Interference-driven structural colors in anodized niobium oxide thin films: mechanism analysis and multi-chromatic optical response

Abstract

Structural colors find wide applications in color printing, intelligent display, and filtering imaging. The perceived color of thin films can be modulated by interference effects. However, the relationship between structural color and film thickness, surface morphology, refractive index, and band gap remains insufficiently explored. Colored niobium oxide-niobium metal dense films with different thicknesses and surface morphologies were fabricated via anodic oxidation under varying durations and voltages, giving rise to two distinct coloration mechanisms: scattering-induced hues in thinner films (<81.2 nm) and interference-driven structural colors in thicker films (81.2–166.6 nm). To capture and quantify these changes, a smartphone-based colorimetric system was developed, enabling real-time monitoring of color evolution with high sensitivity. Using UV-visible–near-infrared spectroscopy, we systematically monitored the evolution of film thickness as a function of anodization time, revealing a pronounced redshift in both absorption and reflection peaks. Additionally, the band gap of the anodized films was precisely determined using the Tauc-plot method, establishing a clear link between anodization parameters and the electronic structure. These findings not only enhance the fundamental understanding of anodic oxidation processes but also underscore their potential for advancing optoelectronic device design and sensor applications.