Se Clusters Research Articles

Photo-induced volume changes in selenium. Tight-binding molecular dynamics study

Tight-binding molecular dynamics simulations of photo-excitations in small Se clusters (isolated Se 8 ring and helical Se chain) and glassy Se networks (containing 162 atoms) were carried out in order to analyze the photo-induced instability in amorphous selenium. In the cluster systems after taking an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital a bond breaking occurs. In the glassy networks photo-induced volume expansion was observed and at the same time the number of coordination defects changed significantly due to illumination.

Aqueous solution chemistry of [Mo3CuSe4]n+ (n = 4, 5) and [W3CuQ4]5+ (Q = S, Se) clusters.

The triangular cluster [Mo3Se4(H2O)9]4+ reacts with Cu turnings to give a new heterometallic cuboidal cluster [Mo3CuSe4(H2O)10]4+(purple; UV/Vis lambda(epsilon): 352(3907), 509(2613)). The reaction of [Mo3Se4(H2O)9]4+ with CuCl afforded the 5+ cube [Mo3CuSe4(H2O)10]5+(red; UV/Vis lambda(epsilon): 356(5406), 500(3477)). In contrast, [W3Se4(H2O)9]4+ both with Cu and CuCl gives the 5+ cube, [W3CuSe4(H2O)10]5+(yellow-green; UV/Vis lambda(epsilon): 312(5327), 419(3256) and 628(680)). Cyclic voltammetry of [M3CuQ4(H2O)10]5+ in 2 M HCl (M = Mo, W; Q = S, Se) shows a reversible one-electron reduction wave for the Mo clusters, but no reduction occurs for the W clusters prior to H+ reduction. In HCl solutions, Cl is coordinated to the Cu site of the clusters, alongside some less extensive coordination to Mo and W, and for [W3(CuCl)S4(H2O)6Cl3]+, isolated as the supramolecular adduct with cucurbit[6]uril, [W3(CuCl)S4(H2O)6Cl3]2Cl2 x C36H36N24O12 x 12H2O, the crystal structure was determined (Cu-W 2.856(4) angstroms, W-W 2.7432(15) angstroms, Cu-Cl 2.167(13) angstroms).

Cluster formation in UV laser ablation plumes of ZnSe and ZnO studied by time-of-flight mass spectrometry

The composition of nitrogen laser (wavelength 337 nm, pulse width 3 ns) induced ablation plumes from ZnSe and ZnO targets was studied at different laser fluences using a time-of-flight mass spectrometer. In the case of ZnSe, abundant ion signals corresponding to Se and ZnSe clusters, some of which were Se-rich, were detected with fluence-dependent distributions. At a laser fluence of 1250 J m-2, clusters with elevated intensity were observed at sizes of 6, 13, 19, 23 and 33 ZnSe molecules (‘magic numbers’), which match quite well with the earlier observation by others of magic numbers of chemically similar ZnS produced by a conventional vaporization and quenching scheme. In the case of ZnO, we detected the presence of atomic Zn and molecular species of ZnO, as well as a series of (ZnO)n-type clusters with fluence-dependent distributions. Unlike the case of ZnSe, no magic numbers were observed for ZnO.

Optical spectroscopy on Se clusters and chains confined in zeolites.

The incorporation of selenium into the supercages of zeolite Y leads to the formation of Se(8) rings and distorted Se chains in a ratio which is influenced by the nature of the zeolitic cations. Here we review Raman (including resonance Raman) and UV/vis absorption spectroscopy results on Se encapsulated into a number of cation-exchanged faujasite zeolites. Both rings and chains give rise to characteristic Raman bands. In particular, low-frequency Raman bands are attributed to localized vibrations in ordered segments of distorted chains. The UV/vis absorption spectra indicate an opening of the band gap of selenium upon confinement in these zeolites. This can be reversed through electronic interaction with zeolite cations.

Medium-range order in amorphous selenium: Molecular dynamics simulations

In this paper we present a theoretical study on a glassy structure and infrared (IR)/Raman spectra of amorphous selenium (a-Se). The vibrational spectra are calculated for two-hundred independent amorphous samples obtained by performing molecular-dynamics (MD) simulations for one linear chain of 216 Se atoms. The interaction potential used in the MD simulation is constructed on the basis of ab initio molecular-orbital calculations (with the Hartree-Fock and second-order M\o{}ller-Plesset methods) for various Se clusters. The parameters in the bond-current and bond-polarizability models needed for the spectral calculations are also determined by the same ab initio approach. The calculated static structure factor, vibrational density of states, and IR/Raman spectra well reproduce experimental results. It is found that there exists a remarkable medium-range order in the disordered Se chain; i.e., a chain segment having four consecutive dihedral angles with alternate signs [i.e., (+, -, +, -) or (-, +, -, +)] tends to be excluded from the chain structure, which is attributed to the steric hindrance effect existing in those configurations. By comparing the polarized Raman spectra for two different random chains with and without the above steric hindrance effect, we find that only the former spectrum exhibits a distinct peak at 80 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which is observed also in the experiment. Therefore the appearance of this peak provides useful information on the medium-range order existing in the disordered Se chain.

New Light on the Stellar Populations in I Zw 18: Deep Near‐Infrared Imaging

We present deep JHK images of IZw18, the most metal-deficient Blue Compact Dwarf galaxy known, and analyze them in conjunction with archival HST/WFPC2 optical images. To investigate the stellar populations, we have divided the main body of IZw18 into eight regions, and fit the optical, near-infrared (NIR), and hybrid optical-NIR colors of these and the C component with evolutionary synthesis models. The composite best fit is obtained for an age for evolved stellar populations of <= 200Myr; fits with an older age of <= 500Myr are less likely but possible. Our data show no evidence for stellar populations in IZw18 older than this, although as much as 22% of the stellar mass in older stars (4% in J light) could remain undetected. The colors of the young and intermediate-age stellar populations are significantly affected by widespread and inhomogeneously distributed ionized gas and dust. Ionized gas emission is important in every region examined except the NW star cluster. Extinction is significant in both the NW and SE clusters. Red H-K, B-H, and V-K colors are not reliable indicators of old stellar populations because ionized gas emission is also red in these colors. V-I, on the other hand, reliably separates stars from gas because the former are red (V-I >= 0.4) while the latter is blue (V-I ~ -0.4).

Theoryof zeolite supralattices: Se in zeolite Linde type A

We study theoretically properties of Se clusters in zeolites, andchoose zeolite Linde type A (LTA) as a prototype system. Thegeometries of free-space Se clusters are first determined, and wereport the energetics and electronic and vibrational properties ofthese clusters. The work on clusters includes an investigation ofthe energetics of C3-C1 defect formation in Se rings andchains. The electronic properties of two Se crystalline polymorphs,trigonal Se and α-monoclinic Se, are also determined.Electronic and vibrational properties of the zeolite LTA areinvestigated. Next we investigate the electronic and opticalproperties of ring-like Se clusters inside the large α-cagesof LTA. We find that Se clusters inside cages of silaceous LTA havevery little interaction with the zeolite, and that the HOMO-LUMOgaps (HOMO standing for highest occupied molecular orbital and LUMOfor lowest unoccupied molecular orbital) are nearly those of theisolated cluster. The HOMO-LUMO gaps of Se6, Se8, andSe12 are found to be similar, which makes it difficult toidentify them experimentally by absorption spectroscopy. We findthat the zeolite/Se8 nanocomposite is lower in energy than thetwo separated systems. We also investigate two types of infinitechain encapsulated in LTA. Finally, we carry out finite-temperaturemolecular dynamics simulations for an encapsulated Se12cluster, which shows cluster melting and formation of nanoscale Sedroplets in the α-cages of LTA.

Mass spectrometric study of Ag’s clustering ability with O, S and Se

Samples Ag/O/S, Ag/O/Se and Ag/S/Se were ablated directly by laser to produce clusters. The detection was performed with a time-of-flight mass spectrometer (TOF MS). The cluster ions produced had the following distribution: Ag/S binary cluster ions for sample Ag/O/S, Ag/Se binary cluster ions for sample Ag/O/Se, and Ag/Se binary cluster ions and Ag/S/Se ternary cluster ions as the main products for sample Ag/S/Se. Laser double ablation reactor was used to study the reaction between Ag/S(Se, O) binary clusters and Se(S) homoclusters. The experiments show that Se clusters can replace S and O in Ag/S and Ag/O clusters while S clusters can replace O in Ag/O clusters. This fact indicates the sequence of the clustering ability of Ag presented as follows: Ag/Se > Ag/S > Ag/O, which was also proved by theoretical calculation. The reason of this tendency was considered to be the different abilities of the elements’ polarization, which acts on the stability of clusters.

Surfactant Templated Assembly of Hexagonal Mesostructured Semiconductors Based on [Ge4Q10]4- (Q=S, Se) and Pd2+ and Pt2+ ions

ABSTRACTMesostructured semiconducting non-oxidic materials were prepared by linking [Ge4Q10]4-(Q=S, Se) clusters with the square planar noble metal cations of Pd2+ and Pt 2+ in the presence of cetylpyridinium surfactant molecules. The use of Pt2+ afforded materials with exceptionally high hexagonal pore order similar to those of high quality silica MCM-41. These materials are semiconductors with energy band gap in the range 1.8&lt;Eg&lt;2.5 eV.

Surfactant Templated Assembly of Hexagonal Mesostructured Semiconductors Based on [Ge4Q10]4- (Q=S, Se) and Pd2+ and Pt2+ ions.

ABSTRACT4-Mesostructured semiconducting non-oxidic materials were prepared by linking [Ge4Q10]4- (Q=S, Se) clusters with the square planar noble metal cations of Pd2+ and Pt2+ in the presence of cetylpyridinium surfactant molecules. The use of Pt2+ afforded materials with exceptionally high hexagonal pore order similar to those of high quality silica MCM-41. These materials are semiconductors with energy band gap in the range 1.8&lt;Eg&lt;2.5 eV.

Hexagonal mesostructured chalcogenide frameworks formed by linking [Ge4Q10]4− (Q = S, Se) clusters with Sb3+ and Sn4+

The synthesis and properties of new hexagonal ordered mesostuctured phases of framework containing adamantane [Ge4Q10]4− (Q = S, Se) clusters linked by Sb3+ and Sn4+ ions and templated with cetylpyridinium surfactant are reported.

Supramolecular Assembly of Hexagonal Mesostructured Germanium Sulfide and Selenide Nanocomposites Incorporating the Biologically Relevant Fe(4)S(4) Cluster Financial support from the National Science Foundation (CHE 99-03706, Chemistry Research Group) is gratefully acknowledged. Part of this work was carried out using the facilities of the Center for Electron Optics of Michigan State University.

Stable hybrid systems are generated by the reaction of adamantane [Ge4Q10]4− (Q=S, Se) clusters with substitutionally labile [Fe4S4Cl4]2− clusters templated by cationic surfactant molecules. The resulting novel hexagonal mesophases have parallel pore tunnels (see schematic representation) with separations of ≈35 Å, which may find applications in shape-selective electro- and photocatalysis, artificial photosynthesis, and biosensors.

Interchain Interaction in Semiconducting Liquid and Amorphous Selenium

Semiconducting liquid and amorphous selenium (l-Se,a-Se), which are representative disordered materials, are regarded as assemblies of two-coordinated chains. The atomic and electronic structures have been studied theoretically using ab initio and tight-binding molecular-dynamics (MD) or Monte Carlo methods. However, the former is difficult to apply to a large system size, and the latter is difficult to deal with intermolecular interactions appropriately. A classical chain model 1) has been also applied to study the atomic structure and dynamics. It assumes, however, an isotropic Lenard-Jones potential (LJ) as the interaction between the nonbonded atoms. Since each Se atom in the chains has a p-lone pair (LP), the interaction between the nonbonded atoms is anisotropic in practice. We construct a chain model including the anisotropy based on an ab initio molecular-orbital (MO) method, and obtain the structural samples by MD simulations. We compute the intra and intermolecular potential of Se clusters using an ab initio MO method (MP2/CEP-31G(d)). Potentials of an isolated Se chain (HSe10H) are calculated around the optimized geometry, and fitted to a valence force field model. The bending force constant is similar to that of Ref. 1). The barrier of internal rotation is 163 meV that is about 2.4 times as large as the corresponding value of Ref. 1). In our MD simulations, we scale the calculated stretching force constant by 0.7 to explain the experimental stretching modes. Intermolecular potentials are calculated for (HSe3H)2 dimer. They are closely related to the way of facing between the LPs on the central Se atom in the monomer, which causes 0.3 A change of the atomic effective radius on the interaction. The interaction potential of the pair of nonbonded atoms i and j is represented by

On the formation and nature of nanometer size clusters on the surface of ZnSe epilayers

ZnSe epilayers were grown on GaAs (001) substrates by metal organic chemical vapor phase deposition, using different group VI–II precursor flow ratios. Atomic force microscopy (AFM) examinations of their surface show that epilayers grown with a high VI/II ratio are not as stable as those grown with a low ratio. When exposed to air, Se clusters would appear and grow on the surface of the unstable epilayers. The ripening process could take as long as 50 days at room temperature. Secondary electron and cathodoluminescence images indicate that the clusters are more likely to emerge from areas of high defect density. Moreover, AFM topographic images of epilayers at an intermediate stage of ripening suggest that clusters near surface depressions would grow in size at the expense of others. By using a low accelerating voltage and allowing the clusters to grow to a size larger than the electron interaction volume, we used energy dispersive x-ray analysis to show that the clusters are made up entirely of Se.

Luminescence due to the indirect band-gap transition activated by the inter-valence transition of Se clusters confined in 13X zeolite

Nanoclusters of Se have been incorporated in the supercage of 13X zeolite. Raman scattering results have established that the Se clusters confined in 13X mainly take the form of chain molecules. Intensive near-infra-red light emission at 1.54 eV is recorded in the photoluminescence measurements. Such a low-energy band-gap transition has not been reported in other forms of selenium. The luminescence is assigned to the exciton of the indirect optical transition between lone-pair and σ * bands of Se chains. The FTIR measurements show an inter-valence-gap transition at 0.38 eV (3064 cm −1) which activates the indirect band-gap transition. The blue shift of the excitonic energy of the indirect band in Se/13X, from that of the bulk t-Se, is very small.

Theoretical studies of Se clusters in zeolite LTA

We study theoretically the properties of small Se clusters in zeolite Linde type A (LTA). We first study free space Se clusters to establish which geometries are preferred without the influence of the zeolite cage. We next investigate the electronic and optical properties of ring-like Se clusters inside the large α-cages of LTA. We find that Se clusters inside cages of silacious LTA have very little interaction with the zeolite, and that the HOMO-LUMO gaps are very nearly those of the isolated cluster. We find that the zeolite/Se 8 nanocomposite is lower in energy than the two separated systems.

Dynamics of glass transition of bulk a-Se and Se clusters incorporated into zeolites

The dynamics of the glass transition of a-Se (both bulk and clusters in zeolites) has been investigated with heat capacity data obtained by differential scanning calorimetry experiments. The heat capacity data were analyzed by the Moynihan formulation with a Kohlrausch–Williams–Watts relaxation function. We found that the confinement of a-Se into the restricted pores in zeolites leads to an increase in the glass transition temperature, a decrease in the activation energy, and a decrease in the nonexponentiality, β. These results suggest that the thermokinetic dimension of the structural relaxation of Se becomes lower owing to the absence of inter-cluster (ring or chain) interaction due to the geometrical restriction in zeolites. These observations are compared with the results observed when the network dimension of bulk a-Se is increased by the doping of three-coordinated As atoms up to 10 at.%.

Nanoclusters in Zeolite

Nanoclusters of Se, Rb-Se, and Cs-Se were incorporated in the supercages of Y zeolite. Anomalous x-ray scattering established that the Se clusters take the form of weakly interacting chains with a disordered structure similar to that of amorphous Se, while the Rb-Se and Cs-Se clusters contain Se{sup 2{minus}}{sub 2} Zintl ions as in crystalline and liquid bulk phases. The Raman spectra of the nanoclusters exhibit features found in the bulk disordered phases. {copyright} {ital 1997} {ital The American Physical Society}

Structural phase stability of Se clusters in zeolites

The effect of confinement on the structural phase of Se clusters in zeolites (LTA and FAU) has been studied in the temperature range of 300 ∼ 550K by Raman scattering and differential scanning calorimetry. It has been found in Raman spectra that the structural unit and phase stability of Se clusters are greatly affected by the geometrical restriction in the zeolites. No clear phase transformation was observed by DSC measurements for Se clusters in LTA zeolites, while Se clusters in FAU zeolites exhibit a distinct endothermic peak which is ascribed to a glass transition. The glass transition temperature of amorphous Se (320 K) is remarkably elevated when Se rings/chains are incorporated in FAU zeolites; the higher glass transition temperature for the smaller window diameter of zeolites. We have also found that the activation energy for the glass transition is decreased drastically in the zeolites; the smaller activation energy for the smaller window size. The observations imply that the kinetics of the structural relaxation at the glass transition of Se is significantly influenced by the absence of inter-cluster interaction and that the thermokinetic dimension becomes lower owing to the geometrical restriction in the zeolites.

Structure, dynamics, and electronic structure of liquid Ag-Se alloys investigated by ab initio simulation.

Ab initio molecular-dynamics simulations have been used to investigate the structure, dynamics and electronic properties of the liquid alloy Ag(1-x)Se(x) at 1350 K and at the three compositions x=0.33, 0.42 and 0.65. The calculations are based on density-functional theory in the local density approximation and on the pseudopotential plane-wave method. The reliability of the simulations is confirmed by detailed comparisons with very recent neutron diffraction results for the partial structure factors and radial distribution functions (RDF) of the stoichiometric liquid Ag2Se. The simulations show a dramatic change of the Se-Se RDF with increasing Se content. This change is due to the formation of Se clusters bound by covalent bonds, the Se-Se bond length being almost the same as in pure c-Se and l-Se. The clusters are predominantly chain-like, but for higher x a large fraction of 3-fold coordinated Se atoms is also found. It is shown that the equilibrium fractions of Se present as isolated atoms and in clusters can be understood on a simple charge-balance model based on an ionic interpretation. The Ag and Se diffusion coefficients both increase with Se content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics reveals surprisingly short bond lifetimes of less than 1 ps. The changes in the density of states with composition arise directly from the formation of Se-Se covalent bonds. Results for the electronic conductivity obtained using the Kubo-Greenwood approximation are in adequate agreement with experiment for l-Ag2Se, but not for the high Se contents. Possible reasons for this are discussed.