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WATER ORDERING BY CONFINEMENT IN CARBON NANOTUBES AT 300K: Implications for possible design of proton-conducting nano-semiconductors

R. J. Mashl, S. Joseph, N. Aluru, E. Jakobsson
University of Illinois, US

Keywords: nanotubes, water dynamics, order disorder transition, proton conduction

Introduction. Molecular dynamics (MD) simulations suggest that water readily enters carbon nanotubes [1]. This is also suggested by the fact that fluorescence in nanotubes is af­fected by the pH of external electrolyte [2]. Distinctive symmetrical water structures emerge when water is energy minimized within nanotubes [3]. In this paper we report on MD simula­tions in which we observe the formation of hexagonal ice at 300 K in a nanotube of critical dia­meter. Simulations, Observations, and Analysis. The simulation systems consisted of single-walled, open-ended carbon nanotubes 40 Å long and of various diameters (eight different dia­meters altogether), embedded in a hy­dro­phob­ic slab and bathed in SPC/E water at 300K. All of the tubes filled with water and remained filled for the entire 2 ns of simulated time. We used the GROMACS simulation package and particle-mesh Ewald sums for the electrostatic interactions. After equilibration of each system, the state of the water in the lumen of the nanotube was analyzed by fluctuation-correlation analysis. Diffusion coefficients for the water inside the nanotubes as a function of channel diameter were computed (Fig. 1). The diffusion co­ef­fi­cient is somewhat reduced from the bulk value in all sizes of nanotubes, with an anomalous re­duc­tion at a diameter of 12.2 Å (carbon atom center-to-center, or about 8.4 Å based on van der Waals surfaces) where the longitudinal motion is effectively eliminated. Rotational auto­cor­re­la­tion functions based on the water molecules' dipole moments for the same simulations show (see Fig. 2) that the water in the 12.2-Å diameter tube is anomalously immobilized rotationally as well. By comparing snapshots of the systems (see Fig. 3), it is seen that the water in the crit­i­cal size tube is in a much more ordered regular array than in either smaller or larger tubes. A cross-section through the critical size tube in Fig. 4 shows that the water assumes a regular hexagonal pattern. By virtue of the translational and rotational immobilization and the hex­ag­onal symmetry, we conclude that confinement of water in a nanotube of particular dimensions induces 300K water to form hexagonal ice [4]. Significance. In considering charge carriers for nanoscale semiconductors, electrons have the disadvantage of tunneling distances of nanometers, rendering distinction between "on" and "off" states problematic. Protons in ice tunnel between adjacent water molecules over the length of a few Ångstroms. Proton-ice conduction might be switched off by a per­tur­ba­tion that disorders the water, dramatically reducing proton mobility. We suggest that pro­tons tunneling through ice created by confinement in nanotubes be considered further as a pos­si­ble conduction mechanism for nanoscale semiconductors.

NSTI Nanotech 2003 Conference Technical Program Abstract

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