Luminescence and scintillation properties of Ce-doped Cs2ZnCl4 crystals

In this study, we have synthesized scintillation materials based on Ce-doped Cs₂ZnCl₄ crystals. The light yield was enhanced by up to 20% by doping Cs₂ZnCl₄ with Ce³⁺ ions. In the scintillation time profiles, fast components exhibited decay time constants on the order of nanoseconds, which was ascribed to Auger-free luminescence (AFL). The light yield of the AFL component decreased at 10 mol% Ce³⁺ concentration, which is mainly attributed to the reabsorption of AFL photons inside the crystals by Ce³⁺ ions, as seen in the scintillation spectra. Long components had decay time constants of approximately 30 ns. In addition, at 10 mol% Ce³⁺ concentration, a prominent band appeared at approximately 500 nm in the scintillation spectrum, which was not observed in the photoluminescence spectra. The long components in the scintillation time profiles and the 500 nm band in the scintillation spectra were tentatively attributed to self-trapped excitons perturbed by Ce³⁺ ions.     

EPR,optical absorption and luminescence studies of Cr3+-doped antimony phosphate glasses

Antimony phosphate glasses (SbPO) doped with 3 and 6 mol% of Cr³⁺ were studied by Electron Paramagnetic Resonance (EPR), UV–VIS optical absorption and luminescence spectroscopy. The EPR spectra of Cr³⁺-doped glasses showed two principal resonance signals with effective g values at g = 5.11 and g = 1.97. UV–VIS optical absorption spectra of SbPO:Cr³⁺ presented four characteristics bands at 457, 641, 675, and 705 nm related to the transitions from ⁴A₂(F) to ⁴T₁(F), ⁴T₂(F), ²T₁(G), and ²E(G), respectively, of Cr³⁺ ions in octahedral symmetry. Optical absorption spectra of SbPO:Cr³⁺ allowed evaluating the crystalline field Dq, Racah parameters (B and C) and Dq/B. The calculated value of Dq/B = 2.48 indicates that Cr³⁺ ions in SbPO glasses are in strong ligand field sites. The optical band gap for SbPO and SbPO:Cr³⁺ were evaluated from the UV optical absorption edges. Luminescence measurements of pure and Cr³⁺-doped glasses excited with 350 nm revealed weak emission bands from 400 to 600 nm due to the ³P₁ → ¹S₀ electronic transition from Sb³⁺ ions. Cr³⁺-doped glasses excited with 415 nm presented Cr³⁺ characteristic luminescence spectra composed by two broad bands, one band centered at 645 nm (²E → ⁴A₂) and another intense band from 700 to 850 nm (⁴T₂ → ⁴A₂).     

Preparation and photoluminescence of Bi-doped strontium choloapatite nanorods

The photoluminescence properties of Bi³⁺-doped Sr₅(PO₄)₃Cl nanocrystals with rod-like structures prepared by a simple precipitation method were investigated. The obtained nanorods had diameters of 10-20 nm and lengths of 50-200 nm observed by transmission electron micrograph (TEM). The photoluminescence spectra revealed that a strong blue emission band centered at 416 nm excited by 385 nm was ascribed to ³P₁-¹S₀ transition in Bi³⁺ ions. The optimum dopant concentration for Bi³⁺ ions was 1.5 mol%, which could be attributed to energy migration to quenching centers.     

Synthesis,EPR and optical spectroscopy of the Cr-doped tetraborate glasses

Electron paramagnetic resonance (EPR), optical absorption and photoluminescence spectra as well as luminescence kinetics of the Li₂B₄O₇:Cr and KLiB₄O₇:Cr tetraborate glasses were investigated at T = 300 K. The Li₂B₄O₇:Cr and KLiB₄O₇:Cr glasses containing 0.4 and 1.6 mol.% Cr₂O₃ of high optical quality were obtained from polycrystalline compounds by fast cooling of the melts. The X-band EPR spectroscopy shows that the Cr impurity is incorporated in the tetraborate glass network as isolated Cr³⁺ centers and Cr³⁺–Cr³⁺ pairs coupled by magnetic dipolar and exchange interactions. The EPR spectral parameters (g_eff and ΔB_pp) of both Cr³⁺ and Cr³⁺–Cr³⁺ centers in the Li₂B₄O₇:Cr and KLiB₄O₇:Cr glasses were measured and analyzed. All transitions in optical absorption, luminescence excitation and emission spectra of these glasses are identified. Broad complex bands that peak near 615, 405, and 350 nm in optical absorption and luminescence excitation spectra correspond to the ⁴A_2g(F) → ⁴T_2g(F), ⁴A_2g(F) → ⁴T_1g(F), and ⁴A_2g(F) → ⁴T_1g(P) spin-allowed transitions of the Cr³⁺ centers in distorted octahedral sites of the tetraborate glass network. The octahedral (cubic) crystal field strength (10Dq) and Racach parameters (B and C) for Cr³⁺ centers in Li₂B₄O₇:Cr and KLiB₄O₇:Cr glasses are estimated. Narrow and broad emission bands in red – NIR regions are assigned to the ²E_g(F) → ⁴A_2g(F) (R₁ line) and ⁴T_2g(F) → ⁴A_2g(F) (electron-vibration) transitions, which correspond to the Cr³⁺ centers in high-field and low-field sites, respectively. All observed emission bands are characterized by non-exponential decay. Measured average lifetimes and local structure of the Cr³⁺ centers in high-field and low-field sites of the Li₂B₄O₇:Cr and KLiB₄O₇:Cr glass network have been discussed.     

Luminescence of Bi3+ in the orthorhombic perovskites CaB4+O3 (B4+=Zr,sn): Crossover from localized to D-state emission

The optical properties of Bi³⁺ in the orthorhombic perovskites CaZrO₃ and CaSnO₃ are investigated. The Stokes shift of Bi³⁺ emission in CaZrO₃ is small (∼0.80 eV) with the peak wavelength of the emission band occurring in the ultraviolet. This emission is attributed to the localized ³P_0,1 → ¹S₀ optical transition. In contrast, the Stokes shift of the Bi³⁺ emission in CaSnO₃ is large (>1 eV) with the emission band peaking in the visible. The emission band is also considerably broadened in CaSnO₃. It is claimed that Bi³⁺ luminescence in CaSnO₃ corresponds with the Bi³⁺ (6s²) -Sn⁴⁺ (5s^°) charge transfer emission (D-state emission). The energy of the ¹S₀→³P₁ (A-band) excitation band in both perovskites are very nearly the same. Physical reasoning is advanced for the occurrence and lack thereof of the D-state emission in these perovskites.     

Synthesis and photoluminescence of Sr5(PO4)3Cl:Mn nanorods

Mn-doped Sr₅(PO₄)₃Cl nanorods were synthesized by a simple precipitation method without any controlling agents. The obtained nanorods were uniform and had diameters of 15–35 nm and lengths of 200–300 nm observed by transmission electron microscopy (TEM). A red emission band centered at 695 nm corresponding to ⁴T₁–⁶A₁ transition of Mn²⁺ ion was seen from the photoluminescence spectra. The optimum concentration for Mn²⁺ luminescence in Sr₅(PO₄)₃Cl host is about 1.5 mol%.     

Photoluminescence and magnetic properties of β-Ni(OH)2 nanoplates and NiO nanostructures

Single-crystalline β-nickel hydroxide (β-Ni(OH)₂) nanoplates of hexagonal structure have been synthesized through hydrothermal process. The β-Ni(OH)₂ nanoplates possess well-defined hexagonal shapes with landscape dimension of 45–140 nm and thickness of 20–50 nm. Post-thermal decomposition of the β-Ni(OH)₂ nanoplates led to the formation of single-crystalline NiO nanostructures with landscape dimension of 25–120 nm including nanorolls, nanotroughs and nanoplates. The sizes of the central hole in NiO nanorolls and the low-lying ground in NiO nanotroughs are in the range of 10–24 nm. Two photoluminescence emission peaks appear at 390.5 nm and 467 nm in the photoluminescence spectrum of NiO nanostructures and were assigned to the ¹T_{1 g} (G) → ³A_{2 g} and ¹T_{2 g} (D) → ³A_{2 g} transitions of Ni²⁺ in oxygen octahedral sites, respectively. Temperature-dependent magnetic measurement results show that an antiferromagnetic-paramagnetic transition occur at 26.3 K in β-Ni(OH)₂ nanoplates.     

Nature of intrinsic and impure luminescence of MAlP2O7 crystals

The results of the photoluminescence (PL) investigation of pure and chromium-doped MAlP₂O₇ (M = Na, K, Cs) compounds are presented. The spectra of the intrinsic luminescence of MAlP₂O₇ crystals consist of a separated UV band at a peak position near 330 nm and a complex wide band which covers the region of visible light up to 750 nm at excitation by VUV synchrotron radiation. The “red” band in 600–1000 nm diapason appears in the PL spectra of crystals doped with chromium ions. The effect of the temperature on the structure, the peak positions and intensities of the luminescence bands was studied. An assumption about the nature of the intrinsic PL was made. The “red” luminescence was considered as a result of the ⁴Т₂ → ⁴А₂ radiation transitions in the impurity Cr³⁺ ions located in the intermediate crystal field.     

Effects of ultraviolet excitation on the spectroscopic properties of Sm3+ and Tb3+ doped aluminophosphate glasses

Li₂O–BaO–Al₂O₃–La₂O₃–P₂O₅ glasses optically activated with rare earth ions with the 4f⁵, and 4f⁸ electronic configuration (Sm³⁺ and Tb³⁺, respectively) were analyzed by Raman spectroscopy, absorption, excitation photoluminescence, decay curves and temperature dependent photoluminescence. The spectroscopic characteristics of the as-prepared and heat treated samples at temperatures below and above T_g were studied as well as their room temperature photometric properties under ultraviolet excitation. All the doped glasses exhibit typical signatures of the lanthanides in their trivalent charge state. For the samarium doped glass heat treated at 250 °C (<T_g) the Sm²⁺ luminescence was also observed. The analysis of the luminescence efficiency was performed in the interval range of 14 K to room temperature, where the integrated intensity of the luminescence was found to decrease for the Sm³⁺ and Tb³⁺ ions in the studied temperature range. Luminescence decay curves were found to be non-exponential for the ⁴G_5/2 → ⁶H_7/2 and ⁵D₃ → ⁷F₄ transitions of the Sm³⁺ and Tb³⁺ ions, respectively. The results strongly suggest the occurrence of energy transfer processes through cross relaxation phenomena, mediated by dipole–dipole interaction in all the studied samples. The decay of the ⁵D₄ → ⁷F₅ emission of the Tb³⁺ ions was found to be single exponential with a time constant of ∼3.1 ms. Based on the spectroscopic characteristics, models for recombination processes are proposed. The room temperature luminance photometric properties with ultraviolet excitation show that the samarium doped glasses have much lower luminance intensity (around 0.3 Cd/m²) when compared with the 6–7 Cd/m² observed for the terbium doped ones.     

Facile synthesis of BaMoO4 and BaMoO4/BaWO4 heterostructures with type -I band arrangement and enhanced photoluminescence properties

A series of BaMoO₄ and BaMoO₄/BaWO₄ phosphors were successfully prepared via a polyacrylamide gel method and low temperature calcination technology. The effects of sintering temperature and mass percentage of BaMoO₄/BaWO₄ on the phase purity, functional group, surface morphology, charge state, photoluminescence properties and photocatalytic activity of the prepared products were studied in detail. The results indicate that the BaMoO₄ phosphor is a scheelite tetragonal structure with high crystallinity. The photoluminescence spectra indicates that the phosphors have a strong blue emission peak at 440 nm with excitation wavelength of 282 nm for the BaMoO₄ phosphor, and three emission peaks at 400, 440 and 460 nm with excitation wavelength of 284 nm for the BaMoO₄/BaWO₄ phosphors. These photoluminescence behaviors can be ascribed to the ¹T₂→¹A₁ transition, Jahn–Teller distorted tetrahedral symmetry of [MoO₄]^2- and surface defect. Photocatalytic experiments further confirmed that the BaMoO₄/BaWO₄ phosphors exhibit a high recombination rate of electron hole pairs. The result further indicates that the type-I band arrangement structure of BaMoO₄/BaWO₄ phosphors is beneficial to enhance the photoluminescent properties of single-phase phosphors. This study provides a novel route for preparing the type-I band arrangement structure composite phosphors with high photoluminescent properties and potential applications in light emitting devices, optoelectronic devices, laser devices and white pigments.     

Controlled oxidative synthesis of Bi nanoparticles and emission centers in bismuth glass nanocomposites for photonic application

Here we demonstrate an oxidative process to control metallic bismuth (Bi⁰) nanoparticles (NPs) creation in bismuth glass nanocomposites by using K₂S₂O₈ as oxidant and enhanced transparency of bismuth glasses. Formation of Bi⁰ NPs has been monitored by their distinct surface plasmon resonance (SPR) band at 460 nm in the UV-visible absorption spectra. It is further confirmed by the transmission electron microscopy (TEM) images which disclose the formation of spherical Bi⁰ NPs whereas the selected area electron diffraction (SAED) pattern reveals their crystalline rhombohedral phase. These glasses are found to exhibit visible and near infrared (NIR) luminescence bands at 630 and 843 nm respectively on excitation at 460 nm of the SPR band. It is realized that the luminescence center of bismuth species is an uncertain issue, however, it is reasonable to consider that the emission band at 630 nm is due to the combination of ²D_5/2 → ⁴S_3/2 of Bi⁰ and ²P_3/2 (1) → ²P_1/2 of Bi²⁺ transitions, and that of NIR emission band at 843 nm is attributed to the ²D_3/2 → ⁴S_3/2 of Bi⁰ transition.     

Luminescence and excitation spectra of Cr:MgAl2O4 nanoceramics

The effect of the applied sintering pressure on luminescence and excitation spectra of Cr³⁺ doped MgAl₂O₄ nanoceramics was investigated. It was found that the intensity of ²E → ⁴A₂ phosphorescence decreased significantly when pumped directly into ⁴T₁ band. The effect was extremely strong for nanoceramics sintered at higher pressures. Moreover, the intensity of ⁴A₂ → ⁴T₁ band in excitation spectra drastically decreased with sintering pressure compared to ⁴A₂ → ⁴T₂ band. This behavior was related to trigonal distortion of O_h symmetry of Cr³⁺ ion which increases with decreasing the size of nanocrystals resulting in enhancement of the ⁴T₁ → ⁴T₂ nonradiative pathway due to decrease of ΔE (⁴T₁, ⁴T₂) energy gap.     

Optical properties of sol–gel derived Sr2SiO4:Dy3+ – Photo and thermally stimulated luminescence

Trivalent dysprosium-doped strontium silicate (Sr₂SiO₄) phosphors were prepared by sol–gel synthesis using tetra ethyl orthosilicate (TEOS) as precursor. The synthesis temperature could be brought down to 600 °C for formation of a single phase sample. The material was characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), photoluminescence (PL), and thermally stimulated luminescence (TSL). The luminescence study revealed strong ⁴F_9/2 → ⁶H_13/2 transition at 577 nm (yellow), strong ⁴F_9/2 → ⁶H_15/2 transition at 482 nm (blue) and weak ⁴F_9/2 → ⁶H_11/2 transition at 677 nm (red), when excited by 250 nm (Charge transfer band, CTB) or 352 nm (f–f band). The concentration of the dopant ion and the temperature of annealing were optimized for maximum PL intensity. The critical energy-transfer distance for the Dy³⁺ ions was evaluated based on which, the quenching mechanism was verified to be a multipole–multipole interaction. The thermally stimulated luminescence studies of Sr₂SiO₄:Dy³⁺ sample showed main TSL glow peak at 413 K. The trap parameters namely activation energy (E), order of kinetics (b), and frequency factor (s) for this peak were determined using glow curve shape method.     

Optical temperature sensing based on the luminescence from YAG:Pr transparent ceramics

The YAG:Pr transparent ceramic was fabricated using a conventional solid-state reactive method to explore its possible application in optical thermometry. Photoluminescence and temperature-dependent luminescence were elaborately investigated under 452 nm excitation. The ceramic showed two intrinsic emission bands at 488 and 594 nm, which were attributed to characteristic Pr³⁺: ³P₀ → ³H₄ and ³P₁ → ³H₆ transitions, respectively. Down-conversion emissions from the two thermally coupled excited states of Pr³⁺ were recorded in the temperature range of 293–593 K. The Boltzmann distribution theory was adopted to interpret the temperature-dependent luminescence of Pr³⁺. The temperature sensitivity exhibited an increasing trend with the increase of temperature, typically, 0.0025 K⁻¹ at 593 K. The results indicated that the present ceramic was a promising candidate for optical temperature sensor.     

Photoluminescence properties of Sr3(PO4)2: Eu2+, Dy3+ double-emitting blue phosphor for white LEDs

Double-emitting blue phosphor Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ was synthesized by solid state reaction under H₂ atmosphere. XRD exhibited the pure hexagonal phase of the prepared phosphor. The photoluminescence results showed that all samples had intense broad absorption band between 250 and 450 nm, which matched well with the near-UV (350–420 nm) emission band of InGaN-based chips. The emission spectrum of Sr₃(PO₄)₂: Eu²⁺, Dy³⁺ consisted of two broad bands, peaked at 485 nm and 410 nm, which originated from two luminescent centers, related to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ in six-coordinated Sr(I) and ten-coordinated Sr(II) sites respectively. The intensity ratio of two emission bands could be easily tuned by adjusting Dy³⁺ co-doping content, which resulted in color-tunable luminescence in bluish green region to purplish blue region.     

Comparative study of nondoped and Eu-doped SrI2 scintillator

Optical and scintillation properties of nondoped and Eu 3% doped SrI₂ crystals grown by the Vertical Bridgman method were investigated. Eu-doped crystal showed an intense single band emission at 430 nm due to the Eu²⁺ 5d-4f transitions in both photoluminescence and scintillation while the nondoped crystal had a complex spectral shape. The latter emission consists of mainly four bands: 360 nm, 540 nm, 410 nm and 430 nm. The origins of 360 nm and 540 nm were self-trapped exciton and unexpected impurity, respectively. The origins of 410 and 430 nm lines were ascribed to F center in different I sites. Under ¹³⁷Cs γ-ray irradiations, both crystals showed a clear photoabsorption peak. The scintillation light yields of the nondoped and Eu-doped SrI₂ resulted 33,000 ph/MeV and 82,000 ph/MeV, respectively. The energy resolution at 662 keV of Eu-doped was 4% while that of the non-doped SrI₂ was 8%.     

Mixed alkali effect in Li2O–Na2O–B2O3 glasses containing Fe2O3—An EPR and optical absorption study

This paper reports the interesting results on mixed alkali effect (MAE) in xLi₂O–(30-x)Na₂O–69.5B₂O₃ (5 ≤ x ≤ 28) glasses containing Fe₂O₃ studied by electron paramagnetic resonance (EPR) and optical absorption techniques. The EPR spectra in these glasses exhibit three resonance signals at g = 7.60, 4.20 and 2.02. The resonance signal at g = 7.60 has been attributed to Fe³⁺ ions in axial symmetry sites whereas the resonance signal at g = 4.20 is due to isolated Fe³⁺ ions in rhombic symmetry site. The resonance signal at g = 2.02 is due to Fe³⁺ ions coupled by exchange interaction. It is interesting to observe that the number of spins participating in resonance (N) and its paramagnetic susceptibility (χ) exhibits the mixed alkali effect with composition. The present study also gives an indication that the size of alkali ions we choose in mixed alkali glasses is also an important contributing factor in showing the mixed alkali effect. It is observed that the variation of N with temperature obeys Boltzmann law. A linear relationship is observed between 1/χ and T in accordance with Curie–Weiss law. The paramagnetic Curie temperature (θ_p) is negative for the investigated sample, which suggests that the iron ions are antiferromagnetically coupled by negative super exchange interactions at very low temperatures. The optical absorption spectra exhibit only one weak band corresponding to the transition ⁶A_1g(S) → ⁴A_1g(G); ⁴E_g(G) at 446 nm which is a characteristic of Fe³⁺ ions in octahedral symmetry.     

Study of the line intensity in the optical and magnetooptical spectra in holmium-containing paramagnetic garnets

Studies of line intensity in the optical and magneto-optical spectra in the holmium-containing paramagnetic garnet Ho³⁺:YAG were carried out within the visible spectrum at T = 85 K. Detailed investigation of the magnetic circularly polarized luminescence spectra at 85 and 300 K on ⁵S₂ → ⁵I₈ emission transition in Ho³⁺:YAG was carried out. A quasi-doublet state in the energy spectrum of the Ho³⁺ ions was observed, characterized by a significant magneto-optical activity, which is caused by a large Zeeman splitting of the quasi-doublet. The measurement of the magnetic circular polarized luminescence spectrum carried out within one of the emission lines of the luminescence band ⁵S₂ → ⁵I₈ in Ho³⁺:YAG at 85 K shows significant magneto-optical effects of the intensity change of the emitted light, compared to that measured for the other emission lines in the same luminescent band.     

Upconversion photoluminescence in GeO2-PbO glass codoped with Nd3+ and Yb3+

Frequency upconversion (UC) photoluminescence (PL) in GeO₂-PbO glass codoped with trivalent ions of neodymium (Nd³⁺) and ytterbium (Yb³⁺) is reported. A diode laser operating at 977.7 nm, in resonance with the ytterbium transition ²F_7/2 → ²F_5/2, was the excitation source. Four PL spectral lines, corresponding to the Nd³⁺ transitions ⁴G_9/2 → ⁴I_9/2 (at 500 nm), ⁴G_7/2 → ⁴I_9/2 (at 550 nm), [⁴G_5/2; ²G_7/2] → ⁴I_11/2 (at 595 nm) and ⁴G_7/2 → ⁴I_13/2 (at 660 nm), were observed and characterized. The quadratic dependence of the PL intensities versus the laser power indicates that two laser photons participate in the UC process. The temperature dependence of the PL emissions from 300 to 390 K was also investigated to identify the contribution of phonons for the UC process. The dependence of the UC intensity with the Yb³⁺ concentration and the time behavior of the UC signal indicated the presence of two energy transfer (ET) pathways involving Nd³⁺-Yb³⁺ pairs and Yb³⁺-Nd³⁺-Yb³⁺ triads. Rate-equations for the population densities of the rare-earth energy levels were used to describe the dynamics of the UC emission and to determine the ET rates.