Electronic Structure Energy Research Articles

Optical features of InAs quantum dots-in-a-well structures

Electronic energy structure and features of optical interband transitions of InAs quantum dots-in-a-well structures are studied via photoreflectance (PR) and temperature-dependent photoluminescence (PL) spectroscopy. InAs dots were grown with and without InGaAs capping layer and embedded in GaAs/AlAs quantum wells. Experimental results revealed that the 5 nm thick InGaAs capping layer significantly improves PR and PL signal intensity. Moreover, a shift of the quantum dot ground-state optical transition to lower energy by about 120 meV was observed. The red-shift of the ground-state transition is associated mainly with an increase of dot size and decrease of strain within quantum dots. Furthermore, the origin of PL intensity quenching with temperature is discussed in terms of electronic energy structure revealed from PR spectra and calculations performed within effective mass approximation.

Controllable electronic energy structure of size-controlled Cu2ZnSnS4nanoparticles prepared by a solution-based approach

Cu2ZnSnS4 nanoparticles with sizes of 2-5 nm, synthesized in hot organic solutions, exhibited size-dependent photoelectrochemical properties due to the quantum size effect. The potentials of the valence band edge and conduction band edge of the nanoparticles, experimentally determined by photoelectrochemical measurements, were shifted more positively and more negatively, respectively, with a decrease in particle size.

Composition-Dependent Photoelectrochemical Properties of Nonstoichiometric Cu2ZnSnS4 Nanoparticles

Size-quantized Cu–Zn–Sn–S quaternary semiconductor nanoparticles having various chemical compositions were synthesized by a solution-phase synthesis method. The chemical composition of the resulting nanoparticles could be controlled by changing the molar fractions of Cu, Zn, and Sn ions in the preparation. The resulting particles had an average particle diameter of 2–3 nm with a tetragonal crystal structure. Their energy gap was enlarged with an increase in Zn content in the particles in the range of 0.98–1.69 eV. Cu–Zn–Sn–S nanoparticles immobilized on ITO electrodes behaved as a p-type semiconductor, in which onset potential of the cathodic photocurrent was ca. +0.25 V vs Ag/AgCl, being independent of the chemical composition. The electronic energy structure of particles, determined from photoelectrochemical measurements, was composed of a valence band edge at ca. +0.25 V vs Ag/AgCl, the potential of which was not influenced by the chemical composition of particles, and a conduction band edge that was negatively shifted from −0.72 to −1.37 V vs Ag/AgCl by increasing Zn content in the particles. Interestingly, the incident photon-to-current efficiency (IPCE) depended on the chemical composition, especially on the Cu/Sn ratio in the particles, and the optimum value was obtained for nonstoichiometric Cu2ZnSnS4 nanoparticles with Cu- and Zn-poor composition.

Specific features of Yb3+ ions in electronic band energy structure and optical functions of RbNd(WO4)2 crystals: Synchrotron ellipsometry measurements and DFT simulations

Abstract The influence of 5% and 10% (in molar units) of ytterbium doping of RbNd(WO 4 ) 2 single crystals on the dielectric permittivity e in the spectral range of electronic excitations was studied. The corresponding differences of the pseudo-dielectric functions 〈 e ( E )〉 for pure and Yb-doped RbNd(WO 4 ) 2 were studied by spectroscopic ellipsometry method including VUV spectral range using synchrotron radiation light source. This method opens an opportunity for spectroscopic diagnostics of the rare earth dopants in crystals using information concerning the electronic inter-band optical transitions. To better understand the experimental results, for the first time the ab initio calculations of band structure, related properties and optical spectra of the centrosymmetric RbNd(WO 4 ) 2 single crystal were performed using the VASP code. The calculated real and imaginary parts of dielectric function dispersions e 1 ( E ) and e 2 ( E ) agree satisfactorily with our experimental results. It was established that differences of optical functions for pure and doped crystals may serve as a powerful tool for spectroscopic diagnostic of localized rare earth ions with respect to the strong inter-band transitions in the wide band dielectrics.

Dielectric Nano-Systems with Unique Optical Properties

The study of optical properties of Al2O3, SiO2, TiO2, and ZnO nanoparticles in weak optical fields is presented. We found that some nanoparticles have unique optical nonlinearity: The values of portions to nanoparticle refractive index and absorption coefficient increase to maxima, and then decrease to zero when the radiation intensity changes from 1 to 500 W/cm2. We estimated electron energy structures of nanoparticles and experimentally determined that such nonlinearity is directly related to peculiarities of the energy structure. We obtained a good fitting of theoretical results to the experimental ones (e.g. absorbance spectra of nanoparticles; the dependence of nanoparticle refractive index and absorption coefficient on radiation intensity).

Finite-Height Effect on Electron Energy Structure of Lead Salts Nanorods

The effect of the finite height of a cylindrical lead salt nanorod on its electronic structure is studied within the effective mass formalism. It is demonstrated that, for most practical purposes, it can be accounted for by sampling the sub-band dispersion dependencies of the infinite cylindrical quantum wire of the same radius at wave numbers kz = πnz/H, where H is the nanorod height and nz is an integer. However, under certain conditions, the otherwise degenerate electron energy levels will repel. A detailed account of these conditions is provided.

Unusual nonlinear optical properties of SiO2 nanocomposite in weak optical fields

We found that SiO2 nanoparticles dispersed in liquid polymethylsiloxane (PMS) exhibit unique optical nonlinearity in low-intensity visible radiation fields. The values of the nonlinear portion of the refractive index and absorption coefficient for nanoparticles increase to maxima, and then decrease to zero when the radiation intensity changes from 1 to 500 W/cm2. We estimated the electron energy structures for SiO2 nanoparticles dispersed in different liquid matrices (PMS, distilled water, and isopropyl alcohol) and explained the influence of the matrix permittivity value on this structure, as well as the optical properties of the nanoparticles.

Electronic energy structure and core-valence luminescence of CsCl crystal

Electronic energy structure and core-valence luminescence of CsCl crystal

XPS and NEXAFS Investigation of Electronic Energy Structure of Ti-Ni and TiNi-Cu Alloys

The electronic energy structure of Ti-Ni and TiNi-Cu alloys has been studied experimentally by XPS and NEXAFS and theoretically by the full-potential local-orbital minimum-basis code. The study has revealed formation in the valence band of the Ti50Ni50 alloy of a common d-band through hybridization of the d-states of Ti with those of Ni, which is localized within a narrow binding-energy interval. This brings about an intra-atomic redistribution of Ti electron density in the Ti50Ni50 alloy compared to the pure metal: decrease of the occupancy of the Ti d-shell is countered by an increase in the number of Ti p-electrons. The occupancy of the Ti d-shell in the TiNi-Cu alloys, where Ni atoms are partially substituted by Cu, and in the Ti-Ni alloys with an excess of the Ni is more than in the equiatomic Ti50Ni50 alloy. The occupancy of the Ni and Cu d-shells in the Ti-Ni and TiNi-Cu alloys is independent of the atomic composition. The Ti d-shell sensitivity to variation of the nearest atomic environment is apparently due to their relatively itinerant behavior in the studied alloys. The results obtained suggest that the increase of the stiffness of an interatomic bond in the TiNi-Cu alloys with increasing Cu content and in the Ti-Ni alloys with increasing Ni content is possibly associated with the filling of Ti bonding d-states.

Peculiarities of electron-energy structure of surface layers of porous silicon formed on p-type substrates

The atomic and electron structure of porous silicon surface layers were investigated by the methods of ultrasoft X-ray emission spectroscopy and X-ray absorption near edge structure spectroscopy. The thicknesses of the surface oxide layer and the degree of distortion of the silicon-oxygen tetrahedron in this layer were estimated. The thickness of the surface oxide layer lying on the amorphous layer, which covers the nanocrystals of porous silicon upon keeping for a year, exceeds severalfold the thickness of the natural oxidation of plates of monocrystalline silicon. The distortions of the silicon-oxygen tetrahedron—the main structural unit of silicon oxide—are accompanied by the strain of Si-O bonds and the increase in Si-O-Si bond angles.

Ethanol adsorption on the Si (111) surface: First principles study

Equilibrium atomic configurations and electron energy structure of ethanol adsorbed on the Si (111) surface are studied by the first principles density functional theory. Geometry optimization is performed by the total energy minimization method. Equilibrium atomic geometries of ethanol, both undissociated and dissociated, on the Si (111) surface are found and analysed. Reaction pathways and predicted transition states are discussed in comparison with available experimental data in terms of the feasibility of the reactions occurring. Analysis of atom and orbital resolved projected density of states indicates substantial modifications of the Si surface valence and conduction electron bands due to the adsorption of ethanol affecting the electronic properties of the surface.

Photoconduction in CDW conductors

Photoconduction study of quasi-1D conductors allows to distinguish between the single-particle and collective linear conduction, investigate the effect of screening on collective transport and obtain interesting new details of the electronic energy structure of pure and doped CDW conductors. Here we present results of photoconduction study in quasi-1D conductors o-TaS3, K0.3MoO3, and NbS3(I).

The influence of magnetic ordering on the electronic energy structure of CuFeS2

The modified LAPW+lo method with Wien2k software package was used to calculate the electronic energy structure of CuFeS2 as a component of chalcopyrite. CuFeS2 was found to be a conductor in the absence of antiferromagnetic ordering. Antiferromagnetic ordering in (001) layers leads to appearance of an energy gap and transforms CuFeS2 into a semiconductor. In the GGA+U approximation, Eg ≈ 0.75 eV was achieved for U = 6 eV, which is close to experimental value.

Electronic excitations and defect creation in wide-gap MgO and Lu3Al5O12 crystals irradiated with swift heavy ions

A comparative study of radiation effects in two groups of single crystals with an energy gap of about 8eV possessing drastically different lattice and electron energy structures − fcc MgO and Lu3Al5O12 with 160 atoms per a unit cell − has been performed using crystal irradiation with vacuum ultraviolet radiation, electrons, fast fission neutrons and, in particular, ∼2.2GeV uranium ions. In MgO with the absence of self-trapping for valence holes, the localization of holes near impurity ions or bivacancies (both as-grown or induced by a plastic stress) has been detected. In LuAG, the peculiarities of the motion of hole polarons and excitons, the radius of which is smaller than the size of a unit cell, have been revealed and analysed. The irradiation of MgO and LuAG with swift heavy ions providing an extremely high density of electronic excitations causes also the nonimpact creation of long-lived Frenkel defects in an oxygen sublattice.

Electron Energy Structure and Electrical Properties of Poly(p-phenylene vinylene) (PPV) with Gold Metal Nanoparticles

The electron energy structures and electrical properties of poly(p–phenylene vinylene) (PPV) and PPV/Au nanocomposite films were investigated to identify an effect of Au metal nanoparticles on a conjugated polymer. The current density in PPV/Au nanocomposite films was enhanced from an increase in the electron affinity with increasing Au nanoparticle content. The roughness of surface morphology was also observed with incorporation of Au nanoparticles. Then, an enhanced applied field could be developed at the thinner region of the film and an increase in the surface area with a resulting increase of electron injection, leading to an extra enhancement of the current.

The conductor-insulator transition as a result of antiferromagnetic ordering in monosulfides of 3d metals

The electron-energy structure of the structure of monosulfides of 3d metals is calculated using a modified LAPW + lo technique that allows for the antiferromagnetic ordering in different crystallographic layers. It is shown that the splitting of d-states of the transition metal with regard to the antiferromagnetic ordering in layers leads to a transition of VS, MnS, CoS, NiS, and FeS monosulfides in the troilite structure from metallic to semiconducting state.

In diffusion and electronic energy structure in polymer layers on In tin oxide

We have investigated the effect of In diffusion on the electronic structure of a polyethylenedioxythiophene-poly (styrenesulfonate) layer deposited on InSn xO 1 − x substrate by photoelectron spectroscopy. We observe additional states near the highest occupied molecular orbital of the polymer layer that are generated by In releases from the substrate. Also, we observe that In diffusion continues into an overlayer of a mixture of poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester. On the basis of the results of the numerical line shape analysis of In 3d 5/2 core level emission, we rule out the presence of metallic In clusters within the organic layers and suggest instead that In/sulfur compounds are present within the organic layers.

Sounding rocket study of two sequential auroral poleward boundary intensifications

[1] The Cascades-2 sounding rocket was launched on 20 March 2009 at 11:04:00 UT from the Poker Flat Research Range in Alaska, and flew across a series of poleward boundary intensifications (PBIs). The rocket initially crosses a diffuse arc, then crosses the equatorward extent of one PBI (a streamer), and finally crosses the initiation of a separate PBI before entering the polar cap. Each of the crossings have fundamentally different in situ electron energy and pitch angle structure, and different ground optics images of visible aurora. It is found that the diffuse arc has a quasi-static acceleration mechanism, and the intensification at the poleward boundary has an Alfvenic acceleration mechanism. The streamer shows characteristics of both types of acceleration. PFISR data provide ionospheric context for the rocket observations. Three THEMIS satellites in close conjunction with the rocket foot point show earthward flows and slight dipolarizations in the magnetotail associated with the in situ observations of PBI activity. An important goal of the Cascades-2 study is to bring together the different observational communities (rocket, ground cameras, ground radar, satellite) with the same case study. The Cascades-2 experiment is the first sounding rocket observation of a PBI sequence, enabling a detailed investigation of the electron signatures and optical aurora associated with various stages of a PBI sequence as it evolves from an Alfvenic to a more quasi-static structure.

Theoretical study and numerical verification of terahertz radiation emitted by carbon nanotubes

Based on the theory of simple tight-binding, the electronic energy structure of carbon nanotubes has been obtained under the quantitative periodic boundary condition of the carbon nanotubes. Terahertz radiation emitted by carbon nanotubes was demonstrated theoretically by adopting the reported experimental results of the interaction between electron and phonon. The numerical results verify that the carbon nanotubs can generate terahertz radiation and the reason of the terahertz oscillation was analyzed. All results provide a sound theoretical basis for using carbon nanotubes to generate terahertz wave in the experiments.

Copper(II), iron(III), and chromium(III) complexes with 5,10-dioxo-4,5,9,10-tetrahydro-4,9-diazapyrene derivatives

Six copper(II), iron(III), and chromium(III) complexes with 5,10-dioxo-4,5,9,10-tetrahydro-4,9-diazapyrene derivatives (H2L1-H2L3) have been synthesized and studied by physical methods (IR and electronic absorption spectroscopy, quantum-chemical calculations). The composition of the complexes has been determined and their stability constants in aqueous dimethylformamide solutions have been calculated. The energy characteristics, electronic structure and geometry of isolated diazapyrenes and their tautomeric forms have been calculated by the PM6 method, and their complexes have been modeled.