Thermoelectric properties and electronic structure of Al-doped ZnO

Impure ZnO materials are of great interest for high temperature thermoelectric application. In this work, we present the effects of Al-doping on the thermoelectric properties and electronic structures of a ZnO system. We find that, with increasing Al concentrations, the electrical conductivity increases and the thermal conductivity decreases significantly, whereas, the Seebeck coefficient decreases slightly. Nevertheless, the figure of merit (ZT) increases owing to high electrical conductivity and low thermal conductivity. On the other hand, the electronic band structures show that the position of the Fermi level is moved upwards and the bands split near the valence-band top and conduction-band bottom. This is due to the interaction between the Al3p and Zn4s orbitals, which drive the system towards semimetal. Besides, the Density Of States (DOS) analysis shows that the introduction of Al atom obviously reduces the slope d(DOS)/dE near the Fermi level. Based on the calculated band structures, we are able to explain qualitatively the measured transport properties of the Al-doped ZnO system.     

Optical properties and electronic structure of US

The optical properties of uranium sulphide single crystals have been determined for the first time. An excellent agreement is found between the structure in the optical spectrum and the results of a self-consistent cellular multiple scattering calculation. The results evidence that the 5? electrons form a resonance state at E_F and that the 5??6d coupling produces a dip in the 6d density of states near E_F which is responsible for many peculiar properties of US.     

Structural and electronic properties of ZnO under high pressures

In this work, we use first-principles calculations based on density-functional theory within the local-density approximation (LDA) to investigate the structural and electronic properties of ZnO under high-pressure. We have calculated the ground-state energy, the lattice constant, the bulk modulus, and its pressure derivative of the B4 (wurtzie), B3 (zinc blende), B2 (CsCl) and B1 (rocksalt) phases of ZnO. Moreover, the electronic structure, density of states (DOS) of the B4 (wurtzite) and B1 (rocksalt) phases of ZnO have been calculated. We show that our calculated values compare acceptably well with values reported in the literature.     

Optical properties and electronic structure of rare-earth ferroborates

The optical absorption spectra of single-crystal ferroborate GdFe₃(BO₃)₄ and GdFe_2.1Ga_0.9(BO₃)₄ are measured and interpreted. It is found that the absorption edge and the absorption bands A, B, and C observed below the edge are close to those for FeBO₃. A many-electron model of the band structure of GdFe₃(BO₃)₄ is suggested including strong electron correlations between the iron d states. It is shown that GdFe₃(BO₃)₄ has a charge-transfer dielectric gap. A rise in pressure is predicted to result in a crossover between the high-spin and low-spin states of the Fe³⁺ ion, collapse of the magnetic moment, a weakening of Coulomb correlations, an abrupt reduction in the energy gap, and an insulator-semiconductor transition.     

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.     

Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis

In this paper, zinc oxide (ZnO) and cerium-doped zinc oxide (ZnO:Ce) films were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors. The effects of Ce concentration on the structural and optical properties of ZnO thin films were investigated in detail. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements. All deposited ZnO layers at the temperature 450 °C are polycrystalline and indicate highly c-axis oriented structure. The dimension of crystallites depends on incorporation of Ce atoms into the ZnO films. The photoluminescence spectra of the films have been studied as a function of the deposition parameters such as doping concentrations and post grows annealing. Photoluminescence spectra were measured at the temperature range from 13 K to 320 K.     

Optical properties and electronic structure of fluorite and corundum

The complete sets of 12 fundamental optical functions for fluorite crystals in the range 5–39 eV and corundum crystals in the range 2–30 eV are determined from the experimental and theoretical spectra known for certain of these functions. The main features and generalities of these functions are revealed. A theoretical analysis of the optical spectra obtained is performed using the known theoretical band diagrams and the spectra of ?₂.     

Optical properties of ZnO nanocrystals embedded in PMMA

We report in this work the preparation of thin films of ZnO nanocrystals synthesized and dispersed in polymethylmethacrylate using a easy route and deposited in class substrate by spin coating technique. Their structural and optical properties were investigated by X-ray, absorption and photoluminescence spectroscopy. The XRD patterns exhibit sharp peaks at $2\uptheta $ corresponding to the hexagonal (wurtzite) phase diffraction planes. The optically characterization, exhibit a wide absorption band in the range of the study and a large emission band with three peaks at 481.5, 531.09 and at 671.28 nm.The crystallites radius (R) was estimated by applying the effective mass approximation model and was about 1.8 nm. From measurements of second order susceptibilities using harmonic generation technique at $\lambda = 1,064\,\text{ nm }$ in picoseconds regime we deduced $\lambda _\mathrm{eff}^{<2>}$ equal to $5.95\times 10^{-10}$  m/V. Obtained $\lambda _\mathrm{eff}^{<2>}$ was four order of magnitude larger compared with ZnO bulk material (2.5 pm/V).     

Structural, electronic, and magnetic properties of Co-doped ZnO

Density functional theory based calculations have been carried out to study structural, electronic, and magnetic properties of Zn1-xCoxO (x = 0, 0.25, 0.50, 0.75) in the zinc-blende phase, and the generalized gradient approximation proposed by Wu and Cohen has been used. Our calculated lattice constants decrease while the bulk moduli increase with the increase of Co 2+ concentration. The calculated spin polarized band structures show the metallic behavior of Co-doped ZnO for both the up and the down spin cases with various doping concentrations. Moreover, the electron population is found to shift from the Zn-O bond to the Co-O bond with the increase of Co 2+ concentration. The total magnetic moment, the interstitial magnetic moment, the valence and the conduction band edge spin splitting energies, and the exchange constants decrease, while the local magnetic moments of Zn, Co, O, the exchange spin splitting energies, and crystal field splitting energies increase with the increase of dopant concentration.     

Optical and structural properties of Eu-diffused and doped ZnO nanowires

Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900 °C for 1 h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was 200 nm with a length of 1 μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515 nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu³ ions was not present. Transmission electron microscopy shows crystalline Eu₂O₃ formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp ⁵D_O–⁷F₂ transition of the Eu³⁺ ion at around 615 nm was observed.     

Optical properties and electronic structure of crossroads material MnTe

Combining an energy level model for the rocksalt manganese chalcogenides with a measurement of 1.3 eV for the optical gap of NiAs-structured MnTe, it is argued that MnTe has extensive energetic p-d overlap and that such overlap is characteristic of NiAs-structured materials.     

Optical properties of p-type ZnO doped by lithium and nitrogen

A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al₂O₃ as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a Li_Zn-N complex acceptor level (eFA), radiative recombination of a donor-acceptor pair and recombination of the Li_Zn-N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The Li_Zn-N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.     

Optical and magnetic properties of laser-deposited Co-doped ZnO thin films

We report the optical and magnetic properties of laser-deposited Zn_1−xCo_xO (x=0.06-0.3) thin films with no intentional electrical carrier doping. The analysis of the high-temperature magnetization data provides an unambiguous evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Co ions in Zn_1−xCo_xO alloy, yielding the value of the effective exchange integral J₁/k_B to be about −27 K. The low-temperature magnetization data reveals a spin glass transition in Zn_1−xCo_xO alloy for the Co content x>0.15, giving the value of the spin freezing temperature T_f to be ∼8 and ∼12 K for x=0.2 and 0.25, respectively. Optical spectra analysis shows a linear increase of the band gap E_g with the increase of the Co content following E_g=3.231+1.144x eV.     

First-principles study of the electronic and optical properties of ZnO nanowires

The geometric, energetic, electronic structures and optical properties of ZnO nanowires (NWs) with hexagonal cross sections are investigated by using the first-principles calculation of plane wave ultra-soft pseudo-potential technology based on the density functional theory (DFT). The calculated results reveal that the initial Zn-O double layers merge into single layers after structural relaxations, the band gap and binding energies decrease with the increase of the ZnO nanowire size. Those properties show great dimension and size dependence. It is also found that the dielectric functions of ZnO NWs have different peaks with respect to light polarization, and the peaks of ZnO NWs exhibit a significant blueshift in comparison with those of bulk ZnO. Our results gives some reference to the thorough understanding of optical properties of ZnO, and also enables more precise monitoring and controlling during the growth of ZnO materials to be possible.     

Optical properties and electronic structure of BiTeCl and BiTeBr compounds

Optical properties of BiTeCl and BiTeBr compounds with a strong Rashba spin–orbit coupling are studied in the 0.08–5.0 eV range using the optical ellipsometry method. Fundamental characteristics of the electronic structure are obtained. Similarly to BiTeI, spectra of the imaginary part of dielectric permittivity constant ε₂(E) in the energy interval between the plasma edge and the threshold of an intense interband absorption (0.7 eV in BiTeCl and 0.6 eV in BiTeBr) display a fine structure of electronic transitions at 0.25 and 0.55 eV in BiTeCl and 0.20 and 0.50 eV in BiTeBr. These features are assigned to electronic transitions between the bulk conduction zones split by the Rashba spin–orbit interaction. The parameters of the electronic structure of BiTeCl and BiTeBr are compared with the BiTeI compound that was studied earlier. In the BiTeCl–BiTeBr–BiTeI row, the absorption edge and main features of the fundamental absorption exhibit a shift to low energies.     

Influence of pressure on electronic and optical properties of phosphorus-doped ZnO

In this work, the geometrical, electronic structure and optical properties of P-doped ZnO under high pressures have been investigated using first-principles methods. The pressure effects on the lattice parameters, electronic band structures, and partial density of states of crystalline P-doped ZnO are calculated up to 8 GPa. Moreover, the evolution of the dielectric function, absorption coefficient (αω)), reflectivity (R(ω)), and the real part of the refractive index (n(ω)) at high pressure are also presented.     

Common electronic band gaps and similar optical properties of ZnO nanotubes

Electronic and optical properties of single-walled zinc oxide （ZnO） nanotubes are investigated from the firstprinciples calculations. Electronic structure calculations show that ZnO nanotubes are all direct band gap semiconducting nanotubes and the band gaps are relatively insensitive to the diameter and chirality of tubes. The origin of the common electronic band gaps of ZnO nanotubes is explained in terms of band-folding from the two-dimensional band structure of graphite-like sheet. Moreover, the optical properties such as dielectric function and energy loss function spectra of different ZnO nanotubes are very similar, relatively independent of diameter and chirality of tubes. The calculated dielectric function and loss function spectra show a moderate optical anisotropy with respect to light polarization.