Antony Lucas W, Jackson Nicholas E, Lyubimov Ivan, Vishwanath Venkatram, Ediger Mark D, de Pablo Juan J
Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States.
The Institute for Molecular Engineering, Argonne National Laboratory, Lemont, Illinois 06349, United States.
ACS Cent Sci. 2017 May 24;3(5):415-424. doi: 10.1021/acscentsci.7b00041. Epub 2017 Apr 14.
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 this work, we present 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 have not been performed before, and this study therefore begins by verifying that the model and method utilized here reproduces key structural features observed experimentally. Having established agreement between several simulated and experimental macroscopic observables, simulations are used to examine the substrate temperature dependence of molecular orientation. The 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 is 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.
通过物理气相沉积形成的有机玻璃薄膜相对于通过传统液体冷却和老化技术形成的薄膜表现出更高的稳定性。最近，实验和计算证据表明，通过控制生长这些“稳定玻璃”的衬底温度，可以调节平均分子取向。在这项工作中，我们使用一种紧密模拟气相沉积过程的计算薄膜形成程序，对典型的稳定玻璃形成体乙苯进行了全面的全原子模拟研究。此前尚未对实验形成的气相沉积玻璃进行过原子研究，因此本研究首先验证此处使用的模型和方法是否能重现实验观察到的关键结构特征。在确定了几个模拟和实验宏观可观测量之间的一致性后，利用模拟来研究分子取向对衬底温度的依赖性。结果表明，乙苯玻璃是各向异性的，取决于衬底温度，并且这种依赖性可以从平衡液体表面存在的取向来理解。通过将乙苯视为分子半导体材料的简单模型，然后进行量子化学分析表明，相对于各向同性的液体冷却薄膜，气相沉积玻璃表现出能量无序性降低和均方转移积分大小增加，这种效应归因于分子薄膜的各向异性有序排列。这些结果提出了一种新颖的结构-功能模拟策略，能够在实验沉积之前调节有机半导体玻璃的电子性质，这对于有机电子材料设计可能具有相当大的潜力。
