Orientational anisotropy in simulated vapor-deposited molecular glasses

Abstract

Enhanced kinetic stability of vapor-deposited glasses has been established for a variety of glass organic formers. Several recent reports indicate that vapor-deposited glasses can be orientationally anisotropic. In this work, we present results of extensive molecular simulations that mimic a number of features of the experimental vapor deposition process. The simulations are performed on a generic coarse-grained model and an all-atom representation of N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD), a small organic molecule whose vapor-deposited glasses exhibit considerable orientational anisotropy. The coarse-grained model adopted here is found to reproduce several key aspects reported in experiments. In particular, the molecular orientation of vapor-deposited glasses is observed to depend on substrate temperature during deposition. For a fixed deposition rate, the molecular orientation in the glasses changes from isotropic, at the glass transition temperature, Tg, to slightly normal to the substrate at temperatures just below Tg. Well below Tg, molecular orientation becomes predominantly parallel to the substrate. The all-atom model is used to confirm some of the equilibrium structural features of TPD interfaces that arise above the glass transition temperature. We discuss a mechanism based on distinct orientations observed at equilibrium near the surface of the film, which get trapped within the film during the non-equilibrium process of vapor deposition.

Influence of Vapor Deposition on Structural and Charge Transport Properties of Ethylbenzene Films

“…All PVD films, except for those deposited at the two coldest T s , start with zero or near-zero correlation to the final orientation. In accordance with what was observed for coarse-grained molecules, 28 as the molecules become more deeply embedded into the film, the rotational correlation to the glass state p 1 ( t ) smoothly approaches unity. The curve shifts to the left as T s is lowered, indicating that the molecular orientation becomes frozen-in at shallower depths, and thus each molecule has a shorter period of time from when it is first introduced to when it is locked into the immobilized glassy state.…”

“…connected this T s dependent molecular orientation to features observed at the liquid–vacuum interface. 28 We performed a similar analysis with ethylbenzene, but we focused on the rotational correlation during the deposition process with respect to the molecular orientation in the immobile state. In other words, where in the previous study the correlation was based on P 2 , here we define for each molecule i a quantitywhere n i ( t ) is the molecular orientation unit vector of molecule i at time t , n i f is the orientation of the final immobilized state, and δ i is the angular displacement between them.…”

“…Keeping with the procedure outlined by Lyubimov et al, 28 we define a rotational arrest time, t a , through the relation 1 – p 1 ( t a ) = 1/ e (horizontal dashed line of Figure 5a). The distance from the free surface at which this occurs is denoted by z a = ⟨ z surf ( t a )⟩.…”

Origin of Ultrastability in Vapor-Deposited Glasses

“…First, additional studies are needed to connect the surface relaxation time measured in this study with the surface mobility inferred from experiments [2,3,5,[21][22][23]26]. Second, the shape and chemical nature of vapor-deposited molecules can result in preferential orientation within the vapor-deposited film [1,13,[27][28][29][30], while such alignment bias is not expected for ordinary liquid-cooled films. Although molecular alignment can be used to tailor glassy properties [29,30], it also inherently leads to vapordeposited films that differ in structure from their liquidcooled counterpart.…”

“…A microscopic explanation for the optimality of 0.85T g , and an estimate of what is a "sufficiently slow" deposition rate are, however, still lacking. Moreover, while simulations and experiments have shown that vapor-deposited glasses may lie lower in the potential energy landscape than liquidcooled glasses [3,[11][12][13][14][15][16], and sometimes have the same structure as glasses of a comparable energy [14], it is not known whether vapor deposition can provide truly equilibrium configurations, especially below T g .…”