Surmounting the inhomogeniety issue of gas sensors and realizing their reproducible ppb‐level gas sensing are highly desirable for widespread deployments of sensors to build networks in applications ...of industrial safety and indoor/outdoor air quality monitoring. Herein, a strategy is proposed to substantially improve the surface homogeneity of sensing materials and gas sensing performance via chip‐level pyrolysis of as‐grown ZIF‐L (ZIF stands for zeolitic imidazolate framework) films to porous and hierarchical zinc oxide (ZnO) nanosheets. A novel approach to generate adjustable oxygen vacancies is demonstrated, through which the electronic structure of sensing materials can be fine‐tuned. Their presence is thoroughly verified by various techniques. The sensing results demonstrate that the resultant oxygen vacancy‐abundant ZnO nanosheets exhibit significantly enhanced sensitivity and shortened response time toward ppb‐level carbon monoxide (CO) and volatile organic compounds encompassing 1,3‐butadiene, toluene, and tetrachloroethylene, which can be ascribed to several reasons including unpaired electrons, consequent bandgap narrowing, increased specific surface area, and hierarchical micro–mesoporous structures. This facile approach sheds light on the rational design of sensing materials via defect engineering, and can facilitate the mass production, commercialization, and large‐scale deployments of sensors with controllable morphology and superior sensing performance targeted for ultratrace gas detection.
A facile approach for designing sensing materials via rational defect engineering to tune the electronic structure of on‐chip MOF‐derived hierarchical ZnOs and thus sensing properties is proposed. The resultant homogeneous ZnO layer with abundant oxygen vacancies exhibits significantly enhanced sensitivity and short response time toward ppb‐level carbon monoxide and volatile organic compounds.
Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have emerged as attractive platforms in next-generation nanoelectronics and optoelectronics for reducing device sizes down ...to a 10 nm scale. To achieve this, the controlled synthesis of wafer-scale single-crystal TMDs with high crystallinity has been a continuous pursuit. However, previous efforts to epitaxially grow TMD films on insulating substrates (e.g., mica and sapphire) failed to eliminate the evolution of antiparallel domains and twin boundaries, leading to the formation of polycrystalline films. Herein, we report the epitaxial growth of wafer-scale single-crystal MoS2 monolayers on vicinal Au(111) thin films, as obtained by melting and resolidifying commercial Au foils. The unidirectional alignment and seamless stitching of the MoS2 domains were comprehensively demonstrated using atomic- to centimeter-scale characterization techniques. By utilizing onsite scanning tunneling microscope characterizations combined with first-principles calculations, it was revealed that the nucleation of MoS2 monolayer is dominantly guided by the steps on Au(111), which leads to highly oriented growth of MoS2 along the ⟨110⟩ step edges. This work, thereby, makes a significant step toward the practical applications of MoS2 monolayers and the large-scale integration of 2D electronics.
Atomically dispersed Au on WO3 exhibited room-temperature coupling of CH4 to CH3CH3 with a high selectivity of 94% under visible light irradiation.
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Gold (Au) as co-catalyst is ...remarkable for activating methane (CH4), especially atomically dispersed Au with maximized exposing active sites and specific electronic structure. Furthermore, singlet oxygen (1O2) typically manifests a mild redox capacity with a high selectivity to attack organic substrates. Peroxomonosulfate (PMS) favors to produce oxidative species 1O2 during the photocatalytic reactions. Thus, combining atomic Au as co-catalyst and 1O2 as oxidant is an effective strategy to selectively convert CH4. Herein, we synthesized atomically dispersed Au on WO3 (Au/WO3), where Au was in the forms of single atoms and clusters. At room temperature, such Au/WO3 exhibited enhanced photocatalytic conversion of CH4 to CH3CH3 with a selectivity, up to 94%, under visible light. The radicals-pathway mechanism of CH4 coupling has also been investigated through detection and trapping experiment of active species. Theoretical calculations further interpret the electronic structure of Au/WO3 and tip-enhanced local electric field at the Au sites for promoting CH4 conversion.
Here a progressive hot deformation procedure that endows the benchmark n‐type V2VI3 thermoelectric materials with short range disorder (multiple defects), long range order (crystallinity), and strong ...texture (nearly orientation order) is reported. Not only it is rare for these structural features to coexist but also these structural features elicit the synergistic compositional–mechanical–thermal effects, i.e., a profound interplay among the counts, magnitude, and temperature of hot deformation in relation to the as formed point defects, dislocations, textures, strain clusters, and distortions. Using progressively larger die sets and relatively low hot deformation temperature, rich multiscale microstructures concurrently with a high level of texture comparable to that of zone melted ingot are obtained. The strong donor‐like effect significantly increases the majority carrier concentration, suppressing the detrimental bipolar effect. In addition, the multiscale microstructures yield an ultralow lattice thermal conductivity ≈0.31 W m−1 K−1 at 405 K. A record zT ≈ 1.3 at 450 K are attained in progressively hot deformed n‐type Bi1.95Sb0.05Te2.3Se0.7 through the synergistic effects. These results not only promise a better pairing between n‐type and p‐type legs in device fabrication but also bring our understanding of n‐type V2VI3 alloys and hot deformation technique to a new level.
Synergistic compositional‐mechanical‐thermal effects through progressive hot deformation creates a delicate balance between strong texture, point defects, and multiscale microstructures in n‐type V2VI3 alloys. The resultant record figure of merit not only addresses the urgent need for high‐performance n‐type alloys toward higher device performance but also constitutes a progress in materials preparation methodology and the understanding of defect engineering.
Exploring exotic interface magnetism due to charge transfer and strong spin‐orbit coupling has profound application in the future development of spintronic memory. Here, the emergence and tuning of ...topological Hall effect (THE) from a CaMnO3/CaIrO3/CaMnO3 trilayer structure are studied in detail, which suggests the presence of magnetic Skyrmion‐like bubbles. First, by tilting the magnetic field direction, the evolution of the Hall signal suggests a transformation of Skyrmions into topologically‐trivial stripe domains, consistent with behaviors predicted by micromagnetic simulations. Second, by varying the thickness of CaMnO3, the optimal thicknesses for the THE signal emergence are found, which allow identification of the source of Dzyaloshinskii–Moriya interaction (DMI) and its competition with antiferromagnetic superexchange. Employing high‐resolution transmission electron microscopy, randomly distributed stacking faults are identified only at the bottom interface and may avoid mutual cancellation of DMI. Last, a spin‐transfer torque experiment also reveals a low threshold current density of ≈109 A m−2 for initiating the bubbles’ motion. This discovery sheds light on a possible strategy for integrating Skyrmions with antiferromagnetic spintronics.
This work presents the topological Hall effect found in a CaMnO3/CaIrO3/CaMnO3//LaAlO3(001) heterostructure, which indicates the presence of magnetic Skyrmions entailing the charge‐transfer induced magnetic superexchange. Thickness variations and atomic structure analyses suggest that the Dzyaloshinskii–Moriya interaction originates from the top CaMnO3/CaIrO3 interface, but is suppressed at the bottom CaIrO3/CaMnO3 interface due to the presence of stacking faults.
Controllable growth of highly crystalline transition metal dichalcogenide (TMD) patterns with regular morphology and unique edge structure is highly desired and important for fundamental research and ...potential applications. Here, single‐crystalline MoS2 flakes are reported with regular trigonal symmetric patterns that can be homoepitaxially grown on MoS2 monolayer via chemical vapor deposition. The highly organized MoS2 patterns are rhombohedral (3R)‐stacked with the underlying MoS2 monolayer, and their boundaries are predominantly terminated by zigzag Mo edge structure. The epitaxial MoS2 crystals can be tailored from compact triangles to fractal flakes, and the pattern formation can be explained by the anisotropic growth rates of the S and Mo edges under low sulfur chemical potential. The 3R‐stacked MoS2 pattern demonstrates strong second and third‐harmonic‐generation signals, which exceed those reported for monolayer MoS2 by a factor of 6 and 4, correspondingly. This homoepitaxial growth approach for making highly organized TMD patterns is also demonstrated for WS2.
Homoepitaxial growth of highly organized transition metal dichalcogenide patterns is realized via chemical vapor deposition. The regular MoS2 dendritic patterns are rhombohedral (3R)‐stacked with the underlying monolayer and possess a unique zigzag Mo edge structure. The highly organized MoS2 pattern/monolayer hybrid heterostructure promises important applications in nonlinear optics and catalysis. This growth approach is also demonstrated for WS2 patterns.
The configurational entropy is an emerging descriptor in the functional materials genome. In thermoelectric materials, the configurational entropy helps tune the delicate trade‐off between carrier ...mobility and lattice thermal conductivity, as well as the structural phase transition, if any. Taking GeTe as an example, low‐entropy GeTe generally have high carrier mobility and distinguished zT > 2, but the rhombohedral‐cubic phase transition restricts the applications. In contrast, despite cubic structure and ultralow lattice thermal conductivity, the degraded carrier mobility leads to a low zT in high‐entropy GeTe. Herein, medium‐entropy alloying is implemented to suppress the phase transition and achieve the cubic GeTe with ultralow lattice thermal conductivity yet decent carrier mobility. In addition, co‐alloying of (Mn, Pb, Sb, Cd) facilitates multivalence bands convergence and band flattening, thereby yielding good Seebeck coefficients and compensating for decreased carrier mobility. For the first time, a state‐of‐the‐art zT of 2.1 at 873 K and average zTave of 1.3 between 300 and 873 K are attained in cubic phased Ge0.63Mn0.15Pb0.1Sb0.06Cd0.06Te. Moreover, a record‐high Vickers hardness of 270 is attained. These results not only promote GeTe materials for practical applications, but also present a breakthrough in the burgeoning field of entropy engineering.
The power of medium‐entropy thermoelectrics is exploited in GeTe by co‐alloying Mn, Pb, Sb, and Cd to optimize the delicate trade‐offs between the carrier mobility, crystal structure, and lattice thermal conductivity. For the first time, a distinguished peak zT ≈ 2.1 and high Vickers hardness HV ≈ 270 are attained in the cubic Ge0.63Mn0.15Pb0.1Sb0.06Cd0.06Te with no phase transition.
Platinum dichalcogenide (PtX2), an emergent group-10 transition metal dichalcogenide (TMD) has shown great potential in infrared photonic and optoelectronic applications due to its layer-dependent ...electronic structure with potentially suitable bandgap. However, a scalable synthesis of PtSe2 and PtTe2 atomic layers with controlled thickness still represents a major challenge in this field because of the strong interlayer interactions. Herein, we develop a facile cathodic exfoliation approach for the synthesis of solution-processable high-quality PtSe2 and PtTe2 atomic layers for high-performance infrared (IR) photodetection. As-exfoliated PtSe2 and PtTe2 bilayer exhibit an excellent photoresponsivity of 72 and 1620 mA W–1 at zero gate voltage under a 1540 nm laser illumination, respectively, approximately several orders of magnitude higher than that of the majority of IR photodetectors based on graphene, TMDs, and black phosphorus. In addition, our PtSe2 and PtTe2 bilayer device also shows a decent specific detectivity of beyond 109 Jones with remarkable air-stability (>several months), outperforming the mechanically exfoliated counterparts under the laser illumination with a similar wavelength. Moreover, a high yield of PtSe2 and PtTe2 atomic layers dispersed in solution also allows for a facile fabrication of air-stable wafer-scale IR photodetector. This work demonstrates a new route for the synthesis of solution-processable layered materials with the narrow bandgap for the infrared optoelectronic applications.
Covalent organic frameworks (COFs) are a promising category of porous materials possessing extensive chemical tunability, high porosity, ordered arrangements at a molecular level, and considerable ...chemical stability. Despite these advantages, the application of COFs as membrane materials for gas separation is limited by their relatively large pore apertures (typically >0.5 nm), which exceed the sieving requirements for most gases whose kinetic diameters are less than 0.4 nm. Herein, we report the fabrication of ultrathin two-dimensional (2D) membranes through layer-by-layer (LbL) assembly of two kinds of ionic covalent organic nanosheets (iCONs) with different pore sizes and opposite charges. Because of the staggered packing of iCONs with strong electrostatic interactions, the resultant membranes exhibit features of reduced aperture size, optimized stacking pattern, and compact dense structure without sacrificing thickness control, which are suitable for molecular sieving gas separation. One of the hybrid membranes, TpEBr@TpPa-SO3Na with a thickness of 41 nm, shows a H2 permeance of 2566 gas permeation units (GPUs) and a H2/CO2 separation factor of 22.6 at 423 K, surpassing the recent Robeson upper bound along with long-term hydrothermal stability. This strategy provides not only a high-performance H2 separation membrane candidate but also an inspiration for pore engineering of COF or 2D porous polymer membranes.
State-of-the-art chemical sensors based on covalent organic frameworks (COFs) are restricted to the transduction mechanism relying on luminescence quenching and/or enhancement. Herein, we present an ...alternative methodology via a combination of in situ-grown COF films with interdigitated electrodes utilized for capacitive benzene detection. The resultant COF-based sensors exhibit highly sensitive and selective detection at room temperature toward benzene vapor over carbon dioxide, methane, and propane. Their benzene detection limit can reach 340 ppb, slightly inferior to those of the metal oxide semiconductor-based sensors, but with reduced power consumption and increased selectivity. Such a sensing behavior can be attributed to the large dielectric constant of the benzene molecule, distinctive adsorptivity of the chosen COF toward benzene, and structural distortion induced by the custom-made interaction pair, which is corroborated by sorption measurements and density functional theory (DFT) calculations. This study provides new perspectives for fabricating COF-based sensors with specific functionality targeted for selective gas detection.
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