Prof. Lupton and his group at the University of Utah, with the support of the Analytical and Surface Chemistry Program in the Chemistry Division of the National Science Foundation, will develop and utilize ultra-sensitive spectroscopic techniques to unravel the structural and electronic properties underlying the function of organic semiconductors. By applying Raman spectroscopy to single semiconducting organic molecules, highly local changes in the intramolecular charge density which affects the interatomic separation can be revealed through the characteristic vibrational fingerprints. The project seeks to identify signatures of molecular charging through a change in vibrational frequency. The surface enhanced Raman scattering (SERS) effect dramatically amplifies the selectivity of Raman spectroscopy. This amplification should make it possible to pick out individual charging events within a working device such as an organic light-emitting diode (OLED), enabling a direct tracking of charge migration.
The project's uniqueness stems from a synergy between the two fields of organic semiconductors and SERS. While SERS is new as an analytical tool in the study of organic semiconductors, conjugated polymers in particular have recently proven to be excellent materials for the investigation of the underlying physics of SERS itself. Further improvements to the reproducibility, stability, selectivity and sensitivity of SERS are therefore anticipated to come out of this project. The highly interdisciplinary nature of the research necessitates a range of collaborations on and off campus, providing a versatile training background for the students engaged with the venture.
This ambitious project aimed at developing a technique to visualize a single electron as it moves between molecules in an electronic device. Rather than using conventional semiconductors such as silicon, the devices to be studied rely on molecules; examples of such devices include organic light-emitting diodes, OLEDs. To make the electron visible, we wanted to display the vibrations of the molecule. A molecule can be thought of like a bunch of balls coupled by springs, which vibrate at a particular frequency. Adding an electron to the balls leads to additional damping in the oscillation, changing the frequency. The trick is to figure out how to actually see these vibrations, and possible changes in vibration frequency. The solution lies in tiny metal nanoparticles, which can serve as antennae for light. These antennae focus incident light on the molecules, and the reflected light contains information on the characteristic vibration of the molecules. If the experiment is done correctly, one single molecule can be identified at a time. In comparison, a typical OLED may contain trillions of molecules. In the course of the project, we developed a series of new spectroscopic techniques to characterize these metal light antennae. An example of such an antenna is given by an antique silver mirror: although the mirror appears uniform to the human eye, under an electron microscope it is seen that it actually contains myriads of tiny metal particles, of different shapes and sizes. Under certain conditions, the metal mirror made out of particles not only reflects light, but also generates light: shine red light at it, and you get blue light coming back. But instead of this light being uniform, like a mirror image, it is highly discrete. Much like in the popular fairy tale, the mirror develops a life of its own and actually shows us something very different than what we put in front of it. Silver nanoparticles can act like little nanobeacons, emitting stable, white light, which can be used as a source in microscopy. We demonstrated this concept by placing a highly diffractive biological specimen, a beetle exoskeleton, on the mirror, and imaging the light transmission. This experiment revealed surprising insight into the internal structure of the beetle skeleton. In the course of the project, several new spectroscopic techniques were developed and employed to investigate seemingly mundane silver mirrors. The techniques proved useful for studying a variety of other material systems, such as investigating how light energy is converted into excitation energy and migrates through single snanostructures, much like in the case of photosynthesis in nature. In the course of the project, a total of four graduate students were supervised and trained. Demonstrations were developed to illustrate the simplicity of metal nanoparticle growth, and these were performed to interested undergraduate and high school students. The key results of the project were disseminated broadly in various journals, and highlighted in both the local media and scholarly outlets such as the Journal of Chemical Education.
