SERS as a Probe of Charge-Transfer Pathways in Hybrid Dye/Molecule-Metal Oxide Complexes

Interfacial charge transfer has been an area of intense interest because of its relevance in molecular electronics, dye-sensitized solar cells, surface-enhanced Raman scattering (SERS), and photocatalysis. Although the chemical natures of both the contact and the linker have been shown to play important roles in determining the properties of hybrid dye/molecule-metal oxide complexes, little is known about the nature of the charge-transfer pathways. In this work, we explore in detail the idea that Raman enhancement and charge transfer are intimately related. To this end, we analyze the vibrational modes of molecules exhibiting the maximum enhancement of the Raman activities when they are adsorbed on semiconducting metal oxide nanoparticles. Our analysis of the potential energy distributions of these modes in the hybrid complexes indicates the significant involvement of bending and torsional modes of atoms deep within the metal oxide nanoparticle. Whereas the individual contribution of each of these oxide bending and torsional modes is very small (similar to 1%), their cumulative contribution (similar to 20-35%) is substantial. We found that the observed Raman enhancement can be correlated to changes in the magnitude of the atomic polarizabilities. More importantly, we note that there is a direct correlation between the observed Raman enhancement and the electron-transfer rates across the molecule-metal oxide interface. Although the current work is a step in our attempts to find a propensity rule connecting Raman enhancement and charge transfer through preferential modes, the involvement of the low-frequency torsional modes of the metal oxide implies that modes involving both the molecule and atoms deep inside the nanoparticle could be responsible for the bulk of charge transfer. The results of the current work are also relevant in understanding the nature of charge-transfer pathways in dye-sensitized solar cells and photoinduced catalysis. The identification of vibrational modes involved in enhancement of the Raman response could lead to interesting insights into interfacial energy transfer and thermoelectric effects in nanosystems.