Structural, morphological and photoluminescence studies on annealed Langmuir-Blodgett films of copper and zinc octakis-octyloxy phthalocyanine and their mixture

Langmuir-Blodgett films of copper(II) 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (CuPcOC8) and zinc 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (ZnPcOC8) and their mixture, CuPcOC8/ZnPcOC8 (1:1) at the air-solid (A-S) interface have been investigated using grazing incidence X-ray diffraction (GIXD), atomic force microscope (AFM), UV-visible absorbance and photoluminescence techniques. At the A-S interface, the GIXD pattern reveals the crystalline nature of the as-deposited and annealed LB films of CuPcOC8. The Bragg peak observed at 3.4 degrees (d = 25.9 angstrom) for the as-deposited and annealed film (323 K) tends to shift to a higher angle (3.8 degrees, d = 23.2 angstrom) at higher temperature (>373 K). Though the absence of Bragg peak in the as-deposited film of ZnPcOC8 suggests an amorphous phase, annealing at 323 K results in a Bragg peak at 3.4 degrees which shifts to a higher angle (3.6 degrees, d = 24.5 angstrom) at higher temperatures (>373 K). The mixed film also exhibited a Bragg peak at 3.4 degrees which shifts to 3.6 degrees with annealing. Surface topographies obtained using AFM reveal nanowire morphology for the as-deposited film of CuPcOC8 whereas a grainy texture is observed for ZnPcOC8. Upon annealing to 373 K, the coexistence of nanowire with thin crystalline flakes for CuPcOC8 and the coexistence of needle-like structures with granular features for ZnPcOC8 were observed. The topography of the as-deposited and annealed (till 373 K) films of the mixture shows the coexistence of two different morphologies (needle and granular, d = 25.9 angstrom) which on further annealing (>373 K) transforms to granular morphology (d = 24.5 angstrom). The UV-visible absorbance studies show the presence of B-bands and Q-bands for the as-deposited films of CuPcOC8, ZnPcOC8 and their mixture. Photoluminescence studies show a clear dependence of emission features with different excitation wavelengths for the as-deposited and annealed films of CuPcOC8, ZnPcOC8 and their mixture. The spectral features for these systems were deconvoluted to obtain the peak parameters at 300 nm excitation. Combining the results obtained using the above mentioned techniques, we infer the occurrence of Form-I polymorph (as-deposited and at 323 K), the coexistence of polymorphs (Form-I and Form-II) at 373 K and Form-III above 373 K for CuPcOC8. For the case of ZnPcOC8, though the spectral features for the as-deposited and annealed films (>373 K) were quite similar, GIXD and AFM results seem to suggest amorphous phase and Form-III polymorph. The new emissions in the blue region observed at 323 K and 373 K is attributed to the presence of Form-I and coexisting polymorphs (Form-I and Form-II). For the mixture, we infer a solid-solid phase separation (below 373 K) and a new crystalline polymorph (above 373 K). Our studies highlight how thermal annealing can modify the structure, packing, morphology of the thin film polymorphs and impact the optical properties.