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Abstract:
Over the past few years Tip-Enhanced Raman spectroscopy (TERS) has been developed into a versatile analytical tool for the detection and identification of (sub)monolayer adsorbates at single crystalline metal surfaces [1, 2]. The illumination of a gold STM tip in close vicinity, i.e. in tunneling contact, to a sample surface generates a strongly enhanced electromagnetic field at the tip apex. However, the size of this enhanced field in relation to the tip curvature is still under debate.
One approach towards the determination of the nearfield dimension is the measurement of the tip-sample distance dependence of the TER signal. This is achieved by applying an external voltage to the z-piezo which controls the tip movement in z-direction perpendicularly to the sample surface. After switching off the feed-back loop of the STM, the z-scan of the tip is controlled by a programmable ramp generator, which can be set to different speeds. During the retraction of the STM tip, a series of Raman spectra is taken subsequently at intervals of approximately 1.5 seconds.
Our experimental results – we monitor the A1 breathing mode of ClO4- at Au(111) – show a fast decay of the TER signal within 10 to 15 nm tip-sample distance for smooth tips of about 20-30 nm radius. These findings are in agreement with a simple theoretical model [3] which assigns an R-10 nearfield distance dependence to the TER profile of a 2-dimensional object, i.e. the adsorbate layer. According to the model, we expect different slopes for the signal distance curves for varying tip radii, e.g. for larger tip curvatures we expect a slower decay of the signal intensity with increasing tip-sample distance. As a first estimate, we find that the radius of the enhanced field is about half the radius of the tip apex.
[1] B. Pettinger, B. Ren, G. Picardi, R. Schuster, G. Ertl, Phys. Rev. Lett. 92, 0906101 (2004).
[2] B. Ren, G. Picardi, B. Pettinger, R. Schuster, G. Ertl, Angew. Chem. Int. Ed. 44, 139 (2005).
[3] B. Pettinger, B. Ren, G. Picardi, R. Schuster, G. Ertl, J. Raman Spectrosc. 36, 541 (2005).
