(688b) Ferroelectric Water Inside Single-Walled Carbon Nanotubes

In this paper, we report novel ferroelectric properties of a new form of ice inside single-walled carbon nanotubes (SWCNTs). The ices are called ?ice nanotubes? (ice NTs) and they consist of polygonal water rings stacked one-dimensionally along the SWCNT axis. We performed molecular dynamics (MD) calculations for the ice NTs under an external electric field and in a temperature range between 100 K and 350 K, using TIP3P water model. First, it is revealed that the present MD simulations in a zero-external electric field reproduce reported x-ray diffraction experiments. From a comparison, it is strongly suggested that ice-NTs consisting of more than 20 polygon units are indeed formed inside real SWCNTs. The calculated polarization indicates significant fluctuation with time and space around the ice-NT phase transition temperature . The fluctuation was more remarkable for thinner SWCNTs and for parallel polarization with respect to the SWCNT axis. Therefore it could be ascribed to quasi-one-dimensional fluctuations in the ice-NT structures. With reduced temperatures well below , a ferro- or antiferro-electric single domain is formed inside the SWCNTs; although the polarization fluctuates between negative and positive polarity with the same magnitude in the intermediate temperature region, it finally freezes along one direction of the SWCNT axis. The polarization as a function of the external field indicates novel hysteresis behavior that cannot be observed in the bulk: The n-gonal ice NT, where n=5, 6, or 7, has (n+1)-polarized structures with different polarizations. The results predict that ice-NTs can be the world's smallest ferroelectrics with different polarized states, as small as 2 nm in diameter and shorter than 4 nm in length. The present findings suggest potential applications of SWCNTs encapsulating dielectric materials to fabricate the smallest ferroelectric devices.

Reference:

F. Mikami, K. Matsuda, H. Kataura and Y. Maniwa, Dielectric Properties of Water inside Single-Walled Carbon Nanotubes, ACS Nano, (2009) DOI: 10.1021/nn900221t