Thickness-Dependent Cross-Plane Thermal Conductivity Measurements of Exfoliated Hexagonal Boron Nitride

Gabriel R. Jaffe; Keenan J. Smith; Kenji Watanabe; Takashi Taniguchi; Max G. Lagally; Mark A. Eriksson; Victor W. Brar

doi:10.1021/acsami.2c21306

Article Thickness-Dependent Cross-Plane Thermal Conductivity Measurements of Exfoliated Hexagonal Boron Nitride

Gabriel R. Jaffe ; Keenan J. Smith ; Kenji Watanabe (National Institute for Materials Science) ; Takashi Taniguchi (National Institute for Materials Science) ; Max G. Lagally ; Mark A. Eriksson ; Victor W. Brar

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Gabriel R. Jaffe, Keenan J. Smith, Kenji Watanabe, Takashi Taniguchi, Max G. Lagally, Mark A. Eriksson, Victor W. Brar. Thickness-Dependent Cross-Plane Thermal Conductivity Measurements of Exfoliated Hexagonal Boron Nitride. ACS Applied Materials & Interfaces. 2023, 15 (9), 12545-12550. https://doi.org/10.1021/acsami.2c21306

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Sub-micron-thick layers of hexagonal boron nitride (hBN) exhibit high in-plane thermal conductivity, useful optical properties, and serve as dielectric encapsulation layers with low electrostatic inhomogeneity for graphene devices. Despite the promising applications of hBN as a heat spreader, the thickness dependence of its cross-plane thermal conductivity is not known, and the cross-plane phonon mean free paths (MFPs) have not been measured. We measure the cross-plane thermal conductivity of hBN flakes exfoliated from bulk crystals. We find that sub-micron thick flakes exhibit thermal conductivities up to 8.1 ± 0.5 W m−1K−1 at 295 K, which exceeds previously reported bulk values by more than 60%. Surprisingly, the average phonon mean free path is found to be several hundred nanometers at room temperature, a factor of five larger than previous predictions. When planar twist interfaces are introduced into the crystal by mechanically stacking multiple thin flakes, the cross-plane thermal conductivity of the stack is found to be a factor of seven below that of individual flakes with similar total thickness, thus providing strong evidence that phonon scattering at twist boundaries limits the maximum phonon MFPs. These results have important implications for hBN integration in nanoelectronics and improve our understanding of thermal transport in two-dimensional materials.

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