and transparent (or mostly transparent) to the electron beam. For
extremely small samples, a sheet of thin, amorphous carbon films have
been used as sample holders. These thin carbon films are suitable
for such applications because they are electron-transparent nanomaterials that possess considerable durability even at a thickness of only one
atom.
There are difficulties associated with using TEM to image nanomaterials,
but it is a useful technique for a variety of applications and boasts
impressive resolutions. Normally TEM is recommended for use in conjunction with other imaging methods to obtain an accurate understanding of the material being studied.
Figure 8.34 shows a TEM image of a carbon nanotube that has been
attached to an AFM tip to serve as a nanoinjector (the operation of a
nanoinjector is discussed in the previous section). The molecular cargo
attached to the surface of the nanotube is clearly visible, thus demonstrating the resolving power of TEM.
8.7.4 Near-field scanning optical microscopy
In our introduction to TEM, we discussed the limited utility of conventional light microscopies in imaging nanostructures. In general, the resolution of an optical microscope is limited by the size of the spot to which
the light beam can be focused using magnifying lenses. This limitation is
often referred to as the diffraction limit. The diffraction limit on resolution is wavelength-dependent, as given by Equation 8.31 For modern
objectives examining samples in an aqueous medium, NA is usually in the
range of 1.3 to 1.5. Therefore, the resolving power of a conventional
optical microscope is approximately half the wavelength of the incident
light, typically ~200 nm for visible light.
The diffraction limit can be avoided using the nonoptical techniques
discussed in the previous sections; however, all of these techniques have
limitations with regard to sample preparation, sample type, or sample
damage. Furthermore, none of these alternative methods offer the types
of information that are available to optical methods—such as spectroscopic information, excellent time resolution, fluorescence detection
capabilities, information about refractive index and reflectance of the
sample, and contrasting power using different staining agents.
Near-field scanning optical microscopy (NSOM or SNOM) is an optical
microscopy that can operate with resolutions below the diffraction limit,
IMAGING NANOSTRUCTURES 323
extremely small samples, a sheet of thin, amorphous carbon films have
been used as sample holders. These thin carbon films are suitable
for such applications because they are electron-transparent nanomaterials that possess considerable durability even at a thickness of only one
atom.
There are difficulties associated with using TEM to image nanomaterials,
but it is a useful technique for a variety of applications and boasts
impressive resolutions. Normally TEM is recommended for use in conjunction with other imaging methods to obtain an accurate understanding of the material being studied.
Figure 8.34 shows a TEM image of a carbon nanotube that has been
attached to an AFM tip to serve as a nanoinjector (the operation of a
nanoinjector is discussed in the previous section). The molecular cargo
attached to the surface of the nanotube is clearly visible, thus demonstrating the resolving power of TEM.
8.7.4 Near-field scanning optical microscopy
In our introduction to TEM, we discussed the limited utility of conventional light microscopies in imaging nanostructures. In general, the resolution of an optical microscope is limited by the size of the spot to which
the light beam can be focused using magnifying lenses. This limitation is
often referred to as the diffraction limit. The diffraction limit on resolution is wavelength-dependent, as given by Equation 8.31 For modern
objectives examining samples in an aqueous medium, NA is usually in the
range of 1.3 to 1.5. Therefore, the resolving power of a conventional
optical microscope is approximately half the wavelength of the incident
light, typically ~200 nm for visible light.
The diffraction limit can be avoided using the nonoptical techniques
discussed in the previous sections; however, all of these techniques have
limitations with regard to sample preparation, sample type, or sample
damage. Furthermore, none of these alternative methods offer the types
of information that are available to optical methods—such as spectroscopic information, excellent time resolution, fluorescence detection
capabilities, information about refractive index and reflectance of the
sample, and contrasting power using different staining agents.
Near-field scanning optical microscopy (NSOM or SNOM) is an optical
microscopy that can operate with resolutions below the diffraction limit,
IMAGING NANOSTRUCTURES 323