Direct visualization of thermal conductivity suppression due to enhanced phonon scattering near individual grain boundaries.

Understanding the impact of lattice imperfections on nanoscale thermal transport is crucial for diverse applications ranging from thermal management to energy conversion. Grain boundaries (GBs) are ubiquitous defects in polycrystalline materials, which scatter phonons and reduce thermal conductivity (kappa). Historically, their impact on heat conduction has been studied indirectly through spatially averaged measurements, that provide little information about phonon transport near a single GB. Here, using spatially resolved time-domain thermoreflectance (TDTR) measurements in combination with electron backscatter diffraction (EBSD), we make localized measurements of kappa within few mu m of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in kappa from , similar to 1000 W m(-1) K-1 at the center of large grains to similar to 400 W m(-1) K-1 in the immediate vicinity of GBs. Furthermore, we show that this reduction in kappa is measured up to similar to 10 mu m away from a GB. A theoretical model is proposed that captures the local reduction in phonon mean-free-paths due to strongly diffuse phonon scattering at the disordered grain boundaries. Our results provide a new framework for understanding phonon-defect interactions in nanomaterials, with implications for the use of high-kappa polycrystalline materials as heat sinks in electronics thermal management.