Water-Assisted Preparation of High-Purity Semiconducting (14,4

Nov 14, 2016 - Up to date, the highest reported content of s-SWNTs as-grown is ∼97%, which is still far below the daunting requirements of high-end ...
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Water-Assisted Preparation of High-Purity Semiconducting (14,4) Carbon Nanotubes Feng Yang,† Xiao Wang,† Jia Si,∥ Xiulan Zhao,† Kuo Qi,‡ Chuanhong Jin,§ Zeyao Zhang,† Meihui Li,† Daqi Zhang,† Juan Yang,† Zhiyong Zhang,∥ Zhi Xu,‡ Lian-Mao Peng,∥ Xuedong Bai,‡ and Yan Li*,† †

Beijing National Laboratory for Molecular Science, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering and ∥Department of Electronics, Peking University, Beijing 100871, China ‡ Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Semiconducting single-walled carbon nanotubes (s-SWNTs) with diameters of 1.0−1.5 nm (with similar bandgap to crystalline silicon) are highly desired for nanoelectronics. Up to date, the highest reported content of s-SWNTs as-grown is ∼97%, which is still far below the daunting requirements of high-end applications. Herein, we report a feasible and green pathway to use H2O vapor to modulate the structure of the intermetallic W6Co7 nanocrystals. By using the resultant W6Co7 nanocatalysts with a high percentage of (1 0 10) planes as structural templates, we realized the direct growth of s-SWNT with the purity of ∼99%, in which ∼97% is (14,4) tubes (diameter 1.29 nm). H2O can also act as an environmentally friendly and facile etchant for eliminating metallic SWNTs, and the content of s-SWNTs was further improved to 99.8% and (14,4) tubes to 98.6%. High purity s-SWNTs with even bandgap determined by their uniform structure can be used for the exquisite applications in different fields. KEYWORDS: carbon nanotube, chirality, semiconducting, chemical vapor deposition, intermetallic catalysts

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approach the daunting requirements of purity for applications such as low-power digital circuits, which may be as high as 99.9999%.3 Another possible route is to realize the selective growth of sSWNTs by using well-controlled catalysts.30,31 Recently, sSWNTs with the purity of 95% were synthesized with Co nanocatalysts partially passivated by carbon under a wellcontrolled CVD condition with suitable H2 supply.32 The growth of SWNTs with enriched semiconducting species such as (6,5), (7,6), (8,4), and (9,8) has also been reported, respectively.33−37 Tuning the composition and the size of the catalysts is essential in those processes. We have developed a method to grow SWNTs with specified chirality by using intermetallic compounds as catalysts, which act as the structural templates for the epitaxial growth of SWNTs.38 This strategy offers serious possibility for growing specific s-SWNT if choosing the catalysts with the right structure. In order to grow s-SWNTs with a specific (n,m), first we need to prepare W6Co7 nanocatalysts presenting a high abundance of desired crystal planes to match the end structure of this chirality.39,40 However, the preparation of nanosized intermetallic compounds is seldom reported, and there is still

tructurally uniform semiconducting single-walled carbon nanotubes (s-SWNTs), which therefore possess identical band structure, are needed for high-end applications in nanoelectronics,1−5 high-resolution multicolor biological imaging,6,7 and high efficiency photovoltaic process.8−10 Great efforts have been devoted to the selective preparation of high purity s-SWNTs. Several aqueous phase separation processes, including dielectrophoresis,11 ultracentrifugation,12 chromatography,13 gel filtration,14 and aqueous two-phase separation15 have been developed and show great success in obtaining chirality-pure s-SWNTs. In addition, relying on the higher activity of metallic SWNTs (m-SWNTs), s-SWNTs on the substrates were retained by removing m-SWNTs via chemical etching16,17 and electrical18−20 or microwave21 breakdown. On the other hand, the direct growth of s-SWNTs is still a great challenge. The most widely used strategy is introducing an oxidative environment by employing water vapor,22 methanol,23 oxygen,24,25 or UV light26 during the chemical vapor deposition (CVD) process to inhibit the growth of mSWNTs, which have lower ionization energies than s-SWNTs. However, to date, the highest reported content of s-SWNTs asgrown by this method is ∼97%.22,27 Because of the complexity in the reactivity of SWNTs, which depends on not only metallicity but also diameters,22,28,29 it is very difficult to achieve ultrapure s-SWNTs with this method. Some new pathways for growing s-SWNTs are highly demanded to © 2016 American Chemical Society

Received: October 12, 2016 Accepted: November 14, 2016 Published: November 14, 2016 186

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of W6Co7 (Figure 1b). The high-angle annular dark-field scanning TEM (HAADF-STEM) corresponding with fast Fourier transform (FFT) pattern and inverse FFT further identified that both the lattice fringes and dihedral angles of the nanocrystal are consistent with the rhombohedral structure of intermetallic W6Co7 (Figure 1c−f). It was reported that the H2O absorption may lower the surface energy of all the exposed Cu facets. The surface energy of facets with higher density of atoms will be affected more, and consequently, the content of these facets will be effectively increased.44 Comparing with products reduced by pure H2 in our previous study,39 the W6Co7 catalyst prepared by H2/H2O at the same temperature (1050 °C) has a remarkably higher ratio of (1 0 10)/(1 1 6) planes. The (1 0 10) plane has a higher density of atoms than that of (1 1 6), therefore the addition of H2O benefits the formation of (1 0 10) planes. This might be the mechanism of producing (1 0 10)-enriched W6Co7 nanocatalysts by adding H2O. Density functional theory (DFT) simulations show that the atomic arrangements in (1 0 10) plane of W6Co7 highly match the circumstance of (14,4) SWNT (Figure 1g, h). All carbon atoms are reasonably bonded with the metallic atoms in (1 0 10) plane of the catalyst, and there is no obvious deformation of the cylindrical structure of (14,4) tube. But none of the SWNTs with similar diameters and different chiralities fits the atomic arrangements of (1 0 10) plane (Figure S2). These theoretical results offer possibilities that such catalysts are able to act as the structural templates for the growth of semiconducting (14,4) SWNTs. Growth of (14,4) Carbon Nanotubes and Measurement of Chirality Purity. We performed CVD growth of SWNTs on SiO2/Si substrates using the catalysts prepared with the presence of H2O vapor. The optimized carbon feed at CVD temperature of 1050 °C was 200 cm3·min−1 Ar through an ethanol bubbler kept in an ice−water bath together with 40 cm3·min−1 H2. Figure 2 shows the characterizations of the sample. The as-grown SWNTs show radial breathing mode (RBM) peaks intensively concentrated at ∼189 cm−1 (Figures 2b and S4a, and Video S1). The height profile of the atomic force microscope (AFM) image indicates a uniform tube diameter (dt) of ∼1.3 nm (Figures 2d and S8), which both closely matches the dt value of 1.29 nm calculated from the RBM frequency (eq S1)38 and ∼1.3 nm measured from the HRTEM image (Figure 2e). The corresponding tangential vibrations (G-band) at 1590 cm−1 all exhibit uniform Lorenzian (G+ and G−) line shapes (Figures 2c and S5), which is a typical feature of s-SWNTs.48 A chirality of (14,4) was assigned by comparing with Kataura plot49 (Figure S6). In order to obtain suspended SWNTs for fluorescence measurements, we also performed CVD growth at the same condition with catalysts supported on silica spheres of ∼400 nm in diameter. When excited with 532 nm laser, the fluorescence emission at 0.77 eV was observed, which corresponds to the first (E11) van Hove optical transition energy of (14,4) SWNT (Figure 2g); meanwhile, Raman bands at ∼189 cm−1 also exhibited accordingly. To further verify the chiral assignment of (14,4) SWNT, we performed electron diffraction on the isolated SWNTs grown across the slits of substrates with RBM at ∼189 cm−1. All these tubes show typical electron diffraction patterns indexed to (14,4) tube (Figures 2f and S7). The abundance of (14,4) SWNTs was obtained by several methods. First we performed the micro-Raman measurements in the different randomly chosen nanotube regions and used six

no feasible and reliable approach available to regulate the structure of intermetallic nanocrystals. Nevertheless, people have already accumulated much knowledge on the structure manipulation of normal metallic nanocrystals, which may offer some hints for controlling the structure of intermetallic W6Co7 nanocrystals. In solution processes, the most widely used strategy is using additives as capping agents or etchants to tune the crystal structure and facets exposed.41−43 Similarly, in the gas−solid system, the presence of certain species may change the morphology of nanocrystals.44,45 For example, moisture can induce surface reconstruction and reshaping of Cu nanoparticles. It was proposed that the presence of water vapor changes the surface energy and thereby the exposed facets of the Cu nanocrystals.44 A similar mechanism was also evidenced in Fe catalysts. The morphological variation of Fe nanoparticles was observed when annealed in different noble gases, which was attributed to the difference of water content in the gases.45 Inspired by the above reports, we tried to manipulate the structure of tungsten-based intermetallic nanocrystals by altering the composition of the gaseous phase during the crystallization process. In this work, we used H2O vapor to adjust the gaseous phase environment during the crystallization process and therefore tune the structure of the resultant intermetallic W6Co7 nanocrystals. Using such produced W6Co7 catalysts which contain dramatically higher abundance of (1 0 10) planes, we realized the direct growth of s-SWNT with the purity of ∼99%, in which ∼97% is (14,4) tubes (Scheme 1). Considering H2O Scheme 1. Water-Assisted Modulating the Structure of the Intermetallic W6Co7 Nanocatalysts and the CVD Growth of (14,4) SWNTs

can also act as an etchant for m-SWNTs,17,22 we further improved the purity of s-SWNTs to 99.8% by post-treatment of water vapor etching. The bandgap of (14,4) tube with a diameter of 1.29 nm was determined to be 1.06 eV by theory46 and 0.95 eV experimentally,47 which is very close to 1.1 eV for crystalline Si and should be suitable for high performance electronic devices.2

RESULTS AND DISCUSSION Water-Assisted Structure Regulation of Intermetallic Catalysts. We started from heteropolyacid molecular cluster containing W and Co (denoted as {W39Co6Ox})38 and added H2O vapor to H2 in the temperature-programmed reduction (TPR) process. The products exhibit similar X-ray diffraction (XRD) patterns to the one reduced by pure H2, showing peaks from W6Co7, W, and SiO2 (Figures 1a and S1). This indicates the presence of H2O vapor does not affect the formation of intermetallic compounds. However, the intensity of (1 0 10) diffraction peak of W6Co7 appearing at 41.5° is remarkably higher, revealing the evident increase of the population of (1 0 10) planes. The high-resolution transmission electron microscope (HRTEM) image shows lattice fringes with intervals of 0.22 nm, which are in good accordance with the (1 0 10) plane 187

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Figure 1. Characterization of catalyst structure. (a) XRD patterns obtained on the fresh catalysts reduced by pure H2 (curve I) at 1030 °C and H2/H2O at 1050 °C (curve II). The W6Co7 standard card from the database is also shown. (b) HRTEM image of W−Co catalyst nanoparticles reduced by H2/H2O at 1050 °C. (c−f) Bright-field (c) and HAADF (d) STEM images, corresponding FFT (e) and inverse FFT (f) of a single W6Co7 nanoparticle, spots corresponding to (1 0 10), (1 1 0), and (2 1 10) planes are indicated in (e). (g, h) DFT simulation. Side (g) and top (h) views of interfaces between (14,4) SWNT and the (1 0 10) plane of W6Co7.

catalysts, which is indeed observed by comparing the XRD patterns of the two catalysts (Figure 1a). This observation indicates that the H2O-mediated process is a very effective pathway for structure modulation of tungsten-based intermetallic compounds. The (1 0 10)-W6Co7 catalyzed (14,4) SWNTs growth also can be extended to lower temperatures than 1050 °C. We used the W6Co7 prepared at the typical condition in the presence of H2O at 1050 °C, at which (1 0 10) planes are already formed, but grew (14,4) SWNTs at lower temperatures of 800−1000 °C (Figure 3a). By optimizing the carbon feeding at each growth temperature (Table 1), we obtained (14,4) SWNTs with the purity of 88−93% (based on counting RBM peaks from Raman measurements) (Figure 3b, c). However, the dominant growth of (14,4) tubes at these temperatures was realized at optimized gas flow and ratio of Ar supported ethanol and H2. Comparison of SWNTs Grown on Catalysts Prepared at Different Conditions. We used CO2 instead of H2O, which is also a weak oxidant, during the H2-TPR process to prepare the W6Co7 catalyst at 1050 °C. The products prepared with such condition show enriched (1 0 10) planes too (Figure 4a). The SWNTs grown with this W6Co7 catalyst under the same CVD conditions show a high percentage of (14,4) species as expected. The content of (14,4) SWNTs was estimated from micro-Raman spectra based on 436 RBM peaks to be 93.3% (Figures 4b, S3, S4b and Table S3). We performed the control experiment in which W6Co7 catalyst obtained by reduction with pure H2 first and annealed in H2/H2O subsequently. By normalizing the XRD peaks with respect to the intensity of (1 1 0) diffraction, it is found that this catalyst shows an obvious weaker (1 0 10) diffraction peak than that directly reduced by H2/H2O (Figure 4c). This result reveals that the presence of H2O during the reduction and crystallization procedure is essential in adjusting the structure of W6Co7. We performed CVD growth at the same condition

excitation wavelengths to detect the same regions for the quantification of (14,4) SWNTs. From the statistics based on 660 RBMs (originated from ∼2500 tubes estimated by the tube density), it was estimated to be 97.4% counting in the coverage of excitation wavelengths (Figure 2i, Tables S1 and S2). The content of s-SWNTs was also obtained by RBM counting to be 98.9%. The as-grown SWNTs were removed from the SiO2/Si substrates and dispersed into the aqueous solution for UV−vis near-infrared (UV−vis-NIR) absorption measurements, and eventually 54 as-grown SWNT samples were collected. Only one absorption peak appears at 2.2 eV. It is assigned to the third (E33) optical transition of (14,4) tube. The intensity of this peak increases with the increase of the number of samples collected (Figures 2h and S9). Though it shows the purity of (14,4) tubes is really high, we could not give a reasonable quantification on the content of (14,4) tubes because the absorptions of tubes with other chiralities are too weak to be detected. Then we deposited nanotubes in the aqueous solution onto the silicon substrate with land markers to identify each tube by SEM and performed Raman measurement of this tube subsequently (Figure S10a−c). Out of 52 investigated tubes, 50 nanotubes were assigned to (14,4). In addition, by using AFM to count all tubes and using Raman to detect all (14,4) tubes, the fraction of (14,4) tube was found to be ∼96% (Figure S10d, e and Table S4). Characterizations of UV−vis-NIR absorption, SEM-Raman, and AFM-Raman all support the result from RBM counting. It is noticed that the purity of (14,4) tubes grown in this work is obviously higher than that of (12,6) tubes previously reported as 92%.38 It has been demonstrated both theoretically50,51 and experimentally52,53 that the growth of tubes with higher chiral angles is kinetically favorable. The chiral angle of (14,4) tube (12.2°) is smaller than that of (12,6) tube (19.1°). The higher selectivity of (14,4) tubes here can only be attributed to the higher degree of enrichment of (1 0 10) planes in this catalysts than that of (0 0 12) planes in the previous 188

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Figure 2. Characterizations of SWNTs grown at 1050 °C. (a) SEM image of SWNTs. (b, c) RBM (b) and G-band (c) region Raman spectra of the SWNTs taking with 1 and 5 μm laser spot, respectively. (d, e) AFM (d) and HRTEM (e) images of SWNTs. (f) Electron diffraction pattern companied by the simulated pattern of a (14,4) SWNT. (g) Photoluminescence spectra of (14,4) SWNTs. (h) UV−vis-NIR absorption spectrum (background subtracted) (open circle) and fitting spectrum (red solid line). (i) Relative abundances of various chiralities from Raman measurements based on ∼2500 tubes denoted in the partial SWNT chiral map.

Figure 3. (a) Scheme showing the growth process. (b, c) Quantification of (14,4) SWNTs grown at different temperatures: Raman spectra (excitation: 532 nm) (b) and the abundance of (14,4) SWNTs grown at 800−1050 °C estimated from Raman measurements (c).

Table 1. Optimized Carbon Feeding Stock Conditions at Different Growth Temperatures growth temperature (°C)

800

850

900

950

1000

Ar-ethanol (ice−water bath)/H2 (cm3·min−1)

300/20

300/25

300/30

240/30

200/35

Further Improvement of s-SWNT Purity by Water Treatment. It has been reported that SWNT samples with high semiconducting selectivity can be obtained by oxidizing metallic tubes, which are more reactive.22−25 Then we treated the as-grown samples with water vapor at 550 °C to remove the

using this catalyst. No selectivity to (14,4) tubes was observed (Figure 4d). All the above comparing experiments further verify the importance of the enrichment of (1 0 10) planes in the catalysts for the selective growth of (14,4) tubes. 189

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Figure 4. (a, b) XRD pattern of W6Co7 catalysts prepared by CO2/H2 (a) and corresponding Raman spectra of as-grown SWNTs (b). (c, d) XRD pattern of W6Co7 catalysts reduced by H2/H2O from 800 to 1050 °C first and then annealed in the H2/H2O at 1050 °C (c) and corresponding Raman spectra of as-grown SWNTs (d). RBM regions that correspond to (14,4) tube are indicated.

Figure 5. (a−e) Multiwavelength Raman spectra of SWNTs after the water vapor treatment. M-SWNTs denoted as M and s-SWNTs denoted as S. (f) The abundance of (14,4) SWNTs and semiconducting SWNTs from Raman measurements based on counting 721 RBM peaks (∼2120 tubes) after the water vapor treatment.

abundance of s-SWNTs was increased to 99.8 ± 0.1% (Figures 5 and S12 and Table S6). We carefully studied 50 G-band region Raman spectra of randomly chosen SWNTs. All the Gbands observed appear at 1590 cm−1 and exhibit typical feature of s-SWNT with the symmetric Lorenzian line shapes. Field-Effect Transistors (FETs) of as-Grown (14,4) SWNTs. We fabricated back-gated FET devices using W6Co7-

m-SWNTs. The as-grown samples contain 96.4 ± 0.4% (14,4) tubes and 98.7 ± 0.3% s-SWNTs according to Raman measurements (Table S5). Based on the statistics of three parallel samples with 721 RBMs (originated from ∼2120 tubes estimated by the tube density) in Raman measurements under five excitation wavelengths, it was found that the purity of (14,4) tubes was further improved to 98.6 ± 0.3% and the 190

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water bubbler (ice−water bath) and another flow of pure hydrogen (200 cm3·min−1) were introduced to reduce the calcined catalyst precursors using a TPR method from 800 to 1050 °C for 4 min. Before the tube growth, a flow of H2 (200 cm3·min−1) was introduced to remove the water vapor. Afterward, a flow of Ar (200−300 cm3· min−1) through an ethanol bubbler and H2 (20−50 cm3·min−1) was introduced into the system to grow SWNTs for 10 min. Last, the system was cooled under the atmosphere of H2 and Ar, respectively. We also used the catalysts prepared at 1050 °C under the typical procedure to perform the CVD growth at temperatures of 800−1000 °C with different optimized carbon feeding conditions. The optimum carbon feeding conditions at different temperatures are shown in Table 1. We also used CO2 instead of H2O vapor to prepare the W6Co7 catalysts. The calcined WCoO precursor was reduced by H2 (200 cm3· min−1) mixed with CO2 (8 cm3·min−1) from 800 to 1050 °C for 4 min. We prepared another W6Co7 catalyst for comparison. The calcined WCoO precursor was first reduced by 200 cm3·min−1 pure H2 using the TPR method from 800 to 1050 °C and then annealed by introducing 200 cm3·min−1 H2 and another 80 cm3·min−1 H2 through a water bubbler (ice−water bath) at 1050 °C for 3 min. Then the SWNTs were grown at exactly the same condition and procedure as that described above. Water Vapor Treatment of SWNTs. A flow of H2 (150 cm3· min−1) through the water bubbler vapor (ice−water bath) mixed with Ar (300 cm3·min−1) was introduced to etch the SWNTs at 550−580 °C for 30 min. Catalyst Characterization. SiO2 microspheres (diameter: ∼400 nm) were used instead of silicon wafers as the catalyst support to prepare catalysts for XRD characterization. The XRD measurements were carried out on a DMAX-2400 X-ray diffractometer using a monochromated Cu-Kα radiation (λ = 0.154 nm, 60 kV, 200 mA). The Si3N4 thin film (thickness: 5 nm) was used as the support for preparing W−Co catalysts and directly applied for HRTEM characterization on a FEI Tecnai F20 microscope operated at 200 kV. HAADF-STEM characterization was taken on a FEI Titan G2 80− 200 ChemiSTEM operated at 200 kV. General Characterization and Chirality Assignment. SEM images of SWNTs were obtained on a Hitachi S4800 SEM operated at 1.0 kV. The Raman spectra of the as-grown SWNTs were collected with four Jovin Yvon-Horiba LabRam systems: an ARAMIS system for 532, 633, and 785 nm laser excitations, a HR800 system for 488 nm, a HR800 system for 514 nm, and another HR800 system for 473 nm. We typically chose the laser beam spot of 1 μm in diameter to collect the Raman signal of the SWNTs. For each sample, more than 600 RBM peaks were collected. Every measurement was performed by moving the laser spot at a step of 1 μm. The Raman spectra at different excitation wavelength were collected from the same regions. Atomic force microscopy (AFM, Veeco diMutiMode V, operated at tappingmode) was used to measure the diameter of SWNTs and count the number of SWNTs. The photoluminescence spectra of the as-grown suspended SWNTs were collected with Jovin Yvon-Horiba LabRam system, an ARAMIS system for 532 nm laser excitation equipped with the InGaAs detector. The Raman measurement was performed simultaneously. The ED measurements were performed on the directly grown suspended SWNTs across the slits (2−50 μm in width and 0.4 mm in length) of the SiO2/Si substrates or holes (1 μm in diameter) of the Si3N4 membrane at 1050 °C under the same CVD conditions as on the normal SiO2/Si substrates. A JEOL JEM2010F microscope operated at 200 kV was used, and the diffraction patterns were recorded with a high-resolution two-dimensional charge-coupled device (CCD) array. The (n,m) of an individually suspended SWNT was determined from ED patterns with a calibration-free intrinsic layer line-spacing method.54 Quantification of Chirality Purity of SWNTs. The as-grown sample was measured by counting the RBM peaks. After Raman measurements, the SWNTs were dispersed in aqueous solution, and the UV−vis-NIR absorption was performed to get the chirality composition. 54 pieces (size ∼1 cm2) of SiO2/Si wafer with SWNTs

catalyzed (14,4) SWNTs. The FETs based on single SWNT and multiple SWNTs with channel length of 1 μm show ratios of on to off current at around 103−106, indicating the typical semiconducting performance (Figure 6).

Figure 6. (a) SEM images of an individual SWNT device with a channel length of 1 μm. (b) The corresponding I−V curves of SWNT transistors. (c) SEM image of a multiple SWNTs device with a channel length of 1 μm. (d) The corresponding I−V curve of multiple SWNTs transistor. Scale bars: 2 μm.

CONCLUSIONS The presence of H2O during the reduction and crystallization procedure of intermetallic W6Co7 nanocrystals may change the nucleation and growth rates of different planes, which eventually leads to the enrichment of the (1 0 10) planes with higher density of atoms. Using such-produced (1 0 10)W6Co7 catalysts, we have realized direct growth of (14,4) SWNTs with the purity of ∼97% and s-SWNTs of ∼99%, which is the highest purity of s-SWNTs ever reported in asgrown samples. In addition, by further removing m-SWNTs with facile H2O vapor etching, the purity of s-SWNTs was eventually improved to 99.8%. Our s-SWNT samples also have the advantage of presenting uniform band structure, which is favorable for high-end applications. The strategy we proposed by using catalysts with well-designed structures has shown to be a powerful and prospective pathway toward the direct growth of pure s-SWNTs. METHODS Catalyst Preparation and SWNT Growth. The molecular cluster Na15[Na3⊂{Co(H2O)4}6{WO(H2O)}3(P2W12O48)3]·nH2O (denoted as {W39Co6Ox}) used as the catalyst precursor is the same as that used in our previous work.38 The SWNT growth was performed in a quartz reactor with the inner diameter of 2.1 cm by using the ethanol CVD method. Silicon wafers with a thermally grown 400 nm-thick SiO2 layer (denoted as SiO2/Si) were used as the substrates for the SWNT growth. The typical growth processes are described as follows. The precursor solution was dropped onto the SiO2/Si substrates and calcined at 700 °C in the tube furnace in air for 3 min. After the system was purged with Ar, a flow of hydrogen (80 cm3·min−1) through a 191

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ACS Nano grown at 1050 °C were collected into the aqueous solution for detection. Then the aqueous dispersion of SWNTs was deposited on silicon wafers with landmarks for further characterization with SEM, AFM, and Raman. The content of (14,4) tubes was obtained by counting the total number of SWNTs with SEM or AFM (for counting the number of the tubes) and number of (14,4) tubes with Raman. Theoretical Simulation. The self-consistent DFT simulations were carried out with the Vienna ab initio Simulation Package (VASP) in combination with generalized gradient approximation (GGA) to fit SWNT to the (1 0 10) plane of a W6Co7 nanocrystal. The simulation method is the same as that used in our previous work.38 We chose different (9,9), (12,6), (16,0), and (14,4) SWNTs with similar diameters (∼1.3 nm) to fit around the (1 0 10) plane of W6Co7. One end of SWNT is passivated by hydrogen atoms. Fabrication of SWNT FET Devices. Back-gated FET devices with channel length of 1 μm were patterned by e-beam lithography on a Si substrate covered with 400 nm SiO2 layer. 40 nm Pd was deposited by e-beam evaporation as the contact metal, followed by a standard lift-off process. Surrounding carbon nanotubes of the FET devices were etched by a reactive ion etching (RIE, oxygen) system to prevent electric leakage during the measurements.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06890. Details of XRD, TEM measurements, DFT simulations, SEM, Raman, AFM, photoluminescence spectroscopy, electron diffraction, UV−vis-NIR absorption characterizations of the SWNTs, including Tables S1−S6, eqs S1 and S2, Figures S1−S12 (PDF) Video S1: as-grown SWNTs showing RBM peaks (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Juan Yang: 0000-0001-5502-9351 Zhiyong Zhang: 0000-0003-1622-3447 Yan Li: 0000-0002-3828-8340 Notes

The authors declare the following competing financial interest(s): Y.L. and F.Y. declare a financial interest: patents related to this research have been filed by Peking University. The University's policy is to share financial rewards from the exploitation of patents with the inventors. The remaining authors declare no competing financial interests.

ACKNOWLEDGMENTS We thank S. Wang, W. Wu, and K. Jiang for the help of Raman and providing Si3N4 specimens for TEM, H. Zhang for the valuable discussions of crystal structure, and R. Li, F. Peng, and H. Sun for the help with AFM. This research is financially supported by Ministry of Science and Technology of China (2016YFA0201904), National Natural Science Foundation of China (grants 21631002, U1632119, and 91333105). REFERENCES (1) Avouris, P.; Chen, Z.; Perebeinos, V. Carbon-Based Electronics. Nat. Nanotechnol. 2007, 2, 605−615. (2) Koswatta, S. O.; Valdes-Garcia, A.; Steiner, M. B.; Lin, Y.-M.; Avouris, P. Ultimate RF Performance Potential of Carbon Electronics. IEEE Trans. Microwave Theory Tech. 2011, 59, 2739−2750. 192

DOI: 10.1021/acsnano.6b06890 ACS Nano 2017, 11, 186−193

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