School of Chemistry, University of Nottingham , University Park, Nottingham NG7 2RD, United Kingdom.
Institute of Process Research and Development, School of Chemistry, University of Leeds , Leeds LS2 9JT, United Kingdom.
Acc Chem Res. 2017 Aug 15;50(8):1797-1807. doi: 10.1021/acs.accounts.7b00078. Epub 2017 Jul 11.
The main objective of this Account is to assess the challenges of transmission electron microscopy (TEM) of molecules, based on over 15 years of our work in this field, and to outline the opportunities in studying chemical reactions under the electron beam (e-beam). During TEM imaging of an individual molecule adsorbed on an atomically thin substrate, such as graphene or a carbon nanotube, the e-beam transfers kinetic energy to atoms of the molecule, displacing them from equilibrium positions. Impact of the e-beam triggers bond dissociation and various chemical reactions which can be imaged concurrently with their activation by the e-beam and can be presented as stop-frame movies. This experimental approach, which we term ChemTEM, harnesses energy transferred from the e-beam to the molecule via direct interactions with the atomic nuclei, enabling accurate predictions of bond dissociation events and control of the type and rate of chemical reactions. Elemental composition and structure of the reactant molecules as well as the operating conditions of TEM (particularly the energy of the e-beam) determine the product formed in ChemTEM processes, while the e-beam dose rate controls the reaction rate. Because the e-beam of TEM acts simultaneously as a source of energy for the reaction and as an imaging tool monitoring the same reaction, ChemTEM reveals atomic-level chemical information, such as pathways of reactions imaged for individual molecules, step-by-step and in real time; structures of illusive reaction intermediates; and direct comparison of catalytic activity of different transition metals filmed with atomic resolution. Chemical transformations in ChemTEM often lead to previously unforeseen products, demonstrating the potential of this method to become not only an analytical tool for studying reactions, but also a powerful instrument for discovery of materials that can be synthesized on preparative scale.
本文的主要目的是评估透射电子显微镜（TEM）在分子方面的应用所面临的挑战，这是基于我们在该领域超过 15 年的工作。此外，本文还概述了在电子束（e-beam）下研究化学反应的机遇。在 TEM 对吸附在原子级薄基底（如石墨烯或碳纳米管）上的单个分子进行成像时，电子束会将动能传递给分子中的原子，从而使它们偏离平衡位置。电子束的撞击会引发键的断裂和各种化学反应，这些反应可以与电子束的激发同时成像，并以定格电影的形式呈现。我们将这种实验方法称为 ChemTEM，它利用了电子束与原子核之间的直接相互作用传递给分子的能量，从而可以准确预测键的断裂事件，并控制化学反应的类型和速率。反应物分子的元素组成和结构以及 TEM 的操作条件（特别是电子束的能量）决定了 ChemTEM 过程中形成的产物，而电子束剂量率则控制着反应速率。由于 TEM 的电子束既是反应的能量源，又是监测同一反应的成像工具，因此 ChemTEM 揭示了原子级的化学信息，例如单个分子的反应途径，一步一步且实时成像；难以捉摸的反应中间体的结构；以及用原子分辨率拍摄的不同过渡金属的催化活性的直接比较。ChemTEM 中的化学转化通常会导致以前未曾预料到的产物，这表明该方法不仅有可能成为研究反应的分析工具，而且还有可能成为发现可在制备规模上合成的材料的强大工具。
