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(a) Structure of a hexagonal TMD monolayer. M atoms are in black and X atoms are in yellow. (b) A hexagonal TMD monolayer seen from above.

Transition metal dichalcogenide (TMD) monolayers are atomically thin semiconductors of the type MX2, with M a transition metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. They are part of the large and family of the so-called 2D materials, name used to emphasize their extraordinary thinness. For example a MoS2 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 and superconductivity.[1]

The discovery of graphene shows how new physical properties emerge when a bulk crystal of macroscopic dimensions is thinned down to one atomic layer. Like graphite, 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 graphene:

The work on TMD monolayers is an emerging research and development field since the discovery of the direct bandgap[2][3] and the potential applications in electronics[4] and valley physics.[8][9][10] TMDs are often combined with other 2D materials like graphene and hexagonal boron nitride to make van der Waals heterostructure. These heterostructures need to be optimized to be possibly used as building blocks for many different devices such as transistors, solar cells, LEDs, photodetectors, fuel cells, photocatalytic 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.[13][14] Others are still being developed and promise to have a huge impact on our technology.

Janus TMD monolayers

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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.[15] Janus TMDs monolayers show an asymmetric structure MXY (M = Mo or W, X/Y = S, Se or Te)[16] exhibiting out-of-plane optical dipole[17] and piezoelectricity[18] due to the imbalance of the electronic wave-function between the dichalcogenides, which are absent in a non-polar TMDs monolayer, MX2. In addition, the asymmetric structure of Janus MoSSe provides an enhanced Rashba spin-orbit interaction, which suggests asymmetrically Janus TMDs monolayer can be a promising candidate for spintronic applications. The synthesis of Janus MoSSe can be achieved via inductively coupled plasma CVD (ICP-CVD). The top layer of sulfur atoms on MoS2 is stripped using hydrogen ions, forming an intermediate state, MoSH. Afterward, the intermediate state is selenized by thermal annealing at 250° C in an environment of hydrogen and argon gases.[15]

Fabrication of TMD monolayers

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Exfoliation

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Exfoliation is a top down approach. In the bulk form, TMDs are crystals made of layers, which are coupled by Van-der-Waals forces. These interactions are weaker than the chemical bonds between the Mo and S in MoS2, for example. So TMD monolayers can be produced by micromechanical cleavage, just as graphene.

The crystal of TMD is rubbed against the surface of another material (any solid surface). In practice, adhesive tape is placed on the TMD bulk material and subsequently removed. The adhesive tape, with tiny TMD flakes coming off the bulk material, is brought down onto a substrate. On removing the adhesive tape from the substrate, TMD monolayer and multilayer flakes are deposited. This technique produces small samples of monolayer material, typically about 5–10 micrometers in diameter.[19]

Large quantities of exfoliated material can also be produced using liquid-phase exfoliation by blending TMD materials with solvents and polymers.[20]

Chemical vapor deposition

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Chemical vapor deposition is a bottom-up approach. For example, the synthesis of MoS2 is made using: SiO2 as a substrate, MoO3 and S powders used as reactants. The reactants are delivered on the substrate and the whole is heated to 650 Celsius degrees in the presence of N2. The size of the sample is larger than obtained with the exfolation technique.[21]

Molecular beam epitaxy

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Molecular beam epitaxy (MBE) is an established technique for growing semiconductor devices with atomic monolayer thickness control. As a promising demonstration, high-quality monolayer MoSe2 samples have been grown on graphene by MBE.[22]

Electronic band structure

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Band gap

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In the bulk form, TMD have an indirect gap in the center of the Brillouin zone, whereas in monolayer form the gap becomes direct and is located in the K points.[3][2]

Spin-orbit coupling

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Theorical energy of the spin-orbit coupling[16][23]
Valence band

splitting (eV)

Conduction band

splitting (eV)

MoS2 0.148 0.003
WS2 0.430 0.026
MoSe2 0.184 0.007
WSe2 0.466 0.038
MoTe2 0.219 0.034

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 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.[24][25][26] The spin orbit splitting in the valence band is several hundred meV.

Spin-valley coupling and the electron valley degree of freedom

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Spin splittings and optical selection rules[23]
 
Photoluminescence (PL) of a MoS2 monolayer at 4 K excited by a σ+ polarized laser. The monolayer absorbs the incident light and re-emits it at lower energy. The energy of the peak center correspond to the optical gap.

By controlling the charge or spin degree of freedom of carriers, as proposed by spintronics, 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.[27]

For TMD monolayers crystals, the parity symmetry 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.[8] 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).[1]

Emission and absorption of light: excitons

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A single layer of TMD can absorb up to 20% of incident light,[6] 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, forming a bound state called an exciton which can be thought as an hydrogen atom (with some difference). This Bosonic-like quasi-particle is very well known and studiend in traditional semiconductors, such as GaAs 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.[28][29][30][31][32]

 
Energy level diagram of an exciton as if it were an hydrogen atom.[33]

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.[28][30]

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.[34][35] 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.[33][36]

At higher excitation powers biexcitons[37][38] 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.[39]

Reference

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