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PhD and Master's students

This page contains descriptions of a selection of current and former PhD and Masters's students' projects.

Former Ph.D. students in Denmark (2007-2011)

Johan Person (May 2009 - )

Ph.D. student funded by the AMON-RA FP7 project and supervised jointly with Jakob Wagner.

Johan is working on the characterization of semiconductor nanowires using advanced electron microscopy techniques.

Eric Jensen (Mar 2009 - )

Ph.D. student funded by the Technical University of Denmark and supervised jointly with Kristian Mølhave.

Eric is working on in situ microfluidic devices for the SEM and TEM.

Robert Pennington (Jan 2008 - )

Ph.D. student funded by the Technical University of Denmark and supervised jointly with Chris Boothroyd and Jakob Wagner.

Robert is working on the characterization of semiconductor nanowires using advanced electron microscopy techniques.

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Former Ph.D. students in Cambridge

Edward T Simpson (Oct 2004 - May 2008)

Ph.D. student funded by the EPSRC.

Dissertation title:

Electron holography of isolated and interacting magnetic nanocrystals.

Abstract:

Off-axis electron holography is used to study both isolated and closely-spaced magnetic nanocrystals, in order to establish how their crystallography, size, shape and spacing influence their magnetic behaviour at the nanometre scale.

The majority of the magnetic crystals that are studied in this dissertation are obtained from different strains of magnetotactic bacteria.By using electron holography, the change in the direction of magnetization of an isolated equidimensional magnetite crystal below the Verwey transition is imaged directly.Linear chains of bacterial magnetosomes and two-dimensional arrangements of crystals from genetically-modified magnetotactic cells are examined.Magnetic states in these crystals, many of which are highly complex, are interpreted with reference to the threshold cellular magnetic moment required for effective magnetotaxis.Interactions between neighbouring crystals are observed to increase the critical size at which the transition from superparamagnetic to single domain behaviour occurs for magnetite.Strains of bacteria that contain elongated and bullet-shaped crystals, as well as carbon nanotubes encapsulating tapering magnetic catalyst particles, are then studied to examine the effect of shape anisotropy on magnetic remanent states.The combined effects of shape, crystallography and interactions are found to stabilise single domain states in crystals that would be expected to support multiple domains if they were isolated.

Finally, the inherently two-dimensional technique of electron holography is extended, by developing aspects of three-dimensional magnetic vector-field recovery, which promises to allow magnetic fields in nanoscale samples to be mapped in all three spatial dimensions.

Yanna Antypas (Oct 2004 - Oct 2007)

Ph.D. student funded by the Gates Cambridge Trust.

Dissertation title:

Theoretical approaches to magnetic induction mapping in the transmission electron microscope.

Abstract:

This dissertation seeks to develop novel theoretical approaches for both two- and three-dimensional magnetic induction mapping in the transmission electron microscope (TEM).As the magnetic induction of the specimen is encoded in the electron optical phase shift, magnetic induction mapping in the TEM is a phase retrieval problem.This work therefore focuses on multiple aspects of phase retrieval, interpretation, and analysis.

Key elements include the critical assessment of existing (unconstrained) intensity transport techniques,the development of improved intensity transport approaches by imposing constraints,and the use of vector field electron tomography to recover three-dimensional magnetic information - otherwise inaccessible, given that transmission electron microscopy is a projection technique.Noise removal from phase images is also considered, as a means to improve phase processing, whether in the context of two or three dimensional magnetic induction mapping.

Ryan K K Chong (Oct 2002 - Jul 2007)

Ph.D. student funded by Chartered Semiconductor Manufacturing Ltd and the Cambridge Commonwealth Trust.

Dissertation title:

The characterisation of nanostructured magnetic materials using image spectroscopy and electron tomography.

Abstract:

For developments in science and engineering, there is a need to visualise and quantify variations in the composition and morphology of nanostructured magnetic materials in relation to characteristics such as their magnetic properties.Most traditional transmission electron microscopy (TEM) techniques only allow two-dimensional projections of three-dimensional structures to be recorded.Few TEM studies provide quantitative information about either absolute or relative chemical compositions.

In this dissertation, two advanced TEM techniques, image spectroscopy and electron tomography, are applied to the quantitative structural and chemical characterisation of nanostructured magnetic materials, in both two and three dimensions (3D), at nanometre spatial resolution.In image spectroscopy, chemical information is obtained by acquiring extended energy-selected series of images, which are interpreted using a combination of computer processing and traditional electron energy-loss spectroscopy (EELS) analysis.In tomography, 3D microstructural and chemical information is obtained by acquiring ultra-high tilt series of high-angle annular dark field (HAADF) or energy-filtered TEM (EFTEM) images, and subsequently applying tomographic reconstruction and visualisation algorithms to the data.

The techniques are applied to the characterisation of materials that include closely-spaced FeNi nanoparticles, chromium carbides in stainless steel, titanomagnetite and an ALH meteorite samples.Biological samples include magnetotactic crystals and bacterial cells, greigite-containing bacterial cells and amyloid plaque cores.The chemical data from image spectrocopy and morphological data from electron tomography are cross-correlated.Furthermoe, experimental tomographic data are compared with tomographic reconstructions of image simulations.

A full assessment of the techniques is provided, including the ability to obtain quantitative microstructural and chemical information, and the possible sources of errors in these measurements.Finally, methods to improve the techniques and integrate them into a coherent analytical methodology are suggested.

Lionel Cervera Gontard (Jan 2004 - Apr 2007)

Ph.D. student funded by Johnson Matthey Plc.

Dissertation title:

The application and development of novel TEM techniques to the study of Pt catalyst particles.

Abstract:

Industrial catalysts usually comprise crystalline particles of high atomic number that have sizes of between 1 and 20 nm and are supported or embedded in a lower atomic number matrix. Electron microscopy is an important tool for the characterisation of their shapes, sizes and crystalline structures, which are, in turn, important for understanding their catalytic properties. In this dissertation, developments in transmission electron microscopy (TEM), including spherical aberration correction and high-angle annular dark-field electron tomography, are applied to the study of 5 - 10 nm platinum nanoparticles supported on carbon.

Indirect methods are used to remove lens aberrations using through-focal series exit- wavefunction restoration (TF-EWR), and at the same time to combine these results with three-dimensional information about particle morphologies obtained using STEM HAADF electron tomography. It is also shown that delocalisation in HRTEM images (the defocus-dependent displacement between dark-field and bright-field images of crystalline specimens) can provide information such as lateral coherence of electron beam or absolute defocus.

A technique for obtaining three-dimensional information from dark-field images or diffraction patterns acquired in a conventional TEM is proposed. The technique is valid for crystalline specimens and can be applied using conventional TEM specimen holders.

The limitations of two existing techniques for characterising particle size distributions are discussed and an approach based on the acquisition of tilt series of annular dark- field images for determining the crystallinity of individual nanoparticles is introduced.

Finally, the application of in situ electron microscopy to the characterisation of catalyst nanoparticles is discussed, and experimental results are obtained using a modified heating holder.

David Cooper (Oct 2002 - Feb 2006)

Ph.D. student funded by the EPSRC and supervised jointly with Paul Midgley.

Dissertation title:

Off-axis electron holography of focused ion beam prepared semiconductor devices.

Abstract:

Off-axis electron holography promises to fulfill the requirements of the semiconductor industry for a technique that can be used to provide quantitative information about dopant potentials in semiconductors with nanometer spatial resolution. The focused-ion-beam (FIB) miller is the preferred method of sample preparation for semiconductor TEM analysis. However, it is increasingly recognised that this method of TEM specimen preparation and subsequent treatments have a profound influence on the phase shifts measured from doped semiconductors.

Specimens containing p-n junctions have been prepared for examination using FIB milling and examined using electron holography. The presence of an electrically 'inactive' thickness resulting from defects introduced during specimen preparation has been shown. The electrically'inactive' regions do not contribute towards the measured phase shift across a p-n junction resulting in an experimental measurement of a built-in potential that is lower than theoretically predicted. In situ annealing has been used to remove these defects, significantly reducing the thickness of the electrically 'inactive' regions. Although an improved signal-to-noise ratio is observed, the theoretical built-in potential is not recovered. By biasing the p-n junctions in situ it is shown that the theoretical built-in potential can be recovered, suggesting that electron holographycan be used to quantitatively measure the dopant concentration in semiconductor devices.

Philippa K Somodi (Oct 2002 - Jan 2006)

Ph.D. student funded by the EPSRC and supervised jointly with Paul Midgley and Crispin Barnes.

Dissertation title:

Interpretation of electron holograms of doped semiconductors and phase retrieval in the transmission electron microscope.

Abstract:

Off-axis electron holography allows the phase shift of an electron wave that has passed through a thin specimen to be recorded directly in the transmission electron microscope (TEM). The measured phase shift can be related to the electrostatic potential. Electron holography studies have shown that built-in potentials across semiconductor p-n junctions in thin TEM specimens are usually lower than predicted and the value measured is highly dependent on the sample preparation technique. Here, finite element methods are used to calculate the electrostatic potential throughout two-dimensional specimens containing p-n junctions. The effects of finite sample thickness and sample preparation are modelled by comparing the results of simulations to experimental data obtained from off-axis electron holography. This comparison shows that the decrease in built-in potential measured in electron holography experiments can be partially accounted for by the electronic state of the specimen surface, which has a more dominant effect for thinner specimens. The built-in potential measured from electron holography is significantly lower than that expected theoretically, possibly because of ion damage during sample preparation and sample charging during examination in the TEM.

The transport of intensity equation (TIE) allows phase retrieval from the intensity gradient of the electron wave in the incident beam direction. However, there are no known analytical solutions to the TIE and all methods presented make approximations that may be incorrect. Here, a method based on finite difference methods is applied to solve the TIE. No approximations about the intensity are required and any desired boundary conditions can be incorporated. The method is applied to both test objects and experimental images.

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