Revealing photoluminescence mechanisms of single CsPbBr3/Cs4PbBr6 core/shell perovskite nanocrystals

CsPbBr₃ nanocrystals (NCs) encapsulated by Cs₄PbBr₆ has attracted extensive attention due to good stability and high photoluminescence (PL) emission efficiency. However, the origin of photoluminescence (PL) emission from CsPbBr₃/Cs₄PbBr₆ composite materials has been controversial. In this work, we prepare CsPbBr₃/Cs₄PbBr₆ core/shell nanoparticles and firstly study the mechanism of its photoluminescence (PL) at the single-particle level. Based on photoluminescence (PL) intensity trajectories and photon antibunching measurements, we have found that photoluminescence (PL) intensity trajectories of individual CsPbBr₃/Cs₄PbBr₆ core/shell NCs vary from the uniform longer periods to multiple-step intensity behaviors with increasing excitation level. Meanwhile, second-order photon correlation functions exhibit single photon emission behaviors especially at lower excitation levels. However, the PL intensity trajectories of individual Cs₄PbBr₆ NCs demonstrate apparent “burst-like” behaviors with very high values of g²(0) at any excitation power. Therefore, the distinguishable emission statistics help us to elucidate whether the photoluminescence (PL) emission of CsPbBr₃/Cs₄PbBr₆ core/shell NCs stems from band-edge exciton recombination of CsPbBr₃ NCs or intrinsic Br vacancy states of Cs₄PbBr₆ NCs. These findings provide key information about the origin of emission in CsPbBr₃/Cs₄PbBr₆ core/shell nanoparticles, which improves their utilization in the further optoelectronic applications.

CsPbBr₃/Cs₄PbBr₆ core/shell perovskite NCs were prepared with a cubic shape. The core CsPbBr₃ are coated by a Cs₄PbBr₆ shell. The XRD, absorption spectra and PLE spectra were different from the simple mixtures of CsPbBr₃ and Cs₄PbBr₆ in bulk.     

Improvement in optical properties of Cs4PbBr6 nanocrystals using aprotic polar purification solvent

We demonstrate the influence mechanism on the optical property of Cs₄PbBr₆ during purification of solution with different protonated levels and polarities. During the purification process, organic groups originating from oleic acid (OA) and PbBr impurity on the surface of Cs₄PbBr₆ nanocrystals can be removed using high polarity aprotic and protonic solvents, and the number of Br vacancies (V_Br) can be reduced. The protonic polar solvent can not only etch the organic groups on the surface of nanocrystals, causing surface reconstruction and particle growth of nanocrystals, but also enter into the lattice of Cs₄PbBr₆ and react with the embedded CsPbBr₃. However, aprotic polar solvent decreases the particle size of Cs₄PbBr₆ nanocrystals with the increase in the solvent polarity. The optical properties of Cs₄PbBr₆ can be effectively improved using aprotic polar solvents as a purification solvent, which is very significant to improve the luminescence efficiency of perovskites.

We demonstrate the influence mechanism on the optical properties of Cs₄PbBr₆ during purification of solutions with different protonated levels and polarities.     

A facile method for preparing Yb3+-doped perovskite nanocrystals with ultra-stable near-infrared light emission

Colloidal all-inorganic cesium lead halide (CsPbX₃, X = Cl, Br, I) nanocrystals (NCs) are very important optoelectronic materials and have been successfully utilized as bright light sources and high efficiency photovoltaics due to their facile solution processability. Recently, rare-earth dopants have opened a new pathway for lead halide perovskite NCs for applications in near-infrared wave bands. However, these materials still suffer from serious environmental instability. In this study, we have successfully developed a facile method for fabricating all-inorganic SiO₂-encapsulated Yb³⁺-doped CsPbBr₃ NCs by slowly hydrolyzing the organosilicon precursor in situ. Experimental results showed that the Yb³⁺ ions were uniformly distributed in the NCs, and the whole NCs were completely encapsulated by a dense SiO₂ layer. The as-prepared SiO₂-encapsulated NCs can emit a strong near-infrared (985 nm) photoluminescence, which originates from the intrinsic luminescence of Yb³⁺ in the NCs, pumped by the perovskite host NCs. Meanwhile, the SiO₂-encapsulated NCs possessed excellent high PLQYs, narrow FWHM, and excellent environmental stability under a room atmosphere for over 15 days. We anticipate that this work will be helpful for promoting the optical properties and environmental stability of perovskite NCs and expanding their practical applications to near infrared photodetectors and other optoelectronic devices.

A facile method for fabricating CsPbBr₃:Yb³⁺@SiO₂ NCs which guarantees high PLQY and excellent stability at the same time.     

Optical and electrical properties of all-inorganic Cs2AgBiBr6 double perovskite single crystals

In this work, we explored the possibility of using Cs₂AgBiBr₆, a double perovskite crystal, for radiation detection. Cs₂AgBiBr₆ crystals were grown using the solution growth technique. The resistivity of the as-grown Cs₂AgBiBr₆ crystal is larger than 10¹⁰ Ω cm, which is high enough to ensure low leakage current for fabrication of semiconductor radiation detectors. Using the temperature-dependent resistivity measurements, we estimated that the Fermi level is at 0.788 eV above the valence band and the material is a p-type semiconductor. From the low-temperature cathodoluminescence measurements, two near band gap energies at 1.917 eV and 2.054 eV were revealed.

Temperature-dependent resistivity and cathodoluminescence (CL) measurements of solution-processed Cs₂AgBiBr₆ double perovskite single crystals.

High performance radiation detectors are widely sought after in many fields, such as national security defense, medical imaging, astrophysical study, industrial monitoring, and basic scientific research.^1–7 As the core component of detector systems, the detection material plays a key role in determining the ultimate device performance. In the past few years, many candidate materials, especially single crystals, have been investigated for radiation detection applications.^2,4,6,7 Even though some of the candidate crystals have achieved success to different extents, many limitations remain untackled. For example, high-purity germanium (HPGe) detectors can offer a superior energy resolution,⁸ but HPGe needs to be cryogenically cooled for routine detector operation. The existence of the bulky cooling system makes HPGe detectors inappropriate for many applications.⁹ Cadmium zinc telluride (CdZnTe) crystals are currently one of the leading semiconductors for room-temperature X-ray and gamma-ray detection. They can provide high energy resolution and high charge collection efficiency without the need for cryogenic cooling. However, the material non-uniformity and the relatively high cost need to be further addressed to promote its large-scale deployment.⁷ Owing to the limitations of existing semiconductor crystals, there is consistently a need for exploring new room-temperature semiconductor-based radiation detector materials. The ideal candidate materials should overcome the aforementioned drawbacks. Simultaneously, this material should be applicable with a low fabrication cost for large-scale deployment.Among the promising candidates, perovskite crystals, which share the structure ABX₃ (e.g., A = methylammonium (MA) or formamidinium (FA), B = Pb²⁺, X = Br⁻, Cl⁻, I⁻), have started to attract interests from the radiation detection community. This type of material has been widely investigated in the area of solar cells over the past few years.^10–18 These efforts have revealed that perovskite crystals have many attractive characteristics, such as high resistivity, large lifetime (τ)–mobility (μ) product, good thermal stability, and low fabrication cost using solution processing methods. These characteristics offer ideal bases for fabricating high performance and cost-friendly radiation devices. However, one major issue related with the development of hybrid (organic–inorganic) perovskite is the long-term instability. MA-based lead halide compounds present better stability in air over months,¹⁹ while FA-based perovskites can undergo phase change within days or even hours.¹⁷ It is known that air exposure may not be the direct driving force for performance degradation of perovskites, but it can accelerate the perovskite decomposition process.³⁴ In this regard, the inorganic perovskites exhibits better long term stability than its hybrid counterparts. Therefore, the inorganic perovskites (e.g., CsPbBr₃ and CsPbI₂Br) possess great potential as high performance radiation detector materials for long-term practical use.³⁵To further explore the possibility of using perovskites for radiation detection, we studied an inorganic double perovskite single crystal Cs₂AgBiBr₆ in this work. This material was chosen for the following reasons: Firstly, Cs₂AgBiBr₆ has large average atomic number (Cs = 55, Ag = 47, Bi = 83) and high density (4.65 g cm⁻³). Large average atomic number implies high mass attenuation coefficients, which is especially important for the development of X-ray and gamma-ray radiation detectors. The attenuation capability of Cs₂AgBiBr₆ is comparable to CdZnTe and CdTe as can be seen in Fig. 1 (data is available online in XCOM photon cross section database). High density leads to a much larger intrinsic efficiency, which is defined as the ratio of counts under the photo-peak to the photons interacting with the detector. Secondly, previously reported humidity and thermal stability tests confirmed the high moisture and heat resistance capabilities of Cs₂AgBiBr₆,²⁰ which offer potential for long-term and stable device operation. More importantly, this crystal does not contain lead, which eliminates the use of toxic heavy metal in the case of hybrid lead halide perovskites. We note the solubility constant of AgBr is approximately four orders of magnitude lower than that of PbI₂.²¹ Finally, previous work by Hoye et al. indicated fundamental carrier lifetime in Cs₂AgBiBr₆ thin films can exceed 1 μs, which is even longer than the carrier lifetime in FAPbI₃ and CdTe bulk single crystals (484 ns and ∼150 ns respectively).^10,24,33 In this work, we grew the Cs₂AgBiBr₆ single crystals and performed detailed structural-, electrical- and optical measurements on as-grown crystals. We report these findings and analyze their correlation with the device performance.Open in a separate windowFig. 1Attenuation coefficient of CdZnTe, Cs₂AgBiBr₆, and CdTe crystals as a function of photon energy, from X-ray to high energy gamma rays. Inset plot shows the attenuation coefficient as a function of photon energy, from 0.1 MeV to 3 MeV.The crystal growth was started by first dissolving 2.0 mmol CsBr (99.999%, Sigma Aldrich) and 1.0 mmol BiBr₃ (99.999%, Sigma Aldrich) in HBr acid (99.9999%, 48% w/w, Alfa Aesar) at room temperature. AgBr (99.998%, Alfa Aesar) was then added and the solution was briefly stirred. Then the vial containing this solution was heated to 120 °C and maintained at this temperature until the solution was clear and transparent. The temperature was then decreased at a controlled rate of 2 °C h⁻¹.²² The solution temperature was unchanged overnights to promote the crystal growth. This process generally takes 4 days to complete. Once the crystal growth ends, the solution was filtered using polytetrafluoroethylene (PTFE) filters. The as-grown crystals were carefully handled and washed using ethanol to remove chemical residues on the surface. In this work, a crystal in one centimeter size was able to be synthesized, however its shape was irregular.X-ray diffraction (XRD) experiment was carried out using a rigaku SmartLab X-ray diffractometer with CuKα X-ray source. We observed the peaks from one set of parallel lattice planes from the typical as-grown bulk single crystal. The obtained diffraction pattern shown in Fig. 2a matches well with Cs₂AgBiBr₆ reference pattern in the database (ICSD collection #252164). This confirms that the as-grown crystals in our solution growth process is indeed Cs₂AgBiBr₆ phase.Open in a separate windowFig. 2(a) X-ray diffraction (XRD) pattern of Cs₂AgBiBr₆ single crystal grown with our solution process. The inset is a picture of a typical as-grown single crystal. (b) Current–voltage (I–V) curve of Cs₂AgBiBr₆. Measurement was conducted at room temperature. Note the curve does not pass origin, probably indicating the existence of weak Schottky barrier. (c) Response of Cs₂AgBiBr₆ single crystals to LED light (wavelength: 472 nm), bias voltage: −5 V.For radiation detector applications, high resistivity of candidate semiconductor materials is an essential requirement since that ensures less leakage current, and ultimately, lower detector noise. Fig. 2b shows the current–voltage (I–V) characteristic of our Cs₂AgBiBr₆ crystal. Silver metal contacts were used as electrodes for Cs₂AgBiBr₆, which shows a good ohmic behavior. The corresponding resistivity is approximately 2.6 × 10¹⁰ Ω cm at room temperature. This is comparable with that of today''s leading room-temperature radiation detector material CdZnTe. Fig. 2c presents the conductivity response of Cs₂AgBiBr₆ to 472 nm LED light. The high resistivity from our as-grown Cs₂AgBiBr₆ in combination with the high photo-response to LED light shows great potential to further explore device fabrication with these crystals.We measured the I–V characteristic of Cs₂AgBiBr₆ crystal at various temperatures and plotted the temperature dependence of the resistivity, as shown in Fig. 3a and b. The resistivity plot in Fig. 3b presents the classical semiconductor behaviors. In other words, the resistivity will decrease with the increase of temperature owing to the presence of more thermally excited carriers. Temperature-dependent resistivity can also help determine the Fermi level. The Fermi level position serves as a good indicator of the n-type or p-type nature of semiconductors. In intrinsic or pure semiconductors (equal concentration of electrons and holes), the position of Fermi level is given bywhere E_g is the band gap energy, k is the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹), T is the temperature in kelvin, N_v and N_c are effective density of states in valence band and conduction band respectively.Open in a separate windowFig. 3(a) I–V curves at different temperatures. (b) Crystal resistivity vs. temperature.Using the obtained temperature-dependent resistivity data, we determined the Fermi level position to be either 0.788 eV below the conduction band (n-type) or above the valence band (p-type), as shown in Fig. 4a and b. N. Guechi et al. reported that Cs₂AgBiBr₆ has larger N_v compared with the magnitude of N_c,²³ which would cause the Fermi level to shift up above the midgap if the material is intrinsic (which does not exist as real). However, this shift is small, as kT is roughly 0.025 eV compared with E_g which is approximately 2 eV as shown in our cathodoluminescence (CL) measurements. Thus, we reasoned the intrinsic defects have caused the difference between Fermi level position and midgap, which ultimately determines the conduction type of the crystal. The crystal defects could serve as traps for the movement of carriers, as revealed in the CL spectrum of Fig. 5. Specifically, more deep electron traps (e.g., antisite substitution Bi_Ag and vacancy V_Br) are likely present in Cs₂AgBiBr₆ single crystals compared with hole traps.³² The above results indicate that as-grown Cs₂AgBiBr₆ is probably a p-type semiconductor, which is consistent with some previously published works.^24,25Open in a separate windowFig. 4(a) Temperature dependence of the resistivity for Fermi level determination. (b) Relative Fermi level position in the forbidden band.Open in a separate windowFig. 5CL spectrum of Cs₂AgBiBr₆ single crystal. Fig. 5 presents the low-temperature CL spectrum of Cs₂AgBiBr₆, measured at 83 K. Lozhkina et al. determined the approximate band gap energies of Cs₂AgBiBr₆ to be 1.946 eV (indirect), 2.095 eV (indirect), and 2.254 eV (direct) from photoluminescence spectrum at T = 1.5 K.²⁶ In the present study, the two peaks at 1.917 eV and 2.054 eV are ascribed to near band gap emissions with low intensities. They are likely from free exciton or bound exciton emissions, which can be induced when the created electrons and holes from band-gap radiation recombine and form free excitons, or the free excitons collide with donors or acceptors and bound to them to form (D⁺, X) or (A⁻, X) complexes.²⁹ The two near band gap energies given in the CL spectrum are lower than the band gap of hybrid perovskite MAPbBr₃ (2.21 eV) and MAPbCl₃ (2.97 eV).^12,27 Rest of the CL peaks (28 Another possible defect, vacancies (e.g., V_Br, V_Ag, V_Bi), can serve as donors (cation vacancy) or acceptors (anion vacancy).^32,36 Recombination may occur at donor or acceptor energy levels (donor-to-valence or band-to-acceptor) and therefore luminescence are likely to occur. In other cases, a donor–acceptor (D–A) pair emission is possible when electrons bound to donors and holes bound to acceptors. The ambiguity will be further solved experimentally using the follow-on thermal annealing method.Peak energies in CL spectrum

Cs2NaGaBr6: a new lead-free and direct band gap halide double perovskite

In this work, we have studied new double perovskite materials, A₂¹⁺B²⁺B³⁺X₆¹⁻, where A₂¹⁺ = Cs, B²⁺ = Li, Na, B³⁺ = Al, Ga, In, and X₆¹⁻. We used the all electron full-potential linearized augmented plane wave (FP-LAPW+lo) method within the framework of density functional theory. We used the mBJ approximation and WC-GGA as exchange–correlation functionals. We optimized the lattice constants with WC-GGA. Band structures were calculated with and without spin–orbit coupling (SOC). Further, band structures for Cs₂LiGaBr₆ and Cs₂NaGaBr₆ were calculated with SOC + mBJ to correct the band gap values with respect to experimental value. We obtained direct bandgaps at Γ-point of 1.966 eV for Cs₂LiGaBr₆ and 1.762 eV for Cs₂NaGaBr₆, which are similar to the parent organic–inorganic perovskite (MAPI) CH₃NH₃PbI₃ (E_g = 1.6 eV). Total and partial density of states were analyzed to understand the orbital contribution of Cs, Na, Li, Ga and Br near the Fermi level. The optical properties in terms of real and imaginary ε, refractive index n, extinction coefficient k, optical conduction σ, absorption I, and reflectivity R were calculated. A study of the elastic and mechanical properties shows that both materials are thermodynamically stable. A stable, direct bandgap and a gap value close to those of MAPI make Cs₂NaGaBr₆ a great competitor in the Pb-free hybrid perovskite solar cells world.

In this work, we have studied new double perovskite materials, A₂¹⁺B²⁺B³⁺X₆¹⁻, where A₂¹⁺ = Cs, B²⁺ = Li, Na, B³⁺ = Al, Ga, In, and X₆¹⁻.     

Synthesis and characterization of Mn-doped CsPb(Cl/Br)3 perovskite nanocrystals with controllable dual-color emission

Metal-halide perovskite nanocrystals (NCs) are considered to be promising types of optoelectronic and photonic materials. The emission colors of the cesium lead halide perovskite (CsPbX₃, X = Cl, Br, I) NCs depend on the joint influence of the emission peaks of the host and its dopant ions. Herein, we report a phosphine-free strategy to synthesize Mn-doped CsPb(Cl/Br)₃ NCs to tune their optical properties in a wide color gamut. Colloidal Mn-doped CsPb(Cl/Br)₃ NCs were synthesized by injecting Cs-oleate solution into the MnCl₂ and PbBr₂ precursor solution. The as-prepared Mn-doped CsPb(Cl/Br)₃ NCs are highly crystalline and uniform sized nanocubes with two emission peaks, including the host emission around 450 nm and the Mn²⁺ dopant emission around 600 nm, which are sensitive to the MnCl₂-to-PbBr₂ molar feed ratio and the reaction temperature. By varying the MnCl₂-to-PbBr₂ molar feed ratio or the reaction temperature, the relative PL intensities of dual color emission can be manipulated, showing their ability in tunable color output.

Colloidal Mn-doped CsPb(Cl/Br)₃ NCs were synthesized at different MnCl₂-to-PbBr₂ molar feed ratios or reaction temperatures to tune their color emission.     

Cs4PbBr6/CsPbBr3 perovskite composites for WLEDs: pure white,high luminous efficiency and tunable color temperature

Cs₄PbBr₆/CsPbBr₃ perovskite composites are fabricated by room-temperature one-pot mixing synthesis, which is short in time, free from inert gases and delivers a high product yield. Temperature-dependent photoluminescence shows that a larger exciton binding energy of 291.1 meV exhibits better thermal stability compared with that of pure Cs₄PbBr₆ and CsPbBr₃ materials. The CIE chromaticity coordinates (0.1380, 0.7236) of green LEDs designed with Cs₄PbBr₆/CsPbBr₃ perovskite composites show almost no variation under driving current changing from 5 to 30 mA. Furthermore, the ground Cs₄PbBr₆/CsPbBr₃ perovskite composites mixed with red emitting K₂SiF₆:Mn⁴⁺ phosphor are dropped and casted on a blue-emitting InGaN chip. The white light emitting diodes (WLEDs) are presented, which have good luminous efficiency of 65.33 lm W⁻¹ at 20 mA, a correlated color temperature of 5190 K, and the white gamut with chromaticity coordinate of (0.3392, 0.3336). According to the state of art, these excellent characteristics observed are much superior to the reported results of conventional perovskite-based WLEDs, which demonstrate the immense potential and great prospect of Cs₄PbBr₆/CsPbBr₃ perovskite composites to replace conventional phosphors in lighting devices.

WLED devices are designed with high luminous efficiency of 65.33 lm W⁻¹ and excellent CIE chromaticity coordinates of (0.3392, 0.3336). The properties of material and the luminous performance of device are calculated and discussed comprehensively.     

Highly (100)-oriented CH3NH3PbI3(Cl) perovskite solar cells prepared with NH4Cl using an air blow method

The effects of adding NH₄Cl via an air blow process on CH₃NH₃PbI₃(Cl) perovskite solar cells were investigated. CH₃NH₃PbI₃(Cl) solar cells containing various amounts of NH₄Cl were fabricated by spin-coating. The microstructures of the resulting cells were investigated by X-ray diffraction, optical microscopy, and scanning electron microscopy. The current density–voltage characteristics of the cell were improved by adding an appropriate amount of NH₄Cl and air blowing, which increased the photoconversion efficiency to 14%. Microstructure analysis indicated that the perovskite layer contained dense grains with strong (100) orientation, as a result of NH₄Cl addition and air blowing. The ratio of the (100)/(210) reflection intensities for the perovskite crystals was 2000 times higher than that of randomly oriented grains. The devices were stable when stored in ambient air for two weeks.

Perovskite solar cells with dense grains with strong (100) orientation were developed by adding NH₄Cl and air blowing.     

A CTAB-mediated antisolvent vapor route to shale-like Cs4PbBr6 microplates showing an eminent photoluminescence

Compared with nanoscale quantum dots (QDs), the large-sized perovskite crystals not only possess better stability but also are convenient for application exploration. Herein, we develop a facile and efficient antisolvent vapor-assisted recrystallization approach for the synthesis of large-sized Cs₄PbBr₆ perovskite crystal microplates. In this method, for the first time, the shale-like Cs₄PbBr₆ microplates with lateral dimensions of hundreds of microns are fabricated by employing cetyltriethylammnonium bromide (CTAB) as a morphology-directing agent. FESEM, TEM, and AFM characterizations indicate that the as-obtained shale-like Cs₄PbBr₆ microplates are actually formed by 6–8 nm thick Cs₄PbBr₆ nanosheets with orientational stacking. Importantly, such highly crystalline Cs₄PbBr₆ microplates with shale-like morphology exhibit a narrow and intense green PL emission with a 59% PL quantum yield. Moreover, the planar structure of shale-like Cs₄PbBr₆ microplates makes it easy to form a preferred orientation on a substrate, which endow them with promising potential in optoelectronic devices such as lighting and displays.

Highly luminescent shale-like Cs₄PbBr₆ microplates with hundreds of microns in lateral dimension and formed by thin nanosheets with orientational stacking.     

Luminescent properties of Eu3+-activated Gd2ZnTiO6 double perovskite red-emitting phosphors for white light-emitting diodes and field emission displays

We synthesized a series of double perovskite Eu³⁺-activated Gd₂ZnTiO₆ red-emitting phosphors by a solid-state reaction route and analyzed their morphology, crystallinity, luminescent properties, and thermal stability. Under 270 nm of excitation, the prominent emission peak of the phosphors was found to be located in the red region with the central wavelength of 613 nm corresponding to the intra-4f transition of Eu³⁺ ions from the ⁵D₀ to ⁷F₂ level. The optimum concentration of the activator was determined to be 7 mol%. The studied phosphors also exhibited good thermal stability with the activation energy of 0.233 eV. The white color emitted from the ultraviolet (UV) light-emitting diode device which was coated by commercial blue-/green-emitting phosphors and Gd_1.86ZnTiO₆:0.14Eu³⁺ phosphors exhibited a high color rendering index of 82.9. Furthermore, the cathodoluminescence performance of the resultant phosphors was also investigated in detail. These characteristics of Gd_2−2xZnTiO₆:2xEu³⁺ phosphors make them potential candidates for UV-based white light-emitting diodes and field emission displays.

We synthesized a series of double perovskite Eu³⁺-activated Gd₂ZnTiO₆ red-emitting phosphors by a solid-state reaction method and analyzed their morphology, crystallinity, luminescent properties, and thermal stability.     

Ligand and adjuvant dual-assisted synthesis of highly luminescent and stable Cs4PbBr6 nanoparticles used in LEDs

We developed a new ligand and adjuvant dual-assisted room temperature colloidal method for the synthesis of highly luminescent and stable Cs₄PbBr₆ nanoparticles, in which acetone, oleamine (OM) and oleic acid (OA) were used as precursors, while water and dimethyl sulfoxide (DMSO) were used as adjuvants. In this process, we explored the influencing factors of process parameters (such as the amount of water, the standing time of precursors, and the molar ratio of raw materials), and found that Cs₄PbBr₆ synthesized by water + DMSO can not only change the morphology and promote crystallization but also improve the lattice strain, reduce the lattice defects and optimize the passivation effect, so as to improve the luminescence properties. Simultaneously, we also found that the pc-LED made of Cs₄PbBr₆ can still emit bright green light after 4344 h of operation, showing excellent stability and making it promising for solid-state lighting application. This method also provides an important reference value for solving the hydrolysis property of perovskites.

We developed a new room temperature colloidal method for the synthesis of highly luminescent and stable Cs₄PbBr₆ nanoparticles, in which acetone, oleamine and oleic acid were used as precursors, while water and dimethyl sulfoxide were used as adjuvants.     

Cu2ZnSn(S,Se)4 thin-films prepared from selenized nanocrystals ink

For the first time, CZTS ink was formulated using low-temperature heating up synthesis of NCs. Besides, the influence of powder concentration on the properties of the films was examined. Subsequently, the CZTS films were annealed under a selenium (Se)/argon (Ar) atmosphere at different temperatures to enhance their properties. The influence of selenization temperature on the properties of CZTS films was examined in detail. Structural analysis showed a peak shift towards lower 2θ values for CZTSSe films because of Se incorporation, resulting in larger lattice parameters for CZTSSe than CZTS. As the selenization temperature increases, an increment in the grain size was observed and the band gap was decreased from 1.52 to 1.05 eV. Hall Effect studies revealed a significant improvement in the mobility and carrier concentration with respect to selenization temperatures. Moreover, film selenized at 550 °C exhibited higher photoconductivity as compared to other films, indicating their potential application in the field of low-cost thin-film solar cells.

For the first time, CZTS ink was formulated using low-temperature heating up synthesis of NCs.     

Upconversion from fluorophosphate glasses prepared with NaYF4:Er3+,Yb3+ nanocrystals

The direct doping method was applied to fabricate upconverter fluorophosphate glasses in the system (90NaPO₃-(10-x)Na₂O-xNaF) (mol%) by adding NaYF₄:Er³⁺,Yb³⁺ nanocrystals. An increase in the network connectivity, a red shift of the optical band gap and a decrease in the thermal properties occur when Na₂O is progressively replaced by NaF. To ensure the survival and the dispersion of the nanocrystals in the glasses with x = 0 and 10, three doping temperatures (T_doping) (525, 550 and 575 °C) at which the nanocrystals were added in the glass melt after melting and 2 dwell times (3 and 5 minutes) before quenching the glasses were tested. Using 5 wt% of the NaYF₄:Er³⁺,Yb³⁺ nanocrystals, green emission from the NaYF₄:Er³⁺,Yb³⁺ nanocrystals-containing glasses was observed using a 980 nm pumping, the intensity of which depends on the glass composition and on the direct doping parameters (T_doping and dwell time). The strongest upconversion was obtained from the glass with x = 10 prepared using a T_doping of 550 °C and a 3 min dwell time. Finally, we showed that the upconversion, the emission at 1.5 μm and of the transmittance spectra of the nanocrystals-containing glasses could be measured to verify if decomposition of the nanocrystals occurred in glass melts during the preparation of the glasses.

The direct doping method was applied to fabricate upconverter fluorophosphate glasses in the system (90NaPO₃-(10-x)Na₂O-xNaF) (mol%) by adding NaYF₄:Er³⁺,Yb³⁺ nanocrystals.     

Scintillation in (C6H5CH2NH3)2SnBr4: green-emitting lead-free perovskite halide materials

We report the optical and scintillation properties of (C₆H₅CH₂NH₃)₂SnBr₄ with excellent absorption length at 20 keV of 0.016 cm, measured bandgap of 2.51 eV, and photoluminescence lifetime of 1.05 μs. The light yield obtained with the ²⁴¹Am source is 3600 ± 600 photons per MeV, which is much smaller than the maximum attainable light yield obtained from the bandgap. Temperature dependent radioluminescence measurements confirm the presence of thermal quenching at room temperature with the activation energy and the ratio between the attempt and the radiative transition rates of 61 meV and 129, respectively. Although thermal quenching affects light yield at room temperature, this green light-emitting perovskite opens an avenue for new lead-free scintillating materials.

A new tin perovskite material with green emission is investigated as a new scintillator for imaging readout. Temperature dependent measurements were studied to understand the mechanism and to improve the future lead-free scintillator.     

Preparation,characterization, and luminescence properties of double perovskite SrLaMgSbO6:Mn4+ far-red emitting phosphors for indoor plant growth lighting

Mn⁴⁺-activated SrLaMgSbO₆ far-red emitting phosphors with double perovskite structure were prepared by traditional solid-state reaction. The research on the crystal structure of the SrLaMgSbO₆:0.8%Mn⁴⁺ (SLMS:0.8%Mn⁴⁺) phosphors showed that the as-prepared sample was made up of two polyhedrons, [SbO₆] and [MgO₆]. Under the excitation of 333 nm, the SLMS:0.8%Mn⁴⁺ phosphors exhibited an intense far-red emission in the 625–800 nm wavelength range with CIE chromaticity coordinates of (0.733, 0.268), which could match well with the absorption spectrum of phytochrome P_FR. The optimal concentration of Mn⁴⁺ ions in the SLMS:Mn⁴⁺ phosphors was 0.8 mol%. Importantly, the as-prepared SLMS:0.8%Mn⁴⁺ phosphors had an internal quantum efficiency of 35%. The thermal stability of SLMS:0.8%Mn⁴⁺ phosphors was also investigated, and the activation energy was found to be 0.3 eV. Thus, the Mn⁴⁺-activated SLMS phosphors have great potential to serve as far-red emitting phosphors in indoor plant growth lighting.

Novel far-red emitting double perovskite SrLaMgSbO₆:Mn⁴⁺ phosphors were prepared and their photoluminescence properties were studied for applications in indoor plant growth lighting.     

Structural designing of Zn2SiO4:Mn nanocrystals by co-doping of alkali metal ions in mesoporous silica channels for enhanced emission efficiency with short decay time

High purity Zn₂SiO₄:Mn crystals were synthesized by impregnating a precursor solution into mesoporous silica followed by sintering process. The effects of doping alkali metal ions (Li⁺, Na⁺, K⁺) on the structural, morphological and photoluminescence properties were investigated. Formation of single phase α-Zn₂SiO₄:Mn crystals was confirmed from X-ray diffraction. The crystal size was significantly decreased from 54 nm to 35 nm with increasing molar concentration of alkali metal ion dopants in Zn₂SiO₄:Mn. Zn₂SiO₄:Mn crystals co-doped with alkali metal ions showed stronger emission and faster decay times compared to the un-doped Zn₂SiO₄:Mn phosphor. The highest emission quantum yields (EQEs) of 68.3% at λ_exc 254 and 3.8% at λ_exc 425 nm were obtained for the K⁺ ion doped samples with Mn²⁺ : K⁺ ratio of ∼1 : 1. With alkali metal ions (Li⁺, Na⁺, K⁺) co-doping, the decay time of Zn₂SiO₄:Mn crystals was shortened to ∼4 ms, whereas the emission intensity was elevated, with respect to un-doped Zn₂SiO₄:Mn crystals. Zn₂SiO₄:Mn crystal growth in silica pores together with selective doping with alkali metal ions paves a way forward to shorten the phosphor response time, without compromising emission efficiency.

Alkali metal ions co-doped Zn₂SiO₄:Mn nanocrystals were synthesized in a mesoporous silica matrix using solution impregnation method. A high PL-QY of 68.3% at λ_exc 254 nm and 3.8% at λ_exc 425 nm with faster decay time of <5 ms is obtained.     

Interleukin 4 induces selective production of interleukin 6 from normal human B lymphocytes

In this paper we have shown that extensively purified human B lymphocytes respond to IL-4 treatment with a marked production of IL-6. Addition of anti-mu potentiated the effect of IL-4 on IL-6 production. Other cytokines tested like TNF-alpha and-beta, IFN-gamma, IL-1, IL-2, and IL-5 did not induce IL-6 secretion when given to resting B cells. Although B cells generally also produced TNF-alpha and TNF-beta upon stimulation, IL-4 did not induce TNF secretion and seemingly had a specific effect on IL-6 production.