ZANG group research

department of chemistry and biochemistry SIU

Near-field Scanning Optical Microscopy (NSOM)

The central part of NSOM is the tapered, aluminum coated, single-mode optical fiber probe (tip), which has a tiny aperture of only a few tens of nanometers (Figure 1).  High optical resolution is achieved by illuminating a sample through the sub-wavelength aperture while the sample is held in close proximity (namely near-field region) to the probe. Within the near-field the optical resolution is limited only by the probe aperture size (~ 50 nm), breaking the diffraction limit (l/2) to the resolution of ordinary optical microscopy. The NSOM tip-sample distance is regulated by feedback mechanisms similarly to those used in AFM, but here sensing the lateral shear force interaction of the probe with the sample during the scanning process. Simultaneous high-resolution optical (~ 50 nm) and topographical (~ 10 nm x, y-resolution, ~ 1-2 nm z-resolution) images are obtained by scanning the sample (Figure 1E). Thus, the extreme utility of NSOM lies in the fact that the nanostructured features (nanodots, nanorods, nanowires) can be directly correlated with the optical and electronic properties. For example, the quantum size effects of semiconductor nanocrystals can be directly investigated by simultaneous imaging of the size distribution and the individual optical spectra. A variety of CW and pulsed laser sources in tandem with high sensitivity detectors such as avalanche photodiodes and liquid nitrogen cooled CCD are used to explore structural and photophysical properties of samples.

NSOM can be used for single-molecule spectroscopy (SMS). At one time, only one molecule on the surface is excited by the laser beam (either polarized or not) from the NSOM tip. The fluorescence is collected and aligned by a fine objective, and then either recorded by a CCD camera for spectra measurement, or counted by an avalanche photodiode (APD) for intensity measurement. Two APD detectors will be used in the case of measuring fluorescence anisotropy. Compared to the confocal measurement, where the focus area is ~ 300 nm, NSOM can deliver the near-field excitation right above the molecule, leading to a much better resolution,  ~ 50 nm. Another advantage with NSOM is the new ScanMaster linearization, which provides precise positioning of the tip (excitation), < 0.5 nm.

The fundamental aspects of NSOM and the practical NSOM instruments were developed in the late 1980s and early 1990s. The first generation of prototype NSOM (Aurora-3, Veeco^TM Microscopes, ) was launched at the beginning of 2002. The new instrument provides advanced features over the early generation of NSOM, including the use of the proven ScanMaster technology for scan linearization, which allows for easier and more accurate tip positioning. Commercial NSOM instruments are amenable to modification for specific research purposes, like applying bias voltage between the aluminum coated tip and the conducting sample substrate.

(updated on june 22, 2004)