Highly crystalline, NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the cooling of the solo NaF flux at a holding temperature of 1100 °C and cooling rate of 5 °C·h−1. The basic forms of NaYF4 and NaYF4:Ln crystals were a sphere. The crystal system, form, and size were affected by cooling rate. The red, green, and blue upconversion fluorescence of NaYF4:Ln crystals were clearly observed under 980 nm laser irradiation by a two- or three-photon upconversion process. The upconversion fluorescence of NaYF4:Ln crystals was successfully controlled by changing the cooling rate and type of dopant. Furthermore, the NaYF4:Yb,Er crystals were successfully grown using a mixed NaF-KF flux cooling method at a relatively low temperature of 800 °C.
YbPO 4 :Er nanocrystals with upconversion fluorescence were successfully prepared using a two-step process, i.e., gelatin gel growth of YbPO 4 :Er 3 nH 2 O precursors and subsequent annealing. The YbPO 4 :Er 3 nH 2 O precursors grown in gelatin gels were milky white, spherical in shape and uniform in size and appearance. The spherical YbPO 4 :Er 3 nH 2 O precursors were an aggregation of numerous, dense nanospheres (10-50 nm). The average diameters were dependent on growth conditions, such as growth period and concentrations of gel and inner and outer solutions. After the precursor spheres were annealed, their shapes changed dramatically to an aggregation of YbPO 4 :Er nanocubes (50-300 nm) in three dimensions (900 °C) and large and wellformed crystals (∼500 nm) of YbPO 4 :Er (1100 °C). The prepared YbPO 4 :Er crystals showed a bright green upconversion emission under 980 nm laser irradiation by a two-photon upconversion process.
Molybdenum dioxide (MoO 2 )/carbon nanofiber composites were successfully fabricated by LiCl-KCl flux cooling. The preparation was obtained by heating mixtures of MoO 3 , LiCl, KCl, and carbon nanofibers at 500 C for 10 h and then carrying out quenching in a furnace with the electric power turned off. The obtained MoO 2 /nanocarbon composites were blackish and had a metallic luster. The obtained composites have lengths of about 5 -150 mm and are in the form of prism. The carbon nanofibers and MoO 2 crystals were interfused densely with each other. Carbon nanofibers were found to act as a reducing agent.
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