Thermal conductivity reduction in silicon nanotubes through phonon localization

Thermoelectric devices must suppress heat conduction while maintaining efficient charge transport. Nanostructuring is widely used to reduce lattice thermal conductivity (κ) by increasing surface-to-volume ratio (S/V) and enhancing phonon-boundary scattering. However, conventional approaches that rely on aggressive downscaling can degrade charge transport, so there is an intrinsic trade-off between reducing κ and maintaining electrical performance. In this study, we reveal a κ suppression mechanism in silicon nanotubes (NTs), which is distinct from boundary scattering and remains effective independent of the absolute size of the structure. While Monte Carlo simulations show that S/V dominates κ across the nanostructures, measurements show a remarkable 33 % reduction in κ for NTs compared to nanowires (NWs) even at the same S/V, suggesting an additional phonon transport regime that cannot be captured by conventional boundary scattering models. To investigate the origin of this discrepancy, harmonic lattice dynamics analysis was performed to quantify phonon localization. A significant improvement in mode localization is observed in NTs relative to NWs, which is consistent with the additional κ reduction in NTs. This previously unexplored form of phonon localization persists beyond hundreds of nanometers, establishing hollow-core architecture as a practical thermoelectric design platform to suppress κ without severe dimensionality reduction.