Optical Microspectroscopy Study of the Mechanical Stability of Empty

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Optical Microspectroscopy Study of the Mechanical Stability of Empty and Filled Carbon Nanotubes under Hydrostatic Pressure Badawi Anis,† F. Börrnert,‡,§ M. H. Rümmeli,‡ and and C. A. Kuntscher*,† †

Experimentalphysik 2, Universität Augsburg, D-86159 Augsburg, Germany IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany § Speziallabor Triebenberg, Technische Universität Dresden, 01062 Dresden, Germany ‡

ABSTRACT: We present a high-pressure optical and infrared spectroscopy study on the mechanical stability of single-walled carbon nanotubes (SWCNTs) filled with C60 and C70 fullerene molecules (C60- and C70-peapods), double-walled carbon nanotubes (DWCNTs) derived from the C60- and C70-peapods (DWCNTs/C60 and DWCNTs/C70), and iodine-filled SWCNTs (I-SWCNTs). High-resolution transmission electron microscopy, Raman, and optical spectroscopy were used to characterize all prepared samples. For the C60- and C70-peapods, we find an anomaly in the pressure-induced shifts of the optical transitions at the critical pressures Pc1 ≈ 6.5 and 7.0 GPa, respectively, compared to Pc1 ≈ 3 GPa in the case of empty SWCNTs. The shift of the anomaly to higher pressure signals the stabilization of the nanotubes by the C60 and C70 filling. The value of Pc1 is in good agreement with theoretical predictions of the pressure-induced deformation for highly filled peapods with similar average diameter. In comparison to SWCNTs, the pressure-induced red shifts of the optical transitions in DWCNTs/C60 are extremely small below ∼10 GPa, demonstrating the enhanced mechanical stability due to the inner tube. The anomaly at the critical pressure Pd ≈ 12 GPa in DWCNTs/C60 signals the onset of the pressure-induced deformation of the tubular cross sections or the collapse of the DWCNTs. For the DWCNTs/C70, the anomaly in the pressure-induced shift is lowered to Pd ≈ 9 GPa compared to 12 GPa in the case of DWCNTs/C60. This behavior signals that the stabilization of the outer tube by the inner tube in DWCNTs/C70 is lower compared to the DWCNTs/C60. Interestingly, the iodine filling shows a stabilization for the outer tubes up to 10 GPa in contrast with the previously published Raman results. For all samples, except I-SWCNTs, the low energy absorbance decreases rapidly with increasing pressure, suggesting the destruction of the SWCNT electronic band structure and hence an increasing carrier localization. In the I-SWCNTs the pressure-induced free carriers localization is partially compensated by the metallization of the iodine chains under pressure.

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INTRODUCTION Carbon nanotubes are one of the well-known allotropes of carbon with a cylindrical structure. Since the discovery of carbon nanotubes, there has been great interest in their structural, mechanical, thermodynamical, and electrical properties. Among others their mechanical properties are attracting great interest. The robust mechanical properties of carbon nanotubes are due to the strong sp2 covalent bonds between the carbon network.1 Theoretical studies2−4 on the mechanical properties of single-walled carbon nanotubes (SWCNTs) predict them to be very strong materials, with an axial Young’s modulus ≥1 TPa. However, bundled SWCNTs are known to be soft materials in their radial direction because they are bound by weak van der Waals forces.5 The investigations demonstrated that applied pressures of a few GPa lead to severe deformations of the SWCNTs’ circular shape and their collapse at high pressure.6−10 According to theoretical calculations,11 the © XXXX American Chemical Society

SWCNT bundles undergo a sequence of structural transformations under pressure, which was confirmed experimentally by optical absorption meaurements:12 SWCNTs with an average tube diameter of 1.4 nm undergo a phase transition at Pc1≈ 3 GPa, where the tubes’ cross section is deformed from a circular to an oval shape. At Pc2≈ 7 GPa a more drastic change in the cross section from oval to racetrack or peanut-type shape occurs, followed by the collapse of the tubes at Pc3 ≈ 13 GPa.12 It has been demonstrated that the homogeneous filling of the nanotubes with an inner wall (resulting in so-called doublewalled carbon nanotubes) or with argon atoms stabilizes the SWCNTs against deformation.10,13−17 In contrast, the inhomogeneous filling with C70 and iodine molecules leads to Received: July 11, 2014 Revised: October 11, 2014

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dx.doi.org/10.1021/jp506922s | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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the destabilization of the nanotubes.10,13,16,18,19 The destabilization of the nanotubes was explained in terms of the inhomogeneous interaction (noncovalent van der Waals forces) between the nanotube wall and the inner molecules, causing a lower critical pressure for the tube’s collapse.10,13 Recently, we carried out high-pressure optical spectroscopy studies on the mechanical stability of C60 fullerene-filled SWCNTs, so-called C60-peapods and C60-peapods-derived double walled carbon nanotubes (DWCNTs/C60).12,20,21 In comparison to SWCNTs, the two anomalies at the critical pressures Pc1 ≈ 3 GPa and Pc2 ≈ 7 GPa in SWCNTs are suppressed in DWCNTs/C60 and only one anomaly at Pd ≈ 12 GPa is observed. The anomaly at Pd ≈ 12 GPa was interpreted in terms of a phase transition, where the cross section of the inner and outer tubes changes from a circular to a hexagonal shape. In the case of C60-peapods, the first anomaly in SWCNTs at Pc1 ≈ 3 GPa is shifted to Pc1≈ 6.5 GPa. Accordingly, the inner wall and the C60 molecules filling has been considered as a case of homogeneous filling and leads to the stabilization of the nanotubes.12,20,21 In general, optical and infrared spectroscopy are powerful techniques to characterize the electronic band structure in terms of the energy position and spectral weight of the excited interband and intraband transitions. As demonstrated recently, the optical response is capable of monitoring small pressureinduced deformations of the tubular cross-section, as the characteristic Van Hove singularities in the density of states in SWCNTs are very sensitive to such deformations.22,23 Here, we present a study of the mechanical stability of carbon nanotubes by filling the tubes with different species−such as inner tubes, fullerene molecules, and atoms−via the pressureinduced changes in the optical response. For this purpose, C60peapods, C70-peapods, DWCNTs/C60, and DWCNTs/C70 were synthesized and characterized, and their optical absorption spectra were studied as a function of pressure. Additionally, iodine-filled SWCNTs (I-SWCNTs) were prepared to extend the comparison by the effect of atom filling on the stability of the SWCNTs and to compare the results with those of the peapods and DWCNTs.

microscope was operated using an electron acceleration voltage of 80 kV to reduce knock-on damage.28 Local energy dispersive X-ray (EDX) spectra were taken with a Bruker XFlash 5030T silicon drift detector attached to the microscope. Raman spectra were excited by 514 nm argon laser at room temperature and the spectra were recorded by a triple Raman spectrometer T64000 (Horiba Jobin Yvon), interfaced to an Olympus BX-40 microscope (100× LWD objective). The laser power impinging on the sample was 0.1 mW. The Raman spectrometer was calibrated by using the mercury line at 435.8 nm. Transmittance spectra of the nanotube films were measured at room temperature in the energy range 200−22000 cm−1 with a frequency resolution of 2 and 4 cm−1 for the far-infrared and mid-infrared/visible frequency range, respectively. Bruker IFS 66v/S Fourier transform infrared spectrometer in combination with an infrared microscope (Bruker IR Scope II) with a 15× Cassegrain objective was used. A clamp diamond anvil cell (Diacell cryoDAC-Mega) was used for generation of pressure in the far/mid-infrared range, where Syassen-Holzapfel type29 DAC was used in the near-infrared/visible range. The ruby luminescence technique was used for pressure measurement. Liquid nitrogen served as hydrostatic pressure transmitting medium (PTM). Nitrogen solidifies at ∼2.4 GPa and undergoes a structural phase transition at at ∼5 GPa.30 However, it remains hydrostatic up to ∼10 GPa according to Klotz et al.,31 and therefore, the pressure transmitting medium should not have a effect on the pressure-induced effects, at least below 10 GPa. Furthermore, according to our earlier works,20,21,23 the choice of the pressure transmitting medium only affects the pressure-induced effects quantitatively (i.e., shift of critical pressures) but not qualitatively. However, subtle effects expecially in the low-pressure region, e.g., due to phase transitions in the pressure transmitting medium, might be beyond the sensitivity of our measurement technique. The intensity Is(ω) of the radiation transmitted through the sample and the intensity Iref(ω) of the radiation transmitted through the PTM in the DAC were measured. From Is(ω) and Iref (ω) the transmittance and absorbance spectra were calculated according to T(ω) = Is(ω)/Iref (ω) and A(ω) = −log10 T(ω), respectively.

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EXPERIMENTAL SECTION All samples investigated here (SWCNTs, SWCNTs filled with C60, C70 or iodine, DWCNTs) were synthesized from the following materials: Arc discharge SWCNTs were purchased from Carbon Solutions, Inc. (Type P2 and average diameter 1.4 nm). C60- and C70-fullerene with purity 99.98% was purchased from Term USA. Triton X-100 (∼10% in H2O) was purchased from Sigma-Aldrich. DWCNTs/C60 and DWCNTs/C70 were prepared from C60and C70-peapods, respectively. SWCNTs were filled with fullerene molecules using the sublimation method.24 The prepared peapods were then transformed to DWCNTs by tempering the peapods at 1250 °C for 24 h under dynamic vacuum and subsequently cooling the furnace to room temperature.25 I-SWCNTs were prepared using the vapor method described by Guan et al.26 Free-standing films from all samples under investigation for IR spectroscopy measurements were prepared from Triton X-100 suspension.27 The samples were characterized in different preparation stages with a JEOL JEM-2010F high-resolution transmission electron microscope (HRTEM) retrofitted with two CEOS third-order spherical aberration correctors for the objective lens (CETCOR) and the condenser system (CESCOR). The

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RESULTS AND DISCUSSION Electron Microscopy. The HRTEM image of the empty SWCNTs is shown in Figure 1 a. The image reveals that the starting SWCNTs are empty with an average diameter of 1.42 nm, which is suitable for the incorporation of the C60 and C70 molecules into the interior of the nanotubes, since the minimum diameter of SWCNTs required for the incorporation is 1.28 nm.32 From the image one can observe comparably low amount of amorphous carbon over the nanotubes surface, this is because the HRTEM can focus on individual nanotube and easily detect such insignificant amount of amorphous carbon. The formation of 1D chain peapods from C60 and C70 molecules inside the SWCNTs, respectively, is illustrated in parts b and c of Figure 1. The images indicate the high filling ratio of the peapods, which is ≥95% and ≥90% in the case of C60- and C70-peapods, respectively. The high filling ratio in this case could be attributed to the higher temperature used and long treatment time (750 °C for 5 days), where more fullerene molecules will arrive to the surface of the SWCNTs and forced to incorporate inside the nanotubes, leading to the high filling ratio.20,24 B



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one can clearly recognize a crystalline or at least an ordered structure. During the measurements, the energy-dispersive Xray (EDX) spectra showed a very clear iodine signal. This means that, due to the low boiling point of iodine and the enlarged interaction cross section due to the low electron energies, the iodine atoms inside the nanotubes are readily evaporated and iodine atoms inside the carbon nanotubes would move too fast to be imaged. In the next subsection, the filling of SWCNTs with iodine atoms will be proved using Raman spectroscopy. Raman Spectroscopy. Radial Breathing Mode. Figure 2 shows the Raman spectra of (a) pristine SWCNTs, (b) C60peapods, and (c) C70-peapods in the low frequency range between 120 and 240 cm−1, where the radial breathing modes (RMBs) of the primary tubes (outer tubes) can be observed. The peaks range from 150 to 187 cm−1. To specify the contribution of the outer tubes, the spectra were fitted using Lorentzian functions. From the theoretical calculations, the diameter d of the tubes is related to the frequency ωr of the RBM according to the relation d = A/ωr, where A is a constant.34 If the small effect of the tube−tube interaction neglected, the value of A would be 234. In this case the uncertainty in the diameter calculations would be around 5%.34 The calculated values of the SWCNTs diameters are in the range from 1.30 to 1.55 nm with an average tube diameter of 1.42 nm. As shown in Figure 2b, the RBMs of the C60-peapods are downshifted by a small amount ∼1−6 cm−1 toward lower frequencies. This small downshift has been attributed to the charge transfer from nanotubes to the C60 molecules, where the charge transfer from the nanotubes induces the C−C bonds softening and hence shifting the RBMs to lower frequencies.35−38 In the case of C70-peapods (see Figure 2c), one can observe that the RBMs of the nanotubes are not affected by the encapsulation of the C70 molecules. This behavior signals that a possible charge transfer between C70 molecules and the SWCNT wall is too small to lead to significant shifts in the RBMs.39−41

Figure 1. HRTEM image of a bundle of (a) SWCNTs, (b) C60peapods, (c) C70-peapods, (d) DWCNTs/C60, (e) DWCNTs/C70, and (f) I-SWCNTs.

Parts d and e of Figure 1 depict the HRTEM images for the DWCNTs/C60 and DWCNTs/C70, respectively. The images reveal that the inner tubes are defect free. This can be attributed to the multistep rotations of the C−C bonds in the sp2 carbon network between the fullerene molecules, the socalled Stone−Wales transformation. This mechanism takes a long time and requires high temperatures, so the obtained inner wall is defect free.33 Based on the HRTEM images (not shown), we estimate a filling ratio of >95% and >90% for DWCNTs/C60 and DWCNTs/C70, respectively. The HRTEM images of the I-SWCNTs under “normal” high-resolution low-voltage conditions are shown in Figure 1f. One can observe dark spots at the center of the images from iodine atoms, especially at the upper part of the image where

Figure 2. Raman spectra recorded with a 515 nm excitation wavelength of the RBM for (a) SWCNTs, (b) C60 peapods, (c) C70 peapods, (d) DWCNTs/C60, (e) DWCNTs/C70, and (f) (I-SWCNTs). In the frequency range 400−700 cm−1, the weak mode at ∼492 cm−1 in (b) corresponds to the radial symmetry mode Ag(1) of the C60 molecules and the Raman modes in (c) correspond to the symmetry mode A′1 of the C70 molecules. Raman spectra in the range 400−700 cm−1 in (b) and (c) were multiplied by a factor of 15 and (d) and (e) by a factor of 2. The dashed-red line is a fit to the data with Lorentzian functions. The Lorentz contributions are shown as well. C



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Figure 3. Raman spectra recorded with a 515 nm excitation wavelength of the G-mode of (a) SWCNTs, (b) C60-peapods, (c) C70-peapods, (d) DWCNTs/C60, (e) DWCNTs/C70, and (f) I-SWCNTs. The Raman mode at 1469 cm−1 in (b) corresponds to the symmetry mode Ag(2) of the C60 molecules. The Raman modes at 1226, 1259, 1446, and 1470 cm−1 in (c) correspond to the symmetry mode E″1 , E′2, E″1 and A′1, respectively, of the C70 molecules. Raman spectra in the range 1200−1500 cm−1 in (a), (b), and (c) were multiplied by a factor of 40. The dashed-red line is a fit to the data with Lorentzian functions. The Lorentz contributions are shown as well.

In the frequency range 250−700 cm−1 no significant Raman features are observed for the empty SWCNTs. In the case of the C60-peapods (see Figure 2b), one can observe the weak Raman radial symmetry mode Ag(1) of the C60 molecules at 492 cm−1 (indicated by small arrow).35,42 In the case of C70peapods, one can observe the appearance of new Raman active peaks (see Figure 2c). These new peaks are due to the A′1 of the C70 molecules Raman active mode at 260, 450, 570, and 699 cm−1.42−44 Compared to the Raman spectra of C60 and C70 solid,45 one can observe that the vibrational frequencies of these modes are downshifted by ∼1−5 cm−1. The softening of the fullerene modes inside the SWCNTs could be attributed to the formation C60 or C70 molecules chain inside the SWCNTs cavity. The low intensities of the C60 and C70 modes in the case of C60- and C70-peapods, respectively, compared to the intensities of C60 and C70 solid, indicate that all the fullerene molecules are encapsulated inside the nanotubes and there are no molecules outside the SWCNTs which signals the high purity of the prepared peapods samples. Figure 2 shows the Raman spectra of (d) DWCNTs/C60 and (e) DWCNTs/C70. In the low frequency range 120−240 cm−1, the RBMs peaks in DWCNTs/C60 and DWCNTs/C70 are shifted toward lower frequencies by 5−7 and 1−10 cm−1, respectively, compared to the empty SWCNTs. The outer tubes diameters were calculated according to d = A/ωr (see above). The diameters of the outer walls in the case of DWCNTs/C60 range from 1.36 to 1.56 nm, with average tubes diameter of 1.43 nm and for the DWCNTs/C70, the diameters of the outer walls range from 1.37 to 1.56 nm, with average tubes diameter of 1.45 nm. The downshift of the RBMs and the corresponding slight increase in the outer tubes diameters compared to the SWCNTs can be attributed to the tube diameter enlargement with the long heat treatments via several Stone−Wales transformation on the SWCNTs surface.33,46 Compared to pristine SWCNTs, several new peaks with quite low intensity are observed in the frequency range 240− 350 cm−1 for DWCNTs/C60 and DWCNTs/C70. These new

S peaks are associated with the small E22 semiconducting 34,35 Generally, the 514 secondary tubes with ∼0.7−0.8 nm. nm excitation is unsuitable for resonant enhancement of secondary tubes; only the smaller secondary tubes with ∼0.7− 0.8 nm are resonant with 514 nm excitation via ES22. Therefore, quite low intensity of the inner tubes’ RBM is expected.34,47 The values for the inner tube diameters amount to 0.72− 0.87 nm with average tube diameter of 0.8 nm in the case of DWCNTs/C60 and 0.74−0.9 nm with average tube diameter of 0.85 nm in the case of DWCNTs/C70. These values are higher than the diameter of the parent fullerene molecules (0.71 and 0.792 nm for C60 and C70, respectively). This difference could be attributed to the tube diameter enlargement due to the long heat treatment; where at the beginning the formed inner tubes have diameters equal to the C60 or the C70 molecules diameter and with long heat treatment, tubes with larger diameters are grown by the transformation of the diameters by several Stone−Wales transformations.33,46,48 One can observe that the average outer and inner tubes diameters in the case of DWCNTs/C70 is slightly higher than the average of the DWCNTs/C60. This behavior is consistent with the previously published data.49 Figure 2f depicts the Raman spectra of I-SWCNTs in the low frequency range 120−400 cm−1. The peak at 176 cm−1 and its overtones at 331 and 351 cm−1 originate after the encapsulation of iodine atoms inside the SWCNTs. Wang et al.50 observed a Raman-active mode at 174 cm−1 for I-SWCNTs excited by 514.5 nm. To clarify the origin of this band, they performed a density functional theory (DFT) calculations for the vibrational properties of the isolated I−3 , I−5 , and I−7 polyiodide chains. From the DFT calculations the expected Raman-active modes are located at 95.6, 148.8, and 152.4 cm−1 for I−3 , I−5 , and I−7 polyiodide chains, respectively. Due to the similarity in the peak position of the I−5 and I−7 Raman bands, Wang et al.50,51 attributed the appearance of the peak at 174 cm−1 to the two polyiodides. The discrepancy between the experimental and theoretical values could be attributed to the interaction between

D



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Figure 4. Background-subtracted absorbance spectra of (a) SWCNTs, (b) C60-peapods, (c) C70-peapods, and (d) I-SWCNTs together with the fit of the absorption bands using Lorentzian oscillators. Inset: absorbance spectrum for the SWCNTs together with the linear background (dashed red line) and the absorbance spectrum after background subtraction.

1576 cm−1 are due to the ATO 1 (S,M) transverse mode of the semiconducting and metallic tubes, respectively. The extra peak 55−57 around 1601 cm−1 originates from the ETO 1 (M,S) modes. For the C60-peapods (Figure 3b), the three components of the G− mode are blue-shifted by 1−3 cm−1, while no significant shift is observed for the G+ component at 1996 cm−1. It is wellknown that the G+ components originate from the carbon atoms vibrating along the nanotube axis, while the G− components originate from the carbon atoms vibrating along the tubes circumference.58 This means that the radial overlapping between the π states of SWCNTs and C60 molecules will hinder the vibrations of carbon atoms along the circumference of the tubes and hence shifts the G− components up. In contrast, in the case of C70-peapods, the G mode components appear at the same positions as in the SWCNTs, which means that the charge transfer between the C70 molecules and the outer tube seems to be too small to be observed in the Raman data.34 Parts d and e of Figure 3 depict the G-mode region of the DWCNTs/C60 and DWCNTs/C70, respectively, in the frequency range 1500−1650 cm−1. The three components of the G mode appear at the same positions compared to the SWCNTs, which means that the interaction between the inner and outer tubes seems to be too small to be observed in the Raman data.34 For I-SWCNTs (see Figure 3f) the G lines are upshifted by ∼9−18 cm−1 compared to the empty SWCNTs. This could be attributed to the electrons transfer from the SWCNTs to the iodine molecules, generating negatively charged polyiodine chains.50 According to Grigorian et al.,51 the transfer of electrons from the π orbitals is expected to induced a contraction of the hexagonal rings, up-shifting the G-mode frequency. This behavior is consistent with the perviously published data on acceptor-intercalated graphite compounds.50,51,59−62 Optical Spectroscopy of Free-Standing Carbon Nanotube Films. The inset of Figure 4a shows the optical absorption spectrum of the free-standing SWCNT film in the frequency range 4000−22000 cm−1. Several pronounced

the SWCNTs and the iodine chains. Accordingly, we interpret the peak at 176 cm−1 due to the presence of the two polyiodide chains I−5 and I−7 inside the SWCNTs. The presence of the I−3 polyiodide chains in our sample cannot be excluded, but we cannot observe any Raman peaks related to these chains due to the detection limit of our Raman instrument (∼100 cm−1). One can observe also that the RBM of the SWCNTs at 151 cm−1 is completely suppressed. This could be attributed to the loss of the Raman resonance because of the removal of the electrons from the SWCNTs walls by hole doping.52 Tangential Mode. The tangential mode region of the Raman spectra for (a) pristine SWCNTs, (b) C60-peapods, and (c) C70-peapods is depicted in Figure 3. Only the D-mode at 1348 cm−1 is observed for the SWCNTs. In contrast, in the case of C60 peapods, in addition to to D-mode, the Ramanactive mode Ag(2) of the C60 molecules at 1469 cm−1 is observed. The average integrated intensity ratio Ag(2)/G-mode = 2 × 10−3 suggests a high filling ratio of the peapods.53,54 For the C70-peapods, several new Raman-active modes can be observed at 1226, 1259, 1446, and 1470 cm−1 (indicated by small arrows) due to the E″1 , E′2, E″1 , and A′1, respectively, of the C70 molecules.42−44 The average integrated intensity ratios between E1″, E2′ , E1″, A1′ , and G-mode are 9.5 × 10−3, 9.0 × 10−3, 15.0 × 10−3, and 13.0 × 10−3, respectively, indicating the high filling ratio of the C70 peapods.53,54 In general, the high intensities ratio of the Raman active modes in the case of C70peapods compared to the C60-peapods (see Figures 3 and 2b,c), is not an indication of the high filling of the C70-peapods but due to the relatively large number of the intratubular C atoms in the C70-peapods compared to the number in the C60peapods.49 In high frequency range 1500−1650 cm−1 the G mode is observed for all samples. In the case of SWCNTs the band centered at 1595 cm−1 is related to the G+ peak, which is mainly due to the ALO 1 (S) longitudinal mode of the semiconducting primary tubes.55,56 The lower frequency bands at 1555, 1569, and 1576 cm−1 are due to the G− peak. The 1555 cm−1 peak is ascribed to the ELO 2 (M,S) longitudinal mode of the metallic and semiconducting tubes,57 and the two bands at 1569 and E



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Table 1. Peak-Position Frequencies (cm−1) of the Lorentzian Contributions from the Different Optical Absorption Bands in Free-Standing Films of SWCNTs, C60-Peapods, C70-Peapods, DWCNTs/C60, DWCNTs/C70, and I-SWCNTs (Error Bar ±10 cm−1 for S11 and S22; Error Bar ±30 cm−1 for M11, S33, and S44) SWCNTs S11 S22 M11 S33/44 S11 S22 M11 S33/44 S22,inner

4800 8300 13300 17763 4830 8300 13250 17435

C60-peapods

5255 6920 8960 9610 14180 15098 18873 19909 I-SWCNTs 5300 8940 14170 18125

6960 9650 15100 19033

10330

10390 20000

4914 8300 13440 16875

C70-peapods

5350 9640 9020 9670 14260 15120 17910 18947 DWCNTs/C60 5185 9580 14100 18780

8930 13270 17700

6870 10300

10340

4800 8300 13320 18150

5240 8970 14140 18725 DWCNTs/C70

4745 8570 13250

5115 9160 14550

6920 9610

10330

19465 6920 9710

10540

19835 16920

According to Table 1, in C60-peapods all the Lorentzian contributions are slightly shifted to higher energies compared to the empty SWCNTs. This could be ascribed to the hybridization between the nearly free electron state (NFE) and the t1u state of the C60 molecules,48 since the hybridization is expected to reduce the effective tube diameter, and hence, the optical absorption bands will be blue-shifted.64 In the case of C 70-peapods, all the Lorentzian contributions are not significantly changed regarding their energy position: all the changes are within the error bar. This could be explained as follows: According to previous HRTEM and theoretical studies on C70-peapods,65−67 the ellipsoidal C70 molecules are arranged in transverse (standing) orientation (with the 0.792 nm axis) for ∼1.49 nm diameter nanotubes and in longitudinal (lying) orientation (with the 0.691 nm axis) for nanotubes with diameter ∼1.36 nm. Since the SWCNTs used in the present work have a diameter distribution in the range of 1.30−1.55 nm (with 1.42 nm average tube diameter), we can assume that a large fraction of the C70 peapods are filled with C70 molecules in the transverse (standing) orientation. This assumption is in agreement with our high pressure results on the C70-peapods (see the following section) and high-pressure Raman study on C70-peapods,18 where the flipping of the C70 molecules from the transverse to the longitudinal orientation was observed at ∼1.5 GPa. In the transverse orientation, the free space between C70 and the tube is 1.4−0.792 nm = 0.608 nm, so the free space in our C70-peapods is comparable to that in 1.31 nm diameter SWCNTs filled with C60 molecules. According to Rochefort,68 the 1.3 nm SWCNT filled with C60 molecules is energetically stable but not the ideal case for the hybridization between the NFE states of the nanotube wall and the C60 states. The effective tube diameter, and hence the energy position of the absorption bands, is not expected to be severely changed in this case. In the case of DWCNTs/C60 and DWCNTs/C70 all contributions are shifted to lower energies compared to the corresponding features in SWCNTs (see Table 1). This could be attributed to the fattening of the tubes with long heat treatment causing the effective tube diameters to increase,33 and hence, the absorption bands are shifted to lower energies, according to the Kataura plot.64 According to Table 1, the peak position frequencies of all contributions in I-SWCNTs are not significantly changed as compared to SWCNTs: all the changes are within the error bar. But according to Figure 4f, the intensity of the S11 is reduced compared to the empty SWCNTs. We attribute this intensity reduction to the removal of electrons from the valence band of

absorption bands are observed on the top of a broad background centered at around 5 eV, which is due to the π−π* electronic interband transition of the graphene sheet. The background was estimated by a linear function. The S11, S22, and S33/44 absorption bands are due to the first, second, and higher optical transitions in the semiconducting tubes, respectively. The M11 absorption band is assigned to the first optical transition in the metallic tubes. All the optical transitions exhibit a fine structure, which reflects the nanotube diameter distribution in the sample. Therefore, all the absorption bands were fitted with several Lorentz oscillators, and the frequencies of the contributions are listed in Table 1. The above-described analysis was also applied to the optical absorbance spectrum of the free-standing (b) C60-peapod, (c) C70-peapod, (d) DWCNT/C60, (e) DWCNT/C70, and (f) I‑SWCNT films. Within the zone-folding scheme,63 the energy between the ith vHS below and above the Fermi level can be written as Eii = 2iaC−Cγo/d, where d is the nanotube diameter and aC−C is the carbon−carbon distance (aC−C = 0.142 nm). The number i denotes the order of the valence and conduction energy bands, where i = 3, 6 for metallic nanotubes and i = 1, 2, 4, 5, 7 for semiconducting tubes.63 From the above relation, the S11 and S22 optical transitions in the case of DWCNTs/C60 and DWCNTs/C70 are expected to be at around ∼0.6 and 1.2 eV, respectively, for outer tubes and for inner tubes at around ∼1.2 and 2.4 eV. This means that the region between 1 and 1.5 eV (see Figure 4d,e) consists of an overlap between the S22 optical transitions of the outer tubes and S11 optical transitions of the inner tubes. Similarly, the region between 1.5 and 2.3 eV consists of an overlap between the M11 optical transitions of the outer tubes and S22 optical transitions of the inner tubes. In the case of DWCNTs/C70, one can observe that the broad band between 1 and 1.5 eV consists of many pronounced contributions from the S22 optical transitions of the outer tubes and S11 optical transitions of the inner tubes, compared to the DWCNTs/C60. Also the obvious band centered at 16920 cm−1 (∼2.1 eV) could be assigned to the S22 optical transitions of the inner tubes. The appearance of the new contributions from the inner tubes of DWCNTs/C70 compared to DWCNTs/C60 could be explained as follows: as discussed above, a large fraction of the C70 peapods are filled with the ellipsoidal C70 molecules in transverse orientation (with the 0.792 nm axis) and the rest of the peapods are filled with C70 molecules in the tangential orientation (with the 0.691 nm axis). Therefore, during the transformation of the C70 molecules to inner walls by heat treatment, the formation of different inner tubes diameters and also different chiralities is expected. F



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Figure 5. Background-subtracted absorbance spectra of (a) SWCNTs, (b) C60-peapods, (c) C70-peapods, (d) DWCNTs/C60, (e) DWCNTs/C70, and (f) I-SWCNTs. The insets depict the absorbance spectrum at the lowest pressure during pressure increase together with the various Lorentzian contributions and the absorbance spectrum at the lowest pressure during pressure release.

fitting curve and the Lorentz contributions. The absorption bands Sii and Mii correspond to the i-th optical transitions in the semiconducting and metallic SWCNTs, respectively. It is clear that the absorption bands at the lowest pressure are broadened and the fine-structure due to different tube diameters is smeared out, as compared to the free-standing films (see Figure 4). In the further discussions only the strong Lorentz contributions will be considered. For SWCNTs, C60peapods, C70-peapods, and I-SWCNTs only one strong contribution for the S11 band and two strong contributions for S22 and M11 bands are observed, which are marked by S22(1), S22(2), M11(1), and M11(2), respectively. In comparison, in the case of DWCNTs/C60, one strong contribution for the S11, S22, and M11 absorption bands is observed and in the case of DWCNTs/C70 one contribution for the S11 band, four strong contributions for S22, and two for the M11 bands are observed. With increasing pressure all absorption bands for all samples under investigation red-shift, broaden, and lose spectral weight. The redshift is generally ascribed to σ*−π* hybridization and symmetry breaking.27,72,73 All optical transitions S11, S22, and M11 are resolvable up to the highest applied pressure. SWCNTs under Pressure. For a quantitative analysis of the pressure-induced red-shifts of the absorption bands, we show in

the nanotubes in I-SWCNTs.69−71 To confirm this idea, we calculated S11/S22 spectral weight ratio for empty SWCNTs, C60-peapods, C70-peapods, DWCNTs/C60, DWCNTs/C70, and I-SWCNTs. The values are 1.35, 1.33, 1.38, 1.38, 1.00, and 1.10, respectively. The S11/S22 spectral weight ratio for all of the samples is around 1.38 except for two samples namely, I‑SWCNTs and DWCNTs/C70. The lower ratio in the case of I‑SWCNTs can be ascribed to the removal of electrons from the first allowed optical transition due the hole doping, and hence, a reduction in the spectral weight of the S11 optical transition is expected. But in the case of DWCNTs/C70 the spectral weight ratio is lower than that of SWCNTs and ISWCNTs. This is due to the increase of the spectral weight of the broad band between 1 and 1.5 eV (see Figure 4c) because of the appearance of the contributions from the S11 optical transitions in the inner tubes. Optical Spectroscopy of Carbon Nanotubes under Pressure. The background-subtracted absorbance spectra as a function of pressure are shown in Figure 5a−f for (a) SWCNTs, (b) C60-peapods, (c) C70-peapods, (d) DWCNTs/ C60, (e) DWCNTs/C70, and (f) I-SWCNTs films. The insets of Figure 5 show the absorbance spectra of all samples under investigation at the lowest pressure together with the total G



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Figure 6 the relative energy shifts of the Lorentz contributions as a function of pressure for SWCNTs. The shift was calculated

Figure 6. Relative energy shift of the optical transitions as a function of pressure in SWCNTs. The shift was calculated as the difference between the absorption frequency of the contribution νpi at a pressure pi and the absorption frequency νp1 at the lowest pressure value p1. All contributions during pressure release are marked as Sii (Re) and Mii (Re) for both semiconducting and metallic tubes, respectively. The shaded gray bars mark the critical pressure regimes as discussed in the text. The proposed sequence of structural deformations in SWCNTs is illustrated on the top.

Figure 7. Relative energy shift of the optical transitions as a function of pressure in (a) C60-peapods and (b) C70-peapods. The shift was calculated as the difference between the absorption frequency of the contribution νpi at a pressure pi, and the absorption frequency νp1 at the lowest pressure value p1. All the contributions during pressure release are marked as Sii (Re) and Mii (Re) for both semiconducting and metallic tubes, respectively. The shaded, gray areas mark the critical pressures regions as discussed in the text. The inset in (b) shows the shifts for the S22(1) and M11(2) band in the low-pressure regime to illustrate the Pflip and Pc1 anomalies. The proposed deformation of the cross sections for the peapods in the different pressure ranges is illustrated on the top.

as the difference between the absorption frequency of the contribution νpi at a pressure pi, and the absorption frequency νp1 at the lowest pressure value p1. The pressure-induced shifts show three anomalies at Pc1 ≈ 3 GPa, Pc2 ≈ 7 GPa, and Pc3 ≈ 13 GPa, followed by a plateau. The second derivative of the evolution of the ith optical transition Eii with pressure was used as a criterium for the values of the critical pressure regimes. The shaded gray bars in Figure 6 mark the critical pressure regime, with an error bar of ±0.5 GPa. According to theoretical predictions and our earlier results,11,12,20−23,74−79 we interpret the anomaly at Pc1 as a structural phase transition from a circular to an oval shape. The anomaly at Pc2 signals a more drastic change in the tubes’ cross section from oval to race-track or peanut-type shape.23 The plateau with an onset at Pc3 ≈ 13 GPa indicates a saturation of the pressure-induced deformation and hybridization effects above this pressure. According to previous studies on SWCNTs,12,13,20,21,23 we interpret the behavior of the SWCNTs above 13 GPa in terms of the collapse of the tubes. The rapid increase in the relative pressure-induced shifts above 11 GPa indicates the onset of the collapse, which is completed at Pc3. The proposed sequence of structural deformations in SWCNTs is illustrated on the top of Figure 6. Peapods under Pressure. Next, the effect of the C60 molecules filling on the mechanical stability of SWCNTs will be considered. Figure 7a shows the relative pressure-induced shifts for the C60 peapods under pressure. First, one notices two anomalies in the pressure-induced shifts at Pc1 ≈ 6.5 GPa and Pc2 ≈ 13 GPa, followed by a plateau. According to an earlier high-pressure X-ray study80 and our recent recent results20 on C60-peapods, we interpret the anomaly at Pc1 ≈ 6.5 GPa in terms of a structural phase transformation from circular to oval shape. Above Pc1 ≈ 6.5 GPa a more drastic ovalization of the outer tubes is expected. Here, the flat or peanut structure is excluded due to steric reasons.10 It is known that intercalated C60 fullerene molecules have a high bulk modulus and can be considered as nondeformable molecules.81 Therefore, the C60-

peapods at high pressure may be transformed to an oval rather than to a flat shape.10 The plateau with the onset at Pc2 ≈ 13 GPa indicates a saturation of the pressure-induced deformation and hybridization above this pressure. In general, the relative energy shifts in the case of C60peapods are quite large compared to the empty SWCNTs (see Figure 6). This could be explained in terms of the formation of close-packed C60 molecules chains inside the SWCNTs cavity.32,82,83 Besides, C60 molecules are known to be very hard molecules with a high bulk modulus;81 for this reason, the pressure-induced symmetry breaking in the outer tubes will be enhanced as compared to the pristine SWCNTs, and hence larger relative pressure-induced shifts are expected. Compared to the empty SWCNTs (see Figure 6), the first anomaly is shifted from ∼3 GPa to ∼6.5 GPa in the C60peapods. This behavior signals the stabilization of the SWCNTs outer wall by the inner fullerene molecules. This finding is in contrast to the results of high-pressure Raman measurements on SWCNTs filled with C70 molecules,10 where an anomaly at ∼2−3 GPa was observed in the pressure dependence of the RBM line width for various pressure transmitting media, i.e., at the same critical pressure as for SWCNTs. Rafailov et al.18 showed that due to the high elastic modulus for deformations of the SWCNTs in a plane perpendicular to the tube axes of ∼33 GPa84 and due to the high bulk modulus of the encapsulated C60 fullerene,81 the applied pressure up to at least 5 GPa is carried predominately by the SWCNTs and then H



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Figure 6), the first structural anomaly is shifted from ∼3 GPa to ∼7 GPa. This behavior signals the stabilization of the SWCNTs outer wall by the C70 fullerene molecules, in contrast to earlier high-pressure Raman results on C70-peapods.10,13,18 Based on our discussion on the C60-peapods, the anomaly at Pc1 ≈ 7.0 GPa could be attributed to a structural-phase transformation of the nanotubes from a circular to an oval shape. The value of Pc1 ≈ 7.0 GPa is close to the predicted value of ∼6.0 GPa according to Pugno et al.85,86 (see above). Above Pc1, the red-shifts of all the optical absorption bands are increased; this could be attributed to to the inhomogeneous interaction, i.e., noncovalent van der Waals forces, between the nanotube walls and the inner molecules. Like in the case of C60peapods, the C70 molecules are also considered as hard molecules. Therefore, the flat or peanut structure of the nanotubes is excluded due to steric reasons.10 DWCNTs under Pressure. The relative energy shifts of the optical transitions S11, S22, and M11 in the DWCNTs/C60 bundles are depicted in Figure 8a. One notices that the redshift

transferred to the encapsulated C60 molecules. Also, a high pressure X-ray study on the C60-peapods by Chorro et al.80 predicted the transformation of the outer walls from the circular to the oval structure at ∼4 GPa. Accordingly, the anomaly at Pc1 ≈ 6.5 GPa could be attributed to a structural phase transformation from circular to oval shape. Above Pc1 ≈ 6.5 GPa a more drastic ovalization of the outer tubes is expected. Here, the flat or peanut structure is excluded due to steric reasons.10 It is known that intercalated C60 fullerene molecules still have high bulk modulus and can be considered as nondeformable molecules.81 Therefore, the C60-peapods at high pressure may be transformed to the oval rather than the flat shape.10 Recently, a theoretical calculation by Pugno et al.85,86 estimated the critical deformation pressure of the peapods. The calculations demonstrated such kind of stabilization in the case of the peapods. For the peapods the critical pressure Pc defined as Pc = (π2NαD/R3)f − γ/R, where N is the number of walls, α is constant and in the case of independent tubes α = 1, D is the stiffness of graphene and equal to ∼0.21 nN nm, γ is the surface tension of the nanotubes, and f is the filling ratio of the nanotubes. The second term is the pressure imposed by the surrounding tubes and the pressure medium; this term is significant only at the nanoscale level. In the present work, the filling ratio f is close to 1; therefore, the expected critical pressure Pc for the C60-peapods is 6.0 GPa. This value is very close to the experimental value of ∼6.5 GPa. According to the above discussion, the proposed sequence of the structural deformations in the C60-peapods is illustrated on the top of Figure 7. Figure 7b shows the relative pressure-induced shifts for the C70-peapods under pressure. One observes the onset of the red shift of the optical transitions at Pflip ≈ 2 GPa. With increasing pressure above 2 GPa, the absorption bands are shifted up to certain critical values, Pc1 ≈ 7 GPa and Pc2 ≈ 13 GPa, followed by a plateau. The inset in Figure 7b shows the shifts for the S22(1) and M11(2) band in the low-pressure regime to illustrate the Pflip and Pc1 anomalies. In a previous high-pressure Raman spectroscopy study on C70-peapods by Rafailov et al.,18 the pressure coefficient of the C70 fullerene pentagonal pinch modes Ag(2) showed an abrupt change of the slope at ∼1.5 GPa. This abrupt change was ascribed to the flipping of the C70 molecules from the transverse to the longitudinal orientation. Raman studies10,13,16 furthermore found that the C70 fullerene molecules filling reduces the collapse pressure of the nanotubes, depending on the pressure transmitting medium. The collapse pressure reduction was attributed the flipping of the C70 molecules at low pressure, which enhances the inhomogeneous interactions between the fullerene molecules and the nanotube and/or alteration of the nanotube symmetry. As discussed in the previous section, a large fraction of the ellipsoidal C70 molecules are arranged inside the SWCNTs in the transverse orientation. Accordingly, at Pflip ≈ 2 GPa the flipping of the C70 fullerene molecules from the transverse to the longitudinal orientation starts the symmetry breaking in the outer tubes, concomitant with the red shift of the absorption bands. The relative energy shifts in the case of C70-peapods are larger than those of the empty tubes (see Figure 6) but comparable to those of the C60-peapods (see Figure 7a). This large relative energy shift can be attributed to the enhanced symmetry breaking in the outer tubes as discuss in the case of the C60-peapods. Compared to the empty SWCNTs (see

Figure 8. Relative energy shift of the optical transitions as a function of pressure in (a) DWCNTs/C60 and (b) DWCNTs/C70. The shift was calculated as the difference between the absorption frequency of the contribution νpi at a pressure pi and the absorption frequency νp1 at the lowest pressure value p1. All contributions during pressure release are marked as Sii (Re) and Mii (Re) for both semiconducting and metallic tubes, respectively. The shaded, gray areas mark the critical pressures regions as discussed in the text. The inset in (b) shows the S22(1) and M11(1) contributions to illutrate the critical pressure regimes in the DWCNTs/C70. The proposed deformation of the cross sections for the DWCNTs in the different pressure ranges is illustrated on the top.

of the absorption bands is very small, with a linear pressure coefficient in the range ∼9−15 cm−1/GPa. According to our previous work on DWCNTs,12,20 we interpret the reduced redshift of the absorption bands in terms of the reduction of the σ*−π* hybridization and symmetry breaking effects in the outer tubes. This finding signals the mechanical stabilization of the outer tube by the inner tube. Above ∼12 GPa the relative energy shifts of the absorption bands in the DWCNTs/C60 bundles steeply increase and seem to saturated above 14 GPa I



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transformation of the C60 molecules to the inner wall. The formed DWCNTs were considered to be inhomogeneously filled compared to the pristine CVD grown DWCNTs. Thus, our finding of an anomaly at Pd ≈ 12 GPa would be in agreement with the critical pressure Ponset ≈ 12 GPa for the c collapse as observed by Raman spectroscopy. According to the HRTEM images (not shown), the filling ratio of the DWCNTs/C70 is ≥90% compared to ≥95% for the DWCNTs/C60. Also, it has been observed from the HRTEM images (not shown) that the DWCNTs/C70 bundles contain many short inner tubes, which are generally considered as defects. Aguiar et al.91 demonstrated that as the number of defects increases in the case of fullerene-derived DWCNTs, the collapse pressure decreases. Also, as the effective tubes diameter increases the collapse pressure decreases.88,89 According to 3 Gadagkar et al., 88 the critical pressure follows a 1/Reff dependence, where Reff is the effective radius of the DWCNT. We calculate the effective radius for our DWCNTs/C70 bundles with dinner ≈ 0.85 nm and douter ≈ 1.5 nm, as estimated from HRTEM images and Raman spectroscopy, to Reff ≈ 0.51 nm and obtain a critical pressure value of Pd ≈ 11 GPa according to ref 88. Accordingly, the critical pressure for the deformation/collapse of the DWCNTs/ C70 bundles is expected to be lower compared to DWCNTs/ C60 bundles, in agreement with our optical data. I-SWCNTs under Pressure. The relative pressure-induced shifts for the I-SWCNTs film under pressure are plotted in Figure 9. The red-shifts of the absorption bands are smaller

(see Figure 8a). This behavior signals strong pressure-induced alterations in the electronic properties of the outer tube at a critical pressure Pd ≈ 12 GPa related to tubular deformation, which is completed at ∼14 GPa. The relative energy shifts of the optical transitions S11, S22, and M11 in DWCNTs/C70 are plotted in Figure 8b: The redshift of the absorption bands is small, with a linear pressure coefficient in the range ∼15−30 cm−1/GPa. The relatively small redshift of the absorption bands in DWCNTs/C70 indicates that the outer tube is mechanically stabilized by the inner tube. The inset in Figure 8b shows the shifts for the S22(1) and M11(1) absorption bands in the low-pressure range to illustrate the critical pressure regimes in DWCNTs/C70. Above ∼9 GPa (see Figure 8b), the relative energy shifts of the absorption bands steeply increase and above 13 GPa only the S11, S22 (1), and S22 (3) are resolvable. This overall behavior is thus the same as in DWCNTs/C60 but with slightly smaller critical pressure values. Accordingly, the stabilization of the outer tube by the inner tube in the case of DWCNTs/C70 is less compared to that in the case of DWCNTs/C60. This could be attributed to the larger inner and outer tubes diameters in the DWCNTs/C70 compared to the DWCNTs/C60 bundles. In a recent theoretical investigation on the stability of DWCNT bundles against pressure,87 a two-step phase transformation has been predicted for bundles of DWCNTs with inner/outer tube diameters similar to the ones of our study: A first phase transition was predicted to occur at 0.8 GPa with a polygonization of the outer tubes, while the cross section of inner tubes remains circular. Above the second phase transition at ∼5.7 GPa the outer and inner tubes have oval or peanut cross-sectional shapes. Our data do not display signs of the predicted two-step phase transformation within the error bar. Thus, our data do not confirm the theoretically predicted phase transitions at low pressures. In our previous work on DWCNTs/C60,12,21 we interpreted the anomaly at Pd ≈ 12 GPa observed for DWCNTs/C60 in terms of a small discontinuous volume change accompanied by a cross sections’ change to two deformed hexagons. This interpretation was based on the very good agreement of the experimentally found critical pressure Pd ≈ 12 GPa with the theoretical prediction by Gadagkar et al.88 for the DWCNTs with the similar inner and outer tube diameters. Also, Yang et al.89 predicted for similar diameter DWCNT bundles under pressure a small discontinuous volume change with a cross section’s change between two deformed hexagons, but for a higher critical pressure of 18 GPa. Our suggestion for the sequence of structural deformations in DWCNTs/C60 is illustrated on the top of Figure 8. In contrast, other theoretical investigations predict that the collapse of the DWCNTs happens already at such low pressures (∼12 GPa). In fact, critical pressure values for the collapse of the DWCNTs and on carbon nanotubes in general are heavily debated in literature and contradictory theoretical predictions exist.90 Furthermore, in a recent high-pressure resonance Raman scattering study of Aguiar et al.91 the mechanical stability of DWCNTs/C60 and pristine, CVD-grown DWCNTs was compared. In the case of DWCNTs/C60, it was found that ≈ the onset of the collapse started at the critical pressure Ponset c ≈ 15 GPa. In 12 GPa with the collapse completion at Pend c contrast, for pristine CVD-grown DWCNTs an onset-pressure ≈ 21 GPa was found. The lower of the pressure of Ponset c collapse pressure for DWCNTs/C60 was attributed to the formation of short inner tubes inside the SWCNTs during the

Figure 9. Relative energy shift of the optical transitions as a function of pressure in I-SWCNTs. All contributions during pressure release are marked as Sii (Re) and Mii (Re) for both semiconducting and metallic tubes, respectively. The shaded, gray areas mark the critical pressures regions as discussed in the text.

than that for the SWCNTs filled with C60 and C70 molecules, but the values are comparable to the DWCNTs/C70. The linear pressure coefficient is in the range ∼30−50 cm−1/GPa. Above ∼10 GPa, the relative energy shifts of the absorption bands steeply increase and saturate. Compared to the empty SWCNTs (see Figure 6), the anomaly is shifted from Pc1 ≈ 3 to 10 GPa. This finding suggests the stabilization of the nanotubes outer walls by the encapsulated iodine atoms. This is in contrast to the behavior of I-SWCNTs according to ref 19. On the basis of the anomalies in the G-mode shifts, it was found that the iodine filling reduces the collapse pressure from 10 to 15 GPa to 7− 9 GPa, depending on the SWCNTs samples’ inhomogeneities and iodine-filling ratio. A previous high-pressure Raman study by Venkateswaran et al.92 on I-SWCNTs bundles, showed that iodine atoms can not J



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Infrared Spectroscopy of SWCNTs and I-SWCNTs under Pressure. Figure 10a shows the absorbance spectra

only reside inside the SWCNTs but also intercalate between the SWCNTs bundles pores. At ambient conditions, the average diameter of the pores between the nanotube bundles is ∼0.3 nm, which is enough to accommodate the short I3− polyiodide chains, where as the long I−5 or I−7 polyiodide chains can reside inside the SWCNTs cavity. Aguiar et al.91 showed that in the case of DWCNTs doped with Br atoms, the lower collapse pressure was attributed to the rearrangement of the Br atoms between the interstitial channels of the DWCNTs bundles which could induced uniaxial stress in the radial direction of the DWCNTs, thus leading to a lower collapse pressure. However, in the present work after the preparation of the I-SWCNTs film the free-standing film was immersed in an ethanol path for 30 min and then heated at 50 °C for 2 h to remove any excess adsorbed iodine atoms. Therefore, after the purification steps it is believed that most of the iodine atoms reside inside the SWCNT cavities. Hence, the destabilization effect of intercalated iodine atoms according to ref 91 can be excluded. A HRTEM study on the I-SWCNTs by Guan et al.26 showed that the 1.4 nm SWCNTs can accommodate three helical chains from the negatively charged iodine atoms. In addition, the study showed that because of the filling with the negatively charged iodine helical chains, the diameter of the nanotubes increased due to the repulsion between the iodine chains. The same behavior was also observed by an HRTEM study on the SWCNTs filled with helical one-dimensional cobalt diiodide by Philp et al.93 A theoretical study by Shanavas et al.17 on the stability of the nanotubes encapsulating argon atoms demonstrated that as the argon atoms density increases to ≥60 atoms/tube, the interactions between the argon atoms and the SWCNT wall will be repulsive adding an outward force against the applied external pressure, thus leading to stabilization of the tubes against the applied pressure. The study suggested that, under high pressure, the nanotubes’ cross section change to two deformed hexagons. Accordingly, the stabilization of the SWCNTs by the iodine filling, found in the present work, can be explained as follows: Since the average diameter of the SWCNTs in the present work is ∼1.4 nm, the SWCNTs will accommodate three negatively charged iodine helical chains (according to ref 17). The repulsive forces between the iodine chains will exert an outward force on the SWCNT, supporting the SWCNTs against the applied pressure, leading to a stabilization of the nanotubes. The onset of the deformation signaled by the anomaly in the pressure-induced frequency shifts of the absorption bands is thus shifted to Pc1 ≈10 GPa in case of I-SWCNTs. The proposed deformation scheme for I-SWCNTs is shown on the top of Figure 9 based on the theoretical prediction of ref 17. However, as for the results on the DWCNTs samples, we cannot exclude the collapse of the I-SWCNTs at ∼10 GPa on the basis of our optical data. Finally, we comment on the reversibility of the pressureinduced structural changes according to our results. For all of the samples under investigation, the pressure-induced frequency shifts of the optical transitions are reversible upon pressure release from the highest applied pressure (see Figures 6−9). But about 50−60% of the original absorbance value is lost. This is illustrated in the insets of Figure 5, where the absorbance spectra at the lowest pressure during pressure release are included. The irreversible changes indicate that a fraction of the tubes has been permanently damaged during pressure loading.

Figure 10. Absorbance spectra of the (a) SWCNTs and (b) ISWCNTs films for selected pressures up to 13 GPa over a broad frequency range (200−7000 cm−1) in FIR-MIR region. The insets show the normalized spectral weights of the S11, S22, and FIR absorption bands as a function of pressure.

of the SWCNT film for selected pressures up to 13 GPa in the MIR and FIR frequency ranges (200−7000 cm−1). One can clearly observe the S11 absorption peak for all the pressure values up to 13 GPa. The behavior of this band under pressure was discussed in detail in the subsection SWCNTs under Pressure. Also, one can observe that the absorbance level in the FIR region decreases monotonically with increasing pressure, and at high pressure values it looks flat. Prior studies have noted the same behavior, but for covalently functionalized SWCNTs,71,94 where the covalent bonds destroy the SWCNTs electronic band structure. Here, the decrease of the absorption level in the FIR region was attributed to the transformation of π-bonds to covalent C−C single bonds by the addition of dichlorocarbene. A theoretical calculation by Bu et al.95 predicted that with increasing deformations of the SWCNTs the localization length of the carriers will decrease, leading to a decrease in the conductivity. According to the above discussion and previous experimental studies,22,96 the monotonic decrease of the absorbance level in the FIR can be explained as follows: equivalent to the nanotubes covalent functionalization is the symmetry breaking and the σ*−π* hybridization with pressure increasing which leads to a destruction of the SWCNTs’ electronic band structure, leading to a decrease in the localization length of the free carriers and consequently a decrease in the low frequency conductivity level in SWCNTs. To support this idea, the inset of Figure 10a shows the spectral weights of the S11, S22, and FIR absorption bands for SWCNTs as a function of pressure, normalized by their lowest pressure values. The spectral weights of S11, S22 absorption bands were calculated from the optical data shown in Figure 5a. The spectral weight in the FIR region was calculated by integrating the area under the spectra between 200 to 2000 cm−1 (see Figure 10a). The spectral weights of the S11 K



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symmetry breaking effects are increased by the filling with fullerene molecules. The anomalies in the pressure-induced shifts of the absorption bands at around Pc1 ≈ 6.5 and 7.0 GPa for C60- and C70-peapods, respectively, are due to the tubular deformation from a circular to an oval structure. The shift of the Pc1 from ∼3.0 GPa for SWCNTs to ∼6.5 and 7.0 GPa for C60- and C70-peapods, respectively, signals the stabilization of the nanotubes walls by the encapsulated fullerene molecules. We attribute the onset of the pressure-induced shifts at Pflip ≈ 2 GPa for the C70-peapods to the flipping of the C70 fullerene molecules from the transverse to the longitudinal orientation. The pressure-induced shifts of the absorption bands in the DWCNTs/C60 are smaller than for pristine SWCNTs, indicating that the outer tube is stabilized by the inner tube regarding its electronic properties. The anomaly in the pressure-induced shift at ∼12 GPa signals the onset of the pressure-induced deformation of the outer tube or the collapse of the DWCNTs. In the case of DWCNTs/C70, the stabilization of the outer tube by the inner tube is smaller than that in the case of DWCNTs/C60, since the anomaly in the pressure-induced shift is lowered to ∼9 GPa. Interestingly, the iodine atoms filling shows a stabilization for the outer tubes up to 10 GPa. This behavior is attributed to the repulsion forces between the negatively charged polyiodide chains, exerting an outward force on the SWCNT, leading to a stabilization of the nanotubes. The pressure-induced alterations of the absorption bands are reversible regarding their frequency position but not completely reversible regarding their intensity, indicating that a fraction of the tubes is permanently damaged under high pressure load. In the low energy range, the absorbance level of the SWCNTs decreases with increasing pressure and at high pressure values it looks flat. The decrease of the absorbance level in the FIR is attributed to the destruction of the nanotubes’ electronic band structure and hence an increase of the carriers localization. In the case of I-SWCNTs, the localization of the free carriers due to the destruction of the SWCNTs band structure is partially compensated by the metallization of the iodine chains under pressure.

and S22 absorption bands show three anomalies at Pc1 ≈ 3.0 GPa, Pc2 ≈ 8 GPa, and Pc3 ≈ 13 GPa. Theses values are consistent with those calculated from the relative pressureinduced energy shifts (see Figure 6). Also, the spectral weight of the FIR band shows two anomalies at Pc1 ≈ 3.0 GPa and Pc2 ≈ 8 GPa. We cannot observe the anomaly at Pc3 ≈ 13 GPa because in the FIR range we were limited to a maximum pressure of 13 GPa. The consistency between the anomalies observed for the S11, S22, and FIR absorption bands indicates that the decrease of the absorbance level in the FIR region is related to the destruction of the SWCNTs’ electronic band structure. All of the samples in the present work show the same behavior in the low-frequency range under pressure as the empty SWCNTs (not shown), except the I-SWCNTs sample. Because of the preparation method of the free-standing films prepared in the present work, all the samples do not have the same thickness; therefore, a quantitative analysis can not be carried out accurately. The absorbance spectra of the I-SWCNT film for selected pressures up to 13 GPa in the MIR-FIR frequency range are depicted in Figure 10b. One can observe that the absorbance level decreases monotonically with increasing pressure. The loss of the spectral weight of the FIR band is less, as compared with the SWCNTs (see Figure 10a). According to Alvarez et al.,19 as the pressure increases the intermolecular interaction between the polyiodide chains becomes more important, leading to a charge delocalization outside the polyiodide chains. This behavior was also observed in iodine single crystal97,98 and ascribed to the metallization transition under pressure. This means that the localization of the free carriers due to the destruction of the SWCNTs band structure will be partially compensated by the metallization of the iodine chains under pressure. The normalized spectral weights of the S11, S22, and FIR absorption bands for I-SWCNTs as a function of pressure are plotted in the inset of Figure 10b. The spectral weights of the S11 and S22 absorption bands show only one anomaly at Pc1 ≈ 11.0 GPa. This values is consistent with the value obtained from the relative pressure-induced energy shifts, Pc1 ≈ 10 GPa (see Figure 9). The spectral weight of the FIR band does not show any clear anomaly with increasing pressure, but only a small decrease up to the highest applied pressure, as compared to SWCNTs (see inset of Figure 10a). This behavior supports our idea that by applying pressure the pressure-induced charge delocalization outside the polyiodide chains will partially compensate for the loss of the spectral weight due to the destruction of the nanotube band structure under pressure and hence increase the conductivity level in the FIR region.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49(0)821 598 3315. Fax: +49-(0)821 598 3411. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge financial support by the German Science Foundation (DFG KU 1432/3-2), the DAAD, and the Egyptian Government.

CONCLUSIONS In conclusion, the mechanical stability of single-walled carbon nanotubes (SWCNTs) filled with C60 and C70 fullerene molecules (C60- and C70-peapods), double-walled carbon nanotubes (DWCNTs) derived from the C60- and C70-peapods (DWCNTs/C 60 and DWCNTs/C 70 ), and iodine-filled SWCNTs (I-SWCNTs) has been investigated by optical and infrared spectroscopy. HRTEM and Raman spectroscopy showed that all the samples under investigation possess a high filling ratio. Compared to empty SWCNTs, the relative energy shifts of the optical transitions for C60- and C70-peapods are large, indicating that the pressure-induced hybridization and the

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