AC Electrokinetics and Nanotechnology Meeting the Needs of the “Room at the Bottom”

AC Electrokinetics and NanotechnologyMeeting the Needs of the “Room at the Bottom” Shaun Elder Will Gathright Ben Levy Wen Tu December 5th, 2004

AC Electrokinetical Theory Device History and Fabrication Case Studies and Current Devices Scaling Laws and Nanotechnology Overview

Dielectrophoresis Electrorotation Traveling-Wave Dielectrophoresis Interaction between induced dipole and electric field AC Eletrokinetics

Dielectrophoresis • Induced dipole on particle • Field gradient generates force on particle • Particle that is more conductive creates attractive force • Inverse for less conductive particle

Dielectrophoresis Force • εm= permittivity of the suspending medium • Delta = Del vector operator • E = Voltage • Re[K(w)] = real part of the Clausius-Mossotti factor

Electrorotation • Rotating electric field • Lag in dipole correction causes torque • Torque causes movement

Electrorotation Torque • Im[K(w)] = imaginary component of the Clausius-Mossotti factor

Dielectrophoresis Function of field gradient Real part of the Clausius-Mossotti factor Electrorotation Function of field strength Imaginary part of Clausius-Mossotti factor Combination Dielectrophoresis and Electrorotation can be applied on a particle at the same time.

Traveling-Wave Dielectrophoresis Linear version of electrorotation.

Electron Beam Lithography High resolution Flexible Slow write speed Expensive Niche Uses Fabrication

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 um 2-3 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 Electron Sources

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

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 Resolution

Feynman, 1959, Nanostructures to manipulate atoms HA Pohl, AC electrokinetic methods for particle manipulation Early 1980’s, crude nanofabrication Current DevicesHistory

DNA separation, extension Bacterium, Cancer cell isolation Virus clumping Colloidal particle translation Non-viable cell extraction Rotation and motor activation Current DevicesVarious Applications

Dielectrophoresis to isolate DNA by length Current Devices DNA molecules Finger electrodes 1st DNA is levitated, elongated, 2nd Measured, viewed OR Solution is dried, collected as uncoiled strands

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

Electrorotationof polystyrene beads to set orientation or conduct experiments Current Devices • 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

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 Nanotechnological Considerations The fundamental challenge facing nanotechnology is the lack of tools for manipulation and assembly from solution.

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.

Dielectrophoresis: Scaling Laws Characteristic electrode feature size must be reduced along with high frequency driving currents for DEP to dominate.

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.

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

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.

Conclusions • Dynamic electric field manipulates particle dipole. • Horizontal, rotational, and directional movement. • Use of EBL enables control to 50 nm • Aberrations limit the resolution

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 and Nanotechnology Meeting the Needs of the “Room at the Bottom”