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
AC ElectrokineticsAC Electrokinetics
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
AC ElectrokineticsAC Electrokinetics
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
AC ElectrokineticsAC Electrokinetics
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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