Screening of Water Dipoles inside Finite-Length Carbon Nanotubes

1. Screening of Water Dipoles inside Finite-Length Carbon Nanotubes Yan Li, Deyu Lu ,Slava Rotkin Klaus Schulten and Umberto Ravaioli Beckman Institute, UIUC

2. 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

3. Rolling up a (10,10) nanotube z Ch By Shigeo Maruyama, University of Tokyo, Japan Carbon Nanotube

4. ~25 Å ~ 20 Å 8 Å < 10 Å Nanotubes & Molecular Channels

5. 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).

6. Water (ions,polymers) Electronic interaction CNT Kolesnikov et al.,PRL, 2004 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.

7. C C H H Model and Methods • 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. S. Reich, et al., PRB,66,035412,2002.

8. Infinitely long armchair CNTs are metallic Density of States of a (6,6) CNT Band structure of a (6,6) CNT

9. Band Gap Oscillation

10. Total electronic potential on a (6,6) CNT of 12 sections Dielectric Response a/2

11. Dielectric Constant (parallel)

12. Dipole Screening • Effective screening near the tube center • Coulomb interaction lower the system energy

13. /ring 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.

14. 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? Don’t forget the  electrons!

15. Charge transfer occurs between C and H atoms qH ~ +0.14e These local dipoles may affect the entering and ordering of polar molecules inside.

16. Length dependence of electronic properties and dielectric behavior by third NN TB andab 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. Conclusion