Working
Model of an Atomic Force Microscope
Relatively
few laboratory experiments are available for introductory
lab courses relating to nanoscience and nanotechnology.
In order to
explain the working principles of an atomic force microscope (AFM) to an
introductory level physics class, we have created an inexpensive
working model of an AFM using a modified phonograph stylus in place of the AFM
cantilever and tip. The sample to be studied is positioned under the stylus using a
micrometer stage, and a 10 mW laser diode is
used to produce a beam, which
reflects off a very small mirror glued
to the end of the stylus. No electronic detection is used, rather
students can measure the deflection of the tip directly from the
movement of the laser beam on a piece of graph paper placed 50 cm from
the mirror. The laser beam is deflected roughly a centimeter for each 10 microns of
stylus deflection, making it simple for students to
collect data.
A one-dimensional trace is typically performed, however
the instrument could be easily modified to produce a full
two-dimensional scan.
Working AFM Demonstration Model
Research
Funds:
National
Science Foundation, Grant Number: DMR - 0348354
College
of Arts and Sciences DeanÕs Office at the University of
Vermont
Above:
Large-scale working model of an Atomic Force Microscope used for class
demonstrations. The model uses an aluminum arm with a plexiglass tip to follow
the contours of a ÔsampleÕ made from plaster of paris. A photocell detects the reflected laser beam
and transmits the signal to an oscilloscope. Note the weight secured to the end
of the arm to ensure the lightest possible contact between the arm and the
sample.
Oscilloscope Reading from Demo
AFM Model
Above:
Complete set-up of the working AFM model to be used in an introductory
nanotechnology lab.
A:
Second
mirror used to reflect laser beam to screen (behind apparatus)
B:
10
mW laser source (see blow up below)
C:
AFM
tip (see blow up below)
D:
Stage
with sample (see blow up below)
E:
Micrometer stages used to adjust x/y and z positions
Above:
Close
up of the AFM tip and sample. By seeing them both together, one
gets a better sense of scale and also can see how well the tip will trace the
sample. In this case, the tip is slightly larger than the deepest parts of the
grooves and so measured values will show slightly shallower channels than are
actually present.
¥Michael J. Hamblin
Above: An example of an oscilloscope reading during a scan. The small regular bumps on the top waves are thought to be due to the grooves in the photocell itself.
Above: Lens which fits over the laser source. The lens is fitted
with a cross-hair made from
quartz fiber .
Above: Model tip and sample. The
tip is a shortened phonograph stylus. A tiny piece of
silicon wafer is glued to the top acting as the primary mirror. The sample is a
regularly grooved piece of plastic (see below).
Above: Close up of the grooved
plastic sample. The picture is taken from the side at a 10
x zoom. The grooves are approximately 65 micrometers deep and each bump is
about 325 micrometers wide.
Above: Graph of the data taken during actual scans. One scan was performed
and then quickly repeated to test for continuity. Converting the laser
deflection (in cm) to depth of the groove (micrometers) gave almost the same
values as those found using the microscope.
Above: Students perform scans and collect data using our working
AFM model. Measurements were taken directly off of
a sheet of graph paper taped to the
screen, as can be seen in the fourth photograph, and graphed in a similar
manner as shown previously. Please
note that the students were given
flashlights and so the glow that is seen (especially in the fourth photo) is
not due to the laser beam alone.
Above: In order to calculate the
height of the bumps on the surface of the sample from
the deflection of the laser point, students learn the following relationships
as illustrated above: the change in angle of the arm is equal to the change
in height of the tip (h) divided by the length of the tip (L). The deflection
of the laser (d) divided by twice the distance from the second mirror to
the screen (D) is also the change in angle of the arm. Therefore, the height
of the bumps equals the deflection distance multiplied by the length of
the arm divided by twice the distance from the second mirror to the screen,
or: h = dL / 2D.
Overall both
our demonstration AFM model and the smaller lab version worked very
well. The design of the small model was especially tricky and several
setups were tried before this final construction was made. We believe it
is the best balance of sensitivity and simplicity. The tip of the AFM was also
a very important decision. We found that a shortened phonograph
stylus works best after trying a tack, bent sheet metal, and a victrola
stylus.
In the lab,
the students were very successful in being able to adjust the sample
stage, align the mirror, and measure the deflection of the laser beam.
Although the instrument itself was small and somewhat delicate, they were
able to take measurements in the dark with little problem.
As for the
future of this project, we tried only one kind of phonograph tip so it would
be interesting to see if other tips, if any, perform better. Also, a small chip
of silicon wafer was by far the best mirror to be used on the tip itself.
However, even careful cutting of the silicon resulted in chips of various
shapes; this and the fact that they were glued on caused slightly different
angles of reflection for each tip. The difference was so small that it could be
ignored for our purpose, but improvements could be made if one was willing
to invest the time and effort into perfecting the model.