Two researchers affiliated with Tsinghua University in Beijing and with Anhui Normal University in Wuhu, China, have reported a relatively simple wet chemistry method for the production of lanthanide-doped hexagonal-phase NaYF4 nanorods of uniform size and with controllable aspect ratios.The aspect ratio of the doped nanorods is a function of the dopant concentration and of the reaction temperature and time. Electron micrographs compare nanorods doped with 25 percent europium and grown at 170 °C for 16 hours (A), doped with 5 percent europium and grown at 180 °C for 24 hours (B), doped with 25 percent terbium and grown at 170 °C for 16 hours (C), doped with 5 percent terbium and grown at 180 °C for 24 hours (D), codoped with 3 percent ytterbium and 2 percent erbium and grown at 205 °C for 24 hours (E), codoped with 3 percent ytterbium and 2 percent erbium and grown at 190 °C for 24 hours (F), codoped with 3 percent ytterbium and 2 percent thulium and grown at 205 °C for 24 hours (G), and codoped with 3 percent ytterbium and 2 percent thulium and grown at 190 °C for 24 hours (H).The developers of the technique, Leyu Wang and Yadong Li, note that hexagonal-phase NaYF4 has been of interest for some time as a host for visible up-conversion materials. Previous means of production, however, have involved high-temperature processes that result in wide variation in the size of the crystalline particles.In contrast, the new method is performed at approximately 200 °C, with the dopant concentration and the reaction temperature and time determining the aspect ratio of the resulting nanorods. Commercially available base materials are used without additional purification, and the entire process is performed in about one day.To demonstrate the versatility of the technique, Wang and Li synthesized both up- and down-conversion nanorods. The former were codoped with ytterbium and erbium or with ytterbium and thulium to produce, respectively, green and blue light visible to the naked eye when stimulated with a 980-nm laser diode from Hi-Tech Optoelectronics Co. Ltd. of Beijing. The latter nanorods were singly doped with europium or terbium to produce red and green light, respectively, when exposed to the output of UV lamps.The scientists suggest that the highly luminescent structures promise to play a role in the development of novel lasers, nanoscale optoelectronic devices, displays, anticounterfeiting measures and biolabels for in vivo imaging. They note that the process should be suitable for the production of other material systems based on hexagonal-phase NaYF4.Nano Letters, August 2006, pp. 1645-1649.