Abstract
In this paper we study the growth of horizontally aligned ultra-long single-walled carbon nanotubes (SWCNTs) (5 mm) on oxidized silicon substrates by the fast-heating chemical vapor deposition (CVD) method. Ferric chloride (FeCl3) and ethanol (C2H5OH) solution are used as catalyst and carbon source, respectively. The influence of growth conditions such as temperature, time and gas flow rates on the SWCNTs' growth is studied in detail. The morphologies and structure of the SWCNTs are analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The experimental results demonstrate that by fast-heating CVD of ethanol at a temperature of 900 °C for 60 min, the SWCNTs have high quality. They are clean, ultra-long (mm), highly dense and as straight as parallel lines.
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1. Introduction
Since carbon nanotubes were found in 1991 by Iijima, they have attracted much attention from many researchers because of their electrical and mechanical properties. Single-walled carbon nanotubes (SWCNTs) are applied for nanoscale electronic devices, such as field-effect transistors, sensors and field emitting devices [1]. Thus, it is very important to synthesized high-purity SWCNTs with control over the diameter, length and orientation. The experimental results demonstrate that the growth rate of nanotubes can be very high under the right growth conditions [2, 3]. Consequently, long nanotubes available for many applications can be grown.
In this paper, horizontally-aligned ultra-long SWCNTs are successfully synthesized on oxidized Si substrates by fast-heating chemical vapor deposition (CVD) under ambient pressure. The catalyst and carbon source are FeCl 3 0.1 M solutions and ethanol C 2 H 5 OH solution, respectively. The mechanism of SWCNTs' growth by the fast-heating CVD method is discussed. The influence of the growth temperature and growth time on the SWCNTs' growth was studied. The morphology and structure of SWCNTs are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy techniques.
2. Experimental
First of all, the oxidized Si substrates were cleaned by ultra-sonication in acetone solution to remove organic impurities. For each CVD process two oxidized Si clean substrates with a size of 0.5×0.5 cm 2 were used. The distance between two oxidized Si substrates is some tens of microns. One substrate was used to contain catalyst FeCl 3 0.1 M solution. The catalyst solution was covered on the surface of substrate containing catalyst by spin-coating. The second oxidized Si substrate was used as a holder of synthesized horizontally aligned SWCNTs (figure 1). The CNTs on this Si holder are studied in this paper.
Figure 1 Simulation of the SWCNTs growth on Si substrates with Fe nanoparticles.
A schema of the CVD setup is shown in figure 2. A 1.2 m long (22 mm ID) quartz tube is horizontally placed inside the hot wall of a horizontal tube furnace. The furnace system allows rapid cooling by opening the furnace cover. In addition, samples can also be cooled/heated rapidly by relatively shifting the oven to the quartz tube reactor through a system of rails.
Figure 2 A CVD system for growing SWCNTs.
The two oxidized Si samples were placed inside a quartz tube reactor. The Ar gas of 200 sccm flowed through the quartz tube reactor during all the working time. Before the CNT growth stage, the two oxidized Si samples were located outside of the heating zone center. When furnace temperature increased to the desired growth temperature, the 'fast-heating' step was executed by shifting the furnace relatively to the quartz tube. The samples in the quartz tube could be located rapidly at the center of the furnace heating zone and were ready for growing CNT.
The CNT growth was carried out by flowing C 2 H 5 OH vapor and Ar/H 2 (200/30 sccm). The C 2 H 5 OH vapor was introduced by conducting Ar gas (30 sccm) through a glass bottle of ethanol solution. After the CVD, the samples in the quartz tube were cooled down to room temperature in argon gas of 60 sccm.
In the CVD method, Ar and H 2 gas flows have to be stably provided and adequate for growing SWCNTs. If Ar gas flow through the ethanol bottle is too small, the ethanol vapor (carbon source) flowed into quartz tube reactor is not enough to conduct the CNTs with high density. On the other hand, if flow rates of H 2 and Ar gas are too strong, they can twist gas flows inside the quartz tube reactor. As a result, the grown nanotubes may be not straight and aligned. The influence of the gas flow rate on the length and direction of SWCNTs was studied but is not indicated here. Our experiments found the optimal gas flow rate of Ar/H 2/(C 2 H 5 OH+Ar) to be 200/30/30 sccm. This value of gas flow rate was used in all experiments presented here.
In this work, the influence of growth temperature and growth time on the CNT growth was studied. The growth temperature and growth time were tested in the range of 700–900 °C and 20–120 min respectively.
The morphology and structure of SWCNTs were analyzed by scanning electron microscope (SEM, HITACHI S-4800, acceleration voltage of 0.7 kV), transmission electron microscope (TEM, JEOL JEM-1010 Instruments) and Raman spectroscopy techniques (λ exc =514 nm, spot size of 1 μm).
3. Results and discussion
The CNTs on the CNT-holder substrate were characterized here. Figure 3 shows the SEM images of CNTs synthesized for a growth time of 60 min at three different temperatures of 700, 800 and 900 °C. From figure 3 it is clear that at a growth temperature of 700 °C there are no CNTs on the Si substrate. At a growth temperature of 800 °C there are some CNTs, but they are not parallel.
Figure 3 SEM images of carbon nanotubes grown for 60 min at different temperatures: (a) 700 °C, (b) 800 °C, and (c) 900 °C.
We can see that at higher growth temperature (900 °C) well oriented CNTs (figure 3(c)) were grown in the gas flow direction as parallel straight lines. It is suggested that due to the high temperature (900 °C), the carbon source augments due to the increases of ethanol decomposition.
On the other hand, the diffusion of carbon into the catalyst is advanced. As a result, the growth of CNTs is favored [4] at a temperature of 900 °C, whereas at lower temperatures of 700 and 800 °C, the ethanol vapor (C 2 H 5 OH) is not completely decomposed, so the carbon amount is small. This is not favorable for growing CNT.
The role of the growth time on the quality of ultra-long CNTs is investigated in this paper. The synthesis of the SWCNTs was executed with different times: 20 60 and 120 min at the same temperature 900 °C. Figures 3(c) and 4 are the SEM images of CNTs grown on Si substrates for different growth times of 20, 60, and 120 min. From these images it is clear that the longer the growth time, the longer the length of the synthesized CNTs. The length of the CNTs synthesized for 20 min (figure 4(a)) is several tens of micrometers, smaller than that of the CNTs synthesized for 60 min (figure 3(c)). But when the growth time is 120 min, from figure 4(b) it is clear that there is a lot of amorphous carbon on the Si surface. It is suggested that if the growth time was too long, the catalyst was poisoned and became inactive for growing CNTs. The supplied carbon formed amorphous carbon on the Si surface. That is the reason for the appearance of amorphous carbon when the growth time was too long. Thus, it is necessary to determine optimal CVD time for nanotube growth. For our experimental setup, growth time of 60 min is appropriate.
Figure 4 SEM images of CNTs on Si substrates with different growth times: (a) 20 min; b) 120 min.
Figure 5 shows SEM images of CNTs grown by conventional and fast-heating CVD methods. It is clear that nanotubes fabricated by conventional method are shorter and randomly oriented (figure 5(a)), while those grown by fast-heating method are some millimeters long and horizontally aligned along the gas flow direction (figure 5(b)).
Figure 5 SEM images of CNTs grown by conventional (a) and fast-heating method (b).
We base on the 'tip-growth' mechanism to explain the orientation and length of CNTs. We suggest that the nanotubes have to be floating to grow across the trench between two Si substrates (figure 1). The existence of the CNTs (shown from SEM images in figures 3 and 4) on the second Si substrate (holder) is evidence of the floating of CNTs, because if CNTs grew along the Si surface, they would be stopped by the trench between two Si substrates.
We think that the initial stage of the CNTs' growth is essential, and it determines the length of CNTs. In fast-heating CVD the samples were heated to growth temperature very quickly (only a few seconds). As a result of fast-heating, the Si substrates and the reactive gases were heated at different speeds, so they had different temperatures. Thus, a convection flow was formed due to the temperature difference. This convection flow can lift the nanotubes up [4]. The horizontal gas flow above the Si surface carried the grown nanotubes and aligned them along the gas flow direction (figure 1). On the other hand, we think that for the ultra-long CNTs the tip-growth mechanism is more preferred to the base-growth mechanism, because, for the base-growth mechanism, during growth process it is necessary to add energy for shifting ultra-long CNTs along gas flow direction. The ultra-long CNTs may be grown by the tip-growth mechanism with the catalyst particles on the nanotube tips as shown in figure 1. During growth process, the CNT tip parts of CNTs were always floating. Near the original position where the catalyst was deposited (the substrate containing catalyst), the CNT parts can form van der Waals contacts with the Si substrate [4–6].
The SEM results demonstrate that the fast-heating CVD is an appropriate method to grow ultra-long and well-oriented carbon nanotubes on Si substrates. Figure 6 is the Raman spectrum of the CNTs synthesized by fast-heating method. The spectrum is measured in the range from 950 to 1900 cm −1 (excitation wavelength of 632.8 nm). The D-peak at 1348 cm −1 and G-peak at 1592 cm −1 are a feature of CNTs grown on Si substrates.
Figure 6 Raman spectra of CNTs grown with FeCl 3 0.1 M as the catalyst, Ar/H 2:30/30 sccm.
It is well-known that radial breathing modes (RBM) of Raman spectra show the characteristic feature of SWCNTs. The diameter of nanotubes can be evaluated from the RBM peak values. Figure 7 is Raman spectrum measured in the range of RBM (from 0 to 400 cm −1). Figure 7 shows obvious RBM peaks at 197, 253 and 310 cm −1. These peaks demonstrate the existence of single-wall carbon nanotubes with diameters of 1.26, 0.98 and 0.8 nm, calculated from the following equation d SWCNTs =248/ω RBM [7–9]. The Raman spectroscopy results prove the successful synthesis of SWCNTs by fast-heating-CVD of ethanol as a carbon feedstock.
Figure 7 Radial breathing modes of Raman spectrum of CNTs on Si substrate surface.
Figure 8 shows the TEM picture of SWCNT grown on Si substrate by the fast-heating method. This individual SWCNT is horizontally aligned in the gas flow direction. We can see that the diameter of SWCNTs is very small, about 1 nm. This value is consistent with the above RBM Raman results. From figure 8 it is clear that the SWCNTs are clean; there is no amorphous carbon on the Si substrates' surface or on the SWCNTs.
Figure 8 TEM image of SWCNTs horizontally aligned on Si substrates, the diameter of nanotubes is about 1 nm.
4. Conclusion
SWCNTs were successfully synthesized on oxidized Si substrates by the fast-heating CVD method using ethanol and iron salt (FeCl 3 0.1 M solution) as carbon source and catalyst, respectively. The important role of the growth conditions to the quality and length of nanotubes is investigated. Among them, the growth temperature, the growth time and particularly the fast-heating CVD are studied in detail. Structure and morphology of the grown SWCNTs were analyzed by SEM, TEM and Raman spectroscopy. We found out the appropriate synthesized conditions for growing horizontally aligned ultra-long SWCNTs by fast-heating CVD: the growth temperature and time are 900 °C and 60 min, respectively. The experimental results demonstrate that SWCNTs were clean, horizontally aligned and ultra-long. The diameter of SWCNTs is small, in the range from 0.8 to 1.5 nm. The obtained horizontally aligned ultra-long SWCNTs may be applied in electronic devices, for example using SWCNTs for STM tips and one-electron transistor.
Acknowledgments
A part of this work was done with the help of the National Basic Research Fund (NAFOSTED). We would also like to thank the Research projects of Science and Technology for providing a grant for this work (VAST and IMS). A part of the work was done with the help of the Key Laboratory for Electronic Materials and Devices, IMS.