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Two-Dimensional Si Nanosheets with Local Hexagonal Structure on a MoS2 Surface
Advanced Materials
Abstract
The structural and electronic properties of a Si nanosheet (NS) grown onto a MoS2 substrate by means of molecular beam epitaxy are assessed. Epitaxially grown Si is shown to adapt to the trigonal prismatic surface lattice of MoS2 by forming two-dimensional nanodomains. The Si layer structure is distinguished from the underlying MoS2 surface structure. The local electronic properties of the Si nanosheet are dictated by the atomistic arrangement of the layer and unlike the MoS2 hosting substrate they are qualified by a gap-less density of states.
... Free-standing silicene has been very difficult to synthesize and requires special techniques [20]. This has led to silicene's synthesis on substrates [21][22][23][24][25]. Initially, silicene was synthesized on metallic substrates [22,23,25]. ...
... This has led to silicene's synthesis on substrates [21][22][23][24][25]. Initially, silicene was synthesized on metallic substrates [22,23,25]. However, lately, the synthesis of silicene on semiconducting substrates has been explored theoretically and experimentally [21,[26][27][28][29]. Consequently, silicene has been successfully synthesized on substrates like MoS 2 , graphene, etc. [21,27]. ...
... This has led to silicene's synthesis on substrates [21][22][23][24][25]. Initially, silicene was synthesized on metallic substrates [22,23,25]. However, lately, the synthesis of silicene on semiconducting substrates has been explored theoretically and experimentally [21,[26][27][28][29]. Consequently, silicene has been successfully synthesized on substrates like MoS 2 , graphene, etc. [21,27]. Besides, its synthesis on different semiconducting substrates, like transition metal-dichalcogenides (TMDs), has opened the possibility of new silicene/semiconductor heterostructures [26]. ...
Two-dimensional (2D) materials have shown promising results for optoelectronic applications with ever-growing applications. The performance of existing III–V optoelectronic devices can be improved by formulating heterostructures between 2D materials and III–V materials. In this work, the first-principles calculation of silicene/GaAs heterostructure is presented. Interfacial effects are very critical for optoelectronic applications. Such effects have not been explored yet for silicene/GaAs heterostructure. Effects like the diffusion/adsorption of Ga and As atoms to/on silicene from the GaAs layer are investigated, and the effect of this diffusion/adsorption on the electronic properties of silicene is analyzed. An opening of the bandgap in silicene is observed under the diffusion of Ga/As to the silicene layer. A bandgap of 0.38 eV/0.44 eV is observed in Ga-doped/As-adsorbed silicene. Both the cases of doping and adsorption are considered in this study. The possibility of diffusion of the Ga atom is more than that of the As atom. Furthermore, carriers (electrons) flow from the GaAs layer to the silicene layer due to a built-in electric field at the interface. This built-in electric field can help separate the photogenerated carriers, which can eventually advance the performance of the GaAs optoelectronic devices.
... Unlike graphene, which can be obtained by mechanical exfoliating graphite, silicene is typically obtained by epitaxial growth on appropriate surfaces with tendency towards sp 3 hybridization. To date, its has been reported to be synthesized on several substrates, primarily metals, including Ag(111) [2], Ir(111) [11], Pb(111) [12], Au(110) [13], Au(111) [14], ZrB 2 [15], MoS 2 [16], and graphite [17]. The characterization of silicene is a complex task, not only due to the necessity of different analysis approach (including scanning tunneling microscopy, angleresolved photoemission spectroscopy, and first principles calculations) but also because the interpretation of these results can be greatly influenced by the substrate [18]. ...
... As a result, many experimental results are still under debate. For instance, the epitaxially strained layer formed on top of MoS 2 [16] has been disputed [20]. The alleged silicene formation on graphite [17] has also proven to be controversial [21], and the results obtained using Ir(111) [11] has been questioned as well [22]. ...
Despite the remarkable theoretical applications of silicene, its synthesis remains a complex task, with epitaxial growth being one of the main routes involving depositing evaporated Si atoms onto a suitable substrate. Additionally, the requirement for a substrate to maintain the silicene stability poses several difficulties in accurately determining the growth mechanisms and the resulting structures, leading to conflicting results in the literature. In this study, large-scale molecular dynamics simulations are performed to uncover the growth mechanisms and characteristics of epitaxially grown silicene sheets on Au(111) and Au(110) substrates, considering different temperatures and Si deposition rates. The growth process has been found to initiate with the nucleation of several independent islands homogeneously distributed on the substrate surface, which gradually merge to form a complete silicene sheet. The results consistently demonstrate the presence of a buckled silicene structure, although this characteristic is notably reduced when using an Au(111) substrate. Furthermore, the analysis also focuses on the quality and growth mode of the silicene sheets, considering the influence of temperature and deposition rate. The findings reveal a prevalence of the Frank-vander Merwe growth mode, along with diverse forms of defects throughout the sheets.
... [24][25][26][27][28][29] and ZrB 2 (0001) substrates. In addition to conductor substrates, including Ag(111) [24][25][26][27][28][29][30][31][32], ZrB 2 (0001) [33,34], Ir(111) [35], ZrC(111) [36], Ru(0001) [37], and graphite [38,39], silicene has formed on semiconductor surfaces, such as two-dimensional (2D) MoS 2 [40]. Although the interaction with the substrate has induced a variety of structural reconstructions, the theoretically predicted properties, including the linear Dirac cone, the buckled structure, and helical edge states, have been experimentally confirmed. ...
... It should be noted that a large staggered potential, defined by λ E = hE z (x, y), can be achieved through various means in experiments. One method is to employ a strong electric field E z , while another effective approach involves increasing the buckling distance h through halogen functionalization [71] or substrates [40], etc. ...
Topological insulators are promising candidates for dissipationless electronics and spintronics. For the design and application of small-sized topological transistors, it is vital to suppress the off-state leakage and keep utilizing the edge or surface states to carry the on-state current. Using the nonequilibrium Green’s function method, the transport properties of clean and disordered silicene were studied in the nanoscale regime. The results revealed the following. (1) The low-energy electron transport across the scattering region included two types of paths, namely, helical edge states and interstate tunnels. The choice of electrons for the two transmission paths was related to the length of the scattering region. When there was a band gap in the ky direction, electrons tended to tunnel between armchair-edge states along the x axis. It was only when the length met Nx=3n+1 that the electrons mainly propagated through the helical edge states. (2) The weak electric field could significantly enhance the wave-function overlap between armchair-edge states and can be used to switch electron-transport paths. (3) The origin of the leakage current in the nanoscale transistors was interstate quantum tunneling; this was promoted—instead of suppressed—by weak or strong disorders. (4) The effect of a strong electric field on the electron transport was opposite to that of the weak field. After reaching a critical value of λE>2λSO, the vertical electric field decreased the interstate tunneling probability and increasing the staggered potential was an effective method to suppress the off-state leakage.
... Silicene growth on MoS 2 has been claimed in 2014. 18 After deposition of 0.8 ML of Si at 473 K, 2D islands are visible in STM images. They display a honeycomb lattice with a lattice constant equal to the one of MoS 2 (0.316 Å). ...
... DFT calculations predicted that the corresponding buckling would be equal to 2 Å, which is in contradiction with the apparent honeycomb lattice seen in high-resolution STM images, that would indeed correspond to nearly planar silicene. 18 It was later suggested from STM and XPS measurements that the silicon layer was in fact intercalated between MoS 2 layers. 141 Moreover, DFT calculations showed that 2D silicon clusters intercalated between MoS 2 layers are stable. ...
Since the breakthrough of graphene, considerable efforts have been made to search for two-dimensional (2D) materials composed of other group 14 elements, in particular silicon and germanium, due to their valence electronic configuration similar to that of carbon and their widespread use in the semiconductor industry. Silicene, the silicon counterpart of graphene, has been particularly studied, both theoretically and experimentally. Theoretical studies were the first to predict a low-buckled honeycomb structure for free-standing silicene possessing most of the outstanding electronic properties of graphene. From an experimental point of view, as no layered structure analogous to graphite exists for silicon, the synthesis of silicene requires the development of alternative methods to exfoliation. Epitaxial growth of silicon on various substrates has been widely exploited in attempts to form 2D Si honeycomb structures. In this article, we provide a comprehensive state-of-the-art review focusing on the different epitaxial systems reported in the literature, some of which having generated controversy and long debates. In the search for the synthesis of 2D Si honeycomb structures, other 2D allotropes of Si have been discovered and will also be presented in this review. Finally, with a view to applications, we discuss the reactivity and air-stability of silicene as well as the strategy devised to decouple epitaxial silicene from the underlying surface and its transfer to a target substrate.
... The crystal structure of MoS 2 NFs comprises of strong interlayer covalent bonds between molybdenum (Mo) and sulfur (S) atoms in the trigonal prismatic, with one atomic plane of Mo sandwiched between two atomic planes of S [42]. These layers are connected to each other with weak Van der Waals forces [43]. ...
... To date, silicene has been successfully synthesized on the surfaces of Ag(111) [24,25], Ir(111) [26], Pb(111) [27], MoS 2 [28], ZrC [29], Ru [30], while germanene was fabricated on the surfaces of Au(111) [31], Pt(111) [32], Al(111) [33], MoS 2 [34] and h-AlN [35]. However, all of the listed substrates are very expensive. ...
Two-dimensional silicon (silicene) and germanium (germanene) have attracted special attention from researchers in recent years. At the same time, highly oriented pyrolytic graphite (HOPG) and graphene are some of the promising substrates for growing silicene and germanene. However, to date, the processes occurring during the epitaxial growth of silicon and germanium on the surface of such substrates have been poorly studied. In this work, the epitaxial growth of silicon and germanium is studied directly during the process of the molecular beam epitaxy deposition of material onto the HOPG surface by reflection high-energy electron diffraction (RHEED). In addition, the obtained samples are studied by Raman spectroscopy and scanning electron microscopy. A wide range of deposition temperatures from 100 to 800 °C is considered and temperature intervals are determined for various growth modes of silicon and germanium on HOPG. Conditions for amorphous and polycrystalline growth are distinguished. Diffraction spots corresponding to the lattice constants of silicene and germanene are identified that may indicate the presence of areas of graphene-like 2D phases during epitaxial deposition of silicon and germanium onto the surface of highly oriented pyrolytic graphite.
... Finally, we briefly discuss the experimental feasibility of our proposed hybrid structure. Silicene sheet has been successfully synthesized on various substrates, including Ag(111) [55], ZrB 2 (0001) [56], Ir(111) [57], and MoS 2 [58]. The typical spin-orbit energy in silicene has been experimentally estimated as λ so ∼ 4 meV and the buckling parameter is approximately 2l ∼ 0.46 Å [44]. ...
We theoretically study the valley-polarized subgap transport and intravalley pairing states in silicene-based antiferromagnet/superconductor (AF/SC) junctions. It is found that in the absence of an electric field, the antiferromagnetic order induced in silicene can give rise to valley-polarized states that strongly affect the subgap conductance. With the increasing antiferromagnetic exchange field, the gap-edge Andreev-resonant peak is replaced by broadened features for the Homo-SC model whereas by a sharp conductance dip for the Bulk-SC one. This significant difference arises from the intravalley Andreev reflection caused by the valley-mixing scattering in the Bulk-SC model, which can be enhanced by the antiferromagnetic order. Particularly, this intravalley pairing process can be switched on or off by adjusting the spin polarization through the electric field applied in the AF region. Our findings not only pave a new road to employ antiferromagnetic materials in valleytronics, but also facilitate the verification and detection of potential intravalley pairing state and valley polarization in silicene.
... substrates [2][3][4]. Transition metal dichalcogenides (TMDs) are another 2D family with a graphene-like hexagonal framework and have been realized experimentally [5,6]. However, these TMDs are layered materials. ...
Valleytronics has emerged as an interesting field of research in two-dimensional (2D) systems and uses the valley index or valley pseudospin to encode information. Spin-orbit coupling (SOC) and inversion symmetry breaking leads to spin-splitting of bands near the valleys. This property has promising device applications.
In order to find a new 2D material useful for valleytronics, we have designed hexagonal planar monolayers of cadmium chalcogenides (CdX, X = S, Se, Te) from the (111) surface of bulk CdX zinc blende structure. The structural, dynamic, mechanical and thermal stability of these structures is confirmed. Band structure study reveals valence band local maxima (valleys) at K and K′ symmetry points. Application of SOC initiates spin-splitting in the valleys that lifts the energy degeneracy and shows strong valley-spin coupling character. To initiate stronger SOC, we have substituted two Cd atoms in the almost planar monolayers by Sn atoms which increases the spin-splitting significantly. Zeeman-type spin-splitting is observed in the valley region and Rashba spin-splitting is observed at the Γ point for Sn-doped CdSe and CdTe monolayers. Berry curvature values are more in all the Sn-doped monolayers than the pristine monolayers. These newly designed monolayers are thus found to be suitable for valleytronics applications. Sn-doped monolayers show band inversion deep in the valence and conduction bands between Sn s and p and X p states but lack topological properties.
... As well as having semiconductor properties, MoS 2 also has good thermal and chemical stability and has good properties as a diamagnetic compound. MoS 2 consists of two layers that can be characterized as typical hexagonal crystal layers in two dimensions [11]. A sandwich plate of S-Mo-S atoms with an average spacing of 0.316 nm is formed by layering two S atoms with one Mo atom. ...
The unique physicochemical properties of MoS2 nanocomposites have drawn escalation in attention for the diagnosis and therapy of cancer. Mostly the 2D forms of MoS2 find application in sensing, catalysis, and theranostics, where it was traditionally applied in lubrication and battery industries as electrodes or intercalating agents. As nanostructures, MoS2 has a very high surface-to-volume ratio, and that helps in the engineering of structures and surfaces to promote absorption of a wide range of therapeutics and biomolecules through covalent or non-covalent interaction. This surface engineering provides excellent colloidal stability to MoS2 and makes them ideal nanomedicines with higher selectivity, sensitivity, and biomarker sensing ability. Furthermore, MoS2 exhibits exceptionally well optical absorption of NIR radiation and photothermal conversion, which helps in the NIR-responsive release of payloads in photothermal and photodynamic therapy. There are several reports that the fabricated MoS2 nanomedicines can selectively counter the tumor microenvironment, which leads to the accumulation of therapeutics or imaging agents in the diseased tissues to improve the therapeutic effects decreasing the adverse effects on the healthy cells. An overview of the basic structure and properties of MoS2 is presented in this article, along with an elaborative description of its morphology. At the same time, an attempt was made in this review to summarize the latest developments in the MoS2 structure, surface engineering, and nanocomposite formulations for improving biocompatibility, bioavailability, biomolecular sensing, and theranostic applications.
... In addition, silicene was grown on ZrB2 thin films located on Si wafers [31], as well as on ZrC [32]. In addition, silicene was obtained by molecular beam epitaxy on bulk MoS2 [33]. A silicon monolayer can be grown on highly oriented pyrolytic graphite at room temperature [34]. ...
Lithium-ion batteries (LIBs) are the main energy storage devices that have found wide application in the electrical, electronics, automotive and even aerospace industries. In practical applications, silicene has been put forward as an active anode material for LIBs. This is facilitated by its high theoretical capacitance, strength, and small volume change during lithiation. Thin-film materials containing two-layer silicene and intended for use in the LIB anode have been studied by the method of classical molecular dynamics. Among the important characteristics obtained is the fillability of the silicene anode (under the influence of an electric field), which was determined depending on the type of vacancy defects in silicene and the type of substrate used. Both metallic (Ag, Ni, Cu, Al) and non-metallic (graphite, silicon carbide) substrates are considered. The behavior of the self-diffusion coefficient of intercalated lithium atoms in a silicene channel as it is filled has been studied. Based on the construction of Voronoi polyhedra, the packing of lithium atoms and the state of the walls in the channel has been studied in detail. The change in the shape of silicene sheets, as well as the stresses in them caused by lithium intercalation, are analyzed. It has been established that two-layer silicene with monovacancies on a nickel substrate is the most optimum variant of the anode material. The results of this work may be useful in the development of new anode materials for new generation LIBs.
... In addition, the azimuthal scan allows for the visualization of the orientation of monolayer MoS2 with respect to the supporting surface's lattice. The streaks together form a hexagonal structure, and since a hexagonal structure in reciprocal space is also a hexagonal structure in real space 20 , we conclude that monolayer MoS2 is hexagonal. Overall, the images support the crystalline nature of the monolayer and its good azimuthal relation with the supporting substrate. ...
Reflection high-energy electron diffraction (RHEED) and ultrafast electron diffraction (UED) are techniques used to characterize crystal structures both statically and dynamically. These experimental methods are of academic interest due to their ability to visualize crystal structures on the atomic level and analyze dynamic changes on the picosecond scale. In this experiment, RHEED and UED are implemented to analyze monolayer molybdenum disulfide (MoS2), a compound that may contribute to the future of microelectronics. Images of various diffraction patterns are presented, and analysis is conducted on diffraction peaks, lattice spacing, and photoinduced intensity changes.
... Since the successful exfoliation of graphene in 2004, two-dimensional layered materials (2DLMs) have been intensively investigated due to their fascinating physical properties [1][2][3][4][5][6][7], such as high carrier mobility, tunable band gap, outstanding photoelectric characteristics, and thermal stability [8][9][10][11][12][13][14]. It is expected to become one of the next-generation optoelectronic materials of the most prominent families of 2DLMs: hexagonal boron nitride (h-BN) [15], transition metal dichalcogenides (TMDCs) [16], and post-transition metal chalcogenides (PTMCs) [17][18][19][20]. ...
Two-dimensional layered materials (2DLMs) have attracted growing attention in optoelectronic devices due to their intriguing anisotropic physical properties. Different members of 2DLMs exhibit unique anisotropic electrical , optical, and thermal properties, fundamentally related to their crystal structure. Among them, directional heat transfer plays a vital role in the thermal management of electronic devices. Here, we use density functional theory calculations to investigate the thermal transport properties of representative layered materials: β-InSe, γ-InSe, MoS 2 , and h-BN. We found that the lattice thermal conductivities of β-InSe, γ-InSe, MoS 2 , and h-BN display diverse anisotropic behaviors with anisotropy ratios of 10.4, 9.4, 64.9, and 107.7, respectively. The analysis of the phonon modes further indicates that the phonon group velocity is responsible for the anisotropy of thermal transport. Furthermore, the low lattice thermal conductivity of the layered InSe mainly comes from low phonon group velocity and atomic masses. Our findings provide a fundamental physical understanding of the anisotropic thermal transport in layered materials. We hope this study could inspire the advancement of 2DLMs thermal management applications in next-generation integrated electronic and optoelectronic devices.
... All these 2D materials have been epitaxially grown on various substrates, including metal and ceramic materials. So far, silicene has been successfully deposited on the surfaces of Ag, Ir, Ru, MoS 2 , ZrC, and ZrB 2 by epitaxy; [20][21][22][23][24][25] while Au, Pt, Al, Cu, and AlN have been employed as templates for the epitaxial growth of germanene. [26][27][28][29][30] Furthermore, the stanene on the Bi 2 Te 3 substrate was accomplished by Zhu et al. using MBE. 31 It is worth noting that Zhuang et al. epitaxially grew the germanene on a Ge (111) thin film predeposited on an Ag substrate. ...
The group IV elemental materials, especially Si and Ge, have been conventionally used as the anodes for lithium-ion batteries. As the research field of two-dimensional (2D) materials was pioneered, the...
... BTDMs are atomically thin crystals possessing hexagonal lattice structures and Dirac-like low-energy excitations, commonly known as silicene, germanene and stanene [28][29][30][31][32][33][34][35][36][37][38][39][40]. Since a stable BTDM sheet prefers a buckled sublattice structure, the low-energy bands and relevant transport properties can be effectively modulated by an electric field perpendicular to the sheet plane [39][40][41][42][43][44][45][46]. ...
We investigate the thermal transport properties in superconductor-antiferromagnet-superconductor and superconductor-ferromagnet-superconductor junctions based on buckled two-dimensional materials (BTDMs). Owing to the unique buckled sublattice structures of BTDMs, in both junctions the phase dependence of the thermal conductance can be effectively controlled by perpendicular electric fields. The underlying mechanism for the electrical tunability of thermal conductance is elucidated resorting to the band structures of the magnetic regions. We also reveal the distinct manifestations of antiferromagnetic and ferromagnetic exchange fields in the thermal conductance. These results demonstrate that the perpendicular electric field can serve as a knob to externally manipulate the phase-coherent thermal transport in BTDMs-based Josephson junctions.
... As is known, silicene and its bulk counterpart do not exist in nature and can be obtained by high vacuum epitaxial deposition on substrates. So far, silicene has been synthesized on several substrates such as Ag (110) [13,14], Ag(111) [15-17], Au(110) [18], ZrB 2 -covered Si(111) [19], Ir (111) [20], and MoS 2 [21]. Thus, silicene is not some kind of virtual material. ...
Development of high-performance lithium-ion batteries (LIBs) is boosted by the needs of the modern automotive industry and the wide expansion of all kinds of electronic devices. First of all, improvements should be associated with an increase in the specific capacity and charging rate as well as the cyclic stability of electrode materials. The complexity of experimental anode material selection is now the main limiting factor in improving LIB performance. Computer selection of anode materials based on first-principles and classical molecular dynamics modeling can be considered as the main paths to success. However, even combined anodes cannot always provide high LIB characteristics and it is necessary to resort to their alloying. Transmutation neutron doping (NTD) is the most appropriate way to improve the properties of thin film silicon anodes. In this review, the effectiveness of the NTD procedure for silicene/graphite (nickel) anodes is shown. With moderate P doping (up to 6%), the increase in the capacity of a silicene channel on a Ni substrate can be 15–20%, while maintaining the safety margin of silicene during cycling. This review can serve as a starting point for meaningful selection and optimization of the performance of anode materials.
... Experimentally, following the successes in the fabrication of graphene [3], a monolayer of carbon atoms packed closely in a hexagonal comblike lattice, much effort has been also paid to the synthesis of the silicon analog of graphene, namely silicene [4]. In recent years, several groups have demonstrated the fabrication of monolayer and multilayer silicene [5][6][7][8][9][10][11][12] onto the metallic and semiconducting substrates, establishing the solid bases for the development of silicene-based electronic devices. ...
We have theoretically investigated the spin-valley asymmetric transport of massive Dirac fermions in the field-controllable bilayer silicene superlattices. The spin-valley dependent ballistic transmission, conductance, and polarization have been systematically calculated by formulating the scattering matrix method for the completed four-band low-energy effective Hamiltonian. Our results uncover that for a single valley transport, near-perfect spin polarization and its perfect switching could be efficiently modulated by the gate field engineering. Under the one-dimensional periodic field modulation, two types of flat bands with less dispersion and, importantly, the perfect contrast in the spin-dependent subbands are observed for the bilayer silicene superlattice. Together with its larger spin-orbit coupling and better stability, these spin-valley asymmetric characteristics engineered by the gate field indicate that the field-controllable bilayer silicene could be a potential component candidate to achieve a fully spin-valley polarized beam for quantum logic applications.
... Thus, the growth of silicene on the semiconducting substrates is expected for its accurate identification, which also facilitates the real application of silicene in nanoelectronic devices. In fact, the 2D epitaxy of highly buckled Si nanosheet on semiconducting MoS 2 substrate with local hexagonal configuration has been reported in 2014, in which the step profile between the MoS 2 and Si domain is 3Å [38], suggesting the weak interaction between them. In 2017, another experiment reveals deposited Si atoms do not reside on the MoS 2 surface, but rather intercalate between the MoS 2 layers at room temperature and low deposition rates [39]. ...
Silicene-based van der Waals (vdWs) heterstructures are expected to design novel nanoelectronic devices due to their intriguing properties. Here, we construct novel silicene/Janus Ga2STe heterobilayers by vertically stacking silicene and Janus Ga2STe monolayer. Employing first-principles calculations, their interfacial electronic properties, Schottky barriers, and contact types are investigated systematically. The silicene/Janus Ga2STe heterobilayers are verified to be favorable energetically and stable dynamically. We also find that the graphene-like Dirac cone is well preserved in the silicene/Janus Ga2STe heterobilayer, suggesting a high carrier mobility. Depending on the stacking orders, an n-type or a p-type Schottky contact can be acquired at the silicene/Janus Ga2STe interface. More importantly, vertical strain and electric field can effectively tune the interfacial electronic properties and contact type in the silicene/Janus Ga2STe heterobilayer. These findings can provide a useful guidance for designing controllable Schottky nanoelectronic devices based on silicene/Janus Ga2STe heterobilayers with high electronic performance.
... The reason is probably related to the difficulty of directly synthetizing silicene on gapped semiconductor surfaces. Typically, MoS 2 has been used as a semiconducting template for silicene, but up to now only small strained patches have been obtained [80], possibly an encouraging result, but which has been disputed [81]. We suggest a new route, which could be promising. ...
In the realm of two-dimensional (2D) materials, besides the ones initially peeled from lamellar crystals, the artificial emerging elemental ones, called Xenes, appear as strong contenders to graphene in a booming new field. The very first synthetic Xene was silicene, created in 2012. On the occasion of its tenth anniversary, this concise review, describes the birth of silicene, in situ, under ultra-high vacuum, and surveys its most tantalizing properties: its Dirac features, its 2D topological insulator character, its easy functionalization, its insertion as atom-thin channel in Field Effect Transistors operating at room temperature. Silicene has striking variants and amazing doubles in the quantum world; these lookalikes are briefly described and their origins discussed. We owe to silicene the legacy of all its descendants from borophene to tellurene, and fascinating prospects for spintronics, the emergence and control of Majorana fermions, possibly for quantum computing.
... [10][11][12][13][14] Following Ag(111), silicene has been successfully synthesized on various substrates. [15][16][17][18][19][20][21] In addition to the novel characteristics of silicene, the growth process of silicene on Ag(111) has a crucial issue. A unique structural transition occurs during silicene growth on Ag(111). ...
We demonstrate the novel growth of silicene grown on Ag(111) using STM and reveal the mechanism with KMC simulation. Our STM study shows that after the complete formation of the first layer of silicene, it is transformed into bulk Si with the reappearance of the bare Ag surface. This dewetting (DW) during the epitaxial growth is an exception in the conventional growth behavior. Our KMC simulation reproduces DW by taking into account the differences in the activation energies of Si atoms on Ag, silicene, and bulk Si. The growth modes change depending on the activation energy of the diffusion, temperature, and deposition rate, highlighting the importance of kinetics in growing metastable 2D materials.
... [78] Additionally, MoS 2 has been shown to interact with Au via strong vdW forces, through changes in its surface density of electronic states [12,79]. Differently, the density of electronic states of MoS 2 was shown not to vary when in MoS 2 -Si structure, indicating no hybridization [80], while weak hybridization is expected at MoS 2 -Au [81]. The adhesive interaction is influenced by the polarizabilities of the interacting substances, which in turn is proportional to their London dispersive forces. ...
Molybdenum disulfide (MoS2) is a two-dimensional material that exhibits unique interfacial interactions with gold (Au) and silicon (Si). These interfaces have gained attention due to their potential to be integrated into a broad range of applications, where they involve direct contact that affects their performance. Here we investigate the nanoscale mechanical contact interaction of MoS2 monolayers with Au and Si, using adhesion measurements and friction force microscopy. MoS2-Au contact exhibits pronounced adhesive interaction, manifested by higher adhesion pull-off forces and energies, compared to MoS2-Si. Influenced by the adhesive contacts, the friction forces recorded demonstrate stronger lateral interactions and stronger interfacial shear strength between MoS2 and Au. Analysis with the Prandtl–Tomlinson model reveals that the MoS2-Au direct interaction is also stiffer, suggesting higher elastic energy at the contact. This information on the mechanistic nature of MoS2-Au and MoS2-Si contacts under shear and normal loads is of potential importance in applications where such interfaces are utilized, such as in the design of coatings, sensors and mechanoelectrical devices.
... A huge effort has been reported to synthetize quasi-free-standing silicene on several metal [27][28][29][30] and inert (i.e. MoS 2 , [31,32], graphite, [33][34][35], epi-Gr [36,37]) substrates, giving rise to debates on the real possibility to obtain an air-stable, large area extended silicene layer on such substrates. Indeed, large lattice parameter differences favor Si sp 3 cluster formation [33,[35][36][37][38] and a number of Si atom superstructures [27,30,39] while the character of outermost substrate atoms dictates the potential interaction with Si [40,41]. ...
In the last years, epitaxial graphene (epi-Gr) demonstrated to be an excellent substrate for the synthesis of epitaxial or intercalated two dimensional (2D) materials. Among 2D materials, silicene has been for a long time a dream for the scientific community, for its importance both from fundamental and application point of view. Despite the theoretical prediction of silicene energetic viability, experimentally substrate proved to play a fundamental role in the Si atom absorption process leading, in case of metal substrates, to a mixed phase formation and, for van der Waals chemical inert substrates, to Si atom intercalation even at room temperature. Such an intercalation has been associated to the presence of surface defects. Very recently it has been shown that hundreds of nanometer area quasi-free standing silicene can be grown on top an almost ideal epi-Gr layer synthesized on 6H-SiC substrate. In the present paper, using scanning tunneling microscopy and Raman analysis, we demonstrate that a non-ideal (slightly defected) epi-Gr network obtained by thermal decomposition of Si-terminated 4H-SiC(0001) enables the Si atoms penetration forming intercalated silicene nanosheets at room temperature, thus opening a path toward controlling intercalated silicene nanosheet formation through pristine graphene defect concentration managing and silicene application in nanotechnology.
... The reason which attracted the scientists to search for possible alternatives of graphene was mainly owing to the zero bandgap of graphene when it was used in in electronic and optoelectronic devices [12]. Silicene, a two-dimensional (2D) hexagonal lattice of silicon (Si) atoms, which is equivalent to graphene [13] with higher atomic bond length because of its larger atoms as compared to carbon atoms [14][15][16], has been extensively studied by the electronic industry [17][18][19][20][21]. It has also been synthesized on substrates, such as Ag(110) [22][23][24], Ag(100) [9], Ag(111) [25], Au(110) [26], MoS 2 [27] and CaSi 2 [28]. The advantages of silicene over graphene, namely, better tunable band gap, stronger spin-orbit coupling and easier valley polarization [29][30][31][32][33] made it a perfect candidate to be used in different applications, such as hydrogen storage [34], super capacitors [35], ion batteries [36][37][38][39][40] and spintronics [41,42]. ...
In this paper, the elastic and plastic properties of 2×2 and 3×3 pristine and transition metal (TM) doped silicene nanosheets are studied using the density functional theory (DFT) calculations. Cr, Co, Cu, Mn, Ti, V, Zn and Ni atoms are selected as doping atoms. It is observed that Young’s and bulk moduli of both 2×2 and 3×3 pristine structures decrease when they are affected by the doping atoms. The highest reduction in the Young’s and bulk moduli of the 2×2 nanosheets occurs for the Ni-doped structure, and the same reduction is observed for the Ni- and Cu-doped structures in the 3×3 nanosheets. In addition, it is shown that all of the investigated structures have an isotropic behavior, since their Young’s moduli have negligible difference along armchair and zigzag directions. Finally, the loading is further increased to investigate the plastic behavior of nanostructures. The results show that the yield strains of all doped nanosheets decrease under uniaxial and biaxial loadings. The highest reduction in the yield strain of the 2×2 nanosheets under biaxial loading is observed for Cu, Zn- and Co-doped nanosheets, and in 3×3 nanosheets, the highest reduction happens for the Cu- and Zn-doped nanosheets under the same condition. For the yield strain of the 2×2 doped nanosheets under the uniaxial loading, the Cu-doped structure experiences the highest reduction, and the highest reduction for the Mn-doped nanosheet under the same condition is observed in 3×3 nanosheets. The findings revealed that how electronic configuration of transition metal atom and its electronegativity difference with silicon atom can control the structural and mechanical properties of the nanosheet.
... After that, silicene was synthesized on Ir(111) [24], Pb(111) [48], ZrB 2 (0001) [26], MoS 2 [27], ZrC [28], Ru [29], and graphite [30] surfaces. For some substrates, to obtain uniform monolayer with honeycomb lattice, post-growth annealing of the synthesized structures was implemented. ...
Today, two-dimensional materials are one of the key research topics for scientists around the world. Interest in 2D materials is not surprising because, thanks to their remarkable mechanical, thermal, electrical, magnetic, and optical properties, they promise to revolutionize electronics. The unique properties of graphene-like 2D materials give them the potential to create completely new types of devices for functional electronics, nanophotonics, and quantum technologies. This paper considers epitaxially grown two-dimensional allotropic modifications of single elements: graphene (C) and its analogs (transgraphenes) borophene (B), aluminene (Al), gallenene (Ga), indiene (In), thallene (Tl), silicene (Si), germanene (Ge), stanene (Sn), plumbene (Pb), phosphorene (P), arsenene (As), antimonene (Sb), bismuthene (Bi), selenene (Se), and tellurene (Te). The emphasis is put on their structural parameters and technological modes in the method of molecular beam epitaxy, which ensure the production of high-quality defect-free single-element two-dimensional structures of a large area for promising device applications.
We investigate the effects of the circularly polarized light (CPL) and the electric field (EF) on the nonlocal transport in a silicene-based antiferromagnet/superconductor/ferromagnet (AF/S/F) asymmetrical junction. For case I (II), the CPL and the EF are applied simultaneously in the antiferromagnetic (ferromagnetic) region, whereas in the ferromagnetic (antiferromagnetic) region, only a constant EF is considered. The spin-valley-resolved conductance can be turned on or off by adjusting the CPL or the EF. The AF/S/F junction can be manipulated as a spin-locked valley filter for case I, while for case II, it can be used not only as a valley-locked spin filter but also as a nonlocal switch between two pure nonlocal processes. Such interesting nonlocal switch effect can be effectively controlled by reversing the direction of the incident energy axis, the handedness of the CPL, or the direction of the EF. These findings may open an avenue to the design and manufacture of the spintronic and valleytronic devices based on the asymmetrical silicene magnetic superconducting heterostructure.
In this study, MoS2/NiFe2O4/MIL-101(Fe) nanocomposite was synthesized by hydrothermal method and used as an adsorbent for the elimination of organic dyes and some antibiotic drugs in aqueous solutions. The synthesized nanocomposite underwent characterization through different techniques, including scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer–Emmett–Teller (BET) surface area analysis, Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), zeta potential analysis, vibrating sample magnetometry (VSM), and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). These results demonstrated the successful insertion of MoS2within the cavities of MIL-101(Fe). The as-prepared magnetic nanocomposite was used as a new magnetic adsorbent for removing methylene blue (MB) and rhodamine B (RhB) organic dyes and tetracycline (TC) and ciprofloxacin (CIP) antibiotic drugs. For achieving the optimized conditions, the effects of initial pH, initial dye and drug concentration, temperature, and adsorbent dose on MB, TC, and CIP elimination were investigated. The results revealed that at a temperature of 25 °C, the highest adsorption capacities of MoS2/NiFe2O4/MIL-101(Fe) for MB, TC, and CIP were determined to be 999.1, 2991.3, and 1994.2 mg g⁻¹, respectively. The pseudo-second-order model and Freundlich model are considered suitable for explaining the adsorption behavior of the MoS2/NiFe2O4/MIL-101(Fe) nanocomposite. The magnetic nanocomposite was very stable and had good recycling capability without any change in its structure.
Two-dimensional (2D) molybdenum disulphide (MoS 2 ) stands out with its unique tunable bandgap and optoelectronic properties, making it a prime focus in transition metal dichalcogenides (TMDs) research. It has wide-ranging applications in energy storage, electronics, optoelectronics and high-performance sensing materials. Synthesis methods fall into top-down (chemical, mechanical and liquid-phase exfoliation) and bottom-up (physical vapour deposition, chemical layer deposition, atomic layer deposition and solvothermal/hydrothermal) categories. Choosing the right synthesis method is pivotal as it significantly impacts the material's properties and application potential. 2D MoS 2 , owing to its natural abundance, adaptability, adjustable bandgap and high surface-to-volume ratio, finds utility in various domains like energy storage, catalysis, composite and sensors. This review delves into recent progress, challenges and future prospects in 2D MoS 2 synthesis and applications.
We investigate the thermal transport properties in superconductor-antiferromagnet-superconductor and superconductor-ferromagnet-superconductor junctions based on buckled two-dimensional materials (BTDMs). Owing to the unique buckled sublattice structures of BTDMs, in both junctions the phase dependence of the thermal conductance can be effectively controlled by perpendicular electric fields. The underlying mechanism for the electrical tunability of thermal conductance is elucidated resorting to the band structures of the magnetic regions. We also reveal the distinct manifestations of antiferromagnetic and ferromagnetic exchange fields in the thermal conductance. These results demonstrate that the perpendicular electric field can serve as a knob to externally manipulate the phase-coherent thermal transport in BTDMs-based Josephson junctions.
Two-dimensional (2D) nanomaterials have emerged as a new class of materials with unique properties and potential applications in various fields, such as electronics, energy, and medicine. This chapter provides precisely an overview of the synthesis, properties, and applications of 2D nanomaterials, including graphene, transition metal dichalcogenides, and black phosphorus. We begin by discussing the each of these
2D materials, their methods to synthesize, such as mechanical exfoliation, chemical vapor deposition, and liquid-phase exfoliation etc. Next, we describe their remarkable properties, such as high electrical conductivity, large surface area, and tunable bandgap that make them suitable for diverse applications. We then explore their various applications, including in flexible electronics, energy storage and conversion, sensing, and biomedicine inside each material description. Moreover, we highlight some of the challenges and limitations that need to be addressed for their commercialization and large-scale production. Finally, the chapter concludes with a summary of the current state of research and suggests possible directions for future work in this exciting field.Keywords2D materialsMXenesGrapheneMetal–organic frameworkNanosheetsNanoparticles
All two-dimensional (2D) materials of group IV elements from Si to Pb are stabilized by carrier doping and interface bonding from substrates except graphene which can be free-standing. The involvement of strong hybrid of bonds, adsorption of exotic atomic species, and the high concentration of crystalline defects are often unavoidable, complicating the measurement of the intrinsic properties. In this work, we report the discovery of seven kinds of hitherto unreported bulk compounds (RO)nPb (R = rare earth metals, n = 1,2), which consist of quasi-2D Pb square nets that are spatially and electronically detached from the [RO]δ+ blocking layers. The band structures of these compounds near Fermi levels are relatively clean and dominantly contributed by Pb, resembling the electron-doped free-standing Pb monolayer. The R2O2Pb compounds are metallic at ambient pressure and become superconductors under high pressures with much enhanced critical fields. In particular, Gd2O2Pb (9.1 μB/Gd) exhibits an interesting bulk response of lattice distortion in conjunction with the emergence of superconductivity and magnetic anomalies at a critical pressure of 10 GPa. Our findings reveal the unexpected facets of 2D Pb sheets that are considerably different from their bulk counterparts and provide an alternative route for exploring 2D properties in bulk materials.
L'objectif ultime de ce travail est l'élaboration puis la caractérisation de nouveaux matériaux utilisés pour réaliser des nanopads destinés à l'électronique moléculaire. Ces nanopads doivent répondre à des critères stricts pour stabiliser, sans déformation et avec une précision atomique, la molécule dans la jonction nanopad-surface-nanopad. En combinant la microscopie à force atomique sans contact (nc-AFM) et la microscopie à sonde Kelvin (KPFM) dans un environnement ultravide (UHV), nous avons mesuré la hauteur et le travail de sortie d’une monocouche de graphène sur la face Si du 6H-SiC (0001). Ces mesures nous ont permis d’identifier trois structures de graphène : La ZLG (couche tampon), la QFMLG et la BLG. Contrairement aux autres méthodes de spectroscopie, le nc-AFM couplé à une sonde KPFM nous a permis ensuite d'identifier directement des nano-îlots de graphène aux premiers stades de nucléation, élaborée par dépôt chimique en phase vapeur (CVD). Le système graphène/SiC/graphène est prometteur pour la réalisation des jonctions en géométrie planaire. Dans la deuxième partie de cette thèse nous avons exploré la croissance de nanopads en silicène dans une chambre d’épitaxie par jets moléculaires (EJM). Nous avons démontré que la croissance du silicium sur graphène n'est pas bidimensionnelle et conduit à la formation des amas 3D ayant des branches fractales. Enfin, nous avons déposé par EJM des nano-îlots d'or sur un film mince d'AlN. Ce système est très prometteur et répond à un grand nombre de critères pour réaliser un dispositif à une seule molécule. Les nano-îlots d'or sont d'épaisseur monoatomique et peuvent être chargés électriquement avec la pointe AFM de manière contrôlée. Il reste alors, à titre de perspective, à synthétiser des nano-rubans moléculaires de graphène sur cette surface pour préparer la mesure de leur conductance à plat.
Silicon (Si) is undoubtedly an excellent material for electronic applications. However, Si is not a suitable material for optoelectronic applications due to its indirect bandgap. Based on the first principles approach, we have studied different stacking (AA and AB) patterns in bilayer silicene. The phonon band structure of AA and AB-stacked “bilayer silicene” reveals that only AB-stacked silicene is stable. Different fluorinated configurations (1F, 2F, 3F and 4F) of AB-stacked bilayer silicene are analyzed in terms of structural parameters and the phonon band structure. Of the four fluorinated configurations, 2F-AB-stacked bilayer silicene has a direct bandgap of 1.23 eV. The direct bandgap allows the carriers to transit from the “valence band” to the “conduction band” with a low probability of phonon generation. Meanwhile, the other configurations either lack stability or have a zero bandgap. The proposed fluorinated bilayer silicene, in addition to having an expected integration with silicon technology, could also be suitable for visible and near-infrared light detection applications.
Two kinds of bronze-graphite-MoS2 self-lubricating materials with copper-coated MoS2 and uncoated MoS2 were prepared by powder metallurgy. Friction and wear experiments were carried out under 4 N and 10 N loads respectively, and the effects of copper-coated MoS2 on the friction performances of the materials were studied. Results showed that the way of copper-coated on the surface of MoS2 could reinforce the bonding between MoS2 and matrix, and inhibited the formation of MoO2. Moreover, both materials formed a MoS2 lubricating film on the surface during the friction process. While the lubricating film formed after copper coating on MoS2 was thicker and had uneven morphology, it was more conducive to improving the friction performance of the material. Compared with conventional materials, the wear rate of copper-coated materials was reduced by one order of magnitude, and the friction coefficient was also reduced by 22.44% and 22.53%, respectively, when sliding under 4 N and 10 N loads. It shows that copper-coated MoS2 can improve friction properties of bronze-graphite-MoS2 self-lubricating materials furtherly.
The successful discovery of graphene has greatly stimulated researchers' enthusiasm for two‐dimensional materials. The research on two‐dimensional materials has shown incredible development in the past 10 years. The two‐dimensional material family has a wide range of composition options, including almost all elements in the periodic table. Such a rich composition makes two‐dimensional materials have a rich electronic structure, such as metals, semi‐metals, insulators, and semiconductors with direct and indirect bandgap in the full spectrum. Therefore, two‐dimensional materials have great potential for the development of new electronic, optoelectronic devices, and novel condensed matter physical phenomena in the future. In this chapter, we classify most of the two‐dimensional materials and make a brief overview.
The electronic structure of a material is the key factor to determine its electronic, magnetic, and optical properties. As an advanced technique to directly observe the electronic structure, angle-resolved photoemission spectroscopy (ARPES) has been widely used for studying the fundamental physical and chemical properties of a material. In recent years, enormous two-dimensional (2D) materials with outstanding performances have been continually discovered, promising for future applications in optoelectronic, electronic, and spintronic devices. In this review article, we briefly introduce the basic components and principles of an ARPES system, and review the frontier progress of the ARPES studies on the electronic structures and fundamental physical properties of 2D materials. The 2D materials reviewed in this article can be categorized in four parts: graphene, h-BN, single element 2D materials, transition metal dichalcogenides (TMDCs). Among them, the ARPES results on graphene are the most fruitful, and have stimulated the expanding researches on the other 2D materials. In present, the studies on 2D van der Waals heterostructures have attracted great attention. We also include some ARPES studies on 2D stacking heterostructures.
Two-dimensional (2D) twisted moiré materials, a new class of van der Waals (vdW) layered heterostructures with different twist angles between neighboring layers, have attracted tremendous attention due to their rich emerging properties. In this review, we systematically summarize the recent progress of 2D twisted moiré materials. Firstly, we introduce several representative fabrication methods and the fascinating topographies of the twisted moiré materials. Specifically, we discuss various remarkable physical properties related to twisted angles, including flat bands, unconventional superconductivity, ferromagnetism, and ferroelectricity. We also analyze the potential applications in various twisted moiré systems. Finally, the challenges and future perspectives of the twisted moiré materials are discussed. This work would spur edge-cutting ideas and related achievements in the scientific and technological frontiers of 2D twisted moiré materials.
Novel memory devices are essential for developing low power, fast and accurate in-memory computing and neuromorphic engineering concepts that can compete with the conventional CMOS digital processors. 2D semiconductors provide a novel platform for advanced semiconductors with atomic thickness, low current operation and capability of 3D integration. This work presents a charge trap memory (CTM) device with MoS2 channel where memory operation arises thanks to electron trapping/detrapping at interface states. We demonstrate transistor operation, memory characteristics and synaptic potentiation/depression for neuromorphic applications. The CTM device shows outstanding linearity of the potentiation by applied drain pulses of equal amplitude. We finally demonstrate pattern recognition by reservoir computing where the input pattern is applied as a stimulation of the MoS2 -based CTMs, while the output current after stimulation is processed by a feedforward readout network. The good accuracy, the low current operation and the robustness to input random bit flip makes the CTM device a promising technology for future high-density neuromorphic computing concepts. This article is protected by copyright. All rights reserved.
For high-performance silicon-carbon (Si-C)-based anode materials used in high-energy-density lithium-ion batteries (LIBs), there is an urgent need to rationally construct a stable solid electrolyte interface (SEI) film and load a high proportion of silicon content, which is closely related to the capacity and cycling stability of the electrode. Herein, composites of Fe/Fe3C-modified carbon nanofiber-coated Si nanoparticles (Fe/Fe3[email protected]) were synthesized via a simple electrospinning method. These composites effectively overcome the volume change effect, poor interfacial compatibility and low conductivity, delivering excellent LIB performance. Tested at 2.0 A g⁻¹, Fe/Fe3[email protected] provides a high reversible capacity of 956.5 mA h g⁻¹ with a coulombic efficiency of more than 99.5% even after 4000 ultra-long stable cycles. The high conductivity of the Fe/Fe3C embedded in the CNF framework can promote e⁻ transfer and boost the Li⁺ diffusion kinetics in the electrode. The catalytic activity of Fe/Fe3C helps to enhance the interfacial compatibility, grow a balanced stable SEI film and promote the long-cycle stability of the electrode at room temperature.
This chapter reviews the optical properties of the so-called Xenes: graphene, silicene, germanene, stanene, and plumbene. Particular emphasis is given to state-of-the-art ab initio methodologies. We revise the key ab initio approaches, namely, density functional theory, and Green’s function-based many-body theory. The rest of the chapter presents examples of calculations at different levels of theory for various two-dimensional materials of current interest, illustrating in each case the interplay between the atomic geometry, electronic band structure, and interband excitations. The effect of the substrate is also discussed. We show that the choice of the substrate is of paramount importance. The interaction with the Xenes across the interface may be negligible or may destroy the Dirac cones. In addition, the substrate may influence the optical spectra.
Within the large family of two-dimensional materials (2DMs), phosphorene has its special place because it stands for the renaissance of monoelemental species. Black phosphorene, a puckered 2DM that can be stabilized in freestanding configuration, exhibiting high hole mobility, direct bandgap, and in-plane anisotropy, has been widely explored in novel nanoelectronics and optoelectronics. In this chapter, we aim to assist researchers from a variety of disciplines to have an insight into this novel material. We review the properties of phosphorene and highlight electronic and bioapplications. More attention is paid to their high-quality synthesis, in particular, substrate engineering of a new phase, the buckled blue phosphorene. We discuss the surface oxidation mechanism and surface functionalization of black phosphorene, as well the future research directions of this promising material.
Pioneered by the tremendous properties of graphene and successively phosphorene, there has been growing interest for other monoelemental 2D materials mainly due to similarities in lattice structure and bandgap dependent on the number of layer and sheet geometry. Arsenene and antimonene are materials which have been mostly theoretically investigated and their properties have been calculated to surpass those of graphene and phosphorene. Hence, there has been an exponential increase over recent years regarding their properties, allotropes and methods of fabrication. Inevitably, these two exotic materials will be researched in the future owing to their huge potential to be implemented in promising applications in optoelectronic, thermoelectric and field-effect transistor devices among others either in their pristine form or after being appropriately functionalized. This chapter aims to give an overview of current knowledge on their structure and allotropes, followed by fabrication methods and then focused in detail about their usability in future in various areas of nanotechnology.
Magnetism is emerging as a key property of two-dimensional (2D) materials and heterostructures. 2D magnetism engenders new quantum and topological phases, endows materials with unexpected functionalities, and inspires new device concepts. The family of Xenes and their derivatives—materials at the core of 2D research—present an important constituent to the library of 2D magnets. Here, we trace out the development of Xene magnetism from general theoretical guidelines to experimental realization of intrinsic 2D magnets in a class of Xene compounds. In particular, we review the synthesis and properties of silicene and germanene coupled with rare earths which evolve from antiferromagnets in multilayer structures to 2D ferromagnets in the monolayer (ML) limit. Unconventional transport properties accompany magnetism in Xene materials to exhibit high carrier mobility in multilayers, colossal exponential negative magnetoresistance in one ML, and layer-controlled laws of electron transport. Furthermore, we demonstrate that the general approach developed for Xenes is applicable to graphene, making it ferromagnetic. Finally, we outline the potential future developments in Xene magnetism.
Silicene is two dimensional material packed by silicon atom in a buckled honeycomb lattice, compared with graphene, the buckled structure weakens the π-π overlaps and turns the hybrid orbitals from sp² to sp³, which enhances the spin-orbit coupling strength while still preserves the Dirac cone near K or K'. Due to its buckled structure silicene is susceptible to external parameters like electric field or substrate, which draws lots of attention both experimentally and theoretically. Recent progress of ferroelectricity in two-dimensional (2D) van der Waals materials found that the spontaneous ferroelectric polarization can be preserved even above room temperature, that inspired us to investigate how to tune the electric properties of silicene through the spontaneous polarization field of 2D ferroelectric substrate. In2X3 (X=Se,S,Te) family recently were found to be ferroelectric down to monolayers with reversible spontaneous electric polarization in both out-of-plane and in-plane orientations, and there were negligible lattice mismatch between silicene and In2S3. So we investigated the stacking and electric properties of silicene and monolayer In2S3 heterostructure by first-principles calculations. The spontaneous polarization field of In2S3 was calculated to be 1.26 μCcm⁻², comparable to the experimental results of In2Se3. We compared the different stacking order between silicene and In2S3, calculated results shown that the AB stacking is the ground state stacking order, and the reversal of the ferroelectric polarization could tune the band structure of heterostructure. When the polarization direction of In2S3 is upward, the layer distance between silicene and In2S3 is 3.93 Å, the polarization field and substrate interaction together break the AB sublattice symmetry and induce a 1.8 meV band gap near Dirac point K and K', while the Berry curvature around K and K' have opposite signs, corresponding to valley Hall effect. When the polarization is downward, the layer distance reduced to 3.62 Å and the band gap around K and K' increase to 30.8 meV. At the same time a 0.04e charge transfer makes some bands moving across the Fermi energy, corresponding to metal state. Our results pave the way for ferroelectric tuning silicene heterostructure and there potential applications in information industry.
Commercial purposes of two-dimensional silicene are usually suppressed in nanoelectronic and optoelectronic devices by its zero-band gap. Here, the halogen atoms are detected to open the bandgap of silicene monolayer by the chemical functionalization, and comprehensive analysis of the effects of halogenation on the structural, electronic, mechanical and optical properties are conducted via first-principles calculations within the framework of density functional theory. Our simulation results prove that the halogenation makes silicene structure more distorted but maintains good stability. Specifically, Full- and Janus-functionalization endow silicene with direct bandgap ranging from 1.536 to 2.123 eV by the HSE06 functional, while half-functionalization renders silicene metallic and retains semiconducting characteristics. In the above three configurations, the bond between the halogen atom and the host Si atom is predominantly ionic and the ionicity of the former two configurations decreases as the period number of the halogen atom increases. Furthermore, the halogenated silicene monolayers exhibit a hard mechanical property and relatively strong ability to resist deformation based on calculated Young's modulus Y and Shear modulus G except for the F–Si monolayer. And the light absorption of pristine silicene monolayer is increased by halogenation in the UV–Vis light spectrum.
The investigation and manipulation of matter on the atomic scale have been revolutionized by scanning tunneling microscopy and related scanning probe techniques. This book is the first to provide a clear and comprehensive introduction to this subject. Beginning with the theoretical background of scanning tunneling microscopy, the design and instrumentation of practical STM and associated systems are described in detail, including topographic imaging, local tunneling barrier height measurements, tunneling spectroscopy, and local potentiometry. A treatment of the experimental techniques used in scanning force microscopy and other scanning probe techniques rounds out this section. The second part discusses representative applications of these techniques in fields such as condensed matter physics, chemistry, materials science, biology, and nanotechnology, so this book will be extremely valuable to upper-division students and researchers in these areas.
The isolation of various two-dimensional (2D) materials, and the possibility to combine them in vertical stacks, has created
a new paradigm in materials science: heterostructures based on 2D crystals. Such a concept has already proven fruitful for
a number of electronic applications in the area of ultrathin and flexible devices. Here, we expand the range of such structures
to photoactive ones by using semiconducting transition metal dichalcogenides (TMDCs)/graphene stacks. Van Hove singularities
in the electronic density of states of TMDC guarantees enhanced light-matter interactions, leading to enhanced photon absorption
and electron-hole creation (which are collected in transparent graphene electrodes). This allows development of extremely
efficient flexible photovoltaic devices with photoresponsivity above 0.1 ampere per watt (corresponding to an external quantum
efficiency of above 30%).
As the Si counterpart of graphene, silicene may be defined as an at least partially sp(2)-hybridized, atom-thick honeycomb layer of Si that possesses pi-electronic bands. Here we show that two-dimensional, epitaxial silicene forms through surface segregation on zirconium diboride thin films grown on Si wafers. A particular buckling of silicene induced by the epitaxial relationship with the diboride surface leads to a direct pi-electronic band gap at the Gamma point. These results demonstrate that the buckling and thus the electronic properties of silicene are modified by epitaxial strain.
The electronic band structure of MoS2 single crystals has been investigated using angle-resolved photoelectron spectroscopy and first-principles calculations. The orbital symmetry and k dispersion of these electronic states responsible for the direct and the indirect electronic band gaps have been unambiguously determined. By experimentally probing an increase of the electronic band gap, we conclude that a MoS2 (0002) surface localized state exists just below the valence band maximum at the Γ point. This electronic state originates from the sulfur planes within the topmost layer. Our comprehensive study addresses the surface electronic structure of MoS2 and the role of van der Waals interlayer interactions.
The electronic properties of two-dimensional hexagonal silicon (silicene) are investigated using first-principles simulations. Though silicene is predicted to be a gapless semiconductor, due to the sp2-hybridization of its atomic orbitals, the weak overlapping between 3pz orbitals of neighbor Si atoms leads to a very reactive surface, resulting in a more energetically stable semiconducting surface upon the adsorption of foreign chemical species. It is predicted that silicene inserted into a graphitelike lattice, like ultrathin AlN stacks, preserves its sp2-hydridization, and hence its graphenelike electronic properties.
Silicene, a two-dimensional (2D) honeycomb structure similar to graphene, has been successfully fabricated on an Ir(111) substrate. It is characterized as a (√7×√7) superstructure with respect to the substrate lattice, as revealed by low energy electron diffraction and scanning tunneling microscopy. Such a superstructure coincides with the (√3×√3) superlattice of silicene. First-principles calculations confirm that this is a (√3×√3)silicene/(√7×√7)Ir(111) configuration and that it has a buckled conformation. Importantly, the calculated electron localization function shows that the silicon adlayer on the Ir(111) substrate has 2D continuity. This work provides a method to fabricate high-quality silicene and an explanation for the formation of the buckled silicene sheet.
Using ab initio methods, we have investigated the structures and stabilities of Si(N) clusters (N ≤ 24) on Ag(111) surface as the initial stage of silicene growth. Unlike the dome-shaped graphene clusters, Si clusters prefer nearly flat structures with low buckling, more stable than directly deposition of the 3D freestanding Si clusters on Ag surface. The p-d hybridization between Ag and Si is revealed as well as sp(2) characteristics in Si(N)@Ag(111). Three types of silicene superstructures on Ag(111) surface have been considered and the simulated STM images are compared with experimental observations. Molecular dynamic simulations show high thermal stability of silicene on Ag(111) surfaces, contrast to that on Rh(111). The present theoretical results constitute a comprehensive picture about the interaction mechanism of silicene on Ag(111) surface and explain the superiority of Ag substrate for silicene growth, which would be helpful for improving the experimentally epitaxial growth of silicene.
Epitaxial growth of topological insulator Bi2Se3 thin films on nominally flat
and vicinal Si(111) substrates is studied. In order to achieve planner growth
front and better quality epifilms, a two-step growth method is adopted for the
van der Waal epitaxy of Bi2Se3 to proceed. By employing vicinal Si(111)
substrate surfaces, the in-pane growth rate anisotropy of Bi2Se3 is explored to
achieve single crystalline Bi2Se3 epifilms, in which threading defects and
twins are effectively suppressed. Optimization of the growth parameters has
resulted in vicinal Bi2Se3 films showing a carrier mobility of ~ 2000 cm2V-1s-1
and the background doping of ~ 3 x 1018 cm-3 of the as-grown layers. Such
samples not only show relatively high magnetoresistance but also a linear
dependence on magnetic field.
QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
Because of its unique physical properties, graphene, a 2D honeycomb arrangement of carbon atoms, has attracted tremendous attention. Silicene, the graphene equivalent for silicon, could follow this trend, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Silicene has been theoretically predicted as a buckled honeycomb arrangement of Si atoms and having an electronic dispersion resembling that of relativistic Dirac fermions. Here we provide compelling evidence, from both structural and electronic properties, for the synthesis of epitaxial silicene sheets on a silver (111) substrate, through the combination of scanning tunneling microscopy and angular-resolved photoemission spectroscopy in conjunction with calculations based on density functional theory.
Topological insulators are an intriguing class of materials with an insulating bulk state and gapless Dirac-type edge/surface states. Recent theoretical work predicts that few-layer topological insulators are promising candidates for broadband and high-performance optoelectronic devices due to their spin-momentum-locked massless Dirac edge/surface states, which are topologically protected against all time-reversal-invariant perturbations. Here, we present the first experimental demonstration of near-infrared transparent flexible electrodes based on few-layer topological-insulator Bi(2)Se(3) nanostructures epitaxially grown on mica substrates by means of van der Waals epitaxy. The large, continuous, Bi(2)Se(3)-nanosheet transparent electrodes have single Dirac cone surface states, and exhibit sheet resistances as low as ~330 Ω per square, with a transparency of more than 70% over a wide range of wavelengths. Furthermore, Bi(2)Se(3)-nanosheet transparent electrodes show high chemical and thermal stabilities as well as excellent mechanical durability, which may lead to novel optoelectronic devices with unique properties.
A new phototransistor based on the mechanically exfoliated single-layer MoS(2) nanosheet is fabricated, and its light-induced electric properties are investigated in detail. Photocurrent generated from the phototransistor is solely determined by the illuminated optical power at a constant drain or gate voltage. The switching behavior of photocurrent generation and annihilation can be completely finished within ca. 50 ms, and it shows good stability. Especially, the single-layer MoS(2) phototransistor exhibits a better photoresponsivity as compared with the graphene-based device. The unique characteristics of incident-light control, prompt photoswitching, and good photoresponsivity from the MoS(2) phototransistor pave an avenue to develop the single-layer semiconducting materials for multifunctional optoelectronic device applications in the future.
Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films or requires high voltages. Although single layers of MoS(2) have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5-3 cm(2) V(-1) s(-1) range are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS(2) mobility of at least 200 cm(2) V(-1) s(-1), similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1 × 10(8) and ultralow standby power dissipation. Because monolayer MoS(2) has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS(2) could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, these puckered structures are ambipolar and their charge carriers can behave like a massless Dirac fermion due to their pi and pi(*) bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.
Scanning tunneling microscopy has repeatedly resolved individual atoms on a number of metal surfaces with atomic distances 2.5-3 Å. This is in sharp contradiction to the resolution limits previously predicted, 6-9 Å. We present a theory of such atomic resolution in terms of actual tip states, for example, d2z tip states on tungsten tips. Quantitative interpretation of the observed images is obtained with no adjustable parameters. We predict that to achieve atomic resolution, the tip material should be either a d-band metal or certain semiconductor.
A new approach to the construction of first-principles pseudopotentials is described. The method allows transferability to be improved systematically while holding the cutoff radius fixed, even for large cutoff radii. Novel features are that the pseudopotential itself becomes charge-state dependent, the usual norm-conservation constraint does not apply, and a generalized eigenproblem is introduced. The potentials have a separable form well suited for plane-wave solid-state calculations, and show promise for application to first-row and transition-metal systems.
The odd-electron molecular theory, which takes into account the electron correlation, has been applied for a comparative consideration of sp2 nanocarbons and their siliceous analogues. Four characteristic quantities that involve the energy misalignment, the number of effectively unpaired electrons, the squared spin value, and CC and SiSi chemical bond lengths have been suggested to classify the extent of the odd electron correlation in both families. Providing a considerable enlargement of the internuclear distances, the electron correlation in siliceous species is extremely strong thus resulting in complete radicalization of the species and proving the impossibility of their existence at ambient conditions in contrast to carboneous ones that are not so strong correlated. © 2012 Wiley Periodicals, Inc.
There has been emerging interest in exploring single-sheet 2D layered structures other than graphene to explore potentially interesting properties and phenomena. The preparation, isolation and rapid unambiguous characterization of large size ultrathin layers of MoS2, GaS, and GaSe deposited onto SiO2/Si substrates is reported. Optical color contrast is identified using reflection optical microscopy for layers with various thicknesses. The optical contrast of these thin layers is correlated with atomic force microscopy (AFM) and Raman spectroscopy to determine the exact thickness and to calculate number of the atomic layers present in the thin flakes and sheets. Collectively, optical microscopy, AFM, and Raman spectroscopy combined with Raman imaging data are analyzed to determine the thickness (and thus, the number of unit layers) of the MoS2, GaS, and GaSe ultrathin flakes in a fast, non-destructive, and unambiguous manner. These findings may enable experimental access to and unambiguous determination of layered chalcogenides for scientific exploration and potential technological applications.
One of the major obstacles to realize various kinds of heterostructures is the lattice matching between constituent materials. This difficulty has been proved to be overcome if one uses the interface having van der Waals nature. This kind of interface can be formed when a layered material is grown on a cleaved face of another layered material. Moreover, it has been found that these heterostructures can be grown on a three-dimensional material substrate such as Si or GaAs, if the dangling bond on its surface is terminated by proper atoms. This idea has also been successfully applied to grow epitaxial films of organic molecular crystals such as metal phthalocyanines and fullerenes.
From the elastic scattering quantum chemistry technique, it is shown that the images of the MoS2 surface obtained with an STM depend on the distance between the tip apex and the surface. At large distances, the tip apex and the Mo d-bands are coupled only through the sulphur surface layer. This leads to image the sulphur sites only. At small distances (< 4 Å), there is a superposition between this through bond tunnelling and the direct tip apex-Mo d-bands tunnelling. This superposition is destructive and the contrast is inverted; however, the two images remain very similar, which may lead to a difficult experimental interpretation of the STM images when the tip apex-to-surface distance is not well controlled.
The tunneling current is measured as a function of voltage, lateral position, and vertical separation between a tungsten probe tip and a Si(111)2 × 1 surface. A rich spectrum is obtained in the ratio of differential to total conductivity, revealing the structure of the surface-state bands. The magnitude of the parallel wave vector for certain surface states is determined from the decay length of the tunneling current. Real-space images of the surface states reveal a phase reversal between those states on either side of the surface-state band gap.
The chemical stability of buckled silicene, i.e., the silicon counterpart of graphene, is investigated then resulting in a low reactivity with O2 when dosing up to 1000 L and in a progressive oxidation under ambient conditions. The latter drawback is addressed by engineering ad hoc Al- and Al2O3-based encapsulations of the silicene layer. This encapsulation design can be generally applied to any silicene configuration, irrespective of the support substrate, and it leads to the fabrication of atomically sharp and chemically intact Al/silicene and Al2O3/silicene interfaces that can be functionally used for ex situ characterization as well as for gated device fabrication.
Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.
Field emission studies are reported for the first time on layered MoS(2) sheets at the base pressure of ∼1 × 10(-8) mbar. The turn-on field required to draw a field emission current density of 10 μA/cm(2) is found to be 3.5 V/μm for MoS(2) sheets. The turn-on values are found to be significantly lower than the reported MoS(2) nanoflowers, graphene, and carbon nanotube-based field emitters due to the high field enhancement factor (∼1138) associated with nanometric sharp edges of MoS(2) sheet emitter surface. The emission current-time plots show good stability over a period of 3 h. Owing to the low turn-on field and planar (sheetlike) structure, the MoS(2) could be utilized for future vacuum microelectronics/nanoelectronic and flat panel display applications.
The interaction of silicene, the silicon counterpart of graphene, with (0001) ZnS surfaces is investigated theoretically, using first-principles simulations. The charge transfer occurring at the silicene/(0001) ZnS interface leads to the opening of an indirect energy band gap of about 0.7 eV in silicene. Remarkably, the nature (indirect or direct) and magnitude of the energy band gap of silicene can be controlled by an external electric field: the energy gap is predicted to become direct for electric fields larger than about 0.5 V Å(-1), and the direct energy gap decreases approximately linearly with the applied electric field. The predicted electric field tunable energy band gap of the silicene/(0001) ZnS interface is very promising for its potential use in nanoelectronic devices.
The MoSe2/MoS2 system is investigated by scanning tunneling microscope (STM) and by its spectroscopic mode, scanning tunneling spectroscopy (STS). STM images show a hexagonal wagon-wheel-like pattern, which is made of bright lines and dark triangles. High-resolution STM images show that the bright line consists of twin lines separated by about 1 nm and that the hexagonal pattern is often skewed. These features are not explained by the simple moiré effect due to the lattice mismatch between the overlayer and the substrate materials. The STS spectra show that the bright area results mainly from electron waves derived from chalcogen orbitals running parallel to the interface between MoSe2 and MoS2 layers. A mechanism of the pattern formation is proposed, in which scattered electron waves produce the bright wagon-wheel-like pattern.
By combining experimental techniques with ab initio density functional theory calculations, we describe the Si/Ag(111) 2D systems in terms of a sp(2)-sp(3) form of silicon characterized by a vertically distorted honeycomb lattice provided by the constraint imposed by the substrate. The Raman spectrum reflects the multihybridized nature of the 2D Si nanosheets (NSs) resulting from a buckling-induced distortion of a purely sp(2) hybridized structure. We show that vibrational and electronic properties of 2D Si-NSs are tightly linked to the buckling arrangement.
We have investigated the Scanning Tunneling Microscopy (STM) of graphite with varying tip-to-surface distance. Using an LCAO type approach we showed that at small separations states are localized between the tip and the surface. The energies and the characters of these Tip Induced Localized States (TILS) depend on the height and the lateral position of the tip. These states play a significant role in the tunneling process and influence the STM corrugations predicted from the local density of states. We have developed a current expression, which includes these local interactions, but differes significantly from earlier theories.
The structural and electronic properties of silicene nanosheets epitaxially grown on Ag(111) are systematically investigated by combining scanning tunneling microscopy and scanning tunneling spectroscopy. By carefully tuning the growth parameters, complex 2D silicon structures are obtained, which evidence the presence of corrugated silicene domains. Local modifications of the density of states are observed throughout reconstructed silicene domains and are attributed to the symmetry breaking induced by the interactions with the Ag lattice, in analogy with the case of graphene.
Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties.
We report on the fabrication of top-gate phototransistors based on a few-layered MoS(2) nanosheet with a transparent gate electrode. Our devices with triple MoS(2) layers exhibited excellent photodetection capabilities for red light, while those with single- and double-layers turned out to be quite useful for green light detection. The varied functionalities are attributed to energy gap modulation by the number of MoS(2) layers. The photoelectric probing on working transistors with the nanosheets demonstrates that single-layer MoS(2) has a significant energy bandgap of 1.8 eV, while those of double- and triple-layer MoS(2) reduce to 1.65 and 1.35 eV, respectively.
Room-temperature, bottom-gate, field-effect transistor characteristics of 2D ultrathin layer GaS and GaSe prepared from the bulk crystals using a micromechanical cleavage technique are reported. The transistors based on active GaS and GaSe ultrathin layers demonstrate typical n-and p-type conductance transistor operation along with a good ON/OFF ratio and electron differential mobility.
In the search for evidence of silicene, a two-dimensional honeycomb lattice
of silicon, it is important to obtain a complete picture for the evolution of
Si structures on Ag(111), which is believed to be the most suitable substrate
for growth of silicene so far. In this work we report the finding and evolution
of several monolayer superstructures of silicon on Ag(111) depending on the
coverage and temperature. Combined with first-principles calculations, the
detailed structures of these phases have been illuminated. These structure were
found to share common building blocks of silicon rings, and they evolve from a
fragment of silicene to a complete monolayer silicene and multilayer silicene.
Our results elucidate how silicene formes on Ag(111) surface and provide
methods to synthesize high-quality and large-scale silicene.
We present a method for synthesizing MoS(2)/Graphene hybrid heterostructures with a growth template of graphene-covered Cu foil. Compared to other recent reports, (1, 2) a much lower growth temperature of 400 °C is required for this procedure. The chemical vapor deposition of MoS(2) on the graphene surface gives rise to single crystalline hexagonal flakes with a typical lateral size ranging from several hundred nanometers to several micrometers. The precursor (ammonium thiomolybdate) together with solvent was transported to graphene surface by a carrier gas at room temperature, which was then followed by post annealing. At an elevated temperature, the precursor self-assembles to form MoS(2) flakes epitaxially on the graphene surface via thermal decomposition. With higher amount of precursor delivered onto the graphene surface, a continuous MoS(2) film on graphene can be obtained. This simple chemical vapor deposition method provides a unique approach for the synthesis of graphene heterostructures and surface functionalization of graphene. The synthesized two-dimensional MoS(2)/Graphene hybrids possess great potential toward the development of new optical and electronic devices as well as a wide variety of newly synthesizable compounds for catalysts.
In order to understand the adsorption mechanism of metal atoms to semiconducting surfaces, we have studied, as a model system, the vapor phase adsorption of Ag, Au, and Cu on the (001) surface of molybdenite (MoS2) and the subsequent surface diffusion of these adsorbates. Our scanning tunneling microscopy (STM) images show that, depending on the type of metal atom that is adsorbed, islands of a characteristic size (2 nm for Ag, 8 to 10 nm for Cu, two distinct sizes of 2 nm and 8 to 10 nm for Au), shape (well rounded in the lateral extension) and thickness (one monolayer for Ag, 1 to 1.5 nm for Cu) are formed during the initial stages of deposition. Whole islands are observed to surface diffuse without loss of size or shape. Despite the relatively large size of the copper islands on molybdenite, these islands surface diffuse extensively, suggesting that the Cu-S interaction is weak. Surface diffusion is only hindered once individual islands start to coalesce. As copper islands accumulate, the size and shape of the original islands can still be recognized, supporting the conclusion that these characteristics are constant and that monolayer growth occurs by the aggregation of islands across the surface.
Employing the generalisation of Van Vechten's cavity model, formation energies of neutral point defects in pyrites (FeS2, RuS2), chalcopyrites (II–IV–V2 and I–III–VI2) as well as molybdenites (MoS2, WS2) have been estimated. As input parameters the fundamental band gaps, work functions, electron affinities, surface energies, coordination numbers, covalent or ionic radii and unit cell parameters were used. The values calculated for tetrahedrally and octahedrally coordinated compounds agreed well with measured values. The data obtained can be used to calculate point defect concentrations and homogeneity ranges as a function of partial pressure and temperature. Introducing charged vacancies, the conductivity type can be predicted.
Sub-monolayer films of layered semiconductor InSe were grown on MoS2 substrates by molecular beam epitaxy, and the change in their growth features with Se/In flux ratio was investigated using scanning tunneling microscope in vacuum. It was found that InSe domains grown at 340°C have a hexagonal shape when the Se/In ratio is about 17. Detailed images of the hexagonal InSe domains have revealed that adjacent sides of the hexagon have different structures; one is a straight edge and the other is a disordered edge. When the Se/In ratio was decreased, the disordered edges became predominant, the straight edges disappeared, and the InSe domain became triangular. On the contrary, when the Se/In ratio was increased, InSe domains became triangular ones consisting of only straight edges. The growth mechanism of InSe domains is discussed by considering the crystal structure of InSe and the reactivity of each side with incoming atoms. It is suggested that the balance of incorporation rate of In and Se atoms determines the structure of InSe domains.
By using ab initio calculations, we predict that a vertical electric field is able to open a band gap in semimetallic single-layer buckled silicene and germanene. The sizes of the band gap in both silicene and germanene increase linearly with the electric field strength. Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed. Therefore, biased single-layer silicene and germanene can work effectively at room temperature as field effect transistors.
We present a theory for tunneling between a real surface and a model probe tip, applicable to the recently developed ‘‘scanning tunneling microscope.’’ The tunneling current is found to be proportional to the local density of states of the surface, at the position of the tip. The effective lateral resolution is related to the tip radius R and the vacuum gap distance d approximately as [(2 Å)(R+d)]1/2. The theory is applied to the 2×1 and 3×1 reconstructions of Au(110); results for the respective corrugation amplitudes and for the gap distance are all in excellent agreement with experimental results of Binnig et al. if a 9-Å tip radius is assumed. In addition, a convenient approximate calculational method based on atom superposition is tested; it gives reasonable agreement with the self-consistent calculation and with experiment for Au(110). This method is used to test the structure sensitivity of the microscope. We conclude that for the Au(110) measurements the experimental ‘‘image’’ is relatively insensitive to the positions of atoms beyond the first atomic layer. Finally, tunneling to semiconductor surfaces is considered. Calculations for GaAs(110) illustrate interesting qualitative differences from tunneling to metal surfaces.
We present a quantitative analysis of the modifications of the scanning-tunneling-microscopy images due to the local perturbations of the electronic states induced by the tip in close proximity to the sample surface. Using an empirical tight-binding method, we have calculated the electronic states of a prototype tip-sample system consisting of a single-atom tip and the graphite surface, as a function of the tip-sample distance. We find that as the tip approaches the sample, their states start to interact and become laterally confined in the vicinity of the tip at small tip-sample separation. These states influence the tunneling phenomenon by connecting the tip and sample surface electronically. The effect of the tip-induced localized states is discussed, and the expression for the tunneling current is reformulated by incorporating the tip-induced states. Calculations using this expression show that the corrugation amplitude obtained from scanning tunneling microscopy is enhanced and deviates from the proportionality to the local density of states of the bare sample at the Fermi level evaluated at the center of the tip.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
We have studied using scanning tunneling microscopy (STM) the atomic-scale realm of molybdenum disulfide ( MoS2) nanoclusters, which are of interest as a model system in hydrodesulfurization catalysis. The STM gives the first real space images of the shape and edge structure of single-layer MoS2 nanoparticles synthesized on Au(111), and establishes a new picture of the active edge sites of the nanoclusters. The results demonstrate a way to get detailed atomic-scale information on catalysts in general.
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