Rational Synthesis of Branched CoMoO₄@CoNiO₂ Core/Shell Nanowire Arrays for All-Solid-State Supercapacitors with Improved Performance

Effectively composite materials with optimized structures exhibited promising potential in continuing improving the electrochemical performances of supercapacitors in the past few years. Here, we proposed a rational design of branched CoMoO₄@CoNiO₂ core/shell nanowire arrays on Ni foam by two steps of hydrothermal processing. Owing to the high activity of the scaffold-like CoMoO₄ nanowires and the well-defined CoNiO₂ nanoneedles, the three-dimensional (3D) electrode architectures achieved remarkable electrochemical performances with high areal specific capacitance (5.31 F/cm² at 5 mA/cm²) and superior cycling stability­(159% of the original specific capacitance, i.e., 95.7% of the maximum retained after 5000 cycles at 30 mA/cm²). The all-solid-state asymmetric supercapacitors composed of such electrode and activated carbon (AC) exhibited an areal specific capacitance of 1.54 F/cm² at 10 mA/cm² and a rate capability (59.75 Wh/kg at a 1464 W/kg) comparable with Li-ion batteries. It also showed an excellent cycling stability with no capacitance attenuation after 50000 cycles at 100 mA/cm². After rapid charging (1 s), such supercapacitors in series could lighten a red LED for a long time and drive a mini motor effectively, demonstrating advances in energy storage, scalable integrated applications, and promising commercial potential

Highly Anisotropic Dirac Fermions in Square Graphynes

We predict a family of 2D carbon (C) allotropes, square graphynes (S-graphynes) that exhibit highly anisotropic Dirac fermions, using first-principle calculations within density functional theory. They have a square unit-cell containing two sizes of square C rings. The equal-energy contour of their 3D band structure shows a crescent shape, and the Dirac crescent has varying Fermi velocities from 0.6 × 10⁵ to 7.2 × 10⁵ m/s along different <i>k</i> directions. Near the Fermi level, the Dirac crescent can be nicely expressed by an extended 2D Dirac model Hamiltonian. Furthermore, tight-binding band fitting reveals that the Dirac crescent originates from the next-nearest-neighbor interactions between C atoms. S-graphynes may be used to build new 2D electronic devices taking advantages of their highly directional charge transport

The evolution of self-driven droplets.

<p>The development process from immobile droplet to self-driven droplet. SEM images in a 45-degree viewpoint are as follows: (a) a “baby” stage droplet with circular contact line; (b) a “child” stage droplet with rounded-rectangular shape contact line, is learning to walk; (c) is a “teenage” droplet with hexagonal-face shape contact line, is in the beginning stage of walking; and (d) is an “adult” droplet with a hexagonal-face-shape contact line, is in the early stage of stick-slip motion.</p

Freshly exposed surfaces from colliding events.

<p>Images (a), (b), and (c) are SEM images of coincident events in top view. Both (d) and (e) are SEM images of coincident events in a 45-degree viewpoint, where roundish deep wells are clearly visible in the A positions of each. Both (f) and (g) are schematic diagrams of the droplet coalescence. (e) In the top view, the orange circle indicates the droplet before the collision; the red dot-dash circle indicates the coalescent droplet, the red dash line in position A indicates the roundish well after collision. (f) In the cross-section view, the orange ellipse indicates the droplet before colliding; the red dot-dash ellipse indicates the coalescent droplet, and the red dash line in position A indicates the roundish well after collision. The wavelike surface indicates the footprints left behind by the droplets.</p

Collision between two running droplets.

<p>The two droplets are coincident with each other: (a) Before colliding, in top view of SEM, both the shape of droplets A and B are confined in their moving trails. (b) After colliding, in the top view, droplet A merges with droplet B. The orange circular shape indicates the original droplet B before colliding resulting in an empty well with a group of small droplets in position A. (c) the top view shows the schematic diagram of the droplet coalescence. The orange circle indicates the droplet before collision; the red dot-dashed circle indicates the coalescent droplet, and the red dash line in position A indicates the roundish well after collision. Both (d1) and (d2) are SEM images which show that the resultant larger droplets from coalescence are moving backward and forward. (e) This shows the AFM 3D image of the coalescent event. The red dash-square area corresponds to the simple topographical AFM mapping (top side) in which the atomic scale layers highlight the atomically flat bottom of the well. The two black dash-lines correspond with the AFM line-scanning profile graphs (right side), outlining the significant depth of the well by comparing them with the trail.</p

The morphology of running Ga droplets.

<p>The typical footprints of Ga droplets motion are: (a) a SEM image of running droplets in microscale; (b) a SEM image of the detailed footprints left behind by the leading droplet in nanoscale; (c) AFM images of the footprint from droplet motion: (c1) the simple topographical AFM mapping; (c2) AFM phase image highlighting the stepping nanostructures of footprints; and (c3) AFM 3D images revealing the depth of the footprints with the central dashed-line corresponding to the AFM line-scanning profile on the right-hand side graph. The multiple dashed-lines on the scanning-line graph indicate the footprint are featured in angles α and β (α = 1.6° and β = 6.4° after ratio normalization). Finally, (d) is the AFM 3D image of a moving droplet, with the central dashed-line corresponding to the AFM line-scanning profile on the right side.</p

High Responsivity Photoconductors Based on Iron Pyrite Nanowires Using Sulfurization of Anodized Iron Oxide Nanotubes

Iron pyrite (FeS₂) nanostructures are of considerable interest for photovoltaic applications due to improved material quality compared to their bulk counterpart. As an abundant and nontoxic semiconductor, FeS₂ nanomaterials offer great opportunities for low-cost and green photovoltaic technology. This paper describes the fabrication of FeS₂ nanowire arrays via sulfurization of iron oxide nanotubes at relatively low temperatures. A facile synthesis of ordered iron oxide nanotubes was achieved through anodization of iron foils. Characterization of the iron sulfide nanowires indicates that pyrite structures were formed. A prototype FeS₂ nanowire photoconductor demonstrates very high responsivity (>3.0 A/W). The presented method can be further explored to fabricate various FeS₂ nanostructures, such as nanoparticles, nanoflowers, and nanoplates

Activating Earth-Abundant Element-Based Colloidal Copper Chalcogenide Quantum Dots for Photodetector and Optoelectronic Synapse Applications

Colloidal copper chalcogenide-based CuAlS2 (CAS) quantum dots (QDs) composed of earth-abundant and benign elements are emerging building blocks for optoelectronic technologies. However, their potential applications in high-performance optoelectronic devices are still unexplored due to limited optical properties and poor charge transfer efficiency. Here, a new class of CAS/ZnSe QDs exhibiting decent visible light absorption/emission was prepared and associative QD-gold nanoclusters (Au NCs) heterostructures were rationally constructed for improved charge separation and transport efficiency. Both experimental and theoretical studies revealed that such metal nanocluster decoration enables effective charge transfer from QDs to Au NCs. Photodetectors (PDs) fabricated using CAS/ZnSe QDs and Au-QD heterostructures were further demonstrated, wherein the Au-QDs PD show enhanced device performance with a responsivity of 7.57 A W–1 and a detectivity of 2.48 × 1011 Jones (405 nm, 2.8 mW cm–2) compared to that of the CAS/ZnSe QD PDs. We found that the Au NCs conjunction is key to setting intermediate energy levels and facilitating photoinduced charge transfer from QDs to the electron-transport layers (TiO2) employed in PD devices. Besides, as-assembled, Au-QD PDs have demonstrated optoelectronic synapse application by emulating the learning and forgetting processes under optical stimulation, representing a promising prototype device to achieve future solution-processed neuromorphic electronics

Transfer-Free Growth of Atomically Thin Transition Metal Disulfides Using a Solution Precursor by a Laser Irradiation Process and Their Application in Low-Power Photodetectors

Although chemical vapor deposition is the most common method to synthesize transition metal dichalcogenides (TMDs), several obstacles, such as the high annealing temperature restricting the substrates used in the process and the required transfer causing the formation of wrinkles and defects, must be resolved. Here, we present a novel method to grow patternable two-dimensional (2D) transition metal disulfides (MS₂) directly underneath a protective coating layer by spin-coating a liquid chalcogen precursor onto the transition metal oxide layer, followed by a laser irradiation annealing process. Two metal sulfides, molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), are investigated in this work. Material characterization reveals the diffusion of sulfur into the oxide layer prior to the formation of the MS₂. By controlling the sulfur diffusion, we are able to synthesize continuous MS₂ layers beneath the top oxide layer, creating a protective coating layer for the newly formed TMD. Air-stable and low-power photosensing devices fabricated on the synthesized 2D WS₂ without the need for a further transfer process demonstrate the potential applicability of TMDs generated via a laser irradiation process

Direct Synthesis of Graphene with Tunable Work Function on Insulators via In Situ Boron Doping by Nickel-Assisted Growth

Work function engineering, a precise tuning of the work function, is essential to achieve devices with the best performance. In this study, we demonstrate a simple technique to deposit graphene on insulators with in situ controlled boron doping to tune the work function. At a temperature higher than 1000 °C, the boron atoms substitute carbon sites in the graphene lattice with neighboring carbon atoms, leading to the graphene with a p-type doping behavior. Interestingly, the involvement of boron vapor into the system can effectively accelerate the reaction between nickel vapor and methane, achieving a fast graphene deposition. The changes in surface potential and work function at different doping levels were verified by Kelvin probe force microscopy, for which the work function at different doping levels was shifted between 20 and 180 meV. Finally, the transport mechanism followed by the Mott variable-range hopping model was found due to the strong disorder nature of the system with localized charge-carrier states