J. S. Pan

Synergism between Cu and Zn sites in Cu/Zn catalysts for methanol synthesis

Abstract Three Cu-based catalysts prepared using RF plasma-sputtering (Cu/ZnO and Cu/Al2O3) and coprecipitation (Cu/Zn/Al oxide) methods were employed for the investigation of the synergism between Cu and Zn sites. The static secondary ion mass spectrometry (SIMS) experiments indicate that, on both Cu/ZnO/Al2O3 and Cu/ZnO catalysts, CO is adsorbed at Cu sites (SIMS peaks appear at 91 and 93 amu), and H is bound to ZnO sites (67, 69, 70, 71 and 81, 83, 84, 85 amu) when the catalyst surfaces are exposed to H2, CO and CO2. Methoxy, the intermediate species of methanol synthesis, is found to be bound at Zn sites (95, 97, 98, 99 amu). CO and CO2 are found to induce Zn migration from sub-layers to the topmost layer while H2 is heterolyzed easier due to the presence of Cu. The coexistence of ZnO with Cu could enhance the capability of Cu to adsorb CO species and itself to adsorb H2 species. Al2O3 shows no synergetic effect with Cu in this experiment. No CO and H2 are detected on the Cu/Al2O3 catalyst exposed to syngas. The synergetic effect between Cu and Zn in the course of methanol synthesis was discussed.

Plasma-induced damage to n-type GaN

The effects of plasma etching on 1/f noise and photoluminescence (PL) characteristics of n-GaN have been investigated. A reduction of 1/f noise was observed after plasma exposure, a result of enhanced passivation of the reactive surface. This is attributed to the removal of carbon and the creation of a Ga-rich surface by the etching process. Nevertheless, the formation of nonradiative recombination centers impaired the PL intensity. Reconstruction of a stoichiometric surface was achieved by annealing. This induced the incorporation of carbon into GaN, deteriorating the PL performance further, but it could be restored by a chemical treatment of 10:1 HF:H2O.

Correlation between the corrosion behavior and corrosion films formed on the surfaces of Mg82−xNi18Ndx (x=0, 5, 15) amorphous alloys

Abstract This study deals with the characterization of the corrosion films formed on the surfaces of amorphous Mg82−xNi18Ndx (x=0, 5, 15) alloys in 3% NaCl solution saturated with Mg(OH)2. Hydrogen evolution test revealed that the addition of Nd has a beneficial effect to reduce the dissolution rate of Mg82−xNi18Ndx alloys. SEM showed that the least severe corrosion occurred for the Mg67Ni18Nd15 alloy which has the highest Nd concentration. XPS revealed that the corrosion film formed on Mg67Ni18Nd15 is the least hydroxidized among the amorphous Mg82−xNi18Ndx (x=0, 5, 15) alloys. The improved corrosion resistance of Mg67Ni18Nd15 is attributed to the high content of oxidized Nd in the corrosion film preventing the detrimental Cl− and CO32− anions from penetrating.

Surface analysis of GaN decomposition

The decomposition of GaN at a range of temperatures has been studied by Rutherford backscattering (RBS), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The development of a surface defect peak is a consequence of preferential N2 loss at elevated temperatures. Additionally, the emergence of a direct scattering peak approximately 0.25 µm beneath the surface at 1100 °C can be attributed to the buildup of extended defects. At such temperatures severe roughening of the surface is observed through AFM scans. Nevertheless, Ga droplet formation is not detected from our samples as verified by XPS. Our results show that GaN remains thermally stable in N2 up to 900 °C. At higher temperatures, significant decomposition occurs and gives rise to degradation of the structural and morphological properties of the film.

Argon incorporation and silicon carbide formation during low energy argon‐ion bombardment of Si(100)

Argon incorporation and the formation of silicon carbide in Si(100) by low energy Ar+ ion bombardment have been studied by angle‐resolved x‐ray photoelectron spectroscopy (XPS). The bombardment was performed at ion energies of 1, 1.5, and 2 keV and various ion fluences in an ultrahigh vacuum chamber equipped with XPS. The XPS measurements showed that the incorporated Ar concentrations achieved saturation in the near‐surface region at ion bombardment fluences ≳1016 cm−2. The surface Ar concentrations decreased with increasing bombardment energy. No Ar bubbles on the surface of Ar+‐bombarded samples were observed by atomic force microscopy under these experimental conditions suggesting that Ar bubble formation was not the main Ar trapping mechanism in our study. The SiC formation was confirmed by characteristic XPS peaks of Si 2p and C 1s for SiC. The carbide formed at lower ion fluence was of a metastable structure as inferred by XPS. Bombardment at higher ion fluence yielded a stable carbide phase through...

Argon incorporation and surface compositional changes in InP(100) due to low‐energy Ar+ ion bombardment

Angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) has been used to study the Ar incorporation and surface compositional changes in InP(100) after 1–5 keV Ar+ bombardment at various ion fluences. The ARXPS measurements showed that the incorporated Ar concentration achieved saturation at ion bombardment fluences of >1016 cm−2. The surface Ar concentration decreased with increasing bombardment energy. No Ar bubbles were observed by atomic force microscopy, suggesting that Ar bubble formation was not the main Ar trapping mechanism. The altered layers were, on average, In rich up to the sampling depth of the ARXPS technique. However, the altered layers were inhomogeneous as a function of depth and appeared more In rich at the surface than in the subsurface region. The results are compared with those obtained by other authors and discussed in the context of preferential sputtering, radiation‐enhanced diffusion and segregation, and Ar incorporation. Although the altered layers were In rich, a P‐rich phase ...

The Effect of Heat Treatment on the Corrosion Behavior of Amorphous Mg–Ni–Nd Alloys

The effect of heat treatment on the corrosion behavior of seven amorphous melt-spun Mg–Ni–Nd alloys containing 10–20 at.% Ni and 5–15 at.% Nd has been studied. Hydrogen evolution testing was used to determine the dissolution rate of the heat-treated specimens immersed in a 3% NaCl solution saturated with Mg(OH) 2 . The dissolution rates of the partially crystallized specimens were found to be lower than those of the untreated specimens, while the fully crystallized specimens exhibited marked deterioration of corrosion resistance. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies on the heat-treated specimens revealed precipitation of Mg 3 Nd, Mg 12 Nd, and Mg 2 Ni phases during the crystallization. TEM results show that the partially crystallized structure consists of uniform dispersion of either Mg 3 Nd or Mg 2 Ni in the amorphous matrix. In contrast, multiple phases precipitate in the fully crystallized specimen.

The alloying effect of Ni on the corrosion behavior of melt-spun Mg-Ni ribbons

Abstract Four binary Mg–Ni ribbons with Ni content up to 18.3 atm% were rapidly solidified by melt-spinning. The microstructure changed from α-Mg plus intermetallic Mg 2 Ni to a fully amorphous structure with increasing Ni content. Both the hydrogen evolution test and anodic potentiodynamic polarization revealed that the corrosion resistance of the amorphous ribbons was superior to that of ribbons with α-Mg plus intermetallic structure in 0.01 M NaCl solution (adjusted to pH 12 by NaOH). The reduced dissolution rate is attributed to the formation of a fully amorphous structure so that no intermetallic Mg 2 Ni is present and all Ni atoms are embedded in the Mg matrix. The enhanced passivity is due to the incorporation of NiO in the surface of the passive film as a consequence of its p-type semiconducting properties, thus preventing the entry of aggressive CO 3 2− and Cl − ions into the passive film.

Kinetics of Ge diffusion, desorption and pit formation dynamics during annealing of Si0.8Ge0.2/Si(001) virtual substrates

Thermal stability of Si(0.8)Ge(0.2)/Si(001) virtual substrates (VS) is studied as a function of annealing temperature and time by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Two regimes describing different Ge behavior are observed when the Si(0.8)Ge(0.2) VS is annealed. Heating the substrate from room temperature to 500 degrees C results in some degree of Ge segregation. The surface morphology however remains relatively smooth and there is no formation of 3D islands on the surface. Above 500 degrees C, Ge is preferentially lost from the surface and microscopic pits with edges aligned along 110 azimuth are formed. As temperature increases, Ge% decreases and the size of pits also increases. The decrease in Ge% and the formation of holes at the surface are attributed to Ge desorption from the surface. A kinetic model involving diffusion and desorption processes is proposed to describe the Ge behavior and pits formation in this regime.

Structural characterization of rapid thermal oxidized Si1−x−yGexCy alloy films grown by rapid thermal chemical vapor deposition

The structural properties of as-grown and rapid thermal oxidized Si1−x−yGexCy epitaxial layers have been examined using a combination of infrared, x-ray photoelectron, x-ray diffraction, secondary ion mass spectroscopy, and Raman spectroscopy techniques. Carbon incorporation into the Si1−x−yGexCy system can lead to compressive or tensile strain in the film. The structural properties of the oxidized Si1−x−yGexCy film depend on the type of strain (i.e., carbon concentration) of the as-prepared film. For compressive or fully compensated films, the oxidation process drastically reduces the carbon content so that the oxidized films closely resemble to Si1−xGex films. For tensile films, two broad regions, one with carbon content higher and the other lower than that required for full strain compensation, coexist in the oxidized films.