Tip‐enhanced Raman spectroscopy (TERS) of malachite green isothiocyanate at Au(111): bleaching behavior under the influence of high electromagnetic fields

Abstract

Tip-enhanced Raman spectroscopy (TERS) was investigated with malachite green isothiocyanate adsorbed at an Au(111) surface. TERS is based on the excitation of localized surface plasmons in the tip apex, producing strongly enhanced electromagnetic fields. The key conditions for giant TERS are sideillumination of the tip, well-prepared single-crystalline surfaces and sharp, smooth gold tips. A TERS enhancement of about 6 × 10 6 has been observed for dye molecules adsorbed at the Au(111) substrate in a region of about 50 nm diameter beneath the tip. This corresponds to a 2500-fold increase in the light intensity at the Au(111)/air interface, which in addition causes fast but local bleaching of the dye. This bleaching behavior was analyzed in detail, giving direct insight into the strength and size of the enhanced field. In addition, the bleaching constant was higher for MGITC in an unperturbed environment than for MGITC in an environment that had been substantially bleached. The MGITC spectra were also different for these two cases. Copyright  2005 John Wiley & Sons, Ltd.KEYWORDS: apertureless scanning near-field optical microscopy; gold(111); tip-enhanced Raman spectroscopy; malachite green isothiocyanate; near-field enhancement INTRODUCTIONThe general use of Raman spectroscopy for surface studies is an old dream of surface scientists. However, with differential Raman cross-sections of the order of d /d ³ 10 31 -10 28 cm 2 sr 1 , surface studies were out of reach in general, although a (small) number of attempts were made to apply (normal) Raman scattering for the investigation of well-defined surfaces.Ł Correspondence to: Bruno Pettinger, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany. E-mail: pettinger@fhi-berlin.mpg. Recently, a new approach has been developed, denoted tip-enhanced Raman spectroscopy (TERS), in some cases also called 'apertureless scanning near-field optical microscopy (SNOM)'. 1 With the advent of TERS, the above-mentioned dream is now becoming a reality. 1 -12 TERS is a particular promising variant of SERS, the well-studied field of surface-enhanced Raman scattering spectroscopy. The latter requires roughened surfaces at which two cooperative enhancement mechanisms for Raman scattering are working. Roughened surfaces can support the excitation of localized surface plasmons leading to the so-called 'electromagnetic enhancement' and provide the peculiar adsorption sites at which the so-called 'chemical enhancement' is operative. In the literature, a large range of (total) enhancement factors are given, starting with the 10 6 -fold enhancement found for pyridine on roughened silver electrodes 13 up to the giant 10 12 -10 14 -fold enhancement reported more recently by Nie and Emory 14 and Kneipp et al. 15 for dyes at colloidal particles. In the latter case, the authors claim to have reached single-molecule detection sensitivity. However, in all these cases the general drawback of SERS investigations remains: the surface enhancement (in particular its ...

Raman analysis of ancient pigments on a tile from the Citadel of Algiers

“…It may also be the result of a small Raman cross section for the pigment in this glaze. Minimum differential Raman cross-sections are in the order of (d /d˝) ≈ 10 −31 -10 −28 cm 2 sr −1 [34]. Although the sampled volume at the focal point is in orders <1 m 3 , if the pigment concentration in the glaze of this tile is in orders of magnitude comparable with molecular monolayers within the sampled volume, the detection of the pigment will be out of reach without any additional forms of Raman signal enhancements such as Surface Enhanced Raman Spectroscopy (SERS) or resonance effects.…”

Performing tip‐enhanced Raman spectroscopy in liquids

“…This is much smaller than the extent of the enhanced field, but the field enhancement is the strongest at the tip's surface and drops rapidly with increasing distance from the tip. [7,46] (2) Direct contact between Ag and analyte is prevented, i.e. additional chemical enhancement is suppressed.…”

Comment on “Scanning-probe Raman spectroscopy with single-molecule sensitivity”

“…[17][18][19] A study on the bleaching behaviour on Malachite Green Isothiocyanate (MGITC), a "sister-dye" of MG, revealed a bleaching time constant τ of 0.7 s for MGITC/Au(111) in the presence of the tip for a simliar incident intensity of ∼ 5 mW and a 10 3 -fold intensity increase near the tip. 20,21 For MG, which lacks only the SCN-group in comparison to MGITC, photodecomposition will occur at a similar or even higher rate, because in the experiment reported in Ref. 1, the estimated 5 x 10 9 TER enhancement is accompanied by a 7 x 10 4 -fold enhanced intensity underneath the tip.…”