Lance Wheeler Ph.D. Thesis Defense
Chemical Engineering and Materials Science Department
IPrime Programs: Renewable Energy Materials (REM)
June 2, 2014, University of Minnesota
Advisor: Uwe Kortshagen (ME)
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Size-tunable optical properties and the ability to process thin films using scalable, cost- efficient printing techniques have long made colloidal nanocrystals (NCs) an attractive candidate for next-generation optoelectronics. Much of the recent progress hinges on the ability to manipulate the NC surface. Conventional solution synthesis yields NCs with ligands bound to metal surface atoms through a labile acid-base complex. The electrically-insulating native ligands are thus routinely exchanged to produce conductive NC arrays for devices such solar cells, light-emitting devices, transistors, photodetectors, and thermoelectrics. Just as surface manipulation has launched metal-based NCs to the forefront of optoelectronic technology, it is the inability to do so with the covalent surface of group IV NCs that has greatly hindered progress. In this work, we demonstrate Lewis acidic surface chemistries of gas-phase synthesized Si NCs provide the same solution-phase versatility as their metal-based counterparts. This represents an immense step toward an abundant, non-toxic alternative to Pb and Cd-based NCs. These Si NCs are also uniquely suited for investigating the mechanism of colloidal stability that has been observed after metal-based NCs are exchanged for "ionic ligands." We find the colloidal forces needed for stability are contrary to the mechanism that has been previously invoked and propose an alternative model for achieving NC colloidal stability. Using this model, we demonstrate stable Si NC colloids in media that runs the gamut from hexane to water.