Postdeposition growth of a uniquely nanostructured organic film by vacuum annealing

Abstract

The organic red pigment N,N-di(3,5-xylyl)perylene-3,4:9,10bis(dicarboximide) is easily vacuum deposited at ambient substrate temperatures to form ordinary looking smooth films. When subsequently annealed in vacuo between ∼160 and 220 °C, an initially 1000 Å uniformly thick film transforms to an extremely dense, random array of discrete, crystalline whiskers, uniformly oriented with their long axes normal to the substrate. Whisker growth occurs at zero supersaturation ratio (no incident flux) and all film material is conserved. Whisker cross sectional dimensions are ∼27×55 nm, lengths 1–1.5 μm, and areal number densities ∼50 per square micron. The high-aspect-ratio whiskers grow purely by surface diffusion to minimum energy binding sites believed to be emergent screw dislocations on the whisker tips, while the uniform cross sectional shapes reflect the ratio of surface free energies of the bounding crystal faces. The transformation has been followed in situ with reflection absorption IR spectroscopy as a function of time and temperature to obtain a dimensionless growth parameter. The surface self-diffusion coefficient and surface entropy to free energy ratio are extracted. The pigment whiskers are readily sputter or vapor-coated to provide conformally coated nanostructure-sized features with metal and semiconductor properties. Vapor coating and annealing of the original pigment films can be done reproducibly in large areas and offer a novel alternative approach to plasma etching and photolithography for producing nanostructured surfaces with new properties.