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Theses

Electric and thermoelectric properties of supported 2D materials

Abstract : Our age has seen an exponential technological growth never recorded before. In the context of this technological development, the availability of clean and renewable energy has become a challenging issue pushing research efforts. Thermoelectric (TE) conversion, namely the ability of a material to generate electric power from a temperature gradient or a thermal current from an applied voltage, aims to recover this wasted energy and the research on new thermoelectric materials is recently experiencing a new enthusiastic boost. Thermoelectric devices are reliable and do not pollute the atmosphere, but their low conversion efficiency remains the limit of an extensive development. New energy recovery solutions are currently highly demanded in particular in the domain of micro- and nano-electronics. The aim of this PhD work is to contribute in finding original solutions to engineer new devices based on 2D materials improving TE performances, particularly considering the on-substrate configuration, actually more appropriate for applications. In particular, I have investigated the electric and thermoelectric properties of hBN/WSe2 heterostructures, where the hBN layer acts simultaneously as spacer, to decouple the TMD from the SiO2 substrate, and as dielectric, to efficiently couple the TMD to a local gate. Tungsten diselenide (WSe2) has been the material of choice since only few works have focused on its thermoelectric properties, revealing, so far, promising results. Moreover, WSe2 owns a particularly low thermal conductivity (1 - 2 W/mK at room temperature), making this material appealing for TE applications. I have performed a detailed analysis of the electric and thermoelectric properties at room temperature of such devices as a function of the metal used for electrical contacts. I found out high values of Seebeck coefficient, up to 200 μV/K, and power factor, up to 4 μW/cm K2, depending on the used metal, revealing the importance of the electronic properties at the electrode/2D material interface for enhanced device performances. Furthermore, I got interested into the complex question of correctly measuring the physical parameters defining the TE performances in actual devices based on supported low dimensional materials. The ZT parameter of a given TE device, which quantifies the energy conversion efficiency, depends on the thermal conductivity of the chosen material, which, at room temperature, is dominated by phonons. In a supported configuration, thermal losses to the substrate strongly dominate heat transport and, phonon boundary and interface scattering can strongly modify the material thermal conductivity. During my PhD work, I have proposed the use of the Joule self-heating method, already used to evaluate the thermal conductivity of supported metallic nanowires, to the case of multilayer graphene nanowires. I chose graphene as a test-bed 2D material for the easiness of its manipulation for device fabrication. I found out that, by using a thick and rough SiO2 oxide layer, thermal losses to the substrate can be considerably reduced and I unveil an effective reduction of the graphene thermal conductivity, with values as low as 40 W/mK. The underlying idea is to extend, in the future, the same approach also to TMDs to achieve a complete in-situ thermoelectric characterization of the studied devices.
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https://hal.archives-ouvertes.fr/tel-03494450
Contributor : Salvatore Timpa Connect in order to contact the contributor
Submitted on : Sunday, December 19, 2021 - 11:59:22 AM
Last modification on : Thursday, April 7, 2022 - 1:58:33 PM
Long-term archiving on: : Sunday, March 20, 2022 - 6:35:06 PM

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Salvatore Timpa. Electric and thermoelectric properties of supported 2D materials. Materials Science [cond-mat.mtrl-sci]. Université de Paris 7 - Denis Diderot, 2021. English. ⟨tel-03494450⟩

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