Electric field enhancement distributions encountered in atomic force microscopy (AFM) tip-enhanced surfaceenhanced Raman spectroscopy (SERS) experiments (AFM-SERS) are simulated using a frequency-domain three-dimensional finite element method to solve Maxwell's equations of electric field distributions. We simulated an electromagnetic field enhancement in the vicinity of an AFM tip in close proximity to silver spherical nanoparticles under the illumination of a laser beam of various incident angles under different geometric arrangements. Maximum electric field enhancement is discussed in terms of the relative position of the tip and nanoparticles, as well as the direction of excitation laser propagation. Our results suggest new approaches for using AFM-SERS tip-enhanced near-field technique to image samples on surfaces.

A near-field optical imaging approach is demonstrated for simultaneous topographic and spectroscopic imaging with spatial resolution beyond the optical diffraction limit. The method combines metallic-tip-based tapping-mode atomic force microscopy (AFM) with fluorescence lifetime imaging microscopy (FLIM). The AFM metallic tip was formed by sputter coating a Si tapping mode tip with Au, in a way that forms a globular tip apex. Such tip apex generates high local electric field enhancement under laser illumination, which provides a strong electric-field interaction between the AFM tip and the fluorescent molecules under the tip. The tip perturbation of fluorescence gives the fluorescence lifetime changes that provide the AFM–FLIM imaging contrast. A finite element method simulation was used to further evaluate the electric near-field enhancement and electric field distribution originating from the metallic Au-coated AFM tapping-mode tip. We have demonstrated that spatially mapping the change in fluorescence lifetime and intensity is a promising approach to spectroscopic imaging at an AFM spatial resolution typically defined by the apex diameter of the AFM tips. The globular Au-coated AFM tip not only gives adequate spatial AFM tapping-mode imaging spatial resolution but also is “environmentally friendly” to soft samples, such as polymeric dye-labeled nanospheres and even biological specimens such as POPO-3 labeled DNA.

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The correlated metallic tip-enhanced Raman spectroscopy and atomic force microscopy (AFM) technique was used to characterize dye-sensitized titanium oxide (TiO2) nanoparticles. We have obtained the near-field Raman spectra that are associated with the photo-induced charge transfer reaction in Ru(4,4’-dicarboxy-2,2’-bipyridine)2(NCS)2-sensitized TiO2 single nanoparticles. This method demonstrates that tip-enhanced near-field Raman spectroscopy is an effective approach for understanding inhomogeneous interfacial electron transfers with nanoscale spatial resolution.

We report on (1) simulations of the influence of different surface morphologies on electromagnetic field enhancements at the rough surfaces of noble metals, and (2) the evaluations of the optimal conditions for the generation of a surface-enhanced Raman signal of absorbed species on a metallic substrate. All simulations were performed with a classical electrodynamics approach using the full set of Maxwell's equations that were solved with the three-dimensional finite element method (FEM). Two different classes of surfaces were modeled using fractals, representing dendritic and sponge-like structures. The simulations depict the high inhomogeneity of an enhanced electromagnetic field as that both a field enhancement and a field attenuation near the surface existed. While the dendritical fractals enhanced the local electromagnetic field, the sponge-like fractals significantly reduced the local electromagnetic field intensity. Moreover, the fractal orders of the fractal objects did not significantly alter the total enhancement, and the distribution of a near-field enhancement was essentially invariant to the changes in the angle of an incoming laser beam.