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Transition Metal Dichalcogenide monolayers

Transition Metal Dichalcogenide (TMDC) monolayers are one atomic layer made of a crystal type MX2, with M a transition metal (Mo,W ...) and X a dichalcogenide (S, Se ...). The most studied at this day are MoS2 and WSe2.

The discovery of graphene's properties raise the interest of the scientific community for studying monolayers. New physical phenomena could appear when a crystal is thinned down to one monolayer.

TMDC's monolayers have not the same properties as graphene, indeed this new type of material differs by his atomic and electronic structure, unlike graphene TMDC's monolayers have :

- a band gap direct, more efficient absorption/emission. For the luminescence quantum efficiency, there is a factor of 104 between monolayer and the bulk material.

- no inversion center, which create a new degree of freedom of charge carriers and open to a new field of physics : valleytronics

- a strong spin-orbit coupling, that allows to control spin's of particles with photon's energy.

TMDC's monolayers are still a curent research topic.

=Crystal structure=

The crystallographic structure change from a bulk state to a monolayer state : in the case of a monolayer a lack of inversion symmetry appears. There is two important consequence of that :

- nonlinear optics phenomena like frequency doubling. Indeed, when the crystal is illuminated by a laser, the output frequency is doubled. It is used to get a variety of laser emission wavelength.

- the coupling of the electronic band structure and the electron's spin. Now there is two valleys, each one is specific to the polarization photon and if a particle passes from one valley to another, its spin reverses. So it creates a relation between the photon polarization and the particle spin which it could be used for new electronics devices.

=Transport properties and devices=

The reduction in the size of electronics components (Moore's law) is a problem when you arrive at very small scales. 3D materials have no longer the same behavior when they are in 2D form, which it could be an advantage as a disadvantage. For example, graphene has a very high carrier mobilitie, so less lose by Joule effect, but it has no bandgap, so a low on/off ratio. What makes of it, a limited material for electronic device.

TMDC's monolayers have a structural stability and electronic mobility comparable to Si, they can be used to make transistor. In 2004, the first FET made of monolayer WSe2 was reported. It showed a mobility about 500 cm2 V-1 s-1 for p-type conductivity at room tempretaure compared to a FET based on silicon which has a mobility about 1000 cm2 V-1 s-1. So a FET based on TMDC heated just a little more than a FET based on silicon. It also showed a high ratio on/off. By maintening a high carrier moblities, a high on/off ratio and because of its thinckness, TMDCs might be interresting materials for new electronic devices.

=Optical properties=

Direct gap = more efficient

Gap in the visible

Spin orbit coupling = two level of absorption/emission, one spin up other spin down

=Manufacturing=

Exfolation
Exfolation is a top down approach. At the bulk state, TMDCs are crystal made of layer which interact between them by Van Der Waals forces, these interactions are weak. So TMDC monolayers can be produce by micromechanical cleavage.

The crystal of TMDC is rubbed against the surface of another material (any solid surface). A way to do that, is to stick a piece of tape on the TMDC, remove it, and stick it on a substract. When you remove the piece of tape from the substract, some TMDC monolayer are deposited. But this technique produces only small sample of monolayer, about ?? nanometer.

Chemical vapor deposition
Chemical vapor deposition is a bottom-up approach. For example, the synthesis of MoS2 is made with : SiO2 used as a substract, MoO3 and S powders used as reactants.

=Changes on electronic band structure=

Band gap
On the bulk state, TMDCs have an indirect gap in the edge of the Brillouin zone whereas at the state of monolayer the gap become direct and is on the center zone.

States in edge zone are due to a combination of pz anti-bonding orbital of S atoms and d orbitals of Mo atoms, these orbitals are a lot delocalized so the interaction between the layers is high in this zone. The states of the center zone are due of d orbitals localized which interact less. As we reduce the layer, we reduce inter-layer interactions so we reduce the band-gap of the edge zone. But states of the center zone are not affected by this modification. When there is only one atomic layer, the band gap moves in the center zone and become direct.

Symmetry breaking
In the case of monolayer, the parity symmetry is broken, there is no more inversion center. K's valleys of the different directions of the Brillouin zone are no more equivalent. They are divide in two part, one corresponding to the positive photon polarization and the other to the negative photon polarization. The transition of one valley to another, is describe by the time reversal operator. So the spin's value is reverse from one valley to another.

The symmetry breaking leads to non linear optic phenomena too.

Absorption of light
Now we have a 2D system so there is a confinement of electrons and holes in the plan. This confinement localizes the electrons and holes orbitals, growing the Coulomb's interaction between them. The energy of this interaction is about 600 meV, which is higher than the heat energy. So the electron and the hole form a particle call exciton that occupy levels of energy similar as hydrogen atom.

Photoluminescence

Spin-orbit coupling
On TMDCs, atoms are heavy, the outer layers electronic are d orbitals which have the particularity of a strong coupling spin-orbit. This spin orbit coupling rises the spin's degeneration of the valence band, so the value of the band gap energy becomes specific of state of spin particles. We can select the spin's value which we want to pass the band gap by adjusting the photon's energy. On the case of MoS2, the rise of degeneration of the conduction band is weak, but it could be more important in other material like WS2.

=References=