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Films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blend have been annealed at a temperature above their glass transition temperatures for up to 48 h. Surface chemical compositions of the cast and annealed films were measured by X-ray photoelectron spectroscopy (XPS) while surface topographical changes were followed by atomic force microscopy (AFM). The blend films spin-cast from chloroform produce nonequilibrium surfaces with a significant excess of PMMA. The polymer component with a lower surface free energy, PS, is shown to segregate to the surface upon annealing. The PS surface concentration of the films, containing 50% PS:50% PMMA in the bulk, was evaluated using the ester peak in XPS C 1s spectra (sampling depth ∼ 9 nm) and found to increase from ∼5% (freshly spin-cast film) to a saturated level of ∼47% after 17 h of annealing. AFM imaging reveals evolution of blend morphology with annealing time. The spin-cast films prior to annealing exhibit pitted topography with typical pit size of ∼1.2 μm and depth of 30−40 nm. As the annealing process proceeds, these pits get continually shallower. Frictional force microscopy with hydroxylated tips recorded surface phase separations for the films of 2−4 h annealing. As the annealing continues to above 14 h, the pitted structure becomes distorted. The surface enrichment and morphology changes upon annealing are explained by dewetting of PMMA relative to PS.

Cathodoluminescence spectra have been measured to determine the characteristics of ubiquitous green luminescence (GL) in nonstoichiometric zinc oxide (ZnO). Zn-and O-rich ZnO were found to exhibit characteristic emissions at 2.53 eV [full width at half-maximum (FWHM) 340 meV] and 2.30 eV (FWHM 450 meV), respectively. Hydrogen was used to probe the physical nature of GL centers. The Zn-rich GL is enhanced upon H incorporation, whereas the O-rich GL is completely quenched as its underlying acceptor-like V Zn centers are passivated by H. The GL emission bands each exhibit remarkably different excitation-power dependencies. The Zn-rich GL follows a close to linear relationship with excitation power, while the O-rich GL exhibits a square-root dependence. Calculations based on bimolecular recombination equations show the defect concentration in Zn-rich ZnO is three orders of magnitude greater than that in O-rich ZnO, indicating V O is more readily formed than V Zn in thermochemical treatments of ZnO.

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