1. Introduction
One-dimensional single-walled carbon nanotubes (SWNTs) are potential materials for future nanoelectronics. Since the electronic and optical properties of SWNTs strongly depend on their structure, such as diameter and chirality, the selective synthesis of SWNTs with desired structures is a major challenge in nanotube science and applications. SWNT growth was first achieved by arc discharge in 1993. Several growth techniques have been developed since then, including laser ablation and chemical vapor deposition (CVD). Since it is possible to grow SWNTs at a specific position on a substrate by patterning a catalyst, CVD has attracted much attention in nanoelectronics applications. In general, CVD can be divided into two types: thermal CVD [1-4] and plasma CVD [5-7]. Due to the strong electric fields in plasma sheaths, nanotubes grown by plasma CVD tend to have an individually- and vertically-freestanding shape [5, 8-10]. Thermal CVD decomposes carbon source gases using thermal energy. In contrast, in plasma CVD, the source gas decomposition is effectively carried out by electron impact with no additional thermal energy; hence, the growth temperature is significantly lower compared to that of thermal CVD. Despite these benefits of plasma CVD, it is difficult to control the structure of SWNTs by plasma CVD because there are many unknown factors in plasma, such as ion density, ion energy, radical species, radical densities, and sheath electric field, which restrict the potential application of plasma CVD in nanotube science. Based on our studies, SWNT growth by plasma CVD has been significantly improved in recent years. In this chapter, we give a brief overview of recent progress in SWNT growth by plasma CVD.
2. Freestanding single-walled carbon nanotube growth
The potential of plasma CVD for nanotube growth was first demonstrated by Ren et al. in 1998 [5]. Vertically- and individually-aligned multi-walled carbon nanotubes (MWNTs) are grown by plasma CVD. Since carbon nanotubes (CNTs) grown by thermal CVD are known to form a spaghetti-like entangled shape, the well-aligned growth of CNTs by plasma CVD makes it an attractive CNT-growth method that may solve the integration issue in CNT-based nanoelectronics. However, plasma CVD is limited to the production of MWNTs; SWNTs, which have superior electrical and optical characteristics compared with MWNTs, have not been successfully produced by plasma CVD. The growth of SWNTs by plasma CVD was first reported by our group in 2003 [11, 12]. SWNTs are grown by plasma CVD using a zeolite as a catalyst support. Zeolites are nanoporous materials known to maintain small catalyst particle sizes on their rough surfaces, even under high-temperature conditions. Thus, certain plasma effects might enhance catalyst particle aggregation during plasma CVD, which could be the main reason why SWNTs could not be grown by plasma CVD. It is thought that catalyst particle aggregation is enhanced due to high-energy ions attacking the catalyst. In general, ions in plasma are accelerated through the potential drop between space potentials in the plasma and substrate biases. The minimum value of this potential drop is determined by the electron temperature in the plasma. Thus, low electron-temperature plasma can significantly decrease the energy of ions arriving at the substrate. Since the diffusion region in plasma is known to have very low electron temperatures, we used the diffusion plasma to decrease the energy of ions attacking the catalyst to below a few eV. SWNT growth under the diffusion plasma region occurs on a flat substrate without using catalyst support materials [13, 14]. Thus, the critical element promoting catalyst aggregation is high-energy ion bombardment. Interestingly, SWNTs grown by diffusion-plasma CVD have the well-aligned freestanding form, i.e., all SWNTs are individually- and vertically-standing on the flat substrate. Figures 1a–d show a typical scanning electron microscope (SEM) image (Figure 1a), low-magnification (Figure 1b) and high-magnification (Figure 1c) transmission electron microscope (TEM) images, and Raman scattering spectra (Figure 1d) of freestanding SWNTs. Relatively high-quality SWNTs were grown with the individually freestanding form, and this alignment can be obtained by the plasma-sheath electric field. Based on numerical calculation, the rotation energy of the dipole moment in SWNTs is much higher than the thermal energy, which disturbs the tube alignment [14]. This indicates that individual SWNTs can be aligned along the electric field. Owing to their unique as-grown state, it is possible to directly detect photoluminescence (PL) spectra from the as-grown freestanding SWNTs on the substrate (Figure 1e) [15]. This is a remarkable advantage for optoelectrical applications and fundamental studies toward chirality control, which will be discussed later.
3. Growth kinetics of SWNTs in plasma CVD
In this section, we report a crucial finding of remarkable etching reaction of SWNTs during the plasma CVD, and key parameters for such etching reaction are also revealed with a numerical analysis of the experimentally established SWNT-growth equation. A reactive ion etching model is also developed to explain the etching reaction of SWNTs in plasmas[16].
Figures 2a and b presents typical Raman scattering spectra of SWNTs as a function of growth time (
In the case of the thermal CVD, it has been reported that the growth kinetics of SWNTs can be expressed with a following equation (normal equation) [17].
Figure 2c shows a comparison between the experimental result of Figure 2b and fitting curve with Eq. (1). The fitting curve gives good agreement with the experimental result, indicating that the advanced equation established enables us to discuss a more detailed correlation between plasma parameters such as ion energy
Based on the advanced growth Eq. (1), we attempt to understand effects of
A further quantitative and practical analysis is also performed upon the fitting of experimental results with the advanced growth equation (Eq. 1). Figure 3c shows a plot of estimated
In addition to the ions, there are many kinds of factors causing significant impacts on the structure of SWNTs in hydrocarbon plasmas. Especially, the density of radicals is much higher than that of ions in reactive plasmas. Thus we attempt to reveal a correlation of
Based on the above mentioned experimental results, a consistent reactive ion-etching model can be established as follows. In our study, the etching efficiency
The linear dependence of
4. Structure-controlled growth of SWNTs
The structure of SWNTs, including diameter and chirality, strongly influences their electrical and optical properties; therefore, it is important to precisely control the structure of SWNTs. Here, we discuss recent progress in the structure-controlled growth of SWNTs by plasma CVD.
4.1. Diameter control
The band gap is known to be inversely proportional to tube diameter; thus, controlling the tube diameter is very important for electrical and optical applications. Here, we present our experimental results for diameter tuning of SWNTs based on gas-phase control in plasma CVD [18].
Figures 4a–d show photoluminescence-excitation (PLE) maps of as-grown SWNTs produced at different gas pressures. Note that all PLE measurements were carried out immediately after the growth process to prevent the freestanding SWNTs from forming bundles, which cause significant PL changes [15]. Peaks in the PLE map at high growth pressures (Figure 4a) appeared in the range of long excitation and emission wavelengths. The peak positions shifted to the region of short excitation and emission wavelengths with decreasing growth pressure (Figures 4b–d). Since each peak corresponds to a different chirality in the sample, and smaller-diameter SWNTs appeared in the shorter wavelength region, the peak-position shifts in the PLE map indicate that the diameter distribution of SWNTs is strongly influenced by growth pressure. Thus, lower pressure results in smaller SWNT diameters. This diameter dependence on the growth pressure is also reflected in Raman scattering spectra of SWNTs grown at different growth pressures. Figure 4e shows that peak positions of the radial breathing mode (RBM) clearly shifted from higher to lower wavenumbers with increasing growth pressure. The RBM peak position and the SWNT diameter are known to have a close correlation,
Since the pressure during the heating and growth were the same in our growth process, the process pressure affected both the heating and the growth process. Based on this systematic investigation, we believe that the catalyst particle size increased due to aggregation after high-pressure annealing, which resulted in the growth of large-diameter SWNTs. The density of reactive hydrocarbon radicals and ions should increase under higher growth pressure conditions. Under high carbon supply conditions, a small catalyst can be deactivated by an oversupply of hydrocarbons, causing the population of small-diameter SWNTs to decrease. Therefore, the heating pressure is an important parameter that controls the catalyst particle size distribution, which directly influences the diameter of SWNTs. The pressure during plasma CVD is also important for a narrow SWNT diameter distribution [18].
4.2. Selective growth of semiconducting SWNTs
Field effect transistors (FETs) are one of the most promising applications of SWNTs. Although the high mobility and flexibility of SWNTs films can provide lots of opportunities to be utilized in various kinds of industrial applications, the low on/off current ratio in SWNT FETs caused by the mixture of metallic and semiconducting SWNTs restricts the practical use of SWNTs in FET applications. Recent progress in chemical separation enables us to fabricate good devices with on/off ratio: ~104 and effective gate mobility: ~ 52 cm2/Vs. However, impurities and defects are sometimes introduced in chemically treated nanotubes during the separation process, which significantly decreases the device performance. Since as-grown SWNTs maintain the original high-quality with low impurity concentration, the selective growth of semiconducting SWNTs is desirable. Dai
Figure 5a shows typical Raman scattering spectra of SWNTs grown under different growth temperatures. The high graphite (G)-peak to defect (D)-peak ratio indicates that the quality of SWNTs is comparable to other conventional CVD grown SWNTs. The RBM in a lower wave number region in Raman spectra exhibits the clear down shift with an increase in the growth temperature. The mean diameter of SWNTs is found to increase with growth temperature. This seems to be due to the catalyst particle size effect. Higher growth temperatures cause particle aggregation and result in the increase of the particle size, which can produce larger diameter SWNTs. A clear dependence is obtained from the plot of on current (
In order to explain the dependence of the working device concentration on the SWNT diameter, devices were irradiated by an Ar plasma, and a defect formation rate is estimated from the current change before and after the plasma treatment. In the case of small diameter SWNTs devices, the on/off ratio does not change, and on and off currents significantly decrease after the Ar plasma irradiation, whereas the on/off ratio increases with an increase in the Ar plasma irradiation time and the off current depression is significant compared to that of the on current in the case of large diameter SWNTs devices. Based on these results, the following model can be developed to explain the dependence of the working device concentration on the diameter. Due to the curvature effect, small diameter SWNTs are more unstable than large diameter ones. Hence, both metallic and semiconducting SWNTs are easily deformed by the Ar plasma irradiation without any difference in the tube metallicity. On the other hand, in the weak curvature range, the dependence of the defect formation rate on a unique metallicity appears, which might correlate with the reactivity, binding energy between carbon and carbon, and healing process. This model is consistent with the selective etching of metallic SWNTs by gas phase reaction, which was previously reported. Further detailed studies relating to the selective damage of metallic SWNTs might provide the possible answer for the preferential growth of semiconducting SWNTs by plasma CVD.
4.3. Narrow-chirality distributed growth of SWNTs
The chirality of SWNTs directly determines their electronic and optical properties; thus, selective synthesis of SWNTs with desired chiralities is a major challenge in nanotube science and applications. In this session, we demonstrate the recent progresses of narrow chirality distributed growth of SWNTs by plasma CVD based on different two approaches, which focus on catalytic reaction and gas phase reaction.
4.3.1. Catalytic reaction control
Narrow chiratliy distributed growth of SWNTs is one of the most critical issue in the scientific field of SWNTs production stage. Some progress has been made with silica-supported CoMo [21] and zeolite-supported FeCo [22] catalysts. FeRu [23] and FeNi [24] catalysts have also been developed to achieve narrow chirality distributions. Interestingly, all syntheses resulting in narrow chirality distributions have involved magnetic catalysts. The main obstacle to research on intrinsic magnetic properties of SWNTs is residual ferromagnetic catalyst particles; thus, SWNT growth with nonmagnetic catalysts is beneficial. Despite recent improvements in SWNT growth with nonmagnetic catalysts [25-28], diameter and chirality (n,m) distribution control with nonmagnetic catalysts is still required for fundamental studies and a variety of applications [29].
Based on this background, we attempto to grow SWNTs with narrow chirality distributions using nonmagnetic catalyst [30, 31]. PLE mapping was used to assign (n,m) of SWNTs grown from the Au catalyst at different H2 concentrations (Figures 6a–c). The total pressure was kept at 50 Pa by adjusting the pumping rate of the rotary pump throughout this experiment. Lower H2 concentrations (0 and 3 sccm) led to larger diameters and wider (n,m) distributions with (6,5), (7,5), (7,6), (8,4), (8,6), and (8,7) (Figures 6a and b). On the other hand, the 7-sccm H2 concentration yielded the narrowest (n,m) distribution with a dominant peak corresponding to the (6,5) tube (Figure 6c). The UV-Vis-NIR optical absorbance spectra of Au-plasma CVD SWNTs grown at the 7-sccm H2 flow rate showed one dominant peak in the first van Hove E11 range (900–1400 nm) corresponding to SWNTs with (6,5) chirality (Figure 6d). Since clear metallic SWNT peaks were not observed in the UV-Vis-NIR spectra (Figure 6d), the concentration of metallic SWNTs was lower than that of the generally grown SWNTs. This is the first result showing narrow chirality distributions for SWNTs grown from a nonmagnetic catalyst [31].
To elucidate the effects of Au and plasma CVD on the narrow chirality distribution, other combinations of catalysts and CVD methods were systematically investigated. Based on the PLE analysis, SWNTs grown by the Fe catalyst with plasma CVD (Fe-plasma CVD) did not show a clear correlation between the H2 flow rate and the chirality distribution, which was broader than that of SWNTs grown by Au-plasma CVD. This indicates that H2-assisted Au catalyzation is a critical factor for achieving narrow chirality distributions, which is in good agreement with theoretical predictions. The first-principle calculation by Yazyev
4.3.2. Gas phase reaction control
Recent progress in the in-situ TEM observation during the SWNT growth revealed that metal-catalyzed SWNT growth is initiated by the formation of a carbon cap structure on the surface of a catalytic nanoparticle with a certain incubation time (
In order to analyze the chirality distribution of the short SWNTs in detail, we carry out the PLE map analysis. It is to be noted that since our plasma CVD grown SWNTs take on the freestanding form due to the strong electric field in the plasma sheath area during their growth, it is possible to observe PL signals from as-grown SWNTs on a substrate [15]. All the PLE measurements are carried out immediately after the growth to prevent SWNTs from forming thin bundles, which leads to causing the PL intensity change by exciton energy transfer between each tube [15]. From the density estimation of SWNTs with the direct TEM observations, it is confirmed that abundant SWNTs exist in the area where the PLE measurement is carried out. Thus, the PLE map gives us macroscopic information in each sample. Figures 8a-c show the PLE maps of the as-grown SWNTs as a function of
To further narrow the initial
The most important factor for the narrow-chirality distributed growth of short SWNTs by TP-PCVD is to control the
As we have discussed above, the growth temperature is one of the key parameters to realize the narrow-chirality distribution of SWNTs by TP-PCVD. In order to realize the further precise control of the
The other possibility to cause the
5. Conclusions
Recent progress in SWNT growth was presented, with a special emphasis on plasma CVD. Due to the strong plasma-sheath electric field, it is possible to grow freestanding individual SWNTs by plasma CVD. Based on the time-evolution study and the detailed plasma parameter measurements, the growth kinetics of SWNTs in plasma CVD were well established. The concentration of semiconducting SWNTs in FET devices can be increased by tuning the mean diameter of SWNTs, and this effect is attributable to selective damage of metallic SWNTs during plasma CVD. Moreover, narrow chirality-distributed growth of SWNTs were also achieved by different two approaches. Au catalyzed plasma CVD with appropriate amount of hydrogen addition can realize preferential gorwth of (6,5) SWNTs. The narrow chirality distributed growth of SWNTs were also demonstrated with precise incubation time control by time-programmed plasma CVD.
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