Compensation of Acceptors in ZnO Nanocrystals by Adsorption of Formic Acid

Zinc oxide (ZnO) nanocrystals have potential applications in optoelectronic devices and are widely used as catalysts for a variety of chemical processes. Using infrared (IR) and photoluminescence (PL) spectroscopies, we provide evidence that ZnO nanocrystals contain hydrogenic acceptors and propose an energy-level scheme to explain the observations. IR absorption peaks at liquid-helium temperatures, which correspond to electronic transitions of the acceptor, disappeared after exposure to formic acid (HCOOH) vapor. This observation is consistent with electrical compensation of the acceptor by the formate ion. The energy level of the formate ion is estimated to be similar to 0.9 eV above the valence-band maximum. Room temperature IR measurements show that the formate species are adsorbed on the surface of ZnO nanocrystals. A broad PL peak centered at 3.2 eV, for samples exposed to HCOOH, is attributed to an exciton bound to a formate species.