▎ 摘　　要

Low-dimensional materials attract extensive interest in electronic applications since the synthesis of graphene. Understanding the thermal transport in low-dimensional materials with shrinking characteristic size where strong confinement effect occurs is of importance for the thermal management of nanoelectronics. Recently, the atomically precise armchair graphene nanoribbons (AGNRs) with well-defined edges have been successfully synthesized. Serving as the fundamental functional elements, AGNRs can potentially make novel nanoelectronics realizable. Here we systematically investigate the thermal property variations of the ultranarrow AGNRs with width without hydrogen termination using the density-functional-based tight binding (DFTB) method, which combines the accuracy of density functional theory and the efficiency of tight-binding approximation. The lattice thermal conductivity increases unexpectedly from 531.7 to 3470.6 W/m-K as the width decreases from 0.97 to 0.35 nm, different from the width dependence in larger scales; the lattice constants, low frequency phonon group velocities and lifetimes, and acoustic phonon contributions also show increasing trends as the width decreases. Such behaviors are attributed to the changes in the lattice constants and the phonon scattering channels of the dominant low frequency acoustic phonons. Further DFTB calculations reveal that planar ultranarrow armchair BN nanoribbons also show analogous trends in thermal properties with the shrinking width. This study unveils the width-dependent phonon transport behaviors of ultranarrow planar nanoribbons and offers guidelines for the thermal design of potential nanoelectronics.