The development of pressure sensor arrays capable of distinguishing the shape and texture details of objects is of considerable interest in the emerging fields of smart robots, prostheses, human–machine interfaces, and artificial intelligence (AI). Here we report an integrated pressure sensor array, by combining solution-processed two-dimensional (2D) MoS₂ van der Waals (vdW) thin film transistor (TFT) active matrix and conductive micropyramidal pressure-sensitive rubber (PSR) electrodes made of polydimethylsiloxane/ carbon nanotube composites, to achieve spatially revolved pressure mapping with excellent contrast and low power consumption. We demonstrate a 10 × 10 active matrix by using the 2D MoS₂ vdW-TFTs with high on-off ratio > 10⁶, minimal hysteresis, and excellent device-to-device uniformity. The combination of the vdW-TFT active matrix with the highly uniform micropyramidal PSR electrodes creates an integrated pressure sensing array for spatially resolved pressure mapping. This study demonstrates that the solution-processed 2D vdW-TFTs offer a solution for active-matrix control of pressure sensor arrays, and could be extended for other active-matrix arrays of electronic or optoelectronic devices.

Direct ethanol fuel cell (DEFC) has received tremendous research interests because of the more convenient storage and transportation of ethanol vs. compressed hydrogen. However, the electrocatalytic ethanol oxidation reaction typically requires precious metal catalysts and is plagued with relatively high over potential and low mass activity. Here we report the synthesis of Pt₃Ag alloy wavy nanowires via a particle attachment mechanism in a facile solvothermal process. Transmission microscopy studies and elemental analyses show highly wavy nanowire structures with an average diameter of 4.6 ± 1.0 nm and uniform Pt₃Ag alloy formation. Electrocatalytic studies demonstrate that the resulting alloy nanowires can function as highly effective electrocatalysts for ethanol oxidation reactions (EOR) with ultrahigh specific activity of 28.0 mA/cm² and mass activity of 6.1 A/mg, far exceeding that of the commercial Pt/carbon samples (1.10 A/mg). The improved electrocatalytic activity may be partly attributed to partial electron transfer from Ag to Pt in the Pt₃Ag alloy, which weakens CO binding and the CO poisoning effect. The one-dimensional nanowire morphology also contributes to favorable charge transport properties that are critical for extracting charge from catalytic active sites to external circuits. The chronoamperometry studies demonstrate considerably improved stability for long term operation compared with the commercial Pt/C samples, making the Pt₃Ag wavy nanowires an attractive electrocatalyst for EOR.

The layered semiconducting transition metal dichalcogenides (s-TMDs) have attracted considerable interest as the channel material for field-effect transistors (FETs). However, the multilayer s-TMD transistors usually exhibit considerable threshold voltage (V_th) shift and ambipolar behavior at high source-drain bias, which is undesirable for modern digital electronics. Here we report the design and fabrication of double feedback gate (FBG) transistors, i.e., source FBG (S-FBG) and drain FBG (D-FBG), to combat these challenges. The FBG transistors differ from normal transistors by including an extra feedback gate, which is directly connected to the source/drain electrodes by extending and overlapping the source/drain electrodes over the yttrium oxide dielectrics on s-TMDs. We show that the S-FBG transistors based on multilayer MoS₂ exhibit nearly negligible V_th roll-off at large source-drain bias, and the D-FBG multilayer WSe₂ transistors could be tailored into either n-type or p-type transport, depending on the polarity of the drain bias. The double FBG structure offers an effective strategy to tailor multilayer s-TMD transistors with suppressed V_th roll-off and ambipolar transport for high-performance and low-power logic applications.

Direct methanol fuel cells (DMFCs) have received tremendous research interests because of the facile storage of liquid methanol vs. hydrogen. However, the DMFC today is severely plagued by the poor kinetics and rather high overpotential in methanol oxidation reaction (MOR). Here we report the investigation of the ultrathin Rh wavy nanowires as a highly effective MOR electrocatalyst. We show that ultrathin wavy Rh nanowires can be robustly synthesized with 2-3 nm diameters. Electrochemical studies show a current peak at the potential of 0.61 V vs. reversible hydrogen electrode (RHE), considerably lower than that of Pt based catalysts (~ 0.8-0.9 V vs. RHE). Importantly, with ultrathin diameters and favorable charge transport, the Rh nanowires catalysts exhibit an ultrahigh electrochemically active surface area determined from CO-stripping (ECSA_CO) of 144.2 m²/g, far exceeding that of the commercial Rh black samples (20 m²/g). Together, the Rh nanowire catalysts deliver a mass activity of 722 mA/mg at 0.61 V, considerably higher than many previously reported electrocatalysts at the same potential. The chronoamperometry studies also demonstrate good stability and CO-tolerance compared with the Rh black control sample, making ultrathin Rh wavy nanowires an attractive electrocatalyst for MOR.

Anisotropic materials are of considerable interest because of their unique combination of polarization- or direction-dependent electrical, optical, and thermoelectric properties. Low-symmetry two-dimensional (2D) materials formed by van der Waals stacking of covalently bonded atomic layers are inherently anisotropic. Layered SnSe exhibits a low degree of lattice symmetry, with a distorted NaCl structure and an in-plane anisotropy. Here we report a systematic study of the in-plane anisotropic properties in layered SnSe, using angle-resolved Raman scattering, optical absorption, and electrical transport studies. The optical and electrical characterization was direction-dependent, and successfully identified the crystalline orientation in the layered SnSe. Furthermore, the dependence of Raman-intensity anisotropy on the SnSe flake thickness and the excitation wavelength were investigated by both experiments and theoretical calculations. Finally, the electrical transport studies demonstrated that few-layer SnSe field-effect transistors (FETs) have a large anisotropic ratio of carrier mobility (~5.8) between the armchair and zigzag directions, which is a record high value reported for 2D anisotropic materials. The highly-anisotropic properties of layered SnSe indicate considerable promise for anisotropic optics, electronics, and optoelectronics.

Lithium-air batteries have attracted significant interest for applications in high energy density mobile power supplies, yet there are considerable challenges to the development of rechargeable Li-air batteries with stable cycling performance under ambient conditions. Here we report a three-dimensional (3D) hydrophobic graphene membrane as a moisture-resistive cathode for high performance Li-air batteries. The 3D graphene membrane features a highly interconnected graphene network for efficient charge transport, a highly porous structure for efficient diffusion of oxygen and electrolyte ions, a large specific surface area for high capacity storage of the insulating discharge product, and a network of highly tortuous hydrophobic channels for O₂/H₂O selectivity. These channels facilitate O₂ ingression while retarding moisture diffusion and ensure excellent charge/ discharge cycling stability under ambient conditions. The membrane can thus enable robust Li-air batteries with exceptional performance, including a maximum cathode capacity that exceeds 5, 700 mAh/g and excellent recharge cycling behavior (> 2, 000 cycles at 140 mAh/g, and > 100 cycles at 1, 400 mAh/g). The graphene membrane air cathode can deliver a lifetime capacity of 100, 000–300, 000 mAh/g, comparable to that of a typical lithium ion battery cathode. The stable operation of Li-air batteries with significantly improved single charge capacities and lifetime capacities comparable to those of Li-ion batteries may offer an attractive high energy density storage alternative for future mobile power supplies. These batteries may provide much longer battery lives and greatly reduced recharge frequency.

Organic–inorganic hybrid halide perovskites, such as CH₃NH₃PbI₃, have emerged as an exciting class of materials for solar photovoltaic applications; however, they are currently plagued by insufficient environmental stability. To solve this issue, all-inorganic halide perovskites have been developed and shown to exhibit significantly improved stability. Here, we report a single-step chemical vapor deposition growth of cesium lead halide (CsPbX₃) microcrystals. Optical microscopy studies show that the resulting perovskite crystals predominantly adopt a square-platelet morphology. Powder X-ray diffraction (PXRD) studies of the resulting crystals demonstrate a highly crystalline nature, with CsPbCl₃, CsPbBr₃, and CsPbI₃ showing tetragonal, monoclinic, and orthorhombic phases, respectively. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies show that the resulting platelets exhibit well-faceted structures with lateral dimensions of the order of 10–50 µm, thickness around 1 µm, and ultra-smooth surface, suggesting the absence of obvious grain boundaries and the single-crystalline nature of the individual microplatelets. Photoluminescence (PL) images and spectroscopic studies show a uniform and intense emission consistent with the expected band edge transition. Additionally, PL images show brighter emission around the edge of the platelets, demonstrating a wave-guiding effect in high-quality crystals. With a well-defined geometry and ultra-smooth surface, the square platelet structure can function as a whispering gallery mode cavity with a quality factor up to 2, 863 to support laser emission at room temperature. Finally, we demonstrate that such microplatelets can be readily grown on a variety of substrates, including silicon, graphene, and other two-dimensional materials such as molybdenum disulfide, which can readily allow the construction of heterostructure optoelectronic devices, including a graphene/perovskite/ graphene vertically-stacked photodetector with photoresponsivity > 10⁵ A/W. The extraordinary optical properties of CsPbX₃ platelets, combined with their ability to be grown on diverse materials to form functional heterostructures, can lead to exciting opportunities for broad optoelectronic applications.

Capacitive deionization is an attractive approach to water desalination and treatment. To achieve efficient capacitative desalination, rationally designed electrodes with high specific capacitances, conductivities, and stabilities are necessary. Here we report the construction of a three-dimensional (3D) holey graphene hydrogel (HGH). This material contains abundant in-plane pores, offering efficient ion transport pathways. Furthermore, it forms a highly interconnected network of graphene sheets, providing efficient electron transport pathways, and its 3D hierarchical porous structure can provide a large specific surface area for the adsorption and storage of ions. Consequently, HGH serves as a binder-free electrode material with excellent electrical conductivity. Cyclic voltammetry (CV) measurements indicate that the optimized HGH can achieve specific capacitances of 358.4 F·g^-1 in 6 M KOH solution and 148 F·g^-1 in 0.5 M NaCl solution. Because of these high capacitances, HGH has a desalination capacity as high as 26.8 mg·g^-1 (applied potential: 1.2 V; initial NaCl concentration: ~5, 000 mg·L^-1).