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  • Conference paper
    Mignuzzi S, Cambiasso J, Vezzoli S, Horsley SAR, Barnes WL, Maier SA, Sapienza Ret al., 2019,

    Dielectric nanocavities with enhanced local density of states

    © 2019 The Author(s) 2019 OSA. We present inverse-designed lossless dielectric nanocavities with enhanced local density of optical states. Photon counting statistics from fluorescent molecules allows determining strong field confinement and single-molecule detection at micromolar concentration in liquid.

  • Journal article
    Mignuzzi S, Vezzoli S, Horsley SAR, Barnes WL, Maier SA, Sapienza Ret al., 2019,

    Nanoscale design of the local density of optical states

    , Nano Letters, Vol: 19, Pages: 1613-1617, ISSN: 1530-6984

    We propose a design concept for tailoring the local density of optical states (LDOS) in dielectric nanostructures, based on the phase distribution of the scattered optical fields induced by point-like emitters. First we demonstrate that the LDOS can be expressed in terms of a coherent summation of constructive and destructive contributions. By using an iterative approach, dielectric nanostructures can be designed to effectively remove the destructive terms. In this way, dielectric Mie resonators, featuring low LDOS for electric dipoles, can be reshaped to enable enhancements of 3 orders of magnitude. To demonstrate the generality of the method, we also design nanocavities that enhance the radiated power of a circular dipole, a quadrupole, and an arbitrary collection of coherent dipoles. Our concept provides a powerful tool for high-performance dielectric resonators and affords fundamental insights into lightmatter coupling at the nanoscale.

  • Journal article
    Jacucci G, Onelli OD, De Luca A, Bertolotti J, Sapienza R, Vignolini Set al., 2019,

    Coherent backscattering of light by an anisotropic biological network.

    , Interface Focus, Vol: 9, Pages: 20180050-20180050, ISSN: 2042-8898

    The scattering strength of a random medium relies on the geometry and spatial distribution of its components as well as on their refractive index. Anisotropy can, therefore, play a major role in the optimization of the scattering efficiency in both biological and synthetic materials. In this study, we show that, by exploiting the coherent backscattering phenomenon, it is possible to characterize the optical anisotropy in Cyphochilus beetle scales without the need to change their orientation or their thickness. For this reason, such a static and easily accessible experimental approach is particularly suitable for the study of biological specimens. Moreover, estimation of the anisotropy in Cyphochilus beetle scales might provide inspiration for improving the scattering strength of artificial white materials.

  • Journal article
    Newman MJ, Speller EM, Barbe J, Luke J, Li M, Li Z, Wang Z-K, Jain SM, Kim J-S, Lee HKH, Tsoi WCet al., 2018,

    Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation

    , Science and Technology of Advanced Materials, Vol: 19, Pages: 194-202, ISSN: 1468-6996

    Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C71 butyric acid methyl ester (BTR:PC71BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV–vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation – rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.

  • Journal article
    Keivanidis PE, Khan JI, Katzenmeier L, Kan Z, Limbu S, Constantinou MK, Lariou E, Constantinides G, Hayes SC, Kim J-S, Laquai Fet al., 2018,

    Impact of structural polymorphs on charge collection and non-geminate recombination in organic photovoltaic devices

    , The Journal of Physical Chemistry C, Vol: 122, Pages: 29141-29149, ISSN: 1932-7447

    The formation of different types of structural polymorphs of poly(3-hexyl-thiophene) (P3HT) affects the performance of organic photovoltaic (OPV) devices that use thermally-annealed P3HT:PCBM[60] blend films as photoactive layer. Here it is demonstrated that, when densely-packed and non-densely packed P3HT polymorphs co-exist in the P3HT:PCBM[60] layer, non-geminate charge recombination is fast; however, in a device non-geminate recombination is effectively overruled by efficient and fast charge carrier extraction. In stark contrast, when only a less-densely packed P3HT polymorph is present in the blend, non-geminate charge recombination losses are less pronounced, and the charge carrier extraction efficiency is lower. The antagonistic non-geminate charge recombination and charge carrier extraction processes in these systems are monitored by time-delayed-collection field (TDCF) and ultrafast transient absorption (TA) experiments. Furthermore, resonance Raman spectroscopy reveals that in the absence of the densely-packed P3HT polymorph, the energetic disorder present in the P3HT:PCBM[60] blend is higher. High-resolution atomic force microscopy imaging further identifies pronounced differences in the layer morphology when the polymorph distribution varies between unimodal and bimodal. These results indicate that less-densely packed P3HT polymorphs increase disorder and impede charge collection, leading to a reduction of the device fill factor.

  • Journal article
    Lee S, Kim DB, Hamilton I, Daboczi M, Nam YS, Lee BR, Zhao B, Jang CH, Friend RH, Kim J-S, Song MHet al., 2018,

    Control of interface defects for efficient and stable quasi-2D Perovskite light-emitting diodes using nickel oxide hole injection layer

    , Advanced Science, Vol: 5, ISSN: 2198-3844

    Metal halide perovskites (MHPs) have emerged as promising materials for light‐emitting diodes owing to their narrow emission spectrum and wide range of color tunability. However, the low exciton binding energy in MHPs leads to a competition between the trap‐mediated nonradiative recombination and the bimolecular radiative recombination. Here, efficient and stable green emissive perovskite light‐emitting diodes (PeLEDs) with an external quantum efficiency of 14.6% are demonstrated through compositional, dimensional, and interfacial modulations of MHPs. The interfacial energetics and optoelectronic properties of the perovskite layer grown on a nickel oxide (NiOx) and poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate hole injection interfaces are investigated. The better interface formed between the NiOx/perovskite layers in terms of lower density of traps/defects, as well as more balanced charge carriers in the perovskite layer leading to high recombination yield of carriers are the main reasons for significantly improved device efficiency, photostability of perovskite, and operational stability of PeLEDs.

  • Journal article
    Kim H, Lee G, Becker S, Kim J, Kim H, Hwang Bet al., 2018,

    Novel patterning of flexible and transparent Ag nanowire electrodes using oxygen plasma treatment

    , Journal of Materials Chemistry C, Vol: 6, Pages: 9394-9398, ISSN: 2050-7526

    We report a novel patterning method using oxygen plasma treatment for flexible and transparent Ag nanowire electrodes. Using a dry film photoresist as a solid-state film-type photoresist, Ag nanowires were selectively oxidized under oxygen plasma treatment. Microstructural analysis revealed that the Ag nanowires were fully oxidized after 30 s of oxygen plasma treatment, which was also reflected in the changes in the optoelectronic properties of the Ag nanowires. The fully oxidized Ag nanowires could be completely dissolved in NH3 solution (aq.), without using a toxic etchant to form sharp patterns of Ag nanowire electrodes. To further confirm the applicability of the patterning technique demonstrated here in electronic devices, MoS2 thin-film transistors (TFTs) with patterned Ag-nanowire source/drain (S/D) electrodes were fabricated and they showed similar performances to typical MoS2 TFTs with thin-film-type Ti/Au S/D electrodes.

  • Journal article
    Das PK, Slawinska J, Vobornik I, Fujii J, Regoutz A, Kahk JM, Scanlon DO, Morgan BJ, McGuinness C, Plekhanov E, Di Sante D, Huang Y-S, Chen R-S, Rossi G, Picozzi S, Branford WR, Panaccione G, Payne DJet al., 2018,

    Role of spin-orbit coupling in the electronic structure of IrO2

    , Physical Review Materials, Vol: 2, ISSN: 2475-9953

    The delicate interplay of electronic charge, spin, and orbital degrees of freedom is in the heart of many novel phenomena across the transition metal oxide family. Here, by combining high-resolution angle-resolved photoemission spectroscopy and first principles calculations (with and without spin-orbit coupling), the electronic structure of the rutile binary iridate, IrO2, is investigated. The detailed study of electronic bands measured on a high-quality single crystalline sample and use of a wide range of photon energy provide a huge improvement over the previous studies. The excellent agreement between theory and experimental results shows that the single-particle DFT description of IrO2 band structure is adequate, without the need of invoking any treatment of correlation effects. Although many observed features point to a 3D nature of the electronic structure, clear surface effects are revealed. The discussion of the orbital character of the relevant bands crossing the Fermi level sheds light on spin-orbit-coupling-driven phenomena in this material, unveiling a spin-orbit-induced avoided crossing, a property likely to play a key role in its large spin Hall effect.

  • Journal article
    Morozov S, Gaio M, Maier S, Sapienza Ret al., 2018,

    Metal−dielectric parabolic antenna for directing single photons

    , Nano Letters, Vol: 18, Pages: 3060-3065, ISSN: 1530-6984

    Quantum emitters radiate light omni-directionally, making it hard to collect and use the generated photons. Here, we propose a three-dimensional metal–dielectric parabolic antenna surrounding an individual quantum dot as a source of collimated single photons, which can then be easily extracted and manipulated. Our fabrication method relies on a single optically induced polymerization step once the selected emitter has been localized by confocal microscopy. Compared to conventional nanoantennas, our geometry does not require near-field coupling, and it is, therefore, very robust against misalignment issues and minimally affected by absorption in the metal. The parabolic antenna provides one of the largest reported experimental directivities (D = 106) and the lowest beam divergences (Θ1/2 = 13.5°) and a broadband operation over all of the visible and near-infrared range together with extraction efficiency of more than 96%, offering a practical advantage for quantum technological applications.

  • Journal article
    Hamilton I, Chander N, Cheetham NJ, Suh M, Dyson M, Wang X-H, Stavrinou PN, Cass M, Bradley DDC, Kim J-Set al., 2018,

    Controlling molecular conformation for highly efficient and stable deep-blue copolymer light-emitting diodes

    , ACS Applied Materials and Interfaces, Vol: 10, Pages: 11070-11082, ISSN: 1944-8244

    We report a novel approach to the achievement of deep-blue, high-efficiency, and long-lived solution processed polymer light-emitting diodes (PLEDs) via a simple molecular-level conformation change whereby we introduce rigid β-phase segments into a 95% fluorene - 5% arylamine copolymer emission layer (EML). The arylamine moieties at low density act as efficient exciton formation sites in PLEDs whilst the conformational change alters the nature of the dominant luminescence from a broad, charge-transfer like emission to a significantly blue-shifted and highly vibronically structured, excitonic emission. As a consequence, we observe a significant improvement in Commission International de L'Eclairage (CIE) (x, y) co-ordinates from (0.149, 0.175) to (0.145, 0.123) whilst maintaining high efficiency and improving stability. We achieve peak luminous efficiency, η = 3.60 cd/A and luminous power efficiency, ηw = 2.44 lm/W; values that represent state of the art performance for single copolymer deep-blue PLEDs. These values are five-fold better than for otherwise-equivalent, β-phase poly(9,9-dioctylfluorene) (PFO) EML PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a vector to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.

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