Tunable Emission of Low-Dimensional Organic Metal Halides by Stoichiometric Ratio and Metal Center.

Low-dimensional (LD) organic metal halides (OMHs) have a bright future due to their excellent photoelectric characteristics and unique structure. However, the synthesis and emission control of LD-OMHs are still unclear. Herein, the different dimensional (zero-dimensional (0D), one-dimensional (1D), and three-dimensional (3D)) of OMHs were obtained by the reaction of 1,4-diazabicyclo (2.2.2) octane with PbBr2 in different stoichiometric ratios. This discovery shows that the structure and properties of OMHs can be regulated while maintaining the functional organic cations of OMHs, which broadens the path for the development of functional LD-OMHs. Among them, 0D-OMH 1 and 1D-OMH 3 have narrow-band (full width at half-maximum (fwhm) = 74 nm) and broad-band (fwhm = 201 nm) emission, respectively. We found that when organic cations have no contribution to the formation of conduction band minimum and valence band maximum, and the distances between polyhedrons are larger than the van der Waals diameter of the halogen atom, the effect of phonons on exciton transitions can be reduced to achieve a narrow-band emission. Further, Cu(I)- and Mn (II)-based 0D-OMHs were synthesized, which have high photoluminescence quantum yield (PLQY) (33.97 and 47.33%, respectively). When the emitting of 0D-OMHs produced by the interaction of the metal-center and halogens, the asymmetric planar metal-halogen structure will result in a higher PLQY.

Hydroclimatic changes on multiple timescales since 7800 y BP in the winter precipitation-dominated Central Asia.

Central Asia (CA) is one of the world's most significant arid regions, which is markedly impacted by global warming. A better understanding of the dynamical processes governing its Holocene climate variability is critical for a better understanding of possible future impacts of climate change in the region. To date, most of the existing CA paleoclimate records are from the summer precipitation-dominated eastern CA (ECA), with few records from the winter precipitation-dominated western CA (WCA). Here, we present a precisely dated (~6‰) and highly resolved (<4-y) record of hydroclimatic variations from the WCA covering the period between 7,774 and 656 y BP. Utilizing multiple proxies (δ18O, δ13C, and Sr/Ca) derived from a stalagmite from the Fergana Valley, Kyrgyzstan, we reveal a long-term drying trend in WCA, which is in contrast with the wetting trend in ECA. We propose that different responses of winter and summer westerly jets to seasonal solar insolation over the past 8,000 y may have resulted in an antiphased precipitation relationship between the WCA and ECA. Our data contain dominant quasiperiodicities of 1,400, 50 to 70, and 20 to 30 y, indicating close connections between the WCA climate and the North Atlantic. We further identified a series of droughts and pluvials on centennial-to-decadal timescales, which may have influenced regional societies and trans-Eurasian culture exchanges during historical and prehistorical times.

Photoluminescence Enhancement in Silica-Confined Ligand-Free Perovskite Nanocrystals by Suppression of Silanol-Induced Traps and Phase Impurities.

The detriments of intrinsic silanol groups in mesoporous silica to the photoluminescence (PL) of lead halide perovskite nanocrystals (LHP NCs) confined in the template have never been determined and clearly elucidated. Here, we disclose that silanol-induced Cs+ and Br- deficiencies prompt the generation of traps and CsPb2Br5 impurities. The temperature-dependent PL spectra verify the higher energetic barrier of trap states in CsPbBr3 NCs confined in silanol-rich mesoporous silica. Femtosecond transient absorption spectra reveal the trapped state mediates a broadband photoinduced absorption and long-lived decay pathway of CsPbBr3 NCs in silanol-rich templates. A remarkable improvement (up to 160-fold) in PL quantum yields is realized by simple silanol elimination. This work demonstrates the detrimental effects of silanol sites on the PL properties of LHP NCs impregnated in mesoporous silica and provides a new perspective for the ligand-free synthesis of high-quality LHP NCs in mesoporous templates by facile impregnation for practical applications.

Ultrasensitive Temperature Sensing Based on Ligand-Free Alloyed CsPbClx Br3-x Perovskite Nanocrystals Confined in Hollow Mesoporous Silica with High Density of Halide Vacancies.

Temperature sensing based on fluorescent semiconductor nanocrystals has recently received immense attention. Enhancing the trap-facilitated thermal quenching of the fluorescence should be an effective approach to achieve high sensitivity for temperature sensing. Compared with conventional semiconductor nanocrystals, the defect-tolerant feature of lead halide perovskite nanocrystals (LHP NCs) endows them with high density of defects. Here, hollow mesoporous silica (h-SiO2 ) template-assisted ligand-free synthesis and halogen manipulation (chloride-importing) are proposed to fabricate highly defective yet fluorescent CsPbCl1.2 Br1.8 NCs confined in h-SiO2 (CsPbCl1.2 Br1.8 NCs@h-SiO2 ) for ultrasensitive temperature sensing. The trap barrier heights, exciton-phonon scattering, and trap state filling process in the CsPbCl1.2 Br1.8 NCs@h-SiO2 and CsPbBr3 NCs@h-SiO2 are studied to illustrate the higher temperature sensitivity of CsPbCl1.2 Br1.8 NCs@h-SiO2 at physiological temperature range. By integrating the thermal-sensitive CsPbCl1.2 Br1.8 NCs@h-SiO2 and thermal-insensitive K2 SiF6 :Mn4+ phosphor into the flexible ethylene-vinyl acetate polymer matrix, ratiometric temperature sensing from 30.0 °C to 45.0 °C is demonstrated with a relative temperature sensitivity up to 13.44% °C-1 at 37.0 °C. The composite film shows high potential as a thermometer for monitoring the body temperature. This work demonstrates the unparalleled temperature sensing performance of LHP NCs and provides new inspiration on switching the defects into advantages in sensing applications.

Fast growth of large-grain and continuous MoS2 films through a self-capping vapor-liquid-solid method.

Most chemical vapor deposition methods for transition metal dichalcogenides use an extremely small amount of precursor to render large single-crystal flakes, which usually causes low coverage of the materials on the substrate. In this study, a self-capping vapor-liquid-solid reaction is proposed to fabricate large-grain, continuous MoS2 films. An intermediate liquid phase-Na2Mo2O7 is formed through a eutectic reaction of MoO3 and NaF, followed by being sulfurized into MoS2. The as-formed MoS2 seeds function as a capping layer that reduces the nucleation density and promotes lateral growth. By tuning the driving force of the reaction, large mono/bilayer (1.1 mm/200 µm) flakes or full-coverage films (with a record-high average grain size of 450 µm) can be grown on centimeter-scale substrates. The field-effect transistors fabricated from the full-coverage films show high mobility (33 and 49 cm2 V-1 s-1 for the mono and bilayer regions) and on/off ratio (1 ~ 5 × 108) across a 1.5 cm × 1.5 cm region.

Dual-Mode of Fluorescence Turn-On and Wavelength-Shift for Methylamine Gas Sensing Based on Space-Confined Growth of Methylammonium Lead Tribromide Perovskite Nanocrystals.

The defect-tolerant nature of lead halide perovskites renders outstanding luminescence by simple space-confined growth in nanopores. The fluorescence turn-on and wavelength-shift phenomena could be found in the formation of methylammonium lead tribromide (MAPbBr3) nanocrystals in hollow SiO2 nanospheres triggered by the reaction between methylamine (MA) gas and HPbBr3/PbBr2@SiO2 nanospheres. The enhanced fluorescence intensity is linear with the MA concentration in the range of 1.0-95 ppm with a limit of detection (LOD) of 70 ppb (S/N = 3). In addition, the maximum emission wavelength is consistently red-shifted from 478.7 to 510.6 nm as the MA concentration increases from 1.0 to 95 ppm, imparting the potential for colorimetric sensing. By combining the fluorescence turn-on and colorimetric sensing modes, the flexible method meets the demands for visual discrimination and point-of-care determination with portable devices.

Development of a sequential injection analysis device and its application for the determination of Mn(II) in water.

A sequential injection analysis device has been developed and applied for the automated determination of Mn(II) in environmental water samples. Differential pulse cathodic stripping voltammetry is selected as the electrochemical detection method. The device consists of several electronic equipment. The electrochemical flow cell was designed for replacing the traditional three-electrode system and introducing high reproducibility. An electrochemical analyzer saddled with laboratory-programmed software written by Embarcadero Delphi 10.2. For higher current response, various determination parameters such as the flow rate, the medium pH, the deposition potential and the thickness of gasket in the electrochemical flow cell have been optimized. Under the optimal conditions, the detection limit (3σ/slope) of 0.63 µg L-1 and a calibration curve (R2 = 0.9987) of current response and Mn(II) concentration from 2.5 µg L-1 to 200 µg L-1 could be achieved. The device was successfully applied to the determination of trace Mn(II) in environmental water samples, and in continuous real-time monitoring of Mn(II) variations in tap water for 14 days. The results are consistent with the reference method and the average recovery is found to be 95.2%-101.4%. The device shows high sensitivity and reproducibility in the determination of Mn(II), and presents a great potential for on-site and real-time detection of metal ions where rapid, low-cost and low-volume analysis is required.

Fluorometric sensing of oxygen using manganese(II)-doped zinc sulfide nanocrystals.

Manganese(II)-doped zinc sulfide nanocrystals (Mn:ZnS NCs) with dual-emission fluorescence (peaks at 445 nm and 590 nm under 330 nm excitation), good water stability and low toxicity were synthesized by hot injection. The fluorescence intensity of both emission bands of the nanocrystals can change rapidly by the content of gaseous and dissolved oxygen. The process is fully reversible. Compared with the maximum intensity of Mn:ZnS sensing film in 100% nitrogen, the emission of the blue emission decreases by 72% in the presence of 100% oxygen, and the yellow emission by 32%. Response is linear in the presence of 3% to 12% of oxygen percentage in gas. For water-dissolved oxygen, the linear response occurs between 0.54 and 11.4 mg·L-1. Graphical abstractMn-doped ZnS NCs with dual-emission fluorescence were synthesized by hot-injection method. The reversible and rapid sensing characteristics of Mn-doped ZnS NCs to oxygen were studied, and the possible sensing mechanism was investigated.

Wavelength-Shift-Based Colorimetric Sensing for Peroxide Number of Edible Oil Using CsPbBr3 Perovskite Nanocrystals.

The peroxide number of edible oil relates to its quality. The classical determination methods for the peroxide number are still unsatisfactory due to their complexity and poor reproducibility in the analytical process and their incapability of field rapid detection. In this study, a novel wavelength-shift-based visual method has been developed for the peroxide number determination of edible oil using halide perovskite nanocrystals (CsPbBr3 NCs). In the analysis, the edible oil sample underwent redox reactions with a part of oleylammonium iodide (OLAM-I) in advance. Then, the halogen exchange occurred between the added CsPbBr3 NCs and the iodide ions from the residual OLAM-I. The resulting wavelength shift of the fluorescence emission reflects the peroxide number in the edible oil sample. Under the ultraviolet light excitation at 365 nm, the apparent color of the photoluminescence could directly be compared with a color chart to determine and qualify the peroxide number. Using the approach, the visual detection of the peroxide number of edible oil samples on site could be realized. The detection process takes only ∼15 min and is convenient and accurate.

Enhancing the Stability of CH3NH3PbBr3 Nanoparticles Using Double Hydrophobic Shells of SiO2 and Poly(vinylidene fluoride).

The instability of lead halide perovskites (LHPs) has tremendously hindered their practical applications. Although some examples on encapsulating LHPs into a SiO2 shell have been reported, these SiO2-coated LHPs still suffer from limited stability. Herein, MAPbBr3 (MA = CH3NH3+) nanoparticles encapsulated in double hydrophobic shells of organic functionalized SiO2 and poly(vinylidene fluoride) (MAPbBr3@SiO2/PVDF) are successfully synthesized by infiltrating the MAPbBr3 precursor solution into hollow siliceous nanospheres and followed by PVDF capping. With the dual protection of SiO2 and PVDF, the MAPbBr3@SiO2/PVDF nanoparticles exhibit drastically improved stability against water and UV-light illumination. A white light-emitting diode with luminous efficiency up to 147.5 lm W-1 and a color gamut encompassing 120% of National Television System Committee in Commission Internationale de L'Eclairage 1931 color space has been demonstrated using the MAPbBr3@SiO2/PVDF nanoparticles as the green light source. This study enlightens new insights into the synthesis of highly stable LHPs-based core-shell-shell architectures toward their practical applications.

Unveiling the interfacial electrochemiluminescence behavior of lead halide perovskite nanocrystals.

In this study, a three-phase heterostructure interface including glassy carbon (conducting medium), CsPbBr3 perovskite nanocrystals (PNCs, emitter) and acetonitrile (electrolyte) is constructed for fully investigating the interfacial electrochemiluminescence (ECL) behavior of CsPbBr3 PNCs. We find that these interfaces serve as bridges for efficient electron-hole transfer during the ECL process. As a proof of concept, the increase of the heterostructure interface area will accordingly enhance the ECL intensity of CsPbBr3 PNCs. About seven-fold enhancement of the ECL intensity could be achieved when the interface area has triple-fold increase, which provides a new perspective to construct more efficient ECL systems via interface engineering.

Enhancing exciton binding energy and photoluminescence of formamidinium lead bromide by reducing its dimensions to 2D nanoplates for producing efficient light emitting diodes.

Formamidinium lead bromide perovskite nanocrystals (FAPbBr3 PNCs) are extremely promising for applications in green light emitting diodes (LEDs) since their photoluminescence maxima locate in the range of 530-535 nm with a narrow full width at half-maximum (<25 nm). However, the low exciton binding energy (EB) of FAPbBr3 (∼25 meV) hinders its development in LEDs. In this study, we have developed a 2-methyl-1-pentanol assisted hot injection method for the synthesis of 2D FAPbBr3 nanoplates with high stability. Due to strong quantum confinement in two dimensions of the nanoplates, the obtained FAPbBr3 nanoplates exhibited a high EB of 168.4 meV. The high EB dramatically enhances the photoluminescence (PL) properties with a high fluorescence quantum yield of 92%, which realizes a long photoluminescence lifetime of 2 ns with negligible trapping processes. Benefiting from the high EB and outstanding PL properties, the FAPbBr3 nanoplate LED device showed a current efficiency of 17.32 cd A-1 at a low voltage of 4 V, and a corresponding external quantum efficiency (EQE) up to 4.5% with the Commission Internationale de L'Eclairage (CIE) 1931 color coordinates of (0.24, 0.73), providing a new pathway for the fabrication of efficient perovskite based LED devices.

MnII-Doped Cesium Lead Chloride Perovskite Nanocrystals: Demonstration of Oxygen Sensing Capability Based on Luminescent Dopants and Host-Dopant Energy Transfer.

The design of photoluminescence-quenching probes for molecular oxygen (O2) is always a large space to explore. Luminescent semiconductor nanocrystals (NCs) have been proposed as emerging oxygen-responsive probes, but the inherent O2 sensing of phosphorescent semiconductor NCs has not been reported so far. Here, we demonstrate the O2 sensing capability of MnII-doped CsPbCl3 nanocrystals (Mn:CsPbCl3 NCs) and reveal the role of O2 on the optical de-excitation process of such perovskite nanocrystals (PNCs). By adjusting the amount and distribution of MnII dopants, as well as the host-dopant energy transfer process in PNCs, we highlight that O2 can reversibly quench the MnII emission due to the temporary disturbance to the ligand field of near-surface MnII dopants in PNCs. In phosphorescence mode, the photoluminescence intensity of the Mn:CsPbCl3 NCs is quenched by 53% on increasing O2 concentration from 0 to 100%. The Stern-Volmer plot shows a good linear in the 0-12% O2 concentration range. High sensing reversibility and rapid signal response are also achieved. In our perception, the mechanism study makes our PNCs candidates for the optical probes of O2, and it is enlightening to explore more possibilities of the inherent O2 sensing based on the semiconductor-doped NCs (not restricted to MnII-doped PNCs) with phosphorescence emission.

Fluorescent carbon nanodots facilely extracted from Coca Cola for temperature sensing.

A novel method for the fabrication of carbon nanodots (CDs) is introduced: extracting CDs from the well-known soft drink Coca Cola via dialysis. The obtained CDs are of good monodispersity with a narrow size distribution (average diameter of 3.0 nm), good biocompatibility, high solubility (about 180 mg ml-1) and stable fluorescence even at a high salt concentration. Furthermore, they are sensitive to the temperature change with a linear relationship between the fluorescence intensity and temperature from 5 °C-95 °C. The CDs have been applied in high stable temperature sensing. This protocol is quite simple, green, cost-effective and technologically simple, which might be used for a range of applications including sensing, catalysts, drug and gene delivery, and so on.