Wettability engineering for studying ion transport in 2D layered materials
Layered materials are widely used for their capacity to incorporate and store foreign ionic species. It has become possible to exploit this process at the level of single crystals consisting of individual atomic layers: 2D materials and their van der Waals heterostructures. Due to the small size of available highquality specimen, however, it remains challenging to probe ion transport and storage in these systems. To promote future advances in this field, wettability engineering is introduced as a strategy for the on-chip integration of electrolytes with micrometer-sized samples of 2D materials. Contact angles are systematically measured for a variety of electrolyte-surface couples to identify a rational device design, and engineer lateral contrasts in surface chemistry to control the rims of drop cast electrolytes with micrometer precision. This allows covering single crystalline flakes of 2D materials only partially, leaving most of the sample surface uncovered and imposing directionality on ion transport. Wettability engineering is used to fabricate few-layer graphene electrochemical micro two-compartment cells that display chemical diffusion of lithium on the order of 10−8 cm2 s−1. While the approach is transferrable to other 2D materials and ionic species, it can also benefit device design for the study of nonlocal electrolyte gating effects.