Dramatic Enhancement of Quantum Cutting in Lanthanide-Doped Nanocrystals Photosensitized with an Aggregation-Induced Enhanced Emission Dye

Activatable Lanthanide Nanoprobes with Dye-Sensitized Second Near-Infrared Luminescence for in Vivo Inflammation Imaging

Lanthanide nanoparticles exhibit unique photophysical properties and thus emerge as promising second near-infrared (NIR-II) optical agents. However, the limited luminescence brightness hampers their construction of activatable NIR-II probes. Herein, we report the synthesis of dye-sensitized lanthanide nanoprobes (NaGdF₄:Nd/ICG; indocyanine green (ICG)) and their further development for in vivo activatable imaging of hypochlorite (ClO^-). Dye sensitization using ICG not only shifts the optimal doping concentration of Nd³⁺ from 5 to 20 mol % but also leads to a 5-fold NIR-II enhancement relative to the ICG-free counterpart. Mechanistic studies reveal that such a luminescence enhancement of NaGdF₄:Nd at high Nd³⁺ concentration is ascribed to an alleviated cross-relaxation effect due to the broad absorption of ICG and faster energy transfer process. Taking advantage of dye oxidation, the nanoprobes enable activatable NIR-II imaging of hypochlorous acid (ClO^-) in a drug-induced lymphatic inflammation mouse model. This work thus provides a simple, yet effective luminescence enhancement strategy for constructing lanthanide nanoprobes at higher activator doping concentration toward activatable NIR-II molecular imaging.

Tuning Phonon Energies in Lanthanide-doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion

Optical applications of lanthanide-doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate the size-controlled synthesis of low-phonon-energy KPb₂ X₅ (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm^-1 . KPb₂ Cl₅ nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb₂ X₅ nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche-like emission from KPb₂ Cl₅  : Nd³⁺ nanoparticles. The realization of nanoparticles with tunable, ultra-low phonon energies facilitates the discovery of nanomaterials with phonon-dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.

Dye-sensitized lanthanide containing nanoparticles for luminescence based applications

Due to their exceptional luminescent properties, lanthanide (Ln) complexes represent a unique palette of probes in the spectroscopic toolkit. Their extremely weak brightness due to forbidden Ln electronic transitions can be overcome by indirect dye-sensitization from the antenna effect brought by organic ligands. Despite the improvement brought by the antenna effect, (bio)analytical applications with discrete Ln complexes as luminescent markers still suffers from low sensitivity as they are limited by the complex brightness. Thus, there is a need to develop nano-objects that cumulate the spectroscopic properties of multiple Ln ions. This review firstly gives a brief introduction of the spectral properties of lanthanides both in complexes and in nanoparticles (NPs). Then, the research progress of the design of Ln-doped inorganic NPs with capping antennas, Ln-complex encapsulated NPs and Ln-complex surface functionalized NPs is presented along with a summary of the various photosensitizing ligands and of the spectroscopic properties (excited-state lifetime, brightness, quantum yield). The review also emphasizes the problems and limitations encountered over the years and the solutions provided to address them. Finally, a comparison of the advantages and drawbacks of the three types of NP is provided as well as a conclusion about the remaining challenges both in the design of brighter NPs and in the luminescence based applications.

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.

NIR-II Upconversion Photoluminescence of Er3+ Doped LiYF4 and NaY(Gd)F4 Core-Shell Nanoparticles

The availability of colloidal nano-materials with high efficiency, stability, and non-toxicity in the near infrared-II range is beneficial for biological diagnosis and therapy. Rare earth doped nanoparticles are ideal luminescent agents for bio-applications in the near infrared-II range due to the abundant energy level distribution. Among them, both excitation and emission range of Er³⁺ ions can be tuned into second biological window range. Herein, we report the synthesis of ∼15 nm LiYF₄, NaYF₄, and NaGdF₄ nanoparticles doped with Er³⁺ ions and their core-shell structures. The luminescent properties are compared, showing that Er³⁺ ions with single-doped LiYF₄ and NaYF₄ nanoparticles generate stronger luminescence than Er³⁺ ions with doped NaGdF₄, despite the difference in relative intensity at different regions. By epitaxial growth an inert homogeneous protective layer, the surface luminescence of the core-shell structure is further enhanced by about 5.1 times, 6.5 times, and 167.7 times for LiYF₄, NaYF₄, and NaGdF₄, respectively. The excellent luminescence in both visible and NIR range of these core-shell nanoparticles makes them potential candidate for bio-applications.

Temperature dependent quantum cutting in cubic BaGdF5:Eu3+ nanophosphors

A task-specific ionic liquid (IL) is employed as a structure directing agent for the synthesis of quantum cutting BaGdF₅:Eu³⁺ nanophosphors.

Quantum cutting-induced near-infrared luminescence of Yb3+ and Er3+ in a layer structured perovskite film

Quantum cutting is an attractive optical phenomenon where one high-energy photon is converted into two low-energy photons, resulting in photoluminescence quantum yields (PLQYs) above 100%. In this report, we demonstrate a novel approach to enhance the quantum cutting energy transfer from an all-inorganic perovskite (CsPbCl₃) to ytterbium (Yb³⁺) and erbium (Er³⁺) ions as near-infrared (NIR) emitters by using the highly orientated crystalline film. Yb³⁺ ions are fixed in the neighborhood of the CsPbCl₃ lattice by preparing a one-to-one layer arrangement consisting of quasi-2D CsPbCl₃ perovskite and Yb³⁺ layers. The successful preparation of layer arrangements resulted in the highly sensitized luminescence of Yb³⁺ by CsPbCl₃ with NIR PLQYs exceeding 130%, which is attributed to quantum cutting. In addition, Er³⁺ luminescence at 1540 nm is acquired by the co-existence of Er³⁺ with Yb³⁺ in a layer, which is a result of the intralayer metal-to-metal energy transfer from Yb³⁺ activated by CsPbCl₃ via the interlayer quantum cutting process. The PLQY of Er³⁺ luminescence reaches to 12.6%, which is the highest value ever observed for Er³⁺ compounds, resulting from the efficient interlayer quantum cutting process over 100% and the following intralayer resonance metal to metal energy transfer with the efficiency over 80%.

Anion Exchange and the Quantum-Cutting Energy Threshold in Ytterbium-Doped CsPb(Cl1- xBr x)3 Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals of CsPbCl₃ doped with Yb³⁺ have demonstrated remarkably high sensitized photoluminescence quantum yields (PLQYs), approaching 200%, attributed to a picosecond quantum-cutting process in which one photon absorbed by the nanocrystal generates two photons emitted by the Yb³⁺ dopants. This quantum-cutting process is thought to involve a charge-neutral defect cluster within the nanocrystal's internal volume. We demonstrate that Yb³⁺-doped CsPbCl₃ nanocrystals can be converted postsynthetically to Yb³⁺-doped CsPb(Cl_{1- x}Br _x)₃ nanocrystals without compromising the desired high PLQYs. Nanocrystal energy gaps can be tuned continuously from E_g ≈ 3.06 eV (405 nm) in CsPbCl₃ down to E_g ≈ 2.53 eV (∼490 nm) in CsPb(Cl_0.25Br_0.75)₃ while retaining a constant PLQY above 100%. Reducing E_g further causes a rapid drop in PLQY, interpreted as reflecting an energy threshold for quantum cutting at approximately twice the energy of the Yb³⁺²F_7/2 → ²F_5/2 absorption threshold. These data demonstrate that very high quantum-cutting energy efficiencies can be achieved in Yb³⁺-doped CsPb(Cl_{1- x}Br _x)₃ nanocrystals, offering the possibility to circumvent thermalization losses in conventional solar technologies. The presence of water during anion exchange is found to have a deleterious effect on the Yb³⁺ PLQYs but does not affect the nanocrystal shapes or morphologies, or even reduce the excitonic PLQYs of analogous undoped CsPb(Cl_{1- x}Br _x)₃ nanocrystals. These results provide valuable information relevant to the development and application of these unique materials for spectral-shifting solar energy conversion technologies.

Probing Energy Migration through Precise Control of Interfacial Energy Transfer in Nanostructure

A novel mechanistic strategy for probing the energy migration through constructing the interfacial energy transfer (IET) in a core-shell-shell nanostructure is reported. In this design, the trilayer nanostructure is composed of a sensitizing core, a migratory interlayer, and a detective shell layer that interact with each other only by IET and the latter two shell layers are nonresponsive to the incident irradiation. This model is well applied in investigating the energy migration over the Tb, Gd, and Yb sublattices, and the results show that the Gd sublattice holds the best energy migratory performance. Moreover, the finding of energy migration over the Yb sublattice enables the 808 nm excited long-lived upconversion of Tb³⁺ and Eu³⁺ , which exhibits unique time-gating performance for information security. The results provide a facile and powerful nanosized model for an in-depth understanding of the fundamentals involving lanthanide interactions, which will further help excite new chances for the frontier applications of lanthanide-based luminescent materials.

Efficient Visible-to-NIR Spectral Conversion for Polycrystalline Si Solar Cells and Revisiting the Energy Transfer Mechanism from Ce3+ to Yb3+ in Lu3Al5O12 Host

The so-called Shockley-Queisser converting efficiency limit of Si solar cells is believed to be surpassed by using the spectral converter. However, searching for efficient spectral converting materials is still a challenging task. In this paper, efficient visible-to-NIR spectral conversion for polycrystalline Si solar cells has been demonstrated in Ce³⁺ and Yb³⁺ codoped Lu₃Al₅O₁₂. Moreover, the underlying energy transfermechanism from Ce³⁺ to Yb³⁺ is systematically re-investigated by the detailed excitation and emission spectra as well as fluorescent decay curves, and our results demonstrate that fast metal-to-metal charge transfer from Ce³⁺ to nearby Yb³⁺ is the dominant energy transfermechanism. Finally, we provide new evidence that Ce⁴⁺-Yb²⁺ charge-transfer state is responsible for the relatively low quantum efficiency of NIR emission in Ce³⁺ and Yb³⁺ codoped system.

Synthesis and photocatalytic activity of δ-doped hexagonal NaYF4:Yb,Tm@TiO2/RGO nanocrystals

Improved photocatalytic activity of δ-doped β-NaYF₄:Yb,Tm@TiO₂/RGO nanocrystals.