Air-Processed MAPbBr3 Perovskite Thin Film with Ultrastability and Enhanced Amplified Spontaneous Emission.

The poor stability, in particular with respect to temperature, moisture, and light exposure, remains a ubiquitous impediment virtually for metal halide perovskite materials and devices in their future practical application. Herein, from the perspective of precursor solution chemistry, ionic liquid solvent methylammonium acetate (MAAc) is introduced to prepare high-quality MAPbBr3 perovskite thin films in a one-step air-processing process without anti-solvent treatment. Due to formation of pinhole-free, uniform, and compact MAPbBr3 perovskite film, excellent amplified spontaneous emission (ASE) with high emission efficiency and low threshold is obtained under nanosecond laser. Furthermore, the prepared MAPbBr3 perovskite exhibits excellent two-photon induced ASE with a low threshold of 100 µJ cm-2 under 800 nm femtosecond laser excitation. More importantly, in comparison with the traditional MAPbBr3 films prepared with N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), the MAPbBr3 film prepared with MAAc shows excellent optical stability: no signs of degradation under more than 2 h pulsed laser excitation, stable ASE emission spectra under the humidity of 95% and ASE spectra can be stimulated when films are kept in air for more than 6000 h without encapsulation.

Three-dimensional metal-organic framework derived porous CoP3 concave polyhedrons as superior bifunctional electrocatalysts for the evolution of hydrogen and oxygen.

Developing low-cost and highly-efficient non-precious metal bifunctional electrocatalysts towards the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is an attractively alternative strategy to solve the environmental pollution problems and energy demands. In this study, metal-organic framework (MOF) derived porous cobalt poly-phosphide (CoP3) concave polyhedrons are prepared and explored as superior bifunctional electrocatalysts for the HER and OER. The prepared MOF derived CoP3 concave polyhedrons show excellent electrocatalytic activity and stability towards the HER and OER in both acidic and alkaline media, with the Tafel slopes of 53 mV dec-1 and 76 mV dec-1 and a current density of 10 mA cm-2 at the overpotentials of -78 and 343 mV for the HER and OER, respectively, which are remarkably superior to those of the transition metal phosphides (TMPs) and comparable to those of the commercial precious metal catalysts. In addition, they also offer efficient catalytic activities and durabilities under neutral and basic conditions for the HER. The results of our study may shed light on the direction towards highly efficient bifunctional TMP electrocatalysts with high phosphorous component.

Linearly Manipulating Color Emission via Anion Exchange Technology for High Performance Amplified Spontaneous Emission of Perovskites.

The most attractive advantages of all-inorganic cesium lead halide perovskites are their optical gain over broad spectral ranges through the visible spectrum, so are well suited to use in tunable lasers or broadband amplifiers. Most reported anion exchange reactions face a challenge to achieve the desired halogen-variable perovskites due to rapid and uncontrollable reactions and difficulty to synthesize directly. In this study, a simple vapor/solid anion exchange strategy is demonstrated for controlling the reaction process and realizing a wide range tuning of band gap and amplified spontaneous emission (ASE) wavelength, which exhibits a temperature-dependent anion exchange rate. By optimizing the reaction temperature at 90 °C, the ASE wavelength can be linearly manipulated by just controlling the reaction time. A clear quantitative relationship between ASE peak position and reaction time is achieved. Compares with the CsPbClBr2 film obtained via the liquid phase anion exchange method, the fabricated perovskite films obtained by vapor/solid anion exchange technology exhibit superior film quality and enhanced ASE performance. This work may have applications in the future using facile and controllable techniques to develop high-quality full-color visible lasers.

Low Threshold and Ultrastability of One-Step Air-Processed All-Inorganic CsPbX3 Thin Films toward Full-Color Visible Amplified Spontaneous Emission.

All-inorganic perovskites (CsPbX3) with the merits of high stability and remarkable optical gain property are attractive for achieving on-chip coherent light sources. Unfortunately, traditional solution-processed CsPbX3 films suffer from inevitable poor surface integrity and pinhole defects, severely hindering their optical properties. Here, from the perspective of precursor solution chemistry, we use an ionic liquid solvent methylammonium acetate (MAAc) to fabricate compact, pinhole-free, and smooth CsPbX3 thin films in a one-step air process without antisolvent treatment. Optically pumped amplified spontaneous emission (ASE) with a straightforward visible spectral tunability (418-725 nm) is achieved under both nanosecond and femtosecond laser excitation. For the representative CsPbBr3 films, the threshold reaches down to 11.4 µJ cm-2 under nanosecond laser pumping, which is comparable to the value under one-photon femtosecond pumping. The long gain lifetime up to 258.2 ps is revealed by transient absorption spectroscopy. Most importantly, the films show excellent optical stability and humidity stability with no obvious degradation under the pulsed laser irradiation for more than 210 min, stable ASE output under 95% high humidity, and conspicuous ASE after 1000 h of storage in air condition without encapsulation. These results demonstrate that the method of fabricating inorganic perovskite films with an ionic liquid solvent is promising in developing high-performance full-color visible lasers.

Visual Cause Analytics for Traffic Congestion.

Urban traffic congestion has become an important issue not only affecting our daily lives, but also limiting economic development. The primary cause of urban traffic congestion is that the number of vehicles is higher than the permissible limit of the road. Previous studies have focused on dispersing traffic volume by detecting urban traffic congestion zones and predicting future trends. However, to solve the fundamental problem, it is necessary to discover the cause of traffic congestion. Nevertheless, it is difficult to find a research which presents an approach to identify the causes of traffic congestion. In this paper, we propose a technique to analyze the cause of traffic congestion based on the traffic flow theory. We extract vehicle flows from traffic data, such as GPS trajectory and Vehicle Detector data. We detect vehicle flow changes utilizing the entropy from the information theory. Then, we build cumulative vehicle count curves (N-curve) that can quantify the flow of the vehicles in the traffic congestion area. The N-curves are classified into four different traffic congestion patterns by a convolutional neural network. Analyzing the causes and influence of traffic congestion is difficult and requires considerable experience and knowledge. Therefore, we present a visual analytics system that can efficiently perform a series of processes to analyze the cause and influence of traffic congestion. Through case studies, we have evaluated that our system can classify the causes of traffic congestion and can be used efficiently in road planning.

High-Performance Deep Ultraviolet Photodetector Based on a One-Dimensional Lead-Free Halide Perovskite CsCu2I3 Film with High Stability.

Lead halide perovskites have received much attention in the field of optoelectronic devices. However, the environment-unfriendly nature and intrinsic instability of these perovskites hamper their commercial applications. In this work, one novel one-dimensional lead-free halide perovskite with high stability, CsCu2I3, was prepared via an antisolvent-assisted crystallization method. The prepared CsCu2I3 bears a high exciton binding energy of ∼105 meV and a high photoluminescence quantum yield of 12.3%. We fabricated a deep ultraviolet photodetector based on a CsCu2I3 film that is nearly blind to 405 nm visible light but is sensitive to 265 and 365 nm illumination. The device exhibits excellent reproducibility and a high Ilight/Idark ratio of 22 under 265 nm illumination. Furthermore, the responsivity, specific detectivity, and external quantum efficiency are as high as 22.1 mA/W, 1.2 × 1011 Jones, and 10.3% under a light density of 0.305 mW/cm2, respectively. These findings demonstrate that CsCu2I3 perovskites should have great potential for future optoelectronics.

Insight into the Superior Electrocatalytic Performance of a Ternary Nickel Iron Poly-Phosphide Nanosheet Array: An X-ray Absorption Study.

Although ternary components and doping with foreign atoms have been widely studied to enhance the electrocatalytic performance of transition metal phosphides, the underlying mechanism is not clear. Here, we fabricated ternary Ni-Fe-P nanosheets on carbon fiber paper as efficient electrodes and studied the local atomic and electronic structure alteration through X-ray absorption spectroscopy. The optimized ternary Ni-Fe-P nanosheet electrode exhibited superior hydrogen evolution activity and stability in 0.5 M H2SO4 with a low overpotential of 56 mV at 10 mA cm-2. X-ray absorption spectroscopy studies revealed that with the Fe ion incorporation into the system, the Ni-P bonds elongated and few electrons transferred from Ni to P which resulted in a reduced oxidation state of Ni and reduced the interaction between the hydrogen atom and the catalyst surface. Our work not only demonstrates the future potential of high-performance electrocatalysts based on ternary Ni-Fe-P but also offers a promising method to explore the unique synergistic effect in ternary compounds.

Self-supported three-dimensional mesoporous semimetallic WP2 nanowire arrays on carbon cloth as a flexible cathode for efficient hydrogen evolution.

The design and development of high-efficiency and non-noble metal hydrogen evolution reaction (HER) electrocatalysts with optimized nanostructures for human clean and sustainable energy systems has attracted significant research interest over the past years. Herein, self-supported semimetallic tungsten diphosphide nanowire arrays on carbon cloth (WP2 NWs/CC) were topotactically fabricated by in situ phosphidation of a WO3 NWs/CC precursor. Such a binder-free flexible HER cathode with integrated three-dimensional nanostructures can not only provide a large surface area to expose abundant active sites, but also facilitate electrolyte penetration for electrons and electrolyte ions. The WP2 NWs/CC electrode exhibits superior catalytic performance, and it needs overpotentials of 109 and 160 mV with a small Tafel slope of 56 mV dec-1 to achieve current densities of 10 and 50 mA cm-2, respectively. High stability in acidic media is also observed for the catalyst for a duration of 20 hours at least. In addition, density functional theory (DFT) calculations indicate a low kinetic energy barrier for H atom adsorption on the WP2 surface which guarantees the excellent catalytic activity of the catalyst, and the influences of phosphidation temperature on the HER activity are also studied. The excellent electrocatalytic activity makes the present 3D structured WP2 NWs/CC a promising catalyst for large scale highly pure hydrogen evolution by electrochemical water splitting.