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Project Topic
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Transition metal dichalcogenides (TMDs) offer a huge flexibility in tuning electronic properties. Their electronic structure is found to change dramatically from bulk to few monolayer samples. Moreover, some TMDs exhibit remarkable bulk thermoelectric (TE) behavior. According to theoretical predictions, these may be further improved in few monolayer thick nanostructures thanks to changes in the electronic structure. On the other hand, TE performance could be also improved by suppressing the thermal conductivity via engineering of the microstructure. Indeed, TMD flakes can be produced by liquid phase exfoliation of their bulk counterpart using scalable and cheap methods. Restacked nano-flake assemblies offer an ideal nanostructured morphology that effectively scatters phonon of different wavelengths, thus suppressing the lattice thermal conductivity, whose value in single flakes is typically in the range of tens Wm-1K-1 at room temperature. The aim of our proposal is to explore the potential of these features – i.e. electronic confinement and nanostructured morphology - in view of enhancing the TE performance of these systems for applications in energy conversion or cooling. We will prepare TMDs, choosing among MoTe2, ZrSe2, MoS2, MoSe2, WSe2, WTe2, HfSe2, SnSe2, HfTe2, in different forms, namely bulk single crystals, epitaxial ultrathin films and heterostructures (grown by molecular beam epitaxy),or nanoflakes (obtained by liquid phase exfoliation and subsequent ink- FLAG-ERA JTC 2017 Full Proposal MELoDICA page 2 of 30 jet printing and drop-casting). In these samples, we will measure electric, thermoelectric and thermal transport properties. In TMD ultrathin films and heterostructures, we will focus on the possibility of tuning TE properties via thickness. In assembled flake patterns, we will further explore the effect of confinement, but also address phonon engineering by nanostructural configurations, in terms of a suitable distribution of thickness and size, as well as proper inter-flake connectivity. In synergy, deeper insight will be gained by carrying out theoretical calculations of electric, thermoelectric and thermal transport properties, focusing on the effects of confinement and the presence of interface thermal resistances. Our investigation will start from the optimization of the sample preparation processes and it will proceed by combining experimental measurements and theoretical calculations, aiming to obtain a comprehensive understanding of the physical mechanisms into play and a realistic assessment of TMDs as TE materials for device applications such as TE micro-coolers, on the basis of our own experimental results, as well as our estimation of the limits of performance we can achieve.
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