Gold nanosphere enhanced green and red fluorescence in ZnO: Al, Eu3+

Gold nanoparticles can generate near field due to surface plasmon resonance (SPR) in the visible region. Such near field has the ability to enhance fluorescence of optimally proximal emitters. We have observed augmented green (intrinsic) and red (Eu3+) emission under UV excitation (375 nm) from an important semiconductor ZnO:Al, Eu3+ when optimally conjugated with gold nanospheres. Local field generated by gold nanosphere (similar to 30 nm) is simulated through finite difference time domain method, and a direct correlation with fluorescence enhancement is established

High temperature carrier controlled ferromagnetism in alkali doped ZnO nanorods

Recent efforts in developing spintronic and magneto-optoelectric material for applications have relied on the use of magnetic semiconductors doped with transition metals and have met with limited success. Using a fresh synthesis approach using alkali ions we demonstrate that alkali doped zinc oxide can provide high temperature magnetic semiconductors. We report studies on nanocrystalline powder and pellets of p-type ZnO:Li and ZnO:Na that exhibit ferromagnetism up to 554 K. The ferromagnetic behavior was confirmed from magnetic hysteresis, ferromagnetic resonance, magnetic force microscopy, and explained by a model where substitutional Li+/Na+ in cation site induce local magnetic moments on oxygen atoms. Optimum dopant concentrations enable ferromagnetic exchange interaction leading to high Curie temperature

ZnSe quantum dots through a facile one pot synthesis process

A novel one pot synthesis approach in oleic acid medium was employed to obtain monophasic ZnSe quantum dots (QD) of average size 3.7nm. The QDs were well crystalline in hexagonal phase as revealed by x-ray diffraction and high resolution transmission electron microscopy (HRTEM) studies. The ZnSe QDs exhibit sharp emission peak in the blue (465nm) with 385picosecond fluorescence decay time. The theoretical band gap corresponding to 3.7nm ZnSe QDs matched well with the measured 3.11eV band gap of synthesized QDs which thus showed quantum confinement effect

Enhanced visible fluorescence in highly transparent Al-doped ZnO film by surface plasmon coupling of Ag nanoparticles

ZnO:Al (AZO) film has been deposited on quartz substrate by Pulsed laser deposition and showed monophasic hexagonal structure of c-axis oriented nanorods upto 80 nm in height. AZO film was optimally conjugated with Ag nanoparticles (Ag NPs) in a hybrid nanostructure to achieve significant enhancement in the visible fluorescence emission. Augmented near field and extinction spectra of shape tailored Ag NPs and their dimers are simulated through FDTD method, and a direct association with fluorescence enhancement is established. Such plasmon-enhanced visible emission from a transparent conducting oxide could be very important for solar cell applications

Time resolved spectroscopic studies on some nanophosphors

Time resolved spectroscopy is an important tool for studying photophysical processes in phosphors. Present work investigates the steady state and time resolved photoluminescence (PL) spectroscopic characteristics of ZnS, ZnO and (Zn, Mg)O nanophosphors both in powder as well as thin film form. Photoluminescence (PL) of ZnS nanophosphors typically exhibit a purple/blue emission peak termed as self activated (SA) luminescence and emission at different wavelengths arising due to dopant impurities e.g. green emission for ZnS : Cu, orange emission for ZnS : Mn and red emission for ZnS : Eu. The lifetimes obtained from decay curves range from ns to ms level and suggest the radiative recombination path involving donor-acceptor pair recombination or internal electronic transitions of the impurity atom. A series of ZnMgO nanophosphor thin films with varied Zn: Mg ratios were prepared by chemical bath deposition. Photoluminescence (PL) excitation and emission spectra exhibit variations with changing Mg ratio. Luminescence lifetime as short as 10(-10) s was observed for ZnO and ZnMgO (100: 10) nanophosphors. With increasing Mg ratio, PL decay shifts into microsecond range. ZnO and ZnMgO alloys up to 50% Mg were prepared as powder by solid state mixing and sintering at high temperature in reducing atmosphere. Time resolved decay of PL indicated lifetime in the microsecond time scale. The novelty of the work lies in clear experimental evidence of dopants (Cu, Mn, Eu and Mg) in the decay process and luminescence life times in II-VI semiconductor nanocrystals of ZnS and ZnO. For ZnS, blue self activated luminescence decays faster than Cu and Mn related emission. For undoped ZnO nanocrystals, PL decay is in the nanosecond range whereas with Mg doping the decay becomes much slower in the microsecond range

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 degrees 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