L. Q. Jiang

Room temperature reaction between polycrystalline Ni/Al bilayers deposited in ultrahigh vacuum

The growth of polycrystalline Ni–Al bilayers deposited in ultrahigh vacuum (P<10−9 Torr), and their modification by annealing are studied by Auger and photoemission spectroscopy. At room temperature, Ni deposition on Al causes a chemical shift of the 68‐eV Al Auger line and photoemission shows that the Ni 3d valence‐band states for thin Ni overlayers are narrower and shifted away from Ef when compared to Ni metal. For Al deposited on Ni, the 68‐eV Al Auger line also undergoes a chemical shift and photoemission shows the Ni 3d valence band is modified by the appearance of a new state at higher binding energy. Based on comparisons with photoemission and Auger data presented by others for Ni–Al intermetallic compounds, and photoemission, ion channeling and ion scattering results published for the chemically similar Pd–Al system, we conclude that polycrystalline Al and Ni films partially intermix and form a NiAl3‐like intermetallic compound at room temperature. We believe this Ni–Al compound may be found at t...

Study of palladium mono‐ and multilayers on Ta(110) between 300 and 1500 K

Core level photoemission was used to study the growth and thermal modification of Pd films deposited on Ta(110) between 300 and 1500 K. The Ta(110) substrate had Ta 4f core level spectral components which could be identified with the Ta(110) terrace and edge sites. The deposition of Pd causes a 175±100 meV shift of these surface‐related core level components. Little or no additional core level shift is seen when the interface is modified by heating. A decrease in the intensity of the bulk Ta 4f component is observed and is consistent with the growth of a Pd–Ta surface alloy. However, changes in the ratio of the components derived from the edge and terrace sites argue for changes in the size of the Ta(110) terraces due to the preferential growth of the alloy phase at the edges or the development of additional surface facets. These changes in surface morphology appear to be reversed when the Ta substrate becomes hot enough to remove the Pd.

Growth and electronic structure of Nb and Ta films on Pd and their interaction with CO

The growth of Nb and Ta films on Pd(111) and their modification of CO chemisorption are studied using ultraviolet photoelectron spectroscopy (UPS), low‐energy electron diffraction, and low‐energy ion scattering (LEIS). At 300 K, LEIS shows significant amounts of Pd in the top layer even after more than a monolayer of Nb or Ta is deposited. This is in contrast to Pd on Nb or Ta(110) where a Pd monolayer completely covers the substrate. UPS shows that the surface electronic structure at monolayer Nb or Ta coverage resembles that of a Pd monolayer on Nb or Ta(110) and is characterized by strong Pd 4d emission 1–4 eV below Ef and a low density of states at Ef. Molecular CO adsorption is observed at submonolayer coverage; there is little CO adsorption at monolayer coverage and dissociative CO adsorption starts at greater than monolayer coverage. It is concluded that the deposited Nb or Ta monolayers intermix with the Pd(111) substrate and it is also shown that this behavior can be understood by examining the e...

Characterization of Rh films on Ta(110)

The surface and electronic structure of Rh films on Ta(110) up to several monolayers thick on Ta(110) are characterized by photoemission, Auger emission, low‐energy electron diffraction (LEED) and low‐energy ion scattering (LEIS). From the variation of the Rh Auger peak‐to‐peak intensity as a function of evaporation time, Rh appears to grow in the Stranski–Krastanov mode at room temperature. However, the LEIS data show that the Rh adatoms begin to cluster on Ta(110) before growth of the monolayer is completed. Diffuse LEED scattering suggests that the Rh films are disordered. Photoemission shows that Rh chemisorption on Ta(110) generates two peaks located at −1.5 and −2.5 eV binding energy during the initial phase of thin‐film growth (0 3.7 ML). CO dissociates on the Rh/Ta(110) surface for Rh coverages<2.5 ML and t...

The electronic structure of YBa2Cu3O7−δ above and below Tc

The electronic structure of YBa2Cu3O7−δ has been investigated using synchrotron radiation above and below the superconducting transition temperature. Valence band spectra show a low density of states at the Fermi Level, a shift of the Cu3d‐O2p derived bands ∼1.5 eV below their calculated binding energy and a Cu 3p‐3d shake‐up satellite that suggest localization of the copper 3d‐states and also suggest that the copper ions have 2+ valence. Corelevel spectra also give evidence of the Cu2+ ion and show at least two distinct oxygen bonding environments. Cooling the surface to 20K, leaves the Cu and Ba corelevels unmodified, may produce a new O 1s corelevel component at 533 eV and causes a small change in the shape of the valence band. The Cu 3p‐3d satellite is weaker and increased photoemission coincident with states derived from the O 2p orbitals at the Cu 3p threshold is noted in the valence band. These changes upon cooling suggest that small changes in the Cu‐O electronic states occur and their possible re...