Abnormally High Thermal Conductivity in Fivefold Twinned Diamond Nanowires

Fivefold twins (5FTs), discovered nearly 200 years ago, are a common multiply twinned structure that usually dramatically deteriorate the thermal transport properties of nanomaterials. Here, we report the anomalous thermal conductivity ($\kappa$) in a novel fivefold twinned diamond nanowires (5FT-DNWs). The $\kappa$ of 5FT-DNWs is effectively enhanced by the defects of 5FT boundaries, and non-monotonically changes with the cross-sectional area ($\textit{S}$). Above the critical $\textit{S}$ = 7.1 nm$^{2}$, 5FT-DNWs show a constant value of $\kappa$, whereas below it, there appears a sharp increase in $\kappa$ with decreasing $\textit{S}$. More importantly, 5FT-DNWs with minimal $\textit{S}$ show a superior $\kappa$ over the bulk diamond. By confirming the Normal-process-dominated scattering event, it is demonstrated that the phonon hydrodynamic behavior plays a determinative role in abnormally high $\kappa$ of 5FT-DNWs with small $\textit{S}$. The super-transported phonon hydrodynamic phenomenon unveiled in the twinned diamond nanowires may provide a new route for pursuing highly thermally conductive nanomaterials.