Northwestern University Institute for NanotechnologyNanoscale Science & Engineering Center

Manipulation of Nanoparticles Using Dielectrophoresis

Matt PappasValparaiso University

Outline

Present talk structure Be brief Broad topics only

Milieu

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…

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

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.

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Device Design

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

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

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

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

We have demonstrated attraction of CNTs to electrode gap, orientation of CNTs correct. We have not, however, detected bridging of a single nanotube.

Summary

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?

needed follow-up work new problems opened by your work

Acknowledgements

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.