Room and low temperature synthesis of carbon nanofibres (2002)

Carbon nanotubes and nanofibres have attracted attention in recent years as new materials with a number of very promising potential applications. Carbon nanotubes are potential candidates for field emitters in flat panel displays. Carbon nanofibres could also be used as a hydrogen storage material and as a filling material in polymer composites. Carbon nanotubes are already used as tips in scanning probe microscopy due to their remarkable mechanical and electrical properties, and could be soon used as nanotweezers. Use of carbon nanotubes in nanoelectronics will open further miniaturisation prospects. Temperatures ranging from 450 to 1000 &deg;C have been a required for catalytic growth of carbon nanotubes and nanofibres. Researchers have been trying to reduce the growth temperatures for decades. Low temperature growth conditions will allow the growth of carbon nanotubes on different substrates, such glass (below 650 &deg;C) and as plastics (below 150 &deg;C) over relatively large areas, which is especially suitable for flat panel display applications. Room temperature growth conditions could open up the possibility of using different organic substrates and bio-substrates for carbon nanotubes synthesis. Carbon nanofibres have been synthesised at room temperature and low temperatures below 250 &deg;C using radio frequency plasma enhanced chemical vapour deposition (r.f PECVD). Previously, the growth of carbon nanofibres has been via catalytic decomposition of hydrocarbons or carbon monoxide at temperatures above 300 &deg;C. To the best of our knowledge, this is the first evidence of the growth of carbon nanofibres at temperatures lower than 300 &deg;C by any method. The use of a transition metal catalyst and r.f. PECVD system is required for the growth of the carbon nanofibre when a hydrocarbon flows above the catalyst. Within the semiconductor industry r.f. PECVD is a well established technique which lends itself for the growth of carbon nanofibres for various electronic and photonic device applications. A new catalytic method for the growth of carbon nanofibres using radio frequency supported microwave plasma-enhanced chemical vapour deposition (PECVD) has been developed. Nickel powder used as a catalyst was placed on a water-cooled sample holder in order to obtain growth at room temperature. Carbon nanofibres grown by our method have shown remarkable characteristics of branching during the growth including the forming of "Y"-shaped junctions and interconnecting networks. A graphite strip heater vacuum system for carbon nanofibres thermal chemical vapour deposition (CVD) has been set up, using methane or acetylene as the carbon containing source gas, and nickel powder as the catalyst. Various carbon nanofibre morphologies have been produced: "whisker-like", helical, branched, bi-directional, and "bead-like". Using this low-pressure thermal CVD synthesis method carbon nanofibres and nanotubes were synthesised at relatively low temperatures from 350 &deg;C. Optimum deposition conditions for the produced fibres with higher graphitic structures at low temperatures have been established by series of experiments varying pressure, temperature, substrate and gas mixture. Optimum growth temperature was found to be around 500 &deg;C. Ropes of roughly aligned carbon nanotubes have been observed after synthesis using nickel catalysed CVD of methane at temperatures of 500 &deg;C, or after radio frequency assisted microwave PECVD. Mixtures of remaining nickel powder and synthesized carbon nanofibres and nanotubes have been treated in 35% nitric acid for periods of 3 to 10 minutes and carbon nanotube ropes have been observed in the dried sediment by scanning electron microscope examination. Rope diameters range from 20?m up to 80?m, and lengths up to few millimeters have been observed. The large size of these ropes means that easy manipulation is possible for their characterisation and applications. A growth model for the room temperature and low temperature produced carbon nanofibres is proposed. Characterisation of produced carbon nanofibres and carbon nanoropes have been performed using scanning electron microscope, Raman spectroscopy and transmission electron microscopy with electron energy loss spectroscopy.