The growth mode and electronic structure of Pb, Tl and Mn overlayers on Cu(100)
The surface sensitive techniques of LEED, Auger electron spectroscopy (AES), ultra-violet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS) were combined in order to study the electronic structure of well characterised overlayers of Pb, T? and Mn on Cu(l00). LEED and AES showed that Pb and T? atoms are adsorbed initially in linear chains lying in the (001) furrows between rows of copper atoms in the substrate surface. Both overlayers form an hop structure at the dense monolayer coverage. Thallium forms an intermediate overlayer structure in which the linear chains have moved closer together. Beyond the dense monolayer coverage both overlayers grow in the Stranski- Krastanov mode. In the case of Pb on Cu(l00) the LEED and AES results confirm the results of earlier work (Sepulveda and Rhead, 1977). Manganese initially forms a c(2 x 2) structure which is interpreted as an overlayer with Mn atoms lying in four-fold hollow sites. Further growth occurs in a disordered layer by layer mode. UPS shows that Pb and T? atoms are bonded mainly by the interaction of their 6s-p electrons with the Cu 4s-p electrons. There is little interaction between the overlayer 6s-p electrons and the Cu 3d electrons. In the case of T?, an analysis of work function measurements shows that the linear chains formed at low coverages do not interact appreciably with each other, whereas there is a strong interaction between the chains when they move closer together to form the next ordered overlayer. The transition from the first ordered T? overlayer structure to the second therefore involves a transition from a one dimensional to a two dimensional electronic interaction. For Mn overlayers there is a strong interaction between the Mn and Cu 3d electrons. At low coverages (?0.25 monolayers) a resonant bound state is observed just above the Cu 3d bands in agreement with a recent calculation. Such a state has not been observed in previous photoemission studies of dilute CuMn alloys. The photoemission spectra of thick Mn overlayers resemble the density of states of bulk Mn. EELS shows that Pb overlayers perturb the Cu 4s band confirming that bonding of the overlayer occurs via the Cu 4s orbitals. The existing models of plasmons in overlayers fail to predict the energies of the discrete losses observed in the EEL spectra of Pb and T? overlayers.