  Heat management becomes more and more critical, especially in miniaturized
modern devices, so the exploration of highly thermally conductive materials
with electrical insulation and favorable mechanical properties is of great
importance. Here, we report that high-quality monolayer boron nitride (BN) has
a thermal conductivity (\k{appa}) of 751 W/mK at room temperature. Though
smaller than that of graphene, this value is larger than that of cubic boron
nitride (cBN) and only second to those of diamond and lately discovered cubic
boron arsenide (BAs). Monolayer BN has the second largest \k{appa} per unit
weight among all semiconductors and insulators, just behind diamond, if density
is considered. The \k{appa} of atomically thin BN decreases with increased
thickness. Our large-scale molecular dynamic simulations using Green-Kubo
formalism accurately reproduce this trend, and the density functional theory
(DFT) calculations reveal the main scattering mechanism. The thermal expansion
coefficients (TECs) of monolayer to trilayer BN at 300-400 K are also
experimentally measured, and the results are comparable to atomistic ab initio
DFT calculations in a wider range of temperatures. Thanks to its wide bandgap,
high thermal conductivity, outstanding strength, good flexibility, and
excellent thermal and chemical stability, atomically thin BN is a strong
candidate for heat dissipation applications, especially in the next generation
of flexible electronic devices.
