Structural and electronic properties of transition-metal chalcogenides Mo_5S_4 nanowires

Transition-metal chalcogenide nanowires(TMCN) as a viable candidate for nanoscale applications have been attracting much attention for the last few decades. Starting from the rigid building block of M_6 octahedra(M = transition metal),depending on the way of connection between M_6 and decoration by chalcogenide atoms, multiple types of extended TMCN nanowires can be constructed based on some basic rules of backbone construction proposed here. Note that the well-known Chevrel-phase based M_6X_6 and M_6X_9(X = chalcogenide atom) nanowires, which are among our proposed structures, have been successfully synthesized by experiment and well studied. More interestingly, based on the construction principles, we predict three new structural phases(the cap, edge, and CE phases) of Mo_5S_4, one of which(the edge phase) has been obtained by top-down electron beam lithography on two-dimensional MoS_2, and the CE phase is yet to be synthesized but appears more stable than the edge phase. The stability of the new phases of Mo_5S_4 is further substantiated by crystal orbital overlapping population(COOP), phonon dispersion relation, and thermodynamic calculation. The barrier of the structural transition between different phases of Mo_5S_4 shows that it is very likely to realize an conversion from the experimentally achieved structure to the most stable CE phase. The calculated electronic structure shows an interesting band nesting between valence and conduction bands of the CE Mo_5S_4 phase, suggesting that such a nanowire structure can be well suitable for optoelectronic sensor applications.     

New type of charged defect in amorphous chalcogenides

We report on density-functional-based tight-binding simulations of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As(4)](-)-[S(3)](+), consisting of a fourfold coordinated arsenic site in a seesaw configuration and a threefold coordinated sulfur site in a near-planar trigonal configuration, was found in several models. The valence-alternation pairs [S(3)](+)-S-1 are converted into [As(4)](-)-[S(3)](+) pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photodarkening in these materials.     

Magnetic and electronic properties of NiAs-type chromium chalcogenides

We have computed spin-dependent energy bands, spin moments and density of states of NiAs-type CrX (X=S, Se and Te) chalcogenides using linear combination of atomic orbitals method within density functional theory as well as full potential augmented plane wave method. In addition, magnetic properties have also been computed using spin polarized relativistic Korringa-Kohn-Rostoker method. We have also obtained the first ever theoretical electron momentum densities of CrX compounds considering linear combination of atomic orbitals and compared the results with the isotropic Compton profiles measured using 20 Ci ¹³⁷Cs Compton spectrometer. The Fermi surface topology and magnetic properties are discussed in terms of majority and minority energy bands and density of states. In addition, to highlight the role of Cr (3d) electrons in such type of chalcogenides, we have also reported the magnetic Compton profile of CrTe using the Korringa-Kohn-Rostoker method.     

Optical properties and electronic structure of europium chalcogenides

The normal incidence reflectivity of europium chalcogenide single crystals and of Gd doped EuO has been measured at room temperature in the spectral region from 250 μm to 12 eV and has been analyzed in terms of the optical constants. In addition, in a reduced spectral region from 0.5 to 6 eV, the optical constants have been evaluated by means of a polarimetric method, as well above as below the magnetic ordering temperature. To enhance the resolution of the magneto-optical transitions, a modulation technique has been applied with a magnetic field as modulating parameter. The Kramers-Kronig relation has been used to analyze the normal incidence reflectivity and the magnetoreflectance spectra in terms of, respectively, the optical constants and the changes in the real and imaginary part of the dielectric response function. For Gd doped EuO the Kramers-Kronig analysis has revealed plasmon and coupled plasmon-phonon modes. The interband transitions of the europium chalcogenides are discussed within the framework of recent APW and OPW energy band calculations. On the other hand we have derived an energy level scheme of the europium chalcogenides from our optical data.     

On the electronic structure of Ag chalcogenides

The electronic structure of Ag chalcogenides in the α phase, which exhibit an interesting, electronic semiconducting behaviour as well as the fast ion transport, is discussed on the basis of an energy band structure calculation. As a simplest way of simulating the effect of the Ag ions on the electronic states, some hypothetical crystalline compounds are constructed such as the perovskite, the sodium chloride and the flourite structures. The absolute magnitude of the calculated conduction electron effective mass is quite small irrespectively of the structures, about 10% of the free-electron mass, in semiquantitative agreement with experiments. A deviation from an effective mass approximation near the conduction band bottom is found to be appreciable, and to explain reasonably well experimental results. An origin of these features of the conduction band is a rather strong hybridization of the Ag 5s band and the chalcogen s band. The calculation also shows that the hybridization of the Ag 4d band and the chalcogen p band can affect the absolute magnitude of the hole effective mass appreciably, and that the energy band gap depends sensitively on these s-s and d-p hybridization effects.     

Chemical bond approach to determining conductivity band gaps in amorphous chalcogenides and pnictides

The minimum energy required for the formation of conjugate pair of charged defects is found to be approximately equal to the experimental activation energy for d.c. conductivity in a number of amorphous chalcoganides and pnictides. This observation implies that the defect pair formation energy represents an intrinsic gap for transport in amorphous chalcogenides.     

Quasistationary current contributions in electronic devices

In an electronic device, the current supplied to the electrodes is related to different types of processes inside the device: current density, change in spontaneous polarization, and change in dielectric properties. Two expressions for the electrode current are derived: one is based on the time derivative of the Shockley-Ramo theorem, the other on the time derivative of the dielectric tensor. This result is illustrated for a switching liquid crystal device and a two-dimensional flux tube.     

Elementary excitations,electronic correlation functions and sum rule in noble-metal chalcogenides

Noble-metal chalcogenides are known as both electronic and ionic conductors. Physics in superionic conductors is investigated on the basis of the idea of elementary excitations. First, the semiconducting properties of noble-metal chalcogenides are investigated by preparing the full Hamiltonian for conduction electrons and phonons. The influence of electron interactions on the longitudinal acoustic wave frequencies and the matrix element for the electron–phonon interaction are investigated. Three cases of ω >> kυ_F, ω < kυ_F and ω = 0 are investigated for polar semiconductors like noble-metal chalcogenides. Next, the structure factors, and the f-sum rule of conductivity are investigated in silver chalcogenides by making use of a continuum model. The structure factors S_ee, S_Ae and S_Be with which electrons are connected are expressed symmetrically in terms of the structure factors S_AA, S_BB and S_AB of ions in the long-wavelength limit using the fluctuation–dissipation theorem and the Kramers–Kronig relation. The obtained conductivity satisfies the f-sum rule.     

Structural amorphous steels

Recent advancement in bulk metallic glasses, whose properties are usually superior to their crystalline counterparts, has stimulated great interest in fabricating bulk amorphous steels. While a great deal of effort has been devoted to this field, the fabrication of structural amorphous steels with large cross sections has remained an alchemist's dream because of the limited glass-forming ability (GFA) of these materials. Here we report the discovery of structural amorphous steels that can be cast into glasses with large cross-section sizes using conventional drop-casting methods. These new steels showed interesting physical, magnetic, and mechanical properties, along with high thermal stability. The underlying mechanisms for the superior GFA of these materials are discussed.     

Structural and electronic contributions to the bandgap bowing of (In,Ga)P alloys

The variation of atomic configurations and their corresponding structural parameters, such as first nearest neighbor distances, is a characteristic feature of ternary semiconductor alloys with zincblende structure. It has a strong influence on important material properties, most prominently the bandgap energy, and can contribute to a nonlinear behavior with changing alloy composition. Using (In,Ga)P as a model system, the atomic-scale structure has been modeled for all possible first nearest neighbor configurations based on experimentally determined structural parameters. While the average position of the P anion corresponds to the ideal lattice site, the average distance from this ideal lattice site does not vanish even in the random alloy. Based on this average anion displacement, the contribution to the bandgap bowing caused by structural relaxation of the alloy was calculated over the whole compositional range. Furthermore, the bowing contribution arising from the overall change of the In-P and Ga-P distances with respect to the binary values was determined. Thus, a clear distinction between the bandgap bowing caused by structural effects on the one hand and by charge redistribution on the other hand is possible.     

Analysis of the electronic polarizabilities of ions in alkaline earth chalcogenides

Electronic polarizabilities of ions in II–VI crystals recently derived by Jain et al. are criticised. It has been demonstrated that the polarizabilities derived by Boswarva in alkaline earth chalcogenides on the basis of the additivity rule using crystal refraction data are consistent with those derived in the present paper employing a theoretical model originally developed by Ruffa.     

Hydrogen diffusion and electronic metastability in amorphous silicon

Hydrogen diffusion and its role in the many electronic metastability phenomena in hydrogenated amorphous silicon (a-Si:H) is reviewed. A-Si:H contains about 10 at% hydrogen, most of which is bonded to silicon. The hydrogen diffuses at relatively low temperatures by releasing hydrogen from the Si-H bonds into interstitial sites. The reactions of hydrogen with the silicon dangling bonds and the weak bonds provide a hydrogen-mediated mechanism for electron-structural interactions, which are manifested as electronic metastability. The annealing of light-induced defects, the equilibration of defects and dopants, the stretched exponential relaxation kinetics, and the atomic structure formed during growth, are all attributed to hydrogen diffusion.     

Investigation of electronic properties of crystalline arsenic chalcogenides: Theory and experiment

The electronic structure of crystalline As₂S₃ and As₂Se₃ has been calculated in this paper. We present the energy bands, density of states (DOS) and the Compton profiles using the linear combination of atomic orbitals (LCAO) scheme based on the density functional theory (DFT). From the calculated total and partial density of states it is seen that the lone-pair p-states of sulphur/selenium contribute closest to the Fermi energy level. To interpret the theoretical data on the Compton line shape, we have measured the Compton profiles on a 100 mCi ²⁴¹Am spectrometer. It is seen that the density functional theory within generalised gradient approximation gives a slightly better agreement with the experimental momentum densities. The nature of chemical bonding in arsenic chalcogenides is studied using Mulliken's population analysis and the experimentally measured equal-valence-electron-density profiles; As₂S₃ is found to be more ionic compared to As₂Se₃.     

Prominent higher-order contributions to electronic recombination

Intershell higher-order (HO) electronic recombination is reported for highly charged Ar, Fe, and Kr ions, where simultaneous excitation of one K-shell electron and one or two additional L-shell electrons occurs upon resonant capture of a free electron. For the mid-Z region, HO resonance strengths grow unexpectedly strong with decreasing atomic number Z (∝Z(-4)), such that, for Ar ions the 2nd-order overwhelms the 1st-order resonant recombination considerably. The experimental findings are confirmed by multiconfiguration Dirac-Fock calculations including hitherto neglected excitation pathways.     

Photo-induced nonthermal melting of amorphous chalcogenides observed by pump and probe x-ray absorption spectroscopy

A new kind of melting phenomenon which is not based on thermal excitation has been observed. X-ray absorption spectroscopy (XAS) experiments under optical pumping provide a “snap-shot” information on the local structure under excitation. We have studied the local structure of chalcogenide glasses such as vitrious selenium and As₂Se₃ under optical excitation and confirmed the local melting phenomenon under light illumination at low temperature. The photo-induced nonthermal melting (PNM) in chalcogenide glasses is interpreted as the result of pairing of excited lone pair electrons during the illumination. Trapped states in this photo-assisted metastable phase result in a local structural disorder which is partially quenched at room temperature. The increased short-range disorder causing Coulomb repulsion is the origin of red shift of the absorption coefficient known as the photodarkening effect. We found that the bond alternation of chalcogens occurs during the photo-excitation.