Extreme reduction of thermal conductivity by embedding Al₂O₃ nanoparticles into single-crystalline Bi nanowires

Al₂O₃ nanoparticles-embedded single-crystalline Bi nanowires were successfully synthesized without using any templates via the spontaneous growth of Bi thin films covered with Al₂O₃ nanoparticles. It is experimentally confirmed that Al₂O₃ nanoparticles were embedded into the single-crystalline Bi nanowires by using the high-resolution transmission electron microscopy and energy dispersive X-ray spectrometry. The temperature-dependent thermal conductivities of individual Al₂O₃ nanoparticles-embedded single-crystalline Bi nanowires were measured directly using suspended micro-devices. The thermal conductivities of the Al₂O₃ nanoparticles-embedded Bi nanowires were found to be extremely low compared with those of pure Bi nanowires of similar diameters. Moreover, the thermal conductivity of the Al₂O₃ nanoparticles-embedded Bi nanowires was not size-dependent, i.e., it did not vary for nanowires with significantly different diameters. This result suggests that the phonon-boundary scattering is not the dominant phonon scattering mechanism in these systems while the phonon-boundary scattering is dominant in the pure single-crystalline nanowires. From the experimental measurements and theoretical calculation, these drastic reduction and unique tendency in the thermal conductivities of Al₂O₃ nanoparticles-embedded Bi nanowires were explained by the combined effect of the phonon-boundary scattering and impurity scattering occurring between the embedded Al₂O₃ nanoparticles and Bi matrix.