Zinc oxide (ZnO) nanostructures are promising platforms for applications in optoelectronics, photocatalysis, and nanoscale transducers, owing to their wide direct bandgap (3.37 eV), high exciton binding energy (60 meV), and large exciton–phonon coupling [1]. In this work, we investigate both ensembles and individual ZnO microrods and nanowires -typically ~10 μm in length and ~2 μm in diameter for microrods, and ~2 μm in length and ~100 nm in diameter for nanowires-, to elucidate their relaxation dynamics following femtosecond excitation near or above the bandgap. Microrods were obtained via a simple hydrothermal method, while nanowires were grown via vapor transport deposition. Structural and morphological characterization was performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical response was probed using steady-state photoluminescence (PL) with a 325 nm He–Cd laser, and ultrafast transient absorption spectroscopy with 325 nm and 400 nm pump pulses and broadband or 800 nm probe.

Preliminary time-resolved measurements on ZnO microrods reveal complete electronic relaxation within the first ~300 ps, consistent with rapid recombination dynamics. Data analysis is ongoing to identify additional relaxation pathways, including phonon-mediated channels.
We aim to resolve coherent acoustic phonons in these nanostructures, expected to appear as oscillatory modulations in the transient signal due to strain-induced refractive index changes. We are conducting pump–probe experiments on ZnO nanowires resting directly on substrates, where mechanical coupling to the underlying material may lead to enhanced acoustic damping. Unlike suspended nanowires, which have been extensively studied in the context of GHz mechanical vibrations, substrate-supported systems remain less explored in terms of their vibrational dynamics. Our ongoing efforts aim to compare different configurations—supported vs. suspended nanowires—on various substrates, in order to elucidate the interplay between electronic losses and phonon coherence. These comparisons are expected to provide insight into substrate-induced dissipation and guide the integration of nanowire-based systems in realistic optoelectronic and phononic devices. We also plan to extend these studies to SnS and SnS₂ nanostructures—semiconductors with strong anisotropy and potentially distinct phonon dynamics, broadening the scope of time-resolved phononics with relevance for future optoelectronic and phononic devices.

[1] Raha, S., & Ahmaruzzaman, M. (2022). ZnO nanostructured materials and their potential applications: progress, challenges and perspectives. Nanoscale Advances, 4(8), 1868-1925.