Nanoscale Probing of Adsorbed Species by Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is based on the optical excitation of localized surface plasmons in the tip-substrate cavity, which provides a large but local field enhancement near the tip apex. We report on TERS with smooth single crystalline surfaces as substrates. The adsorbates were ${\mathrm{C}\mathrm{N}}^{-}$ ions at Au(111) and malachite green isothiocyanate (MGITC) molecules at Au(111) and Pt(110) using either Au or Ir tips. The data analysis yields Raman enhancements of about $4\times {10}^5$ for ${\mathrm{C}\mathrm{N}}^{-}$ and up to ${10}^6$ for MGITC at Au(111) with a Au tip, probing an area of less than 100 nm radius.

Figure 1

Experimental setup for TERS using the $60°$ arrangement. Olympus long distance microscope: $50\times$ magnification, $\mathrm{N}\mathrm{A}=0.5$. He-Ne laser: 5 mW at the sample, ${\lambda }_{\mathrm{e}\mathrm{x}}=632.8\mathrm{n}\mathrm{m}$.

Figure 2

TERS for ${\mathrm{C}\mathrm{N}}^{-}$ at Au(111). Bottom curve for the tip-retracted case and top curve for the tip-tunneling case. Cyanide ions adsorbed from an aqueous ${10}^{-3}M\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{N}+0.1M{\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\mathrm{O}}_4$ solution. Acquisition time: 30 s. Laser power: 5 mW. Adsorption and emersion potential: $-0.75\mathrm{V}$ vs SCE. Tunneling current: 1 nA; voltage: $-150\mathrm{m}\mathrm{V}$.

Figure 3

TERS and RRS for other tip-metal configurations. Top panel: Ir tip/Au(111); acquisition time 30 s. Bottom panel: Au tip/Pt(110); acquisition time 2 s. For both configurations, the MGITC dye is adsorbed from a ${10}^{-6}M$ ethanol solution for 30 min. Laser power: 5 mW. Tunneling current: 1 nA; voltage: $-150\mathrm{m}\mathrm{V}$.

Figure 4

Comparison of RRS and TERS spectra for malachite green isothiocyanate adsorbed at a Au(111) surface. The laser power in the TERS case is reduced to 0.5 mW; the spectral intensities are normalized to full laser power (5 mW) and acquisition time 1 s. The actual acquisition times were TERS, 1 s and RRS, 60 s. The MGITC dye is adsorbed from a ${10}^{-7}M$ ethanol solution for 30 min. Tunneling current: 1 nA; voltage: $-150\mathrm{m}\mathrm{V}$. Inset: Time dependence of the integral intensity of the $1618{\mathrm{c}\mathrm{m}}^{-1}$ band for the reduced laser power.