If you are interested in any of these projects please email: martin.castell 'at' materials.ox.ac.uk
Currently available DPhil projects:
Atomic resolution imaging of ultrathin oxide films
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. In this project oxide (TiOx, CrOx, FeOx) thin films will be imaged with particular emphasis placed on learning about the point and extended defects that occur in the films.
Epitaxial oxide nanocrystals
Very small crystals, nanocrystals, of one type of oxide can be grown onto another oxide substrate. The shape, structure, and electrical / optical properties of these nanocrystals is influenced by the strain that builds up between the substrate and the nanocrystal. The idea is to grow an oxide of one type onto a different oxide substrate that has a slight lattice mismatch. The strain that builds up in the oxide nanocrystals will then affect the electronic properties such as the bandgap. This is called strain engineering, and has been carried out for many years in the semiconductor industry with e.g. germanium on silicon systems. In this project the scope of strain engineering will be expanded into the realm of oxide materials. We have some exciting preliminary data of Mn3O4 and TiO2 nanocrystals on SrTiO3 substrates that show the feasibility of the proposed work.
Chiral networks and the origin of life
In this project 2D molecular networks are synthesized through self-assembly on metal and oxide surfaces. Scanning tunnelling microscopy is used to investigate their ordering. In particular, methods will be studied that influence the chirality (handedness) of the molecular arrangements. Examples of this are networks that consist of DNA and RNA nucleobases such as adenine and uracil. These experiments are motivated by the question of what gives rise to a particular chirality in biomolecules such as DNA and amino acids, and as such are relevant to the origin of life.
Quantum confinement in oxide nanostructures
Crystalline oxides such as SrTiO3 have vast potential as a material to be integrated in the next generation of microelectronic devices. 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. For this research a state of the art microscopy/spectroscopy facility is available.
A new concept for an ultra-sensitive gas sensor
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. A further requirement is that the sensor is cheap and does not require protracted training for the user. In Oxford we have been working on developing such sensors through research into the use of conducting polymer networks operating near the electrical percolation threshold, which we refer to as percolation sensors. A studentship is available to develop 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, building, and testing of the sensor. This project will be integrated into the larger WAFT collaboration, involving a number of UK Universities (www.waftcollaboration.org).
Tailored nanocrystal catalysts
Currently, industrial catalyst nanoparticles used for pollution control and chemical processing are randomly dispersed on their supports with a large variety of sizes and shapes. Within this multi-billion pound industry the main research driver is to find ways of increasing the catalytic efficiency of the precious metals used such as Pt, Pd, and Rh, or increasingly alloys of various metals. One method is to increase the surface to volume ratio of the particles, and much effort has been directed towards that goal. Another method, proposed here, is to recognise that the crystal facets of the catalyst particles all have different chemical properties. This means that highly efficient catalysts can be created by synthesising particles with particularly large fractions of highly active crystal facets. One of the central aims of this project is to develop new processing routes to allow large-scale manufacture of shape and size selected metal and oxide nanoparticles with high catalytic efficiency. The project will also involve the synthesis of core-shell nanocatalysts. Characterisation of the catalyst particles will be carried mainly with scanning tunnelling microscopy.
Surface structure characterisation of iron-based superconductors and topological insulators
(in collaboration with Dr Susannah Speller)
The unexpected discovery in 2008 of a new family of superconductors based on iron promises to lead to substantial progress in understanding the elusive mechanisms responsible for high-temperature superconductivity. However, worldwide efforts to understand the fundamental properties using a wide variety of experimental techniques have so far proved to be inconclusive and contradictory due to the lack of detailed understanding of the complex microstructures of even the best single crystal samples. This project involves using scanning tunnelling microscopy (STM) to investigate the surface structure with atomic resolution in combination with High-Resolution Electron Backscatter Diffraction analysis for mapping local structural variations on the micron-scale.
Iron-based superconductors are only one of several novel quantum state materials of great interest in the scientific community. Another hot topic are the so-called topological insulators, which exhibit bulk insulating properties with special conducting surface states, promising dissipation-less carrier transport at room temperature. There are a wide range of potential applications for these exciting new materials including dramatically faster, almost powerless computer chips. The experimental techniques developed in this project are ideally suited to studying the distribution of ferromagnetic additions needed to exploit the exciting properties of topological insulators in practical devices.
Multi-component molecular 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. Ultimately the idea is to create molecular architectures that can be used in advanced electronic devices.
Growth and spectroscopy of metallic nanocrystals and clusters
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.
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. There is also the likelihood that the experiments will be further expanded through the use of the PEEM instrument at the Diamond synchrotron.