STRUCTURAL AND ELECTRONIC PROPERTIES OF PURE AND K-DOPED C60 ON THE PB(100) SURFACE
Open Access
- Author:
- Huang, Ying Tzu
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 06, 2016
- Committee Members:
- Renee D. Diehl, Dissertation Advisor/Co-Advisor
Renee D. Diehl, Committee Chair/Co-Chair
Eric Hudson, Committee Member
Mauricio Terrones, Committee Member
Miriam A. Freedman, Outside Member - Keywords:
- LEED
STM
alkali metals
fullerene
KxC60 - Abstract:
- In this dissertation, I present the structural and electronic properties of potassium and C60 adsorbed on Pb(100), separately, the potassium-doped C60 monolayer, and C60-doped potassium precovered layer on Pb(100) to understand the interplay between the monolayer structure, molecule-substrate interaction, and doping level, using low-energy electron diffraction (LEED), a low-temperature scanning tunneling microscope (LT-STM), and scanning tunneling spectroscopy (STS). This thesis is organized as follows. Chapter 1 gives a brief review of recent studies on C60 films. Chapter 2 is divided into three subsections: the first part shows the method to prepare a clean surface, the second part introduces the basic principles of LEED and the symmetrized automated tensor LEED (SATLEED) which used for LEED I-V analysis and the third one briefly reviews the principles of STM and STS. Chapter 3 discusses the adsorption of potassium on Pb(100). Studies of the structures of the alkali metal adsorbed on metal surfaces are the foundation for further analysis of the chemical bond and electronic structure. The top and hollow adsorption sites were found to be very common for alkali metals’ adsorption systems. LEED indicates that all submonolayer coverage was found to be the commensurate c(2×2) structure, with potassium atoms located in substitutional sites in the top of the Pb substrate layer. Due to the low vacancy formation energy of Pb, the substitutional adsorption site is more favorable than all on-surface adsorption sites, when potassium is adsorbed on the Pb(100) surface. In chapter 4, after a short review on some properties of C60 molecules, the surface structure of C60 on Pb(100) is discussed. C60 adsorption on several metal surfaces exhibits nearly close-packed hexagonal or quasi-hexagonal features, with a nearest-neighbor distance close to the natural spacing of C60 solid regardless of substrate symmetry. However, a unique commensurate structure is observed for C60 on Pb(100), in which C60 forms a c(4×4) square lattice with a nearest-neighbor distance of 9.9 Å. Based on our LEED calculations, the substitutional adsorption site, which involves a reconstruction to make C60 molecules sink deeper into the surface is more favorable than all on-surface adsorption sites. Chapter 5 describes the structural and electronic properties of potassium-doped C60 monolayer obtained by LEED, STM and STS. With different potassium doping levels, we observed that structural features changed. On the low potassium-doped C60/Pb(100) surface, C60 molecules still adopt a square lattice structure. For intermediate potassium-doped C60/Pb(100), a hexagonal-like structure with an angle of 56◦ was deduced from the LEED pattern. The reorientation and the shifting of the molecules are clearly seen in the STM images. These results suggest that potassium atoms diffuse into C60-induced vacancies, thus C60 molecules are effectively sitting on the K and not the vacancy, therefore they are not as strongly bound to the substrate, which is similar to the system of C60/Ag(111). Further, potassium-doped C60/Pb(100) seems to be a disruptive process. C60 molecules are no longer strongly bound to the substrate. Some of the C60 molecules are mobile to form a bilayer structure. By switching the order of doping, inhomogeneous structures across the surface were observed, including a square structure, a hexagonal-like structure, and a hexagonal structure with two orthogonal domains, when C60 was deposited on a lightly precovered potassium surface. This confirms that the underlying potassium atoms reduce the role played by the substrate. With different potassium doping levels, not only did the structural features change, but the electronic states of C60 molecules also varied. Based on our STS measurement, a C60 monolayer on Pb(100) does not create the optimal doping level corresponding to a C60−3 charge state which is required for superconductivity in bulk alkali-doped C60. By deliberately introducing potassium atoms into the surface, C60 molecules undergo metal-insulator-metal transition which corresponds to C60−3, C60−4 and C60−5 charge states, respectively. Although a C60−3 charge state was achieved by potassium doping, no additional feature appeared around the superconducting energy gap on the optimally doped C60 monolayer, which suggests that the formation of a Cooper pair was suppressed in a C60 monolayer. In the end, conclusions and open problems are given in Chapter 6. Finally, the Appendix A is focused on the structural determination of the high temperature phase of the Al5Co2(001) surface while the Appendix B is aimed at the (2√3×2√3) phase of Sn/Si(111) with the combination of LEED I-V analysis and STM study. Furthermore, the coordinates of the proposed (2√3 × 2√3) structure models are given in Appendix C.