Electronic Properties of Thin Film Organic Superconductors studied using Synchrotron Radiation-based Soft X-Ray Spectroscopies Kevin E. Smith, Boston University, DMR Figure 1: O K-edge SXE spectra reflecting the occupied O 2p partial density of states (PDOS) in VO-Pc, V 2 O 3, and molecular O 2. Also included are the results of a DFT calculation. The O 2p PDOS in VO-Pc bears little resemblance to that from the crystalline 3d 1 vanadate, but is remarkably similar to that obtained from gas phase molecular O 2. This measurement is consistent with the premise that the vanadyl species is spatially isolated in VO-Pc thin films, and is strong evidence for the lack of significant intermolecular interaction. The goal of this program is to use synchrotron radiation-based x-ray spectroscopies to measure the electronic structure of organic superconductors. The spectroscopies used are high resolution soft x-ray emission (SXE), resonant inelastic x-ray scattering (RIXS), high resolution photoemission, and soft x-ray absorption. Figures 1 and 2 show SXE and RIXS spectra from thin films of the organic semiconductor, vanadyl phthalocyanine (VO-Pc). Doped phthalocyanines are predicted to be highly correlated solids, with unusual conductivity properties. For VO-Pc we proved the vanadyl species to be highly localized, and good agreement between the measurements and a density functional calculation is obtained. Both dipole forbidden V 3d to V 3d*, and O 2p to V 3d* charge transfer transitions are observed. These results have recently been published: Y. Zhang, S. Wang, T. Learmonth, L. Plucinski, A.Y. Matsuura, S. Bernardis, C. O'Donnell, J.E. Downes, and K.E. Smith, Chem Phys. Lett. 413, 95 (2005)

Educational Activities: This research program involves two graduate students (Yufeng Zhang and Jason St. John) and one postdoctoral research associate (Dr. Shancai Wang). Note also that the PI has finished a two year appointment as the first Academic Director of the Center for Excellence in Teaching at Boston University. Furthermore, the PI was named the 2001 Massachusetts Professor of the Year by the Carnegie Foundation. Infrastructure Impact: This program uses a custom designed ultra high vacuum organic molecular beam deposition chamber, attached to our multi-technique spectrometer system based at the National Synchrotron Light Source (NSLS). The spectrometer system features both a high resolution angle resolved photoemission spectrometer (100 mm Scienta), and a high resolution soft x- ray emission spectrometer. This is a unique combination of growth and characterization capabilities, and is available to other scientists through the General User Program at the NSLS. Electronic Properties of Thin Film Organic Superconductors studied using Synchrotron Radiation-based Soft X-Ray Spectroscopies Kevin E. Smith, Boston University, DMR Figure 2: Series of V L 3 -edge RIXS spectra from VO-Pc. The spectra are plotted on an energy loss scale relative to the elastic peak. The elastic emission feature lies at 0 eV, and is observed to resonate as the excitation energy is tuned through the L 3 V 2p 3/2 edge. The remaining two features are centered at 2.2 eV (“A”) and 5.6 eV (“B”) in the spectrum recorded with the lowest exciting photon energy, eV. As the excitation energy is increased, these two features disperse to higher loss energies. These features are identified as dipole forbidden V 3d to V 3d* transitions, and O 2p to V 3d* charge transfer transitions.

Organic superconductors have been the subject of intense study for many years due both to the challenge they pose to our understanding of the physical properties of complex solids and due to technological interest in developing advanced carbon-based electronic devices. There is a serious lack of detailed spectroscopic measurements of the electronic structure, primarily due to severe radiation damage problems, and difficulties in preparing ordered and clean surfaces. This program uses a powerful combination of high resolution synchrotron radiation-based soft x-ray spectroscopies to measure the electronic structure of organic superconductors. We intend to solve the beam damage and surface problems associated with earlier studies of single crystals by growing thin films of these materials in-situ, and continuously translating the films in front of the synchrotron radiation beam as spectra are recorded. Program goals are to develop organic molecular beam deposition methods for the in-situ synthesis of ordered thin films of these superconductors and to make definitive measurements of the electronic properties of the films. This project will have a significant impact on the education of students and postdoctoral research associates by exposing them to a highly collaborative research environment at national user facilities. They will be educated in the physics of novel organic materials, and in the application of synchrotron radiation spectroscopies in their study. Electronic Properties of Thin Film Organic Superconductors studied using Synchrotron Radiation-based Soft X-Ray Spectroscopies Kevin E. Smith, Boston University, DMR