11258
J. Phys. Chem. C 2010, 114, 11258–11265
Terahertz Spectroscopy of Nanocrystal-Carbon Nanotube and -Graphene Oxide Hybrid Nanostructures Gyeong Bok Jung, Yoon Myung, Yong Jae Cho, Yong Jae Sohn, Dong Myung Jang, Han Sung Kim, Chi-Woo Lee, and Jeunghee Park* Department of Chemistry, Korea UniVersity, Jochiwon 339-700, Korea
Inhee Maeng and Joo-Hiuk Son Department of Physics, UniVersity of Seoul, Seoul 130-743, Korea
Chul Kang AdVanced Photonics Research Institute, Gwangju 500-712, Korea Downloaded via UPPSALA UNIV on August 3, 2018 at 03:52:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
ReceiVed: March 5, 2010; ReVised Manuscript ReceiVed: May 5, 2010
We employed terahertz (THz) spectroscopy to study the optical/electrical properties of various nanocrystal (NC)-carbon nanotube (i.e., single-, double-, and multiwalled) and -graphene oxide (GO) hybrid nanostructures. The power absorption and conductivity decrease following the order single-walled (SWNT) > double-walled (DWNT) > multiwalled carbon nanotubes (MWNT) ≈ GO showed a strong dependence on the crystallinity of the graphitic layers. The deposition of platinum (Pt), copper sulfide (Cu2S), and tin oxide (SnO2) NC reduced significantly the conductivity of the SWNT and DWNT, irrespective of the nature of the NC but negligibly that of the MWNT and GO. X-ray photoelectron and Raman spectroscopy analyses suggested that the conductivity of the NC hybrid nanostructures was mainly reduced by the electron trapping at the surface defects that formed during the in situ solvothermal growth of the NC. As the conductivity of the graphitic layers increases, such electron trapping effect becomes more significant. 1. Introduction Carbon nanotubes (CNTs) have attracted a great deal of interest because of their potential applications as building blocks in nanoelectronics and photonics.1-5 Lately, significant interest has been directed toward the design of CNT hybrid nanostructures with various nanocrystals (NCs) because of their versatile applications in electronic devices.6-13 For instance, semiconductor NC-CNT hybrid nanostructures exhibit the highly efficient generation of photocurrents, suggesting their importance as building blocks for light harvesting assemblies.6-9 NC-CNT composites were used to build biosensor platforms by making use of their greater ability to promote electron-transfer reactions.7,10-13 Nevertheless, to extend their application area, it is essential to understand more exactly the effect of NC on the optical/electrical properties of CNT. Far infrared (IR) or microwave absorption spectroscopic techniques have been used to obtain the optical conductivities of CNT.14,15 However, these methods have limitations because of the complicated numerical transformation process of the Kramers-Kronig relationship. In contrast, terahertz time-domain spectroscopy (THz-TDS) can provide the exact complex permittivity without Kramers-Kronig analysis, and its high signalto-noise ratio (∼10 000) enables it to yield more accurate values than far IR and microwave spectroscopy.16,17 Therefore, THzTDS has recently been employed to study the optical/electrical properties of CNT films or polymer composites.17-27 Gladden and coworkers used THz-TDS to study a series of carbon nanofibers prepared using different heat treatments.28 Furthermore, they demonstrated how THz-TDS can characterize the * Corresponding author. E-mail:
[email protected].
optoelectronic properties of commercially available MWNT.29 However, there have been no reports on the THz-TDS measurement of NC-CNT hybrid nanostructures as yet. As another important type of nanosized carbonaceous materials, graphene-based materials (e.g., graphene, graphene oxide, exfoliated graphite, chemically modified graphene, etc.) have been used in a variety of applications, such as capacitors, liquid crystalline displays, and nanoelectromechanical resonators.30-32 They also represent a very promising candidate as a new carbonaceous support for various NCs.33-41 For example, the field of application of TiO2-GO composites could conceivably range from photocatalysts to new electrical energy storage materials.40,41 However, so far, there have been no fundamental studies on the optical/electrical properties of NC-GO hybrid nanostructures in the THz region. In this article, we report on the THz-TDS study of various NC-CNT hybrid nanostructures, revealing the effect of the deposition of NC on the optical/electrical properties of the CNT and GO. Our research group previously reported the use of Pt-, Cu2S-, and SnO2-CNT hybrid nanostructures as glucose biosensors.7,42 Herein we extend this work to the investigation of their optical and electrical properties in the THz region and compare the effect of NC deposition in the case of single-walled (SWNT), double-walled (DWNT), multiwalled CNT (MWNT), and GO. Recently, there have been a number of important works on the bottom-up synthesis of graphene, which consists of a monolayer of carbon atoms highly packed into a 2D honeycomb lattice.43-46 Among these works, large-scale graphene oxide (although the authors called it as graphene) was synthesized using the rapid pyrolysis of precursors that were prepared by the hydrothermal reduction of ethanol with sodium.46 We
10.1021/jp1019894 2010 American Chemical Society Published on Web 06/08/2010
NC-CNT and -GO Hybrid Nanostructures
J. Phys. Chem. C, Vol. 114, No. 25, 2010 11259
synthesized graphene oxide (we refer it as “GO”) using the same procedure and deposited Pt, Cu2S, and SnO2 NC by the in situ solvothermal method. Their optical/electrical parameters in the THz region were compared with those of the CNT. Remarkably, we found that the deposition of the NC on the SWNT and DWNT significantly reduced their conductivity, irrespective of the nature of the NC, but negligibly affected that of the MWNT and GO. 2. Experimental Section Purified SWNT (produced by the arc-discharge method, purchased from Iljin), DWNT (produced by the high-pressure HIPCO method, purchased from Unidym), and MWNT (produced by the CVD method, purchased from Sigma-Aldrich) were used as ligands and templates to grow the NC. The detailed solvothermal syntheses of the Pt, Cu2S, and SnO2 NC-CNT hybrid nanostructures were described elsewhere.7,42 GO was synthesized by the low-temperature flash pyrolysis of the solvothermal product of sodium and ethanol.46 All solvothermal reactions were performed in a Teflon-lined reactor having a maximum volume of 23 mL. A typical synthesis consists of heating a 1:1 molar ratio of sodium (2 g) and ethanol (5 mL) in a sealed reactor vessel at 220 °C for 72 h to yield the solid solvothermal product, the GO precursor. This material is rapidly pyrolyzed; then, the remaining product was washed with deionized water (100 mL). The suspended solid is then vacuum filtered and dried in a vacuum oven at 100 °C for 24 h. The final yield of GO is ∼0.1 g per 1 mL at ethanol. The products were characterized by field-emission transmission electron microscopy (FE TEM, FEI TECNAI G2 200 kV and Jeol JEM 2100F) and high-voltage transmission electron microscopy (HVEM, Jeol JEM ARM 1300S, 1.25 MV). Highresolution X-ray diffraction (XRD) patterns were obtained using the 8C2 or 3C2 beamline of the Pohang Light Source (PLS) with monochromatic radiation (λ ) 1.54520 Å). We carried out X-ray photoelectron spectroscopy (XPS) by using the 8A1 beamline of the PLS and a laboratory-based spectrometer (VG Scientifics ESCALAB 250) using a photon energy of 1486.6 eV (Al KR). Thermal gravimetric analysis (TGA, TA Instrument, SDT 2960) was used to measure the wt % of the NC in the hybrid nanostructures. The Raman spectroscopy measurements (Horiba Jobin-Yvon HR-800 UV) were recorded using an Ar ion laser (λ ) 514.5 nm). IR spectroscopy (Thermo Scientific Nicolet iS10 FT-IR spectrometer) was also employed to examine the chemical bonding of the nanostructures. All Raman and IR spectrum measurement were carried at 25 ( 1 °C. For the THz-TDS measurements (at 25 ( 1 °C), we fabricated pellets by mixing the CNT samples (1 wt %) with KBr powder (99.999%, Aldrich-Sigma). The mixture (50 mg) was pressed at 3000 psi for 10 min to form the pellets. The diameter and thickness of the pellets were 8 ( 1 and 0.4 ( 0.1 mm, respectively. A schematic diagram of the setup is provided in Figure S1 of the Supporting Information. A Ti:sapphire laser, operating at a center wavelength of 800 nm with a repetition rate of 80 MHz and a pulse width of 80 fs was used as the light source. The laser beam is split into two beams using a beam splitter; one goes to an InAs emitter generating the THz pulses and the other one goes to the detector. The THz beam is transmitted through the pellet sample and focused onto a lowtemperature-grown (LT) GaAs detector using a pair of parabolic mirrors. The detection antenna was a Hertzian dipole antenna, which had a gap of 5 µm on LT-GaAs. To avoid vapor absorption, we used an airtight acrylic box with a humidity of
Figure 1. TEM images of the (a) SWNT, (b) Pt-SWNT, (c) Cu2S-SWNT, (d) SnO2-SWNT, (e) GO, (f) Pt-GO, (g) Cu2S-GO, and (h) SnO2-GO hybrid nanostructures. The average sizes of the spherical Pt, Cu2S, and SnO2 NC are 3 ( 0.3, 7 ( 1, and 2 ( 0.2 nm, respectively.
DWNT > MWNT ≈ GO series. (2) These optical/electrical constants are reduced by the deposition of the NC irrespective of the nature of the NC. (3) The R(ω) and σ(ω) values are decreased by 50 and 45% for the SWNT and DWNT, respectively, but negligibly for the MWNT and GO. (4) The deposition of the NC decreases the nr(ω) values by 5% for the SWNT and DWNT but negligibly for the MWNT and GO.
NC-CNT and -GO Hybrid Nanostructures
J. Phys. Chem. C, Vol. 114, No. 25, 2010 11261
Figure 3. (a) Power absorption R(ω), (b) real part of refractive index nr(ω), and (c) real conductivity σ(ω) of DWNT, Pt-, Cu2S-, and SnO2-DWNT hybrid nanostructures mixed with KBr.
Figure 4. (a) Power absorption R(ω), (b) real part of refractive index nr(ω), and (c) real conductivity σ(ω) of MWNT, Pt-, Cu2S-, and SnO2-MWNT hybrid nanostructures mixed with KBr.
Oh et al. reported that InAs/GaAs quantum dot layers decrease the THz power absorption and conductivity, suggesting that the quantum dots act as carrier traps and capture the mobile carriers.47 Recently, the electrical conductivity of CdSe/ZnSe quantum dot-MWNT hybrid nanostructures was measured to show that the quantum dots act as carrier traps.48 Therefore, all three types of NC would consistently serve as carrier (electron) traps on the CNT and GO. The electrons confinement in the NCs would decrease the conductivity. In the case of the MWNT and GO, the trapping effect of the NC would be less effective because their conductivity is already much lower than that of the SWNT and DWNT. However, the reduction of the conductivity exhibits no dependence on the nature of the NC (i.e., metallic Pt, semiconducting Cu2S, and insulating SnO2), even for the more highly conductive SWNT and DWNT. Therefore, it is necessary to discuss the electron trapping by considering the surface properties of the carbon nanostructures. For the deposition of the NC on the CNT using the solvothermal method, the surface was acid-modified to produce
electron-withdrawing epoxy or carboxyl groups, which we refer to simply as surface defects hereafter. GO was used without purification, but it also contains these groups, as shown in the IR spectrum (Supporting Information, Figure S2). In fact, the binding of the NC takes place through these chemical bonds at the surface defects. Hull et al. reported that in Pt-CNT hybrid nanostructures, the Pt NC binds with the CNT through a carboxylate or ester-type ionic bonding structure.49 We reported that the surface defects of the DWNT and MWNT increase during the in situ solvothermal growth of the NC, as evidenced by XPS and Raman spectroscopy.42 The increased number of surface defects can decrease the conductivity by increasing the electron trapping at these defects. To explain further the THzTDS results using the surface defects of the graphitic layers, we performed Raman spectroscopy and XPS measurements for all of the samples. Figure 6a shows the Raman spectra of the SWNT and its Pt-, Cu2S-, and SnO2 NC hybrid nanostructures. The spectra reveal the characteristic narrow G band at 1590 cm-1, which
11262
J. Phys. Chem. C, Vol. 114, No. 25, 2010
Figure 5. (a) Power absorption R(ω), (b) real part of refractive index nr(ω), and (c) real conductivity σ(ω) of GO and Pt-, Cu2S-, and SnO2-GO hybrid nanostructures mixed with KBr.
originates from the Raman active A1g, E1g, and E2g axial vibration modes of the graphite sheets.50,51 The D band at 1345 cm-1 involves the scattering from a defect that breaks the basic symmetry of the graphite sheets, as shown on the magnified scale. This band is usually observed in sp2 carbons containing porous, impurities, or other symmetry-breaking defects. The intensity of the D band is very weak relative to that of the G band (ID/IG ) 0.01). Nevertheless, the D band becomes broadened and the ID/IG ratio increases slightly to 0.02, 0.015, and 0.02 for the Pt, Cu2S, and SnO2 NC, respectively. The Raman spectra of the DWNT and NC-DWNT hybrid nanostructures are shown in Figure 6b. The characteristic G and D bands also appear at 1580 and 1340 cm-1, respectively. As the NC deposit, the ID/IG ratio increases from 0.04 to 0.2, 0.15, and 0.2 for the Pt, Cu2S, and SnO2 NC, respectively. For the MWNT series, the characteristic G and D bands also appear at 1580 and 1350 cm-1, respectively (Figure 6c). In comparison with the SWNT and DWNT, the ID/IG ratio of the MWNT is larger, and, consequently, they must contain a higher quantity of
Jung et al. structural defects due to their multiple graphite layers. As the NC deposit, the ID/IG ratio increases from 0.45 to 0.81, 0.81, and 0.85, for the Pt, Cu2S, and SnO2 NC, respectively. Figure 6d shows the Raman spectra of GO and NC-GO hybrid nanostructures. The broad D-band centered at 1345 cm-1 and G-band centered at 1587 cm-1 have similar intensities; that is, ID/IG ≈ 0.9. The ID/IG ratio increases slightly to 0.95, 0.95, and 0.93 following the deposition of the Pt, Cu2S, and SnO2 NC, respectively. The broader band and the higher ID/IG value, compared to those of CNT series, would be ascribed to the more significant C-O and CdO bonding structures. (See the IR spectrum in the Supporting Information, Figure S5.) The ID/IG ratio increases in the order SWNT < DWNT < MWNT < GO. For the hybrid nanostructures, the increase in the ID/IG ratio implies that the deposition of the NC enhances the number of defect sites in the graphitic layers. For the SWNT, DWNT, MWNT, and GO, the deposition of the NC increases the ID/IG ratio by a factor of 2, 5, 2, and 1.1, respectively. The ID/IG ratios are listed in Table 1. Figure 7a shows the fine-scanned XPS C 1s spectra for the SWNT and its Pt, Cu2S, and SnO2 NC hybrid nanostructures, obtained using a photon energy of 630 eV. As the NC deposit, the full width at half-maximum (fwhm) of the asymmetric band, centered at 284.5 eV, remains nearly the same. This asymmetric band could be resolved into two bands at 284.5 (PC1) and 285.5 (PC2) (in average) by fitting it into the Voigt function.52 The PC1 band can be assigned to the C atoms (C-C) binding to the graphite network, and the PC2 band corresponds to the C atoms at the defect sites (e.g., C-O). The widths and fractions of the resolved PC1 and PC2 bands are listed in Table 1. As we previously reported, the fine-scanned XPS C 1s spectra for the DWNT and MWNT exhibit a significant increase in the fraction and broadening of the PC2 band upon the deposition of the NC. The increase in the number of defects is more significant for the DWNT. In contrast, the SWNT exhibits a negligible change upon the deposition of the NC, which is consistent with their Raman spectra. Figure 7b displays the fine-scanned XPS C 1s spectra for the GO and its hybrid nanostructures. The much broader C 1s band (than the CNT series) can be resolved into three components, PC1, PC2, and PC3, located at 284.6, 286.6, and 287.9 eV, respectively. These three components are assigned to the C-C, C-O, and CdO bonding structures, respectively.53 The larger fraction of defects band (PC2 and PC3) compared with that of the CNT series is consistent with the result of Raman and IR spectra. The NC-GO hybrid nanostructures show decreased fractions of the PC2 and PC3 bands, indicating the deoxygenation of GO upon the deposition of the NC. The deposition of the NC would replace the epoxy or carboxyl groups bind with the graphitic layers. However, the Raman spectrum reveals that such a deoxygenation effect is insufficient to reduce significantly the number of defects in the graphitic layers. The survey-scanned XPS spectra also consistently show a significant increase in the number of O atoms in the DWNT hybrid nanostructures but no corresponding increase in the case of the SWNT, MWNT, and GO (Supporting Information, Figure S3). The XPS and Raman spectroscopy analyses indicate that the number of surface defects increases in the order: SWNT < DWNT < MWNT < GO series, which is the opposite order of the R(ω) and σ(ω) values. Therefore, the conductivity is directly related to the number of surface defects existing in the present carbon nanostructures; the higher the number of surface defects, the lower the conductivity. As the NC deposit, the number of
NC-CNT and -GO Hybrid Nanostructures
J. Phys. Chem. C, Vol. 114, No. 25, 2010 11263
Figure 6. Raman spectra of (a) SWNT, (b) DWNT, (c) MWNT, and (d) GO and their Pt, Cu2S, and SnO2 NC hybrid nanostructures.
TABLE 1: FWHM and Area % of Resolved Bands from the XPS C 1s Peak for the NC-SWNT, -DWNT, -MWNT, and -GO Hybrid Nanostructures and Raman Spectroscopy Data fitting parameters PC1 (graphite) CNT
NC
SWNT Pt Cu2S SnO2 DWNTc Pt Cu2S SnO2 MWNTc Pt Cu2S SnO2 GOd Pt Cu2S SnO2
PC2 (defects)
C 1s fwhm (eV)a
fwhm (eV)
area %
fwhm (eV)
area %
1.8 1.6 1.6 1.5 0.7 1.8 1.2 1.7 0.8 1.8 1.5 2.0 3.1 3.4 2.1 2.5
1.7 1.5 1.5 1.3 0.7 0.7 0.7 0.7 0.6 0.6 0.9 0.7 2.0 2.5 1.8 2.2
63 72 77 52 65 30 50 22 62 34 48 22 45 64 66 70
4.6 3.3 2.6 2.8 2.0 1.9 1.5 1.9 1.9 2.5 1.9 1.9 2.3 2.9 2.7 2.2
37 28 23 47 35 70 50 78 38 66 52 78 32 23 28 20
PC3 (defects) fwhm (eV)
3.4 3.6 3.0 2.6
area %
Raman ID/IGb
43 13 6 10
0.01 0.02 0.015 0.02 0.04 0.2 0.15 0.2 0.45 0.81 0.81 0.85 0.9 0.95 0.95 0.93
a XPS C 1s spectra were measured using 630 eV synchrotron radiation. b Intensity ratios of the D versus G bands in the Raman spectra with an error of 5%. c Ref 42. d For GO, the PC1 and PC2 bands correspond to the C-OH and CdO bonding structures, respectively.
surface defects of the DWNT increases more significantly compared with those of the MWNT and GO. The MWNT and
GO already contain many defect sites that can bind to the NC, so the number of defects does not increase as much as that in
11264
J. Phys. Chem. C, Vol. 114, No. 25, 2010
Jung et al.
Figure 7. XPS C 1s spectra of (a) SWNT and (b) GO and their Pt, Cu2S, and SnO2 NC hybrid nanostructures.
the case of the DWNT. This can explain why the observed reduction of the conductivity is more significant in the case of the DWNT than that in the case of the MWNT and GO. In the SWNT series, the deposition of the NC does not increase the number of surface defects much, which explains the six times higher conductivity of the hybrid nanostructures than that of the NC-DWNT nanostructures. The electron trapping effect would be more effective because the conductivity is much higher than that of the others. Another possibility is that the electron trapping of the NC themselves through their ionic type bonding may partially contribute to the reduction of the conductivity when the number of surface defects is negligible. To confirm this, the effect of the density and morphology of the NC on the THz spectrum needs further investigation. It is noteworthy that the SWNT contain less carbon “per tube” than the MWNT, so the same wt % of NCs produces less loading on the individual SWNT. Therefore, if the same loading were made, then the effect of the NC deposition on the conductivity of SWNT can be more significant than the present data. Furthermore, it would be meaningful to investigate the effect of the functionalization (without the NC deposition) on the conductivity of carbon nanostructures. Overall, the electron trapping at the surface defects would play a dominant role in determining the conductivity of the NC hybrid nanostructures. However, it should be noted that the THzTDS may not provide enough signal-to-noise ratio to distinguish the difference between the NC hybrid nanostructures. Further experiment probably needs to investigate the dependence on the nature of NCs with a better sensitivity. Nevertheless, the present results provide the evidence that the solvothermal deposition can reduce the conductivity by forming the defect sites during the process. Finally, on the basis of this result, we can understand their photocatalytic or electrocatalytic properties, which are usually strongly dependent on the nature of NCs and their bonding interaction.
method. The average sizes of the Pt, Cu2S, and SnO2 NC were 3, 7, and 2 nm, respectively. The σ(ω) value of the hybrid nanostructures decreases in the order SWNT > DWNT > MWNT ≈ GO, showing a strong dependence on the crystallinity of the graphitic layers. As the NC (3 atomic %) deposit, the σ(ω) values of the SWNT and DWNT were reduced to a much greater extent (up to 50%), irrespective of the nature of the NC. However, the MWNT and GO show a negligible reduction of the σ(ω) values upon the deposition of the NC. We measured the XPS and Raman spectra to investigate the surface defects of the graphitic layers. The present in situ solvothermal growth of the NC increases the number of surface defects. We suggest that the electron trapping at the surface defects plays an important role in decreasing the conductivity. As the conductivity of the graphitic layers increases, such electron trapping effect can be more effective. Therefore, the THz-TDS spectrum analysis provides us valuable information about the role of the NC in the optical/electrical properties of hybrid nanostructures.
4. Conclusions
References and Notes
We measured the THz-TDS spectra of Pt, Cu2S, and SnO2-SWNT (as well as DWNT, MWNT, and GO) hybrid nanostructures to study the effect of the deposition of NC on their power absorption R(ω), refractive index nr(ω), and conductivity σ(ω). The Pt, Cu2S, and SnO2 NC were grown in situ on the acid-modified CNT and GO by the solvothermal
Acknowledgment. This study was supported by the MKE under the auspices of the ITRC support program supervised by the IITA (2008-C1090-0804-0013). This research was also supported by the WCU (World Class University) program through the NRF funded by the Ministry of Education, Science and Technology (R31-10035). J.-H.S. is grateful for the financial support of the Korea Ministry of Science and Technology under grant no. M10755020002-08N5502-00210. The SEM (Seoul), HVEM (Daejeon), and XPS (Pusan) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH. Supporting Information Available: Experimental setup, THz-TDS, XPS, and IR spectroscopy data of CNT and NC-CNT hybrid nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org.
(1) Iijima, S. Nature 1991, 354, 56. (2) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (3) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (4) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Science 2001, 294, 1317.
NC-CNT and -GO Hybrid Nanostructures (5) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Nature 2003, 424, 654. (6) Robel, I.; Bunker, B. A.; Kamat, P. V. AdV. Mater. 2005, 17, 2458. (7) Lee, H. J.; Yoon, S. W.; Kim, Y. J.; Park, J. H. Nano Lett. 2007, 7, 778. (8) Guldi, D. M.; Rahman, G. M. A.; Sgobba, V.; Kotov, N. A.; Bonifazi, D.; Prato, M. J. Am. Chem. Soc. 2006, 128, 2315. (9) Mu, Y.; Liang, H.; Hu, J.; Jiang, L.; Wan, L. J. Phys. Chem. B 2005, 109, 22212. (10) Hrapovic, S.; Liu, Y.; Male, K. B.; Luong, J. H. T. Anal. Chem. 2004, 76, 1083. (11) Tang, H.; Chen, J.; Yao, S.; Nie, L.; Deng, G.; Kuang, Y. Anal. Biochem. 2004, 331, 89. (12) Yang, M.; Yang, Y.; Liu, Y.; Shen, G.; Yu, R. Biosens. Bioelectron. 2006, 21, 1125. (13) Wen, Z.; Ci, S.; Li, J. J. Phys. Chem. C 2009, 113, 13482. (14) Ugawa, A.; Rinzler, A. G.; Tanner, D. B. Phys. ReV. B 1999, 60, R11305. (15) Tselev, A.; Woodson, M.; Qian, C.; Liu, J. Nano Lett. 2008, 8, 152. (16) Grischkowsky, D.; Keiding, S.; Exter, M.; Fattinger, C. J. Opt. Soc. Am. B 1990, 7, 2006. (17) Jeon, T. I.; Kim, K. J.; Kang, C.; Maeng, I. H.; Son, J. H.; An, K. H.; Lee, J. Y.; Lee, Y. H. J. Appl. Phys. 2004, 95, 5736. (18) Maeng, I.; Kang, C.; Oh, S. J.; Son, J. H.; An, K. H.; Lee, Y. H. Appl. Phys. Lett. 2007, 90, 051914. (19) Jeon, T. I.; Kim, K. J.; Kang, C.; Oh, S. J.; Son, J. H.; An, K. H.; Bae, D. J.; Lee, Y. H. Appl. Phys. Lett. 2002, 80, 3403. (20) Jeon, T. I.; Son, J. H.; An, K. H.; Lee, Y. H. J. Appl. Phys. 2005, 98, 034316. (21) Kro¨ll, J.; Darmo, J.; Unterrainer, K. Acta Phys. Pol., A 2005, 107, 92. (22) Xu, X.; Song, Li.; Shi, Y.; Yang, Y.; Xie, S.; Li, W. Chem. Phys. Lett. 2005, 410, 298. (23) Wu, Z.; Wang, L.; Peng, Y.; Young, A.; Seraphin, S.; Xin, H. J. Appl. Phys. 2008, 103, 094324. (24) Kampfrath, T.; Perfetti, L.; Volkann, K.; Aguirre, C. M.; Desjardins, P.; Martel, R.; Frischkorn, C.; Wolf, M. Phys. Status Solidi B 2007, 244, 3950. (25) Kang, C.; Maeng, I. H.; Oh, S. J.; Lim, S. C.; An, K. H.; Lee, Y. H.; Son, J. H. Phys. ReV. B 2007, 75, 085410. (26) Nishimura, H.; Minami, N.; Shimano, R. Appl. Phys. Lett. 2007, 91, 011108. (27) Rungsawang, R.; Geethamma, V. G.; Parrott, E. P. J.; Ritchie, D. A.; Terentjev, E. M. J. Appl. Phys. 2008, 103, 123503. (28) Parrott, E. P. J.; Zeitler, J. A.; McGregor, J.; Oei, S.-P.; Unalan, H. E.; Tan, S.-C.; Milne, W. I.; Tessonnier, J.-P.; Schlo¨gl, R.; Gladden, L. F. J. Phys. Chem. C 2009, 113, 10554.
J. Phys. Chem. C, Vol. 114, No. 25, 2010 11265 (29) Parrott, E. P. J.; Zeitler, J. A.; McGregor, J.; Oei, S.-P.; Milne, W. I.; Tessonnier, J.-P.; Su, D. S.; Schlo¨gl, R.; Gladden, L. F. AdV. Mater. 2009, 21, 3953. (30) Stoller, M. D.; Park, S. J.; Zhu, Y.; An, J. H.; Ruoff, R. S. Nano Lett. 2008, 8, 3498. (31) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov, K. S. Nano Lett. 2008, 8, 1704. (32) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490. (33) Li, D.; Mu¨ller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101. (34) Xu, C.; Wang, X.; Zhu, J. J. Phys. Chem. C 2008, 112, 19841. (35) Paek, S.-M.; Yoo, E. J.; Honma, I. Nano Lett. 2009, 9, 72. (36) Williams, G.; Kamat, P. V. Langmuir 2009, 25, 13869. (37) Lu, G.; Mao, S.; Park, S.; Ruoff, R. S.; Chen, J. Nano Res 2009, 2, 192. (38) Jasuja, K.; Berry, V. ACS Nano 2009, 3, 2358. (39) Xu, C.; Wang, X. Small 2009, 5, 2212. (40) Peng, W.; Wang, Z.; Yoshizawa, N.; Hatori, H.; Hirotsu, T. Chem. Commun. 2008, 36, 4348. (41) Wang, D.; Choi, D.; Li, J.; Yang, Z.; Nie, Z.; Kou, R.; Hu, D.; Wang, C.; Saraf, L. V.; Zhang, J.; Akasy, I. A.; Liu, J. ACS Nano 2009, 3, 907. (42) Myung, Y.; Jang, D. M.; Cho, Y. J.; Kim, H. S.; Park, J. H.; Kim, J. U.; Choi, Y. M.; Lee, C. J. J. Phys. Chem. C 2009, 113, 1251. (43) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Nature 2009, 457, 706. (44) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 30. (45) Hu, J.; Ruan, X.; Chen, Y. P. Nano Lett. 2009, 9, 2730. (46) Choucair, M.; Thordarson, P.; Stride, J. A. Nat. Nanotechnol. 2009, 4, 30. (47) Oh, S. J.; Kang, C.; Maeng, I.; Son, J. H.; Cho, N. K.; Song, J. D.; Cho, W. J.; Lee, J. I. Appl. Phys. Lett. 2007, 90, 131906. (48) Li, F.; Son, D. I.; Kim, T. W.; Ryu, E.; Kim, S. W. Nanotechnology 2009, 20, 085202. (49) Hull, R. V.; Li, L.; Xing, Y.; Chusuei, C. C. Chem. Mater. 2006, 18, 1780. (50) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature 1997, 388, 257. (51) Dresselhaus, M. S.; Dresselhaus, G.; Jorio, A.; Filho, A. G. S.; Saito, R. Carbon 2002, 40, 2043. (52) Yoon, S. W.; Kim, S. Y.; Park, J.; Park, C. J.; Lee, C. J. J. Phys. Chem. B 2005, 109, 20403. (53) Biniak, S.; Szyman´ski, G.; Siedlewski, J.; S´wiatkowski, A. Carbon 1997, 35, 1799.
JP1019894
