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The success of isolating small flakesof atomically thin layersthrough mechanical exfoliation has triggered enormous research interestin graphene and other two-dimensional materials. For device applications,however, controlled large-area synthesis of highly crystalline monolayerswith a low density of electronically active defects is imperative.Here, we demonstrate the electrical imaging of dendritic ad-layersand grain boundaries in monolayer molybdenum disulfide (MoS2) grown by a vapor transport technique using microwave impedancemicroscopy. The micrometer-sized precipitates in our films, whichappear as a second layer of MoS2in conventional heightand optical measurements, show ∼2 orders of magnitude higherconductivity than that of the single layer. The zigzag grain boundaries,on the other hand, are shown to be more resistive than the crystallinegrains, consistent with previous studies. Our ability to map the localelectrical properties in a rapid and nondestructive manner is highlydesirable for optimizing the growth process of large-scale MoS2atomic layers. [ABSTRACT FROM AUTHOR]

In this work, a novel zirconium-based metal-organic framework (MOF) composite material, UiO-(OH) 2 @RhB, has been solvothermally prepared with zirconyl chloride octahydrate, 2,5-dihydroxyterephthalic acid, and rhodamine B (RhB) for ratiometric fluorescence sensing of Al 3+ ions in an aqueous medium. The luminescence measurement results showed that, at the single excitation wavelength of 420 nm, the fluorescence intensity of the ligand at 500 nm increased significantly in the case of Al 3+ , while that of RhB at 583 nm changed slightly, together with an apparent color change. Under optimal conditions, UiO-(OH) 2 @RhB exhibited an extraordinary sensitivity (10 nM), good selectivity, and a fast response (2 min) for Al 3+ . As far as we know, the limit of detection is superior to that of the current reported MOF-based Al 3+ fluorescence sensors. The response mechanism suggested that -OH could capture Al 3+ in water through coordination and high electrostatic affinity and achieved turn-on ratiometric fluorescence through the excited-state intramolecular proton transfer process and stable fluorescence of RhB. In addition, this sensor was also applied to actual food samples (grain beans), with the recoveries ranging from 89.08% to 113.61%. Such a turn-on ratiometric fluorescence sensor will provide a constructive strategy for the ultrasensitive detection of Al 3+ in practical applications.

The dielectric constant or relative permittivity (ε(r)) of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms but also the specific atomic arrangement in the crystal lattice. In this Letter, we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nanoflakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope is employed to simultaneously quantify the number of layers and local electrical properties. The measured ε(r) increases monotonically as a function of the thickness and saturates to the bulk value at around 6-8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed by first-principles calculations. Our results of the dielectric response, being ubiquitously applicable to layered 2D semiconductors, are expected to be significant for this vibrant research field.

Because of the drastically different intralayer versus interlayer bonding strengths, the mechanical, thermal, and electrical properties of two-dimensional (2D) materials are highly anisotropic between the in-plane and out-of-plane directions. The structural anisotropy may also play a role in chemical reactions, such as oxidation, reduction, and etching. Here, the composition, structure, and electrical properties of mechanically exfoliated WSe2 nanosheets on SiO2/Si substrates were studied as a function of the extent of thermal oxidation. A major component of the oxidation, as indicated from optical and Raman data, starts from the nanosheet edges and propagates laterally toward the center. Partial oxidation also occurs in certain areas at the surface of the flakes, which are shown to be highly conductive by microwave impedance microscopy. Using secondary ion mass spectroscopy, we also observed extensive oxidation at the WSe2-SiO2 interface. The combination of multiple microcopy methods can thus provide vital information on the spatial evolution of chemical reactions on 2D materials and the nanoscale electrical properties of the reaction products.