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-perylenetetracarboxyli c diimide (DMPI) has been incorporated in Langmuir-Blodgett (LB) films mixe d with stearic acid (SA). Steady-state spectroscopic studies of the LB-depo sited 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. Concentra tion dependent studies demonstrate that, with increasing mole fraction of D MPI in the mixed film, the broad band emission at 650 nm increases in inten sity while the higher energy band at 570 nm decreases. Excitation spectra c orresponding to the two emission bands at 570 and 650 nm are observed to be different, indicating that the species corresponding to the emission at 57 0 and 650 nm are different. One plausible explanation is that the band at 5 70 nm corresponds to the monomeric species while the band at 650 nm corresp onds to an aggregate. Near-field scanning optical microscopy (NSOM) of the mixed LB films shows granular structures in addition to flat, irregular-sha ped, terrace-like structures. Detailed studies confirm these structures to be three-dimensional. A close examination of the NSOM topographic and fluor escence scans as well as atomic force microscopy (AFM) studies shows that t hese granular structures are crystallites of DMPI. On the other hand it is confirmed that the flat, irregular-shaped structures, that are nonfluoresce nt and absent in the NSOM fluorescence scans, are indicative of SA multilay ers. 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.