Position-Dependent Properties of Novel Electronic Materials: Tunneling, Infrared, and Electromechanical Measurements

Over the past decades, there has been an explosion of interest in using new electronic materials to replace conventional semiconductors in devices ranging from flexible displays to electromechanical actuators. The application of many of these materials faces not only large technical hurdles but questions in our understanding of their basic conducting and mechanical properties. It is the goal of the proposed research to shed light on many of these problems through measurements of electron tunneling, infrared spectra, response times, and other properties. In particular, the P.Ls of the proposal have developed new tunneling and IR probes that allow these properties to be studied as functions of position in a sample or device. which is important for applications where properties may vary with distance from electrical and/or mechanical contacts. Position dependent tunneling measurements will be done with a scanning tunneling microscope with a "Iong-range positioner" developed in the lab of one of the P.Ls. Use of this coarse position adjustment will allow spectra to be taken on positions as much as 3 millimeters apart without remounting or otherwise repositioning the sample, especially important for cryogenic experiments. Position dependent infrared measurements will be made using a commercial infrared microscope. not only coupled to a Fourier transform infrared spectrometer, as is commonly done, but with tunable infrared diode lasers, which allows detailed measurements at fixed wavelength as functions of frequency (i.e. dynamic response) as well as position. voltage, and temperature. Experiments are planned on two types of materials. Screening-charge and other interface effects near electrical contacts will be measured, using tunneling spectra and electro-optic response, in semiconducting organic thin films, e.g. ofpentacene derivatives, which are of interest for applications in field effect transistors. Position dependent tunneling measurements will also be made on the charge-density-wave conductor blue bronze to search for current induced intragap states, believed responsible for current conversion at the contacts. Tunneling, infrared, and electro-optic measurements will also be used to search for position dependent properties that may be associated with the unique current induced twist that has recently been discovered in the charge-density-wave state of TaS, This torsional effect, with promise for application in micro- and nano-actuators, can also be studied directly with a sensitive helical resonator displacement detector developed in the P.L's laboratory. Broader Impact: Both P.Ls of the proposal have a long history of working closely with graduate students in their research in experimental condensed matter physics and preparing them for a variety of careers in industry, academia, and research labs. Two graduate students will work throughout the academic year on this project; both students will get experience working with both PIs, and hence obtain a broad background in cryogenic, optical, transport, vacuum, chemical synthesis, and thin film techniques. These students will also regularly give departmental seminars and attend national and international meetings. In addition, undergraduate students, both physics majors and education majors, will work in the laboratories in summers. For the education majors, the intent will be for these students to work closely with the other students in the labs, to get a better understanding and appreciation for the scientific process that they can share with their own future pupils. They will also be expected to prepare material (e.g. slide shows, small experiments) that they can use in their future work and/or our own departmental course for preservice teachers, (PHY 160: "Physics and Astronomy for Teachers"). Both P.Ls are experienced and continuing instructors of this course and interact regularly with faculty in the College of Education and teachers throughout the region.