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An asymmetric fiber-shaped supercapacitor with excellent flexibility, ultra-high energy and power density is developed, demonstrating great practical importance.

Due to the rapid advances of Information and Communication Technologies (ICT), especially 5G and Artificial Intelligence (AI), the Internet of Everything is gradually becoming a reality, and human beings living environments are becoming smarter and smarter. Every day there will be generated large amounts of data in Humans-Machines-Things hybrid space, which is also called Cyber-Physical-Social Systems (CPSSs). Today, the city we live in has become a data-driven society. However, how to effectively mine valuable information from these massive data to provide proactive and personalized services for human beings is a challenging problem. Thus, top-k search remains an important topic of ongoing research. In this paper, we focus on a basic problem of geo-tagged data: find the top-k frequent terms among the geo-tagged data in a specific region from the cloud. We first construct a Region Tree Index (RTI) for geo-tagged data. Then the list storage structure is proposed to Store Sorted Terms and Weights (SSTW) in RTI. And then an efficient kTermsSearch algorithm is presented to compute top-k frequent terms in a given region. Finally, extensive experiments verify the validity of the proposed scheme.

Reducing friction and wear is a long-standing concern for machinery. Bulk solid lubricants have been widely adopted to address the tribological challenges at the macroscale, yet their implementation is relatively limited for small-scale applications. Recently, the emergence of two-dimensional (2D) materials brings a new horizon of a range of ultrathin solid lubricants for micro- and nanoscale sliding interfaces. Using ball-on-disk friction tests, we found that monolayer MoS2 possessed an outstanding lubrication performance when the normal load was below a threshold, beyond which wear set in. Further scratch tests on monolayer MoS2 showed that this critical normal load was significantly affected by the residual strain in MoS2. As revealed by finite-element simulations, MoS2 failed primarily because of in-plane stretching during tribological sliding, and the tensile residual strain would add to the in-plane strain inside MoS2, resulting in a noticeably weaker antiwear performance. This work provides guideli...

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with layered structures provide a unique platform for exploring the effect of number of layers on their fundamental properties. However, the thickness scaling effect on the chemical properties of these materials remains unexplored. Here, we explored the chemically induced phase transition of 2D molybdenum disulfide (MoS2) from both experimental and theoretical aspects and observed that the critical electron injection concentration and the duration required for the phase transition of 2D MoS2 increased with decreasing number of layers. We further revealed that the observed dependence originated from the layer-dependent density of states of 2H-MoS2, which results in decreasing phase stability for 2H-MoS2 with increasing number of layers upon electron doping. Also, the much larger energy barrier for the phase transition of monolayer MoS2 induces the longer reaction time required for monolayer MoS2 as compared to multilayer MoS2. The layer-dependen...

The conventional strategies employed for the synthesis of hollow nanofibers (HNFs) require either multistep treatments or special design of the equipment. An additional annealing process is always required, which inevitably consumes more energy and raises the manufacturing cost. In addition, the annealing process may also cause a waste of the matrix materials and the release of toxic gases. Herein, we report for the first time a novel one-step synthesis of hollow hybrid nanofibers via electrospinning. Cellulose was chosen as the polymer matrix, and Fe(OH)3 nanoparticles were grown in situ on the nanofibers during electrospinning. There was no need to remove cellulose to create the hollow nanofiber structure. This can significantly simplify the fabrication process without any negative influence to the air. The obtained Fe(OH)3@cellulose HNF membranes exhibited great mechanical properties and an extremely high water flux of 11 200 L m–2 h–1 bar–1. They could effectively remove various pollutants in water, i...

Mapping local surface charge distribution is critical to the understanding of various surface processes and also allows the detection of molecules binding to the surface. We show here that the optical absorption of monolayer MoS2 is highly sensitive to charge and demonstrate optical imaging of local surface charge distribution with this atomically thin material. We validate the imaging principle and perform charge sensitivity calibration with an electrochemical gate. We further show that binding of charged molecules to the atomically thin material leads to a large change in the image contrast, allowing determination of the charge of the adsorbed molecules. This capability opens possibilities for characterizing impurities and defects in two-dimensional materials and for label-free optical detection and charge analysis of molecules.

Low-dimensional semiconductors provide promising ultrathin channels for constructing more-than-Moore devices. However, the prominent contact barriers at the semiconductor-metal electrodes interfaces greatly limit the performance of the obtained devices. Here, a chemical approach is developed for the construction of p-type field-effect transistors (FETs) with low contact barriers by achieving the simultaneous synthesis and integration of 2D PdTe2 with various low-dimensional semiconductors. The 2D PdTe2 synthesized through a quasi-liquid process exhibits high electrical conductivity (≈4.3 × 106 S m-1 ) and thermal conductivity (≈130 W m-1 K-1 ), superior to other transition metal dichalcogenides (TMDCs) and even higher than some metals. In addition, PdTe2 electrodes with desired geometry can be synthesized directly on 2D MoTe2 and other semiconductors to form high-performance p-type FETs without any further treatment. The chemically derived atomically ordered PdTe2 -MoTe2 interface results in significantly reduced contact barrier (65 vs 240 meV) and thus increases the performance of the obtained devices. This work demonstrates the great potential of 2D PdTe2 as contact materials and also opens up a new avenue for the future device fabrication through the chemical construction and integration of 2D components.

Molecular electronics is a promising subject to overcome the size limitation of silicon-based electronic devices. In the past decades, various micro/nanofabrication techniques have been developed for constructing molecular junctions, and a number of breakthroughs are made in the characterizations and applications of the single-molecule device. The history and progress are reviewed in this article, laying emphasis on the recent works on the combination of micro/nanofabrication techniques with other techniques such as electrochemical deposition and surface-enhanced Raman spectroscopy (SERS). Some prototypical single-molecule devices such as molecular transistors are presented. Finally, the challenges and prospects in the fabrication of single-molecule devices are discussed.

Recent progress in addressing electrically driven single-molecule behaviors has opened up a path toward the controllable fabrication of molecular devices. Herein, the selective fabrication of single-molecule junctions is achieved by employing the external electric field. For molecular junctions with methylthio (-SMe), thioacetate (-SAc), amine (-NH2 ), and pyridyl (-PY), the evolution of their formation probabilities along with the electric field is extracted from the plateau analysis of individual single-molecule break junction traces. With the increase of the electric field, the SMe-anchored molecules show a different trend in the formation probability compared to the other molecular junctions, which is consistent with the density functional theory calculations. Furthermore, switching from an SMe-anchored junction to an SAc-anchored junction is realized by altering the electric field in a mixed solution. The results in this work provide a new approach to the controllable fabrication and modulation of single-molecule junctions and other bottom-up nanodevices at molecular scales.

2D MoS2 has emerged as a promising alternative to Pt-based catalysts for hydrogen evolution reaction (HER) due to its low cost and earth abundance. However, insufficient active sites of basal plane and poor conductivity become the foremost factors restricting the catalytic performance of MoS2 . Here, a facile strategy is presented to enhance the HER performance of MoS2 by converting its 2D structure into 1D/2D heterostructures of Mo6 Te6 /MoS2(1- x ) Te2 x by the in situ tellurization. As-prepared 1D/2D heterostructures exhibit excellent HER performance with the Tafel slope of ≈56 mV dec-1 (only one-third of that for pristine MoS2 ). The enhanced HER catalytic activity is attributed to more Te/S vacancies introduced by tellurization, which serve as the active sites as suggested by theoretical calculations. Besides, the formation of highly conductive well-aligned quasi-1D Mo6 Te6 nanobelts facilitate charge transport in HER. Previous work provides a facile approach to construct mixed dimensional materials, and opens up a new avenue to the properties modulation of 2D transition metal chalcogenides.

van der Waals (vdW) heterostructures synthesized through the chemical vapor deposition (CVD) method allow the creation and tuning of intriguing electronic and optical properties of two-dimensional (2D) materials. Especially, local structures in the heterostructures, such as interfaces, edges, and point defects, are critical for their wide range of potential applications. However, up to now atomic-scale measurements of local structures in as-grown 2D heterostructures on insulating substrates are still rare. Here we report our scanning tunneling microscopy and spectroscopy (STM/STS) study of as-grown MoS2 monolayer and WS2/MoS2 heterobilayer on SiO2. The heterobilayer appears smoother than the MoS2 monolayer, with a root-mean-square roughness of 0.230 nm in the former and 0.329 nm in the latter. For the first time, to our knowledge, we directly observed a novel type of continuous interface between the MoS2 monolayer and the top layer of the heterobilayer with atomic resolution. Our STS results and density f...

Using the MoS2 -WTe2 heterostructure as a model system combined with electrochemical microreactors and density function theory calculations, it is shown that heterostructured contacts enhance the hydrogen evolution reaction (HER) activity of monolayer MoS2 . Two possible mechanisms are suggested to explain this enhancement: efficient charge injection through large-area heterojunctions between MoS2 and WTe2 and effective screening of mirror charges due to the semimetallic nature of WTe2 . The dielectric screening effect is proven minor, probed by measuring the HER activity of monolayer MoS2 on various support substrates with dielectric constants ranging from 4 to 300. Thus, the enhanced HER is attributed to the increased charge injection into MoS2 through large-area heterojunctions. Based on this understanding, a MoS2 /WTe2 hybrid catalyst is fabricated with an HER overpotential of -140 mV at 10 mA cm-2 , a Tafel slope of 40 mV dec-1 , and long stability. These results demonstrate the importance of interfacial design in transition metal dichalcogenide HER catalysts. The microreactor platform presents an unambiguous approach to probe interfacial effects in various electrocatalytic reactions.

Chemical vapor deposition (CVD) has shown great potential in synthesizing various high-quality two-dimensional (2D) transition metal dichalcogenides (TMDCs). However, the nondestruction transfer of these CVD-grown 2D TMDCs at a high yield remains a key challenge for applying these emerging materials in various aspects. To address this challenge, we designed a water-soluble transfer mediator consisting of two polymers, polyvinylpyrrolidone (PVP) and poly(vinyl alcohol) (PVA), which can form strong interactions with CVD-grown 2D TMDCs for the nondestruction transfer of these materials. With this mediator, we realized the physical transfer of CVD-grown MoS2 flakes and several other 2D TMDCs, including 2D alloys and heterostructures to a wide range of substrates at a high yield of90% with well-retained properties as evidenced by various microscopic, spectroscopic, and electrical measurements. Field-effect transistors (FETs) made on thus-transferred CVD-grown MoS2 monolayers exhibited obviously higher mobility than those transferred by chemical method. We also constructed several artificial 2D crystals showing very strong interlayer coupling by the multiple transfer of CVD-grown 2D TMDCs monolayers with this approach. This transfer approach will make versatile CVD-grown 2D materials and their artificial stacks with pristine qualities easily accessible for both fundamental studies and practical applications.

In this work, the vertical electrical transport behavior of bilayer MoS2 under the coupling of force and light was explored by the use of conductive atomic force microscopy. We found that the current-voltage behavior across the tip-MoS2-Pt junction is a tunneling current that can be well fitted by a Simmons approximation. The transport behavior is direct tunneling at low bias and Fowler-Nordheim tunneling at high bias, and the transition voltage and tunnel barrier height are extracted. The effect of force and light on the effective band gap of the junction is investigated. Furthermore, the source-drain current drops surprisingly when we continually increase the force, and the dropping point is altered by the provided light. This mechanism is responsible for the tuning of tunneling barrier height and width by force and light. These results provide a new way to design devices that take advantage of ultrathin two-dimensional materials. Ultrashort channel length electronic components that possess tunneling current are important for establishing high-efficiency electronic and optoelectronic systems.

The controlled synthesis of high-quality multilayer (ML) MoS2 flakes with gradually shrinking basal planes by chemical vapor deposition (CVD) is demonstrated. These CVD-grown ML MoS2 flakes exhibit much higher mobility and current density than mechanically exfoliated ML flakes due to the reduced contact resistance which mainly resulted from direct contact between the lower MoS2 layers and electrodes.

A vertical point heterostructure (VPH) is constructed by sandwiching a two-dimensional (2D) MoS2 flake with two cross-stacked metallic single-walled carbon nanotubes. It can be used as a field-effect transistor with high on/off ratio and a light detector with high spatial resolution. Moreover, the hybrid 1D-2D-1D VPHs open up new possibilities for nanoelectronics and nano-optoelectronics.