Angle-resolved photoelectron spectroscopy and scanning tunnelling spectroscopy studies of the endohedral fullerene Li@C₆₀

Abstract

Angle-resolved photoelectron spectroscopy and scanning tunnelling spectroscopy studies of the endohedral fullerene Li@C 60 .Gas phase photoelectron spectroscopy, TD-DFT calculations and low temperature UHV STM experiments are combined to provide information about the infl uence of the encapsulated Li on the symmetry and energetics of the low lying diff use Super-Atom Molecular Orbitals (SAMOs) of Li@C 60 . PAPER Yao Cai, Yongxin Pan et al. Positive magnetic resonance angiography using ultrafi ne ferritin-based iron oxide nanoparticles Nanoscale rsc.li/nanoscale Volume Gas phase photoelectron spectroscopy (Rydberg Fingerprint Spectroscopy), TDDFT calculations and low temperature STM studies are combined to provide detailed information on the properties of the diffuse, low-lying Rydberg-like SAMO states of isolated Li@C 60 endohedral fullerenes. The presence of the encapsulated Li is shown by the calculations to produce a significant distortion of the lowest-lying S-and P-SAMOs that is dependent on the position of the Li inside the fullerene cage. Under the high temperature conditions of the gas phase experiments, the Li is mobile and able to access different positionswithin the cage. This is accounted for in the comparison with theory that shows a very good agreement of the photoelectron angular distributions, allowing the symmetry of the observed SAMO states to be identified. When adsorbed on a metal substrate at low temperature, a strong interaction between the lowlying SAMOs and the metal substrate moves these states to energies much closer to the Fermi energy compared to the situation for empty C 60 while the Li remains frozen in an off-centre position. † Electronic supplementary information (ESI) available: Temperature-dependent mass spectrometry, laser power ionisation dependence and STM imaging details.

Interfacial Energetic Level Mapping and Nano-Ordering of Small Molecule/Fullerene Organic Solar Cells by Scanning Tunneling Microscopy and Spectroscopy

“…A displacement to the left of the LDOS spectra indicates an electron-conducting (n-type) semiconductor, meanwhile a displacement to the right indicates hole conducting (p-type) semiconductor behavior, with the specific HOMO and LUMO levels located at negative and positive bias voltage, respectively [16,21,22]. By using a cross-sectional scanning tunneling microscope (XSTM) in UHV, Shih et al [16] reported an E g = 2.3 eV for PCBM (TA = 150 • C) with HOMO and LUMO levels located at −1.6 V and 0.7 V, respectively; from Figure 4c, the HOMO level is located at −1.65 V and the LUMO level at 0.76 V for PCBM (TA = 120 • C) leading to an average E g = 2.1 eV, which is consistent with the literature reports [16,34]. Average energy levels from Figure 4c are plotted in Figure 4d.…”

“…• C (open blue diamonds), show a better energy match, which could facilitate the charge transfer between D-A and be manifested as an improvement in the charge extraction [34,35] and, in an enhanced PV device performance.…”

Sublimation of Li@C60

“…Laser-desorption FTICR mass spectrometry of the starting material in both the positive and negative mode showed only minor traces of C 60 with the non-fullerene content being predominantly the stabilising anions and solvent residues. As we have shown previously [8,9] neutral Li@C 60 is evaporated from the oven along with impurities and some C 60 . The material is typically heated to a temperature of ca.…”

“…The material is typically heated to a temperature of ca. 590 K for at least 20 h to remove solvent residue and other impurities before gas phase experiments such as photoelectron spectroscopy [8] are carried out. The neutral species are ionised with an ultrashort pulse laser in the extraction region of a simple linear Wiley-MacLaren time-of-flight mass spectrometer.…”

“…We have recently combined gas phase studies using mass spectrometry and photoelectron spectroscopy with scanning tunnelling microscopy to obtain insight into the socalled Super-Atom Molecular Orbital states of the molecule [8] and showed that it is possible to demonstrate multistate switching in a single Li@C 60 molecule with 14 distinct switching states [9]. These experiments required the preparation of a clean beam of Li@C 60 molecules (containing some proportion of empty C 60 ) that was prepared by heating the commercial [Li + @C 60 ]PF − 6 material in a quartz ampoule at elevated temperatures.…”