Ideal electromagnetic (EM) wave absorbers can absorb all incident EM waves, regardless of the incident direction, polarization, and frequency. Absorptance and reflectance are intrinsic material properties strongly correlated with electrical conductivity; hence, achieving perfect absorptance with zero reflectance is challenging. Herein, we present a design strategy for preparing a nearly ideal EM absorber based on a layered metal that maximizes absorption by utilizing multiple internal reflections and minimizes reflection using a monotonic gradient of intrinsic impedance. This approach was experimentally verified using aluminum nanoflakes prepared via topochemical etching of lithium from Li9Al4, and the impedance-graded structure was obtained through the size-based sorting behavior of aluminum nanoflakes sinking in dispersion. Unlike in traditional shielding materials, strong absorption (26.76 dB) and negligible reflectivity (0.04 dB) with a ratio of >103 can be achieved in a 120 μm thick film. Overall, our findings exhibit potential for developing a novel class of antireflective shielding materials.
Keywords: Aluminum nanoflake; Antireflection; EMI shielding; Electromagnetic absorber; Metamaterial perfect absorber.