Heteropolar Bonds Research Articles

Evolution of structural disorder in amorphous GeSeS thin films by thickness variation

Amorphous GeSeS thin films are deposited for five different thicknesses using thermal evaporation technique and their microstructures are analyzed. The oscillations obtained in the optical transmittance spectra are resulted from the interference between the air-film and film-substrate interfaces. The observed decrease in the optical band gap with increase in the thickness of the film is associated with the change in the bonds formed in the structure of the network. Structural disorder like unsaturated bonds (dangling bonds) are studied by means of Tauc parameter (B1/2), Urbach energy (EU), and thermal diffusivity. The formation of heteropolar bonds become enormous for higher thickness of GeSeS thin films. From Raman spectra the bonds associated with symmetric stretching and tetrahedral units are discussed. Phonon–phonon scattering is the dominating mechanism which determines the phonon mean free path and hence the measured variation in thermal diffusivity.

The feasibility of Sn, In, or Al doped ZnSb thin film as candidates for phase change material

The potentials of Sn, In, or Al doped ZnSb thin film as candidates for phase change materials have been studied in this paper. It was found that the Zn-Sb bonds were broken by the addition of the dopants and homopolar Zn-Zn bonds and other heteropolar bonds, such as Sn-Sb, In-Sb, and Al-Sb, were subsequently formed. The existence of homopolar Sn-Sn and In-In bonds in Zn50Sb36Sn14 and Zn41Sb36In23 films, but no any Al-Al bonds in Zn35Sb30Al35 film, was confirmed. All these three amorphous films crystallize with the appearance of crystalline rhombohedral Sb phase, and Zn35Sb30Al35 film even exhibits a second crystallization process where the crystalline AlSb phase is separated out. The Zn35Sb30Al35 film exhibits a reversible phase change behavior with a larger Ea (∼4.7 eV), higher Tc (∼245 °C), better 10-yr data retention (∼182 °C), less incubation time (20 ns at 70 mW), and faster complete crystallization speed (45 ns at 70 mW). Moreover, Zn35Sb30Al35 film shows the smaller root-mean-square (1.654 nm) and less change of the thickness between amorphous and crystalline state (7.5%), which are in favor of improving the reliability of phase change memory.

8 - N rule and chemical bonding in main-group MgAgAs-type compounds.

The chemical bonding of main-group MgAgAs-type compounds is analyzed with quantum chemical direct-space techniques. A new bonding concept is developed that unites the former ionic bonding and polyanionic network models. Polar and nonpolar contributions to the bonding are extracted by the combined analysis of electron density and electron localizability. A direct-space representation of the 8 - N rule is introduced. In this approach, the anions' heteropolar bonds are treated as a superposition of covalent (nonpolar) and lone-pair closed-shell (polar) contributions. The relation between covalent (nonpolar) and lone-pair (polar) character is obtained with the ELI-D/QTAIM basin intersection technique. This ratio depends on the constituting elements. On basis of this approach, MgAgAs-type compounds are compared with Zintl phases, where covalent bonds and lone pairs are spatially separated.

Experimental observations of photo-induced bond conversions in GeAsS thin films

Photo-induced anisotropy phenomena have been investigated in Ge25As30S45 chalcogenide glassy thin-films prepared by the electron beam evaporation technique. Thin-films of thicknesses ranging from 3 to 7μm were prepared, annealed slightly below the glass transition temperature Tg and characterized by means of optical transmission, thermal analysis, elemental microanalysis through energy dispersive X-ray spectroscopy and by micro-Raman spectroscopy. Photo-induced dichroism (PID) was studied using band gap 514nm laser excitation. Its kinetics was studied by observing the transmission and reflection of the film at different excitation intensities and temperatures. The conversion of homopolar (Ge–Ge, As–As) toward heteropolar (Ge–S, As–S) bonds was confirmed by polarized micro-Raman spectroscopic studies.A phenomenological unipolar model, that suggests the presence of photo induced bond changes through an energetic barrier (estimated at 14kJ/mol), is proposed to explain the experimentally observed non-monotonic behavior of PID.

Evidence of homopolar bonds in chemically stoichiometric GexAsySe1−x−y glasses

We prepared five samples of chemically stoichiometric GexAsySe1−x−y glasses in order to examine whether homopolar bonds exist in these glasses. It is generally accepted that the structure of these chemically stoichiometric glasses should be dominated by GeSe4/2 tetrahedra and AsSe3/2 pyramids with a negligible quantity of homopolar bonds. Analysis of Raman scattering and X-ray photoelectron spectra of the glasses indicates that, while glasses located at the center of the glass-forming region primarily contain heteropolar bonds, a considerable number of homopolar bonds exists in glasses with extremely high Ge or As concentrations. This demonstrates that the formation of homopolar bonds might be intrinsic to chemically stoichiometric chalcogenide glasses.

Optical, photo-acoustic and electrical switching studies of amorphous GeS2 thin films

This paper reports optical, photo-acoustic and electrical switching investigations of GeS2 amorphous thin films of different thicknesses, deposited on glass substrates in vacuum. The Tauc parameter (B 1/2) and Urbach energy (E U) have been determined from the transmittance spectra, to understand the changes in structural disorder; it is found that B 1/2 increases whereas E U decreases as the thickness of the films increases. Based on the results, it is suggested that bond re-arrangement, i.e. transformation from homopolar bonds to heteropolar bonds, takes place with increase in thickness. The thermal diffusivity values of GeS2 thin films also show the presence of a chemically ordered network in the GeS2 thin films. Further, it is found that these films exhibit memory-type electrical switching. The observed variation in the switching voltages has been understood on the basis of increase in chemical order.

Correlation between Structural and Physical Properties in Ge–Sb–Se Glasses

Three groups of Ge–Sb–Se glasses with compositions GexSb10Se90–x, GexSb15Se85–x, and GexSb20Se80–x have been systematically studied with the aim of understanding the role of chemical composition and mean coordination number (MCN) in determining their structural and physical properties. For each group of glasses, it was found that the optical bandgap increases and the refractive index decreases with increasing Ge concentration up to a transition point which corresponds to glasses that have chemically stoichiometric compositions. Raman spectra were measured and decomposed into different structural units. While the relative number of the heteropolar bonds changes in a reasonable manner with chemical composition, the evolution of the optical bandgap and refractive index correlated closely with the number of the homopolar bonds, suggesting that the band-tails formed by homopolar bonds could reduce the optical bandgap. On the other hand, the transitions at the chemically stoichiometric compositions could be attributed to “demixing” of networks above the chemical thresholds. These transition thresholds in the three groups of glasses demonstrated that the chemical composition has significant effects on the physical properties in the Ge–Sb–Se system.

Electron irradiation induced reduction of the permittivity in chalcogenide glass (As2S3) thin film

In this paper, we investigate the effect of electron beam irradiation on the dielectric properties of As2S3 chalcogenide glass. By means of low-loss electron energy loss spectroscopy, we derive the permittivity function, its dispersive relation, and calculate the refractive index and absorption coefficients under the constant permeability approximation. The measured and calculated results show a heretofore unseen phenomenon: a reduction in the permittivity of ≥40%. Consequently a reduction of the refractive index of 20%, hence, suggests a conspicuous change in the optical properties of the material under irradiation with a 300 keV electron beam. The plausible physical phenomena leading to these observations are discussed in terms of the homopolar and heteropolar bond dynamics under high energy absorption. The reported phenomena, exhibited by As2S3-thin film, can be crucial for the development of photonics integrated circuits using electron beam irradiation method.

Structure prediction and targeted synthesis: A new NanN2 diazenide crystalline structure

Significant progress in theoretical and computational techniques for predicting stable crystal structures has recently begun to stimulate targeted synthesis of such predicted structures. Using a global space-group optimization (GSGO) approach that locates ground-state structures and stable stoichiometries from first-principles energy functionals by objectively starting from randomly selected lattice vectors and random atomic positions, we predict the first alkali diazenide compound Na(n)N(2), manifesting homopolar N-N bonds. The previously predicted Na(3)N structure manifests only heteropolar Na-N bonds and has positive formation enthalpy. It was calculated based on local Hartree-Fock relaxation of a fixed-structure type (Li(3)P-type) found by searching an electrostatic point-ion model. Synthesis attempts of this positive ΔH compound using activated nitrogen yielded another structure (anti-ReO(3)-type). The currently predicted (negative formation enthalpy) diazenide Na(2)N(2) completes the series of previously known BaN(2) and SrN(2) diazenides where the metal sublattice transfers charge into the empty N(2) Π(g) orbital. This points to a new class of alkali nitrides with fundamentally different bonding, i.e., homopolar rather than heteropolar bonds and, at the same time, illustrates some of the crucial subtleties and pitfalls involved in structure predictions versus planned synthesis. Attempts at synthesis of the stable Na(2)N(2) predicted here will be interesting.

Single-oscillator description of spectral dependence of the refractive index of new chalcogenide Ge-Sb-(S,Te) glasses

New quaternary telluride glasses based on Ge-S system with addition of Sb and partial substitution of S by Te have been synthesized and their optical properties in dependence on composition have been studied by spectroscopic ellipsometry in 400–820 nm spectral range. The dispersion behavior of the refractive index of these glasses with composition of GexSb40−xS50Te10 and GexSb40-xS55Te5 (x= 10, 20, and 27) was examined applying the single-oscillator theory and using the Wempel-DiDomenico's model. From the data analysis the optical constants (n, k, ε), optical bandgap energy Eog and film thickness, as well as the oscillator energies E0 and Ed have been determined. The observed increase of E0 and decrease of Ed with increasing the Ge content or decreasing the Te content are explained by the change of amount and strength of heteropolar chemical bonds and by the less covalent character of chemical bonding in these materials. Correlation between the oscillator energy E0 and optical band gap energy Eog is established.

Origin of the size-dependence of the polarizability per atom in heterogeneous clusters: The case of AlP clusters

An analysis of the atomic polarizabilities α in stoichiometric aluminum phosphide clusters, computed at the MP2 and density functional theory (DFT) levels, the latter using the B3LYP functional, and partitioned using the classic and iterative versions of the Hirshfeld method, is presented. Two sets of clusters are examined: the ground-state Al(n)P(n) clusters (n=2-9) and the prolate clusters (Al(2)P(2))(N) and (Al(3)P(3))(N) (N≤6). In the ground-state clusters, the mean polarizability per atom, i.e., α/2n, decreases with the cluster size but shows peaks at n=5 and at n=7. We demonstrate that these peaks can be explained by a large polarizability of the Al atoms and by a low polarizability of the P atoms in Al(5)P(5) and Al(7)P(7) due to the presence of homopolar bonds in these clusters. We show indeed that the polarizability of an atom within an Al(n)P(n) cluster depends on the cluster size and the heteropolarity of the bonds it forms within the cluster, i.e., on the charges of the atoms. The polarizabilities of the fragments Al(2)P(2) and Al(3)P(3) in the prolate clusters were found to depend mainly on their location within the cluster. Finally, we show that the iterative Hirshfeld method is more suitable than the classic Hirshfeld method for describing the atomic polarizabilities and the atomic charges in clusters with heteropolar bonds, although both versions of the Hirshfeld method lead to similar conclusions.

Glass-formation in the GeSe 2–Sb 2Te 3–CdSe system

Chalcogenide glasses (ChGs) from the GeSe 2–Sb 2Te 3–CdSe system were synthesized and the glass-forming region was determined by visual examination, X-ray diffraction and electron microscope analyses. The basic physicochemical parameters, such as: density (d), microhardness (HV) and the phase transformation temperatures (glass-transition T g, crystallization T cr and melting T m) were measured. The Hruby's criterion (K G), compactness (C) and some thermomechanical characteristics, such as: volume (V h) and formation energy (E h) of microvoids in the glassy network, as well as the module of elasticity (E) were calculated. The density, microhardness and T g for the investigated ChGs are within the limits of 4.34–5.24 g cm − 3 , 791.8–1078.0 MPa and 509–598 K, respectively. The mean values of the total bond energy <E>, coordination number <Z>, bond energy of the average cross-linking per atom Ē c and that of the “remaining matrix” Ē rm were determined. The average heteropolar bond energy E hb and the degree of “cross-linking per atom” P were also calculated.

Photo induced optical changes in Sb/As 2S 3 multilayered film and (As 2S 3) 0.93Sb 0.07 film of equal thickness

The increase in optical band gap (photo bleaching) due to light illumination was studied at room temperature as well as at low (4.2 K) temperature for Sb/As 2S 3 multilayered film of 640 nm thickness by Fourier Transform Infrared Technique. The interdiffusion of Sb into As 2S 3 matrix results the formation of Sb–As 2S 3 ternary solid solutions which is explained by the change in optical band gap ( E g), absorption coefficient ( α), Tauc parameter ( B 1/2), Urbach edge ( E e). At the same time, photo darkening phenomena was observed in (As 2S 3) 0.93Sb 0.07 film of same thickness both at low and room temperatures. From our X-ray Photoelectron Spectroscopy measurements, we are able to show that some of the As–As, S–S and Sb–Sb bonds are converted into As–S and S–Sb bonds in case of multilayers. We found that the energetically favoured heteropolar bond formation take place by a phonon-assisted mechanism using the lone pair π electrons of S 2 0. But in case of (As 2S 3) 0.93Sb 0.07 film, the homopolar bonds are playing a major role.

Toward a physical understanding of electron-sharing two-center bonds. II. Pseudo-potential based analysis of diatomic molecules

A dozen homo- and heteropolar bonds in a series of diatomic molecules are analyzed by an energy partitioning based on the concepts developed in part I of this series (J Comput Chem 28:411–422, 2007). The different bond types are characterized by various physical contributions to the bond energy, namely classical potential interaction, quantum–mechanical orbital interference, and bond stabilizations by atomic configurational promotion, radial deformation, and angular polarization. The effects of the atomic cores are accounted for by means of pseudo-potential operators. Different atomic cores cause specific bond differences. The various bonding mechanisms can be characterized by several parameters. They describe the quantum–mechanical reduction in the ‘electronic kinetic energy pressure’ due to delocalization (sharing interference) and the increase in the ‘molecular potential energy pull’ due to overlap of atomic electron densities with adjacent atoms in the molecule. In addition, there are one- and two-electron relaxations whose distinctive features depend on the nature of the cores.

Thermal annealing of arsenic tri-sulphide thin film and its influence on device performance

Arsenic tri-sulphide (As2S3) thin film waveguides have been used successfully as nonlinear optical devices for all-optical signal processors. For such devices, low propagation loss is vital if high performance is to be obtained. In this study, thermal annealing was employed not only to stabilize the physical properties of the films, but also to reduce the sources of light attenuation in the as-deposited material. Here we investigated heat-induced changes to the microstructure and optical properties of As2S3 thin films and, based on this information, determined the best annealing conditions. The refractive index of the films rises with annealing due to thermal densification and increased heteropolar bond density. The growth of surface roughness and thermal stress in the film, however, limits the annealing temperature to ∼130 °C. We fabricated and analyzed waveguides produced from as-deposited and annealed films and found that the propagation loss of the guides dropped by ∼0.2 dB/cm as a result of appropriate annealing. Rayleigh scattering and absorption from defects associated with phase separation, homopolar bonds, voids, and dangling bonds in the as-deposited film are shown to contribute to the higher light attenuation in unannealed films.

Thermal and optical analysis of Te-substituted Sn–Sb–Se chalcogenide semiconductors

Bulk amorphous samples of Te-substituted Sn10Sb20Se70−XTeX (0≤X≤12) were prepared using a melt quenching technique. Calorimetric studies of the samples were performed using differential scanning calorimetry (DSC) and the glass transition temperature and crystallization temperature were evaluated from DSC scans. The glass transition temperature Tg exhibits a sharp decrease for small Te substitution of X=2, thereafter increases with increase in Te content up to X=10, and then decreases for further Te substitution. The apparent activation energy for glass transition and the activation energy for crystallization were calculated using Kissinger, modified Kissinger, and Matusita equations. The change in glass transition temperature Tg has been explained based on the bond formation energy of different heteropolar bonds. The optical band gap of thermally evaporated thin films of Sn10Sb20Se70−XTeX (0≤X≤12) was calculated from reflectance and transmittance data. The optical band gap variation with tellurium content exhibits a sharp decrease for an initial tellurium substitution of X=2 similar to that of the glass transition temperature and thereafter a peak is observed in optical band gap around X=4 composition.

Laser induced changes on a-Ga50Se50 thin films

This paper reports on the laser induced changes on the optical properties of a-Ga50Se50 thin films prepared by the thermal evaporation technique under vacuum. Thin film samples, on a glass substrate, were exposed to laser light of wavelength λ = 532 nm for different exposure times, tE (tE = 0, 500, 1500 and 3000 s). Optical parameters such as refractive index, absorption coefficient, optical gap, extinction coefficient, real and imaginary dielectric constants, dielectric loss, optical conductivity of as-deposited thin film and their laser induced changes were studied at three different times of exposure. The mechanism of optical absorption follows the rule of the indirect allowed transition model proposed by Tauc and the optical band gap is calculated by Tauc's extrapolation. It is seen that after laser irradiation there is a shift of the optical absorption edge to larger photon energy. The value of the absorption coefficient of the GaSe thin film decreases on exposing the film to laser irradiation. A microscopic model in which heteropolar bonds are broken by absorption of high energy photons and new homopolar bonds are formed simultaneously has been used to describe the effect.

Charge-shift bonding and its manifestations in chemistry

Electron-pair bonding is a central chemical paradigm. Here, we show that alongside the two classical covalent and ionic bond families, there exists a class of charge-shift (CS) bonds wherein the electron-pair fluctuation has the dominant role. Charge-shift bonding shows large covalent-ionic resonance interaction energy, and depleted charge densities, and features typical to repulsive interactions, albeit the bond itself may well be strong. This bonding type is rooted in a mechanism whereby the bond achieves equilibrium defined by the virial ratio. The CS bonding territory involves, for example, homopolar bonds of compact electronegative and/or lone-pair-rich elements, heteropolar bonds of these elements among themselves and with other atoms (for example, the metalloids, such as silicon and germanium), hypercoordinated molecules, and bonds whose covalent components are weakened by exchange-repulsion strain (as in [1.1.1]propellane). Here, we discuss experimental manifestations of CS bonding in chemistry, and outline new directions demonstrating the portability of the new concept.

Effect of tellurium substitution in Sn10Sb20Se70−XTeX(0 &lt;X&lt; 12) amorphous chalcogenide system

The as-prepared samples of Sn 10 Sb 20 Se Te X chalcogenide system were amorphous as evidenced by X-ray diffraction and Differential scanning calorimetry studies. The different crystalline phases emerged in annealed Sn 10 Sb 20 Se Te X samples have been identified. Glass transition temperature T g of the as-prepared samples decreases sharply with tellurium substitution upto 2 at% and then it starts increasing upto 10 at% and decreases again on further substitution of tellurium. The change in glass transition temperature T g has been explained based on bond formation energy of different heteropolar bonds and crystalline phases obtained in the annealed samples with different tellurium contents.

Charges and bond energies in the analysis of quantum chemistry calculations

It is shown that a point charge model, supplemented by point dipoles and multipoles, can be defined for most molecular wave functions as a form of population analysis which has immediate physical significance. By analyzing the calculated total energies of a set of related molecules the concept of the bond energy of a heteropolar bond can be defined. A comparison between these and the corresponding quantities derived from experimental hydrogenation energies shows much better agreement than might have been expected from the correlation energies. These are two examples of the kind of theoretical analysis of ab initio calculations that is now urgently needed to obtain the maximum return from the computer investment. (auth)