  Using first-principles density functional perturbation theory based
calculations of length-dependent lattice thermal conductivity (\k{appa} L ) and
using our previously calculated results (Phys Rev B 95 085435 (2017)) of
electrical transport, we report results of thermoelectric figure-of-merit (ZT )
of monolayer and bilayer Graphene. We find nearly ten-fold increase in ZT for
the graphene sample doped with boron nitride and reduced sample length. We also
compare \k{appa} L calculated using the iterative real space method with
conventional analytical Callaway-Klemens method and obtain the flexural (ZA)
phonon modes to be dominant in thermal transport unlike in the latter method.
Our calculations are in good agreement with available experimental data.
