Innovations in atomic manipulation with the STM: Chlorobenzene on Si(111) 7x7

扫描隧道显微镜(STM)对单分子的实验代表了纳米技术的极端极限。它们也为我们对表面化学反应的基本理解做出了重要贡献，表面化学反应在从能源到癌症研究的各个领域都有巨大的应用。STM原子操纵方法提供的关键见解是分离和阐明(I)分子结构和成键、(Ii)电子激发、(Iii)热激发和(Iv)分子反应路径中的表面位置的性质。在这个项目中，我们的目标是开发和应用量子和经典动力学方法来模拟STM诱导的反应。基于开放系统密度矩阵理论和第一性原理团簇计算，我们建立了氯苯在Si(111)7×7上的动力学模型，该体系已经成为简单分子反应的范例，所选体系提供了广泛的不同的STM诱导反应，为该体系提供了丰富的实验数据。然而，由于理论工作的明显匮乏，人们对核动力学的细节知之甚少。当偏置电压超过一定的阈值(无论是哪种极性)时，STM电流都能引起C-Cl键的解离和整个分子的脱附。此外，解吸速率与隧穿电流近似成正比，而解离速率与电流的平方成比例关系。此外，最近在新的变温实验中观察到了几个热激活的解离和解吸通道，这两个通道都增强了电流驱动的过程。我们建议应用第一性原理密度泛函理论计算，使用硅团簇来表示重构表面，以确定准确描述这些反应所需的重要中间结构和核坐标。短寿命的负离子和正离子共振被认为驱动了STM诱导的反应，将从这些团簇的电子结构中推导出来。然后，我们将与实验密切合作，建立动力学模型，并将根据实验数据进行测试，这也将完善量子化学模型。这样的开放系统模型能够描述所有不同的实验观察到的反应。因此，可以对这些单分子实验中涉及的基本反应步骤有一个详细的了解。随着时间的推移，预计这一新的理解将转化为控制这些反应及其应用的实际创新。

Scanning tunnelling microscope (STM) experiments on single molecules represent the extreme limit of nanotechnology. They are also making important contributions to our fundamental understanding of chemical reactions on surfaces, which have immense applications across a spectrum from energy to cancer research. The key insights provided by the STM atomic manipulation approach are to separate out and illuminate the nature of (i) molecular structure and bonding, (ii) electronic excitations, (iii) thermal excitations and (iv) surface sites in molecular reaction pathways. In this project, we aim to develop and apply quantum and classical dynamical approaches to the modelling of STM-induced reactions. Based on open system density matrix theory and first principles cluster calculations, we want to establish a comprehensive dynamical model for the system chlorobenzene on Si(111) 7x7, which has become a paradigm in simple molecular reactions.The chosen system offers a wide range of different STM-induced reactions, for which rich experimental data is available. Nevertheless, only little about the details of the nuclear dynamics is known, due to a notable paucity of theoretical work. Both dissociation of the C-Cl bond and desorption of the whole molecule can be induced by the STM current for bias voltages above certain thresholds (with either polarity). Moreover, the desorption rate is approximately proportional to the tunnelling current, while the dissociation rate scales with the square of the current. Also, several thermally activated channels for dissociation and desorption, which both enhance the current-driven processes, have recently been observed in novel variable temperature experiments.We propose to apply first principles density functional theory calculations, using silicon-clusters to represent the reconstructed surface, to identify important intermediate structures and nuclear coordinates needed for an accurate dynamical description of these reactions. The short lived negative and positive ion resonances, which are believed to drive the STM-induced reactions, will be derived form the electronic structure of these clusters.In close cooperation with the experiments, we will then build dynamical models, which will be tested against experimental data and which will also refine the quantum chemical modelling. Such open system models are capable of describing all of the different experimentally observed reactions. Thus, a detailed understanding of the elementary reaction steps involved in these single molecule experiments can be gained. Over time it is expected that this new understanding will be translated into practical innovations in the control of these reactions and their application.