Nanometre sized metal islands on oxide supports are used in diverse applications from catalytic materials to gas sensors. Interaction between the oxide support and the islands, the island shape, the temperature dependence of island ripening, and molecular interactions with the islands are all active areas of study. In this DPhil project a variety of transition metal clusters on single crystal oxide supports will be investigated. The atomic structure of the nanocrystals will be imaged with scanning tunnelling microscopy, and their electronic structure will be probed using optical spectroscopies. For this research a new state of the art microscopy/spectroscopy facility is available.

Molecular alloy crystals

The surfaces of a variety of nanostructured oxides can be used to order molecules, such as fullerenes (e.g. C60, C70), into specific two dimensional patterns. This is called templated molecular ordering. In this DPhil project fullerenes of different sizes will be mixed together to give rise to molecular alloys. Specific concentrations and relative sizes of fullerenes are thought to form ordered systems. The structure of these molecular alloy crystals will be studied at atomic resolution with scanning tunnelling microscopy. Spectroscopic facilities are also available to probe the electronic structure of the alloys.

Quantum confinement in oxide nanostructures

It has recently been discovered in Oxford that certain surface treatments of SrTiO3 produce atomic scale nanostructures by subtly changing the ratio of Ti to Sr in the surface region. The aim of this DPhil project is to investigate the quantum confinement of electrons in these nanostructures, similar to the particle in a box problem in elementary quantum mechanics. Atomic resolution scanning tunneling microscopy will be used to determine the size and distribution of the nanostructures, and spectroscopy techniques will show the degree of quantum confinement. Leading optical, electronic, and structural characterisation facilities are available for this research.

Atomic structure and secondary electron emission

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.

Electrical conductivity of 2D nanoisland arrays

Percolation theory can describe the flow of electric currents through random media such as randomly dispersed metal nanoislands on an insulating support. The sizes and distribution of the nanoislands can be determined accurately via scanning probe 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. A new dedicated ultra high vacuum chamber is available for this project.

Studies of metal nanocrystals on Strontium Titanate
(in collaboration with Professor AI Kirkland, Materials)

It is possible to grow a variety of metal nanocrystals on clean single crystal strontium titanate surfaces. Such particles often adopt novel morphologies which can be controlled and which may provide novel catalysts and gas sensors. For example, silver nanocrystals with fivefold symmetry have been observed, and palladium crystals have been shown to change their shape depending on the detailed atomic structure of the substrate. This project aims to characterize these materials with atomic resolution, using both scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM) in an attempt to understand their growth and structure.