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Surface defect-induced photoluminescence blinking and photodarkening are ubiquitous in lead halide perovskite quantum dots. Despite efforts to stabilize the surface by chemically engineering ligand binding moieties, blinking accompanied by photodegradation still poses barriers to implementing perovskite quantum dots in quantum emitters. To date, ligand tail engineering in the solid state has rarely been explored for perovskite quantum dots. We posit that attractive intermolecular interactions between low-steric ligand tails, such as π-π stacking, can promote the formation of a nearly epitaxial ligand layer that significantly reduces the quantum dot surface energy. Here, we show that single CsPbBr3 quantum dots covered by stacked phenethylammonium ligands exhibit nearly non-blinking single photon emission with high purity (~ 98%) and extraordinary photostability (12 hours continuous operation and saturated excitations), allowing the determination of size-dependent exciton radiative rates and emission line widths of CsPbBr3 quantum dots at the single particle level. Mi et al. report epitaxial surface coverage of single CsPbBr3 quantum dots with size ranging from 3.6 nm to 14 nm using low steric ligand tails with attractive π-π stacking, leading to nearly non-blinking single photon emission with high purity of 98% and photostability over 12-hour irradiation. [ABSTRACT FROM AUTHOR]

Author(s): Dongxin Ma [sup.1] , Kebin Lin [sup.2] , Yitong Dong [sup.1] , Hitarth Choubisa [sup.1] , Andrew H. Proppe [sup.1] , Dan Wu [sup.3] , Ya-Kun Wang [sup.1] , [...], Light-emitting diodes (LEDs) based on perovskite quantum dots have shown external quantum efficiencies (EQEs) of over 23% and narrowband emission, but suffer from limited operating stability.sup.1. Reduced-dimensional perovskites (RDPs) consisting of quantum wells (QWs) separated by organic intercalating cations show high exciton binding energies and have the potential to increase the stability and the photoluminescence quantum yield.sup.2,3. However, until now, RDP-based LEDs have exhibited lower EQEs and inferior colour purities.sup.4-6. We posit that the presence of variably confined QWs may contribute to non-radiative recombination losses and broadened emission. Here we report bright RDPs with a more monodispersed QW thickness distribution, achieved through the use of a bifunctional molecular additive that simultaneously controls the RDP polydispersity while passivating the perovskite QW surfaces. We synthesize a fluorinated triphenylphosphine oxide additive that hydrogen bonds with the organic cations, controlling their diffusion during RDP film deposition and suppressing the formation of low-thickness QWs. The phosphine oxide moiety passivates the perovskite grain boundaries via coordination bonding with unsaturated sites, which suppresses defect formation. This results in compact, smooth and uniform RDP thin films with narrowband emission and high photoluminescence quantum yield. This enables LEDs with an EQE of 25.6% with an average of 22.1 [plus or minus]1.2% over 40 devices, and an operating half-life of two hours at an initial luminance of 7,200 candela per metre squared, indicating tenfold-enhanced operating stability relative to the best-known perovskite LEDs with an EQE exceeding 20%.sup.1,4-6. The efficiency and operating lifetimes of perovskite light-emitting diodes is improved by using a fluorinated triphenylphosphine oxide additive to control the cation diffusion during film deposition and passivate the surface.

Several associations between the built environment and COVID-19 case distribution have been identified in previous studies. However, few studies have explored the non-linear associations between the built environment and COVID-19 at the community level. This study employed the March 2022 Shanghai COVID-19 pandemic as a case study to examine the association between built-environment characteristics and the incidence of COVID-19. A non-linear modeling approach, namely the boosted regression tree model, was used to investigate this relationship. A multi-scale study was conducted at the community level based on buffers of 5-minute, 10-minute, and 15-minute walking distances. The main findings are as follows: (1) Relationships between built environment variables and COVID-19 case distribution vary across scales of analysis at the neighborhood level. (2) Significant non-linear associations exist between built-environment characteristics and COVID-19 case distribution at different scales. Population, housing price, normalized difference vegetation index, Shannon's diversity index, number of bus stops, floor–area ratio, and distance from the city center played important roles at different scales. These non-linear results provide a more refined reference for pandemic responses at different scales from an urban planning perspective and offer useful recommendations for a sustainable COVID-19 post-pandemic response. [ABSTRACT FROM AUTHOR]

Colloidal semiconductor nanocrystals enable the realization and exploitation of quantum phenomena in a controlled manner, and can be scaled up for commercial uses. These materials have become important for a wide range of applications, from ultrahigh definition displays, to solar cells, quantum computing, bioimaging, optical communications, and many more. Over the last decade, lead-halide perovskite nanocrystals have rapidly gained prominence as efficient semiconductors. Although the majority of studies have focused on large nanocrystals in the weak- to intermediate-confinement regime, quantum dots (QDs) in the strongly-confined regime (with sizes smaller than the Bohr diameter, which ranges from 4–12 nm for lead-halide perovskites) offer unique opportunities, including polarized light emission and color-pure, stable luminescence in the region that is unattainable by perovskites with single-halide compositions. In this tutorial review, we bring together the latest insights into this emerging and rapidly growing area, focusing on the synthesis, steady-state optical properties (including exciton fine-structure splitting), and transient kinetics (including hot carrier cooling) of strongly-confined perovskite QDs. We also discuss recent advances in their applications, including single photon emission for quantum technologies, as well as light-emitting diodes. We finish with our perspectives on future challenges and opportunities for strongly-confined QDs, particularly around improving the control over monodispersity and stability, important fundamental questions on the photophysics, and paths forward to improve the performance of perovskite QDs in light-emitting diodes. [ABSTRACT FROM AUTHOR]

Simple Summary: Simple Summary: Mesenchymal stem cells (MSCs) are adult stem cells capable of differentiating into various cell types, including osteoblasts, chondrocytes, and adipocytes. Renowned for their immunomodulatory and regenerative properties, MSCs hold great promise as tools for tissue repair and therapeutic applications. To facilitate the large-scale production of sheep umbilical cord MSCs (UCMSCs), the telomerase reverse transcriptase (TERT) gene was introduced into UCMSCs to promote immortalization. This study provides valuable insights into (a) the proliferation and anti-aging capabilities of transgenic MSCs compared to primary MSCs; (b) the absence of potential tumorigenicity exhibited in TERT-UCMSCs; and (c) the differentiation potential of TERT-UCMSCs. Immortalized MSCs exhibit enhanced proliferation and anti-aging abilities without tumorigenicity concerns, making them a safe immortalized cell line suitable for medical treatments. Mesenchymal stem cells (MSCs) possess significant differentiation potential, making them highly promising in medicine and immunotherapy due to their regenerative capabilities and exosome secretion. However, challenges such as limited cell divisions and complex testing hinder large-scale MSC production. In this study, we successfully established an immortalized MSC line by transfecting the human telomerase reverse transcriptase (TERT) gene into MSCs isolated from pregnant sheep umbilical cords. This approach effectively inhibits cell senescence and promotes cell proliferation, enabling the generation of umbilical cord mesenchymal stem cells (UCMSCs) on a larger scale. Our findings demonstrate that these transfected TERT-UCMSCs exhibit enhanced proliferative capacity and a reduced aging rate compared to regular UCMSCs while maintaining their stemness without tumorigenicity concerns. Consequently, they hold great potential for medical applications requiring large quantities of functional MSCs. [ABSTRACT FROM AUTHOR]

Perovskite quantum dots (QDs) have efficient optical absorption and emission in the visible range, and show a strong quantum confinement effect and high external quantum efficiency. They have been at the forefront of next‐generation photovoltaics and optoelectronics applications. However, two major challenges associated with perovskites and their nanomaterials are poor stability (such as against moisture and polar solvents), as well as the lack of efficient nanopatterning methods. In this work, a promising approach is provided to address both of those major challenges by molecular engineering and integration of QDs with block copolymers (BCP). The BCP thermoplastic elastomers not only substantially improve the stability of perovskite QDs by encapsulating them in a highly stable and soft matrix, but also enable molecular‐level control of the alignment and assembly of perovskite QDs in the microphase‐separated BCP matrix. It is demonstrated that designing and synthesis of compatible polymer ligands for perovskite QDs is key to enabling their selective and strong interaction with the BCP matrix. The structure and molecular weight of the BCP also play an important role in the interfacial structure and optical properties of the QDs‐BCP nanocomposites. Such soft and flexible optical nanocomposites have potential applications in flexible optoelectronics, optical storage, and displays. [ABSTRACT FROM AUTHOR]