Experimental and Simulation Insights into Local Structure and Luminescence Evolution in Eu3+-Doped Nanocrystals under High Pressure

Boosting Upconversion Efficiency in Optically Inert Shelled Structures with Electroactive Membrane through Electron Donation

This study innovatively addresses challenges in enhancing upconversion efficiency in lanthanide-based nanoparticles (UCNPs) by exploiting Shewanella oneidensis MR-1, a microorganism capable of extracellular electron transfer. Electroactive membranes, rich in c-type cytochromes, are extracted from bacteria and integrated into membrane-integrated liposomes (MILs), encapsulating core-shelled UCNPs with an optically inactive shell, forming UCNP@MIL constructs. The electroactive membrane, tailored to donate electrons through the inert shell, independently boosts upconversion emission under near-infrared excitation (980 or 1550 nm), bypassing ligand-sensitized UCNPs. The optically inactive shell restricts energy migration, emphasizing electroactive membrane electron donation. Density functional theory calculations elucidate efficient electron transfer due to the electroactive membrane hemes' highest occupied molecular orbital being higher than the valence band maximum of the optically inactive shell, crucial for enhancing energy transfer to emitter ions. The introduction of a SiO2 insulator coating diminishes light enhancement, underscoring the importance of unimpeded electron transfer. Luminescence enhancement remains resilient to variations in emitter or sensitizing ions, highlighting the robustness of the electron transfer-induced phenomenon. However, altering the inert shell material diminishes enhancement, emphasizing the role of electron transfer. This methodology holds significant promise for diverse biological applications. UCNP@MIL offers an advantage in cellular uptake, which proves beneficial for cell imaging.

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Negative linear compressibility and strong enhancement of emission in Eu[Ag(CN)2]3·3H2O under pressure

The framework material Eu[Ag(CN)2]3·3H2O exhibits a negative linear compressibility (NLC) of -4.2(1) TPa-1 over the largest pressure range yet observed (0-8.2 GPa). High-pressure single-crystal X-ray diffraction data show that the rapid contraction of the Kagome silver layers under compression causes the wine-rack lattice to expand along the c-axis. The hydrogen bonds between the water molecules and the main frameworks constrain the structural deformation under pressure and eventually a weak NLC effect generated. Furthermore, we found that the pressure-induced emission intensity increases almost 800-fold at 4.0 GPa, followed by a gradual decrease and disappearance at 8.1 GPa. Under compression, high pressure significantly tunes the triplet level positions near the Eu3+ ions, and horizontal displacement between a quenching excited state and the excited levels of Eu3+ facilitates the energy transfer process to the 5D0 excited state and limits the nonradiative corssover at elevated pressures, thus increasing the emission intensity. In addition, we observe a gradual band gap reduction with increasing pressure, and the sample could not be returned to the initial state after the pressure was completely released. By controlling the structural flexibility, we observe a coupled NLC and pressure-induced strong enhancement of the emission properties of Eu[Ag(CN)2]3·3H2O, which provides a new route for the design of new optical devices with intriguing luminescence properties under extreme environments.

Recent Developments of Microscopic Study for Lanthanide and Manganese Doped Luminescent Materials

Luminescent materials are indispensable for applications in lighting, displays and photovoltaics, which can transfer, absorb, store and utilize light energy. Their performance is closely related with their size and morphologies, exact atomic arrangement, and local configuration about photofunctional centers. Advanced electron microscopy-based techniques have enabled the possibility to study nanostructures with atomic resolution. Especially, with the advanced micro-electro-mechanical systems, it is able to characterize the luminescent materials at the atomic scale under various environments, providing a deep understanding of the luminescent mechanism. Accordingly, this review summarizes the recent achievements of microscopic study to directly image the microstructure and local environment of activators in lanthanide and manganese (Ln/Mn²⁺ )-doped luminescent materials, including: 1) bulk materials, the typical systems are nitride/oxynitride phosphors; and 2) nanomaterials, such as nanocrystals (hexagonal-phase NaLnF₄ and perovskite) and 2D nanosheets (Ca₂ Ta₃ O₁₀ and MoS₂ ). Finally, the challenges and limitations are highlighted, and some possible solutions to facilitate the developments of advanced luminescent materials are provided.

Arel: Investigating [Eu(H2O)9]3+ Photophysics and Creating a Method to Bypass Luminescence Quantum Yield Determinations

Lanthanide luminescence has been treated separate from molecular photophysics, although the underlying phenomena are the same. As the optical transitions observed in the trivalent lanthanide ions are forbidden, they do belong to the group that molecular photophysics has yet to conquer, yet the experimental descriptors remain valid. Herein, the luminescence quantum yields (ϕ_lum), luminescence lifetimes (τ_obs), oscillator strengths (f), and the rates of nonradiative (k_nr) and radiative (k_r ≡ A) deactivation of [Eu(H₂O)₉]³⁺ were determined. Further, it was shown that instead of a full photophysical characterization, it is possible to relate changes in transition probabilities to the relative parameter A_rel, which does not require reference data. While A_rel does not afford comparisons between experiments, it resolves emission intensity changes due to emitter properties from intensity changes due to environmental effects and differences in the number of photons absorbed. When working with fluorescence this may seem trivial; when working with lanthanide luminescence it is not.

Rare-Earth Doping in Nanostructured Inorganic Materials

Impurity doping is a promising method to impart new properties to various materials. Due to their unique optical, magnetic, and electrical properties, rare-earth ions have been extensively explored as active dopants in inorganic crystal lattices since the 18th century. Rare-earth doping can alter the crystallographic phase, morphology, and size, leading to tunable optical responses of doped nanomaterials. Moreover, rare-earth doping can control the ultimate electronic and catalytic performance of doped nanomaterials in a tunable and scalable manner, enabling significant improvements in energy harvesting and conversion. A better understanding of the critical role of rare-earth doping is a prerequisite for the development of an extensive repertoire of functional nanomaterials for practical applications. In this review, we highlight recent advances in rare-earth doping in inorganic nanomaterials and the associated applications in many fields. This review covers the key criteria for rare-earth doping, including basic electronic structures, lattice environments, and doping strategies, as well as fundamental design principles that enhance the electrical, optical, catalytic, and magnetic properties of the material. We also discuss future research directions and challenges in controlling rare-earth doping for new applications.

Responsive Sensors of Upconversion Nanoparticles

Upconversion nanoparticles are a class of luminescent materials that convert longer-wavelength near-infrared photons into visible and ultraviolet emissions. They can respond to various external stimuli, which underpins many opportunities for developing the next generation of sensing technologies. In this perspective, the unique stimuli-responsive properties of upconverting nanoparticles are introduced, and their recent implementations in sensing are summarized. Promising material development strategies for enhancing the key sensing merits, including intrinsic sensitivity, biocompatibility and modality, are identified and discussed. The outlooks on future technological developments, novel sensing concepts, and applications of nanoscale upconversion sensors are provided.

Local Structure Engineering in Lanthanide-Doped Nanocrystals for Tunable Upconversion Emissions

Upconversion emissions from lanthanide-doped nanocrystals have sparked extensive research interests in nanophotonics, biomedicine, photovoltaics, photocatalysis, etc. Rational modulation of upconversion emissions is highly desirable to meet the requirements of specific applications. Among the diverse developed methods, local structure engineering is fundamentally feasible, through which the upconversion emission intensity, selectivity, wavelength shift, and lifetime can be tuned effectively. The underlying mechanism of the local-structure-dependent upconversion emissions lies in the degree of parity hybridization and energy level splitting of lanthanide ions as well as the interionic energy transfer efficiency. Over the past few years, there has been significant progress in local-structure-engineered upconversion emissions. In this Perspective, we first introduce the principles of upconversion emissions and typical characterization methods for local structure. Subsequently, we summarize recent achievements in tuning of upconversion emissions through local structure engineering, including host composition adjustment, external field regulation, and interfacial strain management. Finally, we propose a few perspectives that should tackle the current bottlenecks. This Perspective is expected to deepen the understanding of local-structure-dependent upconversion emissions and arouse adequate attention to the engineering of local structure for desired properties of inorganic nanocrystals.

Local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for biological applications

This feature article overviews the recent advances in the local-structure-dependent luminescence in lanthanide-doped inorganic nanocrystals for various biological applications.

Strong structural phase sensitive rare-earth photoluminescence color flips in KLaF4:RE3+ (RE3+ = Eu3+, Er3+/Yb3+) nanocrystals

Systematic and strong rare-earth photoluminescence (PL) color flips that are highly sensitive to structural phase transformation in KLaF₄:RE³⁺ (RE³⁺ = Eu³⁺, Er³⁺/Yb³⁺) nanocrystals are demonstrated.