Van der Waals heterostructures

A two-dimensional semiconductor (also known as 2D semiconductor) is a type of natural semiconductor with thicknesses on the atomic scale. The rising research attention towards 2D semiconductors started with a discovery by Geim and Novoselov et al. in 2004, when they reported a new semiconducting material graphene, a flat monolayer of carbon atoms arranged in a 2D honeycomb lattice. A 2D monolayer semiconductor is significant because it exhibits stronger piezoelectric coupling than traditionally employed bulk forms, which enables 2D materials applications in new electronic components used for sensing and actuating. In this emergent field of research in solid-state physics, the main focus is currently on designing nanoelectronic components by the use of graphene as electrical conductor, hexagonal boron nitride as electrical insulator, and a transition metal dichalcogenide as semiconductor.

Graphene's two surfaces are single sheets of carbon atoms arranged in a hexagonal honeycomb lattice. Having two surfaces and lacking bulk makes it the thinnest possible material but also 5 times stronger than steel due to pi and sigma orbital bonds. Graphene has high electron mobility and high thermal conductivity. Although graphene can be used in different applications, one issue regarding graphene is its lack of a band gap, which poses a problem in particular with digital electronics because it is unable to switch off field-effect transistors (FETs).

Monolayer hexagonal boron nitride (h-BN), also known as ‘white graphene’, is structurally similar to graphite and features a honeycomb arrangement with alternating boron and nitrogen atoms in place of carbon. h-BN has a higher energy gap (5.97 eV) than graphene, thus functions as an insulator instead of a semimetal. However, it can also function as a semiconductor with enhanced conductivity due to its zigzag sharp edges and vacancies. h-BN is often used as substrate and barrier due to its insulating property. Furthermore, h-BN also has a large thermal conductivity and mechanical strength. Thus, it can be employed as a support for metal catalyst due to its chemical, thermal, acid-base stability and high thermal conductance.

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