Research papers of the month
Vertically aligned zinc oxide nanowires electrodeposited within porous polycarbonate templates for vibrational energy harvesting
In this Letter, we report a piezoelectric nanogenerator that was fabricated using a simple, fast and scalable template-assisted electrodeposition process, by which vertically aligned zinc oxide (ZnO) nanowires were directly grown within a nanoporous polycarbonate (PC) template. The nanowires, having average diameter 184 nm and length 12 μm, are polycrystalline and have a preferred orientation of the  axis parallel to the long axis. The output power density of a nanogenerator fabricated from the as-grown ZnO nanowires still embedded within the PC template was found to be 151 ± 25 mW m−3 at an impedance-matched load, when subjected to a low-level periodic (5 Hz) impacting force akin to gentle finger tapping. An energy conversion efficiency of ~4.2% was evaluated for the electrodeposited ZnO nanowires, and the ZnO–PC composite nanogenerator was found to maintain good energy harvesting performance through 24 h of continuous fatigue testing. This is particularly significant given that ZnO-based nanostructures typically suffer from mechanical and/or environmental degradation that otherwise limits their applicability in vibrational energy harvesting. Our template-assisted synthesis of ZnO nanowires embedded within a protective polymer matrix through a single growth process is thus attractive for the fabrication of low-cost, robust and stable nanogenerators.
Figure: Schematic of the fabrication procedure for vertically aligned zinc oxide (ZnO) NWs grown within polycarbonate (PC) templates via electrodeposition, and the electrical output of the ZnO-PC nanocomposite nanogenerator when subjected to low-amplitude and low-frequency vibrations.
Francesca L Boughey, Timothy Davies, Anuja Datta, Richard A Whiter, Suman-Lata Sahonta and Sohini Kar-Narayan, 2016, “Vertically aligned zinc oxide nanowires electrodeposited within porous polycarbonate templates for vibrational energy harvesting”, Nanotechnology (Letters) 27 28LT02
The biaxial moduli of cubic materials subjected to an equi-biaxial elastic strain
When dealing with stress in thin film / substrate combinations, it is usual to assume that the film is elastically isotropic and that the substrate on which it lies is also elastically isotropic. The classic Stoney formula can then be used to determine the magnitude of the stress in the thin film. Silicon wafers are widely used in single crystal form as substrates. These can be purchased in a variety of substrate (hkl) surface orientations. Silicon is noticeably anisotropic elastically. Therefore, the stiffness of such substrates is orientation-dependent within the plane of the substrate, unless the wafer has a (001) or (111) surface orientation.
In this work, formulae for the biaxial elastic moduli along the directions of principal stress for general (hkl) interfaces of cubic materials are derived for situations in which there is equi-biaxial strain within the plane, such as when isotropic thin films are deposited on (hkl) silicon single crystal substrates. Within a particular (hkl), the directions defining these principal biaxial moduli are those along which there are the extreme values of both the shear modulus and Poisson’s ratio. Conditions for stationary values of the biaxial moduli have also been derived, from which the conditions for the global extrema of the biaxial moduli in substrates of cubic materials have been established.
Future work will consider the effect of anisotropy for other readily available single crystal substrates of arbitrary surface orientation, such as alumina (trigonal), rutile (tetragonal) and zinc oxide (hexagonal).
Figure: Equi-biaxial elastic strains within isotropic thin film / single crystal substrate combinations introduced during deposition and/or by a change in temperature cause curvature associated with the two principal biaxial elastic moduli in the plane of the substrate. In general, these biaxial elastic moduli are different, so that the curvature induced in a thin film / single crystal substrate combination is not radially symmetric.
Kevin M. Knowles, "The Biaxial Moduli of Cubic Materials Subjected to an Equi-biaxial Elastic Strain", J Elast. (2016) 124:1–25
Xenon oxides Xe2O5 and Xe3O2
Under normal conditions the noble gases, helium, neon and the heavier argon, krypton, xenon and radon, are unreactive. One of the enduring geochemical mysteries is the apparent lack of xenon in the Earth's crust and atmosphere. It has long been speculated that xenon might be locked up in compounds under extreme compression within the Earth - but little is known about xenon compounds, or even whether they can exist.
Combining theoretical structure prediction techniques, with diamond anvil
cell high pressure experiments, two xenon oxides have been synthesised and
characterised below 1 Mbar (100 GPa).  The xenon adopts mixed oxidation
states and forms extended networks that incorporate oxygen-sharing XeO4 squares. Xe2O5 additionally incorporates oxygen-sharing XeO5 pyramids.
In combination with previous theoretical work , xenon's rich chemistry
under extreme conditions is being revealed.
Figure: The crystal structure of the newly discovered xenon oxides, Xe2O5 (top) and Xe3O2 (bottom). The oxidation states of xenon are labelled.
 Agnes Dewaele, Nicholas Worth, Chris J. Pickard, Richard J. Needs, Sakura Pascarelli, Olivier Mathon, Mohamed Mezouar & Tetsuo Irifune, "Synthesis and stability of xenon oxides Xe2O5 and Xe3O2 under pressure", Nature Chemistry (2016)
 Li Zhu, Hanyu Liu, Chris J. Pickard, Guangtian Zou & Yanming Ma, "Reactions of xenon with iron and nickel are predicted in the Earth's inner core ", Nature Chemistry, 6, 644-648 (2014)