Structure and electrical properties of nanoparticulate tungsten oxide prepared by microwave plasma synthesis.

Nanoparticulate WO(3) films were prepared using microwave plasma synthesis and studied with respect to the electrical conductivity in dependence of ambient conditions. The WO(3) films with a monoclinic structure were made from cluster-assembled nanoparticles (diameter 3 nm) by means of dispersion and spin-coating. Above 100 °C a thermally activated decrease of the electrical resistance due to oxygen vacancy donors is found. A reversible increase of the electrical resistance R due to oxygen uptake is observed. The decrease of R in response to reducing H(2)S in the ppm range is studied in dependence of temperature and pre-annealing conditions.

Note: On the deconvolution of Kelvin probe force microscopy data.

In Kelvin probe force microscopy (KPFM) proper interpretation of the data is often difficult because the measured surface potential is affected by the interaction of the cantilever with the sample. In this work, the tip's interaction with a modeled surface potential distribution was simulated, leading to a calculated KPFM image. Although simplified, the calculation is capable of showing the influence of the cantilever in the correct qualitative manner, proven by a comparison with experimental data. Additionally, a deconvolution was performed on the simulated image, showing that for simple geometries revealing the "real" surface potential data is possible in principle.

Stark spectroscopy of excited-state transitions in a conjugated polymer.

Stark spectroscopy, which is well established for probing transitions between the ground and excited states of many material classes, is extended to transitions between transient excited states. To this end, it is combined with femtosecond pump-probe spectroscopy on a conjugated polymer with appropriately introduced traps which harvest excitation energy and build up a sufficient excited state population. The results indicate a significant difference in the effective dipole moments between two short lived excited states.

Intrinsic room-temperature electrophosphorescence from a pi-conjugated polymer.

Electrically induced phosphorescence from a poly(para-phenylene) ladder-type polymer is observed for the first time and characterized using time resolved spectroscopy. Short-lived phosphorescence is also observed in gated fluorescence spectra and is found to be quenched reversibly by oxygen. Thermally activated triplet diffusion to covalently bound palladium sites, which are formed at a concentration of about 80 ppm in a side reaction during polymer synthesis, is believed to be the cause of this novel effect, which suggests a new approach to the design of efficient electroluminescent materials.

Polyfluorenes with polyphenylene dendron side chains: toward non-aggregating, light-emitting polymers.

A polyfluorene 12 has been prepared in which bulky polyphenylene dendrimer substituents suppress formation of long wavelength emitting aggregates, thus giving a polymer with pure blue emission. Absorption- and emission spectra and molecular modeling confirm that the bulky dendrimer side chains do not cause extra torsion between the fluorene units. New polyfluorenes with 9,9-diaryl substituents have been prepared to determine the minimum size of substituent necessary for aggregation suppression. An LED using 12 has been demonstrated to produce blue emission with onset voltages below 4 V.