Scanned probe method boosts Raman sensitivity by factor of 4000
Researchers at the National Institute of Standards and Technology (NIST; Boulder, CO) have achieved spatial resolution on the order of 2 nm for Raman and fluorescence spectroscopy--this represents a 4000-fold sensitivity increase for near-field scanning optical microscopy (NSOM). While 1-nm resolution has been reported previously for interferometric spectroscopy, the NIST researchers say they are the first to achieve this type of resolution with nonresonant spectroscopy.
To achieve the improved sensitivity, the traditional subwavelength-aperture-based NSOM technique was replaced with an apertureless method based on the scanning tip of an atomic force microscope (AFM). The sample, illuminated from below through a prism by a 543-nm laser, was scanned from above with the AFM probe. Evanescent-field interactions between the sample and the AFM probe were measured with a photomultiplier. A key difference from previous scanned-probe efforts was a "liftoff" correction, applied to eliminate the influence of probe scattering from the sample measurement. While the resolution of traditional NSOM techniques is limited by diffraction effects to about 20 nm, resolution of the scanned-probe technique could theoretically proceed down to the atomic level, limited only by the sharpness of the probe.