Paper EN+NS-WeA7
Synthesis of Rare Earth Ion Co-Doped Core-Shell Nanostructures for Improved Energy Generation Efficiency

Wednesday, October 20, 2010, 4:00 pm, Room Mesilla

The development of rare-earth ion (RE) doped phosphors allows for the conversion of photons at various wavelengths to those at energies similar to that of the photovoltaic band gap. Work has been shown that through the incorporation Er³⁺ and Yb³⁺ into complex metal oxides, specifically yttrium based compounds, both energy upconversion and downconversion can be utilized to convert absorbed photons to a more desirable energy for Si based solar cells. However, both photoluminescence and energy transfer mechanisms are highly susceptible to the local crystal environment, including the overall crystal field and surrounding molecules. The addition of a shell layer increases luminescence by decreasing the effect of surface quenching sites while promoting energy transfer between layers.

This work focuses on the synthesis of core-shell nanostructures while controlling the luminescence spectrum through the spatial distribution within the particle architecture using a combination of wet chemical synthesis and atomic layer deposition (ALD). Y₂O₃: Er³⁺,Yb³⁺ core nanoparticles (NPs) were synthesized using the molten-salt synthesis. Enhanced luminescence was observed after deposition of a high quality shell of Y₂O₃: Yb³⁺, roughly 5-10 nm thick, by radical enhanced ALD. However, the deposited shell layer has a slightly lower density, as shown in TEM imaging. The downconversion and upconversion luminescence spectra was collected for Y₂O₃:Er³⁺,Yb³⁺ NPs, 1 mol % Er³⁺ and the Yb³⁺ mol % ranging from 0 to 8 %, with a 980 nm diode excitation. The various NPs cores emitted strongly in the green (532 nm) and red (650 nm) upconversion luminescence and IR (1540 nm) downconversion luminescence, the ²H_11/2, ⁴S_3/2→⁴I_15/2, ⁴F_9/2→⁴I_15/2 and ⁴I_13/2→⁴I_15/2 transitions, respectively. A systematic luminescent red shift, ~2 nm, was observed when comparing the core to core-shell structures for all samples. Visible luminescence occurs via an energy transfer process, requiring two absorbed photons (red) and three absorbed photons (green), between the Yb^{3+ 2}F_5/2 and Er^{3+ 4}I_11/2 and ⁴I_13/2 energy levels. The proposed energy transfer mechanism incorporates the interaction between RE ions at the core-shell interface based on the local binding environment modeled using enhanced x-ray absorption fine structure (EXAFS) spectroscopy. Additionally, the energy transfer coefficients are extracted using the excitation lifetimes and the red-to-green ratio is correlated to key spectroscopic parameters, such as the Judd-Ofelt parameters and branching ratio. These values can be compared to the bulk Y₂O₃ luminescence and the actual measured lifetimes, 200 ms (green) and 600 ms (red), as the figure of merit.