Se Chains Research Articles

Structure and Relaxation in Germanium Selenide Glasses and Supercooled Liquids: A Raman Spectroscopic Study

The structure of the tetrahedral backbone and the nature and time scale of the temperature-dependent structural changes in binary Ge-Se glasses and supercooled liquids with ≤33.33 atom % Ge have been investigated using ambient and high-temperature Raman spectroscopy. The composition dependence of the relative fractions of edge- and corner-shared GeSe(4) tetrahedra and that of the characteristic mean vibrational frequencies of these structural units are shown to be consistent with a structural model for these glasses based on a random interconnection between GeSe(4) tetrahedra and Se-Se chain fragments. The most prominent temperature-dependent structural change in the Ge(20)Se(80) glass and supercooled liquid involves progressive conversion of the edge-shared GeSe(4) tetrahedra into corner-shared tetrahedra, upon lowering of temperature. The time scale of this tetrahedral conversion "reaction" corresponds well with those of enthalpy and shear relaxation near glass transition. Moreover, the temperature dependence of this GeSe(4) tetrahedral speciation is shown to be the most important source for the production of configurational entropy in this supercooled liquid near the glass-transition range, signifying a direct link between structural relaxation, configurational entropy, and viscous flow.

Single crystal structure analysis of the Se-incorporated mordenite, coupled with the anomalous X-ray scattering

Mordenite has one-dimensional 8-ring and 12-ring channels running parallel to the c-axis and Se molecules were successfully incorporated into the mordenite channels through a vapor phase after dehydration. The structure of the Se-incorporated mordenite was analyzed by single crystal X-ray diffraction coupled with anomalous X-ray scattering (AXS). The AXS analysis confirmed the Se distribution in the 12-ring channels only, by analyzing the intensity contrast observed at the Se K-absorption edge. And this environmental structure information allows us to improve the structural analysis of the ordinary single crystal X-ray diffraction. The present analysis suggested that Se molecules such as an infinite chain and its fragments were preferentially formed in the 12-ring channels together with the feasible bonding of Se–Na pairs. The location and probability of Na in the 12-ring channels are suggested to be considered as important factors affecting the probability of Se chain and its fragmentation.

Structural correlations of Ge 20Se 80− xTe x glasses based on reverse Monte Carlo simulation

Bulk chalcogenide Ge 20Se 80− x Te x (where x = 0, 10, 20 and 30 at.%) glasses were prepared using the melt-quench technique. The total structure factors of these alloys are obtained from the X-ray scattering data in the momentum transfer interval 0.61 ≤ K ≤ 16.45 Å −1. From reverse Monte Carlo (RMC) simulations of the X-ray scattering data, the short and intermediate-range order parameters are obtained. The simulations are useful to compute the partial pair distribution functions, g ij ( r), and the partial structure factors, S ij ( K). In Te-rich glass, the first sharp diffraction peak (FSDP) appears as a shoulder, instead of a peak for others, confirms that Se–Se bonds in addition to Ge–Ge bonds are responsible for the intermediate-range order inside these glasses. The partial coordination numbers and the bond angle distributions within the first coordination shell have been calculated. The ratio of the first to second peak positions ( r 1/ r 2) and the corresponding bond angle ( Θ) have confirmed that the Ge(Se 1/2) 4 tetrahedra, connected by Se–Se chains, can be considered as the main building units inside the investigated glasses.

Electrical studies on bulk Se 96Sn 4 semiconducting glass before and after gamma irradiation

Bulk Se 96Sn 4 chalcogenide glass was prepared by melt quenching technique and irradiated by different doses of 4, 8, 12, 24 and 33 kGy using 60Co gamma emitter. I– V characteristics were obtained for this glass, before and after gamma irradiation, in the temperature range 200–300 K. Ohmic behavior was observed at low electric fields (≤1×10 4 V/m), while at higher fields, a deviation from ohmic towards non-ohmic behavior was observed. The plots of ln( I/ V) vs. V were found to be straight lines and the slopes of these lines decrease linearly with temperature indicating the presence of SCLC. In the temperature range of measurements, the dependence of DC conductivity on temperature at low electric field shows two types of conduction channels, one in high temperature range 270–300 K and the other at low temperature range 200–270 K. Analysis of the experimental data shows that the conductivity at room temperature decreases with increase in irradiation dose. This is attributed to rupturing of SnSe 4/2 structural units, upon irradiation, and rebuilt of Se atoms between Se chains. This redistribution of bonds, induced by gamma irradiation, is responsible for the corresponding increase in the activation energy. The obtained values of the activation energy indicate that the conduction occurs due to thermally assisted charge carriers movement in the band tail of localized states. However, in the low temperature range, results obtained from Mott’s variable range hopping (VRH) model reveal that the density of localized states has its maximum value at a gamma dose of 12 kGy, while the disorder parameter T o , hopping distance R hop and hopping energy W have their minimum value at this particular dose.

31P solid-state NMR studies of the short-range order in phosphorus–selenium glasses

The local structure of P-rich and Se-rich phosphorus-selenium glasses was studied using high-resolution (31)P solid-state MAS NMR. Two-dimensional (31)P homonuclear through-bond correlation MAS experiments and 2D homonuclear J-resolved MAS measurements were performed at high spinning frequency to probe P-P and P-Se-P connectivities between the different P sites for the compounds in two glass-forming regions, P(2.5)Se(97.5)-P(50)Se(50) and P(67)Se(33)-P(84)Se(16). Amorphous phosphorus and crystalline alpha-P(4)Se(3) and beta-P(4)Se(3) were also studied as reference materials. Glasses from the Se-rich region contain mainly three- and four-coordinated P sites linked together by Se(n) chains, whereas P-rich glasses contain a mixture of P(4)Se(3) molecular units and possibly other structural units embedded in a red-phosphorus-like polymeric network.

Raman spectroscopy of chalcogenide thin films prepared by PLD

Chalcogenide glasses have many technological applications as a result of their particular optical and electrical properties. Ge–Se and Ag–Ge–Se systems were recently studied and tested as new materials for building non-volatile memories . Following these ideas, thin films of Ge–Se and Ag–Ge–Se were deposited using pulsed laser deposition (PLD). Ag was sputtered over binary films (for a composition between 0.05 and 0.25 Ag atomic fraction) and photo-diffused afterwards. Thus, three kinds of samples were analyzed by means of Raman spectroscopy, in order to provide information on the short- and medium-range order: PLD binary films before Ag doping, after Ag doping and PLD ternary films. Before Ag doping, binary films exhibited Ge–Se corner-sharing tetrahedra modes at 190 cm −1, low scattering from edge-sharing tetrahedra at 210 cm −1, and Se chains at 260 cm −1 (stretching mode). However, after the diffusion process was complete, we observed an intensity reduction of bands centered at 210 cm −1 and 260 cm −1. The spectra of the photo-diffused films were similar to those of films deposited using a ternary target. Relaxation effects in binary glasses were also analyzed. Results were compared with those of other authors.

Negative compressibility of selenium chains confined in the channels of AlPO4-5 single crystals

Pressure-induced structural and electronic variations of Se helical chains confined in nano-channels of zeolite single crystals are studied. Raman and optical absorption spectra provide us with evidence of softened vibration modes and band gap reduction under high pressure. We reveal that the Se chains elongate under hydrostatic compression by ab initio calculations. The changes in morphology lead to a softening of phonons and narrowing of energy gaps consistent with observations from optical measurements. The present investigation demonstrates negative compressibility of Se chains in one-dimensional channels of a zeolite framework.

Photocurrent measurements in chlorine-doped amorphous selenium

Transient photocurrent (TPC) measurements have been carried out on evaporated a-Se films doped with either 12.5 or 67 ppm of Cl and compared with analogous measurements on undoped a-Se samples. The two doping levels lead to strikingly different resulting current traces. While the 12.5 ppm doping level weakens, but does not eliminate the signature of the two hole traps that are observed in the TPC signals of undoped a-Se, no indication for those levels remains in the 67 ppm Cl-doped samples. It is argued that suppression of the trap generated by the negative-U D- centre agrees with the notion that they are being supplanted by doping-induced Cl- centres. The enhanced density of D+ centres and accompanying cross-linking of Se chains by those three-fold coordinated sites leads in turn to the weakening and disappearance of the shallower hole trap ascribed to dihydral angle variations. Results from constant-photocurrent and time-of-flight (TOF) experiments support this interpretation of the TPC observations.

Infrared and Raman studies on Snx–Sb5–Se95−x chalcogenide glasses

Tin–antimony–selenium (Sn–Sb–Se)-based systems belong to the ternary chalcogenide compounds of IV–V–VI group. They have potential applications in infrared region due to their heavy elemental masses, continuous variation of band gap-energies and lattice constants as well as electrical properties, with compositions. Structures of melt quench-synthesized samples of Snx–Sb5–Se95−x system, where x=0, 5, 10 and 12.5-mole% have been studied using Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. FTIR spectra illustrates that addition of Sn-mole% to the system causes a shift in IR-peak’s intensity and width from long to the short wavelength. This change implies the breaking of Se chains that appeared around 210–254cm−1 and the occurrence of pyramidal SbSe3 around 147–210cm−1 and asymmetrical tetrahedral SnSe4 mode around 117–145cm−1 for Sn=5mole% up to 180cm−1 in Sn=12.5mole% spectra. Raman spectra show that a pyramidal SbSe3 peak is cited at 190-cm−1. The intensity of this peak is shifted towards −183cm−1 when Sn-mole% is added to the system. The results confirm the validity of using 4, 3 and 2 as co-ordination numbers of Sn, Sb and Se, respectively, in the amorphous region, which is contained by the average co-ordination number, μ⩽2.4 and the fraction of Sn–Se bonds, fSn–Se<44.3%.

Optical nonlinearities of Se-loaded zeolite (ZSM-5): A molded nanowire system

Nonlinear optical properties of Se-loaded ZSM-5 single crystals have been studied comparatively with those of glassy Se. Two-photon absorption coefficients and intensity-dependent refractive-index of the Se-zeolite are higher by three orders of magnitude than those of glassy Se. The enhancements can be ascribed to electron confinement in single Se chains.

Selenium-loaded zeolite (ZSM-5): II. Photoinduced phenomena

Photoinduced phenomena in Se-loaded ZSM-5 single crystals have been studied through electron-spin resonance, optical absorption, and photoluminescence spectroscopy. Marked exposure effects appear at room temperature. Upon illumination of visible light, an electron-spin signal at g≈2.00 increases by an order, subgap absorption of 103 cm−1 appears, and photoluminescence at subgap regions weakens to a half. Timescales of these photoinduced changes including recoveries are not the same, which suggests different origins for the three changes. The spin increase evinces bond scission in single Se chains.

Structural transformations in amorphous As x Se1−x (0 ≤ x ≤ 0.20) films

Structural transformations are examined by the employment of Raman scattering measurements for amorphous Se-rich As x Se1−x (0 ≤ x ≤ 0.2) alloys. It is found that the molecular structure of amorphous Se (a-Se) on the scale of medium-range order differs from the structure of most inorganic glasses and may be placed between 3-dimensional network glasses and polymeric ones. Further experiments show the existence of successive phases in laser-induced glass—crystalline transition with pronounced threshold behavior. By comparing peak width, peak location and Raman intensity in the range of bond modes it is derived that the changes occur not monotonically with increasing As content. The composition-induced changes of the spectra are explained by cross-linking of Se chains. Under laser irradiation, the changes in the optical transmission, holographic recording properties and Raman spectra of amorphous As x Se1−x films with 0 < x ≤ 0.2 have been examined. The dependence of the transmissivity and diffraction efficiency on the irradiation energy density shows two qualitatively different regions. Below the energy density threshold, E th, only small changes in the local structure of the system can be detected. In the low-energy region, transient changes in transmissivity are observed. Qualitative explanation of this behavior may be based on associating such with alternating of deep defect states. Above E th, the changes were attributed to crystallization transformation. The corresponding Raman spectra reveal transformation of the system from amorphous into the crystalline phase under laser irradiation. Although several articles and texts have provided reviews on various properties and applications of chalcogenide glasses, there is no thorough study of local atomic structure and its modification for Se-rich amorphous As x Se1−x . The present paper is concerned with this problem.

Concentration dependence of structural and bonding properties of liquid Tl-Se alloys byab initiomolecular dynamics simulations

The structural and electronic properties of liquid ${\text{Tl}}_{x}{\text{Se}}_{1\ensuremath{-}x}$ for $0\ensuremath{\le}x\ensuremath{\le}1$ are investigated by means of ab initio molecular dynamics simulations. It is shown that calculated structure factors are in good agreement with experiments over the whole range of composition. It is confirmed from calculated electronic densities of states that the liquid Tl-Se alloys have semiconducting properties for Tl concentrations not exceeding the stoichiometric composition $(x=2/3)$, while they are metallic for $x&gt;2/3$. In Se-rich composition range, Se atoms form chain molecules, whose average length becomes shorter with increasing Tl concentration. In the equiatomic liquid Tl-Se alloy, short Se chains, mostly negatively charged ${\text{Se}}_{2}$ dimers, coexist with a few Se ions, while most of Se atoms exist as ions at the stoichiometric composition. Based on a population analysis, we discuss the concentration dependence of the bonding properties in comparison with that in liquid alkali-metal chalcogenides.

Structure of Se-rich As-Se glasses by high-resolution x-ray photoelectron spectroscopy

To establish the validity of various proposed structural models, we have investigated the structure of the binary As{sub x}Se{sub 100-x} chalcogenide glass family (x{<=}40) by high-resolution x-ray photoelectron spectroscopy. From the composition dependence of the valence band, the contributions to the density of states from the 4p lone pair electrons of Se and the 4p bonding states and 4s electrons of Se and As are identified in the top part of the band. The analysis of Se 3d and As 3d core-level spectra supports the so-called chain crossing model for the atomic structure of Se-rich As{sub x}Se{sub 100-x} bulk glasses. The results also indicate small deviations ({approx}3-8%) from this model, especially for glass compositions with short Se chains (25<x{<=}40). For example, the presence of As-As homopolar defect bonds in the stoichiometric As{sub 40}Se{sub 60} and of Se-Se-Se fragments in a glass with composition x=30 is established.

Voids and Intermediate-Range Order in Network-Forming Liquids: Rb20Se80 and BiBr3

The RMC and Voronoi–Delaunay analyses were applied to study the void structure in the network-forming liquids: Rb 20 Se 80 and BiBr 3 . The distribution function of the relative positions of atoms and void centers in space, the coordination number of atoms and voids around a void and the size distribution of voids were examined using the previous experimental data for S ( Q ) to characterize the connectivity of the bonding network supporting voids and the intermediate-range correlation as manifested in the pre-peak of S ( Q ). The voids in l-Rb 20 Se 80 are formed by the 8-membered ring-like Se chains. A Rb cluster composed of 3–4 Rb atoms replaces one of five void sites and binds ∼4 Se rings involving voids around it through the interactions between Rb and Se and between Rb and Rb. In l-BiBr 3 the hexagonal cages involving voids are formed by the corner-sharing of the trigonal pyramidal BiBr 3 blocks. The strong preference of Bi ion for three-fold coordination helps to bind between the neighboring cages ...

Local structure of liquid Rb–Se mixtures near the miscibility gap

In the liquid Rb–Se mixture there exists a liquid–liquid phase separation (miscibility gap) in the Se-rich concentration region. In order to study how the local structure is associated with the occurrence of the miscibility gap, EXAFS and neutron diffraction measurements for liquid Se, Rb10Se90, Rb15Se85 and Rb20Se80 mixtures have been carried out. The results reveal that the interaction between Rb and Se atoms is very weak and the liquid mixtures are composed of short Se chain-molecules of which chain ends are terminated by Rb atoms. We have carried out detailed structural modeling studies using reverse Monte Carlo simulation. It is found that the chain-like or ring-like Se segments are separated by voids. The Rb atoms adsorbed to Se chain ends segregate in the void and the shortened Se chain-molecules link up with each other when one approaches the miscibility gap.

Structural stability of Se and Te nanowires

Structural stability for selenium and tellurium nanowires is studied by combining first-principles method and empirical approach. This is an interesting topic due to the fact that the Se and Te crystals are composed of triangular Se and Te chains and that the chain-chain interaction is relatively weak. Nanowires of small diameters (&lt;30?) are studied by first-principles calculations. Meanwhile, an empirical method that considers only the interaction between chains is also proposed in order to study the nanowires of larger diameters. The nanowires with hexagonal cross section are more stable, though the hexagon is not always perfect in shape due to the limitation of chain number. This result agrees with the widely accepted stable structures of selenium and tellurium nanowires.

Chain structure and electronic states of liquid Rb-Se mixtures byab initiomolecular-dynamics simulations

The effects of excess electronic charges transferred from alkali metals to Se atoms on the chain structure in liquid ${\mathrm{Rb}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ for $x\ensuremath{\leqslant}0.5$ are investigated by means of ab initio molecular-dynamics simulations. It is found that the interaction between Se chains is enhanced by the transferred electrons, and that the average length of Se chains becomes shorter with increasing alkali-metal concentration. The shortening of Se chains is responsible for an increase of electronic states at the Fermi level, which explains the observed increase of the electrical conductivity on the addition of alkali metals. At the equiatomic concentration, there are almost no electronic states at the Fermi level due to the formation of $\mathrm{Se}_{2}{}^{2\ensuremath{-}}$ dimers. The alkali-metal-concentration dependence of the bonding properties between Se atoms is discussed in comparison with liquid alkali-metal tellurides based on a population analysis.

Spin-coated As33S67−xSex thin films: the effect of annealing on structure and optical properties

The spin-coating method was used for the preparation of amorphous chalcogenide As33S67−xSex films, where x=0, 17, 33.5, 50 and 67 at.%. The films were stabilized in a vacuum furnace at 70°C for 10 hours. The compositions of the films, established by Energy-Dispersive X-ray analysis, were similar to the composition of the glass used for the preparation of the solutions. The optical transmission measurements indicated that the optical band gap decreases with increasing Se-content in the films. Raman spectroscopy showed the films structure i.e. As–S bonds at 344 and 376cm−1, S-rings at 495cm−1 and appearance of As–Se bonds at 225cm−1, Se chains at 254cm−1 and S–Se bonds at 416cm−1 with increasing Se-content. MIR transmission spectra revealed the presence of solvent residua in spin-coated films i.e. N–H bonds at 3395–3300cm−1, C–H bonds at 2980–2850cm−1. Further annealing of the films (150°C, 0–60min) caused decrease of N–H and C–H stretching vibration intensities. Raman spectra showed that the annealed films were more structurally ordered in contrast to the as-deposited material. The shift of the films optical absorption edge to lower energies was observed during annealing procedure. Potential application of the films is expected in optical recording and new types of memories.

Structure of liquid Rb-Se mixtures: void and fluctuation

Neutron diffraction and EXAFS measurements, and the detailed structural modeling by reverse Monte Carlo simulation have been performed in order to study the short-and intermediate-range structures of liquid Rb-Se mixtures. The Se chains in the liquid mixture are strongly bent to form horseshoe-like Se chains with Rb atoms at the chain ends, creating a large amount of voids. The interaction between Rb and Se atoms is weak and Rb atoms come close together. Near the boundary of the miscibility gap at low temperature the short Se chains link up to form a large cluster, and Rb atoms sit around it. The structure factor S(Q) in the liquid mixture has a small pre-peak at 1.3Å-1. The computed partial Sij(Q) for the RMC model indicates that the pre-peak is associated with the Se-Se interchain component of S(Q). From the void analysis using Delaunay simplices it has been revealed that the observed pre-peak corresponds to a pre-peak in the concentration-concentration structure factor due to the chemically ordered packing of the voids and Se chains in the structure.