Two-dimensional transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) represent a large family of layered semiconductor materials of the type MX2, with M a transition metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms.
Many TMDs exhibit tunable band gaps that can undergo a transition from an indirect band gap in bulk crystals to a direct band gap in two-dimensional (2D, i.e. monolayer) nanosheets (e.g., MoS2, WSe2, WS2, MoTe2). This means that by varying the number of layers, the band gaps can be modified.
A distinct feature of TMDs is the widening band gap with decreasing atomic layers because of the quantum confinement effect, which leads to a crossover from an indirect gap in bulk materials to a direct gap at the limit of single layers.
The van der Waals interactions between neighboring layers (similar to how a sticky tape attaches to a flat surface) of TMDs — and other two-dimensional layered materials – may allow much more flexible integration of different materials without the limitation of lattice matching, therefore opening up vast possibilities to nearly arbitrarily combine and control different properties at the atomic scale. These structures are called van der Waals heterostructures.
Due to their extraordinary optical and electrical properties, these 2D TMDs have emerged as a promising class of atomically thin semiconductors for a new generation of electronic and optoelectronic devices. For instance, TMDs possess optical properties that could be used to make computers run a million times faster and store information a million times more energy-efficiently.
Two-dimensional transition metal dichalcogenides
Two-dimensional transition metal dichalcogenides (2D-TMDs). (a) The table shows common TMDs and their band gap. (b) A schematic illustration of the layered structure of MoS2. (c) Energy dispersion in bulk, quadrilayer (4L), bilayer (2L) and monolayer (1L) MoS2 from left to right. The horizontal dashed line represents the energy of a band maximum at the K point. The red and blue lines represent the conduction and valence band edges, respectively. The lowest energy transition increases with the decreasing layer and evolve from indirect to direct (vertical) transitions. (d) The relative valence and conduction band edge of some common TMDs (monolayer).(© The Royal Society of Chemistry) (click on image to enlarge)

Electronic and optoelectronic properties and devices based on 2D TMDs

Initial studies have sparked intense interest in exploring 2D-TMDs as a new generation of atomically thin semiconductors for functional electronics and optoelectronics. Back in 2013, researchers reported the first demonstration of an n-type field-effect-transistor (FET) made of a monolayer of tungsten diselenide (WSe2), showing the tremendous potential of this material for future low-power and high-performance integrated circuits.
With an intrinsic band gap typically in the range 1–2 eV, the 2D-TMDs can overcome the key shortcomings of graphene for electronic applications – an energy gap (band gap) that allows the flow of electrons to be controlled, for the current to be switched on and off. This gap makes TMDs ideal for use in transistors.
Besides functioning as atomically thin channel materials for highly flexible field-effect transistors, TMDs are also very good absorbers of circularly polarized light, so they could be used in detectors.

Synthesis of atomically thin 2D-TMDs

Most of the early research of TMDs, fundamental physics studies and device demonstrations, relied on exfoliated flakes, with the flake size typically limited to the order of 10 µm or less. Consequently, the fabrication of 2D-TMD heterostructure devices has been generally achieved through arduous exfoliation and repeated physical transfer processes, which is clearly not scalable for practical technologies.
To move beyond the initial fundamental physical curiosity and explore the full potential of these materials, it has become necessary to develop robust synthetic strategies for the growth of these atomically thin materials with a well-controlled physical dimension, chemical composition, and heterostructure interface.
To this end, researchers have developed two distinct synthetic approaches: a top-down strategy including various exfoliation approaches to exfoliate bulk 2D crystals into single or few-layered nanosheets, such as mechanical exfoliation, chemical exfoliation and solvent exfoliation; and a bottom-up strategy to chemically synthesize the atomically thin nanosheets either in solution phase or in a chemical vapor deposition process.
Growth of 2D-TMD heterostructures
Growth of 2D-TMD heterostructures. (a and b) A schematic illustration of lateral epitaxial growth of WS2–WSe2 heterostructures. (c) A schematic illustration of successive lateral epitaxial growth of superlattice structure in the lateral dimension. (d) A schematic illustration of van der Waals superlattices through the successive layer-by-layer growth approach. (© The Royal Society of Chemistry) (click on image to enlarge)
Although this research field is still on its early days, it has become clear that the availability of a broad range of 2D-TMDs with variable electronic band structures allows the creation of a wide variety of heterojunctions and superlattices with a designed and optimized band alignment and number of atomic layers and precisely tuned electronic functions, thus creating a powerful material platform for novel, high-performance electronic and optoelectronic devices.