Justification of dipole mechanism over chemical charge transfer mechanism for dipole-based SERS platform with excellent chemical sensing performance

Molecular-dipole-induced surface-enhanced Raman scattering (SERS) has attracted a lot of attention due to its high sensitivity without the need for noble metals. However, its main mechanisms are still unclear. In this work, functional-group-rich graphene oxide (GO)/ZnO (GZ) films are fabricated as SERS platforms. A maximum enhancement factor of 70,000 is achieved by manipulating the chain reactions between the functional groups and ZnO under various annealing conditions. The electronic structures of the GZ composite films, characterized using ultraviolet photoelectron spectroscopy, indicate that the high sensitivity arises from molecular dipole interactions rather than photoinduced charge transfer. Electrostatic force microscopy images show that the amount and spatial uniformity of molecular dipoles on GZ are both significantly improved when appropriate heat treatment is employed, leading to a tremendous 43-fold sensitivity improvement compared to that of pristine GO. Stability tests demonstrate great retention of the dipole-induced enhancement, up to 87%, after more than six months of aging in the dark. The excellent sensitivity, uniformity, and stability make the GZ composite film an excellent SERS platform for practical applications in molecular identification and chemical sensing.