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Towards near-field single molecule surface enhanced Raman scattering detection

Posted on:2001-03-29Degree:Ph.DType:Dissertation
University:University of Toronto (Canada)Candidate:Tay, Li-LinFull Text:PDF
GTID:1461390014959149Subject:Chemistry
Abstract/Summary:
The surface enhanced Raman scattering spectra of crystal violet and R6G and a number of colourless molecules adsorbed on silver colloid aggregates were studied as a function of power and position using a Raman microscope. The anti-Stokes signals of the dye molecules show line intensities much greater than the expected on the basis of the Boltzmann equilibrium population of the excited vibrational state. The large anti-Stokes intensities are attributed to the difference in Stokes and anti-Stokes Raman cross-sections resulting from pre-resonance or resonance Raman processes in the adsorbate-substrate complex rather than to optical pumping of molecular vibrational excited states as proposed by Kneipp and co-workers [Phys. Rev. Lett., 76, 2444, 1996]. Far-field imaging with a Raman microscope shows no significant inhomogeneity of the Raman enhancement in the images of the deposited self-affine silver colloid clusters down to spatial resolutions of I μm2.; A near-field scanning optical microscope (NSOM) operating with a shear-force feedback distance-control mechanism was employed to study the optical response of self-affine, deposited colloidal silver films. In the near-field region, NSOM is capable of an optical resolution down to 100 nm which is beyond the traditional diffraction limit of far field optics. Near-field Rayleigh microscopy with simultaneous topographic imaging indicates that the electromagnetic fields on the surface of the self-affine films are localized in regions smaller than the excitation wavelength. The pattern of high-field regions varies greatly with excitation wavelengths and does not correlate with topography.; Local spectra of self-affine film showed distinct resonances arising with highly location-dependent frequencies. The most highly resolved of these resonances correspond to individual surface plasmon normal modes.; Specific vibrational bands from the SERS spectra of plithalazine adsorbed on self-affine, silver colloid films were mapped over portions of the film surface in the near-field. The near-field Raman intensity maps display patterns of field localization in subwavelength regions. The pattern of field localization varies greatly with scattered wavelength. Observations in the near-field validate recent theories of the optical response of self-affine fractal objects [Phys. Report, 61, 272, 1996].
Keywords/Search Tags:Raman, Near-field, Surface, Self-affine, Optical, Silver
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