DFT studies of surface-enhanced Raman spectroscopy of 1,4-benzenedithiol-Au2 complex under the effect of static electric fields

In this work, we demonstrate that the applied electric-field strength and orientation can multiply modulate the Raman intensity and vibrational wavenumber of small molecule-metal complex, 1,4-benzenedithiol-Au-2 (1,4BDT-Au-2), by density functional theory and time-dependent density functional theory simulations. The polarizabilities are changed by the applied electric fields, leading to enhanced specific vibrational intensity and shifted vibrational wavenumber of the surface-enhanced Raman scattering effect. The applied electric fields perturb the bonds and angles of the 1,4BDT-Au-2 complex. Owing to this reason, the peaks of Raman spectra related to these structures exhibit distinguishable responses in quasi-static field (low-frequency oscillating electric field). We use the visualized method of charge difference density to show that the electric fields tune the traditional excited state to pure charge-transfer excited state. The charge-transfer resonance transition produces enhanced Raman intensities for non-totally symmetric modes and totally symmetric modes. These simulation results of the function of static electric field provide new guidance for the surface-enhanced Raman scattering measurements. Copyright (c) 2015 John Wiley & Sons, Ltd.