Bulk Se Research Articles

Realizing high thermoelectric performance of Cu and Ce co-doped p-type polycrystalline SnSe via inducing nanoprecipitation arrays

Thermoelectric (TE) performance of polycrystalline stannous selenide (SnSe) has been remarkably promoted by the strategies of energy band, defect engineering, etc. However, due to the intrinsic insufficiencies of phonon scattering and carrier concentration, it is hard to simultaneously realize the regulations of electrical and thermal transport properties by one simple approach. Herein, we develop Cu and Ce co-doping strategy that can not only greatly reduce lattice thermal conductivity but also improve the electrical transport properties. In this strategy, the incorporated Cu and Ce atoms could induce high-density SnSe2 nanoprecipitation arrays on the surface of SnSe microplate, and produce dopant atom point defects and dislocations in its interior, which form multi-scale phonon scattering synergy, thereby presenting an ultralow thermal conductivity of 0.275 W·m-1·K-1 at 786 K. Meanwhile, density functional theory (DFT) calculations, carrier concentration, and mobility testing reveal that more extra hole carriers and lower conducting carrier scattering generate after Cu and Ce co-doping, thereby improving the electrical conductivity. The co-doped Sn0.98Cu0.01Ce0.01Se bulk exhibits an excellent ZT value up to ~1.2 at 786 K and a high average ZT value of 0.67 from 300 to 786 K. This work provides a simple and convenient strategy of enhancing the TE performance of polycrystalline SnSe.

Enhanced thermoelectric properties of Cu-Se system via bond-structure adjustment by Ag-doping

Thermoelectric devices have shown attractive application prospects with the emergence of high performance thermoelectric materials in recent years. In the Cu-Se system, bond-structure adjustment was designed to improve thermoelectric properties through simple silver doping, which induces change of phase structure, energy band structure, and thermoelectric transport nature. The polycrystalline Cu2−xAgxSe bulk samples were prepared by the spark plasma sintering (SPS) following vacuum melting and grinding. Appropriate addition of silver makes α-Cu2Se lattice expansion, and β-Cu2Se becomes the room temperature stable phase with further increase of the silver content (>=3%). Ag doping results in significant changes in valence band spectra, which improves thermoelectric transport behave in higher operation temperature. Due to the synergetic effect from electrical conductivity and Seebeck coefficient, the maximum power factor reaches 1456 μWm−1K−2 at 773 K, which increases by 10% compared with the undoped sample. It is notable that the Cu1.99Ag0.01Se bulk sample achieve an low thermal conductivity of 1.1 W m−1K−1 at 773 K, which can be attributed to enhancement of phonon scattering at different frequencies due to the diversified bond lengths and bond energy induced by Ag-doping. Benefiting from reduction of lattice thermal conductivity and enhancement of powder factor, the Cu1.99Ag0.01Se bulk sample shows the better TE transport properties and exhibits the highest ZT value ∼1.12 at 773 K.

Electronic and Lattice Thermal Conductivity Switching by 3D−2D Crystal Structure Transition in Nonequilibrium (Pb1−xSnx)Se

AbstractDynamic control of thermal transport in solid materials is highly desired for thermal management technology. However, the development of a material exhibiting large modulation of thermal conductivity (κ) by external stimuli remains a major challenge. Here, the large κ modulation is reported by the reversible 3D to 2D crystal structure transition in a nonequilibrium solid solution of (Pb1−xSnx)Se, where Pb2+ stabilizes a 3D cubic structure while Sn2+ does a 2D layered structure. The phase boundary of these phases is induced in (Pb0.5Sn0.5)Se bulk polycrystals by thermally quenching the high‐temperature solid solution phase. Through the 3D–2D phase transition, the 1/2.9‐times decrease of lattice κ (κlat.) is achieved by strong phonon scattering in the 2D layered structure, and the electronic κ (κele.) is also decreased by 5 orders of magnitude due to the electronic phase transition from a 3D high conductivity state to a 2D semiconducting state. The total κ (=κlat. + κele.) modulation ratio κ3D phase/κ2D phase = 3.6 is attained at 373 K. The present strategy will lead to a novel concept for designing thermal management materials through crystal‐structure dimensionality switch using nonequilibrium phase boundaries.

Differential protein expression due to Se deficiency and Se toxicity in rat liver

There is a U-shaped dose-response between selenium (Se) status and health outcomes, but underlying metabolic processes are unclear. This study aims to identify candidate proteins in liver regulated by dietary Se, ranging from deficiency to toxic. Male rats (n=4) were fed graded Se concentrations as selenite for 28 days. Bulk Se analysis was performed by ICP-MS on both soluble and insoluble fractions. Soluble fraction samples were chromatographically separated for identification of selenocompounds by SEC-ICP-MS and protein quantification by LC-MS/MS. Bioinformatics analysis compared low-Se (0 and 0.08 µg Se g−1) and high-Se (0.8, 2 and 5 µg Se g−1) with adequate-Se (0.24 µg Se g−1) diets. Major breakpoints for Se were seen at 0.8 and 2 µg Se g−1 in the insoluble and soluble fractions, respectively. Glutathione peroxidase 1 protein abundance reached a plateau at ≥0.08 µg Se g−1diet; Se bound to selenium binding protein 2 was observed with 2 and 5 µg Se g−1 Se. The extreme diets presented the highest number of differentially expressed (P value <0.05, FC ≥1.2) proteins in comparison to the adequate-Se diet (0 µg Se g−1: 45 proteins; 5 µg Se g−1: 59 proteins); 13 proteins were commonly affected in 0 and 5 µg Se g−1 treatments. Network analysis revealed that the metabolism of glutathione, xenobiotics and amino acids were enriched in both 0 and 5 µg Se g−1 diets, indicating a U-shape effect of Se. This similarity is likely due to down-stream effects of lack of essential selenoproteins in Se deficiency and due to toxic effects of Se that exceeds the capacity to cope with excess Se.

Strong bulk photovoltaic effect and second-harmonic generation in two-dimensional selenium and tellurium

Few-layer selenium and tellurium films have been recently prepared, and they provide a new platform to explore novel properties of two-dimensional (2D) elemental materials. In this work, we have performed a systematic first-principles study on the electronic, linear, and nonlinear optical (NLO) properties of atomically thin selenium and tellurium films within the density-functional theory with the generalized gradient approximation plus scissors correction using the band gaps from the relativistic hybrid Heyd-Scuseria-Ernzerhof functional calculations. Interestingly, we find that few-layer Se and Te possess large second-harmonic generation (SHG), linear electro-optic effect, and bulk photovoltaic effect. In particular, trilayer (TL) Te possesses large SHG coefficient, being more than 65 times larger than that of GaN, a widely used NLO material. Bilayer (BL) Te has huge static SHG coefficient, being more than 100 times larger than that of GaN. Furthermore, monolayer (ML) Se possesses large SHG coefficient. Moreover, we predict that TL Te exhibits strong bulk photovoltaic effect (BPVE), being greater than that of GeS, a polar system with the largest BPVE found so far. Although the shift current conductivities of bulk and 2D Se are comparable, the shift current conductivities of TL Te are five times larger than those of bulk Te. Finally, an analysis of the calculated electronic band structures indicates that the strong NLO responses of 2D Se and Te materials are primarily derived from their low-dimensional structures with high anisotropy, directional covalent bonding, lone-pair electrons, and relatively small band gaps. These findings provide a practical strategy to search for excellent NLO and BPVE materials.

Magnetic Gap of Fe-Doped BiSbTe2Se Bulk Single Crystals Detected by Tunneling Spectroscopy and Gate-Controlled Transports.

Topological insulators with broken time-reversal symmetry and the Fermi level within the magnetic gap at the Dirac cone provides exotic topological magneto-electronic phenomena. Here, we introduce an improved magnetically doped topological insulator, Fe-doped BiSbTe2Se (Fe-BSTS) bulk single crystal, with an ideal Fermi level. Scanning tunneling microscopy and spectroscopy (STM/STS) measurements revealed that the surface state possesses a Dirac cone with the Dirac point just below the Fermi level by 12 meV. The normalized dI/dV spectra suggest a gap opening with Δmag ∼55 meV, resulting in the Fermi level within the opened gap. Ionic-liquid gated-transport measurements also support the Dirac point just below the Fermi level and the presence of the magnetic gap. The chemical potential of the surface state can be fully tuned by ionic-liquid gating, and thus the Fe-doped BSTS provides an ideal platform to investigate exotic quantum topological phenomena.

Antagonistic effect of different selenium type on green synthesized silver nanoparticle toxicity on Oreochromis niloticus: oxidative stress biomarkers.

The current investigation pointed to report the negative impacts of silver nanoparticles [Ag]NPs that synthesized by leaf extract of Moringa oleifera on oxidative stress biomarkers of Oreochromis niloticus, as well as the role of different Selenium type to antagonistic that toxicity. Fish were exposed to [Ag]NPs (1.95 and 3.9ppm) as sublethal concentrations alongside 0.1ppm of Se bulk and 0.1ppm biosynthesized [Se]NPs selenium nanoparticles antagonistic effect for 2 and 4weeks. Lipid peroxidation (LPO), DNA fragmentation, total antioxidant capacity (TAOC), catalase (CAT), and superoxide dismutase (SOD) were evaluated as oxidative stress biomarkers,. A significant increase (p < 0.05) in LPO and DNA fragmentation and a significant decrease (p < 0.05) in TAOC, CAT, and SOD were found in [Ag]NPs exposed groups when compared with the control one. Biosynthesized [Se]NPs and Se bulk showed a positive role in [Ag]NPs detoxification. Our investigation suggested that [Ag]NPs showed a toxic effect on oxidative stress biomarkers. However, Se-NP addition gives a good recovery of oxidative biomarkers more than Se bulk in detoxification of [Ag]NPs.

An ab initio study on the structure, energetics and spectra of F(CO2)n−: the observation on the strong F–CO2 bond

Bulk VDE and SE values decrease from fluoride to iodide (F− &gt; Cl− &gt; Br− &gt; I−) in both CO2 and H2O.

VRH and NSPT conduction mechanisms in CuIn0.7Ga0.3Se2 near the liquid nitrogen temperature

Variable range hopping conduction process of Shklovskii-Efros type is identified, for the first time, in CuIn0.7Ga0.3Se2 bulk material in the temperature range [-190 °C, −10 °C]. Two different methods are considered to confirm this mechanism. The corresponding characteristic temperature and the localization length of mobile carriers are estimated. The AC electrical conductivity analysis suggests the predominance of Non-overlapping Small Polaron Tunneling mechanism. In the frequency range [20 Hz, 1 MHz], the equivalent circuit model of this material is established and the microscopic contributions to the total electrical conduction are discussed.

Se diffusion in CdTe thin films for photovoltaics

Manipulating CdSeTe bandgrading to enhance photocurrent and carrier lifetime is an essential part of high-performance CdTe photovoltaics (PVs). In this work, we examine Se diffusion kinetics in single-crystal and polycrystalline CdTe during deposition, thermal annealing, and CdCl2 treatments. Se distributions are determined by dynamic secondary-ion-mass spectroscopy and Auger electron spectroscopy depth profiling and coupled with electron backscatter diffraction images of the crystalline structure. Effective bulk and grain boundary diffusion coefficients are determined by analytical models and discussed in the context of processing and film morphology. Se is found to diffuse in CdTe at much higher rates during CdCl2 treatments than with thermal processing alone. GB diffusion also occurs at a significantly faster rate than bulk diffusion. As a result of these two effects, the near interface bulk and GB Se diffusion during CdCl2 treatments dominates the bandgrading profiles in CdTe PVs.

Effect of minor graphene doping on the microstructure and superconductivity of FeSe

The polycrystalline FeSe bulks were sintered by solid-state reaction using a two-step sintering process to investigate the effect of graphene addition on the superconductivity and microstructure of the FeSe bulks. With the increase of graphene doping, the thickness of tetragonal β-FeSe sheet decreased obviously due to the existence of graphene between the layers. The grain connectivity of FeSe superconductor is significantly improved, and the critical transition temperature (Tc) is apparently enhanced up to 10.57 K in Fe1.01SeG0.05, which increased by 15.3% compared to the Fe1.01Se bulk without doping graphene. The critical current density (Jc) of Fe1.01SeG0.05 reaches 5 × 103 A/cm2 under zero magnetic field. Moreover, the critical current density (Jc) of Fe1.01SeG0.05 is simultaneously increased under high magnetic field due to the refined grains by graphene doping.

Large thermoelectric figure of merit in hexagonal phase of 2D selenium and tellurium

AbstractWe found a large thermoelectric figure of merit in the hexagonal phase of 2D selenium and tellurium from first‐principles calculations. The hexagonal phase (α) is obtained from three atomic layers truncated along the [001] direction of trigonal Te and Se bulk in the equilibrium structure. We found the α‐Se structure dynamically stable. The calculated electronic structures of α‐Se and α‐Te show interesting semiconductor character for both electronic and optoelectronic applications. Furthermore, the obtained elastic properties show that hexagonal tellurene is a softer material than selenene. The thermoelectric figure of merit for hexagonal 2D phase (∼1.0) is larger than those reported for the tetragonal 2D phase (∼0.75) of selenium and tellurium. Additionally, the computed electrical and phonon transport parameters indicate that selenene and tellurene are promising thermoelectric materials; both offer an alternative to recovering residual heat and transforming it into electricity.

Distribution of elements in Ge–Se bulk glasses and optical fibers detected by inductively coupled plasma atomic emission spectrometry

The elemental composition of Ge–Se glasses prepared by different methods and the distribution of Ge and Se along the bulk sample length are investigated by inductively coupled plasma atomic emission spectrometry. The estimated uncertainty of Ge and Se content measurement is 0.02–0.05 at.% (P = 0.95). To date, these results are the most accurate for determining the elemental composition of optical Ge-based chalcogenides. The minimum required mass of the analyzed glass samples does not exceed 1 mg. Partial phase separation in the Ge–Se system leads to a significant (±2 at.%) deviation of the chemical composition of Ge–Se glasses along the length of the bulk sample from the specified value. This is the first time that the elemental composition of single-index Ge–Se fibers along the fiber length has been studied. The rod drawing method allows the fabrication of optical fibers with composition deviation along the length of 0.03–0.05 at.%. In the case of drawing by the crucible method, the deviations of the fiber composition can reach 0.16 at.% compared with the initial glass sample.

Correlation between threshold voltage and its pre-exponential factor in resistive switching

The phenomenon of resistive switching has drawn great attention in last one decade due to its promising applications in the memory technology. The analysis of resistive switching experiments on novel bulk Se–Te–Sn–Ag quaternary glass has been reported in this script. In the present work, we have observed that the pre-exponential factor (Vth)0 of the threshold voltage Vth in thermally governed resistive switching characteristics follows an exponential dependence on the corresponding activation energy Eth as a signature of the compensation effect. Our results indicate that the resistive switching behaviour is influenced by these two parameters. Thus, the correlation between (Vth)0 and Eth may play a significant role in controlling resistive switching. The origin of this correlation is explained in terms of the transition probabilities for recombination/generation of the charge carriers as suggested in the barrier cluster model.The observations show that the threshold voltage of the glassy Se–Te–Sn–Ag system is reduced with the increase in the Ag concentration. An endeavor has been made for the explanation of the observed composition dependence on the basis of nature of bonding of Ag atoms and a decrease in the chemical disorder with composition.

Synthesis of t-Te and a-Se nanospheres using continuous wave visible light

We demonstrate a simple and cost-effective method for the synthesis of elemental Se and Te nanoparticles. The method employs irradiation of bulk Se and Te by visible light, continuous wave sources and results in uniform deposition of nanoparticles on various substrates. The process is fast and avoids the steps of pre- and post-treatment involved in solution-based approaches. The resulting Se and Te nanoparticles have an exclusively spherical morphology and diameters in the order of 50 and 100 nm, respectively. Spectroscopic and electron microscopy characterization reveals that Te nanospheres are single crystalline with trigonal structure and can develop an oxidized surface shell, while Se nanoparticles are identified as purely amorphous. At high-yield growth, Te nanospheres aggregate but preserve their particulate morphology while amorphous Se nanoparticles form fractal-like morphologies. The present work might be important for various areas such as biomedical applications owning to the high antibacterial activity of Te and Se nanostructures and the avoidance of hazardous chemicals.

Magnetotransport Properties in Epitaxial Fe1.1Te0.7Se0.3 Films

Recent discoveries of topological superconductivity and Majorana bound states in Fe(Te,Se) bulk crystals have broadened the research topics in iron-based superconductors. Compared with the bulk samples, thin films possess the advantages in fabrication and manipulation. In the present work, we investigated Fe1.1Te0.7Se0.3 thin films grown by pulsed laser deposition (PLD), focusing on the transport properties, including electrical resistivity and magnetoresistivity in magnetic fields both perpendicular and parallel to the film plane. The activation behavior related to the vortex pinning and the upper critical field were analyzed as well.

Femto- and nano-second laser-induced damages in chalcogenide glasses

We have studied damaging behaviors in bulk As2Se3, Se and As2S3 glasses using fs and ns laser pulses. Threshold fluence of laser damages markedly varies with excitations and materials. Under sub-gap fs pulses with photon-energy of ħω = 1.6 eV, the threshold in As2S3 is roughly twice that of the other two glasses, and photo-crystallization appears in Se. For ns pulses, all the glasses under bandgap (ħω = 2.3 eV) illumination possess higher thresholds than those under mid-gap (ħω = 1.2 eV) illumination. These observations are discussed, taking electronic and thermal effects into account.

Improved Thermoelectric Properties of Hot‐Extruded Bi–Te–Se Bulk Materials with Cu Doping and Property Predictions via Machine Learning

AbstractCu‐doped Bi2Te2.85Se0.15 bulk thermoelectric materials are fabricated using a hot‐extrusion technique. The Cu atoms are found to intercalate into interstitial sites between the Te(1)–Te(1) layers, which results in a reduction in the carrier concentration, and thus increases in the related Seebeck coefficient and electrical resistivity and a decrease in the carrier thermal conductivity. A resulting ZTmax value of 0.86 is obtained in the Cu0.05Bi2Te2.85Se0.15 sample, which is 83% higher than that of the Cu‐free sample. As data‐driven materials science is becoming increasingly important for materials design, quantitative information on the processing, microstructure, and properties of the hot‐extruded materials is also estimated using a machine learning approach, where property predictions are conducted using an artificial neural network model. Moreover, an inverse exploration of the potential best material properties and their corresponding microstructure and processing is further attempted by using a Bayesian optimization algorithm. This study is expected to provide a new route to fabrication of high‐performance Bi2Te3‐based thermoelectric materials with a focus on data‐driven materials design.

The effect of laser pulsewidth on the selenium nanoparticles mass yield

High-throughput synthesis of selenium (Se) nanoparticles in the form of colloidal solutions was carried out by 1030 nm laser ablation of a bulk Se target in deionized water and isopropyl alcohol by laser pulses of variable pulse widths (0.3–8.6 ps). The extinction coefficient of the colloidal solutions exhibits an overall decreasing trend with the increase of the laser pulse width in deionized water, also demonstrating a local minimum at 2–4 ps. This can be explained by the growing ablation thresholds, related to the bigger impact of thermal processes during the transition from femto- to picosecond temporal stages.

Electrical conduction mechanism in films of Se80−xTe20Bix (0 ≤ x ≤ 12) glassy alloys

This paper reports the study of DC electrical conductivity of films of Se80−xTe20Bix (0 ≤ x ≤ 12) glasses prepared using physical vapor deposition method. The films were structurally characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM results indicate the formation of nanorods within the films. The electrical conductivity of the samples was studied using Keithley electrometer in the temperature range 303–373 K. The results show that conduction in these samples takes place via thermally assisted tunnelling and variable range hopping of charge carriers corresponding to higher and lower temperature ranges, respectively. Further, it was found that the conductivity increases with increase in Bi concentration in Se–Te system. This has been explained on the basis of chemically ordered network model. It was also found that nanorod formation improves the electrical conductivity of Se–Te–Bi system compared to bulk Se–Te–Bi system.