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J. Phys. Chem. B 2006, 110, 19804-19809
High-Resolution Apertureless Near-Field Optical Imaging Using Gold Nanosphere Probes† Zee Hwan Kim# and Stephen R. Leone* Department of Chemistry and Physics, UniVersity of California, Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460 ReceiVed: March 7, 2006; In Final Form: May 9, 2006
An apertureless near-field scanning optical microscope (ANSOM) that utilizes the enhanced field around a gold nanosphere, which is attached to the end of an atomic force microscope (AFM) tip, is used to image the local dielectric constant of the patterned metallic surfaces and local electric field around plasmonic nanosphere samples. A colloidal gold nanosphere (∼50 nm diameter) is linked to the extremity of the conventional etchedsilicon probe. The scattering of laser radiation (633 or 532 nm) is modulated by the oscillating nanospherefunctionalized silicon tip, and the scattered radiation is detected. The approach curve (scattering intensity as a function of the tip-sample distance), the polarization dependence (scattering intensity as a function of the excitation polarization direction), and ANSOM image contrast confirm that the spherical nanosphere attached to the silicon tip acts as a point dipole that interacts with the sample surface via a dipole-dipole coupling, in which the dipole created by the field at the tip interacts with its own image dipole in the sample. The image obtained with the nanoparticle functionalized tip provides a dielectric map of the sample surface with a spatial resolution better than 80 nm. In addition, we show that the functionalized tip is capable of imaging the local electric field distribution above the plasmonic nanosphere samples. Overall, the result shows that high-resolution ANSOM is possible without the aid of the lightning-rod effect. With an improved tip-fabrication method, we believe that the method can provide a versatile high-resolution chemical imaging that is not available from usual forms of ANSOM.
Introduction Conventional far-field optical microscopy cannot provide spatial resolution better than half of the optical wavelength (∼300 nm) because of the diffraction of light. The recently developed near-field scanning optical microscopy (NSOM) technique based on metal-clad fiber probes offers resolution below the diffraction limit,1-3 but the practical resolution is limited to 60-100 nm4 because of the finite skin-depth of the metal coating and the low optical transmission of the probe. An alternative form of NSOM that utilizes the local field enhancement at the end of an externally illuminated metallic probe (an “apertureless” probe) has also been demonstrated, and various aspects of this new imaging technique are being explored.5-18 The resolution of the so-called tip-enhanced or apertureless NSOM (ANSOM) is only limited by the radius of curvature of the probe, and therefore it promises unprecedented optical resolution (
