Temporal Fluctuation of Tip-Enhanced Raman Spectra of Adenine Molecules

Temporal fluctuations of spectral shapes have been observed in tip-enhanced Raman scattering (TERS) spectra of adenine molecules. The TERS spectra of a nanocrystalline cluster of adenine molecules showed distinct temporal changes including fluctuations of peak frequencies and peak intensities and extraordinary enhancements of several peaks. The fluctuating TERS signals originate primarily from the molecules in the first layer of the 5 nm thick adenine nanocrystalline cluster. The possible origin of the temporal changes is attributed to the changes of molecular orientation under the metal-coated tip, which is responsible for changing the adsorption angle and the angle of local incident polarization for the molecules adsorbed to the tip. This result suggests the importance of sample−tip interaction at molecular level in the interpretation of TERS spectra and indicates the possibility of the single molecule sensitivity of TERS spectroscopy of DNA base molecules.