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Investigations structural transitions of the cobalt on the Si(111) and Ge(111) surfaces and dicarboxylated viologens on the Cu(100) surface

By 蔡松麟 and Sung-Lin Tsay

Abstract

[[abstract]]In this thesis, the investigations include two parts. The first part (Part I) is the research of the magnetic material cobalt on the Si(111) and the Ge(111) surfaces dependent on annealing temperatures studied by ultra high vacuum-scanning tunneling microscopy (UHV-STM) and low energy electron diffraction (LEED). The second part (Part II) is the research of the surface structures and phase transitions of viologens on the Cu(100) and HOPG surfaces dependent on the applied potentials studied by cyclic voltammetry (CV) and electrochemistry scanning tunneling microscopy (EC-STM). The aims of both subjects are focused on the observation of the surface structure and morphology at stable thermal/kinetics conditions. There are two major topics in part I. The first topic describes the Co absorbed on the pure Si(111)-7×7 and Ge(111)-c(2×8) surfaces. The initial reaction of the Co with the Si substrate happens at the temperature range from 126 to 130K. The defects of the Co-Si compounds are different from the intrinsic defects of the Si(111)-7×7 surface. The Co-Si compounds also decrease the brightness of the neighboring adatoms compared to the intrinsic defects of the Si(111)-7×7 surface. Therefore, the Co on the pure Si(111)-7×7 and Ge(111)-c(2×8) surfaces forms the Co-Si and the Co-Ge compounds at room temperature appeared as the dark region of the defect-like feature. The silicon atoms can separate on top of the Si(111)-7×7 surface after annealing to 400oC, and then the Si(111)-7×7 surface structure disappears. The Co5Ge7 alloy is observed on the Co/Ge(111) surface after annealing to 600K. Further, Co atoms can form the √13×√13 R14° periodic surface structure, but the structure is unfavorably formed compared to the Co5Ge7 alloy. The compound formations of the Co-Si and the Co-Ge result in a lower magnetic property than bulk Co. Therefore, the silver buffer layer is introduced on the intermediate layer between the Co and the Si(111) and Ge(111) surfaces as described at the second topic. At low Co coverage, the Co can form periodic surface structures of the √13×√13 R14° and the 2×2 on the Ag/Ge(111)-√3×√3 surface. For the Co/Ag/Si(111) case, the Co forms a cluster shape both on the Ag/Si(111)-√3×√3 and on the flat Ag/Si(111)-1×1 surfaces at low Co coverage. Further, the average size and height of Co clusters on the flat Ag/Si(111)-1×1 surface are almost independent on annealing temperatures from room temperature to 300oC. The reasons for the Co/Ag/Si(111) surface without periodic surface structure are due to the unsaturated states on the Ag/Si(111)-√3×√3 surface and the weaker interaction of the Co with the Si(111) surface than the Ge(111) surface. Dicarboxylated viologens mixed with a 10 mM HCl on the Cu(100) and HOPG surfaces was studied in different redox states. At the beginning, a 0.1 mM violgens on the Cu(100) surface is investigated. The dicationic viologens show the dot array and the oblique row phases. The radical viologens exhibit the metastable phases, a stripe pattern, the closed stacking stripe pattern, and a dimer phase. The stacking configuration of the dicationic viologen core plane is preferred to be face-on on the surface and that of the radical viologen is formed by π-π stacking with the neighboring viologens. Because dicarboxylated viologens bear long alkyl chains and carboxylic acid groups at the ends of the alkyl chains as illustrated by (HOOC-(CH2)7-V-(CH2)7-COOH), the complex interactions are considered to be the reason of forming various phases on the Cu(100) surface. The experiment of a 0.1 mM dicarboxylated viologens on the HOPG surface without the influence of an anion layer is to confirm the existence of a bilayer formation due to the intermolecular interaction of the hydrogen bonding. The high viologen concentration (1.0 mM) on the Cu(100) surface shows the effects of the chloride and bromide anion layers reflecting on phase transition and the multilayer growth behavior due to the hydrogen bonding interaction and the polarizability, respectively. The effect of the anion layers is consistent with by a 0.1 mM viologens mixed with a 10 mM KBr on the Cu(100) surface.

Topics: 真空掃描電子隧道顯微鏡;電化學掃描電子隧道顯微鏡;鈷;雙羧基紫精;表面結構的變化, UHV-STM;EC-STM;Co;Dicarboxylated viologens;Structure transition, [[classification]]38
Year: 2011
OAI identifier: oai:ir.lib.ntnu.edu.tw:309250000Q/70577
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