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There are a number of ways to analyze a sample.
In nanoscience, with samples that are too small to be seen,
scanning probe microscopy (SPM) becomes invaluable. SPM is a
technique in which one can look at the surface topography of
your sample. There are a number of different SPM modes to choose.
First, let us examine how an atomic force microscope (AFM) functions.
AFM measurements are the results of interaction forces between
a tip and the surface of a sample. These force interactions
are measured as a function of position. The cantilever, which
is attached to the tip, deflects toward the sample’s surface
as the forces begin to act on the tip. In order to control the
interaction between the tip and the surface, it is necessary
to have both high-resolution positioning of the sample and sensitive
detection of the cantilever’s deflection.
The basic design for an AFM system, shown in figure 1, consists
of three main components: the system for sample positioning,
the force detector, and the feedback control system. Each of
these mechanisms will presently be discussed in more detail.
Figure 1: Basic design of an AFM system. [12]
A high-resolution positioning of the sample can
be obtained through piezoelectric ceramic devices. Piezoelectric
ceramics are capable of easily positioning a sample along the
x, y, and z-axes with nanometer resolution. Such devices consist
of two metal electrodes with a piece of ceramic placed between
them. If a potential difference is placed across the electrodes,
the ceramic will expand or contract perpendicular to the applied
electric field.
A common piezoelectric device is the piezotube. This is a piece
of ceramic, shaped into a hollow cylinder, with one electrode
on the inner surface and four electrodes on the outer surface.
By biasing the outer electrodes, the tube will move in the x
and y directions. Biasing the inner electrode will cause motion
of the tube in the z direction. There is a control system that
supplies the high voltage required for motion of the sample
to each position of the scan.
In a typical AFM scan, the forces are explored in numerous positions
within the scan area of the sample surface by sweeping along
the x-axis while moving incrementally along the y-axis. Measurements
of the forces are made at increasing positions along the x-axis
during a sweep. After each sweep is completed, the y position
will change by one step. At this new y position, the forces
are again measured throughout the tip’s movement during
the x-axis sweep. This process repeats until the tip has traveled
the extent of the y-axis for the scanning region. It is the
size of the scanning region, coupled with the size of these
x and y steps that determine the resolution of the scan.
As stated previously, forces interact with the tip as it travels
across the sample surface, causing it to deflect. Because the
cantilever acts as simple harmonic oscillator, the amount of
force acting on the tip can be determined by measuring the amount
of the tip’s deflection. A laser beam is bounced off the
tip onto a segmented photodiode, allowing the deflection to
be detected by the changes in the laser light absorbed by the
different quadrants of the photodiode. The photodiode then supplies
an output voltage proportional to the deflection of the cantilever.
In a typical AFM scan, the cantilever deflection is held constant
for the x and y positions. A feedback system monitors the output
of the deflection sensor in order to control its position by
adjusting the voltage applied to the inner electrode of the
piezotube, thus controlling the motion of the sample in the
z direction. This voltage is adjusted until the deflection of
the tip returns to the set-point value for the feedback system.
The voltage applied to the piezoelectric element is recorded
at every position of the scan to supply information in a Cartesian
coordinate system.
By recording this applied voltage that was required to maintain
a constant deflection of the cantilever, the topographic height
at a particular point on a sample can be determined. The applied
voltage can be converted into an actual height in nanometers
by applying the tube’s z calibration (given in nm/V).
The z calibration is the known expansion of the piezotube for
a given bias voltage. Accordingly, it is possible to determine
the actual x and y distances in nanometers for your scan area
by applying their respective calibration values.
There are two main methods of measuring a
sample’s surface topography as a function of position:
contact mode and non-contact mode. For contact mode, the tip
is in actual contact with the sample surface. Any cantilever
deflection is governed by the repulsive force interactions between
the electron densities of the tip and the sample If the sample
consists of weakly bound particles or soft objects, scanning
in contact mode may cause damage.
A solution for scanning such objects is to scan in non-contact
mode. In this instance, the tip is not in direct contact with
the sample surface, and thus will not damage such items. In
a non-contact scan, the tip is oscillated near its resonance
frequency approximately 5-15 nm above the sample. By driving
the cantilever at this frequency, any damping forces from the
sample surface will cause the tip’s resonance frequency
to shift. This interaction will also cause the amplitude of
oscillations to change at the original frequency. This damping
of the original resonance frequency can be used in order to
maintain a static tip-sample separation distance. To accomplish
this, the amplitude of the original resonance frequency force
can be measured through a phase sensitive detector (PSD). The
feedback system utilizes the output of the PSD to maintain a
constant amplitude for the original resonance frequency. Recording
any adjustments made by the z piezoelectric element to maintain
that constant amplitude generates a topographic profile of the
sample surface.
There is a consequence of using non-contact mode. There is a
significant loss of lateral resolution due to the fact that
the tip is not in physical contact with the surface. Positioning
the tip above the surface exposes it long-range forces generated
from a much larger area of the sample surface. When in contact
mode, the tip-surface interaction dominates the long-range force,
but in non-contact mode the tip-surface interaction becomes
weak so that the long-range electrostatic force becomes dominant.
Though the lateral measurements of the force interactions are
complicated, there have been recent advances in AFM technology
that have minimized this effect.
Of course, the AFM is an extremely flexible instrument. The
two modes mentioned above are not the only possibilities for
this instrument. It is also possible to use the AFM control
system to explore a wide range of surface forces such as friction,
adhesion and electrostatic with different scanning modes.
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