Richert R, Tracy M E, Guiseppi-Elie A, Ediger M D
School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, USA.
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
J Chem Phys. 2024 Jan 21;160(3). doi: 10.1063/5.0187166.
Glassy films of methyl-m-toluate have been vapor deposited onto a substrate equipped with interdigitated electrodes, facilitating in situ dielectric relaxation measurements during and after deposition. Samples of 200 nm thickness have been deposited at rates of 0.1 nm/s at a variety of deposition temperatures between 40 K and Tg = 170 K. With increasing depth below the surface, the dielectric loss changes gradually from a value reflecting a mobile surface layer to that of the kinetically stable glass. The thickness of this more mobile layer varies from below 1 to beyond 10 nm as the deposition temperature is increased, and its average fictive temperature is near Tg for all deposition temperatures. Judged by the dielectric loss, the liquid-like portion of the surface layer exceeds a thickness of 1 nm only for deposition temperatures above 0.8Tg, where near-equilibrium glassy states are obtained. After deposition, the dielectric loss of the material positioned about 5-30 nm below the surface decreases for thousands of seconds of annealing time, whereas the bulk of the film remains unchanged.
间甲基苯甲酸甲酯玻璃膜已通过气相沉积法沉积在配备叉指电极的基板上，便于在沉积期间和之后进行原位介电弛豫测量。已在40K至玻璃化转变温度Tg = 170K之间的各种沉积温度下，以0.1nm/s的速率沉积了厚度为200nm的样品。随着距表面深度的增加，介电损耗从反映可移动表面层的值逐渐变化为动力学稳定玻璃的值。随着沉积温度的升高，这个更易移动层的厚度从低于1nm变化到超过10nm，并且在所有沉积温度下其平均虚构温度都接近Tg。从介电损耗判断，仅在高于0.8Tg的沉积温度下，表面层的类液体部分才超过1nm的厚度，此时可获得近平衡玻璃态。沉积后，位于表面以下约5 - 30nm处的材料的介电损耗在数千秒的退火时间内降低，而膜的主体保持不变。
