AC 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 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 Electrokinetics Overview • • • • AC Electrokinetical Theory Device History and Fabrication Case Studies and Current Devices Scaling Laws and Nanotechnology AC Electrokinetics AC Eletrokinetics • Dielectrophoresis • Electrorotation • Traveling-Wave Dielectrophoresis Interaction between induced dipole and electric field AC 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 AC 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 AC Electrokinetics Electrorotation • Rotating electric field • Lag in dipole correction causes torque • Torque causes movement AC Electrokinetics Electrorotation Torque • Im[K(w)] = imaginary component of the ClausiusMossotti factor AC Electrokinetics Combination Dielectrophoresis Electrorotation • Function of field gradient • Function of field strength • Real part of the ClausiusMossotti factor • Imaginary part of ClausiusMossotti factor Dielectrophoresis and Electrorotation can be applied on a particle at the same time. AC Electrokinetics Traveling-Wave Dielectrophoresis Linear version of electrorotation. AC Electrokinetics Fabrication • Electron Beam Lithography – – – – High resolution Flexible Slow write speed Expensive • Niche Uses AC Electrokinetics Electron Sources • Thermionic Sources • Cold Field Emission • Schottky Emission source type brigh tness (A/c m2/sr ) tungsten thermionic ~105 LaB6 ~106 thermal (Schottky) field emitter cold field emitter sourc e size 25 um 10 um energy spread (eV) vacuu m requir ement (Torr) 2-3 10-6 2-3 10-8 ~108 20 nm 0.9 10-9 ~109 5 nm 0.22 10-10 AC Electrokinetics Electron Lenses • Magnetic Lens – More common – Converging lens only • Electrostatic Lens – Use near gun – Pulls electrons from source AC Electrokinetics 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 AC Electrokinetics Current Devices History • Feynman, 1959, Nanostructures to manipulate atoms • HA Pohl, AC electrokinetic methods for particle manipulation • Early 1980’s, crude nanofabrication AC Electrokinetics Current Devices Various Applications • • • • • • DNA separation, extension Bacterium, Cancer cell isolation Virus clumping Colloidal particle translation Non-viable cell extraction Rotation and motor activation AC Electrokinetics 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 AC Electrokinetics Current Devices Traveling Wave Dielectrophoresis (TWD) to trap human breast cancer cells •spiral shaped electrode electrodes •microfluidic channels Cancer cells •Polarization differences Cancer vs. other cells AC Electrokinetics Current Devices Electrorotation of polystyrene beads to set orientation or conduct experiments •beads rotate Rotating beads electrodes •velocities affected by •frequency of cycles of E •Size, shape •Polarizability •Polystyrene beads coated with protein assays •Micromotors also oriented by electrorotation AC Electrokinetics Nanotechnological Considerations Self-Assembly Scanning Probe Techniques • Relies on non-covalent inter- and • Relies on probes to manipulate intra-molecular interactions such down to the atomic length scale as hydro-phobic/philic, van der with ultimate accuracy Waals, etc. • “Top-down” approach offers active • “Bottom-up” approach is process with a high degree of economical but ultimately passive control • Can be drastically effected by • Impossible to scale to any sort of macro environment, such as massively parallel (economic) temperature, pH, etc. process The fundamental challenge facing nanotechnology is the lack of tools for manipulation and assembly from solution. AC 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. AC Electrokinetics Dielectrophoresis: Scaling Laws Characteristic electrode feature size must be reduced along with high frequency driving currents for DEP to dominate. AC 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. AC Electrokinetics Nanotube Electrode Fabrication 1. 2. 3. 4. 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 AC Electrokinetics Nanotube Electrode Performance Tapping Mode Phase Contact Mode • 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!!! A carbon nanotube electrode has been shown to DEP manipulate particles an order of magnitude smaller than previous work. AC 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 AC 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! AC Electrokinetics ?