ZnS Phosphors Research Articles

Multicolour stretchable perovskite electroluminescent devices for user-interactive displays

Wearable displays require mechanical deformability to conform to the skin, as well as long-term stability, multicolour emission and sufficient brightness to enable practically useful applications. However, endowing a single device with all the features remains a challenge. Here we present a rational material design strategy and simple device-manufacturing process for skin-conformable perovskite-based alternating-current electroluminescent (PeACEL) devices. These devices exhibit a narrow emission bandwidth (full-width at half-maximum, <37 nm), continuously tuneable emission wavelength (468–694 nm), high stretchability (400%) and adequate luminance (>200 cd m−2). The approach leverages a new class of perovskite zinc sulfide (PeZS) phosphors, consisting of ZnS phosphors coated with perovskite nanoparticles for electrical excitation via total intraparticle energy transfer. This strategy results in pure red and green emissions and expands the colour gamut of powder-based ACEL devices by 250%. Moreover, our processing technique facilitates the integration of PeACEL displays with wearable electronics, enabling applications in dynamic interactive displays and visual real-time temperature monitoring. These PeACEL displays offer new routes in flexible electronics and hold potential for the development of efficient artificial skins, robotics and biomedical monitoring devices.

Optical non-linearities and applications of ZnS phosphors

Optical non-linearities play a crucial role in enabling efficient and ultrafast switching applications that are essential for next-generation photonic devices. ZnS phosphor material produces the best results in terms of increased luminescence quantum yield when doped with certain impurities. Nevertheless, the investigation of the third-order non-linear optical susceptibility of the phosphor materials can be exploited for various switching applications. In this regard, we review the recent advancements in the investigation of non-linear optical properties of ZnS phosphors, where the knowledge of absorption and refraction is utilized in various optical and detector applications. Furthermore, the review highlights strategies employed to enhance the non-linear optical response of phosphor materials as well as a general discussion of an attosecond optical switching scheme which can be used to fabricate devices with petahertz speeds. Consequently, we provide a solution to the unsolved problem of the significant extension of optical limiting applications to switching applications by developing design strategies to manipulate conventional ZnS phosphor material. The potential challenges and future prospects of utilizing phosphor materials for switching applications are also addressed. The strategies for manipulating ZnS phosphor can be generalized for a broad range of other materials by minimizing linear and non-linear losses, while enhancing the values of the non-linear refractive index coefficient. We propose that the figure-of-merit of ZnS material can be enhanced by using a suitable combination of pump and probe wavelength values, which can be useful for optical switching applications.

Study of structural, luminescence and Judd-Ofelt calculations for Ce3+ doped ZnS phosphors for display applications

This paper discusses the luminescence properties of novel Zn(1-x)S: xCe3+ (x = 0, 0.01, 0.02, 0.03, 0.1, 0.2 and 0.3) prepared via conventional solid-state reaction method at low temperature. The XRD results revealed that all synthesized phosphors have hexagonal structure. The strain was calculated using size-strain plot (SSP) method. The FTIR study identifies the various vibrational modes present in the synthesized phosphors. The structural and luminescence characteristics of the prepared samples were enhanced by Ce3+ substitution. The optical band gap energies were found to be decreasing with Ce3+ substitution. The optimal Ce3+ content for superior blue emission is found to be x = 10 mol %. The blue emission supports the incorporation of Ce3+ ions within the ZnS lattice, which is again confirmed by Ω2, Judd-Ofelt intensity parameter. The high value of Ω2 indicates the symmetry of site occupies of Ce3+ ions are lower. The obtained critical distance of energy transfer between Ce3+ ions is calculated to be 6.3095 Å, which implies that the exchange interaction mechanism is a multipolar exchange. The high efficiency, branching ratio, stimulated cross section and the optical parameters indicate that the prepared phosphor materials have efficient blue light emission for display applications.

ZnS:Er3+/Yb3+ phosphor for optical temperature sensing and dual-mode anti-counterfeiting

Optical temperature sensors have become a research hotspot in recent years due to their non-contact feature. In this study, Er3+/Yb3+ co-doped ZnS phosphors (1:3, 1:6, 1:9, 1:12, 1:15, 1:19) were prepared by co-precipitation method. The up-conversion and down-conversion luminescence performance of the phosphors was studied. It was found that the luminescence performance is optimized when the doping molar ratio is 1:9. Based on the fluorescence intensity ratio technology of non-thermal coupling energy level, the temperature sensing characteristics of ZnS:Er3+/Yb3+ (1:9) phosphors were studied in the temperature range of 313–533 K. The results show a maximum absolute sensitivity of 1.9 % K−1 and a maximum relative sensitivity of 2.7 % K−1. In terms of multifunctionality, this work also designed a flexible film based on thermoplastic polyurethane (TPU) to explore its anti-counterfeiting properties. The results of this work demonstrate the great potential of ZnS:Er3+/Yb3+ phosphors for optical temperature measurements and anti-counterfeiting.

Effect of Sm3+ ions doping on the structural and optical properties, Judd-Ofelt and radiative parameters of ZnS phosphor materials

Undoped ZnS and Sm3+ doped with varying concentration of ZnS phosphor materials were synthesized via low temperature solid state reaction method. The samples were investigated for the influence of different concentrations of Sm3+ ions on their structural and spectroscopic properties. The presence of various absorption bands in UV–Vis–NIR spectra revealed the transition from ground state to different excited states of Sm3+ ions. The Judd-Ofelt (JO) analyses were evaluated to identify the structure and the bonding around the Sm3+ ions. The high value of Ω2 intensity parameter reveals that the site symmetry is lower. The parameters such as radiative transition probabilities, radiative lifetime, branching ratio, stimulated emission cross-sections and the optical gain parameters are also evaluated. The PL spectra shows five high intensity emission bands centered at 567 nm, 590 nm, 619 nm, 641 nm and 667 nm corresponding to 4G5/2→6Hj (j = 5/2, 7/2,9/2, 11/2, 13/2) transitions and showed optimum for concentration at x = 0.03 in Zn(1-x)S. The quantum yield of the prepared optimal phosphor using integrating sphere method was found to be 10.08%. This indicates that the materials synthesized may be favorable for the applications of laser and other optical devices.

Evaluation of structural, optical, and scintillation characteristics of Ag activated ZnS nanoparticles

The ZnS nanoparticles activated with various concentrations of Ag and co-activated with Cl ions were synthesized hydrothermally and sintered under controlled environment at 960 °C. The transition from cubic to hexagonal phase and presence of some fraction of ZnO were detected with powders X-rays diffraction (PXRD) and confirmed by energy dispersive X-rays (EDX) analysis as well. An increase in particle sized from ∼450 nm to ∼700 nm was observed with increase in Ag doping concentration in the synthesized ZnS nanoparticles. The optical band gap was found lower for the synthesized ZnS nanoparticles as compared to the reported optical band gap for ZnS, which can be assigned to defect states. Emission bands peaking at ∼2.68 eV and 2.35 eV were observed due to the characteristics ZnS, and ZnO luminescence, respectively. A significant improvement in the emission intensity by about 2.3 orders of magnitude with increase in Ag doping concentration were observed as compared to pristine ZnS nanoparticles. For the samples having higher concentrations of O atoms the quenching in luminescence intensity were found which can be assigned to the trapping of charge carriers by O defects states. The scintillation light yield enhanced with increase in Ag doping concentration and showed similar quenching behavior as observed for photoluminescence. A faster decay time response with suppression of afterglow were observed for ZnS nanoparticles as compared to commercial micron size ZnS phosphors.

Performance of phosphor embedded plastic scintillators through thin film coating

A novel method of loading phosphor materials in a plastic scintillator (here on PS) is developed to improve light output. In the present work, phosphor materials ZnS(Ag) and ZnO are loaded into the PS medium through the thin-film coating, and the effect of these phosphors on radiation detection is investigated. PS are prepared using polystyrene as the base, xylene as the solvent, PPO and POPOP as primary and secondary flours. ZnS(Ag) and ZnO phosphors are added to the PS in powder form to investigate their effect on luminescence and scintillation output. The thickness of the prepared scintillators is ∼250 ± 30 µm. Photoluminescence, radioluminescence, luminescence lifetime and their detection efficiencies for the typical alpha, beta, gamma and neutron sources are selected as the tools for investigation, and the synthesized scintillators’ performance is studied. The results obtained showed a notable increase in the performance and the outcomes are compared with commercially available PS.

CaS: Sb3+, Na and ZnS:Sn2+: phosphor materials for the improvement of WLEDs

The remote phosphor configuration, though not the best choice for WLED color quality, is superior to the luminous flux of LEDs compared to the conformal or in-cup structures. Realizing the capability of the remote phosphor design, several studies have been done to overcome the drawback of this structure, which is the color quality. A two-film distant phosphor configuration with improved color rendering index (CRI) and color quality ratio (CQS) for WLEDs is offered in this study paper. The hue temperature of the WLEDs packets employed in this research is 8500 K. This phosphor structure is constructed by layering of a green CaS:Sb3+,Na or red ZnS:Sn2+ phosphor above the yellow YAG:Ce3+ phosphor. The additional phosphor ZnS:Sn2+ concentration will then be modified to get the best color quality. The results demonstrated an increase in CRI and CQS along with the presence of ZnS:Sn2+, indicating that the existence of ZnS:Sn2+ influences significantly on these two aspects. As a result of the increased concentration of red illumination elements inside packets, the higher the concentration of ZnS:Sn2+, the higher the CRI and CQS. The green phosphor CaS:Sb3+,Na, in the meantime, improves the luminous flux.

Flexible Color‐Tunable Electroluminescent Devices by Designing Dielectric‐Distinguishing Double‐Stacked Emissive Layers

AbstractIntrinsically flexible alternating current electroluminescent devices have sparked widespread research interest due to their tremendous potential in bioinspired electronics, smart wearables, and human–machine interfaces. In these applications, it is highly desirable to possess real‐time color tunability but this has not been reported yet. Herein, a flexible color‐tunable electroluminescent device composed of simple double‐stacked emissive layers of ZnS phosphors/dielectric polymer composite is developed by an all‐solution processable method. The color tuning capability can be attributed to the dielectric difference between the two emissive layers, leading to the color combination between two independent emissions originating from different ZnS phosphors at varied electric fields. With the rational selection of the dielectric polymer matrices, a wide‐range color tuning from orange to white and to blue can be realized in a single device just by varying the electric field. It also exhibits high mechanical robustness with well‐maintained performance even after 1000 cycles of bending. Similar natural functionalities like camouflage and visual communication can be further reproduced in the artificial color‐tunable system, thereby opening a general and effective avenue for developing smart wearables and soft electronics.

Coaxial‐Structured Weavable and Wearable Electroluminescent Fibers

AbstractFlexible, lightweight, and wearable devices are currently attracting tremendous interest in the field of advanced electronics. In this work, novel 1D, flexible, coaxial‐structured, bright and colorful alternating current electroluminescent (ACEL) fibers consisting of AgNW‐based electrodes, a ZnS phosphor layer, and silicone dielectric and encapsulation layers are designed and fabricated through a simple protocol. This facile all‐solution‐based fabrication protocol enables scalable production of long ACEL fibers (&gt;12 cm). Stemming from the rational design and facile fabrication process, the as‐prepared ACEL fibers exhibit uniform, bright, and angularly independent luminance (up to 202 cd m−2 @ 195 V and 2 kHz). Benefiting mainly from the robust AgNW‐based electrodes and versatile silicone, the ACEL fibers exhibit additionally excellent flexibility and mechanical stability, being capable to maintain ≈91% of luminance after 500 bending‐recovery cycles, as well as mitigated luminance degradation after continuous work in ambient. The proper length combined with superb mechanical properties makes the ACEL fibers readily weavable. Most notably, the isolating, hydrophobic, and biocompatible silicone encapsulation layer endows the ACEL fibers unprecedented wearability. Eventually, a proof‐of‐concept ACEL fabric is demonstrated by weaving the as‐prepared long ACEL fibers, to show the future perspective of directly weaving ACEL fibers into densely arrayed wearable functional cloths.

Temporal and Remote Tuning of Piezophotonic-Effect-Induced Luminescence and Color Gamut via Modulating Magnetic Field.

Light-emitting materials have been extensively investigated because of their widespread applications in solid-state lighting, displays, sensors, and bioimaging. In these applications, it is highly desirable to achieve tunable luminescence in terms of luminescent intensity and wavelength. Here, a convenient physical approach of temporal and remote tuning of light-emitting wavelength and color is demonstrated, which is greatly different from conventional methods. It is shown that by modulating the frequency of magnetic-field excitation at room temperature, luminescence from the flexible composites of ZnS:Al, Cu phosphors induced by the piezophotonic effect can be tuned in real time and in situ. The mechanistic investigation suggests that the observed tunable piezophotonic emission is ascribed to the tilting band structure of the ZnS phosphor induced by magnetostrictive strain under a high frequency of magnetic-field excitation. Furthermore, some proof-of concept devices, including red-green-blue full-color displays and tunable white-light sources are demonstrated simply by frequency modulation. A new understanding of the fundamentals of both luminescence and magnetic-optics coupling is thus provided, while offering opportunities in magnetic-optical sensing, piezophotonics, energy harvesting, novel light sources, and displays.

Drastic enhancement of blue-to-orange color conversion efficiency in heavily-doped ZnS:Mn2+ phosphor and its application in white LEDs

Heavily-doped ZnS:Mn2+ phosphors showed an extremely enhanced photoluminescence excitation intensity at 464nm peak wavelength originated from 6A1(6S)→4A1(4G) forbidden transition of Mn2+ ions. The best ZnS: Mn2+ phosphors showed the intensive orange color emission of 584nm peak wavelength from 4T1(4G) → 6A1(6S) forbidden transitions with an excellent lumen maintenance of 97% at 200°C, and their quantum efficiency of 75% under the 464nm excitation wavelength. The photoluminescence enhancement was explained in terms of the relaxation of selection rule on the forbidden intra-transitions of Mn2+ ion. It was caused by the spin-spin and the electron-phonon interactions supported by electron spin resonance signal and Raman spectroscopy, respectively. The excellent lumen maintenance was explained in terms of resonance-assistant direct intra-transition caused by the coupling between an excited electron (e*) on the 4A1(4G) state and multi-phonon energy. To evaluate its feasibility for white light-emitting diodes, the optimized ZnS:Mn2+ phosphor and its mixture with a green Lu3Al5O12:Ce3+ phosphor were applied to a 460nm blue-light-emitting diode. It showed high luminous efficiency of 68lm/W, suggesting a possibility on the application of Mn-doped ZnS phosphor for pc-WLED.

Synthesis and influence of ultrasonic treatment on luminescence of Mn incorporated ZnS nanoparticles

Manganese (Mn) doping of ZnS phosphors was achieved by precipitation method using different ultrasound (US) maturation times. The structural and luminescence properties of the samples were carried out by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), photoluminescence (PL), and cathodoluminescence (CL). The real amount of manganese incorporated in ZnS lattice was calculated based on ICP-OES results. According with XRD patterns, the phase structure of ZnS:Mn samples is cubic. EDS spectra reveal deviations of the Mn dopant concentration from the target composition. Both 300 K PL and CL emission spectra of the Mn doped ZnS phosphors display intense orange emission at 590 and 600 nm, respectively, which is characteristic emission of Mn ion corresponding to a 4T1→6A1 transition. Both PL and CL spectra confirmed manganese is substitutionally incorporated into the ZnS host as Mn2+. However, it is suggested that the origin of broad blue emission around 400 nm appeared in CL is due to the radiative recombination at deep level defect states in the ZnS. The ultrasound treatment at first enhances the luminescent intensity by ∼3 times in comparison with samples prepared by classical way. This study gives rise to an optimization guideline, which is extremely demanded for the development of new luminescent materials.

A novel segmented-scintillator antineutrino detector

The next generation of very-short-baseline reactor experiments will require compact detectors operating at surface level and close to a nuclear reactor. This paper presents a new detector concept based on a composite solid scintillator technology. The detector target uses cubes of polyvinyltoluene interleaved with 6LiF:ZnS(Ag) phosphor screens to detect the products of the inverse beta decay reaction. A multi-tonne detector system built from these individual cells can provide precise localisation of scintillation signals, making efficient use of the detector volume. Monte Carlo simulations indicate that a neutron capture efficiency of over 70 % is achievable with a sufficient number of 6LiF:ZnS(Ag) screens per cube and that an appropriate segmentation enables a measurement of the positron energy which is not limited by γ-ray leakage. First measurements of a single cell indicate that a very good neutron-gamma discrimination and high neutron detection efficiency can be obtained with adequate triggering techniques. The light yield from positron signals has been measured, showing that an energy resolution of 14%/√E(MeV) is achievable with high uniformity. A preliminary neutrino signal analysis has been developed, using selection criteria for pulse shape, energy, time structure and energy spatial distribution and showing that an antineutrino efficiency of 40% can be achieved. It also shows that the fine segmentation of the detector can be used to significantly decrease both correlated and accidental backgrounds.

Luminescence of ZnS:Cu particles modified by shungite nanocarbon

This paper presents a study of the luminescence properties of the commercial phosphor ZnS:Cu modified with nanoparticles of shungite carbon. It shows that even a small amount of the modifying additive causes substantial surface darkening of the phosphor and reduces the reflectance at a wavelength of 490 nm. A number of competing processes are observed: a reduction of the luminance of the electroluminescence because part of the emitted light is absorbed, and an increase of the luminance, probably because the electric field is concentrated on the phosphor particles. Besides altering the luminance, such modification alters the electroluminescence spectra. Modification by shungite-carbon nanoparticles is thus promising from the viewpoint of directed adjustment of the characteristics of commercial phosphors, and optimizing the modification conditions makes it possible to obtain luminescence whose luminance exceeds that in unmodified samples.

Fabrication of substrate-free double-side emitting flexible device based on silver nanowire-polymer composite electrode

Flexible light emitting diodes are a promising component for future electronic devices, but require a simple structure and fast fabrication method. Organic light emitting diodes are a viable option as they are lightweight, thin, and flexible. However, they currently have costly fabrication procedures, complicated structures, and are sensitive to water and oxygen, which hinder widespread application. Here, we present a novel approach to fabricate flexible light emitting devices by employing Ag nanowire/polymer composite electrodes and ZnS phosphor particles. The composite electrode was fabricated using inverted layer processing, and used as both a bottom electrode and a dielectric layer. The high mechanical stability of the composite allowed the device to be free standing and mechanically flexible, eliminating the need for any additional support. Using Ag nanowires in both the top and bottom electrodes made a double-sided light emitting device that could be applied to wearable lightings or flexible digital signages.

Enhancing light emission in flexible AC electroluminescent devices by tetrapod-like zinc oxide whiskers.

Flexible alternating current electroluminescent devices (ACEL) are more and more popular and widely used in liquid-crystal display back-lighting, large-scale architectural and decorative lighting due to their uniform light emission, low power consumption and high resolution. However, presently how to acquire high brightness under a certain voltage are confronted with challenges. Here, we demonstrate an electroluminescence (EL) enhancing strategy that tetrapod-like ZnO whiskers (T-ZnOw) are added into the bottom electrode of carbon nanotubes (CNTs) instead of phosphor layer in flexible ACEL devices emitting blue, green and orange lights, and the brightness is greatly enhanced due to the coupling between the T-ZnOw and ZnS phosphor dispersed in the flexible polydimethylsiloxane (PDMS) layer. This strategy provides a new routine for the development of high performance, flexible and large-area ACEL devices.

Color tunable hybrid AC powder electroluminescent devices with organic fluorescent materials

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)] (F8BT) and 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) were drop-casted over screen-printed AC powder electroluminescent (ACPEL) devices to tune the emission color from blue ZnS phosphors. Yellow and red emissions with peak wavelengths of 548 nm and 630 nm were obtained for the hybrid ACPEL devices. By changing the mass ratio and concentrations of the F8BT and DCJTB mixture, colors ranging from green to pink were achieved. White color with the Commission Internationale de l'Eclairage (CIE) coordinates of (0.313, 0.312) was obtained. In electroluminescence (EL) and white light aging studies lasting 114.25 hours, F8BT was found to be more stable than DCJTB, which degraded fast under both conditions.

Spectral-Kinetic Characteristics of ZnS Phosphors Obtained Using the Method of Vapor Transport Synthesis in a Closed System

Samples of crystalline ZnS phosphors are obtained using the method of vapor transport synthesis in a closed system. Employing X-ray phase and structural analysis, two types of the resulting sample structures are revealed. The structural composition of 70% sphalerite+30% wurtzite is obtained under the oxygen excess conditions, whereas a nearly pure wurtzite modification is produced in the deficiency of oxygen. The spectral and kinetic characteristics of the two types of samples with the photoluminescence peaks at 510 and 630 nm are studied. These are attributed to the photoluminescence mechanisms involving self-activated oxygen centers and donor-acceptor pairs. The biexponential form of the photoluminescence decay in the two types of samples is observed, related to the presence of electron capture traps near the level of interstitial zinc. The proposed method of vapor transport synthesis of ZnS in a closed system allows simple monitoring of the thermodynamic parameters of the system and provides chemical stability to the initial and final synthesis products.

Non-Passive Behavior of Equivalent Circuit Components in AC Powder Electroluminescence (ACPEL) Lamps

For the first time, the voltage and frequency characteristics of a single layer AC powder electroluminescent lamp have been examined in detail to reveal the individual contributions of the material components involved. Statistical modelling has been employed to refine the equivalent circuit description of the lamp. DC-blocked resistance-capacitance networks can be reduced to a single effective resistance and capacitance in series. The frequency dependence of these two quantities in the range 4–1600 Hz has been used to unravel the behavior of the different underlying resistance and capacitance components at different voltage amplitudes in the range 25–150 V. The resistive contribution, R, of the activated ZnS phosphor is shown to be non-passive, and obeys the form: R(V,f) = R0(V).f−1/3.e−t.f, where V is the applied voltage, f is the frequency and t is a time constant, at all voltages. For both ZnS and BaTiO3, other characteristics indicate the presence of a thinner, non-polarized region within each semiconducting particle located within the particle's crust. A marked change in the characteristics of the different component values occurs between 25 and 50 V, consistent with the onset of light emission, after which smooth changes in all values are observed up to 150 V.