Comparative study of optical, structural and electrical properties of zinc phthalocyanine Langmuir-Blodgett thin film on annealing

Zinc phthalocyanine (ZnPc) in the mixture of N-methyl pyrroldione (NMP) and chloroform (CHCl3) [1 : 10 (v/v)] spreading solvent was observed to have only a monomeric absorption peak. However, on processing the solution into a thin film using the Langmuir-Blodgett (LB) technique, it shows absorption peaks corresponding to H, J and monomer aggregates, as confirmed by UV-Vis absorption spectroscopy. The Q-band absorption spectra of ZnPc LB thin film was observed to have a sharp change on annealing at 65 degrees C, indicating a change in the aggregation configuration of molecules over the surface. The annealed ZnPc LB thin film is found to have H and monomer aggregates only, which indicates that ZnPc molecules are arranged in an edge on face-to-face conformation. The ZnPc LB thin film remains in this aggregation until the higher annealing temperature. The ZnPc LB thin film shows an alpha-phase characteristic when annealed at 65 degrees C, and it remains intact in this phase up to 200 degrees C. The alpha-beta phase transformation starts occurring from 200 degrees C and is completed at 290 degrees C. The large increase in crystallite size, as obtained from the XRD study, and the change in the shape of the ZnPc nanoparticles from spherical to nanorod structure, as observed from FESEM and AFM images, confirms the transformation of the ZnPc film from metastable alpha phase to stable beta phase. Electrical conductivity is found to be enhanced considerably for beta-phase with respect to alpha-phase in dark and under photoexcitation because of better charge carrier transport.