Research in the Knowles group seeks to develop fundamental understanding of the synthesis, chemistry, electrochemistry, and photophysics of metal oxide semiconductor nanomaterials. Metal oxide semiconductors are a richly diverse class of materials with optical, electronic, chemical, and magnetic properties that vary widely depending on composition and stoichiometry. We aim to contribute to rational strategies for improving the performance of these materials in applications related to solar energy capture and storage.
One of our primary goals is to understand structure-function relationships between the nanostructured morphology and surface chemistry of a metal oxide semiconductor film and its performance as a photoelectrode. In addition to studying nanostructured films fabricated by sol-gel and electrodeposition techniques, we also use colloidal metal oxide nanocrystals as dispersible model systems to study the inter-related impacts of size, shape, and surface chemistry on interfacial redox processes that occur in the dark and under illumination.
To enable these studies, we design and develop rational approaches to controlling the size and shape of metal oxide nanocrystals synthesized by solvothermal methods and explore new single-source molecular precursors that will enable synthesis of previously inaccessible metal oxide crystal phases under ambient pressure. We are particularly interested in developing new synthetic strategies to access ternary metal oxide nanocrystals that have desirable optical, electronic, and chemical properties but are difficult to synthesize in colloidal form. In all of these synthetic projects, we strive to understand the mechanisms by which precursors convert to nanocrystal monomers and monomers nucleate to form nanocrystals.
Students in the Knowles group gain interdisciplinary experience at the interfaces of physical, inorganic, and materials chemistry. In addition to standard techniques for characterization of solution-phase and nanostructured inorganic materials (i.e. electron microscopy, X-ray diffraction, and NMR, infrared, electronic absorption, Raman, and photoluminescence spectroscopy), the core techniques used in our lab include spectroelectrochemistry, photoconductivity, reflectance spectroscopy, and time-resolved optical spectroscopies.
- Brewster, D. A.; Sarappa, D. J.; Knowles, K. E. "Role of Aliphatic Ligands and Solvent Composition in the Solvothermal Synthesis of Iron Oxide Nanocrystals", Polyhedron, 2019, 157, 54-62.
- Knowles, K.E.*; Koch, M. D.; Shelton, J. L. "Three Applications of Ultrafast Transient Absorption Spectroscopy of Semiconductor Thin Films: Spectroelectrochemistry, Microscopy, and Identification of Thermal Contributions." J. Mater. Chem. C. 2018, 6, 11853-11867.
- Marchioro, A.; Whitham P. J.; Nelson, H. D.; De Siena, M. C.; Knowles, K. E.; Polinger, V. Z.; Reid, P. J.; Gamelin, D. R. Strong Dependence of Quantum-Dot Delayed Luminescence on Excitation Pulse Width. J. Phys. Chem. Lett. 2017, 8, 3997-4003.
- Yang, L.; Knowles, K. E.; Gopalan, A.; Hughes, K. E.; James, M. C.; Gamelin, D. R. One-Pot Synthesis of Monodisperse Colloidal Copper-Doped CdSe Nanocrystals Mediated by Ligand-Copper Interactions. Chem. Mater. 2016, 28, 7375-7384.
- Knowles, K. E.*; Hartstein, K. H.; Kilburn, T. B.; Marchioro, A.; Nelson, H. D.; Whitham, P. J.; Gamelin, D. R.* Luminescent Colloidal Semiconductor Nanocrystals Containing Copper: Synthesis, Photophysics, and Applications. Invited Review, Chem. Rev. 2016, 116, 10820-10851. (*denotes co-corresponding Author)
- Knowles, K. E.; Nelson, H. D.; Kilburn, T. B.; Gamelin, D. R. "Singlet-Triplet Splittings in the Luminescent Excited States of Colloidal Cu+:CdSe, Cu+:InP, and CuInS2 Nanocrystals: Charge-Transfer Configurations and Self-Trapped Excitons," J. Am. Chem. Soc. 2015, 137, 13138-13147.
- Knowles, K. E.; Kilburn, T. B.; Alzate, D. G.; McDowall, S.; Gamelin, D. R. "Bright CuInS2/CdS Nanocrystal Phosphors for High-Gain Full-Spectrum Luminescent Solar Concentrators," Chem. Commun 2015, 51, 9129-9132.
- Bradshaw, L. R.; Knowles, K. E.; McDowall, S.; Gamelin, D. R. "Nanocrystals for Luminescent Solar Concentrators," Nano Lett. 2015, 15, 1315-1323.
*Featured in Nature: News and Views: Debije, M. “Renewable Energy: Better luminescent solar panels in prospect.” Nature 2015, 519, 298-299.
- Knowles, K. E.; Tagliazucchi, M.; Malicki, M.; Swenson, N. K.; Weiss, E. A. "Electron Transfer as a Probe of the Permeability of Organic Monolayers on the Surfaces of Colloidal PbS Quantum Dots," J. Phys. Chem. C. 2013, 117, 15849-15857.
- Knowles, K. E.; Malicki, M.; Parameswaran, R.; Cass, L. C.; Weiss, E. A. "Spontaneous Multi-Electron Transfer from the Surfaces of PbS Quantum Dots to Tetracyanoquinodimethane," J. Am. Chem. Soc. 2013, 135, 7264-7271.
- Knowles, K. E.; McArthur, E. A.; Weiss, E. A "A Multi-Timescale Map of Radiative and Nonradiative Decay Pathways for Excitons in CdSe Quantum Dots," ACS Nano 2011, 5, 2026-2035.