Ambient-pressure ozone treatment enables tuning of oxygen vacancy concentration in the La1−x_{1− x}Srx_{x}FeO3−δ_{3− δ } (0 ≤ x ≤ 1) perovskite oxides

Oxygen vacancies in metal oxides can determine their properties. However, it is difficult to reduce the oxygen vacancy concentration in metal oxides without annealing them under high pressure. In this work, we develop a facile approach to control oxygen vacancy content via an ozone treatment under ambient pressure during cooling. This approach is demonstrated for the synthesis of La$_{1−x}$Sr$_{x}$FeO$_{3−δ}$ (0 ≤ x ≤ 1, 0 ≤ δ ≤ 0.5x) perovskite oxides – an important class of energy-related materials due to their wide range of non-stoichiometry, mixed ionic and electronic conductivity, and the presence of a rare Fe(IV) oxidation state. A series of La$_{1−x}$Sr$_{x}$FeO$_{3−δ}$ compounds was initially synthesized using a polymerized complex method. The concentration of oxygen vacancies and Fe(IV) were determined by redox titration, and the crystal structures were derived by analyzing X-ray diffraction patterns using Rietveld refinement. Significant amounts of oxygen vacancies were found in the as-synthesized compounds with x ≥ 0.8: La$_{0.2}$Sr$_{0.8}$FeO$_{3−δ}$ (δ = 0.066) and SrFeO$_{3−δ}$ (δ = 0.195). The ambient-pressure ozone treatment approach was able to substantially reduce the amount of oxygen vacancies in these compounds to achieve levels near the oxygen stoichiometry of 3 for La$_{0.2}$Sr$_{0.8}$FeO$_{3−δ}$ (δ = 0.006) and SrFeO$_{3−δ}$ (δ = 0.021). The oxygenation/deoxygenation kinetics can be tuned by the cooling rate after annealing. As the oxygen vacancy concentration decreases, the structure of SrFeO$_{3−δ}$ evolves from orthorhombic to cubic, demonstrating that the crystal structures in metal oxides can be highly sensitive to the number of oxygen vacancies. The ozone treatment approach developed in this study may thus offer a robust means to tune the properties of a wide variety of metal oxides

Nanoscale Footprints of Self-Running Gallium Droplets on GaAs Surface

In this work, the nanoscale footprints of self-driven liquid gallium droplet movement on a GaAs (001) surface will be presented and analyzed. The nanoscale footprints of a primary droplet trail and ordered secondary droplets along primary droplet trails are observed on the GaAs surface. A well ordered nanoterrace from the trail is left behind by a running droplet. In addition, collision events between two running droplets are investigated. The exposed fresh surface after a collision demonstrates a superior evaporation property. Based on the observation of droplet evolution at different stages as well as nanoscale footprints, a schematic diagram of droplet evolution is outlined in an attempt to understand the phenomenon of stick-slip droplet motion on the GaAs surface. The present study adds another piece of work to obtain the physical picture of a stick-slip self-driven mechanism in nanoscale, bridging nano and micro systems

High operating temperature mid-infrared InGaAs/GaAs submonolayer quantum dot quantum cascade detectors on silicon

Monolithic integration of infrared photodetectors on a silicon platform is a promising solution for the development of scalable and affordable photodetectors and infrared focal plane arrays. We report on integration of submonolayer quantum dot quantum cascade detectors (SML QD QCDs) on Si substrates via direct growth. Threading dislocation density has been reduced to the level of ~10 7 cm -2 with the high-quality GaAs-on-Si virtual substrate. We also conducted a morphology analysis for the SML QD QCDs through a transmission electron microscope strain contrast image and to the best of our knowledge, high quality InGaAs/GaAs SML QDs were clearly observed on silicon for the first time. Photoluminescence decay time of the as-grown SML QD QCDs on Si was measured to be around 300 ps, which is comparable to the reference QCDs on lattice-matched GaAs substrates. With the high-quality III-V epitaxial layers and SML QDs, the quantum cascade detectors on Si achieved a normal incident photoresponse temperature up to 160 K under zero bias

Two-colour In0.5Ga0.5As quantum dot infrared photodetectors on silicon

An InGaAs quantum dot (QD) photodetector is directly grown on a silicon substrate. GaAs-on-Si virtual substrates with a defect density in the order of 106 cm−2 are fabricated by using strained-layer superlattice as dislocation filters. As a result of the high quality virtual substrate, fabrication of QD layer with good structural properties has been achieved, as evidenced by transmission electron microscopy and x-ray diffraction measurements. The InGaAs QD infrared photodetector is then fabricated on the GaAs-on-Si wafer substrate. Dual-band photoresponse is observed at 80 K with two response peaks around 6 and 15 μm.Engineering and Physical Sciences Research Council https://doi.org/10.13039/501100000266Royal Academy of Engineering https://doi.org/10.13039/501100000287National Science Foundation of the U.S.Peer Reviewe

State filling dependent luminescence in hybrid tunnel coupled dot-well structures

A strong dependence of quantum dot (QD)–quantum well (QW) tunnel coupling on the energy band alignment is established in hybrid 'In''As'/'GA''AS'-'IN IND. x''GA IND. 1-x''AS'/'GA''AS' dot–well structures by changing the QW composition to shift the QW energy through the QD wetting layer (WL) energy. Due to this coupling a rapid carrier transfer from the QW to the QD excited states takes place. As a result, the QW photoluminescence (PL) completely quenches at low excitation intensities. The threshold intensities for the appearance of the QW PL strongly depend on the relative position of the QW excitonic energy with respect to the WL ground state and the QD ground state energies. These intensities decrease by orders of magnitude as the energy of the QW increases to approach that of the WL due to the increased efficiency for carrier tunneling into the WL states as compared to the less dense QD states below the QW energy.MWN - Material World NetworkNational Science Foundation of the U.S. (DMR-1008107)Deutsche Forschungsgemeinschaft (Li 580/8-1)Korea Foundation for International Cooperation of Science & Technology (Global Research Laboratory project - K20815000003)

Investigation of electrically active defects in InGaAs quantum wire intermediate-band solar cells using deep-level transient spectroscopy (DLTS) technique

InGaAs quantum wire (QWr) intermediate-band solar cell based nanostructures grown by molecular beam epitaxy are studied. The electrical and interface properties of these solar cell devices, as determined by current–voltage (I–V) and capacitance–voltage (C-V) techniques, were found to change with temperature over a wide range of 20–340 K. The electron and hole traps present in these devices have been investigated using deep-level transient spectroscopy (DLTS). The DLTS results showed that the traps detected in the QWr-doped devices are directly or indirectly related to the insertion of the Si δ-layer used to dope the wires. In addition, in the QWr-doped devices, the decrease of the solar conversion efficiencies at low temperatures and the associated decrease of the integrated external quantum efficiency through InGaAs could be attributed to detected traps E1QWR_D, E2QWR_D, and E3QWR_D with activation energies of 0.0037, 0.0053, and 0.041 eV, respectively

Splitting Arabic Texts into Elementary Discourse Units

International audienceIn this article, we propose the first work that investigates the feasibility of Arabic discourse segmentation into elementary discourse units within the segmented discourse representation theory framework. We first describe our annotation scheme that defines a set of principles to guide the segmentation process. Two corpora have been annotated according to this scheme: elementary school textbooks and newspaper documents extracted from the syntactically annotated Arabic Treebank. Then, we propose a multiclass supervised learning approach that predicts nested units. Our approach uses a combination of punctuation, morphological, lexical, and shallow syntactic features. We investigate how each feature contributes to the learning process. We show that an extensive morphological analysis is crucial to achieve good results in both corpora. In addition, we show that adding chunks does not boost the performance of our system

The impact of cooperative learning strategy in the development of general creative thinking during the course of physical education and sports among middle school students

هدفت هذه الدراسة إلى تقصي أثر إستراتيجية التعلم التعاوني في تنمية مهارات التفكير الإبداعي العام خلال حصة التربية البدنية و الرياضية لدى تلاميذ المرحلة المتوسطة ؛ تكونت عينة الدراسة من (70) تلميذ وتلميذة موزعين بالتساوي على مجموعتين إحداهما تجريبية والأخرى ضابطة، طُبق على المجموعتين قبل المعالجة و بعدها اختبار التفكير الإبداعي العام لإبراهام بعد التحقق من صدقه وثباته. تم تطبيق إستراتيجية التعلم التعاوني بطريقة التعلم معا على المجموعة التجريبية. أظهرت النتائج وجود فروق دالة إحصائيا في القياس البعدي بين المجموعة الضابطة والتجريبية في مهارات التفكير الإبداعي العام لصالح المجموعة التجريبية ؛ أوصت الدراسة باستخدام إستراتيجية التعلم التعاوني بطرقه المختلفة، وبيان أثره في تنمية مهارات التفكير.The aim of this study was to investigate the effect of cooperative learning strategy in the development of creative thinking skills, during the physical education class, for middle school students in Setif-Algeria.The sample consisted of 70 male and female students, equally divided into two groups, experimental and control group. We applied the creative thinking test of "ibraham" to the two groups before and after treatment.The post testing results showed a statistically significant differences between the two groups, in fluency skills, flexibility skills and in the total score of the creative thinking test for the benefit of the experimental group that studied under a strategy of cooperative learning

Investigation of novel inverted NiO@NixCo1-xO core-shell nanoparticles

Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to their different potential application and due to the tunability of their magnetic properties. The antiferromagnetic nature of NiO and high Néel temperature (523 K) makes this material well suited for inverse core-shell nanoparticle applications. Our primary objective in this project has been to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixCo1-xO (FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixCo1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixCo1-xO CSNs have the rock salt cubic crystal structure. SEM-EDS data indicates the presence of Co in the CSNs. Magnetic measurements show that the CSNs exhibit AFM/FiM characteristics with a small coercivity field of 30 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a magnetization axis shift which is attributed to the exchange bias effect between the AFM NiO core and an FiM NixCo1-xO shell of the CSNs. Our ab initio based calculations of the NixCo1-xO rock salt structure confirm a weak FiM character and a charge transfer insulator property of the compound