Two-dimensional capillaries assembled by van der Waals heterostructures

Research on two-dimensional materials in the past decades has brought many insights of low-dimensional science on a wide range of related topics. As a novel two-dimensional structure, the atomic-scale capillaries which can conceptually be seen as the empty space left by removing few layers of two-dimensional materials from their bulk van der Waals crystals offer a unique platform of investigating physical and chemical processes of ions, molecules, and atoms under two-dimensional confinements. Investigation of many important problems, such as capillary condensation and water network structure that are difficult to be explored experimentally in other confinement structures, has now been accessible; two-dimensional migration of ions, water, and gases shows abnormal transport properties beyond conventional theory prediction; influence of quantum effect to molecule permeation is observable even at room temperature. All these discoveries greatly extend our fundamental understandings of nano-science, and stimulate the development of potential applications. We review the fabrication of these two-dimensional capillaries which are created by the assembly of van der Waals heterostructures, and discuss the ultimate steric effects in the smallest possible confinements. Exotic interactions between capillary interior and confined particles are also summarized. When coupled with external stimuli, these channels exhibit tunable mass transport behaviors, which not only gives feedback to the mechanism understanding but in turn guides the channel structure optimization.