Optical polarization anisotropy induced by molecular alignment in pentacene films confined in micron sized grooves etched in SiO₂ substrate

Pentacene thin films grown on SiO₂ substrates with few micron-spaced patterns were found to show anisotropy in their photoluminescence (PL) responses. PL emission was maximum for excitation polarization parallel to the groove direction. Detailed analysis of polarized Raman spectra gave insight on the intermolecular coupling. The anisotropy in PL was found to arise from preferential in-plane molecular alignment of pentacene, induced by the walls of the grooves. This was verified by spatial scan of the polarized micro-PL emission. Though earlier reports have shown anisotropy for nanoscale patterned substrates, to our knowledge, controlled orientation of molecules induced by micron spaced patterning has not been reported so far. This work reports observation of six fold increase in PL response for polarization parallel to the groove of widths 5 μm. (paper)