Department of Materials Science & Metallurgy: Research papers of the month

Department of Materials Science & Metallurgy

Research papers of the month

February 2017

Melt-Quenched Hybrid Glasses

The recent synthesis of several melt-quenched glasses from hybrid porous frameworks and coordination polymers has added a new category of glass to the existing inorganic non-metallic (e.g. oxide glasses, chalcogenide glasses), organic (polymer), and metallic divisions. Specifically, the melting of metal-organic frameworks (MOFs) results in a vitrified molten state consisting of linked inorganic and organic moieties. Courtesy of interchangeable organic and inorganic components, one can now envisage an array of ‘designed’ glass materials, with desired optical, electrical and mechanical properties for future applications, as outlined in this research update.

Figure: The melting of a porous framework of composition [Zn(C3H3N2)2] which lies close to its temperature of decomposition.

H. Tao, T. D. Bennett and Y. Yue, "Melt-Quenched Hybrid Glasses from Metal-organic Frameworks,", Advanced Materials (2017) 1601705.


Microstructural characterisation of metallic shot peened and laser shock peened Ti‒6Al‒4V

Each time an aircraft makes a flight, the moving parts in an aerospace gas turbine engine are subjected to both high cycle vibrational stresses during the flight and low cycle thermal and mechanical stresses, with each flight being a single cycle for the low cycle stresses. As a consequence, these moving parts suffer from a life-limiting condition called fatigue. To prevent failure from fatigue, metallic shot peening and laser shock peening are used to introduce significant compressive surface stresses in the most highly stressed regions of Ti–6Al–4V fan blades. These compressive residual stresses improve the resistance to fatigue crack growth of these blades in service.

Here, a detailed analysis has been conducted of the microstructure of Ti–6Al–4V processed by metallic shot peening and laser shock peening. A notable feature of the material processed by laser shock peening is the almost complete absence of deformation twinning, contrasting with the frequent observation of extensive deformation twinning observed in the material processed by metallic shot peening. Furthermore, very different dislocation structures are produced in surfaces processed by these two techniques - laser shock peening produces more directional planar dislocations and networks of dislocation cells and sub-grains, whereas metallic shot peening produces long wavy tangled dislocation structures and shear bands. These observations have been rationalised as a function of the different strain rates of the two processes and the different physical processes involved.

Figure: Graphic illustrating metallic shot peening (MSP) and laser shock peening (LSP) together with examples of the microstructures produced by the two techniques in Ti–6Al–4V.

S. J. Lainé, K. M. Knowles, P. J. Doorbar, R. D. Cutts and D. Rugg, Acta Materialia 123 (2017) 350‒361


Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires

The ability to generate on-demand single-photons is of vital importance to quantum information technology such as quantum cryptography, linear optical quantum computing and quantum metrology. Due to their high stability, good repetition rates, and practicable incorporation into cavities and electrically pumped structures, quantum dots (QDs) are ideal candidates for the generation of, and interaction with, single photons. Notably nitride QDs offer the advantages of large exciton binding energies and large band offsets which allow higher temperature operation. InGaN QDs, in particular, have some attractive properties for quantum information systems, such as access to the blue spectral range which is both ideally suited to sattelite-based communications and well-matched to efficient and practical fast single-photon detectors.  However, QDs grown on the polar c-plane experience large internal electric fields leading to slow single-photon emission rates and unpolarized emission which limit their applicability to quantum information systems.

Here, we demonstrate single-photon emission from InGaN QDs grown on an alternative, non-polar crystal plane.  The QDs are embedded on the m-plane side-walls of GaN nanowires. The QD exhibits single photon emission up to 100 K. Studies on a statistically significant number of QDs show that these m-plane QDs exhibit very short radiative lifetimes (~260 ps) which would allow high repetition rates in a quantum information system. Moreover, the observed single photons are almost completely linearly polarized perpendicular to the crystallographic c-axis. Such InGaN QDs incorporated in a nanowire system meet many of the requirements for implementation into quantum information systems and could potentially open the door to wholly new device concepts.

Figure. Schematic illustration of self-assembled InGaN QDs-in-nanowire grown by metal-organic vapour phase epitaxy. These QDs exhibit many advantageous properties, such as significantly anti-bunched emission up to 100 K, a strong degree of linear polarisation, and very fast radiative lifetimes.

T. J. Puchtler, T. Wang, C. X. Ren, F. Tang, R. A. Oliver, R. A. Taylor, and T. Zhu, “Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires”, Nano Letters 2016, 16, pp. 7779–7785