Tunable thermal response of ZnO nanowires

ZnO nanowires with the growth orientation undergo a reversible phase transformation from wurtzite (WZ) to a graphitic phase (HX) under tensile loading, leading to a pseudoelastic behavior with recoverable strains up to 16%. Here, we report that this phase transform causes a novel transition in thermal response. Molecular dynamics simulations with the Green–Kubo approach are carried out to determine the thermal responses of wires in the 18.95–40.81 A size range. Results obtained show that the thermal conductivity in the unstressed WZ state is 8.3–8.6 W m−1 K−1 for the sizes considered and is an order of magnitude lower than the corresponding bulk value. Under loading, elastic stretching and the formation of interfaces cause the thermal conductivity to first decrease significantly. As the transformation progresses, the conductivity increases rapidly to 10.1–11.1 W m−1 K−1, or 20.5–28.5% higher than that of the initial WZ-structured wires. The enhancement is primarily due to an increase in atomic packing density, lower anharmonic coupling of phonons and higher surface specularities of the HX-structured wires. This phenomenon offers a means for tuning the thermal behavior of ZnO nanowires.