![]() ![]() In particular, monolayer MoS 2 exhibits a direct bandgap of 1.8 to 1.9 eV ( 15), which makes it promising for optoelectronics applications. Unlike zero-bandgap graphene ( 1, 12), MoS 2 has a bandgap ranging from 1.29 to 1.8 eV, depending on its thickness ( 13, 14). As a typical example of TMDC, MoS 2 has attracted lots of attention because of its unique electronic properties. Back-gate field-effect transistors (FETs) fabricated on the basis of the “Scotch-taped” samples have been extensively applied in devices such as logic circuits ( 3– 5), memory ( 6), heterojunction transistors ( 7, 8), photodetectors ( 9), gas sensors ( 10), and modulators ( 11). Mechanical exfoliation has been widely used so far because of the simple and low-cost preparation of high-quality single-crystal 2D semiconductor samples with the least defects. Monolayer and multilayer 2D materials can be obtained through different approaches, such as lithium-based intercalation ( 2), the hydrothermal method, and the “Scotch tape” method. Since the first report on a back-gate graphene transistor in 2004 ( 1), massive efforts to seek for high-quality materials and device fabrication of graphene and post-graphene TMDCs have been made. Two-dimensional (2D) layered materials, such as graphene and transition metal dichalcogenides (TMDCs), have emerged in recent years as an attractive class of materials for future electronic devices. This opens up a suite of applications of this novel platform in 2D materials research with increasing needs of enhanced gate control. Moreover, MoS 2 and ReS 2 photodetectors fabricated on the basis of the scheme have impressively leading photoresponsivities of 4000 and 760 A W −1 in the depletion area, respectively, and both have exceeded 10 6 A W −1 in the accumulation area, which is the best ever obtained. Both transistors show excellent electrical characteristics, including steep subthreshold swing (62 mV dec −1 for MoS 2 and 83 mV dec −1 for ReS 2), high mobility (61.79 cm 2 V −1 s −1 for MoS 2 and 7.32 cm 2 V −1 s −1 for ReS 2), large on/off ratio (~10 7), and reasonable working gate bias (below 3 V). Two typical 2D materials, MoS 2 and ReS 2, were implemented to demonstrate the enhancement of gate controllability. High-quality exfoliated 2D materials could be easily obtained and recognized on this stack. We introduce a new back-gate transistor scheme fabricated on a novel Al 2O 3/ITO (indium tin oxide)/SiO 2/Si “stack” substrate, which was engineered with distinguishable optical identification of exfoliated 2D materials. However, these devices usually suffer from limited gate control because of the thick SiO 2 gate dielectric and the lack of reliable transfer method. Transistors with exfoliated two-dimensional (2D) materials on a SiO 2/Si substrate have been applied and have been proven effective in a wide range of applications, such as circuits, memory, photodetectors, gas sensors, optical modulators, valleytronics, and spintronics.
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