Co-doped NaLa Research Articles

Nd3+/Sm3+ codoped NaLa(WO4)2 glass ceramic: A novel optical heater

Transparent Nd3+/Sm3+ codoped tungstate silicate glass ceramics were prepared and used for the photothermal conversion process. XRD patterns, TEM image and the enhanced Raman signals confirm the appearance of the tetragonal scheelite NaLa(WO4)2 nanocrystals in the vitreous phase. In comparison to the precursor glass, the enhancement of photoluminescence of Nd3+ ions in the glass ceramics attributes to the enrichment of Nd3+ ion in the precipitated low phonon-energy tetragonal scheelite NaLa(WO4)2 nanocrystals. The rapid reduction of photoluminescence of Nd3+ ions in the Nd3+/Sm3+ codoped glass ceramics demonstrates that a strong energy transfer from Nd3+ to Sm3+ takes place, which provides more non-radiative relaxation channels and is beneficial for the improving the photothermal conversion efficiency. Under the irradiation of 808 nm laser diode, a significant temperature rise is observed in the Nd3+/Sm3+ codoped glass ceramics and may be used as a good optical heater.

Electronic transition pathways in energy transfer processes for upconversion photoluminescence of Yb3+/Ho3+ co-doped NaLa(MoO4)2 microcrystals

In recent years, upconversion photoluminescent micro/nanocrystals with unique anti-Stokes luminescence properties have shown high application value in many relevant fields. In this work, pure tetragonal phase NaLa(MoO4)2:Yb3+/Ho3+ microspheres have been synthesized by a solvothermal method in the presence of oleic acid and 1-octadecene. Under the excitation of 980 nm laser, NaLa(MoO4)2:Yb3+/Ho3+ microcrystals emit red, green and near infrared upconversion luminescence with peak wavelengths at 660/643 nm, 541 nm, and 756 nm. Based on the analysis of excitation power-dependent upconversion luminescence intensities and energy level diagram of Yb3+/Ho3+, electronic transition pathways in the energy transfer processes for upconversion luminescence of NaLa(MoO4)2:Yb3+/Ho3+ microcrystals are studied. For red upconversion luminescence at 660/643 nm, the predominant population pathway to state 5F5 (Ho3+) is “Yb3+ (2F5/2) + Ho3+ (5I7) → Yb3+ (2F7/2) + Ho3+ (5F5)”. While, “Yb3+ (2F5/2) + Ho3+ (5I6) → Yb3+ (2F7/2) + Ho3+ (5F4/5S2)” process accounts for the population of state 5F4/5S2 (Ho3+) for radiative transitions of upconversion luminescence at 541 and 756 nm. Energy back transfer contributes to the intense red upconversion luminescence at 660/643 nm. These electronic transition pathways in energy transfer processes are further confirmed by the analyses based on energy level mismatch and luminescent lifetimes.

Eu3+ doped and Er3+/Yb3+ codoped glass ceramics containing NaLa(WO4)2 nanoparticles: Preparation, optical and temperature sensing properties

Eu3+ doped and Er3+/Yb3+ codoped glass ceramics containing NaLa(WO4)2 nanoparticles have been prepared and their optical and temperature sensing properties have been investigated. Strong diffraction peaks and elliptical NaLa(WO4)2 nanoparticles with well-defined lattice stripes are observed in the X-ray diffraction and transmission electronic microscope results, respectively. Great enhancement of the photoluminescence intensity of Eu3+ ions is observed owing to the selective entering of Eu3+ ions into the precipitated NaLa(WO4)2 nanoparticles. The improvement of visible upconversion emissions and near-infrared photoluminescence emissions of Er3+/Yb3+ ions are also attributable to the enrichment of Er3+/Yb3+ ions in the low phonon-energy NaLa(WO4)2 nanoparticles. The temperature dependence fluorescence intensity ratio was investigated and the temperature sensing characteristics was evaluated. The maximum temperature sensitivity Sa is 0.014K−1 at 476 K, which demonstrates Er3+/Yb3+ codoped NaLa(WO4)2:glass ceramic is one of the candidates for new optical temperature-sensing application.

White light emission and bidirectional energy transfer in an Eu3+/Tb3+ co-doped NaLa(WO4)2 phosphor

Eu3+/Tb3+ co-doped NaLa(WO4)2 phosphor was synthesized by a hydrothermal process. The structure of the samples were investigated by X-ray diffraction. The photoluminescence spectra, photoluminescence excitation spectra, and fluorescence lifetimes of the samples were measured to investigate the luminescence properties. We analyzed the bidirectional energy transfer processes between Eu3+ and Tb3+ in NaLa(WO4)2 and gave out the explanations. By adjusting the doping concentrations of Tb3+ and Eu3+, we obtained a phosphor that emits white light. Under the excitation of 382 nm, the chromaticity coordinate of NaLa0.96(WO4)2:1%Eu3+,3%Tb3+ are (0.31,0.33) and the correlated color temperature is 6472 K, indicating that it can be used as a candidate for white light emission.

Mechanism of morphology formation and energy transfer of NaLa(WO4)2:Sm3+:Eu3+ phosphors

A series of phosphors of Sm3+ and Eu3+ single-doped or co-doped NaLa(WO4)2 matrix are synthesized with the hydrothermal method. The structure, morphology, luminescent properties, quantum efficiency and CIE coordinates of sample are characterized and analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM and EDS), transmission electron microscope (TEM), fluorescence spectrophotometer (FL) and UV–Vis (UV) absorption spectra. The results show that all the obtained products have the scheelite structure with spherical or ellipsoidal shapes. The morphology formation mechanism is proposed as follows: nucleation—dissolution—recrystallization—crystal growth (self-assembly process). Under 393- and 403-nm photoexcitation, NaLa (WO4)2:Sm3+:Eu3+ emits intense red and orange red light. In NaLa (WO4)2: Sm3+:Eu3+ sample, the energy transfer can occur from Sm3+ to Eu3+, and the critical distance of energy transfer is between 1.655 and 2.362 nm. The energy transfer mechanism is electric dipole–electric dipole interaction. The energy transfer efficiency can be over 70%. The best product NaLa(WO4)2:2%Sm3+:4%Eu3+ has a luminous efficiency of 54.70%. The CIE coordinates are X = 0.6585, Y = 0.3404.

Sm3+, Tb3+ co-doped NaLa(MoO4)2 temperature sensing materials based on the fluorescence intensity ratio

Sm3+, Tb3+ co-doped NaLa(MoO4)2 used sodium citrate as chelating agent were coalesced by way of hydrothermal method. In this article, samples exist diverse morphology were obtained by controlling the proportion of sodium citrate in the reaction solution. The microstructure and crystallographic structure of the samples were elaborated through the way of X-ray diffraction (XRD) and Field Emission Scanning Electron Microscope (FE-SEM). Test results indicate that the samples were body-centered tetragonal phase even the proportion of sodium citrate has a measurable influence on morphology. The temperature-dependent emission spectra of Sm3+, Tb3+ co-doped NaLa(MoO4)2 phosphors were tested with different Cit3−/Re3+ at 270 nm fluorescence excitation. It elicits that the fluorescence intensity ratio (FIR) of Sm3+ ion and Tb3+ ion varies with the temperature and can be used to characterize the temperature. In addition, the temperature sensing sensitivity and relative sensitivity increased along with Cit3−/Re3+ increases, which the maximum sensing sensitivity is 0.119 K−1 (603 K, Cit3−/Re3+ = 2). And the maximum relative sensitivity is 0.0193 K−1 (498 K, Cit3−/Re3+ = 2). Simultaneously, from Commission International del’Eclairage (CIE) color coordinates, we can observe that the luminescent color of samples changed obviously with the temperature, so that was used for characterizing the temperature.

Intense red emission and two-way energy transfer in Sm3+, Eu3+ co-doped NaLa(WO4)2 phosphors

NaLa(WO4)2 phosphors with various Eu3+ and Sm3+ concentrations were prepared via a molten salt method. The structure and luminescent properties of the prepared samples were analyzed by the X-ray diffraction (XRD) and fluorescence spectrophotometer. The results show that the crystal phase of all the prepared samples is single tetragonal crystal structure. The energy transfer type between Eu3+ ions is proved to be the exchange interaction and the electron-phonon coupling is weak. With the increase of the Sm3+ concentration, the energy transfer from Sm3+ to Eu3+ in NaLa(WO4)2:0.15Eu3+,xSm3+ is transformed from Eu3+ to Sm3+, and the mechanism for energy transfer is given.

Synthesis and Luminescence Properties of a Novel Yellow-White Emitting NaLa(MoO₄)₂: Dy3+, Li+ Phosphor.

Intense yellow-white NaLa(MoO4)2: Dy3+ phosphors co-doped with Li+ ions have been successfully synthesized via facile sol-gel combustion approach. The dependence of the crystal structure and crystallinity, particle morphology, photoluminescence property, fluorescent lifetime and absolute quantum efficiency of the as-prepared phosphors has been investigated. Stable yellow-white emission from 440 nm to 600 nm and higher absolute quantum efficiency were studied on Dy3+ doped NaLa(MoO4)2, Dy3+ and Li+ co-doped NaLa(MoO4)2, respectively. Surprisingly, only a small amount of Li+ can lead to a remarkable increase of the PL intensity and the quantum efficiency. Especially, along with 0.75 mol% Li+ ions induced in the NaLa(MoO4)2: Dy3+ phosphors, the absolute quantum efficiency increased from 13.8% to 22%, and the possible mechanism has been deeply discussed. Outstanding luminescence properties have certified that NaLa(MoO4)2: Dy3+, Li+ phosphors are promising candidates as new yellow-white components for optical devices.

Blue-Light-Excited Eu3+/Sm3+ Co-Doped NaLa(MoO₄)₂ Phosphors: Synthesis, Characterizations and Red Emission Enhancement for WLEDs.

The system NaLa(MoO4)2:Eu3+/Sm3+ phosphors were prepared by solid-state reaction followed by heat treatment at 450–600 °C. As shown by X-ray powder diffraction, the phosphors had a body-centered tetragonal structure, high phase purity and high crystallinity. The photoluminescence measurements carried out under excitation at 464 nm indicated the main emission at about 615 nm corresponding to the electric dipole transition 5D0→7F2 of Eu3+, which agreed well with the emission wavelengths of the GaN-based blue LED chips. In NaLa(MoO4)2:Eu3+/Sm3+ phosphors, Sm3+ can efficiently transfer excitation energy to Eu3+, which resulted in a significant improvement of the fluorescence intensity, and the fluorescence intensity of the phosphors calcined at 550 °C maximized at the doping concentrations of Eu3+ and Sm3+ of 15.0 and 2.0 mol %, respectively. The decay curves and CIE (Commission Internationale de L’ Eclairage) coordinates of Sm3+ or/and Eu3+ doped phosphors were analyzed for the investigation of the energy transfer mechanism and color variation trend. Thus, NaLa(MoO4)2:Eu3+,Sm3+ can be classified as a potential red-emitting phosphor for white light-emitting diodes (WLEDs).

Highly sensitive up-conversion optical thermometry based on Yb3+-Er3+ co-doped NaLa(MoO4)2 green phosphors

Pursuing highly sensitive thermometry is a permanent goal in recent scientific research, which can be achieved through Fluorescence intensity ratio (FIR) technique due to its fast response, excellent accuracy, high spatial resolution and tunable size. Herein, highly sensitive micro-thermometers based on FIR of the thermal coupled levels (TCLs) 2H11/2/4S3/2 → 4I15/2 from Er3+ in phosphors NaLa(MoO4)2: Yb3+/Er3+ up-conversion (UC) were developed, which emits dazzling green light with the excitation of 980 nm laser and its UC spectra consisted of intense green bands peaked at 530 and 552 nm and a negligible 658 nm red emission (4F9/2 → 4I15/2), respectively. The maximum absolute sensitivity approached to 0.023 K-1 at 490 K through assessing FIR based on thermo-responsive green emissions. Results suggest the present green-emitting phosphors have great potential to serve as UC thermometers with high and stable sensitivity, which greatly meet the growing demand for sensing applications.

Color-tunable luminescence properties of Sm3+/Dy3+ co-doped NaLa(MoO4)2 phosphors and their energy transfer mechanism

The Sm3+, Dy3+ doped and Sm3+/Dy3+ co-doped NaLa(MoO4)2 spherical phosphors were hydrothermally synthesized by the EDTA-2Na mediated method. Under the excitation of 297 nm, the quenching concentration of Sm3+ in NaLa(MoO4)2 host was determined to be 13%, and the concentration quenching mechanism was discussed to be the electric quadrupole–quadrupole interaction. After Sm3+ and Dy3+ ions were co-doped into the NaLa(MoO4)2 host, the energy transfer behaviors resulted from Dy3+ to Sm3+ ions were investigated by the help of the luminescent spectra of the obtained phosphors. By varying co-doping concentrations of Sm3+/Dy3+ ions, the emission color of NaLa(MoO4)2:Sm3+/Dy3+ can be tuned from reddish-orange, pink and white to bluish-green. The CIE chromaticity coordinate, the correlated color temperature and the quantum efficiency of NaLa0.87(MoO4)2:1%Sm3+, 12%Dy3+ were calculated to be (0.356, 0.320), 4353 K and 20%, respectively. Furthermore, in the temperature-dependent analysis, it presented good thermal stability, which can become a promising single-phased white-emitting phosphor for white LEDs devices. Based on these results, the possible energy transfer mechanism between Dy3+ and Sm3+ in NaLa(MoO4)2:Sm3+/Dy3+ was also proposed.

Optical thermometry and heating based on the upconversion fluorescence from Yb3+/Er3+ co-doped NaLa(MoO4)2 phosphor

Abstract

Quadratic general rotary unitized design for doping concentrations and up-conversion luminescence properties of Er3+/Yb3+ co-doped NaLa(MoO4)2 phosphors

It is still a great challenge that designing proper codoping concentrations of rare earth ions for achieving intensest expected emission from the studied phosphor. In this work, the quadratic general rotary unitized design (QGRUD) was introduced into the codoping concentration option of NaLa(MoO4)2: Er3+/Yb3+ phosphor for upconversion (UC) applications, and the optimum doping concentrations of Er3+ and Yb3+ for achieving maximum UC luminescence intensity, which is close to commercial NaYF4:Er3+/Yb3+ phosphor, were obtained. The two-photon process was assigned to the green UC emissions in the optimized NaLa(MoO4)2: Er3+/Yb3+ phosphor. It was also demonstrated that the optimized phosphor presented more stable upconversion emissions than the commercial NaYF4:Er3+/Yb3+ phosphor.

Controlled synthesis and luminescence properties of doped NaLa(WO4)2 microstructures

In this work, well-defined NaLa(WO4)2 microstructures were prepared by a facile hydrothermal process in the presence of oleylamine and oleic acid as a mixed surfactant. The phase, morphology, size and luminescence properties were well characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectra, and decay time, respectively. The results reveal that the morphology and size of the as-synthesized samples can be tuned by altering the reaction time and the amounts of oleic acid. A possible formation mechanism of tetragonal NaLa(WO4)2 is proposed on the basis of oleic acid amount-dependent experiments. Upon ultraviolet (UV) excitation, NaLa(WO4)2:Eu3+ and NaLa(WO4)2:Tb3+ exhibit the characteristic down-conversion emissions of Eu3+ and Tb3+. Yb3+/Er3+ and Yb3+/Tm3+ co-doped NaLa(WO4)2 samples show strong green (Er3+, 4S3/2, 2H11/2→4I15/2) and blue (Tm3+, 1G4→3H6) up-conversion emission under 980nm laser diode excitation. Upconversion emission mechanisms of NaLa(WO4)2:Ln (Ln=Yb/Er, Yb/Tm) are also presented based on the emission spectra and the plot of luminescence intensity to pump power. It should be noted that the intensity of the upconversion emissions can be dramatically enhanced compared with sodium free tungstates, which can be attributed to the charge compensation and means the introduction of Na+ ions in WO42− causes less distortion in the crystal structure.

NaLa(MoO4)2: RE3+ (RE3+ = Eu3+, Sm3+, Er3+/Yb3+) microspheres: the synthesis and optical properties

NaLa(MoO4)2: RE3+ (RE3+=Eu3+, Sm3+) microspheres have been synthesized at 180°C via a facile EDTA-mediated hydrothermal route. X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), photoluminescence (PL) spectra were employed to characterize the samples. It was found that the amount of EDTA in the initial solution was responsible for crystal phase and shape determination. The effect of Eu3+ and Sm3+ doping concentrations on the luminescent intensity was also investigated in details. Furthermore, the up-conversion (UC) emissions have been observed in a series of Er3+/Yb3+ co-doped NaLa(MoO4)2 samples. Concentration dependent studies revealed that the optimal composition was realized for a 2% Er3+ and 10% Yb3+-doping concentration.

Upconversion luminescence of Ho3+/Tm3+/Yb3+ tri-doped NaLa(MoO4)2 phosphors

The polycrystalline Ho3+/Tm3+/Yb3+ tri-doped NaLa(MoO4)2 phosphors were successfully synthesized by the simple solid-state reaction method. Under the laser excitation of a 980nm, Ho3+/Tm3+/Yb3+ tri-doped NaLa(MoO4)2 showed bright white upconversion (UC) emission visible to the naked eyes, which composes of blue emission at 475nm from Tm3+, green and red emissions at 540nm and 651nm from Ho3+. The coordinates of Ho3+/Tm3+/Yb3+ co-doped NaLa(MoO4)2 in the commission international de׳eclairage (CIE) chromaticity diagram could be controlled from cool to warm white color depending on Ho3+ and Tm3+ concentrations. The luminescent properties on Tm3+ and Ho3+ concentrations and UC mechanism based on laser pump power were discussed in detail.

Synthesis and luminescent properties of NaLa(MoO4)2:Eu3+,Tb3+ phosphors by microwave-assisted sol–gel method

A series of NaLa(MoO4)2:xEu3+, 5 %-xTb3+ phosphors were synthesized by microwave-assisted sol–gel method using citric acid as complexing agent, followed by a heat treatment at 700–900 °C. The XRD results indicate that the samples have the tetragonal scheelite-type structure with high purity and crystallinity. The scanning electron microscope images demonstrate that the particles of the phosphors are irregular blocky with the size of 1–6 μm. Photoluminescence spectra show that the emission intensity of the phosphors calcined at 900 °C is the strongest. When the Eu3+ and Tb3+ ions are co-doped into the NaLa(MoO4)2 host lattice, the emission spectra show the characteristic transitions of Eu3+ and Tb3+. The luminescence colors of the phosphors can be turned from red, through yellow and green-yellow, to green by changing the relative doping concentrations of Eu3+ and Tb3+. A series of Eu3+ and Tb3+ co-doped NaLa(MoO4)2 phosphors have been successfully synthesized by microwave-assisted sol–gel method. This method exhibits many advantages of short gelling and drying time, energy saving, uniform composition, and so on. The photoluminescence color of phosphors can be tuned from green, through green-yellow and yellow, to red by controlling the doping concentration of Eu3+ and Tb3+.

Synthesis of scheelite structural Yb3+, Er3+ co-doped NaLa(MoO4)2 and its green upconversion luminescence

The polycrystalline Er3+/Yb3+ co-doped NaLa(MoO4)2 phosphor was synthesized, and its green upconversion (UC) luminescence properties were investigated in detail. Under 980-nm excitation, Er3+/Yb3+ co-doped NaLa(MoO4)2 exhibited weak red emissions near 660 nm and 670 nm and strong green UC emissions at 530 nm and 550 nm corresponding to the intra 4f transitions of Er3+ (4F9/2, 2H11/2, 4S3/2) → Er3+ (4I15/2). The optimum doping concentrations of Er3+ and Yb3+ for the highest emission intensity were determined by using X-ray diffraction (XRD) and photoluminescence (PL) analyses. The Er3+/Yb3+ (7.0/10.0 mol%) co-doped NaLa(MoO4)2 phosphor sample exhibited a strong, shiny, green emission. A possible UC mechanism for Er3+/Yb3+ co-doped NaLa(MoO4)2 depends on the pump power is discussed in detail.

Yellow–green upconversion photoluminescence in Yb3+, Ho3+ co-doped NaLa(MoO4)2 phosphor

Upconversion (UC) properties of Yb3+, Ho3+ co-doped NaLa(MoO4)2 phosphors, synthesized by solid state reaction, were investigated in detail under the excitation of 970nm laser diode. The UC luminescence consists of green emission assigned to the 4F4/5S2→5I8 (Ho3+) transition and red emission assigned to the 5F5→5I8 (Ho3+) transition. The light color of the UC luminescence is dependent on the Yb3+ concentration. UC mechanisms were analyzed based on the pump power dependence. The energy transfer efficiency between Yb3+ and Ho3+ was also investigated and the maximum efficiency of 72% was achieved when the Yb3+ concentration increasing up to 20%. Results have shown that the Yb3+, Ho3+ co-doped NaLa(MoO4)2 powder has a potential as a yellow–green UC phosphor for lighting and signal indication.

Photoluminescence of Li+ and Eu3+ Co-Doped Nala (WO4)2 As Near-UV Excited Red Phosphors

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