Slow water in engineered nano-channels revealed by color-center-enabled sensing

TL;DR

This study uses advanced NV center sensing in diamond to reveal that water molecules confined in nano-channels exhibit significantly reduced diffusivity, providing new insights into interfacial water behavior under nanoscale confinement.

Contribution

It introduces a novel NV-center-based method to measure water dynamics in nanochannels with high sensitivity and spatial resolution, revealing extremely slow water diffusion.

Findings

Confined water shows orders of magnitude lower diffusivity than bulk water.

NV-center sensing enables direct probing of interfacial water dynamics.

Results support theories of slow interfacial water due to trapped carriers and interface effects.

Abstract

Nanoscale confinement of molecules in a fluid can result in enhanced viscosity, local fluidic order, or collective motion. Confinement also affects ion transport and/or the rate and equilibrium concentration in a chemical reaction, all of which makes it the subject of broad interest. Studying these effects, however, is notoriously difficult, mainly due to the lack of experimental methods with the required sensitivity and spatial or time resolution. Here we leverage shallow nitrogen-vacancy (NV) centers in diamond to probe the dynamics of room-temperature water molecules entrapped within ~6-nm-tall channels formed between the diamond crystal and a suspended hexagonal boron nitride (hBN) flake. NV-enabled nuclear magnetic resonance measurements of confined water protons reveal a much reduced H2O self-diffusivity, orders of magnitude lower than in bulk water. We posit the slow dynamics stem from the accumulation of photogenerated carriers at the interface and trapped fluid, a notion we support with the help of molecular dynamics modeling. Our results provide feedback for theories describing interfacial water, and lay out a route for investigating other fluids under confinement.