Probing the wurtzite conduction band structure using state filling in highly doped InP nanowires

We have grown InP nanowires doped with hydrogen sulfide, which exhibit sulfur concentrations of up to 1.4%. The highest doped nanowires show a pure wurtzite crystal structure, in contrast to bulk InP which has the zinc blende structure. The nanowires display photoluminescence which is strongly blue shifted compared with the band gap, well into the visible range. We find evidence of a second conduction band minimum at the gamma point about 0.23 eV above the band edge, in excellent agreement with recent theoretical predictions. Electrical measurements show high conductivity and breakdown currents of 10(7) A/cm(2).     

Electroosmotic flow in nanotubes with high surface charge densities

The ion distribution and electroosmotic flow of sodium chlorine solutions confined in cylindrical nanotubes with high surface charge densities are studied with molecular dynamics (MD). To obtain a more practical physical model for electroosmotic driven flow in a nanoscale tube, the MD simulation process consists of two steps. The first step is used to equilibrate the system and to obtain a more realistic ion distribution in the solution under different surface charge densities. Then, an external electric field is acted to drive the liquids. The simulation results indicate that with the increase of the surface charge density, both the thickness of the electric double layer and the peak height of the counterion density increase. However, the phenomenon of charge inversion does not occur even as the surface charge density increases to -0.34 C/m2, which is rather difficult to reach for real materials in practical situations. This simulation result confirms the recent experimental observation that monovalent ions of sufficiently high concentrations can reduce or even cancel the charge inversion occurred in the case of multivalent ions [F. H. J. van der Heyden et al. Phys. Rev. Lett. 2006, 96, 224502].     

Stress dependent band gap shift and valence band studies in ZnO nanorods

ZnO nanorods are grown on seedless and ZnO seeded glass substrates using chemical solution method and their structural, morphological, optical and valence band studies have been carried out. On seedless substrate horizontal nanorods are observed whereas for the seeded substrates vertically aligned hollow and solid nanorods grows. X-ray diffraction analysis revealed the presence of tensile stress in the vertical nanorods. Blue shift has been observed in the band gap of the vertical nanorods as compared to the horizontal nanorods which is attributed to the presence of tensile stress in the vertically aligned nanorods. Photoluminescence spectra revealed the dominance of Zinc vacancies (V(Zn)) related defects in the nanorods and oxygen defects are found to be higher in the vertically aligned nanorods as compared to the horizontal nanorods. The difference between the Fermi level and valence band maxima for horizontal, hollow vertical and solid vertical nanorods are found to be approximately 0.56 eV, approximately 0.70 eV and approximately 0.92 eV respectively indicating the possibility of p-type of conduction in the nanorods which has been attributed to presence of V(Zn) defects in the ZnO nanorods.     

Materials with cationic valence and conduction bands for photoelectrolysis of water

Two types of materials in which the highest filled and lowest empty electron bands are mainly cationic have been investigated, viz. Bi₄Ti₃O₁₂ and Bi₂WO₆ with a filled Bi 6s band and empty Ti(W) 3d(5d) bands and NiNb₂O₆ and CoNb₂O₆ with filled narrow transition metal bands and empty Nb 4d bands. Bi₄Ti₃O₁₂ shows weak photocurrents in a photoelectrochemical water decomposition cell, whereas Bi₂WO₆ does not. Also NiNb₂O₆ and CoNb₂O₆ behave differently. Whereas NiNb₂O₆ shows photocurrents, CoNb₂O₆ does not. These results are discussed in terms of bulk properties. The absence of a photocurrent is related to the formation of small polarons with high activation energy for migration. In agreement with this the compound Bi₂WO₆ shows luminescence.     

Temperature dependence of AlInAs band gap energy and AlInAs/InP band offsets

Abstract

The temperature variations of the AlInAs photoluminescence (PL) transition energies and the AlInAs/InP interface staggered lineup luminescence (SLL) energy are reported. The S shape appearing from 4 to 90 K on the energy v. temperature curves of these PL energies are owing to extrinsic recombinations. In particular, the S shape of the SLL energy curve v. temperature is probably a result of acceptor impurities localised in AlInAs at the interface (on edge impurities). The band offsets were determined by solving the Schrödinger and Poisson equations with a self consistent calculation program. At 4.5 K, the conduction and valence band offsets are 0.384 and 0.295 eV respectively. Their temperature variation is shown to be important: 35 and 23 meV at 4.5 and 300 K respectively. The Van Vechten and Malloy model (following a thermodynamic approach) for the temperature variation of the band offsets is compared to the results in the present work.     

Measurement of valence band structure in arbitrary dielectric films

A new way of measuring the band structure of various dielectric materials using the secondary electron emission from Auger neutralization of ions is introduced. The first example of this measurement scheme is the magnesium oxide (MgO) films with respect to the application of the films in the display industries. The density of state in the valence bands of MgO film and MgO film with a functional layer (FL) deposited over a dielectric surface reveals that the density peak of film with a FL is considerably less than that of film, thereby indicating a better performance of MgO film with functional layer in display devices. The second example of the measurement is the boron-zinc oxide (BZO) films with respect to the application of the films to the development of solar cells. The measurement of density of state in BZO film suggests that a high concentration of boron impurity in BZO films may enhance the transition of electrons and holes through the band gap from the valence to the conduction band in zinc oxide crystals; thereby improving the conductivity of the film. Secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials.     

Conduction and valence band discontinuities in some new semiconductor heterojunctions

Conduction and valence band discontinuities (band offsets) in the heterojunctions consisting of ZnO/4H-SiC, ZnO/GaAs, MgO/4H-SiC, MgO/AIN, InN/ZnO, InN/GaAs, MgO/InN, and InN/4H-SiC, were precisely determined by X-ray photoelectron spectroscopy (XPS) measurement. As an example, the XPS measurement on the ZnO/4H-SiC heterojunction is described in detail in the beginning of the section of the experiments. We also analyzed several factors which may have impact on the band offset in a practical heterojunction. The deviation in band offset induced by both polar and non-polar heterojunction band bending from the true value was also characterized by taking into account the intrinsic polarization field in the heterojunction consisting of the two polar materials.     

Annealing temperature and oxygen-vacancy-dependent variation of lattice strain,band gap and luminescence properties of CeO2 nanoparticles

CeO₂ nanoparticles are annealed in vacuum at 200°C and in air at 200°C, 600°C and 1000°C, respectively. Vacuum-annealed CeO₂ contains high concentration of oxygen vacancies and exhibits very high lattice strain, whereas the corresponding values decrease on air annealing. Oxygen-deficient CeO₂ has redshift in band gap with high Urbach energy. The magnitude of this energy decreases as CeO₂ is annealed in air at 600°C. At 1000°C, thermal disorder increases the Urbach energy. Photoluminescence property of the samples depends on the presence of radiative and non-radiative oxygen vacancy centres. Vacuum-annealed ceria have large numbers of non-radiative oxygen vacancies that act as emission quencher. CeO₂ annealed at 600°C contains requisite amount of oxygen vacancies to show better luminescence property.     

Nonsteady heat conduction in cells of triangular lattice with central heat sink

A method of numerical solution is outlined for the nonsteady heat conduction in the cells between the tubes of a triangular lattice with a central heat sink for nonlinear boundary conditions. Calculated and experimental data are compared.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 1, pp. 17–21, January, 1978.     

Dielectric constant variation of polystyrene-graphite composite material with temperature

Temperature variation is investigated for the dielectric constant of polystyrene-graphite composite, and variation of normalized capacitance with temperature is studied. Experimental and theoretical results, obtained through modelling, appear to match quite closely.     

二维旋转直柱光子晶体的频率带隙结构分析

设计了一种二维方形旋转正四边形直柱光子晶体,利用平面波展开方法计算了其光子频率带结构,发现在低频和高频区域,该类光子晶体的光子频率禁带明显增大.计算了空气中Al材料的旋转四边形直柱光子晶体的带结构和态密度,当填充比等于0.5时存在绝对带隙,旋转角度为45°时绝对带隙最大,旋转角度为0时,光子频率禁带位于高频区域.利用FDTD方法检验了计算结果,并分析了旋转角度为45°时,正四边形直柱光子晶体的波导特性以及TM模的电场分布.     

Size dependences of lattice constant and magnetic properties of Ni nanoplatelets

Ni plate-like nanoparticles (or nanoplatelets), with a mean diameter in the range from 42 to 130 nm and a thickness of about 10 nm, were synthesized by solution reduction method. Increased crystallite-size dependences of the lattice constant and the coercivity were observed. The cause for the variation of the lattice constant and the coercivity of Ni nanoplatelets with their size was investigated, respectively.     

Core level and valence band investigation of WO3 thin films with synchrotron radiation

In this work, the electronic properties of the surface of WO₃ films with thickness of 150 nm, thermally evaporated in high vacuum onto Si(100) substrates and pre-treated in air by a 24-h-long annealing at 300 °C and 500 °C (obtaining polycrystalline monoclinic samples) have been studied by surface and bulk sensitive core level (W 4f) and angle integrated valence band photoemission using synchrotron radiation (ELETTRA Synchrotron). The photon energy ranged from 50 eV to 200 eV. The line shape analysis of W 4f core level spectra has shown that the surface presents a sub-stoichiometric WO₃ component assigned to oxygen vacancies ultimately responsible for the gas sensitivity of this material. Correspondingly, valence band spectra show well-defined metallic states W 5d in the gap and near the Fermi level. The variations of surface chemical composition caused by Ultra High Vacuum annealing, and prolonged exposure to UV beam has been monitored by changes in spectral line shape. A general consequence of annealing in vacuum is the segregation of oxygen from the bulk toward the surface as confirmed by independent scanning tunnelling spectroscopy measurements.     

Heat conduction in a rod of variable length drawn from a melt with constant velocity

An approximate solution of the one-dimensional heat-conduction problem for a rod of variable length is obtained by the integral method. The result of calculating the temperature in a rod from the obtained formula is given.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 18, No. 2, pp. 333–339, February, 1970.     

Valence band splitting in wurtzite InP nanowires observed by photoluminescence and photoluminescence excitation spectroscopy

We have investigated individual bulk-like wires of wurtzite InP using photoluminescence, photoluminescence excitation spectroscopy and transmission electron microscopy. Using two different methods we find that the top of the valence band is split, as expected theoretically. This splitting of the valence band is peculiar to wurtzite InP and does not occur in zinc blende InP. We find the energy difference between the two bands to be 40 meV.