The practical electromagnetic effect in surface-enhanced Raman scattering observed by the lithographically fabricated gold nanosquare dimers

We experimentally investigated the practical electromagnetic effects in surface-enhanced Raman scattering (SERS), using the precisely fabricated array of gold nanosquare dimers. The array of gold nanosquare dimers was fabricated by electron-beam exposure and the lift-off process. The Raman spectra of 1,4-benzenedithiol and 4-aminothiophenol were obtained at room temperature by exciting the longitudinal mode of the plasmon of gold nanodimers. The gap-size dependency of the enhancement factor of the complete symmetric mode clearly exhibited exponential decay with an increase in the gap size. The exponential decay behavior of each mode and molecule did not show a critical difference in the decay constant. The exponential decay of the enhancement factor was explained by the discrete dipole approximation. The pre-exponential factor obtained by the ν7a modes of ATP was much larger than that of BDT; however, the gap-size dependency of the enhancement factor of the ν8a mode did not exhibit a significant difference in the pre-exponential factors between BDT and ATP that reflects the difference in the chemical effects in SERS between vibration modes. By estimating the gap size dependency of the SERS intensity, we were able to compare the intrinsic chemical effects on the SERS enhancement between two molecules.