Enhanced amplified spontaneous emission performance through effective regulation of phase distribution in Ruddlesden-Popper perovskite films.

Ruddlesden-Popper (RP) perovskites promise next-generation gain media for laser devices. However, most RP perovskite lasers are still suffering from inferior performance characteristics, such as inadequate energy transfer, unstable emission, and short lifetime. To address the above problems, high crystalline quality, compact, and smooth PEA2FA2Pb3Br10 films with uniform phase distribution were successfully prepared by ionic liquid (IL) methylammonium acetate (MAAc) in an air environment. Compared with the PEA2FA2Pb3Br10 film prepared by the traditional solvent dimethyl sulfoxide (DMSO), an enhanced amplified spontaneous emission (ASE) with a lower threshold of 58 µJ·cm-2 from the MAAc-treated film was obtained under nanosecond laser excitation. The transient absorption (TA) spectroscopy revealed that a uniform phase distribution and more efficient energy transfer processes were achieved in the PEA2FA2Pb3Br10-MAAc film, leading to an enhanced band-to-band spontaneous emission process. Furthermore, the films exhibited better stability, showing no signs of degradation under the 120 min pulsed laser pumping in air and stability of ASE spectra at even 95% humidity conditions. This study provides an important foundation for achieving high-performance optically pumped lasers based on the unique RP perovskites.

2D Halide Perovskites for High-Performance Resistive Switching Memory and Artificial Synapse Applications.

Metal halide perovskites (MHPs) are considered as promising candidates in the application of nonvolatile high-density, low-cost resistive switching (RS) memories and artificial synapses, resulting from their excellent electronic and optoelectronic properties including large light absorption coefficient, fast ion migration, long carrier diffusion length, low trap density, high defect tolerance. Among MHPs, 2D halide perovskites have exotic layered structure and great environment stability as compared with 3D counterparts. Herein, recent advances of 2D MHPs for the RS memories and artificial synapses realms are comprehensively summarized and discussed, as well as the layered structure properties and the related physical mechanisms are presented. Furthermore, the current issues and developing roadmap for the next-generation 2D MHPs RS memories and artificial synapse are elucidated.

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.

Phosphorescent-Dye-Sensitized Quantum-Dot Light-Emitting Diodes with 37% External Quantum Efficiency.

Exciton harvesting is of paramount importance for quantum-dot light-emitting diodes (QLEDs). Direct exciton harvesting by the quantum dots (QDs) emitting layer suffers from poor hole injection due to the low conduction bands and valence bands of QDs, leading to unbalanced electron-hole injection and recombination. To address this issue, here, an exciton sensitizing approach is reported, where excitons form on a phosphorescent-dye-doped layer, which then transfer their energies to adjacent QDs layer for photon emission. Due to the very efficient exciton formation and energy-transfer processes, higher device performance can be achieved. To demonstrate the above strategy, red QLEDs with a phosphorescent dye, iridium (III) bis(2-methyldibenzo-[f,h]quinoxaline) (acetylacetonate), Ir(MDQ)2 (acac), doped hole-transporting layer are fabricated and studied. At a doping concentration of 10 wt%, the best device achieves record high current efficiency, power efficiency, and external quantum efficiency (EQE) of 37.3 cd A-1 , 41 lm W-1 , and 37%, respectively. Simultaneously, the efficiency roll-off characteristic is greatly improved, in that 35% EQE can be well retained at a high luminance level of 450 000 cd m-2 . Moreover, the devices also exhibit good stability and reproducibility.

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.

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.

A highly [001]-textured Sb2Se3 photocathode for efficient photoelectrochemical water reduction.

Anisotropic Sb2Se3 is an emerging earth-abundant photocathode for photoelectrochemical water splitting. However, controlling the growth of the Sb2Se3 film with optimal [001] crystallographic orientation is still the most challenging issue. Here, we successfully synthesized [001]-oriented Sb2Se3via a reliable and facile method. The [001]-oriented Sb2Se3 film could provide an excellent carrier-migration efficiency. Consequently, we achieved a record-high photocurrent density of -20.2 mA cm-2 at 0 VRHE and a very high half-cell solar-to-hydrogen efficiency of 1.36% under 1-sun simulated solar illumination in a TiO2/[001]-Sb2Se3 photocathode. This work provides an effective strategy and important guidelines for rationally designing optoelectronic devices based on the [001]-oriented Sb2Se3 film.

Electrospinning Fabricating Au/TiO2 Network-like Nanofibers as Visible Light Activated Photocatalyst.

Exploiting photocatalysts with characteristics of low cost, high reactivity and easy recovery offer great potentials for complete elimination of toxic chemicals and environmental remediation. In this work, Au/TiO2 network-like nanofibers were fabricated using a facile electrospinning technique followed by calcinations in air. Photocatalytic tests indicate that the Au/TiO2 network-like nanofibers possess an excellent photodegradation rate of rhodamine B (RB) under UV, visible and natural light radiation. The enhanced photocatalytic activity can be attributed to the plasmonic resonance absorption of Au nanoparticles, and photogenerated electrons and holes are effectively separated by the Au/TiO2 heterojunction structures. Furthermore, the three-dimensional network structure can provide a large number of active sites for RB degradation.

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.

Fabrication of phosphorus nanostructures/TiO2 composite photocatalyst with enhancing photodegradation and hydrogen production from water under visible light.

As the world faces serious environmental pollution and energy shortage, developing Vis-light-driven photocatalysts for water splitting is highly attractive in clean energy utilization. Fabricating heterostructures has been proposed to be an efficient system to enhance the photocatalytic activity. However, synthesizing heterostructures with good contact and understanding charge transfer dynamics are still unresolved issues. In this work, a facile calcination approach was used to synthesize red phosphorus (RP) nanostructures/TiO2 heterostructured composites. The RP nanostructures were directly grown on the TiO2 nanoparticles with an intimate surface contact. By adjusting the molar ratio of amorphous RP to TiO2 and the synthesizing temperature, thin nanorod-like RP nanostructures with a large exposed surface and a good surface contacting with TiO2 were obtained. The synergetic effect of heterostructured RP/TiO2 composites leads to an enhanced charge separation and transfer, and a better utilization of visible-light. As expected, the RP/TiO2-700 °C composites exhibit good photocatalytic activity of degrading RhB and the optimal H2 evolution rate. This work not only provides a method to prepare earth abundant elemental phosphorus well-contacted heterostructures, expand the well-known UV-active TiO2 photocatalyst to visible active, but also deepens understanding of charge transfer dynamics in heterostructured photocatalyst.

Thermal conductivity of GaAs/AlAs distributed Bragg reflectors in semiconductor disk laser: comparison of molecular dynamics simulation and analytic methods.

GaAs/AlAs distributed Bragg reflectors (DBRs) are widely used in the gain chips of 1 µm wave band semiconductor disk lasers (SDLs) as an end/folded cavity mirror. Because the generated redundant heat in the active region of a gain chip mainly dissipates through the DBR, thermal conductivities of DBRs are crucial for the output performance of SDLs. For the purpose of more reasonable semiconductor wafer design, to improve the thermal management of SDLs, accurate thermal conductivities of DBRs with various layer thicknesses are under considerable requirement. By the use of the equilibrium molecular dynamics (EMD) simulation and the Tersoff potential, thermal conductivities of GaAs/AlAs superlattices with different layer thickness are calculated, and computed results are compared with reported data to verify the validity of the EMD simulation. The computed thermal conductivities of GaAs/AlAs DBRs using the EMD method show significant reduction in contrast to the bulk value. Compared to EMD simulation, analytic methods result in smaller values of thermal conductivities and get close to the bulk value much more slowly with increasing layer thickness. In the layer thickness of interest (60-100 nm), the Matthiessen rule with α=1 for GaAs and α=0.5 for AlAs is a practicable tool for thermal conductivity estimation.

Random lasing in human tissues embedded with organic dyes for cancer diagnosis.

Various nanostructures found in biological organisms are often complex and they exhibit unique optical functions. This study surprisingly found that typical random lasing occurs in cancerous human tissues embedded with the nanotextured organic dye 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7- tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). Hematoxylin and eosin stain images show that there are more laser resonators in cancerous tissues, caused by a large number of disordered scatters. It is also noteworthy that the random lasing thresholds were found to relate to the tumor malignancy grade. Consequently, the resulting typical random lasing resonators differ between cancerous tissues in different malignancy grades. Further studies are warranted to investigate tissue optical spectroscopy in the field of cancer diagnostics.

One step in situ synthesis of core-shell structured Cr2O3:P@fibrous-phosphorus hybrid composites with highly efficient full-spectrum-response photocatalytic activities.

Making full use of solar energy and achieving high charge separation efficiency are critical factors for the photocatalysis technique. In this work, we report core-shell structured fibrous phosphorus (f-P) coated P-doped Cr2O3 (Cr2O3:P@f-P) hybrid composites with a strong optical absorption in the full region of 200-2600 nm. The Cr2O3:P@f-P hybrid composites exhibit a record photocatalytic efficiency under UV, visible and near-infrared light irradiation, demonstrating as promising photocatalysts for the full utilization of solar energy. Systematical investigations combining experimental and theoretical work show that P doping modifies the electronic band structures and creates defective levels in the forbidden gap of Cr2O3 which extends the optical absorption to the visible and near-infrared regions. Highly crystalline fibrous phosphorus in situ grown on the Cr2O3 particles constructs a core-shell hybrid structure which guarantees intimate interfacial contact between f-P and Cr2O3:P and facilitates the separation of photogenerated electron-hole pairs. This study develops a promising system based on earth abundant element P to utilize the overall spectrum of sunlight for photochemical applications.

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.

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.

Amplified spontaneous emission from DCJTB encapsulated in mesostructured composite silica SBA-15.

Amplified spontaneous emission (ASE) characteristics of a red dye 4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7- etramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) encapsulated in a highly ordered mesoporous SBA-15 were studied. The mesoporous composite silica film loaded with organic dye has been successfully synthesized by a solgel reaction process and a simple postgrafting method at room temperature. The spectrum narrowing phenomena has been observed when the composite film is pumped at λp=532 nm by a Nd:YAG ed laser. There is a substantial reduction in the full width at half-maximum of the emitting light, which is one of the signatures of the presence of ASE. The ASE threshold and net gain, respectively, reached 0.03 mJ pulse-1 and 34.7 cm-1 for the DCJTB encapsulated in mesoporous SBA-15 film. The optimized ASE properties owe much to the effects of the better spatial confinement of the molecules in the ordered mesoporous structure of the host SBA-15.

Optimization of lasing in an inverted-opal titania photonic crystal cavity as an organic solid-state dye-doped laser.

Lasing performance of a dye-doped laser by encapsulating orange fluorescent dye 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) with different concentrations in a highly ordered three-dimensional (3D) inverted-opal titania (TiO2) photonic crystal (PC) microcavity was studied. The lasing threshold and laser quality were improved by optimizing the concentration of the laser dye DCM. When the concentration of DCM is optimized to 10-4 mol/l, the photoluminescence (PL) efficiency of DCM is sufficient to achieve lasing emission and meanwhile no fluorescence quantum quenching occurs. Therefore, the emission spectrum was greatly narrowed and the threshold was significantly improved, which reached 0.8 mJ pulse-1 cm-2. Our findings are promising results toward the realization of fabricating a highly efficient low-threshold organic laser.

Optical performance of mesostructured composite silica film loaded with organic dye.

A mesoporous composite silica film loaded with organic dye has been successfully synthesized by a solgel reaction process and a simple postgrafting method at room temperature. The composite film was characterized by x-ray diffraction, transmission electron microscopy, UV-Vis, photoluminescence (PL) spectra, and laser performance, and the results confirmed the existence of dyes in the channels of the silica film. A blue-shift and fluorescence property in the PL spectrum was observed from the composite film compared with that of dye molecules in C2H5OH solution. The spectrum narrowing phenomena has been observed when the composite film is pumped at λp=355 nm by a Nd:YAG pulsed laser. A narrower, higher peak was observed in emission spectra from the mesostructured composite silica film compared with the PL spectrum of dye in C2H5OH solution. There is a substantial reduction in the full width at half-maximum of the emitting light, which results in peaks with linewidths of 26 nm or more. This collapse of the emission spectrum is one of the signatures of the presence of amplified spontaneous emission.

Ultraviolet lasing action in ZnO nanosheets.

We report room-temperature ultraviolet lasing action in large quantities of uniform multilayer ZnO nanosheets grown by a vapor-transport method via thermal evaporation of Zn powder. An excellent multimode lasing emission at a center wavelength of 390 nm with a mode linewidth less than 0.33 nm occurs above an excitation threshold of 8 mJ pulse(-1) cm(-2). The observed multimode lasing action may be attributed to microcavity effect and low concentration of defects in the nanosheets. We believe that the single-mode lasing emission can be obtained by growing completely uniform nanosheets. ZnO nanosheet is an attractive candidate as gain medium to realize ultraviolet semiconductor diode lasers.

Amplified spontaneous emission from an Ag-backed red-fluorescent-dye-doped polymer film.

We demonstrate that the amplified spontaneous emission (ASE) in an Ag-backed red-fluorescent-dye-doped polymer film can be controlled by the effect of the film thickness. Optical losses associated with the metallic contacts necessary for charge injection, an obstacle to the development of an electrically pumped organic solid-state laser, may be possible to be reduced by increasing the gain medium layer thickness. The study of ASE characteristics of Ag-backed 4-(Dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)-doped polystyrene (PS) films with different thicknesses shows that increasing the film thickness can reduce the influence of the Ag layer. The threshold, gain, and loss of the device with a thickness of 800 nm are comparable to those of a metal-free device. Our findings demonstrate that the Ag-backed DCJTB:PS film can still be a good organic gain medium material for the fabrication of solid-state lasers, when the thickness of the DCJTB:PS layer increases to an appropriate value.