The two principal investigators of this award have developed probes to measure the infrared reflectance, tunneling spectra, and mechanical properties of small crystalline and thin film samples as functions of position. These probes, an infrared microscope with tunable diode lasers as light source, scanning tunneling microsocsope with long-distance scanner, and a helical resonator motion detector, will be used to study changes in electronic, molecular, and piezoelectric properties as functions of voltage, frequency, and distance from electrical contacts. The materials to be studied include organic semiconductors based on pentacene derivatives, being developed for applications in field-effect-transistors, and charge-density-wave materials, being studied as nano-torsional actuators, with the goals of improving our understanding of their conducting and piezoelectric mechanisms. Graduate students will gain experience in a wide range of characterizational and synthetic techniques. Undergraduate education majors (pre-service teachers) will work in summers not only to improve their appreciation of scientific techniques but to develop teaching tools for their future careers.
In recent 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, including the effects of charge on the constituent molecules located near electrical contacts. The two principal investigators of this award have developed new optical, tunneling, and electromechanical probes that will be used to study the electronic, molecular, and mechanical properties as functions of distance from contacts. Materials to be studied are new organic semiconductors, to be utilized in transistors, and non-uniform charge distribution materials, with possible applications as mechanical actuators. Graduate students working on the project will gain broad experience in different characterizational and synthetic techniques that will prepare them for careers in industry, academia, and research laboratories. In addition, pre-service teachers will assist in the labs in summers to gain appreciation of scientific laboratory work and also prepare future teaching materials.
This grant was for research in studying infrared, tunneling, and electromechanical properties of charge-density-wave (CDW) conductors and organic semiconductors, being developed for use in transistors, with the emphasis on developing and using probes that could be used to measure position dependent properties. 1) Tunneling: Scanning tunneling microscopes (STMs) are the instruments of choice in studying local (atomic scale) electronic properties. One disadvantage of STMs is that their "depth-of-view" is usually limited, so that if one wants to study properties that vary over hundreds of microns, such as interface effects, one needs to mechanically reposition the STM on the sample, especially disadvantageous for cryogenic applications. We developed an STM with "long-range" (~ 1 mm) lateral motion so that tunneling spectra can be measured as functions of position without warming and mechanically repositioning the STM. 2) Infrared: We have developed a position-dependent electro-optic technique (using infrared lasers and an infrared microscope) to measure the mobility (speed) of charges flowing in organic semiconductor films in field-effect transistors. (Most of our measurements were on "TIPS-pentacene", from which commercial transistors are already being marketed.) While the sensitivity of our technique is limited, it has the advantages that i) mobility can be measured without knowledge of the charge on the transistor, ii) it can be used to study the spatial variation of the mobility (e.g. due to variations in crystal and substrate quality) and iii) it can separate effects due to motion of charges within the semiconducting film from contact effects. 3) Electromechanical: We investigated the recently discovered "voltage-induced torsional strain" (VITS) in CDW conductors: a unique effect in which a crystal turns when a voltage is applied. Our measurements included the time, frequency, voltage, torque, and temperature dependence of VITS in tantalum trisulfide. We found that the VITS speed depends on the current carried by the CDW, but the magnitude and direction depend on whether and how the sample is already twisted. Based on our measurements, we proposed a model for the VITS in which strains in the CDW interact with crystal strains. 4) Broader Impact: Graduate students working on this project obtained wide experience in vacuum, cryogenic, electronic, and optical techniques. In addition, two undergraduate students majoring in elementary or middle school education worked in our labs every summer. This program was designed to help break down the barrier non-technical students often feel about doing science and give them a research experience they could share with their future pupils. The program has been popularly endorsed by our College of Education.
