Ka Wai Wong

CHARACTERIZATION OF TREATED INDIUM-TIN-OXIDE SURFACES USED IN ELECTROLUMINESCENT DEVICES

The influence of oxidative and reductive treatments of indium–tin–oxide (ITO) on the performance of electroluminescent devices is presented. The improvement in device performance is correlated with the surface chemical composition and work function. The work function is shown to be largely determined by the surface oxygen concentration. Oxygen-glow discharge or ultraviolet–ozone treatments increase the surface oxygen concentration and work function in a strongly correlated manner. High temperature, vacuum annealing reduces both the surface oxygen and work function. With oxidation the occupied, density of states (DOS) at the Fermi level is also greatly reduced. This process is reversible by vacuum annealing and it appears that the oxygen concentration, work function, and DOS can be cycled by repeated oxygen treatments and annealing. These observations are interpreted in terms of the well-known, bulk properties of ITO.

Blocking reactions between indium-tin oxide and poly (3,4-ethylene dioxythiophene):poly(styrene sulphonate) with a self-assembly monolayer

In the fabrication of polymeric electroluminescent devices with indium-tin oxide (ITO) as anode, indium contamination of the polymers can greatly degrade the device performance. In the present study, we have used x-ray photoelectron spectroscopy to measure indium incorporation in poly(3,4-ethylene dioxythiophene):poly(styrene sulphonate), referred to as PEDOT:PSS, which were spincast on bare ITO and encapsulated ITO. We found that the deposition of a self-assembled monolayer of alkylsiloxanes on ITO prior to spincasting PEDOT:PSS was effective and practical in blocking the reactions between ITO and PEDOT:PSS.

Electron field emission from silicon nanowires

Silicon nanowires (SiNWs) were synthesized using laser ablation. A continuous SiNW film was prepared by grinding the pieces of sponge-like SiNWs to powder, then dispersing and sticking the powder onto a Si wafer. The field emission characteristics of the SiNW film were studied based on current–voltage measurements and the Fowler–Nordheim equation. The electron field emission increased with decreasing diameter of SiNWs. A hydrogen plasma treatment of the SiNW film aimed at reducing the oxide overlayer improved the emission uniformity of the film.

RE0.6Zr0.4−xYxO2 (RE = Ce, Pr; x = 0, 0.05) solid solutions: an investigation on defective structure, oxygen mobility, oxygen storage capacity, and redox properties

Abstract We have examined the crystal structures, surface textures, oxygen mobility, oxygen storage capacity, and redox behaviors of RE0.6Zr0.4−xYxO2 (RE=Ce, Pr; x=0, 0.05) solid solutions. According to the results of X-ray diffraction (XRD) studies, there are two cubic phases (Ce0.75Zr0.25O2, major; ZrO1.87, minor) in Ce0.6Zr0.4O2 (denoted as CZ hereafter) and Ce0.6Zr0.35Y0.05O2 (CZY), but only one cubic phase in Pr0.60Zr0.40O2 (PZ) and Pr0.60Zr0.35Y0.05O2 (PZY). These nanosized materials are porous and have large surface areas. As revealed by the Ce 3d and Pr 3d results of X-ray photoelectron spectroscopic (XPS) investigations, the doping of Y3+ ions into the CZ and PZ lattices resulted in an increase in concentration of oxygen vacancies and Ce3+ and Pr4+ ions. The results of H2 (or CO)–O2 titration and temperature-programmed reduction (TPR)–reoxidation experiments indicate the presence of a reversible redox behavior of Ce4+/Ce3+ in CZY and Pr4+/Pr3+ in PZY. The results of 18 O / 16 O exchange studies show that, with the presence of oxygen vacancies, the lattice O2− mobility on/in CZY and PZY enhanced. Based on such outcomes, we conclude that, by incorporating Y3+ ions into CZ and PZ, one can enhance (i) lattice oxygen mobility, (ii) Ce3+ and Pr4+ concentrations, and (iii) oxygen uptake capacity. We observed that PZY is superior to CZY in redox behavior, oxygen mobility, and oxygen storage capacity.

Ultralow-Power Alcohol Vapor Sensors Using Chemically Functionalized Multiwalled Carbon Nanotubes

Alcohol sensors, batch fabricated by forming bundles of chemically functionalized multiwalled carbon nanotubes (f-CNTs) across Au electrodes on SiO2/Si substrates using an AC electrophoretic technique, were developed for alcohol vapor detection using an ultralow input power of ~ 0.01 - 1 muW, which is lower than the power required for most commercially available alcohol sensors by more than four orders of magnitude. The multiwalled carbon nanotubes (MWCNTs) have been chemically functionalized with the COOH groups by oxidation. We found that the sensors are selective with respect to flow from air, water vapor, and alcohol vapor. The sensor response is linear for alcohol vapor concentrations from 1 to 21 ppm with a detection limit of 0.9 ppm. The transient response of these sensors is experimentally shown to be ~1 s and the variation of the responses at each concentration is within 10% for all of the tested sensors. The sensors could also easily be reset to their initial states by annealing the f-CNTs sensing elements at a current of 100-200 muA within ~ 100-200 s. We demonstrated that the response of the sensors can be increased by one order of magnitude after adding the functional group COOH onto the nanotubes, i.e., from ~0.9% of a bare MWCNTs sensor to ~9.6% of an f-CNTs sensor with a dose of 21 ppm alcohol vapor.

Effects of ion beam bombardment on electrochromic tungsten oxide films studied by X-ray photoelectron spectroscopy and Rutherford back-scattering

Abstract The effect of ion bombardment on thermally evaporated and magnetron sputtered tungsten oxide films were investigated using X-ray photoelectron spectroscopy (XPS). Results show that irrespective of the porosity and crystallinity of the film samples formed with different techniques and conditions, ion bombardment induced preferential sputtering of oxygen, resulting in a decrease of oxygen/tungsten (O/W) ratio with increasing sputtering time. Samples experienced electrochromic switching cycles also show the same effect, except that a higher O/W ratio is detected because the tungsten oxide film reacts with the LiClO 4 -propylene carbonate electrolyte. Angle-resolved XPS experiments further confirm preferential sputtering of oxygen, suggesting that ion beam sputtering used in XPS for pre-cleaning and depth profile analysis of tungsten oxide must be used with caution. Rutherford back-scattering gives more reliable composition data of tungsten oxide, since it does not involve any sputtering process.

Design and growth of dendritic Cu2−xSe and bunchy CuSe hierarchical crystalline aggregations

Dendritic nanocrystals of copper selenide were fabricated in situ for the first time by using alcohol as the solvent. Cu2−xSe films composed of hierarchically ordered dendritic nanostructures were prepared on Cu substrates at a rather moderate temperature of 190–200 °C for just 1–3 h, while bunchy CuSe nanostructures could be further constructed above the Cu2−xSe dendrites by prolonging the reaction time of solvothermal growth with ethanol as the solvent. The resulting Cu2−xSe nanodendrites display highly symmetric corolitic morphology while the bunchy CuSe aggregations show particular nanostructures with a pronounced trunk and actinomorphic multi-branches. It is also found that the dendritic structures of crystalline Cu2−xSe could never be obtained when the reaction temperature is less than 190 °C, while the temperature needed is 160 °C for Ag2Se nanodendrites and higher than 220 °C for CdSe nanodendrites. These copper selenide nanostructures with hierarchically ordered 3-dimensional (3D) framework exhibited good absorbance and photoluminescence (PL) property and could bear potential applications in solar cell devices in the future.

Dendronized Polymer Organogels from Click Chemistry: A Remarkable Gelation Property Owing to Synergistic Functional-Group Binding and Dendritic Size Effects†

The use of 1,3-dipolar cycloaddition reactions between azides and alkynes (click chemistry) has been extremely successful as a versatile synthetic tool to construct novel polymeric systems. Whereas the main thrust has been focused on building up highly elaborate polymeric architectures, such as block copolymers, star polymers, dendrimers, and hyperbranched polymers, it is also noted that the physical properties of such poly(triazole)-based materials are little studied. Additionally, although there are many examples of the synthesis of dendrimers using click chemistry, only a few concern the preparation of dendronized polymers. These are polymers incorporating multiple dendron segments stemming from a linear polymer backbone and are commonly prepared by graft-to, graft-from, or macromonomer polymerization approaches. The major challenges for these approaches are the difficulty in ensuring complete dendron coverage in the graft-to and graft-from strategies, and the sometimes poor polymerization efficiency in the macromonomer strategy. To improve the synthetic efficacy, it is necessary to make use of reactions that offer perfect conversion efficiency (such as click chemistry). Herein we wish to report a) the successful click synthesis of two different series of dendronized polymers (DPs), AmDP1–AmDP3 and EsDP1–EsDP3, from heterobifunctional amide-linked macromonomers (AmM1–AmM3) and ester-linked macromonomers (EsM1--EsM3), respectively, b) the novel and unique organogelation property of one such poly(triazole)-based dendronized polymer AmDP2, c) the remarkable functionalgroup synergistic effect on polymer interchain H-bonding, owing to the placing of many amide functionalities in close proximity along the polymer chain, and most importantly d) that the macromolecular interactions among the dendronized polymer chains are strongly influenced by the size of dendritic appendages and the nature of the linker functionality. To our knowledge, synthesis of dendronized polymers by AB-type macromonomer polymerization has not been reported before. Moreover, although physical organogels based on dendrimers and linear polymers are known, those based on click poly(triazole) polymers and dendronized polymers are extremely rare.

Copper phthalocyanine film grown by vacuum deposition under magnetic field

Copper phthalocyanine (CuPc) film with good orientation is always required when CuPc is to be used as an electronic functional layer in different device applications. In the present study, a magnetic field (∼6 mT) was applied during the vacuum deposition of CuPc. The CuPc film grown under such a magnetic field showed better orientation and a new stacking behavior as shown by X-ray diffraction study (XRD) and atomic force microscopy (AFM). In addition, the film grown under magnetic field exhibited different absorption characteristics in the UV-visible range. This demonstrates the potential of using magnetic field to optimize the growth of CuPc for various applications.

Growth of highly oriented (110) γ-CuI film with sharp exciton band

The growth of highly oriented (110) γ-CuI films on copper substrates with the best photoluminescence properties among all known CuI films are demonstrated, in the context of improving the current techniques in the preparation of CuI films for electronic and optoelectronic applications. The comparison of the photoluminescence spectra of CuI films with different growth orientations shows that the highly oriented (110) films prepared are less defective and give only a sharp exciton band.