Screening of Water Dipoles inside

Finite-Length Carbon Nanotubes

Yan Li, Deyu Lu ,Slava Rotkin

Klaus Schulten and Umberto Ravaioli

Beckman Institute, UIUC

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

Carbon Nanotube

),(or 21 mnamanCh

030Armchair θ

00 Zigzag θ

Chiral

By Shigeo Maruyama, University of Tokyo, Japan

Rolling up a (10,10) nanotube

Ch

z

2or 1)3,(mod:tingsemiconduc

0)3,(mod :metallic

mn

mn

Nanotubes & Molecular Channels

~ 20

Å

< 10 Å8 Å

~25

Å

Neutron scattering experiments + Molecular Dynamics simulations

Nanotube & Molecular Channels

Hummer et al., Nature 414,188 (2001) Kolesnikov et al, Phys.Rev. Lett. 93, 035503 (2004).

Theory

Electronic interaction

VdW interaction

Motivation: Modeling CNT-Water System

Kolesnikov et al., PRL, 2004

Water (ions,polymers)

CNT Develop a reliable and fast

method with polarization effect

Classic molecular dynamics:non-polarized CNT

ab initio method: e.g. CPMDpolarizable, but slow

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

polarizable

System Finite-length CNTs with ends saturated by H atoms dCC=1.440Å, dCH=1.090Å (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.

Model and Methods

S. Reich, et al., PRB,66,035412,2002.

HC

HC

Infinitely long armchair CNTs are metallic

Band structure of a (6,6) CNT Density of States of a (6,6) CNT

Band Gap Oscillation

Dielectric Response

mid

tot

ext

V

V

||

09.5 ,23.5 3|||| LYPBTB

Total electronic potential on a (6,6) CNT of 12 sections

a/2

Dielectric Constant (parallel)

A 586.0

e 834.0

d

Q

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

Dipole Screening

/ ring

Water Chain in CNT Channel

Partial charge (TYP3P)

H: 0.417

O: -0.834

Induced charges along the axis of a (6,6) CNT

water profile from MD simulation

D. Lu, Y. Li, S. V. Rotkin, U. Ravaioli and K. Schulten, Nano Lett., to be published.

• Gain in Coulomb energy is ~6 kBT.

• Dipole moment from water is screened by more than 50%.

• 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? Don’t forget the electro

ns!

qH ~ +0.14e

Charge transfer occurs between C and H atoms

These local dipoles may affect the entering and ordering of polar molecules inside.

Conclusion

Length dependence of electronic properties and dielectric behavior by third NN TB and ab initio B3LYP methods agree very well.

Example: a short (6,6) CNT at presence of external dipoles, which are substantially screened from image charges on the CNT.

Polarization effect from the channel wall may influence the entering and transport of polar molecules through the Coulomb interaction between the molecules and images charges on the CNT.

Third NN TB method provides a fast and reliable approach to model

this polarization effect in CNT-based channels.

Combine self-consistent TB method and classical MD simulation to study the molecular transport in polarizable CNT channels.