Electrons and Phonons

Advanced Functional Materials

The role of dimensionality on the electronic performance of thermoelectric devices is clarified using the Landauer formalism, which shows that the thermoelectric coefficients are related to the transmission, T(E), and how the conducing channels, M(E), are distributed in energy. The Landauer formalism applies from the ballistic to diffusive limits and provides a clear way to compare performance in different dimensions. It also provides a physical interpretation of the "transport distribution," a quantity that arises in the Boltzmann transport equation approach. Quantitative comparison of thermoelectric coefficients in one, two, and three dimension shows that the channels may be utilized more effectively in lower-dimensions. To realize the advantage of lower dimensionality, however, the packing density must be very high, so the thicknesses of the quantum wells or wires must be small. The potential benefits of engineering M(E) into a delta-function are also investigated. When compared to a bulk semiconductor, we find the potential for ~50 % improvement in performance. The shape of M(E) improves as dimensionality decreases, but lower dimensionality itself does not guarantee better performance because it is controlled by both the shape and the magnitude of M(E). The benefits of engineering the shape of M(E) appear to be modest, but approaches to increase the magnitude of M(E) could pay large dividends.

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Influence of Dimensionality on Thermoelectric Device Performance

Thermal camouflaging metamaterials

Low-resistivity metal-semiconductor (M-S) contact is one of the urgent challenges in the research of 2D transition metal dichalcogenides (TMDs). Here, we report a chloride molecular doping technique which greatly reduces the contact resistance (Rc) in the few-layer WS2 and MoS2. After doping, the Rc of WS2 and MoS2 have been decreased to 0.7 kohm*um and 0.5 kohm*um, respectively. The significant reduction of the Rc is attributed to the achieved high electron doping density thus significant reduction of Schottky barrier width. As a proof-ofconcept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 uA/um, an on/off ratio of 4*106, and a peak field-effect mobility of 60 cm2/V*s. This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nano-electronic devices.

Chloride Molecular Doping Technique on 2D Materials: WS2 and MoS2

The thermoelectric properties (electrical conductivity, Seebeck coefficient, and power factor) of single layer transition metal dichalcogenides (MoS2, MoSe2, WS2, and WSe2) are investigated theoretically on the basis of ab initio quantum transport using the Landauer Buttiker formalism The often used rigid band model is compared to realistic doping, namely substitution and adsorption, it is found that several important physical insights governing the transport are missing in this approximation The rigid band model appears to clearly overestimate the thermoelectic efficiency, hampering its relevance for thermoelectric studies Substitution doping by chloride or phosphorus leads to poor power factor due to drastic changes of the pristine band structure In contrast, adsorption doping by alkalies (Li, Na, K, and Rb) favors larger power factor Realistic treatment of the disorder induced by the dopants is also investigated and reveals that Cl doping leads to very short localization length of 35 nm while K c

First Principle Investigation on Thermoelectric Properties of Transition Metal Dichalcogenides: Beyond the Rigid Band Model

Thermoelectric properties of single layered transition metal dichalchogenide MoS2 are investigated on the basis of ab initio calculations combined with Landauer formalism. The focus is made on sulfur vacancy defects that are experimentally observed to be largely present in materials especially in exfoliated two dimensional MoS2 compounds. The impact of these defects on phonon and electron transport properties is investigated here using a realistic description of their natural disordering. It is observed that phonons tend to localize around defects which induce a drastic reduction in thermal conductivity. For p type doping the figure of merit is almost insensitive to the defects while for n type doping the figure of merit rapidly tends to be zero for the increasing length of the system. These features are linked with a larger scattering of the electrons in the conduction band than that of holes in the valence band.

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First principle investigation of the influence of sulfur vacancies on thermoelectric properties of single layered MoS2

Thermoelectric materials are opening a promising pathway to address energy conversion issues governed by a competition between thermal and electronic transport. Improving the efficiency is a difficult task, a challenge that requires new strategies to unearth optimized compounds. We present a theory of thermoelectric transport in electron-doped SrTiO3 , based on a realistic tight-binding model that includes relevant scattering processes. We compare our calculations against a wide panel of experimental data, both bulk and thin films. We find a qualitative and quantitative agreement over both a wide range of temperatures and carrier concentrations, from light to heavily doped. Moreover, the results appear insensitive to the nature of the dopant La, B, Gd and Nb. Thus, the quantitative success found in the case of SrTiO3 , reveals an efficient procedure to explore new routes to improve the thermoelectric properties in oxides.

https://iopscience.iop.org/article/10.1209/0295-5075/118/67004/pdf

Unified modelling of the thermoelectric properties in SrTiO3

A particularly promising pathway to enhance the efficiency of thermoelectric materials lies in the use of resonant states, as suggested by experimentalists and theorists alike. In this paper, we go over the mechanisms used in the literature to explain how resonant levels affect the thermoelectric properties, and we suggest that the effects of hybridization are crucial yet ill understood. In order to get a good grasp of the physical picture and to draw guidelines for thermoelectric enhancement, we use a tight-binding model containing a conduction band hybridized with a flat band. We find that the conductivity is suppressed in a wide energy range near the resonance, but that the Seebeck coefficient can be boosted for strong enough hybridization, thus allowing for a significant increase of the power factor. The Seebeck coefficient can also display a sign change as the Fermi level crosses the resonance. Our results suggest that in order to boost the power factor, the hybridization strength must not be too low, the resonant level must not be too close to the conduction (or valence) band edge, and the Fermi level must be located around, but not inside, the resonant peak.

Boosting the power factor with resonant states: A model study

Understanding thermal transport in 2D materials and especially in graphene is a key challenge for the design of heat management and energy conversion devices. The high sensitivity of measured transport properties to structural defects, ripples and vacancies is of crucial importance in these materials. Using a first principle based approach combined with an exact treatment of the disorder, we address the impact of vacancies on phonon lifetimes and thermal transport in graphene. We find that perturbation theory fails completely and overestimates phonon lifetimes by almost two orders of magnitude. Whilst, in defected graphene, longitudinal acoustic and transverse acoustic modes remain well defined, the out of plane acoustic (ZA) modes become marginal. In the long wavelength limit, the ZA dispersion changes from quadratic to linear and the scattering rate is found proportional to the phonon energy, in contrast to the quadratic scaling often assumed. The impact on thermal transport, calculated beyond the relaxation time approximation and including first principle phonon-phonon scattering rates as reported recently for pristine graphene, reveals spectacular effects even for extremely low vacancy concentrations.

https://hal.archives-ouvertes.fr/hal-03011389/document

S. Thébaud

Papers

First Principle Investigation on Thermoelectric Properties of Transition Metal Dichalcogenides: Beyond the Rigid Band Model

First principle investigation of the influence of sulfur vacancies on thermoelectric properties of single layered MoS2

Unified modelling of the thermoelectric properties in SrTiO3

Boosting the power factor with resonant states: A model study

Drastic effects of vacancies on phonon lifetime and thermal conductivity in graphene.