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 By Shigeo Maruyama, University of Tokyo, Japan Rolling up a (10,10) nanotube ChCh z

Nanotubes & Molecular Channels ~ 20 Å < 10 Å 8 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, (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. CPMD polarizable, but slow Self-consistent tight-binding method proves to be a good solution. polarizable

System  Finite-length CNTs with ends saturated by H atoms  d CC =1.440Å, d CH =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. H C H C

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 Total electronic potential on a (6,6) CNT of 12 sections a/2

Dielectric Constant (parallel)

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

/ring Water Chain in CNT Channel Partial charge (TYP3P) H: O: 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 k B T. 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  electrons!

q H ~ +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.