Monolayer molybdenum disulfide (MoS2), a semiconductor material with direct band gap, is considered to be an important fundamental material for the future development of the semiconductor industry. In order to apply the material to semiconductor devices, we have to investigate the electrical, optical and thermal properties of MoS2. People have always been concerning about the electrical and optical properties, but pay little attention to the thermal properties of MoS2, especially thermal stability. It is well known that semiconductor device generates a lot of heat when it works, sometimes even running in high temperature environment. The above conditions all require the material which has good thermal stability. So we focus on how to improve the thermal stability of MoS2. In this paper, we report the construction of the van der Waals heterostructures of graphene and MoS2 by encapsulating monolayer MoS2 with graphene, and dissect the thermal stability of encapsulated MoS2 in argon (Ar) and hydrogen (H2) atmosphere respectively. The results show that in Ar atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, while the exposed MoS2 is decomposed almost completely at 1000 ℃. In H2 atmosphere, MoS2 encapsulated by graphene keeps stable when the temperature increases to 1000 ℃, but the exposed MoS2 is decomposed completely at 800 ℃. In conclusion, the thermal stability of MoS2 encapsulated by graphene can be improved significantly. We analyze the reason why MoS2 encapsulated by graphene gains good thermal stability. Firstly, the covered graphene provides additional van der Waals forces, which increases the decomposition energy of MoS2, making it more stable at high temperature environment. Secondly, graphene separates MoS2 from the external environment, preventing MoS2 from contacting and reacting with external gas, which greatly improves the thermal stability of MoS2 at high temperature environment. Meanwhile, graphene covers the active defect site on MoS2, making it difficult to react at defects. In summary, the monolayer MoS2 devices can work normally at high temperature when MoS2 is encapsulated by graphene. In addition, our work also provides a feasible approach to improving the thermal stability of other two-dimensional materials.
The silver nanoparticles of (4.18±0.5) nm in diameter were prepared using two-phase liquid-liquid method.-The size distribution and the stability of the silver nanoparticles were studied by the UV-vis spectrum.-The results showed that the silver colloid solution was stable and monodispersed.-The IR spectra indicated that the nanoparticles were capped by 1-nonanethiol.-Two-dimensional ordered structure of the nanoparticles was formed by self-assembly technique.
Graphene nanostructures are proposed as promising materials for nanoelectronics such as transistors, sensors, spin valves and photoelectric devices. Zigzag edge graphene nanostructures had attracted broad attention due to their unique electronic properties. Anisotropic hydrogen-plasma etching has been demonstrated as an efficient top-down fabrication technique for zigzag-edged graphene nanostructures with a sub-10 nm spacial resolution. This anisotropic etching works for monolayer, bilayer and multilayer graphene and the etching rate depends on substrate temperature with a maximum etching rate at arround 400 C. It has been also founded that the anisotropic etching is also affected by the surface roughness and charge impurities of the substrate. Atomically flat substrates with no charge impurities would be ideal for the anisotropic etching. So far the understanding of hydrogen-plasma anisotropic etching, e.g. whether hydrogen radicals or hydrogen ions dominate the etching process, remains unclear. In this work, we investigated the anisotropic etching of graphene under electrical field modulations. Bilayer graphene peeled off from grahpite on SiO2 substrate was used as the experimental object. 2 nm-Ti (adhesive layer) and 40 nm-Au electrodes was deposited by electronic beam evaporation for electrical contacts. Gate voltates were applied to the bilayer graphene samples to make them either positively or negitively charged. These charged samples were then subjected to the hydrogen anisotropic etching at 400 C under the plasma power of 60 W and gas pressure of 0.3 Torr. The etching rates were characterized by the sizes of the etched hexagonal holes. We found that the etching rate for bilayer graphene on SiO2 substrate depends strongly on the gate voltages applied. With gate voltages sweeping from the negative to the positive, etching rate shows obvious decrease. 45 times of etching rate decrease was seen when sweeping the gate voltages from -30 V (positively charged) to 30 V (negatively charged). This gate-dependent anisotropic etching suggests that hydrogen ions rather than radicals plays a key role during the anisotropic etching process since the negatively charged graphene could neutralize the hydrogen ions quickly thus make them unreactive. The present work provides a strategy for fabrication of graphene nanostructures by anisotropic etching with a controllable manner.
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