Solid-state Lighting Applications Research Articles

Effect of SiO2 shell on photoluminescence enhancement of Eu3+ doped nanophosphor based on monoclinic Y2O3

Surface SiO2 coating strategy was used to enhance the photoluminescent properties of Eu3+ doped nanophosphor based on monoclinic Y2O3. Laser-synthesized m-Y2O3:Eu3+ spherical nanoparticles with high crystallinity and diameter of ca. 18 nm were used as a core for the preparation of m-Y2O3:Eu3+@SiO2 core-shell structures. According to HRTEM and HAADF-STEM data, a thin SiO2 shell uniformly encapsulated the crystal core with a thickness of about 1.5 nm. The SiO2 shell increases the absolute photoluminescence quantum yield of Eu3+ at λ = 395 nm excitation from 14 % to 68 %. To the best of our knowledge, the obtained quantum yield exceeds the known values for Eu3+ doped oxide-based nanophosphors and is comparable to those of some other commercial red microphosphors. The SiO2 coating was shown to reduce the luminescence quenching effect associated with the annealing-induced migration of Eu3+ from the bulk to the surface of nanoparticles. The obtained results demonstrate that m-Y2O3:Eu3+@SiO2 core-shell nanoparticles with intense red luminescence have significant potential for use in LEDs, high resolution displays, bioimaging technologies and in other solid-state lighting applications.

Zero-Dimensional Halides with ns2 Electron (Sb3+) Activation to Generate Broad Photoluminescence.

Organic-inorganic metal halides (OIMHs) have various crystal structures and offer excellent semiconducting properties. Here, we report three novel OIMHs, (PPA)6InBr9 (PPA = [C6H5(CH2)3NH3]+), (PBA)2SbBr5, and (PBA)2SbI6 (PBA = [C6H5(CH2)4NH3]+), showing typical zero-dimensional (0D) structure, octahedra dimers, and corner-sharing one-dimensional chains and crystallized in the monoclinic system with P21, P21/c, and C2/c space groups, respectively. (PPA)6InBr9, (PBA)2SbBr5, and (PBA)2SbI6 have experimental optical band gaps of ∼3.16, ∼2.24, and 1.48 eV, respectively. (PPA)6InBr9 exhibits bright-orange light emission centered at 642 nm with a full-width at half-maximum of 179 nm (0.51 eV) and a Stokes shift of 277 nm (1.46 eV). After Sb3+ doping, the peak position did not change, and the photoluminescence quantum yield increased significantly from 9.2 to 53.0%. The efficient emission of Sb:(PPA)6InBr9 stems from the isolated ns2 luminescent center and strong electron-phonon coupling, making the spin-forbidden 3P1-1S0 observable. By combining commercial blue and green phosphors with orange-red-light-emitting (PPA)6In0.99Sb0.01Br9, a white-light-emitting diode was constructed, with the color-rendering index reaching up to 92.3. Our work highlights three novel 0D OIMHs, with chemical doping of Sb3+ shown to significantly enhance the luminescence properties, demonstrating their potential applications in solid-state lighting.

Ultra-Narrow-Bandwidth Deep-Red Electroluminescence Based on Green Plant-Derived Carbon Dots.

Deep-red light-emitting diodes (DR-LEDs, >660nm) with high color-purity and narrow-bandwidth emission are promising for full-color displays and solid-state lighting applications. Currently, the DR-LEDs are mainly based on conventional emitters such as organic materials and heavy-metal based quantum dots (QDs) and perovskites. However, the organic materials always suffer from the complicated synthesis, inferior color purity with full-width at half-maximum (FWHM) more than 40 nm, and the QDs and perovskites still suffer from serious problems related to toxicity. Herein, this work reports the synthesis of efficient and high color-purity deep-red carbon dots (CDs) with a record narrow FWHM of 21nm and a high quantum yield of more than 50% from readily available green plants. Moreover, an exciplex host is further established using a polymer and small molecular blend, which has been shown to be an efficient strategy for producing high color-purity monochrome emission from deep-red CDs via Förster energy transfer (FET). The deep-red CD-LEDs display high color-purity with Commission Internationale de l'Eclairage (CIE) coordinates of (0.692, 0.307). To the best of the knowledge, this is the first report of high color-purity CD-LEDs in the deep-red region, opening the door for the application of CDs in the development of high-resolution light-emitting display technologies.

Formation and Photophysical Properties of Silver Clusters in Bulk of Photo-Thermo-Refractive Glass

The bright luminescence of silver clusters in glass have potential applications in solid-state lighting, optical memory, and spectral converters. In this work, luminescent silver clusters were formed in the bulk of photo-thermo-refractive glass (15Na2O-5ZnO-2.9Al2O3-70.3SiO2-6.5F, mol.%) doped with different Ag2O concentrations from 0.01 to 0.05 mol.%. The spontaneous formation of plasmonic nanoparticles during glass synthesis was observed at 0.05 mol.% of Ag2O in the glass composition, limiting the silver concentration range for cluster formation. The luminescence of silver clusters was characterized by steady-state and time-resolved spectroscopy techniques. The rate constants of fluorescence, phosphorescence, intersystem crossing, and nonradiative deactivation were estimated on the basis of an experimental study. A comparison of the results obtained for the photophysical properties of luminescent silver clusters formed in the ion-exchanged layers of photo-thermo-refractive glass is provided.

Improved Luminous Efficiency of AgInS2 Quantum Dots and Zeolitic Imidazolate Framework-70 Composite for White Light Emitting Diode Applications.

The application of multiple quantum dots (QDs) in the field of white light emitting diodes (WLEDs) is still an important challenge due to their low luminous efficiency and quenching phenomenon. In this paper, we prepared AgInS2 QDs/zeolitic imidazolate framework-70 (AIS/ZIF-70) composite by a microwave hydrothermal method. Owing to the high porosity and stability of ZIF-70, it could effectively prevent quenching issues due to the aggregation of QDs. Since the ZIF-70 and QDs were chemically bonded, the formation of the ZnS layer could effectively passivate the surface defect and thus the quantum yield reached 21.49 % in aqueous solution. The luminous efficiency (LE) of the assembled AIS/ZIF-based WLED was reinforced by 6.8 times with a molar ratio of AgIn/Zn=18, i. e. at 5.26 % molar fraction of ZIF-70. Moreover, the color rendering index (CRI) and correlated color temperature (CCT) of AIS/ZIF-based WLED were 84.3 and 3631 K, respectively, indicating its potential application in solid-state lighting.

Luminescence properties of Tb3+/Eu3+ ions activated LiLaSiO4 phosphors for solid-state lighting and flexible display applications

The synthesis and luminescence properties of Tb3+ and Eu3+ single- and co-doped LiLaSiO4 (LLSO) phosphors with energy transfer mechanisms were reported. The rare-earth ions doped LLSO phosphors were synthesized via a solid-state reaction method and their physical characterizations such as phase purity, surface morphology, and elemental analysis were investigated systematically. For the Tb3+ and Eu3+ single- and co-doped LLSO phosphors, their photoluminescence (PL) properties were studied in detail. The LLSO:Tb3+ and LLSO:Eu3+ single-doped phosphors showed superior red and green emissions due to 5D4→7F5 and 5D0→7F2 in the optimized characteristic electronic transitions under ultraviolet (UV) irradiation, respectively. The luminescence performances for both the LLSO:Tb3+ and LLSO:Eu3+ phosphors were optimized at 0.125 mol of doping ion concentration. Under UV excitation, by adjusting the Eu3+ ion concentration in LLSO:0.125Tb3+/Eu3+ phosphors, the tunable emissions from green to red via yellow-orange were obtained. Thus, the tunable emission spectra and luminescence decay lifetimes with increasing the doping content of acceptor ions (Eu3+) confirmed an efficient energy transfer from Tb3+ to Eu3+ ions in the co-doped samples. The critical distance between Tb3+ and Eu3+ was determined to be 13.26 Å and the energy transfer mechanism from Tb3+ to Eu3+ ions was demonstrated to be a dipole-dipole mechanism interaction. In addition, the temperature-dependent PL study was carried out for the LLSO:0.125Tb3+/0.025Eu3+ co-doped phosphor sample and it showed better thermal stability against the temperature quenching. The Tb3+ and Eu3+ single- and co-doped LLSO/polydimethylsiloxane (PDMS) flexible composite films were fabricated by mixing the phosphor powders and PDMS solution. Benefiting from the effective protection of the PDMS matrix, the luminescence stability of the composite films was greatly enhanced. Based on the excellent properties of the synthesized phosphors, Tb3+ and Eu3+ single-doped and Tb3+/Eu3+ co-doped LLSO phosphors could serve as a single-phase multicolor-emitting phosphor under UV excitations.

Size-dependent miscibility controls the kinetics of anion exchange in cesium lead halide nanocrystals.

Anion exchange is a facile, post-synthetic method to tune the emission wavelength of colloidal cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals. While colloidal nanocrystals can exhibit size-dependent phase stability and chemical reactivity, the role of size in the mechanism of anion exchange in CsPbX3 nanocrystals has not been elucidated. We used single-particle fluorescence microscopy to monitor the transformation of individual CsPbBr3 nanocrystals to CsPbI3. By systematically varying the size of the nanocrystals and the concentration of substitutional iodide, we observed that smaller nanocrystals exhibit longer transition times in their fluorescence trajectories, while larger nanocrystals undergo a more abrupt transition during anion exchange. Monte Carlo simulations were used to rationalize the size-dependent reactivity, in which we varied how each exchange event affects the probability for further exchange. Greater cooperativity for simulated ion exchange leads to shorter transition times to complete the exchange. We propose that size-dependent miscibility between CsPbBr3 and CsPbI3 at the nanoscale controls the reaction kinetics. Smaller nanocrystals maintain a homogeneous composition during anion exchange. As the nanocrystal size increases, variations in the octahedral tilting patterns of the perovskite crystals lead to different structures for CsPbBr3 and CsPbI3. Thus, an iodide-rich region must first nucleate within larger CsPbBr3 nanocrystals, which is followed by rapid transformation to CsPbI3. While higher concentrations of substitutional anions can suppress this size-dependent reactivity, the inherent differences in reactivity between nanocrystals of different sizes are important to consider when scaling up this reaction for applications in solid-state lighting and biological imaging.

Two Novel Two-Dimensional Organic-Inorganic Hybrid Lead Halides with Broadband Emission for White-Light-Emitting Diodes.

Low-dimensional organic-inorganic metal halides (LOMHs) recently have attracted much attention due to their tunable crystal structures and excellent photoelectric properties. The configuration and arrangement of organic cations in LOMHs have significant effect on the structure of inorganic frameworks and luminescence properties. In this work, we systematically explored the "spatial effect" and "hydrogen bonding effect" of organic cations on the structure and properties of LOMHs, by synthesizing three LOMHs including (N-AD)PbCl4, (N-AD)2Pb2Br7, and (N-AD)4Pb3I12 (N-AD: N-acetylethylenediamine, C4H10N2O). Specifically, (110)-oriented two-dimensional (N-AD)PbCl4 and (N-AD)2Pb2Br7 with manifest blue-white emissions, originating from the free excitons (FEs) and self-trapped excitons (STEs), respectively. The UV-pumped light-emitting diode (LED)-based on (N-AD)2Pb2Br7 was prepared, and the highest color rendering index (CRI) and correlated color temperature (CCT) were up to 80 and 4484 K, respectively. This proves its potential application in solid-state lighting.

Thermodynamic Limit for Excitonic Light-Emitting Diodes.

We derive the thermodynamic limit for organic light-emitting diodes (OLEDs), and show that strong exciton binding in these devices requires a higher voltage to achieve the same luminance as a comparable inorganic LED. The OLED overpotential, which does not reduce the power conversion efficiency, is minimized by having a small exciton binding energy, a long exciton lifetime, and a large Langevin coefficient for electron-hole recombination. Based on these results, it seems likely that the best phosphorescent and thermally activated delayed fluorescence OLEDs reported to date approach their thermodynamic limit. The framework developed here is broadly applicable to other excitonic materials, and should therefore help guide the development of low voltage LEDs for display and solid-state lighting applications.

Warm to neutral white light emissions from Dy3+–Eu3+ co-doped glass ceramics containing NaBSiO4 crystalline phase for W-LEDs applications

This paper reports on the structural, optical and luminescence studies of Dy3+–Eu3+ co-doped glass ceramics that are obtained via thermal treatment method. The initial confirmation on glass ceramics with the formation of NaBSiO4 crystalline phase was made through XRD study. The FTIR study showed the vibrations of network formers (B2O3, SiO2) and other functional groups. The quantity of non-bridging oxygens (NBOs) are increased in the glass ceramics with increase in annealing temperature. The absorption spectra (UV–visible-NIR) showed the possible transitions of Dy3+ and Eu3+ ions in the glass ceramics. A red-shift in the absorption band-edge and reduction in the optical band gap values were obtained for glass ceramics owing to their heating temperatures. Photoluminescence studies showed the excitations of Dy3+ and Eu3+ ions under 575 nm and 613 nm emission wavelengths. The obtained NaBSiO4 crystalline phase in the glass ceramics has enhanced the luminescence intensity, and lifetimes of Dy3+ and Eu3+ ions compared to unheated precursor glass when excited under 350 nm and 393 nm. Color chromaticity diagram and correlated color temperature (CCT) values showed a shift in the color of light from warm white of precursor glass to neutral white of glass ceramics. The overall results justify the possibility of considering the Dy3+-Eu3+ co-doped glass ceramics as efficient materials for solid-state lighting applications like W-LEDs.

Comparative photoluminescence study of MAlB3O7:Dy3+ (M=Ca, Sr) phosphor for solid-state lighting applications

Comparative photoluminescence study of MAlB3O7:Dy3+ (M=Ca, Sr) phosphor for solid-state lighting applications

Wide colour tunability through cross relaxation and energy transfer in a Eu3+/Tb3+:ZnGa2O4 phosphor

In this article, the Tb3+, Eu3+ activated/co-activated ZnGa2O4 phosphors were synthesized using solid state reaction method to study the structural and optical characteristics. The XRD and SEM techniques, were used to study the crystallinity, phase purity and particles size whereas the EDS measurement approved the existence of different elements present in the phosphor samples. The FTIR and diffuse reflectance spectra of phosphor samples were studied to know the phonon frequency and the optical band gap. The PLem spectra of Tb3+ activated ZnGa2O4 phosphor contain blue, green and orange red emissions on 378 nm excitation. A colour tunability from blue to green regions was seen to arise due to cross relaxation on increasing the concentrations of Tb3+ ion. The Eu3+ activated ZnGa2O4 phosphor exhibited a strong red emission under 393 nm excitation. Further, the Eu3+ activated ZnGa2O4:Tb3+ phosphor showed colour tunability from blue to bluish white and from green to red under 260 and 378 nm excitations, respectively. Moreover, an energy-transfer (ET) has been observed from Tb3+ to Eu3+ ions with energy transfer mechanism as dipole-dipole interaction. The ET efficiency for 0.8 mol% Eu3+ in ZnGa2O4:0.05 Tb3+ phosphor has been found 97%. Under 260 nm excitation, the Eu3+/Tb3+ co-activated ZnGa2O4 phosphor also emits blue, green and red bands. Thus, the Tb3+, Eu3+ co-activated ZnGa2O4 phosphor may be useful for solid-state lighting applications.

Bi3+ ions doped double perovskite Ca2MgWO6 phosphor for yellow light emission

To develop new single-phase phosphor materials, a series of Ca2MgWO6:yBi3+ samples were successfully manufactured by high temperature solid state method. The microstructures and optical performances of the phosphors were investigated via X-ray diffraction, electron microscopy, as well as photoluminescence. The crystal lattice and morphology of the samples were adjusted by adding MgF2 flux. And the emission intensity of Bi3+ ions was enhanced by 21.4%. In addition, the characteristics emission of the [WO6] group was explored. Under excitation at 297 nm, [WO6] group possesses a broad emission band (350–550 nm) from W6+→O2− charge transfer band. Under 336 nm excitation, Ca2MgWO6:yBi3+ phosphors exhibit an ultra-wide emission band (400–800 nm) of Bi3+ ions. At the same time, Ca2MgWO6:yBi3+ phosphors have high quantum efficiency and absorption efficiency, and their maximum values reach 42.1% and 89.1%, respectively. All results suggest that Ca2MgWO6:yBi3+ phosphors might have potential applications in solid state lighting.

Synthesis and photoluminescence study of KCaPO4:Eu3+ phosphors for solid state lighting

A series of KCaPO4:Eu3+ phosphors was effectively synthesized utilizing a wet chemical method. The photoluminescence excitation and emission properties of the phosphor were investigated. The KCaPO4:Eu3+ phosphor was efficiently excited at 394 nm, and the PL (Photoluminescence) emission spectra were obtained at 591 and 614 nm. Concentration quenching occurred at a Eu3+ ion concentration of 0.5 mol%. The present work suggests that the KCaPO4:Eu3+ phosphors may be a potential candidate as a near-UV (Ultraviolet) convertible material for solid state lighting applications.

A new Milarite type KMLS:Eu3+ orange-red-emitting phosphor for pc-white LEDs and Forensic applications

In this article, we present the synthesis and characterization of Eu3+-activated KMLS:Eu3+ phosphor for pc-WLEDs, latent fingerprint detection, and security ink applications. The structural phase, luminescent property, and colour purity of the as-prepared phosphor were investigated. XRD profiles and Rietveld refinement analysis confirm the hexagonal crystal system of the as-prepared phosphor. Concentration quenching effect was not observed up to 17 mol% of Eu3+. The site occupancy preferences of Eu3+ were investigated using the bond-valence model. The CIE-coordinate of optimal Eu3+ is estimated to be close to the standard NTSC value, and color purity is found to be above 90% for all the Eu3+ concentrations. Both Judd-Ofelt (J-O) analysis and asymmetric ratios revealed that the local environment around Eu3+ is asymmetric in nature. The radiative properties, including the stimulated emission cross section and optical gain parameters, of the highest concentration of Eu3+ (17mol%) were determined. Further, the results of thermal quenching analysis show that 91% of the luminescence intensity can be maintained at 150 °C, demonstrating good thermal stability. The multilevel ridge characteristics of the latent fingerprint stained with as-prepared phosphor could be clearly visualized under 254 nm UV excitation. All of the above results indicate that as-prepared phosphor materials can be suitable for use in forensic applications as well as solid state lighting applications.

Energy transfer in Sm3+/Eu3+ doped Li6Gd(BO3)3 orange-red phosphor

Sm3+ and Sm3+/Eu3+ doped Li6Gd(BO3)3 (LGBO) powders were prepared by solid-state reaction. The microstructure, luminescent characteristics, energy transfer and fluorescence lifetimes were studied. Gd3+ can absorb energy and transfer it to Sm3+ under 278 nm excitation, which increases the luminescent intensity of Sm3+ at 567, 611 and 649 nm. Under 405 nm excitation, the 4G5/2-6H5/2 transition belonging to Sm3+ and the 5D0-7F2 transition belonging to Eu3+ appear in the emission spectra of Sm3+/Eu3+ co-doped LGBO phosphors, glowing yellow-orange and red light, respectively. The mechanism behind the energy transfer was analyzed, and the energy transfer efficiency was calculated to be 31.2% when the Sm3+ concentration reached 4mol%. The LGBO:Sm3+/Eu3+ phosphors have prospective applications in solid-state lighting, display devices, bio-imaging, etc.

Synthesis and photoluminescence characteristics of Ba2Ca(PO4)4:Dy3+ phosphors for n-UV based solid-state lighting

A series of Ba2Ca(PO4)4:Dy3+ phosphors were synthesized by the wet chemical method and the photoluminescence properties were systematically investigated. Under the 350 nm near ultraviolet excitation, Ba2Ca(PO4)4:Dy3+ phosphor shows emission peak at 475 nm (blue) and 572 nm (yellow). At 0.5 mol% of Dy3+ ions, concentration quenching was shown. The present study implies that the Ba2Ca(PO4)4:Dy3+ phosphors would be a suitable choice for solid-state lighting applications as a near ultraviolet convertible phosphor.

Novel Red-Emitting BaBi2B4O10:Eu3+ Phosphors: Synthesis, Crystal Structure and Luminescence

The novel red-emitting BaBi2−xEuxB4O10 (x = 0.05, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6) phosphors were obtained by a crystallization from a glass. Distribution of the Eu3+ ions over cation sites were refined for x = 0.1, 0.3 and 0.4 from single-crystal X-ray diffraction data. Emission and excitation spectra of the Eu3+-doped BaBi2B4O10 phosphors were investigated for the first time, where it was shown that characteristic lines are attributed to the intraconfigurational 4f-4f transitions. The optimal concentration for the BaBi2−xEuxB4O10 phosphors is x = 0.4, after which a luminescence intensity decreases. The CIE chromaticity coordinates for the BaBi2B4O10:Eu3+ (x = 0.4) phosphor (0.65, 0.35) are close to the NTSC standard values (0.67, 0.33) for commercial red phosphor. The obtained results show that the BaBi2B4O10:Eu3+ phosphors are promising candidates for solid state lighting application.

Dy3+ and Eu3+ co-activated gadolinium aluminate borate phosphor: Synthesis, enhanced luminescence, energy transfer and tunable color

The synthesis of GdAl3(BO3)4 phosphors incorporated with activators of Dy3+ and Dy3+/ Eu3+was successful and achieved through the gel combustion method. Powder X-ray diffraction (XRD) was employed to identify phase purity and the effects of dopant concentration on the crystallographic structure. The results of Photoluminescence (PL) measurements revealed that the intensity and lifetime of luminescence properties varied depending on the concentrations of Dy3+ and Eu3+ ions. The dependence of luminescence intensity on doping concentration is investigated with respect to the energy transfer process between Eu3+ and Dy3+ ions. A decrease in luminescence lifetime occurs with increasing concentrations of Eu3+ co-doping. The energy transfer was also investigated using decay curve analysis. The co-doping of Eu3+ significantly boosts the energy transfer efficiency from 26% to 84%. These findings make GdAl3(BO3)4: Dy3+, Eu3+ phosphors an ideal choice for LED applications in solid state lighting and displays.

Simple fabrication of novel Sm3+ doped BaGd2ZnO5 nanophosphors for flexible displays, improved data security applications, and solid-state lighting applications

In the present work, Urea-assisted combustion synthesis of BaGd2ZnO5:Sm3+ nanophosphors (BGZO:Sm NPs) with orthorhombic symmetry and cool white light luminance. According to the morphological study, the agglomerated particles had non-uniform shapes and crystallite sizes between 30 and 40 nm. The decay lifetime and quantum efficiency are found to be 0.387 ms and 51.4%, respectively. PL emission spectra can gather details about the orange-red emanation from produced NPs since they have a fine peak at 601 nm. Studies on photoluminescence (PL) also demonstrate that exchange-type interrelations are the primary mechanism for the non-radiative relaxing of energy. Finally, outstanding luminous performance is demonstrated by CIE color coordinates (0.5601, 0.3421), color purity (71%), and 1,376 K color-associated temperature (CCT). The multilevel characteristics of the phosphor-attached latent fingerprint (LFP) could be clearly identified in 404 nm light. An optimal observational environment is established by the sharp color contrast between the lines of the valley and ridge. Additionally, the impact of the NP utilized on various substrates for the visualization of LFPs is investigated. The findings demonstrate that the NP is a strong candidate for LFP visualization. Here, a flexible light-emitting material made of polydimethylsiloxane (PDMS) is used as the basis for an anti-counterfeiting (AC) method. Eventually, the ideal sample-convert PDMS flexible films with simple operation and good reusability are created, and their security level is further increased by the collaboration of colorful and distinctive characters.