Electronic structure of Li+@C60: Photoelectron spectroscopy of the Li+@C60[PF6−] salt and STM of the single Li+@C60 molecules on Cu(111)
Graphical abstract
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.
Section snippets
Experimental and computational methods
The Li+@C60[PF6−] salt (>99%) was acquired from Idea International Inc. For the measurements of ultraviolet photoemission spectroscopy (UPS) and X-ray photoabsorption spectroscopy (XAS), the Li+@C60[PF6−] salt powder was loaded on a clean Ag foil. The experiments were performed using synchrotron radiation at the Photon Factory (BL-13B and BL-7A) and UVSOR (BL-2B) facilities. XAS at the Li K-edge was carried out in the SR-center of the Ritsumeikan University (BL-8). The vacuum evaporation of the
Results and discussion
The UPS spectra and the XAS spectra of the Li+@C60[PF6−] salt provided information about the frontier orbitals and are shown in Fig. 1(b) and (c) along with the spectra of pristine C60. In Fig. 1(b), the binding energy from the vacuum level was determined based on the high binding energy cutoff positions of the UPS spectra. The UPS spectra exhibit well-defined occupied molecular orbitals (the highest occupied molecular orbital (HOMO) and HOMO-1) of C60 in both samples. The HOMO of Li+@C60 in
Conclusion
We determined the electronic structure of Li+@C60 both in the form of a Li+@C60[PF6−] salt and as an individual Li+@C60 in the evaporated monolayer on a Cu(111) substrate. For the Li+@C60[PF6−] salt, the electric charges were located mainly on the Li atom and PF6 anion, and the C60 cage was practically neutral. We also demonstrated that Li+@C60 can be selectively deposited on the Cu(111) substrate by vacuum evaporation of the Li+@C60[PF6−] salt. During the deposition process, however, the Li
Acknowledgments
We would like to thank Prof. H. Okada (Tokyo University) and Prof. S. Aoyagi (Nagoya City University) for the fruitful discussions. This study was supported by JSPS KAKENHI Grant Numbers JP26286011, JP16K13678, and JP16H03875. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2016G539 and 2017G030), and UVSOR (Proposal No. 29-259). We thank Prof. M. Takizawa (Ritsumeikan University) and Prof. K. Amemiya (KEK) for experimental cooperation.
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