Premises and potential perspectives of transition metal dichalcogenides as thermoelectrics

Strategies of research on thermoelectric materials

To obtain the largest values of the thermoelectric figure of merit ZT=S²σT/κ and therefore best device performance, thermoelectric materials should have a large Seebeck coefficient (S), large electrical conductivity (σ), and very low thermal conductivity (κ), the latter being comprised of contributions from the lattice (κ_L) and the charge carriers (κ_e).
As κ_e and σ are positively correlated in all known materials, the most effective way to reduce κ is to reduce κ_L. This can be done by enhancing phonon scattering at interfaces, as for example in nanostructured samples.
The power factor S²σ and thermoelectric performance can be maximized by:

very anisotropic electronic transport (generated by very anisotropic electronic bands) with high effective masses across the transport direction and small effective masses along the transport direction;
significant weights of the electronic states participating in transport (multiple degenerate carrier pockets participating in transport);
high carrier mobilities along the transport direction (generated by small effective masses along the transport direction and large relaxation times).

Indeed, contemporary thermoelectric materials research can be classified into two major approaches: (i) find new crystalline materials with unique structure-property relationships that yield the desired combination of properties, and (ii) utilize nanostructured features to tune electrical and thermal transport, quasi-independently.
Both these routes can be pursued to explore and assess the potential of transition metal dichalcogenides (TMDs) as thermoelectrics: on one hand their electronic properties can be optimized through confinement in order to maximize the power factor for enhanced carrier mobility in the transport direction and on the other hand they can be fabricated in the form of nanoflake assemblies to enhance phonon scattering at interfaces.

Aspects of TMDs relevant for potential thermoelectric applications

Remarkable literature results on thermoelectric properties of the akin compound SnSe, a transition metal monochalcogenide:
• ZT up to 2.6 at 923 K and ZT around unity in doped polycrystals
Remarkable very recent literature results on thermoelectric properties of some TMDs, in the form of flakes, films and oriented polycrystals:
    • ZT up to 0.63 at 673K in SnS₂
    • large power factors in MoS₂ and TiS₂
    • low thermal conductivity in disordered MoS₂ and WSe₂
Remarkable very recent literature results on thermoelectric properties of some TMDs, in the form assemblies of exfoliated TMD flakes:
• room temperature power factors of few μWcm^-1K^-2 in TiS₂ and MoS₂ nanosheet assemblies and superlattices
Tunability of electronic properties by electronic confinement in few-atomic-layer thick samples and on the presence of phonon scattering interfaces in nano-flake assemblies.
High tunability of S and S²σ in TMDs by field effect.
p and n type character in TMDs for fabrication of thermoelectric modules.
Possible deposition on technological substrates for flexible and wearable thermoelectric devices.
As graphene-related materials, TMDs could allow to take advantage of the scientific and technological achievements gained with graphene in sample fabrication and in the field of energy generation and storage. Within the energy generation and storage field, the graphene technology is at the readiness level of applied research and development or even of demonstration. Noteworthy, TMDs may even outperform graphene in thermoelectric applications, in that they may exhibit thermal conductivity values significantly smaller than that of graphene.

Perspective potential applications

Considering all the above aspects, TMDs may have a considerable potential as solid state thermoelectric micro-coolers for nanocircuit refrigeration.