Phonon Confinement and Lattice-Level Thermal Energy Storage in Vertically Aligned ZnO Nanorod Arrays

The volumetric heat-storage capacity of a nanostructured solid is not strictly an intensive constant of its parent material. When the rod diameter becomes comparable to the mean free paths of the phonons that carry and store heat, confinement modifies the vibrational density of states, adds surface-localised modes and changes the balance between storage and transport. We report laboratory measurements on vertically aligned wurtzite ZnO nanorod arrays with diameters of 40–120 nm and aspect ratios of 15–40. Differential scanning calorimetry shows a specific-heat excess over bulk ZnO of +3.1 % (80 nm) and +7.8 % (40 nm) at 240 °C. Time-domain thermoreflectance on single rods gives an axial thermal conductivity of 8.6–27.3 W m⁻¹ K⁻¹, compared with 54 W m⁻¹ K⁻¹ for the bulk reference. This strong reduction at diameters of tens of nanometres is consistent with the broad phonon mean-free-path spectrum of ZnO, in which long-mean-free-path phonons (hundreds of nanometres) carry a substantial share of the heat. Isoelectronic point-defect doping lowers the conductivity by up to 60 % at a specific-heat change below 0.5 %, giving a transport lever that is largely independent of capacity. The temperature- and diameter-dependence of the Raman E₂(high) mode provides a non-destructive proxy for rod diameter. Laboratory-scale coupon cycling (4000 cycles between 50 and 350 °C) retained 99.87 % of the initial storage density. We discuss possible origins of the specific-heat excess, including surface and interface modes, adsorbed species and defects, and identify independent confirmation as a priority. All values are laboratory measurements. The laboratory test programme began on 4 April 2025 at laboratories in Tokyo and Incheon; first results were obtained on 13 February 2026.