Diameter-dependent ion transport through the interior of isolated single-walled carbon nanotubes

Abstract

Nanopores that approach molecular dimensions demonstrate exotic transport behaviour and are theoretically predicted to display discontinuities in the diameter dependence of interior ion transport because of structuring of the internal fluid. No experimental study has been able to probe this diameter dependence in the 0.5-2 nm diameter regime. Here we observe a surprising fivefold enhancement of stochastic ion transport rates for single-walled carbon nanotube centered at a diameter of approximately 1.6 nm. An electrochemical transport model informed from literature simulations is used to understand the phenomenon. We also observe rates that scale with cation type as Li þ 4K þ 4Cs þ 4Na þ and pore blocking extent as K þ 4Cs þ 4Na þ 4Li þ potentially reflecting changes in hydration shell size. Across several ion types, the pore-blocking current and inverse dwell time are shown to scale linearly at low electric field. This work opens up new avenues in the study of transport effects at the nanoscale.

Low Dimensional Carbon Materials for Applications in Mass and Energy Transport

“…Several studies have investigated transport through single-SWCNT devices with an applied electric field across the SWCNT. 13,14 Although these devices were not diameter-assigned, we suspect that they had diameters larger than those studied recently in our laboratory 5,6,11 due to their nonstochastic transport characteristics and large Ru(bpy) 3 2+ flux rates. Changing the applied voltage drastically changed the observed ionic transport rates.…”

“…Aligned SWCNTs are first grown on a silicon dioxide substrate and a two-reservoir barrier system built on top. Different studies have customized this setup by studying transport through devices with two SWCNTs, 6 characterizing each SWCNT, 11 or altering the electronic structure with an applied gate voltage. 13,14 These devices are unique in the electrochemical literature in that they allow the study of transport through single nanopores with controllable diameters of less than 2 nm.…”

“…A typical two-reservoir SWCNT device, as used in a variety of studies, 5,6,11,13,14 is shown in Figure 1A. Aligned SWCNTs are first grown on a silicon dioxide substrate and a two-reservoir barrier system built on top.…”

“…We have recently developed device structures to measure ion transport properties through individual well-characterized SWCNTs, 11 illustrated in Figure 2A. Each SWCNT was characterized with Raman spectroscopy allowing for an assignment of the diameter and the electronic type (semiconducting or metallic).…”

“…7 The pores' hydrophobic nature and atomically smooth surface create slip at the wall boundaries, thus enhancing rates of 11 (B) Cartoon of a pore-blocking event, where an accelerated proton flux is reduced by the presence of a larger cation. 11 (C) Three examples of coherence resonance in SWCNT devices, with sustained regular oscillations. 5 (D, E) Diameter dependence of poreblocking current and dwell times, showing a maximum in pore-blocking current for diameters of approximately 1.6 nm.…”