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.

Introduction

The endohedral doping is one of the most investigated aspects of the physics and chemistry of fullerenes because it causes the modification of electronic properties without altering the shape of the fullerene cage [[1], [2], [3]]. However, the low synthetic yields and the enhanced reactivity of the endohedral fullerenes hampers their large-scale production with the adequate purity. This has been particularly observed for metal-endohedral C₆₀ [[1], [2], [3]], including the pioneering investigation of Li@C₆₀ [4]. Recently, a new family of the endohedral C₆₀, namely, metal-ion endohedral C₆₀ (M⁺@C₆₀) has emerged, and an industrial technique for the large-scale synthesis of M⁺@C₆₀ has been developed. A stable compound containing high-purity Li⁺ ion endohedral C₆₀ (Li⁺@C₆₀, >99%) was successfully synthesized by an ion implantation technique called “plasma shower,” and by forming the salt using counter anions such as SbCl₆⁻ and PF₆⁻ [[5], [6], [7]]. The crystalline structure of the Li⁺@C₆₀[PF₆⁻] salt was determined via synchrotron X-ray diffraction, deducing the rock-salt type structure as depicted in Fig. 1(a). Several studies have already reported unique features of the Li⁺@C₆₀ salts, such as the high ionic conductivity in solution, particular optical properties, and enhanced chemical reactivity [[8], [9], [10], [11]]. One of the most important features of Li⁺@C₆₀, which makes it a promising candidate to be applied in the field of organic electronics, is the stabilization of the orbital energy levels of C₆₀ due to the presence of the Li⁺ ion. This causes an enhanced electron-accepting character as compared to that of pristine C₆₀, which is known to be one of the few organic semiconductors that show strong electron accepting (n-type) properties. Such characteristics were recently highlighted in the development of Li⁺@C₆₀ dye-sensitized solar cells with high efficiency [[12], [13], [14], [15]]. Contrastingly, for Li@C₆₀, a significant charge transfer from Li to the C₆₀ cage is expected that causes a small ionization potential [16]. Another important feature of M⁺@C₆₀ is that they can be applied as molecular switches [[17], [18], [19]]. It has been shown that a fraction (approximately 25%) of an external electric field can penetrate into the C₆₀ cage despite efficient screening by the π electrons [20]. Then the field can alter the state of the endohedral atoms or molecules if they are charged or polarized [[17], [18], [19]]. Therefore, it is of considerable importance to completely understand the electronic states of Li⁺@C₆₀ on the metal substrate surfaces in order to control the carrier injection and transport properties for organic electronic devices, and to explore further functionalization for switching applications. Even though a relatively large number of studies related to the applications of Li⁺@C₆₀ and its salt have been reported, their electronic states have not been studied in detail and the information about the electronic structure of Li⁺@C₆₀ on metal substrates is scarce despite its significance.

In this study, the electronic structure of Li⁺@C₆₀ adsorbed on the surface of Cu(111) was determined using the molecular-resolution scanning tunneling microscope (STM) measurements. The sample was prepared using the temperature-dependent evaporation of the Li⁺@C₆₀[PF₆⁻] salt in ultra-high vacuum (UHV). Prior to conducting the STM studies, we also characterized the electronic structures of Li⁺@C₆₀ in the salt. From the film, we were able to identify a small amount of Li⁺@C₆₀ molecules by STM. The electronic structure of Li⁺@C₆₀/Cu(111) was examined with STM, and their results showed a good agreement with the results of the density functional theory (DFT) calculations. However, there were quantitative discrepancies in the empty electronic state that may be caused due to the electric field-induced phenomena during the STM measurements.