Electronic structure of Li+@ C60: Photoelectron spectroscopy of the Li+@ C60 [PF6−] salt and STM of the single Li+@ C60 molecules on Cu (111)

Abstract

We report the scanning tunneling microscope (STM) observation of the Li+ ion endohedral C60 on Cu(111), prepared by means of evaporation of a high-purity Li+@C60[PF6−] salt. The electronic state of Li+@C60 in the Li+@C60[PF6−] salt was also determined using photoemission and X-ray absorption spectroscopy, along with the density functional theory (DFT) calculations. In the salt, Li and PF6 had nearly single positive and negative charge, respectively; thus the C60 cage was practically neutral. The salt decomposed under ultra-high vacuum while heating at 400 °C. This allowed the selective deposition of Li+@C60 on Cu(111). Although secondary-ion mass spectroscopy of the deposited Li+@C60 film showed a decrease in the Li-content during evaporation, Li+@C60 was successfully identified using STM. The DFT calculations of Li+@C60 on Cu(111) suggested that the Li+ ion was singly charged and the location of the Li+ ion was displaced in an upward direction, which altered the local density of states in an upper section of C60, especially for LUMO+2. The calculated results were mostly in agreement with the bias-dependent STM and dI/dV images. However, an inconsistency was observed between the calculation and experiments in case of empty state imaging where tip-induced displacement of the Li+ ion may occur.