Effect of annealing conditions on the luminescence properties and thermometric performance of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphors.

We report on the synthesis, photoluminescence optimization and thermometric properties of Sr3Al2O5Cl2:Eu2+ and SrAl2O4:Eu2+ phosphor powders. The photoluminescence of Sr2.9Al2O5Cl2:0.1Eu2+ phosphors exhibits a blue-shift with an increasing annealing temperature owing to a decrease in the crystal field strength of the host caused by evaporation of Cl from the material. The quenching of the blue band in favour of the red band observed in the luminescence spectra of Sr2.9Al2O5Cl2:0.1Eu2+ with an increased annealing temperature was explained using the mechanism of the Landau-Zener transitions. The quantum yield and the lifetime of the phosphors depend on the annealing temperature. Phosphor samples annealed at 850 °C, 1000 °C, 1200 °C and 1500 °C were found to be potential luminescence thermometers using the luminescence spectral method. For Sr3Al2O5Cl2:Eu2+ annealed at 1000 °C, the temperature-dependent dual-band intensity ratio demonstrated a high-temperature sensitivity of ∼1.47%/°C in the temperature range of 23 °C to 40 °C which is superior to other reported phosphors with a microsecond decay time, suggesting that the material has potential for sensitive thermometry applications at ambient temperatures.

Luminescence Thermometry Based on the Upconversion Luminescence from the Stark Sublevels of BaY2F8:Yb3+, Tm3+ Phosphor.

Visible and near-infrared (NIR) upconversion luminescence (UCL) emissions originating from the BaY2F8: Yb3+, Tm3+ systems were investigated under a laser excitation at 980 nm. The BaY2F8:20 mol% Yb3+, x mol% Tm3+ and BaY2F8: y mol% Yb3+, 0.5 mol% Tm3+ phosphors showed prominent UCL at 800 and 810 nm. The optimized doping concentrations of Yb3+ and Tm3+ in the BaY2F8 host matrix were evaluated, their spectroscopic properties were determined, and studies on their temperature-dependent behaviour were carried out. The temperature-sensing properties were studied by generating the fluorescence intensity ratio (FIR) of the UCL peaks originating from the thermally-coupled energy levels of the Tm3+ ions. The Stark sublevels of 1G4 level of Tm3+ ions were utilized to estimate the temperature-sensing abilities of the phosphor.

Substitution reactivity and structural variability induced by tryptamine on the biomimetic rhenium tricarbonyl complex.

A series of seven fac-[Re(CO)3(5Me-Sal-Trypt)(L)] complexes containing tryptamine on the N,O 5-methyl-salicylidene bidentate ligand backbone and where L is MeOH, Py, Imi, DMAP, PPh3 coordinated to the 6th position have been studied, including the formation of a dinuclear Re2 cluster. The crystallographic solid state structures show marked similarity in structural tendency, in particular the rigidity of the Re core and the hydrogen bond interactions similar to those found in protein structures. The rates of formation and stability of the complexes were evaluated by rapid time-resolved stopped-flow techniques and the methanol substitution reaction indicates the significant activation induced by the use of the N,O salicylidene bidentate ligand as manifested by the second-order rate constants for the entering nucleophiles. Both linear and limiting kinetics were observed and a systematic evaluation of the kinetics is reported clearly indicating an interchange type of intimate mechanism for the methanol substitution. The anticancer activity of compounds 1-7 was tested on HeLa cells and it was found that all compounds showed similar cytotoxicity where solubility allowed. IC50-values between ca. 11 and 22 µM indicate that some cytotoxicity resides most likely on the salicylidene-tryptamine ligand. The photoluminescence of the seven complexes is similar in maximum emission wavelength with little variation despite the broad range of ligands coordinated to the 6th position on the metal centre.

Power-dependent upconversion luminescence properties of self-sensitized Er2WO6 phosphor.

A Yb3+ free self-sensitized Er2WO6 phosphor has been synthesized via a solid-state reaction method. The phosphor material, Er2WO6, has a monoclinic crystal structure with space group P2/c (13). The deconvoluted high-resolution X-ray photoelectron spectra of all the core elements in the Er2WO6 phosphor material were explored. The highly resolved absorption peaks in the ultra-violet, visible and near-infra-red (NIR) regions of the diffuse reflectance spectrum were due to the Stark-splitting of the 4f energy levels of the Er3+ ions. Under 980 nm NIR laser excitation, the Er2WO6 phosphor showed an intense up-converted red emission at 677 nm due to the 4F9/2→4I15/2 transitions of the Er3+ ions. The cross-relaxation and resonance energy transfer process involved in the key intermediate 4F3/2 and 4F5/2 levels of the Er3+ and their role in generating red emissions were investigated. The laser pump power versus upconversion intensity plot showed a slope with an n value <1 and the possible reasons behind this behavior were investigated. The photoluminescence properties of the Er2WO6 phosphor in the visible and NIR region were further analyzed. The potential application of the phosphor as a marker in latent fingerprint detection was also evaluated.

Electronic and Simple Oscillatory Conduction in Ferrite Gas Sensors: Gas-Sensing Mechanisms, Long-Term Gas Monitoring, Heat Transfer, and Other Anomalies.

The early detection and warning of the presence of hazardous gases have been well studied. We present a study that focuses on some fundamental properties of gas sensors for liquefied petroleum gas (LPG) using spinel nanoferrites, namely, CoSm0.1Fe1.9O4, CoCe0.1Fe1.9O4, MgCe0.1Fe1.9O4, and MgFe2O4. A highly sensitive and selective response of 846.34 at 225 °C toward 10,000 ppm concentration of LPG was recorded. Other flammable gases tested were hydrogen, methane, propane, and butane. Electronic conduction of LPG sensors near saturation showed simple electrical oscillations that can be attributed to the self-dissociation of water molecules physically adsorbed on the surface of the chemisorbed oxygen species due to proton transfer. The oscillatory behaviors follow fluctuations in the operating temperature attributed to heat transfer between the physisorbed water molecules and the hot sensor surface. This depends on the LPG concentration because higher LPG concentration gives rise to greater heat transfer from the sensors. The adsorption and desorption of these water molecule multilayers take a few hundreds of seconds at low concentrations, while the adsorption formation process takes longer at higher concentrations. Other parameters such as LPG exposure time, bias voltage, relative humidity, ambient conditions, operating temperatures, and temperature of the gas not only affect electrical oscillations and thermal fluctuations but also switch the dominant charge carriers from p- to n-type or vice versa. The type of sensor surface, either p- or n-type, did not appear to affect the oscillatory behavior, while the exposure time, short or long, determined the appearance and further behavior of the oscillations. The long-time exposure to 10,000 ppm concentration resulted in the resistance gradually decreasing due to the lack of oxygen supply, while at 5000 ppm, this was constant, stable, and oscillated indefinitely. Changing the dry air to argon gas as a carrier and for dilution of the hazardous gas prevented the electrical oscillations and thermal fluctuations and significantly lowered the response values. Both the inert ambient (argon gas) and changing operating temperature flipped the dominant charge carriers of these sensors. The concentration of these chemisorbed oxygen species governs the charge space and depletion layers. In addition, the spinel nanoferrites used contained higher oxygen vacancies than the lattice oxygen and chemisorbed oxygen. When using dry air, the oscillations were observed at 3000 ppm concentration, while using argon gas, they were observed at 7000 ppm concentration. The room-temperature LPG responses were about 35 and 80 under 45% relative humidity using dry air and argon gas, respectively. These room-temperature measurements showed electrical oscillations but did not show any thermal fluctuations or heat transfer phenomena. This study presents a deeper insight into the fundamentals of gas-sensing mechanisms and energy costs involved.

Symmetry correlations between crystallographic and photoluminescence study of ternary ß-diketone europium(iii) based complexes using 1,10-phenanthroline as the ancillary ligand.

This work entails deep red emitting EuIII based complexes with a variety of ternary ß-diketonate ligands and 1,10-phenanthroline as the ancillary ligand in the system. The solid state structure and spectroscopic analysis has been outlaid in terms of photoluminescence and crystallography. A luminescence quantum efficiency of 50% was obtained for the [tris-(4,4,4-trifluoro-1-chlorophenyl-butanedione)mono-(1,10-phenanthroline)europium(iii)] complexes. Moreover, complexes [tris-(2,2,6,6-tetramethyl-heptanedione)mono-(1,10-phenanthroline) europium(iii)] and {[hexa-(benzyl carboxylic acid) bis-(1,10-phenanthroline)di-europium(iii)]-µ-[κ2-O,O'-(benzyl carboxylic acid)]2} were found to also have quantum yields of 9% and 28% with respective sensitization efficiencies of 85%, 15% and 58%. These results were articulated with crystallographic details pertaining to the nature of coordination and the effect of steric and electronic properties thereof which somewhat impacts the Eu-N bond distances. A symmetry correlation was drawn between the crystallographic data and the photoluminescence data.

Energy Transfer Mechanisms and Optical Thermometry of BaMgF4:Yb3+,Er3+ Phosphor.

Motivated from our previous studies on the upconversion properties of BaMgF4:Yb3+,Tb3+ phosphor, here we investigated the upconversion properties of BaMgF4:Yb3+,Er3+ phosphor. We demonstrate a two-way versatile approach for the fine-tuning of emission from green to the red region, by varying the dopant concentration and adjusting the pulse width of an infrared laser. The mechanism involved in tuning the emission color by laser power and pulse width variation was illustrated in detail. The temperature dependent upconversion spectra were studied by analyzing the fluorescence intensity ratio of the thermally coupled levels. The maximum sensitivity obtained is 83.29 × 10-4 K-1 at 583 K, which is much higher than the temperature sensitivity reported for other fluoride based materials. Moreover, the influence of the excitation power density on the ability of the phosphor for temperature sensing was also investigated. We obtained a maximum (∼415 K) temperature detection at 2563 mW laser power. The obtained results illustrate the potential use of BaMgF4:Yb3+,Er3+ phosphor in an optical thermometer due to its highly sensitive temperature detection ability.

Spectroscopic Investigation of Up-Conversion Properties in Green Emitting BaMgF4:Yb3+,Tb3+ Phosphor.

In this work we have comprehensively studied the up-conversion (UC) properties of BaMgF4:Yb3+,Tb3+ phosphor for the first time. BaMgF4:Yb3+,Tb3+ phosphors were prepared by a simple and low cost precipitation method. To determine the influence of dopant concentration on luminescence properties, the corresponding UC luminescence spectra of BaMgF4:Yb3+,Tb3+ phosphors were studied under NIR excitation. Emission spectra under NIR excitation reveal the vital role of Tb3+ concentration in spectral tuning from the blue to green region. The UC decay curves were also studied to explore the possible energy transfer (ET) mechanisms between Yb3+ and Tb3+. The results reported here are expected to provide an approach for better understanding ET mechanisms in many Yb3+/Tb3+ codoped UC phosphors. This study will be helpful in applications where precisely defined optical transitions is an essential criterion.

Structural, optical and photoluminescence properties of Eu3+ doped ZnO nanoparticles.

The structure, particle morphology and luminescent properties of europium (Eu3+) doped ZnO nanoparticles (NPs) prepared by co-precipitation method are discussed. When excited using a 325nm He-Cd laser, undoped ZnO NPs exhibited weakly the well-known ultraviolet excitonic recombination emission (at ~384nm) and strongly broad band visible emissions associated with defects (at ~600nm). In addition, the ZnO NPs exhibited green emission at ~600nm associated with defects when excited using a monochromatized xenon lamp. Upon Eu3+ doping line emissions attributed to 5D0→7F1,2,3,4 transitions of Eu3+ ions were observed when the materials were excited using a monochromatized xenon lamp. The exchange interaction mechanism is identified as the cause for concentration quenching of the luminescence of Eu3+ doped ZnO NPs in this study.