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  • Journal article
    Flasque M, Van Nhien AN, Moia D, Barnes PRF, Sauvage Fet al., 2016,

    Consequences of solid electrolyte interphase (SEI) formation upon aging on charge-transfer processes in dye-sensitized solar cells

    , Journal of Physical Chemistry C, Vol: 120, Pages: 18991-18998, ISSN: 1932-7455

    Solid electrolyte interphase (SEI) layers form on sensitized-TiO2 photoanodes and platinum counter electrodes when dye-sensitized solar cells (DSSCs) are subjected to an accelerated aging protocol (e.g., heating at 85 °C in the dark for 500 h). To understand how this impacts device operation, we conducted an electrochemical impedance spectroscopy study and found that the SEI induces an additional electron-transfer process from the TiO2 to the electrolyte. This is materialized by the onset of a new charge-transfer semicircle at higher frequencies, predominantly visible under bias voltages similar to and greater than the open-circuit voltage. Our results emphasize the detrimental role of SEI formation on device performance and lifetime. Additionally, nanosecond transient absorption spectroscopy showed that SEI formation reduced the rate of oxidized dye regeneration. We also found that a proportion of the photogenerated holes on the dyes were transferred to the SEI itself. A prolonged aging duration led to the electrode’s mesoporosity network being entirely clogged by the SEI, thus impeding efficient transport of the electrolyte redox couple and being responsible for a further decline in photovoltaic performances.

  • Journal article
    Zeissler K, Chadha M, Lovell E, Cohen LF, Branford WRet al., 2016,

    Low temperature and high field regimes of connected kagome artificial spin ice: the role of domain wall topology

    , Scientific Reports, Vol: 6, ISSN: 2045-2322

    Artificial spin ices are frustrated magnetic nanostructures where single domain nanobars act as macrosized spins. In connected kagome artificial spin ice arrays, reversal occurs along one-dimensional chains by propagation of ferromagnetic domain walls through Y-shaped vertices. Both the vertices and the walls are complex chiral objects with well-defined topological edge-charges. At room temperature, it is established that the topological edge-charges determine the exact switching reversal path taken. However, magnetic reversal at low temperatures has received much less attention and how these chiral objects interact at reduced temperature is unknown. In this study we use magnetic force microscopy to image the magnetic reversal process at low temperatures revealing the formation of quite remarkable high energy remanence states and a change in the dynamics of the reversal process. The implication is the breakdown of the artificial spin ice regime in these connected structures at low temperatures.

  • Journal article
    Caixeiro S, Gaio M, Marelli B, Omenetto FG, Sapienza Ret al., 2016,

    Silk-Based Biocompatible Random Lasing

    , Advanced Optical Materials, Vol: 4, Pages: 998-1003, ISSN: 2195-1071

    Biocompatible silk random lasing is obtained by nanostructuring silk proteins into a disordered porous matrix via a self-assembly technique. Lasing action is revealed by spectral narrowing and threshold behavior, and it is sensitive to pH variations in the aqueous environment. Silk random lasing provides a versatile biocompatible system, opening up opportunities for biophotonic applications and biosensing.

  • Journal article
    Gennaro SD, Rahmani M, Giannini V, Aouani H, Sidiropoulos TP, Navarro-Cía M, Maier SA, Oulton RFet al., 2016,

    The Interplay of Symmetry and Scattering Phase in Second Harmonic Generation from Gold Nanoantennas

    , Nano Letters, Vol: 16, Pages: 5278-5285, ISSN: 1530-6992

    Nonlinear phenomena are central to modern photonics but, being inherently weak, typically require gradual accumulation over several millimeters. For example, second harmonic generation (SHG) is typically achieved in thick transparent nonlinear crystals by phase-matching energy exchange between light at initial, ω, and final, 2ω, frequencies. Recently, metamaterials imbued with artificial nonlinearity from their constituent nanoantennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. However, the selection rules of SHG typically prevent dipole emission from simple nanoantennas, which has led to much discussion concerning the best geometries, for example, those breaking centro-symmetry or incorporating resonances at multiple harmonics. In this work, we explore the use of both nanoantenna symmetry and multiple harmonics to control the strength, polarization and radiation pattern of SHG from a variety of antenna configurations incorporating simple resonant elements tuned to light at both ω and 2ω. We use a microscopic description of the scattering strength and phases of these constituent particles, determined by their relative positions, to accurately predict the SHG radiation observed in our experiments. We find that the 2ω particles radiate dipolar SHG by near-field coupling to the ω particle, which radiates SHG as a quadrupole. Consequently, strong linearly polarized dipolar SHG is only possible for noncentro-symmetric antennas that also minimize interference between their dipolar and quadrupolar responses. Metamaterials with such intra-antenna phase and polarization control could enable compact nonlinear photonic nanotechnologies.

  • Journal article
    Gaio M, Moffa M, Castro-Lopez M, Pisignano D, Camposeo A, Sapienza Ret al., 2016,

    Modal Coupling of Single Photon Emitters Within Nanofiber Waveguides

    , ACS Nano, Vol: 10, Pages: 6125-6130, ISSN: 1936-0851

    Nanoscale generation of individual photons in confined geometries is an exciting research field aiming at exploiting localized electromagnetic fields for light manipulation. One of the outstanding challenges of photonic systems combining emitters with nanostructured media is the selective channelling of photons emitted by embedded sources into specific optical modes and their transport at distant locations in integrated systems. Here, we show that soft-matter nanofibers, electrospun with embedded emitters, combine subwavelength field localization and large broadband near-field coupling with low propagation losses. By momentum spectroscopy, we quantify the modal coupling efficiency identifying the regime of single-mode coupling. These nanofibers do not rely on resonant interactions, making them ideal for room-temperature operation, and offer a scalable platform for future quantum information technology.

  • Journal article
    Grinblat G, Li Y, Nielsen MP, Oulton R, Maier SAet al., 2016,

    Enhanced third harmonic generation in single Germanium nanodisks excited at the anapole mode

    , Nano Letters, Vol: 16, Pages: 4635-4640, ISSN: 1530-6992

    We present an all-dielectric germanium nanosystem exhibiting a strong third ordernonlinear response and efficient third harmonic generation in the optical regime. A thin germaniumnanodisk shows a pronounced valley in its scattering cross section close to the dark anapole mode,while the electric field energy inside the disk is maximized due to high confinement within thedielectric. We investigate the dependence of the third harmonic signal on disk size and pumpwavelength to reveal the nature of the anapole mode. Each germanium nanodisk generates a higheffective third order susceptibility of (3) = 4.3 10−9 , corresponding to an associated thirdharmonic conversion efficiency of 0.0001% at a wavelength of 1650 nm, which is four orders ofmagnitude greater than the case of an unstructured germanium reference film. Furthermore, thenonlinear conversion via the anapole mode outperforms that via the radiative dipolar resonancesby about one order of magnitude, which is consistent with our numerical simulations. Thesefindings open new possibilities for the optimization of upconversion processes on the nanoscalethrough the appropriate engineering of suitable dielectric materials.

  • Journal article
    Grinblat G, Li Y, Nielsen MP, Oulton RF, Maier SAet al., 2016,

    Enhanced Third Harmonic Generation in Single Germanium Nanodisks Excited at the Anapole Mode

    , Nano Letters, Vol: 16, Pages: 4635-4640, ISSN: 1530-6992

    We present an all-dielectric germanium nanosystem exhibiting a strong third order nonlinear response and efficient third harmonic generation in the optical regime. A thin germanium nanodisk shows a pronounced valley in its scattering cross section at the dark anapole mode, while the electric field energy inside the disk is maximized due to high confinement within the dielectric. We investigate the dependence of the third harmonic signal on disk size and pump wavelength to reveal the nature of the anapole mode. Each germanium nanodisk generates a high effective third order susceptibility of χ(3) = 4.3 × 10–9 esu, corresponding to an associated third harmonic conversion efficiency of 0.0001% at an excitation wavelength of 1650 nm, which is 4 orders of magnitude greater than the case of an unstructured germanium reference film. Furthermore, the nonlinear conversion via the anapole mode outperforms that via the radiative dipolar resonances by about 1 order of magnitude, which is consistent with our numerical simulations. These findings open new possibilities for the optimization of upconversion processes on the nanoscale through the appropriate engineering of suitable dielectric materials.

  • Journal article
    Leguy AM, Goñi AR, Frost JM, Skelton J, Brivio F, Rodríguez-Martínez X, Weber OJ, Pallipurath A, Alonso MI, Campoy-Quiles M, Weller MT, Nelson J, Walsh A, Barnes PRet al., 2016,

    Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites

    , Physical Chemistry Chemical Physics, Vol: 18, Pages: 27051-27066, ISSN: 1463-9084

    We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm(-1). Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

  • Journal article
    Huidobro PA, Kraft M, Maier SA, Pendry JB, Maier SA, Arroyo Huidobro P, Kraft M, Pendry JBet al., 2016,

    Graphene as a Tunable Anisotropic or IsotropicPlasmonic Metasurface

    , ACS Nano, Vol: 10, Pages: 5499-5506, ISSN: 1936-086X

    We demonstrate a tunable plasmonic metasurface by considering a graphene sheetsubject to a periodically patterned doping level. The unique optical properties ofgraphene result in electrically tunable plasmons that allow for extreme confinementof electromagnetic energy in the technologically significant regime of THz frequencies.Here we add an extra degree of freedom by using graphene as a metasurface, proposingto dope it with an electrical gate patterned in the micron or sub-micron scale. Byextracting the effective conductivity of the sheet we characterize metasurfaces periodicallymodulated along one or two directions. In the first case, and making use of theanalytical insight provided by transformation optics, we show an efficient control ofTHz radiation for one polarization. In the second case, we demonstrate a metasurfacewith an isotropic response that is independent of wave polarization and orientation.

  • Journal article
    Alonso Alvarez D, Ekins-Daukes N, 2016,

    Photoluminescence-Based Current-Voltage Characterisation Of Individual Subcells In Multi-Junction Devices

    , IEEE Journal of Photovoltaics, Vol: 6, Pages: 1004-1011, ISSN: 2156-3381

    We demonstrate a photoluminescence based,contactless method to determine the current-voltagecharacteristics of the individual subcells in a multi-junctionsolar cell. The method relies upon the reciprocity relationbetween the absorption and emission properties on a solarcell. Laser light with a suitable energy is used to excitecarriers selectively in one junction and the internal voltagesare deduced from the intensity of the resultingluminescence. The IV curves obtained this way on 1J, 2Jand 6J devices are compared to those obtained usingelectroluminescence. Good agreement is obtained at highinjection conditions while discrepancies at low injection areattributed to in-plane carrier transport.

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