xSex Alloys Research Articles

Machine-learning guided prediction of thermoelectric properties of topological insulator Bi2Te3−xSex

The random forest model predicts the thermal conductivity, Seebeck coefficient, electrical conductivity and figure of merit of Bi2Te3−xSex alloys aligning closely with experimental and theoretical observations.

Structural and optical characterization of GaS1−xSex layered mixed crystals grown by chemical vapor transport

Alloying is a prominent strategy for tailoring the properties of semiconductors and expanding their usability. Accordingly, an entire series of GaS1–xSex (0 ≤ x ≤ 1) alloys were grown by chemical vapor transport and their structural and optical properties were studied. It was found that, for optimal crystalline quality, the growth temperatures should decrease as x increases. The scanning electron microscopy images of the specimens showed GaS1-xSex are layered crystals. The transmission electron microscopy (TEM) images showed the GaS1−xSex crystals have hexagonal structures with high crystalline quality. Their Raman and X-ray diffraction (XRD) results indicated GaS and GaS0.8Se0.2 are in the 2 H β-phase, GaSe is in the 2 H ε-phase, while those with intermediate x are in the 2 H β-ε mixed phase. The frequencies of the Raman modes decrease continuously as x increases. This single-mode behavior differs from the dual-mode behavior reported by the literature. The lattice constants a and c of the GaS1−xSex crystals, estimated from the TEM and XRD results, increase as x increases. The variation in a with x follows linear Vegard’s law. However, the variation in c with x is quadratic. The indirect and direct bandgaps of GaS1–xSex, determined from the absorption spectra, decrease as x increases. The difference between the indirect and direct bandgaps shrinks as x increases. The minimum difference is only 20 meV, which makes GaS1−xSex with high x a pseudo-direct bandgap semiconductor. The piezoreflectance spectra of the samples showed excitonic signals, which may be attributed to the direct interband transitions. The wavelength of the samples’ photoluminescence peaks increases with x and covers the visible range. Thus, GaS1–xSex shows great promise for optoelectronic applications.

Revealing the relationship of NO2 sensing with energy level in 2D van der Waals SnS1−xSex alloys

Understanding the relationship between sensing behavior and energy level is an important prerequisite for providing rational guidance to predict advanced gas-sensing materials and develop design principles. A tunably compositional van der Waals (vdW) alloy SnS1−xSex (x = 0.2, 0.4, 0.6, 0.8) that reveals the trend of response value with Fermi level is first reported here, which is prepared by the isoelectronic Se atoms substituting the S atoms of SnS via an ion exchange method. The cause-and-effect of the energy level with response value is disclosed by Kelvin probe force microscopy, which reveals that increasing the gap between the Fermi level of alloys and the molecular orbital of NO2 by tuning Se content can enhance the response value for NO2. Compared to a no response to NO2 for pristine SnS crystals at room temperature, the as-prepared SnS1−xSex vdW alloys show a sensitive p-type response, in which the response value reaches 400.3% for SnS0.6Se0.4 alloys toward 20 ppm NO2 with a short response/recovery time (80 s/140 s). The SnS0.6Se0.4 alloys also have excellent selectivity and good stability. As a basic and crucial step, this work could pave the way to develop the gas sensors of vdW layered crystals with a p-type high-sensitive response, and also could be extended to electronics, sensing, and catalysis fields.

Room temperature magnetocaloric effect in CrTe1−xSex alloys

In this work, we report room temperature magnetocaloric properties in CrTe1−xSex (0.00 ≤ x ≤ 0.10) alloys prepared by a conventional solid-state reaction. The Rietveld refinement of the XRD pattern of CrTe1−xSex showed the emerging of pure hexagonal NiAs structure with P63/mmc (194) space group with increasing Se substitution. Upon μ0H = 5 T applied magnetic field, the magnetocaloric effect analysis revealed maximum magnetic entropy change $$(\Delta {S}_{M})$$ in the range 7.9–9.2 J/kg K−1 with a relative cooling power (RCP) value of about 550–694 J/kg. The RCP values exceed the corresponding RCP value of the prototype Gd element near room temperature. The obtained results suggest that CrTe1−xSex (0.00 ≤ x ≤ 0.10) alloys is a viable candidate for a room temperature magnetic cooling application.

Ultralow Lattice Thermal Conductivity and Improved Thermoelectric Performance in Cl-Doped Bi2Te3-xSex Alloys.

Bi2Te3-based alloys are the main materials for the construction of low- and medium-temperature thermoelectric modules. In this work, the microstructure and thermoelectric properties of Cl-doped Bi2Te3–xSex alloys were systematically investigated considering the high anisotropy inherent in these materials. The prepared samples have a highly oriented microstructure morphology, which results in very different thermal transport properties in two pressing directions. To accurately separate the lattice, electronic, and bipolar components of the thermal conductivity over the entire temperature range, we employed a two-band Kane model to the Cl-doped Bi2Te3–xSex alloys. It was established that Cl atoms act as electron donors, which tune the carrier concentration and effectively suppress the minority carrier transport in Bi2Te3–xSex alloys. The estimated value of the lattice thermal conductivity was found to be as low as 0.15 Wm–1 K–1 for Bi2Te3–x–ySexCly with x = 0.6 and y = 0.015 at 673 K in parallel to the pressing direction, which is among the lowest values reported for crystalline materials. The large reduction of the lattice thermal conductivity in both pressing directions for the investigated Bi2Te3–xSex alloys is connected with the different polarities of the Bi-(Te/Se)1 and Bi-(Te/Se)2 bonds, while the lone-pair (Te/Se) interactions are mainly responsible for the extremely low lattice thermal conductivity in the parallel direction. As a result of the enhanced power factor, suppressed bipolar conduction, and ultralow lattice thermal conductivity, a maximum ZT of 1.0 at 473 K has been received in the Bi2Te2.385Se0.6Cl0.015 sample.

Structural Evolution in Thin Films of Ag2Te1−xSex (x~0.3) Alloy Prepared by Thermal Evaporation Technique

Structural Evolution in Thin Films of Ag2Te1−xSex (x~0.3) Alloy Prepared by Thermal Evaporation Technique

MSELD SnS1−xSex alloy thin films towards efficient structural and bandgap engineering for photonic devices

MSELD SnS1−xSex alloy thin films towards efficient structural and bandgap engineering for photonic devices

Structural transformation and phase change properties of Se substituted GeTe

GeTe1−xSex (0 ≤ x ≤ 1.0) alloys have been prepared both in bulk and thin film forms to study the effect of selenium (Se) substitution for tellurium (Te) on the phase change properties. It is observed that with increasing Se substitution in GeTe, the structure transforms from rhombohdral structure to orthorhombic structure. Rietveld Refinement analysis support the phase transformation and show that the short and long bond lengths in crystalline GeTe decrease with increasing Se substitution but the rate of reduction of shorter bond length is more than the longer bond length. The GeTe1−xSex thin films undergo amorphous to crystalline phase change when annealed at high temperatures. The transition temperature shows an increasing trend with the Se substitution. The contrast in electrical resistivity between the amorphous and crystalline states is 104 for GeTe, and with the Se substitution, the contrast increases considerably to 106 for GeTe0.5Se0.5. Devices fabricated with thin films show that the threshold current decreases with the Se substitution indicating a reduction in the power required for WRITE operation. The present study shows that the crystalline structure, resistance, bandgap, transition temperature and threshold voltage of GeTe can be effectively controlled and tuned by the substitution of Te by Se, which is conducive for phase change memory applications.

Synthesis and optical properties of single-crystalline SnS1−xSex nanobelts

In this work, SnS1−xSex ternary nanobelts were synthesized by a facile hydrothermal method without the assistance of surfactants. The structure, morphology, microstructure, compositions, chemical valences, phonon modes, and optical band gaps of the SnS1−xSex nanobelts were characterized in detail. The results indicate that the SnS1−xSex nanobelts have uniform one-dimensional morphology and are single crystals with high crystallinity. Se is incorporated into the SnS lattice to substitute for S-forming ternary SnS1−xSex alloy. With the increase of Se doping concentration, the optical band gaps of the nanobelts gradually decrease from 1.15 to 1.01 eV, confirming the tunable optical property achieved here.

Photoluminescence of Layered Semiconductor Materials for Emission-Color Conversion of Blue Micro Light-Emitting Diode (µLED)

A micro light-emitting diode (μLED) is a key device for the future of advanced information. Owing to expand its application widely, the concept of the emission-color conversion using layered semiconductors as a color converter is proposed. In addition, it is demonstrated that layered semiconductors were transferred directly onto μLED chips, and the emission-color conversion is realized. The layered GaS1−xSex alloy, whose energy bandgap can be controlled by tuning the S and Se compositions, was selected as a color converter. The photoluminescence (PL) measurements using a blue LED as an excitation source revealed that GaS0.65Se0.35 and GaSe can show green and red luminescence with center energies of 2.34 and 1.94 eV, respectively. The emission color of gallium nitride (GaN)-based blue μLEDs covered with GaS0.65Se0.35 and GaSe thin films were clearly converted to green and red, respectively. Furthermore, the emission color could be controlled by changing the film thickness. Thus, these results suggest the possibility of emission-color conversion of blue μLED chips utilizing layered materials.

Scattering Mechanisms and Compositional Optimization of High‐Performance Elemental Te as a Thermoelectric Material

Abstractp‐Type elemental tellurium (Te) has been found to be a promising thermoelectric (TE) material due to its high band degeneracy near the valence band maximum, and has exhibited a high zT ≈ 1.0 above 600 K. However, when forming Te1−xSex solid solutions, the maximal zTs are reduced because of the severely decreased carrier concentration. It is demonstrated that Se alloying is beneficial for enhancing TE performance of elemental Te provided the carrier concentration is optimized. Through Se alloying, the lattice thermal conductivity is remarkably suppressed by the induced large mass and strain field fluctuation, while the power factor can be maintained at a relatively high value as a result of the moderate alloying scattering potential, the unchanged density‐of‐state effective mass, and the optimized carrier concentration in Te1−xSex alloys. Notably, a positive temperature dependence of carrier mobility is observed near room temperature in Te1−xSex, which is proven to be caused by grain boundary scattering. A maximal zT ≈ 1.05 at 625 K is realized in Te0.93Se0.04As0.03 alloys, about 9% higher than the Se‐free Te. The conversion efficiency between 300 and 625 K is also improved ≈18% via Se alloying.

Stable p‐type Cu:CdS1−xSex/Pt Thin‐Film Photocathodes with Fully Tunable Bandgap for Scavenger‐Free Photoelectrochemical Water Splitting

Natural Cd‐based group II–VI semiconductors in photoelectrochemical (PEC) cells are normally used as photoanodes, whose applications are hence hindered by the heavy anodic photocorrosion. Herein, a series of p‐type Cu‐doped CdS1−xSex alloy thin films as photocathodes for water splitting is reported. These alloyed films, synthesized via a simple but well‐controlled physical vapor deposition (PVD) method, are fully tunable in chemical composition, which leads to a tunable bandgap from 1.69 to 2.40 eV. Benefiting from the broadened visible light response, the Cu:CdS0.7Se0.3 photocathode with Pt nanoparticles (NPs) as cocatalysts shows a 37% higher photocurrent density and a higher Faradaic efficiency compared with previously reported Cu:CdS/Pt. More importantly, good stability of the Cu:CdS0.7Se0.3/Pt photocathode in scavenger‐free electrolyte for PEC water splitting is first achieved, with 80% remaining photocurrent and no significant change in 5 h. The released Cd2+ in the electrolyte for the p‐type Cu:CdS1−xSex/Pt photocathode is suppressed by more than three orders of magnitude as compared with a traditional n‐type CdS1−xSex/Pt photoanode. The results reveal the feasibility of Cd‐based group II–VI materials as the thin‐film photocathode for PEC water splitting. The tunable bandgap and durability of p‐type Cu:CdS1−xSex can contribute to future heterojunction structure design in PEC cells.

Effects of ligand and chemical affinity of S and Se precursors on the shape, structure and optical properties of ternary CdS1−xSex alloy nanocrystals

Colloidal ternary CdS1−xSex (0 ≤ x ≤ 1) alloy semiconductor nanocrystals (NCs) with spherical and tetrapodal shapes were synthesized successfully by wet chemical method using cadmium oxide (CdO), selenium (Se), and sulfur (S) as precursors and oleic acid (OA), tri-n-octylphosphine (TOP) as ligands in 1-octadecene (ODE) solvent. For the first time, by using TOP and changing Se content, the shape of ternary CdS1−xSex alloy NCs could be tuned from spheres to tetrapods and the crystal structure could be transformed from zincblende (ZB) to wurtzite (WZ). The photoluminescence (PL) spectra of CdS1−xSex alloy NCs synthesized with and without TOP were also completely different, corresponding to one and two excitonic emission peaks. The causes of these interesting results have been discussed.

Structural and optical properties of a mechanically alloyed thermoelectric lamellar SnSeS solid solution

A chalcogenide SnS2 – xSex alloy with x = 1, synthesized by high-energy mechanical alloying, was characterized by high-resolution transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, Raman spectroscopy, and UV-Vis absorbance. The obtained alloy powder was a lamellar solid solution with nanometric crystalline domain sizes and several types of defects such as stacked faults, discordances, crystal fractures, and local atomic disorders. All of these microstructural features lead to the manifestation of different optical and vibrational properties of this extensively deformed nanostructured sample. Raman spectroscopic measurements suggested a two-mode vibration indicating how the S and Se atoms were distributed in the crystalline lattice. The UV-Vis absorbance spectrum showed multiple bandgaps at 1.99, 2.60, 3.09, 3.66, and 4.56 eV that may well be described as direct allowed interband electronic transitions suggesting inhomogeneous strain and domains.

Substrate temperature dependent physical properties of SnS1−xSex thin films

Recently, researchers showed great interest on SnS1−xSex alloy films because of their tunable physical properties that are suitable as an absorber layer in thin film solar cells. In the present work, SnS1−xSex thin films were deposited by thermal co-evaporation of SnS and Se at different substrate temperatures ranging from 200 to 350 °C. The influence of substrate temperature (Ts) on composition, structure, surface morphology, topography and optical properties of as-deposited films was investigated using appropriate techniques and the results are reported in detail. The EDS analysis of SnS1−xSex films showed that Sn/(S + Se) ratio was changed from 0.84 to 1.16 with increase of substrate temperature. All the films were polycrystalline in nature, exhibiting (111) plane as preferred orientation with orthorhombic crystal structure. From W–H analysis, the crystallite size and lattice strain in the films were evaluated, where the crystallite size varied in the range, 9–22 nm with substrate temperature. The layers showed a change in the shape of grains with the rise of substrate temperature, where the grain size has increased with Ts. The topographical results indicated an indirect relation between surface roughness and average grain size with change in substrate temperature. The band gap energy values of the films was decreased with increase of Ts and varied in the range, 1.59–1.46 eV. In addition, the photoconductivity measurements revealed that the as-deposited SnS1−xSex films had bimolecular type recombination (γ ~ 0.5) of photo-generated charge carriers.

Optical and electrical properties of thermally co-evaporated SnS1−xSex alloy films

The paper presents studies of optical and electrical properties of SnS1−xSex (x = 0.26–0.31) thin films deposited on glass substrates using thermal co-evaporation technique at different substrate temperatures, 200 °C–350 °C. The as-deposited films were characterized by the energy dispersive spectroscopy, X-ray diffractometry, scanning electron microscopy, optical transmission and reflection spectroscopy, four-probe resistivity and photoconductance measurements. The results have shown that deposited films can be used as absorber layers in thin film solar cell applications, and the optimal substrate temperature for the deposition is 300 °C.

First-principles study of the band gap tuning and doping control in CdSexTe1−x alloy for high efficiency solar cell**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0700700), the National Natural Science Foundation of China (Grant Nos. 51672023, 11847043, 11634003, and U1530401), and the Science Challenge Project (Grant Nos. TZ2016003 and TZ2018004).

CdTe is one of the leading materials for low cost, high efficiency thin-film solar cells with a nearly ideal band gap of 1.48 eV. However, its solar to electricity power conversion efficiency (PCE) is hindered by the relatively low open circuit voltage (VOC) due to intrinsic defect related issues. Here, we propose that alloying CdTe with CdSe could possibly improve the solar cell performance by reducing the “ideal” band gap of CdTe to gain more short-circuit current from long-wavelength absorption without sacrificing much VOC. Using the hybrid functional calculation, we find that the minimum band gap of the CdTe1−xSex alloy can be reduced from 1.48 eV at x = 0 to 1.39 eV at , and most of the change come from the lowering of the conduction band minimum. We also show that the formation of the alloy can improve the p-type doping of CuCd impurity based on the reduced effective formation energy and nearly constant effective transition energy level, thus possibly enhance VOC, thus PCE.

Investigations of the structural, optical properties and electronic structure of CdTe1−xSex films fabricated by RF magnetron sputtering

Se alloying CdTe absorber has attracted much attention for enhancing the short current density (Jsc) of CdTe device with high fill factor (FF) and open circuit voltage (Voc). The structural, morphological, optical properties and electronic structure of co-sputtered CdTe1−xSex films (0 ≤ x ≤ 1) are determined in this study. All CdTe1−xSex films are polycrystalline in nature. Phase transition occurs as x increases. The band gap (Eg) of CdTe1−xSex films can be tuned from 1.35 eV to 1.70 eV. CdTe and CdTe0.838Se0.162 films are p-type but the others are n-type, judging from the relative position of Fermi energy (EF) and the intrinsic Fermi energy (EI). Appropriate electric field can form in Se alloying CdTe absorber with a gradient composition, which enhances carrier collection. The barrier height at the emitter/absorber interface can be designed appropriately by changing the composition of CdTe1−xSex films. These results give reasons why CdTe1−xSex absorber is better than CdTe and device performance can be further improved by optimizing the composition distribution of CdTe1−xSex alloys.

GaAsSe Ternary Alloy Nanowires for Enhanced Photoconductivity

Alloyed semiconductor nanowires (NWs) are of great interest for next-generation integrated optoelectronic nanodevices owing to their tunable band gap and emission wavelength. In this study, we synthesized the GaAsSe ternary alloy nanowires (NWs) with various compositions between GaAs and Ga2Se3 using chemical vapor transport method. The band gap was continuously tuned in the range of 1.5–2.1 eV because of the completely miscible solid solution at all compositions. The alloy NWs (including Ga2Se3) consisted of a cubic phase with the [011] growth direction, in contrast with the GaAs NWs grown along the [111] direction. In particular, the GaAs1–xSex (x = 0.3) alloy NWs were grown from Ga-rich Au nanoparticles such as cubic-phase AuGa2 and had a defect-free single-crystalline nature. X-ray photoelectron spectroscopy analysis reveals much less surface oxide layers for x = 0.3, suggesting that Se incorporation at this composition effectively diminishes the surface defects. We fabricated photodetectors using the...

A new procedure to synthesis of ZnS1−xSex nanoparticles by a facile solvothermal method

ZnS1−xSex alloy nanostructures have been successfully synthesized through a simple and effective solvothermal method. Crystal structure, morphology, and optical properties of prepared nanopowders were characterized using X-ray diffraction (XRD), high resolution transmission electron microscopy, energy dispersive X-ray analysis, Field emission scanning electron microscopy, and UV–Vis–IR spectrophotometer. XRD measurements indicate that the ZnS1−xSex alloys have a wurtzite-type hexagonal crystal structure. The lattice constants of ZnS1−xSex nanoparticles have a linear dependence on the Se content and follow Vegard’s law. With the increase of the Se content, their band gap red-shift continuously, further indicating the formation of the alloyed nanostructures. The TEM observations demonstrated that the size of ZnSSe powder is about 30 nm.