Atomic-scale investigation on the electronic properties of graphene using LT-STM

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Atomic-scale investigation on the electronic properties of graphene using LT-STM
Jung, Minbok
Shin, Hyung-Joon
Graphene; STM; STS; Fullerene Anion; Jahn-Teller effect/distortion; Action Spectroscopy
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Graduate School of UNIST
Part 1: Spatially resolved scanning tunneling spectroscopy (STS) for a specific position implies an averaging electronic structure of surface and bulk state. As graphene has an electronic transparent property, the STS spectra on graphene/Cu(111) shows a shifted Shockley surface and bulk state scattering of Cu(111). Here we found that spectra acquired on single layer graphene (SLG) on Cu(111) exhibit complicated scattering traces including intravalley, intervalley, and interband scattering. By applying one-dimensional Fourier transforms to hundreds of STS spectra, we successfully deconvoluted multiple scatterings between graphene and Cu(111). Although SLG has no intravalley scattering due to the pseudospin, our results show singularities in the defect-induced intravalley response near the Γ point similar to theoretical calculations. In this study, we will discuss not only the individual scattering processes, but also what successive STS spectra imply about the STS line orientation. Part 2: A detailed understanding of interactions between molecules and graphene is one of the key issues for tailoring the properties of graphene-based molecular devices, because the electronic and structural properties of molecular layers on surfaces are determined by intermolecular and molecule?substrate interactions. Here, we present the atomically resolved experimental measurements of the self-assembled fullerene molecules on single-layer graphene on Cu(111). Fullerene molecules form a (4 × 4) superstructure on graphene/Cu(111), revealing only single molecular orientation. We can resolve the exact adsorption site and the configuration of fullerene by means of low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) calculations. The adsorption orientation can be explained in terms of the competition between intermolecular interactions and molecule?substrate interactions, where strong Coulomb interactions among the fullerenes determine the in-plane orientation of the fullerene. Our results provide important implications for developing carbon-based organic devices using a graphene template in the future. Part 3: The C60 molecular ions are ideal candidates for analyzing Jahn-Teller effect because the large size of molecule (1 nm) makes it easily discernible through STM. High degeneracies in the electronic and vibrational states are changed by the Jahn-Teller distortion, producing splitting in the electronic states. In this chapter, we will consider the electric field-induced ionization of individual fullerene molecules by the resonance at the image-potential state. And then, we will try to take the JTE to explain how newly generated electronic high structural symmetries are closely linked to electronic degeneracies in electronic energy levels. Breaking the structural symmetry can result in breaking the electronic degeneracy. The C60 anion with degenerated orbitals can lower the energy of occupied electronic levels while raising the energy of empty states with structural distortion, reducing the total energy. Then, we will address why fullerene molecules can be modified, what is the mechanism, and how it is stable by means of fundamental physical theories related to image-potential state, ionization, and the Jahn-Teller effect.
Department of Materials Science Engineering
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