Physiochemical properties of greatly enhanced photoluminescence of aqueous dispersible upconversion CaF2:Yb/Er nanoparticles.

Crystal phase morphological structure and optical properties of the as-prepared upconversion CaF2:Yb/Er(core) and sequential coating of an inert crystalline material and silica layers surrounding the seed core-nanoparticles (NPs) were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), optical absorption, FTIR spectroscopy and upconversion photoluminescence spectroscopy. Owing to the unique properties of CaF2 host matrix, we realized their practical applications in biomedical science to improve the upconversion luminescence property and aqueous dispersibility. The surface coating on the seed core particles will significantly influence the structural, optical band gap energy and upconversion luminescence properties. These NPs were well-dispersed in aqueous and non-aqueous solvents to form clear colloidal solutions. The colloidal solutions of three samples show a characteristic optical absorption band in UV/Visible region. As a result, optical band gap gradually decreases after sequential growth of the inert shell and amorphous silica due to an increase in the crystalline size. Comparative upconversion luminescence analysis showed that after inert shell growth, the upconversion intensity was greatly improved, and such an improvement was found to arise from efficient suppression of surface-related deactivation from the core nanocrystals. Interestingly, growth of an inert (CaF2) shell over the seed core NPs shows intense upconversion emission lines under 980 nm NIR laser excitation, highlighting their promising applications, such as multi-analyte biolabels, staining, displays and other photonic based technological applications.

Multifunctional role of dysprosium in HfO2: stabilization of the high temperature cubic phase, and magnetic and photoluminescence properties.

Hafnium oxide (HfO2) can exist in different crystalline structures such as monoclinic at room temperature, tetragonal at 1700 °C and cubic at 2600 °C. In the present study, nanocrystalline powders of HfO2 synthesized by a Pechini type sol-gel technique show a monoclinic phase, P21/c, at room temperature. By incorporating Dy into the HfO2 lattice, the intensity of all diffraction peaks corresponding to P21/c decreases and at a concentration of 11 at% of Dy, the monoclinic phase transforms completely to the cubic phase, Fm3[combining macron]m, showing a mixed phase of monoclinic and cubic at intermediate concentrations (5-9 at%) of Dy. For the first time, we have stabilized the high temperature cubic phase of HfO2 at room temperature by incorporating Dy. Selected area electron diffraction patterns confirm the monoclinic and the cubic phase as observed from the X-ray diffraction patterns. A mechanism for stabilization of the high temperature cubic phase in Hf1-xDyxO2 has been analyzed based on the substitution of dysprosium for hafnium ions and the formation of oxygen vacancies. While ferromagnetic ordering at room temperature observed in HfO2 nanoparticles is quenched after incorporating 1 at% of Dy, photoluminescence (PL) studies demonstrate excellent emissions in the blue and yellow region after exciting with UV light of wavelength 352 nm. Combining excitation and emission profiles, we have proposed a tentative energy band diagram illustrating the energetic processes taking place in Hf1-xDyxO2.

Influence of Shell Formation on Morphological Structure, Optical and Emission Intensity on Aqueous Dispersible NaYF4:Ce/Tb Nanoparticles.

A highly water-dispersible NaYF4:Ce/Tb (core), NaYF4:Ce/Tb@NaYF4(core/shell) and NaYF4:Ce/Tb@NaYF4@SiO2 (core/shell/SiO2) nanoparticles (NPs) were synthesized via a general synthesis approach. The growth of an inert NaYF4 and silica shell (~14 nm) around the core-NPs resulted in an increase of the average size of the nanopaticles as well as broadening of their size distribution. The optical band-gap energy slightly decreases after shell formation due to the increase the crystalline size. To optimize the influence of shell formation a comparative analysis of photoluminescence properties (excitation, emission, and luminescence decay time) of the core, core/shell, and core/shell/SiO2 NPs were measured. The emission intensity was significantly enhanced after inert shell formation around the surface of the core NPs. The Commission International de l'Eclairage chromaticity coordinates of the emission spectrum of core, core/shell, core/shell/SiO2 NPs lie closest to the standard green color emission at 545 nm. By quantitative spectroscopic measurements of surface-modified core-NPs, it was suggested that encapsulation with inert and silica layers was found to be effective in retaining both luminescence intensity and dispersibility in aqueous environment. Considering the high aqueous dispersion and enhanced luminescence efficiency of the core-NPs make them an ideal luminescent material for luminescence bioimaging and optical biosensors.

Highly aqueous soluble CaF2:Ce/Tb nanocrystals: effect of surface functionalization on structural, optical band gap, and photoluminescence properties.

The design of nanostructured materials with highly stable water-dispersion and luminescence efficiency is an important concern in nanotechnology and nanomedicine. In this paper, we described the synthesis and distinct surface modification on the morphological structure and optical (optical absorption, band gap energy, excitation, emission, decay time, etc.) properties of highly crystalline water-dispersible CaF2:Ce/Tb nanocrystals (core-nanocrystals). The epitaxial growth of inert CaF2 and silica shell, respectively, on their surface forming as CaF2:Ce/Tb@CaF2 (core/shell) and CaF2:Ce/Tb@CaF2@SiO2 (core/shell/SiO2) nanoarchitecture. X-ray diffraction and transmission electron microscope image shows that the nanocrystals were in irregular spherical phase, highly crystalline (~20 nm) with narrow size distribution. The core/shell nanocrystals confirm that the surface coating is responsible in the change of symmetrical nanostructure, which was determined from the band gap energy and luminescent properties. It was found that an inert inorganic shell formation effectively enhances the luminescence efficiency and silica shell makes the nanocrystals highly water-dispersible. In addition, Ce3+/Tb3+-co-doped CaF2 nanocrystals show efficient energy transfer from Ce3+ to Tb3+ ion and strong green luminescence of Tb3+ ion at 541 nm(5D4→7F5). Luminescence decay curves of core and core/shell nanocrystals were fitted using mono and biexponential equations, and R 2 regression coefficient criteria were used to discriminate the goodness of the fitted model. The lifetime values for the core/shell nanocrystals are higher than core-nanocrystals. Considering the high stable water-dispersion and intensive luminescence emission in the visible region, these luminescent core/shell nanocrystals could be potential candidates for luminescent bio-imaging, optical bio-probe, displays, staining, and multianalyte optical sensing. A newly designed CaF2:Ce/Tb nanoparticles via metal complex decomposition rout shows high dispersibility in aqueous solvents with enhanced photoluminescence. The epitaxial growth of inert CaF2 shell and further amorphous silica, respectively, enhanced their optical and luminescence properties, which is highly usable for luminescent biolabeling, and optical bioprobe etc.

Enhanced up-conversion and temperature-sensing behaviour of Er(3+) and Yb(3+) co-doped Y2Ti2O7 by incorporation of Li(+) ions.

Y2Ti2O7:Er(3+)/Yb(3+) (EYYTO) phosphors co-doped with Li(+) ions were synthesized by a conventional solid-state ceramic method. X-ray diffraction studies show that all the Li(+) co-doped EYYTO samples are highly crystalline in nature with pyrochlore face centred cubic structure. X-ray photon spectroscopy studies reveal that the incorporation of Li(+) ions creates the defects and/or vacancies associated with the sample surface. The effect of Li(+) ions on the photoluminescence up-conversion intensity of EYYTO was studied in detail. The up-conversion study under ∼976 nm excitation for different concentrations of Li(+) ions showed that the green and red band intensities were significantly enhanced. The 2 at% Li(+) ion co-doped EYYTO samples showed nearly 15- and 8-fold enhancements in green and red band up-converted intensities compared to Li(+) ion free EYYTO. The process involved in the up-conversion emission was evaluated in detail by pump power dependence, the energy level diagram, and decay analysis. The incorporation of Li(+) ions modified the crystal field around the Er(3+) ions, thus improving the up-conversion intensity. To investigate the sensing application of the synthesized phosphor materials, temperature-sensing performance was evaluated using the fluorescence intensity ratio technique. Appreciable temperature sensitivity was obtained using the synthesized phosphor material, indicating its applicability as a high-temperature-sensing probe. The maximum sensitivity was found to be 0.0067 K(-1) at 363 K.

Structural and up-conversion properties of Er3+ and Yb3+ co-doped Y2Ti2O7 phosphors.

Y(2)Ti(2)O(7) (YTO) and Er(3+)/Yb(3+) co-doped Y(2)Ti(2)O(7) (EYYTO) phosphors have been prepared by solid-state reaction method. Structures of YTO and EYYTO phosphors are identified as face centered cubic pyrochlores. Up-conversion emission spectra of EYYTO under 976 nm excitation is studied, which revealed three prominent emission lines at ~524, 548 and 661 nm originating from (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 of Er(3+) ion, respectively in green and red regions. The power dependence study suggests that these bands arise due to two photon absorption. The monodispersed laser ablated colloidal solution of EYYTO shows strong red and green emissions on excitation with 976 nm laser. The variation of luminescence intensity at different laser excitation powers is observed and thus a color can be tuned. The photoluminescence lifetime of green band at 548 nm ((4)S(3/2) level) has been found to be ~446 µs.

Synthesis and characterization of a stable dispersion of multifunctional Gd2O3:Er,Yb in polyvinyl alcohol.

A stable dispersion of multifunctional Gd2O3:Er,Yb phosphor in polyvinyl alcohol (PVA) was synthesized by varying the concentration of Yb(3+) ions. It had a strong ultraviolet-visible-near infrared (UV-vis) upconversion emission and applications in temperature and magnetic field sensors (e.g., nano-heaters), as well as potential use in bioleveling and bioimaging. Stability of the dispersion was found to strongly depend on the mixing process of the powder in the polymer solution. Spherical shaped nanoparticles in cubic phase of ~ 43 nm diameter were synthesized and characterized by X-ray diffraction; results were confirmed by scanning electron microscopy (SEM). Fourier transform infrared sspectroscopy (FT-IR) and thermal analysis supported the presence of PVA. NIR pumping produced strong UC emission bands in the red and green regions extending up to very high UV (240 nm). This method provides an alternative for synthesizing a highly UC-efficient non-agglomerated pure transparent dispersion from various efficient phosphors for biological applications.

Photoluminescence behavior of rare earth doped self-activated phosphors (i.e. niobate and vanadate) and their applications.

In the present study, the photoluminescence behaviors of rare earth doped self-activated phosphors are discussed briefly. Different techniques were used to develop these phosphor samples. We prepared pure and rare earth doped phosphor samples to look for their various applications. The structural confirmations and surface morphologies were performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements, respectively. The upconversion (UC) phenomenon was investigated in Tm3+/Yb3+ and Ho3+/Yb3+ co-doped niobate and vanadate based phosphors, which gave intense blue/NIR and green/red emissions with a 980 nm diode laser as an excitation source. Pure niobate and vanadate phosphor materials are self-activated hosts which give broad blue emission under UV excitation. Upon UV excitation, intense broad blue emission along with sharp emissions due to Tm3+ and Ho3+ ions are observed via energy transfer between niobate/vanadate and rare earth ions. These self-activated hosts show prominent downshifting (DS) behavior. Broad band quantum cutting (QC) was observed in these self-activated hosts, in which a blue emitting photon is converted into two NIR photons by co-doping Yb3+ ions in it. The multimodal (upconversion, downshifting and quantum cutting) behaviors of these phosphors make them very promising in various applications, such as spectral converters to enhance the efficiency of a c-Si solar cell, security ink and color tunable materials.

Light-into-heat conversion in La2O3:Er(3+)-Yb3+ 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.

Cool white light emission in dysprosium and salicylic acid doped poly vinyl alcohol film under UV excitation.

Dysprosium (Dy) and Salicylic acid (Sal) doped Poly Vinyl Alcohol (PVA) films have been successfully prepared by solution cast technique. The absorption, excitation, emission and lifetime analysis of the samples have been carried out. Judd-Ofelt theory has been used to estimate several parameters for DyCl(3) and Dy(Sal)(3)Phen in PVA polymer film which show fair agreement between the experimental and the theoretical values supporting the J-O theory. A combination of blue and yellow emissions in Dy(x)(Sal)(3)Phen co-doped PVA samples makes one perceive cool white light when excited by ultraviolet light. Energy transfer (ET) from Sal to Dy(3+) is investigated by directly observing the luminescence intensity of Dy(3+) in the Dy(x)(Sal)(3)Phen co-doped PVA samples which is much stronger than that in the DyCl(3) in PVA which is further confirmed by lifetime studies with different concentrations of Dysprosium ion (Dy(3+)). The generation of white light with chromaticity coordinates (0.30, 0.34) makes it potential material for white LED and display devices.

Photoluminescence behavior of Eu3+ doped XAl2O4(X = Mg, Ca, Sr and Ba) phosphors: a comparative study.

In this work, the Eu3+doped stuffed tridymite type structure of alkaline earths aluminate i.e. XAl2O4(X = Mg, Ca, Sr and Ba) phosphor materials have been synthesized by conventional high temperature solid state reaction method at 1623 K. The Samples were structurally and morphologically characterized by x-ray diffraction (XRD) and Scanning electron microscope (SEM) measurements. The vibrational behavior of the phosphor samples were investigated by Fourier transform infrared (FTIR) measurements. The phosphor samples emit intense red emission in 610-615 nm range due to5D0 â†’ 7F2transition of Eu3+ion on excitation with charge transfer band (CTB) wavelength arising due to Eu3+-O2-and also by the discrete bands of Eu3+ions .The decay time of5D0level of Eu3+ion were recorded on excitation with 393 nm and by the CTB wavelength for all the four samples. The optimized 1 mol% Eu3+doped CaAl2O4phosphor exhibits optimum emission intensity and color purity under the excitation with 393 nm than others. The decay time is also found to be larger in the case of Eu3+doped CaAl2O4phosphor sample. Therefore, Eu3+doped CaAl2O4phosphor may be promising material for red color light emitting applications and white light generation.

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.

Gd3+ sensitized enhanced green luminescence in Gd:Tb(Sal)3Phen complex in PVA.

Ternary and tertiary complexes of Tb(Sal)(3)Phen and Gd:Tb(Sal)(3)Phen were synthesized and characterized in PVA polymer. The structural properties of these systems were evaluated on the basis of NMR and FT-IR techniques. The absorption, excitation and emissive properties of the Tb(3+) ion were improved when coordinated with Sal and Phen ligands. Photoluminescence properties of the complexes in solution, crystals and dispersed in PVA film were explored in steady state and in time domain. Selective excitations (487, 355, 310 and 266 nm) of Tb(3+), Sal and Gd(3+) ions reveal an intramolecular energy transfer process. The emission of Gd:Tb(Sal)(3)Phen complex in PVA indicates the contribution of Gd(3+) ion to enhance the emission intensity of Tb(3+) ion. On the basis of these investigations, photophysics was widely discussed in terms of energy transfer and encapsulation effect.

Upconversion based tunable white-light generation in Ln:Y(2)O(3) nanocrystalline phosphor (Ln = Tm/Er/Yb).

We report the generation of efficient white light based on upconversion (UC) in Tm(3+)/Er(3+)/Yb(3+):Y(2)O(3) nanocrystalline phosphor synthesized by simple and cost effective solution combustion technique on 976 nm laser excitation. The calculated color coordinates (using 1931 CIE standard) for samples annealed at different temperatures vary from (0.16, 0.30) to (0.32, 0.33) with dopant concentration, annealing temperature and the pump power; thus providing a wide color tunability including the white one. White emission is observed even at a very low laser power (60 mW). The maximum upconversion efficiency obtained for white emission is 2.79% with the color coordinates (0.30, 0.32) at laser power of 420 mW which is quite close to the standard white color coordinates.

Synthesis, physicochemical and optical characterization of novel fluorescing complex: o-phenylenediamine-benzoin.

The complex of o-phenylenediamine (o-PDA) and benzoin (BN) was synthesized adopting solid state reaction by mixing of their melt together followed by chilling. The phase diagram study shows the formation of a complex in 1:1 molar ratio with congruent melting point and two eutectics lying on either side of complex. The formation of complex was confirmed using the FTIR, NMR, mass spectroscopy, powder XRD and DSC studies. The optical properties of the parent component, their complex and few other compositions nearby the complex were studied using absorption and laser luminescence techniques. The significantly higher green/yellow emission was noted with newly synthesized complex as compared to that of their parents as well as other compositions of o- PDA and BN.

Intense white luminescence from combustion synthesized Ca12Al14O33:Yb(3+)/Yb(2+) single phase phosphor.

The Ca(12)Al(14)O(33):Yb(3+)/Yb(2+) single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb(3+) and Yb(2+) states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb(3+) state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb(2+) ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb(2+) state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb(2+) to Yb(3+) has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb(3+) and Yb(2+) states have been examined and discussed. Quantum yield of emission has also been determined for different samples.

Quality control of herbal medicines by using spectroscopic techniques and multivariate statistical analysis.

Herbal medicines play an important role in modern human life and have significant effects on treating diseases; however, the quality and safety of these herbal products has now become a serious issue due to increasing pollution in air, water, soil, etc. The present study proposes Fourier transform infrared spectroscopy (FTIR) along with the statistical method principal component analysis (PCA) to identify and discriminate herbal medicines for quality control. Herbal plants have been characterized using FTIR spectroscopy. Characteristic peaks (strong and weak) have been marked for each herbal sample in the fingerprint region (400-2000 cm(-1)). The ratio of the areas of any two marked characteristic peaks was found to be nearly consistent for the same plant from different regions, and thus the present idea suggests an additional discrimination method for herbal medicines. PCA clusters herbal medicines into different groups, clearly showing that this method can adequately discriminate different herbal medicines using FTIR data. Toxic metal contents (Cd, Pb, Cr, and As) have been determined and the results compared with the higher permissible daily intake limit of heavy metals proposed by the World Health Organization (WHO).

Overtone spectroscopy of chlorine substituted benzene derivatives.

The absorption spectra of di-, tri- and tetra-derivatives of chlorobenzene have been studied in their pure form in the spectral range 400-20,000 cm(-1). A large number of bands associated with the fundamental, the overtones and the combination frequencies of C-H stretching mode have been observed. Vibrational frequencies, anharmonicity constants and dissociation energies, for the C-H stretch vibrations for the six molecules have been determined using local mode model. The C-H stretch frequencies obtained from experiments are compared with the corresponding frequency determined theoretically using RHF and DFT methods with same 6-31+G* basis set. This information has been used for the assignment of several combination bands as well as some weak overtone bands. Effect of hydrogen atom substitution by chlorine atom has been studied by measuring changes in the vibrational frequency of the C-H stretching mode and the C-H bond length. Frequency changes have been well correlated with the change in charge density on carbon as well as chlorine atoms.

Strong blue emission from Pr3+ ions through energy transfer process from Nd3+ to Pr3+ via Yb3+ in tellurite glass.

Energy transfer excited upconversion emission in Nd3+/Pr3+-codped tellurite glass have been studied on pumping with 800 nm wavelength. The upconversion emission bands from Pr3+ ion are observed at the 488, 524, 546, 612, 647, 672, 708 and 723 nm due to the (3P0 + 3P1)-->3H4, 3P1-->3H5, 3P0-->3H5, 3P0-->3H6, 3P0-->3F2, 3P1-->3F3, 3P0-->3F3 and 3P0-->3F4 transitions, respectively. The addition of ytterbium ions (Yb3+) on the upconversion emission intensity is also studied and result shows an eight times enhancement in the upconversion intensity at 488 nm from Pr3+ ions. The pump power and concentration dependence studies are also made. It is found that Yb3+ ions transfer its excitation energy to Nd3+ from which it goes to Pr3+. No direct transfer to Pr3+ is seen. This is verified by codoping Nd3+ and Pr3+ into the host.

Optical properties of Rh 6G dye in liquid and solid polymer.

Rhodamine 6G dye doped in liquid MMA (monomer) and solid polymer (PMMA) has been studied and its spectra compared with the one dissolved in n-BA solvent. The effect of temperature and laser power on the emissive characteristics of dye in polymer and in solvent has also been compared.