S.B. Rai

Role of Li+ ion in the luminescence enhancement of lanthanide ions: favorable modifications in host matrices

Lanthanide based materials are preferred over other luminescent materials for various applications. Current focus in this area is to exploit the unique luminescence features of lanthanide-based materials for multidisciplinary research and novel applications. Furthermore, efforts are going on to enhance the luminescence of lanthanide ions for better performance. In a broader sense, there are two ways to enhance the luminescence of lanthanide ions. The first is to use a suitable sensitizer, which can absorb excitation energy efficiently, and can transfer it to the lanthanide ions. This method has been known for a long time and is well documented in the literature. The second way is to modify host matrices in such a way that it favors radiative transitions. It is widely reported in the literature that the presence of alkali ions, particularly Li+ ions, in a matrix enhances the luminescence of lanthanide ions significantly. But there are no comprehensive reports available in the literature that summarize how alkali ions help in the luminescence enhancement of lanthanide ions in various host matrices. The prime objective of this review is to highlight various contributing factors that help in the luminescence enhancement of lanthanide ions in the presence of alkali ions.

Light-into-heat conversion in La 2 O 3 :Er 3+ -Yb 3+ phosphor: an incandescent emission

Low-power-threshold cw laser-induced incandescence (CWLII) has been observed in La(2)O(3):Er(3+)-Yb(3+) phosphor on excitation by a 976 nm IR laser. It is suggested that incandescence originates from the extensive heating induced by the nonradiative processes taking place following the laser excitation. Other mechanisms for similar observations have also been suggested in the literature and have been discussed with the present observations. The estimated temperature for the CWLII approaches around 2650 K, and this seems to provide an effective way to rapidly attain high temperature in nano/microvolumes of phosphor. The phosphor exhibited efficient upconversion, and the ratio of the (2)H(11/2)→(4)I(15/2) and (4)S(3/2)→(4)I(15/2) band intensities of Er(3+) permits measurement of the temperature rise, from a distance.

Efficient dual mode multicolor luminescence in a lanthanide doped hybrid nanostructure: a multifunctional material

The present work deals with inorganic-organic hybrid nanostructures capable of producing intense visible emission via upconversion (UC), downconversion (DC), and energy transfer (ET) processes which show the potential of the material as a luminescent solar collector (LSC), particularly to improve the efficiency of silicon solar cells. To achieve this, Gd2O3:Yb3+/Er3+ phosphor (average particle size∼35 nm) and a Eu(DBM)3Phen organic complex have been synthesized separately and then the hybrid structure has been developed using a simple mixing procedure. Intense UC emission (in the red, green, and blue regions) due to Er3+ is observed on near infrared (976 nm) excitation which shows color tunability with input pump power. In contrast, intense red emission of Eu3+ is observed on ultaviolet (UV) (355 nm) excitation. The feasibility of energy transfer from Er3+ ions to Eu3+ ions has also been noted. These excellent optical properties are retained even if the particles of the hybrid nanostructure are dispersed in liquid medium, which also makes it suitable for security ink purposes.

Additional Evidence of Stable EMF-Induced Changes in Water Revealed by Fungal Spore Germination

Spore germination of some fungi was studied in EMF-tieated water samples. Water samples were exposed to 9.575 GHz microwave modulated with square waves of different pulse repetition frequencies. The study indicates varying inhibitory effects of different modulation frequencies on spore germination of Alternaria alternata, A. tenuissima, Fusarium udum, Helminthosorium oryzae, H. turcicumand Ustilago cynodontis. The pulse repetition frequency-dependent variation in spore germination caused by irradiated water illustrates that water may have the capability to store EMF frequency effects. EMF-treated water might have caused the effect by either changing the activities of and/or deactivating the spore enzymes.

Athermal physiological effects of microwaves on a cynobacterium Nostoc muscorum: evidence for EM-memory bits in water

Athermal physiological effects of continuous wave and modulated microwaves were studied on a cynobacterium Nostoc muscorum. The study shows that different microwave frequencies in continuous wave and modulated modes produced significantly different physiological effects on the algae. Water-mediated bioeffects further present additional proof that water has the capability to remember the imposed electromagnetic field characteristics for an extended period of time.

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.

Effect of Water's Microwave Power Density Memory on Fungal Spore Germination

The effect of absorbed microwave power in water on germination of some fungal spores was seen. Water samples were exposed to 2.71 GHz microwave at varying incident power density. Incident and absorbed power by water were determined using power sensor and meter. Germination responses of different fungal spores taken from fresh cultures were studied in the microwave power-absorbed-water samples. The study revealed differential germination responses with variation of the absorbed microwave power in water on different fungi. Results indicated that water has the capability to store microwave power alterations.

X-Ray Determination of Magnetically Treated Liquid Water Structures

X-ray diffraction (XRD) was used to analyze the magnetic field-induced changes in liquid water. Water samples (0.5 × 10−7 Ohm−1 cm−1) were exposed to discrete poles of a 0.3 T permanent magnet for different lengths of time. Study reveals that magnetic exposure breaks as well as coalesces the ice polymorphs of the liquid water, depending on magnetic poles and the length of exposure. Magnetically transformed ice polymorphs are ordered as helical structures, which are candidates for long-term storage of magnetic field information.

Improved photo-luminescence behaviour of Eu3+ activated CaMoO4 nanoparticles via Zn2+ incorporation

Zn2+ (0, 2, 5, 7 and 10 at%) co-doped CaMoO4:2Eu3+ nanophosphors have been synthesized via the polyol method using ethylene glycol (EG) as both capping agent and reaction medium at 150 °C. From XRD analysis, all 900 °C annealed Zn co-doped CaMoO4:Eu3+ nanophosphors have a tetragonal scheelite phase. Some extra phase evolution has been observed for the as-prepared Zn doped samples. The intensity and crystallinity of XRD patterns increase as heat treatment increases to 900 °C. The valence states of the involved compositions (Zn co-doped CaMoO4:Eu) were investigated by X-ray photoelectron spectroscopy (XPS) and it was found that Ca, Mo, Eu and Zn are in their +2, +6, +3 and +2 oxidation states, respectively. TG-DSC studies of the as-prepared samples corroborate their thermal stability. A TEM (Transmission electron microscopy) study reveals that the particles have spherical morphology. Photoluminescence studies have been carried out under ∼266, and 395 nm excitation wavelengths. Zn co-doping in the CaMoO4:Eu matrix produces a high distortion and modifies the crystal field around the Eu3+ ion and improves the PL intensity. CIE co-ordinates of the 900 °C annealed 10 at% Zn co-doped CaMoO4:Eu sample under 266 nm excitation is x = 0.64 and y = 0.35, which are close to the standard of NTSC (x = 0.67 and y = 0.33). These investigations reveal that Zn co-doped CaMoO4:Eu3+ nano-materials can be used as potential red emitting phosphors, an area which is a bottleneck in the development of low cost LEDs.

Host matrix impact on Er3+ upconversion emission and its temperature dependence

By synthesizing Y(1.9−2x)Yb0.1Er2xO3, Y(0.95−x)Yb0.05ErxVO4 and Y(0.95−x)Yb0.05ErxPO4 phosphors, with phonon frequency maxima at 560, 826 and 1050 cm−1, respectively, we present the impact of phonon energy and crystal structure of the host matrix on upconversion and temperature sensing behavior. The spectral upconversion characteristics of all three phosphors reveal noticeable differences. The temperature sensing studies reveal that the phosphors have maximum sensitivity at ∼490 K, which is found to be highest (0.0105 K−1) in Y0.947Yb0.05Er0.003VO4 followed by Y1.894Yb0.1Er0.006O3 and Y0.947Yb0.05Er0.003PO4 phosphors. We found that the temperature sensitivity basically depends on the intensity ratio of two thermally coupled emission bands, 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2, of Er3+. Further, the intensity ratio depends on phonon energy of the host lattice, crystal structure, surface quenching centers and the temperature dependence of non-radiative decay rate.