Probing Raman enhancement in a dopamine-Ti₂O₄ hybrid using stretched molecular geometries

Hybrids consisting of a metal oxide nanoparticle and a molecule show strong enhancement of Raman modes due to an interfacial charge transfer process that induces the formation of midgap states, thereby reducing the effective gap compared to that of the nanoparticle and creating the posibility of an electronic resonance at energies substantially lower than the nanoparticles's band gap. We have developed a simple methodology to mimic the presence of the nanoparticle through a deformation of the bond involved in the chemical binding between the two entities forming the hybrid. The results provide a convincing interpretative frame to the enhancements observed in Raman spectra when all atoms are included. In addition, these enhancements can be correlated to a crossing of excited molecular orbitals that take part in the virtual excitation associated with the Raman process. We illustrate our method for the dopamine-Ti₂O₄ hybrid using the most acidic molecular O-H bond as the control parameter for the deformation.