Luminescence of Nb Oxide Nanoparticles and Its Application to Light Emitting Devices

In transition metal oxides, slight differences in structure, composition, and oxidation state can result in completely different physical properties due to the flexibility of the crystal structure and the diversity of the electronic structure. Therefore, in bulk materials, these different physical properties can appear mixed due to the presence of a large number of different impurities and different types of oxygen defects. However, in nanostructures, the number of atoms is limited, so unique properties can appear due to specific impurities, defects or oxidation states.<br/><br/>It has been difficult to synthesize nano-sized particles of transition metal oxides because of their susceptibility to oxidation and aggregation. Recently, we have successfully synthesized Nb oxide nanoparticles by DMF reduction method. The Nb oxide nanoparticles are 2-4 nm in diameter, passivated in DMF and extremely stable in solvents such as water, ethanol and DMF. The Nb oxide nanoparticles exhibited a broad, bluish-white photoluminescence in solution. The broad emission spectrum could be separated into several emissions. As the concentration of nanoparticles in solution was increased, the peak emission wavelength showed a red shift. This shift was due to Förster-type resonance energy transfer. The results indicate that the Nb oxide nanoparticles have several different emission wavelengths, blue, green and red color regions. Indeed, in EL devices with Nb oxide nanoparticles as the emitting layer, red and blue emission were observed depending on the charge transport (injection) materials sandwiching the Nb oxide nanoparticles. Although the origin of the luminescence of Nb oxide nanoparticles is not yet understood, these results indicate that transition metal oxide nanoparticles have potential applications as light-emitting devices.