Ranvijay Yadav

Enhanced luminescence efficiency of aqueous dispersible NaYF4:Yb/Er nanoparticles and the effect of surface coating

In a general approach, we designed and synthesized monodisperse, well-defined, highly emissive and aqueous dispersible NaYF4:Yb/Er upconversion nanoparticles, and thereafter their surfaces were coated with inert NaYF4 and silica layers. The crystalline phase, morphology, composition and optical properties of the as-synthesized samples were well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-Vis absorption (UV-Vis), optical band gap energy, Fourier-transform infrared spectroscopy (FT-IR) and upconversion luminescence spectra, respectively. It is found that the synthesized hexagonal phase nanoparticles have a highly crystalline spherical shape, and are monodisperse with narrow size distribution. They can easily disperse in nonpolar cyclohexane solvent to form transparent colloid solutions. The optical band gap energy clearly shows the effect of surface coating of inert inorganic and porous silica layers surrounding the surface of seed core-nanoparticles due to the increase the crystalline size. The upconversion luminescence intensity was remarkably improved after the formation of a passive NaYF4 layer due to the decrease of non-radiative rate arising from the surface/defects of the particles in the form of surface dangling bonds and capping agents. The growth of the silica shell after inert shell formation and the emission intensity of Er3+ transitions were little affected with respect to inert shell coated core/shell nanoparticles, indicating that silica has been effectively grafted surrounding the core/shell nanoparticles. Our results indicate that surface coating of inactive and silica shells is a key step in producing upconversion nanocrystals with increased brightness for a variety of upconversion luminescence bioimaging and biosensing applications.

Enhanced white light emission from a Tm3+/Yb3+/Ho3+ co-doped Na4ZnW3O12 nano-crystalline phosphor via Li+ doping

This paper reports white light emission from a Tm3+/Yb3+/Ho3+ co-doped Na4ZnW3O12 nano-crystalline phosphor synthesized through a solution combustion method. The structural measurements reveal the crystalline nature of the synthesized samples. The Tm3+/Yb3+ co-doped Na4ZnW3O12 sample gives intense blue emission due to the 1G4 → 3H6 transition whereas the Ho3+/Yb3+ co-doped Na4ZnW3O12 sample gives intense green and red emissions due to the 5F4/5S2 → 5I8 and 5F5 → 5I8 transitions, respectively, on excitation with 976 nm. When these three ions viz. Tm3+, Ho3+ and Yb3+; are co-doped in Na4ZnW3O12 the sample gives intense upconverted white light. The as-synthesized sample emits larger emission intensity on annealing at higher temperature. Addition of Li+ in the co-doped phosphor further enhances the emission intensity of white light up to two times and the CIE coordinates of the white light are (0.30, 0.41), which is close to the standard white light (0.33, 0.33). The enhancement in the emission intensity has been discussed due to changes in the local crystal field and reduction in the optical quenching centers. Thus, this nano-crystalline phosphor can be a suitable candidate for white light in various optical applications.

Down shifting and quantum cutting from Eu3+, Yb3+ co-doped Ca12Al14O33 phosphor: a dual mode emitting material

We report the quantum cutting (QC) in a Eu3+, Yb3+ co-doped Ca12Al14O33 phosphor synthesized through combustion method. The X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements reveal the crystalline nature of the phosphor sample. The photoluminescence (PL) spectra of the different samples have been studied using 266 and 394 nm radiation, which give an intense red emission at 612 nm due to 5D0 → 7F2 transition. The addition of Yb3+ in the Eu3+ doped sample reduces the emission intensity of Eu3+ bands in the visible region continuously whereas in the NIR region, the intensity of the Yb3+ band first increases up to 2 mol% and then decreases. This is due to cooperative downconversion energy transfer from Eu3+ to Yb3+ ions and concentration quenching. The decay curve analyses of different samples reveal an efficient energy transfer from Eu3+ to Yb3+ ions. The quantum efficiency (QE) has been calculated for different concentrations of Yb3+ ions and is estimated to be 197%. The possible mechanisms involved in different transitions and energy transfer processes can be understood using a schematic energy level diagram. The phosphor sample is a potential candidate for enhancing the efficiency of c-Si solar cells.

Effect of the Li+ ion on the multimodal emission of a lanthanide doped phosphor

The present study probes the multimodal emission: upconversion (UC), photoluminescence (PL) and quantum cutting (QC) processes in a Ho3+/Yb3+ co-doped Y2O3 phosphor and further examines the impact of the Li+ ion on the multi-modal emission, for the first time. The materials give efficient emission in the green region both through UC and PL, while efficient NIR emission is observed through the QC process. Co-operative energy transfer (CET) has been ascribed as the possible mechanism for QC; as a result of which a UV/blue photon absorbed by a Ho3+ ion splits into two near infrared photons (wavelength range 950–1000 nm) emitted by a Yb3+ ion pair. The Yb3+ concentration dependent ET efficiency and QC efficiency has also been evaluated. The energy transfer from Ho3+ to Yb3+ has been calculated and the efficiency is around 82%, and so, the corresponding QC efficiency is 182%. Co-doping of the Li+ ion has been found to enhance the efficiency of QC emission by about 8%, similar to that of UC and PL emission, and hence the multimodal emission is enhanced. Such NIR QC phosphors have great promise in energy conversion for c-Si solar cell applications.

Enhanced green upconversion photoluminescence from Ho3+/Yb3+ co-doped CaZrO3 phosphor via Mg2+ doping

This paper reports enhanced green upconversion photoluminescence from Ho3+/Yb3+ co-doped CaZrO3 phosphor via Mg2+ doping synthesized through a solid state reaction method. The X-ray diffraction measurements confirm a shift in the peak position due to the presence of Mg2+ in the CaZrO3 phosphor. The scanning electron micrographs reveal an increase in the particle size for doping with Mg2+ ions. The Ho3+/Yb3+ co-doped CaZrO3 phosphor gives an intense monochromatic green upconversion emission centered at 543 nm due to 5F4/5S2 → 5I8 transition along with weak UV, blue, red and NIR emissions on excitation at 976 nm. The emission intensity of Ho3+ ions was optimum for 3 mol% Yb3+. The doping of Mg2+ ions slightly changes the band gap of the CaZrO3 phosphor; thereby enhancing the emission intensity significantly. When Mg2+ ions are doped in the Ho3+/Yb3+ co-doped CaZrO3 phosphor the emission intensity of the green band is enhanced by up to 4 times. This enhancement is due to substitution of Ca2+ by the Mg2+ ions, which decreased the lattice parameters and increased the crystallinity. The lifetime of the 5F4/5S2 level increases with the increase in the concentration of Mg2+ ions. Thus, the Ho3+/Yb3+/Mg2+ co-doped CaZrO3 phosphor could be a suitable candidate for intense monochromatic green light and optical devices.

Change in structural morphology on addition of ZnO and its effect on fluorescence of Yb3+/Er3+ doped Y2O3

Yb(3+)/Er(3+) codoped Y(2)O(3) phosphor and its composite with ZnO have been synthesized by combustion method. Morphology of the materials has been investigated using X-ray diffraction pattern (XRD) and scanning electron microscopy (SEM) techniques. XRD confirms the constituents as Y(2)O(3) and ZnO, with average crystallite size of 112 nm. On addition of ZnO, a small shifting in XRD pattern of Y(2)O(3) is observed. SEM pattern suggests that the average particle size lies in micro-range (0.5 μm). A dumble like structure is observed for hybrid material on annealing at 1473 K. A strong green (525, 546 nm) with weak blue (411 nm) and red (657 nm) emissions through upconversion has been observed from the phosphor on excitation with 976 nm diode laser. The observed emissions involve (2)H(9/2)→(4)I(15/2), (2)H(11/2)→(4)I(15/2), (4)S(3/2)→(4)I(15/2) and (4)F(9/2)→(4)I(15/2) electronic transitions, respectively. The upconversion process has been confirmed by power dependence measurements and its slope value was found to be 1.85, 1.72 for green and red emissions, respectively. On addition of ZnO, the intensity of these emissions is enhanced several times. The reason behind the enhancement is discussed with the help of the emitting level lifetime. An interesting dual mode property (upconversion and downconversion) to the same material has been observed on excitation with 532 nm laser source.

A facile synthesis approach and impact of shell formation on morphological structure and luminescent properties of aqueous dispersible NaGdF4:Yb/Er upconversion nanorods

AbstractA general facile synthesis approach was used for fabrication of highly emissive aqueous dispersible hexagonal phase upconversion luminescent NaGdF4:Yb/Er nanorods (core NRs) through metal complex decomposition process. An inert NaGdF4 and porous silica layers were grafted surrounding the surface of each and every NRs to enhance their luminescence efficiency and colloidal dispersibility in aqueous environment. Optical properties in terms of band gap energy of core, core/shell, and silica-coated core/shell/SiO2 nanorods were observed to investigate the influence of surface coating, which was gradually decreased after surface coating because of increase crystalline size after growth of inert and silica shells. The inert shell formation before silica surface grafting, upconversion luminescence intensity was greatly improved by about 20 times, owing to the effective surface passivation of the seed core and, therefore, protection of Er3+ ion in the core from the nonradiative decay caused by surface defects. Moreover, after silica coating, core/shell nanorods shows strong upconversion luminescence property similar to the hexagonal upconversion core NRs. It is expected that these NaGdF4:Yb/Er@NaGdF4@SiO2 (core/shell/SiO2) NRs including highly upconversion emissive and aqueous dispersible properties make them an ideal materials for various photonic-based potential applications such as in upconversion luminescent bioimaging, magnetic resonance imaging, and photodynamic therapy. Graphical abstractᅟ