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The surface of colloidally prepared dodecylamine/trioctylphosphine capped CdTe nanocrystals has been studied by photoelectron spectroscopy. Particles preserved in inert atmosphere, show very high photoluminescence quantum yield which is stable over time. But exposure of these nanocrystal colloids to air reduces the photoluminescence efficiency drastically. Photoelectron spectroscopy study has been performed in order to reveal, if there are any changes occurring in the chemical bonding at the nanocrystal surfaces, which might explain the observed decrease of the photoluminescence efficiency. The photoelectron spectroscopy measurements showed significant oxidation of surface Te-atoms, upon exposure to air. A comparison is drawn between the thioglycolic acid capped CdTe and dodecylamine/trioctylphosphine capped CdTe nanocrystal surfaces.

We propose a high-voltage (>1000 V) pulsed discharge method of preparation of thin anodic films on valve metals under conditions of extremely high rates of the film growth. The peculiarities of this method have been demonstrated on the anodic oxidation of titanium in sulfuric acid solutions. A light flash at the electrode was generated upon the pulsed anodization of titanium. The emission spectra consist of narrow lines assigned to electronically excited O, H and Ti atoms, indicating that a hot plasma is created at the electrode surface. The current efficiency of TiO2 anodic film formation exceeds markedly 100% for the pulsed Ti anodization. This non-faradaic yield was associated with the contribution of the plasma-generated high-energy radicals to the oxide growth processes. We applied various techniques to compare the composition, morphology, structure and semiconducting properties of the anodic films prepared by the high-voltage pulsed discharge and the conventional galvanostatic anodization. The anodic TiO2 films prepared by the highvoltage pulsed discharge have nearly ideal stoichiometric composition and are more amorphous as compared to galvanostatically grown ones. Thin TiO2 films (50–150 nm) formed in the pulsed regime exhibit significantly higher photocurrent quantum yields at k < 310 nm and a significantly lower donor concentration. � 2005 Elsevier B.V. All rights reserved.

Composites of nanoparticles and conjugated polymers are studied via photoluminescence (PL), photoinduced absorption spectroscopy and light induced electron spin resonance (LESR) spectroscopy. Two different types of nanoparticles were investigated namely cadmium selenide and indium phosphide. They were available with a tri- n -octylphosphine-oxide–tri- n -octylphosphine shell or pyridine washed. Nanoparticles were mixed with poly(2-methoxy-5-(3-,7-dimethyloctyloxy)-1,4-phenylenevinylene) (OC 1 C 10 -PPV) in different weight ratios. The quenching of the PL, the formation of polaronic radicals as well as the occurrence of two new absorption bands are explained in the frame of a photoinduced charge transfer between the polymer and the nanocrystal. For a detailed analysis, the lifetime and recombination kinetics are studied by varying the chopper frequency and by changing the laser power. We find a broad distribution of lifetimes, which suggests a dispersive recombination process. With LESR the charge transfer process is confirmed and the g -factor of the polymer cation was determined. Time dependent studies show the involving of long living species in the recombination process.

Semiconductors quantum dots (QDs) possess discrete excitonic and phonon spectra. For a spherical QD of radius R, the excitonic spectrum in the effective mass approximation (EMA) is well known [1]. Quantization of dipole-active optical phonons in very small crystals has received much less attention. Usually it is assumed that, for a spherical QD, there is only one far-infrared (FIR) active mode corresponding to a uniform polarization of the sphere, called the Frohlich mode. Its frequency does not depend on the sphere’s radius. This is approximately correct if the QD radius exceeds 2-3 ran for II-VI materials like CdS and CdSe. For smaller QDs, the quantization of phonon modes is important. As it has been shown theoretically and experimentally in [2], FIR absorption spectrum of such a QD is constituted by a set of (homogeneously broadened) peaks at certain frequencies within the reststrahlen band of the bulk material. These phonon frequencies in function of the QD size can be calculated within a continuum approach similar to the EMA [3] or numerically.

This paper provides an overview of semiconductor nanocrystals, which were synthesized either by an aqueous technique in the presence of different thiols as stabilizing agents (CdS, CdSe, CdTe, Cd x Hg 1− x Te and HgTe) or via organometallic reactions (CdSe, CdTe, InP and InAs). In all cases, successful wide-range tuning of the semiconductor band-gap energies was achieved by the control of the nanocrystal size. In addition, the surface properties of the nanocrystals can be controlled by the exchange of the capping ligand. Blends of nanocrystals and conducting polymers, were prepared using CdTe nanocrystals and polyaniline, polypyrrole or poly(3,4-ethylenedioxythiophene) (PEDT), and considered with respect to possible optoelectronic applications. The photoinduced charge separation and transport in the CdTe–PEDT composites was investigated by photoelectrochemical methods.

CdSe, CdTe and InP nanocrystals were prepared by an organometallic synthesis using mixtures of highly boiling primary amines and trioctylphosphine (TOP) as the coordinating solvent, and were characterized by powder XRD, SAXS, HRTEM, absorption and luminescence spectroscopy. The use of amines allowed us to obtain small crystalline nanoparticles for all materials investigated. In all cases, as-prepared colloids show rather narrow particle size distributions which can be further improved by standard size selective precipitation. Amino-capped II–VI nanocrystals show strong size-dependent band edge photoluminescence (PL). CdSe nanocrystals with the mean particle size in the range of 1.2–3.0 nm exhibit emission from blue to green with room temperature quantum yields of 15–20%. CdTe nanocrystals (2.5–5.0 nm size range) show a PL tunable from green to red with quantum yields up to 65% at room temperature. InP nanocrystals (1.5–4.0 nm size range) possess a weak emission (

This work involves a comparative study of nanocrystalline titanium and indium oxides as well as TiO2–In2O3 composites prepared by a sol–gel method from concentrated hydrous titanium dioxide and indium hydroxide sols. Highly transparent uniform films were fabricated from the sols by a spin-coating technique. TiO2–In2O3 nanocomposites possess pronounced extreme dependencies of both the apparent bandgap and the refractive index on the film composition. No chemical interaction between the oxides was observed during annealing to temperatures between 200 and 700 °C. This behavior was attributed to the effect of surface states formed at the nanocrystallite interfaces. Other possible causes of the pronounced dependence of the optical and electronic properties on the TiO2-to-In2O3 ratio are also discussed. Abrupt changes in resistivity, photoresistivity and photocurrent decay time of the composite films were observed for composites where the TiO2 content ranged from 25 to 50 wt.% and are associated with the formation of a three-dimensional infinite cluster of interconnected In2O3 nanoparticles.

The photovoltage response of a semiconductor electrode to the periodic excitation by short laser pulses has been described in a quasi-steady-state approximation. Measurements with electrically-biased single- and polycrystalline n-TiO2 electrodes illuminated by a N2 impulse laser (337 nm, 10 ns) were used to demonstrate the validity of the proposed model for harmonic components in the frequency spectrum of the photovoltage and to illustrate its application to highly doped semiconductors. It has been shown that the higher harmonics of the ac photovoltage can be used to obtain information on the potential and frequency dependence of space-charge capacitance, which is not distorted by the influence of other elements of the equivalent circuit. The approach developed opens the possibility of determining the flatband potential and doping density in the case of polycrystalline semiconductor electrodes showing quite dispersive non-linear Mott–Schottky behaviour.

Charge transfer processes at the TiO2/electrolyte interface have been studied using periodic short laser pulses (337 nm) as probe. A model has been developed to determine a number of semiconductor characteristics from the Fourier image of the photovoltage response induced by the periodic pulsed excitation. It has been shown that the capacitance of depletion and Helmholtz layers, flatband potential, doping density and dielectric constant of a semiconductor electrode as well as the surface recombination rate can be determined by this method, even for polycrystalline semiconductor films exhibiting non-linear Mott–Schottky dependencies.

Dipole-active optical phonons confined in nearly spherical InP quantum dots (QDs) were studied theoretically and experimentally. Spatial quantization of the phonon modes in an InP QD was considered theoretically by solving the coupled continuous equation of motion and Poisson equation. It is shown that, when the Frohlich frequency falls into the gap between the bulk LO and TO phonon bands, a gap mode appears, which contains most of the dipole oscillator strength. Accordingly, the shape of the far-infrared (FIR) absorption spectra does not change with the QD size. This conclusion has been confirmed by our experimental FIR spectroscopy studies on films of InP QDs prepared by colloidal synthesis.

We report on the efficient photoluminescence up-conversion in colloidally-prepared core-shell CdSe/ZnS quantum dots. We demonstrate that the efficiency of photon energy up-conversion and the magnitude of the spectral shift can be controlled by the thickness of the ZnS shell, the temperature and the excitation intensity. Based on the analysis of the experimental data we propose that intrinsic gap states are involved in the photoluminescence up-conversion as intermediate states rather than nonlinear two-photon absorption or Auger processes.