Molybdenum disulfide most commonly also known as MoS2 is one of the best materials initially belonging to the transition metals. Its structure is unique and all the properties it possesses are therefore unique as well. The building block of MoS2 is its properties as they are the key players in enhancing the productivity of the materials. Its applications being vast and abundant in nature help in maintaining the credibility of this material. However, MoS2 is an excellent material for various purposes and various industries.
Transition metal dichalcogenides' (TMDCs) is the class of materials and molybdenum disulfide belongs to this class. The materials in this class have MX2 as their chemical formula. In MX2, X is a chalcogen (group 16 of the periodic table) and M is a transition metal atom (group 4 to group 12 of the periodic table). MoS2 is molybdenum disulfide's chemical formula.
Molybdenum disulfide's (MoS2) crystal structure takes the shape of S atoms' hexagonal plane on either of the side of Mo atoms' hexagonal plane. There is strong covalent bonding between the S and Mo atoms, and these triple planes stack on each other's top, however, the weak Van Der Waals forcing holds the layers together, which allow the layers to be mechanically separated for forming MoS2's 2-dimensional sheets.
Naturally, the occurrence of MoS2 is as a 'molybdenite' mineral. The appearance of MoS2 in its bulk form is as a shiny, dark solid. MoS2 is also utilized as a lubricant because the sheets can slide over one another easily due to their weak interlayer interactions. MoS2 is also utilized in high-vacuum applications as an alternative to graphite, but its maximum operating temperature is lower as compared to the maximum operating temperature of graphite. With ~1.2eV of an indirect bandgap, bulk MoS2 is a semiconductor and is thus of restricted interest to the optoelectronics industry.
Electrical and Optical Characteristics
In comparison with the bulk, MoS2's layers have radically different characteristics. Eliminating confining electrons and interlayer interactions into a single plane leads to the production of a direct bandgap with ~1.89eV (visible red) of increased energy. 10 percent of incident light with more than the energy of the bandgap can be absorbed by MoS2's single monolayer. An increase of 1000 fold in photoluminescence intensity was observed in comparison with a bulk crystal, however, it stays comparatively weak, with about 0.4% of photoluminescence quantum yield. Although, if we remove the defects that are the reasons for non-radiative combination then this can be increased in a dramatic fashion to over 95%.
The introduction of strain into the structure can tune the bandgap. There have been observations of a 300 meV increase in bandgap per 1% biaxial compressive strain applied to trilayer MoS2. In 2-dimensional TMDCs, the bandgap can be reduced potentially to zero by applying vertical electric field as it has been considered as a method too, therefore switching the semiconducting structure to the metallic structure.
Two excitonic peaks are shown by the photoluminescence spectra of MoS2 monolayers: one peak is at ~1.92eV (the A exciton), and the other peak is at ~2.08eV (the B exciton). Both of the peaks are because of the valence band splitting in the Brillouin zone at the K-point because of the spin-orbit coupling, which enables two optically active transitions. More than 500 meV is the binding energy of the excitons. Therefore, they are stable at high temperatures.
Injection of electrons
Trions can form on the injection of excess electrons through either chemical or electrical doping into MoS2. Trions are charged excitons and they consist of one hole and two electrons. The appearance of trions in the PL spectra and absorption is as peaks, red-shifted by ~40meV. A non-negligible contribution is shared by the trions at room temperature to MoS2 film’s optical characteristics while the trion’s binding energy is way less as compared to the binding energy of excitons (at almost 20 meV).
N-type behavior is generally displayed by the MoS2 monolayer transistors, with almost 350cm2V-1s-1 (or ~500 times lower as compared to graphene) of carrier mobilities. Although, they can exhibit massive on/off ratios of 108 when fabricated into field-effect transistors, making them efficient and attractive for highly efficient logic circuits and switching.
It is shown that when bent to a 0.75 mm radius of curvature, thin-film FETs retain their electronic characteristics, proving that the MoS2 monolayers are flexible. Their stiffness is the same as the steel, and they also have a higher breaking strength as compared to the breaking strength of flexible plastics like polydimethylsiloxane (PDMS) and polyimide (PI), leaving them specifically suitable and appropriate for flexible electronics. As compared to graphene's thermal conductivity, the thermal conductivity of MoS2 monolayers is around 100 times less at around 35 Wm-1K-1.
A route to technologies beyond electronics is offered by the MoS2 and other 2-dimensional TMDCs, where degrees of freedom can be used for storing information or/and processing. MoS2’s electronic bandstructure exhibits the valence band's energy maxima, and conduction band's minima at Brillouin zone's both K and K' (often called -K) points. The same energy gap is possessed by these two discrete 'valleys' but when it comes to position, they are discrete in the momentum space.
Angular momentum changes of -1 for the K’ point and +1 for the K-point need the optical transitions in these valleys. Therefore, it is possible for excitons to be selectively excited into a valley with circularly polarised light - with excitons in the K’ region being excited by left-handed (σ-) polarized light and excitons in the K valley being excited by the right-handed (σ+) polarised light.
Emission of light
Conversely, light that will emit from exciton recombination in the K’ valley will be σ- polarised, and light that will emit from exciton recombination in the K valley will be σ+ polarised. Valley pseudospin, which is a degree of freedom, is represented by these valleys as they can be addressed independently, and valley pseudospin can also be utilized in valleytronic devices.
Spin-orbit valence band
Moreover, for each of the valleys, opposite signs of spin are possessed by the spin-orbit split valence band at the K' and K points. For instance, a spin-down hole and a spin-up electron make up an A-exciton in the K valley, and a spin-up hole and spin-down electron make up a K valley B-exciton. The constituent charge carriers for B and A excitons in the K’ valley have the opposite spin.
Excellent electrochemical characteristics, luminescence characteristics, and semiconducting characteristics are displayed by MoS2 as a remarkable probe for biosensing for observing several analytes. A zero dimension, which is also called inorganic fullerenes, is displayed by the MoS2 quantum dots, and their size is in less than 10 nm of range. Promising electric and catalytic characteristics are contained by MoS2 quantum dots. High photoluminescence at specific wavelengths is exhibited by Mo2 quantum dots due to the quantum confinement effect, and those wavelengths make MoS2 efficient and effective for optical biosensing based on the fluorimetric method.
Processing of Monolayer MoS2
Various techniques have been utilized for the preparation of MoS2’s monolayer films. Here we have mentioned the most common techniques and a brief review of them.
1. Mechanical exfoliation
Mechanical exfoliation is also called the ‘Scotch-tape method’, and it was utilized for the first time for isolating the layers of graphene. If you apply a sticky tape on a bulk crystal sample, it will lead to thin layers of crystal sticking to the tape once you peel the sticky tape off and it is because of its greater mutual adhesion as compared to the interlayer adhesion.
Sticking and peeling process
Until the production of single monolayers, this sticking-and-peeling process repeats again and again. Then, the single monolayers can be transferred on a substrate, for instance through a PDMS stamp. This process forms crystalline monolayers of high quality that are capable of being more than 10's of microns in size, even though this process is with a low monolayer yield. When it comes to TMDC research, this is the most preferred method of processing, despite the method being 'low-tech'.
2. Solvent exfoliation
Sonication of bulk crystals takes place in an organic solvent, breaking them down into thin layers. A distribution is obtained in the thickness and size of the layers, and a surfactant is also obtained which usually is added for stopping the restacking of the layers. This method has a low monolayer yield and a high thin-film yield. The sizes of the flakes are on a 100 nm of scale, making the flakes look small.
Monolayers long MoS2’s intercalation is classed as a form of solvent exfoliation at times. In 1986, it was demonstrated for the first time. A solution that functions as a lithium ions’ source (n-butyllithium commonly, which is dissolved in hexane) has bulk crystals placed in it, and those bulk crystals are diffusing between the layers of the crystal. The addition of water is the next step and then the water forms an interaction with the lithium ions for producing hydrogen, which pushes the layers apart.
Careful control should be done over the parameters of an experiment for obtaining a high monolayer yield in this method. Less needed metallic 1T structure is possessed by the resulting layers instead of thesemiconducting 2H structure. However, potential applications are observed for the 1T structure in the supercapacitor electrodes. Thermal annealing can be used to convert the 1T structure to the 2H.
4. Vapor deposition
Mechanical exfoliation is not a scalable technique however it can give high crystalline monolayers. A reliable and good large-scale method is needed to produced high-quality films if 2-dimensional materials are supposed to find applications in the field of optoelectronics. Vapour deposition is one of the methods with such potential and that's why it is studied in depth. A chemical reaction is involved in the chemical vapor deposition for converting s precursor to the final MoS2. MoO3 is commonly annealed at a high temperature of 1000 degrees celsius for the production of the MoS2 films in sulfur's presence.
Ammonium thiomolybdate and molybdenum metal are the other precursors, and dip coating and e-beam evaporation are used to deposit these before they convert into a furnace. In comparison with those that are made from the exfoliated layers, very low mobility is possessed by the FETs that are made from vapor-grown films. Moreover, the quality, thickness, and size (generally 10’s nm to few microns), of the substrates and films choice.
MoS2 has many promising peculiarities and one of them is that its bandgap has a non-zero value as compared to graphene. MoS2 acts as a semiconductor and due to its conductivity that can be altered, MoS2 is both efficient and effective for electronic and logic devices. Moreover, the indirect bandgap is contained by MoS2's bulk form which is then transformed at the nanoscale into a direct bandgap, suggesting that MoS2's single layer found application in the optoelectronic devices. Low power electronic devices and short channel FETs are also a possibility by MoS2 because of its 2-dimensional structure as it gives us control over the material's electrostatic nature.
The most latest electronic devices have field-effect transistors as their most elementary part. Semiconductor technology has evolved over time. Lithography can particularly lessen the sizes of the transistor in the range of a few nanometres. Their channel size is below 14 nm as compared to many advantages like cost reduction, low power consumption, and fast switching. Quantum mechanical tunneling takes place between the source electrodes and the drain due to the Joule heating effect. For avoiding short channel effects and producing nano-sized devices, exploring thinner channel materials and thinner gate oxides materials is very important. The monolayer of MoS2 is a suitable material for switching nanodevices as it possesses a direct bandgap of 1.8 eV which is appreciable.
A switchable transistor based on MoS2's monolayer was displayed firstly by Radisavljevic. A semiconducting channel with 6.5 A˚ of thickness is contained by this device and a 30 nm thick layer of HfO2 is used to deposit this device on SiO2 substrate as it has been utilized for covering it and also working as a top-gated dielectric layer. The current on/off ratio is displayed by this device at 108 room temperature. Off-state current, for instance, the subthreshold slope of 74 mV/dec, and 100 fA is exhibited by this device. According to this work, MoS2 has promising potential in flexible and transparent electronics, and that MoS2 is a good alternative for low standby power integrated circuits.
When the liquid lubricants fail the requirements of the needed applications, then solid lubricants are used. Oils, greases, and other liquid lubricants are not utilized in various applications because of their weight, sealing problems, and environmental conditions. However, on the other side, as compared to systems that are based on grease lubrication, solid lubricants have less weight and are cheap. In high vacuum conditions, the liquid lubricants cant work thus causing the device to be unfit as in these conditions, lubricants also get evaporated. Decomposition or oxidization of liquid lubricants takes place at high-temperature conditions. At cryogenic temperatures, liquid lubricants get viscous or solidify and are incapable of flowing.
When under the effect of radiation environment conditions and corrosive gas, the liquid lubricants start to decay. Dust or other contaminants are easily taken by the liquid lubricants where the major problem is contamination. The components that are associated with the liquid lubricants are very heavy so handling them in applications where there is a requirement of long storage, is difficult. Thus, these problems are effectively dealt with by solid lubricants. In all aspects, liquid lubricants fail when it comes to space mechanisms. Antennas, rovers, telescopes, vehicles, and satellites, etc., are involved in the space moving systems. In strict environmental conditions, these systems function for a longer period of time with little service. In such environmental conditions, the promising choice is the solid lubricants, MoS2 specifically.
In graphite contrast
Unlike graphite, MoS2 doesn’t need the water’s vapor pressure to exhibit lubrication. Slip rings, gears, ball bearings, and pointing and releasing mechanisms, etc. are the components in the space applications that are dependent on MoS2 lubrication. MoS2's lubricity declines over the effect of a humid environment exhibit a major challenge to its implementation in various terrestrial applications. MoS2's sputtering with Ti involves the improvement of MoS2's mechanical characteristics and it also protects MoS2 against humidity. This improvement in MoS2's mechanical characteristics is significant for dry machining operations.
Serious health issues have significantly affected the lifestyle of the human. Significant effects lead to the increase in the importance of finding new ways and techniques that can observe different and numerous factors that are causing those effects and diseases. A significant and major role is played by the evolution of biosensors in this point of view. There has also been the utilization of biosensing in some elementary ways for efficiently observing the disease-causing factors. Sensitivity and selectivity are the two factors on which the quality of the biosensors depends. The research is being done at a large scale for engineering the sensor matrices for the enhancement of the selectivity and sensitivity of the biosensors.
MoS2 Nanostructures that possess a 2D nature have been used for biosensing based on the electrochemical phenomenon. There has been an extensive exploration of the MoS2's sheets in the form of electrode materials in biosensors. MoS2 nanosheets display strong fluorescence in the visible range because of their direct bandgap, which makes MoS2 a suitable and appropriate candidate for optical biosensors. Optical biosensors are cost-efficient. 1-D MoS2 displays promising electrical characteristics and is analog to carbon nanotubes (CNTs). One of the efficient and effective candidates for biosensors is the electrochemical sensors that are based on carbon nanotubes.
FET based biosensors
Many researchers are fascinated by FET-based biosensors. A drain and two electrodes source are mainly contained by the FET and they electrically associate with each other via a channel that's based on the semiconductor material. The current that's flowing through the channel between the drain snd the source is controlled by the third electrode, the gate that's coupled with a dielectric layer. Biomolecules that create an electrostatic effect are captured by the functionalized channel and are then converted into an observable signal in the form of FET devices' electrical properties. How the characteristics of the devices perform, depends on the gate's biasing strategy.
Right now, it is very much important to trace noxious gases and pollutants, for instance, sulfur dioxide (SO2), hydrogen sulfide (H2S), carbon dioxide (CO2), ammonia (NH3), and nitrogen oxide (NOx). Environment, quality of air, and noxious gas are monitored by a way known as gas sensing. Resistance dependence, field-effect transistor, chemiresistive, Schottky diode optical fibers, etc. and other various semiconductor gas sensors are used for gas sensing but because of their low cost of production and easy operation, the resistivity based gas sensors are the most appreciable one
Evolution of graphene and 2D materials
It is because of their promising characteristics like high sensitivity, selectivity, large surface to mass ratio, and low noise, that the evolution of 2-dimensional materials and graphene helps in the research of gas sensors. Observations were being made on the sensors' sensing behavior at different concentrations and various temperatures. With a 4.6 ppb of detection limit, great sensitivity is showed by this sensor at 60 degrees Celsius temperature. Complete recovery/fast response is showed by the sensor.
Molybdenum disulfide is an excellent material to be used for various industrial purposes. However, its applications are so vast and diverse that they are being observed in various industries and are enhancing the credibility and productivity of the material itself.