Selective Formation of Monodisperse Right Trigonal-Bipyramidal and Cube-Shaped CdSe Nanocrystals: Stacking Faults and Facet-Ligand Pairing.

Formation of monodisperse right trigonal-bipyramidal (rTriBP) and cube-shaped CdSe nanocrystals─both being encased with six (100) facets─is found to be dictated by type of stacking faults along the (111) direction of the zinc-blende structure and an ideal facet-ligand pairing for the (100) facets. During growth with little kinetic overdriving, seeds with single twin boundary (TB) and single intrinsic stacking fault (ISF) grow into rTriBP and cube-shaped nanocrystals, respectively, through two consecutive stages. During the facet-formation stage, each seed would grow rapidly into the smallest faceted one to contain the ∼3 nm seed, with cube-shaped ones growing much faster than rTriBP ones because of the stacking-fault-dependent seed location in the final faceted nanocrystals. In the following facet-growth stage, cube-shaped nanocrystals also grow faster, presumably due to the highly reactive stacking fault edges. Consistent with this hypothesis, growth of rTriBP nanocrystals can become faster than that of cube-shaped ones by intentionally introducing additional intrinsic stacking fault(s) in the seeds. Cube-shaped and rTriBP CdSe nanocrystals exhibit distinctive optical properties, representing two classes of optical materials.

Covalently Functionalized MoS2 Initiated Gelation of Hydrogels for Flexible Strain Sensing.

Transition metal dichalcogenides (TMDs), with superior mechanical and electrical conductivity, are one of the most promising two-dimensional materials for creating a generation of intelligent and flexible electronic devices. However, due to the high van der Waals and electrostatic attraction, TMD nanomaterials tend to aggregate in dispersants to achieve a stable state, thus severely limiting their further applications. Surface chemical modification is a common strategy for improving the dispersity of TMD nanomaterials; however, there are still constraints such as limited functionalization methods, low grafting rate, and difficult practice application. Thus, it is challenging to develop innovative surface modification systems. Herein, we covalently modify an olefin molecule on surface-inert MoS2, and the modified MoS2 can be used as not only a catalyst for hydrogel polymerization, but also a cross-linker in the hydrogel network. Specifically, allyl is covalently grafted onto chemically exfoliated MoS2, and this modified MoS2 can be uniformly dispersed in polar solvents (such as acetone, N,N-dimethylformamide, and ethanol), remaining stable for more than 2 weeks. The allyl-modified MoS2 can catalyze the polymerization of polyacrylamide hydrogel and then integrate in the network, which increases the tensile strength of the composite hydrogel. The flexible sensor based on the composite hydrogel exhibits an ideal operating range of 600% and a quick response time of 150 ms. At the same time, the flexible device can also track the massive axial stretching movements of human joints, making it a reliable option for the next wave of wearable sensing technology.

Shape-Engineered Synthesis of Atomically Thin 1T-SnS2 Catalyzed by Potassium Halides.

Shape engineering plays a crucial role in the application of two-dimensional (2D) layered metal dichalcogenide (LMD) crystalline materials in terms of physical and chemical property modulation. However, controllable growth of 1T phase tin disulfide (SnS2) with multifarious morphologies has rarely been reported and remains challenging. Herein, we report a direct synthesis of large-size, uniform, and atomically thin 1T-SnS2 with multiple morphologies by adding potassium halides via a facile chemical vapor deposition process. A variety of morphologies, i.e., from hexagon, triangle, windmill, and dendritic to coralloid, corresponding to fractal dimensions from 1.01 to 1.81 are accurately controlled by growth conditions. Moreover, the Sn concentration controls the morphology change of SnS2. The edge length of the SnS2 dendritic flake can grow larger than 500 µm in 5 min. Potassium halides can significantly reduce the surface migration barrier of the SnS2 cluster and enhance the SnS2 adhesion force with substrate to facilitate efficient high in-plane growth of monolayer SnS2 compared to sodium halides by density functional theory calculations. More branched SnS2 with higher fractal dimension provides more active sites for enhancing hydrogen evolution reactions. Importantly, we prove that potassium halides are preferable for 1T-phase LMDs structures, while sodium halides are more suitable for 2H-phase materials. The growth mechanism proposed here provides a general approach for controllable-phase synthesis of 2D LMD crystals and related heterostructures. Shape engineering of 2D materials also provides a strategy to tune LMD properties for demanding applications.