COMICS meeting 07/07/2014

* Enhanced thermoelec efficiency in vertical graphene junctions, Viet Hung Nguyen

Présents : Arnaud Bournel, Jérôme Larroque, Rania Lataoui, Huy Viet Nguyen, Van Duy Nguyen, Viet Hung Nguyen, Jongyoon Park, Jérôme Saint Martin, Adrien Vincent, Damir Vodenicarevic.

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Duy in Grenoble next October for a Phd About superconducting devices..

Enhanced thermoelec efficiency in vertical graphene junctions, Viet Hung Nguyen

Principles… Seebeck (heat to electric) / Peltier (electric to heat).

ZT figure of merit. Coupling between the transport quantities… Difficult tradeoff.

Thermoelectric materials… Nanostructuring to both reduce thermal conducante and increase electric conductance.

Graphene : high in-plane thermal transport (due to covalent sp2 bonding), weak out-of-plane heat flow (limited by van der Waals coupling). See Pop et al., MRS Bull. 37, 1273 (2012).

How enhancing thermoelectric properties of graphene ? (i) Enhance the power factor by opening an energy gap (armchair GNR, nanomeshes, zizag GNR + nanoholes, mixed GNR, graphene/hBN heterojunctions), (ii) reduce the thermal conductance (surface roughness, defects, isotopes, nanohole lattices, graphene/hBN…).

Hung’s proposition : vertical graphene junction, based on two stacked graphene sheets parlty overlapped, each connected to an electrode.

Model : tight binding + force constant model for phonons.

Electronic performance weakly affected, while the phonon transmission is importantly reduced. But the reduction of phonon conductance depends on junction length → need of a short enough junction (< 5 nm) for decreasing thermal conductance more than 10 times. But saturation of phonon conductance at high temperature.

Use of armchair GNR to design the vertical devices.

Phonon degradation may be further decreased by inserting some additional layers…

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