Tuning of UV-absorbing performance in high-transmittance ZnO nanostructured thin films: a study of thickness-dependent optical properties

Introduction: Ultraviolet (UV) radiation causes photodegradation in a wide range of materials including polymers, paints, fabrics and solar devices, and it is also widely recognized as a major health hazard. Zinc oxide (ZnO) thin films are promising UV-blocking materials owing to their wide direct bandgap (~3.37 eV), high visible-light transmittance (~80–90%), large exciton binding energy (60 meV) and high electron mobility, making them suitable for protective coating appliances. The main objective of this study was to investigate the thickness-dependent optical properties of ZnO nanostructured thin films deposited by pulsed direct current (DC) magnetron sputtering and to identify the optimal deposition parameters to achieve the best trade-off between UV absorption and visible-light transmission. Materials and methods: ZnO thin films were deposited on a wide range of substrates which included glass, silicon, quartz and fluorinated ethylene propylene (FEP) substrates by pulsed DC magnetron sputtering at three deposition times (5, 10 and 15 min) and two O2 flow rates (1 and 3 sccm) at a constant power of 100 W and bias voltage of ~75 V. Films were characterized using characterization techniques like X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), 3D profilometry, Ultraviolet-Visible-Near Infrared (UV-Vis-NIR) spectroscopy and spectroscopic ellipsometry. Results: All deposited films exhibited near-zero transmittance below 380 nm and average visible transmittance of around 80–90%, confirming effective UV blocking. XRD confirmed dominant (002) preferential c-axis orientation across all samples and substrates. AFM showed a progressive increase in Ra roughness from 11.4 nm (Sample A) to 13.6 nm (Sample C). Spectroscopic ellipsometry gave a refractive index of ~1.95 at 550 nm for the optimized film. The Tauc plot analysis yielded optical bandgaps of 3.06 eV and 3.16 eV, respectively, for sample A and sample B. The film deposited for 10 min and O2 flow rate of 3 sccm (Sample B) achieved the best trade-off between UV absorption and visible-light transmission, while Sample C showed reduced substrate adhesion attributed to residual compressive stress at greater thicknesses. Conclusions: Among the conditions studied, the ZnO thin film deposited with deposition parameters having O2 flow rate of 3 sccm and deposition time of 10 min (Sample B) yielding a thickness of ~225 nm exhibited the optimal UV blocking performance. This thin film demonstrated near-zero transmittance below 380 nm and average visible transmittance of around 80–90%. These results demonstrated that pulsed DC magnetron sputtering with controlled deposition time and oxygen flow rate as an effective route to tunable ZnO UV-blocking coatings for polymer, solar device and optoelectronic applications.