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Schematic of the experimental
setup. The probe of an atomic force microscope is suspended just
above a vibrating cantilever. Electrostatic forces cause the probe
to vibrate, and its vibration is measured by a laser beam.
Image by: Craighead Research
Group/Cornell |
Now Cornell University researchers have come
up with a very simple solution: reach out and touch them. The
vibration of the tiny oscillators can be measured by "tapping" with an
atomic force microscope (AFM).
An AFM uses a tiny probe that moves slowly just
above a surface. Electrostatic attraction or repulsion between the
atoms in the tip of the probe and those in the surface causes the
probe to move up and down, creating an image of the surface so
detailed that individual atoms show up as bumps. Alternatively, the
AFM can be used in "tapping mode," literally bouncing off the surface.
"AFMs are all over the place," said Rob Ilic,
research associate in the Cornell NanoScale Facility and lead author
on a paper about the research published Feb. 23 (2007) in the online
edition of the Journal of Applied Physics. "So this offers a simple
way to study these structures." (Cornell, for example, has at least a
dozen AFMs in various labs.) Moreover, he said, probes similar to
those in an AFM can be built directly into nanofabricated devices.
This would amount to using MEMS to measure NEMS, he
said. MEMS (microelectromechanical systems) are machines with moving
parts measured in microns, or millionths of a meter; NEMS (nanoelectromechanical
systems) are measured in nanometers, or billionths of a meter. A
nanometer is about the length of three atoms in a row. When the NEMS
oscillator is too small to be observed by laser light, it could still
be coupled to a MEMS probe that in turn would be large enough for a
laser readout.
To measure the vibration of a nanomechanical
oscillator, the AFM probe moves along the length of the oscillating
rod. The result is a complex bouncing interaction between the probe
and the oscillator - imagine shaking one end of a spring and watching
the vibrations at the other end - from which the frequency of
vibration of the oscillator can be determined mathematically.
For the experiments just reported, Ilic and
colleagues manufactured a wide variety of silicon cantilevers - strips
of silicon attached at one end with the other free to vibrate - from 5
to 12 microns long, 1/2 to 1 micron wide and about 250 nanometers
thick, which had natural vibration frequencies from 1 to 15 Mhz. The
cantilevers were set into vibration by a piezoelectric device.
The experimenters first measured the resonant
frequencies of the cantilevers by focusing laser beams on them and
observing deflection of the reflected light, then scanned each
cantilever with the AFM probe, both in tapping mode and with the probe
just above the surface. They found the AFM measurements in good
agreement with laser measurements, although the AFM readouts had a
somewhat lower "quality factor," because the oscillator and probe were
interacting. This would make the method somewhat less precise in mass
detection.
Nanomechanical oscillators are often cited as
potential tools for detecting bacteria, viruses or other organic
molecules. An array of tiny cantilevers might be created with
antibodies to many different pathogens attached to them. An
experimental solution could then be washed over the array, allowing
microbes to bind to the cantilevers with matching antibodies. Since
the cantilevers are so tiny, an attached bacterium or virus represents
a significant change in mass, which changes the frequency at which the
oscillator will vibrate.
In a practical device, a MEMS probe could be
mounted above each NEMS oscillator to read out which oscillators in
the array show a change in frequency - and thus identify which
pathogens are present. |
