Martin E. Kordesch

Cobalt tetramethoxyphenyl porphyrin—emission Mossbauer spectroscopy and O2 reduction electrochemical studies

Abstract Cobalt tetramethoxyphenyl porphyrin (Co-TMPP) on a high area carbon support after thermal treatment at 450–900°C has been found to be an effective catalyst for O2 reduction in alkaline electrolytes. The effect of the heat treatment at 850°C on this catalyst has been examine with emission Mossbauer spectroscopy using Co-57 enriched Co-TMPP. The spectra indicate the presence of cobaltous oxides with no evidence of Co-N4 centers or any other form of cobalt. The metal-free complex H2TMPP, given similar thermal treatment on a mineral-free carbon support does not show catalytic activity beyond that of the carbon support, thus confirming the importance of the transition metal to the catalysis.

Visible emission from electroluminescent devices using an amorphous AlN:Er3+ thin-film phosphor

Electroluminescence (EL) studies of AlN:Er alternating-current thin-film electroluminescent (ACTFEL) devices were performed at 300 K. Thin films of Er-doped AlN, ∼200 nm thick, were grown on indium–tin–oxide/aluminum–titanium–oxide/glass substrates using rf magnetron sputtering in a nitrogen atmosphere. The turn-on voltage was found to be around 70–80 and 100 V for ACTFEL devices without and with a top insulator layer. Sharp emission lines in the visible region were observed which correspond to known transitions of the Er3+ ion. Temperature-dependent cathodoluminescence studies corroborate the EL results, and show that optimum device performance is attained near 300 K.

Real-time observation of the nucleation and propagation of reaction fronts on surfaces using photoemission electron microscopy

The catalytic oxidation of CO on Pt{100} has been studied in situ using photoemission electron microscopy (PEEM). The difference in work function between CO-covered regions of the surface and O-covered regions provides image contrast in PEEM using a deuterium lamp. The CO and O2 partial pressures were adjusted to give a CO coverage greater than 0.5 monolayer at a crystal temperature below 500 K corresponding to the low rate branch of the oxidation reaction in which CO inhibits the adsorption of oxygen. The transition to the high rate branch (characterized by low CO coverage) was then initiated by decreasing the CO partial pressure. This allows oxygen adsorption at various types of defect sites, forming holes in the CO film. The holes subsequently enlarge as reaction fronts propagate away from the defects at velocities from 0.5 to over 40 μm s−1 depending on temperature, pressure and directions of steps on the surface. The return to the low rate branch, induced by increasing the CO pressure again, does not give rise to propagating reaction fronts but rather to random adsorption of CO throughout the surface.

Band-gap engineering in sputter-deposited ScxGa1−xN

Reactive sputtering was used to grow thin films of ScxGa1−xN with scandium concentration of 20%–70% on quartz substrates at temperatures of 300–675 K. X-ray diffraction (XRD) of the films showed either weak or no structure, suggesting the films are amorphous or microcrystalline. Optical absorption spectra were taken of each sample and the optical band gap was determined. The band gap varied linearly with composition between 2.0 and 3.5 eV. ScN and GaN have different crystal structures (rocksalt and wurzite, respectively), and thus may form a heterogeneous mixture as opposed to an alloy. Since the XRD data were inconclusive, bilayers of ScN/GaN were grown and optical absorption spectra taken. A fundamental difference in the spectra between the bilayer films and alloy films was seen, suggesting the films are alloys not heterogeneous mixtures.

Electron emission from chemical vapor deposited diamond and dielectric breakdown

After coating with a smooth, nonreactive, sputtered gold layer, previously nonemitting chemical vapor deposited diamond films are observed to emit electrons in an applied field of 30–50 kV/cm in an electron emission microscope. The gold overlayer increases the effective field strength applied to the film. The consequent dielectric breakdown of the film provides conductive channels for the observed electron emission.

A comparison of single-, double-, triple-bonded and aromatic CN compounds on Pd(111) and (100) I. Hreels of NCCN, ethylenediamine and s-triazine

The bonding and geometry of CN, NCCN, ethylenediamine, and s-triazine have been investigated with HREELS in order to compare the characteristics of CN bonds of different order on Pd(111) and (100). The CN bond is found to lie parallel to the metal surface in all cases examined. A π -bonded configuration for these adsorbates is likely. The comparison of the single, double and triple bond CN stretch frequencies, ≈130, 190, and 235 meV, respectively, for the chemisorbed species, allows intermediates and reaction products of the more complex CN adsorbates to be identified. CN is the only species resulting from NCCN decomposition on Pd. Ethylenediamine and s-triazine were used to obtain spectra from a compound modelling the “hydrogenation of NCCN” and a “surface polymer”, respectively. On the basis of the spectra of these model compounds, we conclude that a reaction of molecular NCCN with coadsorbed H or the formation of a surface polymer or paracyanogen-like compound from CN does not take place on Pd(111) or (100).

Band gap engineering in amorphous AlxGa1−xN: Experiment and ab initio calculations

Amorphous alloys of aluminum nitride and gallium nitride deposited at 100 K at compositions ranging from pure AlN to pure GaN with optical band gaps which vary linearly with composition from 3.27 eV (a-GaN) to 5.95 eV (a-AlN) have been synthesized. Ab initio molecular dynamics calculations for these alloys reproduce the band gap versus composition data and give specific information on the electronic localization of the band tail states. There are no midgap states in amorphous AlxGa1−xN alloys. The calculated models have mixed four-fold and three-fold coordination and have no wrong (homopolar nuclear) bonds, demonstrating the strong ionicity in amorphous AlxGa1−xN alloys. It has been found that the valence band tail states are mostly localized on the three-fold coordinated N sites while the conduction band tail states are mostly localized on the three-fold coordinated Ga or Al sites.

Luminescence of Tb ions implanted into amorphous AlN thin films grown by sputtering

We report the observation of visible cathodoluminescence (CL) of Tb ions implanted into amorphous AlN films produced by sputtering. The implanted samples were subjected to thermal annealing treatment up to 1100 °C to optically activate the incorporated ions. The results show that up to 1000 °C annealing temperature the films remain amorphous and the Tb3+ emission intensity increases. The amorphous AlN:Tb films were characterized by x-ray diffraction, CL, and CL kinetics measurements. The sharp characteristic emission lines corresponding to intra-4fn-shell transitions are resolved in the spectral range from 350 to 750 nm, and observed over the temperature range from 7 to 330 K due to the transitions from 5D3 and 5D4 levels toward the 5FJ (J=2 to 6) multiplets. Finally, CL kinetics measurements have revealed that decay times of 5D3→7FJ and 5D4→7FJ transitions are in the range 0.94–0.77 and 0.49–1.61 ms at 300 K, respectively.

Direct ultraviolet excitation of an amorphous AlN:praseodymium phosphor by codoped Gd3+ cathodoluminescence

Sputter deposited thin film amorphous AlN:Pr (1at.%) emits in the blue-green (490–530nm) and red (∼650nm) regions of the visible spectrum under electron excitation. The addition of Gd 1at.% in the film enhances the blue emission by an order of magnitude. The enhancement in the blue region is a result of cathodoluminescence from Gd3+ at 313nm. The optical bandgap of amorphous AlN is about 210nm, so that the film is transparent in the ultraviolet, allowing the Gd emission to excite the Pr3+ ions. No significant quenching of the Gd emission is observed when the Gd and Pr ions are mixed. The blue enhancement is observed even with the two films containing each of the ions that are separated by a 500μm thick quartz spacer, showing that the enhancement is due entirely to UV radiation.

Applications for small photoelectron emission microscopes

Abstract Several applications of simple photoelectron emission microscopes for the observation of real time in situ surface processes are reviewed, including field emission, chemical vapor deposition, tribochemistry and liquids in ultra-high vacuum, and pulsed supersonic molecular beam film growth.