xTex Alloys Research Articles

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.

Tunable Electronic Properties of Two-Dimensional GaSe1−xTex Alloys

In this work, we performed a theoretical study on the electronic properties of monolayer GaSe1-xTex alloys using the first-principles calculations. The substitution of Se by Te results in the modification of a geometric structure, charge redistribution, and bandgap variation. These remarkable effects originate from the complex orbital hybridizations. We demonstrate that the energy bands, the spatial charge density, and the projected density of states (PDOS) of this alloy are strongly dependent on the substituted Te concentration.

Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe1-XTeX Nanocrystals.

ZnSe1-XTeX nanocrystals (NCs) are promising photon emitters with tunable emission across the violet to orange range and near-unity quantum yields. However, these NCs suffer from broad emission line widths and multiple exciton decay dynamics, which discourage their practicable use. Here, we explore the excitonic states in ZnSe1-XTeX NCs and their photophysical characteristics in relation to the morphological inhomogeneity of highly mismatched alloys. Ensemble and single-dot spectroscopic analysis of a series of ZnSe1-XTeX NC samples with varying Te ratios coupled with computational calculations shows that, due to the distinct electronegativity between Se and Te, nearest-neighbor Te pairs in ZnSe1-XTeX alloys create localized hole states spectrally distributed approximately 130 meV above the 1Sh level of homogeneous ZnSe1-XTeX NCs. This forms spatially separated excitons (delocalized electron and localized hole in trap), accounting for both inhomogeneous and homogeneous line width broadening with delayed recombination dynamics. Our results identify photophysical characteristics of excitonic states in NCs made of highly mismatched alloys and provide future research directions with potential implications for photonic applications.

Thermoelectric Properties of Low Te Concentration‐Doped Cu2ZnSnSe4‐Based Quaternary Alloys

Quaternary Cu2ZnSnSe4 alloys have attracted more attentions as promising thermoelectric materials due to their instinct low thermal conductivity. Herein, Cu2ZnSnSe4 and Cu2ZnSn0.98Pb0.02Se4−xTex (x = 0.005, 0.010, 0.015, 0.020, 0.025, 0.030) alloys are successfully synthesized by melting, annealing, and hot pressing. All samples show main kesterite structure with slight Cu0.656Te0.344 impurity phases, high relative densities, and homogeneous element distribution. A higher power factor of 678.3 μW K−2m−1 is achieved at 673 K for Cu2ZnSn0.98Pb0.02Se3.975Te0.025 alloy, which is due to the reduction of the electrical resistivity. Meanwhile, the lattice thermal conductivity is slightly increased because the lattice structure is more symmetrical with Te‐doped samples. Combining the increased point defects, the minimum lattice thermal conductivity of 1.5 Wm−1 K−1 is achieved at 673 K. As the comprehensive result, the maximum figure of merit zT of 0.24 has been obtained at 673 K for Cu2ZnSn0.98Pb0.02Se3.975Te0.025 alloy, which is about 24% higher than that of the Cu2ZnSnSe4.

Optimal performance of Cu1.8S1\u2212xTex thermoelectric materials fabricated via high-pressure process at room temperature

Cu1.8S has been considered as a potential thermoelectric (TE) material for its stable electrical and thermal properties, environmental benignity, and low cost. Herein, the TE properties of nanostructured Cu1.8S1−xTex (0 ⩽ x ⩽ 0.2) bulks fabricated by a facile process combining mechanical alloying (MA) and room-temperature high-pressure sintering (RT-HPS) technique were optimized via eliminating the volatilization of S element and suppressing grain growth. Experimentally, a single phase of Cu1.8S was obtained at x = 0, and a second Cu1.96S phase formed in all Cu1.8S1−xTex samples when 0.05 ⩽ x ⩽ 0.125. With further increasing x to 0.15 ⩽ x ⩽ 0.2, the Cu2−zTe phase was detected and the samples consisted of Cu1.8S, Cu1.96S, and Cu2−zTe phases. Benefiting from a modified band structure and the coexisted phases of Cu1.96S and Cu2−zTe, the power factor is enhanced in all Cu1.8S1−xTex (0.05 ⩽ x ⩽ 0.2) alloys. Combining with a drastic decrease in the thermal conductivity due to the strengthened phonon scatterings from multiscale defects introduced by Te doping and nano-grain boundaries, a maximum figure of merit (ZT) of 0.352 is reached at 623 K for Cu1.8S0.875Te0.125, which is 171% higher than that of Cu1.8S (0.130). The study demonstrates that doping Te is an effective strategy to improve the TE performance of Cu1.8S based materials and the proposed facile method combing MA and RT-HPS is a potential way to fabricate nanostructured bulks.

Structural, Electronic, and Optical Properties of ZnO1 – xTex Alloys

Based on the ab initio evolutionary algorithm, the Universal Structure Predictor: Evolutionary Xtallography (USPEX) code is used to search for the candidate structures of ZnOTe alloys below 20 GPa. Herein, several metastable structures of ZnO1 – xTex alloys are proposed, namely, Zn4O3Te, Zn2OTe, and Zn5O2Te3 with space groups I‐42m, I‐42d, and C2/m structure, to be stable at 0–10, 0–18, and 0–4 GPa, respectively. From the electronic band structures, it is found that the C2/m Zn5O2Te3 is a metal phase, and the other two structures are semiconductors. Further results show that the bandgap of ZnO1 − xTex first decreases and then increases at an ambient condition with the increase in the Te content, which is in agreement with previous theoretical results, and this trend is also confirmed by the imaginary part of the dielectric function. In addition, the calculated dielectric function results show that the polarization capability of the material is enhanced with the increase in the Te content.

A first-principles study of the structural, elastic, electronic, vibrational, and optical properties of BaSe1−xTex

The structural, elastic, electronic, vibrational, and optical properties of BaSe1−xTex alloys are investigated by means of the full-potential linearized augmented plane wave method. The exchange–correlation effects are treated with the local density approximation, as well as the GGA-PBE, GGA-PBEsol, and GGA + mBJ schemes of the generalized gradient approximation. Ternary BaSe1−xTex compounds have not yet been synthesized. Improved predictions of the structural parameters are obtained using the GGA-PBEsol approach. Calculations of the electronic and optical properties with the GGA + mBJ approach yield accurate results. Ternary BaSe1−xTex alloys are wide-band-gap semiconductors with a direct gap Γ–Γ. The upper valence band is mainly due to Se p and Te p states, while the bottom of the conduction band results essentially from Ba d states. The dielectric function, refractive index, reflectivity, absorption coefficient, and energy-loss function are calculated in the range 0–35 eV. The increase in x gives rise to a redshift of the optical spectra. BaSe1−xTex alloys exhibit reflective properties of metals in some energy ranges. The static dielectric constant ɛ1(0) and the static refractive index n0 are calculated. The investigation of the elastic and vibrational properties shows that ternary BaSe1−xTex should be mechanically and dynamically stable, elastically anisotropic, brittle, and relatively soft.

Investigation of pressure-induced phase transitions of the solar cell materials CdSe1−xTex alloys: one- and two-dimensional search DFT calculation

ABSTRACTGibbs script within the all-electron full potential linearized augmented plane wave (FP-LAPW) method has been used to calculate pressure-induced phase transition at zero temperature for selected phases. The calculation exhibits that CdSe is stable at wurtzite phase whereas CdSe0.75Te0.25, CdSe0.5Te0.5, CdSe0.25Te0.75 and CdTe are stable at cubic phase. This prediction is consistent with experimental work of Schenk and Silber at ˂ 800 °C except for CdSe0.75Te0.25. We found that increasing the Te content for both phases of ternary CdSe1−xTex alloys decreases the optimized energy and bulk modulus, hence leads to decrease hardness, while it increases the optimized volume. The density functional theory (DFT) calculation within generalized gradient approximation (PBE-GGA) and the two-dimensional search of equation of state for wurtzite phase predict a possible pressure-induced phase transition from cubic to wurtzite phase for the ternary CdSe0.5Te0.5, CdSe0.75Te0.25 and CdTe at pressures of about 13.6, 1.9 and 10.6 (GPa) with corresponding volume collapses at the phase-transition boundary of about 181.115 and 180.525, 203.839 and 203.485 and 203.337 and 202.692 (bohr3/atom), respectively. However, one-dimensional search of equation of state for wurtzite phases predicts a possible pressure-induced phase transition at pressures 14.5, 1.9 and 11.4 for CdSe0.5Te0.5, CdSe0.75Te0.25 and CdTe, respectively.

Telluriding monolayer MoS2 and WS2 via alkali metal scooter

The conversion of chalcogen atoms to other types in transition metal dichalcogenides has significant advantages for tuning bandgaps and constructing in-plane heterojunctions; however, difficulty arises from the conversion of sulfur or selenium to tellurium atoms owing to the low decomposition temperature of tellurides. Here, we propose the use of sodium for converting monolayer molybdenum disulfide (MoS2) to molybdenum ditelluride (MoTe2) under Te-rich vapors. Sodium easily anchors tellurium and reduces the exchange barrier energy by scooting the tellurium to replace sulfur. The conversion was initiated at the edges and grain boundaries of MoS2, followed by complete conversion in the entire region. By controlling sodium concentration and reaction temperature of monolayer MoS2, we tailored various phases such as semiconducting 2H-MoTe2, metallic 1T′-MoTe2, and 2H-MoS2−xTex alloys. This concept was further extended to WS2. A high valley polarization of ~37% in circularly polarized photoluminescence was obtained in the monolayer WS2−xTex alloy at room temperature.

Physical and optical characterizations of Ge10Se90 − xTex thin films in view of their spectroscopic ellipsometry data

Ge10Se90−xTex (x=10, 20, 30) alloys have been prepared by the conventional melt quenching technique. Several important physical parameters have been calculated to understand the structural changes in the prepared compositions. Spectroscopic ellipsometry technique has been used to investigate the optical properties of the deposited thin films. The extracted optical constants have been utilized to evaluate several interesting optoelectrical parameters. The prepared films have revealed indirect energy gap transition that varied from 1.4 to 1.02eV. Increasing Te content leads to the increase in the joint density of states (JDOS) in the investigated films which is accompanied by the decrease in the energy gap. Profound analysis has been made to study the variation of physical, optical, and optoelectrical parameters of Ge10Se90−xTex as a function of Te content. Moreover, it was fruitful to estimate the changes in the nonlinear parameters of the studied films. An obvious increase in the nonlinear parameters has been observed with the increase in Te percent. This refers to an increase in the nonlinear response of bound electrons in the glassy system to the applied electric field. Believing this we may expect the usage of the samples under investigation in non-linear devices. The studied parameters have enriched our current knowledge regards to the nature of the studied compositions.

Engineering Bicolor Emission in 2D Core/Crown CdSe/CdSe1–xTex Nanoplatelet Heterostructures Using Band-Offset Tuning

Colloidal 2D nanoplatelets (NPLs) are a class of nanoparticles that offer the possibility of forming two types of heterostructures, by growing either in the confined direction or perpendicular to the confined direction, called core/crown NPLs. Here, we demonstrate that bicolor emission can be obtained from 2D NPLs with a core/crown geometry. To date, for CdSe/CdTe NPLs with type-II band alignment, only charge transfer emission has been observed due to the very fast (<picoseconds) transfer of the electrons toward the CdSe core. Here, we show that using CdSe1–xTex alloys crowned with the right composition and lateral extension enables the observation of bicolor emission at the single-particle level. One source of emission originates from recombination at the core/crown interface (Xint), and the other emission source originates from direct recombination in the crown (Xcrown). This crown emission, which is nonvisible in pure CdSe/CdTe core crown NPLs, results from the large binding energy compared to the redu...

Ab initio calculations of the structural, electronic, thermodynamic and thermal properties of BaSe1−xTex alloys

The alkaline earth metal chalcogenides are being intensively investigated because of their advanced technological applications, for example in photoluminescent devices. In this study, the structural, electronic, thermodynamic and thermal properties of the BaSe1−xTex alloys at alloying composition x = 0, 0.25, 0.50, 0.75 and 1 are investigated. The full potential linearized augmented plane wave plus local orbital method designed within the density functional theory was used to perform the total energy calculations. In this research work the effect of the composition on the results of the parameters and bulk modulus as well as on the band gap energy is analyzed. From our results, we found a deviation of the obtained results for the lattice constants from Vegard’s law as well as a deviation of the value of the bulk modulus from the linear concentration dependence. We also carried out a microscopic analysis of the origin of the band gap energy bowing parameter. Furthermore, the thermodynamic stability of the considered alloys was explored through the measurement of the miscibility critical temperature. The quasi-harmonic Debye model, as implemented in the Gibbs code, was used to predict the thermal properties of the BaSe1−xTex alloys, and these investigations comprise our first theoretical predictions concerning the BaSe1−xTex alloys.

Theoretical investigation on thermodynamic properties of ZnO1−xTex alloys

In this study, the formation energy, phase diagram (with/without phonon contribution) and the relationship between bond stiffness and bond length for wurtzite (WZ) and zincblende (ZB) structures of ZnO1−xTex (0 ⩽ x ⩽ 1) alloys have been investigated by combining first-principles calculations and cluster expansion method. The formation energy of ZnO1−xTex alloys is very high in both structures, which means that it is difficult for ZnO and ZnTe to form stable ternary alloys ZnO1−xTex. In the phase diagrams, both structures do not have stable phase of ternary alloys and ZnO1−xTex ternary alloys can only exist in the form of metastable phase. These results indicate that ZnO and ZnTe easily form solid solubility gap when they form alloys. After considering vibrational free energy, we found the solubility of Te in ZnO and O in ZnTe was increased and the vibrational entropy improved the solubility furthermore. The phonon contribution is not ignorable to improve solid solubility. The phonon density of states was analyzed for ZnO1−xTex alloys and the contribution from vibrational entropy was discussed.

Disclosing the structural, phase transition, elastic and thermodynamic properties of CdSe1−xTex(x = 0.0, 0.25, 0.5, 0.75, 1.0) using LDA exchange correlation

The phase transition and structural, elastic, thermodynamic characteristics of CdSe1−xTex alloys for all compositions x (x=0, 0.25, 0.5, 0.75, 1) in both hexagonal wurtzite (WZ) and cubic zinc-blende (ZB) are studied at zero K and zero pressure in emphasis of the Full Potential Linearized Augmented Plane Wave (FP-LAPW) approach, in accordance with the Density Functional Theory (DFT). This was inserted within the WIEN2k code, alongside a local density approximation (LDA) in order to consider the exchange-correlation functional. For all compositions the CdSe1−xTex alloys were found to be mechanically stable for both phases ZB and WZ, and the strongest material among all structures is CdSe. Our findings reveal that the relation between elastic constants and the Te concentrations is not linear. The induced phase transition from ZB to WZ is studied at zero K, and the corresponding volume collapses at the phase transition boundary are calculated for all compositions x (x=0.0, 0.25, 0.50, 0.75). Our results show that for all compositions of the CdSe1−xTex alloys, the stable phase is zinc-blende. Furthermore, the elastic characteristics of ZB and WZ phases of CdSe1−xTex alloys, alongside elastic constants, bulk modulus and shear modulus were determined and assessed in comparison with other theoretical and experimental findings available. A positive relationship was observed.

Ab initio method of optical investigations of CdS1−xTex alloys under quantum dots diameter effect

The indirect energy gap (Γ−X) is calculated using density functional theory (DFT) of the full potential-linearized augmented plane wave (FP-LAPW) method as implemented in WIEN2K code. The Engel–Vosko generalized gradient approximation (EV-GGA) formalism is used to optimize the corresponding potential for energetic transition and optical properties calculations of CdS1−xTex alloys as a function of quantum dot diameter and is used to test the validity of our model of quantum dot potential. The refractive index and optical dielectric constant are investigated to explore best applications for solar cells.

Electronic band structure of ZnO-rich highly mismatched ZnO1−xTex alloys

We synthesized ZnO1−xTex alloys with Te composition x &amp;lt; 0.23 by using pulsed laser deposition. Alloys with x &amp;lt; 0.06 are crystalline with a columnar growth structure while samples with higher Te content are polycrystalline with random grain orientation. Electron microscopy images show a random distribution of Te atoms with no observable clustering. We found that the incorporation of a small concentration of Te (x ∼ 0.003) redshifts the ZnO optical absorption edge by more than 1 eV. The minimum band gap obtained in this work is 1.8 eV for x = 0.23. The optical properties of the alloys are explained by the modification of the valence band of ZnO, due to the anticrossing interactions of the localized Te states with the ZnO valence band extended states. Hence, the observed large band gap reduction is primarily originating from the upward shift of the valence band edge. We show that the optical data can be explained by the band anticrossing model with the localized level of Te located at 0.95 eV above the ZnO valence band and the band anticrossing coupling constant of 1.35 eV. These parameters allow the prediction of the compositional dependence of the band gap as well as the conduction and the valence band offsets in the full composition range of ZnO1−xTex alloys.

Theoretical investigation of band gap and optical properties of ZnO1−xTex alloys (x = 0, 0.25, 0.5, 0.75 and 1)

In this study, the special quasi-random structure (SQS) approach has been considered for structural, electronic and optical properties of rock-salt (RS) and zinc-blende (ZB) phases of ZnO1−xTex (x=0, 0.25, 0.5, 0.75 and 1) using density functional theory. The Wu–Cohen generalized gradient approximation (GGA) has been employed for optimizing the lattice parameters (a0) and bulk moduli (B0) in both phases which show reasonable agreement with numerous theoretical and experimental results. To compute the band gaps with high degree of precision, we employed Engel–Vosko GGA and modified Becke and Johnson local density approximation (mBJLDA) functionals. In the RS phase, metallic nature of the compounds under investigation is evident for 0.25<x<1, whereas direct band gap appears at all concentrations in ZB phase. The density of states (total and partial) are presented to comprehensively analyze the electronic structure of all compounds. Contrary to earlier theoretical studies, the mBJLDA band gap values for the end binaries appear to be significantly improved showing overall better agreement with the experimental data. The optical properties of ZnO1−xTex alloys in the ZB phase are also discussed in terms of dielectric function which show their potential utilization in optoelectronic devices.

Reversible amorphous-crystalline phase changes in a wide range of Se(1-x)Te(x) alloys studied using ultrafast differential scanning calorimetry.

The reversible amorphous-crystalline phase change in a chalcogenide material, specifically the Se1-xTex alloy, has been investigated for the first time using ultrafast differential scanning calorimetry. Heating rates and cooling rates up to 5000 K/s were used. Repeated reversible amorphous-crystalline phase switching was achieved by consecutively melting, melt-quenching, and recrystallizing upon heating. Using a well-conditioned method, the composition of a single sample was allowed to shift slowly from 15 at. %Te to 60 at. %Te, eliminating sample-to-sample variability from the measurements. Using Energy Dispersive X-ray Spectroscopy composition analysis, the onset of melting for different Te-concentrations was confirmed to coincide with the literature solidus line, validating the use of the onset of melting Tm as a composition indicator. The glass transition Tg and crystallization temperature Tc could be determined accurately, allowing the construction of extended phase diagrams. It was found that Tm and Tg increase (but Tg/Tm decrease slightly) with increasing Te-concentration. Contrarily, the Tc decreases substantially, indicating that the amorphous phase becomes progressively unfavorable. This coincides well with the observation that the critical quench rate to prevent crystallization increases about three orders of magnitude with increasing Te concentration. Due to the employment of a large range of heating rates, non-Arrhenius behavior was detected, indicating that the undercooled liquid SeTe is a fragile liquid. The activation energy of crystallization was found to increase 0.5-0.6 eV when the Te concentration increases from 15 to 30 at. % Te, but it ceases to increase when approaching 50 at. % Te.

Composition determination of multicomponent chalcogenide glassy semiconductors with X-ray fluorescence analysis

The standard method-recording of X-ray fluorescence spectra of a standard Ge0.2As0.4Se0.4 alloy followed by evaluation of its component spectral fractions and by building of dependences xXRF = f(x) and yXRF = f(y) for the Asx(GeySe1 − y)1 − x system—was employed to determine the quantitative contents of arsenic, germanium, and selenium in the GetAssSe1 − t − s glassy alloys with X-ray fluorescence analysis. The accuracy of the composition’s determination was ±0.0005 for both x and y. However, it was not possible to use the external standard method for determining the tellurium in AsySe1 − y)1 − xTex alloys because tellurium, arsenic, and selenium had significantly different X-ray fluorescence characteristics and, hence, substitution of the arsenic or selenium for tellurium at a fixed y resulted in tellurium emission fraction changing.

Crystallization kinetics and effect of thermal annealing on optical constants in a-Ge25Se75−xTex glasses

Differential scanning calorimetric (DSC) measurements have been made in glassy Ge25Se75−xTex alloys. DSC thermograms of the samples have been recorded at five different heating rates 5, 10, 15, 20 and 25K/min. From the heating rate dependences of the glass transition temperature (Tg) and crystallization peak temperature (Tc); the activation energy for crystallization (ΔEc) and the activation energy for glass transition (ΔEg) have been derived. The present investigation indicates that both the glass transition and the crystallization processes occur in a single stage. The amorphous Ge25Se75−xTex chalcogenide thin films have been prepared by thermal evaporation onto chemically cleaned glass/Si wafer (100) substrates. Thin films have been thermally annealed at three different temperatures 333, 343 and 353K for 2h. As-prepared and thermally annealed thin films were analyzed by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and UV–VIS–NIR absorption measurements. It is found that the mechanism of the optical absorption follows the rule of non-direct transition. The optical energy gap decreases with increasing Te content and also by thermal annealing. The decrease in the optical gap is discussed on the basis of amorphous-crystalline transformations.