Removal of the Residual Surfactants in Transparent and Conductive

Sep 18, 2009 - YongMing Li,‡ and Kazuhiro Noda‡. State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institu...
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J. Phys. Chem. C 2009, 113, 17685–17690

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Removal of the Residual Surfactants in Transparent and Conductive Single-Walled Carbon Nanotube Films Jiaping Wang,† Jing Sun,*,† Lian Gao,*,† Yan Wang,† Jing Zhang,† Hisashi Kajiura,‡ YongMing Li,‡ and Kazuhiro Noda‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China, and AdVanced Materials Laboratories, Sony Corporation, Atsugi Tec. No. 2, 4-16-1 Okata Atsugi, Kanagawa 243-0021, Japan ReceiVed: June 8, 2009; ReVised Manuscript ReceiVed: August 13, 2009

Photocatalysis and Fenton reaction treatments were first employed to eliminate the residual surfactants in the single-walled carbon nanotubes (SWNTs) films without destroying the intrinsic structure of SWNTs. The removal of the surfactants was verified by Raman and XPS spectra. After the treatments, the conductance of SWNTs films was enhanced by a factor of 2.5. Taking a SWNTs film with transparency of 90.6% at 550 nm and resistance of 4.3 kΩ/0, for example, the sheet resistance decreased to 2.0 kΩ/0, while the transmittance increased by 0.3% after the treatment. Introduction The great demand for solar cells, liquid crystal displays, stillimage recorders, solid-state lighting, and touch panels requires the high performance of transparent and conductive films (TCFs).1 Traditional TCFs are prepared by using semiconductor oxides, like indium tin oxide (ITO) and fluorine-doped tin oxide (FTO), because of their good conductivity and high transparency. However, the inflexibility of these films and insufficiency of rare metal limit their further development. Carbon nanotubes, first discovered by Iijima in Japan, are expected to solve those two problems for their excellent conductivity, flexibility, and easy synthesis. Many methods have been developed to fabricate SWNTs films on flexible polyethylene terepthalate (PET) substrate, such as printing,2,3 dip-casting,4 drop casting,5 spin coating,6 spraying,7 Langmuir-Blodgett,8 filtration,1,9,10 airbrushing,11 and so on. Among them, the filtration method is very feasible for preparing SWNTs films, for it is simple and able to produce homogeneous films without expensive instruments. In this paper, the filtration method developed by Wu et al.1 has been utilized to obtain SWNTs films. Before the filtration process, SWNTs need to be well dispersed with surfactants, which are usually nonconductive. It is generally believed that the surfactants could be removed completely after rinsing by water, as they could be dissolved easily.1,2 However, Geng et al. found that the surfactants could not be rinsed away completely from the films prepared by a spraying process.7 It was also found that surfactants remained in the SWNTs films prepared by the filtration method in our previous work.12 It is essential to remove surfactants because they increase the contact resistance between nanotubes for their insulated nature, resulting in the decrease in film conductivity. Geng et al. reported that SWNTs films could be treated by strong acid to remove insulating surfactants and the conductivity of SWNTs films increased by about 2-3-fold without changing the transmittance obviously.7 However, one shortcoming of this * To whom correspondence should be addressed. Phone: +86-2152412718. Fax: +86-21-52413122. E-mail: [email protected] (J.S.) and [email protected] (L.G.). † Chinese Academy of Sciences. ‡ Sony Corporation.

was the treatment must be carefully controlled at low temperature as the PET substrate might be eroded or even dissolved by strong acid. In our previous work, SWNTs films prepared on glass substrate were annealed at 300 °C to remove the surfactants, and the conductivity was improved greatly.12 However, it was only effective for glass substrate, as PET would be burned out at that temperature. It is of great significance to develop new methods to eliminate surfactants from SWNTs films under mild conditions. We herein report two novel alternative paths, photocatalysis and Fenton methods, to eliminate the insulating surfactants without destroying the integrity of SWNTs films and PET substrate. Surfactants are easily decomposed into carbon dioxide and water by an oxidation or reduction process. Semiconductors, such as TiO2,13,14 ZnO,15,16 and CdS,17 are employed as photocatalysts to degrade volatile organic compounds (VOCs) under UV irradiation by photocatalysis. By using nano ZnO as catalysts to degrade acetaldehyde, the photocatalytical activity was higher than 80% within 1 h.16 ZnO nanoparticles have been utilized to decompose the surfactants remaining in the SWNTs films in this paper. The Fenton reaction is another alternative advanced and effective way to decompose organic compounds, where ferrous ion salts and peroxide are used to produce hydroxyl radicals (OH•) and ferric ions (Fe3+). The oxidation potential of intermediate radicals (OH•) generated in the Fenton reaction is as high as 2.8 eV,18-21 which can destroy most organic pollutants. It was reported that SDS could be degraded by the Fenton reaction more than 95% under optimal conditions.18,22 Inspired by the previous results, the Fenton reaction has also been utilized to eliminate the surfactants remaining in the SWNTs films to improve their conductivity. Experimental Section Chemicals. SWNTs (Chengdu Organic Chemistry Institute, Chinese Academy of Sciences) synthesized by the catalytic vapor decomposition method were purified by refluxing in 2.6 M HNO3 at 140 °C for 48 h and dried at 60 °C for 24 h. A certain amount of SWNTs was added into 1 wt % of SDBS aqueous solution, and then horn-sonicated for 1 h by microsonic

10.1021/jp905353c CCC: $40.75  2009 American Chemical Society Published on Web 09/18/2009

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Figure 1. Sheet resistance versus transmittance at 550 nm pre- and post-treatments: (a) before and after photocatalytical treatment and (b) before and after the Fenton reaction.

ultrasonic cell disruption (22 kHz, 100 W). After sonication, the solution was kept static for 1 h to let the SWNTs bundles precipitate. The supernatant solution was taken out and centrifuged at 14737 g for 30 min twice. After the centrifugation, the supernatant was collected carefully to prepare the SWNTs films on PET substrate by the filtration method.1 Photocatalysis. ZnO nanoparticles with particle sizes between 20 and 50 nm were synthesized by a coprecipitation method. A certain amount of Zn(NO3)2 solution was added to Na2CO3 solution dropwise under magnetic stirring. The precipitation was rinsed with water and ethanol twice, respectively, then dried and calcinated at 400 °C for 2 h to obtain ZnO nanoparticles. A certain number of ZnO nanoparticles were dispersed in 400 mL of H2O by sonication and then the SWNTs films were fixed in the middle of the ZnO suspension, which was irradiated with UV light for 5 h. To prevent the ZnO particles from sedimentation, the whole photocatalytical procedure was operated under magnetic stirring. Then, the films were dipped in 100 mL of 0.1 M HCl solution to remove the byproduct produced in the photocatalytical procedure, rinsed with water, and dried at 90 °C for 5 h. ZnO nanoparticles pre- and post-treatment were characterized by FTIR to confirm the removal of surfactants. Fenton Reaction. A certain amount of FeSO4 or K4[Fe(CN)6] · 3H2O was used as the ferrous source. The pH value of the Fenton solution was adjusted by H2SO4 to be 3-3.5. The SWNTs films were placed at the bottom of the vessel. A certain amount of H2O2 was dripped into the solution to activate the Fenton reaction and the films were soaked for 7 h. After the Fenton treatment, the SWNTs films were rinsed with water and then dried at 90 °C for 5 h. Characterization. Raman spectra of SWNTs films were recorded with a Renishaw micro Raman spectrometer with an excitation wavelength of 633 nm. The elemental analysis of the films and the binding energy of C1s were analyzed by X-ray photoelectron spectroscopy with X-ray source from a scanning auger microprobe, using a Al/Mg dual anode (XPS, Microlab 310F). Infrared absorption spectra of ZnO powders were recorded in the range between 700 and 1300 cm-1 by a FTIR spectrophotometer (FTIR, Bio-Rad FTS-185). The transparency of the films was characterized by UV-vis-NIR spectrometer (Lambda 950). The sheet resistance was measured by four point probe (MCP-T360). Results and Discussion The properties between the as-prepared SWNTs films and the post-treated ones by photocatalysis and the Fenton reaction were summarized in Figure 1. The sheet resistance decreased

by 2.5-3.5-fold after oxidative treatments, without much change on the transmittance after treatments. Similar to ref 7, the reason for the decrease on sheet resistance was that the insulating surfactants remaining in the SWNTs films were eliminated, which decreased the high contact resistance and accelerated the free electron flow among nanotubes, while the integrity of SWNTs was kept intact. The decrease on sheet resistance by the two methods was comparable to that accomplished by Geng et al. by acid treatment,7 while the PET substrate was not damaged after the treatments. With the transmittance higher than 97% at 550 nm, the conductivity could be enhanced by 3.5fold. However, with the transmittance lower than 94%, only 2.5-fold could be reached. It was understandable that SWNTs films with lower transmittance had more SWNTs deposited on the PET substrate, resulting in more remnant surfactants on the SWNTs films. Thus it was more difficult to remove them completely. It was also observed that the decrease on sheet resistance with use of photocatalytical method was a little better than that with the Fenton method. Taking the SWNTs films with transparency about 95% as an example, the sheet resistance could be reduced by 64% after photocatalytical treatment, while the decrease was about 60% after the Fenton reaction. The sheet resistance decreased from 4.3 kΩ/0 to 2.0 kΩ/0 by photocatalytical treatment, while the transmittance at 550 nm increased from 90.6% to 90.8%. SEM was used to explore discriminating information about SWNTs distribution and the remnant surfactants coating state before and after treatments. For the as-prepared SWNTs film, it was shown that some surfactants covered the films in Figure 2a. The surfactants and SWNTs were almost on the same surface, while after treatments, the SWNTs protruded above the substrate and a clearer SWNTs network could be found. Compared with the SWNTs films pre- and post-treatments, SWNTs bundles with smaller diameters could also be observed. These changes were due to the disappearance of the surfactants enwrapped on the nanotubes. The protrusion of SWNTs and diminution of the bundle size after oxidative treatments shown in Figure 2 clearly verified the elimination of surfactants remaining in the SWNTs films. Raman spectroscopy was used to analyze the difference of SWNTs films pre- and post-treatments in Figure 3. There are two main domains for typical Raman spectra of SWNTs. The D-band around 1280 cm-1 corresponds to disordered graphite and the degree of conjugation disruption, and the G-band around 1590 cm-1 corresponds to the tangential carbon-carbon bond stretching vibrations. The intensity ratio of the D-band to the G-band (ID/IG) is widely used as a measure of sidewall covalent

Eliminating Residual Surfactants in SWNTs Films

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17687 TABLE 1: Elemental Analysis of SWNTs Films on PET Substrate Pre- and Post-Treatment by the XPS Method

Figure 2. SEM images of SWNTs films: (a) as-prepared SWNTs film; (b) SWNTs film after photocatalysis; and (c) SWNTs film after the Fenton reaction.

Figure 3. Raman spectra of SWNTs films: (a) as-prepared SWNTs film; (b) SWNTs film after photocatalysis; (c) SWNTs film after the Fenton reaction; and (d) SDBS powder

derivation or defect introduction.23-26 In the present work, both D-band and G-band peaks did not show any frequency shift after treatment, which meant that there was no doping effect on the SWNTs films. It was calculated that ID/IG was 0.031, 0.032, and 0.030 for curves a, b, and c, respectively. The ratio of ID/IG could be regarded as unchangeable, indicating that the structure of SWNTs was kept intact. Besides typical peaks of SWNTs, there were two other peaks at 1330 and 1610 cm-1 in the SWNTs films, which originated from the groups of SDBS. Comparing curves a and d in Figure 3, it could be found that the Raman shifts of SDBS groups remaining in the films were not exactly the same as that of the SDBS powder. The possible reason is the hydrophilic groups of SDBS had been removed after the filtration process; the remnant groups on SWNTs were not identical with those in the pure SDBS powder, which led to the disappearance of the peak at 1445 cm-1 and a frequency shift of the peak at 1310 cm-1 to 1330 cm-1. For the film treated by the Fenton reaction, the peak at 1330 cm-1 diminished a lot, which was the evidence of elimination of SDBS after treatment, while for the SWNTs film treated by photocatalysis, one interesting thing to note was that the stretching vibration mode at 1330 cm-1 disappeared, which meant that photocatalysis decomposed more surfactants than the Fenton method. The Raman spectra results showed that both photocatalysis and the Fenton methods were effective in removing surfactants remaining in the SWNTs films without destroying the structure of SWNTs.

sample

O (atom %)

C (atom %)

Si (atom %)

as-prepared after photocatalysis after Fenton reaction pure PET

17.7 21.4 20.1 19.9

82.3 78.1 79.6 78.3

0.0 0.5 0.3 1.8

More information on the elemental composition of SWNTs films was analyzed by XPS. Table 1 compared the difference between pre- and post-treatments. For the PET substrate, besides C and O elements, the Si element was also found because the Si catalyst was employed in the polymerization process. For as-prepared SWNTs film, the Si element was not detected because the substrate was totally covered by SWNTs and surfactants, while after treatments, 0.5% and 0.3% of Si showed up for post-photocatalysis and post-Fenton reactions, respectively. In our previous work, it was found that the gap between nanotubes would be exposed to the scanning microprobe of XPS after eliminating surfactants, resulting in detectable PET substrate by XPS microprobe. The detection of the Si element from the PET substrate verified less coverage of the surfactant on SWNTs films. Figure 4 showed XPS C 1s spectra fit curves pre- and postphotocatalysis. There were three peaks, with the one at about 285.0 eV attributed to sp2 carbon, which came from SWNTs, the PET substrate, and the benzene group of SDBS (the main structural formula of PET is shown in Figure 5). The peak at about 286.7 eV was ascribed to the sp3 carbon, which was from the alkyl group of the SDBS and PET substrate, while the peak at about 289.2 eV was due to the oxygen related carboxyl group from the PET substrate and defects of SWNTs.27-29 Because the binding energy variation for all three peaks of C 1s was less than 0.35 eV after treatments, it was considered that the valence state of C 1s did not change,27 which meant that no doping and no dedoping effect occurred in the procedure. This result was in accordance with Raman spectra. Table 2 presented the atomic percentage for the three kinds of carbon in detail. It was found that the number of -COO groups increased by more than one-fold after treatments. The reason was that these kinds of groups from the PET substrate were also exposed to the XPS microprobe, while for as-prepared SWNTs films, these groups were only from SWNTs because the PET substrate was totally covered by SWNTs and surfactants, as shown in Table 1. Although sp3 carbons from the PET substrate were also detected, another interesting point to be noted was that the amount of sp3 carbon was reduced by half after both two kinds of treatments. It was obvious that the remnant surfactants were eliminated from SWNTs films after oxidative treatments, resulting in the sharp reduction of sp3 carbon content. Both the increase of -COO groups and decrease of sp3 carbon after treatments verified the elimination of SDBS from SWNTs films, which was in accordance with the detection of the Si element, as shown in Table 1. Scheme 1 illustrated the photocatalytical process of eliminating SDBS from the films with the ZnO concentration of 0.2 mg/400 mL (the ZnO concentration effect on the photocatalytical activity is shown in Figure S1 in Supporting Information). The valid contact for the redox reaction was among oxide, solution, and SWNTs films. When these nanoparticles with a band gap of about 3.17 eV (it was measured by UV-vis-NIR instrument and not shown here) were exposed to UV light with photon energy of 3.40 eV, some electrons jumped out of the valence band, leaving high active holes on the surface of the oxide.

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Figure 4. XPS C 1s spectra of SWNTs films (black lines) with its deconvolution into Gaussian curves (blue lines): (a) as-prepared SWNTs film; (b) the SWNTs film after photocatalysis; and (c) the SWNTs film after the Fenton treatment.

SCHEME 1: Photocatalytical Illustration for Decomposing SDBS Remaining in SWNTs Films with ZnO Nanoparticles As Catalysts Figure 5. The repeated unit of poly(ethylene terephthalate) (PET).

TABLE 2: XPS Analysis Result of the C 1s Peak in the SWNTs Films Based on Gaussian Deconvolution sp2 (atom %) sp3 (atom %) -COO (atom %)

sample as-prepared post-photocatalysis post-Fenton treatment

62.54 62.38 64.48

26.72 13.25 14.36

10.74 24.37 20.05

These holes reacted with water or hydroxyl ion, producing strong oxidative hydroxyl radicals. Hydroxyl radicals then decomposed the SDBS remaining on the films into carbon dioxide and water (solution-solid contact). The reaction equations were shown as below:

H2O + h+ f •OH + H+

(1)

OH- + h+ f •OH

(2)

SDBS + •OH + O2 f low molecular byproducts fCO2 + H2O

(3) In the procedure of photocatalysis, ZnO nanoparticles reacted with CO2 and H2O, and formed Zn(HCO3)2 or ZnCO3. They coated the SWNTs films and decreased the transparency and the conductivity of SWNTs films from their insulated nature. Therefore, it was necessary to remove those byproducts completely. SWNTs films after the photocatalytical process were dipped in HCl solution to eliminate the byproducts. After HCl treatment, the surfactants remaining in the SWNTs films were

eliminated, and the high contact resistance among nanotubes disappeared, resulting in improved conductivity of the SWNTs films. We also verified the removal of SDBS by characterizing ZnO powder pre- and post-photocatalysis by the FT-IR method, as shown in Figure 6. The intensity of the peak at 840 cm-1 in as-prepared ZnO particles decreased a lot and the peak at 920 cm-1 disappeared because ZnO powder was corroded by carbon oxide and water because of its amphoteric nature. Two peaks at 1153 and 1211 cm-1 appeared, corresponding to the stretching vibration of the C-C bond. The existence of the C-C band on ZnO powder verified that the surfactants were removed from the SWNTs films and attached to the ZnO particles. Obviously, the C-C bond could only be introduced from SDBS in the whole experimental procedure. The Fenton reaction is another effective method to decompose organic solutions, which is denoted as RH in this paper. Compared with the photocatalytical method, it was facile to proceed and did not need extra excitation energy because the reaction activation energy between OH• and organics was as low as about 0.01 eV.18 Previous work also had shown that acidic condition was optimal for the Fenton oxidation because

Eliminating Residual Surfactants in SWNTs Films

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17689 particles was produced in the procedure. When K4[Fe(CN)6]/ H2O2 (the ratio is 30 mg to 3.0 mL, as shown in Figure S3 in the Supporting Information) was employed to degrade the residual surfactants in the SWNTs films, the oxidation reaction could proceed directly between SWNTs films and the Fenton solution. With the addition of H2O2, K4[Fe(CN)6] was oxidized as K3[Fe(CN)6], accompanied by the formation of OH•. The complexing agent K3[Fe(CN)6] could be reduced to K4[Fe(CN)6] by organic radicals (R•) again,19 and the whole Fenton reaction will not terminate until H2O2 is used up. The probable reaction equations are shown below:

Figure 6. FT-IR spectra of ZnO nanoparticles before and after photocatalysis.

the right amount of H+ accelerated the decomposition of H2O2 to create the maximum number of hydroxyl radicals.19 The Fenton reaction under acidic condition is shown briefly in the following equations:



Fe(CN)6]4- + H2O2 f [Fe(CN)6]3- + OH- + •OH

(8) •

OH + RH f H2O + •R f further oxidation •

(5)

OH + H+ + e f H2O

(6) (9)

Fe2++H2O2 f Fe3++OH- + •OH

(4)

[Fe(CN)6]3- + •R f [Fe(CN)6]4- + R+

OH + RH f H2O + •R f further oxidation

(5)

After the Fenton treatment, freely soluble [Fe(CN)6]3- and [Fe(CN)6]4- could be easily rinsed with water and no other impurities were introduced. For the SWNTs film with the transparency of 92% at 550 nm, the sheet resistance decreased from 6500 Ω/0 to 2700 Ω/0, as shown in Figure 1b.



RH + H+ + e f H2O

(6)

For traditional removal of RH by the FeSO4/H2O2 system, most of the pollutants were oxidized by the intermediate OH• generated via reaction 4, and some pollutants were captured by several kinds of hydroxo complexes, which were formed by the reaction between the newly formed ferrous ions and hydroxide ions when the pH level was within 3-7. The formation of hydroxo complexes was shown below:21

[Fe(H2O)6]3+ + H2O f [Fex(H2O)y(OH)z]n+ + H3O+ (7) The hydroxo complexes were consistently observed in the form of small particles during the Fenton process (Supporting Information, Figure S2). When degrading organic solutions, this kind of hydroxo complexe could accelerate the removal rate because of its strong coagulation capability.21 However, when it was used to remove the surfactants remaining in SWNTs films, the activity of degrading surfactants from SWNTs films was not as effective as that of the photocatalytical method. Both the concentration and the ratio of FeSO4/H2O2 were optimized, but the maximum reduction of sheet resistance for SWNTs films could only be achieved by 20% after treatment. The reason was that the hydroxo complexes played a negative role. First, the hydroxo complexes coating on the SWNTs films were very detrimental to the transparency and conductivity. Second, the hydroxo complexes hindered the contact between surfactants on SWNTs films and the Fenton solution, resulting in the cessation of the oxidizing process. To overcome the problem mentioned above, freely soluble potassium hexacyanoferrate trihydrate (K4[Fe(CN)6] · 3H2O) was utilized to substitute for FeSO4. The stable complexing agent could restrain the formation of [Fex(H2O)yOHz]n+. The Tyndall phenomenon was not observed even after 1 week at room temperature. This implied that no significant amount of solid

Conclusions Both the photocatalytical and Fenton reactions were first employed to eliminate the remnant surfactants on SWNTs films, while the integrity of the PET substrate and SWNTs remained intact. For the photocatalytical method, ZnO nanoparticles were utilized as photocatalysts because of their high activity in decomposing surfactants and ease of removal. For the Fenton reaction, the K4[Fe(CN)6]/H2O2 system was employed to replace the traditional FeSO4/H2O2 system because this new Fenton solution avoided the formation of hydroxo complexes [Fex(H2O)y(OH)z]n+. The conductance of SWNTs films was enhanced more than 2.5 times after both treatments. The sheet resistance decreased from 4300 Ω/0 to 2000 Ω/0 by photocatalytical treatment. For the SWNTs film treated by the Fenton method, the sheet resistance decreased from 6500 Ω/0 to 2700 Ω/0. Comparing the two methods, the photocatalytical method performed better in eliminating residual surfactants remaing in the SWNTs films, while the Fenton method was preferable because it did not need a special instrument and proceeded in much milder conditions. These two new methods for eliminating surfactants remaining in the SWNTs films will open a new way to improve the performance of transparent and conductive films. Acknowledgment. This work was financially supported by the National Key Project of Fundamental Research (No. 2005CB6236-05), Shanghai Talents Program Foundation, and Shanghai Nano Centre Program (No. 0852 nm01900). Supporting Information Available: Experimental details and discussion along with Figures S1-S3 providing an illustration of ZnO nanoparticles concentration effect on decomposing SDBS from SWNTs films (Figure S1), the Tyndall phenomenon of the FeSO4/H2O2 fenton system (Figure S2), and the sheet resistance of SWNTs films changed with the amount of H2O2

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(Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273. (2) Zhou, Y.; Hu, L.; Gruner, G. Appl. Phys. Lett. 2006, 88, 123109. (3) Moon, J. S.; Park, J. H.; Lee, T. Y.; Kim, Y. W.; Yoo, J. B.; Park, C. Y.; Kim, J. M.; Jin, K. W. Diamond Relat. Mater. 2005, 14, 1882. (4) Spotnitz, M. E.; Ryan, D.; Stone, H. A. J. Mater. Chem. 2004, 14, 1299. (5) Sreekumar, T. V.; Liu, T.; Kumar, S.; Ericson, L. M.; Hauge, R. H.; Smalley, R. E. Chem. Mater. 2003, 15, 175. (6) Meitl, M. A.; Zhou, Y.; Gaur, A.; Jeon, S.; Usrey, M. L.; Strano, M. S.; Rogers, J. A. Nano Lett. 2004, 4, 1643. (7) Geng, H.-Z.; Kim, K. K.; So, K. P.; Lee, Y. S.; Chang, Y.; Lee, Y. H. J. Am. Chem. Soc. 2007, 129, 7758. (8) Kim, Y.; Minami, N.; Zhu, W. H.; Kazaoui, S.; Azumi, R.; Matsumoto, M. Jap. J. Appl. Phys., Part 1 2003, 42, 7629. (9) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513. (10) Zhang, D.; Ryu, K.; Liu, X.; Polikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. Nano Lett. 2006, 6, 1880. (11) Ferrer-Anglada, N.; Kaempgen, M.; Skakalova, V.; Dettlaf-Weglikowska, U.; Roth, S. Diamond Relat. Mater. 2004, 13, 256. (12) Wang, J. P.; Sun, J.; Gao, L.; Liu, Y. Q.; Wang, Y.; Zhang, J.; Kajiura, H.; Li, Y. M.; Noda, K. J. Alloys. Compd. DOI: 10.1016/ j.jallcom.2009.05.139. Published Online: June 9, 2009. (13) Teichner, S. J. J. Porous Mater. 2008, 15, 311.

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