The arc-discharge, laser vaporization, and pyrolysis methods have been reported for the carbon nanotubes synthesis. After carbon nanotube synthesis, very complicate purification process needs to get the high purity carbon nanotube, and also carbon nanotube has been trouble with diameter and vertical growth control.
Recently, CVD method (chemical vapor deposition method) has been proposed to vertically synthesize carbon nanotube, and also many different CVD methods have been reported such as thermal CVD.
For various scientific functional application researches, many difficult technical problems have to solve such as the low temperature synthesis, vertically aligned synthesis, large scale synthesis, high purity synthesis methods. Recently, vapor phase (VP) growth method has been reported for the large scale carbon nanotube synthesis. This method use the reaction of both metal catalyst and carbon contained gas (C2H2, C2H4) in the reaction chamber, but not use substrate. The VP method has a great technical benefit for the large scale carbon nanotube synthesis. We introduce various carbon nanotube synthesis methods in this section.
The figure 1 shows the cross-sectional view of a carbon arc generator that can use to synthesize carbon nanotube. Two graphite electrodes have been used in this system. Once the arc is in operation, a carbon deposit forms on the low temperature negative electrode. This carbon contains carbon nanotube and carbon nanoparticle. In this system, chamber is connected with both vacuum pump and ~several Torr He with ~several ml/s flow rate for cooling purpose. High purity graphite usually use for the electrode. Typically the positive electrode (anode) has 6 mm diameter, and negative electrode (cathode) has 9 mm diameter.For the synthesis of high purity carbon nanotube, cathode cooling requires. Because anode position does not fix, the distance between the anode and cathode can fix constantly during the arc for carbon nanotube synthesis. Typical conditions for operating a carbon arc for the synthesis of carbon nanotubes include the use of carbon rod electrodes separated by ~1 mm with a voltage of 20-40 eV across the electrodes and a dc electric current of 50-100 A flowing between the electrodes.The figure 2 shows TEM image of multi-wall carbon nanotube synthesizing by the arc-discharge. In this figure, the gap between multi-wall coaxial nanotube is around 0.34 nm, and the center of the multi-wall is empty.After the metal catalyst (Co, Ni, Fe, Y) fills into the anode graphite hole, multi-wall nanotube can synthesis. The figure 3 shows TEM image of a bundle of single-wall nanotube which interacts with Van der Walls force.
- Laser Vaporization
Smalley Group synthesized the carbon nanotube using the laser vaporization method (Fig. 4).The condensing vapor of the heated flow tube operated at 1200 ℃ in chamber, laser pulse were used to evaporate a target containing carbon mixed with a small amount of transition metal from target. Flowing argon gas sweeps the entrained nanotubes from the high temperature zone to the water-cooled collector downstream, just outside the furnace. Carrier gas used Ar and He with ~several hundred Torr. A carbon nanotube deposit forms on the cooled collector mixing with both multi-wall carbon nanotube and carbon nanoparticle. The target is mixed with catalyst Co/Ni/Fe instead of pure graphite, and the uniform single wall carbon nanotube synthesized. The figure 5 showed synthesizing a bundle of single-wall carbon nanotube.
The figure 6 shows TEM image of a bundle of single-wall nanotube using laser vaporization synthesis method. This method shows the uniform diameter (0.34 nm) of a bundle of single wall carbon nanotube.
- Plasma Enhanced Chemical Vapor Deposition
The plasma CVD has a good advantage with the low temperature carbon nanotube synthesis comparing with arc-discharge and laser vaporization synthesis methods. The plasma method use a dc electric current or high frequency. Generally, plasma CVD use a dc current, RF(13.56 MHz), and Microwave(2.47 GHz).
Figure 7 shows the scheme of the plasma CVD. To synthesize the carbon nanotube, the substrate is located the bottom electrode and the reaction gas is flowed from the side of top electrode. In this case, thermal resistivity heater sets up the bottom electrode, or between two electrodes. This thermal heater supports the energy of the synthesis of carbon nanotube, or decomposes the reaction gas. For the carbon nanotube synthesis, CH4, C2H2, H2 use for the CNT synthesis.
Figure 8 shows Ren group’s carbon nanotube SEM image using the DC plasma synthesis method on the quartz substrate and growth temperature is below 660 ℃. They used mixed C2H2/NH3 gases (660 ℃). This picture shows Ni catalyst on the carbon nanotube tip and no carbon particle on the surface of the vertically aligned carbon nanotube.
Figure 9 shows Saito group’s carbon nanotube SEM image using the microwave plasma synthesis method on the Fe-Co-Ni substrate and growth temperature is below 600 ℃. In this figure, vertically aligned carbon nanotube shows uniform diameter and Ni catalyst on the tip of the carbon nanotube. This method showed well vertically aligned a bundle of entangled carbon nanotube synthesized, and also Ni catalyst is on the tip of the carbon nanotube.
Figure 10 shows Iljin Nanotech’s carbon nanotube SEM image using the RF plasma synthesis method on the quartz substrate, and growth temperature is below 660 ℃. Vertically well aligned carbon nanotube shows uniform diameter, and Ni catalyst is on the tip of the carbon nanotube.
Figure 11 show the carbon nanotube TEM image using the microwave plasma synthesis method on the Fe-Co-Ni substrate, and growth temperature is 600 ℃. Ni catalyst is on the tip of carbon nanotube, and also bamboo structured carbon nanotube has the herringbone structure which is over 30° angle from the graphite surface axis.
- Thermal Chemical Vapor Deposition
Carbon nanotube synthesis on large area substrate has great advantage for the FED or other various field emission display technology.
Recently, CNT synthesis on the large area substrate has been reported using the thermal CVD. Thermal CVD has various advantages, which this method can use various sources, high purity CNT can synthesize, and also can control CNT structures.
Figure 12 shows the cross-sectional view of a thermal CVD that can use to synthesize carbon nanotube. The quartz reaction chamber used coil on the out side of chamber to keep constant temperature, and also thermometer is in the reaction chamber.The thermal CVD uses Silicon oxide or Al substrates. This substrate is covered by few nanometers metal film (Fe/Ni/Co), and than puts this substrate into the 700 ~ 950 ℃ reaction chamber for the making nanoparticle from the annealing with NH3 gas. After making nanoparticle, carbon nanotube synthesizes using reaction gas such as C2H2, CH4, C2H4, and CO.
Figure 13 shows the SEM image of CNT synthesizing by the thermal CVD.
In this figure, carbon nanoparticle does not exist on the carbon nanotube surface, and high purity carbon nanotube is well aligned vertically with 100 μm length and 120 nm diameter.
Figure 14 shows the SEM image of the CNT synthesizing at 950 ℃ growing temperature. Each tip of CNT is separated, and tip of the CNT is covered.
Figure 15 shows the SEM image of the CNT using the thermal CVD method at the 750 ℃ growth temperature. CNT growing at 950 ℃ does not exist carbon nanoparticle on the CNT surface, and also high purity CNT is aligned vertically which length is 10 μm long and diameter is 60 nm wide.
Figure 17 appears in a SEM image produced at 550 ℃. CNT growing at 950℃ and 750 ℃ do not exist carbon nanoparticle on the CNT surface, and also synthesizing high purity CNT is aligned vertically which length is 1 μm long and diameter is 20 nm wide. Growth temperature decreases decreasing CNT growth rate and diameter with a thermal CVD method.
Figure 18 appears in a SEM image produced at 550 ℃. In this figure, although low temperature synthesizing CNT’s tip shape is very similar with high temperature growing CNT’s tip shape, nanotube density and diameter decrease when CNT growths at low temperature.
Figure 19 shows a SEM image of synthesized CNT which growths on the Ti film coating soda lime glass substrate. Although soda lime glass has low melting temperature (550 ℃), this glass substrate uses various display panels due to low price.
Ti film uses the cathode for the estimation of the electric emission characteristic of CNT. Although CNT synthesizing on the Ti film does no have good vertically aligned, it has high purity CNT without nanoparticle on the CNT surface
Figure 20 appears a high resolution SEM image synthesizing on the soda lime glass at 550 ℃. Electron easily emits, because tip’s growth direction is left on the substrate.
Figure 21 appears a high resolution TEM image synthesizing at temperature range 950 ℃. Multi-wall carbon nanotube shows empty inside of tube, bamboo structure, and also CNT is covered. In the figure, ① shows covered CNT tip structure, ② shows the bottom part of the CNT, and ③ indicates the joint part of the CNT’s bamboo structure.
Figure 23 indicates the a high resolution TEM image synthesizing at temperature range 950 ℃. This CNT has good crystal quality, and the gap of the graphite surface is 0.34 nm.
Inner side of graphite surface has better crystal quality than outside of the graphite surface. Furthermore, inner side graphite surface disappears at the joint part of the bamboo structure, because graphite surface has a incline angle from the CNT’s axis direction.
Figure 24 shows a high resolution TEM image synthesizing at temperature range 550 ℃. This CNT does not have good crystal quality with 0.34 nm the gap of the graphite surface, and CNT contains metal catalyst. Generally, low temperature growth CNT has smaller diameter, shorter length, and worse crystal quality than high temperature growth CNT. CNT synthesizing at 550 ℃ has both multi-wall nanotube and bamboo structures, and also metal catalyst open finds at the tip of the CNT and the joint part of the bamboo structures.
- Vapor Phase Growth
CVD method uses the reaction of both metal catalyst film and reaction gas (C2H2, C2H4, CH4, C2H6). VPG method does not use substrate, but makes reaction in the chamber supporting both reaction gas and metal catalyst in chamber. This method proposes for good advantage of large scale CNT synthesizing. Figure 25 shows the scheme of the VPG that can use to synthesize carbon nanotube.In this system, when reaction gas supports one side, the metal catalyst boat is in the other side. Chamber designed two different temperature regions. Metal catalyst powder sets on the low temperature region, and the CNT synthesized at high temperature region. Although low temperature region does not decompose carbon gas, this region has good enough vaporization temperature of metal catalyst. At low temperature, vaporized metal catalyst is atomic state, and than metal catalyst becomes several nanometer particle after scattering with other metal catalyst. CNT synthesized from the reaction between vaporized atomic state metal catalyst reacts with carbon gas in high temperature region.
Figure 26 shows a synthesizing CNT SEM image which has 1000 ㎛ length. High purity and density carbon nanotube synthesized in the chamber without the substrate.
Figure 27 shows a high resolution TEM image synthesizing by VPG. This CNT has 50 nm diameter, and there is no carbon particle on the CNT substrate.
Figure 28 shows a multi-wall CNT TEM image synthesizing by VPG, which has a 30 nm diameter. Although the center of the multi-wall is empty, there is no joint part of the bamboo structure. Meanwhile, the black part of the CNT indicates the metal catalyst. Most of CNT has high purity, and no nanoparticle on the CNT substrate.
Figure 29 shows a high resolution TEM image synthesizing by VPG. Due to analysis of TEM, the gap between CNT surfaces is 0.34 nm, and it has high crystal quality. The crystal quality of CNT synthesizing by VPG is very similar with the crystal quality of CNT synthesizing by thermal CVD. Out side diameter of CNT is 15 nm, and inner side diameter of CNT is 5 nm. From the high resolution TEM, Large scale CNT is synthesized by VPG, and crystal has very high quality comparing with thermal CVD.