Tunable band gap and doping type in silicene by surface adsorption: towards tunneling transistors

Boosted Lithium-Ion Transport Kinetics in n-Type Siloxene Anodes Enabled by Selective Nucleophilic Substitution of Phosphorus

Doped two-dimensional (2D) materials hold significant promise for advancing many technologies, such as microelectronics, optoelectronics, and energy storage. Herein, n-type 2D oxidized Si nanosheets, namely n-type siloxene (n-SX), are employed as Li-ion battery anodes. Via thermal evaporation of sodium hypophosphite at 275 °C, P atoms are effectively incorporated into siloxene (SX) without compromising its 2D layered morphology and unique Kautsky-type crystal structure. Further, selective nucleophilic substitution occurs, with only Si atoms being replaced by P atoms in the O3≡Si-H tetrahedra. The resulting n-SX possesses two delocalized electrons arising from the presence of two electron donor types: (i) P atoms residing in Si sites and (ii) H vacancies. The doping concentrations are varied by controlling the amount of precursors or their mean free paths. Even at 2000 mA g-1, the n-SX electrode with the optimized doping concentration (6.7 × 1019 atoms cm-3) delivers a capacity of 594 mAh g-1 with a 73% capacity retention after 500 cycles. These improvements originate from the enhanced kinetics of charge transport processes, including electronic conduction, charge transfer, and solid-state diffusion. The approach proposed herein offers an unprecedented route for engineering SX anodes to boost Li-ion storage.

Recent Advances in Surface Modifications of Elemental Two-Dimensional Materials: Structures, Properties, and Applications

The advent of graphene opens up the research into two-dimensional (2D) materials, which are considered revolutionary materials. Due to its unique geometric structure, graphene exhibits a series of exotic physical and chemical properties. In addition, single-element-based 2D materials (Xenes) have garnered tremendous interest. At present, 16 kinds of Xenes (silicene, borophene, germanene, phosphorene, tellurene, etc.) have been explored, mainly distributed in the third, fourth, fifth, and sixth main groups. The current methods to prepare monolayers or few-layer 2D materials include epitaxy growth, mechanical exfoliation, and liquid phase exfoliation. Although two Xenes (aluminene and indiene) have not been synthesized due to the limitations of synthetic methods and the stability of Xenes, other Xenes have been successfully created via elaborate artificial design and synthesis. Focusing on elemental 2D materials, this review mainly summarizes the recently reported work about tuning the electronic, optical, mechanical, and chemical properties of Xenes via surface modifications, achieved using controllable approaches (doping, adsorption, strain, intercalation, phase transition, etc.) to broaden their applications in various fields, including spintronics, electronics, optoelectronics, superconducting, photovoltaics, sensors, catalysis, and biomedicines. These advances in the surface modification of Xenes have laid a theoretical and experimental foundation for the development of 2D materials and their practical applications in diverse fields.

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.

Silicon Nanosheets: An Emerging 2D Photonic Material with a Large Transient Nonlinear Optical Response beyond Graphene

The present work reports on the transient nonlinear optical (NLO) responses of two different types of 2D silicon nanosheets (SiNSs), namely hydride-terminated silicon nanosheets (SiNS-H) and 1-dodecene-functionalized silicon nanosheets (SiNS-dodecene). The main motivation of this study was to extend the knowledge regarding the NLO properties of these Si-based materials, for which very few published studies exist so far. For that purpose, the NLO responses of SiNS-H and SiNS-dodecene were investigated experimentally in the nanosecond regime at 532 and 1064 nm using the Z-scan technique, while the obtained results were compared to those of certain recently studied graphene nanosheets. SiNS-dodecene was found to exhibit the largest third-order susceptibility χ(3) values at both excitation wavelengths, most probably ascribed to the presence of point defects, indicating the importance of chemical functionalization for the efficient enhancement and tailoring of the NLO properties of these emerging 2D Si-based materials. Most importantly, the results demonstrated that the present silicon nanosheets revealed comparable and even larger NLO responses than graphene nanosheets. Undoubtedly, SiNSs could be strong competitors of graphene for applications in 2D-material-based photonics and optoelectronics.

Mechanisms of negative differential resistance in glutamine-functionalized WS2quantum dots

Understanding the mechanism of the negative differential resistance (NDR) in transition metal dichalcogenides is essential for fundamental science and the development of electronic devices. Here, the NDR of the current-voltage characteristics was observed based on the glutamine-functionalized WS₂quantum dots (QDs). The NDR effect can be adjusted by varying the applied voltage range, air pressure, surrounding gases, and relative humidity. A peak-to-valley current ratio as high as 6.3 has been achieved at room temperature. Carrier trapping induced by water molecules was suggested to be responsible for the mechanism of the NDR in the glutamine-functionalized WS₂QDs. Investigating the NDR of WS₂QDs may promote the development of memory applications and emerging devices.

Rise of silicene and its applications in gas sensing

Reviewing a subject is done to provide an insight into theoretical and conceptual background of the study. Looking back into the history of an emerging field and summarizing it in a few pages is a herculean task. Anyway, it was imperative to write a few words about the rise of silicene, its properties, and its applications as gas sensors. Currently, silicene is a growing field of interest. It is probably one of the most studied materials nowadays and scientists and researchers are studying it because of its intriguing electronic properties and potential applications in nanoelectronics. Various experimental and theoretical investigations are going on worldwide to explore the various aspects of this field. It is essential to review the literature based on investigations by various scientists in this field.

Semi-analytical modelling and evaluation of uniformly doped silicene nanotransistors for digital logic gates

Silicene has attracted remarkable attention in the semiconductor research community due to its silicon (Si) nature. It is predicted as one of the most promising candidates for the next generation nanoelectronic devices. In this paper, an efficient non-iterative technique is employed to create the SPICE models for p-type and n-type uniformly doped silicene field-effect transistors (FETs). The current-voltage characteristics show that the proposed silicene FET models exhibit high on-to-off current ratio under ballistic transport. In order to obtain practical digital logic timing diagrams, a parasitic load capacitance, which is dependent on the interconnect length, is attached at the output terminal of the logic circuits. Furthermore, the key circuit performance metrics, including the propagation delay, average power, power-delay product and energy-delay product of the proposed silicene-based logic gates are extracted and benchmarked with published results. The effects of the interconnect length to the propagation delay and average power are also investigated. The results of this work further envisage the uniformly doped silicene as a promising candidate for future nanoelectronic applications.

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.

Strain-engineered black arsenene as a promising gas sensor for detecting SO2 among SF6 decompositions

The adsorption and gas sensing properties of black arsenene (B-As) regarding sulfur hexafluoride (SF₆) and its six decompositions (SOF₂, SO₂F₂, SO₂, H₂S, HF, and CF₄) are investigated using density functional theory combined with the nonequilibrium Green's function. The sensitivity of B-As is evaluated by considering the most stable adsorption configuration, adsorption energy, work function, recovery time, local density of states, and charge transfer between the gas molecules and B-As. It is demonstrated that B-As is more sensitive to the SO₂ molecule than to the other decompositions. Additionally, the adsorption strength can be manipulated by controlling the external electric field (E-field). The application of tensile biaxial strain results in more isotropic electrical conductance of B-As, and it can also effectively enhance the response toward SO₂. For example, under a 1% equibiaxial tensile strain, a 132% response can be obtained along the zigzag direction. This work suggests the promising prospects of B-As-based gas sensors for detecting SO₂ among SF₆ decompositions.

Silicon Nanosheets versus Graphene Nanosheets: A Comparison of Their Nonlinear Optical Response

Silicene, the silicon analogue of graphene, represents a new class of two-dimensional (2D) materials, which shares some of the outstanding physical properties of graphene. Furthermore, it has the advantage of being compatible with the current Si-based technology. However, this 2D material is not stable and is quite prone to oxidation. The hydride-terminated silicene, called silicane, is a more stable form of 2D silicon, if functionalized via, for example, the hydrosilylation reaction. In this work, the third-order nonlinear optical (NLO) properties of two functionalized silicanes, namely hydride-terminated silicon nanosheets (SiNS-H) and 1-dodecene-functionalized silicon nanosheets (SiNS-dodecene), are accessed and compared to those of single-layer graphene, under 35 ps, 532 and 1064 nm excitation. The present results show that the functionalized silicanes exhibit comparable and even higher NLO response than that of single-layer graphene, making them strong competitors of graphene and very interesting candidates for future photonic and optoelectronic applications.

The effects of stacking mode and thickness on the frictional behaviour of multilayer silicene

Understanding the contact behaviour of 2D materials in nanoscale is of great importance for their applications. In the present work, molecular dynamics simulation is employed to study the frictional behaviour of the AA' and AB stacked multilayer silicene for up to 4 layers placed on the weakly adhesive amorphous SiO2 substrate with a sliding AFM tip. During the sliding process, the AFM cantilever represented by virtual atoms moves with the velocity of 2 m s-1 along the zigzag direction under a load of 2 nN at 300 K. The stick-slip frictional behaviour shows high sensitivity to the number of layers. As the thickness increases, the friction force first increases from the monolayer to bilayer and then decreases from the bilayer to 4-layer, which shows an exotic tendency for the first time among all the reported lamellar materials to date where the friction usually decreases monotonically with thickness. For all the investigated thicknesses, the friction on AA'-stacked silicene is slightly larger than the AB stacked counterpart, and the difference diminishes with increasing thickness. The frictional behaviour of AA' bilayer presents the highest peak force with evolving weakening phenomenon induced by a phase transition to the planar structure. Herein, we analyze the frictional force distribution on the tip with kurtosis and skewness as measurement parameters for the commensurability and rigidity components, respectively. The contact area between silicene and the diamond tip is compared for different silicene morphologies. The result shows an affinity between friction and rigidity of multilayer silicene, which is closely related to the interlayer covalent bonds and limited shear between sublayers.

Fundamental Properties of Metal-Adsorbed Silicene: A DFT Study

Sodium, magnesium, and aluminum adatoms, which possess one, two, and three valence electrons, respectively, in terms of 3s, 3s², and (3s², 3p) orbitals, are very suitable for helping us understand adsorption-induced diverse phenomena. In this work, the revealing properties of metal (Na/Mg/Al)-adsorbed graphene systems are investigated by means of the first-principles method. The single- and double-sided chemisorption cases, the various adatom concentrations, the hollow/top/valley/bridge sites, and the buckled structures are taken into account. The hollow and valley adsorptions that correspond to the Na/Mg and Al cases, respectively, create extremely nonuniform environments. This leads to diverse orbital hybridizations in Na/Mg/Al-Si bonds, as indicated by the Na/Mg/Al-dominated bands, as well as the spatial charge density distributions and the orbital-projected density of states (DOS). Out of three types of metal-adatom adsorptions, the Al-adsorption configurations produce the strongest chemical modifications. The ferromagnetic configurations have been shown to survive only in specific Mg and Al adsorptions, but not in the Na cases. The presented theoretical predictions could be verified experimentally, and potential applications are discussed. Additionally, important similarities and differences with graphene-related systems are examined.

Germanene/GaGeTe heterostructure: a promising electric-field induced data storage device with high carrier mobility

Opening up a band gap without lowering high carrier mobility in germanene and finding suitable substrate materials to form van der Waals heterostructures have recently emerged as an intriguing way of designing a new type of electronic devices. By using first-principles calculations, here, we systematically investigate the effect of the GaGeTe substrate on the electronic properties of monolayer germanene. Linear dichroism of the Dirac-cone like band dispersion and higher carrier mobility (9.7 × 103 cm2 V-1 s-1) in the Ge/GaGeTe heterostructure (HTS) are found to be preserved compared to that of free-standing germanene. Remarkably, the band structure of HTS can be flexibly modulated by applying bias voltage or strain. A prototype data storage device FET based on Ge/GaGeTe HTS is proposed, which presents a promising high performance platform with a tunable band gap and high carrier mobility.

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.

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.

Dodecagonal quasicrystal silicene: preparation, mechanical property, and friction behaviour

In this study, we obtained dodecagonal monolayer silicene with three-fold and four-fold coordination by melt quenching via molecular dynamics (MD) simulations. Stretching simulation of the pre-strained dodecagonal silicene showed lower critical stress than the honeycomb silicene and resulted in an increase in six-fold rings during the plastic deformation since the four-coordinated atom sites are less mechanically favoured than the three-coordinated sites. The friction behaviours with an AFM tip sliding on the dodecagonal and honeycomb surfaces under different loads and tip sizes were simulated and compared. For all the investigated cases, the dodecagonal surface always showed a lower mean friction force than the honeycomb surface. The lower friction of the quasicrystal was observed, and the mechanism was illuminated successfully for the first time by MD simulations. The reduced friction of dodecagonal silicene can be explained by the morphology of the one-dimensional potential energy surface (PES). The 1D PES of dodecagonal silicene has longer potential corrugation lengths than honeycomb silicene, which induce mild motion of the tip in the stick process and lower friction force. Considering the close density of the employed dodecagonal and honeycomb structure, the longer potential corrugation length is a consequence of the quasiperiodic morphology rather than the interspace between atoms. Besides, with a larger tip size, the 1D PES on the dodecagonal surface has a flatter area, which contributes further to the reduced friction force on the dodecagonal surface.

Effect of Stone–Wales defects and transition-metal dopants on arsenene: a DFT study

The structural, electronic, and magnetic properties of 3d transition metal (TM) atom (Sc, V, Cr, Mn, Co, and Cu) doped Stone–Wales (SW) defect arsenene were systematically investigated by density functional theory (DFT). The results indicated that the properties of arsenene were effectively changed by the SW-defect and TM-doping. Furthermore, chemical bonds formed between the TM-dopants and the adjacent As atoms of the SW-defect. The dopants Sc, Mn, and Cu induced an indirect-to-direct bandgap transition, and the doping of V, Cr, and Mn in SW-defect arsenene exhibited magnetic states. The magnetic moments of the systems depended on the number of spin-localized valence electrons. The functionalized electronic and magnetic properties of arsenene highlight the applications for electronics, optoelectronics, and spintronics.

The structural, electronic, and magnetic properties of 3d transition metal (TM) atom (Sc, V, Cr, Mn, Co, and Cu) doped Stone–Wales (SW) defect arsenene were systematically investigated by density functional theory (DFT).

Two-dimensional transistors beyond graphene and TMDCs

Two-dimensional semiconductors (2DSCs) have attracted considerable attention as atomically thin channel materials for field-effect transistors. Each layer in 2DSCs consists of a single- or few-atom-thick, covalently bonded lattice, in which all carriers are confined in their atomically thin channel with superior gate controllability and greatly suppressed OFF-state current, in contrast to typical bulk semiconductors plagued by short channel effects and heat generation from static power. Additionally, 2DSCs are free of surface dangling bonds that plague traditional semiconductors, and hence exhibit excellent electronic properties at the limit of single atom thickness. Therefore, 2DSCs can offer significant potential for the ultimate transistor scaling to single atomic body thickness. Earlier studies of graphene transistors have been limited by the zero bandgap and low ON-OFF ratio of graphene, and transition metal dichalcogenide (TMDC) devices are typically plagued by insufficient carrier mobility. To this end, considerable efforts have been devoted towards searching for new 2DSCs with optimum electronic properties. Within a relatively short period of time, a large number of 2DSCs have been demonstrated to exhibit unprecedented characteristics or unique functionalities. Here we review the recent efforts and progress in exploring novel 2DSCs beyond graphene and TMDCs for ultra-thin body transistors, discussing the merits, limits and prospects of each material.

Versatile mechanical properties of novel g-SiC x monolayers from graphene to silicene: a first-principles study

Recently, a series of graphene-like binary monolayers (g-SiC _x ), where Si partly substitutes the C positions in graphene, have been obtained by tailoring the band gaps of graphene and silicene that have made them a promising material for application in opto-electronic devices. Subsequently, evaluating the mechanical properties of g-SiC _x has assumed great importance for engineering applications. In this study, we quantified the in-plane mechanical properties of g-SiC _x (x = 7, 5, 3, 2 and 1) monolayers (also including graphene and silicene) based on density function theory. It was found that the mechanical parameters of g-SiC _x , such as the ideal strength, Young's modulus, shear modulus, Poisson's ratio, as well as fracture toughness, are overall related to the ratio of Si-C to C-C bonds, which varies with Si concentration. However, for g-SiC₇ and g-SiC₃, the mechanical properties seem to depend on the structure because in g-SiC₇, the C-C bond strength is severely weakened by abnormal stretching, and in g-SiC₃, conjugation structure is formed. The microscopic failure of g-SiC _x exhibits diverse styles depending on the more complex structural deformation modes introduced by Si substitution. We elaborated the structure-properties relationship of g-SiC _x during the failure process, and in particular, found that the structural transformation of g-SiC₃ and g-SiC is due to the singular symmetry of their structure. Due to the homogeneous phase, all the g-SiC _x investigated in this study preserve rigorous isotropic Young's moduli and Poisson's ratios. With versatile mechanical performances, the family of g-SiC _x may facilitate the design of advanced two-dimensional materials to meet the needs for practical mechanical engineering applications. The results offer a fundamental understanding of the mechanical behaviors of g-SiC _x monolayers.

Functionalization of group-14 two-dimensional materials

The great success of graphene has boosted intensive search for other single-layer thick materials, mainly composed of group-14 atoms arranged in a honeycomb lattice. This new class of two-dimensional (2D) crystals, known as 2D-Xenes, has become an emerging field of intensive research due to their remarkable electronic properties and the promise for a future generation of nanoelectronics. In contrast to graphene, Xenes are not completely planar, and feature a low buckled geometry with two sublattices displaced vertically as a result of the interplay between sp² and sp³ orbital hybridization. In spite of the buckling, the outstanding electronic properties of graphene governed by Dirac physics are preserved in Xenes too. The buckled structure also has several advantages over graphene. Together with the spin-orbit (SO) interaction it may lead to the emergence of various experimentally accessible topological phases, like the quantum spin Hall effect. This in turn would lead to designing and building new electronic and spintronic devices, like topological field effect transistors. In this regard an important issue concerns the electron energy gap, which for Xenes naturally exists owing to the buckling and SO interaction. The electronic properties, including the magnitude of the energy gap, can further be tuned and controlled by external means. Xenes can easily be functionalized by substrate, chemical adsorption, defects, charge doping, external electric field, periodic potential, in-plane uniaxial and biaxial stress, and out-of-plane long-range structural deformation, to name a few. This topical review explores structural, electronic and magnetic properties of Xenes and addresses the question of their functionalization in various ways, including external factors acting simultaneously. It also points to future directions to be explored in functionalization of Xenes. The results of experimental and theoretical studies obtained so far have many promising features making the 2D-Xene materials important players in the field of future nanoelectronics and spintronics.

Multi-functional spintronic devices based on boron- or aluminum-doped silicene nanoribbons

Zigzag silicene nanoribbons (ZSiNRs) in the ferromagnetic edge ordering have a metallic behavior, which limits their applications in spintronics. Here a robustly half-metallic property is achieved by the boron substitution doping at the edge of ZSiNRs. When the impurity atom is replaced by the aluminum atom, the doped ZSiNRs possess a spin semiconducting property. Its band gap is suppressed with the increase of ribbon's width, and a pure thermal spin current is achieved by modulating ribbon's width. Moreover, a negative differential thermoelectric resistance in the thermal charge current appears as the temperature gradient increases, which originates from the fact that the spin-up and spin-down thermal charge currents have diverse increasing rates at different temperature gradient regions. Our results put forward a promising route to design multi-functional spintronic devices which may be applied in future low-power-consumption technologies.

Tuning the electronic and magnetic properties of InSe nanosheets by transition metal doping

A Cr-doped InSe monolayer that exhibits half metallic transport characteristics can be applied for spintronic devices.

Diaryliodonium salts as hydrosilylation initiators for the surface functionalization of silicon nanomaterials and their collaborative effect as ring opening polymerization initiators

Diaryliodonium salts were found to initiate hydrosilylation reactions on the surface of silicon nanosheets as well as silicon nanocrystals of different sizes. A variety of different functional substrates can be used to stabilize the surface of the photoluminescent materials. Additionally, the combination of hydride terminated silicon nanomaterials with diaryliodonium salts was found to initiate cationic ring opening polymerization, demonstrating the potential of silicon based nanomaterials as coinitiators and enabling a mild, straightforward reaction method.

Electronic and transport behavior of doped armchair silicene nanoribbons exhibiting negative differential resistance and its FET performance

The electronic and transport properties of armchair silicene nanoribbons (ASiNRs) doped with various elements are investigated.

The effect of Rashba spin–orbit coupling on the spin- and valley-dependent electronic heat capacity of silicene

In this work, we have investigated the effect of an electric field and Rashba spin–orbit coupling on the electronic band structure and electronic heat capacity of a ferromagnetic silicene material in three phases at Dirac points.

Strong enhancement of spin–orbit splitting induced by σ–π coupling in Pb-decorated silicene

Electronic properties and spin–orbit (SO) splitting of silicene adsorbed with Cu, Ag, Au and Pb atoms at different coverages are investigated by means of first-principles calculations.

Topotactic synthesis of the overlooked multilayer silicene intercalation compound SrSi2

Silicene, a 2D honeycomb lattice of Si atoms similar to graphene, is expected to be a platform for nanoelectronics and home to novel quantum phenomena. Unlike graphene, free-standing silicene is notoriously difficult to stabilize, while strong hybridization of silicene with substrates destroys its desirable properties. On the other hand, Dirac cones of silicene are effectively realized in a bulk - stoichiometric ionic multilayer silicene intercalation compound CaSi2. Besides, a number of new 2D silicene-based materials are synthesized employing CaSi2 as a precursor. However, the rather complex atomic structure of CaSi2 and fresh opportunities of physical and chemical breakthroughs drive the search for alternative compounds with silicene networks. Here, a new polymorph of SrSi2 is synthesized, enjoying both the structure of intercalated multilayer silicene and the simplest possible stacking of silicene sheets. The MBE-quality synthesis accomplished on Si(001) and Si(111) surfaces leads to epitaxial films of SrSi2 with orientation controlled by the substrate, as revealed by XRD, RHEED and electron microscopy studies. The structural SrSi2/Si relation is mirrored in the transport properties of the films.

First-principles modeling of 3d-transition-metal-atom adsorption on silicene: a linear-response DFT + U approach

By employing DFT  +  U calculations with the linear response method, we investigate the interactions between various 3d transition-metal atoms (Cr, Mn, Fe, Co) and silicene. In the cases of two-dimensional (2D) FeSi2 and CoSi2, the metal atoms tend to penetrate into the silicene layer. While CoSi2 is non-magnetic, FeSi2 exhibits a total magnetic moment of 2.21 μ(B)/cell. Upon the examination of 2D MSi6, a trend in anti-ferromagnetic (AFM) favorability in the z-direction is observed according to our DFT  +  U calculations. In the ferromagnetic (FM) states (less stable), each primary unit cell of CrSi6, MnSi6, and FeSi6 possesses different levels of total magnetization (4.01, 5.18, and 2.00 μ B/cell, respectively). The absolute magnetization given by AFM MSi6 structures varies in the range of 5.33-5.84 μ(B)/cell. A direct band gap in AFM MnSi6 (0.2 eV) is predicted, while the metastable FM FeSi6 structure has a wider band gap (0.85 eV). Interestingly, there are superexchange interactions between metal atoms in the MSi6 systems, which result in the AFM alignments.

The effects of nonmetal dopants on the electronic, optical, and catalytic performances of monolayer WSe2 by a first-principles study

Doping modifies the electronic, optical, and catalytic behavior of materials through the newly formed chemical bonds and the localized electrons.

Energetic stability, STM fingerprints and electronic transport properties of defects in graphene and silicene

Novel two-dimensional materials such as graphene and silicene have been heralded as possibly revolutionary in future nanoelectronics.

Silicene and graphene nano materials in gas sensing mechanism

Silicene, the Si analogue of graphene, has recently extended the short list of existing two-dimensional (2D) atomic crystals.

Indiene 2D monolayer: a new nanoelectronic material

Through first principles calculations, we systematically investigate the structural and electronic properties of indium monolayers in three different allotropic forms: planar, puckered and buckled.

Germanene: the germanium analogue of graphene

Recently, several research groups have reported the growth of germanene, a new member of the graphene family. Germanene is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled and composed of two vertically displaced sub-lattices. Density functional theory calculations have revealed that free-standing germanene is a 2D Dirac fermion system, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, which is the relativistic variant of the Schrödinger equation. Germanene is a very appealing 2D material. The spin-orbit gap in germanene (~24 meV) is much larger than in graphene (<0.05 meV), which makes germanene the ideal candidate to exhibit the quantum spin Hall effect at experimentally accessible temperatures. Additionally, the germanene lattice offers the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene based field-effect devices. In this topical review we will (1) address the various methods to synthesize germanene (2) provide a brief overview of the key results that have been obtained by density functional theory calculations and (3) discuss the potential of germanene for future applications as well for fundamentally oriented studies.

H-Si bonding-induced unusual electronic properties of silicene: a method to identify hydrogen concentration

Hydrogenated silicenes possess peculiar properties owing to the strong H-Si bonds, as revealed by an investigation using first principles calculations. Various charge distributions, bond lengths, energy bands, and densities of states strongly depend on different hydrogen configurations and concentrations. The competition between strong H-Si bonds and weak sp(3) hybridization dominate the electronic properties. Chair configurations belong to semiconductors, while the top configurations show a nearly dispersionless energy band at the Fermi level. Both the systems display H-related partially flat bands at middle energy and the recovery of low-lying π bands during the reduction of concentration. Their densities of states exhibit prominent peaks at middle energy, and the top systems have a delta-funtion-like peak at E = 0. The intensity of these peaks is gradually weakened as the concentration decreases, providing an effective method to identify the H-concentration in scanning tunneling spectroscopy experiments.

Covalent nitrophenyl diazonium functionalized silicene for spintronics: a first-principles study

We predict some novel electronic and magnetic properties of a functionalized silicene sheet by nitrophenyl diazonium (NPD) using first-principles calculations in the framework of density functional theory with dispersion corrections. Our calculations at the HSE06 level show that for the three coverage ratios of NPD considered in this work (i.e., NPD : Si = 1 : 8, 1 : 18 and 1 : 32), spin-polarized electronic structures can be always realized with NPD adsorption although the bandgap decreases upon reducing the NPD coverage ratio. The quasi-localized pz electrons of Si are identified to be responsible for the ferrimagnetism in these two-dimensional systems. Remarkably, the system with the NPD : Si = 1 : 8 ratio is predicted to be a bipolar magnetic semiconductor. As such, half-metallicity can be realized by applying a gate voltage with reversible spin polarization, making NPD-1/8 a potential candidate for future spintronic applications. This work offers a new tailor-made functionalization approach to realize magnetic semiconducting silicene.

Silicene nanomesh

Similar to graphene, zero band gap limits the application of silicene in nanoelectronics despite of its high carrier mobility. By using first-principles calculations, we reveal that a band gap is opened in silicene nanomesh (SNM) when the width W of the wall between the neighboring holes is even. The size of the band gap increases with the reduced W and has a simple relation with the ratio of the removed Si atom and the total Si atom numbers of silicene. Quantum transport simulation reveals that the sub-10 nm single-gated SNM field effect transistors show excellent performance at zero temperature but such a performance is greatly degraded at room temperature.

Density functional theory study on boron- and phosphorus-doped hydrogen-passivated silicene

When silicene is passivated by hydrogen, a bandgap occurs so that it becomes a semiconductor.

Molecular functionalization of silicene/Ag(111) by covalent bonds: a DFT study

Thanks to differential functional theory calculations, we show that a benzene molecule can be chemisorbed in the butterfly configuration on the (3 × 3) silicene/(4 × 4) Ag(111) surface by means of two Si–C covalent bonds.

Band gap opening in silicene on MgBr2(0001) induced by Li and Na

Silicene consists of a monolayer of Si atoms in a buckled honeycomb structure and is expected to be well compatible with the current Si-based technology. However, the band gap is strongly influenced by the substrate. In this context, the structural and electronic properties of silicene on MgBr2(0001) modified by Li and Na are investigated by first-principles calculations. Charge transfer from silicene (substrate) to substrate (silicene) is found for substitutional doping (intercalation). As compared to a band gap of 0.01 eV on the pristine substrate, strongly enhanced band gaps of 0.65 eV (substitutional doping) and 0.24 eV (intercalation) are achieved. The band gap increases with the dopant concentration.

The electronic structure and optical properties of Mn and B, C, N co-doped MoS2 monolayers

The electronic structure and optical properties of Mn and B, C, N co-doped molybdenum disulfide (MoS2) monolayers have been investigated through first-principles calculations. It is shown that the MoS2 monolayer reflects magnetism with a magnetic moment of 0.87 μB when co-doped with Mn-C. However, the systems co-doped with Mn-B and Mn-N atoms exhibit semiconducting behavior and their energy bandgaps are 1.03 and 0.81 eV, respectively. The bandgaps of the co-doped systems are smaller than those of the corresponding pristine forms, due to effective charge compensation between Mn and B (N) atoms. The optical properties of Mn-B (C, N) co-doped systems all reflect the redshift phenomenon. The absorption edge of the pure molybdenum disulfide monolayer is 0.8 eV, while the absorption edges of the Mn-B, Mn-C, and Mn-N co-doped systems become 0.45, 0.5, and 0 eV, respectively. As a potential material, MoS2 is widely used in many fields such as the production of optoelectronic devices, military devices, and civil devices.

Interaction of silicene with β-Si3N4(0001)/Si(111) substrate; energetics and electronic properties

The free-standing, quasi-2D layer of Si is known as silicene, in analogy with graphene. Much effort is devoted in the study of silicene, since, similarly to graphene, it shows a very high electron mobility. The interaction of silicene with a hybrid substrate, β-Si3N4(0001)/Si(111), exposing the β-Si3N4(0001) surface, has been studied by means of Density Functional calculations, with van der Waals interactions included. Once deepened the most important structural and electronic features of the hybrid substrate, we demonstrated that an electron transfer occurs from the substrate to the silicene layer. In turn, such an electron transfer can be modulated by the doping of the substrate. The β-Si3N4/silicene interaction appears to be strong enough to ensure adequate adsorption stability. It is also shown that electronic states of substrate and adsorbate still remain decoupled, paving the way for the exploitation of the peculiar electron mobility properties of the silicene layer. A detailed analysis in both direct and reciprocal space is reported.

Does the Dirac cone exist in silicene on metal substrates?

Absence of the Dirac cone due to a strong band hybridization is revealed to be a common feature for epitaxial silicene on metal substrates according to our first-principles calculations for silicene on Ir, Cu, Mg, Au, Pt, Al, and Ag substrates. The destroyed Dirac cone of silicene, however, can be effectively restored with linear or parabolic dispersion by intercalating alkali metal atoms between silicene and the metal substrates, offering an opportunity to study the intriguing properties of silicene without further transfer of silicene from the metal substrates.