AC ElectrokineticsAC Electrokinetics

AC Electrokinetics and Nanotechnology

Meeting the Needs of the “Room at the Bottom”

Shaun Elder

Will Gathright

Ben Levy

Wen Tu

December 5th, 2004

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Overview

• AC Electrokinetical Theory

• Device History and Fabrication

• Case Studies and Current Devices

• Scaling Laws and Nanotechnology

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AC Eletrokinetics

• Dielectrophoresis

• Electrorotation

• Traveling-Wave Dielectrophoresis

Interaction between induced dipole and electric field

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Dielectrophoresis

• Induced dipole on particle

• Field gradient generates force on particle

• Particle that is more conductive creates attractive force

• Inverse for less conductive particle

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Dielectrophoresis Force

• εm = permittivity of the suspending medium• Delta = Del vector operator• E = Voltage• Re[K(w)] = real part of the Clausius-Mossotti

factor

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Electrorotation

• Rotating electric field• Lag in dipole correction

causes torque• Torque causes movement

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Electrorotation Torque

• Im[K(w)] = imaginary component of the Clausius-Mossotti factor

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Combination

Dielectrophoresis

• Function of field gradient

• Real part of the Clausius-Mossotti factor

Electrorotation

• Function of field strength

• Imaginary part of Clausius-Mossotti factor

Dielectrophoresis and Electrorotation can be applied on a particle at the same time.

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Traveling-Wave Dielectrophoresis

Linear version of electrorotation.

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Fabrication

• Electron Beam Lithography– High resolution– Flexible– Slow write speed– Expensive

• Niche Uses

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Electron Sources

• Thermionic Sources

• Cold Field Emission

• Schottky Emission

source type brightness(A/cm2/sr)

source size

energy spread(eV)

vacuum

requirement(Torr)

tungsten thermionic ~105 25

um2-3 10-6

LaB6 ~106 10 um

2-3 10-8

thermal (Schottky)

field emitter

~108 20 nm

0.9 10-9

cold field

emitter

~109 5 nm 0.22 10-10

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Electron Lenses

• Magnetic Lens– More common– Converging lens only

• Electrostatic Lens– Use near gun– Pulls electrons from

source

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Resolution

• d = (dg2 + ds

2 + dc2 + dd

2)1/2

• Gun diameter

• Spherical aberrations– Outside of lens vs. inside

• Chromatic abberations– Low energy electrons vs. high energy

• Electron wavelength

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Current DevicesHistory

• Feynman, 1959, Nanostructures to manipulate atoms

• HA Pohl, AC electrokinetic methods for particle manipulation

• Early 1980’s, crude nanofabrication

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Current DevicesVarious Applications

• DNA separation, extension

• Bacterium, Cancer cell isolation

• Virus clumping

• Colloidal particle translation

• Non-viable cell extraction

• Rotation and motor activation

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Current DevicesDielectrophoresis to isolate DNA by length

DNA molecules

Finger electrodes

1st DNA is levitated, elongated,

2nd Measured, viewed

OR Solution is dried, collected as uncoiled strands

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Current DevicesTraveling Wave Dielectrophoresis (TWD) to trap human

breast cancer cells

electrodes

Cancer cells

•spiral shaped electrode

•microfluidic channels

•Polarization differences

Cancer vs. other cells

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Current DevicesElectrorotation of polystyrene beads to set orientation or

conduct experiments•beads rotate

•velocities affected by

•frequency of cycles of E

•Size, shape

•Polarizability

•Polystyrene beads coated with protein assays

•Micromotors also oriented by electrorotation

Rotating beads electrodes

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Nanotechnological Considerations

Self-Assembly• Relies on non-covalent inter- and

intra-molecular interactions such as hydro-phobic/philic, van der Waals, etc.

• “Bottom-up” approach is economical but ultimately passive

• Can be drastically effected by macro environment, such as temperature, pH, etc.

Scanning Probe Techniques• Relies on probes to manipulate

down to the atomic length scale with ultimate accuracy

• “Top-down” approach offers active process with a high degree of control

• Impossible to scale to any sort of massively parallel (economic) process

The fundamental challenge facing nanotechnology is the lack of tools for manipulation and assembly from solution.

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Hydroelectrodynamics

• Gravity

• Brownian motion

• Electrothermal forces

• Buoyancy

• Light-electrothermal

• Electro-osmosis

DEP forces must overcome all the above forces for successful manipulation of nanoparticles from solution.

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Dielectrophoresis: Scaling Laws

Characteristic electrode feature size must be reduced along with high frequency driving currents for DEP to dominate.

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Breaking the Barrier

• Single-walled carbon nanotubes are conductive and have diameters on the order of nanometers

• DEP force for a nanotube scales with 1/r3 while electrothermal forces scale with 1/r

For a “nanotube electrode” with such small features, DEP will dominate over all other forces.

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Nanotube Electrode Fabrication1. Optical photolithography

defines catalytic sites for nanotube growth

2. Long, single-walled nanotubes (SWNT) are grown

3. SEM locates nanotubes and optical PL defines electrodes

4. Au/Ti is e-beam evaporated to form electrodes and electrically contact nanotube

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Nanotube Electrode Performance

• 500 kHz to 5MHz AC driving signal

• 20 nm latex particles were easily manipulated out of solution

• 2 nm Au particles were also easily manipulated out of solution!!!Tapping Mode Phase Contact Mode

A carbon nanotube electrode has been shown to DEP manipulate particles an order of magnitude smaller than

previous work.

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Conclusions

• Dynamic electric field manipulates particle dipole.

• Horizontal, rotational, and directional movement.

• Use of EBL enables control to 50 nm

• Aberrations limit the resolution

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Conclusions

• Current Device conclusion here

• Current Device conclusion here

• Fundamental problem in nanotechnology is manipulation tools

• Carbon nanotube electrodes adhere to scaling laws and can manipulate particles down to 2nm!

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