(185b) NSEF Forum Award Winner: Making optical materials from metal oxide nanocrystals

As optical materials, metal oxides usually fulfill basic functions, like transparent window glass or particles that scatter light in bright white paint. We make materials with more sophisticated optical functionality, with highly tunable and even dynamic spectral response, from metal oxides by doping them with deliberate impurities and by shrinking their dimensions to the nanoscale. Replacing a few percent of the metal ions in a wide band gap semiconductor like indium oxide with aliovalent dopants like tin results in the unusual combination of optical transparency with metallic conductivity in indium tin oxide (ITO). When synthesized in colloidal nanocrystal form, ITO maintains its visible transparency, while the confinement of free electrons results in the strong absorption of infrared light and the concentration of energy into nanoscale volumes around the nanocrystals due to localized surface plasmon resonance (LSPR). Based on their LSPR characteristics, ITO nanocrystals can boost sensitivity of molecular detectors or act as remote monitors of biochemical electron transfer reactions. The infrared absorption of ITO and other metal oxide nanocrystals is strongly modulated by electron transfer, enabling fabrication of smart windows that dynamically control solar heat gain and visible glare. Based on their optical tunability and dynamic responsivity, these nanocrystals have the potential to make solar cells and buildings more efficient, drive selective chemical reactions, harness solar energy, and more.