1. “Electron dynamics in organic semiconductors and at organic metal interfaces” Xiaoyang Zhu, University of Minnesota, NSF DMR-0238307 March 1 st, 2003 - February 28 th, 2006 Considerable efforts are underway to develop organic semiconductors as viable alternatives to amorphous silicon for thin film electronics. The figure of merit in these materials is the charge carrier mobility. Besides structural factors, such as defect density and domain boundaries, carrier mobility is ultimately determined by the electronic band structure and the strength of electron-nuclear coupling responsible for carrier localization. The central scheme of this proposal is to use femtosecond two-photon photoemission spectroscopy (2PPE), in conjunction with scanning tunneling microscopy (STM) and attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to establish electron dynamics in organic semiconductor crystals, thin films, and at their interfaces with metals, and to probe structure-property relationship in these systems. Source Drain Gate Dielectric Pentacene Organic thin film transistor

2. Source of shallow trap states in pentacene thin films Left: STM image of local defects in a crystalline domain of pentacene. These molecules are arranged with their long molecular axis nearly perpendicular to the surface. The sliding of pentacene molecules along their long molecular axis gives rise to these defects. Right: Band structure calculation shows that these defects give rise to shallow traps (≤ 100 meV), in agreement with transport measurements. Among the most promising organic semiconductors is pentacene, which has shown the highest charge carrier mobility in OTFTs. We have discovered for the first time a predominant type of defects intrinsic to pentacene thin films. Band structure calculation confirms that these defects are shallow charge carrier traps observed in transport measurements. D. Silva Filho J.-L. Brédas, NSF-MRSEC

3. Probing molecular structure at burier interfaces in molecule-based electronics. Charge transport at or across interfaces is central to the operation of a wide variety of molecule-based devices, including organic light-emitting diodes (OLED), organic thin film transistors (OTFT), organic photovoltaic (OPV) cells, and nanoscopic molecular junctions based on single molecules or a small assemly of molecules. In each of these devices, the critical charge transporting interfaces are buried interfaces (e.g., organic-insulator, organic-metal, or organic- semiconductor heterojunctions), which are not readily accessible to conventional structural or spectroscopic probes. With partial support from this grant, the PI has successfully developed attenuated total-internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to measure the vibrational spectrum (and thus the structure and conformation) of the molecules inside metal-molecule-semiconductor or metal- molecule-metal sandwich structures. This approach can be used to probe molecular tunnel junctions (scheme A) or operating TFTs (scheme B).

4. Broader impact and outreach activities With supplemental fund from the NSF-DAAD international exchange program, the PI has collaborated with Prof. Martin Wolf’s group in Free University-Berlin on Si-based molecular electronics. The PI has hosted the summer research visits of one faculty member (Prof. Shoshana Coon) and one student (Ms. Davenne Mavoure) from an undergraduate institution (University of Northern Iowa). The PI has advised the research of seven undergraduate students (two minority students) during this grant period. The PI has presented a seminar to the 3M Company in St. Paul and a seminar to students and faculty members in the IGERT program on nanoparticle science and engineering at Minnesota. The PI has presented 20 invited talks and lectures on NSF funded projects at professional meetings or academic institutions during this grant period. The PI is organizer of “Symposium on Interfaces in Organic & Molecular Electronics” at the 2005 MRS Fall Meeting. The PI was co-organizer of the “Symp. on Phys. Chem. of Interfaces and Nanomaterials III”.