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Free keywords:
tip-enhanced Raman spectroscopy (TERS); apertureless scanning near-field optical microscopy (aSNOM); parabolic mirror; single-molecule detection; plasmons
Abstract:
Following the Abbe limit, the minimum distance between two point light sources that can be resolved by far-field optical methods is approximately half the wavelength of the light. Thus, for the investigation of chemical processes on a molecular level, the diffraction limit of light has to be overcome. This can be achieved by near-field optical methods, where the spatial resolving capacity is determined by the size of the structures, at which the near-field is generated.
In this work a high-resolution microscope based on tip-enhanced optical processes is presented , which can be used for studies on molecular adsorbates as well as thin layers and nanostructures. The microscope provides chemical and topographic information with a resolution of a few nanometers and can be employed in ultrahigh vacuum as well as in 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 on a rigid frame that also carries a scanning tunneling microscope unit, and to insert a parabolic mirror with high numerical aperture 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 laser light transfered into the UHV system and the scattered light is guided to the spectrograph by two fiber optical wave guides.
Experimental results of tip-enhanced Raman spectroscopic and microscopic measurements on silicon wafers as well as brilliant cresyl blue adsorbates on single crystalline gold and platinum surfaces in ultrahigh vacuum are presented. A Raman enhancement of ~10^6 and a net signal gain of up to 4000 was observed for dye adsorbates. Single dye molecules were imaged by tip-enhanced Raman microscopy with a lateral resolution of ~15 nm.
