Subnanometric Near-Field Raman Investigation in the Vicinity of a Metallic Nanostructure

We present a near-field Raman investigation in the subnanometric vicinity of a metallic nanotip, where the tip-sample distance is precisely controlled by our newly developed time-gated illumination technique. Using this scheme on an isolated carbon nanotube, we have profiled the spatial decay of evanescent light. We also investigated extremely short-ranged chemical and mechanical interactions between the metal on the tip apex and the molecules of an adenine sample, which are observable only within the subnanometric vicinity of the tip. The results show a near-field Raman investigation with an accuracy of better than a few angstroms. Further, this shows strong promise for superhigh resolution in optical microscopy based on this technique.

Figure 1

(a) The tip-sample arrangement is selectively illuminated only for a predecided tip-sample distance. (b) The sinusoidal oscillation of the tip and the synchronized opening of the time gate are illustrated. The tip-sample distance and the resolution in the distance can be selected by choosing certain values of $\tau$ and $\Delta$. (c) The complete system of tip-enhanced Raman spectroscopy using a tapping-mode AFM and acousto-optic modulator for time-gated illumination.

Figure 2

(a) Raman spectra of an isolated SWNT at four different indicated tip-sample distances. (b) A plot of integrated Raman intensity of the $G$-band versus the tip-sample distance $d$. The solid curve represents a profile obtained from FDTD simulation.

Figure 3

(a) Raman spectra of an adenine nanocrystal at indicated tip-sample distances. The peaks marked by ${\omega }_0$ and ${\omega }_1$ represent the unperturbed RBM and the frequency-shifted RBM, respectively. Deconvoluted Lorentzian peaks for each spectrum are also shown by the dotted curves. (b) Tip-sample distance dependence of Raman shift, and (c) of the peak intensity, of the two Raman modes ${\omega }_0$ and ${\omega }_1$. The dashed lines indicate the surface of the sample ($d=0$). An integrated intensity plot is also shown in (c), which represents total enhancement of the RBM. (d) A schematic illustration of 1D scan of an isolated SWNT along a direction perpendicular to its axis. (e) The corresponding TERS intensity increases when the tip is exactly above the SWNT, depicting a spatial resolution of 3 nm.