Doped Luminescence Research Articles

Temperature Promotes Photoluminescence in Lanthanide‐Doped 3D Ceramic Microarchitectures

AbstractTwo‐photon lithography (TPL) is a powerful technique for creating 3D microarchitectures. Applied to high‐refractive‐index materials like ZrO2, it promises advanced optics. This is the case of ZrO2 host matrixes in combination with luminescent dopants. However, due to the nonideal crystallinity attained to the TPL pre‐ceramic replica from a custom‐made photoresin, the emission of lanthanide (Ln) dopants in ZrO2 microarchitectures can be suboptimal. However, crystallinity exacerbated by annealing can promote Ln‐emission, thereby enabling the integration of ceramic micro‐optic into a low‐temperature process. This work presents a photoresin containing a metal‐organic monomer tailored for TPL, enabling the fabrication of Ln‐doped tetragonal ZrO2 (t‐ZrO2) microarchitectures. The emission properties of Ln‐doped microarchitectures with trivalent Ln ions (Ln3+), i.e., Yb3+ (2.5 mol%), Er3+ (0.35 mol%), and Tm3+ (0.35 mol%) are studied. The results demonstrate that Ln emission is absent when annealing the microarchitectures at 600 °C. Annealing at 750 °C activates Ln3+ emissions, including 2F5/2–2F7/2 (infrared), 4S3/2–4I15/2 (green), and 3H4–3F6 (near‐infrared) transitions corresponding to Yb, Er, and Tm species. Transmission electron microscopy (TEM) confirms that t‐ZrO2 crystallinity becomes more prominent at 750 °C, demonstrating the promotion of Ln emissions upon thermal treatment and underscoring the role of crystalline in TPL micro‐optical ceramics.

Near-infrared TADF-type organic afterglow materials

Because of the energy gap law, as well as the spin-forbidden nature of triplet formation and transformation, it remains formidable task to achieve efficient and long-lived organic afterglow materials with long emission wavelengths, especially in the near-infrared region, under ambient conditions. Here we incorporate TADF-type afterglow mechanism in dopant-matrix systems which features a moderate kRISC of 101-102 s−1 to harvest triplet energies, boost afterglow efficiency and maintain afterglow lifetime. Specifically, we design a series of boron difluoride curcuminoid (CurBF2) compounds to serve as luminescent dopants. Organic matrices of crystalline nature and with carbonyl groups are selected to suppress triplet quenching by their rigid microenvironment and populate triplet states via dipole effect developed in our group. The resultant dopant-matrix systems display near-infrared TADF-type organic afterglow with emission wavelength >700 nm, quantum yield around 10 % and afterglow lifetime >10 ms, which can function as deep-penetrating and background-independent bioimaging probes.

Simultaneous modification of emission spectrum and trap distribution for achieving self-trapped excitons-based white afterglow

The incommensurable differences between the feeding and actual doping ratios of luminescent dopants and excessive reliance on rare-earth ions become two key problems in halide perovskite-based afterglow materials, restricting the production cost and further application. Here, we prepare a series of novel lead- and rare-earth-free halide afterglow phosphors Cs2(Na0.9Ag0.1In)1-xZrxCl6 under an ambient condition, in which the Zr dopants modify the lattice thus tailoring the emission spectrum and trap distribution, simultaneously. The focused Cs2(Na0.9Ag0.1In)0.6Zr0.4Cl6 with mainly size distribution of ∼ 0.6–1.8 μm exhibits ∼ 350–850 nm ultrabroad white luminescence under ultra-violet excitation, and the introduced traps endow the white emission with a long-persistence of over 5 h after pre-irradiation. The absorption bands of self-trapped excitons (STEs) are observed in the steady-state absorption spectra during the afterglow process, suggesting the general accepted picosecond-scaled lifetimes of STEs have been successfully prolonged to hour-scale, which provide a potential opportunity for investigating the specific behavior of STEs.

Optical properties of semiconductor nanoparticles doped with 3d and 4f block elements

Doped inorganic nanoparticles (NPs) have enabled researchers across multiple disciplines to create new materials with unique optical, electronic, magnetic, electrical, chemical, and/or compositional properties. In the quest for generating newer functional materials with controlled optical and electronic properties, the use of d- and f-block elements as dopants in the inorganic NPs remain at the forefront of research. This article summarizes experiments that identify the important control parameters, in the form of NP core and surface properties, for promoting host-sensitized dopant emission from d- and f-doped inorganic/semiconductor NPs. We conclude that energy offsets between the dopant and NP electronic levels and an understanding of the charge trapping mechanisms can be used to predict dopant luminescence, and we claim that these features can be used to choose NP host materials by design, rather than by trial and error.

Dopant-Matrix Afterglow Systems: Manipulation of Room-Temperature Phosphorescence/Thermally Activated Delayed Fluorescence Afterglow Mechanism via Mismatch/Match of Intermolecular Charge Transfer between Dopants and Matrices.

Dopant-matrix organic afterglow materials exhibit ease of fabrication and intriguing functions in diverse fields. However, a deep and comprehensive understanding of their photophysical behaviors remains elusive. Here we report manipulation of a room-temperature phosphorescence/thermally activated delayed fluorescence (RTP/TADF) afterglow mechanism via the mismatch/match of intermolecular charge transfer between dopants and matrices. When dispersed in inert crystalline matrices, the luminescent dopants show RTP lifetimes up to 2 s. Interestingly, when suitable organic matrices are selected, the resultant dopant-matrix materials display a TADF-type afterglow under ambient conditions due to the formation of dopant-matrix intermolecular charge transfer complexes. Detailed studies reveal that reverse intersystem crossing from dopants' T1 states to charge transfer complexes' S1 states, which features a moderate kRISC of 101-102 s-1, is responsible for the emergence of a TADF-type organic afterglow in rigid crystalline matrices. Such less reported delicate photophysics reveals a new aspect of the excited state property in dopant-matrix afterglow systems.

Luminescent Hybrid BPA.DA-NVP@Eu2L3 Materials: In Situ Synthesis, Spectroscopic, Thermal, and Mechanical Characterization.

A series of homogeneous hybrid BPA.DA-NVP@Eu2L3 materials were obtained through an in situ approach where the luminescent dopant was formed at the molecular level with different contents (0.1; 0.2; 0.5; 1; and 2% by weight). A Europium(III) complex (Eu2L3) with quinoline-2,4-dicarboxylic acid was applied as a luminescence additive while a polymer matrix consisted of a combination of bisphenol A diacrylate (BPA.DA) and N-vinylpyrrolidone (NVP) monomers. Synthesis steps and the final materials were monitored by NMR and Fourier transform infrared spectroscopy (FTIR). The emission, excitation spectra, lifetime, and quantum yield measurements were applied for the determination of the photophysical characteristics. The thermal and mechanical properties of the obtained materials were tested via thermal analysis methods (TG/DTG/DSC and TG-FTIR) in air and nitrogen atmospheres, dynamic mechanical analysis (DMA), and hardness and bending measurements. Generally, even a small addition of the metal complex component causes changes in the thermal, mechanical, and luminescent properties. Hybrid materials with a greater europium complex content are characterized by a lower stiffness and hardness while the heterogeneity and the flexibility of the samples increase. A very small amount of an Eu2L3 admixture (0.1% wt.) in a hybrid material causes an emission in the red spectral range and the luminescence intensity was reached for the BPA-DA-NVP@1%Eu2L3 material. These materials may be potentially used in chemical sensing, security systems, and protective coatings against UV.

Suppressed Concentration Quenching Brightens Short-Wave Infrared Emitters.

The brightness of doped luminescent materials is usually limited by the ubiquitous concentration quenching phenomenon resulting in an intractable tradeoff between internal quantum efficiency and excitation efficiency. Here, an intrinsic suppression of concentration quenching in sensitized luminescent systems, by exploiting the competitive relationship between light emitters and quenchers in trapping excitation energies from sensitizers, is reported. Although Cr3+ sensitizers and trivalent lanthanide (Ln3+ , Ln = Yb, Nd, and Er) emitters themselves are highly susceptible to concentration quenching, the unprecedentedly high-brightness luminescence of Cr3+ -Ln3+ systems is demonstrated in the short-wave infrared (SWIR) range employing high concentrations of Cr3+ , whereby a record photoelectric efficiency of 23% is achieved for SWIR phosphor-converted light-emitting diodes, which is about twice as high as those previously reported. The results underscore the beneficial role of emitters in terminating excitation energies, opening up a new dimension for developing efficient sensitized luminescent materials.

Luminescent Bimetallic Two-Coordinate Gold(I) Complexes Utilizing Janus Carbenes.

A series of bimetallic carbene-metal-amide (cMa) complexes have been prepared with bridging biscarbene ligands to serve as a model for the design of luminescent materials with large oscillator strengths and small energy differences between the singlet and triplet states (ΔEST). The complexes have a general structure (R2N)Au(:carbene─carbene:)Au(NR2). The bimetallic complexes show solvation-dependent absorption and emission that is analyzed in detail. It is found that the molar absorptivity of the bimetallic complexes is correlated with the energy barrier to rotation of the metal-ligand bond. The bimetallic cMa complexes also exhibit short emission lifetimes (τ = 200-300 ns) with high photoluminescence efficiencies (ΦPL > 95%). The radiative rates of bimetallic cMa complexes are 3-4 times faster than that of the corresponding monometallic complexes. Analysis of temperature-dependent luminescence data indicates that the lifetime for the singlet state (τS1) of bimetallic cMa complexes is near 12 ns with a ΔEST of 40-50 meV. The presented compounds provide a general design for cMa complexes to achieve small values for ΔEST while retaining high radiative rates. Solution-processed organic light-emitting devices (OLEDs) made using two of the complexes as luminescent dopants show high efficiency and low roll-off at high luminance.

Multifunctional Al2O3:Eu2+/Sm2+,3+ + Y2O3:Eu3+ Coatings Created by the Plasma Electrolytic Oxidation

AbstractThe industrial applications of luminescent coatings for temperature sensing and high‐security QR codes have multiple inherent problems in their application (speed, cost, industrial feasibility) or properties (poor adhesion, chemical, thermal stability, or thermal contact). Luminescent coatings created by the electrochemical Plasma Electrolytic Oxidation (PEO) process have superior properties and can be grown directly within minutes, but are limited to the number of possible luminescent dopants. Having the unlimited possibility for doping on the PEO coatings would provide for superior coatings in all industrially appealing features. Herein, it is shown that adding any luminescent powder results in its partial dissociation and incorporation of its constituents, while it also gets incorporated as a whole. In this manner, alumina coatings are grown with various combinations and concentrations of divalent samarium and europium, and yttrium oxide with trivalent europium ions. These polycrystalline coatings whose microhardness is next to that of the diamond produce intense luminescence in the blue and red regions. Al2O3:Eu2+/Sm2+/3+ + Y2O3:Eu3+ coatings show an excellent temperature sensing performance from the cryogenic up to the high temperatures. Ultimately, the proof‐of‐concept of the luminescent QR code by the electrochemical process is realized creating a highly secure code with unprecedented environmental stability.

A site-selective fluorescence spectroscopy study of the crystal phases of KY3F10: Leveraging the optical response of Eu3+ ions

The present work deals with the investigation of the site-selective symmetries in Eu3+-doped KY3F10 with different crystal phases. Employing time-resolved fluorescence line-narrowing (TRFLN), a study of the luminescent response of the dopant is reported in samples presenting a single δ-phase, a single α-phase, and a mixture of both δ and α. A new, simple, very low time-consuming and high-yield method was developed to obtain nanospheres of the δ-phase. For both crystal phases, the main site symmetry is spectrally corroborated with that expected by the crystallographic substitution position (C2v in δ, and C4v in α). In addition, the accuracy of the measurements also unveils the presence of significantly distorted Eu3+ ions in the surface of the nanoparticles in all the samples studied, a spectral behavior commonly found in glassy systems. All these findings contribute to the proper understanding of the optical response of luminescent dopants in this and related types of complex fluorides. It is worth highlighting the fact that the huge potential of the unexplored δ-phase is demonstrated, since the site-selective emission of the Eu3+-doped δ-phase is up to 20 times more intense than that of the α-phase.

Sonication‐Responsive Organic Afterglow Emulsions

AbstractIn contrast to their fluorescence counterparts, stimuli‐responsive organic afterglow materials display distinct advantages to function normally in the presence of strong fluorescence background. However, most of the stimuli‐responsive afterglow materials can only work in their solid states with those in aqueous medium being rarely reported. In addition, the types of these materials are limited to mechanical force‐, pH‐, ion‐, and temperature‐responsive ones. Here, a serendipitous finding of sonication‐responsive organic afterglow emulsions in aqueous medium is reported. The afterglow emulsions are produced by dopant‐matrix strategy and emulsion technique. In‐depth studies reveal that the key to achieve the sonication responsiveness is the selection of a specialized organic matrix that can form microcrystalline particles in aqueous medium with the aid of surfactants. In matrix's microcrystalline state, the triplet excited states of luminescent dopant within the matrix can be protected to exhibit room‐temperature afterglow. Under sonication, matrix's microcrystalline structure can be disrupted and consequently the triplet excited states of luminescent dopants lose protection to show afterglow quenching. To the best of the authors’ knowledge, this study represents the first report of sonication‐responsive afterglow materials in aqueous medium, which show promising biomedical applications.

Structures, photoluminescence, and principles of self-activated phosphors.

Self-activated phosphors without any luminescent dopants, usually display excellent optical properties, such as high oscillator strength, large Stokes shift, and strong luminescence efficiency, and thus have been widely investigated by researchers for several decades. However, their recent advancements in scintillators, white-light illumination, displays and optical sensors compel us to urgently understand the basic principles and significant technological relevance of this worthy family of materials. Herein, we review the structures, photoluminescence principles, and applications of state-of-the-art self-activated phosphors, such as borate, gallate, niobate, phosphate, titanate, vanadate, tungstate, nitrides, oxyfluoride, perovskite, metal halides, and carbon dots. The photoluminescence principles of self-activated phosphors are mainly summarized as transitions between energy levels of rare-earth and transition metal ions, charge transfer transitions of some oxide compounds, and luminescence in all-inorganic semiconductors. The different self-activated phosphors exhibit various structures and site-dependent spectra. Additionally, we discuss the application prospect and main challenges of self-activated phosphors.

Stable chitosan fluorescent nanofiber sensor containing Eu3+ complexes for detection of copper ion

In this work, a luminescent lanthanide complex, namely, Eu(tta)3phen (Eu = europium, tta = 2-thenoyltrifluoroacetone, phen = 1, 10-phenanthroline) is used as a luminescent dopant to prepare fluorescent nanofiber films. The resulting nanofibers (CS/PEO-SiO2@Eu(tta)3phen) are composed of chitosan and polyethylene oxide containing silica nanoparticles, encapsulating the Eu(tta)3phen. The nanofibers are stabilized by treatment with sodium bicarbonate (NaHCO3) aqueous solution as neutralizer and then repeatedly washed with deionized water. A more stable Na-CS/PEO-SiO2@Eu(tta)3phen fiber film is obtained as a fluorescence sensor for Cu2+ ions. The Na-CS/PEO-SiO2@Eu(tta)3phen has high sensitivity, and a wide detection range. The response is linear in the concentration range from 10 to 90 μmol/L, and the detection limit is 0.826 μmol/L. In addition, Na-CS/PEO-SiO2@Eu(tta)3phen nanofiber film can be used as a recyclable sensor for the detection of Cu2+ ions in aqueous solution due to its good recyclability, selectivity and rapid optical response.

Cascade Synthesis of Luminescent Difluoroboron Diketonate Compounds forRoom‐TemperatureOrganic Afterglow Materials

Comprehensive SummaryDifluoroboron β‐diketonate (BF2bdk) compounds represent an important class of luminescent materials, whereas their synthesis requires multiple steps, which restrict their application in diverse fields. Here we report a cascade reaction to prepare BF2bdk from aromatic ketones, carboxylic acids, trifluoroacetic anhydride and boron trifluoride diethyl etherate. The cascade reaction is very simple and straightforward to produce BF2bdk with desired functional groups in reasonable isolation yields. Further, when some specific BF2bdk compounds are used as luminescent dopants, organic room‐temperature phosphorescence with lifetimes >1.0 s and intense afterglow under ambient conditions can be achieved with the assistance by suitable organic matrices. The dopant‐matrix materials exhibit excellent processability into desired patterns and large‐area panels, and function as efficient donors of excited state energy transfer for the fabrication of red afterglow materials. The materials can be processed into aqueous afterglow dispersion that displays unique property of escaping the interference from strong fluorescence background.

Identification of dyes and matrices for dye doped polymer waveguide emitters covering the visible spectrum

Polymer based photonic devices offer the possibility cost effective roll-to-roll manufacture of photonic devices. The incorporation of luminescent dopants within a solid polymer waveguide allows for the generation of light within the device avoiding tedious mechanical light coupling. However, when a dopant is embedded in a solid matrix, depending on its concentration and the nature of materials involved, the emitted light may be quenched due to aggregation effects. In this work, thin films and ridge waveguides processed by UV-photolithography have been successfully obtained from a selection of standard photopolymerizable organic monomers, SU8, EpoCore and OrmoStamp doped with a selection of standard dyes like Rhodamine-B, Coumarin-540A and Pyrromethene-580. All structures were manufactured on glass substrates. An analysis of the solubility and optical properties including band gap energy, absorption coefficient (alpha ) and fluorescence of the doped photoresists at different concentrations has been performed. Photoresists doped with Rhodamine-B shows a higher energy of indirect allowed band gap transition (2.04–2.09 eV) compared to previously reported pure Rhodamine-B thin films (1.95–1.98 eV). Fabrication protocols of dye doped photoresists covering the entire visible spectrum is established.

High‐Efficiency Luminescent Solar Concentrators Based on Antifogging and Frost‐Resisting Fluorescent Polymers: Adding Multiple Functions for Sustained Performance

AbstractLuminescent solar concentrators (LSCs) are light‐management devices able to harvest and downshift sunlight, making it available to edge‐coupled photovoltaic cells for light‐to‐electricity conversion. When operating in real‐life outdoor scenarios, LSCs can be exposed to humid/freezing environments which may yield fogging and frosting, ultimately resulting in detrimental performance decay owing to reduced photon absorption and photon trapping efficiency within the waveguide. To address this issue, the first demonstration of an antifogging/frost‐resisting highly efficient thin‐film LSC is presented in this work, based on a tailored semi‐interpenetrating polymer network serving as the host matrix material, where the antifogging behavior is imparted by the simultaneous presence of hydrophilic and hydrophobic segments in the macromolecular structure, whereas the fluorescent response is achieved via the incorporation of a coumarin‐functionalized methacrylic monomer. By judicious selection and compositional tuning of an additional luminescent guest dopant with excellent spectral match with the fluorescent host matrix, highly efficient (≈98%) energy transfer is achieved. Optimized LSC devices exhibit external and internal photon efficiencies as high as 3.96% and 34.79%, respectively. In addition, persistent and reproducible antifogging/frost‐resisting functionality is demonstrated during aggressive aging tests under UV‐light, with preserved fluorescence emission and prolonged and sustained device efficiency.

Simple Method to Estimate Fractional Numbers of Eu3+ Ions in Different Phases in Highly Luminescent ZrO2–SiO2 Nanocomposites

Luminescent, transparent nanocomposites incorporating very tiny crystals in glassy host matrix are a strategy to control a local structure with low phonon energy and/or adjustable band structure, and the charge‐transfer state of luminescent centers such as rare‐earth ions, resulting in their high photoluminescence (PL) efficiency and controllable absorption–emission route mechanism. For this purpose, it is important to know how many luminescent dopants can be incorporated into the nanocrystal domains. Herein, Eu3+‐doped ZrO2–SiO2 nanocomposites are considered as a test case, together with the starting gel thermal evolution. The progressive sol–gel chemistry enables the preferential introduction of Eu3+ ions into the ZrO2 nanocrystals. An analytical method using the Eu3+ PL decay curves quantitatively reveals for the first time that the number of Eu3+ ions doped in the ZrO2 crystalline nanodomains, which is obtained above 900 °C, exceeds 60%, and increases to 76% when the thermal treatment temperature is 1100 °C. The fractional number of Eu3+ ions and the local asymmetry ratio of Λ = I(5D0 − 7F2)/I(5D0 − 7F1) for each region are estimated as Λ = 2.1–2.9, Λ ≈ 15, and Λ = 4.6–7.4 for the ZrO2, boundary, and SiO2‐rich glass phases, depending on the heat‐treatment temperature.

Doubly Doped BaZnOS Microcrystals for Multicolor Luminescence Switching

AbstractQuaternary oxysulfide semiconductors with a formula of MZnOS (M = Ca, Sr, or Ba) have recently attracted a lot of attention as host materials for luminescent dopant ions. Among these oxysulfides, the BaZnOS remains largely unexplored for luminescence in comparison with the CaZnOS and SrZnOS counterparts. This study describes a highly efficient and repeatable synthesis of BaZnOS microcrystals comprising a variety of lanthanide and transition metal ions. Spectroscopic investigations reveal a host‐sensitized luminescence in the dopant ions that display distinct temperature dependence of emission intensity. Accordingly, a class of BaZnOS crystals doubly doped with Pr3+ and Mn2+ is further constructed, which shows a sensitive switching of luminescence color in response to temperature change. The advances in these luminescent nanomaterials offer exciting opportunities for temperature sensing (relative sensitivity of 8.58% K–1) and electrochromism applications.

Merging photoinitiated bulk polymerization and the dopant-matrix design strategy for polymer-based organic afterglow materials

The polymer produced by photoinitiated bulk polymerization promotes intersystem crossing of luminescent dopants and switches on room-temperature organic afterglow.

Highly Efficient TADF‐Type Organic Afterglow of Long Emission Wavelengths

AbstractDue to the energy gap law, the direct fabrication of efficient organic afterglow materials with long emission wavelengths at ambient conditions remains challenging. Here, luminescent dopants with moderate kRISC values of 100–101 s−1 are designed to harvest triplet energies, simultaneously improving afterglow efficiency and maintaining emission lifetimes >0.1 s. Organic matrices with large dipole moments are selected to populate the triplet excited states of the luminescent dopants and suppress their nonradiative decay and quenching. The dopant‐matrix systems exhibit TADF‐type organic afterglow with quantum efficiency of 20% to 60% and emission wavelengths exceeding 600 nm. Because of their singlet excited state nature, the TADF‐type afterglow emitters can efficiently transfer excited energy to rhodamine B or cyanine 5.5 fluorescence dyes for the construction of red and near‐infrared afterglow materials which display promising bioimaging applications.