Endohedral Fullerene Ce@C-82 on Cu(111): Orientation, Electronic Structure, and Electron-Vibration Coupling
Abstrakt
Structural, electronic, and vibrational properties of the endohedral fullerene Ce@C-82 on Cu(111) have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Ce@C-82 forms islands on the substrate.
Our STM measurements show relatively large differences in morphology and electron spectra of molecules within these islands indicating multiple molecular orientations on the substrate, while the vibrational spectra are more uniform. We have determined molecular orientations by comparing STM and OFT molecular morphology, and we have calculated Ce@C-82 bound to Cu(111) and found that it is chemisorbed.
We show that Ce@C-82 adopts orientations on the surface that enables Ce to remain at its most favorable binding site inside C-82. The effect of chemisorption on the structural and electronic properties of Ce@C-82 is thus small, and the orientations are limited to configurations with Ce in the upper hemisphere of the molecular configurational space.
We show that the variations in the dI/dV spectra between molecules of different orientations is due to Ce-cage orbitals that are localized in space and their involvement in tunneling depends on the molecular orientation on the substrate. The observed electron-vibration coupling modes in the STM-IETS (in-elastic tunneling spectroscopy) of Ce@C-82 arise from cage modes only, and therefore, electron transport properties are expected to be different compared to Ce-2@C-80, which has active Ce-cage vibrations.