Lecturer Renald Schaub, EaStCHEM and School of Chemistry, University of St Andrews, St Andrews, UK

Novel Electronic Materials: From Graphene to Single-Molecule Switches 

Silicon is the base material for current electronic components such as transistors and switches. With progress in solution processing techniques for electronics integration (e.g. nanolithography), every year is accompanied with a reduction in size of individual components, thereby allowing for their increased density on a microchip. Moore’s law predicts that the electronic components are soon reaching scaling limits in the miniaturization process. Hence, research into alternatives to silicon-based components is vital to sustain our ability to devise better performing and cost effective electronic devices that afford greater flexibility and reduced energy consumption. In this talk, I will review our recent progress in two such materials alternatives: graphene (the ultra-thin materials limit) and single-molecule-based electronics (the ultra-small materials limit).

The decomposition and condensation of hydrocarbons on catalytically active metal surfaces is a problem of practical concern for chemists (formation of unwanted carbidic or graphitic byproducts referred to as coke). However, when the same reactions are directed on to a well-defined flat metal substrate, the final product adopts a simple morphology: a one-atom-thin self-passivated carbon overlayer known as graphene. This synthesis method, called chemical vapour deposition (CVD), is currently regarded as most promising for large-scale production of graphene. Hence, the same decomposition and condensation reactions turn out to be a windfall phenomenon for physicists. Indeed, several multinational companies in the consumer-electronics market have gone on record as predicting that CVD graphene could enhance or replace silicon in many electronic devices, due to an unrivalled assortment of formidable mechanical, optical, and electronic properties. To date, however, the detailed atomic-scale mechanisms by which a carbon feed is catalytically driven towards condensing into a detrimental (coke) or an added-value carbon overlayer (graphene) remain elusive. I will present a combined spectroscopic (HREELS), atom-resolved microscopic (STM), and first-principles theoretical (DFT) approach applied in research endeavours that aim at resolving in a systematic step-by-step fashion the atomistic pathways that control the chemical transformation of a simple hydrocarbon into graphene. We expose a Rhodium single crystal of (111) orientation to ethene (H2C=CH2), and probe its catalytic decomposition into reactive intermediates, and its assembly mechanisms into graphene.

A potential end-point in the miniaturization of electronic devices lies in the field of molecular electronics, where molecules perform the function of single components. To date, hydrogen tautomerism in unimolecular switches has been restricted to the central macrocycle of porphyrin-type molecules. The present work reveals how H-tautomerism is the mechanism for switching in substituted quinone derivatives, a novel class of molecules with a different chemical structure. We hence reveal that the previous restrictions applying to tautomeric molecular switches bound to a surface are not valid in general. The activation energy of switching in a prototypical quinone derivative is determined using inelastic electron tunnelling. Through computational modelling, we show that the mechanism underlying this process is tautomerisation of protons belonging to two amino groups. This switching property is retained upon functionalization by the addition of side groups, meaning that the switch can be chemically modified to fit specific applications.