Shibuta Masahiro, Yamamoto Kazuo, Ohta Tsutomu, Nakaya Masato, Eguchi Toyoaki, Nakajima Atsushi
Keio Institute of Pure and Applied Science (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
Sci Rep. 2016 Oct 24;6:35853. doi: 10.1038/srep35853.
Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C 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 sp-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C film: after hot excitons are nascently formed in C 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 C film into the HOPG is evaluated to be 1.31 × 10 s, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C. 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 C layers is 3.3 ± 1.2 × 10 s.
时间分辨双光子光发射（TR - 2PPE）光谱被用于探测在高度取向热解石墨（HOPG）上形成的C富勒烯薄膜的电子态，HOPG作为多层石墨烯的二维（2D）模型材料。由于HOPG的面内sp杂化性质，TR - 2PPE光谱揭示了C薄膜中光载流子的能量和动力学：在通过泵浦光子的分子内激发在C中新生热激子后，它们解离为自由电子和相应空穴的光载流子，随后电子被探测光子作为光电子检测到。从C薄膜到HOPG的光载流子衰减率估计为1.31×10⁸ s⁻¹，表明界面处存在弱范德华相互作用，光载流子暂时占据C的最低未占据分子轨道（LUMO）。对于与覆盖层表现出强相互作用的任何金属衬底，尚未实现有机薄膜中热激子解离后的光载流子电子动力学。此外，LUMO中电子寿命的厚度依赖性表明C层中的电子跳跃率为3.3±1.2×10⁸ s⁻¹。
