Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2.

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Tailoring Broadband Nonlinear Optical Characteristics and Ultrafast Photocarrier Dynamics of Bi2O2S Nanosheets by Defect Engineering.

Low-dimensional bismuth oxychalcogenides have shown promising potential in optoelectronics due to their high stability, photoresponse, and carrier mobility. However, the relevant studies on deep understanding for Bi2O2S is quite limited. Here, comprehensive experimental and computational investigations are conducted in the regulated band structure, nonlinear optical (NLO) characteristics, and carrier dynamics of Bi2O2S nanosheets via defect engineering, taking O vacancy (OV) and substitutional Se doping as examples. As the OV continuously increased to ≈35%, the optical bandgaps progressively narrow from ≈1.21 to ≈0.81 eV and NLO wavelengths are extended to near-infrared regions with enhanced saturable absorption. Simultaneously, the relaxation processes are effectively accelerated from tens of picoseconds to several picoseconds, as the generated defect energy levels can serve as both additional absorption cross-sections and fast relaxation channels supported by theoretical calculations. Furthermore, substitutional Se doping in Bi2O2S nanosheets also modulate their optical properties with the similar trends. As a proof-of-concept, passively mode-locked pulsed lasers in the ≈1.0 µm based on the defect-rich samples (≈35% OV and ≈50% Se-doping) exhibit excellent performance. This work deepens the insight of defect functions on optical properties of Bi2O2S nanosheets and provides new avenues for designing advanced photonic devices based on low-dimensional bismuth oxychalcogenides.

Design, Synthesis, and Ultrafast Carrier Dynamics of Core-Substituted Naphthalene Diimide-Based Covalent Organic Frameworks.

A series of two-dimensional polyimide covalent organic frameworks (2D COF) based on core-substituted naphthalene diimides (cNDIs) were designed and synthesized with the characteristic of tunable bandgap without global structural changes. Cyclic voltammetry (CV) and DFT calculations indicated that COFcNDI-OEt and COFcNDI-SEt possess higher HOMO/LUMO levels and narrower bandgaps than COFNDI-H. Further investigation indicated that the COF bandgaps are not only related to the electron-donating substituents but also varied with respect to the interlayer distances. Moreover, the femtosecond transient absorption (TA) spectra manifested that the electron donor substituents are beneficial to the charge delocalization in the π-columnar unit, resulting in a longer lifetime of charge recombination, which is one of the pivotal prerequisites for high-performance solar cells and photocatalysis.

Unveiling the energy transfer mechanism between aqueous colloidal NIR-II quantum dots and water.

Hydrophilic semiconductor quantum dots (QDs) with emission in the second near-infrared window (NIR-II) have been widely studied in bioimaging applications. In such cases, QDs are usually dispersed in water. As is known, water has strong absorbance in the NIR-II region. However, investigations on the interaction between NIR-II emitters and water molecules are ignored in previous studies. Herein, we synthesized a series of mercaptoundecanoic acid-coated silver sulfide (Ag2S/MUA) QDs with various emissions that partially or completely overlapped with the absorbance of water at 1200 nm. By constructing a hydrophobic interface of cetyltrimethylammonium bromide (CTAB) with MUA on the Ag2S QDs surface via forming an ionic bond, significant enhancement of Ag2S QDs photoluminescence (PL) intensity was observed, as well as a prolonged lifetime. These findings suggest that there is an energy transfer between Ag2S QDs and water in addition to the classical resonance absorption. Transient absorption and fluorescence spectra results revealed that the increased PL intensities and lifetime of Ag2S QDs originated from the suppressed energy transfer from Ag2S QDs to the water due to the CTAB bridged hydrophobic interfaces. This discovery is important for a deeper understanding of the photophysical mechanisms of QDs and their applications.

Strain-Dependent Band Splitting and Spin-Flip Dynamics in Monolayer WS2.

Triggered by the expanding demands of semiconductor devices, strain engineering of two-dimensional transition metal dichalcogenides (TMDs) has garnered considerable research interest. Through steady-state measurements, strain has been proved in terms of its modulation of electronic energy bands and optoelectronic properties in TMDs. However, the influence of strain on the spin-orbit coupling as well as its related valley excitonic dynamics remains elusive. Here, we demonstrate the effect of strain on the excitonic dynamics of monolayer WS2 via steady-state fluorescence and transient absorption spectroscopy. Combined with theoretical calculations, we found that tensile strain can reduce the spin-splitting value of the conduction band and lead to transitions between different exciton states via spin-flip mechanism. Our findings suggest that the spin-flip process is strain-dependent, provides a reference for application of valleytronic devices, where tensile strain is usually existing during their design and fabrication.

Seven-photon absorption from Na+/Bi3+-alloyed Cs2AgInCl6 perovskites.

Nonlinear multi-phonon (2-7) absorption in the Na+/Bi3+-alloyed Cs2AgInCl6 lead-free double perovskites with ∼100% photoluminescence quantum yield and superior stability is observed for the first time, which can be pumped by a femtosecond laser in a wide spectral range (800-2600 nm). First-principles calculations verify that the parity-forbidden transition from the valence band maximum and conduction band minimum (at the Γ point) is not broken by Na+/Bi3+ doping, and strong optical band-to-band absorption occurs at the L&X points. Time-resolved emission spectra evidence that single-photon and multi-photon pumping leads to the same self-trapped exciton transition and high-order nonlinear absorption will not induce a remarkable thermal effect. Finally, we demonstrate that the Cs2Na0.4Ag0.6In0.99Bi0.01Cl6 DP shows great potential for next-generation wavelength-selective and highly sensitive multiphoton imaging applications.

Confined WS2 Nanosheets Tubular Nanohybrid as High-Kinetic and Durable Anode for Sodium-Based Dual Ion Batteries.

Sodium based dual-ion battery (SDIB) has been regarded as one of the promising batteries technologies thanks to its high working voltage and natural abundance of sodium source, its practical application yet faces critical issues of low capacity and sluggish kinetics of intercalation-type graphite anode. Here, a tubular nanohybrid composed of building blocks of carbon-film wrapped WS2 nanosheets on carbon nanotube (WS2 /C@CNTs) was reported. The expanded (002) interlayer and dual-carbon confined structure endowed WS2 nanosheets with fast charge transportation and excellent structural stability, and thus WS2 /C@CNTs showed highly attractive electrochemical properties for Na+ storage with high reversible capacity, fast kinetic, and robust durability. The full sodium-based dual ion batteries by coupling WS2 /C@CNTs anode with graphite cathode full cell presented a high reversible capacity (210 mAh g-1 at 0.1 A g-1 ), and excellent rate performance with a high capacity of 137 mAh g-1 at 5.0 A g-1 .

Reticular Synthesis of One-Dimensional Covalent Organic Frameworks with 4-c sql Topology for Enhanced Fluorescence Emission.

Covalent organic frameworks (COFs) have been extensively investigated due to their unique structure, porosity, and functionality. However, at the topological level, COFs remain as two-dimensional (2D) or three-dimensional (3D) structures, while COFs with one-dimensional (1D) topology have not been systematically explored. In this work, we proposed a synthetic strategy for the construction of 1D-COFs based on non-linear edges and suitable high-symmetry vertices. Compared with their 2D-COFs counterparts, the 1D-COFs with AIEgens located at the vertex of the frame exhibited enhanced fluorescence. The density functional theory (DFT) calculations revealed that the dimensional-induced rotation restriction (DIRR) effect could spontaneously introduce additional non-covalent interactions between the strip frames, which could substantially diminish non-radiative transitions. This work also provides protocols for the design of 1D-COFs and a guidance scheme for the synthesis of emitting COFs.

Pseudocapacitive Vanadium Nitride Quantum Dots Modified One-Dimensional Carbon Cages Enable Highly Kinetics-Compatible Sodium Ion Capacitors.

The kinetics incompatibility between battery-type anode and capacitive-type cathode for sodium ion hybrid capacitors (SIHCs) seriously hinders their overall performance output. Herein, we construct a SIHCs device by coupling with quantum grade vanadium nitride (VN) nanodots anchored in one-dimensional N/F co-doped carbon nanofiber cages hybrids (VNQDs@PCNFs-N/F) as the freestanding anode and the corresponding activated N/F co-doped carbon nanofiber cages (APCNFs-N/F) as cathode. The strong coupling of VN quantum dots with N/F co-doped 1D conductive carbon cages effectively facilitates the ion/electron transport and intercalation-conversion-deintercalation reactions, ensuring fast sodium storage to surmount aforesaid kinetics incompatibility. Additionally, density functional theory calculations cogently manifest that the abundant active sites in the VNQDs@PCNFs-N/F configuration boost the Na+ adsorption/reaction activity well which will promote both "intrinsic" and "extrinsic" pseudocapacitance and further improve anode kinetics. Consequently, the assembled SIHCs device can achieve high energy densities of 157.1 and 95.0 Wh kg-1 at power densities of 198.8 and 9100.5 W kg-1, respectively, with an ultralong cycling life over 8000 cycles. This work further verified the feasibility of kinetics-compatible electrode design strategy toward metal ion hybrid capacitors.

Synthesis of a Low-Cost Thiophene-Indoloquinoxaline Polymer Donor and Its Application to Polymer Solar Cells.

A simple wide-bandgap conjugated polymer based on indoloquinoxaline unit (PIQ) has been newly designed and synthesized via cheap and commercially available starting materials. The basic physicochemical properties of the PIQ have been investigated. PIQ possesses a broad and strong absorption band in the wavelength range of 400~660 nm with a bandgap of 1.80 eV and lower-lying highest occupied molecular orbital energy level of -5.58 eV. Polymer solar cells based on PIQ and popular acceptor Y6 blend display a preliminarily optimized power conversion efficiency of 6.4%. The results demonstrate indoloquinoxaline is a promising building unit for designing polymer donor materials for polymer solar cells.

Novel Binary Ni-Based Mixed Metal-Organic Framework Nanosheets Materials and Their High Optical Power Limiting.

With the rapid advance of laser technology in the photonicera, damage to precision optical instruments caused by exposure to sudden intense laser pulses has stimulated the search for effective optical power limiting materials exhibiting good dispersion, fast response speed, and good visible light transparency. In this study, novel binary Ni-based mixed MOF NSs (M = Mn, Zn, Co, Cd, Fe) were obtained, making the electronic transition more selective and changing the band gap to obtain an excellent reverse saturation absorption signal. The theoretical calculation results show that with the doping of the Fe element, the band gap of Ni-MOF NSs decreases from 3.12 to 0.66 eV of Ni-Fe-MOF NSs, indicating that the doping of the Fe element has a positive effect on the reverse saturated absorption. The experimental results prove that the optical limiting threshold of Ni-Fe-MOF NSs is better than the GNSs, indicating that the Ni-Fe-MOF NSs have a broad application prospect in the field of nonlinear optics and photonics.

Carbon-coated MoS1.5Te0.5 nanocables for efficient sodium-ion storage in non-aqueous dual-ion batteries.

Sodium-based dual-ion batteries have received increased attention owing to their appealing cell voltage (i.e., >3 V) and cost-effective features. However, the development of high-performance anode materials is one of the key elements for exploiting this electrochemical energy storage system at practical levels. Here, we report a source-template synthetic strategy for fabricating a variety of nanowire-in-nanotube MSxTey@C (M = Mo, W, Re) structures with an in situ-grown carbon film coating, termed as nanocables. Among the various materials prepared, the MoS1.5Te0.5@C nanocables are investigated as negative electrode active material in combination with expanded graphite at the positive electrode and NaPF6-based non-aqueous electrolyte solutions for dual-ion storage in coin cell configuration. As a result, the dual-ion lab-scale cells demonstrate a prolonged cycling lifespan with 97% capacity retention over 1500 cycles and a reversible capacity of about 101 mAh g-1 at specific capacities (based on the mass of the anode) of 1.0 A g-1 and 5.0 A g-1, respectively.

Layer-Tunable Nonlinear Optical Characteristics and Photocarrier Dynamics of 2D PdSe2 in Broadband Spectra.

Layered 2D transition metal dichalcogenides (TMDCs) exhibited fascinating nonlinear optical (NLO) properties for constructing varied promising optoelectronics. However, exploring the desired 2D materials with both superior nonlinear absorption and ultrafast response in broadband spectra remain the key challenges to harvest their greatest potential. Here, based on synthesizing 2D PdSe2 films with the controlled layer number, the authors systematically demonstrated the broadband giant NLO performance and ultrafast excited carrier dynamics of this emerging material under femtosecond visible-to-near-infrared laser-pulse excitation (400-1550 nm). Layer-dependent and wavelength-dependent evolution of optical bandgap, nonlinear absorption, and photocarrier dynamics in the obtained 2D PdSe2 are clearly revealed. Specially, the transition from semiconducting to semimetallic PdSe2 induced dramatic changes of their interband absorption-relaxation process. This work makes 2D PdSe2 more competitive for future ultrafast photonics and also opens up a new avenue for the optical performance optimization of various 2D materials by rational design of these materials.

Facile Synthesis of P-Doped Carbon Nanosheets as Janus Electrodes of Advanced Potassium-Ion Hybrid Capacitor.

Potassium-ion hybrid capacitors (PIHCs) shrewdly integrate the merits of the high energy density of battery-type anode and the high power density of capacitor-type cathode, promising prospects for potential application in a diversity of fields. Here, we report the synthesis of P-doped porous carbon nanosheets (P-PCNs) with favorable features as electrochemical storage materials, including ultrahigh specific surface area and rich activity sites. The P-PCN as Janus electrodes show highly attractive electrochemical properties of high capacity and remarkable stability for fast K+ storage and manifest high capacitance for PF6- adsorption. The P-PCNs are applied as both anode and cathode materials to set up dual-carbon PIHCs, which show the capability to deliver a high energy/power density (165.2 Wh kg-1 and 5934.4 W kg-1) as well as remarkable long-life capability.

Engineering Hierarchical Co@N-Doped Carbon Nanotubes/α-Ni(OH)2 Heterostructures on Carbon Cloth Enabling High-Performance Aqueous Nickel-Zinc Batteries.

Searching for high-performance Ni-based cathodes plays an important role in developing better aqueous nickel-zinc (Ni-Zn) batteries. For this purpose, herein, we demonstrate the design and synthesis of ultrathin α-Ni(OH)2 nanosheets branched onto metal-organic frameworks (MOFs)-derived 3D cross-linked N-doped carbon nanotubes encapsulated with tiny Co nanoparticles (denoted as Co@NCNTs/α-Ni(OH)2), which are directly supported on a flexible carbon cloth (CC). An aqueous Ni-Zn battery employing the hierarchical CC/Co@NCNTs/α-Ni(OH)2 as the binder-free cathode and a commercial Zn plate as the anode is fabricated, which displays an ultrahigh capacity (316 mAh g-1) and energy density (540.4 Wh kg-1) at 1 A g-1 as well as excellent rate capability (238 mAh g-1 at 10 A g-1) and superior cycling performance (about 84% capacity retention after 2000 cycles at 10 A g-1). The impressive electrochemical performance might benefit from the rich active sites, rapid electron transfer, cushy electrolyte access, rapid ion transport, and robust structural stability. In addition, the quasi-solid-state CC/Co@NCNTs/α-Ni(OH)2//Zn batteries are also successfully assembled with polymer electrolyte, indicating the great potential for portable and wearable electronics. This work might provide important guidance for constructing carbon-based hybrid materials directly supported on conductive substrates as high-performance electrodes for energy-related devices.

Hierarchical Nanoreactor with Multiple Adsorption and Catalytic Sites for Robust Lithium-Sulfur Batteries.

Developing high-performance cathode host materials is fundamental to solve the low utilization of sulfur, the sluggish redox kinetics, and the lithium polysulfide (LiPS) shuttle effect in lithium-sulfur batteries (LSBs). Here, a multifunctional Ag/VN@Co/NCNT nanocomposite with multiple adsorption and catalytic sites within hierarchical nanoreactors is reported as a robust sulfur host for LSB cathodes. In this hierarchical nanoreactor, heterostructured Ag/VN nanorods serve as a highly conductive backbone structure and provide internal catalytic and adsorption sites for LiPS conversion. Interconnected nitrogen-doped carbon nanotubes (NCNTs), in situ grown from the Ag/VN surface, greatly improve the overall specific surface area for sulfur dispersion and accommodate volume changes in the reaction process. Owing to their high LiPS adsorption ability, outer Co nanoparticles at the top of the NCNTs catch escaped LiPS, thus effectively suppressing the shuttle effect and enhancing kinetics. Benefiting from the multiple adsorption and catalytic sites of the developed hierarchical nanoreactors, Ag/VN@Co/NCNTs@S cathodes display outstanding electrochemical performances, including a superior rate performance of 609.7 mAh g-1 at 4 C and a good stability with a capacity decay of 0.018% per cycle after 2000 cycles at 2 C. These properties demonstrate the exceptional potential of Ag/VN@Co/NCNTs@S nanocomposites and approach LSBs closer to their real-world application.

A thin film of naphthalenediimide-based metal-organic framework with electrochromic properties.

A metal-organic framework (MOF) thin film constructed from Zn nodes and naphthalenediimide (NDI) linkers was grown in-situ uniformly on a transparent conducting glass substrate. This transparent thin film exhibits intriguingly high-contrast electrochromic (EC) switching between canary yellow and dark brown by means of a one-electron redox reaction at its NDI linkers. The findings provide a basic comprehension of the relations between redox state and electrochromism and enrich the application of MOF in the field of optoelectronic materials.

Rapid and Large-Scale Quality Assessment of Two-Dimensional MoS2 Using Sulfur Particles with Optical Visualization.

The efficient nondestructive assessment of quality and homogeneity for two-dimensional (2D) MoS2 is critically important to advance their practical applications. Here, we presented a rapid and large-area assessment method for visually evaluating the quality and uniformity of chemical vapor deposition (CVD)-grown MoS2 monolayers simply with conventional optical microscopes. This was achieved through one-pot adsorbing abundant sulfur particles selectively onto as-grown poorer-quality MoS2 monolayers in a CVD system without any additional treatment. We further revealed that this favorable adsorption of sulfur particles on MoS2 originated from their intrinsic higher-density sulfur vacancies. Based on unadsorbed MoS2 monolayers, superior performance field effect transistors with a mobility of ∼49 cm2 V-1 s-1 were constructed. Importantly, the assessment approach was noninvasive due to the all-vapor-phase and moderate adsorption-desorption process. Our work offers a new route for the performance and yield optimization of devices by quality assessment of 2D semiconductors prior to device fabrication.

Novel One-Dimensional Covalent Organic Framework as a H+ Fluorescent Sensor in Acidic Aqueous Solution.

Covalent organic frameworks (COFs) represent an emerging class of two- or three-dimensional crystalline porous materials with delicate control over topology, composition, and porosity. Here, we develop a new COF made up of 1,3,6,8-tetrakis(p-formylphenyl)pyrene (TFPPy) and 4,4'-diaminobenzophenone (DABP) that exhibits a rare one-dimensional (1D) structure. The resulting frameworks possess good crystallinity, comparatively high Brunauer-Emmett-Teller (BET) surface area (426 m2/g), and good thermal stability (360 °C). Impressively, this 1D COF shows strong fluorescence and can be used as an excellent H+ sensor in an acidic aqueous solution.