Nanomechanical Probing of Confined Water in Graphene-Liquid Cells

S.S. Jugade, S. Sadighikia, P. van Deursen, P. Sen, A.K. Naik
Indian Institute of Science,

Keywords: graphene-liquid cells, nanoconfinement, nanomechanical


Graphene-liquid Cells (GLCs) are femto to nanoliter-sized water pockets sealed between two graphene sheets. GLCs find significance in Transmission Electron Microscopy-based studies of crystal growth, nanoscale chemical reactions, and live biological samples at molecular to atomic-scale resolution [1]. Besides these applications, GLCs can serve as a unique platform to study the mechanical properties of water under confinement. The pressure inside a GLC with a height close to 1 nm is estimated to reach 1 GPa from theoretical calculations owing to the strong van der Waals force between two graphene layers [2]. However, there is no direct experimental evidence of mechanical properties on whether water remains liquid or turns into an ice-like phase when confined in these nanosized pockets sealed by graphene. A deviation in the properties of water from bulk [3] is also critical for samples trapped in GLCs. We fabricated GLCs using a loop-assisted transfer process on 2 ยตm diameter suspended graphene sheets on a holey-carbon TEM grid. Using Torsional Resonance AFM, we found that the formed water pockets are within 100 nm and have a maximum height of less than 5 nm. Peak Force Tapping Nanomechanical measurements showed that the water pockets are at least twice as stiff as suspended bilayer graphene. These results strongly suggest that water behaves like a solid under the nanoconfinement of GLCs. We also probe the mechanical response of water with increasing sealing volumes up to a few hundred 100 nm pocket height and further compare the stiffness of water pockets with the surface tension of water to evaluate its solid or liquid-like response. These insights will help advance our understanding of water confined at the nanoscale.