Dhrubojyoti Roy

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 °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 α-phase characteristic when annealed at 65 °C, and it remains intact in this phase up to 200 °C. The α–β phase transformation starts occurring from 200 °C and is completed at 290 °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 α phase to stable β phase. Electrical conductivity is found to be enhanced considerably for β-phase with respect to α-phase in dark and under photoexcitation because of better charge carrier transport.

Dynamics of roughening and growth kinetics of CdS–polyaniline thin films synthesized by the Langmuir–Blodgett technique

Thin films of cadmium sulphide (CdS) nanoparticle induced polyaniline (PANI) nanocomposites have worked as a better system for application in photovoltaics due to the efficient charge separation and charge transfer. In this communication, we have chosen such a system of varying thickness deposited by the Langmuir–Blodgett (LB) technique in order to study the growth and roughness phenomena by dynamic scaling theory. Different techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and X-ray reflectivity (XRR) are used for characterization of these films. In the study, the growth exponent, β, is found to have a correlation with the invariant logarithmic scaling law. A very large value of β = 1.18 ± 0.23 as calculated is a result of the rapid roughening in the multilayer film growth process. We have investigated the dynamic scaling behavior of the multilayer system which shows a difference in the value of the coarsening factor and this confirms the breakdown of self-affinity as a consequence of some nonlocal growth effects. The normal grain formation with deposition at the initial stage and the increase in grain abnormality with increase in thickness is also verified in support of the dynamic scaling ansatz. In the present study the layer-by layer deposition is confirmed by XRR fitting for a 3 layer film whereas the multilayer shows diffusing behavior due to H-bonding or electrostatic interactions.

Effect of annealing on the growth dynamics of ZnPc LB thin film and its surface morphology

The ZnPc molecules in the thin film prepared by Langmuir-Blodgett (LB) process in asdeposited state has been found to have an edge on orientation with average tilt angle of 64.3 ° as confirmed from the Pressure-Area (π-A) isotherm and X-ray diffraction (XRD) study. The ZnPc LB thin film has been observed to have abnormal growth mode at higher annealing temperature and it is mainly driven by minimization of surface free energy which lead to large increase in crystallinity of the film. Kinetically favored orientational and structural transitions of ZnPc thin film during annealing and their effect on the surface morphology of the thin film has been studied using scaling concepts. The scaling exponents 1) root mean square (RMS) roughness σ, 2) roughness exponent α and, 3) in plane correlation length ξ are calculated from the HDCF g(r) and ACF C(r). The RMS surface roughness σ is found to be dependent on the as defined short wavelength undulations (ρ) and long wavelength undulations (χ). Both ρ and χ are the f...

Study of polymorphism of ZnPc LB thin film on annealing

The average molecular orientation in the as-deposited Zinc Phthalocyanine (ZnPc) Langmuir-Blodgett (LB) film has been obtained to be 47° w.r.t to substrate surface from polarized Raman spectroscopy analysis. Absorption spectroscopy confirms the phase transition of ZnPc thin film from α-phase to β-phase on annealinghowever, such confirmation could not get established from Raman spectroscopy.

Growth and Structural Study of Cadmium Arachidate Multilayers Deposited by LB Technique

Langmuir–Blodgett (LB) multilayer films of cadmium arachidate acid (CdAA) have been studied by X-ray diffraction and X-ray reflectivity techniques, which reveal uniform ordered packing of CdAA molecules with a small tilt angle of long alkyl chain equal to 6.8º. Electron density profiles reveal that the coverage of films remains almost constant with increase in bilayer thickness, which indicates very little presence of pinhole defects. FTIR studies confirm orthorhombic packing in CdAA LB films and the formation of unidentate bridging metal-carboxylate coordination type headgroups consistent with one cadmium metal ion between two carboxylic (COO) groups in each headgroup.

Optical and surface morphology study of zinc phthalocyanine Langmuir Blodgett thin film

The UV-Vis absorption spectroscopy analysis reveals that prominent J-aggregation of ZnPc molecules was observed in the LB films while no such aggregation was found in the solution. Change in fluorescence color of ZnPc LB film from its solution confirms the appearance of new aggregation.

Structural, optical and electrical properties of CdS–polyaniline Langmuir–Blodgett films

Structural, optical and electrical properties study of the cadmium sulphide (CdS) incorporated polyaniline (PANI) thin films with varying layers have been carried out. It is seen that layer variation enhances the particle mean sizes with quenching of photoluminescence and an increase in rectifying nature of current-voltage measurements.