Electric Field Tunable Interlayer Relaxation Process and Interlayer Coupling in WSe2/Graphene Heterostructures

Transition metal dichalcogenides van der Waals (vdWs) heterostructures present fascinating optical and electronic phenomena, and bear tremendous significance for electronic and optoelectronic applications. As the significant merits in vdWs heterostructures, the interlayer relaxation of excitons and interlayer coupling at the heterointerface reflect the dynamic behavior of charge transfer and the coupled electronic/structural characteristics, respectively, which may give rise to new physics induced by quantum coupling. In this work, upon tuning the photoluminescence (PL) properties of WSe₂/graphene and WSe₂/MoS₂/graphene heterostructures by virtue of electric field, it is demonstrated that the interlayer relaxation of excitons at the heterointerface in WSe₂/graphene, which is even stronger than that in MoS₂/graphene and WSe₂/MoS₂ , plays a dominant role in PL tuning in WSe₂/graphene, while the carrier population in WSe₂ induced by electric field has a minor contribution. In addition, it is discovered that the interlayer coupling between monolayer WSe₂ and graphene is enhanced under high electric field, which breaks the momentum conservation of first order Raman‐allowed phonons in graphene, yielding the enhanced Raman scattering of defects in graphene. The interplay between electric field and vdWs heterostructures may provide versatile approaches to tune the intrinsic electronic and optical properties of the heterostructures.     

Improved Morphology and Efficiency of Polymer Solar Cells by Processing Donor–Acceptor Copolymer Additives

A novel wide‐bandgap conjugated polymer PBTA‐FPh based on benzodithiophene‐alt‐benzo[1,2,3]triazole as the main chain and a polar pentafluorothiophenyl (FPh) group in the side chain has been designed and synthesized. In comparison to the pristine polymer PBTA‐BO that consists of nonpolar alkyl side chains, the resulting PBTA‐FPh exhibits less pronounced aggregation while possessing analogous optical and electrochemical bandgaps. Contact angle measurements demonstrate that the surface energy can be enhanced by incorporating FPh moiety, leading to a better miscibility of PBTA‐BO with PC₇₁BM in the presence of a certain amount of PBTA‐FPh. The photoactive layer of PBTA‐BO:PC₇₁BM:PBTA‐FPh with weight ratio of 1:1.2:0.02% exhibits a percolated network with the fibrous features, as revealed by transmission electron microscopy measurements. Of particular interest is the significantly improved photovoltaic performances of polymer solar cell devices for which the power conversion efficiency is enhanced from 6.46% for the control device to 7.91% for device processed with PBTA‐FPh as the polymeric additive. These observations indicate that introducing donor–acceptor type of polymeric additive comprising of polar groups in the side chain can be a promising strategy for the fabrication of high‐performance polymer solar cells.     

Gas Pickering Emulsion Templated Hollow Carbon for High Rate Performance Lithium Sulfur Batteries

A CO₂ in water nanoparticle stabilized Pickering emulsion is used to template micrometer sized hollow porous nitrogen doped carbon particles for high rate performance lithium sulfur battery. For the first time, nanoparticles serve the dual role of an emulsion stabilizer and a pore template for the shell, directly utilizing in situ generated CO₂ bubbles as template for the core. The minimalistic nature of this method does not require expensive surfactants or additional core templates. Upon polymerization of melamine formaldehyde onto CO₂, a robust polymer/silica composite shell is formed and transformed into a porous shell upon washing. The micrometer‐sized hollow morphology in combination with its nitrogen rich porous shell demonstrates impressive rate capabilities of 670 and 500 mAh g^?1 even at a high rate of 7C and 9C, respectively. This material also possesses excellent cycle durability, exhibiting a low capacity decay of 0.088%/cycle over 300 cycles. Measurement of the shuttle current and impedance provides interesting insight into the polysulfide mass transfer mechanism of hollow structured sulfur hosts.     

Criterions of designing codebooks and a relay selection scheme for a relay network

For a relay network system with amplify and forward protocol, in this paper, analysis of performance is conducted, and an upper bound of pairwise error probability is obtained. Moreover, a scheme of relay selection is analyzed under assumptions that (i) the relays amplify and forward received signals without any signal processing; (ii) the receiver only knows channel state information from the relays to the receiver; and (iii) the transmitter and the relays do not have any channel state information. Based on the upper bound, a criterion of designing codebook is proposed. Surprisingly, this bound shows that increasing the number of relays can hardly improve performance. Furthermore, the error probability is decreasing with a speed of P^?1, no matter how large the relay number is, where P is the total energy. Also, for the relay selection, the optimal selecting way is to select only one relay with the largest channel gain, which is very different from existing schemes. These results are all confirmed by simulations. Copyright © 2015 John Wiley & Sons, Ltd.     

An end‐to‐end alternative to TCP PEPs: Initial Spreading,a TCP fast start‐up mechanism

While Transmission Control Protocol (TCP) Performance Enhancing Proxy (PEP) solutions have long been undisputed to solve the inherent satellite problems, the improvement of the regular end‐to‐end TCP congestion avoidance algorithms and the recent emphasis on the PEPs drawbacks have opened the question of the PEPs sustainability. Nevertheless, with a vast majority of Internet connections shorter than ten segments, TCP PEPs continue to be required to counter the poor efficiency of the end‐to‐end TCP start‐up mechanisms. To reduce the PEPs dependency, designing a new fast start‐up TCP mechanism is therefore a major concern. But, while enlarging the Initial Window (IW) up to ten segments is, without any doubt, the fastest solution to deal with a short‐lived connection in an uncongested network, numerous researchers are concerned about the impact of the large initial burst on an already congested network. Based on traffic observations and real experiments, Initial Spreading has been designed to remove those concerns whatever the load and type of networks. It offers performance similar to a large IW in uncongested network and outperforms existing end‐to‐end solutions in congested networks. In this paper, we show that Initial Spreading, taking care of the satellite specificities, is an efficient end‐to‐end alternative to the TCP PEPs. Copyright © 2015 John Wiley & Sons, Ltd.     

Constellation constrained multiuser multiple‐input multiple‐output for high capacity and error performance over correlated fading channels

Channels' correlation has direct impact to degrade the capacity and reliability of multiple‐input multiple‐output (MIMO) systems considerably. In this paper, new signal constellation designs are investigated to mitigate fading correlation and maximize the capacity and error performance of multiuser MIMO (MU‐MIMO) over correlated channels, which is a major research challenge. Two methods are studied in a novel constellation constrained MU‐MIMO approach, namely, unequal power allocation and rotated constellation. Based on principles of maximizing the minimum Euclidean distance (d_min) of composite received signals, users' data can be recovered using maximum likelihood joint detection irrespective of correlation values. Compared with the identical constellation scenario in conventional MU‐MIMO, it is shown that constellation rearrangement of transmitted signals has direct impact to resolve the detection ambiguity when the channel difference is not sufficient, particularly in moderate to high correlations. Extensive analysis and simulation results demonstrate the superiority of proposed technique to capture most of the promised gains of multiantenna systems and application for future wireless communications. Copyright © 2014 John Wiley & Sons, Ltd.     

局部拓扑信息耦合促进网络演化

为了研究局部拓扑信息耦合对网络演化的促进作用,该文提出一种局部拓扑加权方法,用于表征节点间联系的紧密性及拓扑信息的耦合程度,并从演化模型的宏观统计和实际网络数据测试两方面验证了局部拓扑信息耦合促进网络演化的有效性。首先将该加权方法应用于BA模型,提出TwBA模型及局域世界模型TwLW。仿真实验表明,TwBA的度分布随连边数目的增多,迅速从指数分布转变为幂律分布,验证了现实网络加速增长产生幂律分布的现象,并基于此提出一种加速演化的TwBA模型,其在不同的加速率下呈现出幂律分布;而TwLW则展现了从广延指数分布到幂律分布变化的形式。然后将加权方法拓展到链路预测方法,提出3个加权相似性指标。实际网络数据测试表明,该方法能够大幅度地提高基本算法的预测精度,部分甚至高于全局性指标。     

一种无线射频识别系统的设计与实现

本系统是基于数字通信原理、运用电磁耦合进行无线通信、利用单片机编解码的无线识别系统。阐述了无线射频识别系统基本电路的工作原理和硬件设计,并给出了软件设计的程序流程图。最后对该系统进行了调试和测试,得出一组测试数据,结合数据和波形对测试结果进行分析,并给出分析结果。     

U形漏泄同轴电缆的设计与实现

为达到某移动通信系统对漏泄同轴电缆主要电气指标的要求,设计一种U形漏泄同轴电缆,以满足工程应用。阐述了漏泄同轴电缆耦合损耗和传输衰减两个重要电气指标,以U形漏泄同轴电缆实例,利用三维电磁仿真软件HFSS仿真分析900 MHz工作频率下节距及槽孔尺寸的变化对耦合损耗、传输衰减的影响,仿真结果与实测结果吻合良好。结果表明U形漏泄同轴电缆节距越小,漏泄同轴电缆耦合损耗越小。     

基于NFC与Qi的无源电子纸显示设计

传统的电子纸显示装置耗电量大、内容更换繁琐,为增强其便捷性、缩小体积、降低功耗,文中基于LPC824结合近场通信技术与无线充电技术,并配合手机App设计了一款终端无源的电子纸显示装置。将带有NFC功能的手机靠近显示装置,可刷新电子纸上信息,移开手机后,信息可长期保留,且无需电源供电。通过对设计方案进行测试,实验结果验证了该方案的合理性与实用性。