Sculptured thin films: nanorods, nanopipes, nanosmiles

Abstract

Atomic shadowing during physical vapor deposition causes exacerbated growth of surface protrusions and leads to a chaotic 3D layer growth, which can result in the development of well-separated nanorods, nanosprings, or nanopipes, which are surprisingly regular and have potential applications ranging from fuel cell electrodes and pressure sensors to self-lubricating coatings and nanoactuation. Glancing angle deposition (GLAD) causes particularly strong atomic shadowing and can be used to systematically investigate the effect of shadowing on the morphological evolution. These extremely rough layers cannot be described as a chaotic perturbation from a flat surface. However, using a model which describes them as a nanorod array with an average rod width that follows power law scaling results in experimental curves where all metals converge on a single master curve which exhibits a discontinuity at 20% of the melting point, associated with a transition from a 2D to a 3D island growth mode, and a single homologous activation energy of 2.46 for surface diffusion on curved nanorod growth fronts, which is applicable to all metals at all temperatures. Also, under extreme shadowing conditions, the conventional structure zone model is simplified as there is a direct transition from an underdense (zone I) to a dense (zone III) structure at ~50% of the melting point.ïýc