Decoupling the inverse κ – E g trade-off in 2D organic–inorganic magnetic semiconductors

Simultaneously achieving a high dielectric constant (κ) and a wide bandgap (Eg) in a single material remains challenging. Here, we report a charged-building-unit stacking strategy that yields tetragonal β-Fe3Se4-based organic–inorganic semiconductors. These materials exhibit both high κ values and a wide Eg with negligible loss, effectively decoupling the classic κ–Eg trade-off. Specifically, semiconductors intercalated with iron–amine complexes, such as (β-Fe3Se4)4[Fe(teta)1.5] (teta = triethylenetetramine) and (β-Fe2.75Se4)4[Fe(peha)] (peha = pentaethylenehexamine), achieve a high κ of ∼170 and Eg of ∼1.9 eV. First-principles calculations reveal that, in the parent inorganic β-Fe3Se4, the ionic contribution (ɛion) is weak and the electronic contribution (ɛele) is ∼42. In contrast, ɛion is greatly enhanced in the model (β-Fe3Se4)4[Fe(peha)], giving a high-frequency dielectric constant (κ∞) of 145 due to ionic polarization as the dominant contribution. High κ of the present organic–inorganic semiconductors should be attributed primarily to ionic polarization between anionic β-Fe3Se4 fragments and cationic metal–amine complexes. This ionic polarization mechanism is further corroborated by the two-order-of-magnitude suppression of κ observed when amine-uncoordinated metal ions are introduced into the organic spacer layers in (β-Fe2.5M0.5Se4)4[M(peha)]0.7M1.3 (M = Fe3+). Polarization in (β-MN2Se4)4[N(teta)1.5]N (N = Fe2+/Co2+) is further overwhelmed by conductivity. These results establish charged-building-unit stacking as an effective strategy to decouple the κ–Eg trade-off in 2D β-Fe3Se4-based organic–inorganic semiconductors and highlight the potential of them as a versatile platform for tunable dielectric properties.