Superlattice films of semiconducting oxide and rare-earth hydroxide nanosheets for tunable and efficient photoluminescent energy transfer

Abstract

Europium and terbium doped layered gadolinium hydroxides were prepared by microwave assisted hydrothermal precipitation. They were subsequently exfoliated into nanosheets by sonication treatment in formamide. The thickness of the nanosheets (LGdH:Eu and LGdH:Tb) was found to be approximately 1 nm, exemplifying a single-layer feature. Multilayer and superlattice films were prepared through layer-by-layer (LbL) deposition of exfoliated hydroxide nanosheets with a polyanionic electrolyte (polystyrene sulfonate, PSS) and heteroassembly with semiconducting oxide nanosheets (Ti_0.87O₂^0.52− and TaO³⁻), respectively. Compared to the multilayers of (LGdH:Eu/PSS)_n and (LGdH:Tb/PSS)_n, the superlattices of (LGdH:Eu/Ti_0.87O₂^0.52−)_n and (LGdH:Tb/TaO³⁻)_n exhibited significantly enhanced photoluminescence intensity, ∼14 times and ∼5 times, respectively. The photoenergy absorbed by the semiconducting nanosheets can be transferred to the excited states of rare-earth hydroxide nanosheets for enhanced photoluminescence emission. Further investigation on the stacking sequence of the nanosheets revealed that direct neighboring and energy level matching with semiconducting nanosheets was essential for realizing efficient energy transfer across the nanosheet interface. Annealing at 600 °C could further enhance the emission intensity of the superlattice structured films. The current work demonstrates an important strategy for hetero-assembling nanosheets at the molecular level with a carefully designed interface for tunable and enhanced functionalities.