Abstract
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 function of all the three scaling exponents. σ has been observed to be maximum for the ZnPc thin film annealed at 290 °C, since the χ shoot to maximum value at this temperature due to the formation of small domains of ZnPc nanorods. The self affinity of the ZnPc thin film is found to decrease on annealing as obtained from both power spectral density (PSD) and HDCF g(R) & ACF C(R) study, which confirms that the dimension of surface morphology of the ZnPc LB thin film transform towards 2D with increase in annealing temperature.
Highlights
We have characterized the morphology of the LB deposited ZnPc thin film of 40 layers using atomic force microscopy (AFM) and X-ray diffraction (XRD)
The Atomic Force Microscopy (AFM) and FESEM images shows that the grains in asdeposited thin film grows to elongated rod shaped grains on annealing at 290 ◦C which reveals that the temperature dependent asymmetric grain growth of the ZnPc nanoparticles over the thin film matix at higher temperature
The root mean square (RMS) surface roughness (σ ) is observed to be minimum for the ZnPc thin film annealed at 200 ◦C where both the short and long wavelength undulations are at their lowest value, whereas σ is maximum for the ZnPc thin film annealed at 290 ◦C which is due to increase of χ value because of the formation ZnPc nanorods in patch of small domains
Summary
Atomic Force Microscopy (AFM) & Scanning Electron Microscopy (SEM) provides a good insight in understanding the surface and structural morphology of the thin film, while X-ray diffraction (XRD) provides good understanding towards crystallinity and orientational growth of grains in thin film on annealing. Scaling concepts has been used to study and describe the statistical properties of the morphology of the annealed ZnPc thin film surface. The scaling exponents such as (1) root mean square (RMS) roughness σ , (2) roughness exponent α and, (3) in plane correlation length ξ , has been taken up to for this purpose. A brief discussion on the annealing effect towards the growth mechanism (i.e. crystallographic orientation and the grain growth) in ZnPc thin film and the factors controlling it has been put forward
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