Blue PL Research Articles

Manipulating Electron-Phonon Coupling for Efficient Tin Halide Perovskite Blue LEDs.

Low-dimensional perovskites have opened up a new frontier in light-emitting diodes (LED) due to their excellent properties. However, concerns regarding the potential toxicity of Pb limited their commercial development. Sn-based perovskites are regarded as a promising candidate to replace Pb-based counterparts, while they generally exhibit strong electron-phonon coupling and consequently blue emission quenching at room temperature (RT), thus the Sn-based perovskite blue LED devices have not yet been reported. Herein, the luminescence properties are regulated by assembling a rigid organic skeleton within perovskite structure, and the protonated 4-bromobenzylamine (BrPMA+ = C7H9BrN+) is employed as A site cation to synthesize a 100-oriented 2D perovskite (BrPMA)2SnBr4, which exhibits a strong lattice rigidity via strong intermolecular interaction and consequently weak electron-phonon coupling, achieving the excellent blue PL emission at RT. The high quality (BrPMA)2SnBr4 perovskite thin films are obtained by further inhibiting oxidation and promoting crystallization. Finally, the Sn-based perovskite blue emission LED is successfully fabricated for the first time at 467nm with a champion EQE of 1.3% and a maximum brightness of 800cdm-2. This work gives insights into the luminescence mechanism of Sn-based perovskites and provides a new theoretical basis for the development of lead-free blue LEDs.

Photoluminescence Tuning of Stable CaYGaO4: Bi/Eu via Compositional Modulation and Crystallographic Site Engineering for nUV wLEDs.

The olivine-based gallate CaYGaO4 (CYG) with unique cationic ordering, rich lattice sites, and self-photoluminescence (PL) is suitable for application as a host of phosphor. However, research in this area is still in its early stages, especially in high-quality full-spectrum white lighting. Herein, novel CYG: Bi3+/Eu3+ with a controllable PL property is designed based on energy transfer and superposition of emissions from blue self-PL, blue PL of Bi3+, and red-PL of Eu3+. Intriguingly, PL intensity and quantum efficiency could be enhanced via codoping Li+/Zn2+ separately/simultaneously because of their two intentional functions as both charge balancer and flux. Unlike self- and Eu3+ PL, Bi3+ PL is quite sensitive to the lattice environment owing to its exposed 6s2 electronic configuration and is tuned via codoping Sr2+ to regulate the nephelauxetic effect and crystal field splitting concurrently around Bi3+. Meanwhile, for further regulating the PL of Bi3+ and obtaining "warm" white light, La3+ is codoped into the phosphor via crystallographic site engineering to control the substitution trends of Bi3+ at distinct lattice sites. Finally, as a proof-of-concept, a full-spectrum phosphor-converted white-light-emitting diode device under nUV pumping with remarkable color rendering index (Ra), high luminous efficiency, and chemical/thermal stability is achieved by utilizing the individual CYG:Bi/Eu/Li/Zn/Sr/La phosphor via a remote "capping" packaging method.

Combined Nucleophilic and Electrophilic Functionalization to Optimize Blue Phosphorescence in Cyclometalated Platinum Complexes.

Ligand-based functionalization strategies have emerged as powerful approaches to tune and optimize blue phosphorescence, which can involve nucleophilic addition to coordinated ligands or electrophilic functionalization via the coordination of exogenous Lewis acids. Whereas both have been used separately to enhance the photophysical properties of organometallic compounds with high-energy triplet states, in this work, we show that these two strategies can be used together on the same platform. Isocyanide-supported cyclometalated platinum compounds undergo nucleophilic addition with diethylamine to form a strong σ-donor acyclic diaminocarbene-supporting ligand. In a subsequent step, a cyanide ancillary ligand is converted into a more strongly π-acidic isocyanoborate via the coordination of a borane Lewis acid. Importantly, both of these ligand-based functionalization steps improve the quantum yields and lifetimes of the blue-phosphorescent complexes. This synergy results in complexes with photoluminescence quantum yields up to 0.40 for deep blue and 0.75 for sky blue regions and PL lifetimes on the order of 10-5 s.

Microwave-Assisted Grafting of Coal onto Nitrogen-Doped Carbon Dots with a High Quantum Yield and Enhanced Photoluminescence Properties.

The fluorescent nitrogen-doped carbon dots (N-CDs) were synthesized via a facile one-pot solvothermal process using coal (Jin 15 Anthracite and Shaerhu lignite) as raw materials and dimethyl formamide (DMF) as the solvent, employing a microwave pyrolysis method. This approach demonstrates remarkable efficacy in the development of nitrogen-doped carbon dots (N-CDs) with a high quantum yield (QY). The N-CDs prepared have strong photoluminescence properties. Moreover, the obtained N-CDs emit blue PL and are easily dispersed in polymethyl methacrylate (PMMA), preserving the inherent advantages of N-CDs and the PMMA matrix. The JN-CDs exhibit a high quantum yield (QY) of 49.5% and a production yield of 25.7%, respectively. In contrast, the SN-CDs demonstrate a quantum yield of 40% and a production yield of 35.1%. It is worth noting that the production yield and quantum yield of coal-based carbon dots are inversely related indices. The lower metamorphic degree of subbituminous coal favors an enhanced product yield, while the higher metamorphic degree of anthracite promotes an improved quantum yield in the product, which may be attributed to the presence of amorphous carbon within it. Consequently, we propose and discuss potential mechanisms underlying N-CD formation.

Synthesis of Newly Discovered Carbon Nanoframes: A Self‐Assembly Strategy Based on DTAB @ NaCl

AbstractCarbon nanomaterials have attracted much attention in the field of science and technology for their excellent properties; however, developing a simple, environmentally friendly, and versatile synthesis strategy for the preparation of carbon nanomaterials with novel structures remains a great challenge. Herein, a surfactant @ salt (dodecyltrimethylammonium bromide DTAB @ NaCl) self‐assembly strategy for the synthesis of carbon nanomaterials with novel structures, i.e., carbon nanoframes is reported. The synthesis differs from the traditional template method in that it is characterized by the introduction of surfactants for separation and protection, and the salt can be recycled. In addition, the carbon frame size in this system can be adjusted on demand by simply adjusting the concentration of surfactant, thus realizing that the carbon nanoframe size is adjustable in the range of 232.58–322.51 nm. Impressively, the carbon nanoframe has purple, blue, and green PL emission behavior. It is anticipated that this research will provide new insights into the development of novel carbon nanomaterials.

Tuning of structural, optical/ spectral and thermal properties of cubic CeO2 nanoparticles via incorporation of zirconia

In this communication, the influence of Zr4+ ions on structural, microstructural, optical and thermal behavior of CeO2 was investigated. Structural parameters of sol-gel derived samples were assessed by XRD, FTIR and Raman spectroscopies. Refined profile parameters were determined through Rietveld refinement. XRD and Raman studies indicated cubic fluorite phase of CeO2 and occurrence of oxygen vacancies as defects attributed to distortion in local symmetry of O2− ions around Ce4+ ions, hence found applications in oxygen storage devices. UV–visible depicted the blue shifting of absorption edge on increasing Zr4+ ions concentration, into Ce4+ions lattice, hence found utility in photonic devices. Violet, blue and green PL emissions were observed and committed due to various defect states and oxygen vacancies. CIE and CCT coordinates demonstrated their potential applications in display devices. The synthesized materials with enhanced structural and optical characteristics have their wide employability as an active component in sophisticated optoelectronic device fabrication.

Luminescence Characteristics of the MOCVD GaN Structures with Chemically Etched Surfaces.

Gallium nitride is a wide-direct-bandgap semiconductor suitable for the creation of modern optoelectronic devices and radiation tolerant detectors. However, formation of dislocations is inevitable in MOCVD GaN materials. Dislocations serve as accumulators of point defects within space charge regions covering cores of dislocations. Space charge regions also may act as local volumes of enhanced non-radiative recombination, deteriorating the photoluminescence efficiency. Surface etching has appeared to be an efficient means to increase the photoluminescence yield from MOCVD GaN materials. This work aimed to improve the scintillation characteristics of MOCVD GaN by a wet etching method. An additional blue photo-luminescence (B-PL) band peaking at 2.7-2.9 eV and related to dislocations was discovered. This B-PL band intensity appeared to be dependent on wet etching exposure. The intensity of the B-PL was considerably enhanced when recorded at rather low temperatures. This finding resembles PL thermal quenching of B-PL centers. The mechanisms of scintillation intensity and spectrum variations were examined by coordinating the complementary photo-ionization and PL spectroscopy techniques. Analysis of dislocation etch pits was additionally performed by scanning techniques, such as confocal and atomic force microscopy. It was proved that this blue luminescence band, which peaked at 2.7-2.9 eV, is related to point defects those decorate dislocation cores. It was shown that the intensity of this blue PL band was increased due to enhancement of light extraction efficiency, dependent on the surface area of either single etch-pit or total etched crystal surface.

Colloidal CdSe/CdS Core/Crown Nanoplatelets for Efficient Blue Light Emission and Optical Amplification.

The application of CdSe nanoplatelets (NPLs) in the ultraviolet/blue region remains an open challenge due to charge trapping typically leading to limited photoluminescence quantum efficiency (PL QE) and sub-bandgap emission in core-only NPLs. Here, we synthesized 3.5 monolayer core/crown CdSe/CdS NPLs with various crown dimensions, exhibiting saturated blue emission and PL QE up to 55%. Compared to core-only NPLs, the PL intensity decays monoexponentially over two decades due to suppressed deep trapping and delayed emission. In both core-only and core/crown NPLs we observe biexciton-mediated optical gain between 470 and 510 nm, with material gain coefficients up to 7900 cm-1 and consistently lower gain thresholds in crowned NPLs. Gain lifetimes are limited to 40 ps, due to residual ultrafast trapping and higher exciton densities at threshold. Our results provide guidelines for rational optimization of thin CdSe NPLs toward lighting and light-amplification applications.

Efficient Zn2+ doping into CsBr nanocrystals using benzoyl bromide

The potential route to induce effective metal doping into the colloidal nanocrystal (NC) lattice is still limited due to the unfavorable energy cost and needs further investigation for colloidal chemistry involved in the optical properties and device development with optimized NC design. In this study, we reports a bright blue emitting Zn2+-doped CsBr NCs (6.7 at.%) by introducing reactive benzoyl bromide to promote a preferential reaction with metal cations that are not favorably incorporated into the CsBr lattice. In particular, the acyl group (R-C = O-) of benzoyl bromide is more effective to adopt additional metal cations into the crystal lattice during the nucleation and growth process of colloidal CsBr NCs compared to alkylammonium bromides with long hydrocarbon chains, providing a new synthetic route for metal doping into the metal halide perovskites NCs. Zn2+-doped CsBr NCs exhibit a blue PL emission at 425 nm with a Stokes shift of 113 nm for the weak absorption band edge.

Exploring the Links between Photoluminescence and Microstructure in Cs2InBr5·H2O Samples Doped with Pb2+

Precipitation of Cs2InBr5·H2O from HBr(aq) solutions containing Pb2+ ions results in powders that exhibit narrowband green photoluminescence (λmax = 521 nm, fwhm = 89.9 meV). Synchrotron powder X-ray diffraction reveals trace amounts of Cs4PbBr6 and CsPbBr3 that cannot be detected by laboratory powder diffraction measurements. Broadening of the CsPbBr3 diffraction peaks suggests crystals that are tens of nanometers in size. Evidence for the presence of CsPbBr3 can also be seen in diffuse reflectance spectra. Cathodoluminescence imaging shows that luminescence originates from small nanometer-sized regions. Taken together, these observations point to CsPbBr3 nanocrystal inclusions as the source of photoluminescence. Heating these samples to temperatures at or above 80 °C triggers a reversible dehydration process that leads to an irreversible change in the photoluminescence from green to blue (λmax ≈ 480 nm, fwhm = 278 meV), accompanied by significant changes in the microstructure. Cathodoluminescence imaging indicates that the blue emission occurs over much larger micron-sized regions of the sample. The position of blue PL is similar to other hybrid lead bromide compounds where the emission has been assigned to 3P1 → 1S0 transitions on [PbBr4]2– ions. Based on the emission wavelength and cathodoluminescence imaging, the blue emission is assigned to isolated [PbBr4]2– ions that substitute for [InBr5·H2O]2– ions in the parent hydrate phase. This work provides new insight on the spontaneous formation of halide perovskite nanocrystals in an inert matrix, one that does not rely on the use of organic solvents and is stable in ambient atmospheres.

Influence of silver nitrate in obtaining highly luminescent sodium-doped carbon dots for white light emitting diodes

Luminescent metal-doping carbon dots (CDs) possess excellent optical properties for light-converting phosphors of white light emitting diode (LED). However, the photoluminescence quantum yields (PL QYs) of CDs still suffer from the trapping defect at the surface of CDs which may generate the detrimental nonradiative recombination centers. Herein, we developed an N doping method to enhance blue light emission of Na+-doped CDs via a modified solid-state reaction, using silver nitrate as nitrogen source. The blue PL intensity form Na+-doped CDs is improved by the increasing molar ratio of silver nitrate precursor. The obtained Na+ and N co-doping CDs exhibited 5-fold enhancement of PL intensity with a PL QYs up to 24% as compared to their undoped counterpart. Based on the temporal evolution of PL and surface structure characteristics, it suggested that the improvement of PL QYs derived from the elimination of the trapping defects at the surface of Na+-doped CDs. This is realized by the introduction of nitrate groups and Ag+ ions, which can reduce the formation of hydroxyl radicals related nonradiative recombination centers. Furthermore, the Na+ and N co-doped CDs is applied with (Gd,Y)3(Ga,Al)5O12: Ce3+ yellow phosphor on a commercial UV-LED chip, achieving strong cold white emission. Our finding provides a new idea to enhance blue PL QYs of Na+-doped CDs for their further potential applications in optoelectronic devices.

Influence of lanthanum doping on the structural and optical properties of cerium tungstate nanocrystals

In this study, the effect of lanthanum (La) doping on the structural and optical properties of cerium tungstate nanocrystals synthesized by the chemical precipitation method is investigated. The X-ray diffraction studies confirmed the crystalline phase formation and observed no phase change with doping. It is interesting to note that the crystallite size is decreased with La doping. The characteristic vibrational peaks of W-O bonds are confirmed from the FTIR and Raman spectroscopic studies and found no appreciable changes in the spectral positions with doping. The optical properties are studied using UV-Vis spectroscopy and photoluminescence spectroscopy. The characteristic broad blue PL emission from the tetrahedral tungstate group is observed for the pure and doped samples, and the appreciable variation in PL intensity can be associated with the formation of defects that occurred while doping.

Sharp luminescent violet- and blue-emitting stable (Eu3+ → Eu2+: PPA):SiO2 sonogel hybrid glasses: Synthesis, structural and overall photophysical characterizations

Nanostructured Eu2O3:Polyphenylacetylene (Eu2O3:PPA) complexes prepared at different lanthanide content were synthesized as active ligand–to–metal charge–transfer (LMCT) systems. These phosphors were embedded into mesoporous a–SiO2 sonogel hosting networks at different dopant ratios to obtain luminescent (Eu2O3:PPA):SiO2 monolith glasses. The electroactive sonochemical medium of the colloidal sol–phase activates massive Eu3+→Eu2+ self–reduction processes along the polycondensation reaction of the doped emulsions, giving rise to sharp violet (407 nm) and blue (470 nm) PL–emission bands (FWHM ≤18 nm). The unusual violet–lines arise from the 4f65d1→4f7 transitions of Eu2+ ions to produce intense 6P7/2 → 8S7/2 emissions, while the blue–lines are ascribed to overlapping 5D2→7Fj (j = 0, 1) transitions of remanent Eu3+ ions assisted by LMCT and interionic energy transfers. The CIE coordinates of these composites lay within the light violet/blue region and the bandgap energy was reduced up to 2.16 eV for increasing Eu2O3 concentration. Our experimental approach shows that the Eu3+→Eu2+ process occurs at room temperature material processing, without the use of reducing atmospheres, stimulated UV photoreduction, or post–doping functionalization. Results suggest that guest–host redox interactions and Eu/SiO2 surface–assisted mechanisms are responsible for accomplishing a homogeneous self–reduction and the structuring of steady Eu2+–silicates. Extensive morphological, structural, linear and NLO spectroscopic characterizations, CIE–chromaticity, and energy bandgap evaluations, were performed to explore these novel nanostructured composites' physicochemical and photophysical properties, pointing out potential applications in tunable phosphor devices.

Effect of Fluoroalkyl-Substituent in Bistolane-Based Photoluminescent Liquid Crystals on Their Physical Behavior

Photoluminescent liquid crystals (PLLCs) have attracted significant attention owing to their broad applicability in thermosensing and PL switching. Extensive efforts have been made to develop bistolane-based PLLCs containing flexible units at both molecular terminals, and it has been revealed that their PL behavior can switch with the phase transition between the crystalline and LC phases. Although slight modulation of the flexible unit structure dramatically alters the LC and PL behaviors, few studies into the modification of the flexible units have been conducted. With the aim of achieving dynamic changes in their physical behaviors, we developed a family of bistolane derivatives containing a simple alkyl or a fluoroalkyl flexible chain and carried out a detailed systematic evaluation of their physical behaviors. Bistolanes containing a simple alkyl chain showed a nematic LC phase, whereas switching the flexible chain in the bistolane to a fluoroalkyl moiety significantly altered the LC phase to generate a smectic phase. The fluoroalkyl-containing bistolanes displayed a stronger deep blue PL than their corresponding non-fluorinated counterparts, even in the crystalline phase, which was attributed to the construction of rigid molecular aggregates through intermolecular F···H and F···F interactions to suppress non-radiative deactivation.

Synthesis of fluorescent graphene quantum dots from graphene oxide and their application in fabrication of GQDs@AgNPs nanohybrids and sensing of H2O2

Graphene oxide (GO) with hydrophilic properties supports the scalable production of highly desirable material graphene. GO nanosheets were prepared with a modified Hummers method, whereas the oxidative cutting technique was used to synthesize graphene quantum dots (GQDs). The surface and structural morphology of both GO and GQDs were examined and compared. In GO, photoluminescence originated mainly due to the conductive electrons present in the sp2 localized sites and the excimer formation. In contrast, in GQDs, PL is increased due to numerous defects present on their surfaces. The pH plays a crucial role in tuning the blue and green PL of GQDs hence the optical properties. The developed GQDs with functional groups act as a solo reducing and stabilizing agent in producing the GQDs-silver nanoparticles (GQDs@AgNPs) hybrid structure, which exhibits strong surface plasmon resonance along with GQDs absorption. Further, GQDs alone support sensing of hydrogen peroxide (H2O2) in an aqueous medium, thus acting as a sensing probe like the GQDs@AgNPs nanohybrids.

Broad band photoluminescence of g-C3N4/ZnO/ZnS composite towards white light source

We have found one step synthesized g–C3N4/ZnO/ZnS composite to be promising for engineering of white-light sources. The composite was fabricated by simultaneous pyrolytic decomposition of thiourea and zinc acetate with a consequent insitu polymerization of the products at 550–625 °C and characterized by SEM, XRD, EDX and low temperature PL spectroscopy. The composite synthesized at 550 °C demonstrates blue PL with a peak at 2.74 eV while higher temperatures of the synthesis result in a broad spectra with a maximum at 2.22 eV and comprise of the bands of g–C3N4 and mixed ZnO/ZnS as they have been resolved by low temperature PL. The proper selection of the synthesis temperature allows an extension of light emitting spectra of g–C3N4/ZnO/ZnS composites from blue to white light range. The results obtained show ternary systems containing g-C3N4 and wide band-gap semiconductors are suitable for white-light emitting and optoelectronic devices.

Bright aspects of defects and dark traits of dopants in the photoluminescence of Er2X2O7:Eu3+ (X = Ti and Zr) pyrochlore: an insight using EXAFS, positron annihilation and DFT

This work investigated the origin of blue PL in undoped and absence of Eu3+ red PL in the doped Er2X2O7 (X = Ti and Zr) pyrochlore using photon, positron and X-ray as spectroscopic probe and further complimenting with DFT.

Enhancement of photocatalytic degradation of methylene blue dye using Ti3+ doped In2O3 nanocubes prepared by hydrothermal method

Pure and Ti3+ doped In2O3 nanostructures had been synthesized by facile hydrothermal method. The structural, morphological and optical properties were characterized by using XRD, FT-IR, HR-SEM and TEM, EDS, XPS, UV-DRS and PL techniques. X-ray diffraction analysis indicated that pure body-centered cubic crystal structure of In2O3. HR-SEM and TEM analysis showed nanoflakes and porous cube shaped nanostructure for pure and Ti3+ doped In2O3, respectively. XPS spectra revealed composition and elemental state of Ti, In and O. An optical band gap values decreased from 3.40 to 3.30 eV while doping. Violet, blue and green emission PL peaks were observed at room temperature. Photocatalytic activity for Methylene Blue (MB) dye degradation was improved by adding dopant Ti3+ into In2O3 host lattice. Reusability study of Ti3+ doped In2O3 was done to study about the stability of the catalyst.

Near UV/Deep‐Blue Phenanthroimidazole‐Based Luminophores for Organic Light‐Emitting Diodes: Experimental and Theoretical Investigation

AbstractTwo new deep blue phenanthroimidazole base luminophores 2‐(4‐(4‐(4‐(1‐phenyl‐1H‐phenanthro[9,10‐d]imidazol‐2‐yl)phenoxy)butoxy)phenyl)‐1‐phenyl‐1H‐phenanthro[9,10‐d]imid –azole (DPIPOBn) and 2‐(4‐(4‐(4‐(1‐(4‐tert‐butylphenyl)‐1H‐phenanthro[9,10‐d]imidazol‐2‐yl) phenoxy)butoxy)phenyl)‐1‐(4‐tert‐butylphenyl)‐1H‐phenanthro[9,10‐d]imidazole (DPITBOBn) were designed and synthesized by linkage two phenanthroimidazole functional units via the highly flexible alkyl (butane) spacer with different N1 functional groups (benzene and tert‐butylbenzene). The synthesized luminophores were structurally characterized by spectroscopic method (1H, 13C, and mass). The photophysical, electrochemical and electroluminescence properties of the luminophores were systematically studied. These luminophores emit intense blue PL emission in both solution and solid phase. The HOMO‐LUMO energy level of the luminophores was calculated from Cyclic Voltammetry (CV) analysis and which agreed with the results from the density‐functional‐theory (DFT) calculation. The time‐dependent DFT (TD‐DFT) calculation was performed to know the excited states (singlet and triplet) of the luminophores. The solution‐processed OLED was fabricated to test these materials as emissive materials and as emitters. All the doped devices exhibited near‐UV emission with EL peaks centered around 380–395 nm with Commission International deL'Eclairage (CIE) coordinate of (0.16, 0.09 or 0.10). Among all the doped devices, DPITBOBn (3 wt%) based device displayed a power efficiency (PE) of 0.5 lm/W, a current efficiency (CE) of 1.2 cd/A and external quantum efficiency (EQE) of 1.9% at the brightness 100 cd/m2.

Synthesis of novel g-C3N4 microrods: A metal-free visible-light-driven photocatalyst

Novel graphitic carbon nitride (g-C3N4) microrods were prepared by chemical method via pre-treatment of melamine and molar solution of HNO3 at a lower temperature. The phase and morphological structure was examined by XRD and SEM, while elemental composition was found by performing EDX spectroscopy. SEM results revealed that the as-prepared g-C3N4 materials has rods like morphology with average length 3–5 µm. Furthermore, the vibrational modes were studied by FTIR, and optical properties were analyzed by UV–Vis and PL spectroscopy respectively. The optical measurements showed that the band gap of g-C3N4 microrods was 2.70 eV which make them useful for photocatalytic process under visible light. The blue PL emission at 455 nm was ascribed to band-to-band transition which also confirms the activation of g-C3N4 microrods in visible spectrum range. Finally, g-C3N4 microrods were utilized as photocatalyst for the decomposition of hazardous Methyl Orange (MO) dye, and the results showed that they exhibited excellent performance (k = 0.026 min−1) visible light irradiation.