Confocal Raman spectroscopy of -sexithiophene: from bulk crystals to field-effect transistors

We report confocal micro-Raman spectra of the organic semiconductor a-sexithiophene (T6) on bulk crystals and on thin films grown on technologically relevant substrates and devices. We show that the two polymorphs, which are clearly identified by their lattice phonon spectra, may coexist as physical impurities of one inside the other in the same crystallite. Spatial distribution of the two phases is monitored by Raman phonon mapping of crystals grown upon different conditions. Raman microscopy has then been extended to T6 thin films grown on silicon oxide wafers. We identify the crystal phase in thin films whose thickness is just 18 nm. The most intense total-symmetric Raman vibration is still detectable for a two-monolayer thick film. Comparative analysis between micro-Raman and AFM of T6 thin films grown on field effect transistors shows that electrode-channel steps favour the nucleation and growth of T6 molecules on the substrate, at least below 50 nm.