AC Electrokinetics and Nanotechnology Meeting the Needs of the

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AC Electrokinetics and NanotechnologyMeeting
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

• em permittivity of the suspending medium
• Delta Del vector operator
• E Voltage
• ReK(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

• ImK(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

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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  ds2  dc2  dd2)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 1980s, 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 Devices

• Dielectrophoresis 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 Devices

• Traveling Wave Dielectrophoresis (TWD) to trap
  human breast cancer cells

• spiral shaped electrode
• microfluidic channels
• Polarization differences ?
• Cancer vs. other cells

electrodes Cancer cells
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Current Devices

• Electrorotation 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 Fabrication

• Optical photolithography defines catalytic sites
  for nanotube growth
• Long, single-walled nanotubes (SWNT) are grown
• SEM locates nanotubes and optical PL defines
  electrodes
• 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!!!

Phase Contact Mode
Tapping 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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