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.
Apertureless near-field optical spectroscopy is a means of performing spatially resolved analyses of chemical compositions of surface regions of nanostructured materials. In apertureless near-field spectroscopy, it is common practice to utilize nanostructured probe tips or nanoparticles (usually of gold) having shapes and dimensions chosen to exploit plasmon resonances so as to increase spectroscopic-signal strengths. To implement the particular prior approach to which the present approach is an alternative, it is necessary to integrate a Raman spectrometer with an AFM and to utilize a special SERS-active probe tip. The resulting instrumentation system is complex, and the tasks of designing and constructing the system and using the system to acquire spectro-chemical information from nanometer- scale regions on a surface are correspondingly demanding.
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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