What is MoS2?
Crystal Structure
Molybdenum disulfide (MoS2) is a layered material made up of molybdenum and sulfur atoms arranged in a hexagonal crystal structure. It belongs to the family of transition metal dichalcogenides (TMDs) which also includes materials such as tungsten disulfide (WS2), titanium disulfide (TiS2) etc. The crystal structure of MoS2 is composed of layers of Mo sandwiched between two atoms of S, with the Mo atoms forming a hexagonal lattice and the S atoms occupying the positions between the Mo atoms in the top side view. The Mo-S bonds within each layer are strong, while the interactions between individual layers are relatively weak [van der Waals interactions], allowing for easy exfoliation of individual layers.
There are several crystal structures that have been reported for MoS2, including:
- 2H polytype – This is the most common crystal structure of MoS2 and it is characterized by a hexagonal lattice with a symmetry of C6v. The 2H polytype has a layered structure with Mo atoms sandwiched between two atoms of S. In the bulk form, the layers are stacked on top of each other with van der Waals interactions between the layers. The 2H polytype shows semiconducting behavior.
- 1T polytype – The 1T polytype has a distinct crystal structure with a symmetry of D3d, which is different from the 2H polytype. The 1T polytype has a distorted hexagonal lattice with Mo atoms sandwiched between two atoms of S. The 1T polytype shows semiconducting or metallic behavior.
- 3R polytype – The 3R polytype has a symmetry of D3h and it is characterized by a rhombohedral lattice with Mo atoms sandwiched between two atoms of S.
The properties of the different crystal structures of MoS2 can vary depending on the polytype, but in general, the 2H polytype is the most widely studied and has the most robust properties, while the 1T polytype has a direct bandgap and the 3R polytype has a lower symmetry than the other two polytypes. The 2H polytype of MoS2 is the most common crystal structure found in nature and is the one that is most commonly used in electronic and optoelectronic devices.
Crystal growth
There are several methods available for growing MoS2 crystals, including:
- Chemical Vapor Deposition (CVD) – CVD is a widely used technique for growing MoS2 crystals. In this method, Mo and S precursors are introduced into a reactor and heated to high temperatures to form MoS2 crystals. The quality of the crystals can be improved by controlling the growth conditions, such as the precursor concentrations, the growth temperature, and the growth time.
- Molecular Beam Epitaxy (MBE) – MBE is a technique for growing high-quality MoS2 crystals with well-defined crystal structures and low defect densities. In this method, Mo and S atoms are evaporated from separate sources and directed onto a substrate, where they condense to form MoS2 crystals. The growth conditions, such as the evaporation rates, the substrate temperature, and the growth time, can be controlled to improve the crystal quality.
- Pulsed Laser Deposition (PLD) – PLD is a technique for growing MoS2 crystals using a pulsed laser to ablate a MoS2 target and deposit the ablated material onto a substrate. The growth conditions, such as the laser energy, the laser repetition rate, and the substrate temperature, can be controlled to improve the crystal quality.
- Chemical exfoliation – This method is based on the exfoliation of MoS2 crystals using chemical agents such as sulfuric acid (H2SO4) or potassium permanganate (KMnO4).
- Mechanical exfoliation – This method is based on the exfoliation of MoS2 crystals using mechanical force such as scotch tape or by using a mechanical exfoliator.
It is worth noting that the growth method chosen will depend on the specific application and the desired properties of the MoS2 crystals. Each method has its own advantages and limitations, and the optimal method will likely depend on the specific requirements of the application.
As an example, MoO3 powder can be transferred by argon gas carriers to a silicon oxide substrate and form nucleation centers, and simultaneously sulfur vapors carried by argon can join the Mo atoms and the reaction can start.
Defects
The defect density of MoS2 crystals can have a significant impact on their electrical and optoelectrical properties. Defects can be introduced during the growth process, such as vacancies and interstitials, or through exposure to high temperatures or chemical treatments. These defects can act as trap states, affecting carrier transport and recombination in the material.
Some of the most common types of defects found in MoS2 include:
- Vacancies: These are defects that occur when an atom is missing from its normal position in the crystal lattice.
- Interstitials: These are defects that occur when an atom is inserted into the crystal lattice in a position that is not normally occupied by an atom.
- Substitutional impurities: These are defects that occur when an atom is replaced by another atom in the crystal lattice.
- Dislocations: These are defects that occur when the crystal lattice is distorted, causing the atoms to be out of their normal positions.
- Grain boundaries: These are defects that occur at the interface between two grains in a polycrystalline sample.
- Edge defects: These are defects that occur at the edges of a MoS2 flake or at the interface between the MoS2 and substrate.
- Defects induced by defects: These defects can be introduced by other defects, or by external factors such as strain, temperature, and irradiation.
- Point defects: These are defects that occur at specific points in the crystal lattice, such as a missing atom or an extra atom.
- Extended defects: These are defects that cover a larger area in the crystal lattice, such as a missing atom or an extra atom.
- Surface defects: These are defects that occur on the surface of the MoS2 flakes.
These are the most common defects found in MoS2 but there can be more depending on the specific conditions of the sample and the type of fabrication method used.
Grain boundaries
Grain boundaries in MoS2 are the interfaces between two different grains in a polycrystalline sample. The properties of MoS2 can be significantly affected by the presence of grain boundaries, as they can introduce defects and affect the electronic and optical properties of the material.
There are several types of grain boundaries that can be found in MoS2, including:
- Coincidence site lattice (CSL) boundaries: These are grain boundaries that have a specific relationship between the lattice vectors of the two grains, resulting in a lower density of defects than other types of boundaries.
- High angle boundaries (HABs): These are grain boundaries that have a large angle between the lattice vectors of the two grains, resulting in a higher density of defects than CSL boundaries.
- Low angle boundaries (LABs): These are grain boundaries that have a small angle between the lattice vectors of the two grains, resulting in a lower density of defects than HABs.
- Twin boundaries: These are boundaries that occur between two grains that are mirror images of each other, resulting in a lower density of defects than other types of boundaries.
- Random boundaries: These are boundaries that occur between grains that have no specific relationship to each other, resulting in a higher density of defects than other types of boundaries.
The properties of MoS2 are dependent on the specific type of grain boundaries, as well as the density of defects introduced by the boundaries. For example, HABs and random boundaries tend to have a greater impact on the electronic properties of MoS2 than CSL boundaries and twin boundaries. Additionally, CSL boundaries are known to have a positive effect on the mechanical properties of MoS2 and they can be used to enhance the strength and toughness of the material.
It’s worth noting that the specific properties of a sample can vary greatly depending on how it’s been fabricated and the specific conditions it’s been exposed to.
Band structure
The band structure of MoS2 is a fundamental property that describes the energy levels of the electrons in the crystal lattice. It is composed of the valence band and the conduction band, separated by a bandgap. The bandgap is the energy difference between the top of the valence band and the bottom of the conduction band.
The band structure of MoS2 is characterized by a direct bandgap for monolayer in the visible range, making it a suitable material for optoelectronic applications. The bandgap of MoS2 is found to be around 1.8 eV for bulk material and 2.0 eV for monolayer material. This direct bandgap means that the electrons in the conduction band and the holes in the valence band can recombine directly, releasing energy as a photon and resulting in light emission.
In the bulk MoS2, the bandgap is indirect and it is located at the K and Γ points of the Brillouin zone. In the case of monolayer MoS2, the bandgap is direct and it is located at the K point of the Brillouin zone. The direct bandgap in monolayer MoS2 is a result of the reduced dimensionality and the stronger spin-orbit interaction in the monolayer.
The bandgap of MoS2 is also affected by defects, impurities, and strain in the crystal lattice. Defects and impurities can act as trap states for electrons and holes, affecting the carrier transport and recombination in the material. Strain can also affect the bandgap, as it can change the distance between the Mo and S atoms in the crystal lattice. and By changing the periodicity of the crystal lattice, the band energy levels alter.
Carrier transport
MoS2 is a semiconductor with both electron and hole transport properties. The mobility of carriers in MoS2 is found to be relatively low, with electron mobility values of around 10 cm2/Vs and hole mobility values of around 3 cm2/Vs. The low mobility of carriers in MoS2 is due to the presence of defects and impurities in the crystal, as well as the interaction between carriers and phonons. however, external factors such as dielectrics can change these parameters by altering the number of traps in the interface or by coupling to the phononic behavior of the crystal.
In MoS2, carrier transport refers to the movement of electrons and holes through the material. The type of carrier transport in MoS2 can depend on various factors, such as the type of defects present, the thickness of the material, and the temperature.
The main types of carrier transport in MoS2 are:
- Ballistic transport: This occurs when carriers move through the material without experiencing any scattering or resistance, resulting in high mobility and low resistance. This type of transport is typically observed in high-quality, single-layer MoS2 samples.
- Diffusive transport: This occurs when carriers experience scattering and resistance, resulting in low mobility and high resistance. This type of transport is typically observed in samples with a high density of defects or in multi-layer MoS2 samples.
- Tunneling transport: This occurs when carriers tunnel through a potential barrier, such as at a MoS2/metal interface. This type of transport can result in high mobility and low resistance, but is typically observed only at very low temperatures.
- Ambipolar transport: This type of transport occurs when both electrons and holes are present in the material and they can move in both directions. MoS2 is a natural ambipolar material, meaning that it can conduct both holes and electrons. The ambipolar transport is observed in bulk (and some reports in a monolayer) of the material and it’s affected by the density of defects and the type of defects present in the sample.
- Field-effect transport: This type of transport occurs when an external electric field is applied to the material, resulting in the movement of carriers. This type of transport is typically observed in samples that are used in electronic devices, such as transistors, and it’s affected by the density of defects, the type of defects present in the sample and the applied electric field.
Each type of carrier transport can have a different effect on the electronic properties of MoS2 and it’s important to understand the type of transport that is occurring in a specific sample in order to predict and control the electronic properties of the material.
Optoelectrical
The optoelectrical properties of MoS2 are related to its ability to convert light into electrical current and vice versa. These properties are important for a variety of optoelectronic applications, such as photodetection, light-emitting diodes (LEDs), and solar cells.
One of the key optoelectrical properties of MoS2 is its high photoresponsivity, which is a measure of the material’s ability to convert light into electrical current. The photoresponsivity of MoS2 can be as high as 10^7 A/W, making it a suitable material for photodetection applications. This high photoresponsivity is a result of the direct bandgap and the strong absorption of light in the visible range.
Another important optoelectrical property of MoS2 is its high photoluminescence (PL) efficiency, which is a measure of the material’s ability to convert electrical current into light. MoS2 has a high PL efficiency, making it a suitable material for LED applications as low energy-consumption devices.
The optoelectrical properties of MoS2 can be further enhanced by engineering the crystal structure, such as by reducing the thickness of the layers to the monolayer or few layers, or by encapsulating the MoS2 with other materials, such as hBN.
MoS2 has been demonstrated as a suitable material for various optoelectronics applications such as:
- Photodetectors: MoS2-based photodetectors have been demonstrated with high photoresponsivity, fast response time and high detectivity.
- LEDs: MoS2-based LEDs have been demonstrated with high external quantum efficiency and high luminous efficiency.
- Solar cells: MoS2-based solar cells have been demonstrated with high efficiency and high stability.
- Photovoltaics: MoS2-based photovoltaics have been demonstrated with high efficiency and high stability.
Schottky barrier
The Schottky barrier in a MoS2 metal junction refers to the energy barrier that forms at the interface between a metal and a semiconductor. The Schottky barrier is caused by the difference in work function between the metal and the semiconductor, and it can affect the electrical properties of the junction.
In a MoS2 metal junction, the Schottky barrier is formed at the interface between the MoS2 semiconductor and the metal electrode. The Schottky barrier height is the energy difference between the Fermi level of the metal and the conduction band edge of the MoS2. The Schottky barrier height can be measured by various techniques, such as current-voltage (I-V) measurements and capacitance-voltage (C-V) measurements.
The Schottky barrier height in MoS2 metal junctions is found to be around 0.7 eV, which is relatively high compared to other TMDs. This high Schottky barrier height can have a significant impact on the electrical properties of the MoS2 metal junction, such as the current-voltage characteristics and the carrier injection efficiency. This means the higher the Schottky barrier is, the higher the contact resistance will be.
The Schottky barrier height in MoS2 metal junctions can be modulated by various techniques, such as doping the MoS2 semiconductor or modifying the metal electrode, introducing defects in the MoS2 under the electrode area and etc. Lowering the Schottky barrier height can increase the carrier injection efficiency and improve the performance of MoS2-based devices.
The Schottky barrier also affects the leakage current of the junction, a higher Schottky barrier height leads to a lower leakage current, but also a lower injection efficiency. The Schottky barrier can also affect the thermionic emission, which is the process by which carriers are injected from the metal electrode into the MoS2 semiconductor. A lower Schottky barrier height can improve the thermionic emission transfer and increase the carrier injection efficiency.
Contact resistance
One of the main challenges in using MoS2 in electronic devices is its relatively high contact resistance, which can degrade the performance of the device.
There are several ways to improve contact resistance in MoS2 devices. One way is to use a metal with a low work function, such as titanium, as the electrode material. This can help to reduce the barrier for electrons to tunnel from the metal to the MoS2. Although titanium is a good material as a contact, it can be oxidized in air and to prevent oxidation, gold deposition on top of the titanium solves this problem. Another way is to use a metal with a high electron density such as silver.
Additionally, surface modification techniques, such as chemical doping or chemical functionalization, can be used to increase the electron density at the interface between the metal and MoS2, which can also help to reduce contact resistance. For examples introducing more MoS2 edges under the Metal-MoS2 contact area can reduce the resistivity by increasing the charge injection.
Lattice mismatch
Lattice mismatch refers to the difference in the crystal lattice structure between two materials that are in contact with each other. In the case of MoS2, the crystal lattice structure of MoS2 is hexagonal (also known as a “2H” structure) while most metals have a face-centered cubic (FCC) or body-centered cubic (BCC) lattice structure. The lattice mismatch between MoS2 and most metals is significant, which can lead to a high Schottky barrier and high contact resistance.
Some researchers have found that using a metal with a large lattice mismatch with MoS2 can decrease the Schottky barrier and improve contact resistance. For example, it has been reported that using metals such as titanium (Ti) as the electrode material can result in improved contact resistance in MoS2-based devices due to their large lattice mismatch with MoS2.
Additionally, surface modification techniques, such as chemical doping or chemical functionalization, can be used to increase the electron density at the interface between the metal and MoS2, which can also help to reduce the contact resistance. However, the lattice mismatch alone is not the only factor that affects contact resistance, interface engineering and choice of metal electrode material also play a significant role in improving the contact resistance.
Photoluminescence
PL, or photoluminescence, refers to the phenomenon where a material emits light (luminescence) when exposed to light or other forms of electromagnetic radiation. In the case of MoS2, it is a semiconductor material that can exhibit both photoluminescence and electroluminescence (EL), which is the phenomenon where a material emits light when an electric current is applied.
The photoluminescence (PL) of MoS2 depends on the number of layers, the quality of the material, and the environment in which it is measured. In general, the PL intensity of single-layer MoS2 (1L-MoS2) is stronger than that of multi-layer MoS2 (ML-MoS2) because the number of defects and impurities is lower in 1L-MoS2.
The PL of MoS2 is dominated by the A and B excitons. The PL of MoS2 can also be affected by its environment. For example, when MoS2 is placed in a liquid environment, the PL intensity can be quenched due to the interaction of the excitons with the liquid. Additionally, the PL of MoS2 can be enhanced by using different substrates, such as SiO2 or h-BN, which can help to reduce the number of defects and impurities in the material or even change the phononic interactions with the substrates.
Flexibility
The flexibility of MoS2 refers to its ability to bend or deform without breaking or cracking. MoS2 is a relatively flexible 2D material, due to its low density and weak van der Waals interactions between the layers. However, the flexibility of MoS2 can depend on various factors such as the thickness of the material, the number of layers, and the presence of defects.
There are several methods to measure the flexibility of MoS2:
- Nanoindentation: In this method, a small indenter is used to press on the surface of the MoS2 and measure the force required to create a certain amount of deformation. This method can provide information on the elastic modulus and the hardness of the material.
- Flexure tests: In this method, a small beam or cantilever of MoS2 is bent and the deflection is measured. The load-displacement curve can be used to determine the flexural rigidity and the bendability of the material.
- Microscopy: In this method, a microscope is used to observe the deformation of MoS2 under an applied load. This method can provide information on the deformation behavior and the failure mechanisms of the material.
- Raman Spectroscopy: Raman spectroscopy is a non-destructive technique that can be used to measure the mechanical properties of MoS2 such as the Young’s modulus, Poisson ratio, and longitudinal and transverse phonon modes.
- Tensile tests: Tensile tests are a way to measure the strength and elasticity of a material. A sample of MoS2 is pulled in opposite directions while measuring the force applied and the amount of deformation.
The flexibility of a MoS2 sample can vary greatly depending on the specific fabrication method and the conditions under which it was grown. Additionally, the specific properties of a sample can change depending on how it’s been handled and the specific conditions it’s been exposed to. Therefore, it’s important to use different techniques to measure the flexibility of a sample, and to compare the results with theoretical models and simulations.
Exciton
Exciton generation refers to the process by which an electron and a hole are created in the crystal lattice of a semiconductor material. In MoS2, this process typically occurs when an energy source, such as light or heat, is applied to the material. When a photon of energy greater than the bandgap energy of MoS2 is absorbed by the material, an electron in the valence band is excited to the conduction band, leaving behind a hole in the valence band. This creates an electron-hole pair, also known as an exciton.
Exciton recombination refers to the process by which the electron and hole in an exciton recombine and release their energy. In MoS2, exciton recombination can occur through several mechanisms, including radiative recombination and non-radiative recombination. Radiative recombination is the process by which an electron and hole recombine and release their energy as a photon, resulting in light emission. Non-radiative recombination is the process by which an electron and hole recombine and release their energy in the form of heat.
Exciton recombination in MoS2 can also be affected by defects, impurities, and grain boundaries in the crystal lattice, which can act as trap states for electrons and holes. These trap states can affect the efficiency of exciton recombination and the overall performance of MoS2-based devices.
Exciton recombination in MoS2 also depends on the thickness of the MoS2 layers, as the recombination rate in monolayer MoS2 is much higher than in multilayer MoS2. This is because the excitons in monolayer MoS2 are able to be diffused or because of potential fluctuations and trapping of the exciton the recombination is reduced in a monolayer although mostly the recombination is radiative because the monolayer shows a direct band gap, while in multilayer MoS2 excitons can have high amount of traps which are created in the middle layers as a surface trap states and increases the recombination rate, although because of the indirect band structure, the recombination is non-radiative.
Encapsulation
There are several methods available to encapsulate MoS2, including:
- Atomic layer deposition (ALD) can be used to deposit a thin layer of encapsulating material, such as AlO, on top of the MoS2.
- Polymer coating- using a polymer layer as a coating material on MoS2 such as PMMA coating.
- Transfer methods – MoS2 can be transferred onto another substrate and then encapsulated using a thin layer of hBN or other encapsulating material.
- Langmuir-Blodgett (LB) technique – This method involves using a Langmuir trough to deposit a thin layer of encapsulating material on top of the MoS2.
- Wet transfer method – This method involves using a liquid medium to transfer MoS2 onto another substrate, followed by encapsulation using a thin layer of encapsulating material.
- 3D printing – 3D printing technique can also be used for the encapsulation of MoS2.
- Self-assembled monolayers (SAMs) – Using self-assembled monolayers, MoS2 can be encapsulated by self-assembling molecules on the surface of MoS2, this method is useful when the encapsulation material is not available in bulk form.
The chosen encapsulation method will depend on the specific application and the desired properties of the encapsulated MoS2. Each method has its own advantages and limitations and the optimal method will likely depend on the specific requirements of the application.
Heterostructures
MoS2 can form heterostructures with a variety of other materials, including graphene, h-BN, other transition metal dichalcogenides (TMDs) and other semiconductors. Although these heterostructures are vertical, there can be also a possibility of existing heterostructure in the lateral form. The properties of these heterostructures depend on the specific materials involved and how they are stacked or interfaced.
For example, MoS2 can be stacked with graphene to form a van der Waals heterostructure, which can exploit both electronic properties of both materials. This type of heterostructure can exhibit improved carrier mobility and increased optical absorption.
MoS2 can also be stacked with h-BN to form a van der Waals heterostructure, which can significantly reduce the electronic noise and improve the stability of MoS2 transistors.
MoS2 can also form heterostructures with other TMDs, such as WS2 and WSe2, which can lead to new electronic and optoelectronic properties, such as tunable bandgaps, enhanced light-matter interactions, and improved device performance.
Author: Δ Emad Najafidehaghani Δ
