Tantalum diselenide. Tantalum diselenide is a compound made with tantalum and selenium atoms, with chemical formula TaSe2, which belongs to the family of transition metal dichalcogenides. In contrast to molybdenum disulfide (MoS2) or rhenium disulfide (ReS2), tantalum diselenide does not occur spontaneously in nature, but it can be synthesized. Depending on the growth parameters, different types of crystal structures can be stabilized.

In the 2010s, interest in this compound has risen due to its ability to show a charge density wave (CDW), which depends on the crystal structure, up to , while other transition metal dichalcogenides normally need to be cooled down to hundreds of kelvins or even below to observe the same capability.

Structure 

As other TMDs, TaSe2 is a layered compound, with a central tantalum hexagonal lattice sandwiched between two layers of selenium atoms, still with a hexagonal structure. Differently with respect to other 2D materials such as graphene, which is atomically thin, TMDs are composed by trilayers of atoms strongly bounded to each others, stacked above other trilayers and kept together through Van der Waals forces. TMDs can be easily exfoliated.

The most studied crystal structures of TaSe2 are the 1T and 2H phases that feature, respectively, octahedral and trigonal prismatic symmetries. However, it is also possible to synthesize the 3R phase or the 1H phase.

1T phase 

In the 1T phase, selenium atoms show an octahedral symmetry and the relative orientation of the selenium atoms in the topmost and bottommost layers is opposed. On a macroscopic scale, the sample shows a gold colour. The lattice parameters are a = b = 3.48 Å, while c = 0.627 nm.
Depending on the temperature, it shows different types of charge density waves (CDW): an incommensurate CDW (ICDW) between  and a commensurate CDW (CCDW) below . In the commensurate CDW, the resulting superlattice shows a  ×  reconstruction often referred to as star of David (SOD), with respect to the lattice parameter (a = b) of non distorted TaSe2 (above ). Film thickness can influence as well the CDW transition temperature: the thinner the film, the lower the transition temperature from ICDW to CCDW.

In the 1T phase the single trilayers are stacked always in the same geometry, as shown in the corresponding image.

2H phase 
The 2H phase is based on a configuration of selenium atoms characterized by a trigonal prismatic symmetry and an equal relative orientation in the topmost and bottommost layers. The lattice parameters are a = b = 3.43 Å, while c = 1.27 nm. Depending on the temperature, it shows different types of charge density wave: an incommensurate CDW (ICDW) between  and a commensurate CDW (CCDW) below . The lattice distortion below  gives rise to a CCDW that makes a 3 × 3 reconstruction with respect to the non-distorted lattice parameter (a = b) of 2H-TaSe2 (above ).

In the 2H phase the single trilayers are stacked one opposed to others, as shown in the relative image. Through molecular beam epitaxy it is possible to grow one single trilayer of 2H-TaSe2, also known as 1H phase. Basically, the 2H phase can be seen as the stacking of 1H phase with opposed relative orientation with respect to each others.

In the 1H phase the ICDW transition temperature is raised to .

Properties

Electric and Magnetic 
TaSe2 exhibits different properties according to the polytype (2H or 1T), even if the chemical composition remains unchanged.

1T phase 
The resistivity at low temperature is similar to that of a metal, but it starts decreasing at higher temperatures. A peak is exhibited at approximately , which resembles the behavior of semiconductors. 1T phase has almost two orders of magnitude higher resistivity than to the 2H phase.

The magnetic susceptibility of the 1T phases has no peaks at low temperature and remains always nearly constant until  is reached (ICDW temperature transition), when it jumps to slightly higher values. 1T phase is diamagnetic.

2H phase 
Resistivity linearly depends on the temperature when the latter exceeds . On the opposite, below this threshold it shows a non-linear behaviour. This abrupt variation of R(T) at  might be related to the formation of some kinds of magnetic ordering in TaSe2: ordered spins scatter electrons in a less efficient way. This increases electrons mobility and yields a faster drop in resistivity than that ideally corresponding to a linear trend.

The magnetic susceptibility of the 2H polytype slightly depends on the temperature and peaks in the range . The trend is linearly ascending or descending below and above , respectively. This maximum in the 2H phases is related to the formation of the CCDW at . The 2H phase is Pauli paramagnetic. 

The Hall coefficient RH is almost independent of the temperature above , a threshold below which it instead starts to drop to eventually reach a value of zero at . In the range between , the coefficient RH is negative, its minimum being experienced at approximately .

Electronic

1T phase 
Bulk 1T-TaSe2 is metallic, while single monolayer (trilayer Se–Ta–Se in octahedral symmetry) is observed to be insulating with a band gap of 0.2 eV, in contrast with theoretical calculation which expected to be metallic as the bulk.

2H phase 
Bulk 2H-TaSe2 is metallic and so the single monolayer (trilayer Se–Ta–Se in trigonal prismatic symmetry), which is also known as the 1H phase.

Optical 
Investigating the non-linear refractive index of tantalum diselenide can be pursued preparing atomically thin flakes of TaSe2 with the liquid phase exfoliation method. Since this technique requires using alcohol, the refractive index of tantalum diselenide can be retrieved through Kerr's law: , where n0 = 1.37 represents the linear refractive index of ethanol, n2 is the non-linear refractive index of TaSe2 and  is the incident intensity of the laser beam. Using different light wavelengths, in particular λ = 532 nm and λ = 671 nm, it is possible to measure both n2 and χ(3), the third order nonlinear susceptibility.

Both these quantities depend on  because the higher the intensity of the laser, the higher the samples are heated up, which results in a variation of the refractive index.

For λ = 532 nm, n2 =  and χ(3) =  (e.s.u.).

For λ = 671 nm, n2 =  and χ(3) =  (e.s.u.).

Superconductivity 
Bulk 2H-TaSe2 has been demonstrated to be superconductive below a temperature of . However, the single monolayer (1H phase) can be associated with a critical temperature increased by an increment that can range up to .

Despite the 1T phase typically does not show any superconductive behaviour, formation of TaSe2−xTex compound is possible through doping with tellurium atoms. The former compound superconductive character depends on the fraction of tellurium (x can vary in the range ). The superconductive state arises when the fraction of Te ranges within : the optimal configuration is achieved at x = 0.6 and in correspondence of a critical temperature Tc = . In the optimal configuration, the CDW is totally suppressed by the presence of tellurium.

Lubricant 
Opposite to MoS2, which is largely employed as a lubricant in many different mechanical application, TaSe2 has not shown the same properties, with an average friction coefficient of 0.15. Under friction tests, like the Barker pendulum, it shows an initial friction coefficient of 0.2 to 0.3, which quickly increases to larger values as the number of oscillations of the pendulum increases (while for MoS2 it is almost constant during all the oscillations.)

Synthesis 
There are different methods in order to synthesize tantalum diselenide: depending on the growth parameter, different types of polytype can be stabilized.

Chemical Vapor Transport 

In general, TMDs can be synthesized through a chemical vapor transport technique accordingly to the following chemical equation:

 M +  MCl5 + 2 X → MX2 +  Cl2

where M is the chosen transition metal (Ta, Mo, etc.) and X represents the chosen chalcogen element (Se, Te, S). The parameter n, which governs the crystal growth, can vary between 3 and 50, and can be selected appropriately so that the crystal growth is optimized. During such growth, which might last for 2 to 7 days, the temperature is initially increased within a range between Th = . Then, it is cooled down to Tc = . After the growth completion, the crystals are cooled down to room temperature. Depending on the value of Tc, either the 2H or the 1T phase can be stabilized: in particular, using tantalum and selenium with  Tc < , only the 2H phase is stabilized. For the 1T phase, Tc must be larger. This allows to selectively grow the desirable phase of the chosen TMD.

Chemical vapor deposition 
Using powder of TaCl5 and selenium as precursors, and a gold substrate, the 2H phase can be stabilized. The gold substrate has to be heated up to , while TaCl5 and Se can be heated to  and , respectively. Argon and hydrogen gases are used as carriers. Once the growth is complete, the sample is cooled down to room temperature.

Mechanical exfoliation 
Since the single trilayers are kept together only by weak Van der Waals forces, atomically thin layers of tantalum diselenide can be easily separated by using scotch/carbon tape on the bulk TaSe2 crystals. With this method it is possible to isolate few layers (or even a single layer) of TaSe2. Then, the isolated layers can be deposited above other substrates, such as SiO2, for further characterizations.

Molecular Beam Epitaxy 
Pure tantalum is directly sublimated on a bilayer of graphene inside a selenium atmosphere. Depending on the temperature of the substrate Ts (graphene bilayer), the 1T or the 2H phase can be stabilized: in particular, if Ts =  the 2H is favoured, while at Ts =  the 1T is stabilized. This growth method is suitable only for atomically thin/few layers, but not for bulk crystals.

Liquid Phase Exfoliation 
Bulk crystals of TaSe2 (or any other TMDs) are put in a solution of pure ethanol. The mixture is then sonicated in an ultrasonic device with a power of at least 450 W for 15 hours. In this way it is possible to overcome the Van der Waals forces that keep the single monolayers of TaSe2 together, resulting in the formation of atomically thin flakes of tantalum diselenide.

Research

Optoelectronics 
Since 2H TaSe2 has been found to feature very large optical absorption and emission of light at approximately 532 nm, it might be used for the development of new devices. In particular, the possibility of transferring energy between TaSe2 and other TMDs, especially MoS2, has been proved. This process can be accomplished in a non-radiative resonant way by exploiting the large coupling between the TaSe2 emission and the excitonic absorption of TMDs.

Moreover, it is a promising material that may be used for the injection of hot carriers in semiconducting materials and other non-metallic TMDs due to the high lifetime of the generated photoelectrons.

All-optical switch and transferring of information 
Exploiting the dependence of the non linear effects of TaSe2 by the intensity  of the incident laser beam, it is possible to build an all-optical switch by means of two lasers which operate at different wavelengths and intensities. In particular, a high-intensity laser at λ2 = 671 nm is used to modulate a low-intensity signal at λ2 = 532 nm. Since there is a minimum value of  in order to trigger the non-linear effects, the low intensity signal cannot excite alone. On the contrary, when the high-intensity beam (λ1) is coupled with the low intensity signal (λ2), non-linear effects at both λ1 and λ2 arise. So, it is possible to trigger the non-linear effects on the low-intensity signal (λ2) by operating on the high-intensity one (λ1).

Exploiting the coupling between λ1 and λ2 enables transferring information from the high-intensity beam to the low-intensity one. With this method, the delay time for transferring the information from λ1 to λ2 is around 0.6 seconds

Spin-orbit torque devices 
Usually spin-orbit torque and spin to charge devices are built by interfacing a ferromagnetic layer with a bulk heavy transition metal, such as platinum. However, these effects take mainly place at the interface rather than in the platinum bulk, which introduces heat dissipation due to ohmic losses. Theoretical and DFT simulations suggest that interfacing a 1T-TaSe2 monolayer with cobalt might lead to higher performances with respect to the usual platinum-based devices.

Recent experiments showed that the spin-orbit scattering length of TaSe2 is around Lso = 17 nm, which is highly comparable with the one of platinum, Lso = 12 nm. This suggests the possible implementation of tantalum diselenide for the development of new 2D spintronic devices based on the spin Hall effect.

Hydrogen evolution reaction (HER) 
DFT and AIMD simulations suggest that the stacking of flakes of both TaSe2 and TaS2 in a disordered way could be used for the development of a new efficient and cheaper cathode that might be used for the extraction of H2 from other chemical compounds.

See also 
 Molybdenum disulfide
 Molybdenum diselenide
 Molybdenum ditelluride
 Rhenium diselenide
 Rhenium disulfide
 Tungsten diselenide

References 

Transition metal dichalcogenides
Selenides
Monolayers
Tantalum compounds
Nanomaterials