Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

The development of new layered materials has experienced an evolution from graphene to metal oxide and metal chalcogenide nanosheets, and more recently to two-dimensional (2D) covalent organic frameworks, such as conjugated carbon nitride nanosheets (CNNs) with a spectral gap in the band structure. The anisotropic 2D geometric morphology, together with the aromatic π-conjugated framework, endows polymeric CNNs with unique properties, such as an enlarged surface area with a highly opened-up flat structure, reduced thickness with enhanced electron mobility and with intrinsic semiconductive features, which support their attractive bandgap- and surface-engineered applications ranging from energy-related topics to other new emerging fields. In this review, recent research advances in the establishment of two synthetic strategies for CNNs are firstly overviewed, namely, the top-down delamination of graphitic carbon nitride (CN) solids and the bottom-up assembly of molecular building blocks in a 2D manner. Efficient approaches aimed at advancing CNNs for target-specific applications, including hybridizing, doping, sensitization, copolymerization and nanorefinement, are also described as possible solutions.

Two-dimensional covalent carbon nitride nanosheets: synthesis, functionalization, and applications

Owing to the unique mechanism of photoelectron storage and release, long persistent phosphorescence, also called long persistent luminescence or long lasting afterglow/phosphorescence, plays a pivotal role in the areas of spectroscopy, photochemistry, photonics and materials science. In recent years, more research has focused on the manipulation of the morphology, operational wavebands and persistent duration of long persistent phosphors (LPPs). These desired achievements stimulated the growing interest in designing bio-labels, photocatalysts, optical sensors, detectors and photonic devices. In this review, we present multidisciplinary research on synthetic methods, afterglow mechanisms, characterization techniques, materials system, and applications of LPPs. First, we introduce the recent developments in LPPs for the synthesis of nanoparticles from the aspects of particle sizes, monodispersity and homogeneity based on the urgent application of bio-imaging. In the later sections, we present the possible mechanisms, which involve the variation of trap distribution during the trapping and de-trapping process, complicated photo-ionization reaction of trap site levels and impurity centers together with their corresponding migration kinetics of carriers. Meanwhile, we emphasize the characterization techniques of defects, used to qualitatively or quantitatively describe the types, concentrations and depths of the traps. This review article also highlights the recent advances in suggested LPPs materials with a focus on the LPPs' hosts and optically active centers as well as their control, tuning and intrinsic links. We further discuss the classification of LPPs based on the different emission and excitation wavebands from the ultraviolet to the near-infrared region along with an overview of the activation mode of afterglow. Afterwards, we provide an exhibition of new products towards diverse application fields, including solar energy utilization, bio-imaging, diagnosis, and photocatalysts. Finally, we summarize the current achievements, discuss the problems and provide suggestions for potential future directions in the aforementioned parts.

Long persistent phosphors—from fundamentals to applications

Two-dimensional (2D) nanomaterials are ultrathin nanomaterials with a high degree of anisotropy and chemical functionality. Research on 2D nanomaterials is still in its infancy, with the majority of research focusing on elucidating unique material characteristics and few reports focusing on biomedical applications of 2D nanomaterials. Nevertheless, recent rapid advances in 2D nanomaterials have raised important and exciting questions about their interactions with biological moieties. 2D nanoparticles such as carbon-based 2D materials, silicate clays, transition metal dichalcogenides (TMDs), and transition metal oxides (TMOs) provide enhanced physical, chemical, and biological functionality owing to their uniform shapes, high surface-to-volume ratios, and surface charge. Here, we focus on state-of-the-art biomedical applications of 2D nanomaterials as well as recent developments that are shaping this emerging field. Specifically, we describe the unique characteristics that make 2D nanoparticles so valuable, as well as the biocompatibility framework that has been investigated so far. Finally, to both capture the growing trend of 2D nanomaterials for biomedical applications and to identify promising new research directions, we provide a critical evaluation of potential applications of recently developed 2D nanomaterials.

Two‐Dimensional Nanomaterials for Biomedical Applications: Emerging Trends and Future Prospects

During the past few decades, the research on persistent luminescent materials has focused mainly on Eu2+-doped compounds. However, the yearly number of publications on non-Eu2+-based materials has also increased steadily. By now, the number of known persistent phosphors has increased to over 200, of which over 80% are not based on Eu2+, but rather, on intrinsic host defects, transition metals (manganese, chromium, copper, etc.) or trivalent rare earths (cerium, terbium, dysprosium, etc.). In this review, we present an overview of these non-Eu2+-based persistent luminescent materials and their afterglow properties. We also take a closer look at some remaining challenges, such as the excitability with visible light and the possibility of energy transfer between multiple luminescent centers. Finally, we summarize the necessary elements for a complete description of a persistent luminescent material, in order to allow a more objective comparison of these phosphors.

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Persistent Luminescence in Non-Eu2+-Doped Compounds: A Review.

Graphene-like g-C3N4 nanosheet (GCN)/Fe3O4 quantum dot (QD) nanocomposites were successfully synthesized by a facile electrostatic self-assembly method. Characterization shows that the GCN is at least several micrometers in size. The GCN/Fe3O4 nanocomposites were used as photocatalysts for degradation of Rhodamine B (RhB) under visible light irradiation. After irradiation for 1.5 h, the degradation efficiency was 72.5% for pure g-C3N4, 81% for GCN-1 wt% Fe3O4, 95% for GCN-2 wt% Fe3O4, 60.46% for GCN-3 wt% Fe3O4 and 57.2% for GCN-4 wt% Fe3O4, indicating that GCN-2 wt% Fe3O4 nanocomposites had the highest photocatalytic activity. We deduce that the efficient separation of the photogenerated electron–hole pairs and the high specific surface area of GCN play important roles in the photocatalytic activity of the nanocomposites. In addition, the nanocomposites can be loaded with a model drug (Rhodamine B) and the loading capacity was as high as 108.6 mg g−1, making it a potential candidate for photocatalysis and controlled magnetically targeted drug delivery.

Synthesis of graphene-like g-C3N4/Fe3O4 nanocomposites with high photocatalytic activity and applications in drug delivery

Luminescence properties of a new Mn2+-activated red long-afterglow phosphor

In this study, we report here a general protocol for making core-shell magnetic Fe3O4/SiO2-MPS/MIPs (MPS = 3-(methacryloxyl) propyl trimethoxysilane, MIPs = molecularly imprinted polymers, Fe3O4/SiO2-MPS as core, MIPs as shell) via a surface molecular imprinting technique for optical detection of trace λ-cyhalothrin. The fluorescent molecularly imprinted polymer shell was first prepared by copolymerization of acrylamide with a small quantity of allyl fluorescein in the presence of λ-cyhalothrin to form recognition sites without doping. The magnetic Fe3O4/SiO2-MPS/MIPs exhibited paramagnetism, high fluorescence intensity, and highly selective recognition. Using fluorescence quenching as a detecting tool, Fe3O4/SiO2-MPS/MIPs were successfully applied to selectively and sensitively detect λ-cyhalothrin, and a linear relationship could be obtained covering a wide concentration range of 0–50 nM with a correlation coefficient of 0.9962 described by the Stern-Volmer equation. The experimental results of practical detection revealed that magnetic Fe3O4/SiO2-MPS/MIPs as an attractive recognition element was satisfactory for determination of trace λ-cyhalothrin in honey samples. This study, therefore, demonstrated the potential of MIPs for detection of λ-cyhalothrin in food.

A core-shell surface magnetic molecularly imprinted polymers with fluorescence for λ -cyhalothrin selective recognition

Leaf-like BiVO4 nanostructure decorated by nitrogen-doped carbon quantum dots: Binary heterostructure photocatalyst for enhanced photocatalytic performance

Visible-light-driven Ag3PO4/graphite-like carbon nitride/Ag2WO4 photocatalysts with different weight fractions of Ag3PO4 were synthesized. Ag2WO4 nanorods with a scale of 500 nm to 3 μm were prepared by using a hydrothermal reaction. Via a facile deposition–precipitation technique, graphite-like carbon nitride (g-C3N4) quantum dots and Ag3PO4 nanocrystals were then deposited onto the surface of Ag2WO4 nanorods sequentially. Under visible-light irradiation (λ > 420 nm), the Ag3PO4/g-C3N4/Ag2WO4 nanorods degraded Rh B efficiently and displayed much higher photocatalytic activity than that of pure Ag2WO4 and the g-C3N4/Ag2WO4 composite, and the Ag3PO4/g-C3N4/Ag2WO4 hybrid photocatalyst with 30 wt% of Ag3PO4 exhibited the highest photocatalytic activity. The quenching effects of different scavengers demonstrated that reactive h+ and ·O2− played the major roles in Rh B degradation. It was elucidated that the excellent photocatalytic activity of Ag3PO4/g-C3N4/Ag2WO4 for the degradation of Rh B under visible light (λ > 420 nm) can be ascribed to the efficient separation of photogenerated electrons and holes through the Ag3PO4/g-C3N4/Ag2WO4 heterostructure.

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Ag3PO4 nanocrystals and g-C3N4 quantum dots decorated Ag2WO4 nanorods: ternary nanoheterostructures for photocatalytic degradation of organic contaminants in water

Xiu-Ying Li

Papers

Synthesis of graphene-like g-C3N4/Fe3O4 nanocomposites with high photocatalytic activity and applications in drug delivery

Luminescence properties of a new Mn2+-activated red long-afterglow phosphor

A core-shell surface magnetic molecularly imprinted polymers with fluorescence for λ -cyhalothrin selective recognition

Leaf-like BiVO4 nanostructure decorated by nitrogen-doped carbon quantum dots: Binary heterostructure photocatalyst for enhanced photocatalytic performance

Ag3PO4 nanocrystals and g-C3N4 quantum dots decorated Ag2WO4 nanorods: ternary nanoheterostructures for photocatalytic degradation of organic contaminants in water