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Growth of Horizontal Semiconducting SWNT Arrays with Density Higher than 100 tubes/µm using Ethanol/Methane CVD Lixing Kang, Shuchen Zhang, Qingwen Li, and Jin Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03527 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016
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Journal of the American Chemical Society
Growth of Horizontal Semiconducting SWNT Arrays with Density Higher than 100 tubes/µm using Ethanol/Methane CVD Lixing Kang,†,‡,§ Shuchen Zhang,† Qingwen Li,‡ Jin Zhang*,† †
Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡
Division of Advanced Nanomaterials, Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China §
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
Supporting Information Placeholder ABSTRACT: Horizontally aligned semiconducting singlewalled carbon nanotube (s-SWNT) arrays with certain density are highly desirable for future electronic devices. However, simultaneously obtaining s-SWNT arrays with high purity and high density is extremely challenging. We reported herein a rational approach, using ethanol/methane chemical vapor deposition (CVD), to grow SWNT arrays with the sSWNTs ratio over 91% and with the density higher than 100 tubes/μm. In this approach, under certain temperature, ethanol was fully thermal decomposed to feed carbon atoms for Trojan-Mo catalysts growing high density SWNT arrays. While the incomplete pyrolysis of methane provided appropriate active H radicals with the help of catalytic sapphire surface to inhibit metallic SWNTs (m-SWNTs) growth. The synergistic effect in ethanol/methane mixtures resulted in semiconducting SWNTs enriched and no obvious decrease in nanotube density due to their milder reactivity and higher controllability at suitable growth condition. This work represents a step forward in large-area synthesis of high density sSWNT arrays on substrates and demonstrates potential applications in scalable carbon nanotube electronics.
Nowadays silicon transistors are approaching a bottleneck 1 with the devices becoming smaller. Horizontally aligned single-walled carbon nanotubes (SWNTs) have been considered as one of the most viable candidates for the future transistor technology due to their exceptional device 2 performance. To realize SWNTs based transistor applications, the density of SWNT arrays would need to be more than 125 tubes/μm and the impurity concentration of 3 metallic SWNTs (m-SWNTs) less than 0.0001%. Currently, direct growth of high-quality and aligned SWNTs on substrates by chemical vapor deposition (CVD) is a promising strategy to meet the requirement of highperformance devices. The past few decades, notable progress has been made in 4,5 controllable synthesis of SWNT arrays using CVD method. 6 As for the density, through sequentially patterning catalysts, 7 8 multiple-cycle growth and periodic growth, 10-60 tubes/μm aligned SWNTs were obtained on quartz substrates. Recently, our group developed a new designed catalyst called Trojan
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(Fe) and Trojan-Mo (Fe/Mo) catalyst, which realized over 130 tubes/μm on sapphire surface. However, these approaches show no semiconducting (s-) selectivity. In order to increase s-SWNT percent during growth, the most widely used technique was introducing etching species including 11 12 13 14 water, methanol, isopropyl alcohol and hydrogen to remove m-SWNTs. Accordingly, 84−97% semiconducting nanotubes were obtained in the arrays. However, these strategies severely decrease the density of obtained s-SWNT arrays and general results are not exceeding 10 tubes/μm. Based on Trojan-Mo catalyst, we also made an effort to use the usual etchants mentioned above to obtain high density sSWNTs. However, the density and the selectivity also couldn’t coexist. When using water or methanol as etchant, the selectivity was high, while the density decreased significantly. As for using isopropyl alcohol as carbon feedstock, the density had no obvious change, but also no selectivity. And varying the hydrogen flow, there was a decline in the density but no changes in the selectivity. These attempts indicated the very narrow growth window and parameter space of these usual etchants. Thus, how to choose more suitable etchant in our Trojan-Mo system is a key issue for balancing the density and the selectivity. In the CVD system, the carbon precursor is an important factor affecting the properties of the produced SWNT arrays. Methane, as one of the most common used precursors for growing SWNTs, is more stable than other carbon sources like ethanol, ethylene, and acetylene due to its stronger C-H 15 bond. The pyrolysis of methane for synthesis of SWNTs usually requires above 900℃ to become thermodynamically 16,17 allowed. When the temperature decreases to around 800℃ (the normal SWNT growth condition using ethanol), methane fails to supply carbon species to SWNT formation. Whereas, in the presence of metal catalysts or substrate material catalyzing, methane adsorbs and dissociates on their surface. Then some carbon depositions, H radicals and other hydrocarbon are produced from the incompletely 18 decomposition of methane. The activated H radicals from methane plasma or hyperthermal hydrogen (e.g., >1000℃) had been confirmed to selectively react with metallic 19,20 nanotubes through hydrogenation reaction. However, these methane plasma or hyperthermal hydrogen are highly reactive and hard to control, which was always accompanied with a diminished yield of s-SWNTs. By contrast, the
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dissociated H radicals from the catalytic cracking of methane under certain temperature was much milder and more controllable. Therefore, it seems possible to increase s-SWNT percent and maintain high density in the SWNT arrays by introducing appropriate amounts of methane during the CVD process. In this study, we provided a reliable method that combining “Trojan-Mo catalysts” with ethanol/methane CVD, simultaneously achieved high density and high semiconducting selectivity SWNT arrays over large areas. Figure 1a schematically illustrated the procedure for this method. Under certain temperature, ethanol was fully thermal decomposed to feed carbon atoms for Trojan-Mo catalysts growing high density SWNT arrays. While methane was catalytic cracked to provided appropriate active H radicals on the substrate to inhibit m-SWNT growth. Ethanol and methane cooperated to achieve both high density and high selectivity in SWNT arrays.
a)
C2H5OH (thermal decomposed)
CH4 (catalytic cracked ) High Density
& High Selectivity
s-SWNT
H radicals inhibiting m-SWNT growth
Trojan-Mo catalyst
Methane radical
Hydrogen radical
Amorphous carbon
Oxygen atom
Aluminium atom
b)
c) 200 nm
1 mm
100 µm e)
d) 100 nm
1 µm
500nm
Figure 1. (a) Schematic illustration of selective growth of high density s-SWNTs by ethanol/methane CVD. (b-c) SEM images of the as grown SWNT arrays. (d) HRTEM images of the SWNT arrays transferred onto the ultrathin carbon membrane supported on a copper grid. (e) Typical AFM image of the SWNT arrays. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images presented in Figure 1b-d show typical results using an ethanol/methane mixture (50 sccm Ar through an ethanol bubbler and 100 sccm methane) as carbon source. The entire
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sapphire substrate were uniformly covered with high density SWNT arrays (see Figure S1). More than 100 tubes per micron in certain regions were as shown in the high multiplying SEM image (inset in Figure 1c) and TEM image (inset in Figure 1d). But at present, it still meets a big challenge to obtain larger area (wafer scale) with well-distributed density as high as 100 tubes/μm and sub-micron uniformity using ethanol/methane CVD. The atomic force microscopy (AFM) image (Figure 1e) showed a dense array of parallel SWNTs with the main diameter distribution from 0.8 to 1.6 nm (Figure S2). To characterize electronic properties of the as-grown SWNT arrays, we combined three methods including multiple excitation lasers, ultraviolet/visible/near-infrared (UV-Vis-NIR) absorption spectrum and electrical measurements. Raman line mapping with different lasers at the same place were executed to calculate the abundance of semiconducting tubes in the SWNTs array. In Figure 2a, typical radial breathing mode (RBM) of the Raman spectra with 488 nm excitation laser showed that s-SWNT peaks were the overwhelming majority. The statistics of the RBM in the Raman measurements of multi-batch samples (Figure 2b) gave an estimate of 94.6% based on the detected tubes. Other excitation lasers (wavelengths of 514, 633 and 785 nm) representative test results were shown in Figure S3a-c. In addition, the corresponding tangential vibration (G mode) −1 bands at ∼1580 cm did not exhibit the Breit-Wigner-Fano (BWF) line shapes (see Figure S3d), a typical feature of metallic SWNTs, further confirming the semiconducting 21 characteristic of the nanotubes. Meanwhile, unsharp D bands indicated the synthesized s-SWNTs were high quality (Figure S3d). Due to the limitation of Raman measurements, only parts of SWNTs in the arrays resonate with the discrete excitation 21 wavelengths. UV-Vis-NIR absorption spectrograph was a 22 powerful tool to evaluate the abundance of s-SWNTs. However, the low duty ratio of horizontally aligned SWNT arrays on sapphire surface (maximum value
