Chapter 1. Carbon Nanotubes
1.3 Preparing CNTs on Substrate
1.3.2 Chemical Vapor Deposition: For High Density of Nanotube
Substrates that can withstand high temperatures must be chosen, because nanotubes are synthesized at temperatures above 1000 °C. In general, a SiO2/Si wafer is used. The SiO2 layer is formed as a thin film and has a non-crystal form. When synthesizing nanotubes using SiO2/Si wafer, density of random network nanotubes in the Fe particle region is high, but synthesis of the aligned nanotubes is very low. Single crystal SiO2 (Quartz) is used to synthesize high-density nanotubes.
Figure 1.9 SEM images of SWNTs grown on quartz. The quartz substrate was annealed before synthesis of nanotube for a) 10min, b) 4hr, c) 7hr [12].
Figure 1.10 Atomic arrangement of quartz crystals of Y-cut (a), Z-cut (b), X-cut (c).
(d)-(f) The insets show the orientation of quartz surfaces along the X, Y, and Z axes.
(g)-(f) SEM image of SWNTs on Y, Z and X-cut respectively. High density and well aligned SWNTs were synthesized using Y-cut. Z and X-cut didn’t provide any alignment and high density of SWNTs [1-3].
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These studies have been actively studied in the Rogers group. Synthesis of high-density nanotubes has several important requirements. One of them is pre-annealing process of the quartz substrate [12].
Before synthesis of the nanotubes, annealing treatment should be performed at 900 °C for the oxidation of the quartz substrate. There is a difference in the degree of alignment or density of nanotubes depending on the annealing time. Figure 1.9 shows SEM images of nanotubes synthesized according to the pre-annealing time of the quartz substrate. When the annealing time was 10 minutes, it was similar to the synthesis of general Fe catalyst part. The orientation of the synthesized nanotubes was not aligned. If the FET device is fabricated in this state, the on / off ratio would be low. For 4 hours of pre-annealing, nanotubes were synthesized with alignment. However, some nanotubes don’t have orientation and are synthesized as contact with other nanotubes. For pre-annealing for 7 hours, most aligned nanotubes were synthesized except for some nanotubes. The diameter of the synthesized nanotubes was less than 2 nm and seems to be a well-synthesized single tube. However, nanotubes were synthesized on the AT cut quartz crystals after annealing treatment and this method was insufficient for high density nanotube synthesis.
Another important condition for synthesizing high density nanotubes in quartz is to use certain crystal planes of the quartz [1, 59]. The quartz is known to synthesize high density nanotubes following certain crystal. Figure 1.10 shows that nanotubes synthesized on Y-cut quartz were well aligned with high density. On the other hand, when synthesized using Z-cut or X-cut, the nanotubes were not aligned and were almost random networks. So many researchers thought quartz crystals played an important role for high-density nanotubes on quartz. This was supported by the additional experiment. The well aligned nanotubes were synthesized on the crystal quartz, but the randomly orientated nanotubes were synthesized on amorphous Si substrate [1]. As mentioned previously, the nanotubes with high density and high alignment were synthesized after pre-annealing treatment.
However, the conditions were not constant and varied from laboratory to laboratory. For example, AT cuts and Y-cuts of quartz substrate were used [1, 12]. The condition of pre-annealing was reported in various studies ranging from 10 min to 16 hr.
The optimal condition for high density and well-aligned nanotube was to synthesize along the X-axis of quartz crystal using ST-cut [34, 60]. The 8 hours for the pre-annealing was optimal. The density of such synthesized nanotubes is 10 nanotubes per 1 um. In the Rogers group, the flow rates of methane and hydrogen are 1900 sccm and 300 sccm to synthesize high density nanotubes [34]. It was unusual to release a high proportion of methane. The ratio of methane to hydrogen is usually at 1:
2. The synthesis time was also 1 hour. Considering the flow rate, it was a fairly long synthesis time.
One year later, the nanotube synthesis studies on high density were reported, and the density of nanotubes was twice as high as the Rogers group [61]. The difference was that copper was used as a catalyst and ethanol as a carbon source. Using the ST-cut quartz crystal plane, they synthesized more
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than 50 nanotubes per 1um without the pre-annealing treatment. This density was more than twice as dense as the results of a 2 times deposition of catalysts for nanotube synthesis. Representative conditions for dense nanotube synthesis are listed in Figure 1.11.
Finally, the disadvantage of using quartz is that it is necessary to transfer the nanotubes synthesized from quartz onto the desired substrate in order to produce the desired electrical device. To transfer the nanotube using PMMA film, the substrate must be etched. Because it is difficult to etch the quartz substrate, the typical PMMA transfer method can’t be applied. So, Au 100 nm was deposited and mechanically polyimide/Au/SWNTs films transferred to the desired substrate in Rogers group. Using this method, a device with a bottom gate was fabricated.
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900 oC 8 hr
Fe (0.1 - 0.5 nm) (Width = 10 μm, Line = 1cm)
900 oC Time 1 hr ST-cut quartz
In air
Advanced materials22.16 (2010): 1826-1830.
Tw o step SWNT synthesis.
925 oC
20 min 10 min
H2350 sccm H220 sccm Ar 20 sccm
CVD grow th performed for 20 min w ith a flow of argon (20 sccm) and hydrogen (20 sccm) introduced through an ethanol bubbler at 925 8C, yielded aligned arrays of SWNTs.
900 oC 16 hr
Fe (0.2 - 0.8 nm)
(Width = 10 μm, spacing 200 μm)
In air 950 oC Time 1 hr ST-cut quartz
In air
Nature communications2014, 5, 5332.
H2200 sccm
925 oC
30 min H230 sccm Ar 30 sccm
Introducing ethanol (bubbled from a flask at 0 °C by 30 sccm H2and 30 sccm Ar) into the grow th tube at 925 oC for 30 min.
1. Substrate annealing 2. Catalysts annealing 3. SWNTs synthesis
Ferritin catalyst diluted 1:200 (v/v)
900 oC
1 min Purging w ith hydrogen
10 min Methane 2500 sccm Hydrogen 75 sccm 900 oC
10min
In air 900 oC 10min 4 hr 7 hr AT-cut quartz
Nature nanotechnology2.4 (2007): 230-236.
Fe (<0.5nm) (Width = 10 μm, Line = 1cm)
550 oC 900 oC
5 min Purging w ith hydrogen
1 hr Methane 1900 sccm Hydrogen 300 sccm In air
900 oC 8 hr ST-cut quartz
4. Nanotube image
JACS 128.14 (2006): 4540-4541.
Ferritin catalyst diluted 1:20 (v/v)
900 oC 900 oC
1 min Purging w ith hydrogen
10 min Methane 2500 sccm Hydrogen 75 sccm 10min
In air 900 oC 8 hr
ST-cut quartz
Heating the substrate at 900 °C for 10 min oxidized the catalyst.
Cooling to room temperature and then heating to 900 °C in a hydrogen environment reduced the catalyst.
10 min 4 hr 7 hr
~4SWNT / μm
~10SWNT / μm small1.11 (2005): 1110-1116.
The Journal of Physical Chemistry C111.48 (2007): 17879-17886.
In air 900 oC
8 hr
Fe (~0.1 nm) Width 10μm 550 oC
ST-cut quartz
H28 sccm Ar 8 sccm
Ethanol bubbler held a 0 oC in w ater (sccm..?) 925 oC
The CVD process began by flushing the chamber w ith a high flow rate of Ar (3000 sccm) for 2 min H2300 sccm
Time..?
20min
~5SWNT / μm
20~30 SWNT / μm
~0.5 SWNT / μm 15 μm
CuCl2/polyvinylpyrrolidone alcohol solution (spin-coating)
900 oC ST-cut quartz
H2400 sccm
Ar 150 sccmthrough an ethanol bubbler Time 15 min
~50 SWNT / μm
JACS 130.16 (2008): 5428-5429.
No treatment The substrate w as then treated w ith oxygen plasma for 15 min,
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Figure 1.11 Representative nanotube synthesis process for high density and well-alignment. The synthesis sequence is substrate annealing first, then the second step is catalytic deposition, then annealing, and finally synthesis of nanotubes [12].
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