Browsing by Author "Bertoni, Cristina"

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    Cathodic and Anodic Material Diffusion in Polymer/Semiconductor-Nanocrystal Composite Devices
    (John Wiley & Sons, Ltd, 2007-01-01T00:00:00Z) Gallardo, Diego E.; Bertoni, Cristina; Dunn, Steve; Gaponik, Nikolai; Eychmüller, Alexander
    In the present day, the information technologies and telecommunications sector continually increase their demand for low cost, low power consumption, high performance electroluminescent devices for display applications. Furthermore, general lighting applications, such as white light and large array colour displays, would also benefit from an increase in the overall efficiency. Several technologies are being investigated to fulfill these needs, such as organic light emitting diodes (OLED), polymeric light emitting diodes (PLED) and field effect emission devices. A new and promising technology is light emitting devices (LEDs) based on nanostructured materials. With organic LEDs (OLEDs) already making an impact on the market in an increasingly large number of applications, hybrid technologies based on organic/inorganic nano-composites are a potential the next step. The incorporation of highefficiency fluorescent semiconductor nanoparticles has been shown to have a beneficial effect on device performance, [1] modify the colour output from the device 2 and provide a simplified route to generation of LED type devices. [3]
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    Electroforming processes for platinum nanoisland thin films
    (Elsevier Science B.V., Amsterdam., 2006-01-20T00:00:00Z) Bertoni, Cristina; Gallardo, Diego E.; Dunn, Steve
    An investigation of the electroforming processes for platinum discontinuous thin films is detailed. Current–voltage characteristics, for metal nanoislands deposited by sputtering, were obtained in vacuum and air and typically showed voltage-controlled negative resistance (VCNR) behaviour. The current maximum shifted with the electrode separation. Electroforming under high current density regimes was non-regenerative as samples showed irreversible resistance changes. SEM examination of the film revealed a change in the metal microstructure. Such modifications arise as a result of the current flowing through the film inducing electro and thermal migration. Current-induced effects were studied by modelling the metal nanoisland (MN) layer as an array of cubic cells. Plots of current distribution showed that hot-spots develop along conductive paths. Electromigration combined with resistive heating can lead to progressive destruction of current channels at these hot-spots. Hence, current profiles and SEM micrographs were interpreted as evidence of a ‘macroscopic’ electrical breakdown of sample conduction due to microstructural modifications of the thin film. The reduction of ohmic component and consequent resistive heating along the current channels prevented the metal migration and stable current profiles were obt
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    Fabrication and characterization of red-emitting electroluminescent devices based on thiol-stabilized semiconductor nanocrystals
    (American Institute of Physics, 2007-01-15T00:00:00Z) Bertoni, Cristina; Gallardo, Diego E.; Dunn, Steve; Gaponik, Nikolai; Eychmüller, Alexander
    Thiol-capped CdTe nanocrystals were used to fabricate light-emitting diodes, consisting of an emissive nanocrystal multilayer deposited via layer-by-layer, sandwiched between indium-tin-oxide and aluminum electrodes. The emissive and electrical properties of devices with different numbers of nanocrystal layers were studied. The improved structural homogeneity of the nanocrystal multilayer allowed for stable and repeatable current- and electroluminescence-voltage characteristics. These indicate that both current and electroluminescence are electric-field dependent. Devices were operated under ambient conditions and a clear red-light was detected. The best-performing device shows a peak external efficiency of 0.51% and was measured at 0.35mA/cm2 and 3.3V.
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    Light emitting devices based on nanostructured semiconductors
    (2005-07-01T00:00:00Z) Gallardo, Diego E.; Bertoni, Cristina; Dunn, Steve
    Light emitting devices based on high-efficiency photoluminescence (PL) fluorescent nanocrystals have been investigated in terms of the generation of light from the structure using a variety of deposition methods. An automated modified layer-by-layer (LbL) self-assembly technique has been employed to produce multilayers of thiol-capped red fluorescing CdTe nanocrystals. Indium- tin-oxide (ITO) and aluminium electrodes were used as the electrodes. Morphological characterization was carried out through Schottky field effect (SFEG) SEM and atomic force microscopy (AFM). The structures built presented clear red electroluminescence (EL) to the naked eye. Turn on voltages were found to be in the range of 3-6 volts while the onset current was in the order of tens of microamperes. The role of structure homogeneity, the presence of pinholes and lifetime extension were features addressed during this investigation. Samples with a lifetime of continuous operation in air longer than 60 minutes and highly stable EL spectra were achieved; EL was visible to the unaided eye, although the brightness was still below the commercial standards and has not yet been qualified.
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    Using the surface spontaneous depolarization field of ferroelectrics to direct the assembly of virus particles.
    (American Institute of Physics, 2004-10) Dunn, Steve; Cullen, David C.; Abad-Garcia, Estefania; Bertoni, Cristina; Carter, Richard C.; Howorth, Dave; Whatmore, Roger W.
    Tobacco mosaic virus (TMV) particles have been assembled at predetermined locations on the surface of a ferroelectric thin film with the composition PbZr0.3Ti0.7O3. The domain structure, hence the surface depolarization field for the ferroelectric associated with the bending of the ferroelectric band structure, was modified at length scales of around 2 µm. A suspension of TMV particles was deposited onto the surface of the ferroelectric over the region where domain modification had been performed. SPM images (intermittent contact) revealed that the virus particles were attracted to regions of the surface that had either been positively modified or were nominally neutral. Regions of modified negative surface charge effectively repelled the virus particles.