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Research Proposal 1. Proposed title Growth studies of carbon nanotubes. 2. Introduction Nanotubes have attracted a lot of interest since their discovery in 1991 [1]. Their unique structure and properties could have a variety of applications in materials and devices such as electrochemistry, electrodes for fuel cells, composites for coating, filling and structural materials, probes, sensors and memories devices.[1] A lot of literatures discussed the synthesis and properties of carbon nanotubes. But there are still a lot of works need to be done in this field like how to produce the needed nanotubes with special structure by controlling the growth conditions. The structure of carbon nanotubes which are defect-free can be divided into single or multi-walled nanotubes (SWNTs or MWNTs). A SWNT is a hollow cylinder of a graphite sheet whereas a MWNT is a group of coaxial SWNTs. The defective nanotubes have more kinds of structures which include the capped and bent, branched, and helical MWNTs, and the bent, capped, and toroidal SWNTs. [1] The synthesis and growth of carbon nanotubes have been studied a lot using various methods as instance arc-discharge, laser vaporization, or catalytic decomposition of hydrocarbons. SWNTs can grow by adding the appropriate metal catalyst while MWNTs would grow under other conditions [2]. Chemical vapor deposition (CVD) is one of the widely used techniques that can control the growth of nanotubes on patterned substrates at reasonable rates. When a conventional heat source is used, it is called thermal CVD. When a plasma source is used to create a glow discharge, it is called Plasma-enhanced CVD (PECVD). [1] 3. Brief literature review The properties of carbon nanotubes also attract a lot of attentions. Electron field emission from nanotubes has been investigated for many years. Campbell et al. compared the electron field emission of the MWNT film produced by two different methods, thermal CVD and plasma CVD. They found that the film consisting of 10 μm long aligned MWNT was measured. The thermal CVD film shows a lower threshold field than the plasma CVD film. The plasma CVD film gives a more stable and reproducible electron emission or multiple scans and the light emission also starts at higher current densities for the plasma CVD film.[3] Campbell et al. reported a simple method to produce large arrays of aligned nanotubes (ANT) using thermal CVD. The product is entirely MWNTs. Iron catalyst particles are obtained from the decomposition of Fe(CO)5 and C2H2 serves as carbon feedstock. The growth of aligned nanotubes is achieved under both co-deposition and deposition in separate steps of the carbonyl and acetylene. Cold injector set before the furnace is proved to be important. This simple technique allows the production of large arrays of ANT on various substrate materials.[4] Campbell et al. also studied the electric field aligned growth of SWNTs between electrodes using thermal CVD of methane. A straight, aligned individual nanotube is produced between the 10 μm electrode gaps. The diameter of the nanotubes is observed less than 2 nm by Raman spectroscopy and atomic force microscopy.[5] The synthesis of nanotubes under well-controlled conditions is studied a lot nowadays. Lieber et al. discussed the synthesis of carbon nanotubes under diameter-controlled condition. Grown by CVD, they used iron nanoparticles with different average diameters as the growth catalyst to produce different nanotubes. They found that nanotubes produced from small diameter (~ 3 nm average) iron nanoclusters consist primarily of SWNTs with ca. 30% double-walled carbon nanotubes (DWNTs). With the increase of iron nanocluster diameter, the fraction of MWNTs also increases. The large diameter (13 nm average) iron nanoclusters catalyze the growth of thin-walled MWNTs. They also found the partial pressure of reactant gas was important to control the diameter of carbon nanotubes. In order to achieve large size nucleation which is critical to determining diameters of larger (>5 nm) nanotubes, a larger flux of carbon reactant to the nanocluster catalyst is required. The temperature also can affect the growth of nanotubes. Growth at 900 oC using the 9-nm diameter iron nanoclusters produces large diameter, thin-walled MWNTs. Substantial amorphous carbon can be produced on nanotubes under high temperature from ethylene.[6] 4. Research questions What parameters are most important for influencing the nanotubes growth? How can we tune the growth conditions to make the kind of nanotubes we want? How to prepare well characterized materials with different nanotubes lengths/ diameters/ fictionalization etc. for collaborative studies with a group in the medicine faculty on toxicological properties? How to produce dispersed nanotubes is also worth to be studied as the individual nanotube of has very small diameter and tends to form bundles.[7] 5. Methods Either macroscale thermal CVD growth or a local (microscale) heater technique will be used to allow the growth of namotubes on temperature sensitive substrates. The fundamental growth mechanisms and properties under well-controlled conditions will be studied using in situ characterization methods such as micro-Raman spectroscopy, mass spectroscopy etc. The role of external electric fields will be investigated during growth. 6. Predictions The probable conditions that would affect the growth of nanotubes are the catalyst layer thickness, the deposition temperature and the gas flow ratio in feedstock.[8] We can get the kind of nanotubes with particular diameter and density by control these conditions. To produce the dispersed nanotubes, the electronic attraction between nanotubes needs to overcome. They can be dispersed in solutions containing surfactants. 7. References [1] M.Meyyappan, Carbon Nanotubes: Science and Applications, CRS Press, Boca Roton, 2005. [2] S.B. Sinnott, R.Andrews, D.Qian, A.M.Rao, Z.Mao, E.C.Dickey, F.Derbyshire, Chemical Physics Letters, 315, 25 (1999). [3] M.Sveningsson, R.E.Morjan, O.Nerushev, E.E.B.Campbell, Carbon, 42, 1165 (2004). [4] F.Rohmund, L.K.L.Falk, E.E.B.Campbell, Chemical Physics Letters, 328, 369 (2000). [5] S.Dittmer, J.Svensson, E.E.B.Campbell, Current Applied Physics, 4, 595 (2004). [6] C.L.Cheung, A.Kurtz, H.Park, C.M.Lieber, J.Phys.Chem.B, 106, 2429 (2002). [7] P.J.F.Harris, Carbon Nanotube Science: Synthesis, Properties and Applications, Cambridge University Press, Cambridge, 2009. [8] M.Chhowalla, K.B.K.Teo, C.Ducali, N.L.Rupesinghe, G.A.J.Amaratunga, A.C.Ferrari, D.Roy, J.Robertson, W.I.Milne, Journal of Applied Physics, 90, 5308 (2001). |
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