We are working on a new class of hybrid material that is so thin it is both a surface and an interface. These are oxide films that are one atomic layer thick and can be imaged in the scanning tunnelling microscope (STM) with atomic resolution. The structure of the films is unique to the thin film system, is not a bulk termination, and is determined through the interaction with the gold substrate. To date we have explored TiOx, NbOx, VOx, and FeOx films on Au(111) crystal substrates. The new project in this area will concentrate on ternary oxide films such as FeCrOx. These systems are of fundamental importance to understanding the effects of high temperature encapsulation of noble metal catalysts. Particular emphasis will be placed on learning about point and extended defects that occur in the films as well as investigating the properties and atomic structure of preferential molecular adsorption sites.

This project involves the growth of 2D conducting metal-organic frameworks (MOFs) in ultra-high vacuum (UHV) and subsequent characterisation with atomic resolution using scanning tunnelling microscopy (STM). Conductive 2D MOF networks are recently discovered materials, resulting from square planar complexation of late transition metals and some simple polyaromatic hydrocarbon ligands. Within the context of gas sensing we are particularly interested in the interaction of the MOF with small reactive molecules such as ammonia and nitrogen oxides.

The aim of the work in this project is to develop sensing technology that can be readily miniaturised and provide lightweight mobile or networked molecular detection of chemicals in the vapour phase. In Oxford we have been working on developing such sensors through research into the use of conducting polymer networks and metal-organic frameworks (MOFs) operating in the electrical percolation region, which we refer to as percolation sensors. This project involves the development of percolation sensors for the detection of a variety of analytes, especially volatile organic compounds. The student will be involved in a broad range of interdisciplinary activities including design, characterisation, and testing of the gas sensor. The sensors will be grown on a variety of substrates including rigid glass and high-area flexible polymer sheets.

A common method to increase the signal to noise ratio of a data set is to take repeated measurements and average them. This is routinely performed for 2D spectra, where their alignment is straightforward. However, for images the nature and variety of the distortions can severely complicate accurate registration. The usual way to treat images from a scanning tunneling microscope (STM) is to take multiple images of the same area and select the one that appears to be of the highest quality whilst discarding the information contained in the other almost identical images. In this project a step change in the resolving power of the STM will be achieved through automated multi-frame averaging (MFA). Our work so far has shown that images with sub 10 pm height resolution can be routinely obtained.

The most popular method for image creation in the scanning electron microscope (SEM) is to use the secondary electron signal. Until recently it was assumed that secondary electrons are emitted isotropically i.e. with no particular preferred direction, but we now know that the atomic structure of the surface does in fact play a role. This DPhil project is concerned with correlating secondary electron emission using an ultra high vacuum SEM with atomic structure imaged in a scanning tunnelling microscope (STM). Both these techniques are located on the same world-leading instrument in Oxford. The powerful combination of signals will provide a hitherto unexplored path into some very fundamental aspects of nanoscale surface structure. There is also the likelihood that the experiments will be further expanded through the use of the PEEM instrument at the Diamond synchrotron.

Electrical conductivity of 2D nanocrystal arrays

Percolation theory can describe the flow of electric currents through random media such as randomly dispersed metal nanocrystals on an insulating support. The sizes and distribution of the crystals can be determined accurately via scanning probe microscopy and scanning electron microscopy. This allows the electrical behaviour to be correlated with the island size distribution. Once this relationship is established it is possible to follow high speed sintering and island shape change events simply by investigating the change in electrical resistance. Percolation theory is able to set the experiments within a meaningful theoretical context including applications related to neuromorphic signal processing. A dedicated ultra high vacuum chamber is available for this project.