  Materials with high thermal conductivities (k) is valuable to solve the
challenge of waste heat dissipation in highly integrated and miniaturized
modern devices. Herein, we report the first synthesis of atomically thin
isotopically pure hexagonal boron nitride (BN) and its one of the highest k
among all semiconductors and electric insulators. Single atomic layer (1L) BN
enriched with 11B has a k up to 1009 W/mK at room temperature. We find that the
isotope engineering mainly suppresses the out-of-plane optical (ZO) phonon
scatterings in BN, which subsequently reduces acoustic-optical scatterings
between ZO and transverse acoustic (TA) and longitudinal acoustic (LA) phonons.
On the other hand, reducing the thickness to single atomic layer diminishes the
interlayer interactions and hence Umklapp scatterings of the out-of-plane
acoustic (ZA) phonons, though this thickness-induced k enhancement is not as
dramatic as that in naturally occurring BN. With many of its unique properties,
atomically thin monoisotopic BN is promising on heat management in van der
Waals (vdW) devices and future flexible electronics. The isotope engineering of
atomically thin BN may also open up other appealing applications and
opportunities in 2D materials yet to be explored.
