Ion-Engineered Insulator-to-Semiconductor Transition in Natural 2D Biotite
Naturally occurring layered silicates offer an abundant yet unexplored class of 2D materials, but their insulating nature limits their functional utility. Here, we demonstrate a chemical strategy that transforms liquid-phase-exfoliated biotite nanosheets into a tunable 2D semiconductor through controlled NaOH treatment. The resulting insulator-to-semiconductor transition originates from Na incorporation, defect generation, and local structural reconstruction while largely preserving the layered framework. Structural and chemical analyses reveal lattice distortion, interlayer reorganization, hydroxylation, and partial Na+-K+ exchange, establishing the origin of the electronic restructuring. This transformation broadens the optical response, shifting the approximately 221 nm absorption toward approximately 280 and 975 nm, reducing the optical bandgap from approximately 5.2 to 3.2-3.5 eV, and introducing low-energy transitions at approximately 1.12-1.17 eV. Electrical measurements reveal nonlinear transport with currents reaching close to 10 microA, demonstrating activated carrier conduction. Ultrafast transient absorption reveals pronounced excited-state absorption, with carrier cooling (0.16-0.38 ps) followed by fast (35-60 ps) and long-lived (336-491 ps) relaxation associated with trap-mediated recombination. Fluence-dependent dynamics reveal a hot-phonon bottleneck at elevated carrier densities. Together with density functional theory calculations, these results establish chemical defect and ion engineering as a powerful route for converting naturally abundant layered minerals into electronically tunable 2D materials for emerging optoelectronic and ultrafast photonic technologies.