Towards time resolved tip-enhanced Raman spectroscopy: TERS with chopped laser pulses

In this work, the use of a mechanically chopped laser beam as excitation source of an ambient tip-enhanced Raman (TER) spectrometer has been studied. In the experiment, all macroscopic variables are identical except the nature of the laser excitation, with the objective to increase the time resolution of this technique toward the (sub)millisecond time scale. A (sub)monolayer of thiophenol adsorbed on an Au(111) surface was analyzed in terms of the stability of the TER signal as a function of time and laser power. The results show that TER spectra with chopped excitation have similar stability, sensitivity, spectral resolution, peak position and peak width and signal-to-noise ratio as the ones recorded by employing continuous wave (cw) excitation. The proposed methodology allows to successfully overcome the trade-off of spectral and temporal resolution given by ps or fs excitation and increases the pulsed laser power tolerated by the tip/sample junction and the adsorbed molecules before degradation occurs. This works opens the possibility to perform ambient TER measurements with a temporal resolution on the millisecond scale at spectral resolution and SNR comparable to the one achieved with cw excitation without modification of the standard detection system of the TER set-up.