Samanta Subarna, Huang Georgia, Gao Gui, Zhang Yue, Zhang Aixi, Wolf Sarah, Woods Connor N, Jin Yi, Walsh Patrick J, Fakhraai Zahra
Department of Chemistry , University of Pennsylvania , Philadelphia , Pennsylvania 19104-6323 , United States.
J Phys Chem B. 2019 May 9;123(18):4108-4117. doi: 10.1021/acs.jpcb.9b01012. Epub 2019 Apr 30.
Stable glasses are formed during physical vapor deposition (PVD), through the surface-mediated equilibration process. Understanding surface relaxation dynamics is important in understanding the details of this process. Direct measurements of the surface relaxation times in molecular glass systems are challenging. As such, surface diffusion measurements have been used in the past as a proxy for the surface relaxation process. In this study, we show that the absence of enhanced surface diffusion is not a reliable predictor of reduced ability to produce stable glasses. To demonstrate, we have prepared stable glasses (SGs) from two structurally similar organic molecules, 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (TNB) and 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), with similar density increase and improved kinetic stability as compared to their liquid-quenched (LQ) counterparts. The surface diffusion values of these glasses were measured both in the LQ and SG states below their glass transition temperatures ( Ts) using gold nanorod probes. While TNB shows enhanced surface diffusion in both SG and LQ states, no significant surface T diffusion is observed on the surface of α,α-A within our experimental time scales. However, isothermal dewetting experiments on ultrathin films of both molecules below T indicate the existence of enhanced dynamics in ultrathin films for both molecules, indirectly showing the existence of an enhanced mobile surface layer. Both films produce stable glasses, which is another indication for the existence of the mobile surface layer. Our results suggest that lateral surface diffusion may not be a good proxy for enhanced surface relaxation dynamics required to produce stable glasses, and thus, other types of measurements to directly probe the surface relaxation times may be necessary.
在物理气相沉积（PVD）过程中，通过表面介导的平衡过程形成了稳定的玻璃态。理解表面弛豫动力学对于了解这一过程的细节很重要。直接测量分子玻璃体系中的表面弛豫时间具有挑战性。因此，过去表面扩散测量已被用作表面弛豫过程的替代方法。在本研究中，我们表明表面扩散未增强并非产生稳定玻璃态能力降低的可靠预测指标。为了证明这一点，我们用两种结构相似的有机分子，即1,3 - 双（1 - 萘基）- 5 - （2 - 萘基）苯（TNB）和9 - （3,5 - 二（萘 - 1 - 基）苯基）蒽（α,α - A）制备了稳定玻璃态，与它们的液体淬火（LQ）对应物相比，具有相似的密度增加和更好的动力学稳定性。使用金纳米棒探针在低于其玻璃化转变温度（Ts）的LQ和SG状态下测量了这些玻璃态的表面扩散值。虽然TNB在SG和LQ状态下均表现出增强的表面扩散，但在我们的实验时间尺度内，在α,α - A的表面未观察到明显的表面扩散。然而，对低于T的两种分子的超薄膜进行的等温去湿实验表明，两种分子的超薄膜中均存在增强的动力学，间接表明存在增强的可移动表面层。两种薄膜都产生了稳定玻璃态，这是可移动表面层存在的另一个迹象。我们的结果表明，横向表面扩散可能不是产生稳定玻璃态所需的增强表面弛豫动力学的良好替代指标，因此，可能需要其他类型的测量来直接探测表面弛豫时间。
