Peak Wavelength Research Articles

Bright and stable near-infrared lead-free perovskite light-emitting diodes

Long-wavelength near-infrared light-emitting diodes (NIR LEDs) with peak emission wavelengths beyond 900 nm are of critical importance for various applications including night vision, biomedical imaging, sensing and optical communications. However, the low radiance and poor operational stability of state-of-the-art long-wavelength NIR LEDs based on soft materials remain the most critical factors limiting their practical applications. Here we develop NIR LEDs emitting beyond 900 nm with improved performance through the rational manipulation of p doping in all-inorganic tin perovskites (CsSnI3) by retarding and controlling the crystallization process of perovskite precursors in tin-rich conditions. The resulting NIR LEDs exhibit a peak emission wavelength at 948 nm, high radiance of 226 W sr−1 m−2 and long operational half-lifetime of 39.5 h at a high constant current density of 100 mA cm−2. Our demonstration of efficient and stable NIR LEDs operating at high current densities may also open up new opportunities towards electrically pumped lasers.

Silver-TiO2 coated D-shaped photonic crystal fiber based SPR sensor for ultrasensitive refractive index detection: design and FEM analysis

Photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensors are emerging as a promising technology for ultrasensitive detection of various biological and chemical analytes. This paper presents a novel D-shaped PCF based SPR sensor, which has shown great potential for highly sensitive detection of refractive index (RI) changes. The D-shaped configuration is achieved through the polishing of the upper side of the PCF fiber. To enhance sensitivity, a 0.1 μm silver layer is strategically placed between the fiber and analyte, intensifying light–matter interactions. Additionally, a 0.05 μm titanium dioxide (TiO2) layer is employed not only to further boost sensitivity but also to shield the metal from oxidation, ensuring the longevity and stability of the sensor. The finite element method (FEM) is employed to optimize the structural parameters of the sensor design. The findings demonstrate that the proposed SPR sensor is sensitive to RI changes in the 1.31–1.35 range, achieving a peak wavelength sensitivity of 30000 nm RIU−1 and an amplitude sensitivity of −185.33 RIU−1. The sensor holds promise for diverse applications, including chemical and biological sensing, making it a versatile tool with promising implications for advancing sensing technologies in various domains.

Simple method for optical characterization of surface agents on conjugated silver nanoparticles

In this study, a simple formula has been proposed to calculate the refractive index of surface agents of silver nanoparticles (Ag NPs) by using the plasmonic peak of the absorption spectra of dispersed Ag nano-colloids. The basis of the study is the shift in the localized surface plasmon resonance (LSPR) of Ag NPs upon alteration of surface agents. The color changes in a typical metal nano-colloid are mainly due to the shift in the LSPR, which is caused because of electrical interactions of surface agents on the particles. There are some theoretical models to simulate the absorption spectrum, but using these methods to evaluate the plasmonic peak is not facile for a wide range of users. Here, the required simulations were performed for different values of the refractive index of surface agents and particle sizes, and the absorption spectrum and dispersive curves were accordingly plotted. A simple formula was obtained between the wavelength of the plasmonic peak, the refractive index of the shell of surface agents, and the ratio of the hydrodynamic diameter to Feret size of the particles (R0). The refractive index of the surface agents can be calculated by n2=λmax−(202.3R02−748.6R0+947.4)−152.2R02+561.3R0−405.1, where λmax (in nanometer) is the wavelength of the absorption peak due to LSPR. This method can pave the way for experimenters to obtain the refractive index and consequently the type of surface agents around Ag NPs without the need for numerical or mathematical operations. It can also be useful in analyzing the spectral diagnosis of biological agents such as viral antibodies and antigens.

Identification and Limit of Detection of Benzene, Chlorobenzene, Benzoic Acid, Phthalic Acid, and Mellitic Acid in Water Solutions Using Excitation, Emission, and Single-band Synchronous Fluorescence Spectroscopy

Introduction: This paper first introduces the use of computer-simulated single-band synchronous fluorescence (SF) obtained from experimental excitation and emission fluorescence spectra of a pure compound in solution. The simulation produces a single narrow band with a peak wavelength that identifies the compound. Methods: The method is used to show single peak identification of benzene, chlorobenzene, benzoic acid, phthalic acid, and mellitic acid in water solutions. Synchronous fluorescence spectroscopy (SFS) is a variant of fluorescence technique in which excitation and emission scans are simultaneously acquired and multiplied with a predetermined wavelength difference (Δλ) between the two. Commercial instruments have this option to get the SFS signals. Results: In response to the Δλ selected, the result will be an SFS signal producing a series of peaks that could be assigned to compounds. Instead of running the same experiment with different Δλ values to identify the compounds, our simulation program determines a specific Δλ value that generates a narrow SF band with a distinctive peak wavelength for identification purposes. Conclusion: Finally, binary mixtures of chlorobenzene with each compound in water are prepared. The SFS of the solution is acquired and compared with the SFS bands of the components for identification purposes. With the commercial lamp fluorimeter employed, the limits of detection are obtained at the ng/g concentration level with fluorescence emission. Possible limits of detection at lower concentrations are discussed using a laser source. The presence of these molecules in astrochemical studies is discussed.

Solar thermal heat converter design using graphene material for industrial applications

Industrial heat converters are essential to meet the growing demand for thermal energy in industries. This demand can be met by designing solar heat absorbers which can be used as renewable energy sources for these industries. The effective solar absorber is built with the efficient three layers of top and based layer is constructed with the titanium (Ti), and the middle layer is demonstrated with the Gallium Arsenide (GaAs). The resonator (Ti) layer is in the mathematical sign divide and the proposed design can be effectively explored in the four different infrared light regions. The second layer of the GaAs layer is sandwiched between the two Ti-based and resonator layers. In this divide sign solar absorber, the proposed resulting percentage can be identified by using the four peak wavelengths in micrometers of 0.45 for λ1, 0.8 for λ2, 2.2 for λ3, and 2.5 for λ4 respectively. The resulting absorption rate can be presented in three sections of 700 nm bandwidth is above 95% in that rate and the section of 710 nm bandwidth is above 90%. The overall accepted rate from 0.2 to the highest rate of 3 µm is 2800 nm and explores the average rate of greater than 90% (90.16%). The expression of the proposed divided sign solar absorber, the building section's multistep inclusions, and the Air Mass (AM)1.5 situation in the different color sections can also be approved. The different studies of the design section can be expressed by changing the Ti-based and resonator layers, GaAs middle layer. Moreover, the amount of chemical potential and the TE and TM difference can also be analyzed with the degree changes from 0 to 50 degrees. The proposed dividend solar design can be used as a heat converter in industries as effective renewable energy such as cooling techniques, oil refining, recovery systems, air conditioning.

Size Optimization of Spherical Silver Nanoparticles Synthesized with Baicalin and Sodium Dodecyl Sulfate Using Response Surface Methodology

This study aimed to optimize the synthesis of silver nanoparticles mediated by Baicalin and sodium dodecyl sulfonate using Single factor experiment method and response surface methodology, to optimize their process parameters so as to prepare spherical silver nanoparticles with smaller particle size, and to compare the particle size of silver nanoparticles prepared under different conditions by comparing the wavelength of the maximum absorption peak measured by UV-visible spectrophotometer. Thus, the best synthesis program was determined. The average particle size of the silver nanoparticles prepared under the optimal conditions was 32.25 nm with high sphericity, and the silver nanoparticles synthesized under the optimized conditions were characterized by SEM, FTIR, particle size distribution and UV-vis.

Transmission enhancement effect of distributed Bragg reflector structure covered with metal grating

In order to observe the extraordinary optical transmission (EOT) through a metal gratings, induced by Tamm plasmon polaritons (TPPs), a layered structure consisting of a distributed Bragg reflector covered with a one-dimensional metal grating is proposed in this work. When an incident light wave passes through DBR regime and impinges on the DBR-metal interface normally, the generation of TPPs and the resulting highly localized energy on the metal-DBR interface are simulated in detail by the finite element method. As a result, the surface plasmon polariton (SPPs) modes accommodated inside the slits of metal gratings can be excited more effectively by the enhanced electromagnetic field associated with TPPs located on the interface. Furthermore, the enhanced transmission of incident light waves in the structure can be achieved when the SPP mode inside the grating slits satisfies the Fabry-Perot (FP)-like resonance condition, which reveals that the EOT in this structure comes from a TPPs-FP hybrid resonance. This inference can be confirmed by the relationships between the central wavelength and the grating height for the two transmission peaks, and the magnetic field modal profiles associated with the two peaks. Quantitative effects of the slit width and duty cycle on the transmission peak of the metal grating are analyzed numerically, and the results demonstrate that when the period is determined, as the slits width increases, the two peak transmittances first increase and then decrease. On the other hand, when the slit widths are chosen to be 40 nm, 50 nm, and 60 nm respectively, the peak transmittance first increases and then decreases with the duty cycle increasing. Meanwhile, it is found that the center wavelengths of the transmission peaks are related to the duty cycle in a nearly linear manner for three slit widths, which can be used to flexibly adjust the center wavelength of extraordinary optical transmission.

Electroluminescence efficiency and stability of near ultraviolet organic light-emitting diodes based on BCPO luminous materials

To date, in the traditional method of obtaining near-ultraviolet (NUV) light, mercury atoms, which can create a highly toxic heavy metal contaminant, have been used. Therefore, it is an important issue to obtain NUV light by using new environmentally friendly devices. In the last decade, the fabrication of near ultraviolet organic light-emitting diodes (NUV-OLEDs) has become a research hotspot in the field of organic electronics. However, when the electroluminescence wavelength is extended to shorter than 400 nm, higher requirements are put forward for the materials used for each functional layer in these devices. In this work, a wide bandgap small molecule material of BCPO is used as the luminescent layer. The electron-transporting and hole-transporting materials are determined based on the overlaps between absorption spectra of these materials and emission spectrum of BCPO. And NUV-OLEDs with electroluminescent peak wavelength at 384 nm are prepared. By using the optimal device structure, the maximum external quantum efficiency of the device reaches 2.98%, and the maximum radiance of the device reaches 38.2 mW/cm<sup>2</sup>. In the electroluminescence spectrum, NUV light with wavelengths below 400 nm accounts for 57% of the light emission. In addition, the device demonstrates good stability when biased at two different constant voltage modes. The multiple key factors which affect the stability of the device are analyzed in detail. Firstly, it is found that the high glass transition temperature (<i>T</i><sub>g</sub>) of hole-transporting material is very important for the long-time stability of this device. The poor device stability is closely related to the low <i>T</i><sub>g</sub> temperature of hole-transporting material. Secondly, due to the widespread use of PEDOT:PSS as hole injection material in OLEDs, the electron leakage from the hole-transpor layer into the PEDOT:PSS layer may cause significant damage to the conducting polymer. When bombarded with low energy electrons, bond breakage occurs on the surface of PEDOT:PSS, followed by the release of oxygen and sulfur, resulting in changes in conductivity and oxidation reactions with molecules of hole transport material. Thirdly, the photoelectrical stability of organic molecules is the most fundamental reason that restricts the device lifetime. The aging process of material or device is directly relevant to the bond dissociation energy (BDE) of organic molecule. Generally, the BDE value of organic molecule is not high enough. As a result, molecules are prone to chemical bond breakage during electrochemical or photochemical aging. In summary, highly stable NUV-OLEDs should be fabricated by using hole-transporting materials with high <i>T</i><sub>g</sub> temperature, sufficient electron-blocking capacity, and large BDE value.

A broadband near-infrared emission Na3Al2(PO4)2F3:Cr3+ phosphor exhibiting zero photoluminescence quenching at 398 K

A novel Cr3+-doped phosphor is synthesized towards zero thermal quenching and broadband emission for NIR applications.

Optimizing the energy level alignment for achieving record-breaking efficiency in hot exciton deep red OLEDs.

To effectively compete with the quenching process in long-wavelength regions like deep red (DR) and near-infrared (NIR), rapid radiative decay is urgently needed to address the challenges posed by the "energy gap law". Herein, we confirmed that it is crucial for hot exciton emitters to attain a narrow energy gap (ΔES1-T2) between the lowest singlet excited (S1) state and second triplet excited (T2) state, while ensuring that T2 slightly exceeds S1 in the energy level. Two proofs-of-concept of hot exciton DR emitters, namely αT-IPD and βT-IPD, were successfully designed and synthesized by coupling electron-acceptors N,N-diphenylnaphthalen-2-amine (αTPA) and N,N-diphenylnaphthalen-1-amine (βTPA) with an electron-withdrawing unit 5-(4-(tert-butyl) phenyl)-5H-pyrazino[2,3-b]indole-2,3-dicarbonitrile (IPD). Both emitters exhibited a narrow ΔES1-T2, with T2 being slightly higher than S1. Additionally, both emitters showed significantly large ΔET2-T1. Moreover, due to their aggregation-induced emission characteristics, J-aggregated packing modes, moderate strength intermolecular CN⋯H-C and C-H⋯π interactions, and unique, comparatively large center-to-center distances among trimers in the crystalline state, both αT-IPD and βT-IPD emitters exhibited remarkable photoluminescence quantum yields of 68.5% and 73.5%, respectively, in non-doped films. Remarkably, the corresponding non-doped DR-OLED based on βT-IPD achieved a maximum external quantum efficiency of 15.5% at an emission peak wavelength of 667 nm, representing the highest reported value for hot exciton DR-OLEDs.

Measurements of spectral daylight variation in spaces: A case study

This study investigates spectral daylight quality measured in a classroom within a time span of 30-minutes under clouded sky conditions. There is a characteristic difference between vertical and horizontal measured values, where the traditional vertical measurements tend to have greater peak wavelength irradiance reduction. The findings show the importance of considering multi-directional views when considering visual and non-visual light effects. The study indicates that spectral variations and spatiality should be considered when evaluating daylight, in contrast to the static methods commonly used to evaluated the quality of daylight in buildings. The study underscores the need for simulation tools and material databases to enhance the realism of spectral simulations and ultimately contribute to a more effective approach to indoor daylight quality, prioritizing occupant well-being.

Orange to red iridium(iii) complexes possessing good electron mobility with a pyrimidine-4-carboxylic acid ligand for high-performance solution-processed OLEDs with an EQE over 31%

Three rigid pyrimidine-4-carboxylic acid ligand-based iridium(iii) complexes are developed with good electron mobility and the highest solution-processed device efficiency.

Characterization of a new fluorescence lifetime imaging ophthalmoscope

Aims/Purpose: Fluorescence lifetime imaging ophthalmoscopy (FLIO) allows in vivo measurement of autofluorescence intensity decays of endogenous fluorophores in the ocular fundus. So far, only devices from Heidelberg Engineering based on the Spectralis system have been used in FLIO research. Here, we present and characterize a new FLIO device based on the RETImap system from Roland Consult.Methods: The device is based on a confocal scanning laser ophthalmoscope (35° field, 512 × 512 px). A ps diode laser (BDL‐SMN 473 nm, Becker & Hickl GmbH, Berlin, Germany) excites autofluorescence. The fluorescence photons are split into a short (498–560 nm, SSC) and a long (560–720 nm, LSC) spectral channel (one HPM‐100‐40 detector [Becker & Hickl GmbH] each) and are detected by time‐correlated single photon counting (SPC‐160, Becker & Hickl GmbH). We determined the maximum laser power (ILT2400, International Light Technologies, Inc. Peabody, MA, USA) and analysed the instrument's behaviour at three different laser power levels (150 μW, 200 μW and max.) in terms of laser spectrum (CAS140CT, Instrument Systems GmbH, Munich, Germany) and instrument response function (IRF). The IRF was determined using a 25 μM Eosin Y solution, mixed with a 5 M solution of potassium iodide, placed in a flat cuvette (110‐OS, Hellma GmbH & Co. KG, Müllheim, Germany) in front of the objective lens of the FLIO device. Fluorescence measurements of approximately 1‐min duration were performed three times for all three laser powers. The IRF and the full width at half maximum (FWHM) were calculated using FLIMX software (www.flimx.de).Results: The max. laser power was 280 μW. The peak wavelengths of the laser spectra were 467.6 (150 μW), 467.9 (200 μW) and 468.0 nm (280 μW). IRF FWHM in SSC were 298.6 ± 1.1 ps (150 μW), 341.0 ± 2.5 ps (200 μW) and 347.5 ± 6.0 ps (280 μW). In LSC, the IRF FWHM were 290.4 ± 3.8 ps (150 μW), 344.0 ± 3.4 ps (200 μW) and 358.8 ± 1.3 ps (280 μW). Results are mean ± standard deviation.Conclusions: A new fluorescence lifetime imaging ophthalmoscope has been characterized. The device offers a high laser power for fluorescence excitation, a large field of view, a high spatial resolution, and a sufficiently high time resolution. Thus, it is suitable for fluorescence lifetime studies.

Effects of different heat treatment media on odorous constituents, chemical decomposition and mechanical properties of two hardwoods.

With China's increasing dependence on foreign wood, African wood has gradually become a potential imported species, but its use is seriously affected by problems such as unpleasant odors. In this study, we investigate the effect of heat treatment medium on odor-causing VOCs, decomposition of structural polymers, Modulus of Rupture (MOR) and Modulus of Elasticity (MOE) of hardwood. Samples of "Afrormosia" and "Newtonia" wood were heated under air and palm oil for two hours at 160 °C, 180 °C, and 200 °C, respectively. Then, the nature of the odor of each VOC emitted by the wood before and after heat treatment was identified using the GCMS method. The decomposition of hemicelluloses, cellulose and lignin in wood samples was examined using a ThermoGravimetric Analyzer coupled to Fourier Transform InfraRed spectrometry (TGA-FTIR). The 3-point bending test was used to evaluate MOR and MOE. The results indicate that the main VOCs responsible for unpleasant smells are acetic acid and hexanal; the reduction in hexanal emissions after heat treatment is mainly due to the treatment temperature, while the reduction in acetic acid emissions depends on the heat treatment medium and is due to the chemical interactions between palm oil and acetic acid; thus, the heat treatment under palm oil reduces the percentage area of VOCs with unpleasant odors in Afrormosia and Newtonia wood better than the heat treatment under air. Based on TGA-3D FTIR analysis and mechanical results, the reduction in MOR is greater in heat treatment under air because the said treatment induces a greater loss of woody matter, which was characterized by higher H2O and CO emissions during heat treatment of wood under palm oil than during heat treatment of wood under air. On the other hand, palm oil more than air, promotes lignin deacetylation, which is characterized by the fact that the 1050 cm-1 wavelength peak was far higher in samples treated with palm oil than in those treated under air; and this might explain why heat treatment under palm oil reduces MOE more than heat treatment under air.

Multipole Excitation of Localized Plasmon Resonance in Asymmetrically Coated Core–Shell Nanoparticles Using Optical Vortices

AbstractPlasmonic interactions between an asymmetrically coated core–shell (ACCS) nanoparticle and an optical vortex produce a novel engagement of the spin angular momentum (SAM) and the orbital angular momentum (OAM) of the input. Simulations based on a discrete dipole approximation (DDA) indicate that the SAM and the OAM of the incident beam determine the modal order of resonance, correctly identifying the peak wavelength, and both the direction and magnitude of optical torque exerted upon the excited, localized plasmon resonance in the ACCS particle. These simulations also indicate higher‐order resonances, including hexapole and octupole modes, and a zero‐order resonance (expressible as a monopole mode), can be excited by judicious selection of the SAM and OAM. A detailed symmetry analysis shows how the multipoles associated with eigenmode excitations connect to the radiation multipoles at the heart of the multipole expansion. It is also shown how additional, distorted resonance modes due to the asymmetricity of the structure are also exhibited. These specific plasmonic characteristics, which cannot be realized by plane wave excitation, become possible through the ACCS asymmetry engaging with the distinct optical vortex nature of the excitation.

Improving the luminous efficiency of red nanocolumn μ-LEDs by reducing electrode size to ϕ2.2 μm

A red InGaN-based nanocolumn micro μLED with an emission diameter of ϕ2.2 μm was demonstrated to achieve an on-wafer external quantum efficiency (EQE) of 2.1% at the peak wavelength of 615 nm. The LED was fabricated by repeating the electrode process on the same nanocolumn pattern area and reducing the emission diameter from ϕ80 to ϕ2.2 μm. The peak EQE, which was maximized at ∼25 A cm−2, increased by decreasing the emission diameter from 1.2% to 2.1%. This behavior, which differs from that of InGaN-film LEDs, is characterized as a unit of independent nano-LEDs with passivated sidewalls of nanocolumn LEDs.

Test station to characterize the emission of a LiDAR.

A test station setup devised to measure the emission characteristics and beam propagation parameters of a light detection and ranging (LiDAR) system is presented. The main blocks of the station to measure the accessible emission, wavelength peak and FWHM, pulse duration, pulse repetition rate, horizontal and vertical angular resolution, field of view, beam propagation factor M 2, beam waist size, waist location, and divergence are described. The performance of this test station was demonstrated using a commercial spinning LiDAR, a Velodyne VLP-16, which successfully enables these measurements for a laser beam with a wavelength of 913nm.

High‐Efficiency Ultraviolet Electroluminescence from Multi‐Resonance Phosphine Oxide Polycyclic Aromatics

AbstractEfficient ultraviolet (UV) electroluminescent materials remain a great challenge, since short peak wavelength <400 nm and narrow full width at half maximum (FWHM) <50 nm are simultaneously required. In this sense, multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters featuring narrow‐band emissions hold the promise for UV applications. Herein, a novel MR‐TADF skeleton featuring carbazole‐phosphine oxide (P=O) fused aromatics is developed to construct the first two UV MR emitters named CzP2PO and tBCzP2PO. In addition to synergistic resonance effects of P=O and N atom, sp3‐hybrid P atom renders the curved polycyclic planes of CzP2PO and tBCzP2PO, giving rise to their narrowband UV emissions with peak wavelengths <390 nm and FWHM<35 nm. Besides configuration quasi‐planarization for radiation enhancement and quenching suppression, P=O moiety further enhances singlet‐triplet coupling to facilitate reverse intersystem crossing, resulting in the state‐of‐the‐art photoluminescence quantum yield of 62 % in tBCzP2PO doped films. As consequence, tBCzP2PO endowed its UV organic light‐emitting diodes with the peak at 382 nm and FWHM of 32 nm, and especially the record‐high external quantum efficiency (EQE) of 15.1 % among all kinds of UV devices. Our results demonstrate great potential of P=O based MR emitters in practical applications including optoelectronics, biology and medicine science.

High-Efficiency Ultraviolet Electroluminescence from Multi-Resonance Phosphine Oxide Polycyclic Aromatics.

Efficient ultraviolet (UV) electroluminescent materials remain a great challenge, since short peak wavelength <400 nm and narrow full width at half maximum (FWHM) <50 nm are simultaneously required. In this sense, multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters featuring narrow-band emissions hold the promise for UV applications. Herein, a novel MR-TADF skeleton featuring carbazole-phosphine oxide (P=O) fused aromatics is developed to construct the first two UV MR emitters named CzP2PO and tBCzP2PO. In addition to synergistic resonance effects of P=O and N atom, sp3 -hybrid P atom renders the curved polycyclic planes of CzP2PO and tBCzP2PO, giving rise to their narrowband UV emissions with peak wavelengths <390 nm and FWHM<35 nm. Besides configuration quasi-planarization for radiation enhancement and quenching suppression, P=O moiety further enhances singlet-triplet coupling to facilitate reverse intersystem crossing, resulting in the state-of-the-art photoluminescence quantum yield of 62 % in tBCzP2PO doped films. As consequence, tBCzP2PO endowed its UV organic light-emitting diodes with the peak at 382 nm and FWHM of 32 nm, and especially the record-high external quantum efficiency (EQE) of 15.1 % among all kinds of UV devices. Our results demonstrate great potential of P=O based MR emitters in practical applications including optoelectronics, biology and medicine science.

Preparation of Si nanoparticles from bamboo leaves and measurement of their photoluminescence and electroluminescence spectra

In this study, Si nanoparticles were prepared by the Mg reduction of SiO2 extracted from bamboo leaves, and their optical properties were investigated. Si nanoparticles were filtered under reduced pressure to separate the residue and filtrate specimens. Although RT photoluminescence (PL) was observed for both Si nanoparticle specimens, the PL intensity of the filtrate specimens was approximately seven times higher than that of the residue specimens. An electroluminescence (EL) device was fabricated using the Si nanoparticles of the filtrate as the active layer, and its EL properties were evaluated. With an applied voltage of 15 V, red luminescence was observed in the active layer. The Si nanoparticles from the bamboo leaves had relatively broad EL spectra with a peak wavelength approximately 700 nm, similar to that of the PL spectra. These results indicated that Si nanoparticles from bamboo leaves can be recycled for light-emitting devices.