Spatially resolved observation of water transport across nanomembranes using bright-field nanoscopy

Abstract

Gaining a detailed understanding of water transport behavior through ultra-thin membranes including atomically thin graphene layers is increasingly becoming necessary due to their potential applications in water desalination and ion separation. It is important to correlate the nanoscopic architecture of the membrane with the macroscopic properties such as the average water transport rate and the ion selective transport rates. Such correlations are only possible when spatially resolved (in the lateral direction) information of mass transport across the membrane is available. Then, one will be able to identify the relative role of grain boundaries, defects, and other topographical structures of interest in determining the macroscopic parameters which will aid in optimizing the fabrication processes of such membranes. Current techniques do not provide spatially resolved information and only provide macroscopic parameters such as the bulk water transport rate. We describe a technique, referred to here as Bright-Field Nanoscopy (BFN), which provides a spatially resolved measurement of water transport across nanomembranes. Using this technique, we demonstrate how grain engineering of atomically thin chemical vapor deposited graphene membranes can tune the bulk water transport rate across the membranes by orders of magnitude. BFN exploits the strong thickness dependent color response of an optical stack consisting of a thin (∼25 nm) germanium film deposited over a gold substrate and only requires a regular bright-field microscope for data acquisition. To show the generality of this technique, we demonstrate the strong influence of the terminal layer on the bulk water transport rates in thin (∼20 nm) layer-by-layer deposited polyelectrolyte multilayer films by exploiting the spatially resolved nature of the acquired data. We also show that by controlling the ambient conditions, the effect of the terminal layer can be completely suppressed.