Article pubs.acs.org/JPCC
Pressure-Induced Selectivity for Probing Inner Tubes in Double- and Triple-Walled Carbon Nanotubes: A Resonance Raman Study R. S. Alencar,† A. L. Aguiar,†,▽ A. R. Paschoal,† P. T. C. Freire,† Y. A. Kim,‡ H. Muramatsu,§ M. Endo,∥ H. Terrones,†,¶ M. Terrones,⊥,# A. San-Miguel,○ M. S. Dresselhaus,◆ and A. G. Souza Filho*,† †
Departamento de Física, Universidade Federal do Ceará, 60455-900 Fortaleza, Ceará, Brazil School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, buk-gu, Gwangju, 500-757, Korea § Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1, Kamitomioka, Nagaoka 940-2188, Japan ∥ Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-8553, Japan ⊥ Department of Physics, Department of Chemistry, Department of Materials Science and Engineering, Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, United States # Research Center for Exotic Nanocarbons (JST), Shinshu University, 4-17-1 Wakasato, Nagano 380-853, Japan ○ Université Lyon 1, Laboratoire PMCN, CNRS, UMR 5586, F-69622 Villeurbanne Cedex, France ◆ Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, United States ▽ Departamento de Fı ́sica, Universidade Federal do Piauı ́, 64049-550, Teresina, Piauı ́, Brazil ¶ Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, New York 12180, United States ‡
S Supporting Information *
ABSTRACT: The dependence of the radial breathing modes (RBMs) and the tangential mode (G-band) of triple-wall carbon nanotubes (TWCNTs) under hydrostatic pressure is reported. Pressure screening effects are observed for the innermost tubes of TWCNTs similar to what has been already found for DWCNTs. However, using the RBM pressure coefficients in conjunction with the histogram of the diameter distribution, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs. By combining Raman spectroscopy and high-pressure measurements, it was possible to identify these two categories of inner tubes even if the two tubes exhibit the same diameters because their pressure response is different. Furthermore, it was possible to observe similar RBM profiles for the innermost tubes of TWCNTs using different resonance laser energies but also under different pressure conditions. This is attributed to changes in the electronic transition energies caused by small pressureinduced deformations. By using Raman spectroscopy, it was possible to estimate the displacement of the optical energy levels with pressure.
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cross-section (ovalization and nanotube collapse).8−14 Furthermore, it was also reported that SWCNTs, when subjected to high hydrostatic pressures, could undergo pressure-dependent shifts in their electronic transition energies (Eii),15thus making tubes come out of their resonance condition with a given laser excitation energy.4,16 For double-walled carbon nanotubes (DWCNTs), it has been observed that the pressure response of the inner tube is less pronounced than that of the outer tube, which has been
INTRODUCTION
Carbon nanotubes (CNTs) are unique for developing nanoelectromechanical systems due to their remarkable properties, which can be tuned by external parameters, such as doping, strain, and hydrostatic pressure.1−3 Raman spectroscopy has been successfully used for investigating the electronic and mechanical behavior of CNTs under extreme pressure conditions.4−7 It has been observed that the Raman G-band pressure evolution of single-walled carbon nanotubes (SWCNTs) is characterized by their frequency behavior, which is accounted for by theoretical predictions, which are associated with the frequency dependence on pressure and with the pressure-induced structural modifications of the nanotube © 2014 American Chemical Society
Received: December 24, 2013 Revised: March 16, 2014 Published: March 17, 2014 8153
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Figure 1. HRTEM images of the sample studied in this work. Besides the TWCNTs and DWCNTs which are predominant in panels (a) and (b), the sample also contains a small amount of other carbon nanostructures with a higher number of layers (c).
any laser-dependent structural transformations induced by heating effects.18 A National Bureau of Standards (NBS) type diamond anvil cell (DAC) was used to generate high pressures,21 and paraffin oil was used as the pressuretransmitting medium (PTM). Small ruby chips were introduced in the pressure chamber, and the pressure values were calibrated by using the standard ruby luminescence lines.22 Theoretical Methods. We use the SIESTA code,23 which is based on density functional theory (DFT), to describe the electronic properties of SWNTs under uniaxial stress. The general gradient approximation (GGA) as an exchange correlation potential within the pseudopotential framework was used.24,25 Because we simulated one single nanotube unit cell, the Monkhorst-Pack block was set to 1 × 1 × 40 to obtain a good description of the energy levels. We have first calculated all optimized SWCNT unit cells. For each deformation level, we fixed the atomic positions of a line of carbon atoms along the axial tube direction (the length of each nanotube is kept constant because SWCNTs are more sensitive to radial than axial deformations), and the remaining atoms were relaxed by using conjugated gradient techniques when the force was 2 GPa pressures disappears due to structural deformations.6 However, for the RBM peaks localized above 300 cm−1, the relative Raman profile changes upon increasing the pressure with Raman 8155
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the tubes with pressure, which allows some tubes to enter (or leave) the resonance conditions with changing pressure.4,16 Therefore, we separated the data into two regions that are analyzed individually with linear functions. We can clearly observe from the frequency plots in Figure 3a that the G+ band pressure coefficients below and above 2 GPa are quite different, which means that the tubes respond differently to the pressure effect below and above this pressure value, thus leading us to suggest that different tubes are in resonance below and above 2 GPa. Furthermore, it can also be suggested that small variations of the cross-section of the tubes could modify their G-band pressure response.10,11 This behavior is found to be quite similar for all three modes (G+1 , G+2 , G+3 ) using different laser excitation energies. Finally, because the G-band Raman spectra under ambient conditions (outside the DAC) before and after the pressure loading are similar, we observe that any structural transformation of TWCNTs up to 10.4 GPa is largely reversible, regarding their tangential profile (inset of Figure 3a). In Figure 3b, we show results for a Raman measurement of the low-frequency modes for the laser energy of 2.33 eV (six lower spectra) and 2.41 eV (upper spectrum). By comparing the spectrum for ELaser = 2.41 eV with the spectra under pressure (5.1 GPa, for example) using the laser energy 2.33 eV, we can clearly see that the spectra related to the innermost tubes of TWCNTs have a similar intensity profile, thus indicating that the resonant tubes are the same. These results suggest a downshift of the electronic transition energies occurring due to the tube structural deformations induced by pressure. The result would be equivalent to a red shift of the Kataura plot.4,16 In other words, the measurements indicate that the tubes under pressure enter into resonance at lower excitation energies than they would enter under ambient conditions. When the tubes that enter (or leave) the resonant condition are the internal tubes of the TWCNTs, we can be certain that the pressure-dependent effect is caused only by the screened radial pressure induced by the outer tubes because the pressure-transmitting media does not enter any of the tubes. A simple model using DFT calculations can be used to explain this behavior. By fixing the coordinates of a line of C atoms along the axial direction, we can model the nanotube cross section under uniaxial stress in one direction. That means the residual force after optimizing the structure is equal to the force necessary to deform the nanotube cross section, which is higher for nanotubes with small diameters. (See Figure S3 of the Supporting Information.) In Figure 3c, calculations are shown for the valence and conduction bands of a (5,5) SWCNT as the radial deformation is increased. We observe that the differences between their Van Hove singularities decreased with increased deformation. Figure 3d is a plot of those transitions energies (Eii) calculated for several tubes. From this Figure, we observe that the shifts of the Eii values for a (10,10) SWCNT are less pronounced than for smaller diameter tubes ((5,5), (6,6), and (9,0) SWCNTs). These results suggest that small cross-section deformations on small diameter tubes can deeply change their Eii values, which can be used to explain why we observed changes in their resonance profiles for the RBM Raman spectra only for the innermost tubes. We suggest that the pressure effect on the outer tubes of DWCNTs and of TWCNTs is not sufficient to change their electronic structures significantly. For these outer tubes, the effects of line broadening and the disappearance of such RBM modes from the Raman spectra are more pronounced than changes in their mode frequencies. By using the experimental
Figure 3. (a) Evolution of the G-band frequency with pressure for Elaser = 2.33 eV. In the inset, the lowest spectrum refers to ambient pressure outside the pressure cell before compression (OutC), the next four spectra (0, 2.04, 6.45, and 10.41 GPa) refer to different pressure conditions inside the DAC, while the uppermost spectrum refers to the environment after the pressure release (OutD). The Raman band labeled by the star is related to the PTM modes. (b) RBM Raman spectral intensity evolution with pressure from 0.1 to 6.0 GPa including ambient pressure is shown using Elaser = 2.33 eV. A vertical line is used to separate the contribution of the TWCNTs innermost RBM peaks from the spectra for the larger diameter tubes from the DWCNTs and the TWCNTs. Similarities between the RBM innermost tube profiles of the TWCNTs at 5.1 GPa using Elaser= 2.33 eV and the RBM innermost tube profiles of the TWCNTs at ambient pressure Elaser= 2.41 eV laser energy are clearly observed. Calculations for modeling the deformation of the (5,5) tube reveal filled valence bands and empty conduction band states (c), which change as the deformation level is increased. Horizontal red arrows define transition energies (Eii; i = 1, 2, 3) from each corresponding Van Hove singularity, which can be followed in panel (d), where E11, E22, and E33 are plotted as a function of the radial deformation for several nanotubes.
contributions from the intermediate and external tubes of the DWCNTs and the TWCNTs. Similar to what is found for DWCNTs and SWCNTs, the Raman G-band relative intensities for TWCNTs also decrease with increasing pressure (Figure 3a). It has been observed that the frequency of the G+ components of DWCNTs shows a quadratic behavior as a function of pressure.5,7,18 However, for TWCNTs, such a fit to the experimental data does not seem to be appropriate. Indeed, it is clear that there is a change in the profile starting around 2 GPa, which can be better followed in the frequency plot of Figure 3a. This change in behavior is related to the change in the resonance frequency of the electronic transition energies of 8156
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supports from the CLUSTER (second stage) and MEXT grants (No. 19002007), Japan. Y.A.K. acknowledges the support from Global Research Laboratory (K2090300202412E010004010) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (Information and Communication Technologies) and Future Planning, Korea.
Raman results shown in Figure 3b, it is possible to estimate the electronic transition energy shifts caused by the applied pressure. From these spectra, we obtain a variation of (2.33 to 2.41) eV/5.1 GPa = −15.7 meV/GPa, which agrees well with the results found in high-pressure absorption experiments for DWCNTs (−17.4 meV/GPa)30 and SWCNTs (−16.0 meV/ GPa).15,31
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CONCLUSIONS A resonance Raman study of TWCNTs under high-pressure conditions is reported. By using the RBM pressure coefficients, we demonstrated that pressure screening effects are strongly different for the similar diameter intermediate tubes of TWCNTs and inner tubes of DWCNTs. Intermediate tubes of TWCNTs experienced both pressure screening and structural support effects at the same time. Therefore, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs in a simple way, even if their diameter distributions and their corresponding RBM peaks are almost the same. Furthermore, we observed similarities in the RBM profiles for the innermost tubes of TWCNTs using different laser energies but under different pressure conditions. We related this effect to small changes in the electronic transition energies caused by small pressure-induced deformations, and these findings were supported by theoretical modeling. The analysis of Raman spectroscopy data allowed us to estimate the shift of the energy levels as external pressure is applied to the TWCNTs.
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ASSOCIATED CONTENT
S Supporting Information *
Lineshape analysis of Raman spectra in the Radial Breathing Mode and the G band spectral range at different pressures and the residual forces calculated for (9,0), (5,5), (6,6), and (10,10) nanotubes. 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: agsf@fisica.ufc.br. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Brazilian authors acknowledge funding from the Brazilian agency CNPq (Grant No. 307317/2010-2 and INCT NanoBioSimes) and Fundaçaõ Cearense de Apoio ao Desenvolvimento Cientfico e Tecnológico (FUNCAP) through PRONEX (Grant No. PR2-0054-00022.01.00/11) and Cooperação Internacional (CI1-0050-000310100/11), PQ (307317/20102), and CNPq-MIT cooperation grant and CAPES (CAPESCOFECUB grant 608). M.S.D. also acknowledges NSF/DMR 10-04147. M.T. acknowledges funding from the Army Research Office MURI grant W911NF-11-1-0362 on Novel FreeStanding 2D Materials focusing on Atomic Layers of Nitrides, Oxides, and Sulfides; the U.S. Air Force Office of Scientific Research MURI grant FA9550-12-1-0035; JST-Japan for the Research Center for Exotic NanoCarbons under the Japanese regional Innovation Strategy Program by the Excellence; and the Penn State Center for Nanoscale Science Seed grant on 2-D Layered Materials (DMR-0820404). M.E. acknowledges the 8157
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