Intense Emission Band Research Articles

Photoluminescence of divalent cobalt ions in tetrahedral sites of zinc orthotitanate

Zinc orthotitanate (Zn2TiO4) polycrystalline samples doped with Co2+ ions were synthesized by solid state reaction and investigated by X-ray diffraction and photoluminescence spectroscopy at room temperature. Two bands were detected in the photoluminescence spectrum in the near infrared range: the most intense, centered at 714 nm assigned to the T14(P4)→A24(F4) and a less intense band, identified as T14(P4)→T24(F4), both generated from spin-allowed electronic transitions. Photoluminescence excitation spectrum at 714 nm exhibits typical transitions of Co2+ in tetrahedral coordination. From the transitions observed in emission and excitation spectra and using the Tanabe-Sugano theory, the crystal field parameter Dq and Racah parameters B and C were obtained. The intense and broad emission band in the near infrared makes this system a material with high potential for use as tunable media.

Microstructure, crystallographic and photoluminescence examination of Ni doped ZnO nanoparticles co-doped with Co by sol–gel method

Zn0.96−xNi0.04CoxO nanoparticles with different Co concentration from 0 to 6% have been synthesized using sol–gel method. Phase purity of the synthesized samples was confirmed by X-ray diffraction (XRD) and energy dispersive X-ray (EDX) spectroscopy studies. The decreased crystallite size and the increased micro-strain by Co substitution confirmed the substitution of Co2+ instead of Zn2+ ions. Higher optical transmittance and optimal value of energy gap (3.44 eV) found in Zn0.92Ni0.04Co0.04O suggested that it can be taken as possible candidates for the industrial applications especially as transparent electrode. The defects related bluish-green absorption between 483 and 495 nm were due to the existence of oxygen vacancies and intrinsic defects. The strong blue and green band emission was noticed at higher Co-doping concentration due to the generation of new distortion centers in the lattice and surface defects which increases the intensity of green band emission.

Synthesis and properties of furan/thiophene substituted difluoroboron β-diketonate derivatives bearing a triphenylamine moiety

Two novel triphenylamine-modified difluoroboron β-diketonate derivatives containing either a furan or thiophene unit (2c and 2d) have been designed, synthesized and fully characterized. The effects of the triphenylamine substituent and different electron-rich heterocycle units (2-furyl and 2-thienyl) on the photoluminescence and electrochemical properties of these new BF2 complexes were investigated systematically. The two complexes bearing an electron-donating triphenylamine moiety showed broad absorption and intense emission bands in toluene with high fluorescence quantum yields. Significantly larger Stokes shifts were observed for these complexes in comparison to those of the corresponding reference complexes (2a and 2b). Cyclic voltammetry measurements revealed the amphoteric redox properties of the two new BF2 complexes. The results indicated that the introduction of a triphenylamine substituent into BF2 complex could result in large differences in their properties. However, making a contrast between 2c and 2d, it is found that replacing furan with thiophene in the BF2 complex showed slight effects on optical and electrochemical properties. The results of DFT calculations supported the structural and spectroscopic data and confirmed the compositions of frontier molecular orbitals in the BF2 complexes.

Effect of calcium- and/or aluminum-incorporation on morphological, structural and photoluminescence properties of electro-spun zinc oxide fibers

Pure and Al- and/or Ca-added ZnO fibers are synthesized by electro-spinning. Their morphological, structural and optical properties are investigated by means of several complementary characterization techniques, focusing attention on optical properties and fiber surface roughness, which are of relevance in many applications. The comparative analysis of the results obtained demonstrates that all the fibers consist of rounded polycrystalline oxide nanoparticles interconnected to each other, with the (dopant-dependent) nanoparticle size controlling their surface roughness. Non-stoichiometry affects all the samples and strongly influences their photoluminescence properties. Calcium segregates in a secondary phase forming calcite. On the contrary, aluminum well disperses within zincite as an isolated impurity, with Al3+ ions substituting Zn2+ ions in the ZnO lattice. Its content controls the relative exciton-to-defect emission band intensity, as well as the frequency of the main ZnO absorption peak.

Effects of Ho3+ concentration on the photoluminescence of the ZnO Up-conversion films

The ZnO up-conversion films were prepared by sol-gel and spin-coating methods. The results showed that ZnO remained the hexagonal wurtzite structure after rare earth ions were doped. The light transmittance of the films at near-infrared (800–1250nm) wavelengths decreased at first and then increased, and the lowest value appeared when the concentration of Ho3+ was 7mol%. There were two intense emission bands in the up-conversion emission spectrum, namely the 545nm green light and the 660nm red light. The up-conversion luminescence intensity of the films increased at first and then declined, and the maximum was at 7mol%.

Study on luminescence and thermal stability of blue-emitting Sr5(PO4)3F: Eu2+phosphor for application in InGaN-based LEDs

A series of blue-emitting phosphors Sr5(PO4)3F: Eu2+ were synthesized by traditional high temperature solid-state reaction method. The micro-morphology and photoluminescence properties of the phosphors were investigated. The Sr5(PO4)3F: Eu2+ phosphors exhibit broad excitation spectra ranging from 250 to 420nm, and an intense asymmetric blue emission band peaking at 435nm. Two different Eu2+ emission centers in Sr5(PO4)3F: Eu2+ phosphors were confirmed via their fluorescence properties. The concentration quenching mechanism, fluorescence lifetime and thermal stability of Sr5(PO4)3F: Eu2+ phosphors were studied in detail. The thermal stability can be improved obviously by anion substitution. The CIE chromaticity coordinates of Sr5(PO4)3F: Eu2+ phosphors with different Eu2+-doped concentrations were calculated. A blue light-emitting diode was fabricated by combination of a 370nm InGaN chip and the prepared phosphor Sr5(PO4)3F: Eu2+. The present work suggests that Sr5(PO4)3F: Eu2+ is a potential phosphor applied in InGaN-based LEDs.

Luminescence properties, local symmetry and Judd–Ofelt analysis of a Sr2CaWO6:Eu3+, Na+ red phosphor

A series of red phosphors Sr2CaWO6:Eu3+ and Sr2CaWO6:Eu3+, M+ (M=Li, Na, K) were synthesized by conventional solid-state reactions. The optical band gap was calculated through the UV-diffuse reflectance spectrum and strong CT band absorption was observed in compounds in the near ultraviolet region. The optimum doped concentration of Eu3+ was determined to be 20mol%, and the concentration quenching mechanism was found to be electric dipole–dipole interaction. After the introduction of charge compensators Na+, the intensity of the red emission band at 615nm was almost three times than before and shows the strongest emission. Finally, the Ωλ (λ=2 and 4) intensity parameters was calculated according to Judd–Ofelt theory. The CIE colour co-ordinates of the Sr2CaWO6:Eu3+, Na+ was calculated (x=0.661, y=0.339), which are close to NTSC standard values of primary red.

Luminescence characteristics of single-phase white-emitting phosphor Sr2CeO4:Eu3+

A series of (Sr1 − xEux)2CeO4 (x = 0, 0.001, 0.003, 0.005, 0.007, 0.009, 0.01, 0.013 and 0.015) single- phase phosphors have been successfully prepared by solid-state reaction method. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), diffuse reflection, fluorescence decay curves, CIE color coordinates and Correlated Color Temperature (CCT) were used to characterize the prepared phosphors. With the increasing amount of Eu3+, the intensity of the wide blue emission band centered at 475 nm decreased gradually, and the intensity of the sharp peaks of Eu3+ at 615 nm (5D0–7F2) and 585 nm (5D0–7F1) increased gradually at the same time. The influence of Eu3+ concentrations on the properties of the phosphor was discussed. The optimized composition was determined to be (Sr0.99Eu0.01)2CeO4. In addition, the band gap and decay curves of the phosphors were investigated to explore the relationship between the intensity and the content of Eu3+. The enhanced luminescence intensity and good white chromaticity coordinate value of the phosphor are conducive to improve application in the field of white light-emitting diodes (W-LEDs).

980 nm excited Er3+/Yb3+/Li+/Ba2+: NaZnPO4 upconverting phosphors in optical thermometry

The crystalline phase and optical properties of the NaZnPO4: Er3+/Yb3+/M (Li+/Ba2+) phosphors synthesized by conventional solid state reaction method, have been characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman spectra, Diffuse reflectance spectra and Photoluminescence spectra. The effect of co-doping with Yb3+ and non-lanthanide ions (Li+ and Ba2+) on the upconversion (UC) emission intensity has been investigated. The enhancement of about ~ 52 and ~ 132 times respectively in green and red emission bands has been found in the NaZnPO4: Er3+/Yb3+/Li+ phosphors compared to that of the NaZnPO4: Er3+ phosphors. The green colour emitted from the tri-doped phosphors show colour purity ~ 97%. No colour tuning with variation of pump power density in the developed phosphors has been reported. The overall UC emission intensity of the green band of tri-doped phosphors performed in water at ~ 4.0cm of penetration depth reduces to ~ 66% of its original value in the powder phosphors. The maximum sensor sensitivity of the Er3+- Yb3+- Li+ and Er3+- Yb3+- Ba2+ tri-doped phosphors have been obtained as ~ 6.46×10−3K−1 at 603K and ~ 4.32×10−3K−1 at 523K respectively.

Optical emission spectroscopic study of Ar/H2/CH4 plasma during the production of graphene nano-flakes by induction plasma synthesis

Graphene nano-flakes using CH4 precursor were synthesized in a radio frequency inductively coupled plasma reactor with in-situ investigation of Ar/H2/CH4 plasma by optical emission spectroscopy at fixed H2 and Ar flow rates of 4 and 75 slpm, respectively, and at different plate powers (12 to 18 kW), pressures (400 to 700 mbar) and CH4 flow rates (0.3 to 2 slpm). Emissions from C2 Swan band, C3, CH and H2 are observed in the optical emission spectra of Ar/H2/CH4 plasma. Plasma temperature estimated analyzing the C2 Swan band emission intensities is found to be decreased with increasing pressure and decreasing plate power. The decreasing plasma temperature gives rise to increase in production rate due to increase in condensation process. The production rate is observed to be increased from 0 to 0.3 g/h at 18 kW and from 0 to 1 g/h at 15 kW with increase in pressure from 400 to 700 mbar at fixed CH4 flow rate of 0.7 slpm. Broad band continuum emission appears in the emission spectra at specific growth conditions in which the formation of vapor phase nanoparticles due to condensation of supersaturated vapor is facilitated. The production rate at 12 kW, 700 mbar, and 0.7 slpm of CH4 flow rate is found to be 1.7 g/h which is more than that at 15 and 18 kW. Thus, the broadband continuum emission dominates the optical emission spectra at 12 kW due to lower temperature and higher production rate, and is attributed to the emission from suspended nanoparticles formed in vapor phase. The synthesized nanoparticles exhibit flake like structures having average length and width about 200 and 100 nm, respectively, irrespective of the growth conditions. Nano-flakes have thickness between 3.7 to 7.5 nm and are composed of 11 to 22 graphene layers depending on the growth conditions. The intensity ratio (ID/IG) of D and G band observed in the Raman spectra is less than 0.33 which indicates good quality of the synthesized graphene nano-flakes.

Synthesis and photoluminescence properties of Eu doped LiBaPO4 phosphors for solid state lighting

Phosphor LiBaPO4: Eu3+ was successfully synthesized by Pechini (citrate gel) method which is more efficient than conventional high temperature solid state diffusion reaction. X-ray diffraction analysis confirmed the single phase formation of LiBaPO4 with hexagonal crystal structure. Luminescence results showed that the phosphor could be efficiently excited by near UV with intense excitation peak at 393 nm and exhibited two bright red emission narrow bands. In the wavelength range 575–640 nm these bands are centered at 595 and 615 nm, corresponding to characteristics transitions 5D0→7F1 and 5D0→7F2 respectively of Eu3+, with maximum intensity peak at 615 nm. When Eu3+ in LiBaPO4:Eu3+ was reduced by the carbo-thermal reduction process to Eu2+ intense blue broad emission band centered at 472 nm was obtained for LiBaPO4:Eu2+ phosphor when monitored at 350 nm. Hence both of phosphors could be use in solid state lighting application for fabrication of white light emitting diodes.

White Light from a Single Fluorophore: A Strategy Utilizing Excited-State Intramolecular Proton-Transfer Phenomenon and Its Verification

This article is devoted to the development of a strategy for the design of a single fluorophore emitting white light due to the excited-state intermolecular proton-transfer (ESIPT) phenomenon. The key parameters of this strategy include (i) selection of an effective blue emitter, (ii) its structural modification to enable ESIPT, and (iii) adjustment of ESIPT parameters to assess similar fluorescence intensities of both emission bands. The important factor which determines similar intensity of these bands was found to be energetic closeness of the species participating in ESIPT (Δ298G°ESIPT ≈ 0), which makes this phototransformation reversible. Verification of the proposed strategy was carried out by design and synthesis of a new ESIPT fluorophore, which exhibits white light under certain conditions. Spectral features of this compound were investigated in various liquid solutions and solid polymeric films by means of steady-state electronic absorption and steady-state, time-resolved, and temperature-depend...

Biosynthesis, characterization and optical properties of nano-crystalline rosette-shape aragonite and iron(III) chloride–graphite intercalated materials from bacterial cellulose

In this research, well crystalline phase rosette-shape aragonite and iron(III) chloride–graphite intercalated nano-crystals have been prepared by thermal treatment of bacterial cellulose as a promising material.The iron(III) chloride–graphite intercalated nano material was prepared by a novel one-step using bacterial cellulose impregnated directly into the iron(III) chloride with graphitization pyrolysis process at 800 °C under nitrogen. The rosette-shape aragonite was synthesized by thermal treatment at 400 °C in oxygen atmosphere and X-ray diffraction (XRD) results showed the crystal phase of aragonite. The rosette-shape aragonite and iron(III) chloride–graphite intercalated nano materials were characterized extensively by XRD, field emission scanning electron microscopy, fourier transform infrared, and atomic force microscopy techniques. Moreover, the room temperature photoluminescence spectra measurement at excitation band 254 nm of rosette-shape aragonite nano-crystals displayed an intense emission band centred at 500 nm. The nano-structure bacterial cellulose and various its functional groups such as hydroxyls and carboxylates have important effects on nucleation, growth and orientation of prepared crystals. The well crystaline phase rosette-shape aragonite and iron(III) chloride–graphite intercalation compounds have a potential range of application in hydrogen storage and electronic field.

Effect of air flow on the micro-discharge dynamics in an array of integrated coaxial microhollow dielectric barrier discharges

The micro-discharge properties and evolution in a 2D array of integrated coaxial microhollow dielectric barrier discharges are studied by using highly time-resolved electrical and optical diagnostics. The study is focused on the effect of the gas flow rate and gas residence time on discharge properties. The investigated integrated coaxial microhollow discharge geometry allows operating the discharge at exceptionally small residence times, which can be equal to or even smaller than the discharge period, at reasonable gas flow rates. The gas flow has an impact on gas heating, residual humidity, pre-ionization density and the densities of excited and reactive species produced by previous discharges. A unique voltage–charge plot is obtained with elongated periods without discharge activity. A very significant effect of flow on NO emission is observed that relates to the impact of flow on the NO production in these micro-discharges. Using the emission intensities of molecular bands of the second positive system of nitrogen and the first negative system of the nitrogen ion, effective reduced electric field strengths are obtained with a maximum equal to 870 Td. The reduced electric field decreases with increasing gas flow rate. This behavior is consistent with the reduction of the overall discharge intensity due to a reduced amount of charges present in the discharge gap. Both the flow rate and a reduction in water impurity changing the ion mobility can be responsible for the different effective electric field distributions at the highest and no flow conditions.

Near infrared to blue upconverting Tm3+/Yb3+/Li+:Gd2(MoO4)3 phosphors for light emitting display devices

Tm3+-Yb3+/Tm3+-Yb3+_Li+ codoped/tridoped Gd2(MoO4)3 phosphors using chemical co-precipitation method have been synthesized and characterised by XRD, FTIR, diffuse reflectance spectra and FE-SEM analysis. The downconversion emission spectra recorded by using 390nm radiation shows blue colour emission band due to the 1G4→3H6 transition of Tm3+ ion. Frequency upconversion emission study show overall intense blue, red and NIR upconversion emission bands due to the 1G4→3H6, 1G4→3F4 and 3H4→3H6 transitions of Tm3+ ion under 980nm laser diode excitation. The effect of incorporation of Li+ ions in the codoped phosphors produces small shifting in the XRD peaks towards the higher diffraction angle side and also intensity of the blue upconversion emission bands is enhanced around ∼510 times in the Tm3+-Yb3+_Li+:Gd2(MoO4)3 phosphors as compared to the Tm3+ doped phosphors. The confirmation of overall blue colour emission has been identified by colour co-ordinate analysis. The result shows that the developed tri-doped phosphors show improved blue colour emission and hence could be a proper material for near infrared to blue upconverter and blue colour emitting device fabrications.

Improvement for the performance of solar-blind photodetector based on β-Ga2O3 thin films by doping Zn

Highly oriented () Ga2−xZnxO3 thin films with different doping concentrations were grown on (0 0 0 1) sapphire substrates by laser molecular beam epitaxy technology. The expansion of lattice and the shrinkage of band gap with increasing doping level confirms the chemical substitution of Zn2+ ions into the Ga2O3 crystal lattice. The emission intensity of blue-violet emission bands enhanced with the increase of (ZnGa)′ under 254 nm ultraviolet excitation, and the maximum was obtained at x = 0.8. A metal–semiconductor–metal structured solar-blind photodetector based on Ga2−xZnxO3 (x = 0, 0.8) was made, the increasing responsivity and diminishing relaxation time constants for β-Ga2−xZnxO3 (x = 0.8) photodetector were observed with 254 nm ultraviolet illumination.

Effects of calcining temperatures of Eu2+ and Dy3+ ion-codoped calcia-alumina binary compounds on their phase transition and luminescence properties

In this study, phase evolution as a function of calcining temperature in calcia-alumina binary compound phosphors was examined to interpret their luminescence properties. The binary compounds were prepared through a coprecipitation method employing potassium carbonate as the precipitant to obtain precursors with a high-precision stoichiometric composition for the calcination. The results indicate that the morphology, surface properties, and infrared transmittance of the prepared phosphors were affected by the calcining temperature. X-ray diffraction analysis results enabled identification of Ca12Al14O33, CaAl2O4, and CaAl4O7 phase transitions at various calcining temperatures. The amount of the CaAl2O4 phase increased with the calcining temperature within the range of 700–1060 °C. The Ca12Al14O33 and CaAl4O7 phases exhibited trends opposite to that of the CaAl2O4 phase. When the calcining temperature reached 980 °C, the CaAl2O4 phase (60.5%) was determined to be the main phase in the structure, and excellent emission intensity at an emission band of 449 nm was observed as a result of the complete substitution of Eu2+ for Ca2+. The emission intensity corresponding to Eu2+ 4f65d1 → 4f7 decreased slightly when the temperature reached 1060 °C because of more monoclinic reciprocal CaAl4O7 phase (81.5%) formation, causing the transfer of some Eu2+ to Eu3+, during which strong photoluminescence spectra of Eu3+5D0 → 7Fj (j = 0, 1, 2, 3, 4) within the wavelength range of 570–720 nm were observed. Because the strong photoluminescence spectra of the Eu2+ and Eu3+ emissions were together within the wavelength range of 449–720 nm in this phosphor, the photoluminescence was white light. One moderately intense emission band in the infrared region was observed and attributed to the Dy3+4F9/2 → 6Hj (j = 11/2, 13/2, 15/2) transition when the temperature was higher than 900 °C. Finally, the energy transitions of Eu2+, Eu3+, and Dy3+ and the nephelauxetic effect and crystal field are discussed.

Temperature dependence of luminescence intensity in ZnMoO4 crystals

Luminescence of ZnMoO4 crystals (of stoichiometric composition, with excess of molybdenum and doped with tungsten) under X-ray irradiation in the temperature interval from 4 to 400 K was studied. The difference in the composition has little influence on the luminescence properties of the samples. An increase of the 1.93eV emission band intensity was observed in the temperature interval from 8K to 90K. Two mechanisms, concurrence with excitonic emission and concurrence with non-radiative recombination channel, were considered to explain the observation. Theoretical formulas were obtained to describe the temperature dependence of luminescence intensity. The experimental data are well described by the developed theoretical model which assumes recombination of free charge carriers with carriers of opposite sign localized on shallow traps. The high temperature quenching of luminescence is well described by the Mott formula.

Carrier transfer between confined and localized states in type II InAs/GaAsSb quantum wells

Temperature-resolved photoluminescence studies were performed on tensely-strained AlSb/InAs/GaAsSb W-shaped type II quantum wells. They revealed two emission bands: one at lower energy of localized origin resulting from carrier trapping states at interfaces and dominates at low-temperature; and one corresponding to the fundamental optical transition in the type II quantum well. With the temperature increase to 170–200 K the low-energy emission is quenched and the high-energy band dominates and its intensity increases, indicating carrier transfer processes between the respective states at elevated temperatures. In addition, the integrated photoluminescence intensity was measured as a function of excitation power. At high excitation regime the emission intensity of the low-energy emission band saturated, indicating low density of states, thus confirming its localized nature. The depth of the localization potential at the InAs/GaAsSb interface was determined to be 13–15 meV.

Yb3+ sensitized Na2Y2B2O7:Er3+ phosphors in enhanced frequency upconversion, temperature sensing and field emission display

Na2Y2B2O7:Er3+/Yb3+ crystalline phosphors with single monoclinic phase have been synthesized by urea assisted solution combustion method and characterized through X-ray diffraction and Field emission scanning electron microscopy analysis. Frequency upconversion (UC) emission study has been carried out upon 980nm laser diode excitation and maximum enhancement of about ∼1433 times for the UC emission intensity of the green band in the codoped phosphor compared to that of the singly Er3+ doped phosphor has been reported. Optical temperature sensing study performed in the developed codoped phosphor reveals that the material can be used temperature sensing application within 300–613K temperature range with maximum sensitivity ∼7.9×10−3K−1 at 300K. The low-voltage cathodoluminescence measurements have been performed for Na2Y2B2O7:Er3+-Yb3+ phosphors as a function of accelerating voltage and filament current. The developed phosphors exhibit highly intense green emission with some weak emission bands in violet, blue and red regions upon electron beam excitation.