Terahertz dielectric properties and low-frequency phonon Resonances of ZnO nanostructures

Far-infrared optical and dielectric properties of nanostructured ZnO of different morphologies are characterized by terahertz time-domain spectroscopy. Frequency-dependent complex dielectric function, power absorption, and refractive index are experimentally measured in the terahertz regime. The results are analyzed and well fit with dielectric theories combined with effective medium models. Different ZnO nanostructure morphologies exhibit different characteristics due to diverse resonance mechanisms dominated by either free electrons or phonons. The dielectric function of tubular and prismlike ZnO structures exhibits the Drude-like behavior, while it is dominated by the low-frequency phonon resonances for nanowires. Additionally, the overall low-frequency phonon resonances of these nanostructures are measured by Raman scattering spectroscopy, showing good consistency with those of bulk wurtzite single-crystal ZnO.