We report on the local atomic and electronic structures of a nitrogen-doped graphite surface by scanning tunneling microscopy, scanning tunneling spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. The nitrogen-doped graphite was prepared by nitrogen ion bombardment followed by thermal annealing. Two types of nitrogen species were identified at the atomic level: pyridinic-N (N bonded to two C nearest neighbors) and graphitic-N (N bonded to three C nearest neighbors). Distinct electronic states of localized π states were found to appear in the occupied and unoccupied regions near the Fermi level at the carbon atoms around pyridinic-N and graphitic-N species, respectively. The origin of these states is discussed based on experimental results and theoretical simulations.

Ab initio methods for calculating the adiabatic electronic properties of a single isolated molecule interacting with a metal surface are reviewed. First the fundamental approaches of Anderson, Grimley, and Newns for chemisorption, as well as of Zaremba and Kohn, for physisorption, are outlined. Then the density-functional theory and its approximations are considered. The different models for the adsorbate system are described. They comprise those in which the system has a finite volume-i.e., the cluster, the slab, and the supercell models-and those which take into account the semi-infinite nature of the substrate-i.e., the embedding approach based either on the Dyson equation or on Green's-function matching. Those definitions are also introduced that we deem important for the understanding of the physical properties of systems to be presented in this article. The lack of full screening in a localized region around the adsorbate, and hence the existence of long-range Friedel's oscillations induced by the adsorbate in the metal, are discussed. The way in which the lack of full screening influences the calculated adsorption energies is estimated by the grand-canonical functional. Recent ab initio results on physisorption of a noble-gas atom on metals deal mainly with the limits of validity of the simpler effective-medium theory and with the anticorrugating effect of He. Atomic chemisorption is considered in order to deal with the concept of bonding at a metal. Dissociative chemisorption calculations mainly treat the H 2 metal system. Here both the adiabatic electronic properties and the sticking probabilities recently obtained using the ab initio potential-energy surfaces are analyzed. Carbon monoxide chemisorption, lateral interactions between adsorbates, adatom diffusion, and chemisorption on stepped surfaces are presented as prototypes of the large variety of ab initio results currently available. Finally, the conclusions are devoted to the respective merits of the different theoretical approaches and to some future directions. [S0034-6861(99)00501-2] CONTENTS

Single magnetic Co atoms are deposited on atomically thin NaCl films on Au(111). Two different adsorption sites are revealed by high-resolution scanning tunneling microscopy (STM), i.e., at Na and at Cl locations. Using density functional based simulations of the STM images, we show that the Co atoms substitute with either a Na or Cl atom of the NaCl surface, resulting in cationic and anionic Co dopants with a high thermal stability. The dependence of the magnetic coupling between neighboring Co atoms on their separation is investigated via spatially resolved measurement of the local density of states.

We investigated the local influence of the Au(111) herringbone reconstruction on the properties of thin adsorbed NaCl films using cryogenic scanning tunneling microscopy (STM) and spectroscopy. Depending on the local hcp versus fcc character of the reconstruction, NaCl adsorption gives rise to a different shift of the Au(111) surface state toward the Fermi level, in agreement with ab initio calculations. Such lateral modulation may allow for tunable nanostructuring of thin insulating films, which opens up new perspectives for molecular electronics applications. Furthermore, we demonstrate the simultaneous visualization of both the alkali and the halogen atoms in hcp regions of the NaCl/Au(111) surface using a functionalized STM tip. Ab initio calculations relate this simultaneous visualization to the larger electron density in the hcp regions.