Effects of oxygen adsorption on carbon nanotube field emitters

Effects of oxygen adsorption on the field emission of carbon nanotubes are studied through first-principles calculations. Calculated emission currents are significantly enhanced when oxygen is adsorbed at the tip and the underlying physics is explained in terms of the change in the electronic structure by oxidation and the local field increase at the adsorption site. The issue of the current degradation accompanied by the oxidative etching is also addressed. The field-emission-microscopy images on the phosphor screen are simulated, displaying various patterns characteristic of each adsorption configuration.open546

Block copolymer templated synthesis of PtIr bimetallic nanocatalysts for the formic acid oxidation reaction

Arrays of PtIr alloy nanoparticle (NP) clusters are synthesized from a method using block copolymer templates, which allows for relatively narrow NP diameter distributions (~4–13 nm) and uniform intercluster spacing (~ 60 or ~100 nm). Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer micelles were used to create thin film templates of NPs with periodic pyridinium-rich domains that are capable of electrostatically loading PtCl62- and IrCl62- anion precursors for the preparation of NP arrays. The composition of PtIr NPs was specified by the ratio of metal anions in a low-pH immersion bath. Formic acid oxidation, studied by cyclic voltammetry, shows that the arrays of clusters of PtIr alloy NPs are highly active catalysts, with mass activity values on par or exceeding current industrial standard catalysts. The uniformity in the NP population in a cluster and the small diameter range established by the block copolymer template permit an estimate of the optimal Pt:Ir ratio for the direct oxidation of formic acid, where, ~10 nm Pt16Ir84 alloy NPs were the most active with a mass activity of 37 A/g. &nbsp

The Physics of Gaseous Exposures on Active Field Emission Microcathode Arrays

The interaction of active molybdenum field emission microcathode arrays with oxygen, water, carbon dioxide, methane, hydrogen and helium gases was studied. Experiments were setup to measure the emission characteristics as a function of gas exposures. The resulting changes in the surface work function of the tips were determined from the Fowler-Nordheim plots. The kinetics of the FEA-gas interaction were studied by observing the ion species originating from the array during and after gas exposures with a high resolution quadrupole mass spectrometer. With the work function data and the mass spectrometry information, the mechanisms responsible for emission degradation and subsequent device recovery after exposures have been determined. The data obtained was used in estimating the device lifetimes under various vacuum environments. Also it was found that the gas exposure effects are similar in dc and pulsed modes of operation of the arrays, thus permitting the use of dc mode testing as an effective acceleration method in establishing the device lifetimes under various vacuum conditions. The vacuum conditions required for the long term emission current stability and reliability of vacuum microelectronic devices employing FEAs are established. Exposure of Mo field emitter arrays to oxygen bearing species like oxygen, water and carbon dioxide resulted in serious emission current degradation. Whereas, exposure to methane and hydrogen caused a significant increase in emission current. The control of residual gases like 02, C02 and H20 in the vacuum envelope is essential for the emission current stability and long term reliability of vacuum microelectronic devices employing field emission microcathode technology