![Monolayer TMDC structure](https://upload.wikimedia.org/wikipedia/commons/a/a7/Monolayer_TMDC_structure.jpg)
Transition-metal dichalcogenide (TMD or TMDC) monolayers are atomically thin
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
s of the type MX
2, with M a
transition-metal atom (
Mo,
W, etc.) and X a
chalcogen
The chalcogens (ore forming) ( ) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. Group 16 consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioact ...
atom (
S,
Se, or
Te). One layer of M atoms is sandwiched between two layers of X atoms. They are part of the large family of so-called
2D materials
In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some applications but remain the focus of research. Single-layer materials ...
, named so to emphasize their extraordinary thinness. For example, a MoS
2 monolayer is only 6.5 Å thick. The key feature of these materials is the interaction of large atoms in the 2D structure as compared with first-row
transition-metal dichalcogenides, e.g.,
WTe2 exhibits anomalous
giant magnetoresistance
Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter ...
and
superconductivity
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
.
The discovery of
graphene
Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. shows how new physical properties emerge when a bulk crystal of macroscopic dimensions is thinned down to one atomic layer. Like
graphite
Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
, TMD bulk crystals are formed of monolayers bound to each other by
van-der-Waals attraction. TMD monolayers have properties that are distinctly different from those of the
semimetal
A semimetal is a material with a very small overlap between the bottom of the conduction band and the top of the valence band.
According to electronic band theory, solids can be classified as insulators, semiconductors, semimetals, or metals. ...
graphene:
* TMD monolayers
MoS2,
WS2,
MoSe2,
WSe2,
MoTe2 have a
direct band gap
In semiconductor physics, the band gap of a semiconductor can be of two basic types, a direct band gap or an indirect band gap. The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characteriz ...
, and can be used in electronics as
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s and in optics as emitters and detectors.
* The TMD monolayer crystal structure has no inversion center, which allows to access a new degree of freedom of charge carriers, namely the k-valley index, and to open up a new field of physics:
valleytronics
Valleytronics (from ''valley'' and ''electronics'') is an experimental area in semiconductors that exploits local extrema ("valleys") in the electronic band structure. Certain semiconductors have multiple "valleys" in the electronic band structure ...
* The strong
spin–orbit coupling in TMD monolayers leads to a spin–orbit splitting of hundreds meV in the
valence band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
and a few meV in the
conduction band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
, which allows control of the electron spin by tuning the excitation laser photon energy and handedness.
* 2D nature and high spin–orbit coupling in TMD layers can be used as promising materials for
spintronic
Spintronics (a portmanteau meaning spin transport electronics), also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-sta ...
applications.
The work on TMD monolayers is an emerging research and development field since the discovery of the direct bandgap
[ and the potential applications in electronics ][ and valley physics.][ TMDs are often combined with other ]2D materials
In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some applications but remain the focus of research. Single-layer materials ...
like graphene
Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. and hexagonal boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...
to make van der Waals heterostructures A two-dimensional semiconductor (also known as 2D semiconductor) is a type of natural semiconductor with thicknesses on the atomic scale. Geim and Novoselov et al. initiated the field in 2004 when they reported a new semiconducting material graphene ...
. These heterostructures need to be optimized to be possibly used as building blocks for many different devices such as transistors, solar cell
A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. s, LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
s, photodetector
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as Photoelectric effect, photoelectric or photoc ...
s, fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
s, photocatalytic
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the abi ...
and sensing devices. Some of these devices are already used in everyday life and can become smaller, cheaper and more efficient by using TMD monolayers.
Crystal structure
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
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typic ...
phenomena, such as second-harmonic generation
Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of ...
. When the crystal is excited by a laser, the output frequency can be doubled.
* an electronic band structure
In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or '' ...
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
In mathematics and solid state physics, the first Brillouin zone is a uniquely defined primitive cell in reciprocal space. In the same way the Bravais lattice is divided up into Wigner–Seitz cells in the real lattice, the reciprocal lattice i ...
. 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 rule
In physics and chemistry, a selection rule, or transition rule, formally constrains the possible transitions of a system from one quantum state to another. Selection rules have been derived for electromagnetic transitions in molecules, in atoms, in ...
s arise from inversion symmetry breaking. This provides a convenient method to address specific valley states (K+ or K-) by circularly polarized
In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to t ...
(right or left) optical excitation.[ In combination with strong spin-splitting, the spin and valley degree of freedom are coupled, enabling stable valley polarization.]
These properties indicate that TMD monolayers represent a promising platform to explore spin and valley physics with the corresponding possible applications.
Properties
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
Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. has a very high electron mobility, carrier mobility, and accompanying lower losses through the Joule effect Joule effect and Joule's law are any of several different physical effects discovered or characterized by English physicist James Prescott Joule. These physical effects are not the same, but all are frequently or occasionally referred to in the lit ...
. But graphene has zero bandgap
In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in ...
, 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
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
, 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.
Optical properties
A semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
can absorb photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s 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
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
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. The band gaps of TMD monolayers are in the visible range (between 400 nm and 700 nm). The direct emission shows two exciton
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The ...
ic 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. Owing to their direct band gap, TMD monolayers are promising materials for optoelectronics
Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radiatio ...
applications.
Atomic layers of MoS2 have been used as a phototransistor
A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons.
The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packa ...
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. Such a detector has a photoresponsivity reaching , 106 greater than the first graphene photodetector
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as Photoelectric effect, photoelectric or photoc ...
s. 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
A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains ...
, makes possible industrial applications such as high-sensitivity and flexible photodetectors. The only limitation for currently available devices is the slow photoresponse dynamics.[
]
Mechanical properties
Interest in the use of TMD monolayers such as MoS2, 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. 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. 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.
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. 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. 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
"Top Down" is a song by American hip hop record producer and recording artist Swizz Beatz, included as the eighth track from his debut studio album ''One Man Band Man'' (2007). "Top Down" contains Sampling (music), samples of swirls and riotous b ...
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 bond
A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
s 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
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (substra ...
(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. 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.
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.
Molecular-beam epitaxy
Molecular-beam epitaxy
Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the develo ...
(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
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
, including titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
, vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...
, and chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
, tellurides, resulting in extremely clean samples with a thickness of only 0.5 monolayer.
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
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
), or as a rod (for example molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
). Some elements, such as selenium and tellurium
Tellurium is a chemical element with the symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur, all three of which are chalcogens. It is occasionally fou ...
, 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. The evaporation cells are either Knudsen cell In crystal growth, a Knudsen cell is an effusion evaporator source for relatively low partial pressure elementary sources (e.g. Ga, Al, Hg, As). Because it is easy to control the temperature of the evaporating material in Knudsen cells, they are c ...
s or electron beam evaporation Electron-beam physical vapor deposition, or EBPVD, is a form of physical vapor deposition in which a target anode is bombarded with an electron beam given off by a charged tungsten filament under high vacuum. The electron beam causes atoms from the ...
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
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
, 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.
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. 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 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.
Electronic band structure
Band gap
In the bulk form, TMD have an indirect gap in the center of the Brillouin zone
In mathematics and solid state physics, the first Brillouin zone is a uniquely defined primitive cell in reciprocal space. In the same way the Bravais lattice is divided up into Wigner–Seitz cells in the real lattice, the reciprocal lattice i ...
, whereas in monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials.
...
form the gap becomes direct
Direct may refer to:
Mathematics
* Directed set, in order theory
* Direct limit of (pre), sheaves
* Direct sum of modules, a construction in abstract algebra which combines several vector spaces
Computing
* Direct access (disambiguation), a ...
and is located in the K points.[
]
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
Degeneracy, degenerate, or degeneration may refer to:
Arts and entertainment
* ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed
* Degenerate art, a term adopted in the 1920s by the Nazi Party in Germany to descri ...
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. 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
Spintronics (a portmanteau meaning spin transport electronics), also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-sta ...
, 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
Valleytronics (from ''valley'' and ''electronics'') is an experimental area in semiconductors that exploits local extrema ("valleys") in the electronic band structure. Certain semiconductors have multiple "valleys" in the electronic band structure ...
.[
For TMD monolayers crystals, the ]parity symmetry
In physics, a parity transformation (also called parity inversion) is the flip in the sign of ''one'' spatial coordinate. In three dimensions, it can also refer to the simultaneous flip in the sign of all three spatial coordinates (a point refle ...
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.[ 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).][
]
Emission and absorption of light: excitons
A single layer of TMD can absorb up to 20% of incident light,[ 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
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is conventiona ...
, forming a bound state called an exciton
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The ...
which can be thought as a hydrogen atom
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen consti ...
(with some difference). This Bosonic-like quasi-particle is very well known and studied in traditional semiconductors, such as GaAs
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monolithic microwave integrated circui ...
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.
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 EB 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.[
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. 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.]
At higher excitation powers biexciton
In condensed matter physics, biexcitons are created from two free excitons.
Formation of biexcitons
In quantum information and computation, it is essential to construct coherent combinations of quantum states.
The basic quantum operations can b ...
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
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
and piezoelectricity
Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word ''p ...
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
Spintronics (a portmanteau meaning spin transport electronics), also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-sta ...
applications. In addition, Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
TMDs monolayer has been considered as an excellent material for electrocatalysis or photocatalysis
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity depends on the abili ...
.
Janus MoSSe can be synthesized by inductively coupled plasma
An inductively coupled plasma (ICP) or transformer coupled plasma (TCP) is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.
...
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
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contro ...
(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 nanopore
A nanopore is a pore of nanometer size. It may, for example, be created by a pore-forming protein or as a hole in synthetic materials such as silicon or graphene.
When a nanopore is present in an electrically insulating membrane, it can be used a ...
s 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.
To this date, most sensors have been created from MoS2, although WS2 has been explored as well.
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. Molybdenum monolayers can also be stacked due to Van der Waals interactions between each layer.
Electrical
The electrical properties of molybdenum sulfide in electrical devices depends on factors such as the number of layers, the synthesis method, the nature of the substrate on which the monolayers are placed on, 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. However, most electrical devices containing molybdenum sulfide monolayers tend to show n-type behavior.
The band gap of molybdenum disulfide monolayers can also be adjusted by applying mechanical strain or an electrical field. Increasing mechanical strain shifts the phonon modes of the molybdenum sulfide layers. This results in a decrease of the band gap and metal-to-insulator transition. Applying an electric field of 2-3Vnm−1 also decreases the indirect bandgap of molybdenum sulfide bilayers to zero.
Solution phase lithium intercalation and exfoliation of bulk molybdenum sulfide produces molybdenum sulfide layers with metallic and semiconducting character due to the distribution of 1T and 2H geometries within the material. This is due to the two forms of molybdenum sulfide monolayers having different electrical properties. The 1T polymorph of molybdenum sulfide is metallic in character while the 2H form is more semiconducting. However, molybdenum disulfide layers produced by electrochemical lithium intercalation are predominantly 1T and thus metallic in character as there is no conversion to the 2H form from the 1T form.
Thermal
The thermal conductivity of molybdenum disulfide monolayers at room temperature is 34.5W/mK while the thermal conductivity of few-layer molybdenum disulfide is 52W/mK. The thermal conductivity of graphene, on the other hand, is 5300W/mK. Due to the rather low thermal conductivity of molybdenum disulfide nanomaterials, it is not as promising material for high thermal applications as some other 2D materials.
Synthesis
Exfoliation
Exfoliation techniques for the isolating of molybdenum disulfide monolayers include mechanical exfoliation, solvent assisted exfoliation, and chemical exfoliation.
Solvent assisted exfoliation is done by sonicating bulk molybdenum disulfide in an organic solvent such as isopropanol and N-methyl-2-pyrrolidone, which disperses the bulk material into nanosheets as the Van der Waals interactions between the layers in the bulk material are broken. The amount of nanosheets produced is controlled by the sonication time, the solvent-molybdenum disulfide interactions, and the centrifuge speed. Compared to other exfoliation techniques, solvent assisted exfoliation is the simplest method for large scale production of molybdenum disulfide nanosheets.
The micromechanical exfoliation of molybdenum disulfide was inspired by the same technique used in the isolation of graphene nanosheets. Micromechanical exfoliation allows for low defect molybdenum disulfide nanosheets but is not suitable for large scale production due to low yield.
Chemical exfoliation involves functionalizing molybdenum difsulfide and then sonicating to disperse the nanosheets. The most notable chemical exfoliation technique is lithium intercalation in which lithium is intercalated into bulk molybdenum disulfide and then dispersed into nanosheets by the addition of water.
Chemical vapor deposition
Chemical vapor deposition of molybdenum disulfide nanosheets involves reacting molybdenum and sulfur precursors on a substrate at high temperatures. This technique is often used in the preparing electrical devices with molybdenum disulfide components because the nanosheets are applied directly on the substrate; unfavorable interactions between the substrate and the nanosheets that would have occurred had they been separately synthesized are decreased. In addition, since the thickness and area of the molybdenum disulfide nanosheets can be controlled by the selection of specific precursors, the electrical properties of the nanosheets can be tuned.
;Electroplating
Among the techniques that have been used to deposit molybdenum disulfide is electroplating. 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. This allows for synthesis of molybdenum disulfide nanosheets with a defined shape and size. The quality of the nanosheets are determined by the energy of the laser and the irradation angle.
Lasers can also be used to form molybdenum disulfide nanosheets from molybdenum disulfide fullerene
A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...
-like molecules.
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
Tungsten diselenide is an inorganic compound with the formula WSe2. 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 co ...
is an inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...
with the formula . The compound adopts a hexagonal crystalline structure similar to molybdenum disulfide
Molybdenum disulfide (or moly) is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is .
The compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenit ...
. Every tungsten
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...
atom is covalently bonded to six selenium
Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...
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
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
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
External links
* {{Cite web , last=Wood , first=Charlie , date=2022-08-16 , title=Physics Duo Finds Magic in Two Dimensions , url=https://www.quantamagazine.org/physics-duo-finds-magic-in-two-dimensions-20220816/, website=Quanta Magazine , language=en
Semiconductor analysis
Transition metal dichalcogenides
Monolayers