Two-dimensional (2D) materials

Nanomaterials can be classified based on their dimensional characteristics, where a dimension is considered to be within the nanoscale range if it is less than 100 nm.
Zero-dimensional (0D) nanomaterials are those where all dimensions are within the nanoscale range (i.e., no dimension is larger than 100 nm), and they are typically in the form of nanoparticles.
In one-dimensional (1D) nanomaterials, one dimension is larger than 100 nm, and this category includes nanotubes, nanorods, and nanowires.
Two-dimensional (2D) nanomaterials have two dimensions larger than 100 nm and one dimension that is only a few atomic layers thick. This category includes plate-like materials such as graphene, MXenes, black phosphorus (also known as phosphorene), and hexagonal boron nitride.
Three-dimensional nanomaterials (3D) refers to materials that have three-dimensional structures at the nanoscale. This category encompasses a wide range of materials including bulk powders, dispersions of nanoparticles, bundles of nanowires, nanotubes, and multi-layered structures composed of nanoscale materials. These materials can have nanoscale dimensions in any of the three dimensions (length, width, and height), making them different from two-dimensional nanomaterials that are confined to being flat.
nanoscale dimensions
Classification of nanoscale dimensions. (Source: Tallinn University of Technology)
2D materials are of great interest to researchers because of their unique and exceptional physical and chemical properties compared to their three-dimensional bulk counterparts. Some of the most notable properties of 2D materials include high surface area to volume ratio, high mechanical strength, and excellent electronic conductivity, which make them attractive for a wide range of applications.
For example, their high surface area makes 2D materials ideal for catalytic reactions, while their excellent electronic conductivity makes them suitable for use in electronics and energy storage devices.
Additionally, due to their thin and flat structure, 2D materials can be stacked or layered in various combinations (so-called van der Waals heterostructures) to form new materials with tailored properties, opening up the possibility for new and innovative applications.
Inspired by the unique optical and electronic properties of graphene, 2D layered materials – as well as their hybrids – have been intensively investigated in recent years, driven by their potential applications mostly for nanoelectronics.
The broad spectrum of atomic layered crystals includes transition metal dichalcogenides (TMDs), semiconducting dichalcogenides, monoatomic buckled crystals, such as black phosphorous (BP or phosphorene), and diatomic hexagonal boron nitride (h-BN).
This class of materials can be obtained by exfoliation of bulk materials to small scales, or by epitaxial growth and chemical vapor deposition (CVD) for large areas.
Such atomically thin, single- or few-layer crystals are featured with strong intralayer covalent bonding and weak interlayer van der Waals bonding, resulting in superior electrical, optical and mechanical properties.