Direct observation of photocarrier electron dynamics in C60 films on graphite by time-resolved two-photon photoemission

Abstract

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C60 fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp2-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C60 film: after hot excitons are nascently formed in C60 via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C60 film into the HOPG is evaluated to be 1.31 × 1012 s−1, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C60. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C60 layers is 3.3 ± 1.2 × 1013 s−1.

Strong modification of the transport level alignment in organic materials after optical excitation

“…The optical excitation leads to an instantaneous formation of an excitonic state (LUMO+1* exciton, brown curve) which decays within τ LUMO+1,decay = 45 ± 10 fs into the next lower lying excitonic level (LUMO* exciton, red curve). A similar fast decay of the LUMO+1* level has also been observed by Shibuta et al for a C 60 monolayer on highly oriented pyrolytic graphite 46 . The redistribution of spectral intensity between the LUMO+1* and the LUMO* states in our case occurs on identical timescales ( τ LUMO+1*,decay = τ LUMO*,rise ) leading to the shift in spectral intensity discussed before.…”

“…Figure 3a shows the general trend of the optically induced band structure dynamics of the occupied molecular levels by displaying selected EDCs for characteristic time steps of the previously observed exciton dynamics in the excited states. First, before optical excitation ( t = −500 fs), the quasi-static spectral lineshape of the HOMO level as well as the lower lying molecular orbitals (HOMO-X) reflects perfectly the spectroscopic findings of previous studies of C 60 on various surfaces 20,46,48 . Upon optical excitation and creation of excitons, however, the linewidth of all molecular features increases before transforming back to its original state.…”

Ultrafast Multiphoton Thermionic Photoemission from Graphite

“…Further enhancement of photosensitized processes is likely for molecular acceptor states, which lie within the wide σ and π gaps of graphitic surfaces, because quenching of molecule-localized resonances by the reverse charge transfer to resonant states with large k ∥ would be suppressed [97,98]. This scenario is supported by a recent 2PP study of slow relaxation of anion states of C 60 on graphite [100]. Thus, we predict that the strongly enhanced hot electron generation and slow molecular resonance relaxation are likely to promote efficient hot electronmediated surface femtochemistry on graphitic surfaces.…”

Energy Level Alignment at the C60/Monolayer‐WS2 Interface on Insulating and Conductive Substrates

“…4.0 eV. [ 57 ] As displayed in Figure 3b,3e, upon C 60 deposition the ARUPS spectra taken at the Γ‐point for both substrates show an increasing intensity on the high binding energy side of the sharp WS 2 peak. This emission can readily be attributed to stem from the C 60 highest occupied molecular orbital (HOMO) level.…”