  In-plane thermal conduction and phonon transport in both single-crystalline
and polycrystalline Si two-dimensional phononic crystal (PnC) nanostructures
were investigated at room temperature. The impact of phononic patterning on
thermal conductivity was larger in polycrystalline Si PnCs than in
single-crystalline Si PnCs. The difference in the impact is attributed to the
difference in the thermal phonon mean free path (MFP) distribution induced by
grain boundary scattering in the two materials. Grain size analysis and
numerical simulation using the Monte Carlo technique indicate that grain
boundaries and phononic patterning are efficient phonon scattering mechanisms
for different MFP length scales. This multiscale phonon blocking structure
covers a large part of the broad distribution of thermal phonon MFPs and thus
efficiently reduces thermal conduction.
