Department of Physics, University of Warwick, Coventry, CV4 7AL, UK.
Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK.
Nanoscale. 2017 Aug 24;9(33):11959-11968. doi: 10.1039/c7nr03588h.
The ability to control the transition from a two-dimensional (2D) monolayer to the three-dimensional (3D) molecular structure in the growth of organic layers on surfaces is essential for the production of functional thin films and devices. This has, however, proved to be extremely challenging, starting from the currently limited ability to attain a molecular scale characterization of this transition. Here, through innovative application of low-dose electron diffraction and aberration-corrected transmission electron microscopy (acTEM), combined with scanning tunneling microscopy (STM), we reveal the structural changes occurring as film thickness is increased from monolayer to tens of nanometers for supramolecular assembly of two prototypical benzenecarboxylic acids - terephthalic acid (TPA) and trimesic acid (TMA) - on graphene. The intermolecular hydrogen bonding in these molecules is similar and both form well-ordered monolayers on graphene, but their structural transitions with film thickness are very different. While the structure of TPA thin films varies continuously towards the 3D lattice, TMA retains its planar monolayer structure up to a critical thickness, after which a transition to a polycrystalline film occurs. These distinctive structural evolutions can be rationalized in terms of the topological differences in the 3D crystallography of the two molecules. The templated 2D structure of TPA can smoothly map to its 3D structure through continuous molecular tilting within the unit cell, whilst the 3D structure of TMA is topologically distinct from its 2D form, so that only an abrupt transition is possible. The concept of topological protection of the 2D structure gives a new tool for the molecular design of nanostructured films.
在表面有机层生长过程中，从二维（2D）单层到三维（3D）分子结构的转变能力对于功能性薄膜和器件的制备至关重要。然而，从目前对这种转变进行分子尺度表征的能力有限这一事实来看，这一目标极具挑战性。在这里，我们通过创新应用低剂量电子衍射和相衬透射电子显微镜（acTEM），结合扫描隧道显微镜（STM），揭示了在石墨烯上超分子组装两种典型的苯二甲酸 - 对苯二甲酸（TPA）和均苯三甲酸（TMA） - 时，膜厚从单层增加到数十纳米过程中发生的结构变化。这些分子中的分子间氢键相似，在石墨烯上都形成了有序的单层，但它们的结构转变与膜厚有很大的不同。虽然 TPA 薄膜的结构不断向 3D 晶格转变，但 TMA 保持其平面单层结构，直到达到临界厚度，之后发生向多晶薄膜的转变。这些独特的结构演变可以根据两个分子的 3D 晶体学拓扑差异来合理化。TPA 的模板化 2D 结构可以通过单元内分子的连续倾斜平滑地映射到其 3D 结构，而 TMA 的 3D 结构与其 2D 形式在拓扑上不同，因此只能发生突然转变。2D 结构的拓扑保护概念为纳米结构薄膜的分子设计提供了一种新工具。
