[en] In this work, we report that Pt nanoparticles are self-assembled by annealing Pt thin films grown on Si (100) substrates at a high temperature. After growth of Pt thin films, annealing treatment is carried out at 700 and 850 °C respectively, under high purity nitrogen environment. It is found that Pt nanoparticles are formed at annealing temperature of 700 °C, while larger sized and high-quality crystalline Pt nanoparticles are generated at an elevated temperature of 850 °C. It is also found that Pt nanoparticles present plasmon enhanced effect on the optical properties of the polymercrystalline ZnO thin films, and Pt nanoparticles prepared by 20s sputtering time leads to about 5 fold excitation enhancement compared to that without Pt nanoparticles. (paper)

[en] GaN and related III-nitrides have attracted considerable attention as promising materials for application in optoelectronic devices, in particular, light-emitting diodes (LEDs). At present, sapphire is still the most popular commercial substrate for epitaxial growth of GaN-based LEDs. However, due to its relatively large lattice mismatch with GaN and low thermal conductivity, sapphire is not the most ideal substrate for GaN-based LEDs. Therefore, in order to obtain high-performance and high-power LEDs with relatively low cost, unconventional substrates, which are of low lattice mismatch with GaN, high thermal conductivity and low cost, have been tried as substitutes for sapphire. As a matter of fact, it is not easy to obtain high-quality III-nitride films on those substrates for various reasons. However, by developing a variety of techniques, distincts progress has been made during the past decade, with high-performance LEDs being successfully achieved on these unconventional substrates. This review focuses on state-of-the-art high-performance GaN-based LED materials and devices on unconventional substrates. The issues involved in the growth of GaN-based LED structures on each type of unconventional substrate are outlined, and the fundamental physics behind these issues is detailed. The corresponding solutions for III-nitride growth, defect control, and chip processing for each type of unconventional substrate are discussed in depth, together with a brief introduction to some newly developed techniques in order to realize LED structures on unconventional substrates. This is very useful for understanding the progress in this field of physics. In this review, we also speculate on the prospects for LEDs on unconventional substrates. (review)

[en] Ag nanowire (NW)/polydimethylsiloxane (PDMS) pressure sensors with the three-layer and back-to-back structures were fabricated by a coating-peeling method. The bending and pressing responses of the sensors were comparably investigated. The results reveal that two kinds of pressure sensors show similar response linearity in the bending test with a bending angle of 0-180°. However, the response sensitivity of the three-layer structured pressure sensor is superior to that of the back-to-back structural one, which exhibits that the relationship between the capacitance value (Y) and the bending angle (X) is: Y = 0.01244X + 2.9763. On the contrary, in the pressing test, the response sensitivity of the back-to-back structural sensor is better than that of the three-layer structural one. The relationship between capacitance value (Y) and the number of paper clips (pressure, X₂) is Y = 0.09241X₂ + 88.03597. (paper)

[en] Gallium nitride (GaN) films with excellent structural, electrical and optical properties have been epitaxially grown on La_0_._3Sr_1_._7AlTaO_6 (LSAT) (111) substrates by radio-frequency molecular beam epitaxy at low temperature. The GaN films grown at 500 °C exhibits high crystalline quality with the (0002) and (10−12) full width at half maximum of 0.056° and 0.071°. There is a maximum of 1.1-nm-thick interfacial layer existing between the as-grown GaN and LSAT (111) substrate, and the as-grown about 300-nm-thick GaN films are almost fully relaxed only with a 0.0094% in-plane tensile strain. Hall and photoluminescence (PL) measurements also reveal outstanding electrical and optical properties of the as-grown GaN films on LSAT. This achievement brings the prospect for achieving highly-efficient GaN-based optoelectronic devices on LSAT (111) substrates. (papers)

[en] Control of structural modifications inside silica glass by changing the front tilt of an ultrashort pulse is demonstrated, achieving a calligraphic style of laser writing. The phenomena of anisotropic bubble formation at the boundary of an irradiated region and modification transition from microscopic bubbles formation to self-assembled form birefringence are observed, and the physical mechanisms are discussed. The results provide the comprehensive evidence that the light beam with centrosymmetric intensity distribution can produce noncentrosymmetric material modifications.

[en] Highlights: • The framework consisting of nonlinear modeling, strategy optimization and decision making is presented. • Trade-off between accuracy and efficiency is conducted in nonlinear modeling. • The multiple schemes and search corridor are defined in strategy optimization. • Fuzzy analytic hierarchy process is used to select most satisfying non-dominated solution. • The effectiveness and robustness of the framework are shown in extreme conditions. -- Abstract: Pumped storage hydropower system has now become a key support for integration of variable renewable energy. Along with the large-scale development of pumped storage plants all over the world, the importance of operation strategy is rapidly increasing with the fast-growing demand of flexibility. How to direct flexible operation and improve the stability of the system has become a key issue. This paper proposes a novel optimization framework to derive optimal operating policies for pumped storage hydropower system, which is divided into three coordinated stages: nonlinear modeling, strategy optimization and decision making. The real-time accurate equivalent circuit model is proposed in first stage, which can reconcile the conflict between simulation efficiency and accuracy, owing to the novel pump-turbine model and space-time discretization. The search corridor which consists of multiple constraints is defined to improve search efficiency. Reference vector guided evolutionary algorithm is performed to handle non-normalization objectives, multiple constraints and irregular Pareto fronts in second stage. Further, fuzzy analytic hierarchy process is innovatively introduced to select compatible solution under extreme conditions. The originality of the framework is embodied in multiple trade-offs, i.e. trade-off between accuracy and efficiency in the nonlinear model, trade-off between convergence and diversity in the strategy optimization, and trade-off in conflicting objectives of the decision making. Compared with the on-site operation, the maximum water pressure of volute and the vacuum at draft tube can be improved by 5.59% and 9.6%, the rotational speed oscillation also decreases with this framework in load rejection. The proposed framework can specify the optimal policy to enhance the system reliability, which can also serve as the basis for smart operation in various conditions.

[en] Highlights: • A performance assessment of VSPSPs for mitigating wind power variations is done. • A flexible method of integrated modelling for VSPSPs is presented and validated. • The analysis combines technical and economic indicators based on timescale of seconds. • The quantitative results show the power regulation rapidity of variable speed units. • The work could support development of variable-speed pumped storage technology. -- Abstract: Developing the joint operation of hydro and variable renewable energy has emerged as a research trend, for handling the power variability. In recent years, variable-speed pumped storage plants (VSPSPs) have been proposed as an alternative to fixed-speed pumped storage plants, but VSPSPs require a higher investment cost for equipment. Hence, evaluating the advantages and demonstrating the value of VSPSPs are meaningful topics that have seldom been studied by quantitative analysis with a small timescale. In this paper, a performance assessment of VSPSPs in terms of power regulation for mitigating wind power variations is undertaken based on a timescale of seconds, and the assessment combines the analysis of physical features and the economic indicators regarding ancillary service markets. First, a numerical model integrating hydraulic-mechanical-electrical subsystems of VSPSPs with doubly fed induction machines is built with MATLAB/Simulink, and it is validated by on-site measurements of a Japanese VSPSP. Then, based on a Chinese VSPSP, a quantitative comparison between variable-speed units (VSUs) and fixed-speed units (FSUs) is conducted through four indicators based on ancillary service compensation in electricity markets in China and the USA. Twelve scenarios are investigated, including case studies based on measured wind power variations. The results show that the VSU outperforms the FSU by one order of magnitude in the aspect of power regulation performance: the maximum ratios between the VSU and the FSU of the four indicators (average of power difference, standard deviation of power difference, penalty energy, and power delay) are 3.92%, 7.85%, 3.92%, and 5.56%. VSUs not only stand out for contributing to power system stability but can also obtain a significantly higher assessment in the ancillary service of the electricity market from an economic perspective. These results could be an important source of support for the investment and development of variable-speed pumped storage technology.

[en] AlN films with various thicknesses have been grown on Si(111) substrates by pulsed laser deposition (PLD). The surface morphology and structural property of the as-grown AlN films have been investigated carefully to comprehensively explore the epitaxial behavior. The ∼2 nm-thick AlN film initially grown on Si substrate exhibits an atomically flat surface with a root-mean-square surface roughness of 0.23 nm. As the thickness increases, AlN grains gradually grow larger, causing a relatively rough surface. The surface morphology of ∼120 nm-thick AlN film indicates that AlN islands coalesce together and eventually form AlN layers. The decreasing growth rate from 240 to 180 nm/h is a direct evidence that the growth mode of AlN films grown on Si substrates by PLD changes from the islands growth to the layer growth. The evolution of AlN films throughout the growth is studied deeply, and its corresponding growth mechanism is hence proposed. These results are instructional for the growth of high-quality nitride films on Si substrates by PLD, and of great interest for the fabrication of AlN-based devices

[en] Highlights: • A framework integrating novel model, co-optimization, and operation evaluation. • The small signal model is competitive in simulation efficiency and applicability. • The adaptable co-optimization strategy could coordinate the stability-tracking conflict. • The 12 scenarios verify the competitive advantage of proposed method. The fast and stable regulation of pumped storage is a basic guarantee for supporting various scenarios of renewable energy system. The operator pursues sensitive tracking performance, while underestimates the dynamic characteristics of hydraulic system and damping characteristics of pumped storage unit (PSU). These may aggravate the wear-tear of PSU operation, and decrease the frequency stability of power system. Therefore, a systematical study of improving overall regulation performance is conducted by applying PSU modeling, co-optimization and operation evaluation. First, the novel small signal model is proposed and the high-order hydraulic damping model is further derived. Apart from the ultra-low frequency oscillation mode, a new frequency oscillation mode caused by surge tanks is captured. Second, the co-optimization strategy is presented to coordinate the stability-tracking conflict. And then a comprehensive evaluation model, integrating 14 indicators is conducted to quantify the PSU performance and its contribution to power system, driving the favorable decision making of operators. Compared with the original scheme, the overall performance of PSU is improved by 20.76%, at the cost of 10.88% tracking capacity. The comparison of three measures including 12 scenarios verifies the competitive advantage of co-optimization strategy in multi-machine system. This paper supplies a novel tool for performance enhancement of PSU. It may have potential value in the stability of renewable energy systems with multiple hydropower units.

[en] Graphical abstract: - Highlights: • Single-crystalline AlN films have been grown on single-crystalline Cu substrates. • High thickness homogeneity AlN films have been achieved. • Crack free AlN films have been grown single-crystalline Cu substrates. • The as-grown ∼321 nm thick AlN films are only with a compressive strain of 0.48%. - Abstract: The homogeneous and crack free single-crystalline AlN thin films have been epitaxially grown on single-crystalline Cu (1 1 1) substrates with an in-plane alignment of AlN [11–20]//Cu [1–10] by pulsed laser deposition (PLD) technology with an integrated laser rastering program. The as-grown AlN films are studied by spectroscopic ellipsometry, field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), polarized light microscopy, high-resolution X-ray diffraction, and high-resolution transmission electron microscopy (HRTEM). The spectroscopic ellipsometry reveals the excellent thickness uniformity of as-grown AlN films on the Cu (1 1 1) substrates with a root-mean-square (RMS) thickness inhomogeneity less than 2.6%. AFM and FESEM measurements indicate that very smooth and flat surface AlN films are obtained with a surface RMS roughness of 2.3 nm. The X-ray reflectivity image illustrates that there is a maximum of 1.2 nm thick interfacial layer existing between the as-grown AlN and Cu (1 1 1) substrates and is confirmed by HRTEM measurement, and reciprocal space mapping shows that almost fully relaxed AlN films are achieved only with a compressive strain of 0.48% within ∼321 nm thick films. This work demonstrates a possibility to obtain homogeneous and crack free single-crystalline AlN films on metallic substrates by PLD with optimized laser rastering program, and brings up a broad prospect for the application of acoustic filters that require abrupt hetero-interfaces between the AlN films and the metallic electrodes