Paper EM-WeM4
Surface Chemical Control of Charge Transport and Infrared Plasmonic Response in Nanocrystal Thin Films

Wednesday, November 1, 2017, 9:00 am, Room 14

Infrared plasmonic nanocrystal thin films are promising materials for harvesting thermal radiation, but their use requires an ability to simultaneously control nanocrystal carrier density and interfacial charge transport. Here, we combine in situ infrared spectroscopy and nanoscale four-point probe measurements to show the intimate interplay between nanocrystal surface chemistry, the localized surface plasmon resonance (LSPR), and thin film resistivity. Indium tin oxide (ITO) nanocrystals synthesized via colloidal methods exhibit LSPRs tunable in the mid-infrared (from 0.2 to 0.6 eV) and serve as model infrared plasmonic materials with which to study these effects. Removal of surface hydroxyl groups, via reaction with metal alkyl species (e.g., trimethylaluminum), and formation of a monolayer of oxygen-metal surface bonds (e.g., Al-O) reduces the number of nanocrystal surface traps. The corresponding increase in carrier density is evidenced by a clear blue shift and increase in intensity of the LSPR. This monolayer surface treatment decreases nanocrystal film resistivity by two orders of magnitude (to 1 x 10^-2 ohm-cm). We further demonstrate that conformal coatings (e.g., of Al₂O₃ or ZnO) permit control of nanocrystal thin film resistivity without modifying LSPR energy, a capability that allows us to independently engineer light absorption and device properties. We also find that these plasmonic nanocrystal films exhibit a photoconductivity in response to mid-infrared light illumination.