Model Vapor-Deposited Glasses: Growth Front and Composition Effects

TL;DR

This study uses simulations to analyze vapor-deposited glasses, revealing how growth front dynamics and composition influence stability and devitrification, aligning with experimental observations and challenging previous simulation assumptions.

Contribution

It introduces a simulation approach for vapor-deposited glasses that highlights the importance of composition control and growth front effects on stability and transformation mechanisms.

Findings

Vapor-deposited glasses are more stable than cooled glasses.

Composition significantly affects physical properties.

Devitrification propagates via a mobility front from the free surface.

Abstract

A growing body of experimental work indicates that physical vapor deposition provides an effective route for preparation of stable glasses, whose properties correspond in some cases to those expected for glasses that have been aged for thousands of years. In this work, model binary glasses are prepared in a process inspired by physical vapor deposition, in which particles are sequentially added to the free surface of a growing film. The resulting glasses are shown to be more stable than those prepared by gradual cooling from the liquid phase. However, it is also shown that the composition of the resulting glass, which is difficult to control in physical vapor deposition simulations of thin films, plays a significant role on the physical characteristics of the material. That composition dependence leads to a re- evaluation of previous results from simulations of thinner films than those considered here, where the equivalent age of the corresponding glasses was overestimated. The simulations presented in this work, which correspond to films that are approximately 38 molecular diameters thick, also enable analysis of the devitrification mechanism of vapor-deposited glasses. Consistent with experiments, it is found that this mechanism consists of a mobility front that propagates from the free interface into the interior of the films. Eliminating surface mobility eliminates this route of transformation into the supercooled liquid.