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Abstract Iridescent color-shift pigments have been used in some industrial applications, e.g., for cosmetics and packaging. To achieve environmental-friendly and lasting color, thin-film interference is used to generate structural color. By maximizing the refractive index (RI) difference between the thin films (i.e., using an ultralow RI film), super-iridescent structural color can be produced. While the lowest refractive index of a naturally occurring solid dielectric is close to 1.37 (i.e., MgF₂), we synthesized highly porous dielectric SiO₂aerogel to achieve ultralow-RI (n ~ 1.06) and demonstrated a high-refractive index/low-refractive index/absorber (HLA) trilayer structural color. The achieved structural color is highly iridescent and capable of tracing a near-closed loop in CIE color space. By tuning the refractive index, thickness, and geometry of the aerogel layer, we control the reflection dip’s shape, therefore producing a wide range of vivid and iridescent colors. 

Rutile germanium oxide (r-GeO2), an ultrawide bandgap semiconductor, is a promising material for next-generation power electronics. Understanding and controlling the structure and morphology of r-GeO2 thin films are crucial for advancing their integration into future devices. In this work, r-GeO2 thin films were grown using RF magnetron sputtering on r plane sapphire substrates. Postdeposition annealing (PDA) was performed in an oxygen ambient atmosphere to crystallize the films. PDA at 950 °C resulted in the formation of needle-like nanostructures, predominantly originating at the edges of the film and growing inward toward the sample center. Sequential annealing at increasing temperatures indicated that these needle-like structures begin forming at temperatures above 925 °C. Next, the effect of the PDA duration on the structure was studied. It was seen that PDA at 950 °C for durations of 1 to 15 min promoted formation of the rutile phase, and extending the PDA duration allowed greater surface coverage of the nanostructures. However, annealing even longer, i.e., for 120 min, resulted in mixed phases of α-quartz and rutile GeO2. These findings demonstrate that controlling the PDA temperature and duration can effectively modulate the morphology of rutile-phase GeO2 thin films.