Concentration-Dependent SERS of P3HT in the Ag/P3HT/MBA System: A charge transfer study

Organic semiconductors are widely attracting attention as a new type of surface-enhanced Raman scattering (SERS)-active substrate. To design and fabricate high-performance SERS-active materials based on organic semiconductors, enhancing the SERS activity and exploring the SERS enhancement mechanisms are important. In this study, a sandwich structure of metal/organic semiconductor/probe molecules was fabricated via a layered self-assembly method. By changing the concentration of the poly(3-hexylthiophene) (P3HT) solution to control the thickness of the P3HT thin film, the dielectric environment surrounding the Ag nanoparticles could be changed, and the localized surface plasmon resonance phenomenon could be effectively controlled. The enhancement of the SERS performance of the Ag/P3HT composite system was studied using mercaptobenzoic acid (MBA) as a probe molecule. Our results showed that the increase in the SERS intensity tended to be stable, and the degree of charge transfer reached the maximum value at a P3HT concentration of 10^-5 g/mL. Our findings also suggested that the surface plasmon resonance effect at the Ag/P3HT interface promoted transfer of electrons from the conduction band of Ag/P3HT to the lowest unoccupied molecular orbital of MBA, significantly enhancing the SERS intensity. This study, in which the detection limit for MBA molecules was as low as 10^-10 mol/L, confirmed the excellent sensitivity and efficiency of the Ag/P3HT composite as a SERS substrate. The combination of the high electromagnetic field performance of Ag with the efficient charge transfer capability of P3HT significantly improved the SERS activity and detection sensitivity. This study broadens the scope of research on SERS substrates and contributes to the elucidation of the SERS mechanism.