Interface Characterization of Organic Bistable Devices (OBD) Y.Gao, U. Rochester, Y.Yang, UCLA, DMR One of us (YY) observed that inserting a metal interlayer into an organic semiconductor diode produces pronounced current bistability of on/off ratio~10 6, as shown in Fig. 1(a). A current objective of the project is to understand interface effects and mechanisms of electrical bistability in such metal/organic structures. We studied the interface electronic structure of OBD using photoemission spectroscopy. We found that under device fabrication conditions, the Al interlayer is partially oxidized, and the energy levels in the organic material on both sides of the Al interlayer are aligned with each other. Fig. 1(b) shows the energy level diagram. Based on the results shown in Fig. 1a-b, we developed a model to explain OBD. As shown in Fig. 2(a), Vb, the barrier for electrons to enter the organic from the metal interlayer, is reduced by charge trapping inside the nanostructured metal interlayer. The calculated I-V curves are shown in Fig. 2(b), where the current hysteresis and sharp rise to the on-state in Fig. 1(a) are nicely reproduced. The model also qualitatively explains the capacitance increase and voltage redistribution of OBD in the on-state. (a) (b) Fig. 1 (a) The device structure and I-V of OBD. (b) Energy diagram around the metal interlayer. All energies are in eV. (a) (b) Fig. 2 (a) Schematics of model. (b) Calculated I-V curves. organic metal oxide trapped charge

Interface Characterization of Organic Bistable Devices Y.Gao, U. Rochester, Y.Yang, UCLA, DMR We studied doping effects on organic electronic structure. Shown in Fig. 3 is the ultraviolet photoemission spectra (UPS) of Au deposition on Cs doped Alq. As Cs is doped to Alq, the energy levels shift to lower kinetic energy by ~1.0 eV, and a gap state appears (marked G in Fig. 3). It is interesting that the initial deposition of 1 Å Au completely quenches the gap state. The HOMO (marked by arrows) fully recovers its position of the pristine Alq film at high Au coverages. The observations indicate that the doping induced electronic structure modification is mainly due to charge transfer, which can be reversed by charge compensation from the opposite dopant. It offers an effective method for controlling the electronic structure of organic semiconductors. The results have been published in Appl. Phys. Lett. Fig. 3. The UPS spectra evolution as a function of Au thickness deposited on to the Cs-doped Alq film. In OBD study, we found that the organic/metal-nanocluster interface plays an very important role for the electrical bistability. The metal-nano cluster have been formed by codeposition of metal and organic materials, where the best volume ratio between metal (Aluminum) and organic (AIDCN) is around 4:1. This work has been published in MRS Bulletin (2004). The organic/metal-nanocluster generally provides a high contact resistance, when charges are stored in the metal-nanocluster, the interface resistance drops tremendously because of band bending enhanced tunneling effect. As a result only one organic/metal-nanocluster interface is enough for the electrical bistability. Therefore for a regular OBD device, the met-nanocluster layer has been wired out as a third terminal, electrical bistable behavior has been observed between any pair of electrode. This may not only simplify the OBD device structure, but also double the data storage density. This work has been published in JAP (2005). Fig. 4 shows the device structure of the three- terminal OBD and typical IV characteristics (Fig.5). Top-electrode middle-electrode bottom-electrode Fig. 4. device structure of three- terminal OBD. Fig. 5.Typical I-V characteristics of 3-T OBD. G

Outreach: Y. G. participated in the Evaluation of New York State Teacher’s Certificate. The program is an 8 hour event, including evaluation of the examination questions and workshop discussions of improving physics instruction in high schools. Participants include high school teachers, college/university professors, and school administrators. Y.G also instructed children in kindergarten class in Kindercare Center on Chinese traditional art and technology. Y.Y. has volunteered as science teacher to participate in the science program for the University Elementary School. He has taught Solid-Liquid-Phase of materials, as well as arranged tours for kids to visit UCLA machine shop. Interface Characterization of Organic Bistable Devices Y.Gao, U. Rochester, Y.Yang, UCLA, DMR Education: Serkan Zorba (UR), who graduated in 2004, now accepted a tenure track assistant professor position as Whittier College. Another graduate student, Huanjun Ding, who joined the program at UR in the summer of 2004, has already a number of publications, including 2 Appl. Phys. Lett. and 1 Appl. Surf. Sci. as the 1 st author. We have also hosted two undergraduate students in the REU program, Adrew Mosher (SUNY Geneceo) and Kate Green (U. Mich.). At UCLA, Ms. Qianfei Xu obtained her PhD degree and joined Prof. Yong Chen’s group to work on nano-electronics. Through the interface engineering, Dr. Xu has demonstrated the most efficient PLED (38lm/watt). Mr. Zheng Xu joined our group a year ago and has passed his PhD prelim exam, and is currently working on organic memory and XPS/UPS study.