Zhang Yue, Fakhraai Zahra
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323
Proc Natl Acad Sci U S A. 2017 May 9;114(19):4915-4919. doi: 10.1073/pnas.1701400114. Epub 2017 Apr 3.
Tobacco mosaic virus is used as a probe to measure surface diffusion of ultrathin films of ,-Bis(3-methylphenyl)-,-diphenylbenzidine (TPD) (12 nm [Formula: see text] 53 nm, where [Formula: see text] is the film thickness) at various temperatures below the glass transition temperature, [Formula: see text], of all films. As the film thickness is decreased, [Formula: see text] 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 ([Formula: see text]400 nm). For the thinnest films ([Formula: see text]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.
烟草花叶病毒被用作探针，以测量在低于所有薄膜玻璃化转变温度(T_g)的不同温度下，双（3 - 甲基苯基） - 二苯基联苯胺（TPD）（12纳米[公式：见正文]53纳米，其中[公式：见正文]为薄膜厚度）超薄膜的表面扩散情况。随着薄膜厚度的减小，(T_g)迅速降低，且平均薄膜动力学增强了6 - 14个数量级。我们表明，表面扩散与薄膜厚度的减小以及由此产生的整体迁移率增强无关。表面扩散系数的值及其温度依赖性与薄膜厚度无关，并且与相应的体相值（[公式：见正文]400纳米）相同。对于最薄的薄膜（[公式：见正文]20纳米），重排的有效活化能（弛豫时间的温度依赖性）变得小于表面扩散的活化能。这些结果表明，快速表面扩散与薄膜弛豫动力学解耦，是一种仅有的自由表面性质。
