Measurement and analysis of low-temperatures thermal properties of low-dimensional materials -

Abstract

In this work, we report calorimetric measurements of the specific heat of Zinc Oxide nanowires and graphene. We also determine the thermal conductance of the interface between the vertically aligned Zinc Oxide nanowires and Silicon substrate. Heat-pulse calorimetric technique was applied to free-surface Zinc Oxide nanowires and clamped graphene samples. The temperature response of the heat-pulse calorimeter was analyzed by a model that takes into account the effect of the thermal conductance between the nanowires and the substrate. The specific heat of the samples and the thermal conductance of the interface were determined from 1.8 to 300 K using a linear least squares method. It is found that the low temperatures behavior (below 4 K) of the Zinc Oxide nanowires' specific heat admits a two-dimensional crystal contribution in addition to the bulk T³ dependence. Above 25 K, The specific heat of the nanowires is enhanced compared to that of the bulk Zinc Oxide and this enhancement increases as the temperature increases and the nanowires diameter decreases. As for the thermal conductance of the interface between the Silicon substrate and Zinc Oxide nanowires, it is found to be orders of magnitude lower than that between bulk ZnO and Si substrate, suggesting the formation of thin layer of low crystallinity between the nanowires and Silicon substrate. Furthermore, the recorded data demonstrated transition from specular to diffusive elastic transmission, and then from diffusive elastic to diffusive inelastic transmission as temperature increases. Also, the specific heat of clamped graphene at low-temperatures was found to follow a two-dimensional behavior confirming the T² dependence theories reported in the theoretical models. This specific heat of graphene shows an increase in its values at higher temperatures in comparison with graphite.