Screening of Water Dipoles inside Finite-Length Carbon Nanotubes

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Screening of Water Dipoles inside Finite-Length
Carbon Nanotubes

• Yan Li, Deyu Lu ,Slava Rotkin
• Klaus Schulten and Umberto Ravaioli
• Beckman Institute, UIUC

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Outline

• Introduction on carbon nanotubes (CNTs)
• Electronic properties of finite length CNT
• using ab initio and tight-binding methods
• Band gap and dielectric response
• Polarization effect from a CNT channel for water
• Conclusion

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Carbon Nanotube
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Nanotubes Molecular Channels
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Nanotube Molecular Channels
Neutron scattering experiments Molecular
Dynamics simulations
Theory
Hummer et al., Nature 414,188 (2001)
Kolesnikov et al, Phys.Rev. Lett. 93, 035503
(2004).
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Motivation Modeling CNT-Water System

• Classic molecular dynamics
• non-polarized CNT
• ab initio method e.g. CPMD
• polarizable, but slow

VdW interaction
polarizable

• Develop a reliable and fast method with
  polarization effect

Self-consistent tight-binding method proves to be
a good solution.
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Model and Methods

• System
• Finite-length CNTs with ends saturated by H atoms
• dCC1.440Å, dCH1.090Å (no optimization
  effects)
• ab initio method hybrid DFT (B3LYP)
• Mixture of HF exchange with DFT
  exchange-correlation functional
• 6-31G basis sets for C and H atoms
• Semi-empirical method tight-binding
• All ? electrons approximation
• Third nearest-neighbor approximation.
• Self-consistency.

S. Reich, et al., PRB,66,035412,2002.
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Infinitely long armchair CNTs are metallic
Density of States of a (6,6) CNT
Band structure of a (6,6) CNT
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Band Gap Oscillation
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Dielectric Response
a/2
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Dielectric Constant (parallel)
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Dipole Screening

• Effective screening near the tube center
• Coulomb interaction lower the system energy

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Water Chain in CNT Channel
Induced charges along the axis of a (6,6) CNT
Partial charge (TYP3P) H 0.417 O -0.834
water profile from MD simulation
D. Lu, Y. Li, S. V. Rotkin, U. Ravaioli and K.
Schulten, Nano Lett., to be published.
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• Dipole moment from water is screened by more
  than 50.

• Gain in Coulomb energy is 6 kBT.

• For charged molecules, the screening effect will
  be even more substantial.

• Applying electric field or functionalize the CNT
  to facilitate the entering
• of bio-molecules?

Dont forget the ? electrons!
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Charge transfer occurs between C and H atoms
qH 0.14e
These local dipoles may affect the entering and
ordering of polar molecules inside.
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Conclusion