Ediger M D, de Pablo Juan, Yu Lian
Department of Chemistry , University of Wisconsin-Madison , 1101 University Avenue , Madison , Wisconsin 53706 , United States.
Institute of Molecular Engineering , University of Chicago , 5640 South Ellis Avenue , Chicago , Illinois 60637 , United States.
Acc Chem Res. 2019 Feb 19;52(2):407-414. doi: 10.1021/acs.accounts.8b00513. Epub 2019 Jan 23.
The term "organic solids" encompasses both crystals and glasses. Organic crystals are commonly grown for purification and structure determination and are being extensively explored for applications in organic electronics including field effect transistors. The ability to control the packing of one molecule relative to its neighbors is of critical importance for most uses of organic crystals. Often, anisotropic packing is also highly desirable as it enhances charge transport and optimizes light absorption/emission. When compared to crystals, the local packing in organic glasses is highly disordered and often isotropic. Glasses, however, offer two key advantages with respect to crystals. First, glasses typically lack grain boundaries and thus exhibit better macroscopic homogeneity. Second, glass composition can often be varied over a wide range while maintaining homogeneity. Besides electronic materials, many modern plastics used in a wide range of technologies are organic glasses, and the glassy state is being increasingly utilized to deliver pharmaceuticals because of higher bioavailability. In this article, we introduce vapor-deposited organic glasses as hybrid materials that combine some of the useful features of crystals and traditional liquid-cooled glasses. Physical vapor deposition produces glasses by directly condensing molecules from the gas phase onto a temperature-controlled substrate and allows film thickness to be controlled with nanometer precision. Just as liquid-cooled glasses, vapor-deposited glasses have smooth surfaces and lack grain boundaries. These attributes are critical for applications such as organic light emitting diodes (OLEDs), in which vapor-deposited glasses of organic semiconductors form the active layers. In common with crystals, vapor-deposited glasses can exhibit anisotropic packing, and the extent of anisotropy can be comparable to that of the typical organic crystal. For vapor-deposited glasses, in contrast to crystals, anisotropic packing can generally be controlled as a continuous variable. Deposition conditions can be chosen to produce glasses with significant molecular orientation (molecules "standing up" or "lying down" relative to the substrate), and π-stacking can be directed along different directions relative to the substrate. Over the last five years, we have gained a fundamental understanding of the mechanism that controls the anisotropy of vapor-deposited glasses and learned how to control many aspects of anisotropic packing. Two key elements that enable such control are the high mobility present at the surface of an organic glass and the tendency of the surface to promote anisotropic packing of molecules. In contrast to traditional epitaxial growth, for vapor-deposited glasses, the free surface (not the substrate) acts as a template that controls the structure of a growing film. The structure of any given layer is decoupled from those beneath it, thereby providing considerable freedom in producing layered glassy structures.
术语“有机固体”涵盖晶体和玻璃。有机晶体通常用于纯化和结构测定，并且正在广泛探索其在包括场效应晶体管在内的有机电子学中的应用。对于有机晶体的大多数用途而言，控制一个分子相对于其相邻分子的堆积方式至关重要。通常，各向异性堆积也非常理想，因为它能增强电荷传输并优化光吸收/发射。与晶体相比，有机玻璃中的局部堆积高度无序且通常是各向同性的。然而，玻璃相对于晶体具有两个关键优势。首先，玻璃通常没有晶界，因此表现出更好的宏观均匀性。其次，玻璃成分通常可以在很宽的范围内变化，同时保持均匀性。除了电子材料，许多用于各种技术的现代塑料都是有机玻璃，并且由于更高的生物利用度，玻璃态越来越多地被用于输送药物。在本文中，我们将气相沉积有机玻璃作为一种混合材料引入，它结合了晶体和传统液冷玻璃的一些有用特性。物理气相沉积通过将气相中的分子直接冷凝到温度可控的基板上产生玻璃，并允许以纳米精度控制膜厚。与液冷玻璃一样，气相沉积玻璃具有光滑的表面且没有晶界。这些特性对于诸如有机发光二极管（OLED）等应用至关重要，在这些应用中，有机半导体的气相沉积玻璃形成有源层。与晶体一样，气相沉积玻璃可以表现出各向异性堆积，并且各向异性的程度可以与典型的有机晶体相当。然而，与晶体不同的是，气相沉积玻璃的各向异性堆积通常可以作为一个连续变量来控制。可以选择沉积条件来生产具有显著分子取向的玻璃（分子相对于基板“直立”或“平躺”），并且π堆积可以相对于基板沿不同方向定向。在过去五年中，我们对控制气相沉积玻璃各向异性的机制有了基本的了解，并学会了如何控制各向异性堆积的许多方面。实现这种控制的两个关键因素是有机玻璃表面存在的高迁移率以及表面促进分子各向异性堆积的趋势。与传统的外延生长不同，对于气相沉积玻璃，自由表面（而不是基板）充当控制生长膜结构的模板。任何给定层的结构与其下面的层的结构解耦，从而在生产层状玻璃结构方面提供了相当大的自由度。
