Novel lanthanide hybrid functional materials for high performance luminescence application: The relationship between structures and photophysical behaviors

Functional luminescent hybrid materials have emerged as fascinating and promising materials for their versatile applications. In this report, novel efficient luminescent lanthanide (Tb³⁺, Eu³⁺) hybrid materials with a new kind of amide-type β-diketone ligands covalently bonded to the silica gels have been assembled through the sol–gel progresses. The hybrid materials have been characterized by the Fourier transform infrared (FTIR) spectra, UV–vis absorption spectra, powder X-ray diffraction (PXRD), scanning electron microscope (SEM), and thermal analyses. The relationship between structures and photophysical behaviors of these materials was discussed in detail. The materials assembled by the precursors containing aromatic end group (Si–L¹–Ln) exhibited longer luminescence lifetimes and higher quantum efficiencies, suggesting that the existence of a suitable conjugated system should allow a more efficient energy transfer. Under UV irradiation, the materials emitted either bright green light or red light with different intensity which may lead to potential functional applications in optical devices and electronic devices.     

Study on preparation and characterization of MOF based lanthanide doped luminescent coordination polymers

Coordination polymers (metal–organic frameworks or MOFs) offer the opportunity for fine-tuning the luminescence behavior because of the possibility to entrap in the network pores molecules that can influence the lanthanide (Ln) emission. In this study, Zn (II) and polycarboxylate based MOFs were first pre-formed by solvothermal method, then considered as host-matrix for in situ doping of low-input concentration of Eu³⁺ and Tb³⁺ (two most commonly used lanthanides in life science assays), and afterwards lanthanide doped luminescent materials were synthesized. Different characterizations (X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive Spectroscopy (EDS)) were carried out to confirm accordingly MOF's crystallinity, the structure and chemical composition. The study on luminescent properties of the material has revealed an efficient energy transfer from the ligand excited states to the Eu³⁺ and Tb³⁺ f-excited states. With quite low input concentrations (8–15%) of doped rare earth ions, these complexes displayed intense emissions at room temperature and proved to be good candidates for red and green emitter luminescent materials. Generally, this design concept can be extended for the preparation of other rare earth coordination polymers.     

Eu-modification of luminescent hybrid bimodal mesoporous silicas with various anions (NO3, CH3COO,and Cl)

In order to enhance the intensity of luminescent hybrid bimodal mesoporous silica (LHMS), the rare earth Eu³⁺ ion was employed as an enhancer to modify 1,8-naphthalic anhydride (NA) molecules. The modified LHMS were prepared with various anions of europium salts including Cl⁻, NO₃⁻, and CH₃COO⁻. The structure and photoluminescence properties of the obtained samples were characterized by XRD, TEM, N₂-adsorption/desorption, FT-IR, TGA, ICP-OES, and PL measurements. The results showed that the bimodal mesoporous system of BMMs could be preserved after functionalization via grafted Eu³⁺-modified NA molecules onto mesoporous matrix. However, the structure parameters and textural properties of resultant LHMS series were obviously influenced by the anions volume. Meanwhile, the luminescent performances of LHMS series were seriously impacted by both of the binding force between Eu³⁺ and various anions and steric hindrance of the anions, resulting in the intensity of emission peak of obtained LHMS series declining as following order: LHMS-EN (NO₃⁻) > LHMS-EA (CH₃COO⁻) > LHMS-EC (Cl⁻). Finally, the related mechanism was discussed in-depth.     

Hydrothermal synthesis of two photoluminescent nitrogen-doped graphene quantum dots emitted green and khaki luminescence

A simple and effective chemical synthesis of the photoluminescent nitrogen-doped graphene quantum dots (N-GQDs) biomaterial is reported. Using the hydrothermal treatment of graphene oxide (GO) in the presence of hydrogen peroxide (H₂O₂) and ammonia, the N-GQDs are synthesized through H₂O₂ exfoliating the GO into nanocrystals with lateral dimensions and ammonia passivating the generated active surface. Then, after a dialytic separation, two water-soluble N-GQDs with average size of about 2.1 nm/6.2 nm, which emit green/khaki luminescence and exhibit excitation dependent/independent photoluminescence (PL) behaviors, are obtained. In addition, it is also demonstrated that these two N-GQDs are stable over a broad pH range and have the upconversion PL property, showing this approach provides a simple and effective method to synthesize the functional N-GQDs.     

Synthesis of BaMoO4:Er/Yb particles by an MAM method and their upconversion photoluminescence properties

Er³⁺-doped BaMoO₄ (BaMoO₄:Er³⁺) and Er³⁺/Yb³⁺ co-doped BaMoO₄ (BaMoO₄:Er³⁺/Yb³⁺) particles were successfully synthesized by a cyclic microwave-assisted metathetic (MAM) method, and show fine and homogeneous morphology with particle sizes of 0.5–1 μm. At 980-nm excitation, BaMoO₄:Er³⁺ and BaMoO₄:Er³⁺/Yb³⁺ particles exhibited a strong 525-nm emission band and a weak 550-nm emission band in the green region. The Raman spectrum of BaMoO₄:Er³⁺/Yb³⁺ particles indicated the appearance of additional peaks at higher frequencies (390 and 505 cm⁻¹) and at lower frequencies (218 and 255 cm⁻¹).     

Photoluminescence properties of emission-tunable Ca9Y(PO4)7: Tm,Dy phosphor for white light emitting diodes

A white-emitting Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor has been successfully prepared by conventional high-temperature solid-state reaction. X-ray diffraction (XRD) and fluorescence spectrophotometer were used to characterize the as-synthesized phosphors. The excitation and emission spectra show that all the Tm³⁺ and Dy³⁺ co-doped Ca₉Y(PO₄)₇ samples can be effectively excited by UV light and then emit blue and yellow light simultaneously. Furthermore, the emission and color coordinate of as-obtained samples pumped by 365 nm are able to be adjusted around white light by varying the doping concentrations of Tm³⁺ and Dy³⁺. So, the as-fabricated single-composition Ca₉Y(PO₄)₇: Tm³⁺, Dy³⁺ phosphor will have a promising application in the area of white light emitting diodes.     

Synthesis and luminescent properties of organic-inorganic hybrid macroporous materials doped with lanthanide (Eu/Tb) complexes

Visible luminescent macroporous materials covalently bonded with europium and terbium complexes were prepared using polystyrene spheres (PS) as the template. Both of the obtained terbium(Ш) macroporous material (Tb(Ш)-M) and europium(Ш) macroporous material (Eu(Ш)-M) exhibit close-packed spherical porous structure according to the SEM images. Furthermore, their BET surface areas were determined by the N₂ adsorption-desorption isotherms. The two samples show corresponding characteristic green and red luminescence, respectively. Moreover, IR spectra and photoluminescence (PL) properties suggest that the Tb/Eu(Ш) ions were coordinated with organic group grafted on macroporous matrix. The luminescent lifetimes were also measured.     

Unusual broadening of the NIR luminescence of Er-doped Nb2O5 nanocrystals embedded in silica host: Preparation and their structural and spectroscopic study for photonics applications

This paper reports on the preparation of novel sol-gel erbium-doped SiO₂-based nanocomposites embedded with Nb₂O₅ nanocrystals fabricated using a bottom-up method and describes their structural, morphological, and luminescence characterization. To prepare the glass ceramics, we synthesized xerogels containing Si/Nb molar ratios of 90:10 up to 50:50 at room temperature, followed by annealing at 900, 1000, or 1100 °C for 10 h. We identified crystallization accompanying host densification in all the nanocomposites with orthorhombic (T-phase) or monoclinic (M-phase) Nb₂O₅ nanocrystals dispersed in the amorphous SiO₂ phase, depending on the niobium content and annealing temperature. A high-intensity broadband emission in the near-infrared region assigned to the ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ions was registered for all the nanocomposites. The shape and the bandwidth changed with the Nb₂O₅ crystalline phase, with values achieving up to 81 nm. Er³⁺ ions were located mainly in Nb₂O₅-rich regions, and the complex structure of the different Nb₂O₅ polymorphs accounted for the broadening in the emission spectra. The materials containing the T-phase, displayed higher luminescence intensity, longer ⁴I_13/2 lifetime and broader bandwidth. In conclusion, these nanostructured materials are potential candidates for photonic applications like optical amplifiers and WDM devices operating in the S, C, and L telecommunication bands.     

Tunable color emitting of CaAl2Si2O8: Eu,Tb phosphors for light emitting diodes based on energy transfer

A series of single-phased CaAl₂Si₂O₈: Eu, Tb phosphors have been synthesized at 1400 °C via a solid state reaction. The emission bands of Eu²⁺ and Eu³⁺ were observed in the air-sintered CaAl₂Si₂O₈: Eu phosphor due to the self-reduction effect. Tb³⁺ ions that typically generated green emission were added in CaAl₂Si₂O₈: Eu phosphor for contributing for a wider-range tunable emission. Energy transfer from Eu²⁺ to Tb³⁺ and the modulation of valence distribution of Eu²⁺/Eu³⁺ that contributes to the tunable color emitting were elucidated. More importantly, a white emission can be obtained by controlling the codoped contents of Li⁺ as well as suppressing the self-reduction degree of Eu. The white light emitting with the color coordinate (0.326, 0.261) was obtained, which indicates that CaAl₂Si₂O₈: Eu, Tb is a promising tunable color phosphor for application in ultraviolet light emitting diodes (UV-LEDs).     

Electrospinning preparation and characterization of a new kind of composite nanomaterials: One-dimensional composite nanofibers doped with TiO2 nanoparticles and Ru(II) complex

One-dimensional composite nanomaterials, PVP (poly(vinylpyrrolidone)) fibers with diameters in the range of 300–500 nm doped with RuL₂(NCS)₂ (L = 4,4′-Dicarboxy-2,2′-bipyridine) complex and TiO₂ nanoparticles (NPs), were prepared by electrospinning technique. The morphology of the composite nanofibers was systemically studied when the diameters of TiO₂ NPs, the concentrations of TiO₂ NPs and RuL₂(NCS)₂ in the PVP matrix were changed. A new kind of organic–inorganic composite nanomaterials which combine the advantages of one-dimensional nanostructures, organic materials and inorganic materials was obtained and it is desirable for low-weight and flexible photovoltaic devices, especially for dye-sensitized TiO₂ solar cells.     

Structure and luminescence properties of Mn-doped Zn2SiO4 prepared with extracted mesoporous silica

Zn₂SiO₄:Mn powders were prepared by solid-state reaction using extracted SBA-15 as silica source. The well crystalline willemite Zn₂SiO₄:Mn can be obtained at 800 °C, much lower than the conventional solid-state reaction temperature and lower than using the calcined SBA-15. This can be attributed to the high reactive activity of the extracted SBA-15 due to its high density silanol groups, large surface areas, and non-crystalline structure. Ultraviolet (UV) and vacuum ultraviolet (VUV) excitation spectra reveal the host lattice absorption band around 162 nm and the charge transfer transition band around 245 nm. The Zn₂SiO₄:Mn phosphor exhibits a strong green emission around 527 nm. The Zn₂SiO₄:Mn phosphor with an Mn doping concentration of 0.06, i.e., Zn_1.94Mn_0.06SiO₄, shows the highest relative emission intensity. Upon 147 nm excitation, the luminescence decay time of the green emission of Zn_1.94Mn_0.06SiO₄ around 527 nm is 8.87 ms.     

The preparation,characterization and optical properties of Cd2V2O7 and CdCO3 compounds

Cadmium vanadium oxides (Cd₂V₂O₇) and Cadmium carbonates (CdCO₃) were synthesized via a facile hydrothermal method. X-ray diffraction (XRD), Raman spectroscopy, infrared spectrometer (IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) were employed to characterize the structure, morphology and chemical state of the samples, respectively. The photoluminescence (PL) properties of the as-synthesized Cd₂V₂O₇ and CdCO₃ were measured at room temperature using an excitation wavelength of 325 nm. The Cd₂V₂O₇ shows two visible light emission centers located at 589 and 637 nm, which are supposed to be relevant to local defects in Cd₂V₂O₇. The CdCO₃ shows three emission centers located at 408, 530 and 708 nm, which are supposed to be relevant to the electron transition from the conduction band to valence band and defect related energy level.     

Preparation, characterization and luminescence of La2TeO6 phosphor doped with Eu

Phosphors of La₂TeO₆ doped with Eu³⁺ ions have been synthesized by the oxidation of the corresponding rare-earths oxytellurides of formula La_2−xEu_xO₂Te (x = 0.02, 0.06, and 0.1) at 1050 K. Powder X-ray diffraction confirms that the as prepared materials consist of the orthorhombic La₂TeO₆ as main phase. The photoluminescence (PL) of red-emitting La_2−xEu_xTeO₆ powder phosphors is reported. The emission spectrum, exhibits an intense emission peak due to ⁵D₀ → ⁷F₂ transition at 616 nm, which indicates that the Eu³⁺ ion occupies a non-centrosymmetric site in the host lattice. These materials could find application for use as lamp phosphors in the red region.     

Self-activated afterglow luminescence of un-doped Ca2ZrSi4O12 material and explorations of new afterglow phosphors in a rare earth element-doped Ca2ZrSi4O12 system

Un-doped Ca₂ZrSi₄O₁₂ material is prepared using a solid state reaction and it emits intense green afterglow luminescence peaking at 490 nm. The afterglow luminescence can be recorded for about 4800 s (0.32 mcd m⁻²). The thermoluminescence revealed that at least four types of traps existed in the Ca₂ZrSi₄O₁₂ material and the depths of these traps were calculated. The contributions of these traps on the afterglow luminescence were also investigated in detail. Accordingly, a possible afterglow mechanism of Ca₂ZrSi₄O₁₂ was proposed. Moreover, Ca₂ZrSi₄O₁₂ was also doped by rare earth or metal ions for developmental purpose and we arrived at some useful conclusions according to the experimental results.     

Synthesis of fluorescent carbon nanoparticles directly from active carbon via a one-step ultrasonic treatment

Water-soluble fluorescent carbon nanoparticles were synthesized directly from active carbon by a one-step hydrogen peroxide-assisted ultrasonic treatment. The carbon nanoparticles were characterized by transmission electron microscopy, optical fluorescent microscopy, fluorescent spectroscopy, Fourier transform infrared spectroscopy and ultraviolet-visible spectrophotometer. The results showed that the surface of carbon nanoparticles was rich of hydroxyl groups resulting in high hydrophilicity. The carbon nanoparticles could emit bright and colorful photoluminescence covering the entire visible-to-near infrared spectral range. Furthermore, these carbon nanoparticles also had excellent up-conversion fluorescent properties.     

Low-temperature synthesis,phonon and luminescence properties of Eu doped Y3Al5O12 (YAG) nanopowders

This contribution presents two simple and cost-effective routes for the low-temperature and large-scale production of pure and Eu-doped Y₃Al₅O₁₂ (yttrium aluminum garnet YAG) nanopowders. The proposed methodologies combine a mechanically assisted metathesis reaction or coprecipitation from solution followed by crystallization of the obtained precursors from molten sodium nitrate/nitrite. Both procedures allow obtaining pure and/or doped YAG nanopowders at remarkably low temperatures, i.e. already at 350 °C although firing at 500 °C is needed in order to get single phase and fully crystalline materials. As-obtained samples were characterized by XRD, TEM, Raman, IR and luminescence methods. These methods showed that the mean crystallite size is near 23–31 and 51 nm, when crystallization is performed from the amorphous precursor obtained by a mechanically assisted metathesis reaction and coprecipitation, respectively. Raman and IR spectra indicated better crystallinity of the powders prepared at 500 °C. The emission study showed that the intensity ratio between hypersensitive ⁵D₀ → ⁷F₂ and magnetic-dipole ⁵D₀ → ⁷F₁ transitions of Eu³⁺ is significantly larger than expected for well-crystallized YAG. Origin of this behavior is discussed.     

Precipitation based synthesis and luminescence of Ln (Eu,Ce, Dy,Sm, Tb) activated BaCa2Si3O9-Walstromite cyclosilicate phosphors

A series of new phosphors of BaCa₂Si₃O₉:Ln³⁺, (Ln = Eu, Ce, Dy, Sm, Tb) were synthesized by precipitation based method. Good crystallinity was achieved after annealing the sample at 750 °C for 1 h in air. X-ray powder diffraction (XRD) result confirmed the formation of desired BaCa₂Si₃O₉ host. The photoluminescent excitation and emission properties of Ln³⁺, (Ln = Eu, Ce, Dy, Sm, Tb) activated BaCa₂Si₃O₉ were investigated in detail. The photoluminescence (PL) analysis of individual Ln³⁺, (Ln = Eu, Ce, Dy, Sm, Tb) activated BaCa₂Si₃O₉ phosphors exhibits interesting characteristic emission properties in their respective regions. From the measured emission spectra, Judd–Ofelt (J–O) intensity parameters (Ω₂, Ω₄, Ω₆) have been calculated and using these J–O parameters various radiative parameters such as radiative transition probabilities (A_rad), radiative lifetimes (τ_R), branching ratios (β_R) and relative quantum yield have been calculated for the studied ions.     

Hydrothermal synthesis and luminescent properties of LaPO4:Eu 3D microstructures with controllable phase and morphology

Three types of Eu³⁺-doped LaPO₄ three-dimensional (3D) microstructures have been hydrothermally prepared by adjusting pH value and the molar ratio of La³⁺ and Eu³⁺ to phosphorus of Na₅P₃O₁₀ (Ln/P) at 180 °C. As the pH value was 0.15 and the molar ratio of Ln/P was in the range of 1/1 to 1/4, 5 μm urchinlike spheres composed of long nanorods of hexagonal LaPO₄:Eu were obtained. As the pH value was 1.00 and the molar ratio of Ln/P was 1/1, 3 μm hollow spheres consisting of short nanorods of monoclinic LaPO₄:Eu were prepared. Keeping the pH value at 1.0 and the molar ratio of Ln/P at 1/4, 6 μm core-shell spheres of monoclinic LaPO₄:Eu formed. Luminescent measurements were performed. Hollow spheres of monoclinic LaPO₄:Eu had the strongest emission intensity. However, the emission intensity of monoclinic core/shell structure was even lower than that of hexagonal LaPO₄:Eu urchinlike spheres.     

Optical and field emission characteristics of anodic aluminium oxide/ZnO hybrid nanostructure

Arrays of ZnO nanowires (NWs) were fabricated within the well-distributed pores of anodic aluminium oxide (AAO) template by a simple chemical method. The photoluminescence (PL) and field emission (FE) properties of the AAO/ZnO NWs hybrid structure were investigated in detail. The hybrid nanostructure exhibits interesting PL characteristics. ZnO NWs exhibit UV emission at 378 nm and two prominent blue-green emissions at about 462 and 508 nm. Intense blue emission from the AAO template itself was observed at around 430 nm. Herein, for the first time we report the FE characteristics of the ZnO/AAO hybrid structure to show the influence of the AAO template on the FE property of the hybrid structure. It is found that the turn-on electric field of the vertically grown and aligned ZnO NWs within the pores of AAO template is lower than the entangled unaligned ZnO NWs extracted from the template. Although the AAO template exhibits no FE current but it helps to achieve better FE property of the ZnO NWs through better alignment. The turn-on electric field of aligned NWs was found to be 3 V μm⁻¹ at a current of 0.1 μA. Results indicate that the AAO embedded ZnO NW hybrid structure may find useful applications in luminescent and field emission display devices.     

Upconversion emission from Tm:Yb and Tm:Er:Yb doped Y2SiO5 powders prepared by combustion synthesis

Tm³⁺:Er³⁺:Yb³⁺ doped Y₂SiO₅ powders were prepared by combustion synthesis with estimated as-prepared weight (wt.) % concentrations of 0.25:0.0:2.0, 0.25:0.5:2.0 and 0.25:1.0:2.0, respectively. Blue (Tm³⁺: ¹G₄ → ³H₆), green (Er³⁺: ⁴S_3/2, ²H_11/2 → ⁴I_15/2) and red (Er³⁺: ⁴F_9/2 → ⁴I_15/2) upconversion (UC) emissions were observed under 975 nm infrared diode laser excitation. The UC process took place via energy transfer from Yb³⁺ to Er³⁺ and Tm³⁺ ions. The CIE chromaticity coordinates of Tm³⁺:Er³⁺:Yb³⁺ doped Y₂SiO₅ powders were investigated as a function of the diode laser power and Er³⁺ concentration.