L.A. Diaz-Torres

Luminescence and visible upconversion in nanocrystalline ZrO2:Er3+

The structural and luminescence properties of erbium doped zirconium oxide prepared by the sol-gel processes were analyzed. The annealed powders presented a concentration dependent crystallite sizes and crystalline phase, ranging from 28 to 46 nm and from 40 to 96% for the monoclinic phase, respectively. Green (545 nm) and red (680 nm) emissions bands were observed with 489 and 962 nm excitation. Experimental results showed that the emission bands can be tuned by controlling the Er3+ concentration and that the red band is almost quenched with 489 nm whereas it is enhanced with 962 nm excitation. The nature of this behavior is discussed taking into account the nonradiative energy transfer and cross-relaxation process.

Luminescent properties and energy transfer in ZrO2:Sm3+ nanocrystals

The photoluminescence and crystalline structure characterization of undoped and several samarium doped ZrO2 samples are reported. Strong fluorescence emission produced by the transitions 4G5/2→6H5/2,7/2,9/2 of Sm3+ was obtained by the excitation of the host at 320 nm. The energy transfer process from the host to the samarium ion was confirmed by the analysis of the ZrO2 fluorescence decay curve. It is shown that the content of the active ions stabilizes the tetragonal structure of ZrO2 at 1000 °C, being 73% for 2 mol % Sm2O3 doped and 3% for undoped samples. The dependence between the fluorescence emission and the crystalline structure is discussed.

Role of Yb3+ and Er3+ concentration on the tunability of green-yellow-red upconversion emission of codoped ZrO2:Yb3+–Er3+ nanocrystals

Strong green and red visible emissions were obtained from ZrO2:Yb3+–Er3+ nanocrystals synthesized by sol-gel method and annealed at 1000 °C for 5 h. The average crystallite size was ∼70 nm with tetragonal phase for total concentration lower than 3 mol % and cubic phase for concentration higher than 5 mol %. The color coordinate of the upconverted signal was tailored by controlling the dopant composition that change the red/green ratio dominated by the cross relaxation and energy back transfer process as was demonstrated theoretically and confirmed experimentally. Both coefficients were calculated, C51∼1.02×10−16 and C5b∼6.04×10−17, from the theoretical model based on the rate equations. The highest energy transfer efficiency was η∼64% for 2 mol % of Yb and 2 mol % of Er3+. However, for the highest upconverted signal was only η∼29% obtained for 2 mol % Yb and 1 mol % Er with effective decay time τeff∼438 μs for red and τeff∼290 μs for green band.

Luminescence Concentration Quenching Mechanism in Gd2O3:Eu3+.

Luminescence concentration quenching in Gd2O3:Eu(3+) nanocrystals results from strong interactions among O(2-) ions and Eu(3+) ions. Because all synthesized Gd2O3:Eu(3+) nanocrystals present the same cubic crystalline phase regardless of Eu(3+) concentration, it is possible to study the optical properties as a function of the dopant concentration. The emission intensities and lifetime curves for Gd2O3:Eu(3+) were analyzed by a simple rate equation model to study the interaction between the O(2-) ions and Eu(3+) ions. The rate equation model considers that such interaction is driven by the following energy transfer processes: the direct energy transfer (O(2-) → Eu(3+)), back-transfer (Eu(3+) → O(2-)), and direct energy migration (Eu(3+) → Eu(3+)). The exact solution of this model agrees with the experimental results, luminescence concentration quenching is reproduced and the corresponding energy transfer rates are reported. Quantitative results suggest that the direct energy transfer and direct energy migration processes are the main responsible for the luminescence concentration quenching, whereas the back-transfer process promotes the Eu(3+) emission.

Nanocrystalline tetragonal zirconium oxide stabilization at low temperatures by using rare earth ions: Sm3+ and Tb3+

Zirconium oxide was synthesized by the sol–gel method and the tetragonal structure was stabilized up to 1000 °C by doping with different rare earth ions. The evolution of the crystalline structure as a function of the annealing temperature and rare earth concentration of doped and undoped samples were investigated by X-ray diffraction. Our experimental results show that it is possible to obtain up to 73 wt.% of tetragonal content by doping the ZrO2 with 2 mol% of Sm2O3 or Tb2O3 and annealing at 1000 °C. Variations in the lattice parameters and nanocrystallite size were also obtained. Our results suggest that the stabilization of tetragonal structure can be obtained with other rare earth ions.

Concentration enhanced red upconversion in nanocrystalline ZrO2:Er under IR excitation

The effects of Er3+ concentration in ZrO2 on the red (680 nm) and green (545 nm) upconversion emission from Er3+ under near infrared radiation (NIR) excitation (at both 1438 and 962 nm) are presented. For the highest Er3+ concentration used, the red band is almost quenched under VIS (489 nm) excitation, whereas it is enhanced under NIR excitation. Such a difference between photoluminescence and upconversion is explained taking into account the concentration dependent cross-relaxation process (2H11/2 + 4I15/2) → (4I9/2 + 4I13/2). Experimental results suggest that two- and three-photon upconversion processes are taking place under 962 nm and 1438 nm excitation, respectively.

Monoclinic ZrO2 as a broad spectral response thermoluminescence UV dosemeter

Abstract The thermoluminescence characterization of polycrystalline monoclinic ZrO 2 samples prepared by the sol–gel process subjected to ultraviolet irradiation was performed. The results show that sol–gel synthesized ZrO 2 possesses good thermoluminescence efficiency as well as linear dose response. The thermoluminescence excitation spectrum covers the complete UVA, UVB and UVC range indicating a major advantage over other ultraviolet detector and dosemeters currently used.

Energy back transfer, migration and energy transfer (Yb-to-Er and Er-to-Yb) processes in Yb, Er:YAG

A comparison and discussion of the energy back transfer, migration of energy and direct energy transfer process, from Yb-to-Er and Er-to-Yb ions, in the luminescent system Yb, Er: YAG are presented. The sample crystal was excited with a tunable optical parametric oscillator system. The Yb ions as donors were excited at 940 nm and its fluorescence decay measured at 1030 nm. On the other hand, the Er ions as donors were excited at 800 and 670 nm, and the fluorescence decay of Yb ions as acceptors measured at 1030 nm. The analysis is done through the solution to the General Energy Transfer Master Equations. The ytterbium fluorescence decays in both cases were fitted and found that the migration of the energy among Yb ions is neglected when they act as acceptors.

Luminescence and thermoluminescence induced by Gamma and UV-irradiation in pure and rare earth doped zirconium oxide

Abstract Pure and rare earth doped zirconium oxide was prepared by the sol–gel process and annealed at 1000 °C to stabilize the monoclinic phase. Thermoluminescence (TL) and photoluminescence (PL) under Gamma and UV irradiation were performed. A single TL peak was present at 135 °C under gamma irradiation, whereas under UV irradiation the TL presented two peaks at 62 and 130 °C. The PL of pure and rare earth doped samples was also characterized. The experimental spectra suggest the presence of energy transfer processes between the dopant (Sm 3+ ) and the host (ZrO 2 ).

Effects of energy back transfer on the luminescence of Yb and Er ions in YAG

The back-energy-transfer process from acceptors to donors is included to predict and explain the nonradiative energy-transfer processes among Yb–Er ions in a YAG matrix. This process is in addition to the direct Yb-to-Er energy transfer and the migration of energy among Yb ions. The two measured Yb transients are well fitted by the corresponding solution of the master equations that give the dynamics of the energy-transfer processes.