Growth, electronic structure and magnetism of supported metal nanowires
This thesis describes experiments with one-dimensional atomic chains grown by step decoration of vicinal Pt surfaces. The objective of the research was twofold: first, to characterize metal epitaxial growth on stepped substrates; second, to make use of this knowledge to fabricate arrays of monatomic metal wires in order to investigate their electronic and magnetic properties. The first part focuses on molecular beam epitaxy (MBE) of Ag, Cu, Co, and Ni on densely stepped Pt surfaces (one monatomic step every 20 Å). Growth was investigated on the atomic scale as a function of the substrate temperature and coverage by means of thermal energy atom scattering (TEAS) and scanning tunneling microscopy (STM). A wide variety of growth scenarios has been characterized depending on the choice of the overlayer material and on the interplay between surface diffusion, strain and alloying. Uniform arrays of one-dimensional atomic chains of Ag, Cu, and Co can be fabricated in a well defined temperature range. The growth of alternate wires of different metals was also investigated. In the second part we have studied the chemisorption of O2, CO, and H2, at Pt and at Ag-decorated Pt steps. We have identified the Pt edge atoms at the top of the step as the most active sites for O2 dissociation. Controlled decoration of the Pt steps by monatomic Ag wires was used to locally vary the reactivity of the Pt edge atoms and to further elucidate the dissociation process. Ag decoration results in a selective modification of the adsorption rate of O2, CO, and H2 on Pt stepped surfaces. The third part deals with the electronic and magnetic properties of Co monatomic wires grown on Pt(997). We have investigated the wire-induced valence band states by means of angle resolved photoemission (ARPES). Co 3d states attributed to monatomic chains display a double-peaked structure that suggests the presence of a one-dimensional exchange-split band. The magnetic behavior of the Co chains was studied by x-ray magnetic circular dichroism (XMCD). Co monatomic chains are superparamagnetic with blocking temperature between 5 and 10 K. The one-dimensional character of the wires shows up in a pronounced uniaxial magnetic anisotropy and in large values of the orbital magnetic moment compared to bulk Co and thin films.