Deposition and activation of chirality-pure nanotube seeds

The schematic of the chirality-controlled SWCNTs cloning process is illustrated in Fig. 1 (see Methods for details). First, chirality-pure SWCNT seeds obtained from the DNA-based separation were deposited onto the growth substrates using drop coating and incubation with different durations (see Methods). The as-deposited seeds then went through an air and water vapour annealing process. This step removes the DNA wrapped on the nanotube seeds and activates the nanotube ends so that carbon reaction intermediates from decomposed feed stock can be efficiently added. Both methane and ethanol were found to be suitable as the carbon feed source to achieve chirality-controlled growth from these seeds.

Figure 1: Schematic illustration of the VPE process for chirality-controlled SWCNT synthesis. The chirality-pure nanotube seeds obtained from DNA-based separation are first deposited onto various kinds of substrates, including quartz and Si/SiO 2 , followed by air annealing and water vapour annealing processes to activate the ends for the ensuing VPE growth. VPE with either methane or ethanol as the carbon feed stock is then used to achieve chirality-controlled growth from these seeds. Full size image

A critical factor that determines the yield of the VPE cloning process is the air and water vapour annealing process. This was revealed by a systematic study we performed to map out the optimum annealing conditions. As shown in the Supplementary Information (Supplementary Fig. S1), without proper annealing treatment, the cloning yield is extremely low, with only curvy nanotube bundles visible after growth. In comparison, after a proper air and water vapour annealing, individually dispersed, long and straight nanotubes can be grown. We speculate that the air and water annealing processes alter functional groups on nanotube ends, such that the ends are activated and capable of incorporating incoming carbon atoms for growth. We note that the nanotube seeds are very sensitive to the annealing environment, and 30–50% of seeds were etched away during the air and water annealing processes (Supplementary Fig. S2). Extensive studies to compare the VPE cloning results obtained with different temperatures and duration of the annealing steps, as well as gas flow rates (Supplementary Table S1 and Supplementary Fig. S3), led to an optimum recipe described in the Methods section.

Characterization of cloned nanotubes using microscopy

We used atomic force microscopy (AFM) and scanning electron microscopy (SEM) to characterize SWCNTs before and after VPE cloning. Representative AFM images of (7, 6) SWCNT before and after cloning using ethanol on quartz substrates are presented in Fig. 2a, respectively. The average length of the nanotubes after cloning was measured to be 34.5±17.7 μm (Fig. 2d), significantly longer than the average length of 0.34±0.15 μm for the as-purified (7, 6) nanotube seeds (Fig. 2c). The measured diameter of the cloned nanotubes were approximately 0.9 nm, consistent with the (7, 6) nanotube diameter (d=0.89 nm). As control experiments, blank quartz substrates and quartz substrates deposited with DNA solution but without any nanotube seeds were subjected to the air and water vapour annealing, and then the VPE cloning. No nanotube growth was observed after the VPE process (Supplementary Figs S5 and S6). This demonstrates that the long SWCNTs were indeed grown from the nanotube seeds.

Figure 2: AFM and SEM characterization of SWCNTs before and after VPE. (a,b) AFM images of (7, 6) nanotubes before and after VPE using ethanol on quartz substrates. The scale bars in a and b are 1 μm. (c,d) Length distribution of the (7, 6) nanotubes before and after VPE. (e,f) Comparison of (7, 6) nanotubes cloned on quartz and Si/SiO 2 substrates. (g) An SEM image showing (7, 6) nanotubes cloned on the quartz substrate using methane. (h,i) SEM images of (6, 5) nanotubes cloned on quartz substrates using methane and ethanol, respectively. (j,k) SEM images of (7, 7) nanotube cloned on quartz and Si/SiO 2 substrates. Scales bars, 50 μm for (e,f) and j, 30 μm for (g,h) and i, and 10 μm for k. Full size image

We also carefully studied the effect of substrates on the VPE cloning process. On Si/SiO 2 substrates (Fig. 2e), random orientation was observed for the cloned nanotubes irrespective of the gas flow direction. For nanotube cloning performed on the ST-cut quartz substrates (Fig. 2f), the cloned SWCNTs were found to be horizontally aligned along the crystal orientation27. This should be very useful for the fabrication of nanotube transistors and integrated circuits.

Overall, the nanotube VPE process was found to be very robust and highly reproducible. Various carbon feed stocks including methane and ethanol, and different nanotube seeds including (7, 6), (6, 5) and (7, 7) were used successfully. The SEM images of (7, 6) nanotubes cloned with methane (Fig. 2g), (6, 5) nanotubes cloned with methane (Fig. 2h) and ethanol (Fig. 2i), respectively, indicate that similar growth were achieved for all the cases. VPE cloning process is also demonstrated for the (7, 7) armchair metallic nanotube using both Si/SiO 2 (Fig. 2j) and quartz (Fig. 2k) as substrate.

Chirality identification by Raman spectroscopy

To determine whether the original chiralities were preserved during the cloning process, we used micro-Raman to characterize the pristine and cloned SWCNTs. For the (7, 6) case, we used 1.96 eV laser excitation, which is close to the second optical transition E 22 =1.92 eV of the (7, 6) nanotubes28. To further enhance the signal-to-noise ratio, surface-enhanced Raman spectroscopy was used after e-beam evaporation of 5 nm silver on the substrate (Supplementary Fig. S8). The radial breathing mode (RBM) Raman spectra taken at different random locations on the substrates before and after cloning are presented in Fig. 3a, respectively. For small diameter nanotubes such as (7, 6), (6, 5) and (7, 7), adjacent nanotubes have very distinct RBM frequencies (Fig. 3f)29, allowing unambiguous chirality assignment by determining the RBM frequency and the laser excitation energy. Figure 3a show predominant peaks at 265 cm−1, which correspond to the RBM of (7, 6) nanotubes, indicating that the chirality is indeed preserved in the cloned nanotubes. The minority peaks at 300 and 307 cm−1 can be attributed to the (8, 3) and (9, 1) impurities. We have also performed Raman mapping along a specific nanotube as shown in Fig. 3e, confirming that the chirality is preserved along the whole nanotube. Similar studies were carried out for (7, 7) nanotubes using 2.4 eV laser excitation; Raman spectra of (7, 7) before (Fig. 3c) and after VPE growth (Fig. 3d) showed predominant peaks at 249 cm−1, which correspond to the RBM of (7, 7) nanotubes. The weak peak at 303 cm−1 is attributed to the Si/SiO 2 substrate background. The RBM intensity of cloned nanotubes is weaker than that of the nanotube seeds. This is because many nanotube seeds got etched away during the annealing and subsequent VPE step, thus leading to reduced nanotube number density and reduced Raman intensity.

Figure 3: Raman spectroscopy characterization of SWCNTs before and after VPE. (a,b) Raman RBMs of (7, 6) nanotubes before and after VPE. The spectra were taken at different random locations on the quartz substrates with laser excitation energy of 1.96 eV. The peaks at 265 cm−1 correspond to the RBM of (7, 6) nanotubes. (c,d) Raman RBMs of (7, 7) nanotubes before and after VPE. The spectra were taken at different random locations on the Si/SiO 2 substrates with laser excitation energy of 2.4 eV. The peaks at 249 cm−1 correspond to the RBM of (7, 7) nanotubes. (e) Raman RBM spectra along a specific nanotube confirming that the chirality is preserved after the VPE process. Inset, an SEM image (with artificial colour) of the nanotube. (f) Kataura plot shows the RBM frequencies and E ii of different nanotubes. The values were taken from the literature29. Full size image

Electron transport measurement of cloned nanotubes

Back-gated nanotube field-effect transistors were fabricated to further characterize the electrical properties of the cloned (7, 6) and (6, 5) nanotubes. The device schematic and an SEM image of a representative device consisting of an individual (7, 6) nanotube are shown in Fig. 4a, respectively. In brief, the cloned (7, 6) nanotubes grown on quartz substrates were transferred to Si/SiO 2 substrates with 50 nm SiO 2 using methods described in our previous publication9. This was followed by formation of Ti/Pd (0.5/50 nm) source/drain metal contacts using lithography and lift-off techniques. After production, the device had a channel length of L=4 μm and a 50-nm-thick SiO 2 gate dielectric.

Figure 4: Electrical characterization of the cloned (7, 6) SWCNTs. (a) Schematic and (b) an SEM image (with artificial colour) of a back-gated (7, 6) nanotube transistor. The device consists of an individual (7, 6) nanotube, with L=4 μm and 50 nm thick SiO 2 gate dielectric. Scale bar, 5 μm for b. (c) Transfer characteristics (I D –V G ) of the transistor measured at various V D biases. Inset, transfer characteristics plotted in semi-logarithm scale with V D =1 V. (d) Output (I D –V D ) characteristics of the same device measured at various V G biases from −20 to 20 V. Full size image