Seminar: Cherie R. Kagan, University of Pennsylvania
Professor Cherie Kagan of University of Pennsylvania presents a seminar in Chemical & Biomolecular Engineering.
"High-Performance Colloidal Semiconductor Nanocrystal Electronics and Optoelectronics"
Abstract: The long, insulating ligands commonly used in the synthesis of colloidal semiconductor nanocrystals (NCs) inhibit strong interparticle coupling and charge transport once NCs are assembled in the solid state into NC arrays. We introduce ammonium thiocyanate (NH4SCN) to exchange the long, insulating ligands commonly used in the synthesis of colloidal semiconductor NCs. NCs may be exchanged with the new ligand in solution to form dispersions from which NC arrays are deposited or NC arrays with the long, insulating ligands may be exchanged in the solid state with the new ligands. The new compact ligands enhance interparticle coupling in thin film, NC arrays as seen by red-shifts in the optical absorption and concomitant increases in carrier mobilities, consistent with carrier delocalization and band transport. Doping of the QD thin films by thermal diffusion of a low temperature, high diffusivity metal shifts the Fermi energy, fills traps, and provides access to the bands. Thiocyanate-capped CdSe thin film, NC arrays form sensitive photodetectors and n-type field-effect transistors with electron mobilities as high as 27 cm2/Vs and current modulation of >106, while preserving NC quantum confinement. Temperature-dependent transport measurements reveal band-like transport in NC arrays, overcoming carrier hopping that has typified transport in NC arrays until recently. The non-corrosive, chemically benign nature of the ammonium thiocyanate treatment enables the fabrication of NC thin film devices and circuits on flexible plastics.