Fabrication and characterization of nanostructured carbons
by Chen Qihong
xiv, 61 p. : ill. (some col.) ; 30 cm
This thesis is about the preparation and characterization of two carbon nanostructures: graphene magnetoresistance device (EMR) and 0.4 nm carbon nanotubes grown in the linear channels of AFI zeolite crystal. The second part is my focus....[ Read more ]
This thesis is about the preparation and characterization of two carbon nanostructures: graphene magnetoresistance device (EMR) and 0.4 nm carbon nanotubes grown in the linear channels of AFI zeolite crystal. The second part is my focus.
On the first part, graphene EMR devices have shown excellent magnetoresistance effect for potential applications. The MR effect arises from the geometric arrangements of the metallic parts on graphene, together with the placing of the electrodes. It follows that geometric parameters must play an important role in determining the performance of the EMR device. In order to achieve larger MR and to investigate the effect of the size of EMR deveic, I have fabricated several devices with sizes ranging from 1~3 μm and with different metals. Simulations have also been carried out to compare with the measured data. Comparison showed excellent agreement between the experiments and simulations. By using the ideal mobility of graphene, simulation has predicted an MR magnitude as high as 100,000%. After fine tuning, our graphene EMR device has exhibited a MR effect up to 85,000%, close to the prediction.
The 4 Angstrom SWNTs@AFI have exhibited superconductivity. To prepare better samples for transport measurements, I have devoted much effort to improve the sample quality. Different parameters were tested, and it is found that: (1) Neither low temperature nor very high temperature is good for the growth of CNTs; while in the range of 600~650℃, temperature does not have a significant effect on the sample quality. (2) The heating time is not too critical. Four to eight hours should be a good choice. (3) The pressure has an obvious effect on the quality as well as the quantity of carbon in the AFI pores. At low pressure, the RBM signal is clearly seen in the Raman spectra, but the TG analysis shows the carbon content to be on the low side; while at high pressure up to 3 atm, the RBM signal is hard to be seen because of the high intensity of the background PL. The absolute value of G band is stronger under high pressure, indicating that there is more carbon in the AFI channels. The TG analysis has verified our expectation that the higher pressure condition should yield a higher carbon content. More carbon content, but smaller RBM/G ratio, is likely to be indicative of two types of carbon co-existing in the pores of AFI crystals: the 0.4nm CNTs and the byproducts from the decomposition of ethylene. Since the environmental pressure is high, it would be difficult for the byproducts to be dispersed from the channels.
I have also tested the use of catalyst for the growth of 4-Angstrom CNTs. Cobalt carbonyl was introduced as a vapor cobalt source for catalyzing the growth of CNTs. From the Raman data, some positive results have been observed, but it is not as effective as we have expected, mainly due to the fact that no Co, or very little Co, is in the channels to catalyze the CNT growth. As catalyst may be the most promising way to improve the quatliy as well as the quantity of 4-Angstrom CNTs, further tests on catalyzing the growth of CNTs will be carried out.