Implementation of Tip Enhanced Raman Spectroscopy for opaque samples in liquid
NT-MDT Spectrum Instruments has demonstrated enhancement of the Raman scattering (TERS) in application to an opaque sample - layer of organic dye deposed on a golden substrate - in liquid with the upright optical configuration NTEGRA Spectra II.
Implementation of TERS in liquid brings out some difficulties. The main one is the delicate optical alignment to achieve accurate focusing of the laser beam on the tip apex and also to ensure an efficient collection of the scattered signal especially through a layer of liquid. In case of transparent samples set on an inverted optical microscope focusing of the laser beam on the TERS probe is therefore easily achieved directly through the sample. With opaque samples that require top or side illumination/collection, the optical signal is deviated and considerably attenuated by refraction at each interface. One of the possible ways to avoid refraction on air-liquid surface and to achieve stable light path is to use immersion objective. NTEGRA Spectra II is the only instrument which provides such possibility as AFM beam deflection system works through the same objective as Raman laser.
During the experiment in AFM optical head a polarized excitation laser (λexc = 632.8 nm) was focused on the AFM TERS probe apex through a long distance 60× water immersion objective (working distance: 2 mm) with high numerical aperture (N.A. = 1). Therefore, the AFM tip holder can fit between the objective and the sample. Laser spot scanning was performed by scanning mirror.
The enhanced spectrum and the one obtained from tip apex are presented in the figure and confirm the strong enhancement of the signal from around the “hot-spot” location.
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