Broadband Ce(III)-Sensitized Quantum Cutting in Core-Shell Nanoparticles: Mechanistic Investigation and Photovoltaic Application

Near-Infrared Luminescent Materials Incorporating Rare Earth/Transition Metal Ions: From Materials to Applications

The spotlight has shifted to near-infrared (NIR) luminescent materials emitting beyond 1000 nm, with growing interest due to their unique characteristics. The ability of NIR-II emission (1000-1700 nm) to penetrate deeply and transmit independently positions these NIR luminescent materials for applications in optical-communication devices, bioimaging, and photodetectors. The combination of rare earth metals/transition metals with a variety of matrix materials provides a new platform for creating new chemical and physical properties for materials science and device applications. In this review, the recent advancements in NIR emission activated by rare earth and transition metal ions are summarized and their role in applications spanning bioimaging, sensing, and optoelectronics is illustrated. It started with various synthesis techniques and explored how rare earths/transition metals can be skillfully incorporated into various matrixes, thereby endowing them with unique characteristics. The discussion to strategies of enhancing excitation absorption and emission efficiency, spotlighting innovations like dye sensitization and surface plasmon resonance effects is then extended. Subsequently, a significant focus is placed on functionalization strategies and their applications. Finally, a comprehensive analysis of the challenges and proposed strategies for rare earth/transition metal ion-doped near-infrared luminescent materials, summarizing the insights of each section is provided.

Nanocrystals as performance-boosting materials for solar cells

Nanocrystals (NCs) have been widely studied owing to their distinctive properties and promising application in new-generation photoelectric devices. In photovoltaic devices, semiconductor NCs can act as efficient light harvesters for high-performance solar cells. Besides light absorption, NCs have shown great significance as functional layers for charge (hole and electron) transport and interface modification to improve the power conversion efficiency and stability of solar cells. NC-based functional layers can boost hole/electron transport ability, adjust energy level alignment between a light absorbing layer and charge transport layer, broaden the absorption range of an active layer, enhance intrinsic stability, and reduce fabrication cost. In this review, recent advances in NCs as a hole transport layer, electron transport layer, and interfacial layer are discussed. Additionally, NC additives to improve the performance of solar cells are demonstrated. Finally, a summary and future prospects of NC-based functional materials in solar cells are presented, addressing their limitations and suggesting potential solutions.

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.

Core-shell nanostructures: a simplest two-component system with enhanced properties and multiple applications

With the pace of time, synthesis of nanomaterials has paved paths to blend two or more materials having different properties into hybrid nanoparticles. Therefore, it has become possible to combine two different functionalities in a single nanoparticle and their properties can be enhanced or modified by coupling of two different components. Core-shell technology has now represented a new trend in analytical sciences. Core-shell nanostructures are in demand due to their specific design and geometry. They have internal core of one component (metal or biomolecules) surrounded by a shell of another component. Core-shell nanoparticles have great importance due to their high thermal stability, high solubility and lower toxicity. In this review, recent progress in development of new and sophisticated core-shell nanostructures has been explored. The first section covers introduction throwing light on basics of core-shell nanoparticles. Following section classifies core-shell nanostructures into single core/shell, multicore/single shell, single core/multishell and multicore/multishell nanostructures. Next main section gives a brief description on types of core-shell nanomaterials followed by processes for the synthesis of core-shell nanostructures. Ultimately, the final section focuses on the application areas such as drug delivery, bioimaging, solar cell applications etc.

Infrared Photon Pair-Production in Ligand-Sensitized Lanthanide Nanocrystals

This report details spectroscopic characterizations of rare-earth, core-shell nanoparticles decorated with the f-element chelator 3,4,3-LI(1,2-HOPO). Evidence of photon downconversion is corroborated through detailed power dependence measurements, which suggest two-photon decay paths are active in these materials, albeit only representing a minority contribution of the sum luminescence, with emission being dominated by normal, Stokes' shifted fluorescence. Specifically, ultraviolet ligand photosensitization of Nd³⁺ ions in a NaGdF₄ host shell results in energy transfer to a Nd³⁺/Yb³⁺-doped NaGdF₄ nanoparticle core. The population and subsequent decay of core, Yb³⁺²F_5/2 states result in a spectral shift of 620 nm, manifested in a NIR emission displaying luminescence profiles diagnostic of Yb³⁺ and Nd³⁺ excited state decays. Emphasis is placed on the generality of this material architecture for realizing ligand-pumped, multi-photon downconversion, with the Nd³⁺/Yb³⁺ system presented here functioning as a working prototype for a design principle that may be readily extended to other lanthanide pairs.

Design of Lanthanide-Doped Colloidal Nanocrystals: Applications as Phosphors, Sensors, and Photocatalysts

The unique optical characteristics of lanthanides (Ln³⁺) such as high color purity, long excited-state lifetimes, less perturbation of excited states by the crystal field environment, and the easy spectral conversion of wavelengths through upconversion and downconversion processes have caught the attention of many scientists in the recent past. To broaden the scope of using these properties, it is important to make suitable Ln³⁺-doped materials, particularly in colloidal forms. In this feature article, we discuss the different synthesis strategies for making Ln³⁺-doped nanoparticles in colloidal forms, particularly ways of functionalizing hydrophobic surfaces to hydrophilic surfaces to enhance their dispersibility and luminescence in aqueous media. We have enumerated the various strategies and sensitizers utilized to increase the luminescence of the nanoparticles. Furthermore, the use of these colloidal nanoparticle systems in sensing application by the appropriate selection of capping ligands has been discussed. In addition, we have shown how the energy transfer efficiency from Ce³⁺ to Ln³⁺ ions can be utilized for the detection of toxic metal ions and small molecules. Finally, we discuss examples where the spectral conversion ability of these materials has been used in photocatalysis and solar cell applications.

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.

Photophysical characterization of a highly luminescent divalent-europium-containing azacryptate

We report a new luminescent EuII-containing complex. The complex is excited with visible light, leading to emission centered at 447 nm with a lifetime of 1.25 μs. Computational studies suggest that the steric bulk of the ligand is a major factor influencing the wavelength of emission.

Dye-Sensitized Downconversion

Splitting one high-energy photon into two lower energy photons through downconversion has been demonstrated for a variety of combinations of rare earth (RE) ions. However, the low absorption cross section of RE³⁺ 4f-4f transitions hampers practical application. Therefore, enhancing the absorption by sensitization is crucial. We demonstrate efficient dye-sensitized downconversion using a strong blue/UV absorbing Coumarin dye to sensitize downconversion of the Pr³⁺-Yb³⁺ couple in NaYF₄ nanocrystals (NCs). Luminescence spectra and lifetime measurements reveal Förster resonant energy transfer (FRET) from Coumarin to Pr³⁺ in NaYF₄:Pr³⁺Yb³⁺ NCs, followed by downconversion, resulting in Yb³⁺ IR emission with ∼30 times enhancement. The present study demonstrates the feasibility of dye-sensitized downconversion as a promising strategy to engineer strongly absorbing downconversion NCs to enhance the efficiency of photovoltaic cells.

Yb3+-sensitized upconversion and downshifting luminescence in Nd3+ ions through energy migration

A core-shell-shell nanostructure composed of NaGdF₄:Yb/Tm@NaGdF₄:Nd@NaYF₄ is developed to realize Yb³⁺-sensitized upconversion and downshifting luminescence in Nd³⁺ ions. The unusual photon conversion property stems from a gadolinium sublattice mediated Yb³⁺→ Tm³⁺→ Gd³⁺→ Nd³⁺ energy transfer pathway. The energy transfer processes are investigated by varying the dopant concentration and distribution, in conjunction with time decay measurements.