Silicene Layers Research Articles

Layer-sliding-mediated controllable synthetic strategy for the preparation of multifunctional materials

Strategies capable of inducing multifunctionality in materials in a controllable manner are highly desirable. In this work, we have brought multifunctionality to a two-dimensional (2D) van der Waals heterostructure (vdW-HS) consisting of a layer of puckered silicene and indium selenide (InSe) through layer-sliding. Electronic, optical and interfacial properties of silicene/InSe vdW-HS have been varied dynamically by sliding silicene over InSe. The aforementioned layer-sliding is carried out in a gradual manner without and with Mg-intercalation. The buckling parameter relating to the corrugation of silicene drops from 0.438 Å to 0.351 Å on Mg-intercalation, whereas in the case of InSe buckling remains unaffected. The calculated band structure for un-intercalated vdW-HS shows a small band gap opening 205/210 meV with LDA/GGA functional) along high symmetry K direction in the first Brillion zone. The widening of the bandgap makes it suitable for optoelectronics and strain sensor devices, where a small band gap (0.1–1 eV) is required. To look inside into the interface of the vdW-HS, various interfacial electronic parameters such as planar average charge density difference (Δρ), dipole charge transfer (ΔQ) and dipole moment (Δµ) have been determined. It is found that the charge accumulation peaks have appeared near the silicene layer and the charge is transferred from silicene to the InSe layer in both un-intercalated and Mg-intercalated vdW-HSs. Further, we investigated the different optical parameters including real [ε1(ω)] and imaginary [ε2(ω)] parts of dielectric function (DF), energy loss function [L(ω)] and diagonal components of dielectric tensor [ε (iω)] for un-intercalated and Mg-intercalated vdW-HSs. The anisotropic analysis shows that the computed in-plane components significantly dominate the out-of-plane component of the DF, which show minor change as the sliding proceeds. The study endorses that multifunctional electronic 2D materials can be achievable by employing the layer-sliding dynamical strategy.

Bendable Silicene Membranes.

Due to their superior mechanical properties, two-dimensional (2D) materials have gained interest as active layers in flexible devices co-integrating electronic, photonic, and straintronic functions altogether. To this end, 2D bendable membranes compatible with the technological process standards and endowed with large scale uniformity are highly desired. Here, we report on the realization of bendable membranes based on silicene layers (the 2D form of silicon) by means of a process in which the layers are fully detached from the native bulk substrate and then transferred onto arbitrary flexible substrates. The application of macroscopic mechanical deformations induces a strain-responsive behavior in the Raman spectrum of silicene. We also show that the membranes under elastic tension relaxation are prone to form microscale wrinkles displaying a local generation of strain in the silicene layer consistent with that observed under macroscopic mechanical deformation. Optothermal Raman spectroscopy measurements reveal a curvature-dependent heat dispersion in silicene wrinkles. Finally, as compelling evidence of the technological potential of the silicene membranes, we demonstrate that they can be readily introduced into a lithographic process flow resulting in the definition of flexible device-ready architectures, e.g. piezoresistor, and thus paving the way to a viable advance in a fully silicon-compatible technology framework. This article is protected by copyright. All rights reserved.

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.

Plasmon Damping Rates in Coulomb-Coupled 2D Layers in a Heterostructure.

The Coulomb excitations of charge density oscillation are calculated for a double-layer heterostructure. Specifically, we consider two-dimensional (2D) layers of silicene and graphene on a substrate. From the obtained surface response function, we calculated the plasmon dispersion relations, which demonstrate how the Coulomb interaction renormalizes the plasmon frequencies. Most importantly, we have conducted a thorough investigation of how the decay rates of the plasmons in these heterostructures are affected by the Coulomb coupling between different types of two-dimensional materials whose separations could be varied. A novel effect of nullification of the silicene band gap is noticed when graphene is introduced into the system. To utilize these effects for experimental and industrial purposes, graphical results for the different parameters are presented.

Geometric, spintronic, and opto‐electronic properties of 3d transition metals doped silicene: An ab initio study

AbstractThis study shows density functional theory (DFT) investigations that 3d transition metals (TM) doping in silicene can greatly alter the geometric, spintronic, and optoelectronic properties of the pristine silicene (p‐Si) layer. Significant Bader charge transfer from 3d TM atoms to surrounding Si atoms ensures the tight bonding between dopant and substrate; hence, all the 3d transition metal‐doped silicene (3d TM‐Si) systems are said geometrically strong and stable. Sc‐ and Ti‐doped systems show the lowest formation energies of −84.72 and −84.21 eV, respectively, while Zn‐Si bears the highest (−70.89 eV). 3d TMs from V to Co doping induces magnetic moment (MM) in the silicene layer which mainly comes from d‐orbitals of 3d TM atoms and partly from p‐orbitals of Si atoms, meanwhile Mn‐Si has MM as high as 3.0 μB. Among 3d TM‐Si systems studied, Cr‐Si and Mn‐Si systems became half metals, Ti‐Si became indirect semiconductor, whereas rest others convert into metals. Sc and V doping is found to be p‐type doping as the Fermi level shifts into the valence band. Moreover, multiple and broader peaks in the absorption coefficient plot indicate the significant photoabsorption of 3d TM‐Si systems. The present study of electronic, magnetic, and optical properties of 3d TM‐Si systems extend a helpful proposal for further experimental work to fabricate silicene‐based single‐spin electron source and other nano‐electronic devices.

Silicene’s pervasive surface alloy on Ag(111): a scaffold for two-dimensional growth

Silicene, the two-dimensional (2D) allotrope of silicon, is a promising material for electronics. So far, the most direct synthesis strategy has been to grow it epitaxially on metal surfaces; however, the effect of the strong silicon-metal interaction on the structure and electronic properties of the metal-supported silicene is generally poorly understood. In this work, we consider the -silicene monolayer (ML) grown on Ag(111), probably the most illustrious representative of the 2D silicon family, and show that our experimental results refute the common interpretation of this system as a simple buckled, honeycomb ML with a sharp interface to the Ag substrate. Instead, the presented analysis demonstrates the pervasive presence of a second silicon species, which we conclude to be a Si–Ag alloy stacked between the 2D silicene and the silver substrate and scaffolding the 2D silicene layer. These findings question the current structural understanding of the silicene/Ag(111) interface and may raise expectations of analogous alloy systems in the stabilization of other 2D materials grown epitaxially on metal surfaces.

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.

Phonon Thermal Transport in Silicene/Graphene Heterobilayer Nanostructures: Effect of Interlayer Interactions.

Heterostructuring, as a promising route to optimize the physical properties of 2D materials, has attracted great attention from the academic community. In this paper, we investigated the room-temperature in-plane and cross-plane phonon thermal transport in silicene/graphene van der Waals (vdW) heterostructures using molecular dynamics simulations. Our simulation results demonstrated that heat current along the graphene layer is remarkably larger than that along the silicene layer, which suggests that graphene dominates the thermal transport in silicene/graphene heterostructures. The in-plane phonon thermal conductivity of the silicene/graphene heterostructures could be a compromise between monolayer graphene and monolayer silicene. Heterostructuring can remarkably reduce the in-plane thermal conductivity of the graphene layer but increase the in-plane thermal conductivity of the silicene layer in heterobilayers compared with the freestanding monolayer counterparts because of their different structures. We also simulated the interlayer interaction strength effect on the in-plane phonon thermal conductivity and cross-plane interfacial thermal resistance of silicene/graphene heterostructures. Total in-plane phonon thermal conductivity and interfacial thermal resistance both decrease with the increase in the interlayer interaction strength in the silicene/graphene heterobilayers. In addition, the calculated interfacial thermal resistance shows the effect of the thermal transport direction across the interface. This study provides a useful reference for the thermal management regulation of 2D vdW heterostructures.

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.

Thermal boundary conductance of monolayer beyond-graphene two-dimensional materials on SiO2 and GaN

Two-dimensional (2D) materials have emerged as a platform for a broad array of future nanoelectronic devices. Here we use first-principles calculations and phonon interface transport modeling to calculate the temperature-dependent thermal boundary conductance (TBC) in single layers of beyond-graphene 2D materials silicene, hBN, boron arsenide (BAs), and blue and black phosphorene (BP) on amorphous SiO2 and crystalline GaN substrates. Our results show that for 2D/3D systems, the room temperature TBC can span a wide range from 7 to 70 MW m−2 K−1 with the lowest being for BP and highest for hBN. We also show that 2D/3D TBC has a strong temperature dependence that can be alleviated by encapsulating the 2D/3D stack. Upon encapsulation with AlO x , the TBC of several beyond-graphene 2D materials can match or exceed reported values for graphene and numerous transition-metal dichalcogendies which are in the range of 15–40 MW m−2 K−1. We also compute the room temperature TBC as a function of van der Waals spring coupling (K a ) where the TBC falls in the range of 50–150 MW m−2 K−1 at coupling strengths of K a = 2–4 N m−1 for silicene, BAs, and blue phosphorene. We further identify group III–V materials with ultra-soft flexural branches as being promising 2D materials for thermal isolation and energy scavenging applications when matched with crystalline substrates.

Silicene Quantum Capacitance Dependent Frequency Readout to a Label-Free Detection of DNA Hybridization- A Simulation Analysis.

The use of deoxyribonucleic acid (DNA) hybridization to detect disease-related gene expression is a valuable diagnostic tool. An ion-sensitive field-effect transistor (ISFET) with a graphene layer has been utilized for detecting DNA hybridization. Silicene is a two-dimensional silicon allotrope with structural properties similar to graphene. Thus, it has recently experienced intensive scientific research interest due to its unique electrical, mechanical, and sensing characteristics. In this paper, we proposed an ISFET structure with silicene and electrolyte layers for the label-free detection of DNA hybridization. When DNA hybridization occurs, it changes the ion concentration in the surface layer of the silicene and the pH level of the electrolyte solution. The process also changes the quantum capacitance of the silicene layer and the electrical properties of the ISFET device. The quantum capacitance and the corresponding resonant frequency readout of the silicene and graphene are compared. The performance evaluation found that the changes in quantum capacitance, resonant frequency, and tuning ratio indicate that the sensitivity of silicene is much more effective than graphene.

Spin-polarised DFT modeling of electronic, magnetic, thermal and optical properties of silicene doped with transition metals

The geometric, electronic, magnetic, thermal, and optical properties of transition metal (TM) doped silicene are systematically explored using spin-dependent density functional computation. We find that the TM atoms decrease the buckling degree of the silicene structure caused by the interaction between the dopant TM atoms and the Si atoms in the silicene layer plane which is quite strong. In some TM-silicenes, parallel bands and the corresponding van Hove singularities are observed in the electronic band structure without and with spin-polarization. These parallel bands are the origin of most of the transitions in the visible and the UV regions. A high Seebeck coefficient is found in some TM-silicene without spin-polarization. In the presence of emergent spin-polarization, a reduction or a magnification of the Seebeck coefficient is seen due to a spin-dependent phase transition. We find that the preferred state is a ferromagnetic state with a very high Curie temperature. We observe a strong interaction and large orbital hybridization between the TM atoms and the silicene. As a result, a high magnetic moment emerges in TM-silicene. Our results are potentially beneficial for thermospin, and optoelectronic nanodevices.

Computational Modeling of 2D Materials under High Pressure and Their Chemical Bonding: Silicene as Possible Field-Effect Transistor.

To study the possibility for silicene to be employed as a field-effect transistor (FET) pressure sensor, we explore the chemistry of monolayer and multilayered silicene focusing on the change in hybridization under pressure. Ab initio computations show that the effect of pressure depends greatly on the thickness of the silicene film, but also reveals the influence of real experimental conditions, where the pressure is not hydrostatic. For this purpose, we introduce anisotropic strain states. With pure uniaxial stress applied to silicene layers, a path for sp3 silicon to sp3d silicon is found, unlike with pure hydrostatic pressure. Even with mixed-mode stress (in-plane pressure half of the out-of-plane one), we find no such path. In addition to introducing our theoretical approach to study 2D materials, we show how the hybridization change of silicene under pressure makes it a good FET pressure sensor.

Modeling and computation of thermal and optical properties in silicene supported honeycomb bilayer and heterobilayer nanostructures

The compatibility of silicene with the semiconductor technology and its promise in thermoelectric and in optoelectronic devices demands modulation and optimization of its thermal and optical properties using structural modifications. Here we investigate the thermal and optical properties of silicene supported group IV honeycomb bilayer and heterobilayer nanoribbon structures (silicene/germanene and silicene/graphene) using molecular dynamics simulation and first principle calculations. Our modeled silicene/germanene nanoribbons show ~43% lower room temperature average thermal conductivity (2.94 W/m/K) compared to that of similar sized silicene nanoribbons (5.1 W/m/K). Similarly, in silicene/graphene nanoribbons, heterostructuring results in a ~25× reduction of the in-plane thermal conductivity compared to that of a graphene nanoribbon, which is attributed to the phonon confinement in the low frequency region and large mismatch in the phonon frequencies between the monolayers of these heterostructures (~50 THz for the graphene layer and ~11 THz for the silicene layer in the silicene/graphene nanoribbons). We also characterize the temperature and the size dependent thermal conductivity of the modeled structures. The realized low thermal conductivity in these heterostructures could lead to the design of optimized thermoelectrics. Furthermore, we compute the frequency dependent dielectric function and show that the absorption coefficients of our modeled nanostructures are 1.5× to 2× large compared to those of silicene monolayer in the ultraviolet (UV) region, which could be useful in optoelectronic devices such as UV photodetectors. Thus, heterostructuring could be an encouraging route to tune the thermal and optical properties of silicene nanostructures for next generation nano-electronic and optoelectronic devices.

Time-resolved X-ray photoelectron diffraction using an angle-resolved time-of-flight electron analyzer

X-ray photoelectron diffraction (XPD) provides atomic resolution, element sensitive local structure information about the surfaces and interfaces of materials. In the work reported in this paper, a two-dimensional angle resolved time-of-flight (2DARTOF) system is used to perform time-resolved XPD experiments on epitaxial silicene. The two Si 2p peak components from the different atomic sites in the silicene layer allow extraction of the individual XPD patterns. Time-resolved measurements captured small angle shifts in the forward focusing peak, indicating laser-induced changes in the silicene structure. At 10 ns, the XPD patterns appear to relax back to the equilibrium state. This work demonstrates that 2DARTOF systems are well suitable for time-resolved XPD measurements.

Effects of disorder on the transport and thermoelectric properties of silicene superlattices

Abstract Unavoidable structural disorder associated with the variation of the widths of barriers and wells as well as the heights of barriers in superlattices of 2D materials can affect considerably electronic transport and related phenomena. In this work, we study the impact of structural disorder on the electronic transport and thermoelectric properties of gated silicene superlattices (GSSLs). A low-energy effective Hamiltonian has been used to describe electrons in silicene. The transfer matrix approach, the Landauer–Buttiker formalism and the Cutler–Mott formula have been implemented to obtain the transport and thermoelectric properties. In particular, the conductance, Seebeck coefficient and power factor are investigated in ordered and disordered GSSLs. The results show that disorder related to the width of barriers-wells is not equivalent to the one associated to the height of barriers. The former barely changes the transport and thermoelectric coefficients at low energy, while the latter greatly affects them in the whole energy range. We also assess the disorder associated to the on-site potential of silicene layers, finding that its influence is limited. So, our results indicate that in the design and fabrication of GSSLs a precise control of the height of the barriers is required in order to have and preserve good thermoelectric properties.

Undulated silicene and germanene freestanding layers: why not?

Silicene and germanene freestanding layers are usually described as a honeycomb lattice formed by two hexagonal sub-lattices presenting a height difference, namely the layer buckling. In this work, first-principles calculations show that silicene and germanene can be rippled at 0 K with various wavelengths, without any compressive strain of the layer. For germanene, the height difference between two Ge atoms from the same sub-lattice can be as high as 4.7 for an undulation length of 81 . The deformations are related to slight (lower than 1.7°) bond angle modifications, and the energy cost is remarkably low, lying between 0.1 and 0.8 meV per atom. These undulations modify the electronic structure, opening a gap of 15 meV.

Tunable spin-polarized band gap in Si2/NiI2 vdW heterostructure.

Using density functional theory (DFT) calculations we investigate the structural and electronic properties of a heterogeneous van der Waals (vdW) structure consisting of silicene and NiI2 single layers. We observe an interaction between the two layers with a net charge transfer from the ferromagnetic semiconductor NiI2 to silicene, breaking the inversion symmetry of the silicene structure. However, the charges flow in opposite directions for the two spin channels, which leads to a vdW heterostructure with a spin-polarized band gap between the π and π* states. The band gap can be tuned by controlling the vertical distance between the layers. The features shown by this vdW heterostructure are new, and we believe that silicene on a NiI2 layer can be used to construct heterostructures which have appropriate properties to be used in nanodevices where control of the spin-dependent carrier mobility is necessary and can be incorporated into silicon based electronics.

Honeycomb silicon on alumina: Massless Dirac fermions in silicene on substrate

We predict the stability of a graphenelike silicene sheet on one monolayer of aluminum oxide. We find that the honeycomb buckled structure of silicene is not broken upon interaction with one monolayer of ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ in the kagome geometry. As a consequence, the electronic band structure shows unperturbed cones with massless Dirac fermions embedded into ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-derived bands. The heterostructure conserves the topological character of the freestanding silicene. The optical properties of the silicene/${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ bilayers clearly maintain the characteristic infrared universal limit of $\ensuremath{\pi}\ensuremath{\alpha}$. A quantized absorbance $N\ifmmode\cdot\else\textperiodcentered\fi{}\ensuremath{\pi}\ensuremath{\alpha}$ (with $N$ silicene layers) also occurs for stacking of multilayers.

High graphene permeability for room temperature silicon deposition: The role of defects

Graphene (Gr) is known to be an excellent barrier preventing atoms and molecules to diffuse through it. This is due to the carbon atom arrangement in a two-dimensional (2D) honeycomb structure with a very small lattice parameter forming an electron cloud that prevents atoms and molecules crossing. Nonetheless at high annealing temperatures, intercalation of atoms through graphene occurs, opening the path for formation of vertical heterojunctions constituted of two-dimensional layers. In this paper, we report on the ability of silicon atoms to penetrate the graphene network, fully epitaxially grown on a Ni(111) surface, even at room temperature. Our scanning tunneling microscopy (STM) experiments show that the presence of defects like vacancies and dislocations in the graphene lattice favor the Si atoms intercalation, forming two-dimensional, flat and disordered islands below the Gr layer. Ab-initio molecular dynamics calculations confirm that Gr defects are necessary for Si intercalation at room temperature and show that: i) a hypothetical intercalated silicene layer cannot be stable for more than 8 ps and ii) the corresponding Si atoms completely lose their in-plane order, resulting in a random planar distribution, and form strong covalent bonds with Ni atoms.