A synergistic combination of local tight binding theory and second harmonic generation elucidating surface properties...

Zinc oxide (ZnO) in the form of nanoparticles (NPs) is an important nanomaterial due to its catalytic and optoelectronic properties. An interesting aspect of ZnO is that its crystal structure is anisotropic, which leads to a strong 2nd order nonlinear response, such as frequency doubling or second harmonic generation (SHG). In this article we show that not only the bulk but the surface of ZnO NPs in contact with a liquid medium can contribute to the overall SHG. We have developed and applied a synergistic combination of tight binding (TB) theory and optical SHG spectroscopy to determine the surface second order susceptibilities of nearly spherical 33 ± 13 nm crystalline ZnO NPs dispersed in acetonitrile. The corresponding χ and χ were determined to be 0.86 × 10^-8 esu and 1.72 × 10^-8 esu for the O-terminated surface and 3.28 × 10^-8 esu and 6.64 × 10^-8 esu for the Zn-terminated surface. A further application of the TB-SHG approach revealed that adsorption of coumarin based dye, which forms a bidentate attachment between the carboxyl and Zn-terminated surface, does not restructure the NP surface significantly to manifest a change in the SHG polarization profile. However, if the dye acts as an independent source of SHG, its orientation on the surface dictates the overall change in the observed SHG. The results highlighted here bear a strong potential to further our knowledge of molecular interactions at the solid-liquid interface of crystalline nanostructures.