Electric Dipole-dipole Interaction Research Articles

KGaP2O7:Mn4+ deep red emitting phosphor: Synthesis, structure, concentration and temperature dependent photoluminescence characteristics

Mn4+-activated KGaP2O7 red emitting phosphor was successfully synthesized via traditional solid-state reaction method in air. The structural features of KGaP2O7:xMn4+ material were investigated by means of powder XRD & Raman, morphology from SEM, and element analysis, corresponding mapping through EDAX. In conjunction, concentration and temperature dependent spectroscopic properties were analyzed from UV–Vis reflectance, excitation, emission and lifetime decay curves. Powder XRD and photoluminescence profiles revealed that GaO6 polyhedra offer Mn4+ ions to substitute the Ga3+ sites in the crystal lattice resulting in a pure single phase structure with an efficient red emission at 702 nm (Mn4+:2Eg→4A2g) under 452 nm excitation. The emission decay curves exhibited non-exponential nature with lifetimes shortening when Mn4+ concentration is increased. Electric dipole-dipole interaction is identified to be responsible for the concentration quenching beyond 0.07 Mn4+ via energy transfer between Mn4+ ions. The temperature-dependent emission intensity for KGaP2O7:0.07Mn4+ exhibited 66% at 423 K (150 °C) displaying good thermal stability and activation energy (Ea) of ∼0.186 eV. From UV–Vis reflectance spectra using Tanabe-Sugano diagram, crystal splitting factor (Dq), two Racah parameters (B & C), Dq/B, C/B and energy of states are calculated to evaluate nephelauxetic ratio (β). In addition, the effect of Mn4+ content (concentration quenching) and temperature (thermal quenching) on emission intensity are discussed based on the Tanabe-Sugano energy diagram and configurational coordinate scheme of Mn4+. Therefore, KGaP2O7:Mn4+ phosphor can serve as a potential red emitting phosphor under blue light excitation and can also be useful to enhance plants growth.

Static nonlinear Schrödinger equations for the achiral-chiral transitions of polar chiral molecules

In the mean-field theory, the stabilization of polar chiral molecules is understood as a quantum phase transition where the mean-field ground state of molecules changes from the achiral eigenstate of the molecular Hamiltonian to one of the degenerated chiral states as the increase of the intermolecular interaction. Starting from the many-body Hamiltonian of the molecular gases with electric dipole-dipole interactions, we give the static non-linear Schr\"{o}dinger equations without free parameters to explore the achiral-chiral transitions of polar chiral molecules. We find that the polar chiral molecules of different species can be classified into two categories: At the critical point for the achiral-chiral transition, the mean-field ground state changes continuously in one category, and changes discontinuously in the other category. We further give the mean-field phase diagram of the achiral-chiral transitions for both two categories.

Photoluminescence properties and energy transfer mechanisms of Na4CaSi3O9: Sm3+, Eu3+ novel orange-red phosphors

Sm3+ ions doped Na4CaSi3O9 and Sm3+-Eu3+ co-doped Na4CaSi3O9 novel phosphors were prepared via conventional solid-state reaction method. The structure, photoluminescence (PL) properties and thermostability of samples were investigated in detail. Rietveld refinement and XRD patterns are in good agreement with the structure of Na4CaSi3O9. The phosphors can be effectively excited by 405 nm irradiation to produce light emission. Under the excitation of 405 nm irradiation, the Na4CaSi3O9: Sm3+ phosphors show three broadband emission centered at about 564 nm, 603 nm and 650 nm due to the 4G5/2→6HJ (J = 5/2,7/2,9/2) forbidden transition of Sm3+ ions and the Sm3+-Eu3+ co-doped Na4CaSi3O9 phosphors show yellow broadband emission centered at about 564 nm and an orange-red emission band at 592 nm and 613 nm, due to the 4G5/2 → 6H5/2 transition of Sm3+ ions and the 5D0→7F1, 5D0→7F2 transition of Eu3+ ions, respectively. The energy transfer mechanism between Sm3+ and Eu3+ ions is electric dipole-electric dipole interaction. The temperature-dependent luminescent properties of Na4CaSi3O9:Sm3+, Eu3+ ions are measures from 30 °C to 210 °C, and the results show that the luminous intensity of sample at 150 °C is up to 80% of that at room temperature, which indicate that the phosphor has good thermal stability. The above results show that this series of phosphors will have potential applications in near-UV excited white LEDs.

Synthesis via interfacial precipitation, color-tunable photoluminescence and improved thermal stability of (Ce1-xTbx)PO4 (x = 0–1) microspheres by energy transfer

A series of well-dispersed (Ce1-xTbx)PO4 (x = 0–1) microspheres, with an average diameter of ∼1.2 μm, have been originally prepared via two-phase interfacial precipitation and subsequent calcination. Characteristics of the as-synthesized and calcination products were investigated by the combined techniques of XRD, FE-SEM, TEM, FT-IR and TG-DSC. Under 275 nm excitation, tuning the emission color from blue (0.17, 0.02) to green (0.30, 0.56) was realized via varying the Tb3+ content and through Ce3+ → Tb3+ energy transfer. The optimal content of Tb3+ was found to be ∼15 at% (x = 0.15), and the energy transfer was determined to occur via electric dipole-dipole interaction. Moreover, both the emissions of Ce3+ and Tb3+ present gradually higher thermal stability with increasing Tb3+ content owing to enhanced energy transfer. The (Ce0.85Tb0.15)PO4 microspheres, which possess the highest absolute quantum efficiency (∼60.1%) across the series and excellent thermal stability (activation energy for thermal quenching of luminescence: ∼0.312 eV), may potentially serve as a single-phase green phosphor for solid-state lighting and field-emission display.

Crystal structure and luminescence properties of Na2MMg(PO4)2:Eu2+ (M = Ca/Sr/Ba) phosphors

A series of Na2MMg(PO4)2:Eu2+ (M = Ca/Sr/Ba) phosphors were synthesized by the high-temperature solid-state reaction. X-ray diffraction (XRD), photoluminescence (PL) and GSAS structural refinement were used to characterize the samples. Rietveld refinements of powder X-ray diffraction patterns of Na2MMg(PO4)2 confirm that the structure of Na2MMg(PO4)2 consist of stacking along the [001] direction. The M2+ cations are located within the interlayer space, while Na+ is found in a mount of cavities bounded to the layers. Upon excitation at 350 nm, the phosphors exhibit the asymmetric broadband emission peaked at about 392 nm attributed to the 4f65d1→4f7 transitions of Eu2+. It is found that the energy transfer occurred mainly by the electric dipole-dipole interaction according to the Eu2+ concentration quenching mechanism. Moreover, when Ca2+ sites are substituted by Sr2+ or Ba2+, the Eu2+ emission band shifts to a longer wavelength initially and then returns to a shorter wavelength. In particular, 89.5% of the room-temperature emission intensity is still maintained at 500 K, indicating a good thermal stability. The obtained results suggest that the phosphor could be a potential candidate for applications in white light-emitting diodes.

The structural and warm light emission properties of Sm3+/Tb3+ doubly doped strontium bismuth borosilicate glasses for LED applications.

Strontium Bismuth Borosilicate (SrBiBS as N) glasses doped with Tb3+ ions (N: Tb) and co-doped with Sm3+/Tb3+ (N: SmTb) were synthesized through the process of classical melt quenching. The structural and optical properties are investigated through energy dispersive spectroscopy; X-ray diffraction, Differential thermal analysis (DTA) and Fourier transform infrared spectra (FTIR), Optical absorption (OA) and photoluminescence (PL) spectral studies. X-ray diffraction indicates amorphous nature of glass. DTA curves explained the thermal stability and stress parameters of glasses. FTIR spectrum indicates existing vibrations of borate and silicate units. The absorption spectra have shown nine absorption bands of Tb3+ ions in the visible and NIR regions, due to transitions from ground state 7F6. In visible region, absorption spectra of co-doped glasses exhibited two bands of Tb3+ and three bands of Sm3+ with hyper sensitive transitions at 375 nm (7F6 → 5G6) and 401 nm (6H5/2 → 6P3/2). Photo Luminescence (PL) spectra of N: Tb and N: SmTb glasses is recorded at 375 nm excitation. N: Tb glasses have shown green emission corresponding to 5D4 → 7F5 transition. In all N: SmTb glasses, addition of Tb3+ ions has enhanced Sm3+ emission which is connected to quenching of Tb3+ emission. Moreover, 0.8 mol% of Tb3+ ions in co-doped glass (N: SmTb8) has shown maximum emission. The energy transfer process was inferred from shortening of decay times observed in N: SmTb glasses. The decay profiles fitted with Inokuti-Hirayama model suggests that the electric dipole-dipole interaction(S = 6) between Sm3+ and Tb3+ might dominate in energy transfer mechanism with probability PET and energy transfer parameter Q of 113-173 S-1 and 9.9-12.0 respectively. The color chromaticity (CIE) coordinates from PL emission of N: Tb & N: SmTb glasses have covered Green, Orange red and yellow (warm-light) regions with coordinates in ranges of x = 0.39-0.62 and y = 0.34-0.56.The color temperatures in range of 1713 K-4258 K.

Synthesis and photoluminescence of Mn4+ in M4Al14O25 (M = Sr or Mg) compounds as red-light phosphors for white LED

Red-light-emitting phosphors Mn4+-doped M4Al14O25 (M = Sr or Mg) were synthesized by solid-state reaction at 1400 °C in air, and their luminescent properties were initially investigated for their applications in warm white-light-diodes (W-LEDs). The excitation spectra of Mn4+-doped M4Al14O25 (M = Sr or Mg) showed that those phosphors were efficiently excited by near ultraviolet (UV) or blue LED chips, and those phosphors exhibited deep red emission band around 650 nm arising from the spin-forbidden 2E → 4A2 transition of Mn4+. The energy transfer mechanism in Mn4+-doped Mg4Al14O25 phosphor was found to be the electric dipole-dipole interaction and the critical distance of energy transfer between the Mn4+ ions was obtained to be about 1.384 nm. The Commission International de L’Eclairage (CIE) chromaticity coordinates of 0.5at.% Mn4+-doped Mg4Al14O25 were (0.631, 0.252). The temperature-dependent luminescence indicated Mn4+-doped M4Al14O25 phosphor was poor thermal-stable. Further work related to improve its thermal stability need be executed for Mn4+-doped M4Al14O25 (M = Sr or Mg) as a promising candidate of red-component for warm W-LEDs.

Tunable emission and energy transfer in TeO2-GeO2-ZnO and TeO2-GeO2-MgCl2 glasses activated with Eu3+/Dy3+ for solid state lighting applications

The results presented here refer to the tunability of global emission characteristics of Eu3+, Dy3+ and Eu3+/Dy3+ ions, varying the composition glasses; TeO2-GeO2-X (X = ZnO, MgCl2). Also, the influence of exchanging the modifier compound on the global luminescent response of the glasses was analyzed by Raman spectroscopy, excitation and emission spectra, and emission decay profiles. Changes in the Te-O-Te network structure, when MgCl2 substitutes ZnO, were identified from Raman spectrum deconvolution analysis. The energy transfer between Eu3+ and Dy3+ ions, and tunable emission characteristics, were studied under UV excitations that correspond with the emission of InGaN (370–420 nm) based LEDs. The energy transfer process between Eu3+ and Dy3+ ions was studied based on the emission spectra with different excitation wavelengths and time decay curves of the 5D0→7F2 level of Eu3+ at 612 nm. In TGZED and TGMED glasses and upon 351 nm, the time shortening of Dy3+ emission decay in presence of Eu3+ was attributed to an Dy3+→Eu3+ non-radiative energy transfer process. The energy transfer probabilities PD→E and energy transfer efficiency ηD→E were calculated for TGZ and TGM glasses. According to the Inokuti-Hirayama model it might be dominated through an electric dipole-dipole interaction, with efficiency of 5.0% and 1.9% and the obtained γ6 energy transfer parameter values γ6 = 0.082 ± 0.005 and 0.026 ± 0.005 respectively, for TGZED and TGMED glasses. The non-radiative Eu3+→Dy3+ energy transfer was observed too for both glasses TGZED and TGMED, so that transfer of energy is more favored in the oxide vitreous matrix than in the oxide-halide one. The dominated are an electric dipole-dipole interaction with γ6 energy transfer parameter values are 0.072 ± 0.005 (TGZED) and 0.036 ± 0.005 (TGMED), with efficiency 6.9 and 4.1% respectively. In TGM glasses is very low efficiency of non-radiative Dy3+→Eu3+ and Eu3+ →Dy3+ energy transfer that TGZ matrix. Neutral white emissions of 4135, 4567 K were observed in the Dy3+ and 3914 and 242 K, Eu3+/Dy3+ doped TGZ glasses excited at 351 and 388 nm, respectively. Similarly, global emissions of Dy3+ single doped were 4028 and 4123 K, but in Eu3+/Dy3+ double doped TGM glasses are neutral white light of 3929 and 3979 K, respectively. Upon 394 nm excitation, the Eu3+ doped TGZ and TGM glasses displayed reddish-orange global emissions of 1979 and 1964 K, respectively. For excitations at 382 and 394 nm the Eu3+/Dy3+ doped TGZ and TGM glasses emit warm white light of 3414 and 3160 K (382 nm excitation) and reddish-orange of 2123 and 2030 K (394 nm excitation), respectively, depending mostly on the Eu3+ and Dy3+ relative excitation intensity.

Rare‐earth‐free Li5La3Ta2O12:Mn4+ deep‐red‐emitting phosphor: Synthesis and photoluminescence properties

AbstractLi5La3Ta2O12:Mn4+ (LLTO:Mn4+) phosphors are prepared in air via high‐temperature solid‐state method and investigated for their crystal structures and luminescence properties. LLTO:Mn4+ phosphor under excitation at 314 nm shows deep‐red emission peaking at 714 nm due to the 2E→4A2 transition of Mn4+ ion. The excitation bands in the range 220 ‐ 570 nm are attributed to the Mn4+ ‐ O2‐ charge‐transfer band and the 4A2g→4T1g, 2T2g, and 4T2g transitions of Mn4+, respectively. The optimal Mn4+ ion concentration is ~0.4 mol%. The concentration quenching mechanism in LLTO:Mn4+ phosphor is electric dipole‐dipole interaction. The luminous mechanism and temperature quenching phenomenon are explained by the Tanabe‐Sugano energy level diagram and the configurational coordinate diagram of Mn4+ in the octahedron, respectively. The experimental results indicate that LLTO:Mn4+ phosphor has a potential application prospect as candidate of deep‐red component in light‐emitting diode (LED) lighting.

Spectroscopic properties of Dy3+-doped Sr3Y(BO3)3 crystal

A 2.02 at.% Dy3+-doped Sr3Y(BO3)3 crystal was successfully grown by the Czochralski method. Polarized spectroscopic properties of the crystal were investigated at room temperature. The absorption spectra were analyzed based on Judd-Ofelt theory. Then the spontaneous transition probabilities, branching ratios, and radiative lifetime related to the 4F9/2 multiplet of Dy3+ were obtained. The peak emission cross-sections corresponding to the 4F9/2 → 6H13/2 transition are 0.10 × 10−20 and 0.12 × 10−20 cm2 for σ and π polarizations, respectively, and at 576 nm for both polarizations. The fluorescence lifetime and quantum efficiency were estimated to be 0.82 ms and 57%, respectively. The analysis on the fluorescence decay curve of the 4F9/2 multiplet indicated that the cross relaxation induced by the electric dipole-dipole interaction plays an important role in the depopulation of the 4F9/2 multiplet. The results show that the Dy3+:Sr3Y(BO3)3 crystal may be a promising candidate for solid state yellow laser pumped by a blue laser diode around 454 nm.

Luminescence and energy transfer of a single-phased phosphor Y2GeO5: Bi3+, Tm3+, Tb3+, Eu3+ for white UV LEDs

A series of novel Y2GeO5: Bi3+, Tm3+, Tb3+, Eu3+ (YGO: Bi3+, Tm3+, Tb3+, Eu3+) phosphors were synthesized via high-temperature solid-state reaction. The structural, luminescent properties, as well as energy transfer mechanisms, in the phosphors were investigated in detail. The results indicate that all Bi3+, Tm3+, Tb3+ and Eu3+ ions substitute for Y3+ ions in host lattice of YGO without changing its crystal structure. Excitation and emission spectra were evaluated for the compounds. Energy transfer from Bi3+ to Tm3+, Tb3+/Eu3+ and Tb3+ to Eu3+ exist in phosphors YGO: Bi3+, Tm3+, Tb3+, Eu3+. Under the 308 nm excitation, the phosphor YGO: Bi3+, Tm3+, Tb3+ shows the characteristic transitions of Bi3+ (372 nm, 3P1, 0 → 1S0), Tm3+ (455 nm, 1D2→3F4), Tb3+ (544 nm, 5D4→7F5). However, Eu3+ is absent in the composition. Tunable colors from blue to green to red and white are achieved by varying excitation wavelength. The energy transfer from Bi3+ to Tm3+ is proved to be an electric dipole-dipole interaction. A white-light emission with the color coordinate (0.3146, 0.3541), correlated color temperature (CCT) of 6234 K, luminous efficacy of the radiation (LER) of 180 lm/W and 0.2345–0.2424 eV of activation energy for thermal quenching are obtained in the optimum phosphor YGO: 0.005Bi3+, 0.04 Tm3+, 0.07 Tb3+, 0.05Eu3+. The obtained results indicate that the YGO: Bi3+, Tm3+, Tb3+, Eu3+ phosphors can serve as potential color-tunable and single-phase white emission phosphors with impressive color stability as well as good thermal stability for n-UV excited white LEDs.

A single-phase white light emitting phosphor Ba3Y(PO4)3:Ce3+/Eu2+/Mn2+: Luminescence, energy transfer and thermal stability

Abstract Series of Ce3+/Eu2+, Ce3+/Mn2+ and Ce3+/Eu2+/Mn2+ doped Ba3Y(PO4)3 samples were synthesized by a high temperature solid state reaction method. Phase formation, energy transfer, luminescence properties and thermal quenching properties were investigated carefully. The energy transfer from Ce3+ to Eu2+ and Ce3+ to Mn2+ in Ba3Y(PO4)3 were verified and discussed detailed. When Ce3+ and Eu2+ are co-doped, the energy transfer process from Ce3+ to Eu2+ is confirmed with electric dipole-dipole interaction. For Ce3+/Mn2+ co-doped phosphors, the energy transfer process from Ce3+ to Mn2+ is proved with electric dipole-dipole interaction. Besides, due to the existence of defects, the phosphors show excellent thermal stability. For Ce3+/Eu2+/Mn2+ tri-doped Ba3Y(PO4)3, the energy transfer Ce3+→Eu2+ (Mn2+) and Eu2+→Mn2+ are confirmed. The energy transfer priority among Ce3+, Eu2+ and Mn2+ is analyzed in detail basing on the change of emission intensity. The obtained tri-doped phosphors contain red, green and blue component, thus they can emit warm white light basing on the energy transfer. The results show that they may be novel white-light-emitting phosphors for ultraviolet white light emitting diodes.

Broad-band emission and color tuning of Eu3+-doped LiCa2SrMgV3O12 phosphors for warm white light-emitting diodes

In this study, series of Eu3+-doped LiCa2SrMgV3O12 (LCSMV) phosphors with broad-band emission and color tunable feature were prepared via solid phase reaction. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results presented a pure cubic phase product with micron-sized and homogeneous distribution of element. Their spectroscopic properties were investigated systematically by photoluminescence excitation (PLE) and emission (PL) spectra, temperature-dependent PL spectra and luminescence decay curves. The LCSMV phosphors displayed a strong absorption to ultraviolet light and a broad cyan emission. Moreover, in Eu3+-doped LCSMV phosphors, Eu3+ ion characteristic emissions at 589, 610, 651 and 705 nm, attributing to the 5D0→7F1, 7F2, 7F3 and 7F4 transitions, were observed. Along with Eu3+ ion concentrations increasing, the emission colors could be readily tuned from cyan to orange and the decay lifetimes of (VO4)3- became shorter. Meanwhile, electric dipole-dipole interaction was responsible to energy migration from (VO4)3- groups to Eu3+ ions. Further, the quantum efficiency (QE) values were estimated to be 32.5% for LCSMV host and 39.3% for LCSMV: 0.01Eu3+ sample. Finally, a LED lamp was prepared by integrating the blend of the LCSMV: 0.01Eu3+ phosphors and commercial blue-emitting BaMgAl10O17:Eu2+ phosphors with NUV chip (365 nm) and exhibited warm white light (CCT = 3655 K, Ra = 90), which may be applied in lighting and display field.

Photoluminescence Properties of Dy3+ Activated CaWO4 Nanophosphors: a Potential Single Phase near White Light Emitter.

A multicolor tunable CaWO4:xDy3+ nanophosphors have been synthesized via hydrothermal route. X-Ray Diffraction and Fourier transform infrared confirm the formation of CaWO4:Dy3+ nanophosphors. Transmission electron microscopy image and selected area electron diffraction (SAED) reveal the formation of nanosize and crystalline CaWO4:Dy3+. Dependence of energy transfer rate from WO42- to the activator (Dy3+) is observed from the photoluminescence studies. An enhancement of energy transfer efficiency from 36% to 90% is observed after annealing the as-prepared samples at 800°C. The exchange type energy transfer mechanism is observed to be dominant in as-prepared samples while the electric dipole-dipole interaction is dominant in annealed samples. Variation in energy transfer rate from the host to Dy3+ activator ions leads to the tuning of color emission from this nanophosphor. A near white light emission could be achieved with 6at.% Dy3+ doped CaWO4 annealed at 800°C with x = 0.310 and y = 0.327.

Highly efficient and thermal stable La2Si2O7:Ce3+,Tb3+,Eu3+ phosphors: Emission color tuning through terbium bridge

A series of emission color tunable Ce3+, Tb3+, and Eu3+ co-doped La2Si2O7 (LSO) phosphors have been prepared via a solid state reaction. The crystal structure, luminescence properties, and thermal stability of the phosphors were studied in this paper. There exists two kinds of energy transfer (ET) modes under UV excitation, namely, electric dipole-dipole interaction for Ce3+→Tb3+ and terbium bridge model [Ce3+→(Tb3+)n→Eu3+] for LSO:Ce3+,Tb3+,Eu3+ system. The emitting color of the phosphors can be tuned from green to yellow and eventually to orange-red through the Ce3+→Tb3+→Eu3+ energy transfer. Although codoping Tb3+ and Eu3+ into La2Si2O7:Ce3+ leads to the reduction of thermal stability, it still remains 80% for LSO:0.05Ce3+,0.8 Tb3+,0.02Eu3+ at 150 °C when compared to that at room temperature. The absolute quantum yield (QY) increases from 71.04% (LSO:0.05Ce3+) to 95.60% (LSO:0.05Ce3+, 0.6 Tb3+), while those of LSO:0.05Ce3+,yTb3+,0.02Eu3+ samples remain more than 70%. These results indicate that the phosphors can be candidates for application in phosphor-converted white light-emitting diodes (pc-WLEDs).

Near-infrared quantum cutting via energy transfer in Bi3+, Yb3+ co-doped Lu2GeO5 down-converting phosphor

In recent years, down-converting phosphors have grown rapidly and been still growing for their potential to significantly improve the photoelectric conversion efficiency of silicon-based solar cells. However, the discovery of new optical materials with optimized properties (in terms of emission intensity, chemical and thermal stability and quantum yield) would still be of the utmost importance. Herein, we report a novel down-converting phosphor Lu2GeO5: Bi3+, Yb3+. Our research results show that the energy transfer efficiency from Bi3+ to Yb3+ in Lu2GeO5 is as high as 65%. Via such an energy transfer from Bi3+ to Yb3+, an absorbed ultraviolet photon gives rise to almost two emitted infrared photons. The theoretical quantum yield of Lu2GeO5: Bi3+, Yb3+ system is calculated to reach up to 165%. The mechanism of the energy transfer from Bi3+ to Yb3+ is proved to be an electric dipole-dipole interaction. Meanwhile, Lu1.87GeO5: 0.03Bi3+, 0.10 Yb3+ phosphor exhibits an excellent resistance against thermal quenching. The present investigation provides people a new down-converting phosphor Bi3+, Yb3+ co-doped Lu2GeO5 with near-infrared quantum cutting to achieve high photoelectric conversion efficiency of silicon-based solar cells.

An Analogy between the Properties of “Dark Energy” and Physical Vacuum Consisting of Quantum Harmonic Oscillators Characterized by Zero-Point Energy

In quantum field theory, the physical vacuum, free from magnetic and electric fields (without regard to gravitational energy), is defined not as an empty space but as the ground state of the field consisting of quantum harmonic oscillators (QHOs) characterized by zero-point energy. The aim of this work is to show that such physical vacuum may possess the properties similar to the properties of dark energy: the positive density, the negative pressure, and the possibility of so-called accelerated expansion. In the model discussed, the mass of QHOs determines the positive density of dark energy. The observed electric polarization of physical vacuum in an electric field means the existence of electric dipole moment of QHO, which, in turn, suggests the existence inside the QHO of a repulsive force between unlike charges compensating the attractive Coulomb force between the charges. The existence of such repulsive force may be treated as the existence of omniradial tensions inside every QHO. In terms of hydrodynamics, it means that the vacuum with this property may be regarded as a medium with negative pressure. The electric dipole-dipole interaction of QHOs under some condition may result in the expansion of physical vacuum consisting of QHOs. It is shown also that the physical vacuum consisting of QHOs is a luminiferous medium, and based on this concept the conditions are discussed for the emergence of invisiblity of any objects (in particular, dark matter). The existence of luminiferous medium does not contradict the second postulate of special relativity (the principle of constancy of the velocity of light in inertial systems), if to take into account the interaction of photons with QHOs and with virtual photons (the virtual particles pairs) created by quantum entities that constitute the inertial systems.

Tunable photoluminescence properties and energy transfer of Ca5(BO3)3F: Tb3+/Eu3+ phosphors for solid state lighting

A series of Ca5(BO3)3F: Tb3+/Eu3+ phosphors have been prepared via solid-state reaction. Phase purity and crystal structure of as-prepared samples are confirmed by X-ray diffraction measurements. The emission intensity of Ca5(BO3)3F: Tb3+ with Li+, Na+, K+ as charge compensators are remarkably enhanced, and the sample in presence of Na+ exhibits the strongest emission intensity. The emission spectra and fluorescence decays confirm the presence of efficient energy transfers from Tb3+ to Eu3+ in Ca5(BO3)3F: Tb3+/Eu3+ phosphors. The energy transfer mechanism between Tb3+ and Eu3+ has been proven to be resonant type via the electric dipole-dipole interaction. The thermal quenching temperature T50 obtained from the temperature-dependent emission spectra is 478 K, demonstrating a high thermal stability of the as-obtained Ca5(BO3)3F: Tb3+/Eu3+ phosphors. Moreover, by increasing the molar ratio of Eu3+ to Tb3+, the emission colors of Ca5(BO3)3F: Tb3+/Eu3+ phosphors can be tuned from green to orange, and then to red. These results demonstrate that the as-obtained Ca5(BO3)3F: Tb3+/Eu3+ phosphors may have potential applications in solid-state lighting fields.

Mathematical modelling of surface tension of nanoparticles in electrolyte solutions

Nanoparticles (NPs) have been successfully applied to reservoirs for enhanced oil recovery and fines migration mitigation at laboratory scale. Despite the successful implementation at laboratory scale, it is rarely applied at field scale. One of the major reasons for the delay in implementation is the lack of an appropriate model to predict the fluid-particles behaviour in the reservoir. An accurate prediction of the surface tension of the fluid system is particularly important in field design and development in the petroleum sector.The surface tension of NPs in deionized water increases with increase in concentration of NPs. This study exploits the Debye-Hückel constants to calculate the mean activity coefficients of NPs in solution combined with the equation proposed by Li and Lu (2001) for single electrolyte solutions to estimate the change in surface tension.However, NPs behaviour in brine (electrolyte solution) is different from the behaviour of mixture of different electrolytes. In deionized water, the surface tension of the fluid increases with increase in the NPs concentration, but NPs in electrolyte solutions behave as surface active agent decreasing the surface tension with increase in NPs concentration. This work includes the dipole-dipole interaction and structural effects along with the equation developed by Borwankar and Wasan (1988) for surface excess adsorption. This surface excess adsorption is applied to Li and Lu's equation for mixed electrolyte solutions to estimate the change in surface tension.This study is able to model the surface tension of NPs with or without electrolyte. Here, the molecular interaction of NPs in deionized water and electrolyte solutions is considered to predict the fluid-surface tension. The free energy at the interface is affected by the intermolecular interaction of the NPs. This intermolecular interaction includes electrical double layer, dipole-dipole interaction, and structural effects. The proposed model shows a good agreement with the experimental data from previous studies.

Spectroscopic evaluation a new and novel Nd3+/Yb3+ co-doped CdO-V2O5 glass system for 1 μm laser application

Structural and spectral evaluations of Nd3+ and Nd3+/Yb3+ doped CdO-V2O5 glasses based on X-Ray diffraction, Raman spectroscopy, absorption, excitation and emission spectra and decay time profiles, are performed. X-Ray diffraction patterns and Raman spectroscopy revealed that all samples are amorphous (glassy phase) up to 4 mol% of Yb3+, without segregation of any crystalline phase. The optical band gap values determined from the absorption spectra, assuming direct optical band gap, were in the range of 2.13–2.07 eV, depending on the Yb3+ concentration. The emission spectra of the Nd3+ and Nd3+/Yb3+ doped glasses upon Nd3+ excitation at 585 nm, displayed the NIR Nd3+ feature emissions centered at 880, 1060 and 1342 nm, and Yb3+ emission at 977 nm, being the latter one achieved at expenses of energy transfer from Nd3+. According to the shortening of the Nd3+ emission decay profiles such process is not radiative with efficiency in the range of 0.181–0.521, depending on the Yb3+ content. The 880 nm Nd3+ decay fitting using the Inokuti-Hirayama model considering, energy migration among donors, suggested that an electric dipole-dipole interaction might mediate the energy transfer process, within Nd3+-Yb3+ clusters.