Electronic states of adsorbates and insulating overlayers on metal surfaces

Doctoral thesis, 2004

Electronic states of adsorbates and insulating overlayers provide a wealth of scientifically interesting phenomena with great technological importance. Interesting phenomena arise, for example, from the interaction of electronic states of an adsorbate with the continuum of metal electron states, both propagating bulk states and surface states. The electronic states at surfaces and insulating overlayers are of fundamental importance in heterogeneous catalysis, microelectronics, fuel cells and thin film applications. In this thesis we give a detailed account of electronic states at surfaces using electronic structure calculations based on density functional theory. We describe in detail, the chemisorption bond parameters, such as, bonding sites and geometry, the nature and strength of the bonding and the charge state of an adsorbate. In particular, the stabilization of both a neutral and a negatively charged states of an Au adatom on an NaCl film on a Cu(100) surface is analyzed. A unique experimental tool to investigate electronic states at surfaces on the atomic length scales is the scanning tunneling microscope (STM). We simulate STM images and spectra from calculated electronic states using Tersoff and Hamann approximation, where the images and spectra are given by the local density of states (LDOS) of the sample at the tip apex. Comparisons between calculated LDOS and experimental STM images and spectra, give an encouraging agreement. The good agreement allows us to interpret in detail the STM images and spectra in terms of the electronic states of the sample. Shockley surface states and image states at noble metal surfaces presents an interesting example for studies of interactions between two-dimensional electron states and a defect, such as, a single adatom. Based on a wave packet propagation (WPP) method, the scattering of the Shockley surface states and image states from a single Cu adatom, is analyzed. The interaction between the Shockley surface state and the adatom gives rise to a surface state localization at the adsorbate, as revealed by an adatom-induced peak in LDOS and STM spectra. Also adatom-induced effects on the dynamics of image states (population decay and dephasing) are calculated using the WPP method. The calculated adatom-induced population decay and dephasing rates are significant and in agreement with two-photon photoemission experiments.