Electron spectroscopy of transition metal overlayers on low-index silver and copper crystal surfaces.
thesisposted on 19.11.2015, 09:17 by Graham C. Smith
The growth and electronic structure of Fe, Ni, Cu and Pd overlayers on Ag (100) and Pd overlayers on Ag (111) and Cu (100) have been investigated using LEED, Auger electron spectroscopy (AES) and angle resolved photoemission spectroscopy (ARPES). Calculations of the photo-emission spectra from the adsorbed overlayers and the clean substrates are reported. LEED and AES showed that Pd was initially adsorbed on Cu (100) as a c(2 X 2) surface alloy whereas the other overlayer-substrate combinations formed pseudomorphic monolayers. Pd on Ag (100) and Ag (111) showed continuous layer growth but for Fe and Ni islands were formed after three monolayers and for Cu after two monolayers had been deposited. The differences in growth mode can be understood in terms of a classical theory in which lattice mismatch plays an important role. ARPES from the clean Cu (100), Ag (100), Ag (111) and Pd (111) surfaces at 21.2 eV photon energy can be interpreted by direct transitions from calculated ground state band structures. For clean Ag (100) and Pd (111) normal emission photoelectron spectra were recorded over the energy region 0 to 120 eV. Spectra were calculated using the time reversed LEED description and good agreement between theory and experiment obtained. The development of overlayer structure through characteristic phases of different dimensionality was followed using ARPES. At low coverages a single spin-orbit split Pd resonance on Ag (100) occurred, and a similar exchange-split Fe resonance was observed. For Cu and Pd monolayers on Ag (100) the experimental two-dimensional band structures were in fair agreement with calculated isolated monolayer bands. Photo-emission calculations reproduced the spectra from these adsorbed monolayers using bulk atomic potentials, particularly good agreement being obtained for the Cu monolayer, ARPES using synchrotron radiation identified the magnetic state of the Fe monolayer on Ag (100) and a valence band exchange splitting of 1.8 to 1.9 eV was inferred. Magnetic ordering in the Ni monolayer on Ag (100) was also indicated. Throughout the study the orientation and lattice expansion imposed on the overlayer by the substrate was shown to have a significant effect on overlayer electronic structure.