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Reversible Iodine Intercalation into Tungsten Ditelluride
Abstract
The new compound WTe2I was prepared by a reaction of WTe2 with iodine in a fused silica ampule at temperatures between 40 and 200 °C. Iodine atoms are intercalated into the van der Waals gap between tungsten ditelluride layers. As a result, the WTe2 layer separation is significantly increased. Iodine atoms form planar layers between each tungsten ditelluride layer. Due to oxidation by iodine the semimetallic nature of WTe2 is changed, as shown by comparative band structure calculations for WTe2 and WTe2I based on density functional theory. The calculated phonon band structure of WTe2I indicates the presence of phonon instabilities related to charge density waves, leading to an observed incommensurate modulation of the iodine position within the layers.
... The intercalation of van der Waals materials may allow one to effectively adjust the electronic structure and physical properties. Recently, Schmidt synthesized iodine-intercalated tungsten ditelluride (WTe 2 I) and found that its band structures have some differences from that of WTe 2 [32], which are highly significant to further understand the MR behavior in WTe 2 . However, magnetotransport studies on WTe 2 I are still absent. ...
... Å, b = 3.476 Å, and c = 6.327 Å are in good agreement with the previously reported data [32]. The average W:Te:I atomic ratio determined using the EDX is very close to 1:2:1 and no foreign elements were detected within the limitation of instrument resolution, as shown in the left inset of Fig. 1(c). ...
... Our calculated band structure of WTe 2 I is in well agreement with the reported result [32]. As shown in Fig. 6(a), the calculated electronic structures with spin-orbital coupling indicate that WTe 2 I is a metal. ...
We report a systematic study of the magnetoresistance, Hall effect, and the electronic structure in an iodine-intercalated tungsten ditelluride (WTe 2 I) single crystal, which crystallizes in a centrosymmetric structure. WTe 2 I presents significant anisotropic magnetotransport behavior with the magnetic field rotated in the xz and yz planes and violation of the Kohler's rule. Unexpectedly, nonsaturating linear magnetoresistance (LMR) was observed under the magnetic field parallel to the z axis and the current along the x axis. The analysis of the Hall effect reveals that the carrier mobility in WTe 2 I is notably lower than that of WTe 2 and meets the picture of the Parrish and Littlewood model, suggesting that the LMR is likely caused by disorder effects. First-principles calculations show that the electronic structure undergoes a topological phase transition from a Weyl state to a Dirac state due to iodine intercalation. This finding provides a platform to search for physical phenomena related to WTe 2 and study the topological phase transition.
Layered materials with a robust structure and reversible intercalation behavior are highly sought-after in applications involving energy conversion and storage systems, energy converting devices, supercapacitors, batteries, superconductors, photonic materials, and...
Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulk-edge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipation-less nature and spin-momentum locking. Here we introduce a transition-metal dichalcogenide VTe2, that hosts a charge density wave (CDW) coupled with the band inversion involving V3d and Te5p orbitals. Spin- and angle-resolved photoemission spectroscopy with first-principles calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Dirac-type spin-polarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon the CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs’ flexible characters to external stimuli. Manipulating topological states by coupled electronic orders is promising for future dissipation-less electronic devices. Here, Mitsuishi et al. report selective vanishing of Dirac-type topological surface states by the formation of coupled charge density wave in a transition-metal dichalcogenide VTe2.
The new compound (TeI3)2[W6I14] appeared as an unforeseen product from a reaction of WTe2 with iodine at 250 °C, representing an exceptionally low temperature for the formation of an octahedral tungsten iodide cluster. The crystal structure, as refined by single‐crystal X‐ray diffraction, reveals (TeI3)⁺ ions and the well‐known [W6I14]2– cluster. A chemical analysis confirms the identity of the product. The electronic structure of (TeI3)2[W6I14] is analyzed in view of the relative position of tellurium states by density functional theory (DFT) calculations. This cluster compound exhibits an indirect band gap as confirmed by the band structure calculation based on the HSE06 hybrid functional theory.
Using the MoS2‐WTe2 heterostructure as a model system combined with electrochemical microreactors and density function theory calculations, it is shown that heterostructured contacts enhance the hydrogen evolution reaction (HER) activity of monolayer MoS2. Two possible mechanisms are suggested to explain this enhancement: efficient charge injection through large‐area heterojunctions between MoS2 and WTe2 and effective screening of mirror charges due to the semimetallic nature of WTe2. The dielectric screening effect is proven minor, probed by measuring the HER activity of monolayer MoS2 on various support substrates with dielectric constants ranging from 4 to 300. Thus, the enhanced HER is attributed to the increased charge injection into MoS2 through large‐area heterojunctions. Based on this understanding, a MoS2/WTe2 hybrid catalyst is fabricated with an HER overpotential of −140 mV at 10 mA cm⁻², a Tafel slope of 40 mV dec⁻¹, and long stability. These results demonstrate the importance of interfacial design in transition metal dichalcogenide HER catalysts. The microreactor platform presents an unambiguous approach to probe interfacial effects in various electrocatalytic reactions.
Currently, there is a flurry of research interest on materials with an unconventional electronic structure, and we have already seen significant progress in their understanding and engineering towards real-life applications. The interest erupted with the discovery of graphene and topological insulators in the previous decade. The electrons in graphene simulate massless Dirac Fermions with a linearly dispersing Dirac cone in their band structure, while in topological insulators, the electronic bands wind non-trivially in momentum space giving rise to gapless surface states and bulk bandgap. Weyl semimetals in condensed matter systems are the latest addition to this growing family of topological materials. Weyl Fermions are known in the context of high energy physics since almost the beginning of quantum mechanics. They apparently violate charge conservation rules, displaying the 'chiral anomaly', with such remarkable properties recently theoretically predicted and experimentally verified to exist as low energy quasiparticle states in certain condensed matter systems. Not only are these new materials extremely important for our fundamental understanding of quantum phenomena, but also they exhibit completely different transport phenomena. For example, massless Fermions are susceptible to scattering from non-magnetic impurities. Dirac semimetals exhibit non-saturating extremely large magnetoresistance as a consequence of their robust electronic bands being protected by time reversal symmetry. These open up whole new possibilities for materials engineering and applications including quantum computing. In this review, we recapitulate some of the outstanding properties of WTe2, namely, its non-saturating titanic magnetoresistance due to perfect electron and hole carrier balance up to a very high magnetic field observed for the very first time. It also indicative of hosting Lorentz violating type-II Weyl Fermions in its bandstructure, again first predicted candidate material to host such a remarkable phase. We primarily focus on the findings of our ARPES, spin-ARPES, and time-resolved ARPES studies complemented by first-principles calculations.
Recently published discoveries of acoustic- and optical-mode inversion in the phonon spectrum of certain metals became the first realistic example of noninteracting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this paper, we provide strong theoretical and numerical evidence that for a class of metallic compounds (known as triple-point topological metals), the points in the phonon spectrum, at which three (two optical and one acoustic) phonon modes (bands) cross, represent a well-defined topological material phase, in which the hosting metals have very strong thermoelectric response. The triple-point bosonic collective excitations appearing due to these topological phonon band-crossing points significantly suppress the lattice thermal conductivity, making such metals phonon glasslike. At the same time, the topological triple-point and Weyl fermionic quasiparticle excitations present in these metals yield good electrical transport (electron crystal) and cause a local enhancement in the electronic density of states near the Fermi level, which considerably improves the thermopower. This combination of phonon glass and electron crystal is the key for high thermoelectric performance in metals. We call these materials topological thermoelectric metals and propose several compounds for this phase (TaSb and TaBi). We hope that this work will lead researchers in physics and materials science to the detailed study of topological phonon phases in electronic materials, and the possibility of these phases to introduce more efficient use of thermoelectric materials in many everyday technological applications.
Thin layer two-dimensional (2-D) transition metal dichalcogenide (TMD) materials have distinctive optoelectronic properties. Therefore, several methods including mechanical exfoliation, chemical vapor deposition, and liquid-phase exfoliation have been attempted to obtain uniform TMDs. However, such methods do not easily produce high-quality few-layer TMDs with high speed. Here, we report the successful fabrication of few-layer TMD materials by femtosecond laser irradiation. It shows that TMD samples can be exfoliated from bulk to ~3 layers. This method is much faster and simpler than other exfoliation methods. The size and number of the layers were confirmed by atomic force microscopy, scanning electron microscopy, Raman spectroscopy, and photoluminescence experiments. It is expected to be used for the mass production of thin 2-D TMD materials.
We present a density functional theory study of the electronic structure of single-layer TiSe2, and focus on the charge density wave (CDW) instability present on this 2D material. We explain the periodicity of the CDW from the phonon band structure of the undistorted crystal, which is unstable under one of the phonon modes at the M point. This can be understood in terms of a partial band gap opening at the Fermi level, which we describe on the basis of the symmetry of the involved crystal orbitals, leading to an energy gain upon the displacement of the atoms following the phonon mode in a 2 × 1 structure. Furthermore, the combination of the corresponding phonons for the three inequivalent M points of the Brillouin zone leads to the 2 × 2 distortion characteristic of the CDW state. This leads to a further opening of a full gap, which reduces the energy of the 2 × 2 structure compared to the 2 × 1 one of a single M point phonon, and makes the CDW structure the most stable one. We also analyze the effect of charge injection into the layer on the structural instability. We predict that the 2 × 2 structure only survives for a certain range of doping levels, both for electrons and for holes, as doping reduces the energy gain due to the gap opening. We predict the transition from the commensurate 2 × 2 distortion to an incommensurate one with increasing wavelength upon increasing the doping level, followed by the appearance of the undistorted 1 × 1 structure for larger carrier concentrations.
In analogy to various fermions of electrons in topological semimetals, topological mechanical states with two types of bosons, Dirac and Weyl bosons, were reported in some macroscopic systems of kHz frequency, and those with a type of doubly-Weyl phonons in atomic vibrational framework of THz frequency of solid crystals were recently predicted. Here, through first-principles calculations, we have reported that the phonon spectra of the WC-type TiS, ZrSe, and HfTe commonly host the unique triply degenerate nodal points (TDNPs) and single two-component Weyl points (WPs) in THz frequency. Quasiparticle excitations near TDNPs of phonons are three-component bosons, beyond the conventional and known classifications of Dirac, Weyl, and doubly-Weyl phonons. Moreover, we have found that both TiS and ZrSe have five pairs of type-I Weyl phonons and a pair of type-II Weyl phonons, whereas HfTe only has four pairs of type-I Weyl phonons. They carry nonzero topological charges. On the (101¯0) crystal surfaces, we observe topological protected surface arc states connecting two WPs with opposite charges, which host modes that propagate nearly in one direction on the surface.
Recently, a type-II Weyl fermion was theoretically predicted to appear at the contact of electron and hole Fermi surface pockets. A distinguishing feature of the surfaces of type-II Weyl semimetals is the existence of topological surface states, so-called Fermi arcs. Although WTe2 was the first material suggested as a type-II Weyl semimetal, the direct observation of its tilting Weyl cone and Fermi arc has not yet been successful. Here, we show strong evidence that WTe2 is a type-II Weyl semimetal by observing two unique transport properties simultaneously in one WTe2 nanoribbon. The negative magnetoresistance induced by a chiral anomaly is quite anisotropic in WTe2 nanoribbons, which is present in b-axis ribbon, but is absent in a-axis ribbon. An extra-quantum oscillation, arising from a Weyl orbit formed by the Fermi arc and bulk Landau levels, displays a two dimensional feature and decays as the thickness increases in WTe2 nanoribbon.
Recently, the layered semimetal WTe2 has attracted renewed interest owing to the observation of a non-saturating and giant positive magnetoresistance (~105%), which can be useful for magnetic memory and spintronic devices. However, the underlying mechanisms of the giant magnetoresistance are still under hot debate. Herein, we grew the stoichiometric and non-stoichiometric WTe2 crystals to test the robustness of giant magnetoresistance. The stoichiometric WTe2 crystals have magnetoresistance as large as 3100% at 2 K and 9-Tesla magnetic field. However, only 71% and 13% magnetoresistance in the most non-stoichiometry (WTe1.80) and the highest Mo isovalent substitution samples (W0.7Mo0.3Te2) are observed, respectively. Analysis of the magnetic-field dependent magnetoresistance of non-stoichiometric WTe2 crystals substantiates that both the large electron-hole concentration asymmetry and decreased carrier mobility, induced by non-stoichiometry, synergistically lead to the decreased magnetoresistance. This work sheds more light on the origin of giant magnetoresistance observed in WTe2.
Transition metal dichalcogenides (TMDs) are a class of two-dimensional (2D) materials that have attracted growing interest because of their unique electronic and optical properties. Under ambient conditions, most TMDs generally exhibit 2H or 1T structures. Unlike other group VIb TMDs, bulk crystals and powders of WTe2 exist in a distorted 1T structure (Td) at room temperature and have semimetallic properties. There is so far a lack of direct atom-by-atom visualization, limiting our understanding of this distorted 2D layered material system. We present herein atomic resolution images of Td structured WTe2. The Td structure can be distinguished in the three major orientations along the [100], [010], and [001] zone axes. Subtle structural distortions are detected by atomic resolution imaging, which match well with the optimized structure relaxed by ab initio calculations. The calculations also showed that both crystal field splitting and charge density wave (CDW) interactions contribute to the stabilization of WTe2. However, the CDW interaction dominates and leads the Td-WTe2 to be the most stable structure. The combined atomic resolution STEM and ab initio study on WTe2 provided the basis for understanding the correlations between atomic structure and electronic properties in Td structured TMD materials.
Single crystals of CuxTa1+ySe2 were grown by chemical vapor transport. Single crystals of different compositions were obtained at slightly different reaction conditions from mixtures of the reactants of the same nominal composition. It is suggested that different diameters of the ampoules imply different contributions of convection and diffusion to the mass transport, and thus are responsible for different ratios of the amount of Cu, Ta, and Se transported. 2H-Cu0.52TaSe2 (x = 0.52, y = 0) is formed in the narrower ampoule (diameter 15 mm). The crystal structure is based on the MoS2 type of stacking of TaSe2 layers. Partial ordering of Cu over the tetrahedral sites is responsible for a 2a0 × 2b0 × c0 superstructure with hexagonal Pm2 symmetry [a0 = 3.468 (1) Å, c0 = 13.568 (3) Å]. 2H-Cu0.16Ta1.08Se2 (x = 0.16, y = 0.08) is formed in the wider ampoule (diameter 18 mm). It possesses a NbS2-type of stacking. A superstructure is not formed, but the presence of Cu and intercalated Ta in alternating van der Waals gaps is responsible for the reduction of symmetry from P63/mmc to Pm1 [a0 = 3.439 (2) Å, c0 = 12.870 (2) Å]. Single crystals are formed towards the hotter side of the ampoules up to a temperature of 1168 K in both reactions.
Magnetoresistance is the change in a material's electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors, in magnetic memory, and in hard drives at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.
Ta 1+y S 2 (x ≈ 0.23, y = 0, 0.06) Sk Imran Ali, [a] Swastik Mondal, [a] Siriyara Jagannatha Prathapa, [a] Sander van Smaalen,* [a] Steffen Zörb, [b] and Bernd Harbrecht [b] Abstract. The eightfold 2a 0 ϫ2b 0 ϫ2c 0 superstructures of 6R-Cu x Ta 1+y S 2 are reported for crystal A with x = 0.237 and y = 0, and crystal B with x = 0.23 and y = 0.06. Compelling evidence is presented for the self-intercalation of tantalum onto octahedral sites, based on the diffraction data and crystal chemical arguments. The eightfold su-perstructure is formed by partial vacancy ordering within planes of intercalated atoms, while the stacking of TaS 2 layers remains that of the 6R polytype. Temperature-dependent X-ray diffraction experiments
We have studied the dispersion and exfoliation of four inorganic layered compounds, WS(2), MoS(2), MoSe(2), and MoTe(2), in a range of organic solvents. The aim was to explore the relationship between the chemical structure of the exfoliated nanosheets and their dispersibility. Sonication of the layered compounds in solvents generally gave few-layer nanosheets with lateral dimensions of a few hundred nanometers. However, the dispersed concentration varied greatly from solvent to solvent. For all four materials, the concentration peaked for solvents with surface energy close to 70 mJ/m(2), implying that all four have surface energy close to this value. Inverse gas chromatography measurements showed MoS(2) and MoSe(2) to have surface energies of ∼75 mJ/m(2), in good agreement with dispersibility measurements. However, this method suggested MoTe(2) to have a considerably larger surface energy (∼120 mJ/m(2)). While surface-energy-based solubility parameters are perhaps more intuitive for two-dimensional materials, Hansen solubility parameters are probably more useful. Our analysis shows the dispersed concentration of all four layered materials to show well-defined peaks when plotted as a function of Hansen's dispersive, polar, and H-bonding solubility parameters. This suggests that we can associate Hansen solubility parameters of δ(D) ∼ 18 MPa(1/2), δ(P) ∼ 8.5 MPa(1/2), and δ(H) ∼ 7 MPa(1/2) with all four types of layered material. Knowledge of these properties allows the estimation of the Flory-Huggins parameter, χ, for each combination of nanosheet and solvent. We found that the dispersed concentration of each material falls exponentially with χ as predicted by solution thermodynamics. This work shows that solution thermodynamics and specifically solubility parameter analysis can be used as a framework to understand the dispersion of two-dimensional materials. Finally, we note that in good solvents, such as cyclohexylpyrrolidone, the dispersions are temporally stable with >90% of material remaining dispersed after 100 h.
The method of dispersion correction as an add-on to standard Kohn-Sham density functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coefficients and cutoff radii that are both computed from first principles. The coefficients for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination numbers (CN). They are used to interpolate between dispersion coefficients of atoms in different chemical environments. The method only requires adjustment of two global parameters for each density functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of atomic forces. Three-body nonadditivity terms are considered. The method has been assessed on standard benchmark sets for inter- and intramolecular noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean absolute deviations for the S22 benchmark set of noncovalent interactions for 11 standard density functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C(6) coefficients also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in molecules and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems.
Ultrathin sheets of transition metal dichalcogenides have received a great deal of interest in an attempt to develop advanced electrode nanomaterials for electrochemical capacitors, with most of the attention focused on pure transition metal dichalcogenides. We systematically investigated and compared the charge storage capability of tert-butyllithium-exfoliated group 6 transition metal dichalcogenide materials (MX2; M = Mo, W; X = S, Se, Te). The group 6 MX2 2D materials exhibited distinctive charge storage performance as a result of their different chemical compositions and morphological features. MoTe2 nanosheets demonstrated the highest gravimetric capacitance of all materials. Furthermore, we hybridized these 2D materials with nanofibrous polyaniline (PANI) to study the impact of MX2 materials on its capacitance performance. All the fabricated nanocomposites exhibited a greatly enhanced capacitance compared with the pure PANI, with the highest increase observed for MoTe2/PANI (499 F g−1), which represents a 76% increase. This enhancement arises from the promotion of available active surface area of PANI and MoTe2 for electrolyte ions.
A facile environmentally friendly liquid ultrasonic exfoliation of transition metal dichalcogenides (TMDCs) was proposed by using ATP as the exfoliation reagent. A biosensing platform for the detection of adenosine based...
To realize topological superconductor is one of the most attracting topics because of its great potential in quantum computation. In this study, we successfully intercalate potassium (K) into the van der Waals gap of type II Weyl semimetal WTe2, and discover the superconducting state in KxWTe2 through both electrical transport and scanning tunneling spectroscopy measurements. The superconductivity exhibits an evident anisotropic behavior. Moreover, we also uncover the coexistence of superconductivity and the positive magneto-resistance state. Structural analysis substantiates the negligible lattice expansion induced by the intercalation, therefore suggesting K-intercalated WTe2 still hosts the topological nontrivial state. These results indicate that the K-intercalated WTe2 may be a promising candidate to explore the topological superconductor.
Transition metal dichalcogenides are layered materials which are composed of transition metals and chalcogens of the group VIA in a 1:2 ratio. These layered materials have been extensively investigated over synthesis and optical and electrical properties for several decades. It can be insulators, semiconductors, or metals revealing all types of condensed matter properties from a magnetic lattice distorted to superconducting characteristics. Some of these also feature the topological manner. Instead of covering the semiconducting properties of transition metal dichalcogenides, which have been extensively revisited and reviewed elsewhere, here we present the structures of metallic transition metal dichalcogenides and their synthetic approaches for not only high-quality wafer-scale samples using conventional methods (e.g., chemical vapor transport, chemical vapor deposition) but also local small areas by a modification of the materials using Li intercalation, electron beam irradiation, light illumination, pressures, and strains. Some representative band structures of metallic transition metal dichalcogenides and their strong layer-dependence are reviewed and updated, both in theoretical calculations and experiments. In addition, we discuss the physical properties of metallic transition metal dichalcogenides such as periodic lattice distortion, magnetoresistance, superconductivity, topological insulator, and Weyl semimetal. Approaches to overcome current challenges related to these materials are also proposed.
The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe2, in a van der Waals hybrid structure obtained by mechanically transferring NbSe2 onto various thicknesses of WTe2. When the WTe2 thickness (t_(WTe_2 )) reaches 21 nm, the superconducting transition occurs around the critical temperature (T_c) of NbSe2 with a gap amplitude (Δ_p) of 0.38 meV and an unexpected ultra-long proximity length (l_p) up to 7 μm. With the thicker 42nm WTe2 layer, however, the proximity effect yields T_c~1.2 K, Δ_p=0.07 meV and a short l_p of less than 1 μm. Our theoretical calculations, based on the Bogoliubov-de Gennes equations in the clean limit, predict that the induced superconducting gap is a sizable fraction of the NbSe2 superconducting one when t_(WTe_2 ) is less than 30 nm, and then decreases quickly as t_(WTe_2 ) increases. This agrees qualitatively well with the experiments. Such observations demonstrate a new direction in the search for superconducting phases in topological semimetals.
The traditional use of Scotch tape for exfoliating layers of two-dimensional (2D) transition metal dichalcogenides (TMDs) has been compared with a gel-assisted mechanical exfoliation technique, using MoS2 as a representative TMD. The gel-assisted exfoliation process, which makes use of both Scotch tape and a gel film, is superior to the use of Scotch tape alone, as it gives a higher probability of obtaining larger surface area few-layer flakes. A quantitative analysis has been made between the samples prepared by the two techniques. The total density of flakes transferred onto a sample by Scotch tape alone was much higher than when using the gel film. However, most of the transferred flakes were several microns in thickness with lateral dimensions <10 μm. Therefore, the higher percentage of few-layer flakes with large lateral dimensions (> 20 μm) transferred using gel film is very advantageous. Since samples prepared using gel film have fewer flakes, the contacting of potential thin flakes on the sample can be done conveniently. Also, unlike Scotch tape, the gel film does not leave adhesive residue on the substrate. Optical microscopy, contrast difference measurements and Raman spectroscopy were used for identification of the few-layer MoS2 flakes.
ABINIT is a package whose main program allows one to find the total energy, charge density, electronic structure and many other properties of systems made of electrons and nuclei, (molecules and periodic solids) within Density Functional Theory (DFT), Many-Body Perturbation Theory (GW approximation and Bethe–Salpeter equation) and Dynamical Mean Field Theory (DMFT). ABINIT also allows to optimize the geometry according to the DFT forces and stresses, to perform molecular dynamics simulations using these forces, and to generate dynamical matrices, Born effective charges and dielectric tensors. The present paper aims to describe the new capabilities of ABINIT that have been developed since 2009. It covers both physical and technical developments inside the ABINIT code, as well as developments provided within the ABINIT package. The developments are described with relevant references, input variables, tests and tutorials.
Intercalation is a reversible insertion process of foreign species into crystal gaps. Layered materials are good host materials for various intercalant species ranging from small ions to atoms to molecules. Given the recent intense interest in two-dimensional (2D) layered materials in thin limits, this review highlights the opportunities that intercalation chemistry can provide for nanoscale layered materials. Novel heterostructures or emergent electrical properties not found in the intrinsic host materials are possible with intercalation. In particular, we review various exfoliation methods developed for 2D layered nanomaterials based on intercalation chemistry and extensive tuning of electrical, optical, and magnetic properties of 2D layered materials due to intercalation.
Semiconducting two-dimensional transition-metal dichalcogenides (MX2) are attracting much attention as promising materials for a new generation of optical and electronic devices. MX2 compounds are complementary or competitive to graphene because of the existence of a native band gap. The growth of large and high-quality bulk single crystals is one of the critical issues for the application of MX2 compounds, whose bulk crystals are generally grown by the chemical vapor transport (CVT) method. In the present review, I introduce experimental techniques required for the CVT growth of high-quality MX2 single crystals.
Thio-, seleno-, and tellurohalides are classes of compounds with only sparse coverage in the chemical literature. Over the past 20 years, however, there has been a gradual increase in activity in these areas of chemistry. Although the papers published so far have been the almost exclusive preserve of a small number of research groups. This review summarizes the preparation and chemistry of known thio-, seleno-, and tellurohalides of the transition-metal elements, the compounds being dealt with element by element and group by group. For the purposes of the review, we have adopted the definition of transition elements as those which as elements, have partly filled d or f shells in any of their commonly occurring oxidation states. This means that copper, silver, and gold are included. However, whereas the Group I elements may lose one or two d electrons to give ions or complexes in the II and III oxidation states, this is no longer possible for Group 11, and zinc, cadmium, and mercury compounds have been omitted.
Tellurium double layers, between which planar layers of iodine molecules are inserted, characterize the new intercalation compound (Te 2 ) 2 (I 2 ); it contains a tellurium modification that was previously unknown as an independent phase, which is intercalated by elemental iodine. Crystals were obtained by hydrothermal synthesis in concentrated hydrogen iodide.
The structures of AuTe 2Cl and AuTe 2I have been
determined. Both compounds are orthorhombic: for AuTe 2Cl, a
= 4.020 Å, b = 11.867 Å, c = 8.773Å, Z = 4, space
group Cmcm; for AuTe 2I, a = 4.056 Å, b = 12.579
Å, c = 4.741 Å, Z = 2, Pmmb. Intensities were measured on an
automatic diffractometer, and the structures were refined, with
anisotropic temperature factors, to R = 2.1% and R = 3.5%, respectively.
The structures consist essentially of corrugated two-dimensional nets of
gold and tellurium atoms, with interleaving halogen atoms. The tellurium
atoms form pairs coordinated to four gold atoms, and each gold atom is
coordinated to four tellurium atoms.
The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic
modifications, acquire, below some appropriate temperature, phase
conditions that their electromagnetic properties have previously
revealed as ‘anomalous’. Our present electron-microscope
studies indicate that this anomalous behaviour usually includes the
adoption, at some stage, of a superlattice. The size of superlattice
adopted often is forecast in the pattern of satellite spotting and
strong diffuse scattering found above the transition.Our conclusions are
that charge-density waves and their concomitant periodic structural
distortions occur in all these 4d1/5d1 dichalcogenides. We have related
the observed periodicities of these CDW states to the theoretical form
of the parent Fermi surfaces. Particularly for the 1T octahedrally
coordinated polytypes the Fermi surface is very simple and markedly
two-dimensional in character, with large near-parallel walls. Such a
situation is known theoretically to favour the formation of charge and
spin-density waves. When they first appear, the CDW's in the 1T (and
4Hb) polytypes are incommensurate with the lattice. This condition
produces a fair amount of gapping in the density of states at the Fermi
level. For the simplest case of 1T-TaSe2, the room temperature
superlattice is realized when this existing CDW rotates into an
orientation for which it then becomes commensurate. At this first-order
transition the Fermi surface energy gapping increases beyond that
generated by the incommensurate CDW, as is clearly evident in the
electromagnetic properties.For the trigonal prismatically coordinated
polytypes, CDW formation is withheld to low temperatures, probably
because of the more complex band structures. This CDW state (in the
cases measured) would seem at once commensurate, even though the
transition is, from a wide variety of experiments, apparently second
order.A wide range of doped and intercalated materials have been used to
substantiate the presence of CDW's in these compounds, and to clarify
the effect that their occurrence has on the physical properties.The
observations further demonstrate the distinctiveness of the transition
metal dichalcogenide layer compounds, and of the group VA metals in
particular.
A previously unreported compound, Ag0.79VS2, has been synthesized; its structure and elementary properties are reported. Ag0.79VS2 crystallizes in two forms, designated as the α and β, related to the 1s-InTaS2 structure. Single crystal x-ray diffraction shows the α form to have a single layer hexagonal structure with a unit cell of 3.213(3) Å×7.809(6) Å, consisting of layers of edge-shared VS6 triangular prisms separated by layers of Ag. The β form is similar but has an ao(3) supercell in the basal plane, yielding a unit cell of 5.573(5) Å×7.822(6) Å. Both forms have disordered and displaced silver in the basal plane, but the β form has partial ordering of its silver sublattice and in-plane vanadium trimers. Resistivity measurements show metallic temperature dependence with an unusual hysteresis between 210 K and 130 K. Magnetic susceptibility measurements show Pauli Paramagnetic behavior. The Seebeck coefficient at 300 K is 42 µV/K.
The present work reports on the synthesis of indium intercalation compounds InxWS2 (0 ⩽ x ⩽ 1). The material has been characterized by X-ray studies for structure determination and particle size distribution, room temperature magnetic susceptibility, thermoelectric power experiments and conductivity measurements in the temperature range 150–300 K. These results have indicated that, like the host material WS2, the intercalated compounds also possess hexagonal symmetry and are diamagnetic p-type semiconductors. The activation energy for these compounds was determined from the conductivity data.
The structure parameters of TiS2 have been refined using the least squares method. The structure belongs to the space group P3̄ml (164) with nearly ideal octahedral coordination of the sulfur atoms about the titanium. The hexagonal, laminar structure has and with the interlayer sulfur-sulfur distance of 3.462(5) Å.
Recommendations are presented for the nomenclature and terminology of graphite intercalation compounds, i.e. compounds that retain the planarity of the layers in the graphite structure. Not included are compounds with covalent bonding of heteroatoms to the carbon atoms which leads to a sp3 hybridization of the carbon atoms and a corresponding loss of planarity of the layers. Rules for a structural notation are described in addition to those for the naming and the formulation of graphite intercalation compounds.
The new ternary transition-metal tellurides TaIrTeâ and NbIrTeâ are ordered variants of the WTeâ structure, which in turn is based on a distortion of the CdIâ-type layered structure. The layers in WTeâ consist of buckled sheets of Te atoms, with the metal atoms residing in distorted octahedral sites. Through single-crystal X-ray diffraction methods, the structure of TaIrTeâ has been determined and that of WTeâ has been redetermined. The compounds TaIrTeâ and WTeâ belong to the space group Câ·{sub 2ν}-Pmn2â of the orthorhombic system with four formula units in cells of dimensions a = 3.770 (1), b = 12.421 (6), and c = 13.184 (6) â« and a = 3.477 (2), b = 6.249 (4), and c = 14.018 (9) â«, respectively at 113 K. While metal-metal bonding is a structural feature common to all three compounds, Te-Te bonding is observed only in the ternary compounds. The trends of increasing metal-metal and decreasing Te-Te distances on progressing from WTeâ to TaIrTeâ and NbIrTeâ have been rationalized by electronic band (extended Hueckel) calculations. These trends are related to the creation of Te-Te bonds, ensuring the stability of the WTeâ structure type even when addition of more d electrons leads to a weakening of metal-metal bonds. This concept is generalized to an entire series of compounds MMâ²Teâ (M = Nb, Ta; Mâ² = Ru, Os, Rh, Ir). 75 refs., 7 figs., 6 tabs.
Two different charge density wave (CDW) patterns are observed for layered transition-metal compounds ML[sub 2] (e.g., M = transition metal, L = oxygen, chalcogen) of octahedral coordination with d[sup 2] and d[sup 3] electron counts. Tight-binding electronic band structure calculations were carried out for several d[sup 2] ML[sub 2] layers, and factors controlling these patterns were discussed. In general, the 1T-ML[sub 2] systems with short M-L bonds adopt a CDW pattern involving patterns were discussed. In general, the 1T-ML[sub 2] systems with short M-L bonds adopt a CDW pattern involving a weak distortion, while those with long M-L bonds adopt a CDW pattern involving a strong distortion. These calculations show that the metal-atom trimerization in LiVO[sub 2] is energetically favorable and opens a band gap for a small displacement of the metal atoms. This supports the CDW model of weak metal-atom trimerization proposed for the [radical]3 x [radical]3 superstructure of LiVO[sub 2].
The electrical resistivity, Hall coefficient and thermoelectric power of tungsten-ditelluride WTe2 have been determined in the temperature range 4.2 to 600°K using single crystal specimens. The Hall coefficient is of n-type in the whole temperature range, while the thermoelectric power is of p-type below 60°K and of n-type above, with a maximum value at 350°K. The analysis of the Hall coefficient and thermoelectric power based on a three-carrier model leads to the following conclusions: (1) One band of electrons and two bands of holes (light and heavy) contribute to the transport phenomena. (2) At low temperatures heavy holes and electrons play dominant roles and at high temperatures the light holes take part in the conduction. (3) Heavy hole and electron bands overlap by 0.05 eV, and the energy gap between the electron band and light hole band is 0.075 eV.
In this paper we describe the preparation and characterization of ethylenediamine-intercalated 6R-TaS2 and octylamine-intercalated 2H-TaS2. We also describe attempts to intercalate various electron acceptor species into transition metal dichalcogenides.
Solids with low dimensionality are basically chemical compounds with a high anisotropy in the bonding. The structure is built up with such units as |MXn| arranged in slabs or fibers and separated by rather large distances generally of the order of the v.d.w. radii. The process of intercalation result in the pulling apart of these groups.Intercalation compounds can be classified according to the coordination of the intercalated species and their presence in all or only in some of the v.d.w. gaps. Factors affecting the structure, such as the size, the amount of interacalation, the ionicities, are discussed.The mobility of intercalated A+ alkali metal ions between the slabs of a two-dimensional host structure is considered. The effect of different factors is discussed.
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.
We have studied the electronic structure of the layered compound Td-WTe2 experimentally using high-resolution angle-resolved photoelectron spectroscopy, and theoretically using density-functional based augmented spherical wave calculations. Comparison of the measured and calculated data shows in general good agreement. The theoretical results reveal the semimetallic as well as metallic character of Td-WTe2; the semimetallic character is due to a 0.5 eV overlap of Te 5p- and W 5d-like bands along Γ-Y, while the metallic character is due to two classical metallic bands. The rather low conductivity of Td-WTe2 is interpreted as resulting from a low density of states at the Fermi level.
Calculations of electronic structure have been undertaken using ab initio tight-binding (TB) method comparing the group VIb ditellurides WTe2 and MoTe2 in their various guises. The group VIb ditellurides show deviation from a simple band model which predicts semiconducting behaviour due to a trigonal prismatic crystal-field splitting of a filled nonbonding dz2 orbital. For WTe2 and beta -MoTe2 the metal atom is displaced from the centre of an octahedron of Te atoms, and metal-metal chains with bond lengths only slightly longer than the elemental metals run along the layers. The reduced Madelung term for this configuration compensates for loss of the lone dz2-based band and thus results in semi-metallic crystals. Mo but not W occurs with a trigonal prismatic coordination ( beta -MoTe2). beta -MoTe2 differs from WTe2 only slightly-in the stacking of layers; a low-temperature polymorph is believed to stack identically to WTe2.
Methods of growing single crystals of MoS2, MoSe2, WSe2, TaSe2 by direct vapour transport are described. Optimum conditions for growing these crystals and also MoTe2 and WTe2 using Br transport are given. The crystal structures, unit cell dimensions and surface growth features of crystals grown by the two methods are described and compared.
The AB2-type selenides and tellurides of niobium, tantalum, molybdenum and tungsten have been prepared in single crystalline form by means of transport reactions. Lattice parameters were determined on the basis of single crystal patterns and are given for the eight compounds. Pertinent thermoelectric parameters, such as electrical resistivity, Seebeck coefficient and thermal conductivity were measured on compacted polycrystalline aggregates and are given together with the calculated “Figure of Merit”.
The density functional theory (DFT) computation of electronic structure, total energy and other properties of materials, is a field in constant progress. In order to stay at the forefront of knowledge, a DFT software project can benefit enormously from widespread collaboration, if handled properly. Also, modern software engineering concepts can considerably ease its development. The ABINIT project relies upon these ideas: freedom of sources, reliability, portability, and self-documentation are emphasised, in the development of a sophisticated plane-wave pseudopotential code.We describe ABINITv3.0, distributed under the GNU General Public License. The list of ABINITv3.0 capabilities is presented, as well as the different software techniques that have been used until now: PERL scripts and CPP directives treat a unique set of FORTRAN90 source files to generate sequential (or parallel) object code for many different platforms; more than 200 automated tests secure existing capabilities; strict coding rules are followed; the documentation is extensive, including online help files, tutorials, and HTML-formatted sources.
The projector augmented wave method (PAW), introduced for the first time by Blöchl [P. Blöchl, Phys. Rev. B 50 (1994) 17953], has been implemented in the ABINIT code [X. Gonze, J.-M. Beuken, R. Caracas, F. Detraux, M. Fuchs, G.-M. Rignanese, L. Sindic, M. Verstaete, G. Zerah, F. Jollet, et al., Comput. Mater. Sci. 25 (2002) 478]. This implementation allows self-consistent calculations of the electronic structure of a periodic solid within the density functional theory (DFT), including the analytic calculation of forces and stresses. Geometry optimization and molecular dynamics are also available. We present here the details of the implementation, including the analytic formula for forces and stresses. Results concerning the study of iron under pressure are presented to validate the implementation.
Metallic quasi-two-dimensional transition metal dichalcogenides (TMDCs) 1T-TaS2 and 1T-TaSe2 show charge density wave (CDW) at room temperature. These show up as a superstructure in low energy electron diffraction (LEED) and as a distinct splitting of the Ta4f core level and a part of the Ta d(z)2 derived valence band. The changes in this Ta4f CDW splitting are investigated for in situ Cu, Li and Cs intercalated crystals by photoelectron spectroscopy (PES, XPS, SXPS) and LEED. The intercalation effects a certain number of CDW phase transitions, where each CDW phase is related to a characteristic CDW related splitting of the Ta4f core level. We extend the simple electrostatic model of Hughes and Pollak [1] to calculate the form of the splitting for different CDW phases and find a good correlation to the XPS measurements. This agreement confirms that the Ta4f CDW splitting is brought about by an electrostatic interaction between the Ta atoms and the(free) metallic charge density. The comparison of the phase transition sequence for three different intercalants indicates that a three-dimensional rather than a two-dimensional Fermi surface nesting mechanism is responsible for the phase transitions, as originally proposed by Wooley and Wexler [2].
We present a sampling method for Brillouin-zone integration in metals which converges exponentially with the number of sampling points, without the loss of precision of normal broadening techniques. The scheme is based on smooth approximants to the delta and step functions which are constructed to give the exact result when integrating polynomials of a prescribed degree. In applications to the simple-cubic tight-binding band as well as to band structures of simple and transition metals, we demonstrate significant improvement over existing methods. The method promises general applicability in the fields of total-energy calculations and many-body physics.