and therefore is able to offer the advantages of optical microscopies with a
resolution that is actually useful for the study of nanomaterials.
8.7.4.1 History and principles of NSOM
Although the fundamental idea of NSOM is relatively simple, its practical
implementation proved to be rather difficult. The original idea was
developed by Edward Synge and was published in a series of papers
beginning in 1928. Synge realized that if the diffraction limit was imposed
by the practical limit to which a beam of light could be focused, then the
limit might be overcome by shining light through a very small hole (or
aperture) that was smaller than the wavelength of the light itself. If this
hole were placed close to the sample, the light would not have time to
diffract outward and destroy the resolution of the image. Thus, a sample
could be imaged at a resolution below the diffraction limit.
Despite Synge’s development of the theory of NSOM in the early twentieth century, it was not implemented until 1972 when Ash and Nicholls
used an NSOM setup with microwave radiation (l ~ 3 cm) to image a
metal grating sample. They demonstrated that a resolution of 1/60th of
the wavelength of the incident radiation was achievable using their
method. Their results validated Synge’s theory, but practical considerations prevented the development of an NSOM using the much smaller
wavelengths of visible light until the mid-1980s. During this period, scientists overcame the technical difficulties of implementing NSOM with
visible light. Their practical setup serves as the basis for modern NSOM
instruments.
8.7.4.2 Modern NSOM instrumentation and different NSOM
operating modes
The major component of any NSOM microscope is the aperture tip or
NSOM probe. A variety of NSOM probes exist. One common NSOM probe
is manufactured by heating and pulling a fiber-optic cable into a very fine
point, then coating the tapered end with reflective metal, except for a very
small aperture at the point. SEM images of this type of NSOM probe are
shown in Figure 8.36. Laser light shone through the fiber-optic cable
emerges from the aperture as a beam with a diameter that is smaller than
the wavelength of light. For this type of fiber-optic NSOM probe, the
fundamental maximum resolution is ~12 nm, but the practical limit is
typically ~50 nm. Another common NSOM probe can be made by using
electron beam lithography to create a nanometer-scaled aperture through
CHAPTER 8: Surface Characterization and Imaging Methods
324
resolution that is actually useful for the study of nanomaterials.
8.7.4.1 History and principles of NSOM
Although the fundamental idea of NSOM is relatively simple, its practical
implementation proved to be rather difficult. The original idea was
developed by Edward Synge and was published in a series of papers
beginning in 1928. Synge realized that if the diffraction limit was imposed
by the practical limit to which a beam of light could be focused, then the
limit might be overcome by shining light through a very small hole (or
aperture) that was smaller than the wavelength of the light itself. If this
hole were placed close to the sample, the light would not have time to
diffract outward and destroy the resolution of the image. Thus, a sample
could be imaged at a resolution below the diffraction limit.
Despite Synge’s development of the theory of NSOM in the early twentieth century, it was not implemented until 1972 when Ash and Nicholls
used an NSOM setup with microwave radiation (l ~ 3 cm) to image a
metal grating sample. They demonstrated that a resolution of 1/60th of
the wavelength of the incident radiation was achievable using their
method. Their results validated Synge’s theory, but practical considerations prevented the development of an NSOM using the much smaller
wavelengths of visible light until the mid-1980s. During this period, scientists overcame the technical difficulties of implementing NSOM with
visible light. Their practical setup serves as the basis for modern NSOM
instruments.
8.7.4.2 Modern NSOM instrumentation and different NSOM
operating modes
The major component of any NSOM microscope is the aperture tip or
NSOM probe. A variety of NSOM probes exist. One common NSOM probe
is manufactured by heating and pulling a fiber-optic cable into a very fine
point, then coating the tapered end with reflective metal, except for a very
small aperture at the point. SEM images of this type of NSOM probe are
shown in Figure 8.36. Laser light shone through the fiber-optic cable
emerges from the aperture as a beam with a diameter that is smaller than
the wavelength of light. For this type of fiber-optic NSOM probe, the
fundamental maximum resolution is ~12 nm, but the practical limit is
typically ~50 nm. Another common NSOM probe can be made by using
electron beam lithography to create a nanometer-scaled aperture through
CHAPTER 8: Surface Characterization and Imaging Methods
324