Color tunable of Ln-MOFs (Ln = Tb, Eu) and excellent stability for white light-emitting diode.

White light-emitting diodes (WLEDs) possess the advantages of environmental friendliness, long lifetime, and energy saving. Recently, metal-organic frameworks (MOFs) have become one of the hot candidates for LEDs. However, the tunable color and thermal stability of MOFs are urgent problems for their actual applications. In this work, Ln-MOFs (Ln = Eu, Tb) were synthesized by a facile wet chemical route. A series of Ln-MOFs phosphors with tunable luminescence color showed potential applications in white LEDs. The emission color of the phosphors can be easily modulated by changing the molar ratio of the raw materials. The luminescence intensities of Ln-MOFs retained over 90.6% of the initial value, showing excellent thermal stability of Ln-MOFs. In order to explore the potential applications of Ln-MOFs in WLEDs, we mixed them with two kinds of blue phosphors and packaged them to obtain WLEDs. The CIE coordinates of both were (0.31, 0.33) and (0.31, 0.34), which were able to achieve white light emission. The peak shape and peak position in the EL spectrum of the WLEDs device remained stable when increasing the applied current of the device. Meanwhile, the white light with excellent color quality and visual performance was achieved. The results show that Ln-MOFs are potential materials for white light LED, and provide a novel idea for the application of Ln-MOFs materials in the luminescence field.

Enhanced Stability of All-Inorganic Perovskite Light-Emitting Diodes by a Facile Liquid Annealing Strategy.

Ensuring the stability of all-inorganic halide perovskite light-emitting diodes (LEDs) has become an obstacle that needs to be broken for commercial applications. Currently, lead halide perovskite CsPbX3 (X = Br, I, Cl) nanocrystals (NCs) are considered as alternative materials for future fluorescent lighting devices due to their combination of superior optical and electronic properties. However, the temperature of the surface of the LEDs will increase after long-term power-on work, which greatly affects the optical stability of CsPbX3 NCs. In order to overcome this bottleneck issue, a strategy of annealing perovskite materials in liquid is proposed, and the changes in photoluminescence and electroluminescence (EL) behaviors before and after annealing are studied. The results show that the luminescence stability of the annealed perovskite materials is significantly improved. Moreover, the EL stability of different perovskite LED devices under long-term operation is monitored, and the performance of the annealed materials is particularly outstanding. The results have proved that this convenient and low-cost liquid annealing strategy is suitable for large-scale postprocessing of perovskite materials, granting them stable fluorescence emission, which will bring a new dawn to the commercialization of next-generation optoelectronic devices.

Ultrafast Response and High Selectivity toward Acetone Vapor Using Hierarchical Structured TiO2 Nanosheets.

For the development of high-performance gas sensors, ultrafast response and high selectivity are critical requirements for many practical applications. An alternative strategy is to employ hierarchical nanostructured materials in gas sensors. In this work, we report newly synthesized TiO2 hexagonal nanosheets with a hierarchical porous structure, which demonstrate an ultrafast gas response and high selectivity toward acetone vapor for the first time. A simple one-step annealing process to prepare hierarchical TiO2 nanosheets derived from layered TiSe2 nanosheet templates is reported. The hierarchical structure interlaced with anatase TiO2 nanosheets showed an open porous characteristic. The average pore size was about 20 nm examined using a high-resolution TEM. The gas sensing properties toward acetone vapor of the novel hierarchical structured TiO2 nanosheets were characterized in detail including optimal operation temperature, sensitivity, selectivity, response/recovery time, and long-term stability. The gas sensing response and recovery times were 0.75 s and 0.5 s, respectively. We attribute these superior response properties to its unique hierarchical pore structure with a high specific surface area. The results show great potential for acetone vapor detection, particularly in dynamic ultrafast monitoring by using the synthesized hierarchical structured TiO2 nanosheets.

Physisorption-Based Charge Transfer in Two-Dimensional SnS2 for Selective and Reversible NO2 Gas Sensing.

Nitrogen dioxide (NO2) is a gas species that plays an important role in certain industrial, farming, and healthcare sectors. However, there are still significant challenges for NO2 sensing at low detection limits, especially in the presence of other interfering gases. The NO2 selectivity of current gas-sensing technologies is significantly traded-off with their sensitivity and reversibility as well as fabrication and operating costs. In this work, we present an important progress for selective and reversible NO2 sensing by demonstrating an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D) tin disulfide (SnS2) flakes at low operating temperatures. The device shows high sensitivity and superior selectivity to NO2 at operating temperatures of less than 160 °C, which are well below those of chemisorptive and ion conductive NO2 sensors with much poorer selectivity. At the same time, excellent reversibility of the sensor is demonstrated, which has rarely been observed in other 2D material counterparts. Such impressive features originate from the planar morphology of 2D SnS2 as well as unique physical affinity and favorable electronic band positions of this material that facilitate the NO2 physisorption and charge transfer at parts per billion levels. The 2D SnS2-based sensor provides a real solution for low-cost and selective NO2 gas sensing.

Simple Surfactant Concentration-Dependent Shape Control of Polyhedral Fe3 O4 Nanoparticles and Their Magnetic Properties.

The shape and size of monodisperse Fe3 O4 nanoparticles (NPs) are controlled using a chemical solution synthesis in the presence of the surfactant cetylpyridinium chloride (CPC). Cubic Fe3 O4 NPs surrounded by six {100} planes are obtained in the absence of CPC. Increasing the CPC content during synthesis causes the shape of the resulting Fe3 O4 NPs to change from cubic to truncated cubic, cuboctahedral, truncated octahedral, and finally octahedral. During this evolution, the predominantly exposed planes of the Fe3 O4 NPs vary from {100} to {111}. The shape control results from the synergistic effect of the pyridinium cations, chloride anions, and long-chain alkyl groups of CPC, which is confirmed by comparison with NPs synthesized in the presence of various related cationic surfactants. The size of the cubic Fe3 O4 NPs can be tuned from 50 to 200 nm, by changing the concentration of oleic acid in the reaction solution. The Fe3 O4 NPs exhibit shape-dependent saturation magnetization, remanent magnetization, and coercivity.

Large-scale synthesis of NbS2 nanosheets with controlled orientation on graphene by ambient pressure CVD.

We report ambient pressure chemical vapor deposition (CVD) growth of single-crystalline NbS2 nanosheets with controlled orientation. On Si and SiO2 substrates, NbS2 nanosheets grow almost perpendicular to the substrate surface. However, when we apply transferred CVD graphene on SiO2 as a substrate, NbS2 sheets grow laterally lying on the graphene. The NbS2 sheets show the triangular and hexagonal shapes with a thickness of about 20-200 nm and several micrometres in the lateral dimension. Analyses based on X-ray diffraction and Raman spectroscopy indicate that the NbS2 nanosheets are single crystalline 3R-type with a rhombohedral structure of R3m space group. Our findings on the formation of highly aligned NbS2 nanosheets on graphene give new insight into the formation mechanism of NbS2 and would contribute to the templated growth of various layered materials.

Stress perturbation method for the assessment of cathodoluminescence probe response functions.

A method to determine the in-plane cathodoluminescence (CL) probe response function (PRF) (i.e., the function characterizing the in-plane luminescence intensity distribution within the electron probe volume) is proposed, which is based on "perturbing" the spectral position of a selected luminescence band using a highly graded stress field. The method is applied to the stress field developed ahead of the tip of an equilibrium crack in three different cases of CL bands, which arise from different structural phenomena: (i) the F(+) (oxygen excess) defect band in a nominally stoichiometric sapphire (alpha-Al(2)O(3)) single crystal; (ii) the chromophoric R-line in ruby lattice (alpha-Al(2-x)Cr(x)O(3)); and (iii) the near band-gap line in n-type GaN semiconductor crystal. A computer-aided data restoration procedure was applied to rationalize data retrieved from crack-tip line scans performed at different acceleration voltages. For the excitonic band-gap in GaN and for F(+) emission in sapphire the CL probe in the electron focal plane was found to be comparable, but not necessarily coincident, in size to the electron probe. On the other hand, the occurrence of self-absorption in the case of R-line photons in ruby resulted in a significantly broadened CL probe with respect to the average scattering length of electrons.