Results
Sputtering Experiment
Annealing Experiment
Chemical Vapour Deposition Experiment
Sputtering Experiment
The pictures of the samples showed that the carbon nanotubes peaked in height at a slightly different iron catalyst thickness for each (Fig.1) while the relationship between diameter and thickness appeared to be relatively linear, with a possible peak around 9.5 – 10 nm of catalyst thickness (Fig.2). The nanotubes in the first sample (100 W, RTP 550°C, 30°C/s) were shorter than those from the other samples, but they reached their peak height (2.7 μm) at greater thickness of iron catalyst (about 3.5 nm). The second sample’s nanotubes (15 W, RTP 550°C, 30°C/s) grew the highest of all the trials, reaching 5.8 μm at around 3.2 nm of Fe-film thickness. It also had a prominent incline in nanotube height at 9.5 nm of catalyst thickness, going back up to 3.7 μm, unlike the other two samples. The third sample (15 W, RTP 550 °C, 5°C/s) peaked in height (4.8 μm) at the thinnest catalyst thickness, close to 3 nm. All of the samples had nanotubes of nearly the same diameter at the same thickness of catalyst, but the nanotubes of the second sample peaked at around 9.5 nm of film thickness, and then plunged from 100 to 60 nm at the thickest part of the catalyst. Also, all of the samples had a small increase in nanotube height after the initial plateau between 5 and 6.3 nm of catalyst thickness.
When the heights of the nanotubes are compared to previous results obtained by Gary Liu’s experiments with iron catalyst thickness of linear increase from 0 – 5 nm, we see that the growth trend for the first 5 nm of catalyst thickness is more or less the same (Fig.1). Gary’s sample had its peak in height at a thinner film thickness (just below 3 nm), but the diameters of the nanotubes matched those of my samples. At 4 nm of iron catalyst thickness, all of the diameters of the nanotubes are almost exactly the same, with a thickness of 40 – 45 nm. This suggests that the diameters of the nanotubes are directly proportional to the iron film thickness.
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Annealing the iron at 550ºC for longer periods of time allowed vertically aligned nanotubes to grow in locations where the carbon would form tangled nanofibre layers when annealed for less time (Fig.3). It is also interesting to note that when the annealing temperature was increased to 720ºC vertically aligned nanotubes could not be formed below 1.5 nanometres of iron catalyst (Fig.4). The height of the nanotubes from each experiment followed the same basic pattern with smaller nanotubes at smaller iron thicknesses, a plateau at a certain iron catalyst thickness, and short nanotubes when the iron catalyst was between 4 and 5 nanometres thick (Fig.3, Fig.4).
Regarding the diameters of the nanotubes the interesting result is that the longer the iron was annealed for at 550ºC, the greater the consistency in the nanotube diameter (Fig.14). When the annealing temperature was increased to 720ºC all the nanotubes observed had the same diameter (Fig.5). The uniformity in the diameter is potentially very useful.
TopChemical Vapour Deposition Experiment
The carbon structures on the three different samples vary greatly with temperature. The lowest temperature was outside of the range of temperatures at which the carbon nanotubes will form. Little 'blobs' of carbon have formed around the annealed iron nanoparticles, but no carbon nanotubes are formed throughout (Fig.6). This means that the temperature probably wasn't high enough to radicalize the carbon source, and the blobs are indicative of carbonization.
The 750 sample is relatively similar to the results observed to a previous sample made by the Dahn lab which was grown at 720 degrees celcius (Fig.7). On this sample we can see carbon nanotubes, so the temperature was high enough to radicalize the carbon source.
The 850 sample is very interesting. The sample shows evidence of both carbonization and graphitisation. Normal nanotubes appear to have been produced, but other non-graphite carbon species are also apparent. The carbon nanotubes have multiple layers forming around the inner diameters of the nanotubes, like rolls of toilet paper (Fig.8). A layer of indiscriminate blobs appear on the top of the nanotubes, called a crap layer (Fig.9). Both of these species are types of carbonization. The carbonization may appear because the ammonia ratio was not high enough to balance the temperature of the growth. The thermal energy in the furnace produced more radicalized ethylene molecules than the ammonia and catalysts provided active sites for. When there are not enough active sites, the ethylene molecules attach themselves sides of the carbon nanotubes already formed.
Analysis of the lengths of the carbon nanotubes suggests that nanotubes are shortest on the edges. Length increases gradually with catalyst thickness until around 3.5 nanometres and then decreases back down to the original length (Fig.7 and Fig.10). The outer diameter of the carbon nanotubes on the 850 sample decrease with catalyst thickness while the inner diameters stay about the same (Fig.11). As the catalyst thickness increases, the number of active sites for the promotion of graphitisation increases, so less carbon radicals attach to the outside of the carbon nanotubes already formed. The outer diameter of the carbon nanotubes on the 850 sample are significantly larger than those on the 750 sample (Fig.13), but the inner diameter of the carbon nanotubes of the 850 are actually smaller than those of the 750 sample (Fig.11). The carbonization accounts for this significant increase. The inner diameter of the carbon nanotubes is a true measure of nanotube diameter, so if it weren't for carbonization, nanotubes produced at higher temperatures could be thinner than those at lower temperatures. The diameters of the 750 sample seem to follow the inverse trend of catalyst thickness to nanotube diameter than the sample made previously at 720 degrees (Fig.12). Since both samples were made with the same ethylene/ammonia ratio and a different temperature, it is possible that this inversion of the trend is caused by the differences in amount of carbonization caused by the difference in temperature. The 'blob' diameters increase with catalyst thickness (Fig.12). Larger catalyst thickness could attract more carbonization than shorter catalyst thickness because more bigger iron nanoparticles are present.
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