Tailoring the thermal transport properties of monolayer hexagonal boron nitride by grain size engineering

The grain size effect on the thermal transport properties of hexagonal boron nitride (h-BN) thin films was experimentally investigated using the opto-thermal Raman technique. High-quality monolayer h-BN with mean grain sizes ranging from ~7 µm to ~19 nm were successfully synthesized on Pt foil by chemical vapor deposition (CVD). The thermal conductivity (κ) of the single-crystalline h-BN was measured to be ~545 Wm⁻¹ K⁻¹ at 315K, well above the bulk value, and more than a factor of four higher than the value of poly-crystalline h-BN with mean grain size of ~19 nm. The very low thermal boundary conductance (deduced to be ~9.6 GW m⁻² K⁻¹) accounts for the significant reduction of κ for h-BN with small grain size. Molecular dynamics (MD) simulations reveal that due to the disordered vibrations of atoms along/near GB, the phonon scattering in poly-crystalline h-BN is greatly enhanced compared to large-grained or single-crystalline samples. These results provide a deep understanding of the thermal transport in two-dimensional systems as well as the possible technological applications. (paper)