Growth of Semiconducting Single-Walled Carbon Nanotubes by Using

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Growth of Semiconducting Single-Walled Carbon Nanotubes by Using Ceria as Catalyst Supports Xiaojun Qin,†,‡,§ Fei Peng,†,‡,§ Feng Yang,†,‡,§ Xiaohui He,‡ Huixin Huang,† Da Luo,†,‡,§ Juan Yang,†,‡,§ Sheng Wang,† Haichao Liu,‡ Lianmao Peng,† and Yan Li*,†,‡,§ †

Key Laboratory for the Physics and Chemistry of Nanodevices, ‡Beijing National Laboratory for Molecular Science, College of Chemistry and Molecular Engineering, and §State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The growth of semiconducting single-walled carbon nanotubes (s-SWNTs) on flat substrates is essential for the application of SWNTs in electronic and optoelectronic devices. We developed a flexible strategy to selectively grow sSWNTs on silicon substrates using a ceria-supported iron or cobalt catalysts. Ceria, which stores active oxygen, plays a crucial role in the selective growth process by inhibiting the formation of metallic SWNTs via oxidation. The so-produced ultralong sSWNT arrays are immediately ready for building field effect transistors. KEYWORDS: Single-walled carbon nanotubes, semiconducting, ceria, catalyst supports, selective growth

S

More reliable and feasible strategies to acquire s-SWNTs of high purity are expected. If the proposed mechanism is true, the maintenance of a suitable oxidative environment during the tube growth process should be a key factor for producing high purity s-SWNTs. Using oxidative catalyst supports to obtain a steady oxidative environment may be a possible solution for this concern. Cerium, a well-known rare earth element, has two main oxidation states of +3 and +4. Ceria, CeO2, has a high oxygen storage capacity (OSC) because oxygen vacancies can be rapidly created or eliminated on its surface.24−27 Therefore, ceria has found wide applications in automotive exhaust treatments, petrochemical processes, and solid oxide fuel cells (SOFCs).24,28−30 CeO2 also has superior thermal stability. When used in the so-called “three-way catalysts” for automotive exhaust treatments and as electrode materials in SOFCs, the working temperature is normally higher than 500 °C, sometimes even at ∼1000 °C. This indicates that CeO2 might be used as the catalyst support for the CVD growth of SWNTs, which is normally performed at 800−1000 °C. In this research, we tried to apply ceria as the oxidative catalyst support for the selective growth of s-SWNTs by preventing the formation of m-SWNTs. We intend to utilize the unique OSC property of ceria to obtain a feasible oxidative environment during the tube growth process to ensure the reliable elimination of m-SWNTs and thus to produce sSWNTs with high purity. This system also provides us a convenient platform to study the mechanism of selective growth.

ingle-walled carbon nanotubes (SWNTs) can be either metallic or semiconducting depending on their structures.1 Semiconducting SWNTs (s-SWNTs) are used as materials for field effect transistors (FETs)2 and optoelectronic devices.3,4 However, SWNTs are normally synthesized as mixtures of tubes with different structures. In the past decade, great efforts have exerted on obtaining pure s-SWNTs. Various separation processes carried out in aqueous solutions have shown to be very reliable to acquire high purity s-SWNTs.2,5−9 Yet, in order to ensure the high performance of the devices, preparing sSWNTs directly on insulating substrates that are immediately ready for building nanodevices10−13 are highly preferable. A few methods were developed to eliminate metallic SWNTs (mSWNTs) from the SWNT samples grown on substrates in the post-treatment stage.14−16 However, the direct growth of sSWNTs on substrates by chemical vapor deposition (CVD) is a more straightforward and flexible strategy. It is believed that m-SWNTs are more reactive than sSWNTs due to their electronic structures.17−19 Because of the lower ionization energy, the metallic tubes are expected to be oxidized more easily. When methanol was used together with ethanol as carbon feed stocks, s-SWNTs with the content higher than 95% were prepared on quartz.12 It was guessed that the •OH radicals, which were produced from the decomposition of methanol at high temperature, destroyed the formation of m-SWNTs and thus s-SWNTs were obtained. sSWNT-enriched SWNT samples have also been prepared by adding other additives including oxygen20 and water21 or by introducing UV-light22 during CVD growth, as well as by using isopropyl alcohol as carbon feed stock.23 The aforementioned examples proposed a mechanism of destroying the m-SWNTs by certain gaseous-phase oxidative attackers.21 However, some convincing evidence is still needed to verify this mechanism. © 2014 American Chemical Society

Received: September 20, 2013 Revised: December 7, 2013 Published: January 1, 2014 512

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Figure 1. (a−c) Raman spectra of SWNTs synthesized with unreduced Fe/CeO2 measured with 532 nm laser (a) and 633 nm laser (b) and Fe/SiO2 measured by 532 nm laser (c), respectively. (d,e) Statistics of m-SWNTs (denoted as M) and s-SWNTs (denoted as S) by using Fe/CeO2 reduced with H2 for 0 and 10 min and Fe/SiO2 measured with 532 nm laser (d) and 633 nm laser (e), respectively. (f) Photoluminescence (PL) spectra of SWNTs synthesized by using unreduced Fe/CeO2 excited with 532 nm laser. The broad peaks marked with star (*) correspond to the PL spectra of silicon.

shows statistics of m-SWNTs and s-SWNTs under different growth conditions measured with 532 and 633 nm lasers, respectively. When directly using catalyst precursors without prereduction for CVD growth, the abundance of s-SWNTs is ∼96% for samples synthesized with ceria supported catalysts in comparison to ∼72% for silica supported catalysts measured by 532 nm laser. When detected with 633 nm laser, the corresponding abundances are ∼100 versus 58%, respectively, that is, the average percentage of s-SWNTs is 98% for SWNT samples produced with Fe/CeO2 and 65% for samples produced with Fe/SiO2. The above result quantitatively shows that using ceria as catalyst supports can remarkably increase the ratio of s-SWNTs as we expected. When using methanol to obtain an oxidative environment, selectivity of sSWNTs was only observed on quartz substrates and was invalid when using silicon wafers as substrates.12 Principally, using ceria as oxidative catalyst supports to inhibit the formation of mSWNTs presents no substrate-dependence. Therefore, our strategy is much more flexible. Then we carefully compared the G and D bands of SWNTs grown with Fe/CeO2 and Fe/SiO2. It can be found from Figure 2 that SWNTs resultant from Fe/CeO2 do not show BWF band and SWNTs grown with Fe/SiO2 sometimes show obvious BWF bands, which is a typical character of m-SWNTs. What’s more intersting, SWNTs grown with Fe/CeO2 show much weaker D bands than those grown with Fe/SiO2. This indicates that the samples produced with Fe/CeO2 contain less amorphous carbon and defects. It is known that amorphous carbon and defective SWNTs are less stable than well-graphited SWNTs. Therefore they may be oxidized by CeO2 during the CVD process and eventually SWNTs with higher purity and well-graphited structure are obtained. In a conventional SWNT growth process, the catalyst precursors are normally calcined in air and then prereduced with H2 before the carbon feed stocks are introduced. However, in our case the reduction procedure may have a negative effect on the selective growth of SWNTs. In the OSC measurements

It can be envisioned that the tight contact of catalyst nanoparticles with ceria is essential to utilize the oxidizability of ceria. Hence, the lower volatility of the catalyst is preferred. Taking this into consideration, the metals with lower boiling point and more mobile on the substrates such as Cu,31 is not suitable. Therefore, Fe and Co were selected as the catalysts in this study.10,31 The as-prepared ceria by hydrothermal method has an OSC of 310 μmol O g−1. The catalyst precursors of ceria supported iron (Fe/CeO2) or cobalt (Co/CeO2) were prepared by impregnation method with the initial Ce/Fe (or Co) molar ratio of 3:1. The catalyst precursors were put onto silicon wafers and performed CVD with CH4 as carbon feed stocks at 950 °C typically for 15 min. SWNTs were grown with silica supported iron (Fe/SiO2) under exactly the same condition as control samples (see more experimental details in Supporting Information). Scanning electron microscope (SEM) image shows that almost all of the SWNTs were originated from the ceria particles (Supporting Information Figure S2). We characterized the structure of the produced SWNTs by Raman spectroscopy. Using Kataura Plot, the chirality of SWNTs can be assigned based on the radial breathing mode (RBM) band frequencies and the excitation wavelength.32−34 For excitation laser of 532 nm (2.33 eV), SWNTs with RBMs between 100−119 and 207−275 cm−1 are considered to be metallic tubes while SWNTs with RBMs between 119−207 cm−1 are semiconducting tubes. For 633 nm (1.96 eV) excitation, SWNTs with RBMs from 177 to 221 cm−1 are assigned to metallic tubes, and other RBMs appearing at 100 to 275 cm−1 belong to semiconducting ones. As shown in Figure 1a−c and Supporting Information Figure S4, it is obvious that m-SWNTs can hardly be detected for SWNTs grown with Fe/ CeO2, distinctly different from the randomly distributed RBMs of SWNTs grown with Fe/SiO2. The synthesized s-SWNTenriched samples exhibit stronger fluorescence (Figure 1f), indicating that there is a much higher content of s-SWNTs and less impurities in Fe/CeO2 catalyzed samples. Figure 1d,e 513

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Because the CVD process was carried out in the atmosphere of CH4 and H2, the OSC of ceria should be decreased when prolonging the reaction time. Hence the selectivity may be also degraded. As shown in Figure 4 and Supporting Information

Figure 4. (a,b) Raman spectra of SWNTs synthesized with unreduced Fe/CeO2 at growth time of 5 min (a) and 60 min (b) measured by 633 nm laser, respectively. (c,d) Statistics of m-SWNTs (denoted as M) and s-SWNTs (denoted as S) corresponding to (a,b), respectively.

Figure 2. Comparison of the G and D bands between SWNTs grown with Fe/CeO2 (a) and Fe/SiO2 (b). The peaks marked with “*” come from the silicon substrate. The excitation wavelength is 532 nm.

shown in Figure 3, the ratio of the peak area of the four CeO2 samples pretreated with hydrogen for 0, 3, 10, and 60 min

Figure S5, the content of s-SWNTs was decreased from 98 to 69% measured with 633 nm laser when the growth time was extended from 5 to 60 min. Therefore, growth time is also an important factor for optimizing the selectivity to s-SWNTs. It is known that the reactions of SWNTs are normally either diameter-selective or conductivity-dependent and the former is more prevalent.19,35 In order to figure out which pathway is predominant in our case, we carefully compared the diameter distributions of SWNTs grown using ceria-supported iron under different reduction time with Raman spectra taken under the excitation wavelengths of 532 and 633 nm, respectively (Figure 5). For SWNTs with the diameters of 1.2−1.4 nm, semiconducting tubes are resonant with 532 nm excitation and metallic tubes are resonant with 633 nm laser. Along with the shortening of the catalyst reduction time, that is, the increase of OSC of ceria, m-SWNTs in this region were completely eliminated (Figure 5b,d,f) while s-SWNTs still existed in the same samples (Figure 5a,c,e). For SWNTs with larger diameters of 2.0 to 2.3 nm, metallic tubes should be detected with 532 nm laser and semiconducting tubes with 633 nm excitation. The content of m-SWNTs within this diameter range decreases notably with the decreased catalyst reduction time (Figure 5a,c,e); however, no obvious change of the content of s-SWNTs with similar diameters was found (Figure 5b,d,f). Because the longer catalyst precursor reduction time leads to a lower oxidative ability of ceria, we can conclude that m-SWNTs are more sensitive to the oxidative environment. Therefore, the elimination of SWNTs when using ceria as catalyst support is dominated by conductivity-selective pathway rather than diameter-selective route. This behavior is in fact a big advantage for the selective growth of s-SWNTs. Horizontally aligned ultralong SWNT arrays grown on silicon wafers were considered as one big step toward

Figure 3. Oxygen storage capacity measurement of ceria after reduced in H2 for different time from top to bottom: 0 (a), 3 (b), 10 (c), and 60 min (d), respectively. The peak area of each curve stands for the amount of CO2 (obtained from the reaction of CO with active oxygen stored in ceria) detected by mass spectrometry.

respectively is 78.6:9.8:3.1:1. This indicates that the OSC of ceria was significantly decreased upon prolonging the reduction time, leading to the decreased activity for oxidizing m-SWNTs. Then the growth selectivity of s-SWNTs may eventually be degraded. The statistics in Figure 1d,e show that the content of s-SWNTs grown with the catalysts reduced by hydrogen for 10 min decreased from 96% for nonreduced catalysts to 84% when measured with 532 nm laser. Nevertheless, the ratio of sSWNTs for samples grown with the catalyst supported on ceria and prereduced for 10 min is still remarkably higher than that of SWNTs grown with the silica-supported catalysts. 514

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Figure 5. Diameter statistics of SWNTs synthesized with Fe/CeO2 measured by 532 nm laser (a,c,e) and 633 nm laser (b,d,f), respectively. Ten, 3, and 0 min stand for different catalyst precursor reduction time.

integrated nanocircuits.36,37 Therefore the growth of s-SWNT arrays is of great importance for carbon-based electronics. Since ceria-supported iron catalysts are valid for the selective growth of s-SWNTs on silicon substrates, we were able to grow horizontally aligned SWNT arrays by a gas-flow-guided process (Figure 6a).37 The Raman spectra (Figure 6b,c) show that the as-prepared SWNTs array also has a very high semiconductingselectivity. As shown in Figure 6d, eight tubes detected by 633 nm laser are all semiconducting and eight out of total nine tubes observed with 532 nm laser are semiconducting. When using Co instead of Fe, a high selectivity toward sSWNTs was also observed (Figure 7a,b). The content of sSWNTs is 97% for 532 nm excitation and 92% for 633 nm excitation. We fabricated 16 back-gated thin film FETs directly using the Co/CeO2 catalyzed s-SWNT-enriched samples grown on heavily doped silicon substrates (with ∼500 nm insulating silica layer). Each FET typically contains 50−70 nanotubes. Figure 7c and Supporting Information Figure S8a show the SEM images of two devices. The I−V curves in Figure 7d and Supporting Information Figure S8b reveal typical semiconducting behavior of the channel materials, that is, the SWNTs. Table 1 shows the detailed data of the two devices. The contents of the semiconducting tubes in these two devices were estimated16 to be 98.6 and 98.2%, respectively. More I−V curves of the FETs were shown in Supporting Information Figure S9. The estimated contents of s-SWNTs for these devices are all higher than 95%, which is consistent with the ratio obtained by Raman measurements.

Figure 6. (a) SEM image of horizontally aligned SWNT array synthesized by using Fe/CeO2 as catalyts. (b,c) Raman spectra of the as-grown SWNT arrays measured by 532 nm laser (b) and 633 nm laser (c). (d) The corresponding statistics of m-SWNTs and sSWNTs.

In conclusion, ceria-supported iron or cobalt has shown to be a feasible catalyst system to selectively grow s-SWNTs on 515

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ACKNOWLEDGMENTS

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REFERENCES

Letter

This work is supported by Ministry of Science and Technology of China (Project 2011CB933003) and the National Natural Science Foundation of China (Projects 21125103, 91333105, and 11179011).

(1) Saito, R.; Dresselhaus, M. S.; Dresselhaus, G. Physical Properties of Carbon Nanotubes; World Scientific Publishing: Singapore, 1998. (2) Park, H.; Afzali, A.; Han, S. J.; Tulevski, G. S.; Franklin, A. D.; Tersoff, J.; Hannon, J. B.; Haensch, W. Nat. Nanotechnol. 2012, 7, 787−791. (3) Yang, L. J.; Wang, S.; Zeng, Q. S.; Zhang, Z. Y.; Pei, T.; Li, Y.; Peng, L. M. Nat. Photonics 2011, 5, 673−677. (4) Zhang, Z.; Liang, X.; Wang, S.; Yao, K.; Hu, Y.; Zhu, Y.; Chen, Q.; Zhou, W.; Li, Y.; Yao, Y.; Zhang, J.; Peng, L.-M. Nano Lett. 2007, 7, 3603−3607. (5) Lee, H. W.; Yoon, Y.; Park, S.; Oh, J. H.; Hong, S.; Liyanage, L. S.; Wang, H. L.; Morishita, S.; Patil, N.; Park, Y. J.; Park, J. J.; Spakowitz, A.; Galli, G.; Gygi, F.; Wong, P. H. S.; Tok, J. B. H.; Kim, J. M.; Bao, Z. A. Nat. Commun. 2011, 2, 541. (6) Liu, H. P.; Nishide, D.; Tanaka, T.; Kataura, H. Nat. Commun. 2011, 2, 309. (7) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60−65. (8) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338− 342. (9) Krupke, R.; Hennrich, F.; Löhneysen, H. v.; Kappes, M. M. Science 2003, 301, 344−347. (10) Li, Y.; Cui, R.; Ding, L.; Liu, Y.; Zhou, W.; Zhang, Y.; Jin, Z.; Peng, F.; Liu, J. Adv. Mater. 2010, 22, 1508−1515. (11) Peng, F.; Liu, Y.; Cui, R. L.; Gao, D. L.; Yang, F.; Li, Y. Chin. Sci. Bull. 2012, 57, 225−233. (12) Ding, L.; Tselev, A.; Wang, J.; Yuan, D.; Chu, H.; McNicholas, T. P.; Li, Y.; Liu, J. Nano Lett. 2009, 9, 800−805. (13) Liu, B.; Liu, J.; Tu, X.; Zhang, J.; Zheng, M.; Zhou, C. Nano Lett. 2013, 13, 4416−4421. (14) Hong, G.; Zhou, M.; Zhang, R. O. X.; Hou, S. M.; Choi, W.; Woo, Y. S.; Choi, J. Y.; Liu, Z. F.; Zhang, J. Angew. Chem., Int. Ed. 2011, 50, 6819−6823. (15) Li, S. S.; Liu, C.; Hou, P. X.; Sun, D. M.; Cheng, H. M. ACS Nano 2012, 6, 9657−9661. (16) Jin, S. H.; Dunham, S. N.; Song, J.; Xie, X.; Kim, J.-h.; Lu, C.; Islam, A.; Du, F.; Kim, J.; Felts, J. Nat. Nanotechnol. 2013, 8, 347−355. (17) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H. W.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Science 2003, 301, 1519−1522. (18) Wang, J. Y.; Li, Y. J. Am. Chem. Soc. 2009, 131, 5364−5365. (19) Hersam, M. C. Nat. Nanotechnol. 2008, 3, 387−394. (20) Yu, B.; Liu, C.; Hou, P.-X.; Tian, Y.; Li, S.; Liu, B.; Li, F.; Kauppinen, E. I.; Cheng, H.-M. J. Am. Chem. Soc. 2011, 133, 5232− 5235. (21) Zhou, W. W.; Zhan, S. T.; Ding, L.; Liu, J. J. Am. Chem. Soc. 2012, 134, 14019−14026. (22) Hong, G.; Zhang, B.; Peng, B.; Zhang, J.; Choi, W. M.; Choi, J.Y.; Kim, J. M.; Liu, Z. J. Am. Chem. Soc. 2009, 131, 14642−14643. (23) Che, Y. C.; Wang, C.; Liu, J.; Liu, B. L.; Lin, X.; Parker, J.; Beasley, C.; Wong, H. S. P.; Zhou, C. W. ACS Nano 2012, 6, 7454− 7462. (24) Campbell, C. T.; Peden, C. H. F. Science 2005, 309, 713−714. (25) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C. H. J. Phys. Chem. B 2005, 109, 24380−24385. (26) Kaspar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50, 285−298. (27) Ahlers, G.; Araujo, F. F.; Funfschilling, D.; Grossmann, S.; Lohse, D. Phys. Rev. Lett. 2007, 98, 054501.

Figure 7. (a,b) The statistic data of the as-prepared SWNTs by using Co/CeO2 from Raman spectroscopy. (c) A typical SEM image of a thin film FET device fabricated with the as-grown SWNTs from Co/ CeO2 catalysts. The channel length is 1 μm and the width is 35 μm. (d) I−V curve of the device shown in (c).

Table 1. Detailed Data of SWNT-FET Devices and the Estimated Selectivity of s-SWNTs no.

number of SWNTs

Ion (μA)

Ioff (μA)

Ion/Ioff

selectivity

1 2

61 55

9.10 14.2

0.494 0.944

18.4 15.0

98.6% 98.2%

substrates. Raman spectroscopy and FET performance show the remarkable enrichment of s-SWNTs, typically with the content higher than 95%. The selectivity relies highly on the oxygen storage capacity of ceria and the selectivity is dependent on the conductivity rather than diameter of the SWNTs. This indicates that the mechanism of this process is that ceria provides an oxidative environment during the SWNT nucleation and growth process and therefore prevents the formation of m-SWNTs. Because ceria plays the key role in the selective growth of s-SWNTs, it is reasonable to expect the great flexibility of this strategy in choosing substrates and catalysts. It should be applicable for silicon wafers and various other types of substrates. Besides Fe and Co, other catalysts such as Ni, Mn, Cr, and so forth and alloys of these metals would also be effective. Further studies are ongoing in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, more Raman spectra and statistic data, SEM images, I−V curves, and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare the following competing financial interest(s): Y. L., X. Q., and F. P. 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. 516

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(28) Andrade, J. S.; Araujo, A. D.; Filoche, M.; Sapoval, B. Phys. Rev. Lett. 2007, 98, 194101. (29) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935−938. (30) Trovarelli, A. Catal. Rev.: Sci. Eng. 1996, 38, 439−520. (31) Cui, R. L.; Zhang, Y.; Wang, J. Y.; Zhou, W. W.; Li, Y. J. Phys. Chem. C 2010, 114, 15547−15552. (32) Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Phys. Rep. 2005, 409, 47−99. (33) Soares, J. S.; Cancado, L. G.; Barros, E. B.; Jorio, A. Phys. Status Solidi B 2010, 247, 2835−2837. (34) Araujo, P. T.; Doorn, S. K.; Kilina, S.; Tretiak, S.; Einarsson, E.; Maruyama, S.; Chacham, H.; Pimenta, M. A.; Jorio, A. Phys. Rev. Lett. 2007, 98, 067401. (35) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105−1136. (36) Zhou, W. W.; Han, Z. Y.; Wang, J. Y.; Zhang, Y.; Jin, Z.; Sun, X.; Zhang, Y. W.; Yan, C. H.; Li, Y. Nano Lett. 2006, 6, 2987−2990. (37) Jin, Z.; Chu, H. B.; Wang, J. Y.; Hong, J. X.; Tan, W. C.; Li, Y. Nano Lett. 2007, 7, 2073−2079.

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