Detection and manipulation of metallic and magnetic nanostructures : an STM study on (sub)surface atoms, cavities and islands

In the field of metallic and magnetic nanostructures, scanning tunneling microscopy (STM) is nowadays a powerful technique to detect and manipulate nano-objects that are located at or close to the surface of metallic systems. In this research area, we have used STM and scanning tunneling spectroscopy (STS) in order to: (1) manipulate Co atoms incorporated in a Cu(001) surface, potentially leading to formation of stable nanostructures "chapter 3", (2) investigate the mechanism of motion of surfaceembedded atoms, as well as the effect of tip-surface interaction and tip-velocity on the motion of embedded atoms, including experimental demonstrations "chapter 4", (3) study the possibility to detect subsurface argon-filled cavities buried several nanometers in a metallic substrate, including the effect of anisotropic propagation of electrons "chapter 5", and (4) measure and manipulate the conductance properties of triangular Co nano-islands deposited on Cu(111) "chapter 6". Below we will give a further explanation, including a brief overview of the obtained results. Chapter 3 covers the use of STM to study the tip-induced movement of Co atoms in a diluted Co/Cu(001) surface alloy. By varying the sample temperature from 4K up to room temperature, we measured that the threshold temperature at which an incorporated Co atom can be moved is approximately 150K. We propose that a vacancy-mediated mechanism is responsible for the observed movement, in which vacancies in the tip area exchange with an embedded Co atom. Finally we demonstrated for the first time a controlled positioning of single embedded Co atoms in a Cu(001) surface. Related to this in chapter 4, we introduced a microscopic model that successfully describes STM-tip induced motion of surface embedded atoms. Using these calculations, we have investigated the behavior of the system as a function of tip-sample interaction and velocity at which the tip drags atoms along. From the velocity dependence of trace-lengths achieved for Co atoms embedded in a Cu(001)surface, evidence is given for a combined pulling-pushing mode when comparing the calculations with experimental data. We have demonstrated that the tip-induced vacancy model is a most probable candidate to explain recent experiments on moving Co embedded atoms through a Cu(001) surface. To detect nanoscaled objects buried several nanometers below the surface in chapter 5, we used argon-filled nanocavities embedded in a Cu(001) crystal. These nanocavities are capable of reflecting electrons and to induce a localized quantum well between the nanocavity and the atomically flat Cu(001) surface. We have focused on mapping the spatial variation of conductance at the surface above the nanocavity using STM/STS. Our dI/dV spectra showed a quasi-periodic oscillation of electron density versus energy in a point above the nanocavity, from which the depth of the nanocavity can be estimated. We developed a simple electron interference model that is able to describe reasonably all the experimental details, including the possibility to extract a rough estimation of the relevant parameters that govern the shape and depth of the nanocavity. Due to rich details in our observations and the simplicity of our model, this work could open up new opportunities for a more detailed and systematic analysis of buried nano-objects. In chapter 6, we demonstrated the manipulation of electronic properties of triangular Co islands on a Cu(111) surface upon adsorption as well as desorption of adsorbates. When adsorbates are present on the surface of the islands, the surface states were shifted in energy by approximately 200 meV. Using current provided via the STM tip, electrons can stimulate the desorption of the adsorbates from the Co island, which can be clearly recorded by STS by looking at the shift of the surface state towards its original energy value. The recorded differential conductance maps showed the surface of the islands before and after removing the adsorbates. The structure of the adsorbates was resolved as well, which formed a 2×2 reconstruction. A similar behavior is demonstrated in our research group by intentionally introducing H2 to the system, by which the surface state is shifted to a lower energy. Furthermore, the removal of the hydrogen from the islands is done by the STM tip leading to a full recovery of the surface state of the Co islands.