Multi-phonon assisted upconversion emission and power dependence studies in LaF3:Er3+ phosphor

Role of heat treatment on the structural and luminescence properties of Yb3+/Ln3+ (Ln = Tm, Ho and Er) co-doped LaF3 nanoparticles

Hexagonal LaF₃:Yb³⁺/Ln³⁺ and tetragonal LaOF:Yb³⁺/Ln³⁺ (Ln = Ho, Tm, Er) have been successfully prepared via a two-step reaction, which includes a facile aqueous ligand free solution method and the following heat treatment of the as-prepared LaF₃ precursor. The phase formation evolution from LaF₃ to LaOF with different phase structures was characterized by X-ray diffraction (XRD), scanning electron microscopy, Fourier transform infrared, and Raman spectroscopy. At an annealing temperature of 500 °C pure hexagonal LaF₃:Yb³⁺/Ln³⁺ (Ln = Ho, Tm, Er) nanoparticles with an average size of 32 nm were obtained and they showed a strong visible upconversion and a modest infrared emission upon 976 nm laser excitation. Further, using an annealing temperature of 900 °C, tetragonal LaOF:Yb³⁺/Ln³⁺ (Ln = Ho, Tm, Er) nanoparticles with a size of around 44 nm were obtained (obtained from XRD) and an expressive enhancement in the emission of the VIS and near-infrared regions was observed. These results envision applications that require efficient emissions such as fluorescent and thermal images, and LaF₃ nanocrystals have recently been widely explored for applications in biological systems.

Effect of Synthesis Techniques on the Optical Properties of Ho3+/Yb3+ Co-doped YVO4 Phosphor: A Comparative Study

The Ho³⁺/Yb³⁺-codoped YVO₄ phosphors have been synthesized by three different techniques (viz., solution combustion, sol-gel, and solid-state reaction techniques). X-ray diffraction patterns confirm the formation of a pure phase in the samples synthesized by all of the three methods; however, the average crystallite sizes in the three cases are different. The crystallite size increases if they are heated to higher temperature. The particle sizes are measured by scanning electron microscopy, which shows an increase in particle size with increasing the calcination temperature. The vibrational behavior of all of the three synthesized phosphor samples is studied by the Fourier transform infrared (FTIR) technique. The UV-vis absorption measurements give a large number of bands in all of the three samples prepared by three different methods. The upconversion (UC) emissions in all three samples have been monitored using a 980 nm diode laser. It gives an intense red emission in all of the three samples. Upconversion emission intensity is more prominent in the phosphor sample synthesized by the sol-gel technique and heated at 1473 K. The enhancement in UC emission intensity is well understood by the shape and size of the particles and also confirmed by the FTIR and UV-vis measurements. It is interesting to note that whereas UC measurements give red and weak green emissions, downshifting (DS) measurements show intense green, weak red, and broad blue emissions on UV excitation (323 nm). The DS behavior shows the same characteristics of the enhancement in overall emission. Overall, the phosphor sample synthesized by the sol-gel method gives better results in upconversion and downshifting behaviors when heated at 1473 K.

Monochromatic NIR UC emission in Tm3+/Yb3+co-doped GdVO4 phosphor: the effect of the Bi3+ ion concentration and pump power of a diode laser

A Tm³⁺/Yb³⁺/Bi³⁺ co-doped GdVO₄ phosphor sample has been synthesized using a high-temperature solid-state reaction technique. The X-ray diffraction patterns reveal pure phase formation and crystalline behavior of the synthesized samples. Intense blue and NIR upconversion emissions have been observed upon excitation with a 980 nm diode laser. It is found that the addition of the Bi³⁺ ion to the phosphor reduces the intensity of blue emission and enhances the NIR emission intensity to the extent that NIR emission is nearly monochromatic [(I_NIR/I_Blue) ∼ 14]. This ratio is further improved up to 70 times (almost monochromatic) by varying the pump power of the diode laser. Thus, this material can be used as a cheap source of monochromatic NIR emission at 800 nm for bioimaging.