Hierarchical ZnO clusters for high-performance UV photodetection

Hierarchical porous zinc oxide (ZnO) architectures offer a powerful route toward low-voltage visible-blind ultraviolet (UV) photodetectors because they enable simultaneous control over optical attenuation, surface depletion, and carrier transport. Here, we introduce a ZnO-on-ZnO structural optimization strategy that combines self-assembly deposition of a porous dendrite-like nanocluster film (DNC), capillary-force-driven restructuring into nano-micro cluster arrays (NMCAs) and controlled secondary ZnO overcoating. Structural and spectroscopic analyses show that this sequential overcoating preserves the wurtzite ZnO phase, crystallite size, and basic optical characteristics, while profoundly restructuring morphology, light propagation, and charge-transport behavior. Increasing ZnO overcoating promotes visible-range light scattering and enhances photocurrent generation; however, the highest ZnO loading does not yield optimal device performance. Instead, intermediate overcoating conditions produce the optimal balance between transport connectivity and oxygen-mediated surface processes, yielding a responsivity of 910 mA W^-1 at 340 nm, a maximum current contrast of ca. 700, and stable low-voltage cycling performance at 0.5 V. These findings establish hierarchical ZnO-on-ZnO overcoating as an effective design framework for high-performance UV photodetectors and reveal that co-optimization of intercluster percolation and oxygen-mediated surface processes is critical for maximizing device response.