Publication News

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27th July 2017

Mixed-Mode Electro-Optical Operation of Ge2Sb2Te5 Nanoscale Crossbar Devices, Advanced Electronic Materials

This report in Advanced Electronic Materials by Gerardo Rodriguez‐Hernandez in the NanoEng group shows the operation of phase change devices in the mixed optical and electrical domain, and builds a non-volatile mixed-mode switch. It shows that the actual design of the optical stack determines absorption, which can thus be tuned depending on the wavelength.

 

16th June 2017

LiO2: Cryosynthesis and Chemical/Electrochemical Reactivities

Researchers from the Peter Bruce Group have recently studied the synthesis and chemical/electrochemical reactivities of LiO2 species in liquid NH3 and compared its chemical reactivity with those of O2-. The paper in J. Phys. Chem. Lett. reports that O2- and Li2O2 are stable in liquid NH3, while LiO2 can react with liquid NH3 forming LiOH and LiOH.H2O, suggesting it is the most reactive oxygen species in Li-O2 batteries.

16th June 2017

Phenol-Catalyzed Discharge in the Aprotic Lithium-Oxygen Battery

Discharge in the lithium-O2 battery is known to occur either by a solution mechanism, which enables high capacity and rates, or a surface mechanism, which passivates the electrode surface and limits performance. Researchers from the Peter Bruce Group have recently reported in Angewandte Chemie that the introduction of the protic additive phenol to ethers can promote a solution phase discharge mechanism. Phenol acts as a phase-transfer catalyst, dissolving the product Li2O2, avoiding electrode passivation and forming large particles of Li2O2 product; vital requirements for high performance.

Polaron to Fermi Liquid Transition

9th June 2017

Origin of the crossover from polarons to Fermi liquids in transition metal oxides

Understanding the nature of charge carriers in doped oxides is key to engineer the conduction properties of these materials. Angle-resolved photoemission spectroscopy experiments on prototypical metal oxides have shown that the carriers undergo a transition from a polaronic to a Fermi liquid regime with increasing doping. In a paper published in the journal Nature Communications, Carla Verdi, Fabio Caruso and Feliciano Giustino investigate this problem by performing first principles many-body calculations of angle-resolved photoemission spectra of doped anatase TiO
2. They show that this transition is driven by nonadiabatic Fröhlich electron-phonon coupling, and occurs when the frequency scale of plasma oscillations is of the same order as that of longitudinal-optical phonons in the material.

Interferogram - experimental versus theoretical

17th May 2017

Direct Measurement of the Surface Energy of Graphene

A recent publication in Nano Letters reports the successful application of the graphene surface force balance (g-SFB) to directly measure the surface energy of pure graphene. Access to accurate surface energy values of graphene is not only of fundamental interest, but provides a useful reference for anyone involved in research on graphene properties, (surface) modifications, and the implementation of graphene in devices.  

This work is the result of a close collaboration between the Nanomaterials by Design Team lead by Professor Grobert in the Department of Materials and the Surface Forces Research Laboratory led by Prof Susan Perkin in the Department of Chemistry over many years and many years of fine tuning large-area graphene synthesis and transfer in conjunction with an in-depth understanding and fundamental development of the surface-force-balance technique.  The work has been made possible largely through ERC funding.  This publication was also selected as a research highlight in Nanowerk.

Electron spin dephasing

2nd May 2017

Hyperfine and Spin-Orbit Coupling Effects on Decay of Spin-Valley States in a Carbon Nanotube

The two spin states of an electron provide a natural qubit that can be used the basis of a semiconductor quantum computer or as a sensitive probe of device physics on the nanoscale. However, quantum superpositions of the two spin states are delicate and easily destroyed by uncontrolled interactions with the environment. In a new paper in Physical Review Letters, Tian Pei and colleagues have identified the mechanisms that degrade the quantum states of electron spins in a carbon nanotube. Using nanotechnology to fabricate traps for electrons along the nanotube and measuring at ultra-low temperatures well below 1 kelvin, they are able to distinguish the effects of hyperfine coupling and spin-orbit interaction on the spin states. In this device, they find that hyperfine coupling is the main source of spin dephasing, but this effect can be eliminated in future experiments by specially preparing nanotubes that are free of magnetic nuclei. This work is in collaboration with Budapest University of Technology and Economics and uses low-temperature measurement facilities in our recently expanded Ferreras Willetts Laboratory.

Hofmann and Tarleton

20th April 2017

3D lattice distortions and defect structures in ion-implanted nano-crystals

Focussed Ion Beam (FIB) milling is a mainstay of nano-scale machining. By manipulating a tightly focussed beam of energetic ions FIB can sculpt nanostructures via localised sputtering. This ability to cut solid matter on the nano-scale revolutionised sample preparation across the life, earth and materials sciences.  A team of researchers, including Felix Hofmann in Engineering Science and Ed Tarleton in Materials, have recently published in Scientific Reports a study of  gold nanocrystals using bragg coherent x-ray diffraction imaging which showed that even  low ion doses associated with FIB imaging were found to dramatically affect the nanoscale structure, introducing lattice distortions extending far beyond the well known ion-implanted surface layer.  Given the role FIB has come to play in science and technology, there is an urgent need to develop new strategies to properly understand the effects of FIB damage and how it might be controlled.

Oxford University News

 

H atoms in V rich precipitates

18th March 2017

Direct Observation of Individual Hydrogen Atoms at Trapping Sites in a Ferritic Steel

Hydrogen embrittlement (HE) is one of the most devastating and unpredictable, yet least understood, mechanisms of failure experienced by engineering components. The presence of hydrogen leads to a severe degradation in mechanical properties and consequently a loss in structural integrity of a vast range of metals and alloys.  Despite considerable research over decades, the precise mechanisms responsible for the embrittling process are still not understood. A key factor limiting our understanding is the difficulty in experimentally observing the distribution of hydrogen within a material, particularly at the atomic scale. Even ultra-high resolution electron microscopy cannot distinguish hydrogen atoms from the surrounding material in engineering materials. The Oxford Atom Probe group, working with colleagues at Sheffield, Zurich and Brisbane, report in Science the use of isotopic doping for the unambiguous 3D characterisation of individual hydrogen atoms within a ferritic steel.  The research demonstrates the first direct atomic-scale observation of the precise manner in which a microstructural feature acts to trap hydrogen, in this case within the core of carbides. The techniques developed are not limited to steels, and may prove significant for other technologically relevant systems, such as nickel-based superalloys and titanium alloys where hydrogen can play an important role in degrading in-service performance. 

Electron Phonon Interactions

22nd February 2017

Electron-phonon interactions from first principles

The electron-phonon interaction plays an important role in many properties of solids, from superconductivity to the electrical and optical properties of metals and semiconductors.

During the last two decades density-functional theory and many-body perturbation theory methods have made it possible to perform accurate and predictive calculations of the electron-phonon interaction. In this article Feliciano Giustino reviews the state of the art in ab initio theories of electron-phonon interactions in solids and their applications to problems of current interest in materials physics.

Rev. Mod. Phys. 89, 015003 (2017)

Environmentally assisted grain boundary embrittlement

3rd February 2017

Environmentally-assisted grain boundary attack as a mechanism of embrittlement in a nickel-based superalloy

The loss of ductility of high strength polycrystalline superalloy in the mid-temperature regime is of huge importance in both aerospace propulsion and land based power generation applications. Recent systematic work by André Németh and his colleges from Oxford Materials in collaboration with Rolls Royce plc have given new mechanistic insight using the unique range of advanced characterisation techniques available in the department.  The physical factors responsible for the ductility dip were established as embrittlement from internal intergranular oxidation along the g-grain boundaries, and in particular, at incoherent interfaces of the primary g’-precipitates with the matrix phase. These fail under local microstresses arising from the accumulation of dislocations during slip-assisted grain boundary sliding. At high temperatures, ductility is restored because the accumulation of dislocations at grain boundaries is no longer prevalent. The results are published open access in Acta Materialia.

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