Characterization of nanostructures by near-field scanning optical microscopy

This thesis research focuses on applying near-field scanning optical microscopy to characterize designed nanostructures.;The near-field imaging concept is based on Synge's idea that a light source with size smaller than the wavelength can scan a sample point-by-point, sequentially probing its optical property. Near-field scanning optical microscopy (NSOM) is a powerful imaging tool since it provides spectral information at nanoscale in correlation with morphological details. This thesis work utilizes a home-built apertureless NSOM for the investigation of designed nanostructures. In this design, NSOM light source was produced by excitation of commercial silicon nitride (Si3N4) AFM probes with an ultraviolet laser, e.g., 405 nm. Such a light source has the intrinsic advantages of stability, durability and high emission intensity. In addition, utilizing bright photoluminescent (PL) probes simplifies the separation and detection of near-field signals because the PL exhibits different wavelength from the far-field excitation beam.;In terms of the nanostructures fabrication, a method utilizing particles lithography in sequence in conjunction with surface chemistry was developed to produce multicomponent nanostructures. Multicomponent nanostructures with individual geometry have attracted much attention because of their potential to carry out multifunctions synergistically by all components. A film of monodispersed particles serves as a structural mask to guide the deposition of different materials in a designed sequence. After the particle mask is displaced, multicomponent nanostructures are revealed with well-defined sizes and geometries. Such a method has the advantages of simplicity, a high throughput and the capability of patterning a broad range of materials. By changing the size of the particle template and deposition methods, feature size and geometry of the patterns can be well controlled. Furthermore, only one structural mask is applied during the entire patterning process. This method is straightforward and enables designing and constructing two- and three-dimensional structures tailored with designed functionalities.;The aforementioned method can be extended to using different particle masks sequentially. Periodic metal nanostructures of Au and Cu have been produced sequentially using particle lithography, and the overlapped regions serve as Moire patterns at a nanometer scale. NSOM was applied to probe Moire effect directly at the nanometer scale. Moire effect in these regions can be directly visualized from NSOM images, from which periodicity and structural details are accurately determined. In addition, the near-field Moire effect was found to be very sensitive to structural changes, such as lateral displacement and/or rotations of the two basic arrays with respect to each other. Further, nanostructures of Cu exhibited higher photon transmission than Au from NSOM images. Collectively, NSOM enables direct visualization of Moire effect at nanoscale levels from optical read out, and without enhancements or modification of the structures. The results demonstrate the feasibility of extending applications of Moire effect-based techniques to nanometer levels.;The same NSOM setup was utilized to investigate the upconversion metal enhancement effect. Rare-earth upconversion particle (RE-UCP) modified AFM probes were successfully fabricated by attaching RE-UCP to the apex of the AFM probe with glue. The optical properties of the probes were investigated. Under illumination of the 980 nm laser, they emit green light, which is consistent with single crystal behavior. The RE-UCP modified AFM probe was utilized as a NSOM probe to investigate the metal enhancement effect. Upon contact, 1.59-fold enhancements in blue peak and 1.63-fold enhancements in red peak are observed. Such a method provides an alternate tool in the study of the metal enhancement effect within complex surface structures.