Increasing Te Content Research Articles

Compositionally Graded MoS2xTe2(1-x)/MoS2 van der Waals Heterostructures for Ultrathin Photovoltaic Applications.

van der Waals heterojunctions utilizing two-dimensional (2D) transition-metal dichalcogenide (TMD) materials have emerged as focal points in the field of optoelectronic devices, encompassing applications in light-emitting devices, photodetectors, solar cells, and beyond. In this study, we transferred few-atomic-layer films of compositionally graded ternary MoS2xTe2(1-x) alloys onto metal-organic chemical vapor deposition-grown molybdenum disulfide (MoS2) as p- and n-type structures, leading to the creation of a van der Waals vertical heterostructure. The characteristics of the fabricated MoS2xTe2(1-x)/MoS2 vertical-stacked heterojunction were investigated considering the influence of tellurium (Te) incorporation. The systematic variation of parameter x (i.e., 0.8, 0.6, 0.5, 0.3, and 0) allowed for an exploration of the impact of Te incorporation on the photovoltaic performance of these heterojunctions. As a result, the power conversion efficiency was enhanced by approximately 6 orders of magnitude with increasing Te concentration; notably, photoresponsivities as high as ∼6.4 A/W were achieved. These findings emphasize the potential for enhancing ultrathin solar energy conversion in heterojunctions based on 2D TMDs.

Crystal growth and characterization of Fe1+δSe1-xTex (0.5 ≤ x ≤ 1) from LiCl/KCl flux

An eutectic LiCl/KCl flux method in a horizontal configuration has been used to grow a series of homogeneous Fe1+δSe1-xTex single crystals of high quality with 0.5 ≤ x ≤ 1. Compared with previously used melt-growth method, the stable crystallization process in LiCl/KCl flux below their peritectic temperatures results in better homogeneity and crystalline perfection identified by energy dispersive spectrometer and x-ray diffraction. The interstitial Fe value δ remains small within 0.5 ≤ x ≤ 0.85 where the superconducting temperature TC is not sensitive to the Te content with sharp superconducting transition widths ΔTC < 1 K and a maximum of TC = 14.3 K at x = 0.61. The value δ starts to increase quickly accompanied by a deviation of linear behavior of crystal lattice parameters as well as the broadening of ΔTC = 2.1 K at x = 0.91, then suddenly rises up to δ > 0.1 followed by the disappearance of superconductivity and emergence of antiferromagnetic order at x ≥ 0.96. We also observed a metallic to semiconducting transition in the normal state resistivity of Fe1+δSe1-xTex with increasing Te content which is related to a localized electronic state induced by the interstitial Fe. The interstitial Fe value δ might be a key physical parameter to understand various properties of Fe1+δSe1-xTex system.

Enhancement of the response speed of CIGS-based photodetector by Te-doping

Cu(In,Ga)Se2 (CIGS) based devices are promising candidates for high performance photodetector applications. The present work investigated the effects of Te-doping on the properties of CIGS layers and studied the performance of photodetectors (PDs) fabricated on these films. The films were vacuum evaporated on glass substrates and their morphological, structural and optical properties were evaluated after an annealing step at 550ºC. Morphological data indicated that undoped CIGS films had non-uniform surface features with large grains separated by nanoparticles. Addition of 6.6 % Te improved the morphology and yielded a smoother layer. Further increase in Te concentration showed appreciable reduction in surface feature size and shape. X-ray diffraction patterns showed that increasing the Te amount in CIGS caused a shift in the XRD peaks towards smaller angles pointing to replacement of Se atoms by Te. Peak splitting at higher Te samples suggested a graded structure with Se and Te amounts changing through the thickness of the film. Raman analysis demonstrated formation of CuInGa(Se,Te)2 (CIGST) compound. According to Tauc’s approximation, CIGS films with and without Te atoms possessed two energy band gaps of Eg1 and Eg2 that were in the ranges of 1.10–1.28 eV and 1.16–1.36 eV, respectively. Electrical data obtained from photodetector devices showed a responsivity of 4.44×10−1 A/W and a detectivity of 8.01×107 Jones for Te-free material, while the rise/fall times were measured as 34/148 ms. A much faster response speed of 19/20 ms was reached for devices fabricated on films with 10.2 % Te. This work demonstrated, for the first time, the fabrication low-cost and high-performance metal-semiconductor-metal (MSM) type CIGST-based PDs.

Tailoring the surface properties of molybdenum oxytelluride thin films by its compositional ratio

In this work, RF magnetron co‐sputtering was used to fabricate molybdenum oxytelluride thin films with different atomic ratios. Scanning electron micrographs and atomic force micrographs revealed that pure Mo thin films were segregated. Thin films with 40% Te content portrayed needle‐like structures and thus had the largest Rq value of 2.48 nm. Contact angle measurements further supported the findings as the thin films with 40% Te content were the most hydrophobic. X‐ray photoelectron spectroscopy results revealed that the major oxidation states of Mo from Te‐poor to Te‐rich TFs were changed from high (+6) to low (+4 and/or +2). X‐ray diffraction data showed that peaks pertaining to molybdenum oxide species appeared, notably at 33.048° and 33.115°. Ultraviolet photoelectron spectroscopy and Kelvin probe measurements evinced that the work function increases with increasing Te content. Four‐point probe measurements revealed that molybdenum oxytelluride thin films tend to have higher electrical conductivity than pure Mo and Te thin films.

Structural evolution of Ge&lt;sub&gt;20&lt;/sub&gt;Se&lt;sub&gt;80–&lt;/sub&gt;&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Te&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; glass networks and assessment of glass properties by theoretical bandgap

Infrared imaging systems are being updated towards greater performance as well as lighter and smaller devices. Developing infrared materials with special properties is a critical for enhancing the performance of optical systems as well as miniaturizing devices. Chalcogenide glass becomes a popular option for advanced IR materials due to its component-property tunability. Se—based glasses such as Ge&lt;sub&gt;33&lt;/sub&gt;As&lt;sub&gt;12&lt;/sub&gt;Se&lt;sub&gt;55&lt;/sub&gt;, Ge&lt;sub&gt;10&lt;/sub&gt;As&lt;sub&gt;40&lt;/sub&gt;Se&lt;sub&gt;50&lt;/sub&gt;, and As&lt;sub&gt;40&lt;/sub&gt;Se&lt;sub&gt;60&lt;/sub&gt;, which completely cover the mid- and long-wave infrared windows, are the most typical materials used in infrared equipment. However, these classical materials can no longer meet the requirements of high-performance imaging systems, and adding more elements such as Te, Ga, Sb, and Ag to enhance the performance is a reliable way to solve this problem. By analysing the structure and properties of the Ge&lt;sub&gt;20&lt;/sub&gt;Se&lt;sub&gt;80–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Te&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; glass system, the law of its structure and properties evolving with Te content is illustrated. The obtained typical results are shown below. With the increase of Te content, the glass transition temperature (&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;g&lt;/sub&gt;) increases and then decreases, which is caused by the network structure and the average bond energy; the density and refractive index increase in an approximately linear gradient; the Abbe number gradually increases, while the Vickers hardness hardly changes with Te content; the fracture toughness decreases with the Te content increasing. Aiming at the problem that the average coordination number is unable to evaluate the glass systems composed of two or more elements from the same main group, a theoretical bandgap-glass property evaluation system is successfully established. The functional relationships among parameters such as density, refractive index, Abbe number, and fracture toughness, and theoretical band gap are established for Ge&lt;sub&gt;20&lt;/sub&gt;Se&lt;sub&gt;80–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Te&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; glass system as shown in the summary figure, which can be used to rapidly evaluate the glass components and properties.

Phase transition in WSe2-xTex monolayers driven by charge injection and pressure: a first-principles study.

Alloying strategies permit new probes for governing structural stability and semiconductor-semimetal phase transition of transition metal dichalcogenides (TMDs). However, the possible structure and phase transition mechanism of the alloy TMDs, and the effect of an external field, have been still unclear. Here, the enrichment of the Te content in WSe2-xTex monolayers allows for coherent structural transition from the H phase to the T' phase. The crystal orbital Hamiltonian population (COHP) uncovers that the bonding state of the H phase approaches the high-energy domain near the Fermi level as the Te concentration increases, posing a source of structural instability followed by a weakened energy barrier for the phase transition. In addition, the structural phase transition driven by charge injection opens up new possibilities for the development of phase-change devices based on atomic thin films. For WSe2-xTex monolayers with the H phase as the stable phase, the critical value of electron concentration triggering the phase transition decreases with an increase in the x value. Furthermore, the energy barrier from the H phase to the T' phase can be effectively reduced by applying tensile strain, which could favor the phase switching in electronic devices. This work provides a critical reference for controllable modulation of phase-sensitive devices from alloy materials with rich phase characteristics.

Calculation of the localized and extended energy states density for Ge60Se40-xTex alloy prepared by melting point method

The DC electrical conductivity properties of Ge60Se40-xTex alloy with x = 0, 5, 10, 15 and 20). The samples were formed in the form of discs with the thickness of 0.25–0.30 cm and the diameter of 1.5 cm. Samples were pressed under a pressure of 6 tons per cm2 , using a ton hydraulic press. They were prepared after being pressed using a ton hydraulic press using a hydraulic press. Melting point technology use to preper the samples. Continuous electrical conductivity properties were recorded from room temperature to 475 K. Experimental data indicates that glass containing 15% Te has the highest electrical conductivity allowing maximum current through the sample compared to Lu with other samples. Therefore, it is found that the DC conductivity increases with increasing Te concentration. The electrical conductivity properties show non-ohmic behavior due to the effects of temperature on the crystal structure of the samples, which indicates that the samples remain semi-conductive after partial replacement. Three conduction mechanisms are also observed for each sample at high, medium, and low temperatures. The Fermi level local and extended state densities and conductance parameters were calculated, and all were found to change with the change of Te concentration.

Large remnant polarization and improved dielectric, magnetic properties of Te-modified 0.7BiFeO3–0.3BaTiO3 ceramics

BiFeO3–BaTiO3 (BF–BT) ceramics, as a kind of binary ferroelectric material with excellent properties, have a wide range of applications. A novel Te-doped 0.7BiFe1−xTexO3–0.3BaTiO3 (x = 0–0.07) ceramics with enhanced ferroelectric properties were designed and synthesized via a solid-phase reaction. Results of X-ray diffraction and Rietveld refinement revealed a phase transition from the rhombohedra and tetragonal phases to a single tetragonal phase with increasing Te content, indicating the formation of a morphotropic phase boundary (MPB) in samples with x = 0–0.03. The fitting results of the X-ray photoelectron spectra showed that the Te introduction reduced the Fe2+ concentration and improved the insulating performance. Due to the Te-induced structure distortion of the rhombohedral phase and tetragonal phase, a large remnant polarization of 51 μC/cm2 is obtained at x = 0.01, which is superior to those reported in many studies on BF–BT bulk materials. The dielectric properties revealed that Te addition effectively adjusted the relaxation degree of the ceramics, showing an ideal relaxor ferroelectric behavior in samples with x = 0.05 and 0.07. Additionally, Te substitution reduced the Fe–O–Fe bond angle and the grain size, achieving enhanced ferromagnetism and affording the largest remnant magnetization in the case of x = 0.03, which is around six times that of the pure BF–BT ceramic.

Co/GeTe interfacial reactions and Co-Ge-Te phase equilibria

BackgroundGeTe is an important medium-temperature thermoelectric material, and Co is often used as a diffusion barrier layer. This study examines the interfacial reactions between Co and GeTe to verify the suitability of Co as a barrier layer. This study also determines the phase diagram of the Co-Ge-Te ternary system to provide basic material system knowledge and for better understanding the interfacial reactions. MethodsThe Co-Ge-Te phase diagram is determined based on the experimental results of this study, those related in the literature, and the phase diagrams of its three binary constituent systems. The Co/GeTe couples are prepared by plating Co layer on the GeTe substrate, and then reacted at 500 °C and 400 °C. Significant findingsIt is found the Ge0.5Te0.5 is of rhombohedral structure at 500 °C, gradually transforms to a cubic structure with increasing Te content, and the results differ from those in the literature. There are 15 single phases at 500 °C and 400 °C in the Co-Ge-Te ternary system, including two ternary compounds, Co2Ge3Te3 and CoGeTe. In the Co/GeTe couples reacting at 500 °C for 14 days, Co2Te3, Co5Ge3, CoGe and CoGeTe are formed, and the thickness of the reaction layer is about 80 μm. At 400 °C for 50 days, CoGe2, Co5Ge3 and Co2Te3 are formed, and the thickness of the reaction layer is about 12 μm. At both 500 °C and 400 °C, Co is the primary diffusion species in the Co/GeTe reaction couples. The results suggest Co is not a suitable barrier layer for GeTe at 500℃.

Successive, overlapping transitions and magnetocaloric effect in Te doped Ni-Mn-Sn Heusler alloys

Ni 43Mn 46Sn11−xTex (x = 0.1, 0.2, 0.3, 0.4, 0.5, 1) alloys have been prepared by arc melting. Structural studies show the presence of L21 cubic austenite phase. With the increase in Te concentration, the martensite phase evolves and the lattice constant decreases. Magnetic studies show that all the alloys exhibit first-order phase transition. It has been observed that the martensite temperature increases successively with the increase in Te concentration. A non-linear variation of magnetic entropy change, thermal hysteresis was observed in the alloys with increase in Te concentration. Magnetocaloric effect has been observed over a broad range of temperature span from 215 K to 289 K by varying the Te concentration. The magnetocaloric properties with Te doping have been analysed by using quantitative approach. Successive transitions and overlapping magnetocaloric effect occur with Te doping and they can be harnessed as layered magnetocaloric materials to enhance the refrigeration capacity. Our studies manifest that the martensite temperature and magnetocaloric properties are tuneable with the Te concentration and hence can be useful for magnetic refrigeration applications.

Composition-dependent properties and network structure of Ge-Se-Te chalcogenide glasses

Ge12.5Se87.5-xTex (0≤x≤45) glasses were selected for elucidating the composition-dependent properties and network structure of Te-containing glasses. With increasing Te content (x), Vickers hardness (Hv) and glass transition temperature (Tg) initially increased and then decreased, showing a compositional threshold at x=27.5. It is found that the compositional trend of Hv and Tg is in good accordance with the structural evolution studied by Raman spectra. The results suggest that the introduction of Te leads to the evolution of the network connectivity and average bond strength of Ge12.5Se87.5-xTex glass structure, which imposes an opposite impact on the structural properties (Hv and Tg). This work provides a new insight to the structure-property correlation of Ge-Se-Te, which would facilitate the understanding of the structural role of Te in ChGs.

Effect of tellurium concentration on the corrosion and mechanical properties of 304 stainless steel in molten FLiNaK salt

The effect of Te concentration in molten FLiNaK salt on the corrosion and mechanical properties of 304 stainless steel was investigated. The corrosion depth of samples after the immersion tests for 48 h at 600 ℃ decreased with increasing Te concentration. Likewise, the sample after the immersion test containing 1.0 wt% of Te (1.0 wt% Te sample) showed less reduction in mechanical properties than the 0.1 wt% Te sample. We hypothesize that the condition for a Cr-oxide layer that has the protective property is fulfilled underneath the Ni-Fe rich layer which is formed by the selective leaching of Cr.

Thermoelectric properties of a series of polycrystalline Mo(Se1−xTex)2

AbstractTransition metal dichalcogenides have being actively studied on the account of their large density‐of‐state effective mass and low lattice thermal conductivity. Herein, thermoelectric and electrical transport properties of MoSe2–MoTe2 system are investigated. A series of Mo(Se1−xTex)2 (x = 0, .25, .5, .75, and 1) polycrystalline samples was synthesized by conventional solid‐state reaction. Single hexagonal phase was identified for each sample without secondary phase, confirming the formation of complete solid solutions between MoSe2 and MoTe2. The electrical conductivity and magnitude of the Seebeck coefficient increased simultaneously with increasing Te content x at high temperatures, with the power factor following the same trend. As results, the MoTe2 sample exhibited the maximum power factor of .014 mW/(m K2) at 823 K. The lattice thermal conductivities of the alloyed samples (x = .25, .5, and .75) significantly decreased compared to MoSe2 and MoTe2 owing to point defect phonon scattering via anion substitution, which is verified by the Debye–Callaway model. A maximum zT of .0044 was obtained for MoTe2 sample at 773 K; however, the calculated quality factor B values of the samples with x = .75 and 1 at 773 K were almost identical, suggesting that a further enhancement of zT can be achieved by optimizing an nH of the Mo(Se0.25Te0.75)2 sample.

Physicochemical Properties of SnTe Thin Films Dependent on Compositional Ratios

This study introduces physicochemical properties based on the compositional ratio of the SnTe thin films (TFs) fabricated by radio frequency (RF) magnetron cosputtering. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results show that the grain sizes decrease with increasing Te content, with pure Te TFs yielding the lowest Rq value (root mean square value of roughness) of 0.76 nm. Contact angle (CA) measurements show that pure Te TFs pertain to the highest total surface free energy. X‐Ray diffraction (XRD) patterns strongly indicate that pure Te TFs exhibit a hexagonal crystal structure and pure Sn TFs exhibit a tetragonal crystal structure. The dominant oxide species on the surface is TeO2 for Te‐rich films, which is deduced by X‐Ray photoelectron spectroscopy (XPS) results. Pure Te TFs yield a work function value of 4.88 eV when measured with ultraviolet photoelectron spectroscopy (UPS) and 5.46 eV when measured with a Kelvin probe. SnTe TFs with 70% Te content exhibit the lowest electrical conductivity among the fabricated TFs.

Interconnected effects of direct Gd doping and accompanying indirect Te-stoichiometry destroying on the thermoelectric properties of Te-rich Bi2-xGdxTe3+y compounds

Solvothermal synthesis and spark plasma sintering were applied to prepare Te-rich Bi2-xGdxTe3+y compounds with 1; 2; 3 and 4 ​at. % Gd. To synthesize the Te-rich compositions, 3 ​at. % Te excess was introduced into all the starting powders with different x. Under sintering, high-temperature Te evaporation occurred. Owing to difference in electronegativity of Bi and Gd, resulting in increasing of ionicity degree and relevant strengthen of polar covalent Bi(Gd)-Te bond, a rate of the Te evaporation is remarkably decreasing with increasing x. As result, the x (Gd content) and y (Te content) coefficients happen to be interconnected. Specific point defects are formed via direct Gd doping (dopant Gd atoms substituting for Bi) and indirect destroying Te-stoichiometry (interstitial Te atoms and anti-site TeBi defects). Effects of these defects on the thermoelectric properties of the Bi2-xGdxTe3+y compounds with interconnected x and y coefficients were explored in detail. Specific anomalies of all the thermoelectric properties (the specific electrical resistivity, the Seebeck coefficient and the total thermal conductivity), related to onset of intrinsic electrical conductivity, are shifted to higher temperatures with increasing Gd content, which is accompanying by increasing Te content. This x(y)-dependent shifting can be originated from increasing energy gap, which is, in turn, due to strengthen of polar covalent Bi(Gd)-Te bonds in the Gd-doped samples. The best thermoelectric properties are observed in the composition with 1 ​at. % Gd and 59.94 ​at. % Te. This composition is very close to Te-stoichiometric one and is optimal, since it has minimal electron concentration and maximal electron mobility.

Detecting surface-state-dominated electron-phonon coupling in Bi2TexSe3−x crystals at low temperatures by femtosecond transient optical spectroscopy

The structure, transport properties, and ΔR/R ultrafast spectra of Bi2TexSe3-x crystals are studied in this work. The lattice parameter c and saturation temperature of resistance increase with increasing Te concentration. There are two electron-phonon coupling processes in Bi2TexSe3-x crystals corresponding to the interactions between two topological surface states and phonons. The electron-phonon coupling of one surface state is dominant at low temperatures (T ≤ 40 K), and the coupling of the second surface state and phonon is significant. With increasing temperature, electron-phonon coupling in bulk Bi2TexSe3−x is enhanced, which weakens electron-phonon coupling of its surface states. A small amount of Te has little effect on the electron-phonon coupling for Bi2Se3 crystals.

Improving thermoelectric performance of GeSe compound by crystal structure engineering

In the thermoelectric field, GeSe is a two-dimensional layered semiconductor with a large band gap, intrinsically low carrier concentration and poor thermoelectric figure of merit &lt;i&gt;ZT&lt;/i&gt;. In this work, a series of GeSe&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Te&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 0, 0.05, 0.15, 0.25, 0.35, 0.45) polycrystalline samples is prepared by melting and quenching combined with spark plasma activation sintering process. The influences of Te content on the phase structure and thermoelectric transport properties of GeSe are systematically studied. The results indicate that with the increase of Te content, the crystal structure of GeSe gradually changes from orthorhombic to rhombohedral structure. This reduces the band gap of the material, and simultaneously increases the carrier concentration and mobility. Meanwhile, the energy band degeneracy of the compound increases significantly because of enhanced crystal symmetry in this process, thereby considerably improving the effective mass of carriers. Altogether, the power factor of the rhombohedral GeSe is increased by about 2 to 3 orders of magnitude compared with that of the orthorhombic phase GeSe. In addition, the rhombohedral phase GeSe has abundant cationic vacancy defects and softened phonons arising from its ferroelectric feature, leading the lattice thermal conductivity to be 60% lower than orthorhombic one. The GeSe&lt;sub&gt;0.55&lt;/sub&gt;Te&lt;sub&gt;0.45&lt;/sub&gt; sample achieves a peak &lt;i&gt;ZT&lt;/i&gt; of 0.75 at 573 K, which is 19 times that of pristine GeSe. Crystal structure engineering could be considered as an effective way of improving the thermoelectric performance of GeSe compounds.

Ternary Ag2Se1-xTex: A Near-Room-Temperature Thermoelectric Material with a Potentially High Figure of Merit.

Discovering high-performance near-room-temperature thermoelectric materials is extremely imperative to widen the practical application in thermoelectric power generation and refrigeration. Here, ternary Ag2Se1-xTex (x = 0.1, 0.2, 0.3, 0.4, and 0.5) materials are prepared via the wet-mechanical alloying and spark plasma sintering process to investigate their near-room-temperature thermoelectric properties. From density functional theory calculation and single-parabolic-band modeling study, we found that the reduced contribution of Se 4p orbitals to the total density of states decreases the carrier effective mass with increasing Te content, which should enhance the theoretically maximum zT. These calculation results are also verified by the experimental results. Meanwhile, complex microstructures including dislocations, nanograins, high-density boundaries, TeSe substitution, lattice distortions, and localized strain have been observed in ternary Ag2Se1-xTex. These complex microstructures strengthen phonon scattering and in turn lead to ultralow lattice thermal conductivity in the range of 0.21-0.31 W m-1 K-1 in ternary Ag2Se1-xTex at 300 K. Although the increased deformation potential suppresses the carrier mobility, benefiting from the engineered band structures and ultralow lattice thermal conductivity, a high zT of >1 can be potentially obtained in the ternary Ag2Se1-xTex with appropriate carrier concentration. This study indicates that ternary Ag2Se1-xTex is a promising candidate for near-room-temperature thermoelectric applications.

Electronic Transport and Thermoelectric Properties of Te-Doped Tetrahedrites Cu12Sb4-yTeyS13

Tetrahedrite is a promising thermoelectric material mainly due to its low thermal conductivity, a consequence of its complicated crystal structure. However, tetrahedrite has a high hole concentration; therefore, optimizing carrier concentration through doping is required to maximize the power factor. In this study, Te-doped tetrahedrites Cu12Sb4-yTeyS13 (0.1 ≤ y ≤ 0.4) were prepared using mechanical alloying and hot pressing. The mechanical alloying successfully prepared the tetrahedrites doped with Te at the Sb sites without secondary phases, and the hot pressing produced densely sintered bodies with a relative density &gt;99.7%. As the Te content increased, the lattice constant increased from 1.0334 to 1.0346 nm, confirming the successful substitution of Te at the Sb sites. Te-doped tetrahedrites exhibited p-type characteristics, which were confirmed by the positive signs of the Hall and Seebeck coefficients. The carrier concentration decreased but the mobility increased with Te content. The electrical conductivity was relatively constant at 323–723 K, and decreased with Te substitution from 2.6 × 104 to 1.6 × 104 Sm-1 at 723 K. The Seebeck coefficient increased with temperature and Te content, achieving values of 184–204 μVK-1 at 723 K. The thermal conductivity was &lt;1.0 Wm-1K-1, and decreased with increasing Te content. Cu12Sb3.9Te0.1S13 exhibited the highest dimensionless figure of merit (ZT = 0.80) at 723 K, achieving a high power factor (0.91 mWm-1K-2) and a low thermal conductivity (0.80 Wm-1K-1).

Evolution of charge dynamics in FeSe1−xTex : Effects of electronic correlations and nematicity

We systematically studied in-plane optical conductivity of $\mathrm{Fe}{\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{Te}}_{x}$ thin films fabricated on $\mathrm{Ca}{\mathrm{F}}_{2}$ substrates for $x=0$, 0.1, 0.2, and 0.4. This system shows a large enhancement of superconducting transition temperature ${T}_{\mathrm{c}}$ at $x\ensuremath{\approx}0.2$ and a gentle decrease in ${T}_{\mathrm{c}}$ with further increasing $x$. The low-energy optical conductivity spectrum is described by the sum of narrow and broad Drude components, associated with coherent and incoherent charge dynamics, respectively. With increasing Te content, the spectral weight of the narrow Drude component decreases, whereas the total weight of the two Drude components increases. As a consequence, the fraction of the narrow Drude weight significantly decreases, indicating that Te substitution leads to stronger electronic correlations. Below the nematic transition temperature, the narrow Drude weight decreases with decreasing temperature. This indicates the reduction of the coherent carrier density, resulting from the Fermi-surface modification induced by the development of the orbital order. The reduction of the narrow Drude weight with temperature stopped at $x\ensuremath{\approx}$ 0.2, corresponding to the disappearance of the nematic transition. Our result suggests that the increase in the coherent carrier density induced by the suppression of the nematic transition gives rise to the enhancement of ${T}_{\mathrm{c}}$. The decrease in ${T}_{\mathrm{c}}$ with further Te substitution likely arises from too strong electronic correlations, which are not favorable for superconductivity.