CNTs were discovered by Iijima in 1991, and their outstanding physical, chemical, mechanical and electrical properties have been gradually researched due to their unique structure. Because of their nanometer hollow geometry, light weight, large surface area, flexible, high tensile strength, heat transfer and electron mobility, and excellent field emission, many important applications have been demonstrated.
CNT-based sensors may work at room temperature and have a quite low detection limit; however, they have certain limitations such as low sensitivity, lack of selectivity, irreversibility, and long recovery time. To overcome these limitations, plasma modifications have been used for enhancing the sensing sensitivity. In addition, the plasma treatment can also activate the surface of CNTs, and it may be applied to synthesize the CNT composites. Few researches are discussed on the optoelectronic applications of CNTs due to their lack of luminescence. Therefore, the formation of the composites of a wide band-gap semiconductor ZnO and CNTs should extend the optoelectronic application and reinforce the pristine properties of simplex ZnO and CNT materials.
In this work, the multi-wall carbon nanotubes (MWCNTs) or single wall carbon nanotubes (SWCNTs) can be successfully synthesized by decomposition of ethanol over a Fe or Fe+Co/MgO catalyst by CVD in a tube furnace system. After a purification process (nitric acid or nitric acid and hydrochloric acid), purified MWCNTs or SWCNTs can be obtained. The plasma modified CNTs were manufactured by MPECVD, and were developed as novel gas sensor materials. In gas sensing tests, the CNT-based gas sensors have shown a p-type response with resistance enhancement upon exposure to ethanol at room temperature. Oxygen plasma modification can increase the sensor response due to the apparent elimination of amorphous carbon, but it has no effective assistance in decreasing the response and recovery time. By applying fluorine plasma modification, the sensor response can increases, and the response and recovery time also decrease apparently due to the existence of numerous fluorine-included functional groups. The sensitivity also increases 2–4 times and the linear range of measurement can also be extended. The enhancement level of the sensing properties in fluorinated MWCNTs is lower than that in fluorinated SWCNTs because the SWCNTs possess larger surface area, higher electron mobility, and more fluorine-included functional groups. Therefore, the plasma modified CNTs have wide potential to apply on room temperature gas sensor devices, especially fluorinated SWCNTs.
Besides, the ZnO quantum dots can be successfully grown without using catalyst and well attached on the whole surface of MWCNTs with 20 s oxygen plasma treatment (O20). Oxygen plasma treatment to MWCNTs can provide the oxygen-included functional groups to improve the surface activity of MWCNTs, facilitating to grow ZnO quantum dots. O20 shows a small Igreen/IUV ratio, revealing that O20 has a highly crystalline structure with fewer oxygen deficiencies. In addition, O20 also reveals outstanding field emission properties (Eto = 0.27 V/μm, Eth = 3.24 V/μm, and β = 11897). Therefore, ZnO quantum dots attached on oxygen plasma activated MWCNTs successfully combine the particular advantages of ZnO and MWCNTs, and may have potentials to apply on optoelectronic and field emission devices.

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