This article reports on a project that developed an analytical expression for the lowest order non-zero contribution to the surface-enhanced Raman spectrum from a system composed of a molecule adsorbed on a semiconductor nanoparticle.
Researchers considered a combined molecule-semiconductor system and included Herzberg–Teller vibronic coupling of the zero-order Born–Oppenheimer states. This followed a previous derivation for metallic SERS, but instead of a Fermi level, the semiconductor system involved a band gap, and researchers found that the SERS enhancement was maximized at either the conduction or valence band edge. The resulting expression may be regarded as an extension of the Albrecht A-, B-, and C-terms and shows that the SERS enhancement is caused by several resonances in the combined system, namely, surface plasmon, exciton, charge-transfer, and molecular resonances. These resonances are coupled by terms in the numerator, which provide strict selection rules that enabled researchers to test the theory and predict the relative intensities of the Raman lines. Furthermore, by considering interactions of the various contributions to the SERS enhancement, researchers were able to develop ways to optimize the enhancement factor by tailoring the semiconductor nanostructure, thereby adjusting the location of the various contributing resonances. (publisher abstract modified)