Direct Evidence of Surface Charges in n-Type Al-Doped ZnO

The fundamental properties of electrons in the prototypical n-type oxide nanocrystal, Al 3+ -doped ZnO, have been studied at both the ensemble and single-particle levels by spectroscopic and electron force microscopic techniques. We developed and implemented a new synthetic methodology that enables the tunable incorporation of Al 3+ in the ZnO nanocrystal in an “etching–regrowth–doping” (ERD) strategy in a single-pot reaction. The ensemble-averaged properties and evolution of the Al 3+ speciation in ZnO were studied using electronic absorption spectroscopy and powder X-ray diffraction and reveal the successful substitution of Al 3+ only after implementation of the ERD strategy. Characterization of individual ZnO, surface Al 3+ -doped ZnO, and internal Al 3+ -doped ZnO nanocrystals using electrostatic force microscopy reveals strong responses in both the quantity of surface charges and electron polarizabilities, which are dependent on the amount of Al 3+ in the ZnO lattice. These results appear to suggest that an upper limit to the electron polarizability exists for Al 3+ -doped ZnO nanocrystals.