Browsing by Author "Dunn, Steve"

Now showing 1 - 17 of 17
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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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    Impact of Zr/Ti ratio in the PZT on the photoreduction of silver nanoparticles.
    (Elsevier, 2009-06-03) Tiwari, Divya; Dunn, Steve; Zhang, Qi
    Silver nanoparticle deposition from an aqueous solution of silver nitrate onto the surface of PZT thin films of stoichiometric compositions PbZr0.3Ti0.7O3 and PbZr0.52Ti0.48O3 has been investigated. The impact of Zr/Ti ratio on the photochemical properties of PZT is shown by the preferential growth of silver nanoparticles onto the surface. Photoreduction of silver occurs on both c+ and c− domains on PbZr0.52Ti0.48O3 whereas it occurs only on c+ domains on a PbZr0.3Ti0.7O3 surface. The difference in deposition pattern is attributed to difference in magnitude of spontaneous polarization, effective hole concentration and band gap of the two samples which impacts shape and width of space charge layer in the two samples resulting in a change in band bending at the surface.
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    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
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    Influence of annealing on the photochemical deposition of silver onto PZT thin films under UV irradiation
    (Elsevier Science B.V., Amsterdam., 2009-10-31T00:00:00Z) Tiwari, Divya; Dunn, Steve
    Silver nanoparticle deposition from an aqueous solution of 0.01 M silver nitrate solution onto the c+ domain of PZT (30/70) thin films has been investigated for samples annealed at a variety of temperatures from 530 to 690 °C. The impact of annealing was to increase the deposition of photoreduced silver on the surface. When the PZT samples were annealed in air at temperatures ranging from 530 to 690 °C the silver deposition increased by more than 200%. The increase in the deposition of the silver is attributed to increase in the defect concentration due to the volatilisation of components from the PZT, most importantly PbO. Variations in the Pb concentration of the sample are measured using EDX and show a marked change, reduction in Pb, with annealing temperature.
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    Influence of ferroelectricity on the photoelectric effect of LiNbO3.
    (American Institute of Physics, 2008-09) Dunn, Steve; Tiwari, Divya
    A comparison between the influence of domain dependent photochemical and photoelectric cation reduction on the surface of LiNbO3 is presented. The reduction of the photoelectric threshold for LiNbO3 due to the polar nature of the crystal allows high energy UV irradiation to produce free electrons that can participate in photochemical reduction of silver nitrate on the surface. This is in addition to the previously understood phenomenon of domain directed photophysics, where influences on space charge layer due to the internal dipole of a ferroelectric determine the carrier at the surface. We show that the interaction of photoelectric and domain dependent influences can be observed in LiNbO3 due to the low electron affinity (ca 2eV) and large band bending (0.8eV).
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    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
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    Interaction of Stern layer and domain structure on photochemistry of lead-zirconate-titanate.
    (Institute of Physics, 2009-03) Jones, Paul M.; Dunn, Steve
    Perovskite ferroelectric materials such as PZT have long been known to have wideband semiconducting properties. It has also been found that they have interesting spatially controllable surface photochemical effects that are not seen in 'normal' semiconductors. This has led to their being studied as possible tools in areas such as metal salt reduction and oxidation for nanoparticle growth. This paper discusses the effects of incident photon energy on the reduction of Ag0 onto PZT(30/70) surfaces with particular emphasis on the part played by energy band bending and the Stern layer. It was found that for increasing photon energy between 4.4 and 5.0 eV both the [1 1 1] and the [1 0 0] orientations of PZT followed a similar trend in that the average Ag0 cluster cross-sectional area increased by a ratio of ca 1.6 to 1. This increase was put down to the higher energy photons exciting more electrons from deeper in the density of states for the material allowing a greater reduction rate of Ag+ at the surface.
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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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    Photo-reduction of silver salts on highly heterogeneous lead zirconate titanate
    (Institute of Physics, 2007) Jones, Paul M.; Dunn, Steve
    This paper presents the work undertaken to determine the influences on the photo-induced growth of silver nanoclusters on the surfaces of lead zirconate titanate thin films. The lead zirconate titanate films were grown on indium tin oxide coated glass. They exhibited a highly textured surface and can be treated as wide bandgap semiconductors that exhibit ferroelectric behaviour. We show that there is a preferential deposition of silver metal on the ferroelectric films that is related not only to the polarization state of the ferroelectric domains but also to the surface defects such as grain boundaries and defects within the film. The greatest deposition rates are found to occur at grain boundaries where there is an approximately 40:1 ratio of silver clusters when compared to the native positive domains exhibited by the lead zirconate titanate. We propose that the mechanism for cluster growth depends on the availability, and diffusion rate, of electrons into the growing cluster and that the clusters grow from a discrete nucleation point. We also show that the growth of a monolayer of silver is sufficient to prevent the formation of electron–hole pairs by blocking the UV irradiation and that the silver nanoparticles are readily removed from the surface using an ultrasonic bath leading to a possible new method of manufacturing metal nanoparticles.
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    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.
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    Strain behavior of thin film PbZr0.3Ti0.7O3 (30/70) examined through piezoforce microscopy
    (American Institute of Physics, 2003-11) Dunn, Steve
    Using an atomic force microscope (AFM) modified to perform PiezoAFM we have investigated the piezoelectric response of sol-gel thin film lead zirconate titanate (PZT 30/70, PbZr0.3Ti0.7O3) on Pt-Ti/SiO2/Si to a quasi DC electric field. The films were produced by spin coating a PZT 30/70 sol onto a selected substrate and firing at 520ºC. This generated a film that was highly [111] orientated and single phase perovskite. By applying a sinusoidal 2Hz AC waveform between the AFM cantilever and ground we have generated strain-field, or butterfly loops for the PZT film. PiezoAFM butterfly loops show that the localised piezoelectric response for PZT varies depending on the sign of the applied field. The degree of asymmetry in the hysteresis loop has been attributed to charge trapping at the electrode-PZT interface generating a system that can preferentially re-pole in one direction. The charge trapped at the interface has an overwhelming effect on the polarisability of the film. Coercive fields have been calculated from the minimum point of the strain before reversal for the system and are shown to be -30V/μm and +32V/μm in absolute terms and -23V/μm and +39V/μm in terms relative to the charge trapped at the electrode interface. δ33 values obtained for the PZT thin film 2 investigated ranged from 30 to 40pm/V. The maximum strain of the system was shown at applied biases of +/-10V and was 0.3% at a bias of +10V, although at this field the sample was not showing saturated behaviour. By relating the offset of the butterfly loops to the charge density generated by defects at the PZT-electrode interface, calculated to be 0.025Cm-2, an estimate of the number of defects at the interface has been drawn. The concentration of defects at the interface is 1 defect for every 6nm2 of surface, representing 1 defect for every 40 unit cells or 2.5% of unit cells being defected.
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    Substrate effects on domain structures of PZT 30/70 sol-gel films via PiezoAFM
    (Elsevier, 2002-06) Dunn, Steve; Whatmore, Roger W.
    Using an atomic force microscope (AFM) modified to perform PiezoAFM we have investigated the piezoelectric response of sol-gel thin film lead zirconate titanate (PZT 30/70, PbZr0.3Ti0.7O3) on Pt–Ti/SiO2/Si, indium tin oxide (ITO)/glass and Pt/MgO. The films were produced by spin coating a PZT 30/70 sol and firing at 520 °C for Pt electrode systems and 600 °C for the ITO system. By conducting PiezoAFM hysteresis loops we have shown that the localised piezoelectric response varies for PZT on differing substrates. The degree of asymmetry in the hysteresis loops varies for each substrate, as do the coercive fields. The coercive fields have been found to be ±18 V/μm for PZT/ITO/glass, +22 and −18 V/μm for PZT/Ti-Pt/SiO2/Si and +35 and −20 V/μm for PZT/Pt/MgO. The PZT grown on Pt/MgO, Pt-Ti/SiO2 and ITO/glass shows an offset or asymmetric hysteresis loop, which was confirmed by the differing fields required for poling during domain modification experiments performed on PZT/Pt/MgO. δ33 values obtained for the PZT thin films investigated range from 50pm/V for PZT/Pt–Ti/SiO2/Si to 40 pm/V for PZT/ITO/glass.
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    Transformation dependence of lead zirconate titanate (PZT) as shown by PiezoAFM surface mapping of Sol-gel produced PZT on various substrates
    (Taylor and Francis, 2001) Dunn, Steve; Whatmore, Roger W.
    PZT was grown on STP (Si/SiO2/Ti/Pt (Ti 5nm, Pt 100nm)), SAP (Si/Si3N4/Al/Ti/Pt (Al 100nm, Ti 10nm, Pt 15nm)) and GI (Glass/ITO(Indium Tin Oxide)). In each case the PZT underwent the same heat treatment, 200ºC for 2 minutes and then 530ºC for 5 minutes. The extent of perovskite formation was evaluated using Piezoresponse force microscopy (PFM). This provided information about the domain orientation and spatial distribution of ferroelectric material in the PZT film. This showed that PZT/STP underwent complete transformation to the perovskite phase. However, for the PZT/SAP and GI incomplete transformation to the perovskite occurred. A system of rosettes surrounded by an amorphous matrix developed. The size and density of the PZT rosettes on the surface was found to be substrate dependent. For Gi/PZT the density of rosettes formation is ca. 1/μm2, compared to 4/μm2 for PZT/SAP. The rosettes for the PZT on GI grow a maximum size of 2μm, which compares to 1μm for PZT on SAP. Differences in the observed growth rate and nucleation density are associated with the back electrode stability and crystal structure, effects that will be discussed in this paper.
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    Ultrahigh resolution of lead zirconate titanate 30/70 domains as imaged by piezoforce microscopy.
    (2006-11-14T15:31:35Z) Dunn, Steve; Shaw, Christopher P.; Huang, Zhaorong; Whatmore, Roger W.
    Piezoforce microscopy (PFM) has been used to determine the domain structure of lead zirconate titanate (PZT) (30/70) on an indium tin oxide (ITO)/glass substrate with a TiO2 boundary layer. The PZT nucleates into the perovskite form in a random crystallographic manner, which leads to a random domain structure in the final film. Using PFM it has been possible to visualize the domain structure of the PZT and determine that the domain structure has features as fine as 8 nm herringbone patterns. The possible impact of these structures for future devices utilizing nanoscale features of PZT and especially FeRAM developments is highlighted.
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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.