Two-dimensional van der Waals materials based designer fluidic channels

Membranes with physical dimensions approaching hydrated ionic size are crucial for desalination, chemical separation, and sensing technologies. Nanopores, carbon nanotubes and graphene oxide laminates have been widely explored; however large size distribution and lack of true nanoscale structures beyond the surface roughness limit is a challenge for the advancement of nanofluidics. Recently, we reported [1] a novel method of fabrication of thin graphene channels with precise control on the channel dimension down to the ångström level. The transport behavior of ions is relatively unknown in this regime. Here I detail the nature of ion transport through Å-sizechannelsmade from various van der Waals crystals[2]. Contrary to the predictions,the hydrated ions move like soft balls, even though the hydrated ion sizes are comparable or greater than the channel dimensions. Due to larger ion sizes, trivalent cations show significant hydration effects leading to reduction of mobility; the mobility of Al3+ is reduced by 10 times relative to K+. The mobility of monovalent ions is also affected by the presence of water-wall interactions. OH- adsorption on the walls significantly affects the ionic transport properties at this scale. The possibility to choose channel materials from a variety of layered crystals makes them the most promising for future nanofluidic circuits.

[1]	B. Radha, A. Esfandiar, F. C. Wang, A. P. Rooney, K. Gopinadhan, A. Keerthi, A. Mishchenko, A. Janardanan, P. Blake, L. Fumagalli, M. Lozada-Hidalgo, S. Garaj, S. J. Haigh, I. V. Grigorieva, H. A. Wu, A. K. Geim, Molecular transport through capillaries made with atomic-scale precision. Nature538, 222-225 (2016).
[2]	A. Esfandiar, B. Radha, F. C. Wang, Q. Yang, S. Hu, S. Garaj, R. R. Nair, A. K. Geim, K. Gopinadhan, Size effect in ion transport through angstrom-scale slits, Science (2017).