Simulating Ethylbenzene Stable Glass Formation

In the latest issue of ACS Central Science, IME student Lucas Antony, Argonne postdoctoral fellow Nick Jackson, and Professor Juan de Pablo report their new findings on the structure of vapor-deposited organic glasses. Research studies conducted over the last few years have shown that organic glass films formed by physical vapor deposition exhibit enhanced stability relative to those formed by conventional liquid cooling and aging techniques. Recently, experimental and computational evidence has emerged indicating that the average molecular orientation can be tuned by controlling the substrate temperature at which these “stable glasses” are grown.

In their work, IME researchers presented a comprehensive all-atom simulation study of ethylbenzene, a canonical stable-glass former, using a computational film formation procedure that closely mimics the vapor deposition process. Atomistic studies of experimentally formed vapor-deposited glasses had not been performed before, and their study served to verify that the model and methods utilized in their calculations reproduces key structural features observed experimentally. Having established agreement between several simulated and experimental macroscopic observables, the IME group proceeded to use simulations to examine the substrate temperature dependence of molecular orientation.

Their results indicate that ethylbenzene glasses are anisotropic, depending upon substrate temperature, and that this dependence can be understood from the orientation present at the surface of the equilibrium liquid. By treating ethylbenzene as a simple model for molecular semiconducting materials, a quantum-chemical analysis was then used to show that the vapor-deposited glasses exhibit decreased energetic disorder and increased magnitude of the mean-squared transfer integral relative to isotropic, liquid-cooled films, an effect that is attributed to the anisotropic ordering of the molecular film. These results suggest a novel structure–function simulation strategy capable of tuning the electronic properties of organic semiconducting glasses prior to experimental deposition, which could have considerable potential for organic electronic materials design.
 
A link to the paper can be found here.

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