Molecular Functionalisation of Semiconductor Surfaces

Developing the ability to control the flow of charge (electrons) through individual molecules is an exciting field of basic nano-science research that holds promise for the creation of the next generation of technology; from the ultimate level of device miniaturisation (single-molecule device components) to ultrasensitive sensors and novel devices such as a quantum computer. To put this into context, the study of electron transport through micro- and nano-structured metals and semiconductors has been the subject of intense research for decades. This research forms the basis of current electronics technology and the ongoing advancements in this field continue to facilitate dramatic improvements in miniaturisation and performance of electronic devices. At the same time, electron flow within individual molecules is fundamental to a wide range of naturally-occurring chemical and biological processes; chemical reactions occur when there is an energetically favourable path for electrons to flow between and/or within molecules, resulting in a rearrangement of electron density from electron-rich to electron-deficient regions. Important examples of charge transfer in molecules occur in photosynthesis and its technological analogues in organic photovoltaics; and in mechanisms of DNA damage and repair. As such, the research proposed here to study the transport of charge through molecular junctions lies at the interface between traditional areas of research in condensed matter physics and chemistry, and is of both great fundamental interest and technological importance in these and other areas including nano-biotechnology.The overall aim of this proposal is to investigate the nature of charge transport through individual molecules with well characterised electrical contacts, under highly controlled environmental conditions. Individual molecules will be adsorbed to atomically-clean semiconductor surfaces in ultrahigh vacuum (UHV). Scanning tunnelling microscopy (STM) measurements will be performed to elucidate the precise nature of the bonding of the molecule to the surface. Subsequently, scanning tunnelling spectroscopy (STS) measurements will be used to characterise the electronic structure of the resulting molecule/semiconductor/metal(tip) junction. Measurements will be performed at temperatures between 2 Kelvin and room temperature, in externally applied magnetic fields of up to 6 Tesla, and with in-situ generated electric fields created through surface nanolithography. This will allow the investigation of a broad range of possible electron transport phenomena, the nature of which will depend on the properties of the particular molecule and the extent of its coupling to the surface and the STM tip.