Efficient Light Emission Research Articles

Time-correlated luminescence blinking in InGaN single quantum wells

The blinking phenomenon in InGaN single quantum wells is a phenomenon where localized photoluminescence changes over time. Understanding its physics is important for the manufacture of more efficient light emission diodes. We present a study using two InGaN single quantum well samples, emitting at 460 and 510 nm wavelength, respectively. We confirmed that the luminescence intensity fluctuates in localized blinking regions, and we found that these optical variations are not random but are instead correlated in pairs, with either positive or negative coefficient, to a distant reference blinking point. Measurements were performed to obtain standard deviation and cross-correlation maps. Invoking the quantum confined Stark effect, we realized a simple phenomenological model that shows how charge carriers are exchanged among pairs of adjacent opposite correlation regions. As a result, it is suggested that the phenomenon is caused by fluctuations in the number of these exchanged carriers. Our model gives an explanation for the blinking phenomenon in InGaN single quantum wells, and it is important for a deeper understanding to InGaN-based materials.

Surface Passivation for Promotes Bi-Excitonic Amplified Spontaneous Emission in CsPb(Br/Cl)3 Perovskite at Room Temperature.

Perovskite-type lead halides exhibit promising performances in optoelectronic applications, for which lasers are one of the most promising applications. Although the bulk structure has some advantages, perovskite has additional advantages at the nanoscale owing to its high crystallinity given by a lower trap density. Although the nanoscale can produce efficient light emission, its comparatively poor chemical and colloidal stability limits further development of devices based on this material. Nevertheless, bulk perovskites are promising as optical amplifiers. There has been some developmental progress in the study of optical response and amplified spontaneous emission (ASE) as a benchmark for perovskite bulk phase laser applications. Therefore, to achieve high photoluminescence quantum yields (PLQYs) and large optical gains, material development is essential. One of the aspects in which these goals can be achieved is the incorporation of a bulk structure of high-quality crystallization films based on inorganic perovskite, such as cesium lead halide (CsPb(Br/Cl)3), in polymethyl methacrylate (PMMA) polymer and encapsulation with the optimal thickness of the polymer to achieve complete surface coverage, prevent degradation, surface states, and surface defects, and suppress emission at depth. Sequential evaporation of the perovskite precursors using a single-source thermal evaporation technique (TET) effectively deposited two layers. The PL and ASEs of the bare and modified films with a thickness of 400 nm PMMA were demonstrated. The encapsulation layer maintained the quantum yield of the perovskite layer in the air for more than two years while providing added optical gain compared to the bare film. Under a picosecond pulse laser, the PL wavelength of single excitons and ASE wavelength associated with the stimulated decay of bi-excitons were achieved. The two ASE bands were highly correlated and competed with each other; they were classified as exciton and bi-exciton recombination, respectively. According to the ASE results, bi-exciton emission could be observed in an ultrastable CsPb(Br/Cl)3 film modified by PMMA with a very low excitation energy density of 110 µJ/cm2. Compared with the bare film, the ASE threshold was lowered by approximately 5%. A bi-exciton has a binding energy (26.78 meV) smaller than the binding energy of the exciton (70.20 meV).

Raman silicon nanocavity laser with efficient light emission from the edge of an adjacent waveguide.

A Raman nanocavity laser can emit light into free space and into a properly designed waveguide adjacent to the cavity by mode coupling. In common device designs, the emission from the edge of this waveguide is relatively weak. However, a Raman silicon nanocavity laser with strong emission from the waveguide edge would be advantageous for certain applications. Here we investigate the increase in the edge emission that can be achieved by adding photonic mirrors to the waveguides adjacent to the nanocavity. We experimentally compare devices with and without photonic mirrors: the edge emission for devices with mirrors is 4.3 times stronger on average. This increase is analyzed using coupled-mode theory. The results indicate that the control of the round-trip phase shift (between the nanocavity and the mirror) and an increase of the quality factors of the nanocavity are important for further enhancement.

Zero-Dimensional Hybrid Cuprous Halide of [BAPMA]Cu2Br5 as a Highly Efficient Light Emitter and an X-Ray Scintillator.

Lead halide perovskites have been explored as a new kind of promising X-ray with wide applications in radiation-associated fields, but low light yield and serious toxicity extremely restrict further applications. To address these issues, we herein demonstrated one new zero-dimensional (0D) organic-inorganic hybrid cuprous halide of [BAPMA]Cu2Br5 (BAPMA = N,N-Bis(3-aminopropyl) methylamine) containing discrete [Cu4Br10]6- tetramers as excellent lead-free scintillators. Upon UV light excitation, [BAPMA]Cu2Br5 displays highly efficient broadband yellowish-green light emission with one dominant peak at 526 nm, a large Stokes shift of 244 nm, and a high photoluminescent quantum yield of 53.40%. Significantly, this broadband light emission can also be excited by higher-energy X-ray as radioluminescence with a high scintillation light yield of 43,744 photons/MeV. The detection limit of 0.074 μGyair/s is also far less than the required value for regular medical diagnostics of 5.5 μGyair/s. The solution-assembled hybrid structure facilely enables the [BAPMA]Cu2Br5-based scintillation screen to display high-performance X-ray imaging with a spatial resolution of 15.79 lp/mm showcasing potential application in X-ray radiography. In brief, combined merits of low toxicity and cost, negligible self-absorption, a low detection limit, considerable light yield, and spatial resolution highlight the excellent scintillation performance of 0D hybrid cuprous halide.

Cost-effective way of improving the optical properties of phosphor-in-glass by adjusting the particle size of glass powder

The effects of silicate glass particle size and Y3Al5O12: Ce3+ (YAG: Ce3+) phosphor doping concentration on the optical properties of phosphor-in-glass (PiG) samples were investigated. When the average particle size of glass powder was 160, 100, 60, and 30 μm, the transmittance of silicate glass with a thickness of 0.6 mm was 68%, 78%, 62%, and 54%, respectively. With the mixing of 4 wt% YAG: Ce3+ phosphor in each component of silicate glass, the transmitted luminous flux of PiG gradually increased with the increase of transmittance, reaching a maximum of 179.5 lm. Simultaneously, we found that with the increase in transmittance, the photoluminescence (PL) intensity of PiG gradually decreased; however, the total luminous flux of PiG has independent of transmittance. When the doping concentration of YAG: Ce3+ phosphor was 4 wt%, PiG achieved a maximum efficient white light emission of 212.61 lm/W under blue laser excitation at 2.31 W. All results indicate the outstanding feasibility of the prepared YAG-PiG in high-power/high-brightness white light-emitting diodes (WLEDs).

Dipole plasmon vitalized efficient white light emission via charge transfer in all oxide-based heterojunctions

Solid-state white-light emission, which is rapidly becoming the standard in a wide range of lighting applications, can be regarded as the best possible light source for next-generation cost-effective lighting. Herein, we report a simple solution-based technique to get efficient white-light emission using all-oxide semiconductor-based heterojunction. In this work, vertically oriented CuO nanorods were used for outstanding white-light emission by depositing ZnO thin films over CuO nanorods. The resulted bi-layered ZnO/CuO heterojunction arrangement have shown excellent optoelectronic properties. Furthermore, the inclusion of Au layer between ZnO/CuO heterojunction has improved white-light emission significantly and as an integral, a white light photoluminescence (PL) with a chromaticity coordinate of (0.32, 0.34) was obtained. Ab-initio calculations have supported our mechanism of charge transfer from CuO to ZnO especially with Au insertion. The presented work will provide insights into the intriguing optoelectronic properties of oxide hetero-structures.

A Review on the Progress of AlGaN Tunnel Homojunction Deep-Ultraviolet Light-Emitting Diodes

Conventional deep-ultraviolet (UV) light-emitting diodes (LEDs) based on AlGaN crystals have extremely low light-emission efficiencies due to the absorption in p-type GaN anode contacts. UV-light-transparent anode structures are considered as one of the solutions to increase a light output power. To this end, the present study focuses on developing a transparent AlGaN homoepitaxial tunnel junction (TJ) as the anode of a deep-UV LED. Deep-UV LEDs composed of n+/p+-type AlGaN TJs were fabricated under the growth condition that reduced the carrier compensation in the n+-type AlGaN layers. The developed deep-UV LED achieved an operating voltage of 10.8 V under a direct current (DC) operation of 63 A cm−2, which is one of the lowest values among devices composed of AlGaN tunnel homojunctions. In addition, magnesium zinc oxide (MgZnO)/Al reflective electrodes were fabricated to enhance the output power of the AlGaN homoepitaxial TJ LED. The output power was increased to 57.3 mW under a 63 A cm−2 DC operation, which was 1.7 times higher than that achieved using the conventional Ti/Al electrodes. The combination of the AlGaN-based TJ and MgZnO/Al reflective contact allows further improvement of the light output power. This study confirms that the AlGaN TJ is a promising UV-transmittance structure that can achieve a high light-extraction efficiency.

Nanowire dimer optical antenna brightens the surface defects of silicon

Abstract Plasmonic hot spots located between metallic dimer nanostructures have been utilized comprehensively to achieve efficient light emission. However, different from the enhancement occurred in the plasmonic hot spot, the investigation of light emission off the hot spot on submicron scale remains challenge. In this work, we have constructed a plasmonic nanowire dimer (NWD) system to brighten the light emission of the surface defects of silicon off the hot spot on the submicron scale. The NWD can trap light through plasmonic gap, then, the excited emitter on the submicron scale can radiate light efficiently by coupling with the dipole gap plasmonic mode. Furthermore, the coupling of dipole plasmonic mode with the emitters can be tuned by changing the gap size, and then photoluminescence emission was drastically enhanced up to 126 folds. Theoretical simulations reveal the photoluminescence enhancement arises from the combination of the NWD’s high radiation efficiency, Purcell enhancement, efficient redirection of the emitted photoluminescence and the excitation enhancement. In this study, the photoluminescence signal can be effectively enhanced by placing nano-antenna patch on the detected low-quantum-efficiency emitters, which may open up a pathway toward controlling plasmonic gap mode enhanced light emission off the hot spot on submicron scale.

Hybrid Lead‐Free Metal Antimony Oxide Chlorides with Highly Efficient Blue Light Emissions

AbstractHerein, we report two hybrid zero‐dimensional (0D) lead‐free metal oxide halides of [PPip]Sb2Cl6O (1) ([PPip]=1‐(2‐Pyridyl)piperazine) and [PmPip]Sb2Cl6O (2) ([PmPip]=1‐(2‐Pyrimidyl)piperazine). Compounds 1 and 2 are able to exhibit narrow blue light emissions excited by UV light with photo‐luminescence quantum yields (PLQYs) of 25.49 % and 24.71 %, respectively. Through systematical characterizations, these blue light emissions can be attributed to the radiative transition in conjugated organic cations. Benefiting from the highly efficient intrinsic blue emissions, compound 2 can be used as blue fluorophor to fabricate liquid crystal display. This work provides a new to assembly blue emitter through by virtue of the optical active organic cations in hybrid materials.

Tunable Efficient White Emission in Holmium Doped Double Perovskites Cs2KInCl6 via Antimony Sensitization

AbstractRecently, ns2 metal ions (such as Bi3+ and Sb3+) doped double perovskites have captured intense attention for their efficient emission, however, achieving efficient and tunable white light emission is always an enormous challenge. Herein, Sb3+/Ho3+ co‐doped Cs2KInCl6 double perovskites are proposed, and the photoluminescence results show that there are two emission bands, one broad cyan emission band stems from self‐trapped exciton (STE) in [SbCl6]3‐ octahedron, while another red emission band derives from the f‐f transitions of Ho3+. The emission processes for Sb3+/Ho3+ co‐doped Cs2KInCl6 show that Sb3+ can act as the sensitizer to activate Ho3+ emission, which is due to the energy transfer channel from STE to Ho3+. As the Ho3+ concentration increases, the luminous color of Sb3+/Ho3+ co‐doped Cs2KInCl6 can be modulated from cyan to orange under 315 nm light irradiation, and the white emission can even be obtained with a photoluminescence quantum yield of 90 ± 2% when the energy‐transfer efficiency is 50%. Finally, single compound white‐light emitting diodes and fluorescent anti‐counterfeiting labels based on 0.5%Sb3+/60%Ho3+ co‐doped Cs2KInCl6 are fabricated. This work provides an effective strategy to achieve efficient tunable white emission in lead‐free double perovskites through ns2 metal and lanthanide ion co‐doping.

Synthesis and optical properties of light-emitting V2N MXene quantum dots

The outstanding optical properties of MXene quantum dots (MQDs) have attracted significant attention in optoelectronic applications. Recently, light-emitting MQDs have been developed from not only carbide but also nitride MXenes. In this study, we synthesized MQDs using V2N MXene via a hydrothermal method. The average diameter of the water-soluble and mono-dispersed V2N MQDs was 3.26 nm. The V2N MQDs exhibited efficient light emission, with an estimated photoluminescence quantum yield of 11.8%. Our findings open new horizons in the research field of MQDs and their industrial applications.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Strain-Relaxed Nanocrystalline Diamond Thin Films with Silicon Vacancy Centers Using Femtosecond Laser Irradiation for Photonic Applications

Nanocrystalline diamond thin (thickness of 100–500 nm) films with ensembles of negatively charged light-emitting silicon vacancy (SiV) centers might find an application in the field of photonics or biosensing due to their scalable and low-price fabrication process in comparison with monocrystalline diamond. The potential application of diamond thin films is, however, limited due to the presence of radiative and nonradiative defects. The former type is a source of an unwanted background in the photoluminescence spectra, the latter lowers the quantum efficiency of light emission, and both types of defects cause the reabsorption of SiV emission during its propagation within the layer. Here we show that femtosecond (fs) laser pulses focused via a microscope objective on a nanocrystalline diamond thin film can be employed to reduce background photoluminescence intensity by a factor of 5 with respect to the luminescence of SiV centers. Raman spectra show that this decrease is mainly caused by laser ablation of the sp2-related carbon phase present in-between the grains. The reduction of the sp2 carbon phase also leads to a local decrease in the optical absorption followed by an almost twofold increase in the SiV emission peak intensity. Moreover, the fs-irradiation also entails the release of strain inside such a layer as manifested by the observed shift of the Raman diamond peak toward the value measured in the monocrystalline diamond and by a blue-shift of the zero-phonon-line peak position of the SiV centers. Such a finding might enable the improvement of the optical quality of the nanocrystalline diamond-based photonic nanostructures or even might be employed over a larger area by defocusing the irradiation laser in order to create large-scale strain-relaxed nanocrystalline diamond thin films.

Surface Effects on Anisotropic Photoluminescence in One‐Dimensional Organic Metal Halide Hybrids

1D organic metal halide hybrids (OMHHs) exhibit strongly anisotropic optical properties, highly efficient light emission, and large Stokes shift, holding promise for novel photodetection and lighting applications. However, the fundamental mechanisms governing their unique optical properties and in particular the impacts of surface effects are not understood. Herein, 1D C4N2H14PbBr4 by polarization‐dependent time‐averaged and time‐resolved photoluminescence (TRPL) spectroscopy, as a function of photoexcitation energy, is investigated. Surprisingly, it is found that the emission under photoexcitation polarized parallel to the 1D metal halide chains can be either stronger or weaker than that under perpendicular polarization, depending on the excitation energy. The excitation‐energy‐dependent anisotropic emission is attributed to fast surface recombination, supported by first‐principles calculations of optical absorption in this material. The fast surface recombination is directly confirmed by TRPL measurements, when the excitation is polarized parallel to the chains. The comprehensive studies provide a more complete picture for a deeper understanding of the optical anisotropy in 1D OMHHs.

Review: Quantum Dot Light-Emitting Diodes.

Quantum dot light-emitting diodes (QD-LEDs) are one of the most promising self-emissive displays in terms of light-emitting efficiency, wavelength tunability, and cost. Future applications using QD-LEDs can cover a range from a wide color gamut and large panel displays to augmented/virtual reality displays, wearable/flexible displays, automotive displays, and transparent displays, which demand extreme performance in terms of contrast ratio, viewing angle, response time, and power consumption. The efficiency and lifetime have been improved by tailoring the QD structures and optimizing the charge balance in charge transport layers, resulting in theoretical efficiency for unit devices. Currently, longevity and inkjet-printing fabrication of QD-LEDs are being tested for future commercialization. In this Review, we summarize significant progress in the development of QD-LEDs and describe their potential compared to other displays. Furthermore, the critical elements to determine the performance of QD-LEDs, such as emitters, hole/electron transport layers, and device structures, are discussed comprehensively, and the degradation mechanisms of the devices and the issues of the inkjet-printing process were also investigated.

Cyanopyridone-cored fluorophores with triphenylamine peripheries: From molecular design to OLED fabrication studies

• Seven novel cyanopyridone-based D-A-D type molecules with a fixed triphenylamine donor, were synthesized. • Theoretical, optical, solvatochromic, thermal, electrochemical, electrical, and EL studies were performed to evaluate structure-property relationships. • Doped OLEDs using novel fluorophores as dopants outperform non-doped devices. • OLEDs emit yellow light with maximum efficiencies of 10.72 cd/A, 7.87 lm/W, and 5.32%. Amongst many types of fluorescent organic materials, compounds with π-conjugation between an electron donor (D) and electron acceptor (A) have been intensively investigated as effective organic electroluminescence (EL) device components. Herein, we report the synthesis and structural characterization of a new series of D-A-D configured small molecule-based fluorophores ( TPA-CyP1-7 ) having triphenylamine ( TPA ) as a donor unit and 3-cyanopyridine-2-one ( CyP ) as an acceptor core with different architects for use as efficient yellow light emitters in fluorescent organic light-emitting diodes (OLED). The detailed photophysical, solvatochromic, thermal, electrochemical, and EL properties, including quantum chemical calculations, were systematically investigated to study their relation between structure and properties. All the fluorophores show high fluorescent quantum yields in the solid film state and display high thermal stability with decomposition temperatures above 350 o C. The study reveals that they possess appropriate HOMO and LUMO energies level for effective charge injection. Finally, these yellow luminogens were employed to fabricate new OLED devices as sole emitters and dopants with CBP host materials. Interestingly, the host-guest devices doped with CBP host emitters show a remarkable improvement in the overall device performance. Among them, the TPA-CyP3 -based doped device has achieved a maximum current efficiency ( η c ), high power efficiency ( η p ), and good external quantum efficiency ( η EQE ) of 10.72 cd/A, 7.87 lm/W, and 5.32%, respectively.

Highly efficient warm white light emission in Sb3+-doped (NH4)4CdCl6metal halides through A-site Rb-alloying regulation

Novel all-inorganic Sb3+-doped (NH4)4CdCl6metal halides with excellent optoelectronic properties were prepared and efficient single component warm white light emission could be achieved through the A-site Rb alloying.

Luminescent Silicon Nanowires for Light-Harvesting and Environmental Sensing Applications

Silicon nanowires (Si NWs) represent one of the most promising platforms to be integrated into modern nanodevices. The fabrication of a dense array of vertically aligned ultrathin Si NWs using a low-cost, maskless approach and compatible with Si technology will be here demonstrated. Si NWs with efficient light emission at room temperature (RT) represent a great advance industry, paving the way for a wide range of unexpected photonic applications. In this work I will show the realization of a new hybrid material based on the luminescence of Si NWs and two different dyes for light-harvesting antennas applications, without any surface functionalization and with energy transfer efficiencies higher than 90%. The luminescence of Si NWs has also been used for sensing applications by the realization of highly sensitive sensors for the detecting of low concentrations of toxic gases. These sensors allow the detection of toxic gases below the threshold limits for human health, through both optical and electrical transduction. The achievement of light emission from silicon-based materials represents a revolution in the industrial field, as it paves the way for new Si applications.

Plasma etched Ag nanospheres for large increases of ZnO fluorescence emission: A combined theoretical-experimental study

Ag nanoparticles are prepared on p-type silicon substrates by magnetron sputtering, and then Ag nanospheres are constructed using appropriate plasma etching conditions. By comparing and analyzing the AFM and SEM morphologies, the experimental parameters are optimized continuously to achieve Ag nanospheres with uniform size distribution. Finally, ZnO films are deposited on the upper layer to form Ag/ZnO composite films. Compared with Ag/ZnO with unetched Ag nanoparticles, its fluorescence emission is enhanced 140 times by adjusting the optimum coupling distance of Ag nanospheres. In this work, based on the experimental results, the coupling effect of Ag nanospheres and ZnO films is theoretically simulated using COMSOL Multiphysics. According to the plasmon electromagnetic coupling theory, the dielectric function relationship is deduced. At the same time, the relationship between the electric field distribution, scattering cross section, skin depth and fluorescence emission is analyzed, which also confirm theoretically that the plasma resonance of Ag nanospheres enhance the fluorescence emission of ZnO. It provides reliable theoretical and experimental parameters for improving the light response and fluorescence emission efficiency of optical devices.

Iridium(III) Complex Radical and Corresponding Ligand Radical Functionalized by a Tris(2,4,6-trichlorophenyl)methyl Unit: Synthesis, Structure, and Photophysical Properties

Organic radical luminescent materials with doublet excited state character based on tris(2,4,6-trichlorophenyl)methyl (TTM) have attracted extensive attention in recent years. However, how they affect the phosphorescent iridium(III) complex characterized by the triplet excited state has not been studied yet. Herein, a new iridium(III) complex radical (Ir-TTM) and corresponding ligand radical (ppy-TTM) with a TTM unit have been designed and synthesized, and their radical properties were confirmed by the single crystal structure and EPR spectra. Notably, the ligand radical ppy-TTM shows an efficient red light emission, whereas the iridium complex radical Ir-TTM emits no light, which resulted from the intramolecular quenching effect of the TTM radical unit on the iridium luminescence center. DFT calculations demonstrate that the lowest doublet (D1) excited state of ppy-TTM shows an intramolecular charge transfer character from the 2-phenylpyridine moieties to the TTM unit, whereas the D1 of Ir-TTM exhibits a significant charge transfer character from the iridium luminescence center moieties to the TTM unit, which further explains the luminescence quenching mechanism of the phosphorescent iridium complex radical.