Why Does a Neon Light Appear Red but Has Blue Emission Line Red Neon Light

Phosphor-converted LEDs

Govind B. Nair , S.J. Dhoble , in The Fundamentals and Applications of Light-Emitting Diodes, 2021

4.4 Red/orange-emitting phosphors

Eu³⁺ -doped phosphors are the most sought red-emitting phosphors due to their deep red emissions from 593 to 650  nm [14]. A perfect red component with pure color purity is obtained by the ⁵D₀  → ⁷F₂ transition of Eu³⁺, corresponding to the emission wavelength ∼615   nm [14,15]. There is a general belief that Eu³⁺ ions will show their characteristic emission even if they are doped in mere sand. However, not all the Eu³⁺-doped phosphors are suitable for lighting purposes. While considering phosphors for lighting, the major priority must be given to those materials that have a broad excitation band. Although Eu³⁺ ions are known for their characteristic excitation and emission peaks corresponding to forbidden f–f transitions, they have a useful charge transfer band in the UV region that could be utilized to sensitize the red emission [16]. The charge transfer band acts as a sensitizer that absorbs light in the UV region and transfers it to Eu³⁺ ions, thereby reducing them to Eu²⁺ formally. In some cases, the charge transfer band is more intense than the characteristic excitation peaks of Eu³⁺ [17]. Yet, this characteristic has no advantage in NUV-excited red-emitting LEDs, as the charge transfer band of Eu³⁺ seldom covers the NUV region [18]. The only possible way to employ Eu³⁺-doped phosphor for red-emitting NUV-LEDs is by selecting a host in which Eu³⁺ ions shall occupy a site with lower symmetry [19]. When Eu³⁺ ions occupy noncentrosymmetric site in a host lattice, there is a relaxation in the parity-forbidden 4f–4f transition and the efficiency of the 4f–4f excitation band is improved. A vast number of Eu³⁺-doped phosphors have been reported till date and to name a few, these include Y₂O₃:Eu³⁺ [20], YVO₄:Eu³⁺ [21], CaTiO₃:Eu³⁺ [22], Ca₂ZnWO₆:Eu³⁺ [23], LaPO₄:Eu³⁺ [24], and GdAlO₃:Eu³⁺ [25]. Except for the 396-nm excitation peak, no other excitation band of Eu³⁺ lies in the NUV region. This peak cannot be intense unless Eu³⁺ ions occupy a site with lower symmetry and partially permit the parity-forbidden 4f–4f transitions to occur. Hence, we must rethink the possibility of using Eu³⁺-doped phosphors in the pc-LEDs. A similar case goes for the phosphors doped with Sm³⁺: Sm³⁺-doped phosphors give a characteristic excitation peak at 405   nm, which is a forbidden 4f–4f transition [5,14,26–29]. In addition, there are very few cases in which Sm³⁺-doped phosphors turned out to give bright red luminescence [26,30,31]. Otherwise, in most cases, Sm³⁺ is commonly known to give orange-red luminescence [27,28,32–36].

Another approach for developing red-emitting phosphors is to find Eu²⁺-doped nitride or oxynitride phosphors [37]. Nitride-based phosphors are known to exhibit outstanding stability, both thermally and chemically, accompanied by excellent luminescence properties [38,39]. Besides, nitride hosts provide highly covalent surroundings for the Eu²⁺ ions to experience a strong crystal field effect and shift its emission band towards longer wavelengths. Sialon type of hosts has attracted huge attention due to their ability to absorb strongly in the UV to the blue region [40,41]. However, not all sialon materials are known to bring about red luminescence with Eu²⁺ doping [42,43]. Although nitride-based phosphors activated with Eu²⁺ provide excellent red luminescence combined with good thermal and chemical stability, there is a big matter of concern regarding the complexity of its synthesis. This justifies the fact that comparatively very few nitride phosphors are available for discussion. Eu²⁺ ions have also triggered red luminescence in some of the borate phosphors, which is a rare occurrence. Zhang et al. [44] have reported red luminescence in LiSrBO_3:Eu²⁺ phosphors, but with the coexistence of a small amount of LiSr₄(BO₃)₃ phase. The phosphors were prepared by adding 40 and 60   mol% excess of boric acid, and it was inferred that more the amount of boric acid was added, the percentage of LiSrBO₃ phase increased in the sample. Irrespective of the percentage of LiSrBO₃ phase and LiSr₄(BO₃)₃ phase in the sample, a broad emission band was obtained with the peak centered at 618   nm. However, this is in contrast to the report published by Wang et al., wherein LiSrBO₃:Eu²⁺ phosphors were endowed with a yellowish-green light emission peak at 565   nm [45]. On the other hand, Wang et al. and Wu et al. independently reported that LiSr₄(BO₃)₃:Eu²⁺ phosphors produced a broad emission band in the red region of the visible spectrum [46,47]. From this, we may infer that the LiSr₄(BO₃)₃ phase is responsible for the red luminescence, whereas LiSrBO₃ phase produces yellowish-green emission. Ba₂Mg(BO₃)₂:Eu²⁺ phosphors have also exhibited orange-yellow luminescence under 365   nm UV excitation [48]. However, the addition of Mn²⁺ ions to this phosphor has shifted the emission peak to a longer wavelength. Ba₂Mg(BO₃)₂:Eu²⁺, Mn²⁺ phosphors depict a broad excitation ranging from 250 to 450   nm and the red emission is intensified as well as purified until the Mn²⁺ concentration increased up to 0.05   mol. Some halophosphates like K₂Ca(PO₄)F:Eu²⁺ phosphor has also shown broad red emission bands [49]. Alkali earth sulfides also dictate red luminescence with Eu²⁺ ions, but their thermal instability and sensitivity to moisture do not promote their application as red-emitting phosphor materials in LEDs [50].

Yet another approach is to develop phosphor materials that are doped with Mn⁴⁺ ions [51–54]. Mn⁴⁺ ions show a distinct ²E_g  → ⁴A_2g transition in the crystal field that is octahedrally symmetric and this transition corresponds to deep red luminescence with narrow-band emission peaks in the range 600–750   nm with high quantum efficiency [55–60]. Although the emission peaks corresponding to ²E_g  → ⁴A_2g transition is spin-forbidden, their excitation peaks lying in the NUV region and blue region correspond to the spin-allowed ⁴A_2g  → ⁴T_2g and ⁴A_2g  → ⁴T_1g transitions, respectively. The excitation peaks are broad and can be easily excited by InGaN chips [58]. This is a noteworthy feature required for a phosphor to be used in pc-LEDs. However, the main difficulty lies in controlling the valence state of Mn ions that are doped in the host material. Mn can exist in 2+, 3+, 4+, 6+, and 7+ oxidation states, and the synthesis temperature has a huge effect on the occurrence of a particular valence state of Mn ions [61]. Mn⁴⁺ emission is tuned depending on the host material in which it is doped. When the host material is highly ionic, as in the case of fluorides, the prominent emission peak is obtained in the range 600–630   nm [56,60,62–66]. A number of complex alkaline metal fluorides have been reported to be excellent host materials for Mn⁴⁺ doping. Jiang et al. [62] have synthesized the hexagonal phase of BaSiF₆:Mn⁴⁺ phosphors by the hydrothermal method. Its room-temperature emission spectrum consists of narrow-band peaks at 615 nm, 632 nm and 648   nm, with an exceptionally intense and prominent peak at 632   nm. The peak at 615   nm gradually disappears when the emission spectrum is measured at 78 K, and this peak has been associated with the anti-Stokes vibronic sidebands related to the excited state ²E of Mn⁴⁺ ions [62]. On the other hand, the prominent emission band is obtained in the range 630–700   nm when the host material is covalent, as in the case of oxides [58–60,67–69]. The crystal field effect is found to strongly affect the luminescence of Mn⁴⁺ ions doped in oxides such as Sr₄Al₁₄O₂₅, Ba₂GdNbO₆, and Mg₇Ga₂GeO₁₂ [58,59,67,69,70]. Mn²⁺ is also likely to show red luminescence in some cases. Unlike Mn⁴⁺, a broad emission band is obtained for Mn²⁺-doped phosphors [71,72].

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Growth, properties, and applications of β-Ga2O3 nanostructures

Mukesh Kumar , ... R. Singh , in Gallium Oxide, 2019

5.2.3 Optical properties of β-Ga₂O₃ nanostructures

Optical emission of β-Ga₂O₃ nanostructures contains band-to-band emission and usually sub-bandgap emission having blue, green, and red emission bands. The photoluminescence (PL) spectrum of β-Ga ₂O₃ nanostructures at room temperature has been reported by Li et al. [13] showing intrinsic emission at 265   nm (~   4.7   eV) and 278   nm (~   4.5   eV) nm under an excitation wavelength of 230   nm. Anisotropy of the monoclinic phase is responsible for emission of these two peaks. In the case of sub-bandgap emission, it is suggested in a number of reports that the blue emission is related to recombination of electron and hole (due to the presence of donor states resulting from oxygen vacancies and acceptor states resulting from gallium vacancy or gallium-oxygen vacancy), the green emission due to the presence of impurities (such as Be, Ge, Sn, Li, Zr, and Si), and the red emission due to the presence of nitrogen impurity in β-Ga₂O₃ nanostructures [38, 72, 73].

Cathodoluminescence (CL) is also very useful to investigate the optical properties of β-Ga₂O₃ nanostructures. A CL spectroscopy of β-Ga₂O₃ nanostructures containing nanowires/nanosheets in UV-visible range is shown in Fig. 5.7A. The CL spectrum showed a strong broad UV-blue emission centered around 2.64   eV, deconvoluted into three Gaussian peaks at 2.44, 2.60, and 3.01, respectively. Energy peaks at 2.60   eV and 3.01   eV were related to the blue band whereas an energy peak at 2.44   eV was related to the green emission which results from the defect states present inside the bandgap of β-Ga₂O₃ nanostructures. Fig. 5.7(B) is an energy band diagram, demonstrating the UV-blue and red emissions corresponding to β-Ga₂O₃ nanostructures. Guzman-Navarro's group [74] studied UV-blue emission (at 3.31   eV) in β-Ga₂O₃ nanostructures and suggested that the thermal treatment of samples enhance the UV emission with quenching of the blue band due to elimination of point defects in β-Ga₂O₃ nanostructures. Nogales et al. [75] observed a UV-blue emission along with a red emission at 1.73   eV in β-Ga₂O₃ nanowires. In the case of isoelectronic (In, Al) doped β-Ga₂O₃ nanowires, a blue shift in optical emission for In as well as Al doping has been demonstrated using comparisons with undoped β-Ga₂O₃ nanowires [76].

Fig. 5.7. (A) CL spectrum of self-catalyzed β-Ga₂O₃ nanostructures on spin coated Ga₂O₃ film/sapphire substrate showing sub-bandgap emission with deconvoluted emission bands. (B) Energy band diagram for β-Ga₂O₃ explaining the UV-blue and red emissions.

Reproduced with permission from S. Kumar, V. Kumar, T. Singh, A. Hahnel, R. Singh, The effect of deposition time on the structural and optical properties of beta-Ga₂O₃ nanowires grown using CVD technique, J. Nanopart. Res. 16 (2013) with permission of Springer Nature.

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Nanoeffects in Ancient Technology and Art and in Space

In Fundamentals and Applications of Nano Silicon in Plasmonics and Fullerines, 2018

16.3.2.1 Silicon Nanoparticle Carriers

On the basis of experimental data, it was suggested that crystalline silicon nanoparticles with 1.5 nm–5 m diameters as the carrier, that is, cause the ERE, the broad emission band within the 0.54 and 0.95 μm spectral range that is seen in dust-rich objects, such as reflection nebulae as well as in the diffuse interstellar medium [25–27]. The silicon nano-particle hypothesis is also indirectly supported by observation of pre-solar diamond nano-particles in laboratory analysis of meteorites [28,29].

The data not only show that the nanoparticles could provide a close match to the observed ERE spectra, they also satisfy the quantum efficiency requirement. It was estimated that the nanoparticles account for 5% of the total interstellar dust mass and ≈6 Si per 10⁶ H atoms. Modeling [26] the interstellar extinction curve taking SNPs as an interstellar dust component gave ≈18 Si per 10⁶ H atoms, or ∼50% of solar Si abundance.

Recently, there has been extensive research both observation of interstellar medium and solar corona during eclipses in the visible spectrum, as well as in the infrared and laboratory measurements, as well as simulations to study these phenomena. The eventual goal is to identify a nano silicon-based carrier with a (i) plausible mechanism for formation that (ii) exhibits the optical characteristics of the background while (iii) maintaining brightness and withstanding harsh radiation without being destroyed. It was argued that nano silicon-based is a good candidate carrier of the mystery background that exhibits the required attributes. The carrier is a hydrogenated silicon nano cluster or super molecule.

Laboratory measurements showed the availability of a discreet set of clusters ranging from 1 to 3 nm in size. One nanometer clusters exhibit blue emission, while 3 nm particles exhibit red emission shown in Fig. 16.6. High-quality commercial quantities of the nanoparticles with repeatable structure and characteristics are produced in the laboratory (Fig. 16.7).

Figure 16.7. Silicon nanoparticles (A) prototype mode of 1 nm diameter can be assigned a molecular structure Si29H24. (B) Laboratory measurements showed the availability of a discreet set of clusters ranging from 1 to 3 nm in size. 1 nm clusters exhibit blue emission, while 3 nm particles exhibit red emission. (C) High-quality commercial quantities of the nanoparticles with repeatable structure and characteristics are produced in the laboratory.

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NANOSTRUCTURED MATERIALS, MICELLES, AND COLLOIDS

Kerry K. Karukstis , in Handbook of Surfaces and Interfaces of Materials, 2001

5.3.3 Nile Red in Micellar Systems

A recent investigation suggests an alternative interpretation for an observed biexponential decay of Nile red in surfactant-based systems [74 ]. Fluorescence intensity decays (at fixed excitation wavelength) were collected as a function of the Nile red emission wavelength observed in SDS micelles. The concentration of surfactant, 69 mM, is substantially higher than the cmc of 8 mM. A low dye-to-micelle ratio of 1:1272 was maintained to achieve complete partitioning of Nile red within the hydrophobic micellar region [ 74]. Using global analysis for the simultaneous fitting of multiple fluorescence decays, a fit to a double-exponential function was obtained with lifetimes of 0.68 and 2.53 ns. The amplitudes of the components vary with wavelength, with the interesting phenomenon of a negative amplitude for the short-lifetime component at emission wavelengths greater than 640 nm. Krishna [74] interprets the two lifetimes with a negative amplitude for one of the components as evidence for excited-state kinetics that leads to the formation of a new species in the excited state (B*) from the initially excited state (A*). The short lifetime is associated with the initially excited state A*. Species-associated spectra were derived for A* and B* and reveal a 10-nm difference in their emission maxima. Two models for the excited-state reaction—one suggesting an irreversible formation of B* (A* → B*) and the other a reversible one (A* ⇋ B*)—are both consistent with the fluorescence data and could not be distinguished.

As B* is fluorescent and the TICT state of Nile red is nonemissive, TICT formation is not proposed as the origin of the excited-state kinetics. Through additional experiments it was established that only in viscous solvents are dual lifetimes observed with a negative amplitude for the short-lifetime component at longer emission wavelengths. The dependence on solvent viscosity and not on solvent polarity led to the suggestion of excited-state solvent relaxation to form B* from A*. Ordinarily, solvent molecules surrounding a fluorophore reorganize in response to the sudden change in electronic distribution upon promotion of the ground state to the excited state. However, the time scale of the solvent reorganization is dependent on the viscosity of the medium [38]. Using the viscous solvents 1-octanol and glycerol, the viscosity dependence of the excited-state kinetics of Nile red was demonstrated. Double-exponential functions were characterized for both of these solvents with wavelength-dependent amplitudes for both lifetime components as well as negative amplitudes for the short-lifetime component at long wavelengths. These observations, together with the lack of any wavelength dependence of the lifetimes in micelles, 1-octanol, or glycerol, support a two-state solvent relaxation in the excited state (rather than a continuous solvent relaxation). Thus, Nile red has the potential to serve as both a polarity- and a viscosity-sensitive probe of the environments in microheterogeneous systems.

The steady-state fluorescence anisotropy of a fluorophore is viewed as an excellent indicator of the solubilization of the fluorescent probe in organized molecular assemblies. Furthermore, the dynamics of fluorescence depolarization as quantified by the fluorescence anisotropy decay yield information on the local molecular dynamics of the probe in its solubilization site. To assess the nature of the diffusive dynamics of a fluorescent probe in a surfactant micelle, the fluorescence depolarization dynamics of several organic dyes, including Nile red, were studied in aqueous micelles by picosecond time-resolved single-photon counting [76]. Micellar systems with well-characterized structural and physical properties were selected for investigation, including anionic micelles assembled from SDS, cationic micelles from CTAB, and neutral micelles from TX 100. These surfactants yield micelles with core radii and hydrodynamic radii that increase in the order SDS < CTAB < TX 100 [76]. Values of the steady-state anisotropy (r _ss) would be expected to vary from 0 when the dye molecule is solubilized in the bulk solvent water to larger values as the dye is bound to a micelle and tumbles much more slowly. The observed values of r _ss are greater than 0 for all surfactant concentrations, including those well below the cmc, and effectively demonstrate the affinity of Nile red for the selected micelles and the corresponding insolubility of the dye in water. The maximal values of r _ss are in the order SDS < CTAB < TX 100.

The fluorescence anisotropy decay of a dye molecule associated with a surfactant aggregate reflects the coupling of the rotational motion of the dye as well as the translational and rotational dynamics of the aggregate. For all micelles the kinetics of the fluorescence anisotropy decay of Nile red was characterized by a two-exponential function [76]. Such an observation precludes a model for the interaction of the probe with the micelle where the dye molecule and aggregate are viewed as a single unit. Thus, Nile red must be neither rigidly bound to the micellar surface nor incorporated within the homogeneous micellar core. The authors demonstrate that the biexponential anisotropy decay is consistent with overall rotation of the micelle, lateral diffusion of the Nile red molecule on the two-dimensional spherical surface of the micelle, and "free" wobbling of the probe molecule in a restricted space [76]. Furthermore, for neutral TX 100 micelles the lateral diffusion is postulated to be faster and the wobbling diffusion to be slower than in either the cationic CTAB or the anionic SDS micelles. These kinetic differences are attributed to a different mechanism for the transport of a solute over a neutral surface [76].

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White Light-Emitting Novel Nanophosphors for LED Applications

Govind B. Nair , ... Sanjay J. Dhoble , in Nanomaterials for Green Energy, 2018

13.4.3 Tunable Red-Emitting Boron Nitride–Coated Ca_1−xSr_xS:Eu²⁺

Lin et al. synthesized boron nitride (BN)-coated Ca_1−xSr_xS:Eu²⁺ nanowires by a solid–liquid–solid process. The details of the synthesis have been reported in their research article [56]. The CaSrS:Eu nanowires are protected by the BN sheaths, which enhances the chemical stability of nanowires. These nanowires showed a broadband red emission with the peak centered at 645  nm during high-spatial cathodoluminescence (CL) measurements. The red-color emission was tuned effectively by tailoring the composition of the nanophosphor. By increasing the Sr content in this nanowire, a blueshift is observed in the CL emission spectrum. The orange emission has been enhanced by introducing the Ce³⁺ codopant in CaS:Eu²⁺ nanowires. The presence of Ce³⁺ has also enhanced the luminescence intensity of Eu²⁺ emission in CaS nanowires, and this can be attributed to the effectively occurring energy transfer process from Ce³⁺ to Eu²⁺ ions.

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Protected Metal Clusters

Nirmal Goswami , ... Jianping Xie , in Frontiers of Nanoscience, 2015

11.3.2.3 Case Study with Fluorescent Cu NCs

In contrast to Au- and Ag NCs-based cellular imaging, studies on fluorescent Cu NCs are relatively less due to the complicated synthetic procedure of Cu NCs. There are some successful attempts. For example, Zhang et al. ¹⁶⁸ prepared red-emitting BSA-stabilized Cu NCs and used them for cellular imaging. When CAL-27 cells were incubated with the BSA-Cu NCs for 24 h, an intense red emission was seen inside the cells, which suggests an efficient uptake of the NCs by the cells. In addition, the confocal laser fluorescence microscopy analysis suggests that the fluorescence was distributed not only in the cytoplasm but also in the nucleus. This suggests that the BSA-Cu NCs could be used as fluorescent probes for nucleus imaging. Similarly, Feng et al. ¹⁶⁹ synthesized fluorescent Cu NCs by using a peptide as the template. They also used the as-synthesized Cu NCs as a fluorescent probe for the imaging of HeLa cells. More recently, Cao et al. ¹⁷⁰ used tannic acid (TA)-protected Cu NCs (TA-Cu NCs) to image A549 cells. All these examples suggest that fluorescent Cu NCs could also be promising for bioimaging applications.

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Phosphors

Alok M. Srivastava , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

II.C Phosphors for High-Pressure Mercury Vapor Lamps

The high-pressure mercury vapor (HPMV) discharge (pressure exceeding several atmospheres; efficiency of 20–25%) produces visible emission that is particularly deficient in the red spectral region. The discharge also produces significant amounts of longer wavelength 365-nm UV radiation. Since power is dissipated in a small bulb, the operating temperature of the HPMV lamp (HPMVL) exceeds 200 °C. For safety reasons, the small bulb in which the discharge is confined is enclosed in a larger outer bulb. The color rendering of HPMVL can be improved by a phosphor capable of efficiently converting the short- and long-wavelength UV to red emission. Hence, illumination based on HPMVLs requires phosphors capable of red emission at high temperature (200–250  °C).

From the standpoint of the pertinent requirements, three phosphors have been proposed for this application: Mn⁴⁺-activated magnesium fluorogermanate, Eu³⁺-activated Y(P, V)O₄, and Sn²⁺-activated Sr-Mg orthophosphate. The emission spectrum of the three phosphors is shown in Fig. 10.

FIGURE 10. Emission spectrum for (Sr, Mg)₃(PO₄)₂:Sn²⁺ (dashed line), Mg₄GeO_5.5F:Mn⁴⁺ (solid line), and YVO₄:Eu³⁺ (dotted line); excitation at 254   nm.

The emission of Mn⁴⁺-activated magnesium fluorogermanate consists of narrow bands between 600 and 700   nm. The emission is far in the red that enhances the CRI of HPMVL but makes a poor match to the eye sensitivity curve. This coupled with the fact that the phosphor absorbs significantly throughout the visible (the body color of the phosphor is yellow) leads to poor lamp lumen output.

The activation of (Sr, Mg)₃(PO₄)₂ by Sn²⁺ results in a broadband emission centered at 630   nm. As the phosphor does not absorb visible radiation, both the CRI and lumen output is increased.

The emission spectrum of Eu³⁺-activated Y(P, V)O₄ is dominated by strong lines in the proximity of 620   nm. The UV radiation is absorbed by the vanadate group, and the energy is transferred to Eu³⁺ ions. Unlike the other two phosphors, the line emission of Eu³⁺ does not occur in the outlying region of the visual spectrum where the eye is insensitive (see Fig. 2). Consequently, the lumen equivalent of this phosphor is high. The limitation of this phosphor is in the rather poor absorption of the long wavelength UV radiation generated by the discharge. With respect to the effect of variation in temperature, all three phosphors show relatively good thermal stability of their emission intensity.

FIGURE 2. Relative sensitivity of the human eye.

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Phosphors for white LEDs

Zhanchao Wu , Zhiguo Xia , in Nitride Semiconductor Light-Emitting Diodes (LEDs) (Second Edition), 2018

5.3.4 Mn⁴⁺-activated fluoride phosphors

Recently, many researches focused on a kind of important line-type-emitting red phosphors, represented by Mn⁴⁺-activated fluoride phosphor, such as K₂SiF₆:Mn⁴⁺ and BaSiF₆:Mn⁴⁺, ^58–64 which also act as the most promising candidates for improving the color rendering for wLED. Herein, the PLE and PL spectra of typical K₂SiF₆:Mn⁴⁺ phosphor are shown in Fig. 5.5 and this kind of phosphor can absorb the blue light and show line-type red emission originated from the d–d transition of Mn ⁴⁺. ⁵⁸ Since 2008, Adachi's group has reported a series of A₂MF₆:Mn⁴⁺ (A   =   K, Na, Cs; M   =   Si, Ge, Ti, Zr) phosphors, which were prepared by wet chemical etching via mixing the precursors and silicon in the hydrogen fluoride (HF) solution. ^61–68 However, it did not arouse enough attention for the application in LEDs at that time. The main reason is that the previous method by Adachi contains the complex experimental operation and expensive starting materials. In 2014, Zhu et al. successfully synthesized K₂TiF₆:Mn⁴⁺ by coprecipitation method. The high luminous efficacy and excellent optical performance were found for practical wLEDs device fabricated by commercial YAG:Ce and as-prepared line-emitting red phosphor K₂TiF₆:Mn⁴⁺. ⁶⁹ Owing to its high emission intensity and the characteristic line-type red emission of Mn⁴⁺ in these fluorides, it will not only enhance the CRI of the device but also demonstrate application in the back-lighting device. Presently, the Mn⁴⁺ doped hexaflorometallates phosphors have been extensively studied by many groups, focusing on the new fluoride structural types, different synthesis methods, intrinsic luminescence mechanism and wLEDs device evaluation and application. ^70–79 We believe that this is a promising phosphor type for the wLEDs to be able to fulfill the requirement of the practical device package. However, the main problem is that this kind of hexaflorometallates systems were generally prepared by the solution-based method, which will consume plenty of water, HF, and oxidizer, that provides possible pollution in the mass production. Moreover, Mn ion is highly sensitive to different reaction conditions and it can exist in multiple valance states including Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺, and Mn⁷⁺, so that proper synthesis conditions should be controlled, otherwise it will affect the quality of the final phosphor products.

Figure 5.5. Photoluminescent (λ_ex  =   450   nm) and photoluminescence excitation (PLE) (λ_em  =   630   nm) spectra for K₂SiF₆:Mn⁴⁺ phosphor at 300K. ⁵⁸

Modified from Takahashi T, Adachi S. Mn⁴⁺-activated red photoluminescence in K₂SiF₆ phosphor. J Electrochem Soc 2008;155(12):E183–8. https://doi.org/10.1149/1.2993159.

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Rare-Earth Doped Upconversion Nanophosphors☆

F. Wang , X. Liu , in Comprehensive Nanoscience and Nanotechnology (Second Edition), 2011

1.18.4.3 Controlling Dopant Concentration

The UC emission color is dependent upon the concentration of the dopant ions. The dopant concentration, which determines relative amount of the dopant ions in the nanophosphors as well as the average distance between neighboring dopant ions, has a strong influence on the optical properties of the nanophosphors. For example, an increase in the dopant concentration of Yb³⁺ in Y₂O₃:Yb/Er nanoparticles induces enhanced back-energy-transfer from Er³⁺ to Yb³⁺ , thereby leading to a relative increase in intensity of red emission of Er ³⁺ [66]. A similar phenomenon also has been observed by Zhang and coworkers [83] in ZrO₂ nanophosphors co-doped with Yb/Er. In contrast, Li and coworkers [84] have observed a relative decrease in intensity of red emission in NaYF₄:Yb/Er nanophosphors by reducing the concentrations of both Yb³⁺ and Er³⁺ ions.

Recently, a general and versatile approach [25] was developed in our laboratory to fine-tune the UC emission in a broad range of color output based upon a single host source of α-NaYF₄ doped with varied amounts of Yb³⁺, Tm³⁺, and Er³⁺ ions. By precise control of the emission wavelengths and intensity balance through control of different combinations of RE dopants and dopant concentration, the luminescence emission can be deliberately tuned from visible to NIR under single wavelength excitation at 980   nm (Fig. 13). Given the broad range of available dopant/host combinations, this approach should allow for the generation of a large library of emission spectra in the visible and NIR spectral region that are particularly useful in multiplexed staining and labeling. The groups of Capobianco [85], Song [86], Zhang [87], and Lin [88] also utilized the doping approach to modulate UC emission colors in Lu₃Ga₅O₁₂, NaYF₄, Y₂O₃, and Lu₂O₃ hosts, primarily for the generation of white color.

Fig. 13. Upconversion (UC) luminescence photos showing colloidal solutions of (a) NaYF₄:Yb/Tm (20/0.2   mol%); (b–f) NaYF₄:Yb/Tm/Er (20/0.2/0.2–1.5   mol%); and (g–j) NaYF₄:Yb/Er (18–60/2   mol%) particles in ethanol solutions (10   mM). From Wang F and Liu XG (2008) Upconversion multicolor fine-tuning: Visible to near-infrared emission from lanthanide-doped NaYF₄ nanoparticles. Journal of the American Chemical Society 130: 5642–5643.

In addition to the color-tuning strategy by using two types of activators homogeneously doped inside the host lattices, Qian and Zhang [89] have recently demonstrated multicolor tuning in core/shell-structured NaYF₄ nanophosphors. In their design, different types of emitters were incorporated in the core and shell separately. This approach was designed to eliminate cross-talk between different types of activators, thereby allowing for a greater variability in the selection of dopant concentrations. Furthermore, the core/shell structure also blocked energy transfer to the surface quenching sites and resulted in UC luminescence enhancement.

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OPTIMIZED ELECTRONIC POLYMERS, SMALL MOLECULES, COMPLEXES, AND ELASTOMERS FOR ORGANIC ELECTRONIC SYSTEMS

Sulaiman Khalifeh , in Polymers in Organic Electronics, 2020

3.11.3.6 Osmium-based complexes

Osmium-based complexes such as osmium(II) complexes can be prepared by doping the emitter osmium(II) as host material with conductive polymer such as vinyl carbazole. It is considered as the sixth optimized group of heavy-metal complexes for structuring complex-based light-emitting diodes because their structures can form electroluminescent systems of luminance efficiency η_L = 3.0 cd/A at 21 V with the brightness of 615 cd/m ² . Its electroluminescence observed at 640 nm generates saturated red emission. ¹⁶⁹ Regarding the preparation process of osmium-based complexes, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole facilitates the charge balance state and compensates for the poor electron-transporting ability of the host polymers. Osmium-based complexes include ^{169, 173, 238, 267} osmium(III)-1,10-phenanthroline (Os(phen)₃)²⁺ and osmium(III)-tris(2,2′-bipyridine) (Os(bpy)₃)²⁺. Osmium(III)-1,10-phenanthroline of metal-to-ligand charge-transfer triplet state is transition energy state and tridentate splitting of 186.0 cm⁻¹. This complex is a transition metal complex based on 1,10-phenanthroline (phen) ligand, which is used in optical oxygen sensors. ²⁷²

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