Wide-band-gap, dye-sensitized, heterojunctions of the composition n-TiO2/Ru-dye/p-CuSCN undergo large increases in efficiency when subjected to low-power UV illumination (<40 mW/cm2) for periods between 10 and 30 min. Ru-dye refers to any of several ruthenium polypyridyl dyes. The UV illumination increases the incident-photon-to-current-efficiency for light absorbed by the dye by a factor of 5−10 and increases the open circuit voltage by 100−300 mV. This beneficial effect is stable for months after the UV illumination is terminated. Investigation of the effects of varying the UV wavelength and the applied bias show that the reaction(s) responsible for the increase in efficiency begin with the oxidation of some interface species by a photoexcited hole in the TiO2 valence band. Treatment of the TiO2/dye/CuSCN cells with oxidized thiocyanate in solution causes a similar increase in the efficiency of dye sensitization. Results from TiO2 and CuSCN photoelectrochemical cells are also presented. The combined res...

Large Enhancement in Photocurrent Efficiency Caused by UV Illumination of the Dye-Sensitized Heterojunction TiO2/RuLL‘NCS/CuSCN: Initiation and Potential Mechanisms

We have performed ab initio self-consistent calculations based on the full potential linear augmented plane-wave method with the generalized gradient approximation to investigate the structural and the electronic properties of the less known bismuth III-V compounds: BBi, AlBi, GaBi, and InBi. Ground state parameters are computed and compared with available theoretical and experimental works. The zinc-blende phase is found to be the most stable for BBi, AlBi, and GaBi, while InBi prefers the tetragonal PbO structure. The relativistic contraction of the $6s$ orbital of Bi has strong effect on the band structure of III-Bi compounds, which exhibits some features that differ considerably from those of typical III-V semiconductors. In particular, we found an inverted band gap, which reflects a semimetallic character of these systems. Their bonding nature is analyzed in terms of valence charge density transfer, showing three different natures of the bond. Besides, the calculated valence charge density for BBi shows an anomalous behavior characterized by a charge transfer toward the cation B atom, while the others III-Bi behave as the typical III-V compounds with a small charge transfer to the anion bismuth atom.

Structural and electronic properties of III-V bismuth compounds

FP-APW + lo calculations of the elastic properties in zinc-blende III-P compounds under pressure effects

The structural and electronic properties of calcium chalcogenides CaX (X = S,Se,Te) under high pressure have been investigated using the full potential linearized augmented plane wave method within density functional theory. We used both the local density approximation and the generalized gradient approximation (GGA) that is based on exchange?correlation energy optimization for calculating the total energy. Moreover, the Engel?Vosko GGA formalism is applied so as to optimize the corresponding potential for band structure calculations. The equilibrium lattice constant for CaX compounds agrees well with the experimental results. The pressures at which these compounds undergo a structural phase transition from NaCl-type to CsCl-type were calculated. A numerical first-principles calculation of the elastic constants was used to calculate C11, C12 and C44. The energy band gaps at ambient conditions in the NaCl-type structure and the volume dependence of band gaps in the CsCl-type structure up to the band overlap metallization were investigated. Besides this, the nature of the chemical bond in these compounds was analysed in terms of electronic charge density.

https://iopscience.iop.org/article/10.1088/0953-8984/17/26/008/pdf

High pressure study of structural and electronic properties of calcium chalcogenides

The Glenn Research Centre of NASA, USA (www.grc.nasa.gov/WWW/SiC/, silicon carbide electronics) is in pursuit of realizing bulk manufacturing of silicon carbide (SiC), specifically by mechanical means. Single point diamond turning (SPDT) technology which employs diamond (the hardest naturally-occurring material realized to date) as a cutting tool to cut a workpiece is a highly productive manufacturing process. However, machining SiC using SPDT is a complex process and, while several experimental and analytical studies presented to date aid in the understanding of several critical processes of machining SiC, the current knowledge on the ductile behaviour of SiC is still sparse. This is due to a number of simultaneously occurring physical phenomena that may take place on multiple length and time scales. For example, nucleation of dislocation can take place at small inclusions that are of a few atoms in size and once nucleated, the interaction of these nucleations can manifest stresses on the micrometre length scales. The understanding of how these stresses manifest during fracture in the brittle range, or dislocations/phase transformations in the ductile range, is crucial to understanding the brittle–ductile transition in SiC. Furthermore, there is a need to incorporate an appropriate simulation-based approach in the manufacturing research on SiC, owing primarily to the number of uncertainties in the current experimental research that includes wear of the cutting tool, poor controllability of the nano-regime machining scale (effective thickness of cut), and coolant effects (interfacial phenomena between the tool, workpiece/chip and coolant), etc. In this review, these two problems are combined together to posit an improved understanding on the current theoretical knowledge on the SPDT of SiC obtained from molecular dynamics simulation.

The current understanding on the diamond machining of silicon carbide

Using the electronic charge densities obtained by the empirical pseudopotential method for tetrahedrally bonded semiconductors, an ionicity scale is established that is in good agreement with the Phillips ionicity scale.

Correlation between the ionicity character and the charge density in semiconductors

The electronic bands structure and density of states of zinc-blende CuCl, CuBr and CuI have been computed using the tight-binding method. These band structures have been used to calculate the valence and conduction band effective masses. The results are compared with other calculated and experimental values.

Electronic structure of the copper halides CuCl, CuBr and Cul

A computational technique is developed to evaluate electron and electron—positron momentum densities in silicon and germanium and the use of these to provide means of interpreting the k-space densities obtained by experimentalis's using the two-dimensional angular correlation of positron annihilation radiation technique (2D ACAR). As a consequence of the good agreement observed in the momentum densities and the LCW folded data obtained for silicon and germanium the independent particle approximation (IPM) coupled with the use of electron pseudo-wavefunctions represent an excellent starting point for a systematic many-particle description of the process, since significant deviations from the experimental angular correlation distribution can be ascribed to the electron—positron and electron—electron interactions.



Nous presentons dans ce papier le developpement des techniques de calcul computationnelles pour evaluer les densites de moment de l'electron et de l'electron—positron dans le cas du silicium et du germanium. L'utilisation de ces techniques donne les moyens d'interpreter les densitěs dans l'espace des k obtenues par les experimentateurs utilisant la correlation angulaire a deux dimensions de la technique d'annihilation des positrons (2D ACAR). Le bon accord observe entre les densites de moment et les donnees obtenues par le theoreme LCW pour le silicium et le germanium montre que l'approximation de la particule independante (IPM) associee aux pseudofonctions d'onde de l'electron constitue un excellent point de depart pour la description systematique du processus a plusieurs particules. les ecarts entre les resultats calcules et experimentaux de la distribution de la correlation angulaire peuvent ětre attribues aux interactions electron—electron et electron—positron.

Positron Annihilation in Si and Ge

The positron distribution of charge along the (111) direction is calculated for elemental and compound semiconductors. The results show that the positron has a strong affinity for one sort of atom in binary semiconductors. This relative positron affinity may lead to the positron preferentially annihilating with the electrons of that sort of atom in compound semiconductors. The trends in going from Ge to CdTe through GaAs show an increase in positron affinity for the anion over the cation.



La densite de charge des positrons a ete calculee suivant la direction (111) pour des semiconductoeurs simples et composes. Les resultants montrent que le positron a une grande affimite pour un type d'atome dans les semiconducteurs binaires. Cette affinite relative peut amener le positron a s'annihiler de preferrence avec les electrons de ce type d'atome du compose. La densite de charge calculee pour plusieurs semiconducteurs, de Ge a CdTe en passant par GaAs, montre une affinite du positron croissante pour l'anion plutǒt que pour le cation.

Positron Distribution in Semiconductors

The electron and positron charge densities are calculated as a function of position in the unit cell for two diamond and zincblende semiconductors, using wave functions derived from nonlocal pseudopotential band structure calculations for the electronic part, and the positron wave functions are calculated for single-particle approximation in manner which is free of any theoretical assumptions.



Les densites de charge electronique et positronique sont calculees en fonction de la position dans la maille cellulaire pour deux semiconducteurs de structures diamant et zinc-blende. Pour la partie electronique, les fonctions d'onde sont obtenues a partir des calculs de structure de bande utilisant le pseudopotentiel non local. Les fonctions d'onde du positron sont calculees a partir de l'approximation qui consiste a ne considerer qu'une seule particule.

B. Khelifa

Papers

Correlation between the ionicity character and the charge density in semiconductors

Electronic structure of the copper halides CuCl, CuBr and Cul

Positron Annihilation in Si and Ge

Positron Distribution in Semiconductors

Electron and Positron Distribution for the Plane (1110) in Si and GaAs