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Elevated temperature scanning tunneling microscopy of oxide surfaces and thin oxide films to identify atomic scale structure and defects relevant to catalytic processes and nanotechnology. Investigation of patterned oxide surfaces for use as templates in nanoelectronics. High resolution secondary electron imaging in the SEM of semiconductor nanostructures and devices to study local strain, dopant distributions, dopant diffusion and deactivation. Investigation of self-assembled molecular ordering using scanning tunneling microscopy.

The surface structure of SrTiO3 reconstructions
Professor M.R. Castell
Ever increasing miniaturisation of integrated circuit technology continues to be a critical capability for the microelectronics industry in order to increase functionality and fuel market expansion. One of the major challenges identified by the International Technology Roadmap for Semiconductors (ITRS) is the introduction of new materials into the manufacturing process. The purpose of the project is to investigate the surface reconstructions of SrTiO3 (001), (011), and (111), and determine their influence on the growth of silicon and a variety of metals on these surfaces. Atomic resolution imaging through scanning tunnelling microscopy (STM) is the main research tool.

Growth and spectroscopy of metallic nanoislands
Professor M.R. Castell
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. The atomic structures of the islands are imaged with scanning tunnelling microscopy, and a variety of spectroscopic techniques are used to measure their electronic structure.

Structure and electronic properties of titanate materials using TEM and STM
Professor M.R. Castell, Professor A.I. Kirkland, Dr. J. Kim
Complex metal titanates exhibit a variety of unusual structural, surface and electronic properties. This project aims to carry out high resolution studies on a variety of doped titanate systems such as Nb doped SrTiO3 where we expect phase separation to occur at high dopant concentrations. By combining state of the art scanning probe and transmission electron microscopic techniques it will be possible to correlate electronic and structural properties of these materials with atomic scale resolution.

Optical resonances of oxide nanostructures
S. Rahman, Professor M.R. Castell
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. This project investigates the quantum confinement of electrons in these nanostructures, similar to the particle in a box problem in elementary quantum mechanics. Atomic resolution scanning tunnelling microscopy will be used to determine the size and distribution of the nanostructures, and spectroscopy techniques will show the degree of quantum confinement.

Growth of ultra-thin oxide films
C. Wu, Professor M.R. Castell
This project is concerned with growth of BaTiO3 films with atomic level precision using Ba and Ti as elemental sources and single crystal Au and SrTiO3 as substrates. The films are being characterised with scanning tunnelling microscopy, electron diffraction, and electron and optical spectroscopies.

Supramolecular structures for nanoelectronics and quantum computing
Professor M.R. Castell, Professor G.A.D. Briggs, Dr. A. Ardavan*, Dr. S.C. Benjamin, Dr. K. Porfyrakis
Conventional lithographic techniques for surface patterning have powered technological progress for decades and can now reach dimensions down to 20-30 nm. We shall pursue a fundamentally different approach to templating based on a 'bottom-up' nanotechnology in which nanoscale building blocks spontaneously adopt an ordered configuration through a self-assembly process. We shall use these templates to create structures suitable for nanoelectronics and quantum computing. The primary approach will be deposition of a passive molecular 'scaffolding' followed by subsequent deposition of a molecular species that forms an ordered distribution within that scaffolding. The species employed will be an endohedral fullerene with the property that the encapsulated atom can store information in the state of its nuclear and/or electron spin. In this way we shall create ordered arrays of quantum bits (qubits). Experiments will be designed to characterise the qubit-qubit intereactions, and the results will be used to guide further generations of nanoarray synthesis, with the ultimate goal of creating structures suitable for information processing. (*Department of Physics)

Scanning tunneling microscopy of nucleobase pairs on sulfide surfaces
O.Y. Shvarova, Professor M.R. Castell, Professor D.G. Fraser*
We are using scanning tunneling microscopy to investigate the self-assembly of nucleobases and base pairs on inert metal and sulfide surfaces. (*Earth Sciences)

SrTiO3 supported catalysts for carbon nanotube growth
J. Sun, Professor N. Grobert and Professor M.R. Castell.
We are investigating the possibility of using the ceramic oxide SrTiO3 as metal nanoparticle catalyst support for the growth of carbon nanotubes.

Nanostructures on the SrTiO3 (001) surface
Professor M.R. Castell
Atomically resolved scanning tunnelling microscopy of the SrTiO3 (001) surface reveals that certain treatments give rise to many types of self assembled nanostructures. The one dimensional structures type consists of perfectly straight lines that run in <100> directions and have a minimum separation of 2.4 nm. The other structures are dots that on closest packing form 2.4 nm x 1.6 nm arrays. It is proposed that both structure types are formed through nano-crystalline growth of non-perovskite TiOx phases on the surface. Further structural characterization and spectroscopy on these surfaces is currently being carried out.

9 public active projects

Selected Recent Publications:

Surface and defect structure of oxide nanowires on SrTiO₃.

M.S.J. Marshall, A.E. Becerra-Toledo, L.D. Marks and M.R. Castell.

Physical Review Letters, 107, 086102 (2011).

A homologous series of structures on the surface of SrTiO₃ (110).

J.A. Enterkin, A.K. Subramanian, B.C. Russell, M.R. Castell, K.R. Poeppelmeier and L.D. Marks

Nature Materials, 9, 245 - 248 (2010).

Shape transitions of epitaxial islands during strained layer growth: anatase TiO₂ (001) on SrTiO₃ (001).

M.S.J. Marshall and M.R. Castell.

Physical Review Letters, 102, 146102 (2009).

Surface of sputtered and annealed polar SrTiO₃ (111): TiO_x - rich (n x n) reconstructions.

B.C. Russell and M.R. Castell.

Journal of Physical Chemistry C, 112, 6538 - 6545 (2008).

C₇₀ ordering on nanostructured SrTiO₃ (001).

D.S. Deak, K. Porfyrakis and M.R. Castell.

Chemical Communications, 2941 - 2943 (2007).

Template ordered open-grid arrays of paired endohedral fullerenes.

D.S. Deak, F. Silly, K. Porfyrakis and M.R. Castell.

Journal of the American Chemical Society, 128, 13976 - 13977 (2006).

Bimodal growth of Au on SrTiO₃ (001).

F. Silly and M.R. Castell.

Physical Review Letters, 96, 086104 (2006).

Selecting the shape of supported metal nanocrystals: Pd huts, hexagons, or pyramids on SrTiO₃ (001).

F. Silly and M.R. Castell.

Physical Review Letters, 94, 046103 (2005).

Martin Castell's full publication list can be accessed here

Strained layer epitaxy of oxide on oxide islands
M R Castell

We have recently done some innovative work on anatase TiO2 and BaTiO3 epitaxy on SrTiO3 (001). The results of both these streams of work indicate that there is plenty of scope to expand these efforts into a coherent programme related to oxide strained epitaxy. The idea is to grow thin oxide layers on oxide substrates that have a slight lattice mismatch. The strain that builds up in the oxide layer will then affect its electronic properties such as the bandgap. There is a strong foundation of strain induced electronic structure engineering in semiconductors, and this project is to expand these ideas into oxides.

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Hydrogen-bonded chiral supramolecular networks
Dr. M.R. Castell

In this project 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.

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Molecular alloy crystals
M R Castell

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.

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Growth and spectroscopy of metallic nanocrystals and clusters
M R Castell

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.

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Quantum confinement in oxide nanostructures
M R Castell

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 new state of the art microscopy/spectroscopy facility is available.

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Atomic structure and secondary electron emission
M R Castell

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.

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Electrical conductivity through 2D polymer networks
M R Castell

In this project the electrical transport properties of conducting polymer networks are investigated. The polymers are connected via metal nanoparticle nodes. The long term aim is to use these networks to act as highly sensitive gas sensors. 

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Studies of metal nanocrystals on Strontium Titanate
M R Castell / A I Kirkland

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.

Also see homepages: Martin Castell Angus Kirkland

Tailored nanocrystal catalysts
M R Castell

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.

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Also see a full listing of New projects available within the Department of Materials.