Direct monitoring of plasmon resonances in a tip-surface gap of varying width

We have studied the dependence of tip-enhanced Raman (TER) spectra composed of discrete Raman lines and an intense Lorentzian-shaped background on varying interparticle distance. For increasing tip-sample separation, we observe a remarkable blueshift of the background maximum that is ascribed to an energy shift of the localized surface plasmon resonance. In addition, we report on a steep background and Raman band intensity decrease within $5–10\mathrm{nm}$ tip retraction that highlights the underlying near-field enhancement mechanism of TER spectroscopy.

Figure 1

(Color) TER spectra for coadsorbed guanine and $\mathrm{Cl}{\mathrm{O}}_4^{-}$ for different distances $z$ of the STM tip to the surface: Five spectra from a series of 15; (a)–(e) $z=1–5.2\mathrm{nm}$. Retraction speed: $0.16\mathrm{nm}∕\mathrm{s}$. Smooth red lines: bg curves from a Lorentzian fit. Inset: set of nine spectra of the same series and their fit curves (red curves) expanded around $930{\mathrm{cm}}^{-1}$.

Figure 2

(Color) Plot of the energy of the bg maximum vs tip-metal distance. The solid gray line is a guide to the eye. Retraction speeds: (∎) $1.6\mathrm{nm}∕\mathrm{s}$, (▴) $0.32\mathrm{nm}∕\mathrm{s}$, and (●) $0.16\mathrm{nm}∕\mathrm{s}$. Horizontal line: energy of the bg maximum of a free tip at $1.906\mathrm{eV}$.

Figure 3

(Color) Double logarithmic plot of the integral bg and Raman band intensities for varying distances $z$ of the STM tip to the surface in panels (a) and (b), respectively. Panel (a): inset shows the scanning electron microscope image of the STM tip. Panel (b): Raman intensities of the $\mathrm{Cl}{\mathrm{O}}_4^{-}$ stretch are plotted vs $z$. Retraction speeds: (∎) $1.6\mathrm{nm}∕\mathrm{s}$, (▴) $0.32\mathrm{nm}∕\mathrm{s}$, (●) $0.16\mathrm{nm}∕\mathrm{s}$, and (⊡) theoretical data in panel (b) from Fig. 4(b) of Ref. 39. Gray lines in both panels: the same fit curve used as a guide to the eyes.