The distance over which singlet energy is transferred in polycrystalline films of perylene bis(phenethylimide), PPEI, was measured by a surface quenching technique in films ranging in thickness from 0.04 to 2.3 μm. Radiative energy transfer was not observed. Accurate values of the exciton transfer length could be obtained only with quenchers exhibiting rapid surface quenching velocities (>105 cm/s), such as poly(3-methylthiophene). The measured singlet exciton transfer length of 2.5 ± 0.5 μm is apparently the longest yet reported. Its approximate value can be inferred directly from the experimental data and is therefore essentially independent of the assumed theoretical model. Our measurements contain no direct information about the mechanism of the exciton motion; however, if it is assumed to be diffusional, the calculated intermolecular exciton hopping time, τh ≪ 100 fs, is unusually fast. This suggests that excitons, in fact, may be delocalized over a number of molecules and that coherent energy transfer plays some role in the exciton motion. Energy is apparently transferred further and faster in PPEI films than in natural photosynthetic light-harvesting systems.

GaN (Eg = ~3.4 eV) was photoelectrochemically characterized and the energetic position of its bandedges determined with respect to SHE. Electrochemical impedance spectroscopy was employed to analyze the interface, determine the space charge layer capacitance, and, subsequently obtain the flatband potential of GaN in different aqueous electrolytes. The flatband potential of GaN varied at an approximately Nernstian rate in aqueous buffer electrolytes of different pHs indicating acid-base equilibria at the interface.

A mechanism is developed that describes the transient photoelectrochemical response of Ti02 particles in a photoelectrochemical slurry cell. The specific effects of addition of reductants and oxidants are consistent with the proposed mechanism. The relevance of the proposed mechanism to the use of TiO: particles for water photodetoxification is examined.

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The optoelectronic properties of the II-IV-V2 semiconductor ZnSnP2 are studied as a function of the cooling rate of the crystal growth melt. The structure of the material, as studied by x-ray diffraction, is seen to change from chalcopyrite to sphalerite as the cooling rate is increased. Photoelectrochemical measurements show that the bandgap of the material decreases from 1.64 eV for the chalcopyrite to 1.25 eV as the structure approaches sphalerite. The 119Sn Mössbauer spectroscopy shows both an isomer shift and a broadening of the 119Sn resonance as a result of new tin environments produced by disordering of zinc and tin sites at the faster cooling rates. The 31P solid-state nuclear magnetic resonance spectroscopy clearly shows new resonances associated with the additional phosphorus environments produced by metal site disordering. A model based on zinc and tin site exchange with the introduction of compensating donor and acceptor states is proposed and discussed.