STM-induced luminescence study of poly(p-phenylenevinylene) by conversion under ultraclean conditions

Curing-temperature-dependent spectroscopic studies of the conversion process of the sulfonium chloride prepolymer to the conjugated polymer poly(p-phenylenevinylene) reveal an enhancement of the luminescence efficiency when the films are converted under ultrahigh-vacuum conditions. In these experiments, luminescence is excited by electron injection from the tip of a scanning tunneling microscope and the maximum luminescence efficiency is found between 225 °C and 260 °C. The optimum conversion temperature, the total luminescence yield, and the spectral features of the luminescence depend on the substrate material, heating gradient, and composition and purity of the prepolymer. The curing-temperature dependence of the Franck-Condon intensity distribution has a complex behavior. Maximum luminescence efficiency is characterized by a spectrum where the v=0,1 vibronic transition has maximum relative intensity. Images of scanning-tunneling-microscopy-excited luminescence show intensity fluctuations within surface domains as small as a few nanometers in diameter, regions that correlate with the topographic features of the poly(p-phenylenevinylene) surface.