Toward Raman Fingerprints of Single Dye Molecules at Atomically Smooth Au(111)

Abstract

The creation of a highly enhanced electromagnetic (EM) field underneath a scanning tunneling microscope (STM) tip enables Raman spectroscopic studies of organic submonolayer adsorbates at atomically smooth single crystalline surfaces. To study the sensitivity of this technique, tip-enhanced resonance Raman (TERR) spectra of the dye malachite green isothiocyanate on Au(111) in combination with the corresponding STM images of the probed surface region were analyzed. The detection limit for unambiguous identification of the dye and semiquantitative determination of the surface coverage reaches < or =0.7 pmol/cm(2), or approximately five molecules present in the enhanced-field region, which is confirmed by STM images. Because of well-defined adsorption sites at atomically smooth Au(111) surfaces, no variation in band positions or relative band intensities was observed at the single- or few-molecule detection level when employing TERR spectroscopy.

Surface-enhanced Raman spectroscopy at single-molecule scale and its implications in biology

“…To achieve this EF, nanostructures with hot spots are the prerequisite as demonstrated since the first observation of SM-SERS in 1997 [18,19]. A class of silver and gold nanostructures that has provided large EFs is colloidal aggregates [55], as well as metal evaporated films [56] and, most recently, modified scanning tunnelling microscopy (STM) tips [57]. Although the easiest route to achieve SM-SERS is to use the aggregated metal colloid, which could be obtained by simply changing the solvent and the ionic strength, the aggregation state is hard to control because signals from randomly aggregated nanoparticles often suffer from uncertainty in the size of aggregates and geometry.…”

Local field enhancement of an infinite conical metal tip illuminated by a focused beam

“…This could explain the different results reported by two independent groups, in which the Raman spectra from similar molecules were very different. [10,11] The tips used by Neascu et al [11] have an apex diameter r ∼ 10 nm, while in the case of Domke et al, [10] r was >20 nm. This variety could cause a twofold difference in the field enhancement and consequently a fourfold difference in the heat generated by ohmic loss.…”

Tip enhanced Raman scattering: plasmonic enhancements for nanoscale chemical analysis

“…Proper configuration of the gap provides an enhancement that is sufficiently strong to detect the Raman scattering from a single molecule [5]. The use of a metal surface to enhance TERS signals has become known as gap-mode TERS and it has seen use with both STM [39][40][41][42][43][44] and AFM [45,46] based TERS instruments. In the absence of a gap-mode, most TERS measurements require acquisitions on the order of a few seconds to generate appreciable signals from collections of molecules [47,48].…”

“…Similarly, organic molecules with analogous properties have also been studied by TERS. Brilliant cresyl blue, nile blue, rhodamine, and malachite green isothiocyanate are common dye molecules studied [1,5,41,42,65,[108][109][110][111]. The large SERS and TERS signals associated with these molecules lend themselves naturally to single molecule demonstrations [5,108].…”