It is conventionally assumed that the growth of monodisperse colloidal nanocrystals requires a temporally discrete nucleation followed by monomer attachment onto the existing nuclei. However, recent studies have reported violations of this classical growth model, and have suggested that inter-particle interactions are also involved during the growth. Mechanisms of nanocrystal growth still remain controversial. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution onto the existing particles or by coalescence between the particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow. We suggest that nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems.

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

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

The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce3+ and Eu2+. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of hos...

Lanthanide-Activated Phosphors Based on 4f-5d Optical Transitions: Theoretical and Experimental Aspects.

The family of oxide glasses is very wide and it is continuously developing. The rapid development of advanced and innovative glasses is under progress. Oxide glasses have a variety of applications in articles for daily use as well as in advanced technological fields such as X-ray protection, fibre glasses, optical instruments and lab glassware. Oxide glasses basically consist of network formers, such as borate, silicate, phosphate, borosilicate, borophosphate, and network modifiers such as alkali, alkaline earth and transition metals. In the present review article, Raman spectroscopy results for the structures of borate, silicate, phosphate, borosilicate, borophosphate, aluminosilicate, phosphosilicate, alumino-borosilicate and tellurite glasses are summarized.

A review of the structures of oxide glasses by Raman spectroscopy

Site preferences of dopant Eu2+ on the locations of K+, Ba2+, and Ca2+ in the mixed cation phosphate K2BaCa(PO4)2 (KBCP) are quantitatively analyzed via a combined experimental and theoretical method to develop a blue-emitting phosphor with thermally stable luminescence. Eu2+ ions are located at K2 (M2) and K3 (M3) sites of KBCP, with the latter occupation relatively more stable than the former, corresponding to emissions at 438 and 465 nm, respectively. KBCP:Eu2+ phosphor exhibits highly thermal stable luminescence even up to 200 °C, which is interpreted as due to a balance between thermal ionization and recombination of Eu2+ 5d excited-state centers with the involvement of electrons trapped at crystal defect levels. Our results can initiate more exploration of activator site engineering in phosphors and therefore allow predictive control of photoluminescence tuning and thermally stable luminescence for emerging applications in white LEDs.

Eu2+ Site Preferences in the Mixed Cation K2BaCa(PO4)2 and Thermally Stable Luminescence.

Mechanoluminescence (ML) materials with long‐wavelength emission bands are essential for future in vivo bioimaging, non‐destructive testing of solids, etc. The lack of a defined mechanism, however, prevents the application of near infrared ML materials above 650 nm in several new fields. Here, the addition of Ga3+ ions to Y3Al5O12: Cr3+ manipulates matrix microstructure evolution, boosting near‐infrared (NIR) zero‐phonon line (ZPL) stress optical output of the Cr3+ ion at 688 nm. The key factor changing the crystal field intensity Dq/B due to the addition of Ga3+ ions is what causes the luminescence amplification of ZPL. The ML fabricated by composite polydimethylsiloxane and Y3Al4GaO12: Cr3+ (YAGG: Cr3+) may penetrate chicken feet epidermal tissue and 4 mm pork tissue thanks to the strong NIR ZPL emission of YAGG: Cr3+ phosphor. This discovery of enhancing near‐infrared ZPL intensity by solid solution provides us with a new technique for optimizing NIR ML materials, as well as a new prospect for NIR ML materials in biological applications.

NIR Mechanoluminescence from Cr3+ Activated Y3Al5O12 with Intense Zero Phonon line

Real-time evolution of up-conversion nanocrystals from tailored metastable intermediates

High-temperature scintillation detectors play a significant role in oil exploration. However, traditional scintillators have limited ability to meet the requirements of practical applications owing to their low thermal stability. In this study, we designed and developed a one-dimensional (1D) Cs5Cu3Cl6I2 scintillator with high thermal stability. In addition, by preparing Cs5Cu3Cl7I, we proved that the Cs5Cu3Cl6I2 scintillator exhibits high thermal stability because the bridges linking the structural units in the 1D chain structure are only formed by I- ions, which improve their structural rigidity. The scintillator has a high steady-state light yield (59,700 photons MeV-1) and exhibits the highest spatial resolution for powder-based scintillation screens (18 lp mm-1) after cyclic treatment within the temperature range of 298-423 K. The Cs5Cu3Cl6I2 scintillator allows the visualization of alloy melting, indicating that it has significant potential for application in high-temperature environments. This study provides a new perspective toward the design of scintillators with high thermal stability.

High Thermal Stability of Copper-Based Perovskite Scintillators for High-Temperature X-ray Detection.

Strain-sensitive mechanoluminescence with high stability and self-recovery toward stretchable strain sensor

X-ray time-lapse imaging allows for precise radiographic imaging of the item after X-ray has been terminated, which is important for simplifying legacy X-ray detectors and avoiding health hazards caused by ionizing radiation during the imaging process. However, the premature discharge of carriers from shallow traps at room temperature over time leads to the loss of imaging information, making an accurate representation of the recorded information to achieve accurate time-lapse imaging still a challenge. Here, it is demonstrated that monodispersed NaLuF4: Tb3+ nanocrystals with the replacement of Na+ by Cs+ ions in the lattice matrix realize a superior X-ray time-lapse imaging. It is found that the incorporation of Cs+ ions amplifies the deep trap storage capacity, which ensures the stable presence of carriers under thermal perturbation at room temperature. Imaging information with a spatial resolution of 12.0 lp mm−1 can be accurately represented even after the irradiation of the X-ray has been terminated for 192 h. X-ray time-lapse imaging not only allows accurate reading of long-term stored imaging information but also endows the visualization of X-ray imaging independent of the dangerous radiation, which will offer great opportunities for X-ray inspection on a larger scale. 

Xuhui Xu

Papers

NIR Mechanoluminescence from Cr3+ Activated Y3Al5O12 with Intense Zero Phonon line

Real-time evolution of up-conversion nanocrystals from tailored metastable intermediates

High Thermal Stability of Copper-Based Perovskite Scintillators for High-Temperature X-ray Detection.

Strain-sensitive mechanoluminescence with high stability and self-recovery toward stretchable strain sensor

Precision X‐Ray Time‐Lapse Flexible Imaging from Fluoride Nanocrystals with the Constructed Deep Traps