Transcript REU/MIN Orientation - Northwestern University

Manipulation of Nanoparticles
Using Dielectrophoresis
Matt Pappas
Valparaiso University
Northwestern University Institute for Nanotechnology
Nanoscale Science & Engineering Center
Outline
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Present talk structure
Be brief
Broad topics only
Milieu
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Carbon nanotubes have very desirable electrical
and mechanical properties, and are very promising
for a variety of uses. Their discovery has prompted
speculation of uses in everything from nanoscale
electronics to reinforcement where carbon fibers
are used today.
It would be convenient to be able to measure
electrical, mechanical, or electromechanical
properties at will. For example, one might want to
test the conductance of a batch of metallic
nanotubes, or to measure the torsional strength of a
batch made for structural reinforcement.
Near the target…
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Other groups have measured properties such as
conductance, torsional strength, deflection, and
buckling, but experiments are highly specialized.
Many also involve growing nanotubes in situ,
making such procedures unfit for batch testing.
Measuring torsional strength, for example, involved
creating a custom mask for – the nanotubes were
spread on a wafer, found using a scanning electron
microscope, and a mask created to fit the
dispersion.
Dielectrophoresis, Distilled
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In an electric field, a neutral particle becomes
polarized. If the field is non-uniform, the forces on
each end of the dipole are also non-uniform, and
the particle experiences a net force dependent
upon the permittivity and conductivity of the particle
and the media, as well as the field strength and
frequency, but not the field polarity.
F DEP 
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*
1
Re (m( )  ) E
2
1
m( )  v m K ( ) E
3

 p  m
K ( ) 
 p  2 m
~p   p  j
p

~m   m  j
m

Device Design
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We have
constructed a 1 cm2
array of 20
electrodes as
shown
Red represents
electrodes, green
represents
photoresist
patterned on top of
the electrodes, and
blue represents
silicon dioxide.
Trapping Detection
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The circuit below ensures that once a nanotube bridges the
electrodes, the field will diminish substantially, preventing
accumulation of nanotubes and/or other particulate matter in
suspension. The gap can be modeled as a small capacitor.
When the gap is bridged, a high voltage drop is measured
across the resistor.
Measurement of Properties
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Once trapped, the nanotube can
be affixed to the electrode surface
using an electron beam.
The nanotube or electrode can
then be manipulated with an
atomic force microscope tip, or the
electrodes can be deflected using
a light beam.
Versatility
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Different geometries
allow different types of
tests to be performed.
The silicon dioxide can
be chemically etched,
giving deeper wells,
which can be used to
measure large
deflections or large
torsional deformations.
Here and Now
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We have demonstrated attraction of CNTs to electrode gap,
orientation of CNTs correct. We have not, however, detected
bridging of a single nanotube.
Summary
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Dielectrophoresis is a powerful way to
place objects.
Combining dielectrophoresis with a
simple circuit and a versatile electrode
device, virtually any property of
nanotubes can be tested.
Future Work?
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needed follow-up work
new problems opened by your work
Acknowledgements
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Special thanks to Prof. Nicolaie
Moldovan, Professor Horacio
Espinosa, and Mr. Changhong Ke.
Extra special thanks to The National
Science Foundation, whose funding
made this work, and the above
acknowledgements, possible.