Carbon monoxide adsorption on high area platinum fuel cell catalysts was investigated. Isotopic exchange experiments were performed to determine the exchange rate (k) of CO under different partial pressures of CO (pCO) in argon. A linear dependence of ln(k) with ln(pCO) was observed. This pressure dependence of the rate of exchange is explained by considering a change in surface coverage of CO with different CO pressures and a subsequent reduction in the CO binding energy as demonstrated by Density Functional Theory (DFT) calculations. High Pressure Scanning Tunneling Microscopy (HP STM) studies on the Pt(111) surface have also displayed a pressure dependency of the coverage consistent with this data. The relevance of these observations to the Polymer Electrolyte Membrane Fuel Cell (PEMFC) anode reaction is discussed.

A comparative analysis of metal-oxide-semiconductor ͑MOS͒ capacitors by capacitance-voltage ͑C-V͒ and current-voltage ͑I-V͒ characteristics has been employed to characterize the thickness variations of the oxide on different length scales. Ultralarge area ͑1 cm 2 ͒ ultrathin ͑ϳ5 nm oxide͒ MOS capacitors have been fabricated to investigate their functionality and the variations in oxide thickness, with the use as future electron emission devices as the goal. I-V characteristics show very low leakage current and excellent agreement to the FowlerNordheim expression for the current density. Oxide thicknesses have been extracted by fitting a model based on Fermi-Dirac statistics to the C-V characteristics. By plotting I-V characteristics in a Fowler plot, a measure of the thickness of the oxide can be extracted from the tunnel current. These apparent thicknesses show a high degree of correlation to thicknesses extracted from C-V characteristics on the same MOS capacitors, but are systematically lower in value. This offset between the thicknesses obtained by C-V characteristics and I-V characteristics is explained by an inherent variation of the oxide thickness. Comparison of MOS capacitors with different oxide areas ranging from 1 cm 2 to 10 m 2 , using the slope from Fowler-Nordheim plots of the I-V characteristics as a measure of the oxide thickness, points toward two length scales of oxide thickness variations being ϳ1 cm and ϳ10 m, respectively.

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We have made Na (+) and He (+) ions incident on the surface of solid state tunnel junctions and measured the energy loss due to atomic displacement and electronic excitations. Each tunnel junction consists of an ultrathin film metal-oxide-semiconductor device which can be biased to create a band of hot electrons useful for driving chemical reactions at surfaces. Using the binary collision approximation and a nonadiabatic model that takes into account the time-varying nature of the ion-surface interaction, the energy loss of the ions is reproduced. The energy loss for Na (+) ions incident on the devices shows that the primary energy loss mechanism is the atomic displacement of Au atoms in the thin film of the metal-oxide-semiconductor device. We propose that neutral particle detection of the scattered flux from a biased device could be a route to hot electron mediated charge exchange.

Articles you may be interested inThe effects of the in-plane momentum on the quantization of nanometer metal-oxide-semiconductor devices due to the difference between the effective masses of silicon and gate oxide Appl. Phys. Lett. 91, 123519 (2007); 10.1063/1.2789733 Polysilicon metal-insulator-semiconductor electron emitter Determination of electron trap distribution in the gate-oxide region of the deep submicron metal-oxide-semiconductor structure from direct tunneling gate current Effective lifetime of electrons trapped in the oxide of a metal-oxide-semiconductor structure Appl. Phys. Lett. 75, 522 (1999); 10.1063/1.124435Hot electron impact excitation cross-section of Er 3+ and electroluminescence from erbium-implanted silicon metal-oxide-semiconductor tunnel diodes Ultralarge metal-oxide-semiconductor ͑MOS͒ devices with an active oxide area of 1 cm 2 have been fabricated for use as electron emitters. The MOS structures consist of a Si substrate, a SiO 2 tunnel barrier ͑ϳ5 nm͒, a Ti wetting layer ͑3-10 Å͒, and a Au top layer ͑5-60 nm͒. Electron emission from the Au metal layer to vacuum is realized from these devices by applying bias voltages larger than the work function of the Au layer. The emission is characterized for Au layers with thicknesses ranging from 5 to 60 nm nominally. The emission efficiency changes from close to 10 −6 to 10 −10 . The Ti wetting layer is varied from 3 to 10 Å which changes the emission efficiency by more than one order of magnitude. The apparent mean free path of ϳ5 eV electrons in Au is found to be 52 Å. Deposition of Cs on the Au film increased the electron emission efficiency to 4.3% at 4 V by lowering the work function. Electron emission under high pressures ͑up to 2 bars͒ of Ar was observed.