Ir(ppy)(3) phosphorescent microrods and nanowires: promising micro-phosphors

We prepared crystalline Ir(ppy)(3) microrods through a facile self-assembly growth method by employing the so-called reprecipitation technique, and Ir(ppy)(3) nanowires by a solvent-evaporation route. Both have lengths up to several tens of micrometer, but possess significantly different diameters: 1 mu m for microrods and 100 nm for nanowires. The electron diffraction (ED) and X-ray diffraction (XRD) results clarify that both microrods and nanowires preferentially grow along the crystal [001] direction. However, the former have a regular hexagonal geometry and single crystalline in nature, while the latter are polycrystalline with a round cross section. Remarkably, microrods and nanowires of Ir(ppy)(3) present distinct optical properties. The phosphorescence decay of Ir(ppy)(3) microrods and nanowires is much faster than that in degassed solution and polymethylmethacrylate (PMMA) film. The phosphorescence green color of microrods is similar to that of Ir(ppy)(3) molecules doped in PMMA films, while nanowires actually emit yellow light probably from the low-energy trap as a result of its polycrystalline nature. Furthermore, the transverse nanoscale and longitudinal microscale dimensions and well-defined faceting nature of microrods enable the observation of evident optical waveguiding. No optically pumped lasing is observed because of intense triplet-triplet exciton annihilation. Our results afford a novel strategy of phosphorescence emission color tuning by controlling the nano- to microstructure dimensions. The microrod phosphorescence waveguides may be used as building blocks for future miniaturized photonic devices.