Finding the optimal design of a passive microfluidic mixer.

The ability to thoroughly mix two fluids is a fundamental need in microfluidics. While a variety of different microfluidic mixers have been designed by researchers, it remains unknown which (if any) of these mixers are optimal (that is, which designs provide the most thorough mixing with the smallest possible fluidic resistance across the mixer). In this work, we automatically designed and rationally optimized a microfluidic mixer. We accomplished this by first generating a library of thousands of different randomly designed mixers, then using the non-dominated sorting genetic algorithm II (NSGA-II) to optimize the random chips in order to achieve Pareto efficiency. Pareto efficiency is a state of allocation of resources (e.g. driving force) from which it is impossible to reallocate so as to make any one individual criterion better off (e.g. pressure drop) without making at least one individual criterion (e.g. mixing performance) worse off. After 200 generations of evolution, Pareto efficiency was achieved and the Pareto-optimal front was found. We examined designs at the Pareto-optimal front and found several design criteria that enhance the mixing performance of a mixer while minimizing its fluidic resistance; these observations provide new criteria on how to design optimal microfluidic mixers. Additionally, we compared the designs from NSGA-II with some popular microfluidic mixer designs from the literature and found that designs from NSGA-II have lower fluidic resistance with similar mixing performance. As a proof of concept, we fabricated three mixer designs from 200 generations of evolution and one conventional popular mixer design and tested the performance of these four mixers. Using this approach, an optimal design of a passive microfluidic mixer is found and the criteria of designing a passive microfluidic mixer are established

Shape engineering of InP nanostructures for optoelectronic applications

III-V semiconductor nanostructures have been the research focus in the past two decades thanks to the superior properties of the materials themselves and the unique properties produced by reducing the dimension to the nanoscale. In particular, III-V nanowires have drawn much more attention and have been extensively applied in a wide range of devices including solar cells, light-emitting diodes, lasers, transistors, and photodetectors. Despite these great successes, these one-dimensional (1D) nanostructures still face many challenges in terms of synthesis, assembly, fabrication and the ability to form complex nanoarchitectures. Surprisingly, it has been demonstrated that nanostructures with more complex two- and three-dimensional (2D and 3D) shapes can provide possible solutions. These shape-engineered nanostructures can improve material properties and device functionality. Furthermore, recent intensive investigation of nanostructure networks shows a great demand on the shape flexibility, uniformity, structural and optical qualities of epitaxially grown III-V nanostructures. This dissertation presents the shape engineering of InP-based nanostructures grown by metal organic chemical vapour deposition (MOCVD) from 1D nanowires to more sophisticated 2D and 3D shapes. Shape transformation mechanism, crystal structure and optical properties were thoroughly investigated to understand the growth mechanism of these complex III-V nanostructures for electronic and optoelectronic applications. Among the various growth techniques, selective area epitaxy (SAE) has the advantages of producing uniform nanostructure arrays with a high degree of controllability in pattern geometry, such as shape, dimension, site position and spacing, and thus has been used in this work. InP nanostructures grown on {111}A InP substrates was first investigated. Two key growth parameters, temperature and V/III ratio, were studied to optimise the growth conditions. We found that a higher growth temperature was crucial to obtain high quality nanostructures. Under optimal growth conditions, the highly uniform arrays of wurtzite (WZ) InP nanostructures with tunable shapes, such as nanowires, nanomembranes, prism- and ring-like nanoshapes, were simultaneously achieved. Their side facets can be dominated by {101̅0} and/or {112̅0}, making WZ InP a good candidate for tailoring the shape of the nanostructures. In-depth investigation of shape transformation with time and opening geometry/dimension revealed that the shape was essentially determined by pattern confinement and minimisation of total surface energy. A theoretical model was proposed to explain the observed behaviour. Structural and optical characterisation results demonstrated that all the different InP nanostructures grown under optimal conditions have perfect wurtzite (WZ) crystal structure regardless of their shape and strong and homogeneous photon emission. Moreover, we investigated the shape evolution from branched nanowires to vertical/inclined nanomembranes and crystal structure of InP nanostructures grown on InP substrates of different orientations, including {100}, {110}, {111}B, {112}A and {112}B. Two growth models were proposed to explain these observations regarding shape transformation and phase transition. A strong correlation between the growth direction and crystal phase was revealed. Specifically, WZ and zinc-blende (ZB) phases form along the A and B directions, respectively, while crystal phase remains the same along other low-index directions. The polarity-induced crystal structure difference was explained by thermodynamic difference between the WZ and ZB phase nuclei on the {111}A and {111}B planes. Growth from the openings was essentially determined by pattern confinement and minimisation of the total surface energy, similar to growth on {111}A InP substrates. Accordingly, a novel type-II WZ/ZB nanomembrane homojunction array was obtained by tailoring growth directions through alignment of the openings along certain crystallographic orientations. Finally, the incorporation of InAsP quantum well was carried out on pure WZ InP nanostructure templates grown on {111}A InP substrates with two different shapes, i.e. nanowires and nanomembranes. InAsP quantum wells grew both axially and laterally on the InP nanowires and nanomembranes. While the axial quantum well was of ZB phase, the lateral one grown on side facets had a WZ phase. When sidewalls of nanowires and nanomembranes were the nonpolar {11̅00} facets, the radial quantum well selectively grew on the sidewall located at the semi-polar A side of the axial quantum well, leading to the shape evolution of nanowires from hexagonal to triangular cross section and destroying the symmetry of nanomembranes. In comparison, nanomembranes with {112̅0} sidewalls are shown to be an ideal template for growing InP/InAsP heterostructures thanks to the high symmetry and uniformity of quantum well nanomembrane array. EDX results showed that quantum well composition was highly dependent on the crystal facet. Moreover, quantum well nanomembranes with {112̅0} sidewalls gave strong and uniform photon emission around 1.3 µm, showing superior optical properties compared with quantum wells incorporated in InP nanowires and nanomembranes with {11̅00} facets

Production cotonnière chinoise : forces et faiblesses d'une intégration et d'une adaptation à l'économie de marché

La Chine est le premier pays producteur de coton dans le monde mais sa forte consommation la place en position d'importateur structurel. La production chinoise est issue d'une agriculture familiale, avec une sole cotonnière d'environ 0,3 ha par exploitation, dont le niveau d'intensification lui permet d'atteindre l'un des rendements les plus élevés au monde. C'est le résultat d'une volonté politique pendant près d'un demi-siècle, à partir d'une recherche dynamique et d'un soutien aux producteurs à travers subvention aux intrants et garantie de prix d'achat. Dès la veille de l'entrée de la Chine à l'OMC, il n'y a plus de subvention directe aux producteurs de coton et la production cotonnière se poursuit dans une filière de plus en plus libéralisée. Le mode d'intensification persiste en raison du développement du marché des intrants; l'utilisation de variétés de coton génétiquement modifié (CGM) s'inscrit comme une nouvelle étape de l'intensification. L'absence de velléité de coordination pour mener des actions collectives pénalise cependant la rentabilité de la production cotonnière et peut hypothéquer la durabilité de l'utilisation du CGM. Les fortes fluctuations de prix découragent les paysans à s'engager dans une spécialisation cotonnière accrue et elles se répercutent dans une production instable. La Chine devrait rester importatrice nette de coton, surtout dans un contexte de déclin de l'activité agricole.Chine; coton; agriculture familiale; libéralisation; OGM

Induced Ferromagnetic Order of Graphdiyne Semiconductors by Introducing a Heteroatom

To date, the realization of ferromagnetism in two-dimensional carbon semiconductors containing only sp electrons has remained a challenge for spintronics. Here, we utilize the atomic-level functionalization strategy to obtain three carbon matrix materials by accurately introducing different light elements (H, F, Cl) into graphdiyne's benzene ring. Their magnetic and conductive characteristics are thoroughly clarified via physical property measurements and DFT calculations. All of these carbon matrix materials retain their excellent intrinsic semiconductor properties. In particular, compared with the paramagnetism of HsGDY and ClsGDY, a robust ferromagnetic ordering as well as high mobility of up to 320 cm2 V−1 s −1 was observed in FsGDY, successfully realizing a ferromagnetic semiconductor. Through theory calculations, this unique ferromagnetic coupling can be attributed to the most striking charge transfer between carbon and fluorine atoms, demonstrating the advantages of controllable fabrication. These results not only reveal the important role of atomic-scale doping/substitution in optimizing graphdiyne material but also create new possibilities for manipulating spins and charges in 2D carbon materials.This study was supported by the National Natural Science Foundation of China (51802324, 21790050, 21790051, 51822208, 21771187), the Frontier Science Research Project (QYZDB-SSW-JSC052) of the Chinese Academy of Sciences, and the Taishan Scholars Program of Shandong Province (tsqn201812111)

Facet-dependent growth of InAsP quantum wells in InP nanowire and nanomembrane arrays

Selective area epitaxy is a powerful growth technique that has been used to produce III-V semiconductor nanowire and nanomembrane arrays for photonic and electronic applications. The incorporation of a heterostructure such as quantum wells (QWs) brings new functionality and further broadens their applications. Using InP nanowires and nanomembranes as templates, we investigate the growth of InAsP QWs on these pure wurtzite nanostructures. InAsP QWs grow both axially and laterally on the nanowires and nanomembranes, forming a zinc blende phase axially and wurtzite phase on the sidewalls. On the non-polar {1100} sidewalls, the radial QW selectively grows on one sidewall which is located at the semi-polar 〈112〉 A side of the axial QW, causing the shape evolution of the nanowires from hexagonal to triangular cross section. For nanomembranes with {1100} sidewalls, the radial QW grows asymmetrically on the {1100} facet, destroying their symmetry. In comparison, nanomembranes with {1120} sidewalls are shown to be an ideal template for the growth of InAsP QWs, thanks to the uniform QW formation. These QWs emit strongly in the near IR region at room temperature and their emission can be tuned by changing their thickness or composition. These findings enrich our understanding of the QW growth, which provides new insights for heterostructure design in other III-V nanostructures.National Natural Science Foundation of China (No. 61974166, 51702368 and 61874141); Hunan Provincial Natural Science Foundation of China (2018JJ3684); Open Project of the State Key Laboratory of Luminescence and Applications (SKLA-2018-07); and The Australian Research Council (ARC) are acknowledged for financial support

Machine-Learning-Enabled Design and Manipulation of a Microfluidic Concentration Gradient Generator

Microfluidics concentration gradient generators have been widely applied in chemical and biological fields. However, the current gradient generators still have some limitations. In this work, we presented a microfluidic concentration gradient generator with its corresponding manipulation process to generate an arbitrary concentration gradient. Machine-learning techniques and interpolation algorithms were implemented to help researchers instantly analyze the current concentration profile of the gradient generator with different inlet configurations. The proposed method has a 93.71% accuracy rate with a 300× acceleration effect compared to the conventional finite element analysis. In addition, our method shows the potential application of the design automation and computer-aided design of microfluidics by leveraging both artificial neural networks and computer science algorithms

Acoustic Imaging Using the Built-In Sensors of a Smartphone

Thanks to the rapid development of the semiconductor industry, smartphones have become an indispensable part of our lives with their increasing computational power, 5G connection, multiple integrated sensors, etc. The boundary of the functionalities of a smartphone is beyond our imagination again and again as the new smartphone is introduced. In this work, we introduce an acoustic imaging algorithm by only using the built-in sensors of a smartphone without any external equipment. First, the speaker of the smartphone is used to emit sound waves with a specific frequency band. During the movement of the smartphone, the accelerometer collects acceleration data to reconstruct the trajectories of the movements, while the microphones receive the reflected waves. A microphone plus an accelerometer are able to partially replace the functionality of a microphone array and to become a symmetry-imitation system. After scanning, a series of algorithms are implemented to generate a heat map, which outlines the target object. Our algorithm demonstrates the feasibility of smartphone-based acoustic imaging with minimal equipment complexity and no additional cost, which is beneficial to the promotion and popularization of acoustic imaging technology in daily applications