Crystal, electronic and luminescence properties of Eu-doped Sr2Al2−xSi1+xO7−xNx

The crystal and electronic structures, as well as the luminescence properties of Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ are reported. First-principles calculations energetically confirm that the Al and Si atoms are in partial ordering in the 2a and 4e sites in Sr₂Al₂SiO₇. In addition, the band structure calculation shows that Sr₂Al₂SiO₇ has an indirect band gap with an energy gap of about 4.07 eV, which is in good agreement with the experimental data (5.3 eV) obtained from the diffuse reflection spectrum. The crystal structure of Sr₂Al₂SiO₇ can be modified by Si–N substitution for Al–O in the lattice with a maximum solubility of about x=0.6. The average bond length of Eu_Sr^-(O,N) slightly increases although the lattice parameters decrease with the incorporation of Si–N in Sr₂Al₂SiO₇:Eu²⁺. Under excitation in the visible spectral region, Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ emits blue to yellow light with a broad emission band in the range of 480–570 nm, varying with both the Eu concentration and the x value. The red shift of the emission band of Eu²⁺ is associated with an increase in the crystal-field splitting and the covalency, which arise from the incorporation of nitrogen as well as the energy transfer between the Eu ions at high Eu concentrations. Moreover, the Eu ions have a strong effect on both the concentration quenching and the thermal quenching in Sr₂Al_2−xSi_1+xO_7−xN_x. The temperature dependence of photoluminescence indicates that Sr₂Al_2−xSi_1+xO_7−xN_x:Eu²⁺ shows strong thermal quenching due to the dominant nonradiative process at room temperature.     

First-principles studies of the superconductivity and vibrational properties of transition-metal nitrides TMN (TM = Ti,V, and Cr)

The present paper reports the structural, electronic, phonon and thermodynamical properties of some transition-metal nitrides (TMN: TiN, VN and CrN) by means of first-principles calculations. The computed equilibrium lattice constant and bulk modulus agree well with the available experimental and theoretical data. The electronic band structure and density of states calculations show metallic nature. The phonon frequencies are positive throughout the Brillouin zone for these compounds in rocksalt structure indicating dynamical stability. The calculated electron–phonon coupling constant λ and superconducting transition temperature agree reasonably well with the available experimental data. These compounds behave as a conventional phonon-mediated superconductor. Within the GGA and quasi-harmonic approximation, thermodynamical properties are also investigated.     

Structural, electronic and optical properties of Sn1−xSbxO2

The goal of this paper is to undertake a detailed first principle calculation of the structural, electronic and optical properties of Sn_1−xSb_xO₂. The results show that the stability of Sn_1−xSb_xO₂ in the full range of Sb content points to the probability of a continuous solid solution, where the increasing Sb content leads to volume expansion with different variation trends in the lattice constants. The increase of Sb concentration in the semiconductor–metal–semimetal transition occurs in consonance with the corresponding changes in its structural, electronic and optical properties. Two competing mechanisms play essential roles in this transition, namely; the many body effect and the atom disorder. Our calculations concur with previous X-ray diffraction, sheet resistance, resistivity and optical parameters detections. The studies present a practical way of tailoring the physical behaviors of Sn_1−xSb_xO₂ through the alloying technique.     

Electronic structure and transport properties of Co(Si1−xMx) (M = Al, P): First-principles study

In this study, by using the full-potential linear augmented plane wave (FLAPW) method based on the density functional theory (DFT), the lattice parameter of CoSi was calculated theoretically and the calculations of the electronic structures of CoSi and CoSi_1−xM_x (M = Al, P and x = 0.03125, 0.125) were performed. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semi-metal and the density of states (DOS) is very small at the Fermi level. M-doping can tune the Fermi level and the hole pockets and the electron ones, which is very valuable to modulate the transport properties. Based on the calculated electronic structures and our experimental results on CoSi [C.C. Li, W.L. Ren, L.T. Zhang, K. Ito, J.S. Wu, J. Appl. Phys. 98 (2005) 063706], the intrinsic relations between electronic structures and transport properties of CoSi and CoSi_1−xAl_x are discussed in detail. The transport properties along main crystallographic directions of binary CoSi and CoSi_1−xAl_x are experimentally examined. The experimental results show that the electrical resistivity of CoSi-based compounds is anisotropic, while the Seebeck coefficient is almost isotropic. The calculated band structures of CoSi_1−xAl_x can theoretically interpret the anisotropy of the electrical transport properties.     

First-principles study of the (1 1 0) polar surface of cubic PbTiO3

Under GGA, the cleavage energy, surface energy, surface grand potential, surface relaxation, and surface electronic structure have been calculated for five different terminations of PbTiO₃ (1 1 0) surface by using PAW method implemented in VASP. Taking into account the results of two neutral PbTiO₃ (1 0 0) surfaces, the favorable PbTiO₃ (1 1 0) and (1 0 0) surfaces are the TiO₂-terminated (1 0 0) surface, the PbO-terminated (1 0 0) surface, and the O-terminated (1 1 0) surface successively in view of surface energy minimization. The surface grand potential calculations show that two neutral PbO- and TiO₂-terminated (1 0 0) surfaces are favored in the moderate Pb and O chemical potentials, two mutual complementary TiO- and Pb-(1 1 0) terminations are stable in Pb-poor environment and in O- and Pb-rich conditions, respectively. A non-negligible rumpling of O-terminated (1 1 0) surface is found in the third O₂ layer and large lateral displacements between Ti and O atoms on the PbTiO layer lead to the initial O-Ti-O alignment broken. Different from the Fermi levels of the three nonstoichiometric TiO-, Pb- and O-terminations which are located in the band gap, the Fermi level of the PbTiO- termination is located at the bottom of the conduction band and that of the O₂-termination is located at the top of the valence band due to increment and decrement of the occupation states for polarity compensation.     

First principles study on the structural properties and electronic structure of X2B (X = Cr, Mn, Fe, Co, Ni, Mo and W) compounds

First principles calculations are performed to study the stability, electronic and structural properties of X₂B (X = Cr, Mn, Fe, Co, Ni, Mo and W). The calculated cohesive energy and formation enthalpy of these compounds both have negative values, which indicate that they are thermodynamically stable structures. The ground states of Cr₂B and Mn₂B are anti-ferromagnetic; Fe₂B and Co₂B are ferromagnetic; Ni₂B, Mo₂B and W₂B are paramagnetic. The calculated local magnetic of Fe₂B is 1.962μ_B/Fe, and for Co₂B is 1.182μ_B/Co. They are comparable to the values of Fe₃B (1.97μ_B/Fe) and Co₃B (1.18μ_B/Co), but smaller than pure Fe and Co. The observed magnetic behaviors of X₂B compounds can be explained by Stoner’s model. Two main peaks are observed in the calculated PDOS (partial density of states) of these compounds (P1 and P2). P1 is caused by strong covalent X–B bonds and P2 is attributed to metallic X–X bonds.     

Controlled synthesis and improved luminescent properties of (Gd1−x,Eux)3GaO6 phosphors fabricated via spray pyrolysis

Eu³⁺ activated trigadolinium gallate, Gd₃GaO₆:Eu, was successfully synthesized using various aqueous solutions via ultrasonic spray pyrolysis. The corresponding phosphors illuminated an efficient red emission under Hg-discharge condition with enhanced color purity, which is related to prominent peaks centered at 615 and 629 nm induced by the electric dipole transition of ⁵D₀ → ⁷F₂ of Eu³⁺. By using a proper Ga source for aqueous solution, especially gallium sulfate, uniformly distributed Gd₃GaO₆:Eu³⁺ phosphor with controlled morphology and enhanced emission efficiency was obtained.     

Adsorption of H,H2, and H2O inside and outside of (M@Si16F)6 tubelike aggregates and wires (M = V,Ta). A first principles study

We have studied theoretically the adsorption of atomic and molecular hydrogen, as well as H₂O molecules, on porous materials assembled from M@Si₁₆F molecular units (M = V, Ta). Here we consider the (M@Si₁₆F)₆ star-like aggregate formed by two (M@Si₁₆F)₃ triangular supermolecules stacked along the vertical axis and twisted 60° each other, as well as the infinite wire which unit cell is that (M@Si₁₆F)₆ aggregate. That aggregate forms an internal barrel-shaped porous which size is ideal to encapsulate small molecules like H₂ and H₂O. Similarly, the external space between the arms of these star-like systems is also ideal to adsorb small molecules. Our calculations were performed using density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) for the exchange and correlation functional. From these calculations we have shown that adsorption of molecular H₂ occurs preferably outside of (M@Si₁₆F)₆ tube-like structures and when the M dopant is Vanadium rather than Tantalum. The binding energy is higher for the infinite wire than for the finite aggregate. In order to estimate the effect of dispersion interactions, we studied the adsorption of H₂ on a single M@Si₁₆F molecular unit using the non local correlation van der Waals (vdW) functional of Klimes, Bouwler, and Michaelides (KBM). The corresponding H₂ binding energy is about a factor 3 larger than that from the GGA–PBE approach for M = V dopant. From Mulliken population analysis it is shown that GGA–PBE describes the bond between H₂ and M@Si₁₆F as chemisorption whereas vdW–KBM approach leads to the physisorption picture. Considering all the finite ring and infinite tube-like systems studied in this work, the binding energy per H atom is larger than half the dissociation energy of gas phase H₂ only for the V@Si₁₆F finite aggregate, that is, an exothermic dissociative H₂ adsorption is possible only in that case. Finally, we have shown that the encapsulation of a single H₂O molecule inside of these M-doped silicon structures is favored (exothermic) when the H₂O axis is not parallel to the aggregate or wire axis.     

Full-potential study of Fe2NiZ (Z = Al,Si, Ga,Ge)

The first-principles calculations using full-potential in the stable F-43m phase have been performed to investigate the structural, elastic, magnetic, nature of chemical bonding and electronic properties of Fe₂-based inverse Heusler alloys. The structural stability and the lattice constants match well with the experimental results. We have further reported other mechanical, elastic and thermophysical properties for the first time of these Fe₂NiZ (Z = Al, Si, Ga, Ge) materials. Cauchy's pressure and Pugh's index of ductility label these materials as ductile. The spin magnetic moment distributions show that these materials are ferromagnetic in stable F-43m phase. Further, spin resolved electronic structure calculations show that the discrepancies in magnetic moments of Fe-I and Fe-II depend upon the hybridization of Fe with the main group element. The charge density distribution plots present a clear picture of the stronger covalent bonding in Fe₂NiSi and the decreasing trend of covalent bonding in these materials. The main group electron concentration is predominantly responsible in establishing the magnetic properties, formation of magnetic moments and the magnetic order for these alloys. Spin resolved band structure calculations show that these materials are metallic in stable F-43m phase at ambient conditions.     

Thermal behaviour of the O2/TiO2 (110)-(1 × 2) surface

Synchrotron radiation ultra-violet photoemission at different photon energies (17.1, 19.3, and 21.5 eV) has been used to study the interaction of O₂ with the TiO₂ (110)-(1 × 2) surface reconstruction at temperatures between 77 and 320 K. At 77 K the results show a weak molecular chemisorption of the O₂ molecule on the surface. By analysing the thermal behaviour of the O₂/TiO₂ system in a temperature range from 77 to 320 K, it has been found that between 120 and 200 K the O₂ molecule is dissociated.     

Simulation of thermal properties of Ba1−xZrO3 compounds for thermal barrier coating applications

The effect of vacancies on structural properties of the perovskite-type oxide was studied using the full-potential linearized augmented plane wave (FP-LAPW) method, within the density functional theory. In this approach, the generalized gradient approximation was used for the exchange-correlation potential. The ground state properties such as lattice parameter, bulk modulus and inter-atomic distances of cubic Ba_1−xZrO₃ compounds (x = 0, 0.125 and 0.25) were calculated. Additionally, using a set of total energy versus volume obtained with the FP-LAPW method, the quasi-harmonic Debye model was applied to determine the thermal properties including temperature dependence of bulk modulus, thermal expansion coefficient, specific heats at constant volume and constant pressure. No experimental data are available and our results are considered as purely predictive.     

Cathodoluminescence study in AlxGa1 − xN structures

In this work, we consider a 2D model for calculation of cathodoluminescence in GaN-based structures. This model is developed using an extended generation profile and taking into account the influence of the carrier diffusion process, internal absorption and some radiative recombination processes. First, we have investigated the effect of hole diffusion length and the surface recombination velocity on the CL spectra of GaN sample grown at 800 °C by MOVPE method. Then, we have calculated the dependence of CL intensity from AlGaN alloys as a function of Al content and the electron beam energy.

Results show a red shift of the CL peaks when the beam energy is varied from 2 to 10 keV at room temperature. The band-edge emission of Al_xGa_1 − xN shifts about 0.49 eV when the Al composition is increased from x = 0.18 to 0.38. Comparison of the experimental spectra with simulations shows a good agreement.     

Coulomb interaction of electron gas in MQWs Si/Si1 − xGex/Si

We present a theoretical analysis of the conduction and valence-band diagrams of SiGe/Si Multiple Quantum Wells (MQWs), having a specific “W” geometry, and designed for emission or photodetection around the 1.55 μm wavelength. Peculiar features have been extrapolated by solving self-consistent Schrödinger and Poisson equations, taking into account the electrostatic attraction induced by carrier injection. As a result, Coulomb interaction strongly modifies the band profiles and increases the electron probability density at the quantum well interfaces; the injected carrier concentration enhances electron–hole wave functions overlap and the in-plane oscillator strength. These MQWs structures, strain-compensated on relaxed Si_0.75Ge_0.25 pseudo-substrates, are potentially interesting for telecom applications.     

First-principles studies of the electronic and elastic properties of metal nitrides XN (X = Sc, Ti, V, Cr, Zr, Nb)

Electronic and elastic properties of a series of the transition metal ion mononitrides (ScN, TiN, VN, CrN, ZrN, NbN) have been modeled in the framework of ab initio plane wave spin-polarized calculations using the generalized gradient and local density approximations. The calculated band structures are typical for metallic compounds, except for ScN, whose band structure is that one of the gapless semiconductor. Strongly delocalized d states of transition metal ions are spread over a wide region of about 10-12 eV and are strongly hybridized with the nitrogen 2p states. Among the considered nitrides, only CrN exhibits a clear difference between the spin-up and spin-down states, which would manifest itself in magnetic properties. The overall appearance of the calculated cross-sections of the electron density difference changes drastically when going from Sc to Nb in the considered series of compounds. For the first time the calculated tensors of the elastic constants and elastic compliance constants were used for the analysis and visualization of the directional dependence of the Young’s moduli. It was shown that ScN and VN can be characterized as more or less elastically isotropic materials, whereas in TiN, CrN, ZrN, and NbN the Young’s moduli vary significantly in different directions. The maximal values of the Young’s moduli are along the crystallographic axes, the minimal values are along the bisector direction in the coordinate planes; the difference between them in the case of CrN exceeds one order of magnitude. In addition, pressure dependence of the “metal - nitrogen” distance was modeled.     

Structural stabilities, electronic structures and lithium deintercalation in LixMSiO4 (M = Mn, Fe, Co, Ni): A GGA and GGA + U study

Dilithium-orthosilicate oxides Li₂MSiO₄ (M denotes transition metals) have been one of the focuses in the field of new cathode materials for Li-ion batteries recently, due to their possible high capacities and probabilities achieving by experiment. Using the density functional theory within both the generalized gradient approximation (GGA) and GGA + U frameworks, the structural stabilities, electronic structures and delithiation process for the dilithium-orthosilicate oxides Li₂MSiO₄ (M = Mn, Fe, Co, Ni) are systematically investigated. Within the GGA + U approach, LiMSiO₄ is shown to be a stable non-stoichiometric structure, while the compound Li_1.5MSiO₄ are unstable relative to a two-phase form containing Li₂MSiO₄ and LiMSiO₄, which is consistent with the experimental voltage profiles. For Li_0.5MSiO₄, though the formation energies are negative for Mn-system and Ni-system, the absolute values are so small that they would be likely to also undergo phase separation at room temperature. The average deintercalation voltages calculated by the GGA + U scheme are in good agreement with the available experimental data. Furthermore, the possibility of the exchange of two electrons per M in Li₂MSiO₄ is also discussed based on the calculated results.     

Ab initio study of stacking faults and deformation mechanism in C15 Laves phases Cr2X (X = Nb,Zr, Hf)

Based on the synchroshear model, the formation of stacking fault and twinning fault in C15 Laves phases is modeled, then the generalized stacking fault energy curves and deformation mechanism in C15 Laves phases Cr₂X (X = Nb, Zr, Hf) alloys are investigated by ab initio calculations based on the density functional theory. The results demonstrate that the unstable stacking fault and twinning fault energies of C15 Laves phases Cr₂X (X = Nb, Zr, Hf) by the synchroshear are still large while the stable stacking fault and twinning fault energies are low, and the deformation modes by extended partial dislocation and twining are feasible in C15 Laves phases Cr₂X (X = Nb, Zr, Hf). Moreover, the Cr₂Nb has the largest deformation twinning tendency, followed by Cr₂Zr and Cr₂Hf. The evolution of electronic structure during the synchroshear process is further studied to unveil the intrinsic mechanism for the formation of stacking fault and twinning fault in C15 Laves phases Cr₂X (X = Nb, Zr, Hf).     

Thermal expansion properties of Ln2−xCrxMo3O12 (Ln = Er and Y)

Structures and thermal expansion properties of Ln_2−xCr_xMo₃O₁₂ (Ln = Er and Y) have been investigated by X-ray powder diffraction. Rietveld analysis results of Ln_2−xCr_xMo₃O₁₂ indicate that compounds Er_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.3) and Y_2−xCr_xMo₃O₁₂ (0 ≤ x ≤ 0.2) crystallize in orthorhombic structure and exhibit negative thermal expansion, while both monoclinic and orthorhombic compounds Er_2−xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) and Y_2−xCr_xMo₃O₁₂ (1.8 ≤ x ≤ 2.0) possess positive coefficient of thermal expansion. The coefficients of linear thermal expansion of orthorhombic Ln_2−xCr_xMo₃O₁₂ change from negative to positive with increasing chromium content. Thermogravimetric and differential scanning calorimetry have been used to study the hygroscopicity and the phase transition temperature.     

Element-specific electronic structure of Mn dopants and ferromagnetism of (Zn,Mn)O thin films

Element-specific electronic structure of (Zn,Mn)O thin films with various Mn concentrations has been investigated using X-ray absorption and emission spectroscopy. According to comparison between the experimental spectra and the density functional theory calculations (partial density of states and exchange interactions for various Mn defect configurations), the substitutional Mn impurities do not induce ferromagnetism in (Zn,Mn)O samples. The ferromagnetic properties can be obtained when defect configurations consisting of both substitutional and interstitial Mn atoms are present. The ferromagnetism in ZnO-based magnetic semiconductors is favored to be Ruderman-Kittel-Kasuya-Yoshida type and the established theoretical model is in a good agreement with the X-ray spectroscopic measurements.     

Interfacial chemistry of oxides on InxGa(1−x)As and implications for MOSFET applications

The prospect of enhanced device performance from III–V materials has been recognized for at least 50 years, and yet, relative to the phenomenal size of the Si-based IC industry, these materials fulfilled only specific niches and were often referred to as “the material of the future” [1]. A key restriction enabling widespread use of III–V materials is the lack of a high quality, natural insulator for III–V substrates like that available for the SiO₂/Si materials system [2]. The prospect of impending scaling challenges for technologies based on silicon metal oxide semiconductor field effect transistor (MOSFET) devices has brought renewed focus on the use of alternate surface channel materials from the III–V compound semiconductor family. The performance of the traditional MOSFET device structure is dominated by defects at the semiconductor/oxide interface, which in turn requires a high quality semiconductor surface. In this review, reflecting the authors’ current opinion, the recent progress in the understanding of the dielectric/III–V interface is summarized, particularly in regard to the interfacial chemistry that impacts the resultant electrical behavior observed. The first section summarizes the nature of the oxidation states of surface oxides on In_xGa_1−xAs. Then the atomic layer deposition of such oxides on the In_xGa_1−xAs surface is summarized in view of the interfacial chemical reactions employed. Finally the resultant electrical properties observed are examined, including the effects of substrate orientation. Portions of this review have been published previously [3] and [4].     

Synthesis of SrO(SrTiO3)n (n = 1, 2, ∞) compounds and electronic structure analysis

SrO(SrTiO₃)_n compounds were prepared by a modified sol-gel self-propagating combustion, which is a low-temperature combustion synthesis procedure using microwave-assisted sol-gel as precursors. The thermal treating conditions were determined by DTA/TG analysis of the powders. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the sample characterization. The results confirm that the high temperature and long reaction time which occur in classical solid-state reaction method are avoided. The band structure, total density of states (DOS), and partial density of states (PDOS) of SrO(SrTiO₃)_n were calculated in order to study the electronic structures of SrO(SrTiO₃)_n.