Outstanding Thermal Conductivity of Single Atomic Layer Isotope-Modified Boron Nitride

Materials with high thermal conductivities ($\kappa$) are 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 $\kappa$ among all semiconductors and electric insulators. Single atomic layer ($1L$) BN enriched with ${}^{11}\mathrm{B}$ has a $\kappa$ up to $1009\mathrm{W/}\mathrm{mK}$ 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 phonons. On the other hand, reducing the thickness to a single atomic layer diminishes the interlayer interactions and hence umklapp scatterings of the out-of-plane acoustic (ZA) phonons, though this thickness-induced $\kappa$ 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 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.

Figure 1

(a) AFM image of a mechanically exfoliated $1L$ ${}^{10}\mathrm{BN}$ suspended over $\mathrm{Au}/\mathrm{Si}$ substrate with microwells and trenches. (b) AFM height of the $1L$ sample. (c) NEXAFS spectra in the B $K$-edge region of isotopically pure and natural BN at the incidence of 55°. (d) Selected area electron diffraction of a suspended $1L$ ${}^{10}\mathrm{BN}$. (e) The corresponding diffraction spot intensities and (f) the DF-TEM image.

Figure 2

(a) Comparison among Raman spectra of $1L–3L$ and bulk ${}^{10}\mathrm{BN}$, ${}^{\mathrm{Nat}}\mathrm{BN}$, and ${}^{11}\mathrm{BN}$; (b) temperature effect on the Raman $G$ band frequency of $1L$ ${}^{10}\mathrm{BN}$, ${}^{\mathrm{Nat}}\mathrm{BN}$, and ${}^{11}\mathrm{BN}$ sheets with corresponding fittings; (c) experimental $\kappa$ of $1L$ ${}^{10}\mathrm{BN}$ (red) and ${}^{11}\mathrm{BN}$ (blue) with standard deviations as a function of temperature, compared with that of $1L$ ${}^{\mathrm{Nat}}\mathrm{BN}$ (black and dashed); (d) the comparison among $\kappa$ of representative semiconductors and insulators.

Figure 3

(a) Theoretical cumulative $\kappa$ of $1L$ ${}^{10}\mathrm{BN}$ (99.2%), ${}^{11}\mathrm{BN}$ (99.9%), and ${}^{\mathrm{Nat}}\mathrm{BN}$ as a function of sample length. (b) Theoretical cumulative $\kappa$ of the same three materials as a function of phonon frequency, and the two phonon-frequency regions where $\kappa$ splits from different phonon contributions (see text for details) between $1L$ naturally occurring and monoisotopic BN are highlighted. (c) Phonon dispersion of $1L$ ${}^{10}\mathrm{BN}$ (red) and ${}^{11}\mathrm{BN}$ (blue) with the same phonon-frequency regions of $\kappa$ divergence highlighted. (d) Theoretical (open rhombus) and experimental (solid dots with standard deviations) $\kappa$ of $1L$ BN as a function of ${}^{10}\mathrm{B}$ concentration (i.e., 0.1%, 19.9%, 50%, 80%, and 99.2%), and corresponding parabolic fitting (dashed line).