2D Heterostructures for Energy COnversion – 2DHECO
Recovery of wasted heat is a major societal concern in this century focusing important research efforts. Research on new thermoelectric (TE) devices and materials to improve energy conversion is highly demanded in particular in nanoelectronics applications. Energy conversion of TE nanogenerators is ruled by the TE effect, the phenomenon that relates an electrical potential difference created from a temperature gradient in a TE material. The TE efficiency ZT, depending on the Seebeck coefficient, the electrical and thermal conductivity and the temperature, is the relevant parameter that researchers struggle to improve. Values of ZT >> 1 are typically sought for a TE material to be exploitable. Active TE materials must have low thermal conductivity and high electrical conductivity, which is an antonymic behaviour in common bulk materials due to the Wiedemann-Franz law but it can be achieved by nanostructuring. This is why managing and understanding heat at the nanoscale constitutes a major on-going scientific and technological challenge. Improved TE properties have been demonstrated in Si nanowires (ZT=1), in Bi2Te3/ Sb2Te3 superlattices (ZT=2.4), InAs and Sb2Te3 nanowires and carbon nanotubes. More recently, the discovery of two-dimensional (2D) materials has opened new routes of investigation, foreseeing performances that go behind most used TE materials. High ZT values have been predicted in graphene nanostructure and transition metal dichalcogenides (TMDs) have revealed high Seebeck coefficients. Furthermore 2D materials can be precisely assembled layer by layer giving rise to the so-called van der Waals heterostructures (vdW). Stacking of 2D materials ensures decoupling from the substrate and the environment and preserves the material properties.
The thermoelectric performance of graphene, for example, has been demonstrated to be significantly improved by decoupling it from the substrate with hBN, due to reduced substrate-induced random potential fluctuations. Recently a solid-state power generator/refrigerator has been theoretically proposed using vdW heterostructures composen of WSe2 and MoSe2 sandwiched between two graphene electrodes. This device is predicted to harvest waste heat at 400K with efficiencies of about 7-8%. Furthermore nanosheets of SnS2 have revealed a negative correlation between the electrical and the thermal conductivities, highly desirable for TE applications. VdW heterostructures hide possibly surprising properties for thermoelectric performances and applications.
In this context, the 2DHECO project aims to develop new-engineered devices to improve energy conversion efficiency at nanoscale, opening the route to van der Waals heterostructures based thermoelectricity.
The objectives cover different technical and scientific challenges:
1. Fabricating stable and reproducible devices based on vdW heterostructures with different 2D materials (graphene, hBN, WSe2 and SnS2) with the desired sequence and relative orientation between the layers. This heterostructure will act as the active thermoelectric element.
2. Investigating experimentally the electric, thermoelectric and thermal properties of the engineered devices by DC and AC transport techniques and by opthothermal Raman approach in the supported and suspended case, in order to extract reliable figures of merits (ZT higher than 2-3 are expected).
3. Exploring 2 main approaches to engineer the thermal and electrical conductivities:
- nanostructuring the 2D materials in the heterostructure with networks of periodic patterns of different lattice geometries and parameters.
- modifying the 2D material surface in the heterostructure by molecular functionalization, to control the density of charge carriers and defects and to increase phonon scattering.
We are convinced that our investigation will allow to identify new thermoelectric properties of engineered two-dimensional materials and to bring new perspectives for applications.
Madame Maria Luisa Della Rocca (Laboratoire Matériaux et Phénomènes Quantiques)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
MPQ Laboratoire Matériaux et Phénomènes Quantiques
ITODYS Interfaces, Traitements, Organisation et Dynamique des Systèmes
C2N Centre de Nanosciences et de Nanotechnologies
Help of the ANR 459,279 euros
Beginning and duration of the scientific project: February 2021 - 48 Months