Dong Wen, Qianqian Fan, Can Cui, Haodong Jiang, Zhengdong Qiu, Gui-Gen Wang
The development of green MXene inks and inkjet-printed electromagnetic interference (EMI) shielding films has become a hot topic in MXene research. Understanding the "processing-structure-performance" roadmap of MXene ink is essential for the practical implementation of this emerging material. Herein, a green and additive-free γ-valerolactone (GVL)/Ti3C2Tx MXene ink is designed. The GVL/MXene ink exhibits tunable rheology, excellent inkjet printability, and outstanding anti-oxidation stability. It enables the fabrication of highly conductive ultrathin films with precisely controlled thickness (4-51 nm), a DC conductivity of 24,800 S·m-1, and a microwave conductivity of ≈105 S·m-1. A printed film only 51 nm thick delivers an EMI shielding effectiveness (SE) of 11 dB, with an absolute SE per thickness as high as 215.7 dB·μm-1 superior to most reported MXene-based shielding films. By combining Drude and transfer matrix model analyses, we uncover a key mechanism: the conventional DC impedance matching theory completely fails for such ultrathin films. Instead, the microwave response is dominated by the AC sheet conductivity. The thickness modulates only the carrier-concentration-related DC sheet conductivity (σ0) without changing the average electron relaxation time (τ ≈ 22.3 ps), corresponding to ωτ ≈ 1 in the X-band regime. This places the system in a unique critical-damping state, which enables a cooperative reflection-absorption shielding mechanism. This work provides a green, scalable route for MXene printed electronics and offers a refined theoretical framework for microwave shielding of 2D conductive films.