Journal Article

Decoupling of surface diffusion and relaxation dynamics of molecular glasses

Zhang, Yue, Fakhraai, Zahra

2017

Overview

Tobacco mosaic virus is used as a probe to measure surface diffusion of ultrathin films of N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) (12 nm < h < 53 nm, where h is the film thickness) at various temperatures below the glass transition temperature, Tg, of all films. As the film thickness is decreased, Tg decreases rapidly and the average film dynamics are enhanced by 6–14 orders of magnitude. We show that the surface diffusion is invariant of the film thickness decrease and the resulting enhanced overall mobility. The values of the surface diffusion coefficient and its temperature dependence are invariant of film thickness and are the same as the corresponding bulk values (h = 400 nm). For the thinnest films (h < 20 nm), the effective activation energy for rearrangement (temperature dependence of relaxation times) becomes smaller than the activation energy for surface diffusion. These results suggest that the fast surface diffusion is decoupled from film relaxation dynamics and is a solely free surface property.

• Publisher: National Academy of Sciences
• Subject: Activation energy / Decoupling / Diffusion coefficient / Diffusion rate / Film thickness / Free surfaces / Glass transition temperature / Invariants / Molecular structure / Physical Sciences / Physics / Plant diseases / SEE COMMENTARY / Surface diffusion / Surface properties / Temperature / Temperature dependence / Temperature effects / Thin films / Tobacco / Transition temperatures / Viruses