No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
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.
... Monolayer graphene was first realized in 2004 [1] and since then the group IV elemental monolayers have played a crucial role in the field of two-dimensional (2D) materials. In experiments, silicene, germanene, and stanene were all synthesized on different substrates [2][3][4][5][6][7][8][9][10], such as heterostructures of silicene(germanene)/MoS 2 [6,8] and stanene/Bi 2 Se 3 [10], which preserve the hexagonal honeycomb structure of the isolated monolayers. Without spin-orbit coupling (SOC), silicene, germanene, and stanene all have a zero electronic band gap with two bands crossing linearly at the Fermi level and their extremely large Fermi velocity (v F ) makes them ideal materials for high-speed electronic devices [11,12]. ...
... Monolayer graphene was first realized in 2004 [1] and since then the group IV elemental monolayers have played a crucial role in the field of two-dimensional (2D) materials. In experiments, silicene, germanene, and stanene were all synthesized on different substrates [2][3][4][5][6][7][8][9][10], such as heterostructures of silicene(germanene)/MoS 2 [6,8] and stanene/Bi 2 Se 3 [10], which preserve the hexagonal honeycomb structure of the isolated monolayers. Without spin-orbit coupling (SOC), silicene, germanene, and stanene all have a zero electronic band gap with two bands crossing linearly at the Fermi level and their extremely large Fermi velocity (v F ) makes them ideal materials for high-speed electronic devices [11,12]. ...
Two-dimensional (2D) carbon nitride materials play an important role in energy-harvesting, energy-storage and environmental applications. Recently, a new carbon nitride, 2D polyaniline (C3N) was proposed [PNAS 113 (2016) 7414-7419]. Based on the structure model of this C3N monolayer, we propose two new carbon nitride monolayers, named dumbbell (DB) C4N-I and C4N-II. Using first-principles calculations, we systematically study the structure, stability, and band structure of these two materials. In contrast to other carbon nitride monolayers, the orbital hybridization of the C/N atoms in the DB C4N monolayers is sp3. Remarkably, the band structures of the two DB C4N monolayers have a Dirac cone at the K point and their Fermi velocities are comparable to that of graphene. This makes them promising materials for applications in high-speed electronic devices. Using a tight-binding model, we explain the origin of the Dirac cone.
... 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–van der 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.
... Silicene, the silicon equivalent of graphene, has been experimentally synthesized in a vacuum as a promising material and are unstable, likely due to their nonplanar or nonlinear structures. Silicene, the silicon equivalent of graphene, has been experimentally synthesized in a vacuum as a promising material and theoretically studied [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, a significant issue is that silicene sheets oxidize and decompose in air due to their buckled structure [14]. ...
This article provides a comprehensive review of quantum chemical computational studies on the thermal and photochemical reactions of organosilicon compounds, based on fundamental concepts such as initial complex formation, HOMO-LUMO interactions, and subjacent orbital interactions. Despite silicon’s position in group 14 of the periodic table, alongside carbon, its reactivity patterns exhibit significant deviations from those of carbon. This review delves into the reactivity behaviors of organosilicon compounds, particularly focusing on the highly coordinated nature of silicon. It is poised to serve as a valuable resource for chemists, offering insights into cutting-edge research and fostering further innovations in synthetic chemistry and also theoretical chemistry.
... (48)(49)(50), Ag(110) (51,52), Au(110) (53), Ir(111) (54), ZrB 2 (0001)(55), MoS 2 (0001)(56), and highly ordered pyrolytic graphite (HOPG)(57) in a bottom-up approach to make it compatible with contemporary Si-based technology. Due to the nonreactive nature of Ag and the sixfold symmetry of the Ag(111) surface, most early studies focused on the interfacial growth of silicene on Ag(111). ...
Inspired by the success of graphene, two-dimensional (2D) materials have been at the forefront of advanced (opto-)nanoelectronics and energy-related fields owing to their exotic properties like sizable bandgaps, Dirac fermions, quantum spin Hall states, topological edge states, and ballistic charge carrier transport, which hold promise for various electronic device applications. Emerging main group elemental 2D materials, beyond graphene, are of particular interest due to their unique structural characteristics, ease of synthetic exploration, and superior property tunability. In this review, we present recent advances in atomic-scale studies of elemental 2D materials with an emphasis on synthetic strategies and structural properties. We also discuss the challenges and perspectives regarding the integration of elemental 2D materials into various heterostructures.
... Silicene is a monolayer of silicon atoms bonded together on a two-dimensional (2D) honeycomb lattice. Both silicene sheets and ribbons have been experimentally synthesized through synthesis on metal surfaces [1][2][3][4]. Silicene shares almost every remarkable property of graphene; for instance, it exhibits Dirac-like electron dispersion at the corners of the Brillouin zone. Unlike graphene, it has a buckled structure due to the large ionic radius of silicon atoms [5][6][7], which causes different sublattices to sit in different vertical planes with a separation of d ≈ 0.46 Å [5,8], as shown in figure 1. ...
We numerically investigate the effects of disorder on the quantum Hall effect (QHE) and the quantum phase transitions in silicene based on a lattice model. It is shown that for a clean sample, silicene exhibits an unconventional QHE near the band center, with plateaus developing at and a conventional QHE near the band edges. In the presence of disorder, the Hall plateaus can be destroyed through the float-up of extended levels toward the band center, in which higher plateaus disappear first. However, the center Hall plateau is more sensitive to disorder and disappears at a relatively weak disorder strength. Moreover, the combination of an electric field and the intrinsic spin-orbit interaction (SOI) can lead to quantum phase transitions from a topological insulator to a band insulator at the charge neutrality point (CNP), accompanied by additional quantum Hall conductivity plateaus.
... Comparing the lattice constant of the GaSe monolayer (3.804Å), the lattice mismatch between the two monolayers is only 1.0%, which is far smaller than that of the high-buckled silicene on MoS 2 substrate (20.8%) [70], indicating its feasibility for experimental synthesis. Then we calculated the energy and the band structures of the six heterostructures without and with SOC effect. ...
The quantum anomalous Hall (QAH) effect is a topologically nontrivial phase, characterized by a non-zero Chern number defined in the bulk and chiral edge states in the boundary. Using first-principles calculations, we demonstrate the presence of the QAH effect in 1T-YN monolayer, which was recently predicted to be a Dirac half metal without spin-orbit coupling (SOC). We show that the inclusion of SOC opens up a large nontrivial band gap of nearly 0.1 eV in the electronic band structure. This results in the nontrivial bulk topology which is confirmed by the calculation of Berry curvature, anomalous Hall conductance and the presence of chiral edge states. Remarkably, a high Chern number is found, and there are three corresponding gapless chiral edge states emerging inside the bulk gap. Our results open a new avenue in searching for QAH insulators with high temperature and high Chern numbers, which can have nontrivial practical applications.
... 8, 10 Limited by their non-van der Waals structure with strong chemical bonds inside, 2D XIV-group materials are currently available by physical-mechanical/electrochemistry exfoliating methods, molecular beam epitaxy on single crystal substrates or energy-intensive chemical vapor deposition (CVD) preparation methods. 7, [11][12][13] These synthetic processes usually need special apparatus and rigorous conditions with a low production rate. It remains desirable and challenging to develop a synthetic strategy to synthesize 2D XIV-group materials with fast reaction kinetics. ...
Two-dimensional (2D) XIV-group nanosheets (germanene, silicene, andstannene)possessuniquephysical and chemical featurespromisinginfieldsofelectronics,energystorage, and conversions. However, preparing these nanosheets is challenging owing to their non van derWaals structure with strongchemical bonds inside.Herein, abubblingchemical-vapor growth method is proposed to synthesize these XIV-group nanosheet by bubbling XIV-group-element chlorides in molten sodium. During the synthetic process, XIV-groupmaterials are formed by the reaction of XIV-group element chlorides with strong reducing sodium, then nucleated, and finally isolated to 2D nanosheets in the gas−liquid interface. With the collapse of vapor bubbles and subsequent injection, 2D nanosheets are continuously produced. The nanosheets (Ge) possess a thickness of ∼3.8 nm and a lateral size of ∼2.0 μm. Combining with graphene, the hybrid and flexible films are obtained, delivering a volumetric specific capacity of 4785 mAhcm−3 and superior cycling stability (over 4000 cycles) in lithium-ion batteries.
... Although the Xenes can be grown also on non-metallic substrates, e.g. MoS 2 or Al 2 O 3 [50][51][52][53], hitherto a similar approach on the large scale synthesis has not been reported yet. While the physical deposition methodologies described above address the technological challenge of a uniform wafer-scale deposition, chemical-based approaches are considered to directly synthesize Xene nanosheets, and their functionalized forms, in gramscale quantities with low costs ( figure 1(b)). ...
After more than ten years since the silicene discovery, many Xenes, the class of elemental graphene-like lattices, have now enriched the two-dimensional periodic table of elements. Here, we provide a perspective on the future of the Xenes by briefly summarizing their properties and devices reported thus far. Two main challenges are expected to focus the scientists’ attention to bring the Xenes to the next level. To step over the current scenario the Xenes need standardization either in the growth or in the fabrication of devices, aiming at the wafer-scale and the reliability and stability, respectively. The benefits arising from these challenges will enable the concept of hybrid Xenes and hybrid Xenes-based devices, that is a combination of different Xenes with new properties and multifunctional Xenes-based devices, respectively, with potential unexpected fascinating properties to continue the journey.
... As far as the realistic realization of our proposed junction is concerned, it should be possible to fabricate such a geometry with the currently available experimental techniques. Silicene has been fabricated via epitaxial growth on Ag (111), 46 ZrB 2 (0001), 47 Ir (111), 48 and MoS 2 49 substrates. Due to the interaction between the silicene and the different substrates, the various surface reconstructions have been revealed by the theoretical calculations and the ...
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.
... 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...
The synthesis of 2D Xenes and related materials ( e.g. heterostructures and hybrids) is reviewed with respect to physical and chemical methods, covering different epitaxial schemes, topotactic deintercalation and exfoliation techniques.
An extended formation of faceted pit-like defects on Ge(001) and Ge(111) wafers was obtained by thermal cycles to T> 750 {\deg}C. This temperature range is relevant in many surface-preparation recipes of the Ge surface. The density of the defects depends on the temperature reached, the number of annealing cycles performed and correlates to the surface-energy stability of the specific crystal orientation. We propose that the pits were formed by preferential desorption from the strained regions around dislocation pile-ups. Indeed, the morphology of the pits was the same as that observed for preferential chemical etching of dislocations while the spatial distribution of the pits was clearly non-Poissonian in line with mutual interactions between the core dislocations.
Considering the interband correlation, we present a generalized multiple-scattering approach of Green's function to investigate the effects of electron-impurity scattering on the density of states in silicene. The reduction of energy gaps in the case of relatively high chemical potential and the transformation of split-off impurity bands into band tails for low chemical potential are found. The dependency of optical conductivity on the impurity concentration is also discussed for frequency within the terahertz regime.
The fracture toughness, mechanical properties, and crack propagation behaviour of defective single-layer silicon carbide (SiC) nanosheets were investigated through a molecular dynamics (MD) study. Various types of defects were modelled to examine their mechanical properties and toughness under different temperatures. The results indicated that the mechanical properties of both defect-free and defective SiC diminished with increasing temperature and defect size. At room temperature, the failure stress of SiC decreased by approximately 47.22, 48.73, and 52.34 GPa for crack lengths of 25 Å, and circular and square notches with diameters of 25 Å, respectively. Similar trends were observed in Young’s modulus and failure strain. Additionally, higher stress concentrations at the corners suggested that samples with square defects had the weakest properties. As the crack size increased, the stress intensity factor of SiC also increased. Defects propagated in the direction perpendicular to the stress loading, and larger defect sizes mitigated the adverse effect of temperature on failure stress. This research is significant in analysing the mechanical behaviour of SiC, a key wide-bandgap semiconductor structure with substantial potential applications in advanced power devices.
Silicene, a two-dimensional material with potential applications in future technologies, has garnered significant interest in the past decade. Recent attention has focused on modifying silicene's electronic and magnetic properties through adatom adsorption or substitutional doping. While the magnetic, electronic, and optical properties of doped silicene have been extensively studied, a noticeable gap exists in the literature concerning its mechanical properties. In this context, this study addresses this gap by exploring the mechanical characteristics of bilayer silicene doped with aluminum by employing molecular dynamics simulations. The influence of Al concentration on the material’s mechanical response is assessed by tensile tests performed at a strain rate of 1010 s-1. The findings reveal a monotonically decreasing strength with Al concentration in both loading directions, zigzag and armchair. The deformation initiates with the rupture of Si-Al bonds, ultimately leading to a brittle fracture.
2D Materials: Chemistry and Applications, Part 2 addresses the cutting-edge advancements in the synthesis, functionalization, and applications of two-dimensional materials, focusing on graphene and other emerging materials like boron nitride, germanene, silicene, and stanene. This volume explores the potential of these materials in energy storage, nanoelectronics, waste management, and more, while addressing challenges like toxicity and cost-effective production. The book highlights innovative approaches to graphene-based supercapacitors, nanoparticle-functionalized graphene, and the application of 2D materials in diverse fields. It also provides insights into the toxicity and remediation strategies of graphene family materials and outlines the roadmap for sustainable graphene production. This book is ideal for researchers, academics, and professionals in materials science, nanotechnology, chemistry, and environmental engineering. Key Features: Advanced applications of graphene-based supercapacitors. Functionalization and applications of boron nitride, germanene, silicene, and stanene. Insights into graphene toxicity and remediation approaches. Roadmap for cost-effective graphene production and waste management.
Silicene, a two-dimensional allotrope of silicon, has attracted considerable attention due to its distinctive electronic, mechanical, and biochemical properties. This review critically examines the emerging applications of silicene in oncology, emphasising its potential roles in cancer therapy and research. Silicene exhibits exceptional biocompatibility and surface reactivity, positioning it as a promising candidate for oncological applications. This review addresses the current challenges and limitations in the clinical translation of silicene-based technologies, including issues of stability, toxicity, and scalable production. By synthesizing recent research findings, this review aims to provide an assessment of silicene's potential contributions to oncology and delineate future research trajectories in this innovative field.
Layered silicon (L-Si) anodes are celebrated for their high theoretical capacity but face significant challenges regarding safety and material purity during preparation. This study addresses these challenges by employing NH4Cl-CaSi2...
Two-dimensional (2D) molybdenum disulfide (MoS2) has attracted significant attention due to its unique structure and properties, especially following the discovery of graphene. This is mainly attributed due to its flexibility which facilitates the MoS2 phase and properties exploration. MoS2 mainly exists in three phases, 2H, 1T and 3R, where the naturally occurring 2H and 3R phase of MoS2 acts as a semiconductor, with active edge sites and inactive basal planes. Conversely, the metallic 1T phase of MoS2 features active edge sites and basal planes, making it more favorable for various applications. Numerous efforts have been made to modify bulk MoS2 which exists as a 2H phase. By utilizing intercalation, which facilitates the conversion from the 2H to the 1T phase, also improves storage capacity and stabilizes metastable 1T phase, etc. To achieve these unique properties, various intercalation methods have been extensively studied and are thoroughly reviewed in this study. The intercalation process involves introducing foreign materials into a host material with adjustable van der Waals gaps. This allows for the insertion of guest material between the layers, making them excellent candidates for intercalation. In this review, we will explore various methods of intercalating MoS2 material, including the electrochemical, solution-based, wet chemical, and ion-exchange methods.
Two-dimensional carbon nitride materials have been the center of attention for their diverse usage in energy harvesting, environmental remediation and nanoelectronic applications. A broad range of utilities with decent synthetic plausibility have made this family a sweet spot to dive into, whereas the underlying analytical aspects are yet to have prominence. Recently, using the machinaries of first principles, we reported a family of six different structures C3NX (Jana et al., 2023) with a unique dumbbell-shaped morphology, functionalizing the recently synthesized monolayer of C3N (Yang et al., 2017). Here we have critically explored the non-trivial topological phases of the semimetallic Dumbbell C3NX sheets and nanoribbons. Spin–orbit coupling induced gap across the Fermi level, its subsequent tuning via an external electric field, portrayal of band inversion from the Berry curvature distribution and the evaluation of Z2 topological index using the Wannier charge center (WCC) firmly establishes the traces of topological footprint. The real space decimation scheme and Green’s function technique evaluate the underlying spectral information with corresponding transport characteristics. Fascinating features of these quasi-1D systems are observed utilizing the Su-Schrieffer-Heeger (SSH) model where different twisted phases reveal distinct topological signatures even in a low atomic mass system like DB C4N.
Two-dimensional (2D) materials have recently gained much interest as anode materials for lithium ion batteries (LIBs) due to their interesting properties of high surface-to-volume ratio, many adsorption sites, short diffusion paths, fascinating electronic properties and high storage capacity. Despite the diversity of synthesis methods for preparation of 2D materials running from top-down to bottom-up techniques, the experimental investigations of 2D materials in energy storage systems is still in the beginning stage owing to the problems of complicated fabrication process, large scale production and sometime very high cost. First-principles approaches are among the best alternative options in this case since they can significantly reduce the time and cost of the design process while still delivering reliable results in comparison to the experimental measurement. For this regard, in this chapter, we reviewed the theoretical formalism used for calculating important parameters of anode materials such as the electronic and mechanical properties, the adsorption and activation energies, voltage profile and theoretical capacity storage. We discussed also the powerful of density functional theory (DFT) in predicting and designing new 2D materials with high electrochemical performance. After, we reviewed the applicability of Graphene, Phosphorene, Silicene, Germanene, Stanene, Arsenene, Antimonene, h-BX (X=N, P, As, Sb) monolayers and Metal transition dichalcogenides as anode materials for LIBs. Finally, through the discussion, we investigate the external and internal effects on the performance of these materials as anodes for LIBs. Among these effects, we listed structural defects, doping with forging atoms, application of external stain and the chemical composition of the above mentioned materials. Furthermore, the challenges regarding the applicability of these materials in LIBs are also provided.
Two-dimensional (2D) group-IV materials with tetragonal structures have attracted considerable attention recently due to their structural stability and distinctive electronic and optical characteristics. However, topological quantum states, particularly second-order topological insulators (SOTIs), are seldom reported in them. Based on first-principles and tight-binding (TB) model calculations, we find that 2D hydrogenated tetragonal stanene (T-SnH) is an exotic SOTI, identified by the second Stiefel-Whitney number w2=1, along with robust corner states. Intriguingly, the SOTI in T-SnH transforms into a quantum spin Hall insulator under 2.7% tensile strain. The same topological phase transition can also occur by substituting the Sn element with the Pb element, namely, forming a T-PbH monolayer. A three-orbital TB model is constructed to understand the phase transition mechanism for the two schemes, both associated with band inversion between antibonding s and bonding px,y states due to the large Sn-Sn (or Pb-Pb) bond lengths. The phase transition is also rationalized well from topological quantum chemistry theory. These insights are helpful for understanding the SOTI and provide a promising material platform for topological quantum device designs and applications.
Xenes, mono‐elemental atomic sheets, exhibit Dirac/Dirac‐like quantum behavior. When interfaced with other 2D materials such as boron nitride, transition metal dichalcogenides, and metal carbides/nitrides/carbonitrides, it enables them with unique physicochemical properties, including structural stability, desirable bandgap, efficient charge carrier injection, flexibility/breaking stress, thermal conductivity, chemical reactivity, catalytic efficiency, molecular adsorption, and wettability. For example, BN acts as an anti‐oxidative shield, MoS2 injects electrons upon laser excitation, and MXene provides mechanical flexibility. Beyond precise compositional modulations, stacking sequences, and inter‐layer coupling controlled by parameters, achieving scalability and reproducibility in hybridization is crucial for implementing these quantum materials in consumer applications. However, realizing the full potential of these hybrid materials faces challenges such as air gaps, uneven interfaces, and the formation of defects and functional groups. Advanced synthesis techniques, a deep understanding of quantum behaviors, precise control over interfacial interactions, and awareness of cross‐correlations among these factors are essential. Xene‐based hybrids show immense promise for groundbreaking applications in quantum computing, flexible electronics, energy storage, and catalysis. In this timely perspective, recent discoveries of novel Xenes and their hybrids are highlighted, emphasizing correlations among synthetic parameters, structure, properties, and applications. It is anticipated that these insights will revolutionize diverse industries and technologies.
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.
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.
- L Britnell
- R M Ribeiro
- A Eckmann
- R Jalil
- B D Belle
- A Mishchenko
- Y.-J Kim
- R V Gorbachev
- T Georgiou
- S V Morozov
- A N Grigorenko
L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle, A. Mishchenko,
Y.-J. Kim, R. V. Gorbachev, T. Georgiou, S. V. Morozov, A. N. Grigorenko,
A. K. Geim, C. Casiraghi, A. H. Castro Neto, K. S. Novoselov, Science
2009, 340 ( 6138 ), 1311 – 1314.
- E Tekman
- S Ciraci
E. Tekman, S. Ciraci, Phys. Scr. 1988, 38 ( 3 ), 486.
- A J Koma
A. J. Koma, Cryst. Growth 1999, 201, 236 -241.
- D J Late
- B Liu
- H S S R Matte
- C N R Rao
- V P Dravid
D. J. Late, B. Liu, H. S. S. R. Matte, C. N. R. Rao, V. P. Dravid,
Adv. Funct. Mater. 2012, 22, 1894.
- A Molle
- C Grazianetti
- D Chiappe
- E Cinquanta
- E Cianci
- G Tallarida
- M Fanciulli
A. Molle, C. Grazianetti, D. Chiappe, E. Cinquanta, E. Cianci,
G. Tallarida, M. Fanciulli, Adv. Funct. Mater. 2013, 23, 4340 -4344.
- L Meng
- Y Wang
- L Zhang
- S Du
- R Wu
- L Li
- Y Zhang
- G Li
- H Zhou
- W A Hofer
- H Gao
L. Meng, Y. Wang, L. Zhang, S. Du, R. Wu, L. Li, Y. Zhang, G. Li,
H. Zhou, W. A. Hofer, H. Gao, Nano Lett. 2013, 13 ( 2 ), 685 -690.
- G Gao
- W Gao
- E Cannuccia
- J Taha-Tijerina
- L Balicas
- A Mathkar
- T N Narayanan
- Z Liu
- B K Gupta
- J Peng
- Y Yin
- A Rubio
- P M Ajayan
G. Gao, W. Gao, E. Cannuccia, J. Taha-Tijerina, L. Balicas,
A. Mathkar, T. N. Narayanan, Z. Liu, B. K. Gupta, J. Peng, Y. Yin,
A. Rubio, P. M. Ajayan, Nano Lett. 2012, 12 ( 7 ), 3518 -3525.
- R V Kashid
- D J Late
- S S Chou
- Y.-K Huang
- M De
- D S Joag
- M A More
- V P Dravid
R. V. Kashid, D. J. Late, S. S. Chou, Y.-K. Huang, M. De, D. S. Joag,
M. A. More, V. P. Dravid, Small 2013, 9, 2730.