Controlling the Conductivity of Nanocrystal Solids through their Surface Chemistry

University Of Texas At Austin, Austin TX

Investigators

Abstract

In this project funded by the Macromolecular, Supramolecular and Nanochemistry program of the Chemistry Division, Professor Sean T. Roberts of the University of Texas at Austin and his graduate students study very small inorganic particles called semiconductor nanocrystals. Nanocrystals are tiny particles that contain thousands of atoms. While they are much larger than chemical molecules, which contain only a few atoms, they are still much smaller than solid materials that are large enough to see with the naked eye, containing trillions of atoms. Nanoscale materials are unique because they have sizes intermediate between molecules and solids, but sometimes have properties not easily predicted by averaging the two size extremes. The optical properties of these nanocrystals can be tuned by altering their size and shape, which makes them useful for applications such as displays, photovoltaic cells, and photodetectors. However, processing these materials into conductive thin films needed for electronics remains difficult. The researchers approach this challenge by combining the nanocrystals with surface-bound molecules designed to improve the electrical conductivity of nanocrystal thin films. The resulting structures are studied using spectroscopic techniques and photoconductivity measurements. This project also includes an outreach effort entitled GReen Energy At Texas (GREAT) designed to attract Community College students to the physical sciences by introducing them to green energy research. The specific goals of this project are to understand the key factors that facilitate electronic coupling between semiconductor nanocrystals and their surface ligands and to use interactions of this type to improve charge transport in nanocrystal optoelectronic films. The nature of the ligand can impact the bandgap and optical properties of semiconductor nanocrystals. This suggests some degree of electronic coupling between the nanocrystals and the ligands that involves charge transfer. This project quantitatively assesses the degree to which ligands facilitate charge transfer between nanocrystals in thin film. To accomplish this goal, time-resolved Raman spectroscopy is used to map charge density changes within nanocrystal ligand shells following photoexcitation. Two-dimensional electronic spectroscopy and photo-CELIV (charge extraction by linearly increasing voltage) measurements determine how exciton-delocalizing ligands (EDLs) modify charge carrier transport in nanocrystal thin films.