Bright Visible Luminescence of Dimethylformamide-Stabilized Niobium Oxide Nanoparticles

Inorganic metal oxides are expected to be candidates for new functional materials because they exhibit a wide variety of physical properties. For transition metal oxides, slight differences in structure, composition, and oxidation state can cause completely different physical properties, due to the flexibility of their crystal structures and the diversity of their electronic structures. While it is well known that nanostructures exhibit different physical properties than bulk materials due to quantum size and surface effects. Therefore, metal oxide nanoparticles are suitable for novel functional materials. However, it has been difficult to synthesize nano-sized particles of niobium because of its easy oxidation and aggregation.<br/>Recently, we have succeeded in synthesizing niobium oxide nanoparticles (Nb NPs) with a particle size of less than 4 nm by DMF reduction method. The surface of the Nb NPs is terminated with DMF molecules, which is extremely stable and has excellent re-dispersibility in water and alcohol solvents. We have modified the surface of the nanoparticles by adding chlorides. We compared the physical properties of the nanoparticles with and without the addition of chlorides: XPS and XANES analyses showed that the addition of chlorides resulted in a lower average oxidation state of niobium. The both Nb NPs with and without chloride additive showed bluish-white luminescence in the visible light range, but the nanoparticles with chloride additive showed a larger luminescence intensity. The broad emission spectrum could be divided into about four emission origins. As the concentration of nanoparticles in solution was increased, the peak emission wavelength was red-shifted. This shift was due to Förster-type resonant energy transfer. A light emitting device was fabricated by using Nb NPs. The EL devices showed different emission colors depending on the materials in contact with the nanoparticles: red emission was obtained for the Al/p-Si/Nb NPs/ITO structure and blue emission was obtained for the ITO/PEDOT:PSS/Poly-TPD/Nb NPs/TPBi/Al structure. These results suggest that the EL mechanism and origin can be controlled by the interface conditions between Nb NPs and the electron (hole) transport layer.