The controlled synthesis of high-quality multilayer (ML) MoS flakes with gradually shrinking basal planes by chemical vapor deposition (CVD) is demonstrated. These CVD-grown ML MoS flakes exhibit much higher mobility and current density than mechanically exfoliated ML flakes due to the reduced contact resistance which mainly resulted from direct contact between the lower MoS layers and electrodes.

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Developing controlled approaches for synthesizing high-quality two-dimensional (2D) semiconductors is essential for their practical applications in novel electronics. The application of chemical vapor transport (CVT), an old single-crystal growth technique, has been extended from growing 3D crystals to synthesizing 2D atomic layers by tuning the growth kinetics. Both single crystalline individual flakes and continuous films of 1 L MoS were successfully obtained with CVT approach at low growth temperatures of 300-600 °C. The obtained 1 L MoS exhibits high crystallinity and comparable mobility to mechanically exfoliated samples, as confirmed by both atomic resolution microscopic imaging and electrical transport measurements. Besides MoS , this method was also used in the growth of 2D WS , MoSe , Mo W S alloys, and ReS , thus opening up a new way for the controlled synthesis of various 2D semiconductors.

Two-dimensional (2D) metallic transition metal dichalcogenides (TMDCs), such as 1T-TiSe, are ideal systems for exploring the fundamentals in condensed matter physics. However, controlled synthesis of these ultrathin materials has not been achieved. Here, we explored the synthesis of charge density wave (CDW)-bearing 2D TiSe with chemical vapor transport (CVT) by extending this bulk crystal growth approach to the surface growth of TiSe by introducing suitable growth substrates and dramatically slowing down the growth rate. Sub-10 nm TiSe flakes were successfully obtained, showing comparable quality to the mechanically exfoliated thin flakes. A CDW state with 2 × 2 superstructure was clearly observed on these ultrathin flakes by scanning tunneling microscopy (STM), and the phase transition temperature of these flakes was investigated by transport measurements, confirming the existence of CDW states. Our work opens up a new approach to synthesizing 2D CDW and superconductive TMDCs for exploring new fundamentals and applications in novel electronics.

Well dispersed Cu2O hollow microspheres consisted of Cu2O nanoparticles were quickly synthesized in aqueous solution at room temperature (25 °C) with polyvinylpyrrolidone (PVP) as surfactant. The influences of the reaction time, PVP amount and pH value of NaOH solution were studied. The formation mechanism of Cu2O hollow spheres is that, with the modification and steric effect of PVP molecules, Cu2O nanoparticles aggregate to form loose aggregations and then quickly transform to hollow spheres through Ostwald ripening. Formation of loose aggregations is the key to the fast synthesis of hollow spheres at low temperature. The application of Cu2O hollow microspheres in DNA biosensor was also investigated. The hollow Cu2O microspheres greatly enhance the immobilization of the DNA probe on the electrode surface and improve the sensitivity of DNA biosensors.