Using The Probe On The Metal Surface, The Theory Of Single Atom Manipulation

Single-atom manipulation has been the subject of intense research in scientific society during the past years since the pioneer work by Eigler and co-workers, who used a scanning tunnelling microscope(STM) to move and reposition single Xe atoms adsorbed on Ni(110) surface at a cryogenic temperature (4K). This technique is a new contribution to nanoscience since, for example, it makes possible for people to fabricate designed nanostructures in a bottom-up way. STM and atomic force microscope(AFM) are the two main tools for single-atom manipulation. With the tip of a STM or AFM, single atoms can be manipulated laterally or vertically on surfaces. The reliability of the manipulation is naturally a key issue for single-atom manipulation, which is important for the real application of the technique, yet few studies have dealt with this problem so far to our knowledge. In this thesis we focus on the theoretical investigation of the reliability of the lateral single-atom manipulation, and aim to find the method to improve the reliability. Moreover, we present new methods for the reversible vertical manipulation of single atoms on the flat and stepped metal surfaces, respectively.Chapter 1 provides historical background and new developments of single-atom manipulation and raises some important problems that deserve further studies. Chapter 2 describes briefly the theoretical foundations and methods used in our study, including embedded atom method (EAM) potentials, molecular static (MS) method and density functional theory based first-principles methods.In chapter 3, we study the reliability of the lateral manipulation of a single Cu adatom on a Cu(111) surface with single-atom, dimer and trimer apex tips using both semiempirical and first-principles simulations. The influences of the tip-apex geometry, tip height and tip orientation on the manipulation reliability are examined. For the single-atom apex tip the manipulation reliability increases monotonically with decreasing tip height. For the dimer and trimer apex tips the manipulation reliability is greatly improved compared to that for the single-atom apex tip over a certain tip-height range. Two kinds of mechanisms are found responsible for this improvement. One is the so-called enhanced interaction mechanism and the other is the suspended atom mechanism due to the strong vertical attraction of the timer apex tip on the adatom. Both mechanisms occur in the manipulations with the trimer apex tip, while in those with the dimer apex tip only the former is effective. Moreover, we present a method to realize reversible vertical manipulation of a single atom on a Cu(111) surface with the trimer apex tip, based on its strong vertical and lateral attraction on the adatom, without any electric fields.Chapter 4 deals with the reliability of the lateral manipulation of single adatom on five fcc(lll) metal (Pt, Al, Pd, Ni, Au) surfaces, using MS method with EAM potentials. We obtain similar results for these systems and those for Pt/Pt(lll) system are presented. It is found that for the single-atom apex tip the manipulation reliability is sensitive to the tip orientation in a lower tip-height range, and choosing an appropriate tip orientation one can obtain a better manipulation reliability. Moreover, we propose a novel method for the reversible vertical manipulation of single atoms on the stepped surfaces. With the trimer apex tip, we can pick up the adatom adsorbed near the step edge and even extract individual atoms in the step, and in reverse we can release the atom from the tip to the surface in a certain way. This reversible process is simple and very controlled as it only requires to adjust the tip position properly while electric fields are not necessary.Chapter 5 focuses on the single-atom extraction on a stepped Al(111) surface using both MS and molecular dynamics (MD) simulations based on first-principles calculations. We demonstrate a very controlled process about how to extract and reposition single atoms on the stepped Al(111) surface. More interestingly, based on this result we present for the first time a method to modify nanoclusters on surfaces in a reversible way on an atom-by-atom basis. As an illustration, we modify a ten-atom hexagonal cluster to a triangular one. The MD simulations show that the modification process can be completed at a temperature less than or equal to 100K, which indicates that the proposed method can be reliable against the thermal disturbance and provides a theoretical guidance for the real experiments.The last chapter presents a summary of this thesis and some prospects for the ongoing investigations.