Structure and growth of thin films of aniline on silver: nucleation and premelting of nanocrystallites, porosity, and crystallization.
The structure and growth of thin films of aniline vapor deposited on Ag(111) and Ag(110) surfaces have been examined using optical second harmonic generation (SHG) and linear optical differential reflectivity (DR). Aniline thin films deposited at 90 K give a detectable SH signal that arises from small polycrystallites with orientation anisotropy in the film. Upon annealing, the SH signal decreases, first due to premelting (at approximately 145 K) of the polycrystallites and then sublimation (at approximately 180 K) of the film. Quantitative analysis of the SH intensity change by a premelting model [J. Phys. Chem. 1988, 92, 7241] allows the determination of the average size of the crystallites as 1.1 nm in diameter and containing approximately 45 aniline molecules. The existence of the nanocrystalline structure and its premelting are confirmed by DR experiments. The DR signal around 145 K exhibits change corresponding to an order-disorder transition. Quantitative analysis of the DR data results in the same nanocrystallite size. Experimental observations indicate that films deposited at 90 K contain not only nanocrystallites but also approximately 30% porosity, which can be reduced by annealing. At temperatures above 195 K, micron-size crystallites start to form within the amorphous film, causing a large amount of light scattering while the film sublimates. It appears that, for molecules such as aniline with stronger intermolecular interactions, more enthalpy is released, upon adsorption to the local surrounding molecules, causing them to reorient into crystalline form. The low deposition temperature, on the other hand, prevents diffusion for further crystallization beyond nanocrystallites. The refractive index of the amorphous aniline solid can be determined as 1.68 +/- 0.03.