Raman study of pressure screening effects in double-wall carbon nanotubes

Raman study of pressure screening effects in double-wall carbon nanotubes J. Arvanitidis a,b, D. Christofilos a, K. Papagelis c, K. S. Andrikopoulos a, G. A. Kourouklis a, S. Ves b,*, T. Takenobu d,e, Y. Iwasa d,e and H. Kataura f a Physics Division, School of Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece b Physics Department, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece c Materials Science Department, University of Patras, 26504 Patras, Greece d Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577 e CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan f National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan *corresponding author: e-mail: ves@auth.gr Summary The pressure response of double-wall carbon nanotubes prepared by the peapod conversion route has been investigated by using the Raman spectroscopic technique. The pressure dependence of the observed radial breathing bands suggests that the presence of the outer carbon shells results in a screening of the applied pressure on the inner ones, while the latter provide structural support against pressure induced deformation of the external tube crosssection compared to the single-wall carbon nanotubes. The screening effect depends on the combined inner and outer tube structural characteristics and thus on their spacing. Introduction Carbon nanotubes have attracted intense scientific interest due to their fascinating - essentially one dimensional - electronic and vibrational band structure, their unique mechanical properties as well as the prospect for numerous applications. Double-wall carbon nanotubes (DWCNTs), lying between single- (SWCNTs) and multi-wall carbon nanotubes (MWCNTs), constitute a unique system for studying encapsulation effects in these materials. The Raman spectrum of carbon nanotubes allows selective probing of tubes for which the excitation wavelength is in resonance with the energy spacing between the corresponding van Hove singularities (sensitively dependent on the tube diameter and chirality) appearing on either side of the Fermi energy in the 1D electronic density of states (Dresselhaus 2000, Jorio 2002). Moreover, the frequencies of the low-energy Raman peaks, attributed to the radial breathing modes (RBMs) of the nanotubes, are inversely proportional to the tube diameter (Rao 1997), allowing the precise study of individual carbon nanotubes of different structural and electronic properties. The unusually narrow RBMs appearing in the Raman spectrum of the DWCNTs reflect the growth of inner or secondary tubes with a high degree of perfection, being well protected in the interior of the outer or primary rolled graphene shells (Pfeiffer 2003). In this system, the existence of inner-outer tube (intratube) interaction causes the splitting of the Raman peaks, attributed to DWCNTs having the same secondary tube, but included in different diameter

primary tubes (Pfeiffer 2004, Xia 2004). In addition, the resulting frequencies for the RBMs of both inner and outer tubes are upshifted with decreasing intratube distance and thus increasing intratube interaction (Pfeiffer 2004, Rahmani 2005). The application of high pressure on carbon nanotubes is a valuable tool for the investigation of their mechanical and structural stability (Loa 2003). In bundled SWCNTs, the van der Waals tube-tube (intertube) interaction dominates their pressure response, resulting in the hexagonal or oval distortion of their cylindrically shaped cross-section and eventually their collapse (Venkateswaran 1999, Peters 2000, Tang 2000). More specifically, a high pressure Raman study of a wide diameter range HiPCO (high pressure carbon monoxide)-derived SWCNT sample revealed that this deformation becomes more pronounced as the tube diameter increases (Venkateswaran 2003). In this work, we study the effect of high-pressure application on bundled DWCNTs by means of Raman spectroscopy, in order to investigate the pressure response of both the inner and outer tubes, as well as the role played by the intertube and the intratube coupling in the structural stability of the material. Experimental Method The DWCNT sample used in this study was prepared by the C 60 peapod conversion route, following Bandow s procedure (Bandow 2001). Details of the sample preparation and the extensive characterization of the starting SWCNT material, the intermediate step peapods and the resulting DWCNTs by means of transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy can be found elsewhere (Kataura 1999, Kataura 2001, Kataura 2002, Abe 2003). The Raman spectra were recorded using a DILOR XY micro-raman system equipped with a cryogenic charge coupled device (CCD) detector. High pressure was applied by means of a Mao-Bell type diamond anvil cell (Jayaraman 1986). The 4:1 methanol-ethanol mixture was used as pressure transmitting medium and the ruby fluorescence technique for pressure calibration (Barnett 1973). For excitation, the 514.5 nm line of an Ar + or the 676.4 nm line of a Kr + laser was focused on the sample by means of a 20x objective, while the laser power was kept below 2.5 mw -measured directly before the cell- in order to eliminate laser-heating effects on the probed material and the concomitant softening of the observed Raman peaks (Huong 1995, Ando 2001). Results Figures 1 and 2 show the ambient pressure Raman spectrum of the DWCNT material in the low frequency region, where the RBMs of the two concentric rolled graphene sheets constituting the DWCNTs are located. The spectrum presented in figure 1 was excited by means of the 514.5 nm (2.410 ev) line of the Ar + laser, while that in figure 2 with the 676.4 nm (1.833 ev) line of the Kr + laser. The different excitation energies allows resonantly probing of nanotubes with different chiral vector (n,m). In both spectra, the lower energy broad band is attributed to the larger diameter outer tubes, while the narrow peaks at higher frequencies are assigned to the inner tubes. The significantly large number of the observed RBM peaks, providing a wealth of information on the encaged tubes and their interaction, has been previously assigned to specific nanotubes by means of detailed Raman spectroscopic studies at low temperature and ambient pressure (Pfeiffer 2003, Pfeiffer 2004, Kramberger 2003). The assignment was based on the well-known inversely proportional relation in bundled carbon nanotubes between the RBM frequency (ω RBM ) and the tube diameter (d t ), described by the general expression ω RBM (cm -1 )= A/d t (nm) + B (Jorio 2003). Note, however, that this relation is only a rough estimation of the tube diameter, as it does not take into account the intratube interaction (Pfeiffer 2004, Rahmani 2005). Upon pressure application all the observed RBM peaks shift towards higher energies. In addition, the lower energy RBM band associated with the larger outer tubes exhibit strong intensity attenuation and a significant broadening at elevated pressure. In bundled SWCNTs, an abrupt reduction in the intensity of the RBM band and its disappearance near 2 GPa has been attributed to the pressure-induced distortion of the nanotube cross-section due to the van der Waals intertube interaction within a bundle (Venkateswaran 1999, Peters 2000).

However in DWCNTs, although the lineshape changes of the outer tubes RBM band still is ascribed to their distortion at higher pressure, its persistence up to 9 GPa clearly suggests that the internal tubes provide structural support against pressure induced deformation of the externals. On the other hand, the lineshape of the inner tube RBM peaks is only marginally affected by the pressure, reflecting that these tubes are well-protected from external perturbations in the interior of the outer tubes. λ exc = 514.5 nm 400 Raman Intensity (arb. units) Raman Shift (cm -1 ) 300 200 0 5 10 Pressure (GPa) 150 200 250 300 350 400 1 Figure 1: Raman spectrum of DWCNTs in the RBM region recorded at ambient conditions, excited with the 514.5 nm laser line. The vertical lines mark the main peaks followed unambiguously with pressure. Inset: Pressure dependence of the Raman peaks. The open (solid) symbols denote data taken for increasing (decreasing) pressure while solid lines are least square fittings. The pressure dependence of the RBM frequency positions is illustrated in the insets of figure 1 and 2. The RBMs of the external tubes show a sublinear trend with increasing pressure, whereas a more or less linear behavior with substantially smaller pressure slopes is observed for the internal tubes. There is a similarity in the sublinear pressure response of the primary tubes RBM band with that predicted theoretically for bundled SWCNTs (Venkateswaran 1999), where it is assumed that there is no bundle penetration of the pressure-transmitting medium and that the pressure application causes the nanotube faceting mentioned above, eliminating the radial band. This description can be also adopted for the outer tubes in DWCNTs, although in this case the distortion of the outer tubes appears to be smaller as their RBM band persists with pressure. Finally, the pressureinduced blue shift of the RBM frequencies is fully reversible upon pressure release. However, the relative intensity and the width of the outer tubes RBM band do not fully recover after total pressure release. These divergences indicate the presence of residual pressureinduced deformations of the external tubes, in analogy to those earlier observed in SWCNTs (Venkateswaran 1999).

λ exc = 676.4 nm 400 Raman Intensity (arb. units) Raman Shift (cm -1 ) 300 200 0 5 10 Pressure (GPa) 150 200 250 300 350 400 1 Figure 2: Raman spectrum of DWCNTs in the RBM region recorded at ambient conditions, using the 676.4 nm laser line for excitation. The vertical lines mark the main peaks followed unambiguously with pressure. Inset: Pressure dependence of the Raman peaks. The open (solid) symbols denote data taken for increasing (decreasing) pressure while solid lines are least square fittings. Discussion The frequencies of the RBMs appearing in the Raman spectrum (excited with the two different laser lines mentioned above) of the studied DWCNT material are compiled in table 1 along with their normalized pressure derivatives, Γ i = (1/ω i )( ω i / P). The corresponding values for a wide diameter range HiPCO-derived SWCNT sample, taken from the literature (Venkateswaran 2003), are also included for comparison reasons. For SWCNTs the normalized pressure derivatives decrease quasi-quadratically as the tube diameter decreases. This behavior was rationalized in terms of the enhanced cross-section fragility of the larger nanotubes upon pressure and that in bundled nanotube materials the intertube van der Waals interactions govern their pressure response (Venkateswaran 2003). Nevertheless, a high pressure Raman study on solubilized (debundled) SWCNTs has revealed similar RBM s Γ i values to those of the bundled SWCNT sample, indicating that changes in the electronic band structure might also significantly influence the pressure dependence of the Raman peaks (Venkateswaran 2001). As it can be inferred from table 1, the Γ i values for the RBMs of the outer tubes in DWCNTs are comparable to those of the SWCNT sample in this frequency region (similar diameter), but one order of magnitude larger than those for the inner tubes. On the contrary, the Γ's for the inner tubes RBMs in DWCNTs are much smaller than those in SWCNTs appearing in the same frequency region, supporting further the assumption, that the existence of the external tubes in DWCNTs results in a screening of the applied pressure on the internals. This assumption is consistent with the recent results of a high pressure Raman study of C 60

peapods, where a pressure shielding of C 60 inside the tubes is also proposed on the basis of the reduced pressure slope of its A g (2) intramolecular mode (Rafailov 2003). Table 1: The phonon frequencies and their normalized pressure derivatives, Γ i = (1/ω i )( ω i / P), for the RBM peaks in DWCNTs. The corresponding values for a HiPCOderived SWCNT sample are also included for comparison (Venkateswaran 2003). Asterisk marks a peak for which an unambiguous assignment to inner or outer tube cannot be made. DWCNTs SWCNTs λ exc = 514.5 nm λ exc = 676.4 nm λ exc = 514.5 and 632.8 nm ω i (cm -1 ) Γ i (x10-3 GPa -1 ) ω i (cm -1 ) Γ i (x10-3 GPa -1 ) ω i (cm -1 ) Γ i (x10-3 GPa -1 ) primary tubes 187 42.6 175 33.1 179 52.1 191 37.6 186 38.5 193 39.9 secondary tubes 207 25.9 267* 8.2 267 4.5 208 31.7 279 3.4 212 30.9 282 3.6 217 27.6 285 3.8 220 30.5 289 4.1 221 24.4 290 4.2 222 24.9 313 5.4 296 4.9 224 21.5 299 2.8 247 20.3 302 3.4 248 26.1 306 3.9 251 20.5 308 3.6 252 21.9 312 4.0 258 23.7 323 4.5 313 4.3 260 24.6 340 2.5 341 2.3 262 21.8 356 3.9 346 3.2 263 27.7 361 2.6 265 21.0 364 2.8 283 20.5 365 2.4 284 20.9 367 2.9 337 14.2 375 3.3 338 13.0 378 2.6 342 10.6 384 2.7 390 2.9 394 3.3 Finally, let us now concentrate on the normalized pressure derivatives of the RBMs assigned to the inner tubes. There is an overall trend of reducing Γ with increasing frequency (decreasing nanotube diameter), similar but less pronounced to that of the SWCNTs. The effect is more pronounced in the "lower resolution" spectra taken with the green line and thus probing an average behavior. This experimental finding is in accordance with theoretical calculations in the framework of the elastic model, which predict a monotonic increase of individual nanotubes compressibility with increasing tube diameter that is much more pronounced for smaller tubes (Reich 2002). However, the most remarkable point arising from the inner tubes RBM pressure derivatives (more pronounced in the "higher resolution" spectra taken with the red line) is their clustering in groups (included between two horizontal lines in table 1), where the above mentioned Γ i -d t relation is inverted. The chiral vectors of the carbon nanotubes quantize their possible diameter values, leading to discrete intratube spacings. Depending on the chiral vectors involved, inner-outer tube combinations can be found where the minimum interlayer spacing approaches the best fit condition (i.e. close to the turbostratic constraint of graphite), whereas other combinations deviating from this condition, result in DWCNTs with higher intratube distance (Rahmani 2005). These

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