Li Hong-Nian

Electronic structure of PCBM

We have studied the electronic structure of [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) using synchrotron radiation photoelectron spectroscopy (PES) measurements and first-principles calculations. The PES spectrum of the entire occupied valence band is reported, which exhibits abundant spectral features from the Fermi level to ~ 24 eV binding energy. All the spectral features are broadened as compared with the cases of C60. The reasons for the broadening are analysed by comparing the experimental data with the calculated energy levels and density of states. Special attention is paid to the analysis of the C60 highest occupied molecular orbital (HOMO)-1 derived states, which can play a crucial role in the bonding at the interfaces of PCBM/polymer blenders or PCBM/electrodes. Besides the well-known energy level splitting of the C60 backbone caused by the lowered symmetry, C 2p states from the side chain mix or hybridize with the molecular orbitals of parent C60. The contribution of the O 2p states can substantially modify the PES spectrum.

Pits and adatoms at the interface of 1-ML C84/Ag (111)

We prepare a well-defined C84 monolayer on the surface of Ag (111) and study the geometric structure by scanning tunneling microscopy (STM). The C84 molecules form a nearly close-packed incommensurate R30° lattice. The lattice is long-distance ordered with numerous local disorders. The monolayer exhibits complex bright/dim contrast; the largest height difference between the molecules can be greater than 0.4 nm. Annealing the monolayer at 380 °C can desorb part of the molecules, but more than sixty percent molecules stay on the Ag (111) surface even after the sample has been annealed at 650 °C. Our analyses reveal that the 7-atom pits form beneath many molecules. Some other molecules sit at the 1-atom pits. Ag adatoms (those removed substrate atoms, accompanying the pit formation) play a very important role in this system. The adatoms can either stabilize or destabilize the monolayer, depending on the distribution manner of the adatoms at the interface. The distribution manner is determined by the co-play of the following factors: the dimension of the interstitial regions of the C84 overlayer, the number of the adatoms, and the long-distance migration of part adatoms.

Aggregation Behavior of Metal-Ethylene Complexes and Its Effect on Hydrogen Storage Capacity *

The aggregation of Li, Ti decorated ethylene molecules and its effect on hydrogen storage capacity have been thoroughly studied using a first-principles calculation. The results indicate that the aggregation of C2H4-metal complexes substantially impacts on the hydrogen storage capacity. The aggregation of C2H4-Li reduces the hydrogen storage capacity greatly and makes the hydrogen storage capacity of C2H4-Li complexes hardly meet the demands of the U.S. DOE. The C2H4Ti2 dimers can uptake hydrogen as much as 10 wt%, which suggests that the C2H4Ti2 dimer can be a promising candidate for practical applications.

Electronic Structure of Eu6C60

We study the valence band of Eu-intercalated C60 by synchrotron radiation photoelectron spectroscopy to understand the ferromagnetism (FM) and the giant magnetoresistance (GMR) of Eu6C60. The results reveal the semiconducting property and the remarkable 5d6s-π hybridization. Eu-C60 bonding has both ionic and covalent contributions. No more than half the 5d6s electrons transfer from Eu to the LUMO derived band of C60, and the LUMO+1 derived band is not filled. The remaining valence electrons of Eu, together with some π (LUMO, HOMO and HOMO-1) electrons, constitute the covalent bond. The electronic structure implies that the magnetic coupling in Eu6C60 should be through the intra-atomic f-sd exchange and the medium of the π electrons. The possibility of the GMR being tunnelling magnetoresistance is ruled out.

Preparation and photoemission spectra of Rb3C60 single-crystal thin films

Rb3C60 single-crystal thin films were prepared on the cleaved (111) surface of C60 single crystal. The photoemission spectrum line shapes of the lowest unoccupied molecular orbital (LUMO) derived band at room temperature and 150 K were established by synchrotron radiation photoemission spectrum measurements. The density of states near the Fermi level was distinctly affected by temperature. No less than six sub-peaks of the LUMO band were observed even at room temperature. The existence of so many sub-peaks offered the opportunities to analyse in more detail the orientational structure and the electron-Boson interactions of the narrow-band metallic Rb3C60.

Influence of high pressure on emission spectra and crystal field strength for Eu3+ in GdOBr

The emission spectra of the Eu3+ ion in GdOBr:Eu3+ were measured in the range of pressure from 0 GPa to 12.4 GPa. The energy level schemes established from the spectra show that the splittings of the 7F1, 7F3 and 5D1 multiplets decrease obviously with increasing pressure. The crystal-field parameters Bqk and the crystal-field strength parameter S versus Bqk were determined at different pressures. With pressurization S reduces and the J-mixings of the state functions for the Eu3+ ion decrease, which is in agreement with the decrease of the multiplet-splittings. This shows that for Eu3+ in REOX: Eu3+ the diminution of the crystal-field strength with pressure, the authenticity of which is under suspicion for several years, is certain.

Electronic States of IC60BA and PC71BM

We investigate the electronic states of IC60BA and PC71BM using first-principles calculations and photoelectron spectroscopy (PES) measurements. The energy level structures for all possible isomers are reported and compared with those of C60, C70 and PC61BM. The attachment of the side chains can raise the LUMO energies and decrease the HOMO-LUMO gaps, and thus helps to increase the power-conversion efficiency of bulk heterojunction solar cells. In the PES studies, we prepared IC60BA and PC71BM films on Si:H(111) substrates to construct adsorbate/substrate interfaces describable with the integer charge-transfer (ICT) model. Successful measurements then revealed that one of the most important material properties for an electron acceptor, the energy of the negative integer charge-transfer state (EICT−), is 4.31 eV below the vacuum level for PC71BM. The EICT− of IC60BA is smaller than 4.14 eV.

Charge transfer of 1 ML C60/Ag(100)

We have measured the synchrotron radiation photoelectron spectra of monolayer C60 on Ag(100). By calculating the intensity ratios between the LUMO bands and the two deeper bands (HOMO and HOMO-1), we estimate the amount of the charges transferred from Ag(100) to C60 within the range of 1 e to 1.8 e. The results dismay the expectation of surface superconductivity and afford a good reference for further studies of the monolayer C60/Ag(100) system.

Fine structure of K3C60 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, which support the general validity of a quantum chemistry model. However, the experimental data and the theoretical results have some disagreement in the energy positions and magnitudes of the fine structures, and the disagreement is more outstanding than the case of pure C60. So the photoemission data of K3C60 afford valuable experimental reference to further theoretical developments. The LUMO cross-section data, which cannot be discussed with the quantum chemistry description due to strong solid state effects, are also reported for further studies.

Synchrotron radiation photoemission spectrum study on K 3 C 60 film

K3C60, single crystal film was prepared on the cleaved (111) surface of C60, single crystal. Synchrotron radiation angle-resolved photoemission spectra were measured at normal emission with sample temperature at × 150K. Up to four subpeaks of LUMO-derived band were observed. These sub-peaks exhibit distinct energy dispersions which resemble in general the theoretical ones calculated for K3C60 at low temperature with the so-called one-dimensional disordered structure. But there is large deviation of experimental sub-band intervals from the theoretical values. This result is meaningful for the studies of the physical properties of alkali-doped C60 solids, e.g. the mechanism for superconductivity.