Transition metal dichalcogenide monolayers

Transition-metal dichalcogenides (TMDs) are composed of three atomic planes and often two atomic species: a metal and two chalcogens. The honeycomb, hexagonal lattice has threefold symmetry and can permit mirror plane symmetry and/or inversion symmetry. In the macroscopic bulk crystal, or more precisely, for an even number of monolayers, the crystal structure has an inversion center. In the case of a monolayer (or any odd number of layers), the crystal may or may not have an inversion center. == Broken inversion symmetry ==

Two important consequences of that are: • nonlinear optical phenomena, such as second-harmonic generation. When the crystal is excited by a laser, the output frequency can be doubled. • an electronic band structure with direct energy gaps, where both conduction and valence band edges are located at the non-equivalent K points (K+ and K−) of the 2D hexagonal Brillouin zone. The interband transitions in the vicinity of the K+ (or K−) point are coupled to right (or left) circular photon polarization states. These so-called valley dependent optical selection rules arise from inversion symmetry breaking. This provides a convenient method to address specific valley states (K+ or K−) by circularly polarized (right or left) optical excitation. These properties indicate that TMD monolayers represent a promising platform to explore spin and valley physics with the corresponding possible applications. == Properties ==

Transport properties based on a monolayer of MoS2 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. Owing to their direct band gap, TMD monolayers are promising materials for optoelectronics applications. 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. 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. 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. 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. 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%. For WS2, the Young's modulus of chemical vapor deposited monolayer flakes is 272 GPa. From this same study the Young's modulus of CVD-grown monolayer flakes of MoS2 is found to be 264 GPa. 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%. As a result, the emission efficiency of monolayers is expected to decrease for highly strained samples. This property allows mechanical tuning of the electronic structure and also the possibility of fabrication of devices on flexible substrates. == Fabrication of TMD monolayers ==

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. Large quantities of exfoliated material can also be produced using liquid-phase exfoliation by blending TMD materials with solvents and polymers. Chemical vapor deposition Chemical vapor deposition (CVD) is another approach used to synthesize transition-metal dichalcogenides. 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. 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. 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. 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. 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. Geometrically confined-growth techniques are also recently applied to realize wafer-scale single-domain TMD monolayer arrays and their heterostructures. 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, resulting in extremely clean samples with a thickness of only 0.5 monolayer. 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. 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. 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. 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. Colloidal Synthesis A strategy for colloidal synthesis from soluble transition metal and sulfur precursors was found recently, yielding highly defined nanoplatelets and nanosheets with a thickness of 1-2 monolayers. == Electronic band structure ==

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. The spin orbit splitting in the valence band is several hundred meV. Spin-valley coupling and the electron valley degree of freedom 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. s 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. Additionally, quantum light has been observed to originate from point defects in these materials in a variety of configurations. == Radiation effects of TMD monolayers ==

Common forms of radiation used to create defects in TMDs are particle and electromagnetic irradiation, impacting the structure and electronic performance of these materials. Scientist have been studying the radiation response of these materials to be used in high-radiation environments, such as space or nuclear reactors. Damage to this unique class of materials occurs mainly through sputtering and displacement for metals or radiolysis and charging for insulators and semiconductors. To sputter away an atom, the electron must be able to transfer enough energy to overcome the threshold for knock-on damage. Yet, the exact quantifiable determination of this energy still needs to be determined for TMDs. Consider MoS2 as an example, TEM exposure via sputtering creates vacancies in the lattice, these vacancies are then observed to be collected together in spectroscopic lines. Additionally, when looking at the radiation response of these materials, the three parameters that are proven to matter most are the choice of substrate, the sample thickness, and the sample preparation process. == Janus TMD monolayers ==

A new type of asymmetric transitional metal dichalcogenide, the Janus TMDs monolayers, has been synthesized by breaking the out-of-plane structural symmetry via plasma assisted chemical vapor deposition. Janus TMDs monolayers show an asymmetric structure MXY (M = Mo or W, X/Y = S, Se or Te) exhibiting out-of-plane optical dipole and piezoelectricity due to the imbalance of the electronic wave-function between the dichalcogenides, which are absent in a non-polar TMDs monolayer, MX2. In addition, the asymmetric structure of Janus MoSSe provides an enhanced Rashba spin–orbit interaction, which suggests asymmetrically Janus TMDs monolayer can be a promising candidate for spintronic applications. In addition, Janus TMDs monolayer has been considered as an excellent material for electrocatalysis or photocatalysis. Janus MoSSe can be synthesized by inductively coupled plasma CVD (ICP-CVD). The top layer of sulfur atoms on MoS2 is stripped using hydrogen ions, forming an intermediate state, MoSH. Afterward, the intermediate state is selenized by thermal annealing at 250 °C in an environment of hydrogen and argon gases. == Aspirational uses ==

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. 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. 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. However, it has been shown that one can detect nitrogenous bases in DNA when they pass through nanopores made in MoS2. 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. == Specific examples ==

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. The 2H form adopts a trigonal prismatic geometry while the 1T form adopts an octahedral or trigonal antiprismatic geometry. the synthesis method, and mechanical strain. As the number of layers decrease, the band gap begins to increase from 1.2eV in the bulk material up to a value of 1.9eV for a monolayer. Odd number of molybdenum sulfide layers also produce different electrical properties than even numbers of molybdenum sulfide layers due to cyclic stretching and releasing present in the odd number of layers. Molybdenum sulfide is a p-type material, but it shows ambipolar behavior when molybdenum sulfide monolayers that were 15 nm thick were used in transistors. The band gap of molybdenum disulfide monolayers can also be adjusted by applying mechanical strain while the thermal conductivity of few-layer molybdenum disulfide is 52W/mK. Exfoliation Exfoliation techniques for the isolating of molybdenum disulfide monolayers include mechanical exfoliation, The micromechanical exfoliation of molybdenum disulfide was inspired by the same technique used in the isolation of graphene nanosheets. Ultra-thin films consisting of few-layers have been produced via this technique over graphene electrodes. In addition, other electrode materials were also electroplated with MoS2, such as Titanium Nitride (TiN), glassy carbon and polytetrafluoroethylene. The advantage that this technique offers in producing 2D materials is its spatial growth selectivity and its ability to deposit over 3D surfaces. Controlling the thickness of electrodeposited materials can be achieved by adjusting the deposition time or current. Laser ablation Pulsed laser deposition involves the thinning of bulk molybdenum disulfide by laser to produce single or multi-layer molybdenum disulfide nanosheets. Hafnium disulfide Hafnium disulfide () has a layered structure with strong covalent bonding between the Hf and S atoms in a layer and weak van der Waals forces between layers. The compound has type structure and is an indirect band gap semiconducting material. The interlayer spacing between the layers is 0.56 nm, which is small compared to group VIB TMDs like , making it difficult to cleave its atomic layers. However, recently its crystals with large interlayer spacing has grown using a chemical vapor transport route. These crystals exfoliate in solvents like N-Cyclohexyl-2-pyrrolidone (CHP) in a time of just some minutes resulting in a high-yield production of its few-layers resulting in increase of its indirect bandgap from 0.9 eV to 1.3 eV. As an application in electronics, its field-effect transistors has been realised using its few layers as a conducting channel material offering a high current modulation ratio larger than 10000 at room temperature. Therefore, group IVB TMDs also holds potential applications in the field of opto-electronics. Tungsten diselenide Tungsten diselenide is an inorganic compound with the formula . 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. Layers stack together via van der Waals interactions. is a stable semiconductor in the group-VI transition-metal dichalcogenides. The electronic bandgap of can be tuned by mechanical strain which can also allow for conversion of the band type from indirect-to-direct in a bilayer. == References ==