Sub-bandgap trap sites for high-density photochemical electron storage in colloidal SrTiO3 nanocrystals.

We report facile and reversible electron storage in colloidal SrTiO3 nanocrystals using photochemical and redox titration methods. A very high electron storage capacity (∼180 e- per 7 nm nanocrystal) is achieved which we attribute to the localized nature of added electrons at sub-bandgap trap sites in these colloidal SrTiO3 nanocrystals. The rate of electron accumulation is also found to be much faster with ethylene glycol as the sacrificial reductant compared to ethanol. This work provides key insight and establishes a kinetic bottleneck in the charge trapping processes.

On the formation of superoxide radicals on colloidal ATiO3 (A = Sr and Ba) nanocrystal surfaces.

Controlling the surface chemistry of colloidal semiconductor nanocrystals is critical to exploiting their rich electronic structures for various technologies. We recently demonstrated that the hydrothermal synthesis of colloidal nanocrystals of SrTiO3, a technologically-relevant electronic material, provided a strong negative correlation between the presence of an O2-related surface defect and hydrazine hydrate [W. L. Harrigan, S. E. Michaud, K. A. Lehuta, and K. R. Kittilstved, Chem. Mater., 2016, 28(2), 430]. When hydrazine hydrate is omitted during the aerobic hydrothermal synthesis, the surface defect is observed. However, it can be removed by either the addition of hydrazine hydrate or by purging the reaction solution with argon gas before the hydrothermal synthesis. We also propose that the formation of the O2-related defect is mediated by the reduction of dissolved O2 by lactate anions that are present from the titanium precursor. This work helps elucidate the nature of the O2-related defect as a superoxide anion and presents a mechanism to explain its formation during the hydrothermal synthesis of SrTiO3 and related BaTiO3 nanocrystals.