International Commission On Illumination Research Articles

Novel Pyrimidine-Based Iridium Complexes with Bulky Charge-Carrier Groups as Dopants for Highly Efficient Green Polymer Light-Emitting Diodes.

Two new pyrimidine-based iridium complexes with triphenylamine and tetraphenylsilane, namely (TPAPr)2IrAcac and (TPSPr)2IrAcac, were fully synthesized and characterized. Both of the targeted iridium complexes exhibit excellent thermal stability and high photoluminescence quantum yields. Compared to (TPAPr)2IrAcac, (TPSPr)2IrAcac achieved its highest PLQY and current efficiency (CE) at higher dopant concentration probably because of its bulky tetraphenylsilane group, which can effectively suppress the concentration quenching. However, according to DFT studies, (TPSPr)2IrAcac shows faster non-radiative transitions due to the presence of more excited-state distortions than (TPAPr)2IrAcac. As a result, Green phosphorescent polymer light-emitting diodes (PLEDs) containing (TPAPr)2IrAcac and (TPSPr)2IrAcac as dopants exhibit exceptional device performance with peak CE values of 38.24 cd A-1 and 36.06 cd A-1, respectively. (TPAPr)2IrAcac exhibited a superior efficiency than (TPSPr)2IrAcac because of its high Фp, low RMSD value, and efficient energy transfer from the host to the guest. More importantly, the PLEDs based on (TPAPr)2IrAcac and (TPSPr)2IrAcac show stable phosphorescent emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.313, 0.497) and (0.299, 0.483), respectively. This work points out a viable method for creating phosphorescent iridium complexes based on pyrimidine for high-efficiency organic light-emitting diodes (OLEDs).

Development of photochromic carboxymethyl cellulose-based hydrogel toward dual-mode authentication stamp with self-healing properties

Ultraviolet-induced photochromism has proven to be an appealing encoding strategy to improve the anti-counterfeiting of commercial items. However, recent reports have highlighted significant disadvantages with photochromic inks such as high cost, low efficiency and poor durability. Herein, we present a self-healable ink for anti-counterfeiting applications from a hydrogel of carboxymethyl cellulose (CMC) combined with rare-earth activated aluminate (REA) nanoparticles (NPs). Self-healing security inks provide high durability. The REA@CMC hydrogel has the ability to efficiently self-heal under ambient conditions. Using UV light to promote photochromism, the REA@CMC ink showed excellent photostability. To create a composite ink with a wide range of emission characteristics, different quantities of REA NPs were used. CIE (Commission Internationale de L'éclairage) Lab established that stamping a homogeneous film onto a paper surface gives them a transparent layer that transformed into green upon exposure to ultraviolet spectrum. The morphological features of REA NPs were investigated to show a particle diameter of 3–7 nm. The morphology of the photochromic films were studied using different analytical methods. Both ink solution rheological properties and stamped paper sheet mechanical characteristics were examined. The luminous films peaked in green emission at 518 nm, whereas their excitation wavelength was at 370 nm.

Deep-Blue Organic Light-Emitting Diodes Employed Traditional Hole Transporting Material as Emitter for 31-Inch 4K Flexible Display

High-efficiency deep-blue organic light-emitting diodes (OLEDs) play a crucial role in realizing ultra-high-definition (UHD) flat-panel displays and reducing power consumption. Generally, most reported OLEDs with a Commission Internationale de L’Eclairage (CIE) y coordinate < 0.06 are achieved by traditional fluorescent deep-blue emitters. However, it is challenging to obtain deep-blue fluorescent OLEDs with a high external quantum efficiency (EQE) (reaching the theoretical limit of 5%). In this work, we have successfully employed a hole-transporting material for an emitter, which can increase the efficiency in deep-blue OLEDs. The device employed with the proposed hole-transporting material exhibits deep-blue emission peaks at 427.0 nm with CIE coordinates of (0.155, 0.051), a turn-on voltage (Von) of 4.5 V, and an EQE of 4.5%. The performance of the OLED can be improved by 5.0% by optimizing the device structure. Finally, the flexible display when using the OLED devices exhibited a high image quality.

Triazolotriazine-based mixed host for pure-red phosphorescent organic light-emitting diodes exhibiting ultra-low efficiency roll-off

The development of high-performance pure-red phosphorescent organic light-emitting diodes (Ph-OLEDs) especially with low efficiency roll-off at high brightness is still an appealing yet challenging task. Despite success of well-documented 1,3,5-triazine (TRZ)-based thermally activated delayed fluorescence (TADF) hosts in green Ph-OLEDs, the limited electron-withdrawing ability of the TRZ acceptor makes the TADF hosts unsuitable enough for usage in the assembly of high-performance red Ph-OLEDs. Herein, pure-red Ph-OLEDs with ultra-low efficiency roll-off at luminance above 10000 cd m−2 are achieved by developing two triazolotriazine-based TADF hosts (23aIC-TAZTRZ and 32aIC-TAZTRZ) with balanced bipolar transporting property, good morphological stability and high photoluminescence quantum yield in neat film. Particularly, the phosphor-doped 23aIC-TAZTRZ and 32aIC-TAZTRZ emitting layers display high horizontal orientation factors (Θ//) of 84% and 85%, respectively. Ph-OLEDs utilizing 50 wt% 23aIC-TAZTRZ and 50 wt% 32aIC-TAZTRZ as the mixed host exhibit pure-red emission with Commission Internationale de l’Eclairage (CIE) coordinate of [0.66, 0.34], high maximum external quantum efficiency (EQEmax) of 27.4% and ultra-low efficiency roll-off at 10000 cd m−2 (EQE10000 = 23.9%). This work not only discloses the potential of triazolotriazine acceptor for the construction of efficient TADF hosts, but also provides alternative candidates to TRZ-based hosts for the assembly of high-performance red Ph-OLEDs.

A Red‐Emitting La4Ti9O24:Eu3+ Phosphor Excited by Multi‐Wavelength for Healthy Lighting

Herein, an excellent red‐emitting La4Ti9O24:Eu3+ phosphor is produced using eco‐friendly hydrothermal and heat treatment techniques without a high‐temperature and multiple‐sintering process. The phosphor can be excited by multiwavelength light from 382 to 533 nm, and the three main characteristic emission peaks of Eu3+ ions are located at 592, 616, and 709 nm. Its decay time measured at 616 nm is calculated to be 3.32 ms. The color coordinates of La4Ti9O24:Eu3+ phosphor excited by 393 nm ultraviolet (UV) light are (0.665, 0.335), which are near to the Commission International de L'Eclairage (CIE) of red light (0.67, 0.33). A red light‐emitting diode was fabricated by the red‐emitting La4Ti9O24:Eu3+ phosphor encapsulating on a UV‐emitting chip (λ = 395 nm), eliminating the harmful effects of blue light on human eyes. Consequently, it would be a viable source of healthy illumination for display and lighting.

Precise Regulation of Emission Maxima and Construction of Highly Efficient Electroluminescent Materials with High Color Purity

AbstractAdvanced multiple resonance induced thermally activated delayed fluorescence (MR‐TADF) emitters have emerged as a privileged motif for applications in organic light‐emitting diodes (OLEDs), because they furnish highly tunable TADF characteristics and high color purity emission. Herein, based on the unique nitrogen‐atom embedding molecular engineering (NEME) strategy, a series of compounds BN‐TP‐Nx (x=1, 2, 3, 4) have been customized. The nitrogen‐atom anchored at different position of triphenylene hexagonal lattice entails varying degrees of perturbation to the electronic structure. The newly‐constructed emitters have demonstrated the precise regulation of emission maxima of MR‐TADF emitters to meet the actual industrial demand, and further enormously enriched the MR‐TADF molecular reservoir. The BN‐TP‐N3‐based OLED exhibits ultrapure green emission, with peak of 524 nm, full‐width at half‐maximum (FWHM) of 33 nm, Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.71), and maximum external quantum efficiency (EQE) of 37.3 %.

Precisely Slight Regulation of Emission Maxima and Construction of Highly Efficient Electroluminescent Materials with High Color Purity.

Advanced multiple resonance induced thermally activated delayed fluorescence (MR-TADF) emitters have emerged as a privileged motif for applications in organic light-emitting diodes (OLEDs), because they furnish highly tunable bandgap with emission ranges spanning the visible light region, and have the intrinsic properties of outstanding TADF characteristics and high color purity emission. One of the typical construction paradigms of MR-TADF molecule is based on polycyclization of MR parent core and a representative target model molecule BN-TP has been prepared. Herein, based on the unique nitrogen-atom embedding molecular engineering (NEME) strategy, a series of congeners of BN-TP, namely BN-TP-Nx (x = 1, 2, 3, 4), have been customized. The nitrogen-atom anchored at different position of triphenylene hexagonal lattice entails varying degrees of perturbation to the electronic structure. The newly-constructed emitters have demonstrated the precisely slight regulation of emission maxima of MR-TADF emitters to meet the actual industrial demand, and further enormously enriched the MR-TADF molecular reservoir. The BN-TP-N3-based OLED exhibits ultrapure green emission, with peak of 524 nm, full-width at half-maximum (FWHM) of 33 nm, Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.71), and maximum external quantum efficiency (EQE) of 37.3%.

White-light emission from yttrium iron garnet (YIG)

Single-phase phosphors that emit broadband white-light are needed for white-light-emitting diodes (wLEDs) to reach their full potential. However, it is challenging to achieve broad white-light emission from single-phase materials. Consequently, polycrystalline inorganic bulk compounds that emit white-light sans doping are rare. We report on broadband white-light emission from a well-known garnet compound, i.e., yttrium iron garnet (YIG), without activator-ion doping. Upon near-UV excitation at 370 nm, polycrystalline bulk YIG emits broadband white-light with (1931) Commission Internationale de L’Eclairage (CIE) coordinates as (0.28, 0.35) and correlated color temperature (CCT) as 8029 K. Variable excitation wavelengths ranging from 280 to 600 nm enable color-tunable emission as cyan-white-blue-green-yellow-orange-red, including near-white-light emission for a broad range of excitation from 325 to 390 nm. Moreover, a short lifetime (sub-nanosecond) is obtained, which is desirable for LED and other applications. We demonstrated the propriety of YIG as a single-phase converting phosphor for illumination by fabricating prototype wLEDs using commercial InGaN UV-LED chips (λ = 380 nm) for excitation. The CIE coordinates and CCT of prototype wLEDs were obtained as (0.34, 0.37) and 5284 K, respectively. We believe that the reported findings signify the great potential of YIG as a single-phase white-light-emitting phosphor for broadband emission, which offers a new perspective and a viable approach for the development of wLEDs.

Aggregation-induced emission phenanthroline derivatives for ultrahigh color purity deep blue OLEDs with CIEx≤0.15 and CIEy≤0.08 and low efficiency roll-off

Due to the limited molecular design strategy, the development of ultrahigh color purity deep blue aggregation-induced emission (AIE) emitters with excellent photophysical and electroluminescence performances is still facing challenges at present. In this study, an effective design strategy for deep blue emitters is proposed, which can improve the performances of non-doped organic light-emitting diodes (OLEDs) by adjusting the link mode of the phenyl bridge and utilizing the steric hindrance effect of the rigid plane. Two new emitters, 3BCzPT and 4BCzPT, were obtained with phenanthroline acceptor and carbazole donor for deep blue emitting OLEDs. Particularly, 3BCzPT has been proven to have AIE characteristics through photophysical properties. Non-doped OLEDs with 3BCzPT demonstrate prominent electroluminescence (EL) performances reaching a maximum current efficiency (CEmax) of 6.3 cd A−1 and maximum external quantum efficiency (EQEmax) of 3.81%, well as ultrahigh color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.151, 0.053). While the parameters of non-doped OLED based 4BCzPT are 4.2 cd A−1, 1.96% and (0.147, 0.082). Both emitters exhibit narrow EL spectra with the full-width at half maximum (FWHM) of 56 and 52 nm.

The Luminescence Properties of the Interstitial Eu2+ in SrB2O4 for UV Light‐Emitting Diode

SrB2O4:Eu2+ is prepared by the high‐temperature solid‐phase method. The structure is characterized by X‐ray diffraction, and the luminescence properties are analyzed using fluorescence spectroscopy and decay curves. SrB2O4:Eu2+ has strong absorption in the UV region and two broadband emissions in the 350–550 nm, located at 374 and 458 nm. The broadband emission around 374 nm belongs to Eu2+ emission at the Sr lattice position, and the emission around 458 nm belongs to interstitial Eu2+ ions. At low concentrations, Eu2+ replaces the position of Sr2+ first, and the emission peak of 374 nm is stronger than that of 458 nm in the emission spectrum. The luminescence at 458 nm enhances significantly as the concentration of Eu2+ ions increases. The fluorescence lifetime confirms the presence of two luminescent centers, the EuSr and the interstitial Eu2+ ions. The temperature dependence luminescence of SrB2O4:Eu2+ is explored, and the activation energy is calculated to be ΔEa = 0.3622 eV. The Commission Internationale de l'Éclairage (CIE) chromaticity coordinates of SrB2O4:Eu2+ are in the blue area. The results indicate that the interstitial Eu2+ benefits the color‐rendering index when SrB2O4:Eu2+ is used as a blue phosphor in UV light‐emitting diode (LED).

Analyses of ultraviolet-excitable high-efficiency phosphorescent RGB phosphors with various Eu2O3 doping concentrations for fabricating white Light–Emitting diodes

In this study, we fabricated white light–emitting phosphors by applying a 350-nm excitation wavelength to Eu2O3-doped [YBi0.1]VO4 (red), SrAl12O19 (green), and Sr3MgSi2O8 (blue) phosphors. The Eu3+-doped [YBi0.1]VO4 phosphor was synthesized using a high-temperature solid-state reaction method in an air environment, and the Eu2+-doped SrAl12O19 and Sr3MgSi2O8 phosphors were synthesized in a reducing environment. The morphological characteristics, crystal structures, and luminescence performance of the phosphors were investigated using X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM), ultra-high-resolution transmission electron microscopy (UHR-TEM), and photoluminescence emission (PL). The white light–emitting phosphor was successfully fabricated at the molar ratio of [Y0.8 Bi0.1Eu0.1]VO4:[Sr0.88Eu0.12]Al12O19:[Sr0.99Eu0.01]3MgSi2O8 phosphors of 1:1.5:1. Its Commission Internationale de l'Eclairage (CIE) chromaticity coordinates were (0.3422, 0.3398), which is close to the coordinates of pure white light at ([0.33, 0.33]).

Fabrication, Optical Property, and White LED Application of Novel Lanthanide‐Based Family Cs2NaLnX6 (X = Cl, Br, I) Perovskite Nanomaterials

AbstractLead free double perovskites are highly promising optoelectronic materials, but most of them suffer from low photoluminescence quantum yield (PLQY) and poor stability. Herein, a series of lanthanide elements are used to prepare the lead free double perovskite Cs2NaLnX6 (X = Cl, Br, I) nanocrystals (NCs), acting as both the main structural ions and the 4f–4f emission centers. The mechanisms for the broadband emission of Cs2NaLnX6 NCs host are carefully studied based on various spectral results combining ultrafast dynamics detection and first‐principles calculation. The broadband emission of the Cs2NaLnX6 NCs originates from the self‐trapped exciton. The Cs2NaLnX6 NCs show remarkable water stability for more than eight months and favorable temperature and light irradiation stability (500 h). The PLQY for these materials is much higher than most of double perovskite materials, especially for Cs2NaEuCl6, which can approach to 70%. The white light emission from a single host material Cs2NaEuTbCl6 (Tb:Eu = 25:1) is obtained and applied to a light‐emitting diode device, with a good CIE (Commission International de L'Eclairage) coordinate of (0.334, 0.326) and color rendering index of 91.2. Overall, the characteristics of lanthanide ions endow the super performance of Cs2NaLnX6 NCs, which is obviously an important choice for future optoelectronic devices.

Toward Narrowband and Efficient Blue Fluophosphors by Locking the Stretching Vibration of Indolocarbazole Skeletons.

Developing efficient and color-saturated deep-blue emitting molecules with small Commission Internationale de L'Eclairage (CIE) y values is challenging and has great potential for wide-color gamut displays. Herein, we introduce an intramolecular locking strategy to restrain molecular stretching vibrations of the emission spectral broadening. By cyclizing rigid fluorenes and connecting electron-donating groups to the indolo[3,2,1-jk]-indolo[1',2',3':1,7]indolo[2,3-b]carbazole (DIDCz) framework, the in-plane swing of peripheral bonds and stretching vibrations of the indolocarbazole skeleton are restricted due to an increased steric hindrance from cyclized groups and diphenylamine auxochromophores. As a result, reorganization energies at the high-frequency region (1300-1800 cm-1) are reduced, realizing pure blue emission with a small full-width-at-half-maximum (FWHM) of 30 nm by suppressing shoulder peaks of polycyclic aromatic hydrocarbon (PAH) frameworks. The fabricated bottom-emitting organic light-emitting diode (OLED) exhibits an efficient external quantum efficiency (EQE) of 7.34% and deep-blue coordinates of (0.140, 0.105) at a high brightness of 1000 cd/m2. The FWHM of the electroluminescent spectrum is only 32 nm, which is one of the narrowest electroluminescent emissions among the reported intramolecular charge transfer fluophosphors. Our current findings provide a new molecular design strategy to conceive efficient and narrowband emitters with small reorganization energies.

Stable yellow light emission from lead-free copper halides single crystals for visible light communication

Yellow light-emitting diodes (LEDs) as soft light have attracted abundant attention in lithography room, museum and art gallery. However, the development of efficient yellow LEDs lags behind green and blue LEDs, and the available perovskites yellow LEDs suffer from the instability. Herein, a pressure-assisted cooling method is proposed to grow lead-free CsCu 2 I 3 single crystals, which possess uniform surface morphology and enhanced photoluminescence quantum yield (PLQY) stability, with only 10% PLQY losses after being stored in air after 5000 ​h. Then, the single crystals used for yellow LEDs without encapsulation exhibit a decent Correlated Color Temperature (CCT) of 4290 ​K, a Commission Internationale de l'Eclairage (CIE) coordinate of (0.38, 0.41), and an excellent 570-h operating stability under heating temperature of 100 ​°C. Finally, the yellow LEDs facilitate the application in wireless visible light communication (VLC), which show a −3 dB bandwidth of 21.5 ​MHz and a high achievable data rate of 219.2 Mbps by using orthogonal frequency division multiplexing (OFDM) modulation with adaptive bit loading. The present work not only promotes the development of lead-free single crystals, but also inspires the potential of CsCu 2 I 3 in the field of yellow illumination and wireless VLC. The facile pressure-assisted cooling method is proposed to prepare lead-free CsCu2I3 single crystals, which promotes potential applications in both lighting for special occasions and wireless visible light communication.

Suppressing the Undesirable Energy Loss in Solution‐Processed Hyperfluorescent OLEDs Employing BODIPY‐Based Hybridized Local and Charge‐Transfer Emitter

Hyperfluorescent organic light‐emitting diodes (HF‐OLEDs) approach has made it possible to achieve excellent device performance and color purity with low roll‐off using noble‐metal‐free pure organic emitter. Despite significant progress, the performance of HF‐OLEDs is still unsatisfactory due to the existence of a competitive dexter energy transfer (DET) pathway. In this contribution, two boron dipyrromethene (BODIPY)‐based donor‐acceptor emitters (BDP‐C‐Cz and BDP‐N‐Cz) with hybridized local and charge transfer characteristics (HLCT) are introduced in the HF‐OLED to suppress the exciton loss by dexter mechanism, and a breakthrough performance with low‐efficiency roll‐off (0.3%) even at high brightness (1000 cd m−2) is achieved. It is demonstrated that the energy loss via the DET channel can be suppressed in HF‐OLEDs utilizing the HLCT emitter, as the excitons from the dark triplet state of such emitters are funneled to its emissive singlet state following the hot‐exciton mechanism. The developed HF‐OLED device has realized a good maximum external quantum efficiency (EQE) of 19.25% at brightness of 1000 cd m−2 and maximum luminance over 60 000 cd m−2, with an emission peak at 602 nm and Commission International de L'Eclairage (CIE) coordinates (0.57, 0.41), which is among the best‐achieved results in solution‐processed HF‐OLEDs. This work presents a viable methodology to suppress energy loss and achieve high performance in the HF‐OLEDs.

Deep blue fluorescent emitters based imidazole and carbazole for non-doped organic light emitting diodes with CIEy≤0.05

Two fluorescent emitters based imidazole and carbazole were synthesized for deep blue emission organic light-emitting diodes (OLED) with non-doped structure. The photophysical, thermal and electrochemical properties are investigated with high decomposition temperature up to 391 and 380 °C, and strong deep blue emission. Single carrier devices were also fabricated to show that two compounds have good bipolar carrier transport characteristic. The non-doped fluorescent OLED devices using the two compounds as emitting layers are designed among which the devices achieved the maximum external quantum efficiency of 1.66% and 1.56%, respectively. Besides, the deep blue emission peak of 412 nm with Commission Internationale de l’Eclairage (CIE) coordinate of (0.16, 0.04), and 420 nm with CIE coordinate of (0.16, 0.05) were also realized. The small CIEy value of 0.04 and 0.05 obtained in this work are very significant for high color purity full-color display, which is also among the best CIEy value results ever reported as deep blue bipolar emitters.

Accurate luminance and chromaticity controls of digital colors using CIE-based RGBW algorithms.

Emerging high brightness of color displays and high signal-to-noise ratio of camera sensors require an addition of white (W) subpixels to ordinary red, green, and blue (RGB) subpixels. Conventional algorithms converting RGB signals to RGBW signals suffer from reduced chroma of highly saturated colors and complicated coordinate transformationsbetween RGB color spaces and color spaces defined by the Commission internationale de l'éclairage (CIE). In this work, we developed a complete set of RGBW algorithms to digitally code a color in the CIE-based color spaces, making complicated processes including color space transformations and white balancing become largely unnecessary. The analytic three-dimensional gamut can be obtained so that the maximal hue and luminance of a digital frame could be simultaneously obtained. Exemplary applications in adaptive controls of the colors of an RGB display in accordance with the W component of background light validate our theory. The algorithm opens an avenue toward accurate manipulations of digital colors for RGBW sensors and displays.

Polymer-doped perovskite nanocrystals for efficient single active layer white light-emitting diodes through energy transfer

White-light emitting diodes (WLEDs) are mainly constructed by stacking different light-emitting layers with complementary emission spectra. However, this not only requires complex device fabrication processes but also needs comprehensive charge transport regulation, limiting the development and application of WLEDs. In this work, efficient single-layer WLEDs are achieved via doping tiny amount of a red polymer poly[[9-(1-octylnonyl)-9H- carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5 thiophenediy (PCDTBT8) into the perovskite nanocrystals (NCs) matrix. The 490 nm sky blue perovskite nanocrystals (FA0.08Cs0.92PbBr2Cl) are synthesized and passivated with a short-chain ligand 3-methoxyphenethylamine (MPEA), which enhances luminance and stability at high bias. Energy transfer from NCs to PCDTBT8 significantly enhances red-light emitting in the range from 600 to 750 nm, enabling white-light emission. By tuning of the NCs:PCDTBT8 weight ratio from 400:1 to 150:1, a continuous transition of emission from cold white-light to warm white-light is achieved. With the NCs:PCDTBT8 weight ratio of 250:1, a standard white-light emission with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.33, 0.33), a luminance of 752 cd/m2 and an external quantum efficiency (EQE) of 1.59% are obtained at a fairly low voltage of 4.8 V.

Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1−xCaLa0.5Er0.05Yb0.45(MoO4)3 Upconversion Scheelite-Type Phosphors

New triple molybdates LixNa1−xCaLa0.5(MoO4)3:Er3+0.05/Yb3+0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) were manufactured successfully using the microwave-assisted sol-gel-based technique (MAS). Their room-temperature crystal structures were determined in space group I41/a by Rietveld analysis. The compounds were found to have a scheelite-type structure. In Li-substituted samples, the sites of big cations were occupied by a mixture of (Li, Na, La, Er, Yb) ions, which provided a linear cell volume decrease with the Li content increase. The increased upconversion (UC) efficiency and Raman spectroscopic properties of the phosphors were discussed in detail. The mechanism of optimization of upconversion luminescence upon Li content variation was shown to be due to the control of excitation/energy transfer channel, while the control of luminescence channels played a minor role. The UC luminescence maximized at lithium content x = 0.05. The mechanism of UC optimization was shown to be due to the control of excitation/energy transfer channel, while the control of luminescence channels played a minor role. Over the whole spectral range, the Raman spectra of LixNa1−xCaLa0.5(MoO4)3 doped with Er3+ and Yb3+ ions were totally superimposed with the luminescence signal of Er3+ ions, and increasing the Li+ content resulted in the difference of Er3+ multiple intensity. The density functional theory calculations with the account for the structural disorder in the system of Li, Na, Ca, La, Er and Yb ions revealed the bandgap variation from 3.99 to 4.137 eV due to the changing of Li content. It was found that the direct electronic transition energy was close to the indirect one for all compounds. The determined chromaticity points (ICP) of the LiNaCaLa(MoO4)3:Er3+,Yb3+ phosphors were in good relation to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.

The Correlation of Colour and Iron Oxides in Yellow Seal Stones from Northern Laos

The yellow seal stone from northern Laos is one possible substitute for the Tianhuang Stone, the most famous Chinese seal stone, because of its similar yellow to orange-yellow appearance and the same main mineral composition. The colour causation of the yellow seal stone from northern Laos was studied. The samples’ phase, micro-morphology and chemical components were studied by Raman spectroscopy, and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS), respectively. The yellow seal stone from northern Laos is mainly composed of dickite, occasionally with minor impurity minerals, such as hematite, anatase, barite, diaspore and pyrite. Micro- to nano-sized iron oxides/hydroxides were observed and detected by SEM and EDS in the yellow to orange-yellow part of the samples. Moreover, these iron oxides/hydroxides were suggested to cause the yellow to orange-yellow in the seal stone from northern Laos. The UV-Vis spectrum and its second derivative, the Kubelka-Munk spectra, were used to identify and quantify hematite and goethite. The samples’ colour parameters were obtained with the Commission Internationale de l’Eclairage (CIE) 1931 standard space. According to the observation of the samples and the results obtained from experiments and calculations, the colour of the yellow parts (L* = 33.56~47.99, a* = 0.35~3.65, b* = 4.55~9.89) correlated with goethite (goethite is about 0.175~0.671 g/kg, the content of hematite was too low to be figured out in the yellow parts). In contrast, the colour of the orange-yellow parts (L* = 33.99~46.27, a* = 3.98~12.39, b* = 8.04~22.14) was more closely related with the content of hematite (goethite is about 0.096~0.691 g/kg, hematite is about 0.258~2.383 g/kg). The results of correlation analysis also support that the contents of iron oxides or hydroxides influence the samples’ colour. Therefore, it is suggested that micro- to nano-scaled hematite and goethite caused the colour of yellow and orange-yellow in the studied seal stone. Hematite can strengthen the red hue and change the colour from yellow to orange-yellow.