Let’s discuss: When can we call a thin film 2-dimensional?

The understanding of when a thin film is two-dimensional (2D) varies throughout the literature. It was introduced by advances in nanotechnology that allowed the fabrication of structures that are in the nm scale in one dimension. More recently, materials with atomic thickness, such as graphene and other van der Waals materials, allowed us to isolate structures that have reached the ultimate limit of thickness. Their layered structures allow a straightforward identification of the monolayers as 2D structures. Today, 2D structures are reported from a wide class of materials ranging from molecules to thin non-van der Waals layers, generating interest across a large variety of scientific fields. The thickness of these reported 2D films varies from atomic scale to several tens or even hundreds of nm. This puzzling occurrence of several hundred nm thick ‘2D materials’ calls for a critical assessment of when thin films are present as 2D. Here, we explore aspects such as atomic and electronic structure, chemical bonding, composition, and the relation of bulk-to-thin film characteristics to find criteria that describe 2D structures. With that, we aim to fuel an interdisciplinary dialogue towards establishing clear definitions for when a thin film is a 2D structure.