Force-induced molecular dissociation at surfaces via a local probe

• Applicant: Dr. Alexander Saywell
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2013 to 2015
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 242887252

The dissociation of chemical bonds is a fundamental process in all chemical reactions. Scanning tunnelling microscopy (STM) has been used to induce the breaking of single covalent bonds within a molecule via tunnelling electrons. However, it has never been attempted to break a chemical bond within a molecule not via tunnelling electrons but only by applying a force with a local probe. The main objective of this project is to explore whether it is possible to induce such a process within an individual adsorbed molecule by using STM and atomic force microscopy (AFM), which in addition to atomically resolved imaging allows the study of short-range chemical forces between the molecule and the probe as a function of molecule-probe separation. A combination of force-spectroscopy and topographical imaging will allow characterisation of the forces and potential energies during the probe-molecule interaction and the structure of the molecule before and after the applied force with very high resolution.As it has previously been demonstrated that carbon-halogen (C-X) bonds in organic molecules may be cleaved selectively by other methods (e.g. thermal activation - facilitating on-surface synthesis processes), we will focus on breaking such a carbon-halogen covalent bond. Within this project a molecular species containing a reactive C-X moiety will be deposited on a surface held under ultra-high vacuum (UHV) conditions, a metallic probe will be approached towards the C-X bond, and the forces between the molecule and the probe measured. The experiments will (1) ascertain whether a C-X bond within an individual molecule adsorbed on a substrate can be broken by applying a force at low temperatures (120K) and, (2) measure the forces present during the probe-molecule interaction and thus provide information on the potential energy landscape. The breaking of a C-X bond within a molecule that is adsorbed on a surface is typically driven by the interaction between the adsorbed molecule and a metallic substrate (often Cu, Ag, or Au); i.e. a catalytically active environment. Additional experiments will therefore (3) study the dependence of the measured force on the chemical properties of the system by changing the metal atoms at the probe apex (Cu, Ag, or Au) or by changing the halogen species within the molecule (Br or I). Furthermore, as in conventional synthetic processes where thermal energy often initiates the reaction, we will (4) study the effect of the substrate temperature on the force-induced bond dissociation process.This novel approach to inducing and studying molecular bond dissociation by interatomic forces has not previously been explored, and potentially offers an atomic-scale insight into the role of the chemical environment and temperature within this dissociation process.

• DFG Programme: Research Grants