Semiconducting and magnetic lanthanide MXenes from intercalated halides

Two-dimensional (2D) magnetic semiconductors are crucial for next-generation information storage and spintronic technologies^1,2. MXenes, owing to compositional diversity and tunable properties, provide a platform for designing functional materials^3-5. Incorporating lanthanides (Ln) introduces localized 4f electrons with strong spin polarization, while potentially enabling semiconducting behaviour, offering a viable route to magnetic semiconductors^6,7. However, the scarcity of MAX precursors and the susceptibility of Ln to dissolution in common etchants (for example, HF), compared with other M elements such as Mo, hinder the synthesis of lanthanide MXenes (Ln₂CT₂) by conventional 'top-down' etching⁸. Here we propose a general 'bottom-up' methodology for synthesizing Ln₂CT₂ (Ln = Gd, Tb, Dy, Ho, Er, Lu; T = Cl, Br) using layered halides as van der Waals building blocks. Multilayer Ln₂CT₂ exhibits composition-tunable properties, characterized by optical absorption onsets spanning 1.26-1.71 eV, room-temperature resistivity of 0.329-36.1 Ω cm with a negative temperature coefficient, and low-temperature ferromagnetic hysteresis at 2 K accompanied by positive Curie-Weiss temperatures between 6 K and 59 K. Theoretical calculations show that the d-electron states around the Fermi level (E_f) are largely diminished in bare Ln₂C, whereas surface terminals further exhaust these states to open band gaps. Meanwhile, the highly localized 4f electrons in Ln₂CT₂, located far from the E_f, contribute to the spin splitting for the observed ferromagnetic behaviour. This combination of semiconducting and magnetic properties makes Ln₂CT₂ a valuable candidate for spintronic device applications.