Vapor-deposited non-crystalline phase vs ordinary glasses and supercooled liquids: evidence for significant thermodynamic and kinetic differences

Abstract

Vapor deposition of molecules on a substrate often results in glassy materials of high kinetic stability and low enthalpy. The extraordinary properties of such glasses are attributed to high rates of surface diffusion during sample deposition, which makes it possible for constituents to find a configuration of much lower energy on a typical laboratory time scale1,2,7. The exact structure of the resulting phase is often assumed to be identical to that obtained by aging of ordinary glass over exceedingly long times. Using Fast Scanning Calorimetry technique, we show that out-of-equilibrium relaxation kinetics and possibly the enthalpy of vapor-deposited films of toluene, an archetypical fragile glass former, are distinct from those of ordinary supercooled phase even when the deposition takes place at temperatures above the glass softening. These observations provide support to the conjecture that the vapor-deposition may result in formation of non-crystalline phase of unique structural, thermodynamic, and kinetic properties.