Intrinsic carrier multiplication in layered Bi2O2Se avalanche photodiodes with gain bandwidth product exceeding 1 GHz

Emerging layered semiconductors present multiple advantages for optoelectronic technologies including high carrier mobilities, strong light-matter interactions, and tunable optical absorption and emission. Here, metal-semiconductor-metal avalanche photodiodes (APDs) are fabricated from Bi₂O₂Se crystals, which consist of electrostatically bound [Bi₂O₂]²⁺ and [Se]²⁻ layers. The resulting APDs possess an intrinsic carrier multiplication factor up to 400 at 7 K with a responsivity gain exceeding 3,000 A/W and bandwidth of ~ 400 kHz at a visible wavelength of 515.6 nm, ultimately resulting in a gain bandwidth product exceeding 1 GHz. Due to exceptionally low dark currents, Bi₂O₂Se APDs also yield high detectivities up to 4.6 × 10¹⁴ Jones. A systematic analysis of the photocurrent temperature and bias dependence reveals that the carrier multiplication process in Bi₂O₂Se APDs is consistent with a reverse biased Schottky diode model with a barrier height of ~ 44 meV, in contrast to the charge trapping extrinsic gain mechanism that dominates most layered semiconductor phototransistors. In this manner, layered Bi₂O₂Se APDs provide a unique platform that can be exploited in a diverse range of high-performance photodetector applications.

     

Optimization of Root Section for Ultra-long Steam Turbine Rotor Blade

This study presents the comparison of aerodynamic performances of two successive designs of the root profiles for the ultra-long rotor blade equipped with a straight fir-tree dovetail. Since aerodynamic and strength requirements laid upon the root section design are contradictory, it is necessary to aerodynamically optimize the design within the limits given by the foremost strength requirements. The most limiting criterion of the static strength is the size of the blade cross-section, which is determined by the number of blades in a rotor and also by the shape and size of a blade dovetail. The aerodynamic design requires mainly the zero incidence angle at the inlet of a profile and in the ideal case ensures that the load does not exceed a limit load condition. Moreover, the typical root profile cascades are transonic with supersonic exit Mach number, therefore, the shape of a suction side and a trailing edge has to respect transonic expansion of a working gas. In this paper, the two variants of root section profile cascades are compared and the aerodynamic qualities of both variants are verified using CFD simulation and two mutually independent experimental methods of measurements (optical and pneumatic).     

第2届南方电网技术论坛主旨报告荟萃

[编者按]中国南方电网有限责任公司科技工作会议暨第二届南方电网技术论坛于2005年10月11日至12日在广州成功举行.大会邀请了有关专家作论坛主旨报告,内容涉及大电网的协调控制、灾变防御体系、安全评估策略、特高压技术和设备等,对中国拟构建的特高压电网研究有一定的参考价值.<南方电网技术研究>将其编辑成论坛主旨报告荟萃,发表于2005年第6期.本刊特转载该文,以飨读者.     

3D flow past transonic turbine cascade SE 1050 — Experiment and numerical simulations

This paper is concerned with experimental and numerical research on 3D flow past prismatic turbine cascade SE1050 (known in QNET network as open test case SE1050). The primary goal was to assess the influence of the inlet velocity profile on the flow structures in the interblade channel and on the flow field parameters at the cascade exit and to compare these findings to results of numerical simulations. Investigations of 3D flow past the cascade with non-uniform inlet velocity profile were carried out both experimentally and numerically at subsonic (M 2is = 0.8) and at transonic (M 2is = 1.2) regime at design angle of incidence. Experimental data was obtained using a traversing device with a five-hole conical probe. Numerically, the 3D flow was simulated by open source code OpenFOAM and in-house code. Analyses of experimental data and CFD simulations have revealed the development of distinctive vortex structures resulting from non-uniform inlet velocity profile. Origin of these structures results in increased loss of kinetic energy and spanwise shift of kinetic energy loss coefficient distribution. Differences found between the subsonic and the transonic case confirm earlier findings available in the literature. Results of CFD and experiments agree reasonably well.     

大城市地区的供电——从规划到解决

大城市面临的主要挑战是,用电需求的大幅增长,特别是在城市中心.变电站设备自由空间的日益减少更促使了这种需求增长.但是某些占用空间大的超高压设备需要安装在市中心,以便减少输电损失. 当今,世界上几乎有一半的人口居住在城市,城市化尤其是大城市的发展还在继续,虽然大城市地区对于国民经济极其重要,但是我们还面临提供相应基础设施的挑战,如人口的增长导致了用电量的增加,建筑物的高度集中导致了空间的限制和通行权的缺乏.     

Solution‐Based Processing of Optoelectronically Active Indium Selenide

Layered indium selenide (InSe) presents unique properties for high‐performance electronic and optoelectronic device applications. However, efforts to process InSe using traditional liquid phase exfoliation methods based on surfactant‐assisted aqueous dispersions or organic solvents with high boiling points compromise electronic properties due to residual surface contamination and chemical degradation. Here, these limitations are overcome by utilizing a surfactant‐free, low boiling point, deoxygenated cosolvent system. The resulting InSe flakes and thin films possess minimal processing residues and are structurally and chemically pristine. When employed in photodetectors, individual InSe nanosheets exhibit a maximum photoresponsivity of ≈5 × 10⁷ A W^?1, which is the highest value of any solution‐processed monolithic semiconductor to date. Furthermore, the surfactant‐free cosolvent system not only stabilizes InSe dispersions but is also amenable to the assembly of electronically percolating InSe flake arrays without posttreatment, which enables the realization of ultrahigh performance thin‐film photodetectors. This surfactant‐free, deoxygenated cosolvent approach can be generalized to other layered materials, thereby presenting additional opportunities for solution‐processed thin‐film electronic and optoelectronic technologies.     

Layered Platinum Dichalcogenides (PtS2, PtSe2, and PtTe2) Electrocatalysis: Monotonic Dependence on the Chalcogen Size

Presently, research in layered transition metal dichalcogenides (TMDs) for numerous electrochemical applications have largely focused on Group 6 TMDs, especially MoS₂ and WS₂, whereas TMDs belonging to other groups are relatively unexplored. This work unravels the electrochemistry of Group 10 TMDs: specifically PtS₂, PtSe₂, and PtTe₂. Here, the inherent electroactivities of these Pt dichalcogenides and the effectiveness of electrochemical activation on their charge transfer and electrocatalytic properties are thoroughly examined. By performing density functional theory (DFT) calculations, the electrochemical and electrocatalytic behaviors of the Pt dichalcogenides are elucidated. The charge transfer and electrocatalytic attributes of the Pt dichalcogenides are strongly associated with their electronic structures. In terms of charge transfer, electrochemical activation has been successful for all Pt dichalcogenides as evident in the faster heterogeneous electron transfer (HET) rates observed in electrochemically reduced Pt dichalcogenides. Interestingly, the hydrogen evolution reaction (HER) performance of the Pt dichalcogenides adheres to a trend of PtTe₂ > PtSe₂ > PtS₂ whereby the HER catalytic property increases down the chalcogen group. Importantly, the DFT study shows this correlation to their electronic property in which PtS₂ is semiconducting, PtSe₂is semimetallic, and PtTe₂ is metallic. Furthermore, Pt dichalcogenides are effectively activated for HER. Distinct electronic structures of Pt dichalcogenides account for their different responses to electrochemical activation. Among all activated Pt dichalcogenides, PtS₂ shows most accentuated improvement as a HER electrocatalyst with an exceptional 50% decline in HER overpotential. Knowledge on Pt dichalcogenides provides valuable insights in the field of TMD electrochemistry, in particular, for the currently underrepresented Group 10 TMDs.     

Transition Metal‐Depleted Graphenes for Electrochemical Applications via Reduction of CO2 by Lithium

Graphene has immense potential for future applications in the electrochemical field, such as in supercapacitors, fuel cells, batteries, or sensors. Graphene materials for such applications are typically fabricated through a top‐down approach towards oxidation of graphite to graphite oxide, with consequent exfoliation/reduction to yield reduced graphenes. Such a method allows the manufacture of graphenes in gram/kilogram quantities. However, graphenes prepared by this method can contain residual metallic impurities from graphite which dominate the electrochemical properties of the graphene formed. This dominance hampers their electrochemical application. The fabrication of transition metal‐depleted graphene is described, using ultrapure CO₂ (with benefits of low cost and easy availability) and elemental lithium by means of reduction of CO₂ to graphene. This preparation method produces graphene of high purity with electrochemical behavior that is not dominated by any residual transition metal impurities which would dramatically alter its electrochemical properties. Wide application of such methodology in industry and research laboratories is foreseen, especially where graphene is used for electrochemical devices.