Hong-Nian Li

Fine structure of C60 photoionization cross-section oscillations

The synchrotron radiation angle-resolved photoemission data of K3C60 single crystal film are analyzed, aiming at the photoionization cross-sections of the LUMO, HOMO and HOMO-1 bands in low photon energy region(14.5—27.5 eV).Many fine structures of the HOMO/HOMO-1 intensity ratios are observed, whic...

Valence band of metal europium studied with synchrotron radiation photoemission spectroscopy

We have measured the photoemission spectra of Eu film prepared in an ultra-high vacuum chamber within the photon energy range from 21.2 to 177.7 eV. The 4f core level is located at ~2.1 eV below the Fermi level with some fine structures. The occupied part of the 5d6s valence band distributes between the Fermi level and ~6.5 eV binding energy. The 5d state extends from the Fermi level to the bottom of the valence band, while the 6s state has its maximum of the density of states at ~5.2 eV. The 5p–5d resonance has significant effects on the spectral lines acquired with photon energies lower than ~30 eV. The 5d photoemission is enhanced and accompanied by Auger emission around the threshold of the 5p–5d resonance.

Fullerene film on metal surface: Diffusion of metal atoms and interface model

We try to understand the fact that fullerene film behaves as n-type semiconductor in electronic devices and establish a model describing the energy level alignment at fullerene/metal interfaces. The C60/Ag(100) system was taken as a prototype and studied with photoemission measurements. The photoemission spectra revealed that the Ag atoms of the substrate diffused far into C60 film and donated electrons to the molecules. So the C60 film became n-type semiconductor with the Ag atoms acting as dopants. The C60/Ag(100) interface should be understood as two sub-interfaces on both sides of the molecular layer directly contacting with the substrate. One sub-interface is Fermi level alignment, and the other is vacuum level alignment.

Electronic structure of Eu?C70 fullerides

The electronic structure of Eu-intercalated C(70) has been studied by a synchrotron radiation photoemission spectroscopy technique. At low intercalation levels (below the stoichiometry of Eu(3)C(70)), the photoemission data clearly exhibit charge transfer from Eu 6s states to the lowest-unoccupied-molecular-orbital (LUMO) and the LUMO + 1 of C(70). The amount of charge transfer reaches its maximum far before intercalation saturation. Detailed analysis reveals that most of the 5d6s electrons of Eu occupy the so-called interstitial states in the saturation phase (Eu(9)C(70)). The interstitial states are induced by a Eu sub-lattice formed at heavy intercalation levels, and comprise substantial 6s-π hybridized states. The π states participating in the hybridization are mainly the HOMO - n (n = 6-10) orbitals. The PES data also reveal the semiconducting property for both Eu(3)C(70) and Eu(9)C(70). The 6s-(HOMO - n) hybridization and the semiconducting property should play important roles in understanding the ferromagnetic mechanism for Eu(9)C(70).

Hydrogen bonds in PC61BM solids

We have studied the hydrogen bonds in PC61BM solids. Inter-molecular interaction is analyzed theoretically for the well-defined monoclinic (P21/n) structure. The results indicate that PC61BM combines into C–H⋯Od bonded molecular chains, where Od denotes the doubly-bonded O atom of PC61BM. The molecular chains are linked together by C–H⋯Os bonds, where Os denotes the singly-bonded O atom of PC61BM. To reveal the consequences of hydrogen bond formation on the structural properties of PC61BM solids (not limited to the monoclinic structure), we design and perform some experiments for annealed samples with the monoclinic (P21/n) PC61BM as starting material. The experiments include differential scanning calorimetry, X-ray diffraction and infrared absorption measurements. Structural phase transitions are observed below the melting point. The C–H⋯Od bonds seem persisting in the altered structures. The inter-molecular hydrogen bonds can help to understand the phase separation in polymer/PC61BM blends and may be re...

Electronic states of a C70 monolayer on the surface of Ag(111)

We have investigated the electronic states of a C(70) monolayer on the surface of Ag(111) (1 ML C(70)/Ag(111)) using synchrotron radiation photoelectron spectroscopy and soft x-ray absorption spectroscopy techniques. The experimental data exhibit metallic properties and at least 2.6 e(-) charge transfer per C(70) molecule. The screening effect of Ag(111) on the electronic structure of C(70) is remarkable; it greatly reduces or even eliminates the on-site Hubbard energy. The work functions of the C(70) multilayer and monolayer are determined as 4.53 eV and 4.52 eV respectively. The energy levels of C(70) align with the Fermi level of the Ag(111) substrate, and the shift of the vacuum level caused by C(70) adsorption is negligible. Potassium doping indicates that 1 ML C(70)/Ag(111) can still accommodate about nine electrons and that the sample remains metallic at any doping level.

Valence state evolution of C60 deposited on Sm film

Ultraviolet photoemission spectra of C60 deposited on Sm film were measured. The results indicate that C60 and Sm can combine with each other to form fulleride in the interface at room temperature. The bonding is mainly ionic with some covalent contribution. Both the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 orbital of C60 are occupied at coverages less than about one monolayer. As for the second overlayer, only the LUMO orbital is occupied. The first and second C60 overlayers are metallic and semiconducting, respectively.

Metallic Rb4C60 and Rb5C60 on the top layer of C60 single crystal

Photoemission studies reveal the existences of metallic Rb4C60 and Rb5C60 surface phases on the top layer of C60 single crystal. After Rb3C60 thin film with thickness of nanometers has formed on the (1 1 1) surface layers of the C60 single crystal, the excess deposition of Rb atoms do not induce the bulk-like face-centered cubic to body-centered tetragonal or body-centered cubic structure transitions at room temperature. The large size of C60 molecule offers the surface vacancies for the formation of Rb4C60 and Rb5C60 monolayer that is further verified by Rb 3d core-level photoemission measurements. Valence band photoemission results exhibit the surface phases are metallic.

Electronic structure and orientational phase transition of K3C60

Synchrotron radiation angle resolved photoemission spectra (PES) were measured on the (111) surface of K3C60 single crystal film at three different temperatures, namely. 190. 220 and 300 K. Dispersions for the subbands of the conduction band along the [111] direction could be observed clearly at 190 K. and the band structure was nearly the same as that at 150 K corresponding to the one-dimensional-disordered orientational structure (1DDS) of K3C60, Most subpeaks smeared and the dispersions could no longer be observed when the sample temperature was raised to 220 K. The PES at room temperature differed remarkably from that at lower temperatures. The change of spectra with temperature is consistent with an orientational phase transition at 200 K. and can be explained by the theory of the fcc anti ferromagnetic Ising model, assuming that the 1DD structure at lower temperatures converts to be a mixture of bi-directional structure domains and disordered molecules at higher temperatures

Electronic states and molecular orientation of ITIC film

ITIC is the milestone of non-fullerene small molecule acceptors used in organic solar cells. We have studied the electronic states and molecular orientation of ITIC film using photoelectron spectroscopy and X-ray absorption spectroscopy. The negative integer charge transfer energy level is determined to be 4.00 plus or minus 0.05 eV below the vacuum level, and the ionization potential is 5.75 plus or minus 0.10 eV. The molecules predominantly adopt the face-on orientation on inert substrates as long as the surfaces of the substrates are not too rough. These results provide physical understanding of the high performance of ITIC-based solar cells, also afford implications to design more advanced photovoltaic small molecules.