Study Of Self-assembly And Quantum Diffusion Of Gd On Ag(111)

Ordered arrays of magnetic nanostructures are of special interests due to their rich physical properties and potential applications such as magnetic data storage. With the advance of modern growth and imaging techniques, it is possible to fabricate structures and investigate their unique properties down to the atomic level. Atomic manipulation through scanning tunneling microscopy (STM) and self-assembly growth are two main routes to fabricate the atomic-scale structures. Atomic manipulation is of advantage in building structures in arbitrary geometry, while self-assembly can provide well-ordered structures with relatively large area homogeneity and in an economic way. Self-assembly through substrate mediated long-range interactions (LRI) between adatoms on noble metal (111) surfaces represents one of these typical samples. On the other hand, in nano-size corrals built by atomic manipulation, two-dimensional (2D) quantum confinement can be used to engineer atom diffusion and build novel atomic structures. In this thesis, I will introduce these works during my Ph.D. program:(1) Self-organized Gd atomic superlattice on Ag(111):Scanning tunneling microscopy and kinetic Monte Carlo simulations,(2) Spectroscopy of one-dimensional atomic strings:the role of step edges,(3) Two-dimensional quantum diffusion of Gd adatoms in nano-size Fe corrals.(1) With STM, we experimentally demonstrate the self-organized Gd atomic superlattice on Ag(111) surface at low temperature. The diffusion barrier of a single Gd adatom and the LRI between Gd adatoms on Ag(111) are experimentally determined. With these parameters, the formation mechanism is discussed and we confirm the validity of the previous discussed preconditions for formation of a good superlattice. The well-ordered structure is reproduced by the kinetic Monte Carlo (KMC) simulations with the experimentally determined parameters. As well-ordered superlattice is realized in two different lanthanide adatoms on Ag(l11), we predict that similar self-organization could form in other lanthanide adatoms because of their identical electron configuration in the outer shell and similar atomic radii.(2) One-dimensional atomic strings, Gd strings created by self-assembly and Fe strings fabricated by atomic manipulation on Ag(111) surface, are studied by STM and scanning tunneling spectroscopy (STS) at low temperature. Gd adatoms form atomic strings with a periodicity of3.0nm and the strings are located2.5nm away from the step edges on the upper terraces. STS measurements show a characteristic peak of+65meV in the Gd atomic string in sharp contrast with the tight-binding (TB) calculations of a string on a flat terrace. To clarify the discrepancy, Fe strings, both on a flat terrace and near the step edges, are constructed by atomic manipulation. The obtained spectroscopies of both are in good agreements with the calculations when detailed local environments are included. Our findings demonstrate that the electron scattering by both the adatoms of the string and step edges play important roles in the electronic spectra of the self-assembled strings guided by step edges.(3) Gd atom diffusion in30-nm diameter Fe quantum corrals is studied utilizing STM and KMC simulations. The Gd adatom probability distribution inside the corral forms several trajectories and is closely related to oscillations of the local density of states at the Fermi level, as revealed by spectroscopy measurements. With increasing coverage, the Gd adatoms form a ring-like structure within the vicinity of the quantum corrals and finally quantum onion with the optimal coverage. The results are explained with KMC calculations utilizing the experimentally determined LRI and the quantum confinement. The findings demonstrate that the diffusion of Gd adatoms is significantly influenced by the quantum corrals, and that novel atomic structures can be created via control of the coverage.