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We present composition-controlled synthesis of ZnO-Zn composite nanoparticles by laser ablation of a zinc metal target in pure water or in aqueous solution of sodium dodecyl sulfate (SDS). By SDS concentration, composition and size of the nanoparticles can be controlled in a wide range. Relative amounts of the components Zn and ZnO, the particle size, and the microstructure can evolve with SDS concentration in solution. High SDS concentration corresponds to high relative amount of Zn nanoparticles existing as the core in the core/shell nanostructures, whereas low SDS concentration leads to high ZnO amount. This was explained by a dynamic mechanism on the basis of the competition between aqueous oxidation and SDS capping protection. Correspondingly, optical absorption spectra evolve from the excitonic peak of ZnO (about 350 nm) to the Zn surface plasmon resonance (about 242 nm) with rise of SDS concentration. A blue (about 450 nm) photoluminescence was observed in the obtained ZnO nanoparticles, which was attributed to existence of interstitial zinc in ZnO lattices. This study has revealed that laser ablation of active metal in liquid media is an appropriate method to synthesize a series of metal oxide semiconductor-metal composite nanoparticles with controlled composition and size.

Graphene with a sp 2 -honeycomb carbon lattice has drawn a large amount of attention due to its excellent properties and potential applications in many fields. Similar to the structure of graphene, two-dimensional semiconductors are its two-dimensional and isostructural counterparts based on the typical layerstructured semiconductors, such as boron nitride (h-BN) and transition metal dichalcogenides (e.g. MoS 2 and WS 2 ), whose layers are bound by weak van der Waals forces. Unlike the semi-metal features of graphene, the two-dimensional semiconductors are natural semiconductors with thicknesses on the atomic scale. When one of the dimensions is extremely reduced, the two-dimensional semiconductors exhibit some unique properties, such as a transition from indirect to direct semiconductor properties, and hence have great potential for applications in electronics, energy storage, sensors, catalysis and composites, which arise both from the dimension-reduced effect and from the modified electronic structure. In this feature article, recent developments in the synthesis, properties and applications of two-dimensional semiconductors are discussed. The reported virtues and novelties of two-dimensional semiconductors are highlighted and the current problems in their developing process are clarified, in addition to their challenges and future prospects.

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