Se Crystals Research Articles

Reflectivity spectrum and electrical properties of TlIn0.98Ce0.02Se2 single crystals

The room-temperature reflectivity spectrum of TlIn0.98Ce0.02Se2 crystals is measured at photon energies from 1 to 6 eV, and their conductivity, Hall coefficient, and thermoelectric power are determined in the temperature range 80–460 K. The results are used to evaluate the ionization energy of impurities (ΔE a = 0.09 eV), the band gap of the crystals (E g = 1.30 eV), and their refractive index (n = 2.5).

Effect of TeI 4 content on the thermoelectric properties of n-type Bi–Te–Se crystals prepared by zone melting

n-Type (Bi 2Te 3) 0.93(Bi 2Se 3) 0.07 thermoelectric materials doped with various content of TeI 4 (0, 0.05, 0.10, 0.13, and 0.15 wt.%) have been fabricated through the zone melting method. Electrical conductivity ( σ), Seebeck coefficient ( α) and thermal conductivity ( κ) were measured along the crystal growth direction in the temperature range of 300–500 K. The influence of the variations of TeI 4 content on thermoelectric properties was studied. The undoped (Bi 2Te 3) 0.93(Bi 2Se 3) 0.07 exhibited p-type conduction and it translated to n-type when TeI 4 was doped. The increase of TeI 4 content increased the carrier concentration and thus resulted in an increase of σ and a decrease of | α|. The maximum figure of merit ZT (ZT = α 2 σT/ κ) of the zone-melted crystals in the direction parallel to the growth direction showed a value of 0.90 for the sample containing 0.10 wt.% TeI 4.

Conduction through Localized States in a Single Crystal of the TlGa[sub 0.5]Fe[sub 0.5]Se[sub 2] Alloy

Layered single crystals of the TlGa0.5Fe0.5Se2 alloy in a dc electric field at temperatures ranging from 128 to 178 K are found to possess variable-range-hopping conduction along natural crystal layers through states localized in the vicinity of the Fermi level. The parameters characterizing the electrical conduction in the TlGa0.5Fe0.5Se2 crystals are estimated as follows: the density of states near the Fermi level NF = 2.8 × 1017 eV−1 cm−3, the spread in energy of these states ΔE = 0.13 eV, the average hopping length Rav = 233 A, and the concentration of deep-lying traps N t = 3.6 × 1016 cm−3.

A rapid route for the synthesis of submicron Se and Te rod-like crystals

Submicron Se and Te rod-like crystals were successfully synthesized via a rapid polyol process by refluxing Na 2EO 3 (E Se, Te) and (NH 3) 2S 2O 3 in ethylene glycol at 180 °C for 0.5 h under vigorous stirring. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), and XPS procedures. Studies found that reaction time and temperature have great influences on the morphology of the final products.

Changes in the Short-Range Order of Chalcogenide Glasses upon Crystallization

The crystallization of the chalcogenide glass formers Se, As2Se3 , and As2S3 is studied by x-ray fluorescence analysis (measurements of the relative integrated intensity of the last x-ray emission line). The results demonstrate that the integral parameter R describing the short-range order is larger in the glasses compared to the corresponding crystals. This finding correlates with earlier reports that glass crystallization is accompanied by a decrease in coordination number and also with the assumption that chalcogenide glasses contain hypervalent configurations.

Nonlinear absorption of light by Zn0.37Cd0.63Se solid solutions

Nonlinear absorption spectra of Zn0.37Cd0.63Se crystals were measured using the method of two light sources. At high levels of excitation by neodymium laser pulses, both fundamental two-photon absorption and absorption by excess free carriers are observed. At low excitation, impurity centers with energy levels spaced 0.72, 0.63, 0.53, 0.42, 0.36, 0.25, and 0.20 eV from the edge of the band gap are detected.

Unusual Crystallization Behavior of Selenium in the Presence of Organic Molecules at Room Temperature

Abstract Amorphous Se was converted into fibrous crystals of hexagonal Se or polyhedral crystals of α-monoclinic Se when exposed to organic fluids at room temperature for several days. The growth phase was dependent upon the dielectric constant of the organic liquids. It is notable that hexagonal Se and α-monoclinic Se formed under vapors of d- and l-camphor, respectively. It is proposed that the organic compounds do not act as solvents, but play a catalytic role in the reactions.

Effect of structural and compositional inhomogeneities on spin-glass transition in Hg 1 −x−yCr xMn ySe crystals

We report experimental results on the growth of Hg 1 −x−y Cr x Mn y Se crystals and their magnetic susceptibility χ( T) in dependence on the crystal structure and composition. It was found that the crystals with the Mn composition y=0.01–0.08 exhibit the spin-glass transition temperature T g=100–110 K. An increase of y value leads to the saturation of the χ max and T g characteristics in the composition ranges of y>0.06 and y>0.02, respectively. This phenomenon is explained as a result of phase-separated magnetic behavior caused by the formation of HgCr 2Se 4 inclusions and textures.

Occurrence of native selenium in Yutangba and its environmental implications

Rare native Se has been discovered in Yutangba, Enshi City, Hubei Province, China, where a sudden incidence of Se poisoning occurred in the early 1960s. It was first found in a small area where native Se was produced on a large scale and was diverse in forms due to different formation mechanisms. Genetically, native Se can be divided into 3 categories: the primary native Se occurring in carbonaceous-siliceous rocks and tiny Se crystals formed in cracks of rocks during tectonic activities; micro-Se crystals formed in the weathering processes of Se-rich rocks; and larger Se crystals derived from natural burning of stone coal on the subsurface of abandoned stone coal spoils. The different forms of native Se found in Yutangba demonstrate that Se can be activated, transformed, remobilized and enriched at special sites such as in the unsaturated-subsurface zone or in the saturated zone. This discovery of native Se has not only resolved a long-standing controversial question on modes of occurrence of Se, which has puzzled many researchers in China, but has provided new possible evidence for the pathogenic explanation of the sudden prevalence of Se poisoning in Yutangba. The research results also have important implications for studying the mineralogy, ore geochemistry and environmental geochemistry of Se.

Hydrazine route to one-dimensional structural metal selenides crystals

One-dimensional metal selenides Bi 3Se 4, Cu 2− x Se, SnSe 2, and MSe (M=Cd 2+, Ni 2+, Pb 2+) crystals were produced through the reactions of selenium powder with metal salts under solvothermal conditions in hydrazine hydrate. X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy characterized the products. Solvent hydrazine hydrate plays a crucial role in the formation of these selenides. The UV–Vis absorption and photoluminescence spectra of SnSe 2 were presented.

Sonochemical Synthesis of Trigonal Selenium Nanowires

This paper describes a solution-based method for the large-scale synthesis of selenium nanowires. Sonication was used as the driving force for both nucleation and dispersion. The trigonal Se seeds were formed during sonication and grew over the course of hours at the expense of the amorphous Se colloids in a process similar to Ostwald ripening. The resultant nanowires were typically single crystals of trigonal Se whose morphologies could be tuned by adjusting the reaction conditions. A variety of solvents are demonstrated for use with this process. We also demonstrate patterned growth of these nanowires on substrates to form extensive 2D networks and isolated clusters. The morphologies of the products were examined with a combination of microscopic and spectroscopic techniques.

Correlation of resistivity with zinc content in a vapor grown (Cd,Zn)Te:Se

We report the possibility to grow semi-insulating (Cd,Zn)Te:Se crystals by the modified Markov method (MMM) from the vapor phase. When oriented Cd(Te,Se) seed material was used for the growth, lattice matching and doping with Se resulted in an increase of resistivity up to several units of 109 Ω cm. A homogenous Zn distribution was also observed. The content of Se and Zn in the seed and grown crystals was determined by photoluminescence and energy dispersive analysis by x ray measurements. Resistivity and Zn concentration maps show a good spatial correlation in which the highest values of resistivity correspond to the areas of the highest Zn concentration. The main part of the crystals demonstrate a low extinction coefficient in the IR spectral region, except for the Zn inhomogeneity region.

Ellipsometric, photoluminescence and Auger electron spectroscopy studies of Zn 1− xBe xSe and Zn 1− x− yBe xMn ySe crystals

Zn 1− x− y Be x Mn y Se and Zn 1− x Be x Se crystals, investigated in the present paper, were grown from the melt by the high pressure Bridgman method. Spectroscopic ellipsometry was used for determination of the complex dielectric function ε( ω) of Zn 1− x Be x Se and Zn 1− x− y Be x Mn y Se samples. On the basis of ε( ω) dependence, the energy gaps E 0 of alloys containing different beryllium and manganese concentrations have been evaluated. It was found that for Zn 1− x Be x Se, the value E 0 linearly increases with the beryllium content. Measurements of luminescence spectra allowed us to find the excitonic energy gap in the investigated alloys. The results were compared with E 0 values obtained from ellipsometric investigations. Auger electron spectroscopy and argon ion sputtering were used for determination of composition of investigated samples in the sub-surface region. Surface concentrations were compared with the values obtained in the bulk.

Auger depth profile analysis and photoluminescence investigations of Zn 1− xMg xSe alloys

Zn 1− x Mg x Se crystals grown by the high-pressure Bridgman method for 0< x<0.38 were investigated. Composition of the samples was determined by electron microprobe analysis. Photoluminescence (PL) spectra were measured in the temperature range from 40 K up to room temperature. It was found that PL spectra measured at 40 K, consist of the exciton, edge and deep level emission bands. The broadening of PL bands due to compositional and structural disorder was observed. Auger electron spectroscopy (AES) together with argon ion sputtering was used to determine sub-surface composition of Zn 1− x Mg x Se crystals. AES depth profiles, obtained by ion milling of the samples and consecutive Auger analysis, allowed to obtain distribution of Zn, Mg and Se near the crystal surface.

Magnetization and EPR Studies in Zn1-x-yBexMnySe Crystals

Magnetisation and EPR investigations were performed in mixed Zn 1-x-y Be x Mn y Se crystals. For crystals containing about 3% of Mn the transition from paramagnetic to antiferromagnetic behaviour is observed with temperature decreasing from 300 to 12 K. EPR spectra of all samples show the g-factor value in the range of 2.0065-2.0220. If Mn atoms are not introduced to the lattice, the EPR signal shows only g = 1.999.

Formation of semiconductor Cu 2− xSe rod-like crystals through a solvothermal reaction

Micron-sized rod-like crystals of Cu 2− x Se were successfully grown for the first time by a hydrothermal reaction from CuCl 2·2H 2O and Se powder at 60°C for 2 h. X-ray powder diffraction patterns (XRD), X-ray photoelectron spectra (XPS), scanning electron microscopy (SEM) images, and transmission electron microscope (TEM) images show that the product is cubic Cu 2− x Se rod-like crystal and well crystallized. Hydrazine hydrate was used as solvent in the process and played an important role in the formation of rod-like Cu 2− x Se crystals. The optical properties of Cu 2− x Se, such as absorption spectra, photoluminescence (PL) spectra, and Raman spectrum are also reported for the first time.

Crystalline–amorphous transitions of Ge–Se alloys by mechanical grinding

X-ray diffraction and differential scanning calorimetry (DSC) measurements of Se and GeSe 2 during ball milling have been investigated. In this study, amorphous Se and GeSe 2 are synthesized by mechanical grinding (MG) recognized as a method of inducing a crystalline–amorphous phase transition. The amorphization of Se is caused from distortion between helical chains connected by van der Waals forces. The enthalpy ΔH for crystallization of Se by rapid quench is lower than that by the MG. This result suggests that the difference of ΔH corresponds to that of the structures in intermediate range. Monoclinic GeSe 2 is amorphized by disordering between the connected tetrahedra. It is considered that the tetrahedral unit of GeSe 2 by the MG is distorted with wide distributions. GeSe 2 is completely amorphized after 4 h of grinding, and Se after 30 h in the present milling. The difference indicates that disordering between the connected tetrahedral units in monoclinic GeSe 2 is induced more easily than that between chains in trigonal Se.

Transition metal 3d states in HgSe-based diluted magnetic semiconductors

The contributions of Mn3d, Fe3d and Co3d electrons to the valence band electronic structure of Hg 1− x TM x Se (TM=Mn, Fe, Co) crystals were investigated by means of resonant photoemission. The comparison of photoemission spectra with the results of theoretical calculations made by the CPA (coherent potential approximation) method shows that the valence band of Hg 1− x TM x Se diluted magnetic semiconductors is strongly influenced by TM3d electrons. We find out that for all TM incorporated in the HgSe matrix, 3d electrons with t 2g symmetry hybridize strongly with the valence band of the crystal, whereas 3d electrons with e 2g symmetry are not hybridized with the valence band. The contribution of 3d t2g electrons is spread over the valence band with the maximum at 3.4–3.8 eV below the Fermi level. The contributions of 3d e2g electrons were found in the middle part of the valence band and, in the Fe and Co case, also at the edge of the valence band.

Giant magnetoresistance of semimagnetic semiconductors and applications for magnetic field sensors

Magnetotransport properties of Hg 1− x Cr x Se and Hg 1− x Eu x Te crystals grown by a modified Bridgman method have been studied. Structural characterization of the Hg 1− x Cr x Se crystals showed good homogeneity for compositions x<0.05, while samples with higher chromium content included macroscopic needle-like inclusions. It has been found that the inner part of each inclusion is amorphous CrSe and the boundary regions are ternary HgCr 2Se 4. Depending on the magnetic field direction different kinds of the temperature dependence of the magnetoresistance Δ R/ R have been found. At room temperature the value of Δ R/ R can reach 100% for the Hg 1− x Eu x Te crystal. This makes the studied semimagnetic semiconductors an attractive magnetoresistive material for magnetic field sensors.

Thermal Expansion and Isothermal Compressibility of TlIn1 – xNdxSe2 (0 ≤ x≤ 0.08) Crystals

The thermal expansion coefficient (α) and isothermal compressibility (χ T ) of TlIn1 – x Nd x Se2(0 ≤ x≤ 0.08) crystals were measured between 77 and 400 K. In the range 77–160 K, both α and χ T increase with temperature, the increase in α being much steeper. At higher temperatures, α and χ T change very little. The observed composition dependences of α and χ T are interpreted in terms of energy-band structure.