Doping and Dedoping in SWCNT Film by the Spontaneous Redox

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Doping and Dedoping in SWCNT Film by the Spontaneous Redox Process Dong-Wook Shin,† Xianhui Meng,‡ Jong Hak Lee,† Seong Man Yu,† Jin Hyoung Yoo,‡ Kwang Soo Lim,† Shashikant P. Patole,†,‡ and Ji-Beom Yoo*,†,‡ †

SKKU Advanced Institute of Nanotechnology (SAINT) and ‡School of Advanced Materials Science and Engineering (BK21), Sungkyunkwan University, 300, Chunchun-dong, Jangan-gu, Suwon, 440746, Republic of Korea

bS Supporting Information ABSTRACT: Single-walled carbon nanotube (SWCNT) film was deposited with MnO2 using a spontaneous redox reaction between MnO4̅ ions and the SWCNTs. Doping and dedoping had a significant effect on the sheet resistance of the resulting SWCNT film. The injection of holes in the SWCNT film S ) observed bleached the van Hove singularity transitions (E11 by optical absorption spectroscopy. The observed doping was not permanent, and spontaneous reduction under ambient conditions restored the defects and dedoped the SWCNT film. Dedoping increased the Fermi level and restored the van Hove S ). The doping and dedoping mechanism of these films was investigated. Transmission electron singularity transitions (E11 microscopy, X-ray photoelectron spectroscopy, and optical transmission spectroscopy were used to characterize the films.

’ INTRODUCTION Single-walled carbon nanotubes (SWCNTs) are promising candidates for a range of applications owing to their outstanding properties.1,2 SWCNTs can be either metallic or semiconducting depending on their diameter and chirality. The semiconducting nature of SWCNTs make them a promising material for a variety of electronics applications, such as field-effect transistors, single electron memories, sensors, and touch screen panels.3 6 The further improvement in SWCNT electronics requires modulation of the electrical properties of SWCNTs, which can be achieved by doping it with impurities.7 10 In contrast to conventional semiconductors, where doping is mainly interstitial, doping in SWCNTs can be substitutional or surface bound. As all carbon atoms in SWCNTs are seated on the surface, substitutional doping becomes a great challenge. Nevertheless, some research groups have doped SWCNTs with nitrogen, boron, and phosphorus in SWCNTs during the growth process.11 14 In surface-bound doping or chemical doping, an adsorbate or functional group attached to the surface of semiconducting SWCNTs facilitates charge transfer, resulting in n-type or p-type doped SWCNTs.15 The direction of electron transfer is generally determined by the redox potential difference between the SWCNTs and adsorbates.16 p-type and n-type SWCNTs are presumably caused by electron-accepting and electron-donating adsorbates, respectively. Until now, a range of transition metals (Au, Pt), alkaline metals (Li, K, Cs), alkaline-earth metals (Sr), and Brønsted acids (H2SO4, HNO3, and HCl) have been used for surface-bound doping.17 22 The spontaneous redox reaction between metal ions (AuCl4̅ and PtCl4 2 ion) and SWCNTs doped the SWCNTs with p-type dopants. For strong Brønsted r 2011 American Chemical Society

acids, H2SO4, HNO3, and HCl, p-type doping is observed by X-ray-induced photoelectron spectroscopy. In contrast, n-type doping is observed for the several alkali metals, such as Li, K, and Cs. The amount of electron transfer determines the doping level or Fermi level shift in the SWCNTs.23 In p-doped SWCNTs, the Fermi level shifts toward the valence band and affects the Schottky barrier between the SWCNT random network. In a random SWCNT network, two SWCNTs can have metal semiconductor or semiconductor semiconductor contact. The spontaneous redox reaction between metal ions (AuCl4̅ and PtCl4 2 ion) and SWCNTs also deposited Au and Pt nanoparticles on the outer wall of SWCNTs. The deposited metal particles may add additional metal metal or metal semiconductor contact to the SWCNT network. A larger extent of doping can remove the Schottky barriers and allow a low sheet resistance in the SWCNT network. In this case, the sheet resistance of SWCNT film is determined only by the carrier concentration of metallic and semiconducting tubes near the Fermi level. In addition, the shift in the Femi level of SWCNTs also bleaches the van Hove singularity (vHs) transitions near the Fermi level.17 Nevertheless, there is no report explaining the p-doping mechanism by metal oxides in SWCNT film. A spontaneous redox reaction between MnO4̅ ions and SWCNTs was used to deposit the MnO2 on the surface of SWCNTs. The SWCNTs were doped as a p-type by MnO2 Received: May 23, 2011 Revised: August 8, 2011 Published: August 13, 2011 18327

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deposition. The observed doping was not permanent, and the spontaneous reduction under ambient conditions restored the defects and dedoped the SWCNTs. The electrical and optical properties of doped and dedoped SWCNT films were investigated. The doping and dedoping mechanism is proposed on the basis of the observed results.

’ EXPERIMENTAL SECTION The SWCNTs with a mean diameter of 1.2 1.6 nm and a purity of 90 vol %, which were synthesized by an arc discharge process, were purchased from Hanwha Nanotech. To obtain a well-dispersed SWCNT solution, 10 mg of SWCNT powder was dispersed in a 1 wt % aqueous solution of sodium dodecylbenzene sulfonate (SDBS) surfactant. The mixture was sonicated in a horn-type sonicator (VC 505, Sonics & Materials Inc.), and all sonication processes were performed under ambient conditions for 10 min. The SWCNT solution was centrifuged in 12 000g for 1 h and used for further experiments. The SWCNT film was fabricated on a range of substrates using a polydimethysiloxane (PDMS) printing method.6 In the case of the PDMS printing method, SWCNT film deposited by filtration on the membrane (OMNIPORE 0.2 um, Millipore) was placed on a PDMS stamp and pressed down. A PDMS stamp with the SWCNT film was obtained after simply removing the membrane. The PDMS stamp with the SWCNT film was placed in contact with the quartz substrate and put in a dry oven at 80 °C for 10 min. All SWCNT films on the stamp were transferred to a quartz substrate. The PDMS stamp can also be recycled for additional film transfer. The SWCNT films fabricated on the quartz substrate were treated with an aqueous KMnO4 solution (1 mM) under ambient conditions for different times, after which the samples were rinsed with copious amounts of deionized water, dried in a N2 stream, and used for further characterization. UV vis NIR absorption spectroscopy (Shimadzu UV-3600) was used to characterize the change in intensity in the vHsrelated transition peaks, that is, ES11, ES22, and EM 11, of SWCNT films before and after doping. The change in sheet resistance of the as-prepared SWCNT film and KMnO4-treated SWCNT film was measured using a four-point probe resist meter (AIT CMTSR1000N). X-ray photoelectron spectroscopy (XPS, VG Microtech ESCA2000) was used to detect the MnO2 deposited onto the surface of the SWCNTs. Transmission electron microscopy (TEM, JEM-3010) was used to characterize the formation of MnO2 on the SWCNTs surface. ’ RESULTS AND DISCUSSION A spontaneous redox reaction between metal ions and SWCNTs leads to the reduction of metal nanoparticles on the sidewall of the SWCNTs.16 18 Efficient charge transfer from the SWCNTs to the metal ions resulted in p-type doping. The difference in reduction potential between the metal ions and SWCNTs is mainly responsible for this type of spontaneous redox reactions. Concomitantly, the spontaneous formation of MnO2 from MnO4̅ ions on the surface of the SWCNTs can be explained by the difference in the reduction potential between the SWCNTs and MnO4̅ ions, as shown in Figure 1. In Figure 1a, the Fermi level of a SWCNT was approximately +0.5 V above the potential of a standard hydrogen electrode (SHE) and was well above the reduction potentials of MnO4̅ (+1.692 V vs SHE).16 According to Figure 1a, MnO2 formation from the MnO4̅ ions requires both protons and electrons. Ma et al.24 reported that the rate of

Figure 1. (a) Schematic showing the Fermi energy (EF) of a SWCNT, and the reduction potentials of MnO4̅ vs SHE. (b) Formation of MnO2 and doping in SWCNTs due to the spontaneous redox reaction between SWCNTs and MnO4 .

MnO4̅ ion reduction to MnO2 by CNTs increases with decreasing initial pH of the aqueous KMnO4 solution. The protons released from the solution and the electrons ejected from the CNTs were responsible for the spontaneous reduction of MnO4̅ to MnO2 by the CNTs. Therefore, in an aqueous KMnO4 solution, electrons can be withdrawn from the SWCNTs, and as a result, the SWCNTs can be p-type doped (Figure 1b). To determine the effect of doping, the sheet resistance of the KMnO4-treated SWCNT film with time was investigated. Figure 2a shows the change in sheet resistance in terms of the percentage change defined as 100  (Rs before Rs after)/Rs before (%) as a function of the treatment time. The absolute sheet resistance was also plotted on the same graph. The change in sheet resistance increased sharply for the initial 60 min treatment and decreased with time thereafter. A maximum 21% change in the sheet resistance was recorded for the initial 60 min of treatment. The value reached 1.9% for a further 1080 min of treatment. In the initial 60 min of treatment, the spontaneous redox reaction might have caused p-type doping in the SWCNTs. The increase in doping with time increased the carrier concentration, resulting in a decrease in sheet resistance. Interestingly, a peculiar change in the sheet resistance was observed after 60 min of treatment. A further KMnO4 treatment increased the sheet resistance instead of decreasing it. This suggests that another mechanism dominates the sheet resistance of the longer-treated films. The formation of MnO2 on the SWCNTs affected the electrical properties of the SWCNTs. The formation of MnO2 acts as a barrier for the percolation of electrons in a SWCNT film, which might have increased the sheet resistance. p-type doping and the MnO2 barrier layer formation occurred concomitantly, where the former dominates the sheet resistance in the initial 60 min of treatment, whereas the latter dominates after 60 min of treatment. Figure 2b shows the absorption spectra of the as-prepared and doped SWCNT films. The vHs-related transition peaks corresponding to the intrinsic excitonic transitions (ES11 and ES22) of the semiconducting SWCNTs from the valence band to the conduction band can be observed in these spectra. The as-prepared 18328

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Figure 2. (a) Change in sheet resistance (Rs) in percentage and absolute values as a function of KMnO4 treatment time. (b) Optical absorption spectra of a SWCNT film (arrow indicates the bleaching of vHs transitions ES11). (c) Transmittance of the SWCNT film before and after 60 min of KMnO4 treatment (in the inset, the arrow indicates the change in transmittance below 600 nm). (d) Photographs showing a SWCNT film before and after 60 min of KMnO4 treatment.

SWCNT film showed the maximum intensity for the ES11 band, which gradually bleached with increasing KMnO4 treatment time. Kim et al. examined the doping of SWCNTs with various AuCl3 concentrations.17 They reported that, with increasing p-doping concentration in the SWCNTs, the vHs transitions disappear gradually and the work function increases. In the present absorption spectra, the intensity of the transition peaks related to the vHs, such as the ES11 band (shown by the orange arrow in Figure 2b), decreases with increasing treatment time, suggesting that the Fermi level was shifted near the first vHs in the valence band of the semiconductor nanotube. In the case of the transition peak of ES22 and EM 11, the intensity was not changed significantly with increasing treatment time. In the present experiment, the maximum treatment time was 1080 min, which was not sufficient to achieve the extent of p-type doping that can change the ES22 and EM 11 peak intensity. Nevertheless, the bleaching in the ES11 peak intensity clearly confirmed that the SWCNTs are p-type doped. Although the SWCNT film was doped more with the KMnO4 treatment, the sheet resistance of the SWCNT film rarely decreased after 60 min of treatment, as shown in Figure 2a. This means that the formation of MnO2 on the SWCNTs affected the electrical properties of the SWCNTs. Figure 2c,d shows the transmittance and a photograph before and after doping for 60

min. After immersing the SWCNT film in the aqueous KMnO4 solution for 60 min, the change in transmittance was negligible. On the other hand, the transmittance below ∼600 nm decreased gradually for a further treatment time, that is, 360 and 1080 min (inset of Figure 2c). The change in transmittance can be assigned to MnO2 formation on the surface of the SWCNTs. MnO2 formation on the surface of the SWCNTs was confirmed by TEM analysis. Figure 3a c shows TEM images of the SWCNT film treated for different times in a KMnO4 solution. The brown circles in the images highlight the MnO2 formation on the surface of the SWCNTs. Higher-magnification images (see Figure 3e g) confirmed that the presence of MnO2 on the SWCNTs increases with time. This was also confirmed using energy-dispersive X-ray spectroscopy in that the atomic percentage of Mn in the SWCNT films increases with increasing treatment time (see the Supporting Information, Figure S1). Figure 3d shows high-resolution XPS Mn 2p spectra of the SWCNT film treated for 1080 min in KMnO4 solution. The spectra for Mn 2p are split into 2p3/2 and 2p1/2 peaks due to the spin orbit interactions. The peaks of Mn 2p3/2 and 2p1/2 were centered at 642.5 and 654.3 eV, respectively. A spin-energy separation of 11.8 eV is in good agreement with the values reported for Mn 2p3/2 and Mn 2p1/2 in MnO2.25 MnO2 formation on the surface of the SWCNTs leads to heterogeneous junctions and suppresses 18329

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Figure 3. TEM images of the SWCNT film treated for (a) 60, (b) 360, and (c) 1080 min in a KMnO4 solution (scale bar is 100 nm). The corresponding high-magnification TEM images (e g) with a scale bar of 10 nm are shown in the bottom. (d) XPS Mn 2p spectra in the doped SWCNT film. (h) Schematic diagram showing the relationship between the sheet resistance and the formation of MnO2 on SWCNTs.

Figure 4. (a) Increase in Rs as a function of time. (b) Normalized absorbance as a function of the photon energy. (c) Schematic diagram of the doping and dedoping process in SWCNTs by a spontaneous redox reaction.

the percolation of electrons. The SWCNT/MnO2/SWCNT junction has very low conductivity (10 5 10 6 S/cm).26 The observed increase in sheet resistance (see Figure 2a) was attributed to the heterogeneous junctions in the SWCNT network. In the initial 60 min treatment time, MnO2 formation through nucleation results in p-type doping in SWCNT film. An increase in p-type doping decreases the sheet resistance. The further treatment favors MnO2 particle growth rather than nucleation, which increases the sheet resistance. In addition,

the formation of MnO2 on the surface of SWCNTs might have created a barrier for the electrons to percolate into the external probe of the measurement device. This is illustrated in the schematic shown in Figure 3h. The favorable extent of p-doing, as revealed by the sheet resistance, is limited by the MnO2 nucleation factor. The further treatment of the KMnO4 solution favors MnO2 particle growth and increases the sheet resistance. The MnO2 species formed do not have any covalent bonds with the SWCNTs, indicating 18330

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The Journal of Physical Chemistry C that they are physisorbed on the outer wall of the SWCNTs. This suggests that the loss of electrons from the SWCNTs may not have a permanent effect as p-type doping. The lack of any covalent bonds and the loss of electrons lead to unstable doping. The doped SWCNT films exposed to ambient conditions should eventually lead to spontaneous reduction by receiving the electrons from air. Figure 4 shows the effect of the exposure of doped SWCNT films under ambient conditions. Figure 4a shows the change in sheet resistance with ambient exposure. The SWCNT films were treated with an aqueous KMnO4 solution for 60 min, and the sheet resistance was measured (shown by an arrow and “Doping” in Figure 4a). The sheet resistance increased to 43% in 4 days, and its original value, that is, before doping, was restored (see Figure 4a). The 10-day sheet resistance was increased to 50% and became saturated at 54% in 60 days. Figure 4b shows the absorption spectra of the as-prepared SWCNT film. The KMnO4-treated SWCNT film (for 1080 min) was then exposed to ambient conditions for 60 days. After 60 days under ambient conditions, the intensity with a transition peak related to the vHs, such as the ES11 band, was fully restored. Under ambient conditions, the hole vacancies within SWCNTs were filled by receiving the electrons from the air. This suggests that, eventually, the doped SWCNTs become dedoped by a spontaneous reduction process. The higher sheet resistance than the original value can be assigned to MnO2 formation, which still exists on the surface of the SWCNTs. Figure 4c shows the doping and dedoping mechanism in SWCNTs. Spontaneous redox reaction between MnO4̅ ions and SWCNTs leads to the formation of MnO2 on the surface of the SWCNTs. The donation of three electrons from SWCNTs leads to the generation of three holes within the SWCNTs. The SWCNTs were doped p-type. In the MnO2 deposition process, nucleation dominates in the initial 60 min of the reaction and later MnO2 particle growth dominates the reaction. Under ambient conditions, hole vacancies become filled by accepting electrons from the air by spontaneous reduction. The process of dedoping neutralizes the SWCNTs and restores the sheet resistance. The spontaneous reduction does not affect the MnO2, which is physisorbed on the surface of SWCNTs. Although the SWCNTs are dedoped and restore their original charge state, the presence of MnO2 on the surface of the SWCNTs causes a higher sheet resistance. These results are expected to provide the guidelines for future SWCNT-based electronic device architectures.

’ CONCLUSION SWCNT films were coated with MnO2 by a spontaneous redox reaction between MnO4̅ ions and SWCNTs. MnO2 deposition acts as a surface-bound p-type doping in SWCNTs. The doping level affects the sheet resistance of the SWCNT film. In the doping process, the sheet resistance decreased with doping but increased after a certain limit. The decrease in sheet resistance was attributed to the increase in carrier concentration through MnO2 nucleation, whereas the increase in sheet resistance was attributed to MnO2 particle formation on the surface of the SWCNTs. The heterogeneous junctions in the SWCNT network hinder the percolation of charge carriers, resulting in an increase in sheet resistance. p-doping decreases the Fermi level and bleaches the vHs transitions (ES11) observed near the Fermi level. Spontaneous reduction under ambient conditions restores the defects and dedopes the SWCNT film. Dedoping increases the Fermi level and restores the vHs transitions (ES11). The dedoped

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SWCNT film gained more sheet resistance than its original value and also lost its transparency below 600 nm due to the residual MnO2.

’ ASSOCIATED CONTENT

bS

Supporting Information. EDX spectra of SWCNTs after doping. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20110006268) and Ministry of Knowledge and Economy (Project No. 10037449). This work was partly supported by the GRRC program of Gyeonggi province [(GRRC Sungkyunkwan 2010-B10), Development of Carbon nano composite material for lightweight vehicle]. D.-W.S. is grateful for the “Seoul Fellowship” and S.P.P. is grateful to the Korean Government for the BK 21 fellowship. ’ REFERENCES (1) Ijima, S.; Ichihashi, T. Nature 1993, 363, 603–605. (2) Zhou, W.; Bai, X.; Wang, E.; Xie, S. Adv. Mater. 2009, 21, 4565–4583. (3) Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Nano Lett. 2004, 4, 35–39. (4) 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, 1273–1276. (5) Cattanach, K.; Kulkarni, R. D.; Kozlov, M.; Manohar, S. K. Nanotechnology 2006, 17, 4123–4128. (6) Zhou, Y.; Hu, L.; Gruner, G. Appl. Phys. Lett. 2006, 88, 123109. (7) Zhou, C.; Kong, J.; Yenilmez, E.; Dai, H. Science 2000, 290, 1552–1555. (8) Javey, A.; Tu, R.; Farmer, D. B.; Guo, J.; Gordon, R. G.; Dai, H. Nano Lett. 2005, 5, 345–348. (9) Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, P. Appl. Phys. Lett. 2002, 80, 2773–2775. (10) Bockrath, M.; Hone, J.; Zettl, A.; McEuen, P. L.; Rinzler, A. G.; Smalley, R. E. Phys. Rev. B 2000, 61, R10606–R10608. (11) Xu, Z.; Lu, W. G.; Wang, W. L.; Gu, C. Z.; Liu, K. H.; Bai, X. D.; Wang, E. G.; Dai, H. J. Adv. Mater. 2008, 20, 3615–3619. (12) Terrones, M.; Ajayan, P. M.; Banhart, F.; Blase, X.; Carroll, D. L.; Charlier, J. C.; Czerw, R.; Foley, B.; Grobert, N.; Kamalakaran, R.; et al. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 355–361. (13) Maciel, I. O.; Campos-Delgado, J.; Cruz-Silva, E.; Pimenta, M. A.; Sumpter, B. G.; Meunier, V.; Lopez-Urias, F.; Munoz-Sandoval, E.; Terrones, H.; Terrones, M.; et al. Nano Lett. 2009, 9, 2267–2272. (14) Liu, Y.; Jin, Z.; Wang, J.; Cui, R.; Sun, H.; Peng, F.; Wei, L.; Wang, Z.; Liang, X.; Peng, L.; et al. Adv. Funct. Mater. 2011, 21, 986–992. (15) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Nature 1997, 388, 257–259. (16) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc. 2002, 124, 9058–9059. (17) Kim, K. K.; Bae, J. J.; Park, H. K.; Kim, S. M.; Geng, H. Z.; Park, K. A.; Shin, H. J.; Yoon, S. M.; Benayad, A.; Choi, J. Y.; et al. J. Am. Chem. Soc. 2008, 130, 12757–12761. (18) Kong, B. S.; Jung, D. H.; Oh, S. K.; Han, C. S.; Jung, H. T. J. Phys. Chem. C 2007, 111, 8377–8382. 18331

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