Propietats termoelèctriques i tèrmiques d'aïllants topòlogics i materials bidimensionals
- Autores: Aloïs Jean Guillaume Arrighi
- Directores de la Tesis: Clivia Sotomayor Torres (dir. tes.), Sergio Valenzuela (codir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2020
- Idioma: español
- ISBN: 9788449094743
- Tribunal Calificador de la Tesis: Andres Castellanos Gomez (presid.), David Jimenez Jimenez (secret.), Georgios Deligeorgis (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universidad Autónoma de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Thermal management is becoming a critical issue in the packaging and design of nanoelectronics. Advanced cooling solutions and efficient energy harvesting are key aspects to help keep the trend for ever smaller and faster electronics. This thesis is focused on thermal management and the use of heat waste in emerging materials for electronics. In particular, two-dimensional materials (2DM), and related heterostructures, are amongst the most intriguing prospects for future electronics and are being intensively investigated. Here, two main subjects were explored. First, the thermal transport of suspended 2DMs, including CVD graphene, transition metal dichalcogenides (TMDCs) and heterostructures of TMDCs with hexagonal boron nitride (hBN) and, second, the thermal properties and thermoelectricity of (Bi1-xSbx)2Te3 (BST) thin films. These materials are being considered for interconnects and THz transistors (graphene), digital electronics (TMDCs) and electrical insulation (hBN) and are well known as thermoelectric generators, as are also materials that have recently been identified as topological insulators (BST).
In the first part, the objective was to demonstrate the measurement of the thermal conductivity of 2DMs using the recently developed two-laser Raman spectroscopy method. Its implementation was rendered difficult by the relatively small exfoliated flakes of the materials investigated and their high thermal conductivity. The thermal conductivity of CVD graphene was found to be about 300 W/(m·K). Although smaller than exfoliated graphene, it is argued that this could be due to grain boundaries and disorder. Exfoliated MoS2 and MoSe2 (two well-known TMDCs) presented thermal conductivities of 12 to 24 W/(m·K) and 60 W/(m·K). Measurements on different membranes of MoS2 further showed that the conductivity increases with the thickness in thin membranes (few monolayers). Furthermore, stacking an exfoliated hBN membrane on top of a previously characterized MoS2 sample allowed us to demonstrate a notorious increase of the thermal conductivity in the hBN/MoS2 heterostructure, when heat is introduced on MoS2. Indeed, when compared with MoS2 alone the thermal conductivity is found to be almost one order of magnitude larger, 185 W/(m·K).
For the second part, BST thin films were grown by molecular beam epitaxy. The main objective was to investigate the correlation of the thermoelectric properties of these materials with the Fermi level, which would tune the relative weight of bulk and topological surface state (TSS) transport. It was first demonstrated that controlling the concentration of Sb we could engineer the band structure and tune the Fermi level from the valence to the conduction band. Such demonstration was achieved by using angle-resolved photoemission spectroscopy in combination with conductivity and Hall measurements in relatively thin (10 nm) films. The Sb concentration at which TSS dominated the transport was also identified. Thermoelectric experiments on the same films were then carried out but no clear correlation between the thermopower and the carrier nature was found when the TSSs were dominant. These results indicate that TSS transport has limited influence on the thermoelectric properties. Further studies should be carried our using even thinner films. Finally, a side characterization of the BST thin films using Raman spectroscopy demonstrated specific variations in the behaviour associated to Sb concentration. An increase of the laser power showed the emergence of non-active Raman peaks of undetermined origin. However, they can indicate the presence of broken structural symmetries, surface phonon modes or other effects such as plasmonic resonances. This interesting response is worthy of for further investigation.
