silicene-substrate interaction for the single layer silicene and if the multi-layer silicene fabricated on the substrate has the features expected for the free-standing silicene.

Comparative study on the nonlinear properties of bilayer graphene and silicene under tension

Atomic structures and nonlinear properties of single layer graphene (SLG), bilayer graphene (BLG), single layer silicene (SLS), and bilayer silicene (BLS) under equiaxial tension and uniaxial tensions along armchair and zigzag directions have been investigated comparatively using first-principles calculations. First, we have calculated the dependences of atomic structures (bond length, interlayer distance, and buckling height) of BLG and BLS on strain under three types of tensions. There exists the weak Van der Waals interaction between two layers of BLG and the interlayer distance is not variable with strain for three types of tensions. However, the interlayer of BLS is the covalent bond interaction, and the distance decreases with the increasing strain for three types of tensions. The continuum description of elastic response is formulated by expanding the elastic strain energy density in a Taylor series in strain truncated after the third-order term. The in-plane second- and third-order elastic constants of BLG and BLS have been obtained by fitting to the strain energy density versus Lagrangian strain relationships. The results show the in-plane stiffnesses of BLG and BLS become slightly larger than those of their single layer counterparts. In spite of the interlayer Si–Si covalent bond between two layers of BLS, its stiffness is still much less than BLG and SLG. Poisson’s ratios of BLG and BLS basically maintain unchanged compared to their single layer counterparts.