  Graphene, in addition to its unique electronic and optical properties,
revealed unusually high thermal conductivity. The fact that thermal
conductivity of large enough graphene sheets should be higher than that of
basal planes of bulk graphite was predicted theoretically by Klemens. However,
the exact mechanisms behind drastic alteration of material's intrinsic ability
to conduct heat as its dimensionality changes from 2-D to 3-D remain elusive.
Recent availability of high-quality few-layer graphene materials allowed us to
study dimensional crossover experimentally. Here we show that the
room-temperature thermal conductivity changes from K~3000 W/mK to 1500 W/mK as
the number of atomic plains in few-layer graphene increases from 2 to 4. We
explained the observed evolution from 2-D to bulk by the cross-plane coupling
of the low-energy phonons and corresponding changes in the phonon Umklapp
scattering. The obtained results shed light on heat conduction in
low-dimensional materials and may open up few-layer graphene applications in
thermal management of nanoelectronics.
