Dye sensitization of a large band gap semiconductor by an iron(III) complex

The Fe(III) complex, [Fe^III(HQS)₃] (HQS = 8-hydroxyquinoline-5-sulfonic acid), is found to effect sensitization of the large band gap semiconductor, TiO₂. The role of interfacial electron transfer in sensitization of TiO₂ nanoparticles by surface adsorbed [Fe^III(HQS)₃] was studied using femtosecond time scale transient absorption spectroscopy. Electron injection has been confirmed by direct detection of the electron in the conduction band. A TiO₂-based dye-sensitized solar cell (DSSC) was fabricated using [Fe^III(HQS)₃] as a sensitizer, and the resulting DSSC exhibited an open-circuit voltage value of 425 mV. The value of the short-circuit photocurrent was found to be 2.5 mA/cm². The solar to electric power conversion efficiency of the [Fe^III(HQS)₃] sensitized TiO₂-based DSSC device was 0.75 %. The results are discussed in the context of sensitization of TiO₂ by other Fe(II)-dye complexes.     

Effect of surface photoreactions on the photocoloration of a wide band gap metal oxide: probing whether surface reactions are photocatalytic

A nonphotocatalytic reaction occurring on the surface of an irradiated wide band gap metal oxide, such as ZrO2, can affect the process of photoinduced formation of Zr3+, F- and V-type color centers. The effect of such reactions is seen as the influence of photostimulated adsorption on the photocoloration of the metal oxide specimen. In particular, photoadsorption of electron donor molecules leads to an increase of electron color centers, whereas photoadsorption of electron acceptor molecules leads to an increase of hole color centers. Monitoring the photocoloration of a metal oxide during a surface photochemical reaction probes whether the reaction is photocatalytic: accordingly, the influence of simple photoreactions on the photocoloration of ZrO2, reactions that involved the photoreduction of molecular oxygen, the photooxidation of molecular hydrogen, the photooxidation of hydrogen by adsorbed oxygen, and the photoinduced transformation of ammonia and carbon dioxide. Kinetics of the photoprocesses are reported, as well as the photoinduced chesorluminscence (PhICL effect) of ammonia. Thermoprogrammed desorption and mass spectral monitoring of the photoreaction involving NH3 identified hydrazine as an intermediate and molecular nitrogen as the final product. The photoreactions involving NH3 and CO2 are nonphotocatalytic processes, in contrast to the photooxidation of hydrogen which is photocatalytic. Carbon dioxide and carbonate radical anions are formed by interaction of CO2 with Zr3+ centers and hole states (OS-*), respectively. Mechanistic implications are discussed.     

Dimensional reduction of a layered metal chalcogenide into a 1D near-IR direct band gap semiconductor

Reducing the dimensionality of inorganic lattices allows for the creation of new materials that have unique optoelectronic properties. We demonstrate that a layered metal chalcogenide lattice, TiS(2), can form a dimensionally reduced crystalline one-dimensional hybrid organic/inorganic TiS(2)(ethylenediamine) framework when synthesized from molecular precursors in solution. This solid has strong absorption above 1.70 eV and pronounced emission in the near-IR regime. The energy dependence of the absorption, the near-IR photoluminescence, and electronic band structure calculations confirm that TiS(2)(ethylenediamine) has a direct band gap.     

Origins of hole doping and relevant optoelectronic properties of wide gap p-type semiconductor, LaCuOSe

LaCuOSe is a wide band gap (～2.8 eV) semiconductor with unique optoelectronic properties, including room-temperature stable excitons, high hole mobility ～8 cm(2)/(Vs), and the capability of high-density hole doping (up to 1.7 × 10(21) cm(-3) using Mg). Moreover, its carrier transport and doping behaviors exhibit nonconventional results, e.g., the hole concentration increases with decreasing temperature and the high hole doping does not correlate with other properties such as optical absorption. Herein, secondary ion mass spectroscopy and photoemission spectroscopy reveal that aliovalent ion substitution of Mg at the La site is not the main source of hole doping and the Fermi level does not shift even in heavily doped LaCuOSe:Mg. As the hole concentration increases, the subgap optical absorption becomes more intense, but the increase in intensity does not correlate quantitatively. Transmission electron microscopy indicates that planar defects composed of Cu and Se deficiencies are easily created in LaCuOSe. These observations can be explained via the existence of a degenerate low-mobility layer and formation of complex Cu and Se vacancy defects with the assistance of generalized gradient approximation band calculations.     

Ultraviolet-emitting conjugated polymer poly(9,9'-alkyl-3,6-silafluorene) with a wide band gap of 4.0 eV

An ultraviolet-emitting conjugated polymer, poly(9,9'-alkyl-3,6-silafluorene) with a wide band gap of 4.0 eV, has been synthesized and characterized.     

Nitrogen defects in wide band gap oxides: defect equilibria and electronic structure from first principles calculations

The nitrogen related defect chemistry and electronic structure of wide band gap oxides are investigated by density functional theory defect calculations of N, NH, and as well as and in MgO, CaO, SrO, Al(2)O(3), In(2)O(3), Sc(2)O(3), Y(2)O(3), La(2)O(3), TiO(2), SnO(2), ZrO(2), BaZrO(3), and SrZrO(3). The N acceptor level is found to be deep and the binding energy of NH with respect to and is found to be significantly negative, i.e. binding, in all of the investigated oxides. The defect structure of the oxides was found to be remarkably similar under reducing and nitriding conditions (1 bar N(2), 1 bar H(2) and 1 × 10(-7) bar H(2)O): NH predominates at low temperatures and predominates at higher temperatures (>900 K for most of the oxides). Furthermore, we evaluate how the defect structure is affected by non-equilibrium conditions such as doping and quenching. In terms of electronic structure, is found to introduce isolated N-2p states within the band gap, while the N-2p states of NH are shifted towards, or overlap with the VBM. Finally, we assess the effect of nitrogen incorporation on the proton conducting properties of oxides and comment on their corrosion resistance in nitriding atmospheres in light of the calculated defect structures.     

A generalized analytical model for the elastic deformation of an adhesive contact between a sphere and a flat surface

A new method to calculate the elastic deformation of a sphere on a flat surface is presented. The model considers the influence of short-range as well as long-range attractive forces both inside and outside the actual contact area. In contrast to earlier models, this theory describes the nature of these deformations in the intermediate regime between the so-called JKR and DMT limits by simple analytic expressions. Equations for the calculation of the contact radius, the deformation, and the pressure distribution are given. In all equations, the critical force that might vary between the limiting values found in the DMT and the JKR model acts as transition parameter.     

Rare earth oxides in zirconium dioxide:How to turn a wide band gap metal oxide into a visible light active photocatalyst

In the present study, we investigated the effect of cerium and erbium doping of the zirconium dioxide matrix. We synthesized doped samples using hydrothermal process. The amounts of dopant used were 0.5%, 1% and 5% molar(rare earth oxide over zirconium dioxide) respectively. The samples have been studied via X-ray Diffraction measurements for the structural characterization. UV visible diffuse reflectance was used for the optical analysis, Brunauer-Emmett-Teller(BET) model for the measurement of the surface area. Finally the samples have been analysed via electron paramagnetic resonance(EPR) for the electronic characterization. Then we tested the new synthetized materials to determine their photocatalytic activity in the reaction of degradation of methylene blue performed under irradiation by diodes(LEDs) emitting exclusively visible light.     

Theoretical ab initio study of the intrinsic band gap in semiconductor oxides based on modified titanium dioxides

The results of Hartree–Fock calculations mimicking the effect of transition metal ions on electronic and structural properties of rutile and anatase structures have been presented. The vertical excitation energies of modified structures were estimated by performing single point configuration interaction calculations including only singlets (CIS). The main discussion concerns the excitation shifts into a longer wavelengths region and their relation to the frontier orbitals responsible for the observed band gap changes. The results obtained allow us to give some insight into understanding the target phenomenon as well as point out on the importance of boundary frontier orbitals. Particularly, it was shown that the transition metal modified anatase structures prepared by modern ion-engineering techniques are superior compared to those prepared by the use of conventional ones.     

Surface-assisted one-dimensional self-assembly of a perylene based semiconductor molecule

Chloroform-vapor annealing of thin films of propoxyethyl perylene tetracarboxylic diimide (PE-PTCDI, an n-type semiconductor) deposited on glass or mica leads to formation of well-defined one-dimensional self-assemblies (e.g. nanobelts), which show optically uniaxial properties as demonstrated by the linearly polarized emission.     

Sensitization of wide band gap photocatalysts to visible light by molten CuCl treatment

Cu(i)-substituted metal oxide photocatalysts were prepared using molten CuCl treatment of wide band gap photocatalysts. The Cu(i)-substituted metal oxide photocatalysts possessed a new absorption band in the visible light region and showed photocatalytic activity for hydrogen evolution from an aqueous solution containing sulfur sacrificial reagents under visible light irradiation. Notably, the Cu(i)–K₂La₂Ti₃O₁₀ and Cu(i)–NaTaO₃ photocatalysts showed relatively high activities for hydrogen evolution and gave apparent quantum yields of 0.18% at 420 nm. These photocatalysts responded up to 620 nm. Thus, Cu(i)-substitution using a molten CuCl treatment was an effective strategy for sensitizing a metal oxide photocatalyst with a wide band gap to visible light.     

Theoretical treatments of ultrafast electron transfer from adsorbed dye molecule to semiconductor nanocrystalline surface

In studying ultrafast electron transfer from a dye molecule to a nanosized semiconductor particle, pump-probe experiments are commonly used. In this system the electron transfer (ET) rate is faster than vibrational relaxation so that the ET rate should be described by a single-level rate constant and the probing signal (often in the form of time-resolved spectra) contains the contribution from the dynamics of both population and coherence (i.e., wave packet). In this paper, we shall present the theoretical treatments for femtosecond time-resolved pump-probe experiment and the dynamics of population and coherence by the density matrix method, and the calculation of single-level ET rate constant involved in a pump-probe experiment. As an application, we show the theoretical results using parameters extracted from experiments on a specific dye/semiconductor system.     

Effect of molecular binding to a semiconductor on metal/molecule/semiconductor junction behavior

Diodes made by (indirectly) evaporating Au on a monolayer of molecules that are adsorbed chemically onto GaAs, via either disulfide or dicarboxylate groups, show roughly linear but opposite dependence of their effective barrier height on the dipole moment of the molecules. We explain this by Au-molecule (electrical) interactions not only with the exposed end groups of the molecule but also with its binding groups. We arrive at this conclusion by characterizing the interface by in situ UPS-XPS, ex situ XPS, TOF-SIMS, and Kelvin probe measurements, by scanning microscopy of the surfaces, and by current-voltage measurements of the devices. While there is a very limited interaction of Au with the dicarboxylic binding groups, there is a much stronger interaction with the disulfide groups. We suggest that these very different interactions lead to different (growth) morphologies of the evaporated gold layer, resulting in opposite effects of the molecular dipole on the junction barrier height.     

Neutralization of ions at an electronically excited semiconductor surface

Neutralization of positive ions colliding with a semiconductor surface is studied. It is shown that the neutralization probability can be significantly enhanced if the surface exposed to the impinging ions is electronically excited. The basic reason behind this is that the impinging ions have easier access to the excited surface electrons than to bulk electrons.     

Three-phase contact line profile between a large air bubble and a flat solid surface

The spreading of the three-phase contact (TPC) line after rupture of the thin film between a large air bubble and a flat solid surface was experimentally studied by means of a CCD high-speed video technique. The present study shows that the TPC line profile is noncircular for a deformed bubble.     

Relation between surface preconditioning and metal deposition in direct galvanic metallization of insulating surfaces

The relation between surface preconditioning and metal deposition in the direct galvanic metallization of different insulating polymer surfaces by the so-called PLATO technique was studied using electrochemical and surface analytical methods. AFM, XPS and contact angle measurements show that the chromic acid etching of original polymer surfaces leads to an increase of the surface energy and hydrophilicity of polymer substrates due to both surface roughening and the formation of -COOH and/or -COH surface groups. However, decisive for the subsequent surface activation with cobalt sulfide is the increase in surface roughness. The influence of the degree of activation and the electrode potential on the kinetics of Ni metallization was studied by current transient measurements on activated line-shaped structures. The results suggest that the electrochemical reduction of cobalt sulfide to cobalt is a necessary step to induce the process of Ni electrodeposition. Successful Ni metallization could be obtained on ABS (acrylonitrile-butadiene-styrene) and PEEK (poly-ether-ether-ketone) surfaces. The lateral propagation rate, V _x , of the depositing Ni layer depends exponentially on the applied potential and was found to be several orders of magnitude higher than the Ni deposition rate, V _z , in the normal z-direction (V _x /V _z =10²–10⁴). It was demonstrated that the approach involving cobalt sulfide pre-activation can also be applied successfully for metallization of oxidized porous silicon surfaces.Presented at the 3rd International Symposium on Electrochemical Processing of Tailored Materials held at the 53rd Annual Meeting of the International Society of Electrochemistry, 15–20 September 2002, Düsseldorf, Germany