Layered Chalcogenides Research Articles

Deintercalation of Zero-Valent Metals from Two-Dimensional Layered Chalcogenides

A general solution-based approach to deintercalate zero-valent tin and copper from two-dimensional layered chalcogenides is presented using a one-step comproportionation reduction–oxidation reaction. The reaction is performed between the intercalated zero-valent metal and high oxidation state metal cations (Sn4+ and Cu2+) dissolved in acetone. This chemistry is shown to work for a variety of layered chalcogenides with differing morphologies and crystallinity. Copper and tin are deintercalated from powders of MoS2, MoSe2, NbSe2, and WS2 and crystalline nanoribbons of Bi2Se3, In2Se3, and GeS. This chemistry achieves a general route to remove zero-valent tin and copper from 2D layered chalcogenides.

Oxidation and oxidative vapor-phase etching of few-layer MoS2

Understanding oxidation of layered chalcogenide semiconductors is important for device processing, as oxidation can be both an intentional and unintentional result of processing steps. Here, the authors investigate chemical and morphological changes in mechanically exfoliated few-layer MoS2 in oxidizing and inert environments using different microscopies (optical, scanning electron, and atomic force) and spectroscopy (Raman, x-ray photoelectron, and Auger electron) techniques. The environments studied were oxygen, oxygen and water vapor, argon, argon and water vapor, and ultraviolet-generated ozone at temperatures from 25 to 550 °C. Oxidation at low temperatures resulted in the formation of a condensed molybdenum oxide phase and sulfur trioxide gas. At sufficiently elevated temperatures, all the products of oxidation volatilize, resulting in a vapor-phase etch. The kinetics of oxidation and etching depended upon the annealing gas, temperature, time, and the number of layers of MoS2. Conditions can be selected to create isolated etch pits, smooth oxide layers, oxide islands, or flakes of reduced lateral dimensions (etching from the flakes' edges). These results can provide useful guidance for MoS2 device processing.

Layered tetragonal zinc chalcogenides for energy-related applications: from photocatalysts for water splitting to cathode materials for Li-ion batteries.

Two-dimensional (2D) materials with tunable direct bandgaps are attractive for energy-related applications such as visible-light optical devices and cathode materials in metal-ion batteries. Here we perform first-principles calculations to investigate the structural, electrical, optical, and electrochemical properties of 2D tetragonal (t-) ZnX (X = S, Se) with layered structures and we also explore their applications in photocatalysts and Li-ion batteries. We find that t-ZnX layers prefer the AA stacking pattern when forming multi-layer (ML) structures. We also show that t-ZnX MLs and 3D bulks are all stable according to phonon calculations and ab initio molecular dynamics (MD) simulations. The band edge positions of these layered materials can be tuned by modifying the number of layers to transform them into being more suitable for photocatalysis. We further show that the t-ZnX layered structures, in particular t-ZnS single-layer (SL), are promising cathode materials for Li-ion batteries exhibiting a strong adsorption of Li atoms without reducing the Li mobility. Finally, we find that the most favorable adsorption configuration of Li atoms on t-ZnX SL strongly depends on the Li concentration. It is worth pointing out that the almost barrierless feature of Li diffusion on t-ZnS SLs makes t-ZnS SL a good candidate for a fast-charging device. Our work opens a promising avenue for the modulation of novel t-ZnX layered structures for a wealth of potential applications in energy conversion and storage.

Transport Properties and High Thermopower of SnSe2: A Full Ab-Initio Investigation

Motivated by the observation that many known layered chalcogenides show promising thermoelectric properties, we investigate the similar properties of SnSe2 by solving the Boltzmann transport equation for both lattice and electron. A self-consistent single parabolic band model (SPB) is employed to compute the electron relaxation time rigorously. The obtained intrinsic lattice thermal conductivities in a and c directions are 6.78 and 0.79 W/m·K at 300 K. The results show that acoustic phonon branches play the dominant role in heat transport. Thermoelectric properties of n-type SnSe2 are found to be significantly better than those of p-type doping for temperatures between 200 and 800 K and carrier concentrations between 1017 and 1020 cm–3. At n = 1020 cm–3 and 300 K, we find σa = 4.97 × 105 Ω–1·m–1 and σc = 3.39 × 104 Ω–1·m–1 and the ratio σa/σc = 14.67 for n-type SnSe2. Both electrical and lattice thermal conductivities show a strong anisotropic feature. A high thermoelectric figure of merit is revealed in ...

Metastable Layered Cobalt Chalcogenides from Topochemical Deintercalation.

We present a general strategy to synthesize metastable layered materials via topochemical deintercalation of thermodynamically stable phases. Through kinetic control of the deintercalation reaction, we have prepared two hypothesized metastable compounds, CoSe and CoS, with the anti-PbO type structure from the starting compounds KCo2Se2 and KCo2S2, respectively. Thermal stability, crystal structure from X-ray and neutron diffraction, magnetic susceptibility, magnetization, and electrical resistivity are studied for these new layered chalcogenides; both CoSe and CoS are found to be weak itinerant ferromagnets with Curie temperatures close to 10 K. Due to the weak van der Waals forces between the layers, CoSe is found to be a suitable host for further intercalation of guest species such as Li-ethylenediamine. From first-principles calculations, we explain why the Co chalcogenides are ferromagnets instead of superconductors as in their iron analogues. Bonding analysis of the calculated electronic density of states both explains their phase stability and predicts the limits of our deintercalation technique. Our results have broad implications for the rational design of new two-dimensional building blocks for functional materials.

Achieving ZT=2.2 with Bi-doped n-type SnSe single crystals.

Recently SnSe, a layered chalcogenide material, has attracted a great deal of attention for its excellent p-type thermoelectric property showing a remarkable ZT value of 2.6 at 923 K. For thermoelectric device applications, it is necessary to have n-type materials with comparable ZT value. Here, we report that n-type SnSe single crystals were successfully synthesized by substituting Bi at Sn sites. In addition, it was found that the carrier concentration increases with Bi content, which has a great influence on the thermoelectric properties of n-type SnSe single crystals. Indeed, we achieved the maximum ZT value of 2.2 along b axis at 733 K in the most highly doped n-type SnSe with a carrier density of −2.1 × 1019 cm−3 at 773 K.

Self-directed channel memristor for high temperature operation

Ion-conducting memristors comprised of the layered chalcogenide materials Ge2Se3/SnSe/Ag are described. The memristor, termed a self-directed channel (SDC) device, can be classified as a generic memristor and can tolerate continuous high temperature operation (at least 150°C). Unlike other chalcogenide-based ion conducting device types, the SDC device does not require complicated fabrication steps, such as photodoping or thermal annealing, making these devices faster and more reliable to fabricate. Device pulsed response shows fast state switching in the 10−9s range. Device cycling at both room temperature and 140°C show write endurance of at least 1 billion.

Chemical vapor transport and characterization of MnBi2Se4

Layered metal chalcogenides such as MnBi2Se4 are interesting candidates for a wide field of applications such as for thermo- and photoelectrics. High-quality single crystals are necessary in order to investigate their properties which can be prepared by chemical vapor transport (CVT). The CVT of MnBi2Se4 has not been investigated until this point and is subject of the presented paper. We obtained needle-like MnBi2Se4 single crystals with a length up to 15mm. The magnetic characterization has shown an antiferromagnetic transition around 14K. Additionally, electrical transport described MnBi2Se4 as a narrow band-gap semiconductor (EGap=0.15eV). Thermodynamic data for MnBi2Se4 at room temperature were determined to H°=−305KJ·mol−1, S=321JK−1·mol−1 and Cp=167.568+25.979·10−3·TJ·K−1·mol−1, respectively. Our results on CVT-grown single crystals confirm reported data from literature and complete the data set for MnBi2Se4.

The role of the alkali and chalcogen atoms on the stability of the layered chalcogenide (A = alkali-metal; M = metal-cations; Q = chalcogen) compounds: a density functional theory investigation within van der Waals corrections

There is a great interest to design two-dimensional (2D) chalcogenide materials, however, our atomistic understanding of the major physical parameters that drive the formation of 2D or three-dimensional (3D) chalcogenides is far from satisfactory, in particular, for complex quaternary systems. To address this problem, we selected a set of quaternary 2D and 3D chalcogenide compounds, namely, (A = Li, K, Cs; Q = S, Se, Te), which were investigated by density functional theory calculations within van der Waals (vdW) corrections. Employing experimental crystal structures and well designed crystal modifications, we found that the average atomic radius of the alkali-metal, A, and chalcogen, Q, species play a crucial role in the stability of the 2D structures. For example, the 2D structures are energetically favored for smaller and larger average atomic radius, while 3D structures are favored at intermediate average atomic radius. Those results are explained in terms of strain minimization and Coulomb repulsion of the anionic species in the structure framework. Furthermore, the equilibrium lattice parameters are in excellent agreement with experimental results. Thus, the present insights can help in the design of stable quartenary 2D chalcogenide compounds.

Manifestations of Quasi-Two-Dimensional Metallicity in a Layered Ternary Transition Metal Chalcogenide Ti2PTe2

Quasi-two-dimensional (2D) metallicity in a ternary layered transition metal chalcogenide of single crystalline Ti2PTe2 is studied by transport prop-erties measurements and electronic structure analyses. The logarithmic dependence of resistivity in the low temperature region appears as a charac-teristic feature of the quasi-2D transport in a weakly disordered metallic system. A large Sommerfeld coefficient value of 6.94 mJ/mol·K2 and negative temperature dependence of the Hall coefficient indicate that electron-electron interaction is dominant in a phase-breaking mechanism originating from bipolar behavior. Angle-resolved photoemission spectroscopy combined with theoretical calculations successfully proves that quasi-2D elec-tron pockets encompassing a small three-dimensional hole pocket mainly determine the quasi-2D metallic transport properties. We also adopt an orbital character calculation to verify that Ti character is dominant in valence bands, revealing that 2D Ti-networks form quasi-2D conducting ...

Multi-layered Chalcogenides with potential for magnetism and superconductivity

Layered thallium copper chalcogenides can form single, double, or triple layers of Cu–Ch separated by Tl sheets. Here we report on the preparation and properties of Tl-based materials of TlCu2Se2, TlCu4S3, TlCu4Se3 and TlCu6S4. Having no long-range magnetism for these materials is quite surprising considering the possibilities of inter- and intra-layer exchange interactions through Cu 3d, and we measure by magnetic susceptibility and confirm by neutron diffraction. First principles density-functional theory calculations for both the single-layer TlCu2Se2 (isostructural to the ‘122’ iron-based superconductors) and the double-layer TlCu4Se3 suggest a lack of Fermi-level spectral weight that is needed to drive a magnetic or superconducting instability. However, for multiple structural layers with Fe, there is much greater likelihood for magnetism and superconductivity.

High visible light sensitive MoS2 ultrathin nanosheets for photoelectrochemical biosensing

For the purpose of effectively utilizing the visible light, photoelectrochemical (PEC) detection represents a unique detection signal and different energy form of the excitation source. In this work, ultrathin MoS2 nanosheets with narrow band gap were successfully fabricated by a facile C3N4 sacrificial template assisted thermolytical approach. Upon immobilizing glucose oxidase, excellent photocatalytic activity towards glucose is achieved in neutral buffer solution. As a novel visible light sensitive photocatalytic material, ultrathin MoS2 nanosheets present a detection limit of ~0.61nM, which is much lower than those with the similar configurations reported previously. Based on the excellent anti-interference property, the feasibility of applying the proposed sensor to determine glucose in human serum is further demonstrated. This work provides new insight into the fabrication of promising visible light sensitive two-dimensional layered transition-metal chalcogenides nanostructures for construction of photoelectrochemical biosensors.

Few-Layer Nanosheets of n-Type SnSe2.

Layered p-block metal chalcogenides are renowned for thermoelectric energy conversion due to their low thermal conductivity caused by bonding asymmetry and anharmonicity. Recently, single crystalline layered SnSe has created sensation in thermoelectrics due to its ultralow thermal conductivity and high thermoelectric figure of merit. Tin diselenide (SnSe2 ), an additional layered compound belonging to the Sn-Se phase diagram, possesses a CdI2 -type structure. However, synthesis of pure-phase bulk SnSe2 by a conventional solid-state route is still remains challenging. A simple solution-based low-temperature synthesis is presented of ultrathin (3-5 nm) few layers (4-6 layers) nanosheets of Cl-doped SnSe2 , which possess n-type carrier concentration of 2×1018 cm-3 with carrier mobility of about 30 cm2 V-1 s-1 at room temperature. SnSe2 has a band gap of about 1.6 eV and semiconducting electronic transport in the 300-630 K range. An ultralow thermal conductivity of about 0.67 Wm-1 K-1 was achieved at room temperature in a hot-pressed dense pellet of Cl-doped SnSe2 nanosheets due to the anisotropic layered structure, which gives rise to effective phonon scattering.

Highly active and stable layered ternary transition metal chalcogenide for hydrogen evolution reaction

Layered ternary transition metal chalcogenides (TTMCs) material has great potentials that can overcome to the limitation of active sites which is challenging in binary transition metal chalcogenides (BTMC), such as MoS2, towards electrochemical hydrogen production. Here, we demonstrate TTMC material which contains two transition metals Cu and Mo with chalcogen S. The TTMC, Cu2MoS4 has been successfully synthesized by a facile solution-processed method. Moreover, by anion doping such as Se in as the synthesized Cu2MoS4, it has been found that TTMC can be exfoliated into single layer nanosheets. Furthermore, by controlling the number of layers, single layers TTMC exhibit the highest electrocatalytic activity towards hydrogen evolution reaction (HER) because the single layers can provide more catalytic active sites than multilayers and bulk. The onset potential for hydrogen generation is −96mV for single layer TTMC electrode material with corresponding Tafel slope 52mV/decade. After 1000 cycles with continuous electrolysis in acid electrolyte for 15h, the electrode material preserves its structure and robust catalytic activity perfectly. Our new TTMC materials show highly active electrocatalytic performance and high stability which overcome the intrinsic limitation of BTMC. As a result, our work can guide new strategy for the developments of real applications of TMCs in HER.

Layer-Confined Excitonic Insulating Phase in Ultrathin Ta2NiSe5 Crystals.

Atomically thin nanosheets, as recently realized using van der Waals layered materials, offer a versatile platform for studying the stability and tunability of the correlated electron phases in the reduced dimension. Here, we investigate a thickness-dependent excitonic insulating (EI) phase on a layered ternary chalcogenide Ta2NiSe5. Using Raman spectroscopy, scanning tunneling spectroscopy, and in-plane transport measurements, we found no significant changes in crystalline and electronic structures as well as disorder strength in ultrathin Ta2NiSe5 crystals with a thickness down to five layers. The transition temperature, Tc, of ultrathin Ta2NiSe5 is reduced from its bulk value by ΔTc/Tc(bulk) ≈ -9%, which strongly contrasts the case of 1T-TiSe2, another excitonic insulator candidate, showing an increase of Tc by ΔTc/Tc(bulk) ≈ +30%. This difference is attributed to the dominance of interband Coulomb interaction over electron-phonon interaction and its zero-ordering wave vector due to the direct band gap structure of Ta2NiSe5. The out-of-plane correlating length of the EI phase is estimated to have monolayer thickness, suggesting that the EI phase in Ta2NiSe5 is highly layer-confined and in the strong coupling limit.

Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit.

Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

Electron-Beam Induced Transformations of Layered Tin Dichalcogenides

By combining high-resolution transmission electron microscopy and associated analytical methods with first-principles calculations, we study the behavior of layered tin dichalcogenides under electron beam irradiation. We demonstrate that the controllable removal of chalcogen atoms due to electron irradiation, at both room and elevated temperatures, gives rise to transformations in the atomic structure of Sn-S and Sn-Se systems so that new phases with different properties can be induced. In particular, rhombohedral layered SnS2 and SnSe2 can be transformed via electron beam induced loss of chalcogen atoms into highly anisotropic orthorhombic layered SnS and SnSe. A striking dependence of the layer orientation of the resulting SnS-parallel to the layers of ultrathin SnS2 starting material, but slanted for transformations of thicker few-layer SnS2-is rationalized by a transformation pathway in which vacancies group into ordered S-vacancy lines, which convert via a Sn2S3 intermediate to SnS. Absence of a stable Sn2Se3 intermediate precludes this pathway for the selenides, hence SnSe2 always transforms into basal plane oriented SnSe. Our results provide microscopic insights into the transformation mechanism and show how irradiation can be used to tune the properties of layered tin chalcogenides for applications in electronics, catalysis, or energy storage.

Bulk Fabrication of WS2 Nanoplates: Investigation on the Morphology Evolution and Electrochemical Performance.

Two-dimensional layered chalcogenide WS2, similar to graphene, is considered to be very interesting for materials scientists. However, to make it a useful material platform, it is necessary to develop sophisticated synthesis methods to control its morphology. In this paper, we present a simple approach to prepare various morphologies of WS2 nanostructures by direct thermal evaporation of WO3 and S powders onto Si substrates sputtered with W film without using any nanostructured W-contained precursors and highly toxic sulfide gases. This method can produce bulk quantities of pure hexagonal, horizontally grown WS2 nanoplates, vertically grown nanoplates, and nanoplate-formed flowers simply by tuning the distance between the substrate and source powders. The synthesis mechanism and morphology evolution model were proposed. Moreover, when employed as a thin-film anode material, the Li-ion battery with as-prepared, vertically grown WS2 nanoplates presented a rechargeable performance between 3 and 0.01 V with a discharge capacity of about 773 mAh/cm(3) after recycling three times, much better than its already-reported counterparts with randomly distributed WS2 nanosheet electrodes, but the battery with horizontally grown WS2 nanoplates could not show any charge-discharge cycling property, which could be attributed to the different structures of WS2 anodes for Li(+) ion intercalation or deintercalation.

ChemInform Abstract: Recent Advances in Layered Metal Chalcogenides as Superconductors and Thermoelectric Materials: Fe‐Based and Bi‐Based Chalcogenides

AbstractReview: 97 refs.

Superior Electrical Conductivity in Hydrogenated Layered Ternary Chalcogenide Nanosheets for Flexible All‐Solid‐State Supercapacitors

AbstractAs the properties of ultrathin two‐dimensional (2D) crystals are strongly related to their electronic structures, more and more attempts were carried out to tune their electronic structures to meet the high standards for the construction of next‐generation smart electronics. Herein, for the first time, we show that the conductive nature of layered ternary chalcogenide with formula of Cu2WS4 can be switched from semiconducting to metallic by hydrogen incorporation, accompanied by a high increase in electrical conductivity. In detail, the room‐temperature electrical conductivity of hydrogenated‐Cu2WS4 nanosheet film was almost 1010 times higher than that of pristine bulk sample with a value of about 2.9×104 S m−1, which is among the best values for conductive 2D nanosheets. In addition, the metallicity in the hydrogenated‐Cu2WS4 is robust and can be retained under high‐temperature treatment. The fabricated all‐solid‐state flexible supercapacitor based on the hydrogenated‐Cu2WS4 nanosheet film shows promising electrochemical performances with capacitance of 583.3 F cm−3 at a current density of 0.31 A cm−3. This work not only offers a prototype material for the study of electronic structure regulation in 2D crystals, but also paves the way in searching for highly conductive electrodes.