A numerical study of thermoelectric properties of layered platinum chalcogenides and oxide

Mohammed, Hamza Adam Haroun
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The research reported in this thesis was motivated by the pressing need for novel materials for thermoelectric and photovoltaic applications. We numerically explored properties of Platinum dichalcogenides dioxide to examine if these compounds have potential as active components in thermoelectric and photovoltaic devices. Transition metal dichalcogenides (TMDs), in layered structures, have diverse properties that complement and potentially extend beyond promising properties of graphene, the prototypical layered material. Structural, stabilities (mechanical and dynamical), electronic, optical and thermoelectric properties of the bulk, bilayer and monolayer trigonal platinum dioxide and dichalcogenides PtX2, (X = O, S, Se and Te) were investigated based on density functional theory (DFT) and many-body perturbation theory (MBPT) as implemented in the Vienna ab-initio simulation package. The structural properties of the bulk, bi-layer and monolayer PtX2 (such as optimized lattice parameters, cohesive and formation energies) were extracted from relaxed structures. Elastic coefficients and phonon dispersion studies showed that the relaxed structures are mechanically and dynamically stable. Investigation of the electronic band structure and densities of states of the bulk, bilayer and monolayer PtX2 show that, at the DFT level of approximation, all the com-pounds are indirect band gap semiconductors apart from bulk PtO2, PtS2, PtSe2 and PtTe2 which are semi-metals. To calculate optical properties, we implemented the Bethe-Salpeter equation (BSE) calculations on top of non-self-consistent G0W0 calculations to determine the dielectric matrix. The obtained results for absorbance in the visible light range for single layers in bulk, bilayer and a monolayer PtX2, are from 0.9−35.27% for in-plane absorbance. which is higher than 2.3% for graphene and 5−10% for layered MoS2, MoSe2and WS2of similar thickness. The BSE optical gaps are in the range 0.37 to 2.75 eV for monolayer and bilayer PtX2, while there is no gap bulk PtO2, PtS2, PtSe2 and PtTe2. Monolayer and bilayer PtX2 may have potential for application in tandem solar cell applications. Lattice and electronic transport coefficients were obtained within the relaxation-time approximation to the Boltzmann transport equations as implemented in PHONO3PY package and BoltzTraP2 packages. We calculated the lattice thermal conductivity for PtX2 structures per layer, for ease of comparison between few layer and bulk systems. The obtained results are in range from 24.61×10−8 to 0.07×10−8Wm−1K−1 at 300 K for bulk PtO2 to monolayer PtTe2, respectively. The out-of-plane coefficients of bilayer and monolayer are zero. The obtained values of the figure of merit (ZT) were in a range from 0.04 to 0.74, with the highest ZT values achieved by bilayer and monolayer PtO2 (0.62 and 0.74), while the lowest ZT value was obtained by bulk PtS2 of 0.04. Also, we observed the increase of figure of merit from bulk to monolayer which was expected due to their low lattice thermal conductivity. Most, the highest values of the calculated ZT were dominated by the electron charge carriers. The investigation suggests that of the compounds explored, n-type monolayer PtO2 has the most promise for thermoelectric applications
A thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2020