Publication News

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Flexible OTFTs

24th June 2014

A high-yielding evaporation-based process for organic transistors based on the semiconductor DNTT

Ziqian Ding, Gamal Abbas and Hazel Assender from the Polymers group, along with collaborators from Bangor and Manchester Universities have published an article in Organic Electronics that demonstrates high-performing organic transistors made by a high-speed evaporation process on flexible polymer substrates suitable for large-area roll-to-roll processing. Using the semiconductor DNTT, hole mobility of around 1cm2/Vs can be achieved using a non-polar modification of the dielectric surface, and the resulting transistors show excellent stability unencapsulated, but in addition, the successful encapsulation of the transistors either with an off-line lamination process or an in-line deposition of an oxide encapsulation layer is reported. The paper makes a first report of organic vapour jet printing of DNTT semiconductor to manufacture transistors with a high semiconductor deposition rate.

Slip band measurements

5th June 2014

Slip Band-Grain Boundary Interactions: local stresses and slip transfer

Yi Guo and Angus Wilkinson of the Oxford Micromechanics Group along with Ben Britton at Imperial College London have just published a paper in Acta Materialia using EBSD strain mapping to characterise the interactions of slip bands with grain boundaries in Ti. Three broad classes of grain boundary interactions were identified: (i) blocked slip bands with no obvious long range slip feature in the neighbour grain but intense localised stress, (ii) slip bands in both neighbouring grains which connect across the grain boundary which show negligible local stresses, and (iii) blocked slip bands with no significant local stress field. The three classes of interaction were rationalised in terms of the geometry and alignment of (active) slip systems in the neighbouring grains and the shear stress on the slip systems from the applied far field loading. Identifying in this way the local grain morphologies and crystallography that lead to stress ‘hot spots’ is important in understanding and controlling crack initiation and twin nucleation processes. Such studies of fundamental micromechanics issues underpin development of improved microstructures and new alloys all of which are being pursued within the HexMat programme grant that seeks to improve future Ti alloys for jet engine and Zr alloys for nuclear fuel technologies.

NanoSIMS image of stable isotopes

23rd May 2014

Stable isotope imaging of biological samples with high resolution secondary ion mass spectrometry and complementary techniques

Professor Grovenor's Nanosims Group together with colleagues at the National Physical Laboratory Biotechnology Group and the University of California Los Angeles have published an article in Methods describing the methodologies that they have developed to correlate atomic force microscopy and backscattered electron imaging with NanoSIMS experiments to illustrate the imaging of stable isotopes at molecular, cellular, and tissue scales. These studies make it possible to address 3 biological problems: (1) the interaction of antimicrobial peptides with membranes; (2) glutamine metabolism in cancer cells; and (3) lipoprotein interactions in different tissues.

In-situ view of dendrite fragmentation

13th April 2014

A synchrotron X-ray radiography study of dendrite fragmentation induced by a pulsed electromagnetic field in an Al–15Cu alloy

Patrick Grant’s group working with colleagues from Tsinghua University, the Norwegian University of Science and Technology  and the Diamond Light Source have published a study on the effect of a pulsed electromagnetic field on dendrite growth and fragmentation during solidification. Although magnetic fields are used in the continuous casting industry for macro-segregation minimisation, the way in which the moving magnetic field also affects grain growth and microstructure has not been well understood. By passing synchrotron X-rays through a thin sample of an Al-Cu alloy as it is carefully solidified, and subjecting the sample to a pulsed magnetic field, the team were able to show for the first time that dendrite fragmentation was increased in all areas on the application of the pulsed magnetic field - and not only at the dendrite tips as was usually thought. The team showed that even deep in mushy zone well behind the advancing dendrite tips, solute-rich liquid was effectively stirred by the magnetic field, and this caused fragmentation. This work may help understand how to optimise the use of electromagnetic field to control grain size in commercial processes.

N3 molecule on surface

22nd February 2014

Atomic-Scale Observation of Multiconformational Binding and Energy Level Alignment of Ruthenium-Based Photosensitizers on TiO2 Anatase

A joint experimental-computational study performed as a collaboration between Feliciano Giustino's group and researchers from the Max Planck Institute for Solid State Research has been published in Nano Letters. Through a combination of electrospray ion beam deposition, high resolution scanning tunnelling microscopy (STM) and spectroscopy (STS) measurements and density-functional theory (DFT) calculations, the work provides an unprecedented view of the interface formed between titanium dioxide and the ruthenium-based "N3" dye molecule (pictured), considered the prototypical interface for dye-sensitized solar cells. The STM experiments observe the dye molecule binding to the surface in a wide variety of different geometries, and the STS shows that the specific geometry can have a large effect on the expected performance of the solar cell. The work suggests that optimization strategies based solely on the electrochemical properties of the dye should be replaced by a more comprehensive approach where the focus is on the engineering of the chromophore–semiconductor interface at the atomic scale.

H edges in Graphene

10th February 2014

Hydrogen Free Graphene Edges

Graphene edges and their functionalization influence the electronic and magnetic properties of graphene nanoribbons. Theoretical calculations predict saturating graphene edges with hydrogen lower its energy and form a more stable structure. Despite the importance, experimental investigations of whether graphene edges are always hydrogen-terminated are limited.

New results (Nature Communications, 5, 3040 (2014)) published byDr Jamie Warner and DPhil student Kuang He from the Materials Department at Oxford University reveal that C–C bond lengths at the edges of clean graphene in vacuum can have ~86% contraction relative to the bulk and confirm that non-functionalized graphene edges can exist. The research utilized the Oxford-JEOL aberration-corrected transmission electron microscope with sub-Angstrom spatial information to obtain ultra-high resolution images of suspended graphene. The graphene samples were grown by chemical vapour deposition in the labs of Dr Warner and exploited their unique approach to transferring graphene onto special TEM grids to achieve clean surfaces. Collaboration with Prof. Gun-Do Lee and Prof. Euijoon Yoon from Seoul National University enable the experimental results to be corroborated with density functional theory (DFT). The DFT revealed the bond contraction at the edge of graphene is attributed to the formation of a triple bond and the absence of hydrogen functionalization. Time-dependent images revealed temporary attachment of a single atom to the arm-chair C–C bond in a triangular configuration, causing expansion of the bond length, which then returns back to the contracted value once the extra atom moves on and the arm-chair edge is returned. These results provide important insights into the nature of graphene edge states and can be used to build a better understanding of the unique structure of graphene edges.

GPILRUFT reconstruction

8th February 2014

Deterministic electron ptychography at atomic resolution

A collaboration between researchers at the Department of Materials at the University Oxford and the Department of Physics at the University of Melbourne has developed a new method for deterministic ptychographic reconstruction of a specimen exit wave to solve the phase problem.

PHYSICAL REVIEW B 89, 064101 (2014)

This work suggests that a dedicated instrument designed with long-term optical and sample stability as key criteria could be expected to come closer in resolution to the fundamental limit imposed by the wavelength of the electrons used.

Graph

1st February 2014

Aluminium oxide barrier films on polymeric web and their conversion for packaging applications

A recent paper by Hazel Assender and co-workers in Thin Solid Films demonstrates the value in an acrylate overcoat to infiltrate defects in an oxide barrier layer and hence improve the barrier to water vapour. For the first time, this paper decouples the effect of a mechanical protection during winding from the more intrinsic value of the overcoat in modifying the oxide defects.

Thin Solid Films 553 153-156 (2013)

TEM of dislocation loops

4th January 2014

Dislocation instabilities and microstructure in anisotropic alpha-Fe

Steve Fitzgerald of the Defect Dynamics Group, has compared theoretical predictions of the shapes of dislocation shear loops in highly anisotropic crystals with TEM of irradiated alpha-Fe. The microstructures observed provide experimental evidence for a new mechanism for high-temperature plastic deformation which may contribute to the severe loss of strength observed above 500C.

Phil. Mag. Lett. 93(11) 2013 625-630

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