Raman frequency shift in oxygen functionalized carbon nanotubes 7 0 Z. X. Guo1, J. W. Ding1,2, Y. Xiao1, and D. Y. Xing2 0 1Department of physics,Xiangtan University, 2 Xiangtan 411105,Hunan,China 2National Laboratory of Solid State Microstructures, n a Nanjing University, Nanjing 210093,China J email: [email protected] (Dated: February 6, 2008) 8 1 In terms of lattice dynamics theory, we study the vibrational properties of the oxygen- functionalized single wall carbon nanotubes (O-SWCNs). Due to the C-O and O-O interactions, ] l manydegeneratephonon modesare split and evensome newphononmodes areobtained, different al from the bare SWCNs. A distinct Raman shift is found in both the radial breathing mode and h G modes, depending not only on the tube diameter and chirality but also on oxygen coverage and - adsorption configurations. With the oxygen coverage increasing, interesting, a nonmonotonic up- s and down-shift is observed in G modes, which is contributed to the competition between the bond e expansion and contraction, there coexisting in thefunctionalized carbon nanotube. m . PACSnumbers: 63.22.+m;78.30.-j;61.46.Fg t a m Chemical functionalization of carbon nanotubes could As two types of the most important Raman modes, - d offer new and promising avenues to process and assem- the radial breathing mode (RBM) and G modes are n ble tubes, add sensing capabilities, or tune their elec- widely used in the estimation of diameter distribution o tronic properties, which are the subject of intensive of nanotubes.[11, 12] To assign the Raman peaks of an c research.[1, 2] When the functional groups are chemi- O-SWCN, the frequency shift in these modes should be [ cally bonded to the nanotube wall, the tube geometry fully taken into account. In addition, an O-SWCN can 3 can be significantly changed. As a result, the C-C force form several possible configurations, leading to different v constantandthus the vibrationalpropertiesofnanotube electronic properties[10], which depend not only on the 9 willbe largelymodified. Forexample,anobviousupshift adsorbed sites and coverage but also on whether the O- 5 3 in the tangential mode vibrations (so-called G modes) O bond breaking or not.[9, 10] It was reported that an 1 wasobservedexperimentallyinbothsingle-walledcarbon SWCNwithhighadsorbedoxygenhasahigherelectrical 0 nanotubes (SWCNs) and multi-walledcarbon nanotubes conductancethanonewithlessadsorbedoxygen.[13]How 7 (MWCNs) in the acid treatment.[3, 4] For the physically about the influence of the adsorption configurations and 0 doped nanotubes, such a frequency shift was usually at- coverage on the vibrational properties of an O-SWCN? / t tributedtothe variationsintheC-Cbondsonnanotube, In the lattice dynamics, it is still an open question and a induced by the charge transfer.[5, 6, 7, 8] In the chem- should be further explored. m ically processed nanotubes, however, there exists much Inthispaper,westudythevibrationalpropertiesofthe - stronger interaction between the functional groups and d O-SWCNs in terms of lattice dynamics theory. Due to n nanotubes,whichisofparticularimportanceinthechar- the C-O and O-O interactions, many degenerate phonon o acterizationofnanotube-baseddevice. Sofar,therewere modes are split and even some new phonon modes are c few, if any, works reported on how functional groups af- : obtained,differentfromthebareSWCNs. AdistinctRa- v fect Raman shift in the nanotube. For nanotube-based man shift is found in RBM and G modes, depending not i device applications, this is an outstanding issue and its X only on the tube diameter and chirality but also on oxy- clarification is greatly desirable. gen coverage and adsorption configurations. With the r a An oxygen molecule (O2) can be regarded as a sim- oxygen coverage increasing, interesting, a nonmonotonic up-anddown-shiftisobservedinGmodes,whichiscon- plestkindoffunctionalgroups. Itwasreportedthatoxy- tributed to the competition between the bond expansion gen molecule is very reactive to SWCNs with diameter and contraction, there coexisting in the functionalized below 1nm, and the oxidation can even occur at room carbon nanotube. temperature,[9, 10] forming an oxygen-functionalized SWCN (O-SWCN). Such oxidation can dramatically in- As typical example, we consider the cycloaddition fluence the nanotubes’ electrical resistance, thermoelec- structure for the O2 adsorption on achiral tube. From tric power, and local density of states. The Raman shift previous report, the two configurations, O2 on top of an and even some new vibrational modes may be expected axial C-C bond (site TA) and on top of a zigzag C-C insuchanO-SWCN,whichmaybe characterizedby Ra- bond (site TZ), can lead to chemisorbed structures via man spectroscopy. Therefore, the study of the oxygen cycloaddition (also see Fig. 1), which shall be the fo- chemisorptioneffects onRamanmodes canbe veryhelp- cus of present work. To achieve the optimum structure ful to explore the underlying mechanism of the Raman oftheO-SWCNs,first-principleplane-wavepseudopoten- shift in the functionalized nanotubes. tial density functional theory (DFT) is used, performed 2 in the CASTEP code.[14] For a simplification, we choose 300cm−1withnearlylineardispersion,whichcanbecon- aunitcellofabout4.26˚Atobeasupercellinzigzagtube, tributed to the O-O bond stretching vibrations and the while two unit cell of about 4.92˚A in armchair tube.[15] coupling vibrations between the oxygen and the tube. The validity of the optimum results has been confirmed Moreover, the highest frequency mode of 1607cm−1 is by using even larger supercell. newly obtained at Γ point, higher than 1600cm−1 of the baretube,whichisascribedtothe strongC-Ostretching Fig. 1 shows the optimized geometries of a (10,0) O- vibrations. Similar results are also obtained in TZ con- SWCNinbothTAandTZconfigurations,whicharesim- figuration. The splitting of Raman modes and the new ilar with the previous results.[9, 10] Interestingly, both vibrationalmodesareexpectedtobeobservedbyRaman the expansion and contraction of the C-C bonds are spectroscopic experiment. obtained in the O-SWCN, different from the physically doped SWCNs only with a bond expansion or contrac- Also shown in Table 1 are the RBM and G mode fre- tion. In TA configuration, for example, three C-C bonds quencies of the (10,0)O-SWCN in both TA and TZ con- at the adsorbed site are markedly expanded to be 1.500, figurations. Forthebare(10,0)tube,theRBMfrequency 1.498 and 1.521˚A, respectively, while the bonds near the isobtainedtobeof294cm−1,ingoodagreementwiththe adsorbed site is contracted, there appearing a minimal experiment and the first principle results.[25, 26] In the length of about 1.374˚A, shorter than 1.417˚A of the bare TA (TZ) configuration of the O-SWCN, the RBM fre- tube. Far from the adsorbed site, little change in the quency downshifts to be 278cm−1(276cm−1), decreased C-C bonds is observed, almost not affected by the oxi- by 16(18)cm−1, while the upshifts are obtained in the dation. The bond expansion can be understood by the G modes, increasing by 8(4)cm−1 in E1g, 6(13)cm−1 in fact that a strong C-O covalent bond has been formed A1g, and 7(3)cm−1 in E2g, respectively. It is shown that by the introduction of holes into the π∗ orbital,[7] form- the frequency shift in both RBM and G modes depends ing a sp3-like rehybridization at the adsorbed site. This strongly on the adsorption configurations, which should leadstothebondlengthincreasing,differentfromtheBr2 be considered in the nanotube characterization by the donorgraphiteintercalationcompounds,in whichthe C- Raman spectroscopic experiment. C bonds still keep sp2 hybridization. As for the bond To explore the dependence of the Raman shift on the contraction,it is mainly contributed to the chargetrans- tube diameter and chirality, we calculate the RBM of fer from SWCN to oxygen,[5, 6, 7, 8] similar to that in armchair (5,5) and zigzag (8,0) O-SWCNs, which have the physically doped SWCNs, where the gain (removal) the similar tube diameter. For the (5,5) O-SWCN, the of electrons induces the C-C bond expansion (contrac- O-O bond direction is perpendicular to the tube axis. In tion) and thus resulting in a downshift (upshift) in G the case of the (8,0) O-SWCN, the TA configuration is modes.[7, 8] The coexistence of the bond expansion and considered. For the bare and oxygenated (8,0) SWCNs, contraction may indicate a new mechanism of Raman the RBM frequencies are obtained to be 359 cm−1and shift in the functionalized SWCNs. 329cm−1, respectively. The frequency shift in RBM, For the lattice dynamic calculations, the vibrations ∆ω = −30cm−1 is much lager than that of (10,0) O- of the carbon atoms on the nanotube can be modeled SWCN.ThisindicatesthedependenceoftheRamanshift by the force constant model in terms of the force con- ontube diameter, i.e., the largerthe frequency shift, the stants of graphene, up to the fourth next neighbor- smallerthe tube diameter. Inthe (5,5)O-SWCN, onthe ing interaction.[16] To incorporate the oxygen adsorp- otherhand,theRBMfrequencyofabout272cm−1 shifts tion effects, the C-C force constants have been corrected from336cm−1ofthebare(5,5)tube. Thefrequencyshift through the distorted bond lengths obtained above.[17, (∆ω = −64cm−1 ) is larger than two times that of the 18] As for the C-O and O-O bond interactions, we use (8,0)O-SWCNs. Atagiventube diameter,obviously,an the well-knownTersoff-Brennerbondorderpotential,[19] armchair tube is more sensitive to the oxidation than a and the potential parameters in Ref.20. Shown in Fig. zigzagone,showingup a chirality-dependenceof the Ra- 2 are the phonon dispersions of the (10,0) bared SWCN manshift. The resultsgiveapossibilitytodetermine the and O-SWCN in TA configuration. For the O-SWCN, tube diameter and chirality of the O-SWCNs by Raman therearethreeacousticphononmodesataroundΓpoint: spectroscopic experiment. two transverse, one longitudinal, similar with the bared Now we further study the dependence of Raman shift SWCN. However, the twisting mode has a non-zero fre- on the coverage (defined by O/C ratio x). As a typical quency of about 20cm−1 at the center of Brillouin zone, example, Fig.3 shows the frequency of E mode (one of 1g different from the bared SWCN, which is attributed to G modes) in the (10,0) O-SWCN with TA configuration the tube geometry distortion and the oxygen vibrational as a function of x. For a given x, there may be much interaction. Due to the tube symmetry destruction, in- many configurations. To obtain the qualitative relation terestingly, many doubly degenerate phonon modes are betweenRamanshiftandthecoverage,theoxygenatoms split. For example, the lowest E2g mode is split into two are simply considered to be averagelyadsorbed as possi- absolute subbands at 58cm−1 and 68cm−1, shifted from ble, especially for a large x. From Fig. 3, E mode first 1g 62cm−1 of the bared SWCN. Such splitting will have upshifts and then downshifts with x increasing. There an important influence on the low-temperature specific exists a maximum of 1611cm−1 at x = 0.25, showing a heat.[21] Also, some new phonon modes appear below nonmonotonic behavior of the frequency shift. At half 3 coverage (x = 0.5), it even downshifts to be 1580cm−1, x=0.1. In this case, the O-SWCN is considered to have lower than 1585cm−1 of the bare SWCN. For other G itshighestsymmetryofmirrorsymmetry. Tounderstand modes, the frequency shift in both A1g and E2g is also this local minimum, we recalculate the E1g mode at the calculated, which has the similar dependence on cover- same coverage but an asymmetrical configuration. The age to that in E . In order to explore the origin of the obtainedfrequencyof1592cm−1ishigherthan1586cm−1 1g G mode shift, we remove the C-O vibrational interac- in the symmetrical case, which further shows the depen- tions and recalculate the E mode in terms of the same dence of Raman shift on the configurations. 1g optimum geometry above. As a result, the frequencies Insummary,the vibrationalpropertiesofoxygenfunc- are overalldownshifted, lower than those in the presence tionalized nanotubes are studied in terms of lattice dy- of the C-O vibrational interactions, as shown in Fig. 3. namics theory. Due to the C-O and O-O vibrational in- Therefore, it is shown that there exists a large contribu- teractions, many degenerate phonon modes are split and tion of the C-O vibration to the upshift in G modes. evensomenewphononmodesareobtained,whichcanbe IntheabsenceoftheC-Ovibrationalinteractions,also, observed in the Raman spectroscopic experiment. Also, the nonmonotonic behavior is still obtained for the fre- the Raman shift is observed in the O-SWCNs, depend- quency shift in E modes. This may be due to the ing not only on the tube diameter and chirality but also 1g competition between the rehybridization induced bond on oxygen coverage and adsorption configurations. It is expansion in the bonds at the adsorbed sites and the found that the bond expansion and contraction coexist charge transfer induced bond contraction in the bonds in the functionalized carbon nanotube. With the cover- near the adsorbed site. At low coverage, the bond con- age increasing, a nonmonotonic behavior of the G mode traction effect is dominant, leading to the upshift in G shift is obtained, which is contributed to the competi- modes, while the bond expansion effect would become tion between the bond expansion and contraction. The predominant at high coverage, resulting in an decrease results can be helpful to the characterizationand practi- of G mode frequency with x. Therefore, there may exist cal application of functionalized carbon nanotube-based a new competition mechanism between the bond expan- device. sionandcontraction,coexistinginthefunctionalizednan- Acknowledgments: This work was supported by otube,differentfromthephysicallydopedSWCNs.[7,22] National Natural Science Foundation of China (Nos.: In some alkali-metal doped nanotube bundles, the simi- 10674113 and 10374046), Hunan Provincial Natural lar changes in G modes with the coverage had been ex- Science Foundation of China (No.: 06JJ5006), and par- perimentally observed via vapor phase doping or redox tially by Scientific Research Fund of Hunan Provincial reaction.[23,24]Consequently,ourresultscancontribute Education Department (No.: 06A071). to the understanding of the origin of the G mode shift in the functionalized carbon nanotube. In the case of neglecting the C-O vibrational interactions, in addition, there exists a locally minimal frequency of E1g mode at [1] C.A.DykeandJ.M.Tour,J.Phys.Chem.A108,11151 [12] M. Milnera, J. Kurti, M. Hulman, and H. Kuzmany, (2004). Phys. Rev.Lett. 84, 1324 (2000). [2] X.Lu and Z. Chen,Chem. Rev.105, 3643 (2005). [13] Philip G. Collins,Keith Bradley, Masa Ishigami,and A. [3] U. J. Kim, C. A. Furtado, X. Liu, G. Chen, and P. C. Zettl, Science 287, 1801 (2000). Eklund,J. Am. Chem. Soc. 127, 15437 (2005). [14] M. D. Segall, P. L. D. Lindan, M. J. Probert, C. J. [4] H.Murphy,P.Papakonstantinou,andT. I.TOkpalugo, Pickard, P. J. Hasnip, S. J. Clark, and M. C. Payne, J. J. Vac. Sci. Technol. B 24, 1071 (2006). Phys.: Condens. Matter14, 2717 (2002). [5] R.A.JishiandM.S.Dresselhaus,Phys.Rev.B45,6914 [15] H. J. Liu, C. T. Chan, Z. Y. Liu, and J. Shi, Phys. Rev. (1992). B 72, 075437 (2005). [6] C. T. Chan, W. A. Kamitakahara, K. M. Ho, and P. C. [16] R.Saito,G.Dresselhaus,andM.S.Dresselhaus,Physical Eklund,Phys.Rev.Lett. 58, 1528 (1987). Propertiesof Carbon Nanotubes (Imperial College Press, [7] A. M. Rao, P. C. Eklund, S. Bandow, A. Thess, and R. London, 1998). E. Smalley, Nature(London) 388, 257 (1997). [17] Y. Xiao, Z. M. Li, X. H.Yan,Y.Zhang, Y. L.Mao, and [8] B.Akdim,X.Duan,D.A.Shiffler,andR.Pachter,Phys. Y. R.Yang, Phys.Rev. B 71, 233405 (2005). Rev.B 72, 121402(R) (2005). [18] Z.X.Guo,J.W.Ding,Y.Xiao,andY.L.Mao,J.Phys. [9] S. P. Chan, G. Chen, X. G. Gong, and Z. F. Liu, Chem. B,110,21803 (2006). Phys.Rev.Lett. 90, 086403 (2003). [19] D. W. Brenner, Phys.Rev. B 42, 9458 (1990). [10] Y.F.Zhang,andZ.F.Liu,J.Phys.Chem.B108,11435 [20] Y. Yamaguchi, and J. Gspann, Phys. Rev. B 66, 155408 (2004). (2002). [11] S.Bandow, S.Asaka,Y.Saito, A.M.Rao, L.Grigorian, [21] V. N.Popov, Phys. Rev.B 66, 153408 (2002). E. Richter,and P. C. Eklund, Phys. Rev. Lett. 80, 3779 [22] G.U.Sumanasekera,J.L.Allen,S.L.Fang,A.L.Loper, (1998). A.M.Rao,andP.C.Eklund,J.Phys.Chem.B103,4292 4 (1999). Table. 1. Frequencies of RBM and G modes of the [23] N. Bendiab, E. Anglaret, J. L. Bantignies, A. Zahab, J. bare (10,0) SWCN and O-SWCN in both TA and TZ L.Sauvajol,P.PetitC.MathisandS.Lefrant,Phys.Rev. configurations (in units of cm−1). B 64, 245424 (2001). Figure. 1. Optimized structures of (10,0) O-SWCNs [24] G.Chen,C. A.Furtado,S.Bandow, S.Iijima, andP.C. in (a) TA and (b) TZ configurations. Eklund,Phys.Rev.B71, 045408 (2005); G.Chen,C.A. Figure. 2. Phonon dispersion relations of the (10,0) Furtado, U.J. Kim, and P. C. Eklund, Phys. Rev.B 72, SWCN (a) and O-SWCN in TA configuration (b). 155406 (2005). [25] H. M. Lawler, D. Areshkin, J. W. Mintmire, and C. T. Figure. 3. E1g mode frequency of the (10,0) O-SWNT White, Phys.Rev.B 72, 233403 (2005). in TA configuration as a function of the coverage x in [26] J. Maultzsch, H. Telg, S. Reich, and C. Thomsen, Phys. boththeabsenceandthepresenceoftheC-Ovibrational Rev.B 72, 205438 (2005). interactions. 1.474 1.500 1.521 (b) 1.586 1.506 1.472 FIG. 1 Table. 1. Frequencies of RBM and G modes of the bare (10,0) SWCN and O-SWCN in both TA and TZ configurations (in units of cm-1). G Modes RBM E A E 1g 1g 2g SWCN 294 1585 1566 1535 Site TA 278 1592 1572 1543 Site TZ 276 1588 1579 1539 (a) (10,0) SWCN (b) (10,0) O-SWCN 1600 1600 1200 1200 ) ) 1 1 - - m m c c ( 800 (800 w w 400 400 0 0 G G X X wave vector wave vector FIG. 2 1610 presence of C-O vibrational interactions absence of C-O vibrational interactions for an asymmetrical configuration at x=0.1 1600 ) 1 - m c ( w 1590 1580 0.0 0.1 0.2 0.3 0.4 0.5 O/C ratio, X FIG. 3