Layered Chalcogenide Materials Research Articles

All-Inorganic Open-Framework Chalcogenides, A3Ga5S9·xH2O (A = Rb and Cs), Exhibiting Ultrafast Uranyl Remediation and Illustrating a Novel Post-Synthetic Preparation of Open-Framework Oxychalcogenides

Fast and effective uranyl sequestration is of interest to the nuclear industry. Recently, layered chalcogenide materials have demonstrated fast, selective, and efficient sorption properties toward uranyl cations, and the development and investigation of new types of chalcogenide materials continues to be of interest and represents an intriguing option for uranyl remediation. Three new all-inorganic A3Ga5S9·xH2O (A = Rb, Rb/Cs, and Cs) open-framework chalcogenides were obtained via an in situ alkali carbonate to alkali sulfide conversion process achieved under mild hydrothermal conditions. The structures of the all-inorganic open-framework chalcogenides consist of a twofold interpenetrated diamond-like 3D framework containing pseudo-T3 [Ga10S20]10– supertetrahedra. Forty-eight percent of the structural volume is occupied by A+ cations and water species, as established by single-crystal X-ray diffraction (SCXRD), infrared (IR) spectroscopy, and energy-dispersive spectroscopy (EDS). The dynamic nature of the A+ cations and water molecules within the pores was investigated via SCXRD as well as by IR spectroscopy-monitored H2O-to-D2O exchange experiments. Framework stability was probed with post-synthetic treatment of A3Ga5S9·xH2O (A = Rb and Cs) samples in acidic solutions that resulted in the formation of the oxysulfide (A/H)3Ga5S9–yOy·xH2O (A = Rb and Cs; y = 0–1), as shown by SCXRD and IR. Ion-exchange studies on A3Ga5S9·xH2O (A = Rb and Cs) samples were carried out utilizing a uranyl acetate solution. The presence of the UO22+ species in the ion-exchange product was supported by IR spectroscopy and EDS. Batch method ion-exchange experiments on Cs3Ga5S9·xH2O powder demonstrated fast kinetics with 95% uranyl removal from the uranyl acetate solution during the first minute, a maximum uranyl uptake capacity of 15 mg/g, and the subsequent elution of uranyl species with KCl solution. The porous and dynamic nature of the A3Ga5S9·xH2O framework coupled with effective UO22+···S2– bonding interactions makes it a good potential sorbent for uranyl remediation from aqueous media.

Dimensional transformation of chemical bonding during crystallization in a layered chalcogenide material

Two-dimensional (2D) van der Waals (vdW) materials possess a crystal structure in which a covalently-bonded few atomic-layer motif forms a single unit with individual motifs being weakly bound to each other by vdW forces. Cr2Ge2Te6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications. Here, we provide evidence for a dimensional transformation in the chemical bonding from a randomly bonded three-dimensional (3D) disordered amorphous phase to a 2D bonded vdW crystalline phase. A counterintuitive metastable “quasi-layered” state during crystallization that exhibits both “long-range order and short-range disorder” with respect to atomic alignment clearly distinguishes the system from conventional materials. This unusual behavior is thought to originate from the 2D nature of the crystalline phase. These observations provide insight into the crystallization mechanism of layered materials in general, and consequently, will be useful for the realization of 2D vdW material-based functional nanoelectronic device applications.

In Situ Characterization of Transformations in Nanoscale Layered Metal Chalcogenide Materials: A Review

AbstractLayered metal chalcogenide materials, such as MoS2 and Bi2Te3, have found important applications as 2D materials in emerging electronic devices. To create nanoscale layered chalcogenide materials with precisely controlled structures and properties, it is critical to understand and control how they evolve and transform under various conditions. This Minireview presents an overview of recent research focused on using in situ characterization to understand the atomic‐scale details of transformations during growth, under exposure to reactive chemical and thermal environments, and on interfacing with other materials. These efforts have used techniques including in situ transmission electron microscopy (TEM), X‐ray spectroscopy, and optical methods to understand nanoscale transformations. These in situ studies have provided a substantially improved understanding of transformation mechanisms in layered chalcogenide materials, which is an important step toward the use of these interesting materials in a variety of electronic and electrochemical devices.

High-quality sputter-grown layered chalcogenide films for phase change memory applications and beyond

This paper summarizes recent progress on thin film growth of chalcogenides by sputtering. The materials discussed include Sb–Te, Bi–Te, Ge–Te, and their superlattices, materials that are technologically important particularly for non-volatile phase change memory. In this work, the sputter-growth behavior of high-quality layered chalcogenide films is discussed. Sputtering is one of the most commonly used thin-film growth techniques in the semiconductor industry, however, the complex interrelationship between growth parameters can lead to difficulty in fabricating high-quality films although the deposition method itself is relatively simple. Here, we successfully demonstrate the fabrication of highly-oriented layered chalcogenide materials by sputtering. The selection of the appropriate sputtering target is important. In particular, it was found that a Te-rich Sb–Te alloy target such as Sb33Te67 is necessary in order to obtain a stoichiometric Sb2Te3 film. Moreover, the growth temperature is also a key factor in obtaining a highly-oriented film, namely the ideal growth temperature for an Sb2Te3 film is between 230 °C and 250 °C after the growth of an amorphous seed layer at room temperature. Furthermore, it was found that this technique is also useful to grow films epitaxially on Al2O3 or Si(111) substrates even though there are some misoriented grains as well as twins present. Finally, we demonstrate the growth of highly-oriented Sb2Te3 films on a flexible substrate. The versatility of sputtering will become technologically more and more important for the various applications represented by phase change memory.

(Invited) Chemical Functionalization and Transformation of 2D Layered Chalcogenide Materials

Layered chalcogenide materials are a diverse class of crystalline materials that consist of covalently bound building blocks held together by van der Waals forces. Among these materials are the transition metal dichalcogenides (TMDCs) which can be exfoliated into two-dimensional (2D) nanosheets, and the pnictogen chalcogenides (PCs) which can be exfoliated into one-dimensional (1D) nanoribbons and 2D nanosheets. Controlling the electronic and optical properties of these materials, as well as forming interfaces with other materials structures, is important for a wide range of potential applications, and can be achieved using a set of strategies for chemical modification that has been pursued in our laboratory. This talk will highlight our recent work on covalent chemical functionalization, structural and geometric conversion, and self-assembly of organic van der Waals heterostructures.In the first strategy, semiconducting MoS2 is covalently functionalized using aryl diazonium salts that form out-of-plane bonds on the basal plane. Detailed imaging studies of the reaction under a range of parameters combined with density functional theory (DFT) calculations show that the functionalization initiates at point defects but then propagates in a chain-like morphology across the MoS2 surface. The reaction is also shown to be well-described by Freundlich and Temkin isotherm models, and to have pseudo-second-order kinetics. The diazonium salts can also be modified to enable tethering of proteins to the MoS2 surface. The same reaction scheme is also extended across the TMDC and PC layered chalcogenides including MoSe2, WS2, WSe2, Bi2S3, and Sb2S3. In the second strategy, MoO3 and WO3 nanoscrolls are formed by transformation from 2D MoS2 and WS2 nanosheets, respectively, via atmospheric plasma treatment. The combination of different radicals in the plasma simultaneously oxidize the layered disulfides and initiate delamination from the substrate, so that the strain of the altered crystal structure is relieved by rolling into nanoscroll geometries. In the final strategy, we form a self-assembled monolayer of fullerene molecules on WSe2, and use atomic resolution scanning tunneling microscopy to show that the molecules exhibit a 2x2 rotational superstructure due to the interplay of molecule-molecule, molecule-substrate, van der Waals, and Coulomb interactions. The system forms a charge redistribution network with long-range ordering.

Substitutional doped GeSe: tunable oxidative states with strain engineering

Layered chalcogenide materials have a wealth of nanoelectronics applications like resistive switching and energy-harvesting. The work shows dynamic oxidative states of the dopants under strain in GeSe, an emerging 2D chalcogenide with potential nanoelectronics applications.

Enhancement of Thermoelectric Properties of Layered Chalcogenide Materials

AbstractThermoelectric materials have long been proven to be effective in converting heat energy into electricity and vice versa. Since semiconductors have been used in the thermoelectric field, much work has been done to improve their efficiency. The interrelation between their thermoelectric physical parameters (Seebeck coefficient, electrical conductivity, and thermal conductivity) required special tailoring in order to get the maximum improvement in their performance. Various approaches have been reported in the research for developing thermoelectric performance, including doping and alloying, nanostructuring, and nanocompositing. Among different types of thermoelectric materials, layered chalcogenide materials are unique materials with distinctive properties. They have low self-thermal conductivity, and their layered structure allows them to be modified easily to improve their thermoelectric performance. In this review, basic knowledge of thermoelectric concepts and challenges for enhancing the figure of merit is provided. It discusses briefly different groups of layered chalcogenide thermoelectric materials with their structure and thermoelectric properties. It also reports different approaches in the literature for improving their performance and the recent progress done in this field. It highlights graphene as a promising nano additive to layered chalcogenide materials’ matrix and shows its effect on enhancing their figure of merit.

Covalent chemical functionalization of semiconducting layered chalcogenide nanosheets

The covalent functionalization of the surfaces of transition metal dichalcogenide and pnictogen chalcogenide materials is demonstrated using aryl diazonium chemistry.

Facilitated Lithium Storage in Hierarchical Microsphere of Cu2S‐MoS2 Ultrathin Nanosheets

AbstractConsidering the high energy density Lithium ion batteries have become one of the best option for next‐generation energy‐storage technologies. Transition metal chalcogenide nanomaterials are promising electrodes for Lithium‐ion batteries. Molybdenum based layered chalcogenide materials wisely studied for rechargeable Li‐ion batteries, due to its two‐dimensional (2D) layered structure and better specific capacity. The three‐dimensional (3D) synthesis of microsphere by ultrathin nanosheets is necessary for practical applications. Herein, the fabrication of a unique hierarchical 2D layered Cu2S‐MoS2 nanostructure was (ultrathin nanosheets) demonstrated via in situ assembling of two‐dimensional (2D) growths in facile solvothermal technique. The structural study reveals the existence of Cubic Cu2S and Rhombohedral MoS2 phase. Morphological study by FESEM and TEM shows unique ultrathin nanosheets of ∼10 nm thickness self‐assembled in the form of layered microsphere. In Li‐ion storage testing, Cu2S‐MoS2 electrode exhibited good specific discharge capacity of 651 mAhg−1at 50 mAg−1 applied current and maintained 320 mAhg−1 after 100 cycles. The facilitated advanced electrochemical performance attributed to layered ultrathin Cu2S‐MoS2 composite nanosheets.

A facile way to control phase of tin selenide flakes by chemical vapor deposition

Although two-dimensional (2D) tin selenides are attracting intense attentions, studies on its phase transition are still relatively few. Here we report a facile way to control the phase growth of tin selenide flakes on mica and SiO2/Si by only adjusting nominal Sn:Se ratio, which refers to the amount of loaded SnO2 and Se precursors. High normal Sn:Se ratio induced SnSe flakes, conversely SnSe2 flakes formed. It could be used as a practical guide to selectively synthesize pure phase of single crystalline 2D layered chalcogenide materials similar to tin selenides.

(Invited) Atomic Scale Investigations of Extended Crystal Defects in Tetradymite-Structured Thermoelectric Materials

Dislocations and other extended defects, such as grain boundaries and stacking faults, can impart strong effects on the thermal and electronic transport properties of thermoelectric materials. Yet, our understanding of the structure of such defects, even for such widely used materials as bismuth telluride, is still quite limited. In this presentation, I will discuss our work investigating the structure of extended crystal defects in Bi2Te3 and related tetradymite-structured thermoelectric materials. As will be discussed, the anisotropic, layered crystal structure of Bi2Te3 has important implications for the resulting atomic-scale core structures of defects in this material. I will begin with a brief overview of the tetradymite system, placing the structure of Bi2Te3 in relationship to other layered chalcogenide materials and discussing the dependence of the layering sequence on the metal to chalcogen ratio. Next, I will discuss HAADF-STEM observations of the atomic structure of dislocations in Bi2Te3. I will focus on two very different types of dislocation: one with Burgers vector of type (1/3)<2 -1 -1 0>, which lies parallel with the basal plane, and the other with Burgers vector of type (1/3)<0 1 -1 -1>, which has a large dislocation component normal to the basal plane of c/3, or one full quintuple unit. The first type of dislocation possesses a stoichiometric core that is glissile on the basal plane. The second type possesses a much more complex dislocation core consisting of bismuth-rich faulted region consistent with Bi3Te4. I will also discuss our current understanding of grain boundaries in this class of materials. Arrays of discrete dislocations can accommodate small misorientations between adjacent crystallites of a few degrees. Examples will be shown for low angle boundaries composed of both (1/3)<2 -1 -1 0> and (1/3)<0 1 -1 -1> type dislocations. For large misorientations, however, simple dislocation descriptions break down and it is more physically meaningful to consider a given interface with reference to a low-energy, reference grain boundary structure. Perhaps the simplest such interface in Bi2Te3 is the (0001) basal twin. Our electron microscopic observations and density functional theory calculations have shown that the structure of the Bi2Te3 basal twin consists simply of a reversal of the basal plane stacking sequence and that this reversal occurs at the Te(1) type planes. As I will discuss it is also possible to go beyond the simple structure of an ideal (0001) twin by considering the introduction of disconnections (defects possessing both dislocation and step content) to this interface. Finally, I will discuss a more complex mode of twinning, on {1 0 -1 5} and {0 1 -1 10}, which has previously been reported in electroplated films of Bi2Te3. Our HAADF-STEM observations provide insight concerning the atomic-scale structure of such twinning, pointing to the formation of structural units consistent with local packets of rock-salt structured material. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Poisoning of MoO3 Precursor on Monolayer MoS2 Nanosheets Growth by Tellurium-Assisted Chemical Vapor Deposition**Supported by the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China under Grant No 14XNLQ07.

We obtain molybdenum disulfide (MoS2) nanosheets (NSs) with edge sizes of by direct sulfuration of MoO3 powder spread on the SiO2/Si substrates. However, the undesirable MoO3 nanoparticles (NPs) left on the surface of MoS2 NSs poison the MoO3 precursor. Introducing Te vapors to react with MoS2 to form low melting point intermediate MoSxTe2−x, the evaporations of MoO3 precursor recover and MoO3 NPs disappear. Thus Te vapor is effective to suppress poisoning of the MoO3 precursor. Selecting the appropriate amount of Te vapor, we fabricate monolayer MoS2 NSs up to in edge length. This finding can be significant to understand the role of Te in the Te-assisted chemical vapor deposition growth process of layered chalcogenide materials.

Research Update: Van-der-Waals epitaxy of layered chalcogenide Sb2Te3 thin films grown by pulsed laser deposition

An attempt to deposit a high quality epitaxial thin film of a two-dimensionally bonded (layered) chalcogenide material with van-der-Waals (vdW) epitaxy is of strong interest for non-volatile memory application. In this paper, the epitaxial growth of an exemplary layered chalcogenide material, i.e., stoichiometric Sb2Te3 thin films, is reported. The films were produced on unreconstructed highly lattice-mismatched Si(111) substrates by pulsed laser deposition (PLD). The films were grown by vdW epitaxy in a two-dimensional mode. X-ray diffraction measurements and transmission electron microscopy revealed that the films possess a trigonal Sb2Te3 structure. The single atomic Sb/Te termination layer on the Si surface was formed initializing the thin film growth. This work demonstrates a straightforward method to deposit vdW-epitaxial layered chalcogenides and, at the same time, opens up the feasibility to fabricate chalcogenide vdW heterostructures by PLD.

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.

Molecular Dynamics Simulation of Bismuth Telluride Exfoliation Mechanisms in Different Ionic Liquid Solvents.

Bismuth telluride (Bi2Te3) is a well-known thermoelectric material with potential applications in several different emerging technologies. The bulk structure is composed of stacks of quintuple sheets (with weak interactions between neighboring sheets), and the performance of the material can be significantly enhanced if exfoliated into two-dimensional nanosheets. In this study, eight different imidazolium-based ionic liquids are evaluated as solvents for the exfoliation and dispersion of Bi2Te3 at temperatures ranging from 350 to 550 K. Three distinct exfoliation mechanisms are evaluated (pulling, shearing, and peeling) using steered molecular dynamics simulations, and we predict that the peeling mechanism is thermodynamically the most favorable route. Furthermore, the [Tf2N-]-based ionic liquids are particularly effective at enhancing the exfoliation, and this performance can be correlated to the unique molecular-level solvation structures developed at the Bi2Te3 surfaces. This information helps provide insight into the molecular origins of exfoliation and solvation involving Bi2Te3 (and possibly other layered chalcogenide materials) and ionic liquid solvents.

A Spatially Resolved Study of the Role of Surface Motifs in the Photoelectrochemical Conversion on Prospective 2-D Layered Chalcogenide Photoelectrodes for Solar-Fuel Generation

2-D Layered chalcogenide materials, with the general formula, MX2(where, generally, M= Mo, W and X=S, Se), have been of sustained research interest, over the past few decades, in the area of solar-fuel generation. They have been looked at as light absorbers as they exhibit favorable bandgaps, high absorption coefficients for capturing solar energy and have good photoelectrochemical stability. More recently, they have been also found to be highly active electrocatalysts for hydrogen evolution reaction (HER). The surfaces of these materials are highly heterogeneous displaying macroscopic terraces and step edges; along with other micro/nano-scale defects. The optimal role of these materials in solar-energy conversion is decisively determined by their surface morphology. For HER catalysis, material forms such as nanoparticles and amorphous powders that have high concentration of step edges are preferable as the dangling bonds/exposed atoms at the step edges are instrumental in catalyzing the reaction. On the other hand, in their role as light absorbers, the surfaces of these materials should, ideally, be composed of large flat terraces since the absorption of light and the separation of carriers at the semiconductor/electrolyte interface occur most efficiently at the terraces. The step edges and other defects are thought to be detrimental as they act as sites for carrier recombination. It is clear that, in employing these materials for solar-fuel generation, it is of fundamental importance to delineate the specific roles of the surface motifs of 2-D layered chalcogenides in the photoelectrochemical conversion processes. Addressing the above-mentioned knowledge gap, we were able to spatially probe the local photoelectrochemical response of the surface motifs on the layered chalcogenide photoelectrodes by employing the technique of laser beam induced current microscopy (LBIC). LBIC involves the photogeneration of carriers locally using a laser beam and the resulting photocurrent could then be spatially correlated to the excitation spot on the surface of the electrode. Hence, a detailed spatial map of the local photoconversion efficiencies on the photoelectrodes could be obtained. An important observation of the spatially resolved photocurrent measurements was that there were significant variations in the photoconversion efficiencies of different terraces and that, more than the macroscopic step edges, it was the presence of low performing terraces that crucially determined the overall performance of these materials as light absorbers. The source of the low efficiencies on some terraces were further explored by studying and comparing the topography of the various terraces at the micro/nano-scale using AFM and STM. These studies revealed that the terraces were invariably textured at the micro/nano-scale and that the texturing differed from terrace to terrace. Further, local spectral response measurements at terraces of differing photo-conversion efficiencies provided evidence for the existence of sub-bandgap surface states on the low-performing terraces; possibly, explaining the variations in the efficiencies between terraces. Differences in work-function values across terraces measured by Kelvin probe force microscopy also supported the prevalence of surface states on the low performing terraces. In conclusion, employing a suite of spatially resolved techniques to study in-depth the surface motifs of layered chalcogenide photoelectrodes, we have been able to construct a detailed picture of the photoconversion processes occurring on these materials at the microscopic level. The results from our study crucially highlight the sources of performance losses in these materials and will serve to guide efforts in optimization of this material class for application in solar-fuel generation.

Designing the shape evolution of SnSe2 nanosheets and their optoelectronic properties.

Layered chalcogenide materials (LCMs) are emerging materials in recent years for their great potential in applications of electronics and optoelectronics. As a member of LCMs, SnSe2, an n-type semiconductor with a band gap of ∼1.0 eV, is of great value to explore. In this paper, we develop a facile CVD method, for the first time, to synthesize diverse shaped SnSe2 and square SnSe nanosheets (NSs) on SiO2/Si substrates. To the best of our knowledge, the thickness of as-grown SnSe2 is among the thinnest ones synthesized by CVD methods on various substrates. What's more, photodetectors are fabricated to investigate the optoelectronic properties of SnSe2. The on/off ratio of photoswitches reaches 100 under the illumination of an 800 nm laser. This work will pave a new pathway to synthesize LCM nanostructures, shed light on the shape evolution during the growth process and expand the candidates for high performance optoelectronic devices.

Layer-Structured Copper Antimony Chalcogenides (CuSbSexS2–x): Stable Electrode Materials for Supercapacitors

The ever-growing need for energy generation and storage applications demands development of materials with high performance and long-term stability. A sizable number of chalcogenide-based materials have been investigated for supercapacitor applications. Layer-structured chalcogenides are advantageous in terms of providing large surface area with good ionic conductivity and ability to host a variety of atoms or ions between the layers. CuSbS2 is a ternary layered chalcogenide material that is composed of earth abundant and less-toxic elements. For the first time, we have developed a simple colloidal method for the synthesis of CuSbSexS2–x mesocrystals over the whole composition range (0 ≤ x ≤ 2) by substitution of S with Se. Our approach yields mesocrystals with belt-like morphology for all the compositions. X-ray diffraction results show that substitution of sulfur with selenium in CuSbS2 enables tuning the width of the interlayer gap between the layers. To investigate the suitability of CuSbSexS2–x mesoc...

Basic properties and thin film preparation methods of layered chalcogenide materials for atomic-layer electronics

Basic properties and thin film preparation methods of layered chalcogenide materials for atomic-layer electronics