Search

This paper proposes a classification for the relationships between non-critical activities. And then, we explore the influence of a change in a non-critical activity's duration to its early (later) realization time, and give sensitivity analysis of a non-critical activity's early (later) realization time to other activities' total floats. The results of a computational example manifest the efficiency and effectiveness of the proposed methods.

Critical activity and inflexible activity are two important types of activities in the Activity-on-Arc network with generalized precedence relations (AOA-GPRs), which are very useful for project managers and practitioners to allocate and adjust resources in practice. This paper discusses and defines some important concepts, and studies the effect on the project makespan of changes in the duration of a given activity, the classification procedure of critical activities, the relations between activity criticality and activity inflexibility in AOA-GPRs. Furthermore, we deduce that there are some necessity relations between the activity inflexibility and the activity criticality just under some strict restrictions.

The network with generalized precedence relations (GPRs) is very useful for project managers. However, it is limited to be used widely in real projects due to its very complex graphical representation which makes it difficult to be understood by the project managers and practitioners. Therefore, some methods are proposed to make the GPRs model structure more clear and concise, which include the conditions of adding backward arcs and the methods of eliminating cycles. And then, an illustrative example manifests that the improved GPRs model is helpful for searching and discriminating all network paths which can be searched by the Depth-First-Search algorithm efficiently.

Self-assembled SiGe quantum rings (QRs) on Si(001) are capped with Si layers at temperatures varying from 200 to 550 °C; their shape changes after Si capping are investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The SiGe QR shape can be well preserved in the Si capping layer when the substrate temperature at Si capping is lower than 350 °C, whereas the SiGe QR shape transforms from a ring to a mound when the substrate temperature is higher than 480 °C. The SiGe QR shape could also be well preserved in the Si capping layer with an initial low temperature (300 °C) Si capping followed by a relatively high temperature (550 °C) Si capping. A comparison of the photoluminescence (PL) spectra of the SiGe QRs and the SiGe quantum dots (QDs) is also reported.

The morphologies of self-assembled Ge/Si(001) islands with initial Si capping at a temperature of 640 °C are investigated by atomic force microscopy. Before Si capping, the islands show a metastable dome shape with very good size uniformity. This dome shape changes to a pyramid shape with {103} facets at a Si capping thickness of 0.32 nm, and then changes to pyramid shapes with {104} and {105} facets at Si capping thicknesses of 0.42 and 0.64 nm, respectively. Noteworthy is that islands with one side retained their dome shape while the other three sides that changed to {103} facets are observed at a Si capping thickness of 0.18 nm. These observations indicate that island shape change with Si capping is a kinetic rather than thermodynamic process. The atomic processes associated with this island shape change are kinetically limited at a low temperature of 400 °C, and no significant change in size and shape of islands is observed when Si capping layers are deposited at this temperature.

Despite quick development of perovskite light-emitting diodes (PeLEDs) during the past few years, the fundamental mechanisms on how ion migration affects device efficiency and stability remain unclear. Here, it is demonstrated that the dynamic redistribution of mobile ions in the emissive layer plays a key role in the performance of PeLEDs and can explain a range of abnormal behaviours commonly observed during the device measurement. The dynamic redistribution of mobile ions changes charge-carrier injection and leads to increased recombination current; at the same time, the ion redistribution also changes charge transport and results in decreased shunt resistance current. As a result, the PeLEDs show hysteresis in external quantum efficiencies (EQEs) and radiance, that is, higher EQEs and radiance during the reverse voltage scan than during the forward scan. In addition, the changes on charge injection and transport induced by the ion redistribution also well explain the rise of the EQE/radiance values under constant driving voltages. The argument is further rationalized by adding extra formamidinium iodide (FAI) into optimized PeLEDs based on FAPbI(3), resulting in more significant hysteresis and shorter operational stability of the PeLEDs. Funding Agencies|ERC Starting GrantEuropean Research Council (ERC) [717026]; Swedish Energy Agency EnergimyndighetenSwedish Energy Agency [48758-1, 44651-1]; Swedish Foundation for International Cooperation in Research and Higher Education [CH2018-7736]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; NSFCNational Natural Science Foundation of China (NSFC) [61774077]; Research and Development Program in Key Areas of Guangdong Province [2019B1515120073, 2019B090921002, 2019B010132004]; China Scholarship CouncilChina Scholarship Council

Orientational manipulation of transition dipole moment (TDM) plays an important role in controlling the polarization of excited states in light emission as well as lasing actions. The present work discovers vertically aligned TDMs in hybrid perovskite films through angle-resolved photoluminescence (PL) measurements, which show enhanced emission through the film edge. With increasing excitation intensity, the edge emission induced by these vertically aligned TDMs becomes dominant and eventually leads to amplified spontaneous emission (ASE) through the edge view. Meanwhile, polarized emission of both PL and electroluminescence (EL) provides further evidence for vertically aligned TDMs. Surprisingly, the degree of polarization (DOP) through the film edge is increased when grain boundary defects are passivated through either stochiometric engineering or self-passivation by mobile ions under working conditions. With increasing DOP, ASE threshold of the perovskite film is reduced owing to enhanced collective behaviors of light-emitting states. This work presents a useful method to manipulate TDMs in organic-inorganic hybrid perovskites. Funding Agencies|ERC Starting GrantEuropean Research Council (ERC) [717026]; Swedish Energy Agency EnergimyndighetenSwedish Energy Agency [48758-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]