15659
2006, 110, 15659-15662 Published on Web 07/27/2006
Angular and Local Spectroscopic Analysis to Probe the Vertical Alignment of N-doped Well-separated Carbon Nanotubes T. M. Minea,*,† B. Bouchet-Fabre,‡ S. Lazar,§ S. Point,† and H. W. Zandbergen§ Institut des Mate´ riaux Jean Rouxel-IMN, LPCM, UniVersite´ de Nantes, CNRS-UMR 6502, 44322 Nantes Cedex, France, Laboratoire Francis Perrin, SPAM/DRECAM, CEA-Saclay, 91191 Gif sur YVette Cedex, France, and KaVli Institute of Nanoscience, TU Delft, Lorentzweg 1, 2628 CJ Delft, The Netherlands ReceiVed: June 14, 2006; In Final Form: July 17, 2006
Vertically aligned well-separated N-doped multiwalled carbon nanotubes (CNTs) were grown on a silicon substrate by plasma enhanced chemical vapor deposition (PECVD). Angular near-edge X-ray absorption fine structure (NEXAFS) was used to investigate the vertical alignment of as-grown CNTs. In addition, both individual tubes and tube bundles were characterized by high-resolution electron energy loss spectroscopy (HREELS). Simultaneous analysis of both spectroscopic techniques provides information on chemical environment, orbital orientation between carbon and heteroatoms, and local curvature effects. We demonstrate the utility of NEXAFS as an in situ probe of CNTs.
Interest in carbon nanotubes (CNTs) is mainly due to their extreme and very promising properties in terms of elasticity, stress resistance, electrical and thermal conductivities, reduced dimension with high aspect ratio (quasi 1D systems), chemical inertness but possible rich chemistry based on the carbon hybridization, and allotropy. Many applications of CNTs taking advantage of these properties have been proposed. However, the successful use of CNTs in many applications requires control of the growth direction, such as sensors,1 probe tips,2 supercapacitors,3 and especially nanoelectronics devices such as cold field emitters4 or field-effect transistors (CNT-FET).5 Moreover, the emerging field of nanoelectromechanical systems (NEMS)6 will involve CNTs following a three-dimensional (3D) design. Although the control of the growth direction of CNTs has seen important advances in recent years, very few studies have been performed dealing with the characterization of the degree of alignment. The synthesis of CNTs by catalytic chemical vapor deposition (CCVD) often produces a carpetlike structure composed of vertically aligned multiwalled nanotubes (MWCNTs), which can be seen as a crowding end result. Note that it was only very recently that a quantitative study was reported on the alignment in such CNT carpets.7 Very few groups have been successful in producing CNT orientation parallel to the substrate surface,8 and there is a lack of studies in the literature on horizontal alignment of CNTs. Such studies are also missing for 2D architectures9 or self-assembled CNTs.10 The primary purpose of this letter is to characterize the vertical alignment of well-separated as-grown MWCNTs synthesized by plasma-enhanced chemical vapor deposition (PECVD) as deduced from the angular analysis of near-edge X-ray * To whom correspondence should be addressed. E-mail:
[email protected]. Current address: Universite´ Paris-Sud, LPGP, Bat. 210, 91405 Orsay. E-mail:
[email protected]. † Universite ´ de Nantes. ‡ Laboratoire Francis Perrin. § Kavli Institute of Nanoscience.
10.1021/jp0637072 CCC: $33.50
Figure 1. Schematic diagram of the ECR PECVD reactor used for the synthesis of N-doped CNTs.
absorption fine structure (NEXAFS). The effect of nitrogen incorporation into the CNT walls is deduced by comparing global NEXAFS results with local high-resolution electron energy loss spectroscopy (HREELS), focused on selected nanoscale areas such as well-crystallized walls or bunches of CNTs. N-doped MWCNTs have been grown by PECVD using a low-pressure-high-density plasma excited via electron cyclotron resonance (ECR) in the microwave range (2.45 GHz, 200 W). The synthesis reactor is presented in detail elsewhere, and its schematic drawing is shown in the Figure 1.11,12 Briefly, the plasma gas is a mixture of C2H2/NH3 (2:1) at 0.2 Pa, and the substrate (500 nm SiO2/Si) is heated to 700 °C. The substrateholder was grounded, but a weak positive bias (100 V) is applied on the stainless steel grid separating the ECR plasma source from the deposition chamber. Previous to CNT synthesis, a ∼4 nm thin film of Ni was deposited by plasma sputtering. © 2006 American Chemical Society
15660 J. Phys. Chem. B, Vol. 110, No. 32, 2006
Figure 2. Vertically oriented N-doped CNTs synthesized by ECRPECVD.
Figure 3. MWCNT structure by TEM. The black zone shows the catalyst (Ni). A slight shift in the nanotube axis is visible (inset).
Scanning electron microscopy (SEM) observations show that the product is clean and the CNTs are well separated from each other. CNTs are vertically oriented, and the average growth axis is perpendicular to the substrate. According to Figure 2, the MWCNTs appear well aligned. Different specimens of carbon nanotubes deposited on a carbon holey film grid (300 meshes) have been imaged using a Hitachi HF 2000 field emission gun transmission electron microscope (FEG-TEM) operating at 200 kV and a FEITECNAI G2 monochromated microscope at 200 kV. TEM images show that the multiwalled structure of the CNTs consist of about 20 walls (Figure 3). The inner walls join occasionally, and they replicate the shape of the catalytic particle (black spot, Figure 3). Long (∼1 µm) and straight CNTs can be grown using the PECVD process. However, typical “shoulders” are visible by TEM (inset of Figure 3). We emphasize that only the nanotube axis shifts at these shoulders; the growth direction does not change. Figure 3 clearly shows that ECR grown CNTs exhibit tip growth. NEXAFS experiments were performed around the inner shell (1s) binding electron energy of carbon using synchrotron radiationsSACEMOR beam line 22 at LURE-SUPERACO (Orsay, France). The C1s K-edge spectra have been recorded in the total electron yield (TEY) mode at different incidence angles, from grazing (∼1°) to almost perpendicular at the substrate. The electric field vector of the X-ray beam is perpendicular to the propagation direction and then nearly parallel to the CNT axes at ∼1°. Energies were normalized with respect to pure highly ordered pyrolytic graphite (HOPG). The energy resolution was fixed to 0.1 eV.
Letters
Figure 4. Angular dependence of NEXAFS spectra of aligned wellseparated PECVD grown CNTs. The NEXAFS spectrum of HOPG is shown with a bold line. The inset shows the light polarization (E B denotes the associated electric field) and the incidence angle (θ) with respect to the sample.
As shown in Figure 4, the intensity of the normalized NEXAFS spectra increases drastically when the incidence angle varies from 1° to 45°. The transitions of 1s inner shell electrons to the antibonding π* states are located between 284 and 290 eV. The transitions to σ* states extend over the ionization potential (IP) which is close to 291 eV for aromatic carbon. Since the π* bonding states are perpendicular to the σ* states, the angular dependence of each resonance may help to determine both the degree of alignment of the nanotubes and the different chemical environments into the walls. The main π* resonant feature both in CNTs and HOPG is C1: ∼284.7 eV, assigned to sp2 graphitic carbon. Its intensity and shape are correlated to that of the resonance at ∼292 eV. Changing the incidence angle, NEXAFS spectra reveal additional localized π* states: C2, 286.8 ((0.1) eV; C3, ∼287.3 eV; C4, 288.5 ((0.1) eV. C2 is less localized but already present in HOPG, C3 can be related to pentagonal topological defects as observed in nitrogen-free fullerene-like material,13 while C4 involves manifold C-N bonds.11,14 In other studies, a feature close to C4 was attributed to nickelcarbon bonds in Ni-diamond-like carbon thin films.15 For HiPCO grown CNTs, a feature energetically close was proposed as the NEXAFS signature of the helicity of the N-free single-walled CNTs (SWCNT).16 We are very skeptical of this last assignment for C4. However, one recent work on ozonized SW-CNTs shows a very sharp resonance in this π* region certainly due to carbon-metal oxide bonds.17 For ECR PECVD grown CNTs, a set of samples was previously methodically analyzed by XPS and the signature of metal-carbon bonds was never observed.18 Due to the synthesis method, the interface between nickel particles and graphene sheets is free of oxygen (Figure 3). Therefore, the strongly localized state C4 cannot be the mark of C-Ni-O atomic configurations. The recent availability of electron-beam monochromators for transmission electron microscopes (TEM) allows the possibility of near-edge fine structure (ELNES) of core-loss spectra with a resolution comparable with X-ray absorption spectroscopy, with the advantage of nanometric spatial resolution.19 By the use of an FEI Tecnai G2 FEG microscope equipped with a prespecimen monochromator and a high-resolution electron spectrometer, HREELS spectra of the C K-edge have been acquired from different regions of the specimen. The measurements were performed in diffraction mode with an energy resolution of ∼0.14 eV and a collection semiangle of ∼0.3 mrad. The collection angle of the EELS signal was quite small leading to
Letters
Figure 5. Comparison between NEXAFS at 45° (line + symbol) and HREELS of a clean MWCNT (line). The inset shows a TEM image of the walls and the catalyst particle in black.
preferential collection of the scattered electrons characterized by the momentum transfer direction parallel with the beam direction. As a consequence, an enhancement of the features given by the transitions through the states mainly generated by orbitals that are oriented parallel with the beam direction can occur. This effect could significantly affect the shape of the EEL spectra acquired from anisotropic materials such as nanotubes. Let us discuss the main points of the analysis of the angular dependence of the NEXAFS C K-edge spectra with respect to those of the HREELS. The variation of the C1 feature (Figure 4) of our samples can be compared with that of HOPG. For the latter, the C1 intensity drastically decreases from ∼3 for θ ) 90° to 0.1 for θ ∼ 0°, when the electric field becomes perpendicular to the HOPG π* orbitals.16,20 For our samples, C1 intensity also weakens when the electric field becomes parallel to the CNTs axis but simultaneously with the σ* contribution. This behavior clearly shows the lack of the sp2 orbital orientation. Therefore, we associate the decrease of π* and σ* resonance intensities with the variation of the depth and sample area from where the secondary electrons are emitted. At 45° (Figure 5; line + symbol), the TEY signal originates mainly from the graphitic walls since the PECVD process used ensures the synthesis of individual CNTs. Local HREELS measurements validate this assumption, as can be seen from the good agreement of both spectra reported in Figure 5. The EEL spectrum comes from the clean and well-graphitized region of one CNT (inset of Figure 5). Contrarily, at 1° (Figure 6; line + symbol), the NEXAFS signal comes primarily from the CNT tips. Indeed, for this small angle, the penetration depth is about 40 times shorter than that at 45°. Once more, this assumption is sustained by the similarity between HREELS and NEXAFS spectra, except that the EEL spectrum is recorded through a “bunch” of distorted CNTs (inset of Figure 6). Consequently, the angular evolution of the NEXAFS spectra (Figure 4) characterizes different parts of the same sample. One can extend the analysis in terms of the nanoscale structure of the samples as obtained from the direct comparison of local EELS and averaged NEXAFS signals, two by two, and the good agreement found between them. NEXAFS spectroscopy provides evidence for the chemical environment of elements composing the samples. Thus, the variation of several features mentioned above (C1 to C4) in the π* zone indicates changes of the CNT structure from the wellgraphitized walls (45°) to the primarily top part (1°).
J. Phys. Chem. B, Vol. 110, No. 32, 2006 15661
Figure 6. Comparison between NEXAFS at the grazing angle (line + symbol) and HREELS of a “bunch” of CNTs (line). The inset shows a TEM image of the “bunch” (black spots are the catalyst particles).
Surprisingly, Figure 5 shows equivalent intensities of the C1 peak by NEXAFS and EELS. This result is surprising because NEXAFS averages the signal coming from a large number of more or less organized CNTs (beam area ∼1 mm2), while EELS analyzes a well-organized zone of an individual CNT. One can note the good agreement of the full width at halfmaximum (fwhm) of the C1 peak, since both spectra have been recorded using the same energy resolution. This agreement emphasizes the effectiveness and the reproducibility of our PECVD synthesis process to grow individual MWCNTs. The C1 peak intensity is lower for CNTs than for HOPG (Figure 4) due to the curvature of the graphene walls into the tubes. In addition the first σ* features (∼293 eV) show close intensities but different fine structures. The splitting exhibited by the σ* EEL spectrum (Figures 5 and 6) as in HOPG reveals the good graphitization of the as-deposited CNT walls. The NEXAFS (line + symbol) σ* feature is slightly broadened and smooth due to the multiplicity of local configurations. However, the superposition of these two types of spectra reveals several differences of the additional resonances (C2, C3, and C4). Most of these can be identified on the “bunch” spectrum (Figure 6, line). The feature labeled C2 (∼286.8 eV) has been identified by NEXAFS on an HOPG sample (Figure 4, bold line) and on CNTs only at the grazing angle (Figure 6, line + symbol). EELS measurements are in good agreement since only the “bunch” sample shows this peak but less intense (Figure 6, line). The presence of C2 on our HOPG sample can be related to localized π* states of CdO bonds induced by superficial adsorption of water vapor.21,22 This feature is also present in the 45° NEXAFS spectrum (Figure 5, line + symbol), but the other features C3 and C4 are broader and strengthened. The absence of C2 from the EEL spectrum of the individual CNT (Figure 5, line) can be explained by our choice to locally examine one perfect multiwalled nanotube. Concerning C3, it has been identified on two different spectra, the bunch of CNTs by EELS (Figure 6, line), and the analysis at 45° by NEXAFS (Figure 5, line + symbol). As previously mentioned, we associate this feature with topological defects of graphite CdC bonds, such as were recorded from amorphous carbon-based thin films,25 carbon nitride nanopowders,14 or fullerenes.13 The absence of this feature from the individual MWCNT EEL spectrum indicates the good graphitization quality of the nanotubes walls. On the other hand, the grazing angle NEXAFS spectrum does not show any sharp resonance at 287.3 eV corresponding to C3
15662 J. Phys. Chem. B, Vol. 110, No. 32, 2006 but rather a broad feature with the maximum intensity shifted to higher energies, at ∼288.4 eV, the so-called C4. Associating C4 with the π* orbital of C-N bonds,11,14 this result reveals the high concentration of nitrogen at the extreme surface of nanotubes, that is, their most top carbon layers. But the intensity of C1 is very close to that of the C4 peak (Figure 6, line + symbol). Therefore, the regular graphite rings are as frequent as nitrogen-doped ones, defining thus the structure of the top CNT layers. The broadness of C4 points to a large variety of chemical environments of nitrogen in the CNTs. We emphasize that 45° NEXAFS analysis (average over ∼1 mm2 of the sample) confirms the presence of C-N bonds, because C4 is present on this spectrum too but shifted by 0.2 eV in agreement with previous results.11 Note that the major part of the 45° analysis comes from the CNTs walls, so the nitrogen is well incorporated in the walls. The opposite angular evolution of C1 and C4 (Figures 4, 5, and 6) signifies that the π orbitals of the C-N bonds are twisted with respect to the C-C one, showing thus their curvature and defective nature.23,24 The comparison between NEXAFS and XPS has shown the preferential positioning of nitrogen in conjugated configurations bridged by one or two C atoms (e.g., N-C-N, N-C-C-N).11,25 From the angular analysis of the NEXAFS spectra (Figure 5), it is also possible to estimate the average alignment of PECVD nanotubes. At 45°, the normalized intensity of C1 reaches ∼1.7. Considering the normalized C1 intensity for HOPG tilted at 45° about 2.1, we can roughly estimate the deviation of the CNTs axis from the substrate normal of (25° as deduced from geometric assessments taking into account the cylindrical distribution of π orbitals in CNTs. In a recent but pioneering work, carpet-like CNTs grown by CCVD 7 have been characterized by NEXAFS with respect to XRD. The average deviation from the growth axis is found to be (15°, despite the relatively low level of the C1 peak intensity that never exceeds 1.35.15 Thus, we think that the as-grown wellseparated CNTs analyzed here are fairly well aligned despite the ∼4 at. % nitrogen incorporated.11,12 Finally, we comment on the absence of nitrogen signature in the EEL spectra. The measurement of the C4 feature by EELS seems to be very difficult, due to the low concentration of N and very limited volume of analyzed sample. However, a slight difference can be seen at ∼288.6 eV between the samples of the individual MWCNT and the “bunch”, but no peak rises up. Another reason could be the different orientation of C-N π orbitals with respect to C-C ones, making them less sensitive to the diffraction mode operation of TEM. In addition, the NEXAFS π* signal (from 287 to 289 eV) partially originates from the traces of turbostratic carbon deposited among wellseparated CNTs, while samples investigated by EELS are only nanotubes. Conclusions As-grown PECVD synthesized nitrogen-doped multiwalled carbon nanotubes were successfully investigated by two spectroscopic techniques. HREEL spectra recorded on individual MWCNTs show the good quality of plasma grown nanotubes. The direct comparison of local HREELS and global NEXAFS characterization techniques is very effective for nanomaterials
Letters analysis. Capabilities of both spectral techniques compliment each other. The organization and structure of CNTs can be understood from the good agreement found between the NEXAFS and EELS spectra, pairwise. We deduced the nanotube angular deviation from the growth axis induced by the heteroatom (nitrogen) bonding carbons. NEXAFS comes out as a very promising nondestructive technique, as an in situ characterization tool for industrial application of CNTs grown on large area substrates. Acknowledgment. This work has been performed within the framework of the GDRE No. 2756 Science and applications of the nanotubessNANO-E. C. Ewels is acknowledged for fruitful discussions. References and Notes (1) Star, A.; Han, T.-R.; Joshi, V.; Gabriel, J.-C. P.; Gru¨ner G. AdV. Mater. 2004, 16 (22), 2049. (2) Dai, H. J.; Hafner, J. H.; Rinzler, A. G.; Colbert, D. T.; Smalley R. E. Nature 1996, 384, 147. (3) Du, C.; Yeh, J.; Pan, N. Nanotechnology 2005, 16, 350. (4) Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A.; Milne, W. I.; Hasko, D. G.; Pirio, G.; Legagneux, P.; Wyczisk, F.; Pribat, D. Appl. Phys. Lett. 2001, 79, 1534. de Heer, W. A.; Chaˆtelain, A.; Ugarte, D. Science 1995, 270, 1179. (5) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. J. Nature 2003, 424, 654. Dresselhaus, M. S.; Dai, H. J. (Guest Editors) MRS Bull. 2004, 29 (4), 237. (6) Hierold, C. J. Micromech. Microeng. 2004, 14 (9), S1. Clelnad, A. N. Fundations of Nanomechanics; Springer: Berlin, 2002 (ISBN: 3-54043661-8). Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (7) Pichot, V.; Launois, P.; Pinault, M.; Mayne-L’Hermite, M.; Reynaud, C. Appl. Phys. Lett. 2004, 85 (3), 473. (8) Lacerda, R. G.; Teh, A. S.; Yang, M. H.; Teo, K. B. K.; Rupesinghe, N. L.; Dalal, S. H.; K. Koziol, K. K. K.; Roy, D.; Amaratunga, G. A. J.; Milne, W. I.; Chhowalla, M.; Hasko, D. G.; Wyczisk, F.; Legagneux, P. Appl. Phys. Lett. 2004, 84 (2), 269. (9) Chakrapani, N.; Wei, B. Q.; Carrillo, A.; Ajayan, P. M.; Kane, R. S. Proc. Natl. Acad. Sci. U.S.A. 2004, 101 (12), 4009. (10) Franklin, R. N.; Dai, H. J. AdV. Mater. 2000, 12, 890. (11) Point, S.; Minea, T.; Bouchet-Fabre, B.; Granier, A.; Turban, G. Diamond Relat. Mater. 2005, 14, 891. (12) Minea, T. M.; Point, S.; Granier, A.; Touzeau, M. Appl. Phys. Lett. 2004, 85 (7), 1244. (13) Pichler, T.; Knupfer, M.; Golden, M. S.; Fink, J.; Cabioc’h, T. Phys. ReV. 2001, 63, 155415. (14) Marino, E.; Bouchet-Fabre, B.; Porterat, D.; Reynaud, C. Diamond Relat. Mater. 2005, 14, 881. (15) Benchikh, N.; Garrelie, F.; Donnet, C.; Bouchet-Fabre, B.; Wolski, K.; Rogemond, F.; Loir, A. S.; Subtil, J. L. Thin Solid Films 2005, 482, 287. (16) Tang, Y. H.; Sham, T. K.; Hu, Y. F.; Lee, C. S.; Lee, S. T. Chem. Phys. Lett. 2002, 366, 636. (17) Banerjee, S.; Hemraj-Benny, T.; Balasubramanian, M.; Fischer, D. A.; Misewich, J. A.; Wong, S. S. Chem. Commun. 2004, 7, 772. (18) Minea, T. M.; Point, S.; Gohier, A.; Granier, A.; Godon, C.; Alvarez, F. Surf. Coat. Technol. 2005, 200, 1101. (19) Lazar, S.; Botton, G. A.; Wu, M.-Y.; Tichelaar, F. D.; Zandbergen, H. W. Ultramicroscopy 2003, 96, 535. (20) Chen, J. G.; Eng, J., Jr.; Kelty, S. P. Catal. Today 1988, 43, 147. (21) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992. (22) Kuznetsova, A.; Popova, I.; Yates, J. T., Jr.; Bronikowski, M. J.; Huffman, C. B.; Liu, J.; Smalley, R. E.; Hwu, H. H.; Chen, J. C. J. Am. Chem. Soc. 2001, 123, 10699. (23) Terrones, M.; Terrones, H.; Grobert, N.; Hsu, W. K.; Zhu, Y. Q.; Hare, J. P.; Kroto, H. W.; Walton, D. R. M.; Kohler-Redlich, Ph.; Ru¨hle, M.; Zhang, J. P.; Cheetham, A. K. Appl. Phys. Lett. 1999, 75, 3932. (24) dos Santos, M. C.; Alvarez, F. Phys. ReV. B 1998, 58, 13918. (25) Bouchet-Fabre, B.; Zellama, K.; Godet, C.; Ballutaud, D.; Minea, T. Thin Solid Films 2005, 482, 156.
