Red Spectral Region Research Articles

Critical Review—Narrow-Band Nitride Phosphors for Wide Color-Gamut White LED Backlighting

To achieve a brilliant image with high color saturation in white LED backlighting technology, innovative phosphor materials with narrow-band emission in the green and red spectral region are continuously pursed. Nitride phosphors are so far accepted as the most suitable phosphors for white LED backlights due to their high efficiency and excellent robustness. In this perspective, we will present an overview about the recent developments of state-of-the-art and newly-emerging nitride phosphors with a narrow emission band, and the emphasis would be placed on the relationships between the crystal structure and luminescence properties. Finally, some empirical rules for designing novel narrow-band phosphors for backlighting technologies would be summarized.

Continuously tunable organic solid-state laser based on the adjustment of effective refractive index

We report a facile way for continuously tuning the lasing wavelength of an organic solid-state laser by varying the effective refractive index. The effective refractive index of the organic gain medium can be fine controlled by the dopant concentration in the DCJTB:PS film. Continuous wavelength tuning of laser and amplified spontaneous emission (ASE) are achieved in the distributed feedback (DFB) gratings and planar waveguides, respectively. In the DFB laser, laser emission is demonstrated in the red spectral region with a tunable wavelength range of 30 nm, centered at 612 nm. The laser exhibits a well-defined pump threshold and good emission characteristics. Furthermore, ASE with a large wavelength range of 40 nm is also achieved by varying the dopant concentration, thus giving one the freedom to set or tune the lasing wavelength to meet a particular purpose with a common structure. Our results allow for a simple, broadband continuous tuning of the laser emission wavelength.

DNA mimics of red fluorescent proteins (RFP) based on G-quadruplex-confined synthetic RFP chromophores.

Red fluorescent proteins (RFPs) have emerged as valuable biological markers for biomolecule imaging in living systems. Developing artificial fluorogenic systems that mimic RFPs remains an unmet challenge. Here, we describe the design and synthesis of six new chromophores analogous to the chromophores in RFPs. We demonstrate, for the first time, that encapsulating RFP chromophore analogues in canonical DNA G-quadruplexes (G4) can activate bright fluorescence spanning red and far-red spectral regions (Em = 583−668 nm) that nearly match the entire RFP palette. Theoretical calculations and molecular dynamics simulations reveal that DNA G4 greatly restricts radiationless deactivation of chromophores induced by a twisted intramolecular charge transfer (TICT). These DNA mimics of RFP exhibit attractive photophysical properties comparable or superior to natural RFPs, including high quantum yield, large Stokes shifts, excellent anti-photobleaching properties, and two-photon fluorescence. Moreover, these RFP chromophore analogues are a novel and distinctive type of topology-selective G4 probe specific to parallel G4 conformation. The DNA mimics of RFP have been further exploited for imaging of target proteins. Using cancer-specific cell membrane biomarkers as targets, long-term real-time monitoring in single live cell and two-photon fluorescence imaging in tissue sections have been achieved without the need for genetic coding.

Up-conversion luminescent properties of Er3+/Yb3+ co-doped transparent glass ceramics containing Na3.6Y1.8(PO4)3 crystals

Er3+/Yb3+ co-doped transparent glass ceramics containing Na3.6Y1.8(PO4)3 crystals were prepared by high-temperature melting and subsequent heat treatment the precursor glass, and their up-conversion luminescent properties were investigated. The optimum heat treatment condition of glass at 630 °C for 2 h were determined by a series of experimental results, and confirmed the existence of Na3.6Y1.8(PO4)3 crystals in glass ceramics. The up-conversion luminescence spectra of the glass ceramic samples under pulsed excitation at 980 nm revealed luminescence bands corresponding to Er3+ ion transitions in the green (4F7/2-4I15/2, 2H12/2-4I15/2,4S3/2-4I15/2) and red (4F9/2 - 4I15/2) spectral regions. The fluorescence spectra reaches a maximum when the molar ratio of Er2O3 to Yb2O3 is 1:7. Luminescence quantum yield of the samples was measured by using an integrating sphere. The transmission spectra proved that the glass ceramics sample still maintained a high transparency up to 83% in the visible region.

Optical and Structural Properties of Zn2TiO4:Mn2+

Polycrystalline Zn2TiO4 samples with Mn2+ doping level of 0%, 0.1%, 1.0%, and 5.0% have been produced by conventional solid-state method and their optical and structural properties investigated. Rietveld refinement of x-ray diffraction patterns revealed the formed phases and the crystallographic parameters. The chemical composition was obtained by x-ray fluorescence measurements. The optical properties were studied by photoluminescence, excitation, reflectance, and photoacoustic spectroscopy. All measurements were performed at room temperature. The photoluminescence spectrum of the pure sample (0% Mn2+) showed a band in the red region associated with Zn2TiO4, while the sample with 0.1% Mn2+ exhibited two bands, in the green and red spectral regions, assigned to Mn2+ ions at tetrahedral and octahedral sites. No emission was observed for the samples with 1.0% or 5.0% Mn2+. The excitation results for the sample with 0.1% Mn2+ ions showed characteristic peaks of Mn2+ transitions. Tanabe–Sugano theory was used to obtain the crystal field Dq, B, and C Racah parameters from the energy peak positions in the excitation spectrum of the sample with 0.1% Mn2+. Photoacoustic measurements revealed a broad band, characteristic of semiconductor materials, hiding the Mn2+ transitions.

Coumarin-Rhodamine Hybrids-Novel Probes for the Optical Measurement of Viscosity and Polarity.

A comprehensive systematic study of absorption and fluorescence properties in solvents of varying viscosity and polarity of three novel and red-emitting coumarin-rhodamine hybrid derivatives with differences in the rigidity of their substituents is presented. This includes ethanol-polyethylene glycol, toluene-polyethylene glycol, and toluene-paraffin mixtures. Moreover, protonation-induced effects on the spectroscopic properties are studied. A viscosity-induced emission enhancement was observed for all coumarin-rhodamine hybrid derivatives. MCR2 bearing a julolidine donor showed the expected low sensitivity to viscosity whereas MCR3 with its freely rotatable diphenylamino substituent revealed a particularly pronounced sensitivity to this parameter. Moreover, MCR2 shows an enhancement in emission in the open, i.e., protonated form in conjunction with a largely Stokes shift fluorescence in the deep red spectral region. This enables the application of these dyes as viscosity sensors and as far red emitting pH-sensitive probes.

Persistent luminescence nanothermometers

Persistent phosphorescence nanoparticles emitting in the red and near-infrared spectral regions are strongly demanded as contrast nanoprobes for autofluorescence free bioimaging and biosensing. In this work, we have developed Sr4Al14O25:Eu2+, Cr3+, Nd3+ nanopowders that produce persistent red phosphorescence peaking at 694 nm generated by Cr3+ ions. This emission displays temperature sensitivity in the physiological temperature range (20–60 °C), which makes these nanoparticles potentially useful as fluorescence (contactless) nanothermometers operating without requiring optical excitation. Nd3+ ions, which act as shallow electron traps for the red Cr3+ persistent emission, also display infrared emission bands, extending the fluorescence imaging capability to the second biological window. This unique combination of properties makes these nanoparticles multifunctional luminescent probes with great potential applications in nanomedicine.

Phytochrome B-dependent regulation of reductive phase of photosynthetic carbon assimilation

The experiments were conducted with 10-day-old seedlings of wheat (Triticum aestivum L). Phytochrome B was activated using an array of light diodes emitting light in the red spectral region (RL) and inactivated by an array of light diodes emitting far-red light (FRL). At the end of the night dark period (8 h), activity of the chloroplastic GAP-dehydrogenase complex (the sequence of reactions: 3-PGA → 1,3-PGA → 3-GAP) was 1.0−1.2 μmol of oxidized NADPH/(min g fr wt of the leaf). When the leaves of intact plants were exposed to a maximal dose of RL (20 min at 17.5 kJ/m2), enzyme activity rose by 100–120%. Longer exposure to RL (30 and 40 min) did not cause further activation. Successive exposure to RL and FRL (20 min at 3.0 kJ/m2) completely negated a stimulatory effect of RL. It was shown that as little as 5-min-long exposure to RL increased the rate of 3-GAP formation by 20–25%, and enzyme activity rose linearly when radiation dose was elevated. Determination of the lifetime of RL-activated state by its decrease in plants placed in darkness showed that decay occurred with τ1/2 of 50−60 min when RL was switched off. Thus, a phytochrome B-induced regulation of reducing enzyme complex governing the reductive pentose phosphate cycle was discovered. Judging from the kinetics of attenuation of the activated state, phytochrome B apparently does not affect de novo synthesis of the enzyme. Since the investigated metabolic process consists of two coupled reactions controlled by kinase and dehydrogenase, the place and mechanism of action of the phytochrome system remain unknown.

New, thermally stable Gd11(GeO4)(PO4)3O10-based upconversion phosphors

A series of Gd11–x–y Yb x Er y GeP3O26 germanate phosphates differing in the ratio of the Yb3+ and Er3+ active ions have been synthesized, and their luminescence spectra have been measured. According to X-ray diffraction characterization results, all of the synthesized germanate phosphates are single-phase and have a triclinic structure (sp. gr. P1). We have measured upconversion luminescence spectra due to the Er3+ 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 radiative transitions in the synthesized gadolinium ytterbium erbium germanate phosphates and determined the luminescence upconversion energy yield (B en) in Gd11–x–y Yb x Er y GeP3O26. The effects of the concentrations and ratio of the dopants in the Gd11(GeO4)(PO4)3O10 germanate phosphate host on B en and the ratio of the luminescence intensities in the red and green spectral regions (R/G) have been assessed.

Luminescent properties of Pr3+-doped SrZrO3 phosphors

In this work Sr1-1.5xPrxZrO3 phosphors with different Pr3+ contents were synthesized by polymerizable complex and high-temperature solid-state methods. The samples were calcined between 700–1400°C. The crystal structure, microstructure and photoluminescence properties of the prepared powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence spectroscopy, respectively.The relationship between luminescent properties and the different sintering temperatures was studied. It was found that calcination at high temperature may remarkably increase the luminescence for samples synthesized by both methods. Emission spectra for all samples show dominant bands ascribed to transitions from the 3P0,1 energy levels to lower levels, particularly that one in the greenish-blue region, corresponding to the 3P0 → 3H4 transition. However, the bands ascribed to the 3P0,1 → 2S+1LJ transitions corresponding to the red spectral region are not inhibited at all, in contrast to recent reports. The samples prepared by solid state method exhibit a charge transfer band (CTB) due to the presence of Ti4+ impurities.

Doping β-Ga2O3 with europium: influence of the implantation and annealing temperature

β-Ga2O3 bulk single crystals were doped by ion implantation at temperatures from room temperature to 1000 °C, using a 300 keV Europium beam with a fluence of 1 × 1015 at cm−2. Rising the implantation temperature from room temperature to 400–600 °C resulted in a significant increase of the substitutional Eu fraction and of the number of Eu ions in the 3+ charge state as well as in a considerable decrease of implantation damage. Eu is found in both charge states 2+ and 3+ and their relative fractions are critically dependent on the implantation and annealing temperature, suggesting that defects play an important role in stabilizing one of the charge states. The damage recovery during post-implant annealing is a complex process and typically defect levels first increase for intermediate annealing temperatures and a significant recovery of the crystal only starts around 1000 °C. Cathodoluminescence spectra are dominated by the sharp Eu3+ related intra-ionic 4f transition lines in the red spectral region. They show a strong increase of the emission intensity with increasing annealing temperature, in particular for samples implanted at elevated temperature, indicating the optical activation of Eu3+ ions. However, no direct correlation of emission intensity and Eu3+ fraction was found, again pointing to the important role of defects on the physical properties of these luminescent materials.

Determining the True Optical Gap in a High-Performance Organic Photovoltaic Polymer Using Single-Molecule Spectroscopy.

Low-gap conjugated polymers have enabled an impressive increase in the efficiencies of organic solar cells, primarily due to their red absorption that allows harvesting of that part of the solar spectrum. Here we report that the true optical gap of one prototypical material, PTB7, is in fact at significantly higher energy than has previously been reported, indicating that the red absorption utilized in these materials in solar cells is entirely due to chain aggregation. Using single-molecule spectroscopy we find that PL from isolated nanoscale aggregates consists of multiple independently emitting chromophores. At the single-molecule level, however, straight single chains with a high degree of emission polarization are observed. The PL is found to be ∼0.4 eV higher in energy, with a longer lifetime than the red aggregates, and is attributed to single chromophores. Our findings indicate that the impressive light-harvesting abilities of PTB7 in the red spectral region arise solely from chain aggregation.

Brown carbon absorption in the red and near-infrared spectral region

Abstract. Black carbon (BC) aerosols have often been assumed to be the only light-absorbing carbonaceous particles in the red and near-infrared spectral regions of solar radiation in the atmosphere. Here we report that tar balls (a specific type of organic aerosol particles from biomass burning) do absorb red and near-infrared radiation significantly. Tar balls were produced in a laboratory experiment, and their chemical and optical properties were measured. The absorption of these particles in the range between 470 and 950 nm was measured with an aethalometer, which is widely used to measure atmospheric aerosol absorption. We find that the absorption coefficient of tar balls at 880 nm is more than 10 % of that at 470 nm. The considerable absorption of red and infrared light by tar balls also follows from their relatively low absorption Ångström coefficient (and significant mass absorption coefficient) in the spectral range between 470 and 950 nm. Our results support the previous finding that tar balls may play an important role in global warming. Due to the non-negligible absorption of tar balls in the near-infrared region, the absorption measured in the field at near-infrared wavelengths cannot solely be due to soot particles.

Allelopathic effects of exogenous phenylalanine: a comparison of four monocot species.

Exogenous phenylalanine stunted annual ryegrass but not switchgrass or winter grain rye, with deuterium incorporation up to 3% from phenyalanine-d 8 . Toxicity to duckweed varied with illumination intensity and glucose uptake. Isotopic labeling of biomolecules through biosynthesis from deuterated precursors has successfully been employed for both structural studies and metabolic analysis. Phenylalanine is the precursor of many products synthesized by plants, including the monolignols used for synthesis of lignin. Possible allelochemical effects of phenylalanine have not been reported, although its deamination product cinnamic acid is known to have deleterious effects on root elongation and growth of several plant species. The effects of phenylalanine and its deuterated analog phenylalanine-d 8 added to growth media were studied for annual ryegrass (Lolium multiflorum), winter grain rye (Secale cereale), and switchgrass (Panicum virgatum) cultivated under hydroponic conditions. Growth of annual ryegrass was inhibited by phenylalanine while switchgrass and rye were not significantly affected. Growth was less affected by deuterated phenylalanine-d 8 than by its protiated counterpart, which may be a typical deuterium kinetic isotope effect resulting in slower enzymatic reaction rates. Deuterium incorporation levels of 2-3% were achieved in biomass of switchgrass and annual ryegrass. Both protiated and deuterated phenylalanine were moderately toxic (IC25 values 0.6 and 0.8mM, respectively) to duckweed (Lemna minor) grown using a 12h diurnal cycle under photoautotrophic conditions. A significant increase in toxicity, greater for the deuterated form, was noted when duckweed was grown under higher intensity, full spectrum illumination with a metal halide lamp compared to fluorescent plant growth lamps emitting in the blue and red spectral regions. Supplementation with glucose increased toxicity of phenylalanine consistent with synergy between hexose and amino acid uptake that has been reported for duckweed.

Crystal structures and photoluminescence properties of two novel sorosilicates with an unprecedented ratio of di- and trisilicate groups: Ba2Ho10(Si2O7)3(Si3O10)2 and isotypic Ba2Er10(Si2O7)3(Si3O10)2

Ba2Ho10(Si2O7)3(Si3O10)2 and isotypic Ba2Er10(Si2O7)3(Si3O10)2 represent a new mixed-framework structure type with an unprecedented ratio of di- and trisilicate groups among silicates. The atomic arrangement features asymmetric zigzag chains of edge-sharing Ln3+O6 (Ln = Ho, Er) octahedra corner-linked by isolated Si2O7 and horseshoe-shaped Si3O10 groups in the ratio of 3:2. Single-crystal X-ray diffraction investigations yielded the triclinic space group P1¯ (no. 2), with a = 5.5494(11)/5.5360(11)Å, b = 6.8795(14)/6.6870(13)Å, c = 23.703(5)/23.691(5)Å, α = 83.60(3)/83.64(3)°, β = 88.80(3)/88.77(3)°, γ = 73.43(3)/73.38(3)°,V = 861.9(3)/835.2(3)Å3, R(F) = 0.0289/0.0559, respectively. The laser-induced photoluminescence emission of trivalent holmium und erbium in both framework silicates was studied using a dispersive single-stage spectrometer and compared to the emission spectra of other framework Ba-Er and Ba-Ho silicates. The 4S3/2→4I15/2 transition of Er in Ba2Er10(Si2O7)3(Si3O10)2 causes a photoluminescence emission in the visible green spectral range whereas its Ho representative with the 5F3→5I7 transition in the red spectral region.

All-quantum-dot emission tuning and multicolored optical films using layer-by-layer assembly method

In this report, all-quantum-dot multilayer photoluminescent films (PLFs) based on two types of modified quantum dots (MA-C8-QDs and PEI-QDs) were fabricated through the layer-by-layer (LBL) self-assembly method, providing a new kind of luminescent material with emission color covering blue to red spectral region. Aqueous QDs with high stability and photoluminescence properties were obtained by an efficient phase transfer, followed by fabrication of (PEI-QDs/MA-C8-QDs)n PLFs with alternate adsorbing the layer of the MA-C8-QDs endowed with negative (COO−) charges and PEI-QDs endowed with positive (NH3+) charges using electrostatic interactions between each layer. The resulting single color films preserved good color purity and strong luminescence of original QDs, PL intensities increased linearly with the number of bilayers n, which indicated that growth of the film is regular and uniform. In addition, the uniform and smooth PLFs as prepared have high visible light transmittance. Furthermore, by emission tuning, multicolor and white light-emitting PLFs with the color coordinates at (0.3292, 0.3418) have been easily obtained by assembly of two types of modified red, green, blue QDs. Therefore, these PLFs (especially white-light PLFs) show promise for the development of novel multiplexed biological sensors, full-color displays, intelligent response, photonic, and optoelectronic devices.

Luminescence properties of Dy3+, Eu3+ co-doped Ca7Si2P2O16 single host phosphor

ABSTRACTDy3+/Eu3+ ions co-doped Ca7Si2P2O16 phosphors were successfully prepared by the sol-gel method. The characterization results confirmed that the rare earth ions occupied Ca2+ sites in Ca7Si2P2O16 crystal. Meanwhile, the luminescent properties of Ca7Si2P2O16:Eu3+, Dy3+ upon excitation at 361 nm were discussed and its sharp emissions were in blue and red spectral regions, respectively. The luminescence intensity and CIE varied with the doping ratio of Dy3+, Eu3+ and the emission region is close to the white light. All the results showed that Ca7Si2P2O16: Dy3+, Eu3+ as a single host phosphor could be applied in the field of UV-excited white LEDs.

Reversible pH switchable aggregation-induced emission of self-assembled benzimidazole-based acrylonitrile dye in aqueous solution

We report the pH switchable fluorescence and aggregation-deaggregation behaviour of a 2-benzimidazolyl substituted acrylonitrile dye (BIA). From the single molecular entity, three prototropic species (tri-state system) are derived that respectively emit in the red, green and cyan spectral regions. The neutral form is capable of self-assembly in an aqueous environment exhibiting stable red-orange aggregation-induced emission (AIE) at 600 nm in the physiologically relevant pH range. The aggregation and emission are pH switchable and fully-reversible. The molecule is shown to similarly function in the solid state where its emission can be modulated by exposure to acidic and basic vapours. Multicolour and reversibly pH switchable tri-state emission from a single fluorophore containing a biologically active benzimidazole moiety can find application in ratiometric optical chemosensing, bioimaging and functional materials.

Luminescence properties of a novel red-emitting phosphor LaBMO6: Pr3+ (M = W, Mo)

A novel blue InGaN-chip-based red-emitting phosphor Pr3+: LaBMO6 (M = W, Mo) in pure phase were synthesized via conventional solid-state reaction in air and the photoluminescence properties of the phosphor were investigated for the application in white LEDs. The as-synthesized phosphors were characterized by the X-ray diffraction; diffuse reflection spectra, photoluminescence excitation and emission spectra, the Commission International de L'Eclairage (CIE) chromaticity coordinates and temperature-dependent emission spectra. Orangish red emission band around 575–625 nm was observed in Pr3+-doped LaBMO6 (M = W, Mo) upon 445 nm excitation. Fluorescence concentration quenching in Pr3+-doped LaBMoO6 were observed and the critical distance between Pr3+ ions for energy transfer was calculated to be 8.369 nm. The CIE chromaticity coordinates of Pr3+-doped LaBMoO6 were located in the red spectral region and the temperature-dependent luminescence spectra indicated that Pr3+-doped LaBMoO6 show good thermal stability. All results demonstrated the developed Pr3+-doped LaBMO6 (M = W, Mo) was a novel red phosphor.

Mechanisms of Up-Conversion Luminescence in Glass-Ceramics Containing Er:PbF2 Nanocrystals

Transparent oxyfluoride germanosilicate glass-ceramics containing Er:PbF2 nanocrystals is synthesized on the basis of SiO2–GeO2–PbO–PbF2 initial glass doped with Er2O3. The glass-ceramics is characterized by yellow-green luminescence, the intensity of which is ~12-times higher than that for the initial glass. The redistribution of the intensity between green and red emission bands after the glass heat-treatment is explained using data of the lifetime measurements for six excited states of Er3+ ions from 4 I 11/2 to 2 H 11/2. A substantial increase of the lifetime of the 4 F 9/2 state (from 0.6 to 71 μs) is detected for the glass-ceramics as compared to the initial glass. UV up-conversion luminescence is observed for the glass-ceramics. The mechanisms of the up-conversion for 11 emission lines in the UV, blue, red, and deep-red spectral regions are discussed.