Browsing by Author "Gallardo, Diego E."

Now showing 1 - 8 of 8
  • Thumbnail Image
    ItemOpen Access
    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]
  • Thumbnail Image
    ItemOpen Access
    Dynamic effects in CdTe quantum-dot LEDs
    (Cranfield University, 2006-08) Gallardo, Diego E.; Dunn, S. C.
    In this work the electrical and electroluminescence properties CdTe nanocrystal films were analysed. The structure consisted of a multilayer of CdTe nanocrystals deposited by the layer-by-layer technique, sandwiched between an ITO anode and an aluminium cathode. The first part of this work was dedicated to structural and process improvement. Earlier devices, produced through a layer-by-layer (LbL) manual procedure, had an average thickness of 30nm per nanocrystal monolayer, with a roughness near 30% of the overall thickness. Electrical tests showed current densities over 100 mA/cm^2, with a frequent occurrence of pinholes and short-circuits that caused erratic sample behaviour and device rupture. SEM and AFM microscopic analysis showed that the nanoparticles were aggregated in 15-20nm thick clusters bound by polymer. A high porosity and non- uniformity of the multilayer was observed, explaining the formation of short-circuits. Luminescence was obtained from a very small number of samples, and with a very short duration that did not make spectral analyses possible. A robotic arm was programmed to carry out the LbL deposition, in an attempt to reduce the inhomogeneities of the multilayer. A key factor was the introduction of a special routine to remove the samples from the solutions. The sample withdrawal was designed to be in the vertical at a rate of 1.18 mm/s. The idea was to use gravity and the surface tension of the aqueous solutions to remove all the excess liquid from the surface. Additionally, poly(ethylenimine) (PEI) was eliminated from the process to improve homogeneity. These modifications produced multilayers with a thickness of 3nm per layer, average roughness below 5nm and CdTe packing density of 27%. Electrical measurements showed a stabilisation of the current-voltage (I-V) characteristics. A significant improvement in luminescence occurrence frequency and intensity was also achieved, enabling first spectral analyses. Once a reliable manufacturing procedure was developed, the electrical characterisation commenced with the analyses of samples with a different number of layers, operated in air. A field dependency of the I-V curves was found. Optimal performance was obtained from 30-layer samples, and this number of layers was adopted for subsequent analyses. Best samples showed external quantum efficiencies of 0.51%, with a photometric response of 0.8 lm/W and peak brightness of 1.42 cd/m^2. However, current and electroluminescence (EL) degradation with voltage and operation time were found in the device. Single carrier devices revealed a barrier for electron injection higher than predicted by the band diagram of the structure. The presence of an aluminium oxide layer at the multilayer/cathode interface was postulated, and confirmed through experiments in nitrogen. It was proposed that the growth of this oxide layer is the cause of device degradation during operation in air. However, it was demonstrated that the presence of the oxide favoured radiative recombination prior to degradation, with device efficiencies nearly 10 times higher than in devices without the oxide film. This was justified through three effects: charge accumulation at both sides of the oxide, field concentration across the oxide barrier and a reduction in leakage current. Unequal behaviour of samples with different electrode materials revealed that charge injection was the limiting mechanism for current flow, with a current onset field in the range of 2-3x10^7 V/m. Fowler-Nordheim plots showed that field emission was responsible for hole and electron injection into the device. Also, Fowler-Nordheim plots provided evidence of the dynamic nature of the cathode oxidation process. Dielectric breakdown of the aluminium oxide barrier at a rupture field of 3x10^7 V/m was found as a possible triggering mechanism of oxide layer growth. It was found that a critical field around 4.5x10^7 V/m caused irreversible loss of photoluminescence (PL) of the nanocrystals. This loss was attributed to an avalanche effect within the multilayer. The operational range for the devices is then found to be between 2x10^7 V/m and 4.5x10^7 V/m.
  • Thumbnail Image
    ItemOpen Access
    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
  • Thumbnail Image
    ItemOpen Access
    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.
  • Thumbnail Image
    ItemOpen Access
    In situ antimony doping of solution-grown ZnO nanorods
    (Royal Society of Chemistry, 2009-01-01T00:00:00Z) Briscoe, Joe; Gallardo, Diego E.; Dunn, Steve
    ZnO nanorods are doped with Sb during the aqueous chemical synthesis by addition of Sb acetate dissolved in ethylene glycol. The reliable production of p-type ZnO has been a challenge for some time. This is because the as-grown ZnO is nominally n-type due to intrinsic defects, 1 so holes introduced by acceptor dopants are generally compensated by the high intrinsic free electron density. Despite these difficulties, there have been increasing examples in recent years of p-type ZnO thin films and nanostructures. There is significant interest in producing p-type ZnO nanostructures due to many potential device applications. For example, transistors or diodes based on ZnO nano-homojunctions could be used for transparent electronics, UV optoelectronics and photonic
  • Thumbnail Image
    ItemOpen Access
    Insights into the relationship between inherent materials properties of PZT and photochemistry for the development of nanostructured silver.
    (Royal Society of Chemistry, 2007-09-25) Dunn, Steve; Tiwari, Divya; Jones, Paul M.; Gallardo, Diego E.
    Recently there has been great interest in using patterned ferroelectric materials for the photochemical growth of metal nanostructures. Variations in surface and sub-surface structure influence the photochemical processes. Here we show that crystallography, and hence remnant polarization, of the ferroelectric affects photo-deposition. The ratio of metal growth on c– and c+ domains varies from 1 : 2 for [100] to 1 : 100 for [111]. This is shown to be dependent on the variations in the band structur
  • Thumbnail Image
    ItemOpen Access
    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.
  • Thumbnail Image
    ItemOpen Access
    Photochemical growth of silver nanoparticles on c- and c+ domains on PZT thin films
    (American Chemical Society, 2007-06-23) Dunn, Steve; Jones, Paul M.; Gallardo, Diego E.
    The photochemical growth of silver nanoparticles on the negative domains of lead zirconate titanate thin films is reported. A sample of highly [100] orientated lead zirconate titanate, with a ratio of 30:70, that was 65−70 nm thick grown on Pt-coated MgO was poled by use of piezoresponse force microscopy to produce defined regions of surface positive and negative polarization. A comparison between the growth of silver nanoparticles on the surface of the lead zirconate titanate when illuminated with two sources of super band gap UV is given. In both cases the wavelength of illumination leads to growth on the positive domains but only illumination with a Honle H lamp, with a high photon output over 250−200 nm, caused significant growth of silver nanoparticles on the negative domain. The deposition on the negative domain is explained in terms of changed band bending due to the excitation of electrons into the conduction band, the rate of decay to the ground state, and dimensions of the ferroelectric film. The rate of deposition of silver nanoparticles on the negative domains is approximately half that on the positive domains.