  The enhancement of thermoelectric figure of merit ZT requires to either
increase the power factor or reduce the phonon conductance, or even both. In
graphene, the high phonon thermal conductivity is the main factor limiting the
thermoelectric conversion. The common strategy to enhance ZT is therefore to
introduce phonon scatterers to suppress the phonon conductance while retaining
high electrical conductance and Seebeck coefficient. Although thermoelectric
performance is eventually enhanced, all studies based on this strategy show a
significant reduction of the electrical conductance, most often leading to a
lower electronic performance. In this study we show that appropriate sources of
disorder, including isotopes and vacancies at lowest electron density
positions, can be used as phonon scatterers to reduce the phonon conductance in
graphene ribbons without degrading the electrical conductance, particularly in
the low-energy region which is the most important range for device operation.
By means of atomistic calculations using semi-empirical Tight-Binding and Force
Constant models in combination with Non-Equilibrium Green function formalism,
we show that the natural electronic properties of graphene ribbons can be fully
preserved while their thermoelectric efficiency is strongly enhanced. For
ribbons of width M = 5 dimer lines, room-temperature ZT is enhanced from less
than 0.26 for defect-free ribbons to more than 2.5. This study is likely to set
the milestones of a new generation of nano-devices with dual electronic,
thermoelectric functionalities.
