Two-dimensional transition metal transitional compounds (TMDCs) are two-dimensional crystalline materials composed of transitional metal atoms and chalcogenide non-metallic atoms. Most of them have semiconductor properties (such as MoX2 and WX2, where X=S, Se , Te) or superconducting properties (such as NbX2 and TaX2, where X = S, Se) greatly enrich the physical properties and applications of two-dimensional crystal materials. Unlike multilayer materials, single-layer semiconductor TMDCs are direct-gap semiconductors with high quantum yields and significant valley polarization effects. They have important application prospects in the field of flexible electronic/optoelectronic devices and valley electronics and spintronic devices. .
Single-layer WX2 is an important semiconductor two-dimensional TMDCs. Currently, single-layer WX2 is prepared by mechanical stripping, liquid stripping, and chemical vapor deposition (CVD) using an inert non-metallic material (such as SiO2/Si, Al2O3, etc.) as a growth substrate. Compared with the former two methods, although CVD method can realize the preparation of large-area two-dimensional WS2, due to van der Waals epitaxial growth mechanism, the controllability of the number of layers of the obtained material is poor, and it is difficult to obtain a uniform single-layer material with large area, and the material crystal The small particle size (usually less than 100 microns), there are a large number of structural defects such as sulfur atom vacancy, resulting in its carrier mobility is very low. In addition, rigid non-metallic growth substrates are incompatible with existing roll-to-roll flexible film production processes and flexible printed electronics processes, which limits their application in flexible electronics/optoelectronics.
Recently, the Ren Wencai Research Group of the Advanced Carbon Materials Research Department of the Shenyang National Institute of Materials Science and Technology, Chinese Academy of Sciences Institute of Metal Research, has made new progress in the preparation of large-area uniform monolayer WX2 and flexible electronic applications. Based on binary phase diagram analysis and theoretical calculations, they found that gold is the only metal that does not react with sulfur to form sulfides at high temperatures, and that gold is catalytically active, which can effectively reduce the barrier of the tungsten trioxide vulcanization process, and that gold is heated at high temperatures. The solubility of tungsten atoms is extremely low. On this basis, they proposed a surface catalytic atmospheric CVD method using gold as a growth substrate to realize the preparation of high-quality, uniform monolayer millimeter-sized WX2 single crystals and large-area thin films. It was found that, similar to the growth of graphene on copper, the catalytic activity of gold and the very low solubility of tungsten in gold make the growth of WX2 on gold follow a self-limiting surface catalytic growth mechanism, thereby ensuring a homogeneous monolayer of high quality WX2 crystals. The growth. In addition, the monolayer WX2 prepared under atmospheric pressure has weak binding to the gold matrix, so the electrochemical bubbling method can achieve high-quality transfer of WS2 without damaging the gold matrix. The single-layer WX2 produced by this method has a very high crystalline quality and exhibits optical and electrical properties that are comparable to those produced by mechanical lift-off methods (far superior to those grown in an inert matrix CVD).
In addition, they used a non-destructive transfer method combining roll-to-roll and electrochemical bubbling, using gold foils with good flexibility and chemical stability. A large-area single-layer WX2 film was realized without destroying the gold matrix. Low cost continuous transfer to flexible transparent substrates such as PET, PEN, etc. Through layer transfer, it is also possible to prepare a large-area flexible and transparent double-layered, multi-layered WX2 thin film and a functional thin film of WX2/graphene laminated heterostructure. With the roll-to-roll electrochemical bubbling lossless transfer method, they also achieved the preparation of large-area flexible transparent single-layer WS2 thin-film transistor arrays. The electrical properties of the flexible devices are comparable to those of the SiO2/Si substrate, and they are bent. Electrical performance does not decay after a hundred times.
The high quality, uniform, and strictly monolayer WX2 single crystals and thin films are cost-effective, large-area preparations, and lay the material foundation for their application in the field of flexible electronic/optoelectronic devices and valley electronics and spintronic devices. This achievement was funded by the National Natural Science Foundation of China's Outstanding Youth Fund, Major Projects, Innovation Groups, and Key Deployment Projects of the Chinese Academy of Sciences. It was published online at Nature Communications on October 9 (Nature Communications, 6: 8569, DOI: 10.1038/ Ncomms9569, 2015).
Researchers from the MA Xiuliang Research Group, Sun Dongming and Yin Lichang of the Materials Science National (Joint) Laboratory and the Peng Lianmao Research Group of the Department of Electronics of Peking University also participated in this work.
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