Thermoelectric Transport in Transition Metal

**THERMOELECTRIC TRANSPORT IN TRANSITION METAL DICHALCOGENIDES**:

- Réf **ABG-125712**
- Sujet de Thèse- 06/09/2024- Contrat doctoral- Université- Lieu de travail- Gif sur Yvette - Ile-de-France - France- Intitulé du sujet- THERMOELECTRIC TRANSPORT IN TRANSITION METAL DICHALCOGENIDES- Champs scientifiques- Sciences de l’ingénieur
- Energie
- Sciences de l’ingénieur
- Mots clés- thermoelectricity, low dimension materials, transport measurement**Description du sujet**:
**Context and framework**
The heat waste is an under-exploited source of energy that could be harvested directly and reversibly through thermoelectric (TE) transducers thus providing a cleaner form of useable energy. The efficiency for the energy conversion i.e., the TE figure of merit of a material is defined by the dimensionless ZT = S2σ/(κe+κl) where S is the Seebeck coefficient, σ is the electrical conductivity T is the absolute temperature, κe is the electronic thermal conductivity and κl is the lattice thermal conductivity. High performance TE materials should ideally exhibit : a high S, a high σ and a low κ. However, these transports factors (S, σ, κ) are strongly entangled to each other through charge carrier concentration
(n) and mobility (μ), thus, it is still a great challenge to enhance the ZT of TE materials. Therefore, there is an urgent need to develop a portfolio of thermoelectric materials offering thermal stability, especially for the temperature range 300-400 K, where a large amount of heat is wasted into the environment, but no standalone harvesting methods are suitable to effectively generate electrical power. Several routes have been proposed to achieve high ZT materials e.g., band convergence, low dimensionality, and energy filtering. Low-dimensional materials such as transition metal dichalcogenides (TMDs) are particularly interesting as they encompass the alternative routes owing to quantum confinement and their peculiar band structures. They typically present higher S values than 3D bulk materials and are prone to energy filtering (EF) whereby the energy dependence of the density of states (DOS) presents singularities, substantially enhancing S and σ. The goal of current TE research is to disentangle S and σ. Low dimensionality increases the DOS near the Fermi level EF, leading to an enhancement of S. These improvements are mainly observed in chalcogenides such as Bi2Te3, PbT e, and Sb2Te3 that exhibit a ZT close to unity and have been widely integrated into TE generators. Recently, Yang et al 1. have measured a PF of 0.150 mWm 1K 2 on 8 nm Bi2O2Se at 300 K. They observed a high mobility
which independently enhances σ without compromising S. Similarly, GEEPS and NTU also measured a high value, i.e. S -250 μV/K for a 14 nm film at 300 K. However, their PF 0.9 mWm 1K 2, figure 1, is 6-fold higher and presents a stable temperature window centered on 280 K, in striking contrast with the observation of Yang et al. While the mobility decreases, it remains high (350 cm 2V 1s 1 at 400 K) with a transition at 120 K from acoustic phonon to piezoelectric scattering regime below which we observe the decoupling of S and σ, departing from the Mott equation.

**Objectives of the research project**
Our work on 2D TMDs and the large PF of Bi2O2Se observed is very encouraging and, at the same time, raises fundamental questions on the underlying mechanisms that led to such an enhancement. Understanding these factors could, eventually, lead to the design of new TE systems outperforming current high ZT materials. Our recent experimental observations revealed a substantial enhancement of the power factor (S2σ) on 2D TMDs and highlight the importance to investigate transport mechanisms at low dimensions, particularly 1D nanowires (NW). These later could be formed with nano inclusions whereby, lower energy electrons scattered by the potential barrier of the interface, may also induce a significant improvement. In this PhD project, we propose to benchmark the TE properties of 3D, 2D and 1D configurations of the same selected material grown by chemical vapor transport (CVT), using the same measurement techniques.

1. F. Yang et al., Adv. Mater. 33, 2004786 (2021)
2. J. Ge et al., Nat. Commun. 12, 3146 (2021)
3. H. Moutaabbid et al., Inorg. Chem. 55, 6481-6486 (2016)
4. C. Euaruksakul et al, Journal of Electronic Materials, 1543-186X (2020)
5. K. Fukumoto et al., J. Phys. Appl. Phys. 53, 405106 (2020)

**Experimental methods**

**Prise de fonction**:

- 01/02/2025**Nature du financement**:

- Contrat doctoral**Précisions sur le financement**:

- extension concours doctoral 2024**Présentation établissement et labo d'accueil**:

- Université**Site web**:
**Intitulé du doctorat**:

- doctorat sciences de l'ingénieur**Pays d'obtention du doctorat**:

- France**Etablissement délivrant le doctorat**:

- Sorbonne Université**Ecole doctorale**:

- Sciences mécanique, acoustique, électronique et robotique de Paris (