A two-dimensional 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 materialgraphene, 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.
Materials
Graphene
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. Nanosheets of other group-IV elements present structural and electronic properties similar to graphene.
Monolayer hexagonal boron nitride, 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 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.
Transition metal dichalcogenides
Transition metal dichalcogenide monolayers are a class of two-dimensional materials, which have the chemical formula MX2, where M represents transition metals from group VI, V and VI, and X represents a chalcogen such as sulfur, selenium or tellurium. MoS2, MoSe2, MoTe2, WS2 and WSe2 are TMDCs. TMDCs have layered structure with a plane of metal atoms in between two planes of chalcogen atoms as shown in Figure 1. Each layer is bonded strongly in plane, but weakly in interlayers. Therefore, TMDCs can be easily exfoliated into atomically thin layers through various methods. TMDCs show layer-dependent optical and electrical properties. When exfoliated into monolayers, the band gaps of several TMDCs change from indirect to direct, which lead to broad applications in nanoelectronics, optoelectronics, and quantum computing
Synthesis
2D semiconductor materials are often synthesized using a chemical vapor deposition method. Because CVD can provide large-area, high-quality, and well-controlled layered growth of 2D semiconductor materials, it also allows synthesis of two-dimensional heterojunctions. When building devices by stacking different 2D materials, mechanical exfoliation followed by transferring is often used. Other possible synthesis methods include chemical exfoliation, hydrothermal synthesis, and thermal decomposition.
Device applications
Some devices applications include electronic devices, photonic and energy harvesting devices, and flexible and transparent substrates. Furthermore, applications on quantum computing qubit devices have been proposed.
Electronic devices
2D Semiconductors can be used as transistors for digital electronics. The impure charges at the interfaces that are free of dangling bonds, allow for 2D semiconductors to run low-power devices. The 2D semiconductor interface has future potential in nano circuits due to its ability to optimize and regulate thermal transfer.
Energy and harvesting devices
2D semiconductors have potential for application in the harvesting of solar energy. The atomically thin structure allows for lower surface recombination velocity, which leads to better photocurrent conduction. An improvement on solar cell performance has been shown, while stacking 2D semiconductors with multilayers of graphene.
Flexible and transparent substrates
The thin layer of 2D materials can be used for flexible electronics. In particular, 2D MoS2 can be used to create thin displays and wearable electronics due to its out of plane flexibility, strong covalent bonds, and diverse electronic properties.
Magnetic NEMS
2D layered magnetic materials are attractive building blocks for nanoelectromechanical systems : while they share high stiffness and strength and low mass with other 2D materials, they are magnetically active. Among the large class of newly emerged 2D layered magnetic materials, of particular interest is few-layer CrI3, whose magnetic ground state consists of antiferromagnetically coupled ferromagnetic monolayers with out-of-plane easy axis. The interlayer exchange interaction is relatively weak, a magnetic field on the order of 0.5 T in the out-of-plane direction can induce spin-flip transition in bilayer CrI3. Remarkable phenomena and device concepts based on detecting and controlling the interlayer magnetic state have been recently demonstrated, including spin-filter giant magnetoresistance, magnetic switching by electric field or electrostatic doping, and spin transistors. The coupling between the magnetic and mechanical properties in atomically thin materials, the basis for 2D magnetic NEMS, however, remains elusive although NEMS made of thicker magnetic materials or coated with FM metals have been studied.
