NANO LETTERS
Dynamic Growth Rate Behavior of a Carbon Nanotube Forest Characterized by in Situ Optical Growth Monitoring
2003 Vol. 3, No. 6 863-865
Do-Hyung Kim,*,† Hoon-Sik Jang,† Chang-Duk Kim,† Dong-Soo Cho,† Hee-Sun Yang,‡ Hee-Dong Kang,† Bong-Ki Min,§ and Hyeong-Rag Lee*,† Department of Physics, Nanophyscis and Technology Laboratory, Kyungpook National UniVersity, Daegu, 702-701, Korea Received April 8, 2003
ABSTRACT We characterize the dynamic growth rate behavior of a carbon nanotube (CNT) forest grown by means of optical interference phenomena. The CNT growth rate increased with an increase in CNT length at the initial stage and became stabilized after the CNT length was about 2 µm. Then the growth rate started to decelerate, passing the critical growth length in an almost linear manner. The termination length of the carbon nanotube was also precisely estimated by fitting the data of growth rate of carbon nanotubes to time. It was found that the CNTs show a transition from straight to curly nanotubes that is related to the decrease in the growth rate. The use of an in situ optical monitoring method has made possible the delicate length control of carbon nanotubes independent of the growth rate.
Carbon nanotubes (CNTs) have shown a variety of applications such as scanning probes,1,2 field emitters,3-5 and nanoelectronic devices.6 The nanoscale structuring of carbon nanotubes with respect to growth7-9 has rapidly progressed. The in situ control of CNT growth will be required in order to optimize and realize their applications in the area of vacuum electronics. In addition, the in situ control of the length of CNTs is needed for optimal performance as well as for wider applications involving nanobased device structures. However, in situ characterization and control methods are difficult to apply during the CNT growth period due to unfavorable conditions such as no vacuum, gas ambient conditions, and high temperatures used in the chemical vapor deposition (CVD). In this letter, we describe an in situ method for monitoring the growth of aligned carbon nanotubes based on an optical interference technique. Using this method, it was possible to investigate the growth behavior of carbon nanotubes during the growth period. Furthermore, the length of carbon nanotubes can now be precisely controlled. A typical plasma-enhanced chemical vapor deposition (PECVD) technique was used to grow the aligned CNTs.9 The substrate consisted of silicon wafers with 10 nm Ni sputtered film. Flows of C2H2 and NH3 were kept constant at 60 sccm and 180 sccm, respectively. A mixture of * Corresponding authors. E-mail:
[email protected]; dhkim@ nptl.knu.ac.kr † Kyungpook National University. ‡ Korea Basic Science Institute, Daegu branch. § Instrumental Analysis Center, Yeungnam University, Kyongsan. 10.1021/nl034212g CCC: $25.00 Published on Web 05/17/2003
© 2003 American Chemical Society
acetylene (C2H2) and ammonia (NH3) was used as the gas source at ∼2 Torr. CNT growth was performed at 700 °C. A -450 V bias was applied to the substrate in order to create a dc plasma. We counted about 100 nanotubes per square micrometer. In situ optical interferences were measured by means of a focused 650 nm laser diode with a 1 mm beam diameter and a photodiode detector. Tempered glass was used for the PECVD growth chamber in order to transmit the coherent laser beam from the laser source to the substrate and the reflected beam from the substrate to a detector. We confirmed that the plasma had negligible influence on the transmittance of the laser light to the photodiode. Measurements and growth experiments were automatically controlled by a personal computer and in-house prepared software. Figure 1a shows the interference oscillations measured at a 30° angle to the beam incidence angle during the growth of nanotubes under previously optimized growth conditions. The interference patterns can be varied by experimental conditions such as laser wavelength, beam incidence angle, and growth conditions, which also provides a very effective way to investigate growth behavior. The interference oscillations in Figure 1a represent interference phenomena between the reflected beam from the top of carbon nanotubes and from the surface of a nickel-coated substrate. The diminution in the interference oscillations is mainly due to the absorption of laser light through the CNTs. The reflectance remains nearly unchanged during NH3 pretreatment prior to CNT growth in the presence of C2H2 and NH3. The starting intensities of the reflected laser beam were slightly
Figure 2. (a)-(c) Carbon nanotube images controlled at the points indicated by gray arrows in inset of Figure 1. (d),(e) Carbon nanotube images grown for 6 and 30 min, respectively. (f) TEM image showing a bundle of nanotubes with the upper part straight and the lower part curled.
Figure 1. (a) Interference oscillation behaviors as a function of growth time. The oscillation periods were gradually increased with increasing growth time. The inset shows the length of carbon nanotubes as a function of the oscillation period and the linear fit of experimental data has the relation Y ) (0.649 ( 0.003)X. (b) Average length of a carbon nanotube forest as function of growth time.
different than the experimental setup and the initial laser intensity. However, the unique interference oscillations were shown in each run. The four curves in Figure 1a were obtained from the growth of well-aligned CNTs with identical growth conditions. However, the oscillations show slightly different behaviors, which means the growth always brings the unintentional slight fluctuations of growth conditions. Meanwhile, the as-grown CNTs do not show significant differences, and the growth lengths were ruled by the interference formula. Constructive interference by a thin film is described by dsin θ ) nλ/2, where λ and θ are the wavelength and incidence angle of the laser beam, d the film thickness, and integral n () 0, 1, 2,...). Our experimental results indicate that this simple equation can be used to successfully describe the growth of the carbon nanotubes. Figure 1b shows the average length of CNTs as function of growth time. The average growth length of CNTs increased in the initial stage, then gradually stabilized with increasing growth time. The lengths of the carbon nanotubes were measured by scanning electron microscopy (SEM) as shown in Figure 2, and these data are in agreement with interference oscillations. The lengths of the nanotubes are plotted as a function of the interference oscillation in the inset of Figure 1. The growth rate per oscillation was determined to be about 649 nm ( 3 nm, which coincides with the 650 nm wavelength of the laser used here. Various shapes for the interference oscillations were measured in order to confirm the consistency between 864
Figure 3. Growth rate of carbon nanotubes as function of the growth length, calculated from the data in Figure 1.
the measured oscillation patterns and laser source conditions such as the beam incidence angles and the laser wavelengths. Figure 3 shows the growth rate as a function of the CNT length calculated from the data in Figure 1. The growth rate increases with an increase in CNT length at the initial stage, which can be due to the involvement of a surface diffusion process suggested by Louchev et al.10,11 The carbon species collide with CNT surfaces and diffuse to the CNTs wall edge where they incorporate directly into the wall or via diffusion through a catalyst particle. These papers show that the growth rate increases as long CNT length is smaller than the diffusion length. In our experiment, the growth rate increase continues up to a length of ∼2 µm. After the maximum growth rate is reached, it becomes stabilized. The growth rate becomes almost constant and then finally starts to decrease at the critical CNT length in an almost linear manner, as shown in Figure 3. This termination behavior is attributed to the change of the critical growth condition passing periods with stable growth rate. O. Zhou et al.12 have suggested that the termination is due to the encapsulation of catalyst metal at the bottom of the nanotubes after an extended period of growth. However, diverse experimental parameters and lack of in situ methods to characterize CNT growth have so far hindered a complete understanding of the precise nature of the termination of CNT growth. Figure Nano Lett., Vol. 3, No. 6, 2003
the curly CNTs were formed in the region of decreased growth rate. It is clear that the growth termination brings about the relatively curly form of CNTs under changed growth conditions after the critical growth length is reached. The straight part of the CNTs in Figure 2f is about 4 µm long, which corresponds to the point where the growth rate decreases. More sophisticated studies are under way to unveil the termination mechanism for the growth of carbon nanotubes. An extrapolation of the linear fitting results in Figure 3 indicates that the stabilized lengths of the carbon nanotube have values ranging from 10 to 11.3 µm under our optimized growth conditions. The carbon nanotubes were allowed to grow for 6 and 30 min for comparison, which results in a length of about 10.5 µm in both cases as shown in Figure 2d,e, respectively. This is in agreement with the stabilized length of a nanotube as calculated from linear fitting data. In summary, we have presented an in situ optical monitoring method for carbon nanotube growth. Growth rate behaviors can be characterized and the length of carbon nanotubes can be precisely controlled by in situ monitoring of the interference oscillations. The termination length of the CNTs was well predicted by the interpretation of interference behavior. The upper portions of CNTs were straight and the lower portions were curly , as related to the decrease in the CNT growth rate based on a “base growth” model. This in situ technique can be used as a method for such growth control in future applications of nanotubes for use in functional devices. Figure 4. (a) Cross-sectional TEM image for the bottom of a wellaligned MWNT. (b) Magnified HRTEM image for the white circle region of Figure 4a. The straight dotted line indicates the discontinuous carbon nanotubes at the bottom. The Ni particle was not fully enclosed at the bottom region marked in black dotted circle.
Acknowledgment. This study was performed under the Daegu City Government 2002 Nano Project and was also supported by the Korea Ministry of Information and Communication. References
4 is the transmission electron microscopy image of the sample. Our CNT was grown via a “base model” mechanism with Ni particles at the bottom of CNTs as shown in Figure 4a. The Ni particle was not fully enclosed at the bottom of the CNT, as shown in Figure 4b. It was also confirmed that the additional growth of nanotubes per a quarter or half periods was found for some CNTs growth, showing a drag in the interference oscillations curve. This can indicate that the encapsulation of catalyst metal can be processed after finishing the growth. It may be difficult to estimate the dynamic change during growth using a TEM measurement taken after finishing the growth period. In our CNTs based on the “base growth” mechanism, bottom parts of CNTs were shown to be relatively curly whereas the upper parts of the CNTs appear in an aligned manner, as shown in Figure 2d,e,f. This growth behavior indicates that CNTs in the constant growth rate region were well aligned as well as straight, and
Nano Lett., Vol. 3, No. 6, 2003
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