Electric Field Modulated Near-Field Photo-Luminescence of Organic Thin Films

The concentrated electric field (E-field) in the vicinity of a voltage-biased near-field optical probe is used to modulate the photoluminescence of organic thin films on the ∼100 nm scale. The samples are bilayers comprising a self-organized-thin-film (50−500 nm) layer of zinc-octakis (β-decoxyethyl) porphyrin (ZnODEP ) on top of an indium tin oxide (ITO) coated glass electrode. The Al coated-optical-fiber-near-field probe functions simultaneously as a noncontacting moveable electrode and a local source of optical excitation (30−70 nm aperture). When the ITO electrode is charged positive relative to the probe (yielding a field on the order of 1MV/cm -1 ), the photoluminescence (fluorescence) intensity decreases ∼5% and the probe−sampleprobe−sample distance (under shear-force feedback control) increases ∼7 Å. Opposite effects for both signals occur when the ITO is charged negative. The E-field effect on the near-field luminescence properties of thin films of ZnODEP is discussed in terms of the following physical mechanisms: (i) direct field induced dissociation of excitons, (ii) exciton/charge carrier electron/hole transfer processes, and (iii) interfacial electron/hole transfer processes. The potential for using the observed effects as an indirect means of imaging charge injection efficiencies in organic thin film devices is explored.