Interface formation energy, bonding, energy band alignment in α-NaYF₄ related core shell models: For future multi-layer core shell luminescence materials

To break through the bottle-neck of quantum yield in upconversion (UC) core-shell system, we elucidated that the energy transfer efficiency in core-shell system had an evident contribution from the charge transfer of interface with related to two factors: (1) band offsets and (2) binding energy area density. These two variables were determined by material intrinsic properties and core-shell thickness ratio. We further unraveled the mechanism of non-radiative energy transfer by charge transfer induced dipole at the interface, based on a quasi-classical derivation from Förster type resonant energy transfer (FRET) model. With stable bonding across the interface, the contributions on energy transfer in both radiative and non-radiative energy transfer should also be accounted together in Auzel's energy transfer (ETU) model in core-shell system. Based on the discussion about interface bonding, band offsets, and formation energies, we figured out the significance of interface bonding induced gap states (IBIGS) that played a significant role for influencing the charge transfer and radiative type energy transfer. The interface band offsets were a key factor in dominating the non-radiative energy transfer, which was also correlated to core-shell thickness ratio. We found that the energy area density with related to core/shell thickness ratio followed the trend of Boltzman sigmoidal growth function. By the physical trend, this work contributed a reference how the multi-layered core-shell structure was formed starting from the very beginning within minimum size. A route was paved towards a systematic study of the interface to unveil the energy transfer mechanism in core-shell systems.