Technical: This project addresses synthesis, processing and characterization of PbSe nanowires. PbSe nanowires with controlled size and shape (straight, zig-zag, helical, and branched topologies) will be synthesized via wet chemical methods and studied electronically and optically to probe size and shape dependent charge transport and excitonic interactions. Absorption and emission linewidths and energies will be collected to test effective mass models, probe wire homogeneity, and measure expected one-dimensional polarizations. Exposing the nanowires to selected charge transfer dopants will enable n- and p-doping of the wires and allow the transport of both electrons and holes to be probed. Using, in combination, temperature-dependent, spatially and temporally resolved photoconductivity and nanoscale electrical measurements, uniformity in electronic structure along the lengths of the nanowires and the influence of correlated, nanoscale roughness on a length scale (~10 nm) smaller than that of the PbSe Bohr radii for excitons (~46 nm), electrons (~23 nm), and holes (~23 nm) will be investigated. These measurements will be used to characterize in different topologies, exciton and charge trapping and scattering that impact the length scale for ballistic transport, the electron and hole carrier mobilities, and the absorption, emission, and diffusivity of excitons.
The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. The research activities will help to expand the knowledge base in nanomaterials and the teaching of low-dimensional materials physics and properties, and provide important interdisciplinary research opportunities for undergraduate and graduate students. The PI places emphasis on teaching courses that span chemical and physical concepts important to bridging between Materials, Chemical, Physical, and Electrical communities where exciting research opportunities exist. She is developing a new course on semiconductor physics and on the electronic, optical, magnetic, and biological properties of materials and devices. Additionally she is actively involved in speaking with students in forums that encourage them to think about their careers. She also promotes informal lunch and coffee meetings among departmental and interdisciplinary women chemist groups at U. Penn and other academic institutions.
In this project we explored the materials physics of the semiconductor lead selenide in the form of one-dimensional nanowires with diameters approximately 10 nm. At this length scale, the behavior of electrons and holes are modified by the small size of the structure and the nearby influence of the surface that profoundly affects the materials electronic and optical properties. We prepared highly uniform lead selenide nanowires by simple, wet-chemical synthetic methods. We incorporated them in device architectures to form arrays of nanowires or to measure one single nanowire at a time to unmask their electronic properties. We showed that the dominance of electrons versus hole transport in these nanowires could be switched by modifying the surface of the nanowire with different molecules, by gas adsorption, and by changing nanowire stoichiometry (introducing more lead versus selenium). By changing the electron versus hole transport we could design the properties of the nanowires to form n-type (more electrons), p-type (more holes), or ambipolar (both electron and hole transport) devices. Engineering the materials properties are important to the application of these nanowires, which are of particular interest in optoelectronics, thermoelectrics, and electronics. For example, we demonstrated the first lead selenide nanowire circuits by combining n-type and p-type nanowire devices. The room temperature processing of these materials allowed their devices to be fabricated on flexible plastics, for bendable electronic applications. The methods for nanowire assembly, device fabrication, and the manipulation of their electronic properties through surface chemistry is more broadly applicable to other compositions of nanoscale semiconductors. We have applied our understanding to the exploration of questions of controlling the number of electrons (known as doping) in cadmium selenide nanostructures. This project was instrumental in the education of two graduate students and two undergraduate students. One of the graduate students defended his thesis and will continue as a postdoctoral fellow at ETH in Zurich, Switzerland. Our undergraduate students have continued their education and have each now begun their first year as PhD students in nanoscience and materials programs at Cambridge University and University of California, Santa Barbara. We have also shared the enthusiasm and research of nanoscale materials with K-12 students, their teachers, and the community. We have for three years worked with the Franklin Institute in Philadelphia for Nano Day. We run all-day demonstrations on nanoscale materials and phenomena and their applications or experiences people will know from their daily lives. I and my students have given presentations and demos at "Philly Materials Day" in the last two years since its inception. We were "Make it Cleaner" to coincide with the Nova Series "Making Stuff." We have also presentations, off-site and hosted in our lab demonstrations as part of public school teachers continuing education programs.
