Santa Chawla

Observation of Nd3+ visible line emission in ZnO : Nd3+ prepared by a controlled reaction in the solid state

Visible emission from ZnO is usually broadband and realizing line emission from rare earth f?f transitions in ZnO at room temperature has proven to be difficult. A controlled solid-state reaction process with different standing times to synthesize rare earth doped (Nd3+) zinc oxide (ZnO) has shown that sharp and intense line emission in the orange region (595?nm) could be realized under blue excitation, where both excitation and emission transitions occur between Nd3+ levels situated intragap in ZnO. In addition, host-to-Nd3+ energy transfer is also observed under band-to-band excitation. Incidentally, undoped ZnO synthesized by an identical process exhibits a broad green emission. Detailed characterization of the ZnO?:Nd3+ powder by x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, x-ray photoelectron spectroscopy, photoluminescence spectroscopy and time-resolved decay indicates a hexagonal phase with the presence of Nd3+ in the ZnO lattice, forming spherical particles (?200?nm) having a sharp visible emission with decay time in the microsecond range (9.60??s).

Efficient multiphoton upconversion and synthesis route dependent emission tunability in GdPO4:Ho3+, Yb3+ nanocrystals

Double rare earth ion doped GdPO4 nanocrystals with multiform morphologies, such as nanoparticles, nanospheres, and nanorods have been synthesized via four different routes namely a facile solid-state reaction (SSR) method, low temperature co-precipitation (CPP), pechini type sol–gel (SG) and an auto combustion process (ACP). All the methods produce crystalline nanoparticles with dimensions ranging from a few nm to tens of nm but with different morphologies. We systematically investigated the photoluminescence properties of GdPO4:Ho3+, Yb3+ nanocrystals under IR (980 nm) excitation and observed that the emission colour is green for the sample synthesized by the SG route whereas all other samples predominantly emit in the red due to efficient energy transfer from Yb3+ to emitter Ho3+ in the GdPO4 host. The upconversion was found to be a two photon process in GdPO4:Ho3+, Yb3+ nanocrystals and synthesis route dependent emission tunability suggests different intra level phonon relaxation routes.

Intense red-emitting multi-rare-earth doped nanoparticles of YVO4 for spectrum conversion towards improved energy harvesting by solar cells

Yttrium vanadate nano-particles doped with single and multi ions (Sm3+, Eu3+, Bi3+) have been successfully synthesized at room temperature by optimized co-precipitation method. Doped orthovanadate forms monophasic nanocrystals in the 10–50 nm size range. Photoluminescence (PL) excitation shows broad band in the range 250–350 nm due to vanadate absorption and sharp peaks in the range of 390–470 nm due to f–f transitions of Sm3+/Eu3+ and emission in intense red/orange (614, 645, 699 nm). The nanoparticles can efficiently convert UV and blue photons (250–470 nm) to intense red and orange light that can be harnessed by both Si and dye sensitized solar cells for photovoltaic conversion. PL and time-resolved decay suggest that excitation and charge transfer between host, dopant and co-dopants play a profound role in the photophysical processes of multi-ion doped yttrium vanadate nanophosphor. Thin films of such nanophosphor exhibit 80–90% transparency in the visible range. Nanophosphor films convert UV to visible leading to better photon harvesting by solar cells.

Enhanced UV emission in ZnO/ZnS core shell nanoparticles prepared by epitaxial growth in solution

Co-precipitation in highly alkaline environment resulted in well crystalline ZnO nanoparticles over which ZnS shell was grown in situ in solution at room temperature. Formation of epitaxial hexagonal ZnS over wurtzite ZnO core particles have been confirmed by x-ray diffraction and transmission electron microscopy studies. The core ZnO nanoparticles show prominent UV emission which enhances appreciably due to ZnS shell formation in ZnO/ZnS core/shell particles. Experimental evidence indicates the quenching of non radiative recombination pathways due to shell formation. The results suggest that synthesized epitaxial core/shell particles have charge confinement mainly in the core region with type I characteristics of band alignment and can be useful for device applications.

Introducing dual excitation and tunable dual emission in ZnO through selective lanthanide (Er3+/Ho3+) doping

We have introduced dual excitation properties in the multifunctional semiconductor ZnO by controlled solid state diffusion of dopant lanthanide ions like Er3+ and Ho3+ into the lattice at 500 °C. So far light emission from doped ZnO has been explored either under UV or IR excitation. Our results show that the emission colour can be tuned from cyan to red under UV (band edge, 377 nm) excitation and from green to red under IR (980 nm) excitation in ZnO through selected doping of lanthanide ions. Doping lanthanide ions in ZnO changes its morphology and emission characteristics. Whereas down conversion emission under UV excitation is due to across band gap excitation and subsequent donor–acceptor pair recombination, the dependence of up conversion emission yield on pump laser power indicates that two to three photon processes may be more effective in ZnO hosts for frequency upconversion.

Förster's resonance energy transfer between Fullerene C60 and Coumarin C440

The interaction between Coumarin C440 with Fullerene C60 has been studied by fluorescence and time resolved spectroscopic techniques. The Coumarin C440-Fullerene C60 pair shows Forsters resonance energy transfer (FRET) from Coumarin C440 (donor) to Fullerenes C60 (acceptor). The FRET efficiency of this pair increases with the increase of the acceptor concentration. The critical energy transfer distance (R0) at which transfer efficiency is 50% is found to be 34Ǻ. Stern-Volmer plot indicates static as well as dynamic quenching. However, the FRET studies show highest efficiency at the critical stage of dimer formation.

Fabrication of dual excitation dual emission phosphor with plasmonic enhancement of fluorescence for simultaneous conversion of solar UV and IR to visible radiation

A dual excitation, dual emission phosphor has been fabricated by simultaneous doping of lanthanide ions Er3+, Yb3+, Eu3+ in a highly efficient host YVO4. YVO4 doped with Er3+, Yb3+, Eu3+ showed dual excitation i.e., simultaneously excitable by UV and IR radiation and dual emission i.e., bright red fluorescence under UV excitation and intense green under IR excitation. Red DC fluorescence arises due to 5D0–7FJ transitions of Eu3+. UC emission spectra indicate that Yb3+ not only sensitizes Er3+ to emit predominantly in the green but also Eu3+ to produce a signature red emission. A direct assembly of Ag nanoparticles (NPs) and YVO4:Er3+, Yb3+, Eu3+ on a suitable substrate showed enhancement of fluorescence of Eu3+ red emission under UV excitation. Such studies indicate that a two dimensional conformal transparent layer of a Ag NP–phosphor combined on a silicon solar cell may be used as a DC and UC solar spectrum converter from UV/IR to the visible region where the spectral response of Si is high.

Red upconversion luminescence and paramagnetism in Er/Yb doped SnO2

Red upconversion luminescence has been introduced in functional semiconductor SnO2 by doping rare earth ions Er3+ and Yb3+ and using 980 nm diode laser as pump source. High temperature solid state reaction leads to incorporation of trivalent lanthanide ions in SnO2. For high concentrations of lanthanide doping, partial reduction of Sn4+ to Sn2+ occurs for charge compensation making the material system SnO2-SnO alloy. Upconverting SnO2 exhibits paramagnetic characteristics with magnetization increasing with introduction of trivalent rare earth ions Er3+/Yb3+.

Plasmonic enhancement of dual mode fluorescence in a silver nano-antenna–ZnO:Er3+ hybrid nanostructure

Tuning of surface plasmon resonance (SPR) of silver nanoparticles (Ag NPs) through shape tailoring make them frequency tunable multipolar optical nano antennas that can be harnessed for optical enhancements in a fluorophore placed in optimal proximity. Such SPR tuning has been achieved with Ag nano-hexagons in which enhancements in both down (under UV excitation) and up (under IR excitation) conversion fluorescence from rare earth Er3+–doped ZnO nanoparticles are realised. The near field generated by the pure Ag NPs and their hybrids under UV and IR incident light is simulated using a finite difference time domain method, and a direct correlation with the observed fluorescence enhancement is established.

Red enhanced YAG:Ce, Pr nanophosphor for white LEDs

Rare earth ion Pr3+ was co doped in yellow emitting YAG:Ce nanophosphor to introduce red emission for improving colour rendering property of white LED. Nanocrystalline YAG:Ce, YAG:Ce, Pr was synthesised by a single step auto combustion process. Photoluminescence (PL) emission spectra of YAG:Ce, Pr nanophosphor, when excited by 460 nm blue light, exhibited sharp red emission peaks at 610 nm from Pr3+ ions in addition to broad yellow emission from Ce3+ peaking at 540 nm due to charge transfer from excited Ce3+ 5d band to 3P J /1D2 level of Pr3+. Ce3+ (yellow) and Pr3+ (red) emission decays in tens of nanoseconds and microseconds, respectively. Electroluminescence spectra of blue LED with developed phosphor layer shows colour coordinates (0.30, 0.39) close to ideal white light.