D. N. E. Buchanan

Valence bands of layer structure transition metal chalcogenides

Abstract The valence band structure of representative MX 2 layer structure compounds has been obtained by X-ray photoemission with monochromatized radiation. Chalcogen s and p and metal d band are identified and their width and position obtained. The results are compared with u.v. and He II photoemission and with recent band structure calculations.

Mössbauer Effect in Co1−xFexSi

It is shown on the basis of experiments with CoSi(57Co) in a high magnetic field and with magnetically ordered Co0.5Fe0.5Si that iron does not develop a magnetic moment greater than 0.1 μB at low temperature in the system Co1−xFexSi. The results also indicate that the magnetic properties are not directly related to the electron and hole bands pertinent to transport properties. The isomer shift shows that the configuration of Fe is independent of composition over the entire range of composition. The electron concentration dependence of the quadrupole splitting is discussed.

Core electron binding energy shifts in metal clusters: tin on amorphous carbon

Abstract Small tin clusters supported on amorphous carbon substrates were studied by X-ray photoelectron spectroscopy for comparison with earlier work on noble and transition metals in order to assess the role of the d -band in core electron binding energy shifts. Comparable shifts are found in all cases, indicating that they have their origin in the net unit positive charge due to the emission of a photoelectron, which remains on cluster during the life time of the core hole. The limiting value of this shift in the smallest clusters is given by the metallic screening energy.

Reaction of C60 with metals: W

Abstract Layers of C 60 deposited on W(100) show charge donation into the LUMO of the molecules in contact with the metal surface, but there is no evidence for formation of a bulk compound. We present a simple formulation which accounts for the known successes and failures in metal−C 60 compound formation. Hypothetical compounds with the transition metals are unstable against precipitation of the metal clusters.

X‐ray photoemission spectroscopy study of surface oxidation of Nb/Al overlayer structures

We identify and characterize the chemical nature of the oxide formed by the air oxidation of thin Al overlayers on Nb. Take‐off angle experiments were performed to determine the sequential layer configuration. The thickness of the oxide did not exceed 20 A, regardless of the thickness of the metallic Al originally deposited. Grain‐boundary diffusion is suggested as a mechanism for the removal of most of the excess metallic Al. The observation of a small amount of residual metallic Al near the interface is in accord with Miedema’s theory of surface segregation which indicates that Al will segregate to the surface of a Nb‐Al alloy. An Al overlayer as thin as 9.5 A is sufficient to protect the underlying Nb film from oxidation and to form an effective tunnel‐junction barrier.

X-ray photoemission spectra of palladium

Abstract X-ray phtoemission spectra of palladium prepared by vacuum evaporation and by argon ion sputtering are compared. The former show a close correlation with calculated band structures but the latter exhibit much less detail, strongly suggesting that the sputtered surface is amorphous. The narrow core lines of the evaporated samples show the asymmetry predicted to arise from the hole-conduction electron interaction.

Electronic structure of CsAu

X-ray photoemission spectra of CsAu prepared in vacuum are in good agreement with the ionic character expected for this material. The Cs 5p doublet lies well below flat Au 5d bands. The Au 6s valence band is located somewhat closer to the 5d states than recent band structure calculations indicate. The charge transfer to gold is in the range 0.6 to 0.8 electrons.

Electron‐Bombardment Damage in Oxygen‐Free Silicon

Electron‐bombardment damage in oxygen‐free silicon is compared with that found in pulled crystals which have been previously reported. It is found, in agreement with conclusions drawn from spin resonance experiments, that the level 0.16 ev below the conduction band is associated with oxygen. In oxygen‐free silicon a deep level, 0.38 ev below the conduction band is produced in dominant density, and a broad shallow level in lesser concentration. The 0.16‐ev level is not produced. The difference between this level scheme and that found in pulled crystals which contain oxygen points up the hitherto unsuspected importance of impurities in the formation of the stable defects, during the rearrangement of the vacancies and interstitials produced by the primary displacement.

Surface effects in photoemission from alkali-fullerides

Abstract Photoemission data on alkali-doped C 60 are reinterpreted in the light of the finding that the electron mean free path (escape depth) in typical experiments is comparable to or less than the diameter of the molecular unit. Data are consequently dominated by the signal from the surface layer of molecules which have distinct electronic properties. This accounts for the weakness of the Fermi cut-off found in all photoemission studies.

Charge Donation by Calcium into the t1g Band of C60.

Photoemission spectra of compounds prepared by the reaction of C60 films with calcium show two distinct metallic phases, whereas alkali-doped C60 films have only one. In the first phase the bulk t1u band, derived from the lowest unoccupied molecular orbital of C60, is partially occupied. This is followed by an insulating phase that has the composition Ca3C60 in which the t1u band is filled and has properties analogous to those of K6C60. Continued exposure to calcium produces a second metallic phase in which electrons are donated into the t1g band. The superconductivity of Ca5C60 is associated with the t1g band.