Light-emitting structures based on nanocrystalline (Si/CaF2) multiquantum wells

Periodic nanocrystalline (Si/CaF₂) multilayers, deposited on (111) silicon by Molecular Beam Epitaxy (MBE) at room temperature, were used to fabricate simple light emitting structures and to study their electrical and optoelectronic properties. Photoluminescence (PL) and electroluminescence (EL) spectra from the same area of the devices are approximately the same, indicative of the same emission mechanism. Current–voltage characteristics reveal important phenomena in vertical carrier transport. Regions of negative differential resistance and current oscillations were observed and were tentatively attributed to resonant tunneling at high electric fields under field domain formation.     

Thermoluminescent properties of a Li2O-B2O3-Al2O3 glass system doped with CaF2 and Mn

A Li₂O-B₂O₃-Al₂O₃ glass system doped with CaF₂ and Mn was synthesized by fusion and its physical properties were investigated using thermoluminescence (TL) and differential thermal analysis (DTA) techniques. The TL glow curve peaks, resulting from this analysis, are characteristic of metastable levels intrinsic to CaF₂ crystals that have undergone γ-ray irradiation from a colbalt-60 (⁶⁰Co) source. This provides evidence of CaF₂ crystal formation in the glass system. Furthermore, the TL glow peak at about 480 K was stable at room temperature, sensitive to ⁶⁰Co γ-rays and showed good linearity with doses ranging from 3 Gy to approximately 100 Gy, and consequently could be used to quantify radiation doses. High quality synthesis of these crystals permits control of their thermoluminescent properties.     

Synthesis and luminescence of SrZnO2 phosphors

SrZnO₂ phosphors have been synthesized by two new methods viz. carbonate decomposition at 1000 °C and combustion synthesis. Phosphors activated with Pb²⁺, Sm³⁺, Tb³⁺, Bi³⁺ and Pr³⁺ could be prepared in one step using the combustion synthesis. Characteristic emission and excitation were observed for Bi³⁺. For the remaining activators excitation spectra always contained a band at 283 nm. Presence of this band for all these different types of activators was interpreted as host sensitization.     

Preparation and luminescence property of Dy-doped YPO4 phosphors

The co-precipitation reaction soft chemistry method was employed to prepare fine YPO₄:Dy³⁺ phosphor particles calcined at 950 °C. Adjusting appropriate pH and introducing lithium may greatly affect the particle size of the product and further affect the luminescence intensity. The as-prepared samples were characterized by X-ray diffraction (XRD), transmission electron micrograph (TEM) and photoluminescence (PL) spectroscopy. The obtained YPO₄ nanocrystals appeared to be spherical with some agglomeration and their sizes ranged from 20 to 40 nm. The characteristic transitions of Dy³⁺ due to ⁴F_9/2→⁶H_15/2 (blue) and ⁴F_9/2→⁶H_13/2 (yellow) were detected in the emission spectra.     

Up-conversion white light of Tm/Er/Yb tri-doped CaF2 phosphors

Tm³⁺/Er³⁺/Yb³⁺ tri-doped CaF₂ phosphors were synthesized using a hydrothermal method. The phosphors were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and up-conversion (UC) emission spectra. After annealing, the phosphors emitted white light under a 980 nm continuous wave diode laser (CW LD 2 W) excitation. As the excitation power density changed in the range of 20-260 W/cm², the chromaticity coordinates of the UC light of the phosphor Ca_0.885Tm_0.005Er_0.01Yb_0.1F₂ fell well in the white region of the 1931 CIE diagram. For the proportion of red, green and blue (RGB) in white light is strict, key factors for achieving UC white light, such as host materials, rare earth ions doping concentrations, annealing temperatures, as well as the excitation power densities, were investigated and discussed.     

Preparation and characterization of tunable YVO4: Bi, Sm phosphors

Yttrium vanadate phosphors co-doped with Bi³⁺- and Sm³⁺ ions have been prepared via the solid-state reaction as well as via the sol-gel method. The luminescence studies demonstrate the potential of the prepared phosphors as multi-color emitters, which can be achieved by adjusting the excitation wavelengths. The excitation spectra of Bi³⁺- and Sm³⁺ co-doped phosphors clearly revealed energy transfer from Bi³⁺ to Sm³⁺ ions. When the co-doped phosphors were excited at 254 nm, the emission from Bi³⁺ was dominant. Upon excitation at 365 nm, the emission from both Bi³⁺ and Sm³⁺ was detected. With 410 nm excitation, Sm³⁺ ions were selectively excited to yield intense red emission. It is shown that the prepared phosphors with optimal concentrations of Bi³⁺ and Sm³⁺ can be excited at 254, 365 and 410 nm to yield yellow, orange and red emissions, respectively.     

Preparation and luminescence properties of Tb doped ZrO2 and BaZrO3 phosphors

ZrO₂:Tb³⁺ and BaZrO₃:Tb³⁺ powders are prepared by combustion synthesis method and the samples were further heated to 500, 700 and 1000 °C to improve the crystallinity of the materials. The structure and morphology of materials have been examined by X-ray diffraction, Raman spectra and scanning electron microscopy. It is remarkable that all the samples of ZrO₂:Tb³⁺ and BaZrO₃:Tb³⁺ have similar morphology. These images exhibited homogeneous aggregates of varying shapes and sizes, which are composed of a large number of small cuboids and broken cuboids. The cuboids and broken cuboids size of all the samples are less than 0.5 μm. Photoluminescence for both materials increases with increase of temperature and found maximum for the samples heated to 1000 °C with 5 mole% doping of Tb³⁺ ions. Luminescence is almost double for the zirconia compared to that of barium-zirconate.     

Preparation and characterization of Zn2SiO4: Mn phosphors with hydrothermal methods

Mn²⁺-doped Zn₂SiO₄ phosphors had been prepared by hydrothermal method in stainless-steel autoclaves. Effects of synthesized methods, reaction temperature, ambience of heat treatment on the structure and the luminescence properties of this silicate were studied with X-ray diffraction apparatus (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) and fluorescence spectrum. Results show that Zn₂SiO₄ nanocrystalline can be obtained by hydrothermal method at relatively low temperatures. The absorption pattern shows an absorption edge at about 380 nm originated from ZnO crystals and two absorption bands at about 215 and 260 nm. Mn²⁺-doped Zn₂SiO₄ has a luminescence band with the wavelength at about 522 nm under 255 nm excitation, and the luminescent intensity increases after being heat treated.     

Synthesis and luminescence properties of red-emitting phosphors NaY0.87Eu0.13(WO4)1.2(MoO4)0.8

A series of NaY_1−yEu_y(WO₄)_2−x(MoO₄)_x (x=0−2 and y=0.06−0.15) phosphors have been prepared by a combustion route. X-ray powder diffraction, photoluminescence excitation and emission spectra were used to characterize the resulting samples. The excitation spectra of these phosphors show the strongest absorption at about 396 nm, which matches well with the commercially available n-UV-emitting GaN-based LED chip. Their emission spectra show an intense red emission at 616 nm due to the ⁵D₀→⁷F₂ electric dipole transition of Eu³⁺. As the Mo content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm increases and reaches a maximum when the relative ratio of Mo/W is 2:3. The intense red-emission of the tungstomolybdate phosphors at near-UV excitation suggests that the material is a potential candidate for white light emitting diode (WLEDs).     

Estimation of conductivity parameters in Sr1 − xThxF2 + 2x solid solution with fluorite-type structure

Concentration of nn-dipoles, nnn-dipoles and charge carriers as well as parameters related to their reorientation and mobility have been estimated from an analysis of a.c. conductivity data of polycrystalline samples of the Sr_{1 − x}Th_xF_{2 + 2x} solid solution. The molar fraction of charge carriers has been evaluated as a function of composition. Results obtained confirm, in agreement with the clustering process model, that the (F_i)_m ions responsible for long range motions in Sr_{1 − x}Th_xF_{2 + 2x} are interstitial fluoride ions of F type.     

Combustion synthesis of Sr3MgSi2O8:Eu and Sr2MgSi2O7:Eu phosphors

Sr₃MgSi₂O₈:Eu²⁺ and Sr₂MgSi₂O₇:Eu²⁺ phosphors find uses in applications such as plasma display panel (PDP), solid-state lighting, longafter glow. Preparation of these phosphors by a modified combustion synthesis is described in this paper. As-prepared samples did not show photoluminescence. After reducing the samples at 900 °C, characteristic Eu²⁺ emission was observed. Preparation of these phosphors by using similar methods helped clarifying various results obtained for Sr₃MgSi₂O₈:Eu²⁺ by different investigators.     

Photoluminescence of cubic and monoclinic Gd2O3:Eu phosphors prepared by flame spray pyrolysis

In this paper, europium-doped gadolinium phosphor, which is a potentially bifunctional material with both fluorescent and magnetic properties, has been prepared in a one-step procedure via flame spray pyrolysis, and its crystal structure, morphology, and PL intensity were investigated. All the prepared phosphors were submicron-sized with spherical shapes and either a pure cubic or pure monoclinic phase. In order to observe the effects of temperature on the crystal phases of the prepared phosphors, we applied a H₂ vs. N₂/O₂ diffusion flame, with the maximum flame temperature ranging from T_max=1375 to 2050 K. The temperature profiles under various flame conditions are also reported herein to further elucidate the rapid synthesis process. The PL intensity in the cubic phase improved linearly with increasing flame temperature until the transition to a monoclinic phase. The peak of the photoluminescence(PL) spectrum from the phosphors prepared at T_max=1733 K in the cubic phase was narrower and twice as strong as the peak of the PL spectrum from the phosphors prepared at T_max=2050 K in the monoclinic phase. This paper provides important data showing the relationship between the synthesis temperature and the phase transition in Gd₂O₃:Eu in the continuous one-step use of flame spray pyrolysis.     

Photoluminescence of Eu single doped and Eu/Dy codoped Sr2Al2SiO7 phosphors with long persistence

This paper reports the preparation of long persistent Sr₂Al₂SiO₇:Eu²⁺ and Sr₂Al₂SiO₇:Eu²⁺, Dy³⁺ phosphors and the comparison of their photoluminescent properties. The silicate phosphors prepared by solid-state reaction routine showed a broad blue emission peaking at 484 nm when activated by UV illumination. Such a bluish-green emission can be attributed to the intrinsic 4f-5d transitions of Eu²⁺. After the UV source was switched off, long persistent phosphorescence could be observed by naked eyes for both samples in darkness. Afterglow measurements revealed that Eu/Dy codoped phosphor possesses better afterglow properties than the Eu single doped one, since the maximum lifetime (τ_max=99 s) of the photons calculated from the decay profile is much larger than that of the Eu single doped phosphor (τ_max=82 s). TSL results suggested that the difference in afterglow properties was caused by the difference in the electron traps within the crystal lattice. For Eu/Dy codoped phosphor, the doping of Dy ions produced electron traps with trap depth of 0.52 eV, which is suitable and therefore leads to good persistence. However, in the case of Eu single doped phosphor, the trap depth is 0.88 eV, which is really too deep an energy barrier to overcome, and therefore a poor persistence was observed in the experiment.     

White light-emitting Mg0.1Sr1.9SiO4:Eu phosphors

Divalent europium-activated strontium orthosilicate Sr₂SiO₄:Eu²⁺ and Mg_0.1Sr_1.9SiO₄:Eu²⁺ phosphors were synthesized through the solid-state reaction technique. Their luminescent properties under ultraviolet excitation were investigated. The X-ray diffraction (XRD) results show that these phosphors are of α′-Sr₂SiO₄ phase with a trace of β-Sr₂SiO₄. Doping of Eu²⁺ ion into the crystal lattice results in the lattice constant being expended, while Mg²⁺ makes the lattice constant shrinking. A solid solution with the same crystal structure is formed when Eu²⁺ or Mg²⁺ substitutes part of Sr²⁺ ions and occupies the same lattice sites. The Sr₂SiO₄:Eu²⁺ phosphors show two emission spectra peaked at 535 and 473 nm originated from the 5d-4f transition of Eu²⁺ ion doped in two different Sr²⁺ sites in the host lattice. By substitution of 0.1 mol of Sr²⁺ with Mg²⁺, these two emission bands are tuned to be in the blue and yellow region (459 and 564 nm for Mg_0.1Sr_1.88SiO₄:Eu_0.02), respectively. The tuning effect is discussed. With a combination of the blue and yellow emission bands the phosphors show white color, indicating that these phosphors may become promising phosphor candidates for white light-emitting diodes (LEDs).     

Synthesis and optical properties of Ce-doped Y3Mg2AlSi2O12 phosphors

Y_3−xMg₂AlSi₂O₁₂:Ce_x³⁺ (x=0.015, 0.03 and 0.06) phosphors possessing garnet crystal structure were synthesized by the sol–gel combustion technique. The samples were characterized by application of powder X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, thermal quenching (TQ) and scanning electron microscopy (SEM). Moreover, luminous efficacies (LE), color points and quantum efficiencies (QE) were calculated. Optical properties were studied as a function of Ce³⁺ concentration and annealing temperature. XRD analysis revealed that sintering of polycrystalline Y₃Mg₂AlSi₂O₁₂:Ce³⁺ powders at 1550 °C results in nearly single-phase garnet materials. Phosphors showed broad emission band in the range of 500–750 nm and had the maximum intensity at 600 nm, which results in strongly red-shifted phosphors compared with conventional YAG:Ce phosphors emitting at 560 nm. However, strong concentration quenching has also been observed, probably due to increased Stokes shift.     

Photoluminescence and energy transfer of Tm doped LiIn (WO4)2 blue phosphors

Room temperature steady and time resolved emission spectra of LiIn_1−xTm_x(WO4)2 (where thulium concentration is 0, 0.5, 1, 5 and 10 at%) blue phosphors, under UV excitation energy have been investigated. The concentration quenching effect on the blue emission, due to the (WO₄)⁻² groups and ¹G₄→³H₆ emission transition of Tm³⁺ were studied. Two energy transfer mechanisms are shown. The first takes place between excited (WO₄)⁻² groups and the ¹G₄ energy level of Tm³⁺, and is mainly analyzed by phonon-assisted energy transfer. The second mechanism is due to an energy transfer from the excited Tm³⁺ ions to the surrounding ground state Tm³⁺ ions. The non-exponential decay curves of the ¹G₄ level observed for higher concentrations are analyzed by the Inokuti–Hirayama model. We think that the quenching effect between Tm³⁺ ions is mainly linked to the dipole–dipole interactions.     

Preparation of long-afterglow colloidal solution of Sr2MgSi2O7: Eu, Dy by laser ablation in liquid

We have successfully prepared a novel nanoparticle solution of Sr₂MgSi₂O₇: Eu²⁺, Dy³⁺ with afterglow properties by means of laser ablation in liquid. This process also produced by-products of different kinds, depending on the liquid used. The amount of by-product and the size of the nanoparticles were controlled by the energy density of laser ablation. The amount of by-product was reduced by a decrease in the energy density, which also decreased the particle size of the nanoparticles. The PL spectrum of the nanoparticles was the same as that of the target materials used for laser ablation. The afterglow properties deteriorated with a decrease in particle size. We concluded that an increase in specific surface area caused by a decrease in particle size resulted in the decrease of luminescent intensity.     

Enhanced luminescence and degradation of SiO2:Ce,Tb powder phosphors prepared by a sol-gel process

Enhanced green photoluminescence and cathodoluminescence (CL) from Tb³⁺ ions due to co-doping with Ce³⁺ ions were observed from SiO₂:Ce,Tb powder phosphors prepared by a sol-gel technique. Blue emission from the Ce³⁺ ions was completely suppressed by Tb co-doping, presumably due to energy transfer from Ce³⁺ to Tb³⁺. In addition, the green CL intensity from SiO₂:Ce,Tb degraded by ∼50% when the powders were irradiated for 10 h with a 2 keV, 54 mA/cm² beam of electrons in an ultra-high vacuum chamber containing either 1×10⁻⁸ or 1×10⁻⁷ Torr O₂. Desorption of oxygen from the surface was observed during the decrease of CL intensity. The mechanisms for energy transfer from Ce³⁺ ions to Tb³⁺ ions to enhance the green luminescence, and mechanisms for desorption of oxygen from the phosphor surface that would result in decreased CL intensity are discussed.     

Luminescence of Ni doped CaF2

Luminescence measurements of CaF₂ : Ni are reported. Before X-irradiation an emission band at 680 nm is observed with the corresponding excitation band at 255 nm. After RT X-irradiation the emission spectrum consists of three peaks at 290, 375 and 680 nm, all of them with an excitation band at 255 nm. The same emission spectrum together with the electron-hole recombination band is obtained by X-ray excitation.A comparison with previous EPR and optical absorption measurements indicates that the emission bands are due to different kinds of Ni²⁺ centers. The emission processes are similar to those found in CaF₂ : Co and CaF₂ : Mn.     

Growth Mechanism and Characterization of Single-crystalline Ga-doped SnO2 Nanowires and Self-organized SnO2/Ga2O3 Heterogeneous Microcomb Structures

Ga掺SnO2单晶纳米线和SnO2/Ga2O3自组织异质微米梳是通过简单的热蒸发沉淀法一步制得的,并通过X射线粉末衍射(XRD)、场激发扫描电子显微镜(FE-SEM)、透射电子显微镜(TEM)、X射线能量散射谱仪(EDS)、选区电子衍射谱(SAED)进行表征．从FE-SEM的图片上可以看出生成的产物具有纳米线和一种新的微米梳状形貌．XRD、SAED和EDS显示他们是单晶四角形的SnO2．产物的主干呈带状,纳米带阵列均匀的分布在主干的一侧或两侧．大量的Ga2O3纳米颗粒沉积在微米梳的表面．主干纳米带主要沿着[100]方向生长, 自组织的纳米带分支则在主干的(100)面上沿着[110]或者[110]方向生长．由于Ga的大量掺杂,光致发光谱的衍射峰发生红移并严重变宽．针对SnO2:Ga2O3异质微米梳的生长过程进行了解释,并讨论了实验条件对形貌的影响．