Patrick J. Moyer, Assistant Professor

The significance of NSOM is that it allows spatial resolution with more than an order of magnitude improvement over the best conventional optical methods, including laser scanning confocal microscopy. Although optical characterization is the most widespread method to anlayze materials from biology to the semiconductor industry, it suffers from one inherent problem: the diffraction limit provides a spatial resolution limit of about half of the wavelength of light. Thus, features smaller than 250 nm can not be imaged or spectrally characterized with visible light. NSOM combines scanning probe microscopy instrumentation with optical microscopy and spectroscopy to provide optical characterization with, in some cases, 15 nm resolution using visible light. The technique employs a sharpend optical fiber that is coated with metal such that a small aperture (approximately 25 nm diameter) is formed at the tip of the fiber. This aperture serves to illuminate a small spot on the sample which is much smaller than the conventional diffraction limit. The sample is then scanned beneath the tip and the image is formed in the same fashion that a dot matrix printer prints a picture.

Professor Moyer and his former thesis advisor, Professor Michael Paesler of North Carolina State University have just published a comprehensive book on NSOM. The book is entitled NEAR-FIELD OPTICS: THEORY, INSTRUMENTATION, AND APPLICATIONS and has been published by Wiley-Interscience.

Schematic diagram of an NSOM used for either microscopy or spectroscopy.

NSOM investigations of liquid crystals

NSOM/shear force has been used to modify the optical properties of liquid crystals and then subsequently read the modifications with resolution of better than 65 nm. The liquid crystal material is 4'-octyl-4-biphenylcarbonitrile (8cb) and the film thickness is roughly 300 nm. Using polarization-mode NSOM, the birefringence of the liquid crystal material which results from a preferential LC chain alignment is imaged. Clearly, the optical properties of the material are modified while the topographic characteristics are relatively unchanged. Such a material modification experiment has implications in lithography and data storage.

FIGURE CAPTION: Simultaneously acquired reflection-mode and polarization-mode NSOM (left) and shear force topography (right) images. Data acquired using the TopoMetrix Aurora NSOM.

NSOM operation in liquids

A number of our experimental interests lie in the fields of biology and surface photochemistry. Such experiments require NSOM operation in liquid environments. We've demonstrated NSOM operation under water with optical resolution characteristic of a typical NSOM experiment. Shown here are NSOM and corresponding shear force operation results of the same standard NSOM calibration grid shown above. The resolution of the NSOM image is roughly 70 nm. Shown below these images is a third image, which is a shear force topography image acquired elsewhere on this sample while the sample was still under water. Notice that some of topographic images show the standard hexagonal pattern while others do not. Independent of the topography, the same high resolution NSOM imaging results. Such results indicates that the shear force method is sensitive to soft contaminants that settle onto the surface from the relatively impure tap water. As a comparison, we also imaged the same sample under the same liquid conditions. These images are shown below as well. Notice that the contact AFM image (left) shows little variation from the hexagonal pattern, and the non-contact ac-mode AFM image (right) shows a bit more of the contamination. Clearly, neither AFM experiment is as sensitive to soft surface materials as is the shear force method. This indicates that the shear force method may be better suited than AFM to image soft biological materials for surface profiling.

FIGURE CAPTION: Transmission NSOM (top left) and corresponding shear force topography image (top right) of aluminum pattern on glass obtained while the sample and tip are immersed in water. The image on the bottom is a larger topography image of the same sample also acquired under water. Data acquired using the TopoMetrix Aurora NSOM.

FIGURE CAPTION: Atomic force microscope (AFM) images of the same calibration sample under the same liquid conditions as used for NSOM experiments. On the left is the contact AFM image and on the right is the ac-mode non-contact AFM image. Data acquired using the TopoMetrix AFM.

Thursday, December 19, 1996 1:58 PM