Tip-enhanced Raman spectroscopy and microscopy on single dye molecules with15 nm resolution

Our recently developed approach of UHV-tip-enhanced Raman spectroscopy permits us to acquire Raman spectra of a few single brilliant cresyl blue (BCB) molecules and even a single one adsorbed on a Au(111) surface. This is substantiated by simultaneously recorded STM images. Furthermore, due to the reduced photobleaching in UHV, the time frame for spectral acquisition is sufficiently extended to allow tip-enhanced Raman imaging of a single BCB molecule with a lateral resolution of 15 nm

High-resolution microscope for tip-enhanced optical processes in ultrahigh vacuum

An optical microscope based on tip-enhanced optical processes that can be used for studies on adsorbates as well as thin layers and nanostructures is presented. The microscope provides chemical and topographic informations with a resolution of a few nanometers and can be employed in ultrahigh vacuum as well as gas phase. The construction involves a number of improvements compared to conventional instruments. The central idea is to mount, within an UHV system, an optical platform with all necessary optical elements to a rigid frame that also carries the scanning tunneling microscope unit and to integrate a high numerical aperture parabolic mirror between the scanning probe microscope head and the sample. The parabolic mirror serves to focus the incident light and to collect a large fraction of the scattered light. The first experimental results of Raman measurements on silicon samples as well as brilliant cresyl blue layers on single crystalline gold and platinum surfaces in ultrahigh vacuum are presented. For dye adsorbates a Raman enhancement of ~10^6 and a net signal gain of up to 4000 was observed. The focus diameter (~λ/2) was measured by Raman imaging the focal region on a Si surface. The requirements of the parabolic mirror in terms of alignment accuracy were experimentally determined as well

Tip-enhanced Raman spectroscopy – How far can the near-field reach?

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)