Multiphoton excited singlet/triplet mixed self-trapped exciton emission

Multiphoton-Excited Upconversion Luminescence and Amplified Spontaneous Emission from Te4+-Doped Cs2SnCl6 Nanocrystals

High-order nonlinear multiphoton absorption (MPA) is technologically important for a variety of photonic and biological applications owing to its superior advantages over linear absorption and low-order MPA such as greater spatial confinement, larger penetration depth, reduced autofluorescence, and enhanced imaging resolution. However, practical implementation beyond three-photon processes remains notoriously difficult due to the sharp reduction of absorption cross sections with increasing nonlinearity and inherent material instability under high-density irradiation. Herein, we address these challenges through rationally designed Te4+-doped Cs2SnCl6 nanocrystals (NCs), which demonstrate wideband nonlinear responsiveness across 800-2600 nm, allowing achievement of two- to seven-photon absorption (PA) with cross sections outperforming conventional nonlinear optical materials. Particularly, the engineered NCs enable 3PA-excited amplified spontaneous emission (ASE) with an ultralow excitation threshold of 0.22 μJ cm-2 under a 1300 nm femtosecond-pulsed laser excitation, representing 1-4 orders of magnitude improvement compared to existing nonlinear ASE systems. This work presents the excellent 7PA properties in metal halide NCs, positioning lead-free metal halide NCs promising as efficient light-emitting materials for extreme nonlinear nanophotonics.

Circularly Polarized Luminescence in Achiral Tin-Based Perovskites via Structural Isomer-Driven Coordination Interaction

A chiral bidentate ligand, (R)-(-)-1-amino-2-propanol (denoted as R1) or (R)-(-)-2-amino-1-propanol (denoted as R2), was used to modify achiral 2D tin-based perovskite HDASnBr4 (HDA: 1,6-hexamethylenediamine) to form R1-HDASnBr4 or R2-HDASnBr4 by an acid precipitation method. R1-HDASnBr4 exhibits a near-unity photoluminescence quantum yield (PLQY) and strong yellow circularly polarized luminescence (CPL) with a luminescence asymmetry g-factor (|glum|) of 8.3 × 10-3, while R2-HDASnBr4 shows a PLQY of 95% and |glum| of 3.2 × 10-3. Both exhibit strong CPL activities, attributed to the significant centro-asymmetric distortion induced by the interaction between the chiral ligand and the inorganic lattice of 2D perovskites. The |glum| of R1-HDASnBr4 is 2.6× that of R2-HDASnBr4, resulting from the direct coordination of the hydroxyl group attached to the chiral carbons in R1 with the [SnBr6]4- inorganic framework, which induces a higher degree of distortion than the amino group in R2. Furthermore, we explored the potential of R1-HDASnBr4 as a chiral inducer and a CPL source to facilitate asymmetric polymerization. This work offers a simple strategy to introduce chirality to achiral perovskites.

Directed Anion Diffusion Induces Defect Passivation in 0D/3D Perovskite Single-Crystal Heterostructures

0D/3D perovskite heterostructures have been extensively utilized in optoelectronic devices. However, synthesizing 0D/3D heterostructures with well-defined interfaces, high-exposure crystal facet orientations, and high-purity phases remains challenging due to their soft ionic nature, which complicates the understanding of ion defect passivation mechanisms in these heterostructure systems. In this study, a one-step vapor-phase epitaxial growth of pure-phase 0D/3D single-crystal heterostructures is reported with well-defined interfaces. Through a combination of various in situ measurements and theoretical calculations, it is elucidated that defect passivation in the heterostructure arises from the diffusion of bromide anions in the 0D phase across the interface, effectively compensating for vacancies in the 3D component. The 0D/3D heterostructures exhibit enhanced stability in their physical and optical properties compared to pure 3D perovskites. Furthermore, the observed decrease in bromide anion kinetic energy (from 0D to 3D heterointerface) in 0D/3D provides direct evidence of the bromide-rich form of the 3D component. As anticipated, the bulk defect density in the 0D/3D heterostructures is estimated to be an order of magnitude lower than that of pure 3D single crystals. As the first systematic study utilizing a 0D/3D heterostructure model to explore defect self-passivation mechanisms, this work offers crucial insights for designing high-performance perovskite devices.

Superior Multimodal Luminescence in a Stable Single-Host Nanomaterial with Large-Scale Synthesis for High-Level Anti-Counterfeiting and Encryption

Multimode luminescent materials exhibit tunable photon emissions under different excitation or stimuli channels, endowing them high encoding capacity and confidentiality for anti-counterfeiting and encryption. Achieving multimode luminescence into a stable single material presents a promising but remains a challenge. Here, the downshifting/upconversion emissions, color-tuning persistent luminescence (PersL), temperature-dependent multi-color emissions, and hydrochromism are integrated into Er3+ ions doped Cs2NaYbCl6 nanocrystals (NCs) by leveraging shallow defect levels and directed energy migration. The resulting NCs display strong static and dynamic colorful luminescence in response to ultraviolet, 980-nm laser, and X-ray. Additionally, the NCs exhibit distinct luminescent colors as the temperature increases from 330 to 430 K. Surprisingly, it also demonstrates the ability of the reversible emission modal and color in response to water. Theoretical calculations and experimental characterizations reveal that self-trapped exciton state (STEs), chlorine vacancy defects, and ladderlike 4f energy levels of Er3+ ions contribute to multimodal luminescence. More importantly, it has extremely remarkable environmental stability, which can be stored in the air for more than 18 months, showing promising commercial prospects. This work not only gives new insights into lanthanide-based metal halide NCs but also provides a new route for developing multimodal luminescent nanomaterials for anti-counterfeiting and encryption.

Deviatoric stress-induced transition of self-trapped exciton emissions

Self-trapped exciton (STE) emissions, featured by broad spectral band and minimal self-absorption, have garnered considerable attention for advanced lighting and imaging applications. However, developing strategies to facilitate multiple STE states, modulate the emission energy and extend the emission range remains a great challenge. Here, we introduce deviatoric stress to induce another intrinsic STE state (STE-2) and enable transitions between the intrinsic STE state (STE-1) and STE-2 in pyramidal ZnO nanocrystals. This approach results in a remarkable shift in emission energy, from yellow-green (2.34 eV) to deep-blue (2.88 eV). Combined in-situ stress monitoring and optical experiments show that the STE-2 state originates from a potential well generated by the deviatoric yield deformation of the pyramidal crystals under deviatoric stress. Spectroscopic and dynamical characterizations of the two STE emissions reveal a transition process in the carrier's relaxation pathway from STE-2 to STE-1, and conversely at much higher pressures. These findings demonstrate that deviatoric stress serves as a robust tool for modulating STE emissions and provide new insights into the evolution of carrier dynamics of STE emissions.

Hot Carrier Cooling and Multiphoton Absorption of Quasi-Type II ZnSeTe-Based Quantum Dots

Ternary ZnSeTe quantum dots (QDs) are regarded as the most promising Cd-free blue emitters, while their fundamental optical properties such as hot carrier (HC) cooling process and multiphoton absorption (MPA) remain unclear, which will hinder their potential application. In this work, we compare the HC cooling processes of ZnSeTe/ZnSeS and ZnSeTe/ZnSeS/ZnS QDs and find that the HC cooling times of ZnSeTe/ZnSeS/ZnS QDs are insensitive to excitation intensity as a result of the suppressed hot-phonon bottleneck and Auger effect. Importantly, we have determined the two- to five-photon absorption cross sections of two kinds of QDs, highlighting the advantages of ZnS shells in enhancing MPA cross sections of ZnSeTe-based QDs. These insightful findings shed light on the underlying optical properties of ZnSeTe-based QDs and emphasize the critical role of the shell in engineering their HC cooling and MPA performance.

Recent strategies for triplet-state emission regulation toward non-lead organic-inorganic metal halides

Organic-inorganic metal halides (OIMHs) have strengthened the development of triplet-state emission materials due to their excellent luminescence performance. Due to the inherent toxicity of lead (Pb) significantly limiting its further advancement, numerous studies have been conducted to regulate triplet-state emission of non-Pb OIMHs, and several feasible strategies have been proposed. However, most of the non-Pb OIMHs reported have a relatively short lifetime or a low luminescence efficiency, not in favor of their application. In this review, we provide a summary of recent reports on the regulation of triplet-state emissions in non-Pb OIMHs to provide benefits for the design of innovative luminescent materials. Our focus is primarily on exploring the internal and external factors that influence the triplet-state emission. Starting from the luminescence mechanism, the current strategies for regulating triplet-state emissions are summarized. Moreover, by manipulating these strategies, it becomes feasible to achieve triplet-state emissions that span a range of colors from blue to red, and even extend into the near-infrared spectrum with high luminescence efficiency, while also increasing their lifetimes. This review not only provides fresh insights into the advancement of triplet-state emissions in OIMHs but also integrates experimental and theoretical perspectives to illuminate the trajectory of future research endeavors.

Improving Three-Photon Fluorescence of Near-Infrared Quantum Dots for Deep Brain Imaging by Suppressing Biexciton Decay

Three-photon fluorescence microscopy (3PFM) is a promising brain research tool with submicrometer spatial resolution and high imaging depth. However, only limited materials have been developed for 3PFM owing to the rigorous requirement of the three-photon fluorescence (3PF) process. Herein, under the guidance of a band gap engineering strategy, CdTe/CdSe/ZnS quantum dots (QDs) emitting in the near-infrared window are designed for constructing 3PF probes. The formation of type II structure significantly increased the three-photon absorption cross section of QDs and caused the delocalization of electron-hole wave functions. The time-resolved transient absorption spectroscopy confirmed that the decay of biexcitons was significantly suppressed due to the appropriate band gap alignment, which further enhanced the 3PF efficiency of QDs. By utilizing QD-based 3PF probes, high-resolution 3PFM imaging of cerebral vasculature was realized excited by a 1600 nm femtosecond laser, indicating the possibility of deep brain imaging with these 3PF probes.

Combinational antimicrobial activity of biogenic TiO2 NP/ZnO NPs nanoantibiotics and amoxicillin-clavulanic acid against MDR-pathogens

The development of effective strategies against multidrug-resistant (MDR) pathogens is an urgent need in modern medicine. Nanoantibiotics (nABs) offer a new hope in countering the surge of MDR-pathogens. The aim of the current study was to evaluate the antibacterial activity of two attractive nABs, TiO2 NPs and ZnO NPs, and their performance in improving the antimicrobial activity of defined antibiotics (amoxicillin-clavulanic acid, amox-clav) against MDR-pathogens. The nABs were synthesized using a green method. The physicochemical characteristics of the synthesized nanoparticles were determined using standard methods. The results showed the formation of pure anatase TiO2 NPs and hexagonal ZnO NPs with an average particle size of 38.65 nm and 57.87 nm, respectively. The values of zeta potential indicated the high stability of the samples. At 8 mg/mL, both nABs exhibited 100 % antioxidant activity, while ZnO showed significantly higher activity at lower concentrations. The antibiofilm assay showed that both nABs could inhibit the formation of biofilms of Acinetobacter baumannii 80 and Escherichia coli 27G (MDR-isolates). However, ZnO NPs showed superior antibiofilm activity (100 %) against E. coli 27G. The MIC values were determined to be 8 (1), 2 (2), and 4 (4) mg/mL for amox-clav, TiO2 NPs, and ZnO NPs against A. baumannii 80 (E. coli 27G), respectively. The results showed that both nABs had synergistically enhanced antibacterial performance in combination with amox-clav. Specifically, an 8-fold reduction in MIC values of antibiotics was observed when they were combined with nABs. These findings highlight the potential of TiO2 NPs and ZnO NPs as effective nanoantibiotics against MDR-pathogens. The synergistic effect observed when combining nABs with antibiotics suggests a promising approach for combating antibiotic resistance. Further research and development in this area could lead to the development of more effective treatment strategies against MDR infections.

Engineering Sizable and Broad-Spectrum Antibacterial Fabrics through Hydrogen Bonding Interaction and Electrostatic Interaction

Long-lasting and highly efficient antibacterial fabrics play a key role in public health occurrences caused by bacterial and viral infections. However, the production of antibacterial fabrics with a large size, highly efficient, and broad-spectrum antibacterial performance remains a great challenge due to the complex processes. Herein, we demonstrate sizable and highly efficient antibacterial fabrics through hydrogen bonding interaction and electrostatic interaction between surface groups of ZnO nanoparticles and fabric fibers. The production process can be carried out at room temperature and achieve a production rate of 300 × 1 m2 within 1 h. Under both visible light and dark conditions, the bactericidal rate against Gram-positive (S. aureus), Gram-negative (E. coli), and multidrug-resistant (MRSA) bacteria can reach an impressive 99.99%. Furthermore, the fabricated ZnO nanoparticle-decorated antibacterial fabrics (ZnO@fabric) show high stability and long-lasting antibacterial performance, making them easy to develop into variable antibacterial blocks for protection suits.

Nonlinear optical properties in chiral copper oxide nanosheets

Chiral transition metal oxides (TMOs) are in the forefront of research as potential active materials in various optoelectronic applications. However, the nonlinear optical (NLO) properties of the chiral TMOs have not been fully understood. Here, several kinds of copper oxide nanosheets capped with different chiral amino acids are synthesized. Notably, we investigate the NLO activities of these materials, including broadband second harmonic generation and transformation of nonlinear optical properties from saturable absorption to reverse saturable absorption. This work will broaden the use of chiral TMO materials in nonlinear photonic devices.