Nanoscale investigation of the organic semiconductor tris-8-hydroxyquinoline aluminum by near-field scanning optical microscopy (NSOM)

We have used the high-resolution optical microscopy technique near-field scanning optical microscopy (NSOM) to probe the local optical and morphological properties in thin films (10 to 500 nm thick) and clusters of the luminescent molecule tris-8-hydroxyquinoline aluminum (Alq₃). Organic semiconductors such as Alq₃ are attractive materials for light-emitting diode (LED) and flat panel display technology due to their desirable properties: facile wide-area deposition, self-emission, and versatile color selection. Despite the numerous spectroscopic studies being conducted on Alq₃ and Alq₃-based devices, few studies examine the relationship of the morphology of the film to its optical properties. Using NSOM our typical optical and topographical resolution is better than 100 nm, the length scale of important optical processes and interesting structural domains. We use the combination of NSOM and concurrent shear force (SF) microscopy (analogous to atomic force microscopy or AFM) to correlate the morphology of different regions to intensity variations in film fluorescence as well as variations in localized fluorescence spectra. We have examined the fluorescence and topography variations of as-deposited vacuum-evaporated Alq₃, drop-cast Alq₃, spin-cast Alq₃, thermally annealed Alq ₃ films, and Alq₃ clusters. In addition, we have used the near-field optical probe tip as an active probe to examine localized photo-oxidation as a function of time, position and environment free from the limits of far-field spatial averaging. From these experiments we obtain a direct measurement of excited carrier or exciton diffusion. Finally, as a means of understanding the nanoscale properties of Alq₃, we have examined the topography and fluorescence of single clusters of five to ten Alq₃ molecules deposited on glass from solution. At higher concentrations, we observed unexpected film morphologies due to highly favorable Alq₃-Alq₃ interactions that dominated Alq₃-substrate interactions. At sufficiently low concentrations, we were able to spatially isolate Alq₃ clusters and measure their sizes, collect their spectra, measure their photo-oxidation, and compare the relative intensity levels of a variety of Alq₃ clusters dispersed on glass in order to estimate the number of chromophores or molecules per cluster.