Ultra-broadband Epsilon-Near-Zero Ultrathin Metafilms Assembled by 2D Nanosheets Overcoming Size Limitation in Metamaterials.

Broadband epsilon-near-zero (ENZ) materials are crucial for 6G (sixth generation mobile networks) signal stability, biochemical sensor accuracy, and intelligent weapon systems. Here, the ultrathin (d∼10 µm) 2D metafilms achieving low-frequency ultra-broadband ENZ (10 kHz-1 MHz, d/λ∼10-10) are constructed via the self-assembly of nanosheets (MXene and graphene oxide) and interface engineering. High-resistance interfaces and wrinkles can block free carriers, suppressing long-range plasma and Drude-type negative dielectric response, while these carriers tend to accumulate locally at the interfaces and enhance Debye-type positive dielectric response. These two pathways achieve the ultra-broadband ENZ behavior via the synergistic cancellation effect between these two responses with comparable intensity and dispersion characteristics. The ENZ (|ε'|<1) with a 100-fold ultrabroad bandwidth (10 kHz-1 MHz) is obtained, 10 kHz of ENZ also updates the low-frequency limit of ENZ in materials. Besides, the thickness of 10 µm is achieved in metafilms, a smallest d/λ value (∼10-10), overcoming the size limitation of ENZ media. The effectiveness of the ultra-broadband ENZ is verified in electromagnetic tunneling devices, where metafilms can replace complex artificial arrays. This work establishes a new material-genesis strategy for ultra-broadband ENZ rather than artificial metamaterial arrays, easily applicable across the entire electromagnetic spectra.