ac electrokinetics ac electrokinetics and nanotechnology meeting the needs of the “room at the...
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![Page 1: AC Electrokinetics AC Electrokinetics and Nanotechnology Meeting the Needs of the “Room at the Bottom” Shaun Elder Will Gathright Ben Levy Wen Tu December](https://reader030.vdocuments.site/reader030/viewer/2022032800/56649d4e5503460f94a2d33d/html5/thumbnails/1.jpg)
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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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 ElectrokineticsAC Electrokinetics
AC Eletrokinetics
• Dielectrophoresis
• Electrorotation
• Traveling-Wave Dielectrophoresis
Interaction between induced dipole and electric field
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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
Electrorotation
• Rotating electric field• Lag in dipole correction
causes torque• Torque causes movement
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AC ElectrokineticsAC Electrokinetics
Electrorotation Torque
• Im[K(w)] = imaginary component of the Clausius-Mossotti factor
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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
Traveling-Wave Dielectrophoresis
Linear version of electrorotation.
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AC ElectrokineticsAC Electrokinetics
Fabrication
• Electron Beam Lithography– High resolution– Flexible– Slow write speed– Expensive
• Niche Uses
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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
Electron Lenses
• Magnetic Lens– More common– Converging lens only
• Electrostatic Lens– Use near gun– Pulls electrons from
source
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AC ElectrokineticsAC Electrokinetics
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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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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
Dielectrophoresis: Scaling Laws
Characteristic electrode feature size must be reduced along with high frequency driving currents for DEP to dominate.
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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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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AC ElectrokineticsAC Electrokinetics
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