STM Combined with Raman Spectroscopy

Pettinger et al. observed a TERS spectrum of monolayer-thick brilliant cresyl blue (BCB) adsorbed on a smooth Au film surface by using a Ag tip, while no STM image of the adsorbed surface was shovm [23]. The Raman intensity increased when the tip was in the tunneling position, meaning that the tip-surface distance was around 1 nm. They calculated the field enhancement factor by the method described by 

Demming et al. [17], and compared it with their experimental results. The observed increase in Raman intensity by a factor of 15 within the laser focus of about 2 pm, corresponds to a maximum local enhancement at the center of the tip apex by a factor of about 12 000 under the assumption that the tip radius is 100 nm. More precisely, about 20% of the Raman intensity is considered to originate from an area with a radius of 14 nm under the tip, which is roughly equivalent to 400 molecules out of 2 X 10 in the total focused area. 

Ren et al. reported a method to prepare a gold tip with a tip apex radius of 30 nm reproducibly [27]. They observed the TERS of a Malachite Green isothiocyanate (MGITC) monolayer on an Au(lll) surface and obtained an enhancement factor of about 1.6 X 10, by using the relation, q= /TERs/lRRs=g /l focus where q is the net increase in the signal. Iters snd rrs the signal intensities for TERS and RRS (resonance Raman scattering), respectively is the TERS enhancement (gis the field enhancement), a denotes the radius of the enhanced field, and Rfocus the radius of the laser focus. 

Pettinger et al. observed TERS with a sharp Au tip for MGITG dye on Au(l 11) with a side illumination [28]. They studied the bleaching of the dye and fitted the data by taking into account the radial varying intensity distribution of the field as a Gaussian profile instead of a Heaviside profile. For the former profile, the TERS radius is smaller by a factor of 1/2 than for the latter profile. They obtained a TERS enhancement factor of 6.25 x 10. The radius of the enhanced field Rfieid is about 50 nm, which results in a TERS radius of 25 nm, which is smaller than the radius of the tip apex (about 30 nm). 

Picardi et al. introduced a method to fabricate a sharp Au tip for STM by electrochemical etching [31]. The efficiency of TERS for a thin BCB dye layer using the etched sharp tip was then compared with that using an Au-coated AFM tip. 

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STM-Raman spectroscopy utilizes the effect that Raman scattering is enhanced for a molecule in the vicinity of a metal nanostructure. This enhancement effect is generally called surface-enhanced Raman scattering (SERS). When a sharp scanning probe, such as a tunneling tip for STM, is used as a metal nanostructure to enhance Raman intensity, it is called tip-enhanced Raman scattering (TERS). The concept of STM combined with Raman spectroscopy is presented in Figure 1.1. 

Figure 1.1 Concept of STM combined with Raman spectroscopy.

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