Implementing a NAND gate from SWCNTs

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Implementing a NAND gate from SWCNTs

• Ben Gojman, Happy Hsin, Joe Liang, Natalia
  Nezhdanova, Jasmin Saini

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Motivation

• SWCNT Transistor
• SWCNT Inverter

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SWCNT Field-Effect Transistor
A semiconducting carbon nanotube supported on two
electrodes can replace the silicon channel in a
field-effect transistor.
A CNT about one nanometer in diameter bridges
source and drain electrodes, and a gating voltage
applied to the silicon substrate induces carriers
onto the nanotube to turn the transistor on.
Conductance G vs. gate voltage Vg of a p-type
semiconducting SWCNT FET
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Intramolecular Inverter

• Inverter built out of SWCNT
• Protect p-type FET part of nanotube with PMMA and
  dope the exposed area to create the n-type FET

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Experimental Results

• Vin/Vout measured
• Results indicate high gain as well as inverter
  behavior

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Y-junction CNTs

• Synthesis
• Electronic Properties

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Fabrication of Y-Junction Carbon Nanotubes
Pyrolysis Synthesis Method Proposed by B. C.
Satishkumar, et al

240 sccm
Outlet
Thiophene
10 sccm
Yield 70
First Furnace is heated at a rate 10 degree/min
until it reaches 623K.
Second Furnace is the Pyrolysis zone (1273K)
Pyrolysis is a form of incineration that
chemically decomposes organic materials by heat
in the absence of oxygen.
The output has carbon nanotubes, and these tubes
have multiple Y junction and can be examined
with Transmission Electron Microscope (TEM).
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Template Synthesis Method Proposed by Jingming
Xu, et al
3 . Stem 90 nm branches 50 nm in diameter
1 .forming a Y-branched nanochannel alumina
template
2. electrochemically deposited some cobalt
catalyst in the bottom of the template channels
and reduced the catalystat 600 oC for 45 hours
under a carbon monoxide flow (100 cm3/min).
Some addition Pictures
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Mechanical Properties Carbon Nanotubes

• Small size
• Low density
• Stiff and strong
• Conduct electricity
• Conduct heat

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Defining characteristics/properties
Two important features chirality diameter of
tube ? Determine whether metallic or
semiconducting
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CNTs as nanoelectronical devices

• High mechanical strength (tensile strength 60GPa)
  and modulus (Youngs modulus 1TPa)
• High electrical conductivity (106 ohm-1
  typically)
• Exhibit ballistic transport
• High thermal conductivity (1750-5800 W/mK)
• covalently bonded and are electrical conductors
  dont suffer from electromigration or atomic
  diffusion and thus can carry high current
  densities (107 -109 A/cm2 ) 
• Single wall nanotubes can be metallic or
  semi-conducting
• Chemically inert, not attacked by strong acids or
  alkali

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Necessary properties/attributes

• Requirements
• No rectification
• High conductance
• Low power dissipation

• Current challenges
• Fabrication
• separation
• Positioning
• Physical models

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Electronic Structure

• Look at graphene (fused benzene)
• Bonding p-band and anti-bonding p-band form from
  overlap between two Pz AOs
• Expression 2-D energy states
• W2D(kx,ky)
• /-?014cos(?3kxa/2)cos(kya/2)4cos²(kya/2)½

• Density of States
• -high number in axial direction
• -limited in circumferential direction by chiral
  vector Ch
• Allowed (1-D quantum states) along tube axis
• Chk 2pj j0,1,2,
• ? Different allowed wave vectors (kmomentum e-)
  according to CNT

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Molecular Simulation Methods-the physics-

• Ab Initio methods
• Obtain accurate solutions to Schrodinger equation
• Not many exact solutions
• Use various approximations
• Tight binding model

• Classical molecular dynamics
• Solve Newtons 2nd law, particle dynamics
• Continuum and multiscale models

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Physics for Y junction CNTs

• Transmission function obtained using Greens
  function formalism
• Using tight-binding formalism for Hamiltonian and
  Greens function
• From transmission function derive current
  properties using Landauer and Buttiker formalism
  for Quantum conductivity

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Y junction results
Current through nanotube i
Nonlinear conductance matrix normalized to e2/h
reads
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Symmetric Y-junction
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...and now, our Feature Presentation
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NAND gate proposal
Au microwires
VDD
B
A
Vout
A
B
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250nm
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Future Directions

• DNA Self-Assembly

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DNA-Assisted Disperion and Separation of Carbon
Nanotubes
WHY DNA-Assisted?

• Carbon nanotubes are polydispersive and possess
  poor solubility in aqueous solution and
  non-aqueous solution.
• It is discovered that DNA(ssDNA) binding to
  nanotubes is EFFECTIVE and FACILE.
• DNA-coated nanotubes are stable for months at
  room temperature
• Free DNA can be removed by using anion-exchange
  chromatography without disturbing the bound tube
  (strong binding).

polyT sequence
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How DNA-Assisted?

• ssDNA molecule are flexible within the
  sugar-phosphate backbone therefore, the molecule
  can find the low-energy conformations that
  maximize base-nanotube stacking interactions
  while exposing the sugar-P groups to water.
• Some thermodynamics simulations were done to show
  that the binding of ssDNA onto tubes can compete
  effectively with nanotubes clinging to each other
  to form ropes.
• Binding allows the backbone to be solvated by
  solvent and thus reduce the surface tension.
• DNA is charged molecule and thus changes the
  charge of the bound tube. An anion-exchanged
  column could be used to separate bound tube and
  free tube.

Metallic tubes
Semi-conducting
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References

• Y-Junction Carbon nanotuebe, B. C. Satishkumar,
  P. J. Thomas, A. Govindraj, and C. N. R. Rao,
  Appl.Phys. Lett. 77, 2530 2000
• Ballistic switching and rectification in single
  wall carbon nanotube Y junctions, Antonis N.
  Andriotis, APPLIED PHYSICS LETTERS VOLUME 79,
  NUMBER 2
• Sticking of Carbon Nanotube Y Junction Branches,
  L. A. Chernozatonskii and I. V. Ponomareva, JETP
  letter, Vol.78, No.5, 2003, pp. 327-331
• Growing Y-junction carbon nanotube, J. Li, C.
  Papadopoulos, and J. Xu, Nature London! 402, 253
  1999!.

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Acknowledgements

• Prof. Marc Bockrath

• Prof. Erik Winfree

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Questions?
JOE
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Have a Great Summer!!!