Spectroscopic, AFM, and NSOM studies of 3D crystallites in mixed Langmuir-Blodgett films of N,N′-bis(2,6-dimethylphenyl) 3,4,9,10-perylenetetracarboxylic diimide and stearic acid

Nonamphiphilic N.N'-bis(2,6-dimethylphenyl)-3,4,9,10-perylenetetracarboxylic diimide (DMPI) has been incorporated in Langmuir-Blodgett (LB) films mixed with stearic acid (SA). Steady-state spectroscopic studies of the LB-deposited films on quartz substrates indicate an extensive broadening and shift of the absorption and emission spectra, suggesting molecular aggregation. Perhaps the most interesting spectral feature observed in this study is the observation of a broad emission band with its maximum at 650 nm. Concentration dependent studies demonstrate that, with increasing mole fraction of DMPI in the mixed film, the broad band emission at 650 nm increases in intensity while the higher energy band at 570 nm decreases. Excitation spectra corresponding to the two emission bands at 570 and 650 nm are observed to be different, indicating that the species corresponding to the emission at 570 and 650 nm are different. One plausible explanation is that the band at 570 nm corresponds to the monomeric species while the band at 650 nm corresponds to an aggregate. Near-field scanning optical microscopy (NSOM) of the mixed LB films shows granular structures in addition to flat, irregular-shaped, terrace-like structures. Detailed studies confirm these structures to be three-dimensional. A close examination of the NSOM topographic and fluorescence scans as well as atomic force microscopy (AFM) studies shows that these granular structures are crystallites of DMPI. On the other hand it is confirmed that the flat, irregular-shaped structures, that are nonfluorescent and absent in the NSOM fluorescence scans, are indicative of SA multilayers. These studies confirm the formation of three-dimensional crystallites of the dye formed in the LB films due to phase separation of the components, which suggests low miscibility of the components.