UNCORRECTED PROOF ARTICLE IN PRESS. 2 Electronic properties of Ag- and CrO 3 -filled single-wall. 3 carbon nanotubes

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1 Chemical Physics Letters xxx (2005) xxx xxx 2 Electronic properties of Ag- and CrO 3 -filled single-wall 3 carbon nanotubes 4 Solange B. Fagan a, A.G. Souza Filho b, *, J. Mendes Filho b, 5 P. Corio c, M.S. Dresselhaus d 6 a Departamento de Ciências Exatas, Centro Universitário Franciscano, Santa Maria-RS, Brazil 7 b Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, Campus do Pici, Fortaleza, CE, Brazil 8 c Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, São Paulo , Brazil 9 d Department of Physics and Department of Physics and Department of Electrical Engineering and Computer Science, 10 Massachusetts Institute of Technology, Cambridge, MA , USA 13 Abstract Received 4 January 2005; in final form 14 February The structural and electronic charge distributions of single-wall carbon nanotubes (SWNTs) chemically modified with Ag and 15 CrO 3 were investigated by ab initio methods. Using first-principles spin-polarized calculations, we studied the structural and elec- 16 tronic behavior of Ag atoms and CrO 3 molecules interacting with an (8,0) semiconducting SWNT. We have found that the Ag atom 17 behaves as an electron donor and the CrO 3 as an electron acceptor in the presence of the SWNT. Resonance Raman experiments 18 performed on Ag and CrO 3 -adsorbed SWNTs confirm the donor and acceptor behavior, respectively. 19 Ó 2005 Elsevier B.V. All rights reserved Introduction 22 Carbon nanotubes [1] have promise for potential 23 applications in molecular electronic devices due to their 24 unique electronic and structural properties [2,3]. The 25 strong dependence of the electronic properties of sin- 26 gle-wall carbon nanotubes (SWNTs) on their atomic 27 structure, meaning their diameter and chirality, makes 28 a SWNT a versatile system. The preparation methods 29 are not yet developed sufficiently to produce tubes with 30 desired electronic properties. The possibility of tuning 31 the physical chemistry properties of these nanostruc- 32 tures is of great interest for optimizing their applica- 33 tions. Several theoretical and experimental approaches 34 have been proposed to modify the SWNT electronic 35 band structure [3,4]. * Corresponding author. Fax: address: agsf@fisica.ufc.br (A.G. Souza Filho). One method used for achieving this type of control is by carrying out donor or acceptor doping experiments where either electrons or holes are added to the SWNTs [5]. By doing this in a controlled way, it is possible to assess how specific chemical species perturb the SWNT electronic properties. In addition, these modified systems also open up the opportunity for studying the basic properties of SWNTs. In this work, we have investigated the effects on the electronic properties of SWNTs interacting with Ag atoms and CrO 3 molecules by using ab initio simulations. Following what we have learned from the rich chemistry of graphite intercalation compounds, the SWNT/metal and SWNT/metal oxide systems are of special interest because these systems may find applications in batteries and open the possibility, due to their high specific area, of using intercalated SWNT systems for the oxidation of primary alcohols. We have found from our calculations that the Ag atom acts as an elec /$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi: /j.cplett

2 2 S.B. Fagan et al. / Chemical Physics Letters xxx (2005) xxx xxx 55 tron donor and CrO 3 as an acceptor, independent of 56 whether they are outside or inside the SWNTs. We have 57 observed different electronic band structures, depending 58 on whether the Ag atom is inside or outside the SWNT, 59 and the differences are attributed to curvature effects 60 and quantum confinement of the 5 s orbitals. We have 61 found that an Ag atom (CrO 3 molecule) interacts 62 weakly (strongly) with the SWNT. The predictions are 63 confirmed by resonance Raman scattering experiments 64 performed on Ag- and CrO 3 -filled SWNTs Theoretical procedures 66 The theoretical calculations are based on first-princi- 67 ples spin-polarized density-functional theory using 68 numerical atomic orbitals as basis sets. We have used 69 the SIESTA code [6], which solves the standard 70 Kohn Sham (KS) equations. The calculations are done 71 using the generalized gradient approximation for the 72 exchange-correlation term, as proposed by Perdew 73 Burke and Ernzerhof [7]. The standard norm-conserving 74 Troullier Martins [8] pseudopotentials are used. The KS 75 orbitals are expanded using a linear combination of 76 numerical pseudoatomic orbitals, similar to the ones 77 proposed by Sankey and Niklewski [9]. In all procedures 78 we have used a split-valence double-f basis set with a 79 polarization function [10]. A cutoff of 150 Ry for the 80 grid integration was utilized to represent the charge 81 density. 82 Our calculations were performed using a (8, 0) semi- 83 conducting SWNT. Periodic boundary conditions and 84 a supercell approximation with a lateral separation of nm between tube centers are used to ensure that 86 the SWNTs plus the atom or molecule dopant do not 87 interact with their periodic images. The supercells that 88 were used have 64 C atoms, with a total length 89 of nm. Along the tube axis, 15 Monkhorst-Pack 90 k-points for the Brillouin zone integration were used 91 [4]. The relaxed atomic structures of the tubes were ob- 92 tained by a minimization of the total energy using Hell- 93 mann Feynman forces including Pullay-like corrections. 94 Structural optimizations were performed until the resid- 95 ual forces were smaller than 0.05 ev/å Results and discussion 97 We analyze the interaction between a SWNT and 98 an Ag atom and a CrO 3 molecule by having them ap- 99 proach the outer and the inner SWNT surfaces. In the 100 case of an Ag atom, we have performed the approach 101 to the inner surface in three ways as follows: (i) close 102 to the center of a carbon hexagon, (ii) over a C atom, 103 and (iii) in the middle of the C C bond. The resulting 104 configuration shows that the Ag, in all the three cases, moves to the center of the tube, as can be seen in Fig. 1a. For the configuration shown in Fig. 1b, we observe that the equilibrium position for the Ag atom is close to the C ring with an Ag C distance of 0.32 nm. The same approach used for placing the Ag atom in the interior of the tubes was also used for the interaction of the Ag atom with the outer surface. The energy difference between the most stable configuration (the Ag close to the center of the hexagon) and the less stable configurations (the Ag atom over the C atom and in the middle of the C C bond) is 0.05 ev. The Ag atom, when placed either inside or outside the SWNT, interacts with the tube surface through a weak adsorption. The binding energies for both the inside and outside configurations are 0.18 ev. The binding energies are calculated using the difference between the total energy of the tube + X system, and the isolated tube and isolated X systems, where X stands for the Ag atom or CrO 3 molecule. The CrO 3 molecule, approaching the outer and inner surface of the tube, was also studied. In Fig. 1c, d we show a schematic view of the totally relaxed structures of the SWNT/CrO 3 arrangement. The most stable configuration for the CrO 3 molecule is the same for both Fig. 1. Schematic view of an Ag atom [CrO 3 molecule] interacting with the SWNT through the (a) [(c)] inner and (b) [(d)] outer surface. The Ag atoms, in both cases, are close to the center of the carbon hexagon. For the CrO 3 molecule in both cases, the Cr atom remains aligned with a C atom and the three Cr O bonds are aligned with the three C C bonds of the hexagon

3 S.B. Fagan et al. / Chemical Physics Letters xxx (2005) xxx xxx the inside and outside positions. The Cr atom remains 130 aligned with a C atom and each of the three Cr O bonds 131 are aligned with each of the three C C bonds of the 132 hexagon.the minor difference in the distance between 133 the CrO 3 molecule inside (outside) and the carbon atoms 134 of the tube is 0.24 nm (0.22 nm). The binding energy va- 135 lue of the SWNT/CrO 3 system thus obtained is 1.4 ev, 136 being much larger than the corresponding value for Ag. 137 The electronic band structures for the SWNT/Ag sys- 138 tem, with the Ag atom placed inside and outside the 139 (8, 0) SWNT, are shown in Fig. 2b, c, respectively. For 140 comparison, we show the band structure of the pristine 141 semiconducting (8,0) SWNT in Fig. 2a. In Fig. 2b, a 142 half-filled level in the original band gap is observed, 143 which indicates that electron charge transfer occurs in 144 the system from the Ag atom to the SWNT, and this 145 is also consistent with the change in the Fermi level en- 146 ergy. This half-filled level which has a (5s)-Ag character, 147 containing the majority (spin up) and minority (spin 148 down) levels, can be probed in more detail when we ana- 149 lyze the difference in the electronic density resulting 150 from the Ag addition denoted by the projected density 151 of states (PDOS) onto the Ag atom shown in Fig. 3a. 152 By performing the Mulliken population analysis [5], 153 we observed that the Ag atom inside the tube transfers electrons while staying in an effective electronic con- 155 figuration 5(sp) 0.8 4d 10 with zero magnetization because 156 of the filled 4d levels. When inside the nanotube, the 157 Ag electronic configuration due to charge transfer is dif- 158 ferent from that of the isolated Ag atom where the elec- 159 tronic configuration is 5s 1 4d In Fig. 2c, we show the electronic band structure for 161 the Ag atom interacting with the SWNT through the 162 outer surface. A flat (5s)-Ag majority (spin up) level is 163 also observed close to the Fermi level and a (5s)-Ag 164 minority (spin down) level is observed somewhat above 165 the Fermi level. The latter is delocalized and hybridized 166 with the tube levels. From analysis of the Mulliken population, an effective resulting electronic configuration of 5(s) 0.8 4(d) 10 is found, for the Ag atom outside the tube. A charge transfer of 0.2 electrons from the Ag atom to the tube is observed, similar to when the Ag atom is inside the tube. Only the (5s)-Ag majority level contributes to the charge occupation with 0.8 electrons, which differs from what we observed for the Ag atom inside the tube surface where the spin up and spin down states are equally occupied. The occupied levels for Ag placed inside (outside) occurs for electrons with wavevectors near the C(X) point in the Brillouin zone. This is a consequence of the different dispersion of the minority and majority levels shown in Fig. 2b, c. The PDOS for the Ag atom outside the tube is plotted in Fig. 3b for majority and minority carriers. Here, we can clearly observe that the PDOS for the Ag atom addition outside the tube exhibits an asymmetry between the majority and minority carrier plots up to the Fermi level, being more pronounced for the (5s)-Ag spin up levels. According to Fig. 3a, b the Ag d levels are located between 3.0 and 4.0 ev for both Ag inside and outside the SWNTs, so these levels can not be seen in the energy range where the electronic energy levels are plotted in Fig. 2b, c. It should be emphasized that Ag does not intercalate in graphite but it does in SWNTs. Our prediction, that is further confirmed through the experiments to be discussed later, suggests that the Ag intercalation into SWNTs should be related to the curvature of the graphene sheet. The different effects on the SWNT electronic band structure observed when Ag is inside or outside the tube points out the role of the curvature effect and the effect of the Ag environment on the SWNT/Ag interaction. Besides the curvature effect, the quantum confinement of the electronic states is important when the atom is inside the SWNT [11]. This confinement is evident in the electronic band structure shown in Fig. 2b where the 5s level is dispersive. When Ag is outside, there is no confinement and the electronic structure is affected in a different way when compared with Ag inside the tube, as can be noticed by comparing the electronic band structures shown in Fig. 2b, c. A detailed analysis of the curvature effect should be performed in order to elucidate this behavior further. This study is underway as a function of tube diameter, which will shed light on why Ag does not intercalate into graphite. In Fig. 4b, c, we have shown the electronic band structure for the CrO 3 approaching the inner and outer tube surface, respectively. In contrast to the SWNT/Ag system, the interaction is stronger in the CrO 3 case where the oxygen and Cr levels appear delocalized and strongly hybridized with the SWNT levels, as can be observed in Fig. 4b, c. The Cr levels are located near the SWNT conduction bands, whereas the O levels are mixed with the SWNT valence levels. We can observe the appearance of an empty level in the conduction band Fig. 2. Electronic band structure for a pristine (8,0) SWNT (a) and for an Ag atom interacting with the SWNT through the (b) inner and (c) outer surface. The horizontal dashed lines correspond to the Fermi level

4 4 S.B. Fagan et al. / Chemical Physics Letters xxx (2005) xxx xxx Fig. 3. Projected density of electronic states onto the Ag atom inside (a) and outside (b) the tube. The upper and lower traces indicate the majority and minority carriers, respectively. The dashed vertical lines correspond to the Fermi level and E = 0.0 ev stands for the Fermi level of the pristine (8,0) SWNT. 223 (just above the Fermi energy) showing an acceptor 224 behavior because this level crosses the Fermi energy of 225 the pristine (8, 0) SWNT (denoted by zero energy). This 226 empty level is characterized by a mixing of the CrO 3 and 227 SWNT levels with a strong Cr (4s level) character, as we 228 can observe in the smaller dispersion of this level when 229 the molecule is adsorbed on the tube (Fig. 4b, c) com- 230 pared with the corresponding level in the pristine SWNT 231 (Fig. 4a). An electronic charge transfer of 0.12 electrons 232 from the tube to the CrO 3 molecule is observed accord- 233 ing to the Mulliken population analysis for both SWNT/ 234 CrO 3 configurations. From our analysis, we could con- 235 clude that it is the Cr atom that is responsible for the 236 charge transfer in the SWNT/CrO 3 system for both con- 237 figurations of the CrO 3 molecule with respect to the 238 SWNT. 239 By comparing the electronic changes of SWNT/Ag 240 and SWNT/CrO 3, we can conclude that the CrO 3 inter- 241 calation causes much stronger changes than Ag interca- 242 lation to the SWNT electronic structure. This is clearly 243 seen by observing the electronic levels for both the 244 SWNT/Ag system (Fig. 2b, c) and the SWNT/CrO 3 sys- 245 tem (Fig. 4b, c) when compared with the pristine 246 SWNT. In the former system, the SWNT levels are per- 247 turbed only near the Fermi level while for the latter sys- 248 tem, the SWNT levels are highly perturbed, thus being 249 strongly hybridized with the CrO 3 levels. In this case, 250 the CrO 3 interacts with the SWNT via a chemical bond 251 between the Cr and C atoms (chemisorption process). 252 We can observe that the electronic band structure, 253 mainly in the valence bands, is very different when 254 CrO 3 is placed inside or outside the tube. However, 255 the levels involved in the charge transfer are those near 256 the Fermi level and as we can observe by comparing Fig b, c which are very similar for both placement of CrO 3 inside and outside the tube. Therefore, it is concluded that the charge transfer itself is very similar for both cases. 4. Comparison to experiments In order to give support to our theoretical predictions, we now discuss the resonance Raman results obtained for SWNTs interacting with Ag and CrO 3 [5]. The resonance Raman scattering technique has been of particular importance because it allows one to assess the electronic properties of the tubes through study of their vibrational modes, due a strong and selective coupling between photons and phonons [12]. As a consequence, the Raman spectrum is very sensitive to Fig. 4. Electronic band structure for a pristine (8,0) SWNT (a) and for a CrO 3 molecule interacting with the SWNT through the (b) inner and (c) outer surface. The horizontal dashed lines correspond to the Fermi level

5 S.B. Fagan et al. / Chemical Physics Letters xxx (2005) xxx xxx modifications of the nanotube surface, such as by the 272 introduction of surface species and the charge transfer 273 effects resulting from the chemical modifications of 274 nanotubes. For the case of SWNT bundles doped with 275 acceptors (for example, Br 2 ), frequency upshifts are ob- 276 served for both the radial breathing mode and tangential 277 modes, respectively, which upshift of 74 and cm 1, relative to the corresponding frequencies in 279 the undoped SWNT bundles [13,14]. On the other hand, 280 doping with donor alkali metals, like K or Rb, results in 281 frequency downshifts [14]. 282 In Fig. 5a we show the Raman spectra of both pris- 283 tine SWNT and SWNT/Ag obtained with an excitation 284 energy of 2.41 ev. The details of the sample preparation 285 and Raman experiment have been reported elsewhere 286 [4]. It should be pointed out that the tangential band 287 (at about 1590 cm 1 ) is composed of several peaks, (a) (b) but the two most intense features are labeled G (lower frequency) and G + (higher frequency). The downshift in the tangential G + -mode from 1582 to 1575 cm 1 indicates that electrons are transferred from the silver to the SWNTs [4]. A downshift of 10 cm 1 is also observed for the G 0 -band located at about 2650 cm 1. The Raman spectrum of SWNT/CrO 3 is shown in Fig. 5b for the excitation energy of 1.96 ev. In contrast, significant changes to the resonance Raman spectra due to the chemical intercalation of CrO 3 into SWNTs are observed when compared with the corresponding spectrum for the pristine SWNTs. The large upshifts of the G + - band and the G 0 -band modes resulting from CrO 3 intercalation indicates a hardening of the C C bonds, suggesting that electrons are flowing from the nanotubes to the CrO 3 species, whose strong oxidizing nature is well established [15]. Besides the shift of the G + -band mode frequencies, further confirmation for the charge transfer between the chemical species and the SWNTs can be gathered by analyzing the profile of the G -band located below 1600 cm 1. The profile of this G -band tells us about the metallic and semiconducting behavior of the SWNTs whose electronic transitions are in resonance with the laser energy [16]. By considering the diameter distribution of the sample (d 0 = 1.25 ± 0.20 nm), it is expected that when excited with 2.41 ev, the Raman spectra will have mainly contributions from semiconducting SWNTs [4]. This is indeed the case for the lower trace in Fig. 5a where the G -band exhibits a profile typical of contributions from mainly semiconducting SWNTs. The G - band for the SWNT/Ag system, however, exhibits a strong Breit Wigner Fano profile, which is typical of metallic SWNTs [17]. This feature is associated with the coupling between the phonons and the free electrons available in metallic tubes. However, the change from a semiconductor to metallic-like profile observed for the SWNT/Ag samples indicates that the conduction band states are populated by charge transferred electrons from the Ag atom to the SWNT, thus moving the Fermi level up in energy. An opposite behavior is observed for the SWNT/ CrO 3 sample. Here, the G-band profile for the pristine SWNT (lower trace in Fig. 5b) when excited with 1.96 ev photons is typical of metallic SWNTs. However, for the SWNT/CrO 3 sample, the G -band profile looks as if it is originating from mostly semiconducting SWNTs. This change in profile is consistent with the removal of electrons from the metallic tubes to the CrO 3, thus depopulating the conduction bands and moving the Fermi level down in energy and thereby weakening the Breit Wigner Fano lineshape for the G feature. The upshifts of the G + -band and G 0 -band are very large in the SWNTs/CrO 3 system compared with the downshifts found in the SWNT/Ag system. It is also remarkable that the G 0 -band intensities in the case of the SWNTs Fig. 5. Raman spectra of (a) SWNT/Ag obtained with 2.41 ev and (b) SWNT/CrO 3 obtained with 1.96 ev. The lower traces in both panels stand for the Raman spectra obtained for pristine SWNTs, shown for comparison

6 6 S.B. Fagan et al. / Chemical Physics Letters xxx (2005) xxx xxx 344 interacting with Ag and CrO 3 are so different. In the first 345 case, the intensity is increased by only a small amount 346 but in the latter case a drastic reduction in the intensity 347 was observed. The intensity involves matrix elements for 348 the radiation phonon interaction and a detailed study 349 of this phenomenon should be performed in order to 350 understand the G 0 -band intensity in doped SWNTs. 351 The observation of larger effects for CrO 3 intercalation 352 than in the case of Ag is however consistent with our 353 theoretical prediction, where CrO 3 interacts with the 354 SWNTs more strongly by a chemisorption process. This 355 is further supported by analyzing the disorder-induced 356 (D-band) mode located at about 1348 cm 1 for pristine 357 SWNTs excited with 1.96 ev. This mode comes from a 358 double resonance process and its intensity and linewidth 359 depends on the symmetry-breaking effects in the SWNT 360 crystalline lattice [18 20]. When the CrO 3 is attached to 361 the SWNT sidewall, the translational symmetry is bro- 362 ken and this contributes to enhancing the D band inten- 363 sity and linewidth when compared with the pristine 364 SWNT. The linewidth of all the modes increases, thus 365 indicating that the system becomes disordered due to 366 the CrO 3 attachment to the tube walls. The effect of 367 Ag intercalation on the D band linewidth is weaker, as 368 can be observed in Fig. 5a, thus supporting the weaker 369 interaction regime for Ag intercalation. 370 Our theoretical calculation predicts a donor behavior 371 for metallic Ag addition and an acceptor behavior for 372 CrO 3 addition in good agreement with the experimental 373 results. Furthermore, the magnitudes of the changes 374 introduced in the Raman spectra of SWNTs when they 375 interact with Ag and CrO 3 are consistent, respectively, 376 with their weaker and stronger adsorption interaction 377 process, respectively, as predicted by our modeling. It 378 should be pointed out here that our predictions for the 379 SWNT/CrO 3 system were performed by using a semi- 380 conducting SWNT and the resonance Raman experi- 381 ments are most sensitive to what happens to metallic 382 SWNTs when CrO 3 is added. However, the charge 383 transfer effect should not be directly observed by look- 384 ing at the (8, 0) calculation. Nevertheless, the charge 385 transfer trend is the same, independent of whether the 386 SWNT is metallic or not. Even for semiconducting 387 SWNTs the charge transfer is predicted to be from the 388 tube to the CrO 3. However, for a metallic tube, this ef- 389 fect should be stronger because a metallic SWNT has its 390 Fermi level populated and therefore the electrons are 391 more easily transferred from the metallic SWNT to the 392 acceptor species Conclusions 394 In summary, we studied the interaction of an Ag 395 atom and a CrO 3 molecule with an (8, 0) semiconducting 396 SWNT by using first principles calculations. The band structure of the pristine SWNT is affected by the SWNT interaction with both an Ag atom and a CrO 3 molecule. In the first case it is observed that the Fermi level moves up in energy whereas in the second case it moves down in energy. The calculations indicate that the Ag atom interacts with the SWNT surface through a weak adsorption process, while the CrO 3 molecule interacts with the SWNT through a stronger adsorption process. Both Ag and CrO 3 when placed either inside or outside the SWNTs are predicted to behave as a donor and an acceptor, respectively. This prediction is in agreement with the resonant Raman data for SWNTs filled with silver and CrO 3. Acknowledgments We thank the CENAPAD-SP for the computer time. A.G.S.F. acknowledgesfinancial support from Universidade Federal do Ceará and FUNCAP (Grant PPP- 985/03). This work was partially supported by CNPq and FAPESP. P.C. acknowledges a research fellowship from CNPq. M.S.D. acknowledges support from NSF DMR References [1] S. Iijima, Nature 354 (1991) 36. [2] M.S. Dresselhaus, G. Dresselhaus, P. Avouris, Carbon Nanotubes, Springer, Berlin, [3] H. Dai, Surf. Sci. 500 (2002) 218. [4] S.B. Fagan, A.J.R. da Silva, R. Mota, R.J. Baierle, A. Fazzio, Phys. Rev. B 67 (2003) [5] P. Corio, A.P. Santos, P.S. Santos, M.L.A. Temperini, V.W. Brar, M.A. Pimenta, M.S. Dresselhaus, Chem. Phys. Lett. 383 (2004) 475. [6] P. Ordejón, E. Artacho, J.M. Soler, Phys. Rev. B 53 (1996) 10441; D. Sánchez-Portal, E. Artacho, J.M. Soler, Int. J. Quantum Chem. 65 (1997) 453. [7] J.P. Perdew, K. Burke, M. 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