Research papers of the month
Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system
An international collaboration, including the University of Cambridge, has shown that quantum mechanical effects are central to the high temperature (up to 203K, -70 degrees Celsius) superconductivity that has been measured in hydrogen sulphide under extreme compression.
Earlier results of the collaboration addressed the superconducting mechanism and the structure and decomposition of hydrogen sulphide under pressure. In the current article it is shown that the crystal structure that exhibits the highest temperature superconductivity is adopted a lower pressures than would be expected in a purely classical picture.
 "High-pressure hydrogen sulfide from first principles: a strongly anharmonic phonon-mediated superconductor", I Errea, M Calandra, CJ Pickard, J Nelson, RJ Needs, Y Li, H Liu, Y Zhang, , Yanming Ma, Francesco Mauri Physical review letters 114 (15), 157004, 2015
 "Dissociation products and structures of solid H2S at strong compression", Y Li, L Wang, H Liu, Y Zhang, J Hao, CJ Pickard, JR Nelson, RJ Needs, Wentao Li, Yanwei Huang, Ion Errea, Matteo Calandra, Francesco Mauri, Yanming Ma Physical Review B 93 (2), 020103, 2016
University of Cambridge news article: http://www.cam.ac.uk/research/news/quantum-effects-at-work-in-the-worlds-smelliest-superconductor
Figure: The crystal structure of H3S - the highest temperature superconductor currently known.
Ion Errea, Matteo Calandra, Chris J. Pickard, Joseph R. Nelson, Richard J. Needs, Yinwei Li, Hanyu Liu, Yunwei Zhang, Yanming Ma and Francesco Mauri, "Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system", Nature 532, 81-84 (07 April 2016)
Single-Layered Hittorf’s Phosphorus: A Wide-Bandgap High Mobility 2D Material
The field of two-dimensional materials has seen enormous growth since the discovery of graphene, largely driven by the promise of exotic electronic properties that can be exploited in novel applications. Unfortunately many of the two-dimensional materials studied thus far exhibit either large band gaps or high mobilities but not both.
Scientists at Cambridge and EPFL have studied the properties of a single layer of violet, or Hittorf’s, phosphorus using first-principles density functional theory (DFT).
The single-layered form of Hittorf’s phosphorus is found to exhibit quite exceptional electronic properties. This two-dimensional material is predicted to have a large, direct band gap of around 2.5 eV and to have a very high hole mobility with an upper bound lying between 3000-7000 cm2 V-1 s-1. Being a direct semiconductor makes it very attractive for optical applications. In addition, the binding energy per layer is found to be very small (similar to graphite), suggesting that exfoliation may be experimentally possible.
The rare combination of properties in single-layered Hittorf’s phosphorus render it an exceptional candidate for use in future applications spanning a wide variety of technologies, in particular for high-frequency electronics and optoelectronic devices operating in the low-wavelength blue colour range.
Figure: The structure of single-layered Hittorf’s phosphorus and the band gap for different thicknesses of Hittorf’s phosphorus, going from bulk to its single-layered form
Georg Schusteritsch, Martin Uhrin and Chris J. Pickard, "Single-Layered Hittorf’s Phosphorus: A Wide-Bandgap High Mobility 2D Material", Nano Lett. 16 (5), (2016) pp 2975–2980
Template-Assisted Hydrothermal Growth of Aligned Zinc Oxide Nanowires for Piezoelectric Energy Harvesting Applications
A flexible and robust piezoelectric nanogenerator (NG) based on a polymer-ceramic nanocomposite structure has been successfully fabricated via a cost-effective and scalable template-assisted hydrothermal synthesis method. Vertically aligned arrays of dense and uniform zinc oxide (ZnO) nanowires (NWs) with high aspect ratio (diameter ~250 nm, length ~12 μm) were grown within nanoporous polycarbonate (PC) templates. The energy conversion efficiency was found to be ~4.2%, which is comparable to previously reported values for ZnO NWs. The resulting NG is found to have excellent fatigue performance, being relatively immune to detrimental environmental factors and mechanical failure, as the constituent ZnO NWs remain embedded and protected inside the polymer matrix.
Figure: Schematic of the fabrication procedure for vertically aligned zinc oxide (ZnO) NWs grown within polycarbonate (PC) templates via hydrothermal synthesis, and the design and implementation of a flexible nanogenerator based on the ZnO-PC nanocomposite.
Canlin Ou, Pedro E. Sanchez-Jimenez, Anuja Datta, Francesca L. Boughey, Richard A. Whiter, Suman-Lata Sahonta, Sohini Kar-Narayan, "Template-Assisted Hydrothermal Growth of Aligned Zinc Oxide Nanowires for Piezoelectric Energy Harvesting Applications", ACS Appl. Mater. Interfaces, Article ASAP (2016)