單壁奈米碳管合成與燃燒效應並探討其在場發射及電晶體之應用

Investigations on synthesis and ignition effect of single walled carbon nanotubes toward field emitter and thin film transistor

曾士豪(Shih-Hao Tseng)

指導教授 ： 戴念華

國立清華大學/工學院/材料科學工程學系/博士(2009年)

https://doi.org/10.6843/NTHU.2009.00785

摘要

Vertically aligned multi-walled carbon nanotubes (VA-MWCNTs) were synthesized through the cold-walled chemical vapor deposition (CVD) process by using a Ta plate as the primary heater welded with Ta wires of different diameters (0.8, 0.6, and 0.4 mm) as the assisted heater. Extra heat generated by the Ta wires with diameters of 0.6 and 0.4 mm reduces the catalyst size and increases the catalyst density. The longest VA-MWCNTs were estimated as 270 μm when 0.6 mm Ta wires were used, while amorphous carbon was generated by using 0.4 mm Ta wires, resulting in shorter VA-MWCNTs with lower structural integrity (ID/IG = 1.76). The transmission electron microscope images show the reduction in the CNT diameter with a decrease in the Ta wire diameter. Using the proposed method, vertically aligned single-walled carbon nanotubes (VA-SWCNTs) can also be synthesized readily. For fabricating field emission devices, the VA-MWCNTs were transferred to a substrate pre-coated with silver paste and then subjected to curing at an extremely low temperature of 100 oC. Due to the reduced screen effect and the proper CNT density, the device with a low turn-on electric field (E0= 0.41 V/μm) and a high field enhancement factor (β= 13,689) was achieved.
Furthermore, the design and electric behavior of nanocomposites reinforced by VA-MWCNTs with different heights are presented in this study. Nanocomposites with uni-directional fillers can be easily achieved through a self-designed immersion process, which utilizes the infiltration of polymethylmethacrylate (PMMA) to fill up the inter-space of VA-MWCNTs via capillary attraction. VA-MWCNTs with larger heights limit the infiltration length of polymer matrix, which resulted in porosities remain on the surface of the nanocomposite. Raman spectrum indicated the existence of PMMA on the surface of the nanocomposite. Nearly linear I-V curve observed in raw VA-MWCNTs, like ohmic behavior, was changed to non-ohmic behavior for the nanocomposite due to the insulating polymer blocks the conductive path and provides an extra barrier for electron transfer. Furthermore, ohmic behavior can reappear via modifying the PMMA with Ni powders, which provides additional conduction pathways.
On the other hand, unpurified SWCNTs synthesized by the floating catalyst method using ferrocene as the catalyst precursor were subjected to varying numbers of photoflashes and the resulting products were studied. In addition to the remaining SWCNTs, Fe2SiO4 particles covered with amorphous carbon were found to be attached to the SWCNTs, the size of which increased with increasing number of flashes. Fe2SiO4 arose from the oxidation of Fe3C, a ferrocene-induced catalyst particle embedded in the SWCNTs, and SiO2 released from the mullite tube at 1200°C during SWCNT growth. The carbon coating had insufficient time to crystallize during rapid cooling after the flash. The change in the Raman ID/IG ratio from an initial value of 0.035 to 0.025 after one hundred flashes was due to competition between the removal of carbon from the nanotubes and the formation of amorphous carbon on the Fe2SiO4 particle surface. The electrical resistance of the SWCNT film increased with the number of flashes but the change became progressively smaller, with the increment decreasing from 17.5 to 0.2%. Similar experiments using purified SWCNTs were performed, and no such particles were observed.
The effect of twisting on the electrical resistance of a raw SWCNT rope was investigated and found to increase with increasing number of twists, varying from 28.7 Ω initially to 35.2 Ω after twelve twists. There are two reasons for this. One is that the twisting generated more contact points between the SWCNTs and catalytic nanoparticles, resulting in a high density of high local resistance points. The other is that the protrusion of SWCNTs in a rope twisted a large number of times (12–15 twists) partially interrupted the conducting path. By using a 9-V battery, ignition of the rope could be produced at a threshold resistance between 17.5 and 21.1 Ω, and this could be used to ignite ferrocene with the process lasting for several minutes. A two-step method for the purification of single-walled carbon nanotube (SWCNT) rope containing substantial catalyst particles embedded in carbonaceous shells was developed. The first step was the triggering of rope ignition using a 9-V battery, which resulted in pre-oxidization of the carbon shells on the Fe3C catalyst and oxidation of the exposed Fe3C to form Fe2O3. In addition, SWCNT fragments with open-end structures due to ignition-induced cutting remained. In the second step, either oxalic acid (H2C2O4) or hydrochloric acid (HCl) was used as the reactant to remove the Fe2O3 particles. No damage on the SWCNT walls after H2C2O4 purification was found, whereas after HCl treatment, slight breakage on SWCNT walls was observed. In addition, adsorption of H2C2O4 was also found on the H2C2O4 purified SWCNT rope and it can be effectively removed by heating the rope at 200oC in vacuum for 60 min.
The Taguchi method is used to obtain the optimum parameters for growing SWCNT films with the highest quality in our CVD system. Furthermore, the fabrication of transparent and flexible thin film transistors (TFTs) using SWCNT networks as both bottom gates and conducting channels and polymethylmethacrylate (PMMA) as an insulating layer by the direct transfer method is demonstrated. The fabricated SWCNT-TFTs exhibited a mobility of 23.4 cm2/Vs and an ON/OFF current ratio of ~103. A minor influence of ~7 % on the performance of the SWCNT-TFTs after bending to a radius of curvature of ~6 mm was observed. The differences in the performance of devices fabricated with SWCNTs on SiO2/Si and those created by transferring SWCNTs to a polycarbonate (PC) substrate are also discussed.

關鍵字

奈米碳管 ； 合成 ； 燃燒 ； 場發射 ； 電晶體

並列摘要

無資料

並列關鍵字

carbon nanotube ； synthesis ； ignition ； field emission ； transistor

參考文獻

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