It is not necessary to integrate an AFM with a spectrometer.
An alternative approach to apertureless near- field optical spectroscopy involving an atomic-force microscope (AFM) entails less complexity of equipment than does a prior approach. The alternative approach has been demonstrated to be applicable to apertureless near-field optical spectroscopy of the type using an AFM and surface enhanced Raman scattering (SERS), and is expected to be equally applicable in cases in which infrared or fluorescence spectroscopy is used.
In the present alternative approach, unlike in the prior approach, one does not integrate a spectrometer with the AFM; that is, the spectrometer and the AFM are separate instruments. Moreover, instead of using a special SERS-active AFM/spectrometer probe tip, one fabricates SERS-active regions at locations of interest on the specimen surface by using an AFM tip to deposit gold nanoparticles at those locations.
The first step is to image the specimen by use of the AFM to establish the locations of interest for high-resolution spectro-chemical analysis. Then SERS-active regions are fabricated at those locations by a form of dip-pen nanolithography: The AFM tip is dipped into a colloidal gold solution and used to deposit a single gold nanoparticle or a cluster of gold nanoparticles at each affected location (see figure). Then the AFM is disengaged, the deposited nanoparticles are illuminated in the spectrometer excitation beam, and the locally enhanced spectrum is acquired. Optionally, the AFM tip or the cantilever on which it is mounted can be moved above the deposited nanoparticles to modulate the light to enhance discrimination between the particle-enhanced components of the signal and the components from illuminated areas surrounding the particles.
This work was done by Mark S. Anderson of Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-44033, volume and number of this NASA Tech Briefs issue, and the page number.