Selenium Chains Research Articles

Endohedral selenium chains in carbon, boron nitride, and BC2N nanotubes

Endohedral selenium chains in carbon, boron nitride, and BC2N nanotubes

Structure of high temperature fluid selenium

Monte Carlo simulations based on a semi empirical tight binding model including dispersion forces were performed to study liquid selenium at temperatures between 600 and 2000 K. The atomic structures obtained are in agreement with the X-ray scattering and extended X-ray absorption fine structure (EXAFS) data in a range of temperatures and densities. A correlation between the conductivity of high temperature fluid selenium and the degree of branching and breaking of the selenium chains is observed.

Structure of high-temperature fluid selenium

A semiempirical tight-binding energy model is developed for selenium. It includes s and p electrons as well as an empirical description of the dispersion forces that proves necessary at the liquid densities under study. The band-structure parameters are obtained by fitting ab initio calculations. The simulated liquid structures are in very good agreement with the most recent x-ray scattering and extended x-ray absorption fine-structure experiments in a broad temperature and density range. The Monte Carlo simulations performed show that the complex liquid structures observed result from the breaking and branching of the selenium chains. The total coordination number is shown to result from the balance between one-, two-, and threefold coordinated atoms. The role of these defects is discussed in relationship with the electrical conductivity of the liquid, i.e., the semiconductor-metal and metal-nonmetal transitions observed at high pressures.

Structure of single selenium chains confined in nanochannels of zeolites: a polarized X-ray absorption study.

Local structure of selenium confined in single crystal zeolites with one-dimensional nanochannels of ~7,~ in diameter (cancrinite, mordenite, ALPO-5) has been studied by polarized EXAFS and XANES. In cancrinite, selenium is found to form dimers strongly oriented along the channel axis and with the bond lengths of 2.40/~. XANES spectra exhibit two bound-state-like peaks one of which is strongly polarized parallel to the channel whilst the other one is isotropic. Peak positions and their dependence on the sample orientation indicate negative charge on the dimers. In mordenite and .ALPO, no strong polarization dependence is observed suggesting that selenium forms continuous helical chains.

Dimerization of single selenium chains confined in nanochannels of cancrinite: An x-ray absorption study

Dimerization of single selenium chains confined in nanochannels of cancrinite: An x-ray absorption study

Enthalpy relaxation in Ge xSe 1− x chalcogenide glasses

Enthalpy relaxation in selenium samples and Ge x Se 1− x alloys with x=0.08 and 0.12 has been investigated by differential scanning calorimetry. All the samples have been submitted to comparable aging temperatures 10°C below the glass transition temperature for a period from 0.5 to 800 h. The apparent activation energy and the Narayanaswamy parameter, X, were calculated and the non-exponentiality parameter, β, was estimated for the samples. The introduction of the germanium atoms in the selenium chain changes the distribution of relaxation times, increases the relaxation rate per decade and decreases the apparent activation energy. No differences were observed in the Ge x Se 1− x alloys in the evaluation of these parameters.

Holographic recording in ultrathin amorphous selenium films: recording mechanism and apparent erasability

Holographic recording has been performed for the first time in ultrathin (20 nm thick) amorphous selenium films. Two different experimental configurations leading to spatial frequencies of 360 and 1000 lines/mm are studied. Whereas a high diffraction efficiency (> 1 %) is achieved in the low-frequency regime, partial erasability is feasible in the high-frequency case. Electron microscopy based techniques show that the whole grating is in the amorphous phase, the dark fringes are denser than the bright ones, and there are no traces of oxidation. Transmittance measurements show a blue shift in the absorption edge of the films upon laser irradiation. The results are discussed in terms of film densification, accomplished through rearrangements in the amorphous selenium chains, the heat flow along the direction of the grating vector playing an important role.

An X-ray absorption study of selenium confined in channels of cancrinite: Evidence for dimerisation in highly oriented chains

Local structure of one-dimensional selenium chains in cancrinite channels have been studied by polarised X-ray absorption. Polarised EXAFS reveals that selenium atoms are one-fold coordinated and well-aligned along the c-axis of cancrinite. The obtained SeSe bond length is 2.40 ± 0.01 A ̊ . The results provide direct evidence for dimerisation of selenium chains and suggest incommensurate intercalation of dimers which do not form a triplet (π-bond) state or a dangling bond state. Highly anisotropic polarisation dependence of XANES demonstrate that antibonding 4p(σ ∗) states are partially filled suggesting a charge transfer from Na cations located in the channels.

Structure of inorganic nanoclusters embedded in solid matrixes: an X-ray absorption study

Results of X-ray absorption studies of copper nanoclusters embedded in SiO2 matrix and selenium chains confined in channels of cancrinite are reported. Initially randomly distributed copper atoms mainly surrounded by silicon species in an as-made film upon annealing form copper nanoclusters with a characteristic size of 80 A and the structure identical to that of bulk copper. The characteristic size of the clusters does not increase with further annealing. Selenium in cancrinite is shown to form strongly oriented linear dimerised chains with a bond length much larger than that in bulk forms of selenium.

The Formation of Se2-: A New Resonance Raman Feature in the Photochemistry of Zeolite-Encapsulated Selenium

This Raman spectroscopic study on selenium incorporated in Nd−Y zeolite presents new aspects of the photochemistry of selenium. The formation of the Se2- radical anion has been established from its resonance Raman spectrum, consisting of ten almost equidistant bands between 328 and 3220 cm-1 on the low-energy side of the 476.24-nm laser excitation line. The derived spectroscopic constants of the electronic ground state are ω0 = 328 cm-1 and x0ω0 = 0.71 cm-1. Another broad band at 260 cm-1 resembles features in the Raman spectra of amorphous selenium and selenium chain structures encapsulated in zeolites. Continuous irradiation at sufficiently high laser power levels leads to depletion of the 260 cm-1 band and a simultaneous enhancement of the Se2- bands. These observations are discussed with respect to the defect states conventionally invoked for reversible photoinduced changes in amorphous selenium.

Dynamics of Selenium Chain Melts by MD Simulation

Abstract A molecular dynamics (MD) study of liquid selenium modelled by linear chains is presented. The simulated thermodynamic state corresponds to the experimental density of 3570 Kgm−3 at 873 K. We have studied melts with chains of different lengths, varying in the range of 10–80 atoms, just up to the limit where the melts relax in the temporary and spatial length window (101 ns and 28.6 A) of the numerical experiment. The flexibility of the chains at the thermodynamic state considered enabled us the observation of different spatio temporal dynamics regimes, which can be described by the Rouse model for dense polymer solutions.

圧力誘起相転移の研究への放射光の応用 高圧下のＸＡＦＳ

Recent advance in X-ray Absorption Fine Structure (XAFS) study under pressure is reviewed. After a brief introduction to XAFS and its applications to high-pressure studies, recent development of XAFS measurements under pressure at the Photon Factory is presented. We successfully obtained X-ray absorption spectra at pressures up to 10 GPa and temperatures up to 700°C using a multi-anvil type apparatus, MAX90. Results of our studies on crystalline selenium, amorphous selenium and isolated selenium chains are presented.

Structure of one-dimensional selenium chains in zeolite channels by polarized Raman scattering

Mordenite and cancrinite single crystals containing one-dimensional selenium chains with compositions Ca 2Na 4Al 8Si 40O 96,Se 9.5 and Na 8Al 6Si 6O 24OH 2Se 0.54, respectively were studied by polarized Raman scattering. Structural models of the chains, which are helix-like with varying dihedral angle signs for mordenite and linear for cancrinite, are proposed. Temperature-dependent structural changes were found for the selenium chains in the cancrinite.

Structural investigation and dielectric properties of Na-mordentte-encapsulated selenium

Abstract A crystal chemical study has shown that both small pore and large pore Na-mordenite exposed to Se vapour incorporate this element into their framework. The maximum amounts of selenium inserted are equal to 8.3 and 23.5 wt % respectively. EXAFS results have allowed to conclude that Se-Se bond lengths are 0.04 Å shorter in Na mordenite with respect to the value in the hexagonal selenium. Dielectric results are compatible with incorporation of selenium chains within the pores of the mordenites.

Structure and dynamics of selenium chain melts: A molecular dynamics study

A molecular dynamics (MD) study of liquid selenium modeled by 16 linear chains of 40 monomers each is presented. The simulated thermodynamic state corresponds to the experimental density of 3570 Kg m−3 at 873 K. The structural and force constant data of the chains were obtained from previous studies of neutron diffraction experiments, lattice dynamics, and first principles calculations. The computed structural properties show a good agreement with available neutron scattering data. The flexibility of the chains and the high temperature thermodynamic state of the liquid enabled the observation of fast torsional motions and different spatiotemporal dynamic ranges, which can be described by the Rouse model for dense polymer solutions. We identify the crossover from an atomic to an intermediate or ‘‘universal’’ chain regime, and subsequently to global chain behaviors. The dynamics of the system is discussed in terms of time and space-dependent transport coefficients. The generated MD trajectory thus provides information on the single particle motions, the collective dynamics of one chain, and the dynamics of the global system. This separation is useful for understanding the low frequency collective motions which can be measured by inelastic neutron scattering. The spectra are interpreted in terms of existent dynamical models, which imply a degree of trapping of the atoms in some spatial regions of the liquid (‘‘chain cages’’) defined by atomic crosslinks, plus a slow diffusive process which modifies the shape of the cage according the renewal of the atomic crosslinks.

A molecular dynamics study of amorphous selenium

The structure of amorphous selenium has been studied using molecular dynamics. Ten chains of selenium consisting of 30 atoms each have been generated by a new algorithm with glass density as a criterion. A covalent bond has been simulated using harmonic potential and non-bonded closed shell interactions have been treated using Lennard-Jones potentials both for intra-and inter-chain atoms. Good agreement of bond distances, bond angles and nearest neighbour coordinations with experimental data has been observed. Absence of any preferred dihedral angle has been noticed. Diffusion of selenium has been interpreted as a result of coupled motion of atoms in the chains whose overall dynamics is considered as jiggling. It has been shown that the activation barrier for this kind of a displacement is fairly low, although atomic displacements give a diffusion constant of around 2×10 −7 cm 2/s at 200 K.

Photorelaxation in amorphous Ge xSe 1− x (0 < x < 0.08) thin films

By means of differential thermal analysis, the variations of glass transition temperature, T g, with aging time, of amorphous thin films, have been measured. After illumination by light, the ratio between the variation of T g with aging duration under illumination, to the same variation in the dark shows a wavelength dependence similar to that of amorphous selenium absorption coefficient. The equilibrium relaxation-time under illumination, τ e, may be divided into two terms: one that characterizes the relaxation in the dark, the other that characterizes the influence of light. Finally, it is shown that relaxation processes under illumination are essentially due to the selenium chain rearrangements.

Reversible dismutation and coordination of bis(2,4,6-triisopropylphenyl) diselenide with iodine. A model study that relates to iodine intercalation between selenium chains

Reversible dismutation and coordination of bis(2,4,6-triisopropylphenyl) diselenide with iodine. A model study that relates to iodine intercalation between selenium chains

Absorption spectra of selenium clusters and chains incorporated into zeolites

Absorption spectra are reported for Se adsorbed in faujasite (X or Y) as a function of x, where x is the Al-to-(Al+Si) ratio. The amount of adsorbed Se increases with increasing x for small x and is saturated for x>or approximately=0.1. The results are interpreted using the model in which the finite number of Se atoms are physically adsorbed by each permanent dipole which consists of (AlO4)- in a zeolite framework and cation. The energy of the lower absorption edge changes with x and the spectral shapes are classified into three types. Spectral differences are also observed for Se incorporated into Linde type A and Linde type L lattices. From a comparison with the spectrum of the isolated Se chain in mordenite, the structures of Se in A and L are expected to be isolated clusters and double parallel chains, respectively.

Photo-Induced Phenomena in Isolated Selenium Chains

Photoacoustic spectroscopy, ESR and EXAFS measurements have been carried out on the Se chains isolated in the channels of mordenite crystal with 6.7 A diameter. The optical gap is estimated to be 2.3 eV at room temperature, which is much larger than that of trigonal Se. At low temperatures, illumination with the light having energy near or above the optical gap induces new absorption bands at 1.85 and 2.25 eV, which are associated with paramagnetic defects. Higher energy light (>2.9 eV) shifts the absorption edge to lower energy. The fluctuations in the covalent bond length increase with illumination, though the helical chain structure is preserved. These changes vanish by annealing at room temperature. The phenomena are reversible in the illumination and annealing cycle. The photo-induced absorption and the edge shift are reduced when the induced bands are excited optically.