Transport properties

At submicron scales, 3D materials no longer have the same behavior as their 2D form, which can be an advantage. For example, graphene has a very high carrier mobility, and accompanying lower losses through the Joule effect. But graphene has zero bandgap, which results in a disqualifyingly low on/off ratio in transistor applications. TMD monolayers might be an alternative: they are structurally stable, display a band gap and show electron mobilities comparable to those of silicon, so they can be used to fabricate transistors.

Although thin-layer TMDs have been found to have a lower electron mobility than bulk TMDs, most likely because their thinness makes them more susceptible to damage, it has been found that coating the TMDs with HfO₂ or hexagonal boron nitride (hBN) increases their effective carrier mobility.^[25]

Optical properties

A semiconductor can absorb photons with energy larger than or equal to its bandgap. This means that light with a shorter wavelength is absorbed. Semiconductors are typically efficient emitters if the minimum of the conduction band energy is at the same position in k-space as the maximum of the valence band, i.e., the band gap is direct. The band gap of bulk TMD material down to a thickness of two monolayers is still indirect, so the emission efficiency is lower compared to monolayered materials. The emission efficiency is about 10⁴ greater for TMD monolayer than for bulk material.^[4] The band gaps of TMD monolayers are in the visible range (between 400 nm and 700 nm). The direct emission shows two excitonic transitions called A and B, separated by the spin–orbit coupling energy. The lowest energy and therefore most important in intensity is the A emission.^[2][27] Owing to their direct band gap, TMD monolayers are promising materials for optoelectronics applications.

Atomic layers of MoS₂ have been used as a phototransistor and ultrasensitive detectors. Phototransistors are important devices: the first with a MoS₂ monolayer active region shows a photoresponsivity of 7.5 mA W⁻¹ which is similar to graphene devices that reach 6.1 mA W⁻¹. Multilayer MoS₂ show higher photoresponsivities, about 100 mA W⁻¹, which is similar to silicon devices. Making a gold contact at the far edges of a monolayer allows an ultrasensitive detector to be fabricated.^[5] Such a detector has a photoresponsivity reaching 880 A W⁻¹, 10⁶ greater than the first graphene photodetectors. This high degree of electrostatic control is due to the thin active region of the monolayer. Its simplicity and the fact that it has only one semiconductor region, whereas the current generation of photodetectors is typically a p–n junction, makes possible industrial applications such as high-sensitivity and flexible photodetectors. The only limitation for currently available devices is the slow photoresponse dynamics.^[5] Utilizing WSe₂ the photoresponse was improved to a bandwith of over 230 MHz by device symetry optimization^[28]

Mechanical properties

Interest in the use of TMD monolayers such as MoS_2, WS₂, and WSe₂ for the use in flexible electronics due to a change from an indirect band gap in 3D to a direct band gap in 2D emphasizes the importance of the mechanical properties of these materials.^[29] Unlike in bulk samples it is much more difficult to uniformly deform 2D monolayers of material and as a result, taking mechanical measurements of 2D systems is more challenging. A method that was developed to overcome this challenge, called atomic force microscopy (AFM) nanoindentation, involves bending a 2D monolayer suspended over a holey substrate with an AFM cantilever and measuring the applied force and displacement.^[30] Through this method, defect free mechanically exfoliated monolayer flakes of MoS₂ were found to have a Young's modulus of 270 GPa with a maximum experienced strain of 10% before breaking.^[31] In the same study, it was found that bilayer mechanically exfoliated MoS₂ flakes have a lower Young's modulus of 200 GPa, which is attributed to interlayer sliding and defects in the monolayer.^[31] With increasing flake thickness the bending rigidity of the flake plays a dominant role and it is found that the Young's modulus of multilayer, 5- 25 layers, mechanically exfoliated MoS₂ flakes is 330 GPa.^[32]

The mechanical properties of other TMDs such as WS₂ and WSe₂ have also been determined. The Young's modulus of multilayer, 5-14 layers, mechanically exfoliated WSe₂ is found to be 167 GPa with a maximum strain of 7%.^[33] For WS₂, the Young's modulus of chemical vapor deposited monolayer flakes is 272 GPa.^[34] From this same study the Young's modulus of CVD-grown monolayer flakes of MoS₂ is found to be 264 GPa.^[34] This is an interesting result as the Young's modulus of the exfoliated MoS₂ flake is nearly the same as that of the CVD grown MoS₂ flake. It is generally accepted that chemically vapor deposited TMDs will include more defects when compared with the mechanically exfoliated films that are obtained from bulk single crystals, which implies that defects (points defects, etc.) that are included in the flake do not drastically affect the strength of the flake itself.

Under the application of strain, a decrease in the direct and indirect band gap is measured that is approximately linear with strain.^[35] Importantly, the indirect bandgap decreases faster with applied strain to the monolayer than the direct bandgap, resulting in a crossover from direct to indirect band gap at a strain level of around 1%.^[36] As a result, the emission efficiency of monolayers is expected to decrease for highly strained samples.^[37] This property allows mechanical tuning of the electronic structure and also the possibility of fabrication of devices on flexible substrates.

Exfoliation

Exfoliation is a top down approach. In the bulk form, TMDs are crystals made of layers, which are coupled by Van-der-Waals forces. These interactions are weaker than the chemical bonds between the Mo and S in MoS₂, for example. So TMD monolayers can be produced by micromechanical cleavage, just as graphene.

The crystal of TMD is rubbed against the surface of another material (any solid surface). In practice, adhesive tape is placed on the TMD bulk material and subsequently removed. The adhesive tape, with tiny TMD flakes coming off the bulk material, is brought down onto a substrate. On removing the adhesive tape from the substrate, TMD monolayer and multilayer flakes are deposited. This technique produces small samples of monolayer material, typically about 5–10 micrometers in diameter.^[38]

Large quantities of exfoliated material can also be produced using liquid-phase exfoliation by blending TMD materials with solvents and polymers.^[39]

Chemical vapor deposition

Chemical vapor deposition (CVD) is another approach used to synthesize transition-metal dichalcogenides.^[40] It has been used broadly to synthesize many different TMDs because it can be easily adapted for different TMD materials. Generally, CVD growth of TMDs is achieved by putting precursors to the material, typically a transition-metal oxide and pure chalcogen, into a furnace with the substrate on which the material will form.^[41] The furnace is heated to high temperatures (anywhere from 650 to 1000 °C) with an inert gas, typically N₂ or Ar, flowing through the tube.^[41] Some materials require H₂ gas as a catalyst for formation, so it may be flowed through the furnace in smaller quantities than the inert gas.^[42]

Outside of traditional CVD, metal organic chemical vapor deposition (MOCVD) has been used to synthesize TMDs. Unlike traditional CVD described above, MOCVD uses gaseous precursors, as opposed to solid precursors and MOCVD is usually carried out at lower temperatures, anywhere from 300 to 900 °C.^[43] MOCVD has been shown to provide more consistent wafer-scale growth than traditional CVD.

CVD is often used over mechanical exfoliation despite its added complexity because it can produce monolayers ranging anywhere from 5 to 100 microns in size as opposed to the surface areas of roughly 5-10 microns produced using the mechanical exfoliation method.^[44] Not only do TMD monolayers produced by CVD have a larger surface area than those flakes produced by mechanical exfoliation, they are often more uniform. Monolayer TMD flakes with very little or no multilayer areas can be produced by chemical vapor deposition, in contrast to samples produced by mechanical exfoliation, which often have many multilayered areas.^[38][41] Geometrically confined-growth techniques are also recently applied to realize wafer-scale single-domain TMD monolayer arrays and their heterostructures.^[45]

Molecular-beam epitaxy

Molecular-beam epitaxy (MBE) is an established technique for growing semiconductor devices with atomic monolayer thickness control. MBE has been used to grow different TMDs, such as MoSe₂, WSe₂, and early transition metals, including titanium, vanadium, and chromium, tellurides,^[46][47][48] resulting in extremely clean samples with a thickness of only 0.5 monolayer.^[46][48]

The growth takes place in ultra-high vacuum (UHV). Precursors for the target materials are placed into evaporation cells, usually as powder (for example selenium), or as a rod (for example molybdenum).^[46] Some elements, such as selenium and tellurium, both of which are chalcogens, can be used in pure solid form as precursors. Some elements, however, can only be used when extracted from solid compounds, such as sulfur from FeS₂. The compound materials are broken down by heating up the material at UHV pressures.^[49] The evaporation cells are either Knudsen cells or electron beam evaporation based, depending on the materials; electron beam evaporation works with rods and can be used to reach high temperatures without overheating heating filaments, while Knudsen cells are suitable for powders and materials with a lower evaporation point. The evaporated materials are then directed towards the substrate; some common ones are MoS₂, HOPG, mica, or a sapphire substrate, such as Al₂O₃.^{[46][47][48][50]} A specific substrate is chosen to fit the targeted growth the best. The substrate is kept heated during the process to enhance the growth, with the temperatures ranging from 300 °C to 700 °C. The temperature of the substrate is one key factor of the growth, and altering it can be used to grow different phases, such as 1T and 2H, of the same material.^[46]

MBE holds some advantages in regards to both manual exfoliation and CVD. Use of reflection high-energy electron diffraction (RHEED) enables the in-situ monitoring of the growth, and this additionally with UHV and slow growth speed allows one to create clean, atomically thin monolayers.^[46][51] The improvement in sample quality is considerable when compared to exfoliation, as MBE is more effective in getting rid of the large flakes and impurities. In contrast to CVD, MBE proves beneficial when single-layerd TMDs are required.^[48][51] The disadvantage of MBE is that it is a relatively complicated process that requires large amounts of specialized equipment. Maintaining UHV can be difficult, and the preparation of samples is slower than in the other two methods.

Electrochemical Deposition

Electrodeposition is among the techniques that have emerged to produce TMDC semiconductors such as MoS₂, WS₂ and WSe₂. Several reports have shown controlled electrodeposition of TMDC layers down to a monolayer.^{[52][53][54][55]} The materials have so far shown continuous films of good uniformity but typically require annealing temperatures > 500 °C. Electrodepositions of TMDC films have been successfully reported over conducting films such as graphene and TiN, and over a SiO₂ insulator by growing the TMDC laterally starting from a conductive film.^[56]

Band gap

In the bulk form, TMD have an indirect gap in the center of the Brillouin zone, whereas in monolayer form the gap becomes direct and is located in the K points.^[57][2]

Spin–orbit coupling

For TMDs, the atoms are heavy and the outer layers electronic states are from d-orbitals that have a strong spin–orbit coupling. This spin orbit coupling removes the spins degeneracy in both the conduction and valence band i.e. introduces a strong energy splitting between spin up and down states. In the case of MoS₂, the spin splitting in conduction band is in the meV range, it is expected to be more pronounced in other material like WS₂.^[60][61][62] The spin orbit splitting in the valence band is several hundred meV.

Spin-valley coupling and the electron valley degree of freedom

By controlling the charge or spin degree of freedom of carriers, as proposed by spintronics, novel devices have already been made. If there are different conduction/valence band extrema in the electronic band structure in k-space, the carrier can be confined in one of these valleys. This degree of freedom opens up a new field of physics: the controlling of carriers k-valley index, also called valleytronics.^[22][63]

For TMD monolayers crystals, the parity symmetry is broken, there is no more inversion center. K valleys of different directions in the 2D hexagonal Brillouin zone are no longer equivalent. So there are two kinds of K valley called K+ and K−. Also there is a strong energy degeneracy of different spin states in valence band. The transformation of one valley to another is described by the time reversal operator. Moreover, crystal symmetry leads to valley dependent optical selection rules: a right circular polarized photon (σ+) initializes a carrier in the K+ valley and a left circular polarized photon (σ-) initializes a carrier in the K− valley.^[7] Thanks to these two properties (spin-valley coupling and optical selection rules), a laser of specific polarization and energy allows to initialize the electron valley states (K+ or K−) and spin states (up or down).^[1]

Emission and absorption of light: excitons

A single layer of TMD can absorb up to 20% of incident light,^[5] which is unprecedented for such a thin material. When a photon of suitable energy is absorbed by a TMD monolayer, an electron is created in the conduction band; the electron now missing in the valence band is assimilated by a positively charged quasi-particle called a hole. The negatively charged electron and the positively charged hole are attracted via the Coulomb interaction, forming a bound state called an exciton which can be thought as a hydrogen atom (with some difference). This Bosonic-like quasi-particle is very well known and studied in traditional semiconductors, such as GaAs and ZnO but in TMD it provides exciting new opportunities for applications and for studying fundamental physics. Indeed, the reduced dielectric screening and the quantum size effect present in these ultrathin materials make the binding energy of excitons much stronger than those in traditional semiconductors. Binding energies of several hundreds of meV are observed for all the four principal members of the TMD family.^{[21][27][64][65][66]}

As mentioned before, we can think about an exciton as if it were a hydrogen atom, with an electron bound to a hole. The main difference is that this system is not stable and tends to relax to the vacuum state, which is here represented by an electron in the valence band. The energy difference between the exciton 'ground state' (n=1) and the 'vacuum state' is called optical gap and is the energy of the photon emitted when an exciton recombines. This is the energy of the photons emitted by TMD monolayers and observed as huge emission peaks in photoluminescence (PL) experiments, such as the one labelled X⁰ in the figure. In this picture the binding energy E_B is defined as the difference between the free particle band gap and the optical band gap and represent, as usual, the energy needed to take the hole and the electron apart. The existence of this energy difference is called band gap renormalization. The analogy with hydrogen atom doesn't stop here as excitonic excited states were observed at higher energies and with different techniques.^[21][64]

Because of the spin–orbit splitting of the valence band two different series of excitons exist in TMD, called A- and B-excitons. In the A series the hole is located in the upper branch of the Valence band while for the B-exciton the hole is in the lower branch. As a consequence the optical gap for B-exciton is larger and the corresponding peak is found at higher energy in PL and reflectivity measurements.

Another peak usually appears in the PL spectra of TMD monolayers, which is associated to different quasi-particles called trions.^[68][69] These are excitons bound to another free carrier which can be either an electron or a hole. As a consequence a trion is a negative or positively charged complex. The presence of a strong trion peak in a PL spectrum, eventually stronger than the peak associated with exciton recombination, is a signature of a doped monolayer. It is believed now that this doping is extrinsic, which means that it arises from charged trap states present in the substrate (generally SiO₂). Positioning a TMD monolayer between two flakes of hBN removes this extrinsic doping and greatly increase the optical quality of the sample.^[67][70]

At higher excitation powers biexcitons^[71][72] have also been observed in monolayer TMDs. These complexes are formed by two bound excitons. Theory predicts that even larger charge-carrier complexes, such as charged biexcitons (quintons) and ion-bound biexcitons, are stable and should be visible in the PL spectra.^[73] Additionally, quantum light has been observed to originate from point defects in these materials in a variety of configurations.^{[74][75][76][77][78][79]}

Electronics

A field-effect transistor (FET) made of monolayer MoS₂ showed an on/off ratio exceeding 10⁸ at room temperature owing to electrostatic control over the conduction in the 2D channel.^[91] FETs made from MoS₂, MoSe₂, WS₂, and WSe₂ have been made. All show promise not just because of their electron mobility and band gap, but because their very thin structure makes them promising for use in thin, flexible electronics.^[92]

Sensing

The band gap TMDs possess makes them attractive for sensors as a replacement for graphene. FET-based biosensors rely on receptors attached to the monolayer TMD. When target molecules attach to the receptors, it affects the current flowing through the transistor.^[93]

However, it has been shown that one can detect nitrogenous bases in DNA when they pass through nanopores made in MoS₂.^[94] Nanopore sensors are based upon measuring ionic current through a nanopore in a material. When a single strand of DNA passes through the pore, there is a marked decrease in ionic current for each base. By measuring the current flowing through the nanopore, the DNA can then be sequenced.^[94]

To this date, most sensors have been created from MoS₂, although WS₂ has been explored as well.^[95]

Molybdenum disulfide

Molybdenum disulfide monolayers consist of a unit of one layer of molybdenum atoms covalently bonded to two layers of sulfur atoms. While bulk molybdenum sulfide exists as 1T, 2H, or 3R polymorphs, molybdenum disulfide monolayers are found only in the 1T or 2H form.^[96] The 2H form adopts a trigonal prismatic geometry^[97] while the 1T form adopts an octahedral or trigonal antiprismatic geometry.^[96] Molybdenum monolayers can also be stacked due to Van der Waals interactions between each layer.

Tungsten diselenide

Tungsten diselenide is an inorganic compound with the formula WSe
₂. The compound adopts a hexagonal crystalline structure similar to molybdenum disulfide. Every tungsten atom is covalently bonded to six selenium ligands in a trigonal prismatic coordination sphere, while each selenium is bonded to three tungsten atoms in a pyramidal geometry. The tungsten – selenium bond has a bond distance of 2.526 Å and the distance between selenium atoms is 3.34 Å.^[113] Layers stack together via van der Waals interactions. WSe
₂ is a stable semiconductor in the group-VI transition-metal dichalcogenides. The electronic bandgap of WSe
₂ can be tuned by mechanical strain^[114] which can also allow for conversion of the band type from indirect-to-direct in a WSe
₂ bilayer.^[115]