Dielectric-environment tuning of surface plasmon resonance in ZnO/Au-NPs /ZnO nanostructures: Refractive-index sensitivity and optoelectronic figure of merit

Abstract

Localized surface plasmon resonance (LSPR) engineering has emerged as an effective strategy for tailoring the optical response of plasmonic nanostructures. In this work, ZnO/Au nanoparticle/ZnO nanostructures were fabricated by RF magnetron sputtering and thermal annealing, followed by the deposition of ZnO capping layers with thicknesses of 0, 5, 15, and 25 nm. Thermal treatment at 500 °C for 30 min induced the formation of plasmonically active Au nanoparticles through solid-state dewetting of the Au layer. The influence of ZnO capping-layer thickness on the structural, morphological, optical, electrical, and plasmonic properties was systematically investigated using XRD, SEM, UV–Vis spectroscopy, and Hall-effect measurements. Increasing the ZnO capping thickness promoted nanoparticle encapsulation and dielectric confinement, resulting in a pronounced LSPR red shift from 597 to 633 nm and an increase in peak absorption from 28.67% to 62.91%, in good agreement with the numerical data. The structures preserved the wide-bandgap characteristics of ZnO, with optical bandgaps close to 3.9 eV, while an apparent refractive-index sensitivity of approximately 43.7 nm/RIU was achieved. Moreover, the sheet resistance decreased from 95 to 58 Ω/sq and the electron mobility increased from 26 to 34 cm² V⁻¹ s⁻¹ using effective Hall mobility. A new optoelectronic–plasmonic figure of merit (FoM_OP) was introduced to evaluate the combined optical and electrical performance, showing an approximately 3.6-fold improvement for the 25 nm ZnO-capped structure. These findings demonstrate that ZnO capping-layer engineering provides a simple and scalable route for optimizing plasmonic and electrical properties, making these nanostructures promising for sensing, photonic, and optoelectronic applications.