Moiré Superlattices of Zn ²⁺ Mediated 2D Assembly of Mn ²⁺ -Cysteine Complex Nanoparticles Alter Electron Spin Transitions in the X-Band

We report the generation of 2D moiré superlattices in the aqueous medium and the influence of a new field arising at the interfaces of the crystals on the electronic spin states of constituent Mn2+ ions. Ambient reaction-mediated assembly of Mn2+-cysteine complex (MnCys) nanoparticles (≤4 nm) with Zn2+ ions led to the generation of nanoscale 2D films with moiré and supermoiré patterns. Transmission electron microscopy revealed the moiré pattern generation by stacking crystalline 2D nanoparticle films rotated relatively at angles that varied from 1° to 13°. Contrary to the weakly fluorescent precursor nanoparticles (NPs), the moiré superlattices exhibited visible delayed fluorescence with a lifetime of 35.6 ± 5.0 μs. Electron spin resonance (ESR) spectroscopy indicated the presence of two chemically different types of Mn2+ ions–one possibly belonging to the nanoparticles and the other to the film. The ESR signal of the Mn2+ ion was found to be altered, and the nuclear spin selection rule was relaxed, resulting in the hyperfine splitting of the central electronic transition to split further. Four noncentral electronic transitions in the X-band ESR spectrum were observed upon the formation of moiré superstructures, indicating alteration in the zero field splitting (ZFS) term (|D|) of Mn2+ to be as low as 0.026 ± 0.0015 cm–1.