Ytterbium-Doped CsPbCl3 Quantum Cutters for Near-Infrared Light-Emitting Diodes

Near-infrared two-photon excited photoluminescence from Yb3+-doped CsPbClxBr3-x perovskite nanocrystals embedded into amphiphilic silica microspheres

Nonlinear absorption of metal-halide perovskite nanocrystals (NCs) makes them an ideal candidate for applications which require multiphoton-excited photoluminescence. By doping perovskite NCs with lanthanides, their emission can be extended into the near-infrared (NIR) spectral region. We demonstrate how the combination of Yb3+ doping and bandgap engineering of cesium lead halide perovskite NCs performed by anion exchange (from Cl- to Br-) leads to efficient and tunable emitters that operate under two-photon excitation in the NIR spectral region. By optimizing the anion composition, Yb3+-doped CsPbClxBr3-x NCs exhibited high values of two-photon absorption cross-section reaching 2.3 × 105 GM, and displayed dual-band emission located both in the visible (407-493 nm) and NIR (985 nm). With a view of practical applications of bio-visualisation in the NIR spectral range, these NCs were embedded into silica microspheres which were further wrapped with amphiphilic polymer shells to ensure their water-compatibility. The resulting microspheres with embedded NCs could be easily dispersed in both toluene and water, while still exhibiting a dual-band emission in visible and NIR under both one- and two-photon excitation conditions.

Speciation of Lanthanide Metal Ion Dopants in Microcrystalline All-Inorganic Halide Perovskite CsPbCl3

Lanthanides are versatile modulators of optoelectronic properties owing to their narrow optical emission spectra across the visible and near-infrared range. Their use in metal halide perovskites (MHPs) has recently gained prominence, although their fate in these materials has not yet been established at the atomic level. We use cesium-133 solid-state NMR to establish the speciation of all nonradioactive lanthanide ions (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Sm2+, Eu3+, Eu2+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+, Lu3+) in microcrystalline CsPbCl3. Our results show that all lanthanides incorporate into the perovskite structure of CsPbCl3 regardless of their oxidation state (+2, +3).

Trap-Mediated Sensitization Governs Near-Infrared Emission from Yb3+-Doped Mixed-Halide CsPbClxBr3-x Perovskite Nanocrystals

Understanding the photosensitization mechanisms in Yb3+-doped perovskite nanocrystals is crucial for developing their anticipated photonic applications. Here, we address this question by investigating near-infrared photoluminescence of Yb3+-doped mixed-halide CsPbClxBr3-x nanocrystals as a function of temperature and revealing its strong dependence on the stoichiometry of the host perovskite matrix. To explain the observed experimental trends, we developed a theoretical model in which energy transfer from the perovskite matrix to Yb3+ ions occurs through intermediate trap states situated beneath the conduction band of the host. The developed model provides an excellent agreement with experimental results and is further validated through the measurements of emission saturation at high excitation powers and near-infrared photoluminescence quantum yield as a function of the anion composition. Our findings establish trap-mediated energy transfer as a dominant photosensitization mechanism in Yb3+-doped CsPbClxBr3-x nanocrystals and open up new ways of engineering their optical properties for light-emitting and light-harvesting applications.

Enabling Yb3+ Luminescence with Visible Light Response in Mg2GeO4 via Energy Transfer

The growing demand for spectroscopy applications in the areas of bioimaging, food quality analysis, and temperature sensing has led to extensive research on infrared light sources. It is crucial for the design of cost-effective and high-performance systems that phosphors possess the ability to absorb blue light from commercial LEDs and convert the excitation energy to long-wavelength infrared luminescence. In this work, we obtained Yb3+ luminescence with visible light response by utilizing the energy transfer from Cr3+ to Yb3+ in Mg2GeO4. After the introduction of Yb3+, intense NIR luminescence peaking at 974 nm can be achieved with an increasing intensity. The local structure analysis was performed to investigate the preferential occupation of Yb3+ ions and the energy transfer process in Mg2GeO4. Considering the properties of thermally coupled anti-Stokes and Stokes emissions of Yb3+ and the sensitive variation of the emission intensity, the potential application of Mg2GeO4:Cr3+, Yb3+ as thermometers was demonstrated.

Deep red emission from rare-earth-free calcium aluminozincate phosphor with the substitution of Cr3+ ion

Chromium-doped calcium aluminozincate phosphor with a distinct amount of chromium was prepared via the sol-gel technique. The phase analysis and morphological study along with optical properties were conducted on the prepared material. The room temperature luminescent traits of the sample were studied in detail under 540 nm excitation wavelength. The deep red emission was confirmed from the CIE coordinates calculated using emission data. The decay curves were recorded to calculate the lifetime values for the aforementioned powder samples. The temperature-dependent luminescent characteristics were also investigated to identify the activation energy and thermal stability. The quantum yield was also calculated using luminescence spectra and found to be relatively good for the present phosphor. All of the investigated studies specified above signify that the synthesized phosphor is well suited as a red emitter in lighting and display devices.

A generic lanthanum doping strategy enabling efficient lead halide perovskite luminescence for backlights

Metal halide perovskites films have attracted great interest because of their solution-proceed fabrication and potential applications for next-generation displays. However, their non-ideal photoluminescence quantum efficiency (PLQE) and stability still does not meet the requirements of displays. Here, we adopt lanthanum ion (La³⁺) doping strategy to enhance its luminescence performance and the possibility of taking it to commercialization. In addition to the entry of lanthanum ions into the lattice to greatly improve the crystal quality, the excess La³⁺ gives rise to the formation of newly developed formamidinium cesium lanthanum bromine, (FA, Cs)₂LaBr₅, which provides additional energy transport pathways, therefore concentrating more energy onto the perovskite host. Consequently, a near-unity PLQE of 99.5% is realized in standardized green-emission cesium (Cs)/formamidinium (FA) mixed FA_0.7Cs_0.3PbBr₃ perovskite films. The introduction of La³⁺ can also lead to markedly stability with nearly 1000 days' shelf storage half-lifetime and 400 hours' light-irradiation T₉₀ lifetime as well as much-enhanced color purity. Moreover, the films with La³⁺ doping enable full-visible spectral enhancement and high-performance white light emission and trichromatic luminescence with 98.9% coverage of the color gamut area required for the Rec. 2020 standard, which suggest that perovskite films have great application potential for backlights in liquid crystal display technologies.

Ytterbium-Doped Lead-Halide Perovskite Nanocrystals: Synthesis, Near-Infrared Emission, and Open-Source Machine Learning Model for Prediction of Optical Properties

Lead-halide perovskite nanocrystals are an attractive class of materials since they can be easily fabricated, their optical properties can be tuned all over the visible spectral range, and they possess high emission quantum yields and narrow photoluminescence linewidths. Doping perovskites with lanthanides is one of the ways to widen the spectral range of their emission, making them attractive for further applications. Herein, we summarize the recent progress in the synthesis of ytterbium-doped perovskite nanocrystals in terms of the varying synthesis parameters such as temperature, ligand molar ratio, ytterbium precursor type, and dopant content. We further consider the dependence of morphology (size and ytterbium content) and optical parameters (photoluminescence quantum yield in visible and near-infrared spectral ranges) on the synthesis parameters. The developed open-source code approximates those dependencies as multiple-parameter linear regression and allows us to estimate the value of the photoluminescence quantum yield from the parameters of the perovskite synthesis. Further use and promotion of an open-source database will expand the possibilities of the developed code to predict the synthesis protocols for doped perovskite nanocrystals.

Near-Infrared LEDs Based on Quantum Cutting-Activated Electroluminescence of Ytterbium Ions

Cesium lead halide perovskite nanocrystals (PNCs) exhibit promising prospects for application in optoelectronic devices. However, electroactivated near-infrared (NIR) PNC light-emitting diodes (LEDs) with emission peaks over 800 nm have not been achieved. Herein, we demonstrate the electroactivated NIR PNC LEDs based on Yb³⁺-doped CsPb(Cl_1-xBr_x)₃ PNCs with extraordinary high NIR photoluminescence quantum yields over 170%. The fabricated NIR LEDs possess an irradiance of 584.7 μW cm^-2, an EQE of 1.2%, and a turn-on voltage of 3.1 V. The ultrafast quantum cutting process from the PNC host to Yb³⁺ has been revealed as the main mechanism of electroluminescence (EL)-activated Yb³⁺ for the first time via exploring how the trend between the EL intensity of PNC and Yb³⁺ varies with different voltages along with the tendency of temperature- and doping-concentration-dependent PL and EL spectra. This work will extend the application of PNCs to optical communication, night-vision devices, and biomedical imaging.

Highly efficient and thermally stable broadband NIR phosphors by rationally bridging Cr3+–Yb3+ in LiScGe2O6 for optical bioimaging

A broadband NIR phosphor covering the 750–1200 nm region with high luminescence efficiency and good thermal stability was realized by the energy transfer of Cr3+ → Yb3+.

Förster and nanometal surface-energy transfer in CsPbCl3/Yb3+ quantum-cutting multilayer structures

Recently, in nanophotonics, thin metal films owing to the plasmon modes they support and their perovskite nanostructures exhibit novel optical properties, which have attracted considerable interest. Both the Förster resonant energy transfer (FRET) of the dopant-induced right-angled Yb³⁺-V_Pb-Yb³⁺ defect state and a pair of Yb³⁺ ions in all-inorganic perovskite nanocrystal (PeNC) CsPbCl₃:Yb³⁺ quantum-cutting (QC) materials and the nanometal surface-energy transfer (NSET) of the excitons of PeNC-Ag nanoparticles (NPs) were investigated experimentally in CsPbCl₃:Yb³⁺/PMMA/Ag/Si (CYA_ii = 1, 2, 3, 4, 5, 6), CsPbCl₃:Yb³⁺/PMMA/Si (CY_i), and CsPbCl₃/PMMA/Ag/Si (CA_i), representing three species of multilayer structures. It was found that due to the mediation of the Ag film and an increase in the interaction volume of donors-acceptors, FRET efficiencies increased from 26% to 66% as the spacer (or wave-guiding layer) thicknesses decreased from 63.7 to 17.8 nm. The energy-transfer efficiencies of CA_i in the NSET in the surface-surface scheme followed a d^-1.6-distance dependence. This distance dependence approached the d^-2-distance dependence expected of a point-to-surface or 0D-2D energy transfer (ET). The ET in quantum cutting (QC) modulated by plasmons undoubtedly paves a way for improving the FRET and NSET performances of materials.

Cesium Manganese Bromide Nanocrystal Sensitizers for Broadband Vis-to-NIR Downshifting

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible-to-NIR spectral conversion. However, lead-based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr₃ and Cs₃MnBr₅ nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr₃. The correlation of the lanthanide emission to the CsMnBr₃ visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr₃ matrix to the lanthanides was understood by density functional theory calculations. This study shows that lead-free metal halides with an appropriate phase are effective sensitizers for lanthanides and offer a route to efficient downshifting applications.

Blue LED-pumped intense short-wave infrared luminescence based on Cr3+-Yb3+-co-doped phosphors

The growing demand for spectroscopy applications in the areas of agriculture, retail and healthcare has led to extensive research on infrared light sources. The ability of phosphors to absorb blue light from commercial LED and convert the excitation energy into long-wavelength infrared luminescence is crucial for the design of cost-effective and high-performance phosphor-converted infrared LEDs. However, the lack of ideal blue-pumped short-wave infrared (SWIR) phosphors with an emission peak longer than 900 nm greatly limits the development of SWIR LEDs using light converter technology. Here we have developed a series of SWIR-emitting materials with high luminescence efficiency and excellent thermal stability by co-doping Cr³⁺-Yb³⁺ ion pairs into Lu_0.2Sc_0.8BO₃ host materials. Benefitting from strong light absorption of Cr³⁺ in the blue waveband and very efficient Cr³⁺→Yb³⁺ energy transfer, the as-synthesized Lu_0.2Sc_0.8BO₃:Cr³⁺,Yb³⁺ phosphor emits intense SWIR light in the 900-1200 nm from Yb³⁺ under excitation with blue light at ~460 nm. The optimized phosphor presents an internal quantum yield of 73.6% and the SWIR luminescence intensity at 100 °C can still keep 88.4% of the starting value at 25 °C. SWIR LED prototype device based on Lu_0.2Sc_0.8BO₃:Cr³⁺,Yb³⁺ phosphor exhibits exceptional luminescence performance, delivering SWIR radiant power of 18.4 mW with 9.3% of blue-to-SWIR power conversion efficiency and 5.0% of electricity-to-SWIR light energy conversion efficiency at 120 mA driving current. Moreover, under the illumination of high-power SWIR LED, covert information identification and night vision lighting have been realized, demonstrating a very bright prospect for practical applications.

Optimizing the Performance of Perovskite Nanocrystal LEDs Utilizing Cobalt Doping on a ZnO Electron Transport Layer

Metal halide perovskite nanocrystal (PNC) light-emitting devices (LEDs) are promising in the future ultra-high-definition display applications due to their tunable bandgap and high color purity. Balanced carrier injection is indispensable for realizing highly efficient LEDs. Herein, cobalt (Co) was doped into ZnO to modulate the electron mobility of a pristine electron transport layer (ETL) and to inhibit exciton quenching at the ZnO/EML interface due to the passivation of oxygen vacancies and the reduction of electron concentration resulting from the trapping of electrons by the Co²⁺-induced deep impurity level. Also, the bandgap was widened due to the size confinement effect. All of those were beneficial to achieve a balanced charge injection during the operating process. Consequently, the maximum luminance increased from 867 cd m^-2 for ZnO LEDs to 1858 cd m^-2 for Co-doped ZnO LEDs, and there was a 70% increase of external quantum efficiency (EQE). By further inserting a polyethylenimine (PEI) layer in the Co-doped ZnO LEDs, the EQE reached 13.0%.