Measuring Hole Transfer across the Single Nanowire/Liquid Interface

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Semiconductor nanowires can be thousands of times thinner than a strand of hair. These extremely small sizes give rise to unique properties that can be harnessed for applications ranging from light emitting diodes to solar energy conversion devices. However, as the wire diameter gets smaller and smaller, defects at the nanowire surfaces, which trap charge and degrade device performance, play an ever-increasing role. With support from the Macromolecular, Supramolecular, and Nanochemistry program in the Division of Chemistry, Professor Mario Alpuche is studying the light-induced transfer of electrons between nanowires and molecules in solution. Working with his students, Professor Alpuche is using extremely small electrodes to detect the electrical current resulting from the absorption of light by a single nanowire. Their discoveries could have important implications for the use of nanomaterials in solar energy conversion applications. In addition, the project is broadening participation by recruiting students of Hispanic descent through engagement with educational programs throughout northern Nevada, and collaborations in the United States and abroad motivate students from underrepresented groups to pursue careers in chemistry.Professor Alpuche's research group is developing new methods to study charge transfer across the nanowire semiconductor surface. CdSe nanowires are attached to ultramicroelectrodes and excited with light, producing photogenerated electron-hole pairs. The hole in the valance band can be transferred from the nanowire to a molecule in solution, or trapped at surface defect sites, while the electron is transferred to the electrode and detected as a photoinduced current. The dependence of photocurrent on electrochemical potential and irradiation power enables a separation of the contribution of hole traps from that of charge transfer, and finite element simulations enable extraction of the rate constant for hole transfer out of the nanowire. The electrochemical instrumentation can detect photocurrent from individual nanowires, allowing the group to study the effects of nanowire heterogeneity across an ensemble of structures.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.