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Martin Castell

Professor Martin Castell
Professor of Materials
Fellow of Linacre College

Department of Materials
University of Oxford
16 Parks Road
Oxford OX1 3PH
UK

Tel: +44 1865 273786 (Room 276.40.24)
Tel: +44 1865 273700 (switchboard)
Fax: +44 1865 273789 (general fax)

Surface Nanoscience Group website
Personal Homepage

Summary of Interests

I am interested in the atomic structure of the surfaces of materials, in particular the surfaces of oxide crystals, atomically thin films of oxides on metal substrates, and the networks that form when molecules self-assemble on surfaces. This research is directed at solving problems relevant to catalytic processes and nanotechnology. I am also involved in the development of a sensor for the detection of low concentrations of gas-phase analytes.

We have two scanning tunneling microscopes (STM) in the group that can routinely produce images of surface structure with atomic resolution. We also carry out characterisation using electron diffraction and photoemission spectroscopy. Most of the experimental results are complimented by theoretical modelling carried out by international collaborators.

Current Research Projects

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. 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.

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 is 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
X. Hu, Dr 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. Molecular adsorption on the films is also being investigated.

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.

7 public active projects

Research Publications

Below lists recent publications. The full list is on my group website.

2015

110. Scanning tunnelling microscopy of surfaces and nanostructures.
M.R. Castell
In the book Nanocharacterisation: 2nd edition, chapter 3, pages 80 - 107 (2015).
Royal Society of Chemistry Publishing, Cambridge, U.K., edited by A.I. Kirkland and S.J. Haigh.

109. Smart Align - a new tool for robust non-rigid registration of scanning microscope data.
L. Jones, H. Yang, T.J. Pennycook, M.S.J. Marshall, S.V. Aert, N.D. Browning, M.R. Castell and P.D. Nellist.
Advanced Structural and Chemical Imaging, 1, 8 (2015).

108. Transition from order to configurational disorder for surface reconstructions on SrTiO3 (111).
L.D. Marks, A.N. Chiaramonti, S.U. Rahman and M.R. Castell.
Physical Review Letters, 114, 226101 (2015).

107. Stoichiometry engineering of ternary oxide ultrathin films: BaxTi2O3 on Au (111).
C. Wu, M.R. Castell, J. Goniakowski and C. Noguera.
Physical Review B, 91, 155424 (2015).

106. Defects on strontium titanate.
M.S.J. Marshall, A.E. Becerra-Toledo, L.D. Marks and M.R. Castell.
In the book Defects at Oxide Surfaces, chapter 11, pages 327 - 349 (2015).
Springer International Publishing, edited by J. Jupille and G. Thornton.


2014

105. Out- versus in-plane magnetic anisotropy of free Fe and Co nanocrystals: tight-binding and first-principles studies.
D. Li, C. Barreteau, M.R. Castell, F. Silly and A. Smogunov.
Physical Review B, 90, 205409 (2014).

104. Scanning tunnelling microscopy of epitaxial nanostructures.
M.S.J. Marshall and M.R. Castell.
Chemical Society Reviews, 43, 2226 - 2239 (2014).

103. The effect of the size of surface Pd island ensembles on electron transfer of adsorbed perchlorate ions on Au (111).
C. Wu, F. Xu, M.R. Castell and S.C.E. Tsang.
Chemical Communications, 50, 1198 - 1201 (2014).


2013

102. Initial growth stages of titanium and barium oxide films on SrTiO3 (001).
C. Wu, K. Kruska and M.R. Castell.
Surface Science, 618, 94 - 100 (2013).

101. Synthesis of carbon nanocoil forests on BaSrTiO3 substrates with the aid of a Sn catalyst.
J. Sun, A.A. Koos, F. Dillon, K. Jurkschat, M.R. Castell and N. Grobert.
Carbon, 60, 5 - 15 (2013).

100. Controlled growth of Ni nanocrystals on SrTiO3 and their application in the catalytic synthesis of carbon nanotubes.
J. Sun, C. Wu, F. Silly, A.A. Koos, F. Dillon, N. Grobert and M.R. Castell.
Chemical Communications, 49, 3748 - 3750 (2013).

Projects Available

*Vapour sensing of explosives, chemical agents, and drugs
Prof M R Castell

There is significant civil and military interest in the accurate detection of hazardous and illicit materials. Current methods for detecting explosives and narcotics include the olfactory sense of a trained sniffer dog and ion mobility spectrometry (IMS).

The aim of the work in this project is to develop vapour sensing technology that can be readily miniaturised and provide lightweight mobile or networked molecular detection of threats. 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 DPhil (PhD) studentship is available to develop percolation sensors for the detection of explosives, chemical agents, and drugs. The student will be involved in a broad range of interdisciplinary activities including design, building, and testing of the sensor. This studentship is an Industrial CASE award funded jointly by EPSRC and the UK Defence Science and Technology Laboratory (Dstl www.dstl.gov.uk). The student will have regular contact with the Dstl Explosives Detection Group, based near Sevenoaks in Kent and Porton Down in Wiltshire. Some extended visits to Dstl of a few weeks per year may also be required.

Candidates are considered in the January 2016 admissions cycle, which has an application deadline of 22 January 2016. This 4 year EPSRC iCASE studentship will provide full fees and maintenance. The stipend is expected to be at least £16,057 per year. Only UK nationals are eligible to apply for this studentship.

Any questions concerning the project can be addressed to Professor Martin Castell (martin.castell@materials.ox.ac.uk). Some information on the Surface Nanoscience research group is available on the website: http://users.ox.ac.uk/~stm/ General enquiries on how to apply can be made by e mail to graduate.studies@materials.ox.ac.uk. You must complete the standard Oxford University Application for Graduate Studies and further information and an electronic copy of the application form can be found at http://www.ox.ac.uk/admissions/graduate/applying-to-oxford.

Also see homepages: Martin Castell

Atomic resolution imaging of ultra-thin oxide films
Professor Martin Castell

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 with atomic resolution. The structure of the films is unique to the tin film system, is not a bulk termination, and is determined through the interaction with the gold substrate. In this project oxide thin films will be imaged with particular emphasis placed on learning about the point and extended defects that occur.

Also see homepages: Martin Castell

Quantum confinement in oxide nanostructures
Professor Martin 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.

Also see homepages: Martin Castell

Surface structure characterisation of iron-based superconductors and topological insulators
Dr Susannah Speller/Professor Martin Castell

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.

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Tailored nanocrystal catalysts
Professor Martin 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. The project will also involve the synthesis of core-shell nanocatalysts. Characterisation of the catalyst particles will be carried mainly with scannning tunnelling microscopy.

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Ultra-sensitive molecular detection
Professor Martin Castell

The aim of this project is to develop a novel ultra-sensitive sensor for the detection of low concentrations of gases, especially concentrating on volatile organic compounds. The sensor design is based on our recently submitted 'percolation sensor' patent. It is made of a network of metal nanoparticles connected by conducting polymers and is optimised for high sensitivity and functionalised for selectivity. The student will be involved in a broad range of interdisciplinary activities from sensor design and construction to testing and evaluation.

Also see homepages: Martin Castell

Epitaxial oxide nanocrystals
Professor Martin Castell

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 feasability of the proposed work.

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Chiral networks and the origin of life
Professor Martin Castell

In this project 2D molecular networks are synthesized through self-assembly on metal and mineral 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.

Also see homepages: Martin Castell

Growth and spectroscopy of metallic nanocrystals and clusters
Professor Martin 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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Multi-component molecular crystals
Professor Martin 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. Ultimately the idea is to create molecular architectures that can be used in advanced electronic devices.

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Atomic structure and secondary electron emission
Professor Martin 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. There is also the likelyhood that the experiments will be further expanded through the use of the PEEM instument at the Diamond synchrotron.

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