Co-doped Calcium Research Articles

Mode locked Nd3+ and Gd3+ co-doped calcium fluoride crystal laser at dual gain lines

Based on a novel disordered fluoride crystal—Nd3+ and Gd3+ co-doped CaF2, we demonstrate a dual-wavelength synchronously mode locked laser in a single cavity, for the first time. Two gain lines at 1064.2 nm and 1064.7 nm were synchronously mode locked by gain spectrum splitting method, corresponding to a 0.15 THz repetition rate. The mode locked laser shows a 6.5 ps separated pulse duration.

Sol-gel synthesis of biocompatible Eu3+/Gd3+ co-doped calcium phosphate nanocrystals for cell bioimaging

Herein, a polyacrylic acid sol-gel method was presented to synthesize rare earth doped calcium phosphate nanocrystals (RE-CaP) as cell bioimaging agent. The effects of calcination temperature, calcination time and PAA concentration on phase composition, morphology, size and luminescence were discussed. In addition, the hemolysis, cytotoxicity, cell uptake and cell imaging of resulting RE-CaP nanocrystals were investigated. Results show that the increases in calcination temperature and time promote the transformation of product from impure crystalline phase (CaCO3, CaO) to pure CaP phase. The rise of calcination temperature results in significant enhancement of luminescence intensity. PAA concentration does not influence the crystalline phase composition and its increase leads to a little increase in luminescence intensity. In addition, the co-doping of Gd3+ improves the luminescence intensity of Eu3+ emissions. This method achieves the doping of Eu3+ ions in the crystallographic Ca2+ I and Ca2+ II sites of HAP with a preferable occupation at Ca2+ II site. The resulting RE-CaP nanocrystals (PAA: 0.6mg/mL, Eu/Gd = 2:1.5, 600°C, 3h) show good biocompatibility and can be easily uptaken by cancer cells. The internalized nanocrystals show stable and strong red luminescence under visible wavelengths excitation. In summary, PAA sol-gel method can be used to synthesize biocompatible RE-CaP luminescent nanomaterials for cell bioimaging.

Ce3+-sensitized red Mn2+ luminescence in calcium aluminoborate phosphor material

Ce3+ doped and Ce3+,Mn2+ co-doped calcium aluminoborate (CAB) phosphors were synthesised by solid-state reaction method and their optical properties were studied. X-ray powder diffraction, SEM and TEM studies indicated the crystallization of the main trigonal CaAl2B2O7 phase and the presence of an additional non-crystalline phase. It was also observed that increasing dopant concentration promotes phase separation. Hence, both series of phosphors demonstrated the changes in luminescence properties via activator concentration variation. Upon UV excitation (λex = 310 nm) Ce3+ doped and Ce3+,Mn2+ co-doped materials yielded intensive blue and pinkish luminescence, respectively. The spectra of CAB:Ce3+ samples showed a broad emission band due to 5d→4f transition of Ce3+, which broadened and shifted to longer wavelengths with increasing dopant content. Mn2+ co-doping caused appearance of another broad-band emission with a maximum of 680 nm, resulting from the 4T1(4G) →6A1(6S) transition of Mn2+. Detailed analysis of the emission and excitation spectra as well as decay time traces as a function of dopant concentration showed that efficient resonant energy transfer mainly occurs between Ce3+ and Mn2+ incorporated in the non-crystalline phase in CAB material. The estimated values of energy transfer efficiency of CAB:Ce3+(3%),Mn2+(4%) is close to 52%.

Photoluminescence properties of Ce3+/Mn2+ doped calcium zirconium silicate phosphors with energy transfer for white LEDs

A series of Ce3+ doped and Ce3+/Mn2+ co-doped calcium zirconium silicate CaZrSi2O7 (CZS) phosphors have been synthesized via conventional high temperature solid state reactions. The luminescence properties, energy transfer between Ce3+ and Mn2+ have been investigated systematically. Under 320nm excitation, the phosphor CZS: 0.05Ce3+ exhibit strong blue emission ranging from 330nm to 500nm, attributed to the spin-allowed 5d-4f transitions of Ce3+ ions. There are two different emission centers of Ce3+ ions, Ce3+(I) and Ce3+(II). The emission spectra of Ce3+, Mn2+ co-doped phosphors shows a broad emission around 550nm corresponding to the 4T1(4G)-6A1(6S) spin-forbidden transition of Mn2+. The energy transfer between Ce3+ and Mn2+ is detected and the transfer efficiency of Ce3+(II) to Mn2+ is faster than that of Ce3+(I) to Mn2+. The resonant type is identified via dipole-dipole mechanism. Additionally, a blue-shift emission of Ce3+ and a red-shift emission of Mn2+ have been observed following the increase of Mn2+ content in relation to the energy transfer. Thermal quenching has been investigated and the emission spectra show a blue-shift with the temperature increases, which have been discussed in details. CZS: 0.05Ce3+, yMn2+ phosphors can be tuned from blue to white and even to yellow by adjusting the Mn2+ content. All the results indicate that CZS: Ce3+, Mn2+ phosphor have a potential application for near-UV LEDs.

Pulsed Laser Synthesis of Er3+/Yb3+ Co-Doped CaMoO4 Colloidal Nanocrystal and Its Upconversion Luminescence.

We report a novel synthetic route for Er3+Yb3+ co-doped calcium molybdate (CaMoO4) nanoparticles by pulsed laser ablation in ethanol. The crystalline phase, particle morphology, laser ablation mechanism and upconversion (UC) luminescent properties are investigated. Stable colloidal suspensions consisting of well-dispersed Er3+/Yb3+ co-doped CaMoO4 nanoparticles could be obtained without any surfactant. Under 980 nm excitation, Er3+/Yb3+ co-doped nanocolloidal CaMoO4 suspension had a weak red emission near 670 nm and strong green UC emissions at 530 and 550 nm, corresponding to the intra 4f transitions of Er3+ (4F(9/2), 2H(11/2), 4S(3/2)) --> Er3+ (4I(15/2)). The Er3+/Yb3+ co-doped nanocrystalline CaMoO4 suspension exhibited strong green emission visible to the naked eyes and a possible UC mechanism depending on the pump power dependence is discussed in detail.

A clear effect of charge compensation through Na+ co-doping on the luminescence spectra and decay kinetics of Nd3+-doped CaAl4O7

We present a detailed analysis of luminescence behavior of singly Nd3+ doped and Nd3+, Na+ co-doped calcium aluminates powders: Ca1−xNdxAl4O7 and Ca1−2xNdxNaxAl4O7 (x=0.001–0.1). Relatively intense Nd3+ luminescence in IR region corresponding to typical 4F3/2→4IJ (J=9/2–13/2) transitions with maximum located at about 1079nm was obtained in all samples on direct excitation into f–f levels. The effect of dopant concentration and charge compensation by co-doping with Na+ ions on morphology and optical properties were studied. The results show that both, the Nd3+ concentration and the alkali metal co-doping affected the optical properties but had no influence on the powders morphology. The studies of luminescence spectra (298 and 77K) in a function of dopant concentration showed an increasing distortion of the local symmetry of Nd3+with raising activator content due to certain defects created in the crystal lattice. On the other hand Na+ addition led to significant narrowing of absorption and luminescence bands and also a reduction of the number of their components, showing smaller disturbance of Nd3+ ions local symmetries. Consequently, charge compensated by Na+ co-doping materials showed significantly enhanced Nd3+ luminescence. The decrease of emission intensity and luminescence lifetimes with increase of activator concentration was attributed mainly to phonon-assisted cross-relaxation processes between Nd3+ ions. Analysis with Inokuti–Hirayama model indicated dipole–dipole mechanism of ion-ion interaction. Na+ addition led to much smaller concentration quenching due to smaller clustering of dopant ions in CaAl4O7 lattice.

An outlook of rare-earth activated persistent luminescence mechanisms

AbstractBy studying energy transfer and persistent energy transfer properties in Eu, Mn co-doped calcium aluminate CaAl2O4, it was found that traditional persistent luminescence mechanisms had difficulty in explaining persistent energy transfer property between Eu2+ and Mn2+ ions. Based on this question, a mechanism was suggested in which vibrational energy levels were considered as a Morse potential, and trap centers were distinguished into barriers and wells. Then, this phenomenon was explained by allowing energy transfer and persistent energy transfer could have different preference to different vibrational energy levels. Properties of trap centers were also discussed by using the concept of Madelung energy.

Pulsed-laser-assisted synthesis of a Tm3+/Yb3+ co-doped CaMoO4 colloidal nanocrystal and its upconversion luminescence

We report a novel synthetic route for the synthesis of Tm3+/Yb3+ co-doped calcium molybdate (CaMoO4) nanoparticles by using pulsed laser ablation in ethanol. The crystalline phase, particle morphology, particle size distribution, laser ablation mechanism, and upconversion (UC) luminescent properties are investigated. Stable colloidal suspensions consisting of well-dispersed Tm3+/Yb3+ co-doped CaMoO4 nanoparticles with a narrow size distribution could be obtained without any surfactant. Under 980-nm excitation, a Tm3+/Yb3+ co-doped nanocolloidal CaMoO4 suspension showed bright blue emission at a wavelength near 475 nm, which was generated by the 1G4 → 3H6 transition, and a weak red emission at a wavelength near 650 nm due to the 3F2 → 3H6 transition. The Tm3+/Yb3+ co-doped nanocrystalline CaMoO4 suspension exhibited a strong blue emission visible to the naked eyes, and a possible UC mechanism that depends on the pump-power dependence is discussed in detail.

Pulsed laser assisted synthesis of Ho3+/Yb3+ codoped CaMoO4 nanocolloid and its upconversion luminescence

The authors report a novel synthetic route for Ho3+/Yb3+ codoped calcium molybdate (CaMoO4) nanoparticles by pulsed laser ablation in ethanol. Well-dispersed Ho3+/Yb3+ codoped CaMoO4 nanocolloid was prepared without any surfactant. Crystallographic information, microstructure, and the upconversion (UC) luminescent properties of the nanocolloid were investigated. Under the excitation at 980 nm, Ho3+/Yb3+ codoped CaMoO4 nanocolloid exhibited strong yellow emission visible to the naked eyes, which is the combination of green emission near 542 nm and strong red emission around 656 nm. Based on the pump power dependence, UC mechanism of Ho3+/Yb3+ codoped CaMoO4 nanocolloid was discussed in detail.

Cooperative energy transfer in Yb3+–Tb3+ co-doped CaAl4O7 upconverting phosphor

Tb3+ and Yb3+ co-doped calcium aluminate powders: Ca1−x−yYbxTbyAl4O7 (x = 0.02–0.06; y = 0.02), Ca1−x−yYbxTbyAl4O7 (x = 0.03, y = 0.01–0.07), Ca1−x−yYbxTbyAl4O7 (x = 0.01–0.05, y = 0.01) were synthesized by a modified Pechini citrate process and their optical properties were investigated. Upon excitation at 245 nm all studied samples yielded relatively strong Tb3+ luminescence corresponding to the 5D4 → 7FJ and 5D3 → 7FJ transitions with the dominant component around 542 nm. The direct excitation of two Yb3+ ions at 980 nm led to cooperative sensitization of the Tb3+5D4 level giving rise to its green up-conversion luminescence. The up-conversion emission was also observed from higher 5D3 level of Tb3+, which is rather rare in high phonon frequency host materials. The effect of activator concentration and charge compensation by co-doping with Na+ ions on luminescence properties were studied. It was found that Na+ addition lowers disturbance/distribution of activator ions local symmetries and enhanced Stokes Tb3+ photoluminescence brightness, while the up-conversion emission intensity does not appear to change noticeably then.

Low temperature microwave solid-state synthesis of CaCO3: Eu3+, K+ phosphors

An Eu3+ singly doped material and a K+–Eu3+ co-doped calcium carbonate phosphor were synthesized successfully by microwave assisted sintering at low temperature. The sintering condition was set at 270°C for 3h in air. The structure and micro-morphology of samples were characterized and analyzed by X-ray powder diffraction (XRD) and scanning electronic microscope (SEM). Results showed that Eu3+ and K+ ions were uniformly introduced into the host lattice of CaCO3, taking place of Ca2+ ions. The photoluminescence (PL) properties were characterized by PL, PL excitation spectroscopy. The maximum photoluminescence intensity was obtained when the molar concentration of Eu3+ was 0.08. Under the excitation of 394nm, the transitions of 5D0→7FJ (J=0–5) for Eu3+ were observed, and the luminescent intensities of Eu3+–K+ co-doped CaCO3 phosphors could be greatly increased.

Synthesis and luminescent properties of Eu3+/Eu2+ co-doped calcium aluminosilicate glass–ceramics

Eu3+ and Eu2+ co-doped calcium aluminosilicate glass–ceramics have been prepared by devitrification of calcium aluminosilicate glass using heat-treatment. Control of crystallization in the glass–ceramics was studied by X-ray diffraction (XRD) and Raman spectroscopy. The results showed that crystalline phases in glass–ceramic belong to the family of melilite Ca2Mg0.25Al1.5Si1.25O7 as the major phase and anorthite CaAl2Si2O8 as the minor phase. Luminescent properties were investigated by emission; lifetime and the color points were calculated. Emission spectra showed that Eu2+ entered into the crystalline phase in a two steps mechanism: first as Eu3+ which is then reduced to Eu2+. This incorporation in the crystal enhanced Eu2+ emission with increasing time of heat-treatment and therefore crystallization.

Roles of doping ions in afterglow properties of blue CaAl2O4:Eu2+,Nd3+ phosphors

Eu2+ doped and Nd3+ co-doped calcium aluminate (CaAl2O4:Eu2+,Nd3+) phosphor was prepared by a urea-nitrate solution combustion method at furnace temperatures as low as 500°C. The produced CaAl2O4:Eu2+,Nd3+ powder was investigated in terms of phase composition, morphology and luminescence by X-Ray diffraction (XRD), Scanning Electron Microscope (SEM), Fourier Transform Infra Red spectroscopy (FTIR) and Photoluminescence (PL) techniques respectively. XRD analysis depicts a dominant monoclinic phase that indicates no change in the crystalline structure of the phosphor with varying concentration of Eu2+ and Nd3+. SEM results show agglomerates with non-uniform shapes and sizes with a number of irregular network structures having lots of voids and pores. The Energy Dispersive X-ray Spectroscopy (EDS) and (FTIR) spectra confirm the expected chemical components of the phosphor. PL measurements indicated one broadband excitation spectra from 200 to 300nm centered around 240nm corresponding to the crystal field splitting of the Eu2+ d-orbital and an emission spectrum in the blue region with a maximum on 440nm. This is a strong indication that there was dominantly one luminescence center, Eu2+ which represents emission from transitions between the 4f7 ground state and the 4f6–5d1 excited state configuration. High concentrations of Eu2+ and Nd3+ generally reduce both intensity and lifetime of the phosphor powders. The optimized content of Eu2+ is 1mol% and for Nd3+ is 1mol% for the obtained phosphors with excellent optical properties. The phosphor also emits visible light at around 587 and 616nm. Such emissions can be ascribed to the 5D0–7F1 and 5D0–7F2 intrinsic transition of Eu3+ respectively. The decay characteristics exhibit a significant rise in initial intensity with increasing Eu2+ doping concentration while the decay time increased with Nd3+ co-doping. The observed afterglow can be ascribed to the generation of suitable traps due to the presence of the Nd3+ ions.

CaMoO4:Dy3+, K+ near White Light Emitting Phosphor: Structural, Optical and Dielectric Properties

Undoped, Dy 3+ doped and Dy 3+ , K + codoped calcium molybdate phosphors have been synthesized by solid-state reaction method. X-ray diffraction studies reveal the tetragonal structure of the prepared phosphors having crystallite size 15–50 nm. Scanning electron microscopy (SEM) studies reveal the morphology and crystallite size of the prepared phosphors. Photoluminescence studies indicate that there are blue and yellow emissions at 489 and 576 nm, respectively corresponding to Dy 3+ ion. The introduction of K + ion significantly influences the blue and yellow emissions which causes the near white light emission from this codoped phosphor. The intense absorption peak of the codoped phosphor at 210 nm is attributed to the band gap and a shoulder at 240 nm appears due to charge transfer from oxygen ions to neighbouring molybdenum ions. The band gap of the codoped phosphor is calculated as 5.5 eV from the absorption studies. The dielectric properties such as permittivity and dielectric loss are studied as a function of frequency.

Optimization and characterization of trap level distribution in γ-irradiated doped/codoped CaMoO4 phosphors

Different thermoluminescence techniques were applied to optimize and characterize the trap level distribution in γ-irradiated doped/codoped CaMoO4 phosphors. Thermoluminescence glow curves of undoped, Dy3+ doped and Dy3+/K+ codoped calcium molybdate phosphors were recorded showing one glow peak except for the undoped phosphor. The doping and codoping of the phosphor influence the peak temperature drastically. Trapping parameters such as trap depth, order of kinetics and frequency factor were calculated. The effect of heating rate on the glow curves and trapping parameters were investigated in detail.

Intense upconversion luminescence and origin study in Tm3+/Yb3+ codoped calcium scandate

Doping concentration optimized CaSc2O4:0.004Tm3+/0.1Yb3+ shows stronger upconversion luminescence (UCL) than doping concentration also optimized typical oxide upconverting phosphor Y2O3:0.004Tm3+/0.1Yb3+ upon 980 nm laser diode pump. The two-step up converted 3H4 → 3H6 near infrared emission peaked around 800 nm and the three-step up converted 1G4 → 3H6 blue emission around 480 nm of Tm3+ is enhanced by a factor of 3.5 and 2.2, respectively. On analyzing the 2F5/2 → 2F7/2 emission intensities and decay curves of Yb3+ in the two hosts, we reveal that Yb3+ in CaSc2O4 exhibits both a larger absorption cross section at 980 nm and Yb3+ → Tm3+ first step energy transfer coefficient (9.29 × 10−17 cm3 s−1) than that (2.87 × 10−17 cm3 s−1) in Y2O3, indicating that CaSc2O4 is an excellent host for achieving very intense UCL in Tm3+/Yb3+ codoped oxide upconverting materials.

Fluorescent properties of a blue-to green-emitting Ce3+, Tb3+ codoped amorphous calcium silicate phosphors

Ce3+, Tb3+ codoped amorphous calcium silicate phosphor was prepared by heating (830°C for 30min) Ce3+, Tb3+ codoped calcium silicate hydrate phosphor formed by liquid-phase reaction. The excitation peak wavelength of the resulting phosphor was 330nm and the emission peak wavelengths were at 544nm, attributed to the 5D4→7F5 transition of Tb3+, and at 430–470mm, attributed to Ce3+. The intensity ratio of the two peaks could be freely controlled by varying the Tb/Ca atomic ratio of the Ce3+, Tb3+ codoped amorphous calcium silicate phosphor, allowing light to be emitted over a wide range from blue to green. It was clarified that energy transfer exists from Ce3+ to Tb3+.

Fluorescent properties of a green- to red-emitting Eu3+, Tb3+ codoped amorphous calcium silicate phosphor

A Eu3+, Tb3+ codoped amorphous calcium silicate phosphor was prepared by heating a Eu3+, Tb3+ codoped calcium silicate hydrate phosphor formed by liquid-phase reaction for 30min at 900°C. The excitation peak wavelength of the resulting phosphor was 379nm and the emission peak wavelengths were at 542nm, attributed to the 5D4→7F5 transition of Tb3+, and at 613mm, attributed to the 5D0→7F1 transition of Eu3+. The intensity ratio of the two peaks could be freely controlled by varying the Eu/Tb atomic ratio of the Eu3+, Tb3+ codoped amorphous calcium silicate phosphor, allowing light to be emitted over a wide range from green to red. It was clarified that electron transfer from Tb3+ to Eu3+ is occurring.

Laser induced optical heating from Yb3+/Ho3+:Ca12Al14O33 and its applicability as a thermal probe

Abstract Yb3+/Ho3+ co-doped calcium aluminate phosphor has been synthesized using solution combustion process. Multicolored (blue, green and red) strong upconversion emission (λexc=980 nm) due to Ho3+ ion is observed which shows a color tunability (from green to red) with a variation in input laser power. The color tunability has been attributed to be due to the induced heating in the local volume of the sample and the temperature produced has been estimated using the fluorescence intensity ratio (FIR) method. The sample shows temperature sensing behavior and more importantly the temperature could be sensed through two pairs of thermally coupled levels, one lying in the green region (5F4/5S2→5I8) and the other in the blue region (5G4/5G5→5I8). The temperature sensing through the blue pair of levels is novel in itself. The material thus prepared serves as temperature sensor as well as a source for the production of heat in a localized volume.

The influence of SiO2 content on spectroscopic properties and laser emission efficiency of Yb3+-Er3+ co-doped calcium aluminosilicate glasses

We have studied the influence of SiO2 content on the spectroscopic properties and laser emission efficiency of Yb3+-Er3+ co-doped calcium aluminosilicate glasses. An increase in SiO2 content resulted in higher phonon energy, which reduced the up-conversion emission, enhanced the energy transfer efficiency up to 70 % from Yb3+ to Er3+, and enhanced the optical quality. All these results led to an increase from 20 to 30 % in the laser emission efficiency.