In this CAREER award, co-funded by the Experimental Physical Chemistry (CHE) and Electronic Materials (DMR) programs, Prof. Masaru K. Kuno of the University of Notre Dame and his graduate and undergraduate students will use single particle spectroscopy to study the optical and electronic properties of linear and branched semiconductor nanowires (CdSe, CdTe, and PbTe). Experiments will focus on scanning tunneling microscopy (STM) spectroscopy, photoluminescence excitation, and the fluorescence intermittency of single nanowires. The aim of this work is to explore the effects of disorder (including phase defects), heterogeneity, and the surrounding medium on the behavior of solution-based nanowires.
Understanding the links between intrawire defects and the optical and electronic properties of nanowires will impact the further development of semiconductor nanowires as optical sensors and as photovoltaics. The proposed work is integrated with educational activities involving undergraduate and graduate students at the University of Notre Dame. These activities will lead to the development of a series of student-tested, undergraduate experiments and include the construction of an inexpensive, single-molecule microscope. Prof. Kuno also plans to publish an introductory textbook on nanoscience.
This CAREER proposal focused on developing a program to investigate the detailed optical and electrical properties of chemically synthesized semiconductor nanowires. Nanowires are essentially wires with radii on the nanometer scale that have potential uses in a number of next generation technologies. Among questions asked were: What is the appreance of a single wire's absorption spectrum? How do individual wires interact with their surroundings? Are there any instances of unexpected behavior that can be seen when examining individual wires instead of a collection of them? We therefore carried out the following experiements. First, we successfully synthesized high quality nanowire using solution chemistry. The wires were made from popular compound semiconductors such as CdSe and CdTe. Next, we constructed an apparatus that could examine individual wires optically. Initially the aparatus could only look at them through their emission. We were successful doing this and were able to record images, spectra and movies of individual nanowires. We found that the wires exhibited some unusual behavior. Among them, the nanowire emission intensity fluctuated. This is a little unexpected because the thinking is that if you add light to a material in a consistent fashion, you should get light back in the same consistent manner. However, the wires would sometimes give you back less light and sometimes more in a seemingly random way. Next, we also found that if we subjected the wires to an external electric field that we could start to move the nanowire emission around. In fact, if we alternately applied the field pointing in opposite directions we could get the nanowire to emit from different ends. Again, this is very unexpected because small electric fields shouldn't alter where light is emitting from in a material. We since have explained this latter phenomenon as due to the motion of electrons on the nanowire surface. At the same time, we speculate that perhaps this explanation also addresses the origin of the strange emission intensity fluctuations we saw earlier. When no electric field is present, the electrons on the nanowire can still move. Perhaps their random motion causes these random intensity flucutuations being seen. Continuing on this train of thought, it stands to reason that perhaps emission fluctuations seen in other nano-objects arise because of a similar reason. We would then have a "universal" explanation for unexpected emission intensity fluctuations of single emitters -something that is currently an open question in science. Following these studies, we embarked on modifying our instrument to look at the absorption of individual nanowires. This is a much harder experiment because in it you are looking for a small change in light intensity caused by something absorbing light. By contrast, in the earlier emission experiment you simply look for light emitted from the object over a black background. I would compare this to trying to see Venus occlude the sun versus looking at the moon on a dark night in Indiana. We were ultimately successful in carrying out the absorption experiment and can now report for the first time the absorption spectrum of individual nanowires. We have also begun to probe the origin of features seen in the acquired spectra through comparisons to theory. This is an exciting development because it now means that we can begin to investigate the optical properties of other nanostructures, not just nanowires, which are not necessarily emissive. To expand the reach of these results, I have simultaneously developed a program to instruct undergraduates and the general public on current topics in nanoscience. For undergraduates, I have developed a small compact optical microscope that can image individual molecules and nanostructures through their emission. I have reported details of this instrument in a mainstream chemical education journal. Next, I have participated in developing new experiments for the undergraduate chemistry curriculum at Notre Dame. Among the more successful experiments is one where students actually synthesize what are called semiconductor "quantum dots". The students are therefore involved in cutting edge chemistry at the forefront of today's nanoscience. Finally, I spent the last two years writing a nanoscience book. This book recently came out in the fall of 2011 and is titled "Introductory Nanoscience: Physical and Chemical Concepts". You can find it on Amazon.com. My hope is that it will provide future generation of undergraduates and beginning graduate students a solid foundation from where to being their careers in nano. Bulleted summary of tangilble outcomes from this award: Training/development of five graduate students, one postdoctoral associate and two undergraduates. 3 successful PhDs One nanoscience book for upper level undergraduates and entering graduate students 19 papers in mainstream scientific journals 1 paper in a mainstream chemical education journal Report of the first direct absorption measurements of semiconductor nanowires, which, in turn, opens the door to future experiments with other nanostructures. 1 nanoscience display at the University
