Characterization of molecular organization in pentacene thin films on SiO2 surface using infrared spectroscopy, spectroscopic ellipsometry, and atomic force microscopy

Abstract

Thin films of pentacene of 32 and 100nm thickness obtained by organic molecular beam deposition (OMBD) in high vacuum conditions onto silicon/native silica (Si/SiO2) and fused silica substrates were examined. Alignment, anisotropic optical properties and morphology were studied in ambient conditions using infrared (IR) transmission and polarized grazing angle attenuated total reflection (GATR) techniques, variable angle spectroscopic ellipsometry (VASE), UV–VIS absorption, and atomic force microscopy (AFM). For the first time dichroic GATR IR spectra were recorded for such thin films and conclusions on pentacene orientation were deduced on the basis of dichroic ratio of the IR-active vibrations. The symmetry assignment of the vibrational transitions is also discussed. The films exhibit continuous globular texture with uniaxial alignment of pentacene molecules and strongly anisotropic optical properties evidenced in the ellipsometric measurements. The results revealed that there are some quantitative differences in the orientation and in the dielectric properties between the two pentacene films of different thickness.