MoS2 Interlayer Research Articles

Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity, Stability, and Scalability via Molten-Metal-Assisted Intercalation.

The crystalline phase of layered transition metal dichalcogenides (TMDs) directly determines their material property. The most thermodynamically stable phase structures in TMDs are the semiconducting 2H and metastable metallic 1T phases. To overcome the low phase purity and instability of 1T-TMDs, which limits the utilization of their intrinsic properties, various synthesis strategies for 1T-TMDs have been proposed in phase-engineering studies. Herein, a facile and scalable synthesis of 1T-phase molybdenum disulfide (MoS2 ) via the molten-metal-assisted intercalation (MMI) approach is introduced, which exploits the capillary action of molten potassium and the difference between the electron affinity of MoS2 and the ionization potential of potassium. Highly reactive molten potassium metal can readily intercalate into the MoS2 interlayers, inducing an efficient phase transition from the 2H to 1T crystal structure. The ionic bonding between the intercalated potassium and sulfur lowers the energy barrier of the 1T-phase transition, enhancing the phase stability of the 1T crystals. Owing to the high purity and stability of the 1T phase, the electrocatalytic performance for the hydrogen evolution reaction is significantly higher in 1T-MoS2 (MMI) than in 2H-MoS2 and even in 1T-MoS2 synthesized using n-butyllithium.

Efficient Raman Enhancement in Molybdenum Disulfide by Tuning the Interlayer Spacing.

Two-dimensional nanomaterials, such as graphene and molybdenum disulfide (MoS2), have recently attracted widespread attention as surface-enhanced Raman scattering (SERS) substrates. However, their SERS enhancement is of a smaller magnitude than that of noble metal nanomaterials, and therefore, the detection sensitivity still needs to be substantially improved for practical applications. Here, we present the first detailed studies on the effect of the (MoS2) interlayer distances on the SERS intensity enhancement. We find that MoS2 with smaller interlayer distances achieves an SERS enhancement factor as high as 5.31 × 105, which is one of the highest enhancement factors to date among the two-dimensional nanomaterial SERS sensors. This remarkable SERS sensitivity is attributed to the highly efficient charge transfer from MoS2 to probe molecules. The charge-transfer ability directly determines the variable quantity dz2 orbitals of Mo elements in the MoS2-molecule system and then tunes the Raman intensity of probe molecules. Our work contributes to reveal the influence of MoS2 interlayer spacing on SERS detection and to open a new way for designing a highly sensitive nonmetal SERS technology.

Tunable Molybdenum Disulfide-Enabled Fiber Mats for High-Efficiency Removal of Mercury from Water.

The application of molybdenum disulfide (MoS2) for water decontamination is expanded toward a novel approach for mercury removal using nanofibrous mats coated with MoS2. A bottom-up synthesis method for growing MoS2 on carbon nanofibers was employed to maximize the nanocomposite decontamination potential while minimizing the release of the nanomaterial to treated water. First, a co-polymer of polyacrylonitrile and polystyrene was electrospun as nanofibrous mats and pretreated to form pristine carbon fibers. Next, three solvothermal methods of controlled in situ MoS2 growth of different morphologies were achieved on the surface of the fibers using three different sets of precursors. Finally, these MoS2-enabled fibers were extensively characterized and evaluated for their mercuric removal efficiency. Two mercury removal mechanisms, including reduction-oxidation reactions and physicochemical adsorption, were elucidated. The two nanocomposites with the fastest (0.436 min-1 mg-1) and highest mercury removal (6258.7 mg g-1) were then further optimized through intercalation with poly(vinylpyrrolidone), which increased the MoS2 interlayer distance from 0.68 nm to more than 0.90 nm. The final, optimal fabrication technique (evaluated according to mercuric capacity, kinetics, and nanocomposite stability) demonstrated five times higher adsorption than the second-best method and obtained 70% of the theoretical mercury adsorption capacity of MoS2. Overall, results from this study indicate an alternative, advanced material to increase the efficiency of aqueous mercury removal while also providing the basis for other novel environmental applications such as selective sensing, disinfection, and photocatalysis.

Optimal film thickness of rGO/MoS2 @ polyaniline nanosheets of 3D arrays for carcinoembryonic antigen high sensitivity detection

A three-dimensional array of reduced graphene oxide/MoS2 @ polyaniline-layered nanosheets (denoted as rGO/MoS2 @ PANI) has a novel structure of intercalated MoS2/PANI nanosheets arrays arranged on rGO that can be used as carcinoembryonic antigen (CEA) electrochemical immunosensors (detectors). In this graded nanosheet, the PANI was embedded in the MoS2 interlayer. The MoS2/PANI nanosheets themselves limit volumetric changes when arranged as MoS2/PANI nanosheet arrays on rGO nanosheets to provide tight and sufficient heterogeneity. PANI locates on the interface itself and contains a large amount of –NH2 that can bind many antibodies. Simultaneously, we investigated the optimal amount of aniline to add. We found that these arrays had excellent film thickness and conductivity for improving CEA detection sensitivity. The tested immunosensor achieved sensitive and specific detection of CEA, with a detection limit of 0.3 pg mL−1. Under the optimal conditions, the prepared immunosensor exhibited acceptable reproducibility and selectivity and excellent stability. Satisfactory results of human serum samples indicated the potential application of the immunosensor for clinical monitoring of tumor markers.

A MoS2/Carbon hybrid anode for high-performance Li-ion batteries at low temperature

Traditional lithium-ion batteries (LIBs) suffer from tremendous electrochemical performance fading at low temperatures (LT). This work reports a novel MoS2/C hybrid electrode that shows excellent LT performance as LIB anodes. At −20 °C, it delivers a stable discharge capacity as high as 854.3 mAh g−1 which is 72.8% of that at room temperature (RT). At a current density of 3 A g−1, a capacity of 140.9 mAh g−1 can still be achieved. The enlarged MoS2 interlayer spacing favors the fast Li+ diffusion, which in turn improves the lithiation/delithiation kinetics and the rate performance at LT. In-situ transmission electron microscopy (TEM) and galvanostatic intermittent titration technique (GITT) showed the fast Li+ diffusion kinetics at LT and the lithiation experiences a domain-by-domain reaction process, that is a multistep insertion-conversion reaction started from the outside domains to the neighboring inside domains, which is markedly different from that the two-step insertion-conversion process occurs at RT. Our results enrich the understanding of Li intercalation and diffusion kinetics of MoS2-based materials and provide a material design strategy for high-performance LIBs suitable for cold climate environment.

RhIr@MoS2 nanohybrids based disposable microsensor for the point-of-care testing of NADH in real human serum

Dihydronicotinamide adenine dinucleotide (NADH) is an important enzyme in all living cells, which is found to be abnormally expressed in cancer cells. Since it is redox-active, an electrochemical detection method would be suitable for monitoring its concentration in biological fluids. Here we present a strategy for specific determination of NADH in real human serum by using RhIr@MoS2 nanohybrids based microsensor. To implement the protocol, RhIr nanocrysrals are in-situ grown onto MoS2 interlayers forming a nanohybrid structure (RhIr@MoS2). After being locally deposited on an electrochemical microsensor, it could be used for the analysis of NADH. The developed RhIr@MoS2 nanohybrids based microsensor possesses the ability for analyzing NADH at the applied potential of 0.07 V (much lower than most reported values). The detection limit is evaluated as low as 1 nmol/L even in bovine serum albumin (BSA) media. In addition, the sampling analysis of human serum from cancer patients and health controls shows that the microsensor displays good diagnostic sensitivity and specificity, illustrating that this developed detection technique is a relatively accurate method for measuring NADH in biological fluids. The proposed electrochemical microsensor assay also owns the benefits of convenience, disposable and easy processing, which make it a great possibility for future point-of-care cancer diagnosis.

Metallic MoS2 nanosphere electrode for aqueous symmetric supercapacitors with high energy and power densities

Metallic 1T-MoS2 nanospheres composed of nanosheets were facilely synthesized by a hydrothermal method, and the moderate polyvinyl alcohol (PVA) and K+ ions were intentionally introduced in the subsequent exfoliation process. The as-prepared electrode (1T-MoS2/PVAK+), collected on the available carbon paper through vacuum filtration, exhibits a large specific capacitance of 488 F g−1 at a current density of 1 A g−1 in an aqueous electrolyte (3M KCl) because of the crosslink effect PVA to integrate the MoS2 nanospheres and interaction of K+ cation into the interlayers of MoS2. An aqueous symmetric supercapacitor was further prepared with 1T-MoS2/PVAK+ as the positive and negative electrodes. 1T-MoS2/PVAK+//1T-MoS2/PVAK+ device can work with an operating voltage of 1.6 V and manifests a high energy density of 49.0 Wh kg−1 at power density of 800 W kg−1. Remarkably, the maximum power density of 1T-MoS2/PVAK+//1T-MoS2/PVAK+ device reaches as high as 8000 W kg−1 with a corresponding energy density of 38.2 Wh kg−1. Furthermore, this type of aqueous symmetric device evidences the outstanding cycling stability with capacitance retention of 96% after 16,000 charge–discharge cycles at 1 A g−1. These excellent electrochemical performances of 1T-MoS2/PVAK+ electrode demonstrate a promising potential for application in supercapacitor with high energy density and high power density.

Ultralow Schottky Barrier Height Achieved by Using Molybdenum Disulfide/Dielectric Stack for Source/Drain Contact.

Energy barrier formed at a metal/semiconductor interface is a critical factor determining the performance of nanoelectronic devices. Although diverse methods for reducing the Schottky barrier height (SBH) via interface engineering have been developed, it is still difficult to achieve both an ultralow SBH and a low dependence on the contact metals. In this study, a novel structure, namely, a metal/transition-metal dichalcogenide (TMD) interlayer (IL)/dielectric IL/semiconductor (MTDS) structure, was developed to overcome these issues. Molybdenum disulfide (MoS2) is a promising TMD IL material owing to its interface characteristics, which yields a low SBH and reduces the reliance on contact metals. Moreover, an ultralow SBH is achieved via the insertion of an ultrathin ZnO layer between MoS2 and a semiconductor, thereby inducing an n-type doping effect on the MoS2 IL and forming an interface dipole in the favorable direction at the ZnO IL/semiconductor interfaces. Consequently, the lowest SBH (0.07 eV) and a remarkable improvement in the reverse current density (by a factor of approximately 5400) are achieved, with a wide room for contact-metal dependence. This study experimentally and theoretically validates the effect of the proposed MTDS structure, which can be a key technique for next-generation nanoelectronics.

The influences of Mg intercalation on the structure and supercapacitive behaviors of MoS2

Metallic 1T MoS2 greatly benefits the supercapacitance of MoS2 due to its considerably higher conductivity as compared to semiconducting 2H MoS2. Alkali metals, such as Li, Na and K, are always used to prepare 1T MoS2 through intercalation of alkali metal ions into the interlayer of 2H MoS2. Nevertheless, the influences of the alkali-earth metals as the guest in the interlayer of MoS2 on its structure and electrochemical capacitor performance are rarely investigated. Herein, we introduced hydrated Mg ions as the guest into MoS2 nanosheets. The interlayer spacing was increased to 1.144 nm after the introduction of hydrated Mg ions, larger than that of the pristine MoS2 (0.620 nm) and restacking MoS2 (0.626 nm). The enlarged interlayer spacing can accommodate more ions during intercalation process. Moreover, the 1T phase concentration after the introduction of hydrated Mg ions was as high as ~ 90%, which benefits the charge transfer during the charging/discharging processes. Consequently, the specific capacitance of the MoS2 with Mg guest ions as well as the energy densities and power densities was greatly improved as compared to those of the restacking MoS2 or pristine MoS2 counterparts. The present work not only demonstrated a new strategy for engineering the physical properties and improving the electrochemical performance of MoS2 supercapacitor electrode through pre-intercalation of alkali-earth metal ions in the interlayer of MoS2, but also has directive significance for the design of other layered electrode materials for energy storage systems.

MoS2/RGO hybrids prepared by a hydrothermal route as a highly efficient catalytic for sonocatalytic degradation of methylene blue

Abstract In this paper, the MoS2/RGO hybrids were successfully prepared via a one-pot hydrothermal. The MoS2/RGO was used for sonocatalytic degradation of methylene blue (MB) for the first time. It exhibited high sonocatalytic performance under ultrasound (US) irradiation than that of pure MoS2. Then the MoS2/RGO hybrids are well analyzed by various spectroscopic techniques such as TGA, X-Ray Diffraction, XPS, TEM, Raman, PL, UV–Vis and DRS etc. Furthermore, the sonocatalytic-degradation mechanism is discussed. The enhanced sonocatalytic performance of MoS2/RGO composites can be put down to that the heterojunction structure of MoS2/RGO can heighten the separation efficiency of the electron-hole pairs. The TEM images of the MoS2/RGO reveal that the RGO nanosheets had inserted into MoS2 layers. Thus, the template growth method based on GO expanded the interlayers of MoS2 so that it exposed a greater fraction of the catalytically active sites to offer a large surface area and improve the electronic conduction efficiency. This work provides an evidence that the MoS2/RGO possesses enormous potential in sonocatalytic applications.

Dynamically Optimized Multi-interface Novel BiSI-Promoted Redox Sites Spatially Separated n–p–n Double Heterojunctions BiSI/MoS2/CdS for Hydrogen Evolution

Novel BiSI promoted n–p–n double heterojunction multi-interface photocatalyst BiSI/MoS2/CdS was constructed. BiSI is applied to the photocatalytic hydrogen evolution. It possesses a small band gap and a strong optical absorption coefficient; therefore, the optical absorption scope and coefficient of MoS2/CdS have been effectively enhanced by compounding with BiSI. The continuous heterojunctions strengthened the function of the single junction and guided the carriers’ transfer direction; thus, the redox reactions occur at spatially separated sites. The built-in electric field along the radial direction of the BiSI nanorod and MoS2 interlayer helps to transport carriers within the lifetime. Carrier dynamics is optimized by the multi-interface structure. In general, a new material BiSI is introduced to construct a multi-interface structure to optimize carrier dynamics, which resulted in a 46-fold increase in hydrogen production efficiency.

A polymer-direct-intercalation strategy for MoS2/carbon-derived heteroaerogels with ultrahigh pseudocapacitance

The intercalation strategy has become crucial for 2D layered materials to achieve desirable properties, however, the intercalated guests are often limited to metal ions or small molecules. Here, we develop a simple, mild and efficient polymer-direct-intercalation strategy that different polymers (polyethyleneimine and polyethylene glycol) can directly intercalate into the MoS2 interlayers, forming MoS2-polymer composites and interlayer-expanded MoS2/carbon heteroaerogels after carbonization. The polymer-direct-intercalation behavior has been investigated by substantial characterizations and molecular dynamic calculations. The resulting composite heteroaerogels possess 3D conductive MoS2/C frameworks, expanded MoS2 interlayers (0.98 nm), high MoS2 contents (up to 74%) and high Mo valence (+6), beneficial to fast and stable charge transport and enhanced pseudocapacitive energy storage. Consequently, the typical MoS2/N-doped carbon heteroaerogels exhibit outstanding supercapacitor performance, such as ultrahigh capacitance, remarkable rate capability and excellent cycling stability. This study offers a new intercalation strategy which may be generally applicable to 2D materials for promising energy applications.

Efficient elimination of Cr(VI) from aqueous solutions using sodium dodecyl sulfate intercalated molybdenum disulfide

In recent years, the heavy metal ions have been immoderately released into the ecological system and result in potential hazardous to public health. Herein, the sodium dodecyl sulfate intercalated molybdenum disulfide (SDS-MoS2) was synthesized for the adsorption of Cr(VI). The SDS molecule was flat and vertically intercalated into the interlayer of MoS2, which was further evidenced by density functional theory calculations. The capture of Cr(VI) on the sphere-like SDS-MoS2 relied on solution pH. The retention of Cr(VI) on SDS-MoS2 attained 63.92 mg/g, and the removal process was endothermic, spontaneous and increased with temperature increasing. The main removal mechanism of Cr(VI) onto SDS-MoS2 was Cr(VI) fixing on the surface of the composites by chemisorption involving possible Cr-S coordination bonding. More importantly, Cr(VI) passed into the increased interlamination and reacted at the interlamination of SDS-MoS2, which was further proved at molecular level. The results can provide critical information for the application of SDS-MoS2 in Cr(VI) elimination or other kinds of pollutants removal in natural aquatic environment.

Controlling the growth of ultrathin MoS2 nanosheets/CdS nanoparticles by two-step solvothermal synthesis for enhancing photocatalytic activities under visible light

Heterostructure MoS2 nanosheets decorated CdS nanoparticles were successfully prepared by a facile two-step solvothermal method. The growth of ultrathin MoS2 nanosheets on CdS surface in terms of thickness and interlayers of (002) MoS2 was controlled by changing solvothermal reaction temperature. A series of MoS2 nanosheets decorated CdS nanoparticles were prepared at different temperature. The synthesized composites were characterized with XRD, FESEM, TEM, UV–vis DRS, BET, PL, XPS, EPR and I-T. TEM observations revealed the growth of ultrathin MoS2 nanosheets over the CdS nanoparticles, the thickness of MoS2 sheets and interlayers distance of (002) in composite was found to decrease with increasing the reaction temperature to 220 °C, under the same conditions. Photocatalytic activities of the as-prepared MoS2/CdS nanocomposites for the MO degradation were evaluated under the visible light irradiation. The ultrathin MoS2/CdS nanocomposites 1–220 with thinner sheets prepared at 220 °C exhibits higher photocatalytic efficiency for degradation of MO under visible light than all the other synthesized samples at different temperature. Furthermore, it was found that this catalyst retains remarkable photocatalytic activity after five cycles. The PL spectrum and photocurrent responses results suggest that the MoS2/CdS nanocomposite 1–220 possess higher separation efficiency and minimum recombination of photo-generated electrons and holes than pure CdS. The photocatalytic degradation mechanism of the MO over MoS2/CdS nanocomposite was proposed.

Nanoconfined Construction of MoS2@C/MoS2 Core–Sheath Nanowires for Superior Rate and Durable Li-Ion Energy Storage

Achieving fast and stable Li-ion energy storage in two-dimensional MoS2 materials has become a formidable challenge due to their sluggish electrochemical reaction kinetics and large structural change. In this study, a rational synthesis approach based on the nanoconfinement effect is reported to construct MoS2@C/MoS2 nanowires with a unique core–sheath configuration. The nanocomposite exhibits a great surface area of 170.1 m2 g–1, mesoporous nanotexture, along with expanded MoS2 interlayers. The porous core–sheath architecture and the electrically conductive carbon are of great benefit for swift transportation of Li-ions/electrons to enable enhanced reaction kinetics, and supply a great number of electroactive sites for more efficient energy storage. Additionally, the outer carbon nanoshells could maintain the structure integrity of the nanocomposite after a long-term cycle test. As a consequence, the MoS2@C/MoS2 nanowire anodes exhibit a high reversible capacity of 443 mA h g–1 at 10 A g–1 (53.2% retention of the capacity at 0.1 A g–1), and display superior stability over 500 cycles at both 1 and 5 A g–1. The electrochemical properties bestow the MoS2@C/MoS2 core–sheath nanocomposite a potential promise for high-rate and durable anodes of lithium-ion batteries.

Polyaniline-intercalated molybdenum disulfide composites for supercapacitors with high rate capability

Polyaniline-intercalated MoS2 composites with different morphologies (aggregated particles, nanowires, and nanotubes) were successfully prepared by a three-step method. The intercalation structure enhances electron transfer, and the expanded interlayer of MoS2 is beneficial for ion diffusion kinetics. Among the three composites, the tubular composite shows the best capacitive property in terms of a high specific capacitance of 375 F g−1 at a current density of 50 A g−1 and good stability for charge-discharge cycling. This indicates that the hollow structure, compared with the other nanostructures, facilitates the exposure of the active sites of the material and the penetration of electrolyte. Moreover, this work helps to understand the intrinsic issue of the electrochemical behavior of the polyaniline-intercalated MoS2 composite system, especially the influence of the morphology on the electrochemical properties.

Dual-Functional Interlayer Based on Radially Oriented Ultrathin MoS2 Nanosheets for High-Performance Lithium–Sulfur Batteries

The polysulfide shuttle is a major challenge for practical application of lithium–sulfur (Li–S) batteries. To solve this issue, a thin layer (3 μm) of radially oriented 3D MoS2 nanospheres (3D MoS2) constructed by 2D MoS2 nanosheets is developed as an interlayer coating overlaid on the sulfur cathode, which can simultaneously prevent polysulfide migration and facilitate Li+ diffusion. The trapping and catalytic conversion of polysulfides would benefit from the abundant active edge sites and the large contact surface in the well-preserved MoS2 nanosheets. Meanwhile, fast Li+ diffusion can be realized by the expanded (002) interlayer of the MoS2 nanosheets along with the sufficient electrolyte wettability due to the open 3D structure. Thus, the Li–S batteries with a 3D MoS2 interlayer and the pure sulfur cathode exhibit high specific capacity, excellent cycling stability (62.5% capacity retention with a 0.06% capacity decay per cycle and >99% Coulombic efficiency over 600 cycles), and satisfactory areal cap...

Mitigation of Shuttle Effect in Li–S Battery Using a Self-Assembled Ultrathin Molybdenum Disulfide Interlayer

Lithium-sulfur batteries are promising for low-cost and high-energy storage, but their applications are still limited by poor cycling stability owing to soluble lithium polysulfide shuttling during battery operation. Avoiding shuttle effect is challenging but it is essential to avoid active material loss and prevent performance decay. Here, we use an ultrathin layer of MoS2 with thickness of 10-40 nm, which is 1-2 orders of magnitude thinner than conventional interlayers, for Li-S batteries to mitigate polysulfide shuttling. The MoS2 layer formed by exfoliated nanoflakes is deposited by the scalable liquid-based self-assembly method. With less than 1% of additional weight in the cathode, the MoS2 interlayer with complete coverage inhibits polysulfide diffusion across the separator and therefore remarkably improves the battery performances. Reversible specific capacity reaches 1010 and 600 mA h g-1 at 0.5 and 2C rates, respectively, which decay slowly over 400 cycles (0.11% per cycle). Moreover, the MoS2 films with high density of catalytic active flake edges enable high areal sulfur loading up to 10 mg cm-2 and areal capacity up to 9.7 mA h cm-2 at a current density of 3.2 mA cm-2.

Reversible Intercalation of 1‐Ethyl‐3‐methylimidazolium Cations into MoS2 from a Pure Ionic Liquid Electrolyte for Dual‐Ion Cells

AbstractHerein, we present a novel dual‐ion cell with a molybdenum disulfide (MoS2) anode, a graphite cathode, and a pure 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) electrolyte. During charging, the EMIm+ cations intercalate into the anode‘s MoS2 interlayers and the TFSI− anions synchronously penetrate into the cathode‘s graphite lattice. During discharging, both the cations and anions deintercalate from the electrodes. The reversible intercalation of EMIm+ into MoS2 is systematically evaluated by cyclic voltammetry, galvanostatic cycling, XRD, Raman, and SEM tests. The results reveal that within 0.5–3.6 V and at 0.5 C, the batteries present a high medium discharge plateau of ∼2.5 V, a decent discharge capacity of 77.0 mAh g−1, and a good energy density of 175.6 Wh kg−1; at 4 C, the cells exhibit a superior cyclability with 84.3 % capacity retention for 300 cycles and a high Coulombic efficiency of 96.1 %. Electrode characterizations indicate that both the MoS2 anode and the graphite cathode display a high stability over a long‐term cycling process. This novel cell configuration opens a new avenue to further develop promising rechargeable dual‐ion energy storage systems.

Polyaniline intercalated with MoS2 nanosheets: structural, electric and thermoelectric properties

In this report, we present the synthesis of few layer MoS2 and polyaniline/MoS2 nanocomposites with a complete structural characterization by atomic force microscopy, small-angle X-ray scattering, X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy followed by AC impedance spectroscopy and thermoelectric characterizations. Our structural studies revealed that the intercalation of MoS2 nanosheets into polyaniline fibers produces a decrease in the MoS2 interlayer distance and a slight increase in the degree of order of the nanocomposites. In addition, an increment in the polaron Raman signature was observed with the addition of MoS2 nanosheets into polyaniline nanocomposites in agreement with the enhancement of the electrical conductivity, i.e. up to ~ 2.5 S cm−1. The addition of low amounts of MoS2 also lead to an increment of the seebeck coefficient (up to 5.3 µV K−1) and power factor (up to 0.0070 µV m−1 K−2). This report showed excellent synergistic effect between PANI and MoS2 nanosheets which could be of great importance for future applications of these nanocomposites, mainly as thermoelectric materials.