Study of growth defect in GaN nanowires *

GaN nanowires have been produced by MBE for use as an intermediate layer in the growth of high quality GaN epilayers. It has been found that the growth of the nanowires is controlled by an extended threading defect. It is believed that this defect acts as a growth step, but its structure is uncertain. The SuperSTEM is being used to characterise the structure and displacement associated with this unique defect, along with how it interacts with stacking faults also present in the nanowires.

* In collaboration with Prof. David Cherns and Ian Griffiths, University of Bristol

Atomic structure of the NiSi2/ Si (001) interface

Silicides, such as NiSi2, are used as a conducting material for poly-Si gate electrodes in MOSFET devices. Several investigations have focused on the NiSi2/ Si (001) interface. A theoretical study had shown that at this interface the Ni atoms are 7-fold coordinated and Si 5-fold coordinated to give a (2x1) superstructure. By acquiring HAADF images along two orthogonal <110> directions parallel to the interface direct experimental evidence confirming the theoretical interface structure was obtained for the first time.
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Composition measurement of buried quantum dots

Quantum dots, such as In(Ga)As in a GaAs matrix, have many potential applications in optoelctronic devices and hence there is a need to characterise their size, shape and composition. A major obstacle is the fact that the quantum dot lies buried in a matrix containing identical chemical elements (e.g. Ga and As for the In(Ga)As/ GaAs system). The HAADF image was therefore first used to estimate the size and shape of the quantum dot from which the number of In atom sites can be estimated at any given position. From EELS measurements of the In areal density the In occupancy is easily determined and hence a 2D projection of the composition constructed for the entire quantum dot.
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Distribution of Au atoms in Si nanowires *

Si nanowires are produced using the vapour-liquid-solid (VLS) method and Au-Si eutectic. The presence of metals such as Au is deleterious to the electrical properties of Si; hence it is important to characterise the distribution of any Au atoms on the surface or within the bulk of Si nanowires produced by the VLS technique. The unique optical sectioning properties of aberration corrected STEM was used to acquire HAADF images at various depths through a Si nanowire so that the 3D distribution of Au atoms could be determined for the first time.
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* In collaboration with Prof. Lincoln Lauhon, Northwestern University.

Interfacial bonding in HfO2 dielectric TiN/metal electrode gate stacks for MOSFET devices (ongoing)

HfO2 has been proposed as a high dielectric material to replace Si(O,N) in Si-based MOSFET devices. As device size continues to diminish any interfacial reactions between the chemically dissimilar layers will have an increasingly important effect on the overall electrical performance. The near-edge structure of an EELS core loss signal is dependent on the local bonding environment and can be used to probe any interfacial reactions, particularly at the TiN/ poly-Si gate electrode.

Low-loss EELS spectroscopy of graphene (ongoing) *

Graphene is technically a single layer of graphite and has potential use as future transistor devices. For obvious reasons determining whether the material is true graphene or (say) bi-layer ‘graphene’ is extremely difficult. However, mono-layer graphene shows unique behaviour in the low-loss region of the EELS spectrum, compared to multi-layer ‘graphene’ or graphite, which comprises the pi-> pi* transition and (pi + sigma) plasmon. Using aberration corrected STEM allows us to acquire these measurements at the atomic scale.
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* In collaboration with Dr Ursel Bangert and Dr. Andre Geim, University of Manchester.

Reactive element (RE) segregation to grain boundaries in FeCrAl(RE) alloys

FeCrAl alloys are widely used commercially in high temperature applications. They form a protective layer of alpha-Al2O3 on the surface and it is known that the addition of reactive elements (RE) such as Y and Hf improve the oxide spallation resistance. One hypothesis for a more adherent oxide is the modification of its growth by REs. EELS measurements have revealed Y and Hf segregation to the grain boundaries in the bulk region of the oxide scale which have a columnar grain structure. In the outer part of the oxide scale, where the grains are equiaxed, Hf-rich precipitates are detected. Further study of this segregation behavior is to reveal the 3-dimensional distribution of the individual RE atoms in the grain boundary using various techniques, for example, through focal series.

Atomic structure of bcc-Mo screw dislocation cores

Bcc-screw dislocations are a classic example of sessile dislocations due to core spreading. Conclusive experimental evidence confirming the core spreading is extremely difficult to achieve through (say) lattice imaging since much of the spreading takes place along the dislocation line direction. The near-edge structure of the EELS core-loss signal is however, sensitive to the full 3D bonding environment. Theoretically simulated core structures can therefore be validated by comparing the FEFF generated core-loss edge with the experimental EELS spectrum.

EELS fine structure of O and Mn in perovskite structural materials

The Ruddlesden-Popper (RP) perovskite-like manganates [AO•(A’BO3)n] (n = 1, 2, 3) and [A’BO3] (n = 8) have recently become subjects of interest due to their complex magnetic and electrical transport properties and colossal magnetoresistance (CMR). In previous work, the epitaxial growth of n = 4, 5, 6 RP phases CaO.[(CaMnO3)]n on SrTiO3 (001) substrate by PLD and the dislocations incorporated at the interface/substrate have been studied .Now, we focus our studies on the structure and chemical composition of the CaO rock salt blocks in the n = 4 RP phase at the atomic scale using HAADF imaging/ EELS in the aberration-corrected STEM. Simultaneously, EELS can be recorded at the electron probe positions with atomic precision. EELS is able to provide an insight on the local elemental composition, electronic states, bonding, and coordination near the RP phase.

Detection of single-walled carbon nanotubes (SWNT) in cells *

As the application of nanotechnology becomes more widespread its effect on human health, such as for example the interaction between SWNTs and cells, needs to be better understood. Since SWNTs and cellular material are largely carbon-based they show very little contrast difference in conventional imaging modes. Here the chemical shift of the plasmon peak due to the different carbon bonding environments is used to image SWNTs within the cellular material. This allows the distribution of carbon based nanomaterials within a cellular structure to be determined, which in turn will aid the understanding of their potential toxicity.
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* In collaboration with Alexandra Porter, Imperial College London

Iron mineral cores of ferritin (ongoing)

Iron is an essential element for biological processes such as oxygen transport, cellular respiration, and DNA synthesis. Disorders in the human metabolism and storage of iron include the liver disease haemochromatosis and iron is increasingly being recognized as important in neurodegenerative disorders including Alzheimer’s disease. The protein ferritin assembles into a multi-subunit cage and stores most of the iron that is not bound to heme in a mineral form. Atomic resolution HAADF imaging and EELS are being used to investigate the morphological and mineral structure of ferritin cores from multiple tissues to reveal their biological roles.
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Single atom detection of Pt-group metals on catalyst supports *

Due to the high cost of Pt-group metals optimising the performance of each atom in the catalyst support is of extreme commercial importance. One approach to understanding their role in detail is to find where they are located. Imaging a single Pt-group metal atom on a thick catalyst support is only feasible using HAADF in an aberration-corrected STEM. Vacuum transfer and ex-situ methods are being developed. Multislice simulations have also shown that atoms at the bottom surface of the catalyst support have a higher visibility than those at the top surface, complicating the interpretation.
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* In collaboration with Johnson Matthey.

3D atomic scale structure of Au 309-nanoclusters *

Nanoscale Au particles, consisting of as few as 309 atoms, have important applications as catalysts for a number of chemical reactions. Determining the geometric shape of these nanoclusters is challenging, since being so small they are electron beam sensitive. Hence traditional tomography methods, which involve acquiring several images of the specimen at different projections, are inapplicable. The HAADF signal in the STEM however, depends monotonically on the local thickness and can be used together with multislice image simulations to build a 3D model of the Au nanocluster. The shape of the 309- nanoclusters was identified with Ino-decahedral, cuboctahedral and icosahedral geometries.
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* In collaboration with Prof. Richard Palmer, University of Birmingham.

What causes the colour of diamonds? *

Diamonds are most often thought of as colourless precious stones. However, many diamonds do not conform to the trademark colour to which we have become accustomed and in fact can vary from brown/green to yellow and blue. Given that all diamonds have a common building block, that is tetrahedrally bonded carbon, the source of the colouration must lie elsewhere. Research has shown that the diamond crystal lattice can contain impurities, which originate from both natural and artificial sources. This work concentrates on the use of high-resolution electron microscopy and electron energy loss spectroscopy to better understand the nature of the crystal defects that are associated with these impurities. As the origin of gem quality diamonds becomes harder to trace, new methods of identifying their signature characteristics become increasingly important.

* Sponsor and collaborator: Diamond Trading Company

Atomic-resolution mapping of chemical elements using core-loss EELS *

Lattice imaging using inelastically scattered electrons provides information on the site occupancy of chemical elements within the crystal. With a conventional electron microscope this information is generally not accessible since the current within the finely focused electron probe is too low to produce good counting statistics. Aberration corrected STEM probes however, have a current that is an order of magnitude larger and hence chemical mapping at atomic resolution becomes feasible. Inelastic lattice images for (Bi,Sr)MnO3, a material that shows colossal magnetoresistance, have recently been acquired for different crystallographic orientations using SuperSTEM.
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* research carried out by Dr. Michael Bosman at SuperSTEM.

‘Smart’ EELS acquisition for beam sensitive materials (ongoing)

Due to larger inelastic scattering cross-sections acquiring EELS data with a reasonable signal-to-noise ratio requires a higher dose compared to conventional imaging. However, in many cases the analysis volume need not be spatially localised but can be extended to include many chemically identical volume elements. One such example is the EELS signal from periodically repeating metal atoms in a protein structure. By computer control of the electron beam over the positions of interest a statistically significant EELS signal can be generated while at the same time satisfying low-dose conditions for each analysis volume element.
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Discrete electron tomography of nanoparticles

Catalyst nanoparticles are of great promise in many fields of industries and science. As the particle size is reduced in an effort to reduce the catalyst usage without compromise in the activity, an understanding of the 3D structure of the particles is crucial. A novel discrete electron tomographic algorithm: multiplicative backprojection method, using five HAADF images from SuperSTEM2 is developed. In contrast to the other electron tomographic methods, which requires over 100 images, our algorithm greatly reduced the number of projections for reconstruction and hence data acquisition time and electron dose to the specimen. Using 2D atomic resolution images holds the possibility of improving 3D volume resolution towards atomic resolution. The algorithm demonstrated successful and accurate reconstructions of catalyst nanoparticles with a resolution of 4x10-3nm-3 voxels (0.16nm/pixel) and show potential applications to beam-sensitive specimens and particles whose properties are dominated by surface structure.
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Ptychographic iterative phase reconstruction (ongoing)

A new principle of TEM/STEM, suitable for all forms of radiation which does not rely on the use of a lens or any other form of far-field interfereometry has been proposed by J.M. Rodenburg. The technique could provide wavelength-limited resolution of transparent objects over a wide field of view, with potential applications largely with various radiations. We are able to apply this technique to SuperSTEM instrument by using high resolution diffractive imaging (Ronchigram) and combining with phase iterative algorithm to solve phase problem and reconstruct object wave function at very high resolution.

Elastic scattering of high-energy electrons by dopant atoms

Bloch waves provide a physically intuitive framework for understanding elastic scattering in a perfect crystal. They can be extended to imperfect crystals, containing slowly varying strain fields, through the the Howie-Whelan equations and column approximation. However, there is as yet no suitable method to account for changes in chemistry, such as for example in the case of a dopant atom buried within a crystal. The dopant atom will contribute an 'excess' potential to the periodic potential of the host atom lattice. This 'excess' potential represents a small change in the system Hamiltonian so that standard perturbation techniques can be used to deduce the wavefunction in the imperfect crystal (containing the dopant atom) in terms of the Bloch wave solutions for the perfect crystal, similar to the Howie-Whelan method.
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