Growth, electronic, magnetic and spectroscopic properties of transition metals on graphite

Abstract

This report presents a combined theoretical and experimental study of the electronic structure of nanostructures of transition-metals on graphite. In the last few years a variety of techniques have been used to prepare mesoscopic metal islands on this substrate and novel effects associated with the electronic structure in nanoscale systems have been reported. These include magnetism in the bulk paramagnetic metals V and Ru. In the present report, following the introduction in Section 1, Section 2 is devoted to review the growth and spectroscopic properties of 3d and 4d metals on graphite. A description of the experimental techniques and conditions used by the authors is presented. The morphology of deposited MBE films of Cr, V, Mn and Fe and pre-formed nanoscale clusters of Mn and Fe, determined by XUV reflectivity, SEM and STM is discussed. Results from XPS and other photoemission-based techniques, used to examine the electronic and magnetic properties of the films, are shown and several novel effects expected in nanoscale systems are demonstrated. These include enhanced magnetic moments in Mn nanoclusters, size-dependent screening effects in islanded Mc, Cr and V films and evidence for increased magnetic disorder in Cr and Fe particles. Section 3 is dedicated to the theoretical investigation of magnetism for hexagonal 3d and 4d metal monolayers, epitaxially adsorbed on a (0001) graphite surface, by spin-polarized electronic structuer calculations using semi-empirical and ab-initio methods. It is shown that, when they exist, the magnetic moments of the adsorbed monolayers are substantially reduced from their unsupported monolayer values. A comparison with one and two adatoms is discussed. Also in this chapter, we present calculations of the 3s-XPS of 3d metal atoms chemisorbed on graphite, using a cluster model that takes into account intra-atomic dd and d-core electron exchange and Coulomb interactions as well as hybridization between 3d metal orbitals and graphite p states. The ability of the system to undergo a low-to-high spin transition is discussed along the 3d series with respect to the metal atom-graphite distance and is clearly evidenced in the evolution of the 3s-core-XPS line shape. All the transition metal films on graphite presented in this report grow as islands with a thickness of a few nanometers. However, the monolayer growth of 3d or 4d metals on graphite might be achieved in certain cases, even if this corresponds to thermodynamically metastable phases. In this way our report presents interesting and systematic results for new epitaxial systems but still remains a challenge for both experimentalists and theoreticians.