Highly luminescent ultranarrow Mn doped ZnSe nanowires.

Colloidal Mn-doped ZnSe nanowires with diameters of 1-3 nm and lengths up to 200 nm were prepared from Li(4)[Zn(10)Se(4)(SPh)(16)] clusters and manganese stearate. The nanowires exhibit optical properties that depend on size, shape, and doping level. The manganese photoluminescence is slightly polarized perpendicular to the long axis and reaches a quantum yield of 40% after passivating the crystals with a CdSe shell.

White-light emitting hydrogen-bonded supramolecular copolymers based on pi-conjugated oligomers.

Three different pi-conjugated oligomers (a blue-emitting oligofluorene, a green-emitting oligo(phenylene vinylene), and a red-emitting perylene bisimide) have been functionalized with self-complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) units at both ends. The molecules self-assemble in solution and in the bulk, forming supramolecular polymers. When mixed together in solution, random noncovalent copolymers are formed that contain all three types of chromophores, resulting in energy transfer upon excitation of the oligofluorene energy donor. At a certain mixing ratio, a white emissive supramolecular polymer can be created in solution. In contrast to their unfunctionalized counterparts, bis-UPy-chromophores can easily be deposited as smooth thin films on surfaces by spin coating. No phase separation is observed in these films, and energy transfer is much more efficient than in solution, giving rise to white fluorescence at much lower ratios of energy acceptor to donor. Light emitting diodes based on these supramolecular polymers have been prepared from all three types of pure materials, yielding blue, green, and red devices, respectively. At appropriate mixing ratios of these three compounds, white electroluminescence is observed. This approach yields a toolbox of molecules that can be easily used to construct pi-conjugated supramolecular polymers with a variety of compositions, high solution viscosities, and tuneable emission colors.

Cluster synthesis of branched CdTe nanocrystals for use in light-emitting diodes.

Highly luminescent cadmium telluride (CdTe) nanocrystals were synthesized using Li(2)[Cd(4)(SPh)(10)] as a reactive Cd cluster compound at relatively low temperature, making it a safe precursor for the large scale synthesis of CdTe nanocrystals. Transmission electron microscopy (TEM) showed that the shape of the CdTe nanocrystals changes from nanorods to branched structures with increasing reaction time. The nanocrystals show high luminescent quantum yields up to 37% for CdTe branched nanostructures, and as high as 52% for CdTe/CdS core-shell heterostructures. CdTe/CdS nanocrystals were used to make light-emitting diodes in combination with organic layers for electron and hole injection. The devices show a maximum luminance efficiency of 0.35 cd A(-1).

Colloidal nanoparticles of Ln3+-doped LaVO4: energy transfer to visible- and near-infrared-emitting lanthanide ions.

Colloidal, organic solvent-soluble Ln3+-doped LaVO4 nanoparticles have been synthesized by a precipitation reaction in the presence of (C18H37O)2PS2- as ligand, that coordinates to the surface of the nanoparticles. The materials are well soluble in chlorinated solvent such as chloroform. Energy transfer of excited vanadate groups has been observed for Ln3+ ions that emit in the visible and the near-infrared (Eu3+, Tm3+, Nd3+, Er3+, Ho3+, Dy3+, Sm3+, Pr3+), thus making it a very generic sensitization mechanism. The LaVO4 nanoparticles have a different crystal structure than bulk LaVO4 ones (xenotime instead of monazite), similar to YVO4 nanoparticles. This xenotime crystal structure results in a more asymmetric crystal field around the Ln3+ ions that is advantageous to their luminescence, for it increases the radiative rate constant, thus reducing quenching processes.

Improvement in the luminescence properties and processability of LaF3/Ln and LaPO4/Ln nanoparticles by surface modification.

The surface of lanthanide(III)-doped LaPO4 nanoparticles was modified by reaction with an alcohol, leading to a covalent bond between the ligand and the particle surface. The surface of lanthanide(III)-doped LaF3 nanoparticles was modified to alter the solubility of the nanoparticles and study the influence of surface effects on the luminescence of lanthanide ions doped in the nanoparticles. The coordinated organic ligands can be modified by a quantitative exchange reaction in solution or by using functionalized ligands during the synthesis. Variation of the ratio of ligand to core reagents had a significant influence on the size of the nanoparticles. Smaller nanoparticles were formed with a higher ligand ratio. The optical properties of these nanoparticles show a strong dependence on nanoparticle size, indicating the influence of quenching probably by CH and OH groups at or near the surface of the nanoparticle cores. The luminescence lifetime of LaF3/Eu nanoparticles varied from 6.5 to 7.4 ms for nanoparticles with an average size of 7.1 to 8.4 nm. A significant reduction of the quenching from the surface of the nanoparticles was obtained by the synthesis of core-shell nanoparticles, in which a shell of LaF3 was grown epitaxially around the doped core nanoparticles. This leads to an increase in the luminescence lifetime of the Eu3+ ion and the observation of emissions from the 5D2 energy level, in addition to emissions from the 5D1 and 5D0 levels. The quantum yield of LaF3/Ce,Tb nanoparticles could be increased from 24 to 54% by the growth of a LaF3 shell around the nanoparticles.