Tunable White Light Emission Research Articles

Influence of Gd3+ on the Optical Properties of Dy3+-Activated CaF2 Single Crystal for White LED Application

The spectroscopic properties of 0.6%Dy:Ca1−xGdxF2+x (x = 0.003, 0.006, 0.012, 0.03) crystals were investigated and compared. The energy transfer process between Gd3+ and Dy3+ was also studied. Increasing the proportions of Gd3+ ions could enhance the visible emission intensity significantly for the blue (478 nm) and yellow (573 nm) emissions; the intensity of the yellow emission was especially increased to be 77 times greater than the 0.6%Dy:CaF2 crystal. Gd3+ ions were not only used to enhance the emission intensity, but also played an important role in regulating the local lattice structure of Dy3+ ions. As a consequence, tunable white-light emission colors could be obtained by adjusting the doping concentration of Gd3+ ions.

Strongly luminescent inorganic–organic hybrid semiconductors with tunable white light emissions by doping

Tunable white light emissions from copper bromide based inorganic–organic hybrid semiconductors have been observed upon doping.

Tunable white-light emission from Pr3+/Dy3+ co-doped B2O3 - TeO2[sbnd] PbO - ZnO [sbnd] Li2O - Na2O glasses

In this paper, structural and optical analysis of boro-tellurite based glasses co-doped with Dy3+ and Pr3+ ions has been carried out. The glasses were fabricated by traditional melt quenching technique. XRD results confirm the amorphous-like nature of the glasses. FTIR spectroscopy was applied to explore the functional groups and interactions for the samples. The transition temperatures of glasses were identified by TGA/DSC analysis. Optical absorption and emission spectra of all the glasses have been recorded in the UV Vis- NIR spectral range. Tauc's method was applied to determine the direct and indirect optical band gaps. Luminescence spectra show intense emission bands at ∼482 nm and ∼574 nm wavelengths under λexc = 388 nm corresponding to Dy3+: 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions. Decay time data revealed the occurrence of a non-radiative energy transfer from Dy3+ to Pr3+ ions. Intense emission bands are identified at ∼602 nm (orange), ∼487 nm (blue) and ∼574 nm (yellow) observed upon λexc = 437 nm, corresponding to Pr3+:1D2→3H4, Pr3+:3P0,1,2 → 3H4/Dy3+: 4F9/2 → 6H13/2 and Dy3+: 4F9/2 → 6H13/2 transitions, respectively, where the Dy3+ emissions are due to energy transfer from Pr3+ to Dy3+ ions. The CIE chromaticity results confirm that different colors can be generated by tuning the exciting wavelength, where all the glasses are capable to emit white light under λexc = 388 nm, while at λexc = 437 nm the samples (S2S4) emit reddish orange light. Accordingly, it is plausible to suggest these glasses for white LED and SSL applications.

Design and energy transfer mechanism for single-phased Gd2MgTiO6: Bi3+, Eu3+ tunable white light-emitting phosphors

In recent years, numerous efforts have been made to develop single-phased white light-emitting phosphors for the near-UV region to solve problems of color reabsorption and ratio regulation between different phosphors. In this work, we have designed Bi3+- and Eu3+-codoped single-phased Gd2MgTiO6 phosphors to achieve tunable white light emission based on multi-luminescence center energy transfer. The structural analysis showed that all the samples were crystallized as a monoclinic double perovskite with the P21/n symmetry space group (No. 14), with HRTEM images showing clear lattice fringes between the lattice planes. The single Bi3+-doped Gd2MgTiO6 sample exhibits two obvious emission peaks at 417 and 508 nm, which correspond to a characteristic 3P1 → 1S0 transition for the Bi3+ ions under near-UV excitation due to two types of Bi3+ emission centers, with their relative emission intensity depending closely on the value of the excitation wavelength. In this case, a suitable choice of excitation wavelength can achieve tunable emission for Gd2MgTiO6: Bi3+ between blue and green. Eu3+ is codoped into Gd2MgTiO6 as a red emission component and shows sharp emission lines that correspond to the characteristic 5D0 → 7FJ (J = 1, 2, 3, and 4) transitions of Eu3+ ions. Energy transfer in Gd2MgTiO6: Bi3+, Eu3+ has been confirmed by the electric dipole–dipole (d–d) interaction from Bi3+ to Eu3+. Our experiments show that it is straightforward to create tunable white light emission by adjusting the excitation scheme and Eu3+ concentration. Moreover, a schematic for the energy transfer mechanism and simplified spectral levels based on Bi3+ and Eu3+ ions has also been established.

Tunable white light emission of a large area film-forming macromolecular complex with a high color rendering index

The technological route of pre-polymerization and post-coordination was employed to obtain a rapid film-forming macromolecular complex emitting white light by simultaneously combining poly (styrene-co-glycidyl methacrylate) with three primary color complexes [r(R)/g(G)/b(B)]. The white light emission of the macromolecular complex was realized by tuning the ratios of the trichromatic complexes or altering excitation wavelengths. Upon excitation at 365 nm, macromolecular complex 5 with R:G:B = 1:10:1 exhibits three discrete characteristic peaks at 458 (blue emission), 543 (green emission), and 612 (red emission) nm. The corresponding CIE coordinates of (0.332, 0.337) are in close proximity to pure white light (0.330, 0.330). Macromolecular complex 3 (R:G:B = 1:8:0.5) achieves white emission being excited at the wavelength of 375 nm. Macromolecular complex 5 was melted at 140 °C, dropped on a 365 nm UV chip, and then cooled to room temperature to fabricate an LED device with CIE coordinates of (0.35, 0.34), CCT of 5306 K, and CRI of 95.2. The prepared macromolecular complex film further shows vivid color reproduction for real objects, confirming the high rendering index of the macromolecular complex. In addition, the macromolecular complex remains stable up to 272.6 °C, meeting the requirements of operating temperature for LEDs. This work establishes a rapid film-forming macromolecular complex with potential application in NUV-WLEDs.

Highly efficient and tunable white light emission of Sn2+-Dy3+ co-doped fluorophosphate glasses

A series of Sn2+-Dy3+ co-doped fluorophosphate glasses (FPGs) were prepared by the melt quenching method. The luminescent properties and energy transfer mechanisms of the FPGs glasses were investigated through photoluminescence and decay lifetime analysis. By controlling the concentration of Dy3+, the FPGs present a white light with a CIE chromaticity coordinate of (0.311, 0.330), which is very close to the standard equal energy white light illumination. The corresponding quantum efficiency, CRI and the brightness are 56.3%, 75 and 6706 cd/m2, respectively. Furthermore, the physical and chemical stability and thermal properties were also analyzed using differential scanning calorimetry and a thermal conductivity detector. The corresponding values of the ∆T (Tx-Tg) and thermal conductivity are 155 °C and 3.02~3.31 W/m·K, respectively. These results demonstrate that the FPGs can be a promising candidate for tunable white light phosphors.

Tunable and white luminescence from mixed lanthanide with aza-macrocycles through multistimuli responses

In this work, two aza-macrocycles (1 and 2) were designedly synthesized as hosts for sensitizing lanthanide luminescence in aqueous solution to explore new light emitting material. It is expected that macrocycle molecules can encapsulate lanthanide centers into their cavity to form new lanthanide fluorescent materials based on their excellent host–guest binding abilities. In addition, macrocycle molecules are blue emitters, so it will be possible to construct white light-emitting and color-tunable materials through the doping of both Tb3+ ions and Eu3+ ions into aza-macrocycle hosts with an appropriate molar ratio. All of experimental results verify that aza-macrocycle based lanthanide materials realized tunable luminescence and white light emissions through changing relative concentration (Tb/Eu = 1:0, 1:1, 1:2, 1:4, 1:6, 1:8, 0:1) of the constituent Ln3+ ions and the excitation wavelength from 351 nm to 395 nm. As far as we know, it is very rare that the color-tunable and white-light emission can be controlled by two types of strategies and on the basis of macrocycle hosts.

Enhanced and tunable white light emission from Ag nanoclusters and Eu3+-co-doped CaBAl glasses.

Noble metal embedded glasses have been studied as promising candidates for a variety of technological applications, mainly due to their ability to enhance rare earth luminescence properties. In this work, Ag:Eu-co-doped calcium boroaluminate glasses were prepared and submitted to further heat treatment to form different Ag species. The optical and luminescence properties were investigated in terms of heat treatment times. Absorption spectra showed a successful Eu and Ag ion incorporation in the host, as well as Ag nanoparticle precipitation induced by heat treatment. Upon UV-light excitation, the co-doped glasses exhibited an intense wide emission band centered at about 500 nm, attributed to molecule-like silver species, which combined with the Eu3+ characteristic emission reaches a white light resultant emission. A new excitation band for Eu3+ at 335 nm and a silver luminescence lifetime decrease suggest an energy transfer process from molecule-like Ag to Eu3+ as being responsible for the enhanced PL properties in these glasses. An appropriate combination of a violet LED with the sample emission provides a route to achieve the ideal white light CIE color parameters. The relevant quality color results qualify these glasses as phosphors with high potential for white light emitting devices.

Eu3+/Tb3+ functionalized Bi-based metal-organic frameworks toward tunable white-light emission and fluorescence sensing applications.

The rational design strategy to construct lanthanide ion functionalized metal organic frameworks (MOFs) has attracted tremendous attention as they are promising candidates for developing novel luminescence materials and optical sensors owing to their intense, long-lived and tunable luminescence performances. In this work, a series of Eu3+/Tb3+ singly doped or codoped bismuth-based MOFs were prepared under in situ facile solvothermal conditions using BiOBr nanoplates as bismuth resources. A red-green-blue-based trichromatic white-light emission can be finely achieved by modulating the doping ratio of Eu3+/Tb3+ in the blue-emitting Bi-MOF host material and such photoluminescence tuning can also be realized by controlling the excitation wavelength. In addition, the Eu3+/Tb3+ singly incorporated Bi-MOF can serve as a multifunctional fluorescent probe for sensing Fe3+ and Cr2O72- ions in aqueous solution, as well as small organic pollutants of acetone molecules, and their sensing mechanisms have also been analyzed. These lanthanide ion functionalized Bi-MOF materials exhibit great potential in exploring white-light emitting devices and construction of supersensitive multifunctional sensors.

Tunable white-light emission by supramolecular self-sorting in highly swollen hydrogels.

Fluorescence-tunable hydrogels especially emitting white-light were achieved by swelling hydrogels in solutions containing two kinds of dyes. The fluorescence of the dyes was enhanced by the orthogonal supramolecular complexation with different binding sites in the hydrogels.

A color tunable and white light emitting Ca2Si5N8:Ce3+,Eu2+ phosphor via efficient energy transfer for near-UV white LEDs.

Achieving tunable color and white light emission in a single phase phosphor is a challenging issue. Here, a series of Ce3+-Eu2+ co-doped Ca2Si5N8 phosphors were successfully synthesized by a high temperature solid state method. Luminescence properties, energy transfer (ET) and thermal quenching of the as-synthesized samples were investigated in detail. The emission color of as-prepared Ca1.94Na0.03Si5N8:0.03Ce3+,yEu2+ (0.0000 ≤ y ≤ 0.0035) could be tuned from blue to white light and eventually to orange via ET by changing the Ce3+/Eu2+ ratio. ET efficiency from Ce3+ to Eu2+ could reach up to 68.0% and the ET mechanism was demonstrated to be a non-radiative dipole-dipole interaction. More importantly, when y = 0.0012, approximate standard white light was generated with CIE coordinates of (0.335, 0.341). A photoluminescence (PL) mechanism was proposed to understand PL properties and thermal properties of the as-prepared phosphors. Additionally, the achieved white light phosphor Ca1.9388Na0.03Si5N8:0.03Ce3+,0.0012Eu2+ had good thermal stability, exhibiting about 75% at 150 °C and 64% at 200 °C of the emission intensity at 25 °C, respectively. The chromaticity shifts of Ca1.9388Na0.03Si5N8:0.03Ce3+,0.0012Eu2+ were 0.0032 at 150 °C and 0.0152 at 200 °C, respectively, which were only 27% and 42% of the commercial white-emitting phosphor mixture at the corresponding temperature. Furthermore, a proof-of-concept white LED was fabricated by combining the single component phosphor Ca1.9388Na0.03Si5N8:0.03Ce3+,0.0012Eu2+ with a near UV LED chip. All results demonstrate the promising application of the Ca2Si5N8:Ce3+,Eu2+ single phosphor for near-UV white LEDs.

Reversibly Tunable White-Light Emissions of Styrylpyridiniums with Cucurbiturils in Aqueous Solution.

The different excimer emissions based on a series of host-guest complexes of dipolar dyes styrylpyridiniums (An and Bn) with cucurbit[n]urils (CB[n]) are reported. The syn- and anti- excimers due to the opposite stacking orientations have been well investigated both in experiments and DFT calculations. Furthermore, reversibly color-tunable luminescent supramolecular assemblies including near white-light emission with CIE coordinates (0.32, 0.29) were achieved by competitive inclusion of An-CB[7] with ADA.

Tunable White-Light Emission in Single-Cation-Templated Three-Layered 2D Perovskites (CH3CH2NH3)4Pb3Br10-xClx.

Two-dimensional (2D) hybrid halide perovskites come as a family (B)2(A)n-1PbnX3n+1 (B and A= cations; X= halide). These perovskites are promising semiconductors for solar cells and optoelectronic applications. Among the fascinating properties of these materials is white-light emission, which has been mostly observed in single-layered 2D lead bromide or chloride systems (n = 1), where the broad emission comes from the transient photoexcited states generated by self-trapped excitons (STEs) from structural distortion. Here we report a multilayered 2D perovskite (n = 3) exhibiting a tunable white-light emission. Ethylammonium (EA+) can stabilize the 2D perovskite structure in EA4Pb3Br10-xClx (x = 0, 2, 4, 6, 8, 9.5, and 10) with EA+ being both the A and B cations in this system. Because of the larger size of EA, these materials show a high distortion level in their inorganic structures, with EA4Pb3Cl10 having a much larger distortion than that of EA4Pb3Br10, which results in broadband white-light emission of EA4Pb3Cl10 in contrast to narrow blue emission of EA4Pb3Br10. The average lifetime of the series decreases gradually from the Cl end to the Br end, indicating that the larger distortion also prolongs the lifetime (more STE states). The band gap of EA4Pb3Br10-xClx ranges from 3.45 eV (x = 10) to 2.75 eV (x = 0), following Vegard's law. First-principles density functional theory calculations (DFT) show that both EA4Pb3Cl10 and EA4Pb3Br10 are direct band gap semiconductors. The color rendering index (CRI) of the series improves from 66 (EA4Pb3Cl10) to 83 (EA4Pb3Br0.5Cl9.5), displaying high tunability and versatility of the title compounds.

Tunable white-light emission of lanthanide(III) hybrid material based on hectorite

In this work, we present novel trivalent lanthanide ions (Ln3+)-based luminescent hybrid materials, in which the organic ligands are covalently grafted on the hectorite templates and the Ln3+ ions can be well immobilized by the ligands through coordination bond. The hybrid materials exhibit tunable emission colors by varying the molar ratio of Eu3+ to Tb3+, and the one with Eu3+:Tb3+=1:1 exhibits excellent co-ordinate of (0.327, 0.328) located in the “white region” of the CIE 1931 chromaticity diagram (under 300nm UV illumination). These properties make the hybrid composites suitable for fabricating optoelectronic devices such as full-color displays and white LED.

Introduction of Red-Green-Blue Fluorescent Dyes into a Metal-Organic Framework for Tunable White Light Emission.

The unique features of the metal-organic frameworks (MOFs), including ultrahigh porosities and surface areas, tunable pores, endow the MOFs with special utilizations as host matrices. In this work, various neutral and ionic guest dye molecules, such as fluorescent brighteners, coumarin derivatives, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), and 4-(p-dimethylaminostyryl)-1-methylpyridinium (DSM), are encapsulated in a neutral MOF, yielding novel blue-, green-, and red-phosphors, respectively. Furthermore, this study introduces the red-, green-, and blue-emitting dyes into a MOF together for the first time, producing white-light materials with nearly ideal Commission International ed'Eclairage (CIE) coordinates, high color-rendering index values (up to 92%) and quantum yields (up to 26%), and moderate correlated color temperature values. The white light is tunable by changing the content or type of the three dye guests, or the excitation wavelength. Significantly, the introduction of blue-emitting guests in the methodology makes the available MOF host more extensive, and the final white-light output more tunable and high-quality. Such strategy can be widely adopted to design and prepare white-light-emitting materials.

Tunable white light emission from hafnium oxide films co‐doped with trivalent terbium and europium ions deposited by Pyrosol technique

In this paper, the photo and cathodoluminescent properties of HfO2 films optically activated with different atomic concentrations of Tb3+ and Eu3+ ions, deposited by the Pyrosol technique, are reported. These films were deposited at temperatures from 400 to 600 °C, using chlorides as raw materials. The surface morphologies of all deposited films were rough and dense. X‐ray diffraction analysis showed that the films deposited at 600 °C were polycrystalline exhibiting the HfO2 monoclinic phase. The tuning by the means of the excitation wavelength generates photoluminescence spectra, for co‐doped films, in several emissions from blue to yellow (including white light) due to the characteristic electronic transitions of Tb3+ (green), Eu3+(red) ions and the violet‐blue emission associated to the host lattice (HfO2). According to the chromaticity diagram, the best white light is reached for the sample S2 excited with 382 nm (x = 0.3343, y = 0.3406). The cathodoluminescence emission spectra for co‐doped films showed emissions from green to red (including yellow, orange and other intermediate emissions). The averaged quantum efficiency values of the sample labeled as S2 resulted between 47 and 78% depending on the excitation wavelength. In addition, XPS, TEM, SEM and decay times were performed to characterize these films.

Self-Assembly of Conjugated Metallopolymers with Tunable Length and Controlled Regiochemistry.

Self-assembled materials can be designed to express useful optoelectronic properties; however, achieving structural control is a necessary precondition for the optimization of desired properties. Here we report a simple, metal-templated polymerization process that generates helical metallopolymer strands over 75 repeat units long (28 kDa) from a single bifunctional monomer and CuI . The resulting polymer consists of a double helix of two identical conjugated organic strands enclosing a central column of metal ions. The length of this metallopolymer can be controlled by adding monofunctional subcomponents to end-cap the conjugated ligands. The use of ditopic and bulky monotopic subcomponents, respectively, allows a head-to-head or head-to-tail double helix to be generated. Spectroscopic measurements of different polymer lengths demonstrate how control over polymer length leads to control over the electronic and luminescent properties of the resulting material, thereby enabling tunable white-light emission.

Mixed-Lanthanide Metal–Organic Frameworks with Tunable Color and White Light Emission

Three isostructural lanthanide metal–organic frameworks (Ln-MOFs, Ln = Eu3+, Tb3+, Dy3+) containing P-terphenyl-2,2″,4,4″-tetracarboxylate ligand (H4L) with red, green, and blue luminescence were solvothermally synthesized. Thus, a series of mixed Ln-MOFs, (EuxTbyDy1-x-y)(HL)(H2O)(DEF) (DEF, N,N-diethylformamide), were designed and obtained, which displayed highly temperature-tuned emission in the visible region, including white light emission. Additionally, tunable luminescence can also be achieved by changing the excitation wavelength.

A family of doped lanthanide metal–organic frameworks for wide-range temperature sensing and tunable white light emission

A series of binary and ternary mixed LnMOFs with high stability showing potential applications in wide-range thermosensors and white LEDs are reported.

Tunable white-light emission PMMA-supported film materials containing lanthanide coordination polymers: preparation, characterization, and properties.

A series of lanthanide coordination polymers(LnCPs) containing both light and heavy rare-earth elements, namely {[Eu2(pydc)3(H2O)]·2H2O}n (1-Eu, H2pydc = pyridine-2,3-dicarboxylic acid), [Ln(pyc)2(Hpyc)(NO3)]n (Ln = Nd (2-Nd), Sm (3-Sm), Eu (4-Eu), Gd (5-Gd), Tb (6-Tb), Ho (7-Ho), and Er (8-Er), Hpyc = pyridine-3-carboxylic acid), has been synthesized under hydro(solvo)thermal conditions and fully characterized. The crystal structure analysis indicates that in situ decarboxylation of H2pydc occurred in the synthesis process of 2-Nd-8-Er. Coordination polymer 1-Eu displays a 3-D pcu network with central-symmetric quad-core structural units [Eu4(COO)6] linked by 1-D chains. 2-Nd-8-Er are of triple helical chain enantiomeric pair 61/65 axis, and can be further linked through two separate kinds of Hbonding interaction to form a mirror symmetrical 3-D framework; the final topological symbol of the jointly connected network is rare {47·68}. Solid-state luminescence studies show that the emission spectra of these LnCPs cover both the visible and near-infrared luminescence region. 2-Nd exhibits characteristic 4F3/2 → 4IJ/2 (J = 9, 11, 13) transition NIR emission. 1-Eu and 4-Eu provide characteristic 5D0 → 7FJ intense and bright red luminescence, while 4-Eu exhibits better luminescence performance because of the presence of the O-H oscillators within 1-Eu. 6-Tb exhibits characteristic 5D4 → 7FJ intense and bright green luminescence. Furthermore, through doping with PMMA polymer, the luminescence properties of 4-Eu and 6-Tb are all improved. The results show the best doping concentration is 4%. The thermal stabilities of 4-Eu-PMMA and 6-Tb-PMMA increased from 270 to 315 °C when compared with single coordination polymers 4-Eu and 6-Tb. The co-doping of 4-Eu, 5-Gd, and 6-Tb (0.92/0.04/0.04) with PMMA at a total concentration of 4% resulted in a tunable luminescence material W(4-Eu,5-Gd,6-Tb)-PMMA film. When excited at 285 nm, W(4-Eu,5-Gd,6-Tb)-PMMA shows white-light emission with the CIE chromaticity coordinates of (0.33, 0.35). The lifetime of Eu3+ ions in W(4-Eu,5-Gd,6-Tb)-PMMA increased from 1032.24 μs to 1272.26 μs when compared with 4-Eu-PMMA, and the efficiency of energy transfer (ηET) from Tb3+ to Eu3+ within the co-doping PMMA film could be estimated to be 29.9%.