Quantifying Structural Order and Its Effect on Absorption in Structurally Colored Colloidal Assemblies

Structural colors arise from wavelength-selective scattering in micro/nanostructures. The structural order governs the color brightness, saturation, and iridescence, but there is not yet a generalized framework to quantify the structural order. It remains unclear how the structural order quantitatively affects the reflection and absorption in structurally colored materials. Here, we co-assemble binary silica nanoparticles to tune the degree of order and quantify its impact on reflection and absorption. By varying compositions and particle size ratios, we can adjust the degree of order from long-range crystalline to short-range ordered states. Combining reflectance spectroscopy with ultra-small-angle X-ray scattering measurements, we establish an experimentally order parameter to distinguish long-range ordered, transitional, and short-range ordered regimes. Correlating structural order with optical absorption reveals that increasing disorder enhances scattering events and leads to larger absorption. In addition, increasing particle size further increases absorption by reducing the transport mean free path. This work demonstrates that the particle-size-ratio engineering serves as an effective route to tune the structural order, offering a general approach for tailoring both color appearance and absorption efficiency for designing novel photonic materials.