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

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.