Matter in ångström-scale two-dimensional confinement.

Matter exhibits strikingly different structures and behaviours in strongly confined spaces compared with its bulk form. These differences become particularly pronounced when the confinement is reduced to the ångström scale, comparable with the characteristic size of atoms, ions and molecules. Advances in layered two-dimensional (2D) materials - including both van der Waals and non-van-der-Waals materials - have enabled the precise fabrication of ångström-scale 2D confinement systems, opening a new frontier for investigating matter properties and mass transport at this extreme scale. In this Review, we summarize the methods used for precisely fabricating such confined systems, the novel structural transformations of matter that arise and their associated physicochemical properties, and the unique molecular and ionic transport phenomena observed and their underlying mechanisms. We also critically assess the knowledge gaps, technological implications and untapped potential of this burgeoning field.