Glassy Se Research Articles

Toward Ideal Glassy Selenium at Kauzmann Temperature

To get insights into glassy Se at Kauzmann temperature, Se clusters including helical and disordered H–nSe–H chains with n ≤ 20 are constructed, using an ab initio molecular orbital package. The study demonstrates the existence of kinked chains with bistable dihedral‐angular conformations, which have similar total energies to that in straight helical chains. Electronically, such kinked chains possess wider optical gaps with no gap states, which is less efficient in localized electronic excitation. This would predict that the photodarkening cannot appear in fully stabilized glasses at Kauzmann temperatures, the behavior being similar to that demonstrated for liquid Se lying in thermal equilibrium.

A DFT study of Se n Te n clusters.

First principles calculations have been performed to study the characteristic properties of SenTen (n = 5–10) clusters. The present study reveals that the properties of these small clusters are consistent with the properties of Se–Te glassy systems. Several hundred equilibrium structures obtained from a genetic algorithm based USPEX code are relaxed to their minimum energy using the VASP code. Most of the lowest energy buckled ring-like structures are formed from Se–Te heteropolar bonds. Detailed structural analysis and distance energy plots unveil that many equilibrium structures are close in energy to their global minimum. The computed Raman and IR spectra show the dominance of Se–Te heteropolar bonds, consistent with earlier simulation and experimental findings in Se1−xTex glass materials. Low frequency vibrational modes observed in small clusters are characteristic features of amorphous materials. Non-bonding orbitals (lone pair) are observed in the HOMO, whereas the LUMO is formed from purely antibonding orbitals. The dielectric functions corroborate the bonding mechanism and slightly polar nature of SenTen clusters. The energy loss and absorption coefficient indicate the presence of π–plasmons in the UV-visible region. Furthermore, it is ascertained that the use of a hybrid functional (B3LYP) does not affect the properties of small clusters appreciably, except causing a blue shift in the optical spectra. Hence, we find that the small clusters have bearing on the formation of glassy Se–Te systems.

Invariance of Meyer-Neldel compensation rule in thermally activated d.c. and a.c. conduction for as-prepared and aged glassy Selenium and As2Se3 glass

This paper reports the invariance of the Meyer-Neldel Rule for thermally activated d. c. conduction under the aging effects in glassy Se (g-Se) and As2Se3 glass. For this purpose, we have analyzed the electrical data of the samples (i.e. glassy Se and As2Se3 glass) in bulk form. The fresh samples of both materials were prepared by the well-known melt-quench technique. Particularly, we have done current-voltage (I–V) measurements at different temperatures. Using the I–V characteristics, we calculated d. c. conductivity and the activation energy involved in d. c. conduction for both fresh as well as aged samples. Finally, we observed that the Meyer-Neldel rule holds and Meyer-Neldel characteristic energy are almost the same in both fresh and aged samples of g-Se as well as As2Se3 glass. This indicates that there is no time-dependent variation in the Meyer-Neldel parameter in the present chalcogenide glasses.

Kinematics of glass to crystal phase transformation in novel multi-component glassy Se–Te–Sn–M (M = Sb, In, Cd) alloys

The kinetics of the thermally induced glass / crystal phase transformation of chalcogenide glasses plays an important role in determining their candidacy for optical phase change memory applications. The rate of crystallization and the corresponding activation energy are the two crucial kinetic parameters that reflect the durability and quality (i.e., storage properties) of phase change materials. This script deals with metal-induced effects on thermally regulated non-isothermal crystallization in a new glass alloy of Se-Te-Sn using calorimetric measurements. The elements Antimony (Sb), Cadmium (Cd) and Indium (In) have been used as structural modifiers for this purpose. The crystallization and glass transition kinetics of these glass alloys have been investigated by thermal analysis of several kinetic parameters such as the parameter of order n, the maximum crystallization temperature Tc, the crystallization rate K and the consequent activation energy Ec). A DSC is used in non-isothermal mode for the present studies. The values of the activation energy Ec are determined using the data obtained from the displacement of the exothermic peaks of crystallization in non-isothermal DSC plots at various heating rates. The role of the additives Sb, Cd and In in the variation in the rate of crystallization K of and the Avrami index (n) for each glass alloy is also examined. Detailed thermal analysis of the kinetic data confirms the superiority of Cd over the other two additives (In and Sb) for optimization of the kinetic properties of the main SeTeSn glass.

Морфология поверхности и оптические свойства стеклообразных пленок селена после лазерной модификации

Abstract The results of studies of the effect of low–power He-Ne laser radiation at a wavelength of 632.8 nm on the surface morphology of glassy (amorphous) Se films with a thickness of ~ 1 µm synthesized by vacuum-thermal evaporation are presented. It is shown that depending on the intensity and time of radiation exposure, laser modification of films is accompanied by various changes in their roughness and the appearance of structures on the surface of the films under study that differ in size and phase composition. The formation of submicron ridge chains on the surfaces of laser-modified films leads to an increase in their reflection coefficient and refraction index and a decrease in their optical thickness.

A new approach for nano-structuring of glassy selenium (g-Se) using silver nanoparticles (AgNPs) as precursor

Various routes have been proposed for the synthesis of chalcogen based nanostructured materials but the search of conventional and cost-effective method for the nanostructuring of chalcogenides is an ongoing research activity. This unique work reports the synthesis and the characterization of a novel kind of glassy nanocomposite material belonging to the nano-structuring of g-Se with the help of silver nanoparticles. The local structure is analyzed with the help of X-ray diffraction, Scanning electron microscopy, Raman spectroscopy, and Differential Scanning Calorimetry. It was found that the addition of AgNPs in glassy Se creates fine nano-wires of trigonal Selenium (t-Se) in the glass matrix of g-Se. The electrical, mechanical, and thermal characterizations of the present glassy nanocomposite material have been done in terms of current-voltage characteristics, d.c. conductivity, micro-hardness, and kinetic parameters of glass/crystal phase transformation. A comparative analysis of present glassy composites with a conventional Ag containing glassy alloy (g-Se98Ag2) has also been done to understand the role of incorporation of silver nanoparticles over the normal silver powder. The detailed analysis shows that different physical properties [e.g., thermal stability, modulus of elasticity, micro-hardness, the threshold voltage (i.e., turn-on voltage for resistive switching), and d.c. conductivity] are enhanced more effectively when the nano-structuring of g-Se by using AgNPs is executed as compared to the routine way of alloying.

Some novel results of physical aging studies in glassy selenium

Physical aging (PhA) of disorder materials is still an important problem in areas of physics, chemistry and materials science for the study even nowadays. This work reports the results of some novel observations during the devitrification of the aged and as-quenched glassy selenium. Investigation of physical aging effect induced by natural storage up to 10 years aged Selenium glass has been carried out using the state of art experimental techniques (DSC, XRD, SEM, TEM and Raman spectroscopy). Further, the thermal stability, micro-hardness, density, fragility index and compactness of the aged and as-quenched glassy selenium have been determined to see the effect of aging on the glassy Se. The results show that there are significant changes in themechanical behavior of g-Se due to physical aging.

Laser-induced crystallization of monoclinic nanowires in glassy selenium films

The authors investigate influence of 632.8 nm laser irradiation on structural properties of thin glassy (amorphous) Se films synthesized on quartz substrates using vacuum thermal evaporation. They first propose the fact that low power laser irradiation of Se films (power density PS=80 W cm−2) initiates phase transformations, accompanied by growth of monocrystalline nanowires of metastable phase of monoclinic beta-Se8 in the amorphous matrix at room temperature. New structural properties of Se films are proven to be long term stable at room temperature. It is shown that laser irradiation at PS ≥ 800 W cm−2 induces synthesis of a mixture of nanocrystallites of trigonal Se and monoclinic alpha-Se8 in the amorphous films and their mass crystallization with growth of spherulites at room temperature.

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.

Effect of high-energy mechanical milling on the FSDP-related XRPD correlations in Se-rich glassy arsenic selenides

The X-ray powder diffraction (XRPD) patterns related to the first sharp diffraction peak (FSDP) are registered for Se-rich AsxSe100-x glasses (5 ≤ x ≤ 40), subjected to high-energy mechanical milling in a dry mode with 500 min−1 rotational speed. The results are treated in terms of structural model assuming broad diffraction halos in the XRPD patterns appeared due to remnants of inter-planar “quasi-crystalline” correlations, disturbed by overlapping with diffuse halos originated from strongest inter-atomic correlations, in part, the nearest-neighbor correlations between cation-like atoms. Milling does not influence the FSDP in a vicinity of glassy Se (x < 20), while causes increase in the FSDP width ΔQ (the reduced correlation length L) in the glasses with 20 ≤ x < 40, these changes being ascribed to disturbed inter-planar arrangement in covalent-bonded network due to structural defects generated under attrition ball milling. Assuming contribution of the Ehrenfest diffraction from coordination spheres in the detected XRPD patterns, the latter is ascribed to broadening in the distribution of characteristic FSDP-related inter-atomic distances, thus testifying in a favor of more defective structure of milled glass.

Glass-forming ability and thermal stability of Se100−x(Ge2Sb2Te5)x glassy alloys

Glassy Se100−x(Ge2Sb2Te5)x (x = 5, 10, 15 and 20) bulk alloys were prepared by melt-quenched technique and studied by using differential scanning calorimetry at different heating rates under non-isothermal condition. The detailed thermal analysis shows that the glass transition temperature (Tg) depends on heating rates and x content. In particular, it is found that the glass-forming ability, thermal stability (Tc − Tg) and crystallization activation energy (Ec) increase with increased x content in amorphous Se, whereas glass transition activation energy (Eg) and fragility index (F) decrease with increased x contents. Variation in these parameters can be explained on the basis of network-forming ability of Se and bonding arrangement among the constituent atoms of alloys.

Thermal analysis of cadmium addition on the glass transition and crystallization kinetics of Se–Te–Sn glassy network

Calorimetric measurements have been performed in quaternary glassy system Se78−xTe20Sn2Cdx (x = 0, 2, 4, and 6) to study the thermally induced glass transition and crystallization phenomena. The characteristics kinetic temperatures (glass transition temperature $$T_{\text{g}}$$ , onset crystallization temperature $$T_{\text{o}}$$ , peak crystallization temperature $$T_{\text{c}}$$ ) and static melting temperature $$T_{\text{m}}$$ of the quaternary glassy series have been determined using their non-isothermal DSC curves. These temperatures were utilized to calculate the thermal stability $$S$$ and the Hruby parameter $$H_{\text{R}}$$ for the studied glasses. Both the activation energy of glass transition $$E_{\text{t}}$$ and the activation energy of crystallization $$E_{\text{c}}$$ for all compositions were evaluated using the well-known Kissinger's relation and the method of Augis–Bennett. Values of the fragility index $$m$$ and the crystallization rate factor $$K_{\text{p}}$$ were estimated using the activation energy values $$E_{\text{t}}$$ and $$E_{\text{c}}$$ , respectively. The compositional dependence of the above-mentioned parameters was discussed on the basis of rigidity models and according to the formation of iono-covalent bonds when our glassy Se–Te–Sn system is doped with a heavy element as Cd.

Comparative Thermal Analysis of Antimony and Arsenic Addition on the Glass Transition Kinetics of Glassy Se80 Ge20 Alloy

Comparative Thermal Analysis of Antimony and Arsenic Addition on the Glass Transition Kinetics of Glassy Se₈₀ Ge₂₀ Alloy

Theoretical Prediction of Some Physico–Chemical Parameters in Glassy Se80–xTe20Cdx Alloys

Theoretical Prediction of Some Physico–Chemical Parameters in Glassy Se_{80–<i>x</i>}Te₂₀Cd<i>_x</i> Alloys

Applicability of Single-Scan Differential Scanning Calorimetry Technique for Determination of Kinetic Parameters of Crystallization in Glassy Se–Te–Ga System

Applicability of Single-Scan Differential Scanning Calorimetry Technique for Determination of Kinetic Parameters of Crystallization in Glassy Se–Te–Ga System

Thermal analysis of specific heat measurements in glassy Se80−xTe20Sbx alloys in glass transition region

In the present report, we have done specific heat measurements in glassy Se80−xTe20Sbx (0 ≤ x ≤ 15) alloys in glass transition region. Differential scanning calorimetry (DSC) technique is used for this purpose. We have observed a tremendously huge increase in the specific heat (Cp) values at the glass transition temperature. The thermal analysis shows that the values of Cp below glass transition temperature and the difference of Cp values before and after glass transition (∆Cp) are highly composition-dependent.

A.C. conduction in glassy alloys of Se90Sb10-xAgx

Temperature and frequency dependence of a.c. conductivity is studied in glassy Se90Sb10-xAgx (x=2, 4, 6 and 8) in the temperature range (288–358K) and frequency range (1–500kHz). An agreement between experimental and theoretical results suggests that the behavior of glassy Se–Sb–Ag system can be successfully explained by correlated barrier hopping (CBH) model. The results show that bipolaron hopping dominates over single polaron hopping in present case, which is explained in terms of lower value of potential barrier. By fitting the experimental data to CBH model, the density of charged defect states is calculated for all the glasses studied. The composition dependence of the density of these states is studied and found to show a maxima at 6at% of Ag which is explained in terms of the mechanically stabilized structure at a particular average co-ordination number.

Crystallization kinetics of glassy Se–Te–Sn alloys

The present article deals with the differential thermal analyses (DTA) study of Se–Te glasses containing Sn. DTA runs are taken at six different heating rates (5, 10, 15, 18, 20, and 22 K min−1). The crystallization data are examined in terms of modified Kissinger, Mahadevan method, and Augis and Bennett approximation for the non-isothermal crystallization. Results of DTA under non-isothermal conditions on the glasses of the Se80Te20--xSnx (x = 3 and 9) are reported and discussed at different heating rates. The glass transition temperatures (Tg), the onset crystallization temperatures (Tc), and the peak temperature of crystallization (Tp) were found to be dependent on the compositions and the heating rates. From the dependence on heating rates of (Tg) and (Tp) the activation energy for glass transition (Eg) and the activation energy for crystallization (Ec) are calculated and their composition dependence discussed.

Study of zinc incorporation on the non-isothermal crystallization in glassy selenium using iso-conversional approach

The crystallization kinetics of glassy Se (g-Se) and glassy Se98Zn2 alloy has been studied by means of differential scanning calorimetric (DSC) technique. Iso-conversional methods [Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), Tang and Straink] were used to determine the various kinetic parameters (i.e. crystallization temperature Tα, activation energy of crystallization Eα) of crystallization process in non-isothermal mode. The monotonous decrement in activation energy, Eα with the extent of conversion, α confirms the complex kinetic mechanism of g-Se and Se98Zn2 alloy. The reaction model, ‘Avrami–Erofeev’ g(α)=[−ln(1−α)]1/n has been followed to describe the crystallization process with n=1.5 for g-Se and n=2.0 for Se98Zn2 alloy.

Determination of Some Thermo-Mechanical Properties of Glassy Se and Binary Glassy Se&lt;SUB&gt;98&lt;/SUB&gt; M&lt;SUB&gt;2&lt;/SUB&gt; (M&lt;SUP&gt;=&lt;/SUP&gt; Ag, Cd, and Zn) Systems

Determination of Some Thermo-Mechanical Properties of Glassy Se and Binary Glassy Se&lt;SUB&gt;98&lt;/SUB&gt; M&lt;SUB&gt;2&lt;/SUB&gt; (M&lt;SUP&gt;=&lt;/SUP&gt; Ag, Cd, and Zn) Systems