My master thesis consists of a DFT study on the thermal transport properties of graphene and single-layer transition metal dichalcogenides. Calculations have been performed using the SIESTA DFT package based on atomic orbitals. In a first part, the phonon dispersion relation of graphene, MoS2, MoSe2, WS2 and WSe2 were calculated. By means of the Landauer formalism, the ballistic phonon transmission function and the thermal conductance were computed and compared. Using the atomistic Green’s function technique, the influence of a disordered distribution of sulphur vacancies on the thermal conductivity was analysed. The thermoelectric figure of merit of MoS2 in presence of sulphur vacancies was obtained as well. Finally, a procedure was developed to include in the calculations the effect of phonon-phonon Umklapp scattering and scattering due to roughness of the boundaries. This method was successfully applied to compute the lattice thermal conductivity of graphene, allowing to evaluate the impact of scattering as a function of temperature and size of the samples.
La mia tesi di laurea magistrale consiste in uno studio computazionale sulle proprietà di trasporto termico in grafene e monostrato dicalcogenuri dei metalli di transizione (TMDCs). Le simulazioni si basano su calcoli DFT eseguiti mediante il metodo "SIESTA". Nella prima parte, si mostrano le relazioni di dispersione dei fononi in grafene, MoS2, MoSe2, WS2 and WSe2. La funzione di trasmissione dei fononi e la conduttanza termica sono calcolate in regime di trasporto balistico nel contesto del formalismo di Landauer. Mediante un metodo basato sulle funzioni di Green, si valuta l'influenza di una distribuzione disordinata di vacanze di zolfo sulla conduttività termica del MoS2 monostrato e se ne calcola la figura di merito termoelettrica. Infine si mostra una procedura sviluppata per includere l'impatto del fenomeno di scattering fonone-fonone (processo Umklapp) sul trasporto termico. Il metodo è applicato per calcolare la conduttività termica (contributo fononico) del grafene.
Ab-initio study on the thermal transport properties of bidimensional materials : application to thermoelectricity
PECORARIO, STEFANO
2018/2019
Abstract
My master thesis consists of a DFT study on the thermal transport properties of graphene and single-layer transition metal dichalcogenides. Calculations have been performed using the SIESTA DFT package based on atomic orbitals. In a first part, the phonon dispersion relation of graphene, MoS2, MoSe2, WS2 and WSe2 were calculated. By means of the Landauer formalism, the ballistic phonon transmission function and the thermal conductance were computed and compared. Using the atomistic Green’s function technique, the influence of a disordered distribution of sulphur vacancies on the thermal conductivity was analysed. The thermoelectric figure of merit of MoS2 in presence of sulphur vacancies was obtained as well. Finally, a procedure was developed to include in the calculations the effect of phonon-phonon Umklapp scattering and scattering due to roughness of the boundaries. This method was successfully applied to compute the lattice thermal conductivity of graphene, allowing to evaluate the impact of scattering as a function of temperature and size of the samples.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/147272