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31st March 2012
Novel SEM-based diffraction method for characterising nitride thin films
Researchers at Strathclyde (Carol Trager-Cowan and Naresh-Kumar) and Oxford (Angus Wilkinson) Universities have been collaborating for some time on the development of novel SEM-based diffraction methods for characterising nitride thin films. Electron channelling contrast imaging (ECCI) has a long association with the Department of Materials at Oxford. Modern FEG SEM instruments and digital imaging capabilities make it possible to fully realise the possibilities identified and pursued in much earlier pioneering work by Profs Hirsch and Booker at Oxford. The recent Physical Review Letter shows that threading dislocations in GaN films with the hexagonal wurtzite structure can be reliably imaged and characterised as edge, screw, or mixed types using a very simple and unambiguous analysis of the black-white contrast associated with each defect when imaged using controlled diffraction conditions. The innovation offers improved quantification of dislocation densities and speed of analysis – two factors of concern to crystal growers we are working with.
13th March 2012
Fracture toughness of Individual Grain Boundaries
Researchers from the Department of Materials have developed new method for fracture toughness testing of single, selected, grain boundaries in normal polycrystalline materials. Measuring the fracture properties of single grain boundaries has until now required macroscopic bi-crystals. These are expensive and usually not even available for most materials. Dave Armstrong, Angus Wilkinson and Steve Roberts have developed a method for fracture testing of single, selected, grain boundaries in normal polycrystalline materials. Focussed ion beam machining is used to cut micron-scale cantilevers each containing a grain boundary, which are tested using nanoindenter. The paper published in Phil Mag. Letters describes a "test-case" set of experiments to prove the method by measuring the fracture toughness of selected grain boundaries in bismuth-embrittled copper. More information is available on the Materials for Fusion & Fission Power website.
12th March 2012
Chemistry at the nanoscale: synthesis of an N@C60-N@C60 endohedral fullerene dimer
Researchers from the Department of Materials have synthesized a coupled electron-spin dimer system. The team led by K. Porfyrakis have achieved two important milestones: They are the first to have synthesized a covalently-linked endohedral fullerene dimer. They are also the first to have shown evidence of dipolar coupling between the adjacent electron spins in the fullerene cages. In their paper published in Angewandte Chemie Int. Ed. B.J. Farrington et al., report a method for the rapid one pot double 1,3-dipolar cycloaddition reaction of the rare endohedral fullerene N@C60 to an oligo(p-phenylene polyethylene) bis-aldehyde using a novel amino acid derivative; creating the first example of a chemically linked two spin centre N@C60-N@C60 molecule. N@C60 has got an extremely long electron spin lifetime and has been proposed as a molecular quantum bit (qubit). This work shows for the first time, that using chemistry at the nanoscale it is possible to link together these extraordinary molecules in larger arrays: an important milestone for any molecular quantum computer
24th February 2012
Understanding structures of amorphous materials
Michael Treacy from the Department of Physics, Arizona State University and Konstantin Borisenko from the Department of Materials, University of Oxford, have used computer models to show that atomic structure of amorphous silicon has more order to it than previously thought.
Since the 1930s, most chemists have represented the structure of amorphous silicon by what is known as the continuous random network model. This model agrees well with the electron diffraction data. However, it is inconsistent with the intensity speckle variance observed by scanning an electron microscope probe over the sample of material. An inhomogeneous paracrystalline structure is consistent with both observations. A paracrystalline structure model has small ordered clusters of silicon atoms, about 1 to 2 nanometres in size, embedded in a less-structured matrix.
These structural insights can be useful for improving solar cells and other many products where the material has applications, as well as provide more fundamental understanding of phase transitions in materials in general: Science, doi:10.1126/science.1214780
3rd February 2012
Researchers from the Department of Materials have synthesised a dyad of two radical centres
An international team, led by K. Porfyrakis, have managed to covalently link the spin-active molecule N@C60 to another spin-active molecule: a Cu-Porphyrin. In their paper published in JACS Liu et al., have shown that the coupling between the adjacent spin centres leads to suppression of the electron spin resonance (ESR) signal of N@C60. Demetalation of the metalloporphyrin moiety of the dyad, which effectively turned the two-radical-center dyad into a single-radical-center dyad, recovered 82% of the ESR signal of N@C60. In this way Liu et al., were able to use Chemistry to effectively switch the spin state of N@C60 on and off. The work could find use in molecular spintronics applications where N@C60 and other endohedral molecules can be used as spin valves.
10th January 2012
Testing the reality of quantum mechanics
This research offers an answer to the question, "How can we know that the strange quantum effect called superposition is REALLY happening in a particular system, rather than being merely a mathematical trick in the theory?". The paper in Nature Communications by an international collaboration describes a new approach to confirming the existence of quantum superpositions. It applies a test developed by Nobel prize-winner Anthony Leggett and his collaborator Anupam Garg in the 1980s - achieving for the first time the "ideal negative result" measurement the pair proposed. The test relies on measuring a quantum system in such a way that the results can't be dismissed as influenced by the measurement. To achieve this we used at exceptional material: cooled, ultra pure silicon with a small scattering phosphorous atoms within it -- it was these "impurities" that whose quantum nature was tested and proven.
1st January 2012
Nomenclature of sp2 carbon nanoforms
Carbon’s versatile bonding has resulted in the discovery of a bewildering variety of nanoforms which urgently need a systematic and standard nomenclature. A recent guest editorial in Carbon 50 (2012) 741-747 co-written by Nicole Grobert aims to raise awareness of the issue and stimulate debate, providing a platform for future development of an exhaustive, definitive, yet flexible carbon nomenclature.
15th December 2011
Atomistic modelling of semiconductor-sensitized solar cells
Researchers from the Materials Modelling Laboratory have conducted the first atomic-scale investigation of a novel type of solar cell. The solid-state semiconductor-sensitized solar cell is an evolution of the concepts of dye-sensitized solar cells and hybrid nanocrystal/polymer solar cells. During the past two years semiconductor-sensitized cells based on mesoporous TiO2 and quantum dots of stibnite (Sb2S3) have been fabricated, and cell efficiencies above 5% have been demonstrated. In the article by C. E. Patrick and F. Giustino published on December 20 in Advanced Functional Materials, the authors used atomistic computational materials modelling to investigate semiconductor-sensitized TiO2 surfaces. By virtually screening several potential semiconductor sensitizers the researchers have found that TiO2 films sensitized with antimonselite (Sb2Se3) may lead to higher power conversion efficiencies than those reported for the TiO2/stibnite system. The work constitutes the first step in the direction of engineering semiconductor-sensitized solar cells using rational design at the nanoscale.
31st October 2011
Three-dimensional observations of a growing fatigue crack
The lifetime of a fatigue crack, before it causes failure, is mostly when it is smallest, which is also when it is most sensitive to the material's microstructure. So, to design fatigue-resistant materials we must study how cracks may be slowed by microstructure features. Yet this is the most difficult regime to examine. A collaborative effort, led by James Marrow and published recently in Acta Materialia (http://dx.doi.org/10.1016/j.actamat.2011.07.034), studied short fatigue cracks for the first time in 3-dimensions and in-situ at the European Synchrotron Radiation Facility. A 3D map of grain shapes and crystallographic orientations was produced using non-destructive X-ray diffraction contrast tomography (DCT) . A focused ion beam instrument was used to introduce small notches in selected grains and synchrotron X-ray computed microtomography was used to study the evolution of the fatigue crack through the microstructure. The observed crack retarding interactions between the crack and grain boundaries could be explained in terms of the crystallography of cracking and deformation in the magnesium alloy.
15th October 2011
Observation of individual carbon nanotubes and their electronic, structural and chemical behaviour
Researchers at the Department of Materials have been investigating the impact of current passing through specially synthesised carbon nanotubes. Multi-walled carbon nanotubes (MWCNTs) have long been anticipated as candidates for electrical components in an increasingly miniaturized electronics industry due to their inherent electrical properties. It is possible to manipulate and control these properties by introducing dopants such as N, B, and P. To understand how the tubes will behave in electronic circuits, a study has been carried out looking at the changes in electrical and chemical behaviour in the presence of flowing current. The findings by Aslam & Grobert et al., which are due to be published in Advanced Functional Materials http://dx.doi.org/10.1002/adfm.201101036, show that N-MWCNTs not only undergo current-induced structural transformations, but also - and most importantly—the complete removal of the dopant causing a significant change in the electronic behaviour. This has serious implications for the use of doped CNTs as electronic components, potentially limiting their applications.
3rd October 2011
Resolving strain in carbon nanotubes at the atomic level
Details of how atomic structure responds to strain are essential for building a deeper picture of mechanics in nanomaterials. The article by Jamie H. Warner*, Neil P. Young, Angus I. Kirkland, G. Andrew D. Briggs published on October 2 in Nature Materials http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3125.html provides the first experimental evidence of atomic displacements associated with shear strain in single-walled carbon nanotubes (SWNTs) by direct imaging using aberration-corrected transmission electron microscopy. The atomic structure of a zig-zag SWNT is resolved with unprecedented accuracy and the strain induced by bending is mapped in two dimensions. We show the existence of a dominant non-uniform shear strain that varies along the SWNT axis. The direction of shear is opposite to what would be expected from a simple force applied perpendicular to the axis to produce the bending. This highlights the complex atomistic strain behaviour of beam-bending mechanics in highly anisotropic SWNTs.
3rd February 2011
Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials
If they could be easily exfoliated, layered materials would become a diverse source of two-dimensional crystals whose properties would be useful in applications ranging from electronics to energy storage. Coleman and Nicolosi et al. in Science http://dx.doi.org/10.1126/science.1194975 show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, and Bi2Te3 can be efficiently dispersed in common solvents and can be deposited as individual flakes or formed into films. Electron microscopy strongly suggests that the material is exfoliated into individual layers. By blending this material with suspensions of other nanomaterials or polymer solutions, we can prepare hybrid dispersions or composites, which can be cast into films. We show that WS2 and MoS2 effectively reinforce polymers, whereas WS2/carbon nanotube hybrid films have high conductivity, leading to promising thermoelectric properties.
