Enhanced Raman Scattering by ZnO Superstructures: Synergistic Effect of Charge Transfer and Mie Resonances.

A remarkable enhancement of Raman scattering is achieved by submicrometer-sized spherical ZnO superstructures. The secondary superstructures of ZnO particles with a uniform diameter in the range of 220-490 nm was formed by aggregating ca. 13 nm primary single crystallites. By engineering the superstructure size to induce Mie resonances, leading to an electromagnetic contribution to the SERS enhancement. Meanwhile, a highly efficient charge-transfer (CT) contribution derived from the primary structure of the ZnO nanocrystallites was able to enhance the SERS signals as well. The highest Raman enhancement factor of 105 was achieved for a non-resonant molecule by the synergistic effect of CT and Mie resonances. The Mie resonances scattered near-field effect investigated in the present study provides not only an important guide for designing novel SERS-active semiconductor substrates, but also a coherent framework for modelling the electromagnetic mechanism of SERS on semiconductors.