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

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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.