Polythiophene, oligothiophene, and thiophene interfaces and their applications for electronic devices

This thesis includes research topics related to oligo- and polythiophenes and thiophene containing alkanethiols. These comprise an important class of molecules with important organic device applications, including light-emitting diodes, photovoltaics, field-effect transistors, and sensors.; A novel means of lithographically forming fluorescent oligothiophene patterns is demonstrated. MgKalpha X-ray and low energy electron irradiation of 3-hexylthiophene monomer condensed on clean gold at 145 K results in formation of an organic photoluminescent film that does not desorb when the sample is warmed to room temperature. C 1s and S 2p X-ray photoelectron spectra (XPS) of the X-ray and electron-formed films are similar to those of chemically-synthesized poly(3-hexylthiophene). Ultraviolet photoelectron spectroscopy (UPS) indicates the presence of thiophene ring electronic states, consistent with retention of conjugation. UPS spectra of 3-hexylthiophene monomer and beam-formed films have been compared with theoretically simulated density-of-states spectra of 3-hexylthiophene, thiophene, bithiophene, terthiophene, and quaterthiophene. The radiation-induced changes in the 3-hexylthiophene UPS valence spectra are explained by delocalization of electrons along the oligomer backbone. Comparison of the experimental UPS and simulated spectra suggests that the average conjugation length of the beam-formed films is less than six. Photoluminescence measurements show that the excitation maxima for the X-ray- and electron-formed samples are 350 and 405 nm, respectively, with corresponding emission maxima of 430 and 525 nm, indicating that the films are oligomeric rather than polymeric. This is consistent with the UPS results. Electron bombardment with 5 keV electrons results in films of micron thickness, as determined by profilometry. The ability to lithographically oligomerize the monomer is demonstrated by forming a dot with an electron beam. FTIR spectroscopy of electron-formed films shows vibrations consistent with the formation of linearly oligomerized 3-hexylthiophene. Fluorescence and atomic force microscopies of electron-formed samples show the presence of oligomerized 3-hexylthiophene islands residing on a less fluorescent organic background. (Abstract shortened by UMI.)...