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Two different carbon sources, sucrose and glucose, were used as raw materials for constructing a nanoscale carbon coating on the surface of LiNi0.8Co0.15Al0.05O2 (NCA). They were mixed with a certain mass ratio and then were annealed to form a nanoscale even coating, which showed a C@NCA shell-core structure. The core layer was NCA and the shell layer was nanoscale carbon coating. The cathode coating material using nanoscale carbon layer has better electrical conductivity and high diffusivity of Li-ion. It can effectively reduce the mechanical stress, prevent the formation of micro-cracks on secondary particle surface and maintain the integrity of the cathode material. Obtained by comparison, the results show that the material with sucrose as carbon source has better performance. When the mass ratio of coating is 1%, it has the best electrochemical performance. The initial discharge specific capacity is 250 mAh / g at the current density of 0.1C. The capacity retention rate increased to 88.3% after 200 cycles at current density of 1C, higher than uncoated materials significantly (51.1%). While at current density of 5C, the capacity retention rate increased to 63.7% after 500 cycles, higher than uncoated (46.4%) again.

A series of Tb 3+ -doped europium-based molybdate red-emitting phosphors (LiEuMo 2 O 8 ) were prepared by solid-state reaction method, their crystal structure and luminescence properties have been investigated by X-ray diffraction, micro-Raman spectra, diffuse reflectance spectra, photoluminescence and photoluminescence excitation spectra. Partial substitution of Li + by Tb 3+ slightly distorts the host lattice, red-shifts the absorption edge of host and improves Eu 3+ red-emitting intensity in LiEuMo 2 O 8 compound. The red-emitting intensity of LiEuMo 2 O 8 powder is enhanced by 30% at the doping level of 0.02 under excitation at 395 nm. In present study, we found that the contribution of Tb 3+ → Eu 3+ energy transfer to the enhanced Eu 3+ emission is little since the Tb 3+ ions are not responsive to the near ultraviolet (∼395 nm), the enhanced red emission is tentatively ascribed to the relaxation of parity-forbidden selection rules of Eu 3+ ions. White light-emitting diodes are fabricated by using near ultraviolet light-emitting diode chips (∼395 nm) and the blends of the commercial blue, green and as-prepared red Li 0.98 Tb 0.02 EuMo 2 O 8 phosphor, the resulting white light-emitting diode shows a color-rendering index of 93.5, luminous efficacy of 14 lm/W and chromaticity coordinates of (0.292, 0.329).

Spectrally tunable phosphors have been extensively studied for their potential applications in phosphor-converted white light-emitting diodes and display devices. Herein, we synthesize a series of color-tunable (Sr2.95-xCax)Ce(PO4)3:0.05Eu2+ (x = 0–2.95) phosphors via solid-state reaction, which are designed by using eulytite-type phosphate with different PO4 structural ordering, viz., Sr3Ce(PO4)3:Eu2+ with disordered state of PO4 tetrahedrons, and Ca3Ce(PO4)3:Eu2+ with ordered state of PO4 tetrahedrons. In Sr3Ce(PO4)3:Eu2+, the successive substitution of Sr by Ca causes microstructural transformation of PO4 tetrahedrons from disordered state to ordered state. Strikingly, the disorder-to-ordered microstructural variation of PO4 anion-groups remotely controls Eu2+ emissions, yielding adjustable emission from yellow-green to yellow and then green emission and fine tailoring of photoluminescence excitation spectra, which are demonstrated by an in-depth local structure analysis of X-ray, nuclear magnetic resonance and electron paramagnetic resonance. This work provides a deeper understanding on how structural ordering degree affects the local structures of Eu2+ centers.

A series of color-tunable (Ca9−x Sr x )Mg1.5(PO4)7:Eu2+ (0 ≤ x ≤ 9) phosphors are designed by using mineral-inspired structural design strategy and synthesized by solid-state reaction. The mineral structure β-Ca3(PO4)2-type compounds, Ca9Mg1.5(PO4)7 and Sr9Mg1.5(PO4)7 are chosen as structural prototypes. Both of them are hexagonal structure but have different space groups. The former has R3c space group, while the latter possesses R3m space group. When Ca9Mg1.5(PO4)7 compound is used as maternal materials, the powder X-ray diffraction results demonstrate that the successive substitution of Ca2+ with Sr2+ ions expands and distorts the lattice parameters and unit cell volume of Ca9Mg1.5(PO4)7 compound, rendering phase transition from the (Ca9−x Sr x )Mg1.5(PO4)7:Eu2+ (phase 1, 0 ≤ x < 3) with R3c space group to (Ca9−x Sr x )Mg1.5(PO4)7:Eu2+ (phase 2, 5 < x ≤ 9) with R3m space group. The phase transition causes noticeable spectral redshift from the original blue-violet emission of x = 0 to bright yellow emission of x = 9 sample. Importantly, at x = 1, a bright white-light emission with a full width at half maximum of ∼255 nm is discovered in Ca8SrMg1.5(PO4)7:Eu2+ phosphor, which is beneficial to achieve phosphor-converted white-light-emitting diodes with high color-rendering index. These findings indicate that the mineral-inspired structural design strategy is very helpful for the development of new phosphors.

High efficiency red phosphors, showing spectral range of 610–620 nm and a narrow-line emission, are helpful for achieving high color rendering index (CRI) and low correlated color temperature (CCT) phosphor-converted white-light diodes. Red phosphors activated by trivalent europium (Eu3+) thus have attracted a renewed interest for pc-WLEDs. In our case, we prepare a series of Pb2+-doped LiEuMoO8 red-emitting phosphors by solid-state reactions. The introduction of Pb2+ ions expands and distorts slightly the unit cell volume of LiEuMo2O8 matrix, and red-shifts the charge transfer absorption (CTA) band of LiEuMo2O8, which are demonstrated by X-ray powder diffraction, absorption spectra and micro-Raman spectra. Photoluminescence and photoluminescence excitation spectra results show that the excitation efficiency of Eu3+ is improved significantly in the near ultraviolet region when Pb2+ ions are doped into the host lattice. At x=0.015, the red-emitting intensity of LiEuMo2O8 is enhanced by 50%, and the internal quantum yields is 53% under excitation at 395 nm. The enhanced red emission is discussed in term of the relaxation of parity-forbidden selection rules for trivalent europium luminescent center, and the sensitization effect of Pb2+ as a sensitizer for Eu3+ ions is negligible in LiEuMo2O8 under near ultraviolet excitation.

White lighting converted by tricolor phosphor and near-ultraviolet light-emitting diode demonstrates great potential in general lighting. However, a major problem with this approach is the lower luminous efficiency of the resulting white lighting by the reabsorption among tricolor phosphors. In this work, we propose a general but effective method for achieving high-efficacy white light-emitting diodes (WLEDs) by using two-color phosphors having the same host that can lower energy loss by reabsorption. Color-tunable Sr7.95Si4O12Cl8:0.05Eu2+ (SSO_Cl:0.05Eu2+) phosphors are synthesized by cationic substitution strategy, and the emission colors can be tuned from blue-greenish to blue/yellow by Sr → Mg/Ca substitution. The red-shifted emission is attributed to the increased Eu2+ d-orbital splitting owing to the smaller size of Ca2+, whereas blue-shifted Eu2+ emission is unusual in Mg-substitution for Sr2+. WLEDs with luminous efficacies (ηL) of 28.9–56 lm/W and CRIs of 79–90.2 are demonstrated using the two-co...

Efficient modulation of the site-selection of activators is of great significance for designing high-performance phosphors. Herein, we report that site-selective occupation of Eu2+ activators in mixed anion-group realizes full visible spectrum emission in borophosphate K2BaCa(PO4)2−x(BO3)x:0.03Eu2+ phosphors, which are designed by using K2BaCa(PO4)2:0.03Eu2+ cyan-emitting phosphor. In the pristine sample, there are four crystallographic sites (Ca, Ba, K1 and K2), yet, Eu2+ ions only occupy the K1 and K2 sites. By anion-group cosubstituting (PO4)3− with (BO3)3− units, the most strikingly discoveries here are that a Eu2+-substitution new pathway is opened up, rendering partial Eu2+ ions enter Ca crystallographic site that could never be occupied before, yielding an intense Eu2+ red emission band at 660 nm, and consequently realizing spectral tuning from original cyan-emitting to white-light emission. The structural analysis and DFT calculations support the concept that Eu2+ ion preferably occupy the Ca site, and revealing impressive insights of site-selective occupation of Eu2+ in the borophosphate white-light phosphors. Importantly, using only this full-visible-spectrum phosphor (x = 0.4), as-prepared WLED shows excellent color quality (Ra = 90.4 and R9 = 93.8). This work not only presents an in-depth investigation of site-selection of Eu2+, but also opens a new way to design full-visible-spectrum phosphors.

We have synthesized and investigated a series of yellow-emitting Sr3Ce(PO4)3:Eu2+ phosphors, showing an extreme broadband at around 535 nm (with a FWHM of ca. 200 nm/6760 cm−1) and a large Stoke shift (ca. 8265 cm−1), which is attributed to the 4f–5d transition of Eu2+ without the contribution from Ce3+ emission under excitation at long wavelengths (>370 nm). However, such broadband Eu2+ emission is rather surprising, considering that there is only one cation site for the Eu2+ luminescent center. Herein, we investigated the crystal structure by performing X-ray Rietveld refinement on the synchrotron X-ray diffraction data, and demonstrate that both cation and anion sites in Sr3Ce(PO4)3 are disordered in the host lattice. These unusual structural characters generate a variety of distinct Eu2+ sites, which is verified by decay lifetime analysis and electron paramagnetic resonance spectra, and thus result in astonishing broadband yellow-emission. Moreover, a white LED device with a color-rendering index of 86.5, a color temperature of 5996 K, and chromaticity coordinates of (0.32, 0.38) was obtained by combining a 405 nm near-UV LED chip and the phosphor blends of yellow-emitting Sr3Ce(PO4)3:Eu2+ and the commercial blue-emitting BaMgA10O17:Eu2+ phosphor. These results indicate that the as-prepared yellow-emitting Sr3Ce(PO4)3:Eu2+ phosphor has potential applications in the dual-color-phosphor-converted WLEDs.

This work reports the fabrication of a luminescent planar plate based on stable aqueous quantum dots (QDs) and poly(diallyldimethylammonium chloride) (PDDA) via a layer-by-layer (LBL) assembly technique. Preparation of aqueous QDs with facile monoalkyl maleate amphiphilic surfactants as the coating agent is conducted by a robust and efficient phase-transfer method. The as-prepared aqueous QDs exhibit bright emission, and their surface has very large negative zeta potential values, which are useful for electrostatic LBL assembly. Red, green, and blue luminescent planar plates are successfully fabricated via LBL assembly of the monocolor QDs, respectively. Through accurately adjusting the relative proportion of each monocolor luminescent component, we obtain an inspiring luminescent planar plate, which emits bright white light with a color coordinate of (0.3509, 0.3483), a correlated color temperature (CCT) of 4766 K, and a high color rendering index (CRI, Ra) of 89.5 under the irradiation of UV light. Therefore, this paper reports a facile process for the design and preparation of luminescent planar plates, which have potential applications in display and solid-state lighting devices.

A series of Yb 3+ -Er 3+ coactivated La(Nb 1-x V x )O 4 (0≤ x ≤1) upconversion (UC) emission phosphors are prepared by high-temperature solid-state reaction method. Experimental results demonstrate that the introduction of V 5+ expands and distorts the unit cell volume of LaNbO 4 , and induces structural transformation of LaNbO 4 from Phase 1 with monoclinic fergusonite structure (0 x ≤0.2) to Phase 2 with tetragonal scheelite structure (0.25≤ x ≤0.5) to Phase 3 with monoclinic monazite structure ( x =1). Under 976 nm excitation, continuously enhanced green UC emission is observed in Phase 1 with gradual replacement of Nb 5+ with the V 5+ (0 x ≤0.2). At x =0.2, the luminescence intensity is enhanced 1.3 times than that of the pristine sample. In Phase 2 and Phase 3, the intensity of green UC emission is decreased gradually with increasing the V 5+ concentration. Based on the XRD results and crystal structural analysis, the enhanced luminescence is attributed to the lowered symmetry around La 3+ ions (Er 3+ ions), which relaxes the parity-forbidden selection rules of Er 3+ luminescent centers. This study provides a useful approach in developing UC luminescence materials with controllable optical properties based on variations in local coordination environments through composition substitutions.

Mn2+-activated red-emitting phosphors have emerged as promising candidates for phosphor-converted white light-emitting diodes (WLED). Unfortunately, optical center Mn2+ suffers from weak emission and inefficient excitation due to its forbidden d-d transition. Often, a sensitizer (e.g., Eu2+/Ce3+ or Tb3+) is required to pair with Mn2+ (mono-sensitization strategy) to enhance emission/excitation efficiency of Mn2+ (e.g., Eu2+/Ce3+/Tb3+→Mn2+), with the energy transfer (ET) principle. However, at present, sufficiently sensitizing Mn2+ by using the above strategy to yield a nearly pure, efficient Mn2+ red emission still remains challenging. Herein, we demonstrate a sufficient sensitization of Mn2+ in Sr3La3(PO4)3 compounds by utilizing two sensitizer ions, Eu2+ and Tb3+, to synergistically sensitize one Mn2+, which brings out a new, high-efficient Eu2+→Tb3+→Mn2+ ET, achieving almost pure, bright Mn2+ red emission without seeing the emissions from Eu2+ and Tb3+ in a composition-optimized sample. The intensity of the resulting Mn2+ red emission is better than that of each mono-sensitization strategy. Moreover, a high-quality WLED with high color-rendering-indexing of 91 and low correlated color temperature of 4436 K is achieved by using as-prepared red-emitting phosphor. We believe that such a strategy can be easily expanded to other systems, initiating a new pathway in designing the Mn2+-activated red phosphors.

Single-component, single-activator-converted high-color-rendering index (CRI) white-light phosphors have sparked much interest for phosphor-converted white light-emitting diodes. However, manipulating the distribution and locations of single activators to target desired sites in a given host lattice is still a challenge. Herein, we report the remote regulation of the distribution of Eu2+ ions by engineering and controlling the structural ordering in Sr3(Ce1−xLax)(PO4)3:0.05Eu2+ (SCLP:Eu2+) compound, which are designed by using two-color phosphors with different anionic structural ordering, viz., Sr3Ce(PO4)3:Eu2+ (yellow) with disordered state of PO4 tetrahedra, and Sr3La(PO4)3:Eu2+ (blue) with ordered state of PO4 tetrahedra. The successive substitution of Ce by La triggers a pronounced disorder-to-order structural transformation of PO4 tetrahedra within SCLP:Eu2+ compounds and guides remotely the distribution of Eu2+ activators, which is demonstrated by a detailed analysis of synchrotron X-ray diffraction refinement, electron paramagnetic resonance spectra, electron spin-echo envelope modulation spectra, Raman spectra, nuclear magnetic resonance spectra and micro-cathodoluminescence spectrum. Benefitting from the controllable structural ordering transformation, as-prepared samples form a unique disorder-order crystal structure, exhibiting controllable microstructure acting on Eu2+ ions, tunable-color emissions including a high CRI (Ra = 90) white-light, increased structural rigidity and thermal stability. These interesting findings in the local structure-dependent luminescence properties demonstrate that structural ordering engineering may be an effective approach to regulate the migration of Eu2+ activators and improve the thermal stability of phosphors.

Pyrochlore-structured Y2Ti2O7 doped with Eu3+ activators via high-temperature solid-state reaction shows thermally stable orange-red emission under near ultraviolet excitation. The concentration of Eu3+ activators was observed to influence both the intensity and color point of photoluminescence of Y2−xTi2O7:xEu3+ phosphors. The optimal doping concentration is around x = 0.45 determined by photoluminescence intensity; at this concentration, the luminescence branch ratio R/O=I(5D0−7F2)/I(5D0−7F1), named as the luminous intensity ratio of 5D0–7F2 (R) and 5D0–7F1 (O) transition is also at maximum, about 0.46. This orange–red emitting phosphor Y1.55Ti2O7:0.45Eu3+ shows excellent thermal stability in luminescence intensity, retaining 84% of room-temperature luminescence at 250°C. Therefore, Y1.55Ti2O7:0.45Eu3+ phosphors may have great potential as an orange–red component for white light-emitting diodes, especially for the high-power devices.

Si 4+ was introduced to the lattice of LiEuMo 2 O 8 by solid-state reaction to prepare a new kind of red-emitting LiEuMo 2− x Si x O 8 (0 x ≤0.5) phosphors. The introduction of Si 4+ ion caused the distortion and slight shrinking of the unit cell of LiEuMo 2 O 8 material, and the blue-shift of the charge-transfer-absorption (CTA) band of LiEuMo 2 O 8 . Photoluminescence excitation (PLE) and photoluminescence measurements showed that the introduction of Si 4+ was able to enhance the excitation efficiency of LiEuMo 2 O 8 in NUV spectral region (360–400 nm). Consequently the red emission of LiEuMo 2 O 8 phosphor was improved by 30% at x =0.2 under 395 nm light excitation, without loss of color purity. The enhanced red emission of LiEuMo 2− x Si x O 8 was discussed in terms of the blue-shift of CTA band and the relaxation of parity-forbidden selection rules for trivalent europium luminescent centers.

A series of single-phased (Sr3-x,Ca1-y-z,Ba)(PO4)3Cl (SCBPO_Cl):xEu(2+), yTb(3+), zMn(2+) phosphors were synthesized by high-temperature solid-state reaction, and luminescent properties of these phosphors were investigated by means of photoluminescence and microcathode luminescence (μ-CL). Under UV excitation, white-light emission was obtained from triactivated SCBPO_Cl phosphors via combining three emission bands centered at 450, 543, and 570 nm contributed by Eu(2+), Tb(3+), and Mn(2+), respectively. White-light emission with the three emission bands is further demonstrated in the fluorescence microscope images, CL spectrum, and μ-CL mappings, which strongly confirm that the luminescence distribution of as-prepared SCBPO_Cl:xEu(2+), yTb(3+), zMn(2+) phosphors is very homogeneous. Both spectral overlapping and lifetime decay analyses suggest that dual energy transfers, that is, Eu(2+)→Tb(3+) and Eu(2+)→Mn(2+), play key roles in obtaining the white emission. The International Commission on Illumination value of white emission as well as luminescence quantum yield (51.2-81.4%) can be tuned by precisely controlling the content of Eu(2+), Tb(3+), and Mn(2+). These results suggest that this single-phased SCBPO_Cl:xEu(2+), yTb(3+), zMn(2+) phosphor may have a potential application as a near-UV convertible white-light emission phosphor for phosphor-converted white light-emitting diode.

Single-phased, high-color-rendering index (CRI) white-light phosphors are emerging as potential phosphor-converted white-light-emitting diodes (WLEDs) and as an alternative to blends of tricolor phosphors. However, it is a challenge to create a high CRI white light from a single-doped activator. Here, we present a high CRI (Ra = 91) white-light phosphor, Sr5(PO4)3-x(BO3)xCl:Eu2+, composed of Sr5(PO4)3Cl as the beginning member and Sr5(BO3)3Cl as the end member. This work utilized the solid-solution method, and tunable Eu2+ emission was achieved. Color-tunable Eu2+ emissions in response to structural variation were observed in Sr5(PO4)3-x(BO3)xCl solid solutions. This was further confirmed using X-ray Rietveld refinement, electron paramagnetic resonance spectroscopy, and in the photoluminescence spectra. The color-tunable emissions included the white light that originated from the combination of the blue emission of Sr5(PO4)3Cl:Eu2+ and an induced Eu2+ yellow emission at approximately 550 nm in the solid solution. Importantly, the white-light phosphors showed a greater R9 = 90.2 under excitation at 365 nm. This result has rarely been reported in the literature and is greater than that of (R9 = 14.3) commercial Y3Al5O12:Ce3+-based WLEDs. These findings demonstrate the great potential of Sr5(PO4)3-x(BO3)xCl:0.04Eu2+ as a white-light phosphor for near-UV phosphor-converted WLEDs. These results also provide a shortcut for developing a high CRI white-light phosphor from a single Eu2+-doped compound. A white light-emitting diode with a very high colour rendering index is made using a phosphor based on a europium-doped oxychloride material. A team of scientists from China, Switzerland and the USA investigated the white-light phosphor Sr5(PO4)3−x(BO3)xCl:Eu2+. Its structural and electronic properties were characterized by X-ray diffraction analysis, photoluminescence spectroscopy, and fluorescence decay analysis of the phosphor. When excited by ultraviolet light (365 nanometres) from a light-emitting diode, the phosphor emitted broadband photoluminescence with peaks in the blue (446 nanometres) and yellow (550 nanometres) regions. These emission characteristics resulted in high-quality white light with a very high colour rendering index that was significantly better than those of existing commercial white light-emitting diodes based on the phosphor YAG:Ce3+, indicating the potential of this new phosphor.

Two series of K1−xNaxSrPO4:0.005Eu2+ and K0.4Na0.6Sr0.995−yPO4:0.005Eu2+, yTb3+ phosphors are synthesized via a high-temperature solid-state reaction. Their emission color can be tuned from deep blue to blue–green by modulating the crystal field strength and energy transfer. Partial substitution of K+ with Na+ results in a contraction and distortion of the unit cell of the K1−xNaxSr0.995PO4:0.005Eu2+ host, tuning the emission from 426 to 498 nm. The red-shifted emission is attributed to an increased crystal field splitting for Eu2+ in a lowered symmetry crystal field. The tunable emission is further demonstrated in the cathodoluminescence spectra, which indicates that the luminescence distribution of the K1−xNaxSr0.995PO4:0.005Eu2+ phosphor is very homogenous. Additionally, utilizing the principle of energy transfer, the emission color can be further tuned by co-doping with Tb3+. The chromaticity coordinates for the co-doped phosphor, K0.4Na0.6Sr0.995−yPO4:0.005Eu2+, yTb3+, can be adjusted from (0.202, 0.406) for y = 0 to (0.232, 0.420) for y = 0.09. The energy transfer processes from the sensitizer (Eu2+) to the activator (Tb3+) are studied and demonstrated to have a resonance-type dipole–dipole interaction mechanism, with the critical distance of the energy transfer calculated to be 12.46 A using a concentration quenching method.