Relative Photoluminescence Intensity Research Articles

P‐253: Late‐News Poster: The Evaluation on the Stability of Electroluminescent Materials Using Polaron Pair Formation Kinetics

We developed the method for evaluation of the stability of electroluminescent materials by using QEMS (quantum efficiency measurement system). Relative PL (photoluminescence) intensity decay of BDs (blue dopants) with BHs (blue hosts) in solution under UV light irradiation were measured and the rate constants for polaron pair formation by electron transfer were determined through first‐order kinetics analyses. The rate constants of polaron pair formation (6.487E‐04 –1.805E‐03 min‐1 ) were inversely proportional to the lifetime of OLED (organic light‐emitting diode) devices. We expect that this evaluation method can be applied to the development of new OLED materials and longer lifetime OLED devices.

Defects assisted the enhancement of optical and ferromagnetism in Zn1-xCoxS quantum dots: Toward efficient optoelectronic and spintronic applications

Colloidal Co-doped ZnS Quantum dots (QDs) were successfully synthesized through a facile and effective chemical co-precipitation method using polyethylene glycol (PEG) as a capping agent. A comprehensive study on the structure, optoelectronic, and magnetic properties of ZnS: Co2+ QDs was thoroughly carried out using diverse characterization techniques. Transmission electron micrographs reveal spherical QDs with a mean particle size of 5 ± 0.5 nm, whereas the X-ray diffraction (XRD) analysis and Raman spectroscopy confirm the exclusive presence of cubic ZnS phase structure, without any signs of Co-related impurity phases. Energy-dispersive x-ray (EDX) analysis verifies the presence of only three elements: Zinc, Cobalt, and Sulphur, with nearly stoichiometric proportions. X-ray photoemission (XPS) analysis confirmed the presence of Co2+ ions in the divalent oxidation state as well as the fair existence of sulphur vacancies. Optical measurements demonstrate that Co-doping in ZnS leads to a decrease in the optical band gap owing to the presence of defects that create localized states within the band structure. In addition, the refractive index (n) is found to increase with Co-doping level due to the increase in the polarizability. The outcomes of optical properties reveal that the tunability of the optical band gap and dispersive oscillator parameters in Co-doped ZnS QDs results in an enhancement of the non-linear optical parameters, such as third-order non-linear optical susceptibility χ(3), and non-linear refractive index n2, rendering them well-suited for applications in optoelectronic devices. Furthermore, the room-temperature photoluminescence (PL) spectra revealed that the Co-doped ZnS QDs have two broad emission bands, including the blue emission, and the green emission. By varying the level of Co-doping, it becomes possible to effectively control the relative PL intensities of dual-color emissions, highlighting their potential for producing adjustable color outputs. The room temperature ferromagnetic behaviour of the QDs has been observed and analyzed in accordance with the bound magnetic polarons model. These findings suggest that Co-doped ZnS QDs can be utilized effectively in the design of spintronic devices.

Addressing optical dynamics in CdS quantum dots through composition regulation strategy

Colloidal quantum dots (QDs) have received intensive attention over the past few decades because of their size-dependent properties, which enable them to be used in various applications such as displays, light-emitting diodes, and solar cells. Controlling the synthesis conditions to produce QDs with the desired size and distribution is becoming an exciting area of focused research. In addition, surface passivation of QDs is critical to increase their photoluminescence (PL) quantum yield. Because of their higher surface-area-to-volume ratio, nanocrystals contain abundant surface defects that can be reduced by the presence of excess cations. In this work, CdS QDs with different Cd:S ratios (1:1, 2:1, 3:1, and 4:1) are synthesized by the hot-injection method. Absorption and PL spectra show that the optical peaks at the beginning of growth are shifted to the shorter wavelength region with increasing Cd concentration. In particular, the sizes, size distributions, relative PL intensities, and optical bandgap properties of the synthesized QD samples are characterized to demonstrate the effect of the cation–anion ratio on the optical properties of CdS QDs.

Intermediate Color Emission via Nanographenes with Organic Fluorophores.

Photoluminescence (PL) color can be tuned by mixing fluorophores emitting the three primary colors in an appropriate ratio. When color tuning is achieved on a single substrate, we can simplify device structures. We demonstrated that nanographenes (NGs), which are graphene fragments with a size of tens of nanometers, could be utilized as carriers of fluorophores. The addition of red- and blue-light-emitting fluorophores on the edge successfully reproduced the purple light. The relative PL intensities of the fluorophores could be regulated by the excitation wavelength, enabling multicolor emission between blue and red light. Owing to the triphenylamine units of the fluorophores, the NGs showed PL enhancement due to aggregation. This characteristic was valuable for the fabrication of solid polymer materials. Specifically, the functionalized NGs can be dispersed into polyvinylidene difluoride. The resultant polymer films emitted red, blue, and purple color. Our study demonstrated the potential applicability of NGs for fluorophore carriers capable of reproducing intermediate colors of light.

Selective fluorescence sensor for Fe3+ based on in situ synthesis of CsSnCl3 perovskite quantum dots with bone gelatin

The sensors of perovskite quantum dots (QDs) based on Pb have gained more popularity in sensing applications. However, severely restricts any more practical uses because of the toxicity of Pb. In this study, we created an efficient in situ method to make bone gelatin‐CsSnCl3 QDs in 1‐allyl‐3‐methyl imidazole chloride ([AMIM]Cl), which were both very stable and environmentally benign. As a consequence, the relative photoluminescence intensity of bone gelatin‐CsSnCl3 QDs retained over 96% in water for 55 h, showing negligible degradation. The oxidation of Sn2+ in CsSnCl3 QDs was effectively prevented. The interaction strength of carboxyl, amino, and long molecule chains in bone gelatin and CsSnCl3 QDs was evaluated; the strongest binding force is between ‐COO− and CsSnCl3 QDs; and the weakest is between long molecular chains and CsSnCl3 QDs. Furthermore, bone gelatin‐CsSnCl3 QDs demonstrated good selectivity and a low detection limit for Fe3+ in aqueous solution (150 μM). The findings indicate that bone gelatin‐CsSnCl3 QDs have a bright future in the realm of fluorescence sensing.

Surface plasmon coupling effects on the photon color conversion behaviors of colloidal quantum dots in a GaN nanoscale hole with a nearby quantum-well structure.

To improve color conversion performance for color display application, we study the near-field-induced nanoscale-cavity effects on the emission efficiency and Förster resonance energy transfer (FRET) under the condition of surface plasmon (SP) coupling by inserting colloidal quantum dots (QDs) and synthesized Ag nanoparticles (NPs) into surface nano-holes fabricated on a GaN template and an InGaN/GaN quantum-well (QW) template. In the QW template, the inserted Ag NPs are close to either QWs or QDs for producing three-body SP coupling to enhance color conversion. Time-resolved and continuous-wave photoluminescence (PL) behaviors of the QW- and QD-emitting lights are investigated. The comparison between the nano-hole samples and the reference samples of surface QD/Ag NP shows that the nanoscale-cavity effect of the nano-hole leads to the enhancements of QD emission, FRET between QDs, and FRET from QW into QD. The SP coupling induced by the inserted Ag NPs can enhance the QD emission and FRET from QW into QD. Its result is further enhanced through the nanoscale-cavity effect. The relative continuous-wave PL intensities among different color components also show the similar behaviors. By introducing SP coupling to a color conversion device with the FRET process in a nanoscale cavity structure, we can significantly improve the color conversion efficiency. Simulation results confirm the basic observations in experiment.

Extended dimensionality of the density of states in InGaAs coupled quantum dot-ring structures as evidenced by radiative decay times

We observed radiative decay times as a function of temperature by analyzing the thermal relaxation from the excited exciton states in InGaAs coupled quantum dot (QD)–quantum ring (QR) structures. To investigate the confinement of electron-hole pair wavefunctions in the coupled QD-QR structures, we measured radiative decay times with temperature, where the photoluminescence decay time of electron-hole pairs was corrected based on the relative photoluminescence intensity at 4 K to discriminate the radiative processes. For decoupled QRs and QDs, quasi-one dimensional (for QRs) and zero-dimensional (for QDs) densities of states have been reported. However, dimensional densities of states from decoupled QRs and QDs do not apply when the distance between QRs and QDs is sufficiently close in the coupled QD-QR structure owing to coupling interactions (both short- and long-range). In the present coupled structure, QD and QR is connected at the bottom, and the top side peak-to-peak distance is approximately 24 nm. Based on our observations, the power order of temperature dependence increased from 0 to 0.33 (for QDs) and from 0.7 to 1.32 (for QRs), corresponding to the quasi-zero-dimensional (∼Tα=0.33) and quasi-two-dimensional (∼Tα=1.32) confinement state, respectively, owing to amended wavefunctions of electron-hole pairs and dark states effect.

Photoluminescence Performance and Photocatalytic Activity of Modified Carbon Quantum Dots Derived from Pluronic F127.

The photocatalytic degradation of organic dyes in waste water using carbon quantum dots (CQDs) remains a hot topic due to the importance of environmental protection. However, identifying suitable carbon resources and successful surface modification are still challenging. Herein, the hydrothermal method and surface modification of ammonia and thionyl chloride were applied to synthesize CQDs with different surface groups using PEO106PPO70PEO106 (Pluronic F127) as a carbon source. The average particle size of the as-prepared CQDs was in the range of 2.3-3.5 nm. The unmodified CQDs had the highest relative photoluminescence intensity, while all as-prepared CQDs exhibited abnormal photoluminescence located outside the scope of the visible spectrum. Interestingly, CQDs modified with ammonia achieved a degradation rate of 99.13% (15 d) for 50 mg/L indigo carmine solution, while CQDs modified with thionyl chloride reached a degradation rate of 97.59% (15 d) for light green SF yellowish solution. Therefore, in this work, two typical organic dyes can be effectively photocatalytically degraded by as-prepared CQDs, with suitable surface modification.

Electrical control of Förster resonant energy transfer across single-layer graphene

Abstract In artificial structures of molecular or quantum dot emitters in contact with single-layer graphene (SLG) Förster-type resonant energy transfer (FRET) can occur unconditionally due to the gapless band structure of SLG. A significant breakthrough for applications, however, would be the electrical modulation of FRET between arbitrary FRET pairs, using the SLG to control this process and taking advantage of the particular band structure and the monatomic thickness of SLG, far below the typical Förster radius of a few nanometers. For a proof of concept, we have therefore designed a Sandwich device where the SLG was transferred onto holey Si3N4 membranes and organic molecules were deposited on either side of the SLG. The relative photoluminescence (PL) intensities of donor and acceptor molecules changed continuously and reversibly with the external bias voltage, and a variation of about 6% of FRET efficiency has been achieved. We ascribe the origin of the electrical modulation of FRET to important doping-dependent nonlocal optical effects in the near field of SLG in the visible range.

Emission properties of sequentially deposited ultrathin CH3NH3PbI3/MoS2 heterostructures

Hybrid organic-inorganic perovskite materials have obtained considerable attention due to their exotic optoelectronic properties and extraordinarily high performance in photovoltaic devices. Herein, we successively converted the ultrathin PbI2/MoS2 into the CH3NH3PbI3/MoS2 heterostructures via CH3NH3I vapor processing. Atomic force microscopy (AFM)、Scanning electron microscopy (SEM) and X-ray photoemission spectroscopy (XPS) measurements prove the high-quality of the converted CH3NH3PbI3/MoS2. Both MoS2 and CH3NH3PbI3 related photoluminescence (PL) intensity quenching in CH3NH3PbI3/MoS2 implies a Type-II energy level alignment at the interface. Temperature-dependent PL measurements show that the emission peak position shifting trend of CH3NH3PbI3 is opposite to that of MoS2 (traditional semiconductors) due to the thermal expansion and electron-phonon coupling effects. The CH3NH3PbI3/TMDC heterostructures are useful in fabricating innovative devices for wider optoelectronic applications.

Highly Efficient and Thermally Stable Eu2+-Doped Ca9Nd(PO₄)7 Phosphor for Near-Ultraviolet White-Emission LED Applications.

Various Eu2+-based Ca9Nd(PO₄)7 (CNP:xEu2+, with different x values) materials are prepared via facile solid-state reaction. Their crystal structures are investigated in detail by means of the Rietveld refinement. The structure of CNP:Eu2+ with a trigonal lattice is analogous to that of β-Ca₃(PO₄)₂. Therefore, Eu2+ ions tend to incorporate calcium sites in the host. All the obtained samples can be excited using near ultraviolet (nUV) light to present blue-green emission. An optimal dopant concentration is verified at x = 0.8 with a large critical interaction radius (11.21 Å). The mechanism of the concentration quenching effect is assigned to the multipole-multipole interaction. CNP:xEu2+ possesses a short decay lifetime of ∼60 μs and can endure severe working conditions thanks to its great thermal stability. The relative photoluminescence (PL) intensity of CNP:0.8Eu2+ can retain 84.75% of the pristine intensity measured at room temperature, and the relative intensity remains as high as 69.97% at 423 K. The CNP:Eu2+ phosphors also show great performance in the WLED demonstration. The correlated color temperature (CCT) of the prototype device is 3404 K, with an extremely high Ra (97.6). Therefore, CNP:xEu2+ could be regarded as a promising alternative to blue green phosphors in nUV chip-based WLED applications.

Twisted Intramolecular Charge Transfer—Aggregation-Induced Emission Fluorogen with Polymer Encapsulation-Enhanced Near-Infrared Emission for Bioimaging

Fluorescence probes with strong near-infrared (NIR) emission and water solubility are considered useful visualization tools for localization marking as well as investigating cell migration and tran...

Highly UV Resistant Inch-Scale Hybrid Perovskite Quantum Dot Papers.

Halide perovskite quantum dots (PQDs) are promising materials for diverse applications including displays, light‐emitting diodes, and solar cells due to their intriguing properties such as tunable bandgap, high photoluminescence quantum yield, high absorbance, and narrow emission peaks. Despite the prosperous achievements over the past several years, PQDs face severe challenges in terms of stability under different circumstances. Currently, researchers have overcome part of the stability problem, making PQDs sustainable in water, oxygen, and polar solvents for long‐term use. However, halide PQDs are easily degraded under continuous irradiation, which significantly limits their potential for conventional applications. In this study, an oleic acid/oleylamine (traditional surface ligands)‐free method to fabricate perovskite quantum dot papers (PQDP) is developed by adding cellulose nanocrystals as long‐chain binding ligands that stabilize the PQD structure. As a result, the relative photoluminescence intensity of PQDP remains over ≈90% under continuous ultraviolet (UV, 16 W) irradiation for 2 months, showing negligible photodegradation. This proposed method paves the way for the fabrication of ultrastable PQDs and the future development of related applications.

High-stability inorganic perovskite quantum dot–cellulose nanocrystal hybrid films

Inorganic perovskite quantum dots (IPQDs) such as cesium lead halide (CsPbX3, X = Cl, Br and I) quantum dots have attracted much attention for developing cadmium-free quantum light-emitting displays (QLEDs) based on outstanding light emission properties including narrow full width at half maximum (FWHM), tunable bandgap and ultrahigh (>90%) photoluminescence quantum yield (PLQY). Nevertheless, their poor stability under ambient conditions, at high temperature or under continuous light irradiation is the main problem for practical applications. In this study, a new method is proposed to effectively stabilize CsPbBr3 IPQDs by synthesizing them with sulfate-functionalized cellulose nanocrystals (CNCs) at room temperature without using traditional quantum dot stabilizers such as oleylamine (OLA) and oleic acid (OA). The as-prepared CsPbBr3 IPQD/CNC hybrid paper-like films are highly stable and the relative photoluminescence (PL) intensity can be maintained at 92% under continuous UV light (306 nm, 15 W) illumination for 130 h, >99% at high temperature (100 °C) for 130 h, and >99% in ambient conditions for 15 d. Additionally, the PLQY and FWHM of IPQD/CNC are 45.69% and 22 nm, respectively. The ultrahigh stability and narrow FWHM characteristics proposed here for IPQD/CNC hybrid films can provide new possibilities for practical applications in the future development of IPQD-related devices.

Large area chemical vapour deposition grown transition metal dichalcogenide monolayers automatically characterized through photoluminescence imaging

Chemical vapour deposition (CVD) growth is capable of producing multiple single-crystal islands of atomically thin transition metal dichalcogenides (TMDs) over large areas. Subsequent merging of perfectly epitaxial domains can lead to single-crystal monolayer sheets, a step towards scalable production of high quality TMDs. For CVD growth to be effectively harnessed for such production it is necessary to be able to rapidly assess the quality of material across entire large area substrates. To date, characterisation has been limited to sub-0.1-mm2 areas, where the properties measured are not necessarily representative of an entire sample. Here, we apply photoluminescence (PL) imaging and computer vision techniques to create an automated analysis for large area samples of monolayer TMDs, measuring the properties of island size, density of islands, relative PL intensity and homogeneity, and orientation of triangular domains. The analysis is applied to ×20 magnification optical microscopy images that completely map samples of WSe2 on hBN, 5.0 mm × 5.0 mm in size, and MoSe2–WS2 on SiO2/Si, 11.2 mm × 5.8 mm in size. Two prevailing orientations of epitaxial growth were observed in WSe2 grown on hBN and four predominant orientations were observed in MoSe2, initially grown on c-plane sapphire. The proposed analysis will greatly reduce the time needed to study freshly synthesised material over large area substrates and provide feedback to optimise growth conditions, advancing techniques to produce high quality TMD monolayer sheets for commercial applications.

Synthesis of Pt-Loaded Y2WO6:Eu3+ Microspheres and Their Hydrogen-Sensitive Turn-Off Luminescence.

Pt nanoparticles were loaded on Y2WO6:Eu3+ phosphor microspheres to enhance hydrogen sensitivity at low temperatures through turn-off luminescence. Mesoporous Y2WO6:Eu3+ microspheres were synthesized first by a hydrothermal method. Pt loading on the surface of the Y2WO6:Eu3+ microspheres was then carried out by a method of the self-regulated reduction of surfactants using aqueous K2PtCl4 solutions. Structural and morphological properties of unloaded and Pt-loaded Y2WO6:Eu3+ microspheres were investigated by various techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The relative photoluminescence intensity of the Pt-loaded Y2WO6:Eu3+ microspheres, due to 5D0 → 7FJ (J = 0, 1, 2, 3, 4) electronic transitions of doped Eu3+ ions, was found to decrease as low as 22% after the microspheres were kept in the hydrogen gas atmosphere at a low temperature of 150 °C for 15 min. Meanwhile, mechanisms underlying this turn-off luminescence of the Pt/Y2WO6:Eu3+ microspheres in response to hydrogen gas are illustrated in detail.

Synthesis of hydrophilic P(VDF-TrFE) chloride sensitive polymer films for fluorescence sensing

Hydrophilic lucigenine doped P(VDF-TrFE) based chloride sensitive films were prepared by three different methods. These include increased pore size and distribution, introduction of alumina nanoparticles and addition of amphiphilic copolymer. The properties of the films were characterized by scanning electron microscope (SEM), contact angle meter (CAM), fourier transform infrared absorption spectroscopy (FT-IR) and photoluminescence (PL) spectroscopy. The PL intensity of chloride sensitive films is chloride concentration-dependent and the evolution of the lucigenine related PL intensity as a function of the chloride concentration follows the Stern–Volmer equation. From all the results, addition of amphiphilic copolymer shows the best effect on the hydrophilic modification of the film as well as the chloride sensitivity, and one of the optimal linear calibration formula I0/I=1+15.069[Cl-] (R2=0.981) was obtained in the concentration range 0.02 M-0.10 M of a standard sodium chloride solution with a detection limit of 0.01M (S/N=3).

Fiber-based optical thermocouples for fast temperature sensing in extreme environments

We have developed fiber-based optical thermocouples (OTCs) for fast temperature sensing in extreme environments. Our OTCs consist of a thin film of dysprosium-doped yttrium aluminum garnet (Dy:YAG)—a well-known two-color thermometry phosphor—deposited on the end of a sapphire fiber using pulsed laser deposition. Temperature sensing is achieved by comparing the relative intensities of photoluminescence arising from two closely spaced Dy3 + excited states. Using a combination of time-gated detection and blackbody background subtraction, we are able to measure Dy:YAG’s photoluminescence up to 2033 K, which is one of the highest temperatures obtained in literature. However, we are only able to use the photoluminescence spectra for temperature sensing up to 1773 K due to poor signal-to-noise ratio for higher temperatures. These results suggest the possibility of measuring higher temperatures with time-gated detectors designed for low-light levels. After characterizing the fiber-based OTCs’ temperature response, we next demonstrate their functionality using subsecond pulsed CO2 laser heating using both intensified charge-coupled device detection and a photodiode-based software time-gating technique. In the lab, we have utilized this technique to measure temperatures at rates up to 80 kHz. In addition, we comment on the applicability of OTCs to fast temperature sensing in turbulent flows and estimate rise times on the order of several hundred microseconds for a 1-μm OTC film.

Effect of Mg2+ co-doping on the photo- and thermally stimulated luminescence of the (Lu,Gd)3(Ga,Al)5O12:Ce epitaxial films

Photoluminescence and thermally stimulated luminescence of Ce3+ - doped (Lu,Gd)3(Ga,Al)5O12 epitaxial films co-doped with different concentrations of Mg2+ ions (varying from 0 to 0.09 at.%) are investigated in the 60–500 K temperature range under selective photoexcitation in the Ce3+ - related 4f → 5d2 and 4f → 5d1 absorption bands and in the absorption bands arising from the 8S7/2→6IJ and 8S7/2→6PJ electronic transitions of Gd3+ ions. Influence of Mg2+ ions on the Ce3+ - related photoluminescence intensity, spectrum, decay kinetics, temperature dependence of the emission intensity, and the activation energy of the luminescence thermal quenching is studied. The shortening of the luminescence decay time and simultaneous reduction of its light yield are explained by the presence of {Ce3+ - Mg2+} centers in the Mg2+ - containing films, where the luminescence optical quenching occurs in the 5d1 excited state of Ce3+ in these centers. Co-doping with Mg2+ results in the reduction of the afterglow and thermally stimulated luminescence and shortening of the afterglow decay kinetics. It influences also defects creation spectra and the activation energy of the photostimulated defects creation. The optimum Mg2+ content in the (Lu,Gd)3(Ga,Al)5O12:Ce epitaxial films is estimated to be around 0.005 at.%.

Enhanced electrical performance by modulation-doping in AlGaN-based deep ultraviolet light-emitting diodes

Through the silicon modulation-doping (MD) growth method, the electrical performance of AlGaN-based deep ultraviolet light-emitting diodes (DUV-LEDs) is improved by replacing the commonly uniform-doped (UD) method of n-AlGaN layer. The electroluminescence characterisic measurements demonstrate the MD growth method could effectively enhance the light emission intensity. Both the forward voltage and reverse leakage current of the MD samples are obviously reduced compared to those of the UD sample. Due to the existence of periodic Si-MD superlattices in n-AlGaN layers, which may behave like a series of capacitors, the built-in electric fields are formed. Both the measured capacitance–voltage (C–V) characteristics, and related photoluminescence (PL) intensity with the Si-MD growth method are enhanced. In detail, the effects of these capacitors can enhance the peak internal capacitance up to 370 pF in the MD sample, whereas the UD sample is only 180 pF. The results also mean that with better current spreading ability in the MD sample, the MD processes can effectively enhance the efficiency and reliability of DUV-LEDs. Thus, the investigations of the Si-MD growth methods may be useful for improving the electrical performance of DUV-LEDs in future works. Meanwhile, this investigation may partly suggest the minor crystalline quality improvements in the epi-layers succeeding the MD n-AlGaN layer.