Tsu Yi Fu

Noble Metal/W(111) Single-Atom Tips and Their Field Electron and Ion Emission Characteristics

We have developed a simple, reliable and reproducible method for preparing single-atom tips. With electrochemical techniques, a very small amount of a noble metal is plated on the surface of a clean W tip. Upon annealing the tip at an appropriate temperature in vacuum, a three-sided {211} pyramid with a single-atom sharpness is formed spontaneously at the tip apex by adsorbate-induced faceting. This tip is both thermally and chemically stable, and can be regenerated several ten times when accidentally damaged. We use a field ion microscope to examine the atomic structure of the tip apex layer by layer and characterize the corresponding electron emission in the field emission mode. Some properties of Ne+ ions emitted from a single-atom tip are also measured, indicating a high brightness and a small extension angle. Many desirable features make the single-atom tips very promising for future particle beam applications in nanoscience and nanotechnology.

Gas field ion source from an Ir∕W⟨111⟩ single-atom tip

We show that a thermally stable Ir∕W⟨111⟩ single-atom tip is a very good point ion source for rare gases (He, Ar) and reactive gases (H2, O2). The ion beams are emitted from the topmost atom with a very small opening angle (<1°) and, most importantly, they exhibit high brightness. In addition, the ion currents are very stable. These good properties together with the long lifetime of the tip and the reliable tip preparation method make this tip especially suitable for applications in gas field ion source focused ion beam systems.

Microscopic interfacial structures and magnetic properties of ultrathin Co∕Si(111) films

The relation between magnetic properties and microscopic structure for a metal/semiconductor system is described. Cobalt films on a CoSi interface possess an in-plane easy axis of magnetization as the result of magnetocrystalline anisotropy of the Co∕CoSi interface. On a Si(111)-7×7 surface, direct evidence for the formation of CoSi2 compounds at the interface was found by the appearance of doubled spot defects in scanning tunneling microscopic images. The interfacial effects cause the easy axis of magnetization of a Co∕Si interface to be canted out of plane.

Alloying of Co ultrathin films on Pt(111) with Ag buffer layers

The structure at the interfaces of Co/Ag/Pt(111) was studied by low-energy electron diffraction, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, and depth profiling. An atomic exchange occurs between Co and Ag before the formation of a Co–Pt alloy. Ag atoms start moving to the top at 425 K when the coverage of Co is one monolayer. The temperature of the complete exchange between Ag atoms and Co atoms is dependent on the thickness of the Ag buffer layer. The Co–Pt alloy develops after the atomic exchange is complete. The especially small surface free energy of Ag and large strain energy in this system are proposed as the driving force for the exchange.

Structure and diffusion of small Ir and Rh clusters on Ir(001) surfaces

It is known that Ir adatoms diffuse on Ir(001) surface by atomic-exchange mechanism, whereas Rh adatoms diffuse on this surface by atomic-hopping mechanism. The question is how about their clusters, and how the mechanisms can affect their diffusion behavior and energetics. Using the field ion microscope, we have measured diffusion parameters of individual Rh and Ir adatoms and small clusters on Ir(001) surfaces. We also show how the activation energy changes as a function of the cluster size and shape. From the probability of observing different atomic configurations during diffusion, different diffusion mechanisms are investigated. By considering the energetics of different atomic processes, it appears that atomic-exchange is still favored for Ir dimers. But for clusters larger than trimers, the exchange mechanism is no longer favored.

Electron and atom dynamics at solid surfaces and relation to epitaxy

Abstract At the surface, the three dimensional symmetry of a solid is broken. Electrons and atoms near the surface may rearrange to lower the free energy of the system. Scattering by defects and confinement by boundaries of electrons may produce long-range charge density oscillations. Adatoms interact with each other via mutual perturbation of the surface, known as indirect electronic and elastic interactions. These interactions are very weak and are also oscillatory. For some systems, formation of adsorption layer superstructures can be directly correlated to adatom–adatom interactions. When the temperature is raised, adatoms and admolecules can start to diffuse, interact, or react. They may aggregate into clusters and islands, and grown into a thin film. The stability of clusters may exhibit magic numbers in size and thickness. When the temperature is changed, island shape transitions may occur. The growth of islands and ultra-thin films can also be influenced by electronic effects as well as by the addition of a surfactant layer. All these growth behaviors in epitaxy can be understood from the mechanisms and energetics of elementary surface atomic processes, and atom and electron dynamics. They, in turn, can be studied in details using atomic resolution microscopy.

Magnetic Properties of Co/Si(100) Thin Films Studied using Magnetooptic Kerr Effect Technique

Magnetic properties of cobalt films grown on a Si(100) substrate were investigated using the magnetooptic Kerr effect technique. The longitudinal coercivity for the films with a thickness range from 2.4 to 17.7 nm is observed to be approximately 55 Oe. The easy axis of magnetization is in the surface plane. Because of silicide formation at the interface and a short absorption length for light in condensed matter, the Kerr intensity is not completely proportional to the film thickness. Thermal evolution for the Co/Si(100) system was systematically investigated. A higher thermal stability is concluded for a thicker cobalt film. At a low temperature, the Kerr intensity is nearly maintained constant upon annealing treatment. This is expected to be due to the Co–Si compound at the interface blocking further interdiffusion.

Initial growth of a silver thin film on a Pt(110)-(1 × 2) surface

Abstract The growth mode of a silver ultrathin film on a Pt(110)−(1 × 2) reconstructed surface was studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and ultraviolet photoelectron spectroscopy (UPS) at room temperature. The Auger uptake curve of this system shows a premonolayer break and a monolayer break with an initial linear growth. An unusual increase in intensity of integer beams of LEED was observed after silver atoms filled the missing-row sites in an out-of-phase scattering. This property can be used to calibrate the coverage of an adsorbate. UP spectra show that the d-band and the Fermi edge have some interesting changes. These results indicate that the growth of this system follows the Stranski-Krastanov mode. The growth mechanism is discussed.

Step edge diffusion and the structure of nanometer-size Ir islands on the Ir(111) surface

We report an FIM study of the structure of nanometer-size Ir islands on the Ir(111) surface. In this experiment, the number of atoms in an island is carefully controlled by field evaporation and vapor deposition. When this number can be fitted to a hexagonal atomic arrangement, the stable structure is found to be a perfect hexagon. In other cases, an addition of one ledge atom can reverse the symmetry of a small island or change its shape. We also compare diffusion of adatoms on the Ir(311) and (331) surfaces to that of ledge-atoms along the A- and B-type steps of the (111) layer, and the relative binding energies of a ledge atom at these steps.

Interaction transfer of silicon atoms forming Co silicide for Co/ 3×3R30°-Ag/Si(111) and related magnetic properties

Combined scanning tunneling microscopy, Auger electron spectroscopy, and surface magneto-optic Kerr effect studies were employed to study the microscopic structures and magnetic properties for ultrathin Co/ 3×3R30°-Ag/Si(111). As the annealing temperature increases, the upward diffusion of Si atoms and formation of Co silicides occurs at temperature above 400 K. Below 600 K, the 3×3R30°-Ag/Si(111) surface structure persists. We propose an interaction transferring mechanism of Si atoms across the 3×3R30°-Ag layer. The upward transferred Si atoms react with Co atoms to form Co silicide. The step height across the edge of the island, a separation of 0.75 nm from the analysis of the 2 × 2 structure, and the calculations of the normalized Auger signal serve as strong evidences for the formation of CoSi2 at the interface. The interaction transferring mechanism for Si atoms enhances the possibility of interactions between Co and Si atoms. The smoothness of the surface is advantage for that the easy axis of magne...