Free Electron Concentration in Colloidal Indium Tin Oxide Nanocrystals Determined by Their Size and Structure

We report the synthesis and separation of colloidal indium tin oxide (ITO) nanocrystals in the stable cubic bixbyite (bcc-ITO) and metastable corundum (rh-ITO) phase under identical conditions, based on the size-structure correlation. Both phases are obtained in the same reactions, with nanocrystals below ca. 5 nm in size having corundum crystal structure. This bimodal size distribution allows for the separation of the nanocrystal phases by size selective precipitation. A comparative study of bcc-ITO and rh-ITO nanocrystals reveals a dramatic difference in their optical and electrical properties. Unlike smaller rh-ITO nanocrystals, bcc-ITO nanocrystals exhibit a strong absorption in the near-infrared (NIR) region arising from the plasmon oscillations due to the presence of free electrons. The difference in the free electron concentration in bcc-ITO and rh-ITO nanocrystals is related to the different electronic structure of the donor states, associated with Sn4+ dopants, in these two nanocrystal allotropic modifications. The donor activation energy is significantly higher in rh-ITO NCs, prohibiting any appreciable concentration of free electrons in the conduction band. The increased replacement of organic protective ligands by anions in the solution leads to the oriented attachment of larger sized bcc-ITO nanocrystals and the formation of flowerlike clusters. These results demonstrate tuning of the optical and electrical properties of complex oxide nanocrystals. by selecting their crystal and electronic structures through size and composition and allow for a designed preparation and controlled self-assembly of ITO nanocrystals.