AC Electrokinetics AC Electrokinetics and Nanotechnology Meeting the Needs of the “Room at the Bottom” Shaun Elder Will Gathright Ben Levy Wen Tu December 5 th, 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 ε m = 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 Clausius- Mossotti factor

AC 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.

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 typebrigh tness (A/c m 2 /sr ) sourc e size energy spread (eV) vacuu m requir ement (Torr) tungsten thermionic ~ um LaB 6 ~ um thermal (Schottky) field emitter ~ nm cold field emitter ~ nm

AC Electrokinetics Electron Lenses Magnetic Lens –More common –Converging lens only Electrostatic Lens –Use near gun –Pulls electrons from source

AC Electrokinetics Resolution d = (d g 2 + d s 2 + d c 2 + d d 2 ) 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 1 st DNA is levitated, elongated, 2 nd Measured, viewed OR Solution is dried, collected as uncoiled strands

AC Electrokinetics Current Devices Traveling Wave Dielectrophoresis (TWD) to trap human breast cancer cells electrodes Cancer cells spiral shaped electrode microfluidic channels Polarization differences  Cancer vs. other cells

AC Electrokinetics 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

AC 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.

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/r 3 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.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

AC 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.

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 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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