Nanoparticle White-Light Emitter Based on Up-Conversion Demonstrated

Daniel S. Burgess

Alternative sources of white light, such as LEDs and organic LEDs, are under investigation for applications ranging from the backlighting of displays to general lighting. Now scientists at the University of Victoria and at the University of British Columbia in Vancouver, both in Canada, have demonstrated the generation of bright white light by the up-conversion of near-infrared radiation in lanthanide-doped nanoparticles suspended in thin films. They suggest that the approach may be suitable for the development of planar waveguides, flat panel displays and fiber amplifiers.

Thin-film matrix

In the experiments, the researchers used a wet chemical process to produce 5- to 6-nm LaF₃nanoparticles co-doped with Yb³⁺ and Eu³⁺, Er³⁺ or Tm³⁺, for red and green, red, and blue emission, respectively. They dispersed the nanoparticles in water and added tetraethoxyorthosilane, ethanol and hydrochloric acid to obtain a sol. Spin-coating this sol onto a quartz substrate and heating it at temperatures of 25 to 800 °C over a period of nearly 16 hours yielded a robust SiO2 thin-film matrix in which the doped nanoparticles were suspended. A similar process was used to prepare ZrO₂ matrices.

The up-conversion of 980-nm radiation in lanthanide-doped LaF₃ nanoparticles yields bright white light. The phenomenon has potential applications in the development of planar waveguides, flat panel displays and fiber amplifiers. Courtesy of Frank C.J.M. van Veggel, University of Victoria.

Four trial runs enabled the investigators to establish the proper proportion of doped nanoparticles to generate white light, with the best color coordinates falling at (0.37, 0.32) on the 1931 CIE chromaticity diagram. The excitation source was a 980-nm CW laser diode from Coherent Inc. of Santa Clara, Calif., that had a maximum power of 800 mW, although the researchers observed white-light emission at pump powers as low as 200 mW. Changing the pump power did not affect the color coordinates of the response.

Control films in which lanthanide and ytterbium ions at the same ratio were directly implanted, rather than incorporated as co-dopants of the nanoparticles, did not produce white light. Instead, the spectra from these films displayed a contribution only from the Er³⁺ ion, suggesting the importance of spatial isolation of the lanthanides afforded by the nanoparticles in preventing internal energy transfer. X-ray diffraction data of the nano- particle films further revealed the presence of a lanthanum silicate phase and La₂Zr₂O₇, suggesting that solid-state chemistry had occurred in the heating process.

The researchers suggest that the efficiency of the up-conversion process can be improved with the capping of the doped nanoparticles by a layer of undoped LaF₃.

Journal of the American Chemical Society, online Aug. 18, 2005, doi:10.1021/ja052583o.