Chapter IV Results and discussions
4.2 Morphologies and Raman spectra of the carbon nanostructures
4.2.2 Effect of specimen stacking sequences on carbon nanostructures.79
The schematic diagram of specimen placement in MPCVD is shown in Fig 3.4, and different placement gets different result. Sample A obtains the highest IG/ID ratio of as-grown SWNTs, because of the relative higher temperature by directly plasma bombardment to backside. But in sample C, only short and entangled SWNTs can be observed, as shown in Fig. 4.11. This is probably caused by plasma etching to SWNTs during growth process. Sample D gets the highest SWNTs because of the rough backside surface of Si wafer which gives more space for SWNTs growth. Although the placement of specimen will affect the result of SWNTs growth, the degree is relative slight comparing with other conditions, such as catalyst precursor, gas ratio, etc.
Fig. 4.11 Top-view SEM image of short and entangled SWNTs of sample C.
4.2.3 Morphologies of the two-step grown carbon nanostructures
Two-step growth of SWNTs using 1st-grown SWNTs removed substrate is attempted under the same H-plasma pretreatment and growth conditions of section 4.2.1. Figure 4.12 show the SEM image and Raman spectrum of two-step grown carbon nanostructure, and the results shows that well-aligned SWNTs are successfully grown again and indicate the base growth mechanism.
From the low magnification SEM top view, it is observed that vertical well-aligned SWNTs are successfully synthesized again, but randomly oriented cracks appear on all two-step SWNT samples, as shown in Fig. 4.12(a). In contrast, cracks were never observed on previous samples. The most probably reason for these cracks is the insufficient density of two-step SWNTs by poisonous catalyst which amorphous carbon atoms are not completely removed in 10 minutes H-plasma pretreatment time. Therefore, the Van der Waals attraction among SWNTs creates cracks by contraction when the samples are cooled from 600°C to room temperature. Hence, we extend the H-plasma pretreatment time to 15 minutes to eliminate the amorphous carbon atoms on the catalysts, and then vertical well-aligned SWNTs without cracks are synthesized.
The Raman spectrum of two-step SWNTs with 15 minutes H-plasma pretreatment time is shown in Fig. 4.12(b). It is of interest that the RBM peak becomes singular centralization at Raman shift of 281 cm-1, indicates that the
100 200 300 400 1200 1300 1400 1500 1600 1700
diameter of two-step SWNTs are highly uniform. This is probably resulted from the uniform catalyst particle size by longer H-plasma pretreatment time. Thus, extremely dense and vertical well-aligned SWNTs (40~50 μm) are successfully re-grown and the average yield rate one cycle is ~0.2 mg/cm2.
Fig. 4.12 (a) Top-view SEM image of re-grown SWNTs with insufficient pretreatment time. (b) Raman spectrum of re-grown SWNTs with suitable pretreatment time.
4.3 Effect of scanning local laser heating pretreatment on CNTs growth 4.3.1 Morphologies of the laser-pretreated catalyst precursors
Figure 4.13(a) shows the morphology of the 1-nm-thick catalyst precursor deposited PC substrate after local laser heating pretreatment. The laser heating region which rises above other area is about 1-μm-diameter size. Figure 4.13(b) is high magnification image of Fig. 4.13(a), shows there are many tiny seams on
(a) (b)
the catalyst surface, which is probably result from the exploding reduction of PtO2.The morphology of laser heating region after 10 minutes 350 °C H-plasma pretreatment is shown in Fig. 4.13(c). It is apparent that the catalyst precursor at pattern region becomes well-distributed nano-particles and other area only exist a small amount disorder and bigger particles. We firmly believe that these tiny seams increase the plasma etching area and many defects occur on their surface during the process of PtO2 exploding reduction which enhances the etching ability of H-plasma. Thus, the catalyst nano-particles are successfully synthesized at 350 °C by laser heating and then H-plasma pretreatment. AlON buffer layer was not employed here, because it was thoroughly damaged after local laser heating pretreatment.
Fig. 4.13 Top-view SEM images of catalyst precursor (a), (b) after laser ablation pretreatment and (c) after 10 min. 350 °C H-plasma pretreatment.
(c)
(a)
(b)100 200 300 400 1200 1300 1400 1500 1600 1700
190 Si RBM
G band D band
Intensity (arb. units)
Raman shift (cm-1)
4.3.2 Morphologies and Raman spectra of carbon nanostructures
Top-view SEM image and Raman spectrum of low temperature grown SWNTs at 373 °C are shown in Figs. 4.14(a) and (b). From these, we confirm that SWNTs are successfully synthesized at laser heating region. Although many outcomes must be improved like the entangled SWNTs, weak RBM peak and low IG/ID ratio, etc, the feasibility for selective growth of well-aligned SWNTs and for compatibility with IC processes at low temperature is expectable in the future. The lowest temperature to synthesize SWNTs is 373 °C and MWNTs is 262 °C. And these are never reached in the past research in the world.
Fig. 4.14 (a) Top-view SEM image and (b) Raman spectrum of low temperature grown SWNTs at 373 °C
(a) (b)
4.4 Growth mechanisms of SWNTs
4.4.1 High temperature growth mechanism
The EDX analysis is used to study the growth mechanism of as-grown vertically well-aligned SWNTs by novel CoCrPtOx catalyst precursor, and the result [Fig. 4.15] shows the catalysts existed in the SWNTs removed substrate, indicates the growth mechanism of as-SWNTs is base-growth mechanism. This is confirmed by the interestingly two-step growth method which 1st-grown SWNTs is removed from substrate to grow SWNTs again under longer H-plasma pretreatment and the same growth conditions. The surface and body diffusion were both expected in the high temperature growth mechanism
[Hofmann-2005-036101].
The TEM results indicate that each SWNT is grown from individual catalyst nano-particle and tube diameter is determined by catalyst size. With respect to the fabrication of the extremely dense SWCNT film, highly dense very fine catalytic particles is proposed to be formed uniformly by H-plasma pre-treatment and aligned tubes are then grown vertically, constrained among the tubes. By contrast, if the distribution densities of fine catalytic particles are low, entangled SWCNT are formed.
4.4.2 Low temperature growth mechanism
Hofmann et al. proposed the surface diffusion: The low activation energy path for nanotube growth [Hofmann-2005-036101], find low activation energy of 0.4 eV for carbon surface diffusion on Ni and Co (111) planes, much lower than for bulk diffusion. In MPCVD, the plasma ionizes the gas and creates new and more reactive species, such as radicals, in the gas phase and/or the catalyst surface, as well as to cause a local surface heating that enables an efficient adsorption and diffusion of carbon atoms at low substrate temperatures. The plasma atmosphere may influence the detailed catalyst surface kinetics in many ways and may also supply carbon from the gas phase for SWNTs growth. This allows a reduction of the growth activation energy, as, e.g., atomic carbon can chemisorb directly on the catalyst. Therefore, we suggest that diffusion of carbon on the catalyst surface is the rate determining step at low temperatures. The effect of the plasma is to increase the dissociation of CH4, and etch any a-C, which may deposit on top of the catalyst particle, thereby providing a steady supply of carbon atoms at the top surface of the catalyst particle. At low temperatures, the solubility of C in Co is low, so the amount of carbon diffusing through the particle is very limited. However, carbon atoms adsorbed at the top surface of the catalyst particle can diffuse along the surface, where their motion is much faster. Carbon then segregates at the bottom of the particle, forming graphitic planes. This process allows SWNTs to grow at such low temperatures.
Fig. 4.15 EDX analysis of as-grown SWNTs removed substrate.
4.5 Properties analyses of SWNTs 4.5.1 Field emission properties
The field emission current density as a function of electric field for the as-grown well-aligned SWNT of sample A5 (CoCrPtOx 1 nm) is shown in Fig.
4.16. It shows a turn on voltage of 4.6 V/μm (0.01 mA/cm2) and the current
density of 6 mA/cm2 (7.2 V/μm). In our experiments, the tube packing density is obviously higher and results in lower intertube distances. In order to avoid the field screening effect, a lower packing density of CNTs is favorable.
Fig. 4.16 Field emission current density as a function of electric field for the SWNTs
4.5.2 Oxidation resistance
Thermo-gravimetric analysis (TGA) is implemented on 5 mg of the as-grown SWNTs (ramp rate, 10°C/min in air, 30 sccm/min), as shown in Fig.
4.17. The combustion range of the SWNTs was 586°C to 691°C, with the peak weight reduction occurring at 650°C, a result very similar to that of purified, high-quality SWNTs synthesized by a laser-oven method. We believe that the small initial weight increase is due to physisorption,supported by the fact that the weight returns to its initial value by subsequent annealing. No measurable residue remained after heating above 700°C, indicating very high purity.
800.00 600.00
400.00 200.00
0.00 TGA
%
50.00
0.00 100.00
Temperature 0C
Fig. 4.17 Thermo-gravimetric properties of as-grown SWNTs. (5 mg, ramp rate, 10°C/min)
Chapter V Conclusions
In this work, the CoCrPtOx film was successfully used as catalyst precursor to grow the well-aligned SWNTs with high tube number density by MPCVD.
The main mechanism to form SWNTs is due to the fact that the PtOx in the CoCrPtOx precursor can be decomposed during pretreatment to promote miniaturization of the Co-catalyst particle due to explosive effect of the reaction.
Moreover, Cr2O3 in the precursor can act to separate the Co-catalyst nanoparticles from agglomeration. The process also takes the advantage of recycling the catalyst-coated substrate to minimize the processing cost.
These SWNTs exhibit competitive field emission properties, i.e. turn on voltage of 4.6 V/μm (at 0.01 mA/cm2) and the current density of 6 mA/cm2 (at 7.2 V/μm). Raman spectra indicate that the IG/ID ratio of these SWNTs can be reached to ~ 43, indicating a good quality. The results of TGA analysis in air show that these SWNTs can resist oxidation up to 586°C ~ 691°C, which are much higher than the reported temperatures (~ 350°C) in the literature, and are comparable with that for the purified SWNTs synthesized by a laser-oven method.
The scanning local laser heating pretreatment was successfully used to
decrease the CNTs deposition temperature to 373°C, which is based on heating the local area to a higher temperature to form nanoparticles and maintaining the substrate at lower temperature.
Chapter VI Future prospects
1. The effect of catalyst composition on SWNTs growth.
2. Feasibility study of SWNTs growth with higher length at large area.
3. Synthesize SWNTs on plastic substrate.
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