Showing papers by "Lei Zhang published in 2019"

Large plasticity in magnesium mediated by pyramidal dislocations

Abstract: Lightweight magnesium alloys are attractive as structural materials for improving energy efficiency in applications such as weight reduction of transportation vehicles. One major obstacle for widespread applications is the limited ductility of magnesium, which has been attributed to [Formula: see text] dislocations failing to accommodate plastic strain. We demonstrate, using in situ transmission electron microscope mechanical testing, that [Formula: see text] dislocations of various characters can accommodate considerable plasticity through gliding on pyramidal planes. We found that submicrometer-size magnesium samples exhibit high plasticity that is far greater than for their bulk counterparts. Small crystal size usually brings high stress, which in turn activates more [Formula: see text] dislocations in magnesium to accommodate plasticity, leading to both high strength and good plasticity.

Space-Time-Coding Digital Metasurfaces

Abstract: We study space-time modulated digital coding metasurfaces that enable simultaneous manipulations of electromagnetic waves in both space and frequency domains, including harmonic beam steering/shaping and scattering-signature control. Our theoretical predictions are in good agreement with experimental results at microwave frequencies, and may find interesting applications to a variety of fields, including wireless communications, cognitive radars, adaptive beamforming, and holographic imaging.

Breaking Reciprocity with Space-Time-Coding Digital Metasurfaces.

Abstract: Metasurfaces are artificially engineered ultrathin structures that can finely tailor and control electromagnetic wavefronts. There is currently a strong interest in exploring their capability to lift some fundamental limitations dictated by Lorentz reciprocity, which have strong implications in communication, heat management, and energy harvesting. Time-varying approaches have emerged as attractive alternatives to conventional schemes relying on magnetic or nonlinear materials, but experimental evidence is currently limited to devices such as circulators and antennas. Here, the recently proposed concept of space-time-coding digital metasurfaces is leveraged to break reciprocity. Moreover, it is shown that such nonreciprocal effects can be controlled dynamically. This approach relies on inducing suitable spatiotemporal phase gradients in a programmable way via digital modulation of the metasurface-elements' phase repsonse, which enable anomalous reflections accompanied by frequency conversions. A prototype operating at microwave frequencies is designed and fabricated for proof-of-concept validation. Measured results are in good agreement with theory, hence providing the first experimental evidence of nonreciprocal reflection effects enabled by space-time-modulated digital metasurfaces. The proposed concept and platform set the stage for "on-demand" realization of nonreciprocal effects, in programmable or reconfigurable fashions, which may find several promising applications, including frequency conversion, Doppler frequency illusion, optical isolation, and unidirectional transmission.

Installing earth-abundant metal active centers to covalent organic frameworks for efficient heterogeneous photocatalytic CO2 reduction

Abstract: Photocatalytic conversion of CO2 into energy carriers has been recognized as a highly promising strategy for achieving the virtuous carbon cycle in nature. The realization of this process depends on an efficient catalyst to reduce the reaction barrier. Herein, we report a series of transition metal ion modified crystalline covalent organic frameworks (COFs) for the heterogeneous photocatalytic reduction of CO2. By coordinating different kinds of open metal active species into COFs, the resultant DQTP (2,6-diaminoanthraquinone - 2,4,6-triformylphloroglucinol) COF-M (M = Co/Ni/Zn) exerts a strong influence on the activity and selectivity of products (CO or HCOOH). Significantly, DQTP COF-Co exhibits a high CO production rate of 1.02 × 103 μmol h−1 g−1, while DQTP COF-Zn has a high selectivity (90% over CO) for formic acid generation (152.5 μmol h−1 g−1). This work highlights the great potential of using stable COFs as platforms to anchor earth-abundant metal active sites for heterogeneous CO2 reduction.

Creating Well-Defined Hexabenzocoronene in Zirconium Metal-Organic Framework by Postsynthetic Annulation.

Abstract: The incorporation of large π-conjugated ligands into metal-organic frameworks (MOFs) can introduce intriguing photophysical and electrochemical properties into the framework. However, these effects are often hindered by the strong π-π interaction and the low solubility of the arylated ligands. Herein, we report the synthesis of a porous zirconium-based MOF, Zr6(μ3-O)4(μ3-OH)4(OH)6(H2O)6(HCHC) (PCN-136, HCHC = hexakis(4-carboxyphenyl)hexabenzocoronene), which is composed of a hexacarboxylate linker with a π-conjugated hexabenzocoronene moiety. Direct assembly of the Zr4+ metal centers and the HCHC ligands was unsuccessful due to the low solubility and the unfavorable conformation of the arylated HCHC ligand. Therefore, PCN-136 was obtained from aromatization-driven postsynthetic annulation of the hexaphenylbenzene fragment in a preformed framework (pbz-MOF-1) to avoid π-π stacking. This postsynthetic modification was done through a single-crystal-to-single-crystal transformation and was clearly observable utilizing single -crystal X-ray crystallography. The formation of large π-conjugated systems on the organic linker dramatically enhanced the photoresponsive properties of PCN-136. With isolated hexabenzocoronene moieties as photosensitizers and Zr-oxo clusters as catalytic sites, PCN-136 was employed as an inherent photocatalytic system for CO2 reduction under visible-light irradiation, which showed increased activity compared with pbz-MOF-1.

CuCo2S4 Nanosheets@N‐Doped Carbon Nanofibers by Sulfurization at Room Temperature as Bifunctional Electrocatalysts in Flexible Quasi‐Solid‐State Zn–Air Batteries

Abstract: The performance of quasi-solid-state flexible zinc-air batteries (ZABs) is critically dependent on the advancement of air electrodes with outstanding bifunctional electrocatalysis for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), together with the desired mechanical flexibility and robustness. The currently available synthesis processes for high-efficiency bifunctional bimetallic sulfide electrodes typically require high-temperature hydrothermal or chemical vapor deposition, which is undesirable in terms of the complexity in experimental procedure and the damage of flexibility in the resultant electrode. Herein, a scalable fabrication process is reported by combining electrospinning with in situ sulfurization at room temperature to successfully obtain CuCo2S4 nanosheets@N-doped carbon nanofiber (CuCo2S4 NSs@N-CNFs) films, which show remarkable bifunctional catalytic performance (Ej = 10 (OER) - E 1/2 (ORR) = 0.751 V) with excellent mechanical flexibility. Furthermore, the CuCo2S4 NSs@N-CNFs cathode delivers a high open-circuit potential of 1.46 V, an outstanding specific capacity of 896 mA h g-1, when assembled into a quasi-solid-state flexible ZAB together with Zn NSs@carbon nanotubes (CNTs) film (electrodeposited Zn nanosheets on CNTs film) as the anode. The ZAB also shows a good flexibility and capacity stability with 93.62% capacity retention (bending 1000 cycles from 0° to 180°), making it an excellent power source for portable and wearable electronic devices.

Biocompatible Conductive Polymers with High Conductivity and High Stretchability.

Abstract: Stretchable electronic materials have drawn strong interest due to their important applications in areas such as bioelectronics, wearable devices, and soft robotics. The stretchable electrode is an integral unit of stretchable systems. Intrinsically conductive polymers such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) can have high mechanical flexibility and good biocompatibility. However, their electrical conductivity and mechanical stretchability should be greatly improved for its applications as the stretchable electrode. Here, we report highly conductive and highly stretchable PEDOT:PSS by incorporating biocompatible d-sorbitol. d-Sorbitol can serve as both the secondary dopant and plasticizer for PEDOT:PSS. It can not only significantly improve the conductivity but also the stretchability. d-Sorbitol-PEDOT:PSS (s-PEDOT:PSS) can have a conductivity of >1000 S/cm, and the conductivity could be maintained at a strain up to 60%. The resistance of s-PEDOT:PSS remains almost constant during repeated stretching-releasing cycles. The mechanism for the stretchability improvement by d-sorbitol is ascribed to the softening of PSSH chains. d-Sorbitol can position among the PSSH chains and thus destructs the hydrogen bonds among the PSSH chains. This makes the conformational change of the PSSH chains under stress become easy and thus increases the mechanical flexibility of PEDOT:PSS. This conductivity is the highest for biocompatible intrinsically conductive polymers with high stretchability.

Trends in and Predictions of Colorectal Cancer Incidence and Mortality in China From 1990 to 2025

Abstract: Colorectal cancer (CRC) has emerged as a major public health concern in China during the last decade. In this study, we investigated the disease burden posed by CRC and analyzed temporal trends in CRC incidence and mortality rates in this country. We collected CRC incidence data from the Cancer Incidence in Five Continents, Volume XI dataset and the age-standardized incidence rate (ASIR) and age-standardized mortality rate (ASMR) of CRC by sex and age, from the 2016 Global Burden of Diseases Study. We used the average annual percentage change (AAPC) to quantify temporal trends in CRC incidence and mortality from 1990 to 2016 and found the ASIR of CRC increased from 14.25 per 100,000 in 1990 to 25.27 per 100,000 in 2016 (AAPC = 2.34, 95% confidence interval [CI] 2.29, 2.39). Cancer cases increased from 104.3 thousand to 392.8 thousand during the same period. The ASIR increased by 2.76% (95% CI 2.66%, 2.85%) and 1.70% (95% CI 1.64%, 1.76%) per year in males and females, respectively. The highest AAPC was found in people aged 15-49 years (2.76, 95% CI 2.59, 2.94). Cancer deaths increased from 81.1 thousand in 1990 to 167.1 thousand in 2016, while the ASMR remained stable (-0.04, 95% CI -0.13, 0.05), A mild increase (AAPC = 0.42, 95% CI 0.34, 0.51) was found among males and a significant decrease (AAPC = -0.75, 95% CI -0.90, -0.60) was found among females. Between 2016 and 2025, cancer cases and deaths are expected to increase from 392.8 and 167.1 thousand in 2016 to 642.3 (95% CI 498.4, 732.1) and 221.1 thousand (95% CI 122.5, 314.8) in 2025, respectively. Our study showed a steady increase in the CRC incidence in China over the past three decades and predicted a further increase in the near future. To combat this health concern, the prevention and management of known risk factors should be promoted through national polices. Greater priority should be given to CRC prevention in younger adults, and CRC screening should be widely adopted for this population.

Multichannel direct transmissions of near-field information.

Abstract: A digital-coding programmable metasurface (DCPM) is a type of functional system that is composed of subwavelength-scale digital coding elements with opposite phase responses. By configuring the digital coding elements, a DCPM can construct dynamic near-field image patterns in which the intensity of each pixel of the image can be dynamically and independently modulated. Thus, a DCPM can perform both spatial and temporal modulations. Here, this advantage is used to realize multichannel direct transmissions of near-field information. Three points are selected in the near-field region to form three independent channels. By applying various digital phase codes on the DCPM, independent binary digital symbols defined by amplitude codes (namely, weak and strong amplitudes) are transmitted through the three channels. The measured near-field distributions and temporal transmissions of the system agree with numerical calculations. Compared with the conventional multichannel transmission, the proposed mechanism achieves simultaneous spatial and temporal modulations by treating DCPM as an energy radiator and information modulator, thereby enduing DCPM with high potential in near-field information processing and communications. Scientists have developed a programmable metasurface capable of controlling both the spatial and temporal modulations of light, opening the door for tunable optical components that are flat and easily integrated into other devices. Metasurfaces are ultrathin materials made from metals or semiconductors that can modulate the amplitude, phase, and polarization of light waves. However, most metasurfaces currently available are static in nature, with their optical properties fixed during fabrication. Now, Xiang Wan and colleagues from Southeast University in China have developed a digital-coding programmable metasurface (DCPM) that can dynamically control light in real time. Made from subwavelength-scale digital coding elements with opposite phase responses, the proposed DCPM is capable of multi-channel direct transmissions of near-field information and could be used in a range of applications, including wearable devices, autonomous vehicles, and sensing, imaging, and display technologies.

Magnetic Transition in Monolayer VSe 2 via Interface Hybridization

Abstract: Magnetism in monolayer (ML) VSe2 has attracted broad interest in spintronics, while existing reports have not reached consensus. Using element-specific X-ray magnetic circular dichroism, a magnetic transition in ML VSe2 has been demonstrated at the contamination-free interface between Co and VSe2. Through interfacial hybridization with a Co atomic overlayer, a magnetic moment of about 0.4 μB per V atom in ML VSe2 is revealed, approaching values predicted by previous theoretical calculations. Promotion of the ferromagnetism in ML VSe2 is accompanied by its antiferromagnetic coupling to Co and a reduction in the spin moment of Co. In comparison to the absence of this interface-induced ferromagnetism at the Fe/ML MoSe2 interface, these findings at the Co/ML VSe2 interface provide clear proof that the ML VSe2, initially with magnetic disorder, is on the verge of magnetic transition.

Hydrophobic Polyoxometalate-Based Metal-Organic Framework for Efficient CO2 Photoconversion.

Abstract: A novel polyoxometalate (POM)-based metal-organic framework, TBA5[P2Mo16VMo8VIO71(OH)9Zn8(L)4] (NNU-29), was in situ synthesized and applied into CO2 photoreduction. The selection of porous materia...

Double Switch Biodegradable Porous Hollow Trinickel Monophosphide Nanospheres for Multimodal Imaging Guided Photothermal Therapy.

Abstract: Due to the limitation of inorganic nanomaterials in present clinical applications induced by their inherent nonbiodegradability and latent long-term side effects, we successfully prepared double switch degradable and clearable trinickel monophosphide porous hollow nanospheres (NiP PHNPs) modified with bovine serum albumin (BSA). Attributed to their acidic and oxidative double switch degradation capacities, NiP PHNPs can be effectively excreted from mice without long-term toxicity. Moreover, because of the paramagnetic and high molar extinction coefficient property resulting from the strong absorption in the second near-infrared light (NIR II) biowindow, NiP PHNPs have potential to be used for photoacoustic imaging (PAI) and T1-weighted magnetic resonance imaging (MRI) guided photothermal ablation of tumors in the NIR II biowindow. Specifically, it is interesting that the hollow structure and acidic degradation property enable NiP PHNPs to act as intelligent drug carriers with an on-demand release ability. These findings highlight the great potential of NiP PHNPs in the cancer theranostics field and inspire us to further broaden the bioapplications of transition metal phosphides.

Topologically Protected Edge State in Two-Dimensional Su–Schrieffer–Heeger Circuit

Abstract: Topological circuits, an exciting field just emerged over the last two years, have become a very accessible platform for realizing and exploring topological physics, with many of their physical phenomena and potential applications as yet to be discovered. In this work, we design and experimentally demonstrate a topologically nontrivial band structure and the associated topologically protected edge states in an RF circuit, which is composed of a collection of grounded capacitors connected by alternating inductors in the x and y directions, in analogy to the Su–Schrieffer–Heeger model. We take full control of the topological invariant (i.e., Zak phase) as well as the gap width of the band structure by simply tuning the circuit parameters. Excellent agreement is found between the experimental and simulation results, both showing obvious nontrivial edge state that is tightly bound to the circuit boundaries with extreme robustness against various types of defects. The demonstration of topological properties in circuits provides a convenient and flexible platform for studying topological materials and the possibility for developing flexible circuits with highly robust circuit performance.

Ultrahigh Hole Mobility of Sn-Catalyzed GaSb Nanowires for High Speed Infrared Photodetectors.

Abstract: Owing to the relatively low hole mobility, the development of GaSb nanowire (NW) electronic and photoelectronic devices has stagnated in the past decade. During a typical catalyst-assisted chemical vapor deposition (CVD) process, the adopted metallic catalyst can be incorporated into the NW body to act as a slight dopant, thus regulating the electrical properties of the NW. In this work, we demonstrate the use of Sn as a catalyst and dopant for GaSb NWs in the surfactant-assisted CVD growth process. The Sn-catalyzed zinc-blende GaSb NWs are thin, long, and straight with good crystallinity, resulting in a record peak hole mobility of 1028 cm2 V-1 s-1. This high mobility is attributed to the slight doping of Sn atoms from the catalyst tip into the NW body, which is verified by the red-shifted photoluminescence peak of Sn-catalyzed GaSb NWs (0.69 eV) compared with that of Au-catalyzed NWs (0.74 eV). Furthermore, the parallel array NWs also show a high peak hole mobility of 170 cm2 V-1 s-1, a high responsivity of 61 A W-1, and fast rise and decay times of 195.1 and 380.4 μs, respectively, under the illumination of 1550 nm infrared light. All of the results demonstrate that the as-prepared Sn-catalyzed GaSb NWs are promising for application in next-generation electronics and optoelectronics.

Ppb-level photoacoustic sensor system for saturation-free CO detection of SF6 decomposition by use of a 10 W fiber-amplified near-infrared diode laser

Abstract: A photoacoustic sensor system for the ppb-level CO detection of sulphur hexafluoride (SF6) decomposition was developed by use of a 10 W fiber-amplified near-infrared (NIR) diode laser. A photoacoustic detection module with symmetrical construction was designed to allow a high power laser beam and a large gas flow. A theoretical model was developed to describe the dynamic equilibrium processes of CO molecular activation and deactivation in the presence of gas flow. The 10 W optical excitation power effectively compensates the CO weak absorption line strength in the NIR spectral region and realizes a saturation-free CO detection with the assistance of the gas flow, resulting in a ppb-level CO detection sensitivity, which is comparable with the detection sensitivity obtained using a mid-infrared CO sensor.

Ni@C composites derived from Ni-based metal organic frameworks with a lightweight, ultrathin, broadband and highly efficient microwave absorbing properties

Abstract: The Ni@C composites were successfully synthesized by thermal decomposition of a Ni-based metal organic framework (Ni-MOF). By varying the pyrolysis temperature, Ni@C composites with different morphology and microstructure can be easily synthesized. Benefiting from the synergistic effect between the Ni core and carbon shell, the composite possess good impedance matching and excellent microwave absorption performance. Interestingly, the strongest absorption of the Ni@C composite reaches −55.7 dB corresponding to 6.0 GHz absorption bandwidth (<−10 dB) at a thickness of 1.85 mm. The results demonstrate that Ni@C composites may be promising microwave absorption materials because of their strong absorption, being lightweight, thin thickness and large absorption bandwidth.

Calculation model of dense spot pattern multi-pass cells based on a spherical mirror aberration

Abstract: We report a novel calculation model for dense spot pattern multi-pass cells consisting of two common identical spherical mirrors. A modified ABCD matrix without the paraxial approximation was developed to describe the ray propagation between two spherical mirrors and the reflection on the mirror surfaces. The intrinsic aberration from the spherical curvature creates a set of intricate variants with respect to a standard Herriot circle spot pattern. A series of detailed numerical simulations are implemented to verify that the input and output beams remain the same and, hence, retrace the same ray pattern. The set of exotic spot patterns obtained with a high fill factor improves the utilization efficiency of the mirror surfaces and produces a longer total optical path length with a low mirror cost.

Data mining new energy materials from structure databases

Abstract: New energy materials that act as clean power sources and data science are developing rapidly in the past decades and the advancement of the two research areas have significantly benefited the development of each other. At the meantime, structural information of materials have been obtained and stored in various structure databases, such as the Cambridge Structure Database (CSD) and the Inorganic Crystal Structure Database (ICSD). Researchers have developed various structure-property relationships of the energy materials, which could be applied to screen the potential suitable materials from structure databases; this has become an efficient route to explore and design new energy materials. In this article, we review recent progresses on the data mining study of new energy materials based on structure databases such as CSD and ICSD, in the context of dye-sensitized solar cells and perovskite solar cells, and also include other energy systems such as water splitting systems, lithium batteries, thermoelectric devices and gas adsorbent materials. The structure descriptors that are more fundamental in the data mining procedure employing the structure-properties relationships are focused; the structural descriptors are complementary to the quantum descriptors and are efficient in the materials design process. We believe that with the successful formulation of more advanced and case-by-case structure-property relationships of energy materials, many new energy materials could be efficiently identified with much lower cost and shorter design period via the data mining process.

A Novel Workflow-Level Data Placement Strategy for Data-Sharing Scientific Cloud Workflows

Abstract: Cloud computing can provide a more cost-effective way to deploy scientific workflows than traditional distributed computing environments such as cluster and grid. Due to the large size of scientific datasets, data placement plays an important role in scientific cloud workflow systems for improving system performance and reducing data transfer cost. Traditional task-level data placement strategy only considers shared datasets within individual workflows to reduce data transfer cost. However, it is obvious that task-level strategy is not necessarily good enough for the situation of multiple workflows at the workflow level. In this paper, a novel workflow-level data placement model is constructed, which regards multiple workflows as a whole. Then, a two-stage data placement strategy is proposed which first pre-allocates initial datasets to proper datacenters during workflow build-time stage, and then dynamically distributes newly generated datasets to appropriate datacenters during runtime stage. Both stages use an efficient discrete particle swarm optimization algorithm to place flexible-location datasets. Comprehensive experiments demonstrate that our workflow-level data placement strategy can be more cost-effective than its task-level counterpart for data-sharing scientific cloud workflows.

Mechanism of Single-Photon Upconversion Photoluminescence in All-Inorganic Perovskite Nanocrystals: The Role of Self-Trapped Excitons.

Abstract: The efficient single-photon upconversion photoluminescence (UCPL) feature of lead halide perovskite semiconductors makes it promising for developing laser cooling devices. This is an attractive potential application, but the underlying physics still remains unclear so far. By using the all-inorganic CsPbX3 (X = Br, I) nanocrystal samples, this phenomenon was investigated by photoluminescence (PL) and time-resolved PL under different temperatures and various excitation conditions. A broad emission band located at the low-energy side of the free exciton (FE) peak was detected and deduced to be from the self-trapped exciton (STE). The lifetime of STE emission was found to be 171 ns at 10 K, much longer than that of FE. The UCPL phenomenon was then attributed to thermal activation of transformation from STEs to FEs, and the energy barrier was derived to be 103.7 meV for CsPbBr3 and 45.2 meV for CsPb(Br/I)3, respectively. The transformation also can be seen from the fluorescence decay processes.

Enhanced photocatalytic performance of porous ZnO thin films by CuO nanoparticles surface modification

Abstract: Improving the response of ZnO photocatalyst to visible light is a hot topic. In this work, the porous ZnO thin films were first prepared by sol-gel method, and then CuO nanoparticles were deposited on the porous ZnO thin films to form CuO-ZnO heterojunction; the effect of CuO content on the photocatalytic activity of ZnO thin films were investigated. The results show that the formation of CuO-ZnO heterojunction effectively separates the photogenerated electron-hole pairs, leading to significant decrease of ZnO ultraviolet emission. In addition, the ZnO thin films with CuO nanoparticles modified on the surface increase the absorptive capacity in the visible range. Photocatalytic degradation of methylene blue shows that the photocatalytic activity of all CuO/ZnO thin films is greatly improved. The improvement of photocatalytic performance should be attributed to the enhanced separation efficiency of photogenerated electron-hole pairs by the CuO-ZnO heterojunction and the increased light absorption in the visible range by CuO nanoparticles. This kind of CuO/ZnO thin films also shows good photocatalytic reusability.