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

Abstract

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 µm 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.