ellipsometry provides topographical data with excellent vertical resolution. However, because the lateral resolution is on the order of
microns, nanostructures cannot be resolved in the xy plane. In order to
image nanostructures at atomic-level resolutions in all three dimensions,
methods such as the scanning probe microscopies must be used.
8.7.2 Scanning probe methods
The scanning probe microscopies are powerful imaging techniques
in which a very sharp tip is scanned across a surface, producing an
image with near atomic-level resolution. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are the most common
examples of scanning probe methods. Both STM and AFM produce threedimensional images of surfaces that approach atomic or molecular resolution, making them ideal for the study of nanostructures on surfaces.
8.7.2.1 Scanning tunneling microscopy
STM operates by monitoring the “tunneling” current that is produced
when a sharp tip is brought extremely close to a surface that is able to
conduct electricity. In order to bring and maintain the tip so close to the
surface, a piezoelectric transducer is used. As a refresher, we recall that
piezoelectric transducers possess the ability to physically expand or
contract in response to an applied voltage (for a more complete discussion, see the discussion of QCM techniques in Section 8.3). Therefore, a
sharp metal STM tip, usually made of platinum, is attached to a piezoelectric scanner that can cause the tip to move small distances in the x, y,
and z directions. Depending on the type of piezoelectric material used to
make the scanner (usually some form of ceramic) and its dimensions, the
contraction or expansion of the material can be as small as 1 nm per volt
applied, allowing for the metal tip to be brought close to the surface being
imaged. In early models of STMs, the piezoelectric scanners consisted of
transducers arranged in the x, y, and z directions, as shown in Figure 8.29,
but more recent models have made use of a tubelike piezoelectric
transducer to achieve better resolution.
Using a piezoelectric scanner, the STM tip can be brought close (within
1 nm) to a conducting surface. A small voltage, generally between 2 mV
and 2 V, is then applied between the conducting substrate and the metal
tip, causing electrons to tunnel between the tip and the surface, creating a
current. The magnitude of this tunneling current depends exponentially
on the distance (h) between the conducting surface and the STM tip. For a
IMAGING NANOSTRUCTURES 311
microns, nanostructures cannot be resolved in the xy plane. In order to
image nanostructures at atomic-level resolutions in all three dimensions,
methods such as the scanning probe microscopies must be used.
8.7.2 Scanning probe methods
The scanning probe microscopies are powerful imaging techniques
in which a very sharp tip is scanned across a surface, producing an
image with near atomic-level resolution. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are the most common
examples of scanning probe methods. Both STM and AFM produce threedimensional images of surfaces that approach atomic or molecular resolution, making them ideal for the study of nanostructures on surfaces.
8.7.2.1 Scanning tunneling microscopy
STM operates by monitoring the “tunneling” current that is produced
when a sharp tip is brought extremely close to a surface that is able to
conduct electricity. In order to bring and maintain the tip so close to the
surface, a piezoelectric transducer is used. As a refresher, we recall that
piezoelectric transducers possess the ability to physically expand or
contract in response to an applied voltage (for a more complete discussion, see the discussion of QCM techniques in Section 8.3). Therefore, a
sharp metal STM tip, usually made of platinum, is attached to a piezoelectric scanner that can cause the tip to move small distances in the x, y,
and z directions. Depending on the type of piezoelectric material used to
make the scanner (usually some form of ceramic) and its dimensions, the
contraction or expansion of the material can be as small as 1 nm per volt
applied, allowing for the metal tip to be brought close to the surface being
imaged. In early models of STMs, the piezoelectric scanners consisted of
transducers arranged in the x, y, and z directions, as shown in Figure 8.29,
but more recent models have made use of a tubelike piezoelectric
transducer to achieve better resolution.
Using a piezoelectric scanner, the STM tip can be brought close (within
1 nm) to a conducting surface. A small voltage, generally between 2 mV
and 2 V, is then applied between the conducting substrate and the metal
tip, causing electrons to tunnel between the tip and the surface, creating a
current. The magnitude of this tunneling current depends exponentially
on the distance (h) between the conducting surface and the STM tip. For a
IMAGING NANOSTRUCTURES 311