Enhanced Raman scattering based on a ZnO/Ag nanostructured substrate: an in-depth study of the SERS mechanism

Combining semiconductor and noble metal nanostructures into a hybrid system has shown many complementary advantages in the optical properties, making them more attractive in practical applications. Herein, we prepared a semiconductor/noble metal hybrid system composed of Ag nanoparticles decorated on ZnO nanoplates acting as a surface-enhanced Raman scattering (SERS) substrate for probing methyl red. The tuning of the optical characteristics of the hybrid system was demonstrated through the changes in the absorption, fluorescence, and Raman spectra. The formation of the local electromagnetic field at the heterostructure interface plays a pivotal role in its SERS activity. Thanks to density functional theory calculations, methyl red's vibrational modes and symmetry properties were assigned to be consistent with the contribution of the neutral trans conformer and protonated state. Then, using Herzberg-Teller-surface selection rules, these assignments strongly support the realization that the SERS mechanism based on the ZnO/Ag substrate has a significant electromagnetic contribution versus the Ag substrate in which charge transfer plays a pivotal role. To the best of our knowledge, this is the first investigation that has clarified the mechanism and advantage of semiconductor/metal (ZnO/Ag nanostructures) even over noble metals (Ag nanoparticles) in SERS applications. Moreover, the SERS behavior based on the ZnO/Ag substrate was also examined and the results indicated high sensitivity and good repeatability. Using Herzberg-Teller-surface selection rules in combination with the results of DFT calculations, the SERS mechanism based on the ZnO/Ag substrate has indicated a significant electromagnetic contribution versus the Ag substrate, in which charge transfer plays a pivotal role.