2d materials

What are 2D materials?

Nanomaterials can be classified by the total number of their nanosized dimensions:

• If 1 dimension is nano-sized, it would be a 2D material – resembling a large, but very thin sheet (like a piece of paper).
• If 2 dimensions of a material are nano-sized, with the other dimension much larger, then this is a 1D material.
• If 3 dimensions of a material are nano-sized, it would be called a 0D material.

Graphene was the first ‘modern’ 2D material to be isolated in 2004. Since then, there have been literally hundreds of other examples, with an extensive range of properties.

Graphene is a covalently-bonded hexagonal lattice of carbon atoms just one atom thick (about 0.14 nm). It is a semimetal (its conduction and valence bands both touch). Graphene's unique band structure means that electrons move through it at extremely high speeds (about 1/300 the speed of light), giving it fascinating properties - such as unparalleled thermal conductivity.

Hexagonal boron nitride (h-BN) is an isomorph of graphene (has the same crystallographic appearance), but has boron and nitrogen atoms instead of carbon. In contrast to graphene, it is a wide-bandgap insulator.

Transition metal dichalcogenides (commonly reffered to as TMDCs) have the chemical formula MX₂, where M is a transition metal (such as molybdenum (Mo) or tungsten (W)) and X is a chalcogen (such as sulfur (S), selenium (Se) or Tellurium (Te)). Bulk TMDCs are van der Waals materials with each layer being three atoms thick, consisting of the metal layer sandwiched between two chalcogenide layers.

TDMCs can take various crystal structures. The most common is the 2H-phase with trigonal symmetry, which results in semiconducting characteristics such as possessed by MoS₂, WS₂, MoSe₂. These semiconductors have an indirect bandgap when in the bulk. For monolayers, the bandgap becomes direct and in the visible spectrum, making them attractive for optoelectronics. Charge mobilities of ~100-1000 cm²V^-1s^-1­ make them a popular choice for 2D transistors. Another possible structure is the metallic 1T phase, which is the most stable polymorph of WTe₂.

Phosphorene is a single layer of black phosphorus – a layered, stable allotrope of elemental phosphorus. It is a direct bandgap semiconductor with a puckered honeycomb structure. The bandgap can be tuned throughout the visible region by stacking layers on top of each other. It has good charge mobility (~1000 cm²V^-1s^-1­), therefore making it suitable for optoelectronic devices and transistors. Phosphorene's corrugated structure means that its properties can vary significantly, depending on which direction the material is measured along.