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 600 K, while other transition metal dichalcogenides normally need to be cooled down to hundreds of kelvin degrees, or even below, to observe the same capability.

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.

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 A, while c = 0.627 nm.
Depending on the temperature, it shows different types of charge density waves (CDW): an incommensurate CDW (ICDW) between 600 K and 473 K and a commensurate CDW (CCDW) below 473 K. In the commensurate CDW, the resulting superlattice shows a

13

×

13

{\displaystyle {\sqrt {13}}\times {\sqrt {13}}}

reconstruction often addressed as star of David (SOD), with respect to the lattice parameter (a = b) of non distorted TaSe2 (above 600 K). 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.

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 A, while c = 1.27 nm. Depending on the temperature, it shows different types of charge density wave: an incommensurate CDW (ICDW) between 122 K and 90 K and a commensurate CDW (CCDW) below 90 K. The lattice distortion below 90 K 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 122 K).

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 130 K.

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

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 473 K, 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 473 K temperature is reached (ICDW temperature transition), when it jumps to slightly higher values. 1T phase is diamagnetic.

Resistivity linearly depends on the temperature when the latter exceeds 110 K. On the opposite, below this threshold it shows a non-linear behaviour. This abrupt variation of R(T) at 110 K 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 110-120 K. The trend is linearly ascending or descending below and above 110 K, respectively. This maximum in the 2H phases is related to the formation of the CCDW at 120 K. The 2H phase is Pauli paramagnetic.

The Hall coefficient

R

H

{\displaystyle R_{H}}

is almost independent of the temperature above 120 K, a threshold below which it instead starts to drop to lastly reach a value of zero at 90 K. In the range included between 4 and 90 K, the coefficient

R

H

{\displaystyle R_{H}}

is negative, its minimum being experienced at approximately 35 K.

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.

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.

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 the Kerr’s law:

n
=

n

0

+

n

2

I

{\displaystyle n=n_{0}+n_{2}I}

where n0=1.37 represents the linear refractive index of ethanol,

n

2

{\displaystyle n_{2}}

is the non-linear refractive index of TaSe2 and

I

{\displaystyle I}

is the incident intensity of the laser beam. Using different light wavelengths, in particular l=532 nm and l=671 nm, it is possible to measure both

n

2

{\displaystyle n_{2}}

and

x

(
3
)

{\displaystyle \chi ^{(3)}}

, the third order nonlinear susceptibility.

Both these quantities depend on

I

{\displaystyle I}

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

l
=
532

{\displaystyle \lambda =532}

nm,

n

2

=
8
×

10


7

c

m

2

/

W

{\displaystyle n_{2}=8\times 10^{-7}cm^{2}/W}

and

x

(
3
)

=

1.37

×

10


7

(
e
.
s
.
u
)

{\displaystyle \chi ^{(3)}={1.37}\times 10^{-7}(e.s.u)}

For

l
=
671

{\displaystyle \lambda =671}

nm,

n

2

=

3.3

×

10


7

c

m

2

/

W

{\displaystyle n_{2}={3.3}\times 10^{-7}cm^{2}/W}

and

x

(
3
)

=

1.58

×

10


7

(
e
.
s
.
u
)

{\displaystyle \chi ^{(3)}={1.58}\times 10^{-7}(e.s.u)}

Bulk 2H TaSe2 has been demonstrated to be superconductive below a temperature of 0.14 K. However, the single monolayer (1H phase) can be associated with a critical temperature increased by an increment that can range up to 1 K.

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 varies in the range

0
< x < 2 {\displaystyle 0

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