Visible Light Emission Research Articles

Fabrication of Large-area, Planar, UV Lighting Devices Based on flat Cs3Sb Photocathode Emitters

Planar lighting devices are ideal for many applications. Requirements for planar lighting devices are high luminance and good luminance uniformity, together with low power consumption. In this article, a large-area, planar lighting device with high luminance, excellent luminance uniformity, and controllable luminance characteristic that was successfully demonstrated for visible light emission is discussed. The same device with sufficient UV intensity and tunable intensity for UV light emission is also discussed. The design concept involves a diode panel structure that utilizes photocathodes operated in a reflection mode with an external excitation light source. The excitation light source is in the blue-visible light range. Commercially available phosphors are used as light sources for both visible and UV emissions. The light emission characteristic of the devices is investigated by measuring the optical and the electrical parameters as functions of the applied anode voltage. The results show that the luminance is high for visible light emission and that the intensity is sufficient for UV light emission. In particular, UV emission exhibits a characteristic of cold-UV light with a near-monochromatic spectrum. The optical and the electrical properties of the large-area planar device are good enough for many industrial applications.

AIE-active shape-persistent macrocycles for the efficient discrimination of toluene

Efficient discrimination of toluene from volatile organic compounds is of great significance in the field of environmental protection. Herein, the novel AIE-active shape-persistent macrocycles (SPMs) were successfully synthesized by incorporating the propeller-shaped tetraphenylethylene (TPE) moieties into the framework. The macrocycle M1 displayed unique toluene-dependent fluorescence emission behavior, an emission peaking at 508 nm was observed in the absence of toluene, while first decreasing and then increasing fluorescence intensity along with blue-shift emission was witnessed with increasingly addition of toluene, accompanied by a visually discernible green-to-blue fluorescence color change. Furthermore, tunable visible-light emission including white-light was successfully achieved by the macrocycle M1 and rhodamine B two-component molecular system under toluene stimuli. The present system provided an efficient approach for discriminating toluene from volatile organic compounds.

Nanoscale mapping of ZnO–TiO2 up-conversion phosphor containing Yb3+ and Er3+

Up-conversion phosphors have attracted considerable attention for their visible-light emission. In this study, a ZnO–TiO2 up-conversion phosphor containing Yb3+ and Er3+ ions was prepared; its emission characteristics and crystal structure were analyzed, and its nanoscale elemental mapping was examined. The metal organic decomposition (MOD) method was used to fabricate the samples. After firing the sample at 1000 °C, the emission intensity showed a maximum when the molar ratio was Ti/Zn/Yb/Er = 1/1/0.06/0.02. Finally, the function of each element was considered from the viewpoint of the crystal structure and nanoscale mapping.

Hydrogen non-equilibrium ionisation effects in coronal mass ejections

Context. A new generation of coronagraphs used to study solar wind and coronal mass ejections (CMEs) are being developed and launched. These coronagraphs will heavily rely on multi-channel observations where visible light (VL) and UV-EUV (ultraviolet-extreme ultraviolet) observations provide new plasma diagnostics. One of these instruments, Metis on board ESA-Solar Orbiter, will simultaneously observe VL and the UV Lyman-α line. The number of neutral hydrogen atoms (a small fraction of coronal protons) is a key parameter for deriving plasma properties, such as the temperature from the observed Lyman-α line intensity. However, these measurements are significantly affected if non-equilibrium ionisation effects occur, which can be relevant during CMEs. Aims. The aim of this work is to determine if non-equilibrium ionisation effects are relevant in CMEs and, in particular, when and in which regions of the CME plasma ionisation equilibrium can be assumed for data analysis. Methods. We used a magneto-hydrodynamic (MHD) simulation of a magnetic flux rope ejection to generate a CME. From this, we then reconstructed the ionisation state of hydrogen atoms in the CME by evaluating both the advection of neutral and ionised hydrogen atoms and the ionisation and recombination rates in the MHD simulation. Results. We find that the equilibrium ionisation assumption mostly holds in the core of the CME, which is represented by a magnetic flux rope. In contrast, non-equilibrium ionisation effects are significant at the CME front, where we find about 100 times more neutral hydrogen atoms than prescribed by ionisation equilibrium conditions. We find this to be the case even if this neutral hydrogen excess might be difficult to identify due to projection effects. Conclusions. This work provides key information for the development of a new generation of diagnostic techniques that aim to combine visible light and Lyman-α line emissions. The results show that non-equilibrium ionisation effects need to be considered when we analyse CME fronts. Incorrectly assuming equilibrium ionisation in these regions would lead to a systematic underestimate of plasma temperatures.

YVO4 cascaded Raman laser for five-visible-wavelength switchable emission.

We report switchable emission of visible light at five wavelengths generated in a $\text{YVO}_4$YVO4 cascaded Raman cavity driven by an acousto-optic ${Q}$Q-switched Nd:YAP laser at 1080 nm. The second-harmonic generation and sum-frequency generation of the fundamental, first-Stokes, and second-Stokes waves were realized based on the angle-tuned BBO crystal. The phase-match angle tuning span of a BBO crystal is only approximately 2.3° to allow the frequency mixing to produce five visible wavelengths on demand. At an absorbed pump power of 5.96 W and a pulse repetition frequency of 10 kHz, average output powers of 800, 340, 460, 190, and 326 mW were obtained for 539.9, 567.2, 597.4, 631.0, and 668.5 nm lasers, respectively. The results show that the cascaded Raman laser system with an angle-tuned BBO crystal can provide a convenient and efficient method to achieve wavelength-switchable visible emission for multi-wavelength laser applications.

Low-cost radiative cooling blade coating with ultrahigh visible light transmittance and emission within an “atmospheric window”

Radiative cooling is a passive means of cooling that does not require the consumption of additional energy and has broad application potential. While experimental spectral measurements have been conducted, the daylighting of buildings, photovoltaic (PV) cells, and vehicles has seldom been considered. Here, we use spectrum-selective coatings to resolve the competing demands of daylighting (or appearance) and radiative cooling. A simple liquid blade-coating method that was inexpensive, convenient, and scalable was used to achieve radiative cooling with ultrahigh transmittance in visible light and excellent cooling performance. Resonant polar dielectric SiO2 microparticles with optimized volumetric fractions and diameters were randomly mixed into an acrylic resin and manually bladed at room temperature with natural evaporation drying. Spectrometry results showed that a mean visible light transmittance >91.3% was achieved, which was much higher than reported in previous studies; a mean emittance >93.7% within the “atmospheric window” was also reached. Over four days testing of a silver-plated aluminum sheet in a populous area at sea level, we achieved a maximum sub-ambient decrease in temperature of ~8.7 °C and a maximum cooling power of 108.49 W/m2.

Solution-processed upconversion photodetectors based on quantum dots

Upconversion photodetectors convert photons from the infrared to the visible light spectrum and are of use in applications such as infrared detection and imaging. High-performance upconversion devices are, however, typically based on vacuum-deposited materials, which are expensive and require high operating voltages, which limits their implementation in flexible systems. Here we report solution-processed optical upconversion photodetectors with a high photon-to-photon conversion efficiency of 6.5% and a low turn-on voltage of 2.5 V. Our devices consist of a colloidal lead sulfide quantum dot layer for harvesting infrared light that is monolithically coupled to a cadmium selenide/zinc selenide quantum dot layer for visible-light emission. We optimized the charge-extraction layers in these devices by incorporating silver nanoparticles into the electron transport layers to enable carrier tunnelling. Our photodetectors exhibit a low dark current, high detectivity (6.4 × 1012 Jones) and millisecond response time, and are compatible with flexible substrates. We also show that the devices can be used for in vitro bioimaging. By embedding silver nanoparticles in the electron transport layer, solution-processed quantum-dot-based photodetectors with a high photon-to-electron conversion efficiency of 6.5% and a low turn-on voltage of 2.5 V can be created for use in infrared imaging applications.

Diagnostics of ion and electron diffusion in pulsed plasma thrusters using neutral gas injection

Pulsed plasma thrusters (PPTs) have been widely used in space flight applications due to their low cost and robustness. Recently, it has been observed that during the operation of a PPT, the plasma plume exhibits an in-plume acceleration phenomenon where the leading-edge ions continue to accelerate in the plume even in the absence of electromagnetic forces. The physical mechanism behind the phenomenon may be ambipolar diffusion in nature. However, details regarding the propagation of electrons within the plume are still unknown. To better understand the complete plasma diffusion process, it is important to understand the behavior of both ions and electrons. To investigate this mechanism, a diagnostic method using neutral gas injection is proposed to estimate the velocity of fast electrons within the plasma plume. This is a new method where neutral gas is injected at the plasma plume edge to increase the local visible light emission and ionization rate of electrons. In addition to this, a triple Langmuir probe, bandpass filter, and spectrometer were also used to study the plume characteristics. From the experimental results, we estimate that the exhaust velocity of the electrons is over 200 km s−1, roughly an order of magnitude higher than the ion velocity (26 km s−1). The acceleration of ions and the corresponding deceleration of electrons with downstream distance was observed using time-of-flight probe data. Furthermore, the spectral data was also used to identify electron deceleration within the plume. These results further suggest that ambipolar diffusion of electrons and ions may exist in the PPT plume, and that the in-plume acceleration of ions is due to the ambipolar electric field in the plume. Another implication is that depending on the length scales, the plasma plume of the PPT can be non-quasineutrally distributed during the initial period of plume formation.

Lanthanide-Based Nanocomposites for Photothermal Therapy under Near-Infrared Laser: Relationship between Light and Heat, Biostability, and Reaction Temperature.

In this research, typical organic/inorganic photothermal therapy (PTT) agents were designed with a combination of upconversion luminescent (UCL) or near-infrared (NIR) II imaging rare-earth nanomaterials for photo-acoustic (PA)/UCL/NIR II imaging-guided PTT under NIR laser irradiation. The results show the following: (1) The PTT effect mainly comes from NIR absorption and partly from UCL light conversion. (2) Visible UCL emission is mainly quenched by NIR absorption of the coated PTT agent and partly quenched by visible absorption, indicating that excitation may play a more important role than in the UCL emission process. (3) The biostability of the composite might be decided by the synthesis reaction temperature. Among the five inorganic/organic nanocomposites, UCNP@MnO2 is the most suitable candidate for cancer diagnosis and treatment because of its stimuli-response ability to the micro-acid environment of tumor cells and highest biostability. The composites generate heat for PTT after entering the tumor cells, and then, the visible light emission gradually regains as MnO2 is reduced to colorless Mn2+ ions, thereby illuminating the cancer cells after the therapy.

Light-Driven Halide Exchange Facilitates Complete Crystal Transformation in Nanostructured Perovskites.

Colloidal perovskite nanocrystals (NCs) have shown promise in visible and near-infrared light emission compared with conventional quantum dots because of their sharp and highly efficient emission. Light-driven halide exchange offers high degrees of tunability of perovskite NC composition and photoluminescence properties. A remaining challenge is the incomplete change between bromide and chloride due to the presence of radical quenchers. Herein, an optimized in situ halide exchange method of perovskite NCs is reported. Chloroform is used as the solvent and the precursor to effectively generate halide radicals and ions under ultraviolet light irradiation in an inert atmosphere. This strategy achieves complete crystal transformation in colloidal perovskite NCs such as CsPbBr3-to-CsPbCl3 and Cs4PbBr6-to-CsPbCl3. The exchange reaction rate can be further tuned by the presence of B-site dopants such as Zn2+ and Ce3+. The dopant cations are partially remained in the exchanged perovskite NCs, indicating the predominance of thermodynamic control in the crystal transformation.

White emissive europium complex with CRI 95%: butterfly vs. triangle structure

The spectral window of dual visible light emissions of the molecular europium complex that leads to white light can be modulated by design strategy.

Effects of different polymers and solvents on crystallization of the $$\hbox {NaYF}_{4}$$:Yb/Er phase

Up-converting $$\hbox {NaYF}_{4}$$ :Yb,Er nanoparticles were obtained by polymer-assisted solvothermal synthesis using a common solution of hydrated RE nitrates in ethanol or ethylene glycol. It was shown that polymer choice (polyacrylic acid—PAA, polyvinylpyrrolidone—PVP and chitosan—CS) controls the size and shape of $$\hbox {NaYF}_{4}$$ :Yb,Er nanoparticles, while the solvent type and pH value affect their crystallinity. Consequently, the spherical nanoparticles of a cubic ( $$\upalpha $$ ) phase, the average size of which ranged from 60 to 140 nm, were obtained either when PVP/ethanol or PVP/ethylene glycol were used solely during synthesis, whereas NaOH addition induced hexagonal ( $$\upbeta $$ ) phase nucleation. The formation of the hierarchically organized spherical aggregates and nanofoils was observed when CS and PAA were used during synthesis, respectively. The average crystallite size, microstrain, doping level, lattice parameters, as well as, the presence of the certain ligands on the particle surface were determined and correlated with the intensity of visible-light emission observed under 980 nm laser-diode excitation.

Controlling spontaneous emission via electronic correlations and temperature in transparent oxides

We study the spontaneous emission of agglomerates of two-level quantum emitters embedded in a correlated transparent metal. The characteristic emission energy corresponds to the splitting between ground and excited states of a neutral, nonmagnetic molecular impurity (F color center), while correlations are due to the existence of narrow bands in the metal. This is the case of transition metal oxides with an ABO3 Perovskite structure, such as SrVO3 and CaVO3, where oxygen vacancies are responsible for the emission of visible light, while the correlated metallic nature arises from the partial filling of a band with mostly d-orbital character. For these systems we put forward an interdisciplinary, tunable mechanism to control light emission governed by electronic correlations. We show that not only there exists a critical value for the correlation strength above which the metal becomes transparent in the visible, but also that strong correlations can lead to a remarkable enhancement of the light-matter coupling. By unveiling the role of electronic correlations in spontaneous emission, our findings set the basis for the design of controllable, solid-state, single-photon sources in correlated transparent metals.

Optical characterization of type-II ZnO/ZnS multiple quantum wells grown by atomic layer deposition

ZnO/ZnS multiple-quantum wells (MQWs) were grown on Si substrates by atomic layer deposition, and their optical properties were investigated. Light emission well below the band-to-band transition energies of constituent materials was observed, which was attributed to the type-II band alignment of ZnO/ZnS MQWs. The type-II transition showed a significant blue shift with increasing excitation power, which originated from modified band bending caused by spatial separation of electrons and holes as electrons are confined in the ZnO conduction band quantum well and holes are confined in the ZnS valence band quantum well. The conduction band offset of the ZnO/ZnS heterojunction was determined to be 1.58 eV simulated from the type-II emission of the MQWs with controlled layer thicknesses. The type-II transition provides an important opportunity to utilize visible light emission and absorption for various optical and energy harvesting devices using the ZnO/ZnS material system.

Transmission electron microscope imaging of plasma grown electroformed silicon nitride-based light emitting diode for direct examination of nanocrystallization

We report for the first time a direct transmission electron microscope (TEM) imaging of a cross-section of a silicon nitride-based light emitting diode (LED), produced via a method patented by our research group. Grown by plasma enhanced chemical vapor deposition (PECVD) technique the LED structure (glass/Cr/p+-nc-Si:H/i-SiNx:H/n+-nc-Si:H/ITO) was then subjected to a high forward voltage stress for one time only, i.e. electroforming process. After electroforming the LED exhibited a boosted visible light emission and memory effect. To study the structural effect of the electroforming on the as-deposited LED the cross-section was extracted by focused ion beam (FIB) technique directly from the electroformed diode and thus prepared for TEM imaging. Since the electroforming process caused crystallization of ITO and its breakup in some parts of the diode surface, the FIB was conducted for the cross-section containing some regions with ITO layer and some without ITO. TEM examination revealed the nanocrystalline phase formation within the intrinsic layer (i-SiNx:H) caused by the electroforming process. The average size and distribution of Si nanocrystallites formed inside i-SiNx:H was determined. The Si nanocrystallization within i-SiNx:H was compared for the regions with and without ITO layer. The previously proposed model describing the changes taken place in the diode during electroforming process was reconsidered in the light of this TEM analysis.

Investigation into the surface chemical analysis and luminescent properties of La7O6(BO3)(PO4)2:Dy phosphor

The exploitation of high-quality borophosphate-based phosphor products is currently garnering a significant research focus in solid-state light source. The interest is due to its low-cost and environment-friendly synthesis. In this paper, a new La7O6(BO3)(PO4)2 (LBP):Dy phosphor was developed via sintering at 1673 K for 3 h under an air atmosphere. Structural characterization indicates that all of the prepared products are the monoclinic phases with the space group P21/n. A comparison of the surface chemical states of the elemental compositions for the LBP host versus optimal LBP:13%Dy product was conducted via X-ray photoelectron spectroscopy. Optical characterization shows that the Dy-doped products exhibited visible light emissions of Dy3+ from 4F9/2 to 6H15/2, 6H13/2, and 6H11/2 upon exposure to 348 nm near-ultra violet irradiation. The dominant emission (yellow, 576 nm) belongs to the 4F9/2 → 6H13/2 transition. The best Dy content for doping was 13 mol%, and content quenching occurred due to dipole – dipole interaction. The measured decay lifetime and calculated chromatic properties together manifest that the optimal LBP:13%Dy phosphor shows potential as a yellow ingredient in solid-state lighting.

High-performance all-solution-processed quantum dot near-infrared-to-visible upconversion devices for harvesting photogenerated electrons

All-solution-processed quantum dot (QD) upconversion devices were realized by constructing a colloidal PbS QD near-infrared (NIR ∼ 1 μm)-sensitive layer on the cathode side of a red CdSe quantum dot light-emitting diode (QLED). Different from conventional upconversion devices, we utilized the photogenerated electrons rather than the holes created in the NIR-sensitive layer to inject into the QLED for visible light emission. Consequently, we obtained high-performance devices with a maximum photon-to-photon conversion efficiency of 3.35% and a luminance on-off ratio of approximately 8 × 103. The results indicate that the device structure could be used to realize low-cost and portable high-performance QD-based upconversion devices via solution-processing.

A near-infrared fluorescent probe for the ratiometric detection and living cell imaging of β-galactosidase.

β-Galactosidase (β-gal) has captured the attention of biologists, chemists, and medical researchers as an important biomarker for cell senescence and primary ovarian cancer. Therefore, many fluorescent probes with visible light emission have been developed for the detection and imaging of β-gal in living cells. However, near-infrared (NIR) ratiometric probes are more suitable for bioimaging because near-infrared light can effectively avoid the interference of autofluorescence and the ratiometric approach can improve sensitivity and accuracy of the detection. In this work, we designed an NIR ratiometric probe (TMG) for the highly sensitive detection of β-gal. Using a spontaneous degradation mechanism based on the ICT effect, the change in ratio (F650/F580) exhibited a prominent β-gal-dependent performance and proved a strong linear response to the activity of β-gal at an enzyme concentration between 0 and 200UL-1, with a limit of detection as low as 0.86UL-1, and the response speed is much faster than the same type of probes previously reported. The probe also revealed an excellent biocompatibility and a large Stokes shift. Moreover, fluorescence microscopy imaging experiments confirmed that this probe could be successfully used for the detection of endogenous β-gal in living cells. Graphical abstract.

Microscopic origin of multiple exciton emission in low-dimensional lead halide perovskites.

Low-dimensional halide perovskites exhibit intriguing excitonic properties and emerge as an important class of self-activated luminescent materials. However, the ability to manipulate and optimize their luminescent properties is limited by the lack of the microscopic understanding of the exciton relaxation and emission and the inconsistency in the theoretical results in the literature. In this work, based on first-principles calculations, we studied excitons in 1D lead halide perovskites, C4N2H14PbBr4 and C4N2H14PbCl4, which are both bright visible-light emitters. We find that, in both compounds, the polaron-pair exciton (EX-PP) is more stable than the onsite exciton (EX-OS) and only the EX-PP emission energy from the calculation is close to the main photoluminescence (PL) peak observed in the experiment. The EX-OS is found to emit UV light in both compounds. Therefore, the EX-PP is responsible for the experimentally observed visible light emission in both C4N2H14PbBr4 and C4N2H14PbCl4. Furthermore, the calculated small energy difference between the EX-PP and EX-OS in C4N2H14PbBr4 suggests that the metastable EX-OS can be thermally populated at room temperature (RT); the calculated EX-OS emission energy agrees well with the energy of a minor PL peak observed at RT but not at 77 K. The validity our approach in the exciton calculation is supported by the benchmark of the calculated exciton emission energies against the experimental results in 13 0D and 1D metal halides. The discrepancies between this work and a recent theoretical study in the literature are also discussed.

Study on hydrothermal synthesis and photoelectric properties of nano-ZnO

The Nano-ZnO material was synthesized by using the hydrothermal method. The structure, morphology and optical properties of Nano-ZnO material was studied by changing the concentration ratio of Zn(CH3COO)2•2H2O and NaOH. The experimental results showed that, as the concentration ratio of Zn2+ and OH− decreased gradually, the crystallinity of Nano-ZnO sample would increase gradually. The morphology of the sample firstly changed from sheet to rod-like, and finally changed from rod-like to sheet. It indicated that the concentration ratio of Zn2+ and OH− was the main factor determining the morphology of Nano-ZnO. The optical property test results showed that, there appeared a distinct ultraviolet emission peak of Nano-ZnO near 380 nm; there appeared an extremely strong visible light emission band in the wavelength range of 450–650 nm; the luminescence intensity of Nano-ZnO would significantly increase with a decrease in the concentration ratio of Zn2+ and OH−; both the ultraviolet emission peak and the visible light emission band were blue-shifted.