J. Phys. Chem. C 2007, 111, 10353-10358
10353
ERDA and Structural Characterization of Oriented Multiwalled Carbon Nanotubes A. Gohier,* S. Point, M. A. Djouadi, and A. Granier IMN, UMR 6502 CNRSsUniVersite´ de Nantes, BP 32229, 44322 Nantes, France
T. M. Minea LPGP, UMR 8578 CNRSsUniVersite´ Paris Sud, Bat. 210, 91405 Orsay, France
U. Kreissig, G. Abrasonis, A. Kolitsch, and W. Mo1 ller Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf ReceiVed: March 2, 2007; In Final Form: May 18, 2007
Oriented multiwalled carbon nanotubes have been synthesized by distributed electron cyclotron resonance plasma enhanced chemical vapor deposition. Elastic recoil detection analysis measurements on multiwalled carbon nanotubes are reported here for the first time. On the basis of the recorded depth profiles, we have developed a simple model to estimate the surface densities of as-grown nanotubes. Besides, nitrogen and hydrogen contents into MWNT, typically less than 6.5 and 8 atom %, respectively, have been characterized as a function of the chemical nature of the catalyst, the synthesis temperature, and the hydrogen carrying diluent gas. These results are discussed with respect to the structural characterization performed by electron energy loss spectroscopy measurements, transmission electron microscopy and X-ray photoelectron spectroscopy.
1. Introduction In the past few years, intensive studies have been devoted to carbon nanotubes (CNTs) growth, characterization, or integration into nanoscale devices.1-7 Indeed, thanks to its mechanical and electronic properties, carbon nanotube has become one of the most promising elementary structure in the emerging field of nanotechnology. Among CNTs growth processes, plasma-enhanced chemical vapor deposition (PECVD) appears very attractive due to its ability to grow self-oriented CNTs,8 perpendicular to the substrate, with a precise location control.9 Furthermore, it is the unique method capable to grow carbon nanofibers close to room temperature.10 Carbon nanotubes, especially N-doped CNTs, grown by PECVD or CVD techniques, have been widely characterized by near-edge X-ray absorption fine structure (NEXAFS),11-14 Electron energy loss spectroscopy (EELS),15-17 X-ray photoelectron spectroscopy (XPS),18,19 or Raman spectroscopy.3,20 However, very few works have focused these spectroscopic studies on the impact of the main parameters that govern CNTs growth process (catalyst, temperature, gas precursor, ...). Moreover, the elastic recoil detection analysis (ERDA), usually dedicated to thin films characterization, has never been used for CNTs investigation, even if PECVD as-grown CNTs often organize as “carpet-like” or compact thin films.18,20,21 This paper reports on ERDA and EELS characterization of oriented multiwalled carbon nanotubes (MWNT) grown by distributed electron cyclotron resonance (DECR)-PECVD using several catalysts (Ni, Pd, Co), temperatures and gas mixtures (C2H2 with NH3 or H2). Note that both techniques check out all CNTs walls, EELS at local scale (individual CNT), and * Corresponding author. E-mail address:
[email protected].
ERDA at macroscopic scale (∼1 mm2). From the ERDA concentration depth profile of light elements (hydrogen, nitrogen, oxygen and carbon) contained into the CNTs samples, the atomic contents have been calculated and compared with XPS measurements.18 Furthermore, from the ERDA carbon spectra, the porosity and hence the MWNT surface density has been evaluated. EELS investigations were conducted in order to estimate the MWNT local order degree (graphitization) as a function of the synthesis temperature. 2. Experimental Section DECR-PECVD process has been developed for CNTs growth, and it is detailed elsewhere.22 At low pressure (0.2 Pa), highdensity plasma (ne ≈ 1010 cm-3) can be generated, thanks to the resonant transfer of the microwave power (2.45 GHz) to electrons trapped by an additional magnetic field (permanent magnets) distributed at the top of the chamber. During the plasma discharges, the heating substrate holder was grounded while the plasma potential was fixed at 100 V via a stainless grid situated at the mid-gap between the DECR source and the substrate. A thin film of catalyst was deposited by PVD (Physical Vapor Deposition) on thermally oxidized silicon substrates (SiO2-500 nm/Si). Catalyst deposition was in situ performed for palladium by sputtering a wire-shaped electrode with ions coming from pure ammonia plasma (Pµw ) 200 W). Nickel was ex situ deposited in another PVD plasma reactor.18 Film thicknesses, evaluated by XPS, were 2 nm for Pd and 4 nm for Ni. The substrate was heated up to the synthesis temperature (500-700 °C). This promotes the restructuring of the metallic thin film in isolated catalyst nanoparticles with diameters ranging from 5 to 40 nm. Acetylene diluted in ammonia C2H2/NH3 (1:
10.1021/jp071719x CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007
10354 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Gohier et al.
Figure 1. SEM (tilt 45°) and TEM images (insets) of oriented N-doped carbon nanotube synthesized using nickel (4 nm) and C2H2/NH3 1:2 plasma at (a) 500, (b) 550, (c) 600, and (d) 700 °C).
Figure 2. EELS spectra of nanotubes synthesized at 550, 600 and 700 °C and turbostratic carbon obtained at 500 °C using nickel as catalyst with C2H2/NH3 1:2 plasma at the (a) C-K edge and (b) N-K edge.
2) or in hydrogen C2H2/H2 (1:10) was used as feedstock gas for CNTs growth. Carbon nanotubes were characterized by scanning electron microscopy (SEM), performed on JEOL JSM 6400 F1 apparatus. Samples were micro-delaminated with a diamond tip and directly transferred onto the Transmission Electron Microscope (TEM) grid. The CNTs structure was analyzed by field emission gun transmission electron microscope (FEG-TEM; Hitachi HF 2000) operating at 200 kV. EELS spectra were recorded on a GATAN 666 parallel spectrometer in the above TEM (working at 100 kV), focusing the e-beam on the rim of the MWNT. ESCA Leybold spectrometer for X-ray photoelectron Spectroscopy (XPS) was used for ex situ analysis. XPS spectra were recorded at ∼10-7 Pa. with X-ray Al KR (1486.6 eV) line and the pass energy of the analyzer at 50.4 eV for narrow scans. The CNTs composition was determined by Elastic Recoil Detection Analysis at the Rossendorf 5 MV tandem accelerator. The measurements were performed with 35 MeV Cl7+ ions hitting the samples under an angle of 15° relative to the surface. The light recoiled atoms and the scattered chlorine ions were detected by a Bragg-ionization-chamber under a forward direc-
tion of 30°. The H-recoils are detected by a Si-detector which was shielded by 18 µm thick Al foil against all other particles. Further details of the overall experimental setup can be found elsewhere.23 The energy spectra of recoils and scattered ions were converted into concentrations versus depth profiles by means of a computer code24 using the stopping power data from Ziegler et al.25 3. Results and Discussion 3.1. CNTs Growth and Structural Characterization. a. Impact of the Synthesis Temperature. First, CNTs were grown during 60 min with C2H2/NH3 (1:2) plasmas using nickel as catalyst at several synthesis temperatures (Figure 1). For temperatures bellow 550 °C, no nanotube can be observed, nor by SEM, neither by TEM (Figure 1a). Note that only nickel nanoparticles embedded in turbostratic carbon can be discerned by TEM at 500 °C (Figure 1a, inset). At 550 °C, SEM observations show very short (600 °C (Figure 5a). This provides evidence that higher the temperature, higher the amount of sp2 bonding. It denotes an improvement in the local order (graphitization degree) of carbon nanotubes. EELS spectra were also acquired at the N-K edge (Figure 2b). They undoubtedly validate that MWNT grown with C2H2/ NH3 mixture are N-doped. As for the C-K edge, two components can be discerned: (i) a peak at ∼398 eV, relative to the transitions from 1s to π* band and (ii) a wide feature starting at ∼402 eV, due to the transitions from 1s to the σ* band. From the integration of the EELS carbon and nitrogen peaks, nitrogen content up to 5 atom % has been calculated. Even if these spectra are quite noisy (due to the low nitrogen content into CNTs walls, one can note that the π* peak intensity increases with the temperature (T > 550 °C). Therefore C-N π bonding systems (pyridine-like, graphitic-like) are favored at higher synthesis temperatures. In addition, σ* resonances become much more defined as the temperature increases, emphasizing a better cristallinity of the nanostructures. Trying to correlate the structure and the local environment of the carbon atoms in these CNTs, TEM investigations reveal some specific defects in the graphitic network common for our growth process. Curvatures at right angles in the shell structures were discerned in Figure 3b,c (solid arrow). One can speculate about their origin which can lie in the nitrogen doping. Indeed, it is well-known that whatever their hybridization, carbon atoms do not form stable right angles bonds. Moreover, it has been shown that pairs of nitrogen atoms inserted in the lattice as azapentalene units may stabilize right angles in fullerene-like structures.26 Our intention to analyze precisely this defective region by HR-STEM has been unfruitful due to the very noisy nitrogen signal. However, carbon nanotubes grown by ECRPECVD have already been studied by NEXAFS and XPS. These two techniques, averaging many CNTs (∼1 mm2) at the N 1s
10356 J. Phys. Chem. C, Vol. 111, No. 28, 2007
Gohier et al.
TABLE 1: Structural Characteristics of MWNT Synthesized by DECR-PECVDa
a
samples
structure
aspect/ ength (nm)
mean diameter (nm)
max diameter (nm)
Ni C2H2/NH3 1:2 700 °C Ni C2H2/NH3 1:2 600 °C Ni C2H2/NH3 1:2 550 °C Ni C2H2/H2 1:10 700 °C Pd C2H2/NH3 1:2 700 °C
nanotubes nanotubes nanofibers nanotubes nanofibers
uniform 380 < lCNTs < 600 uniform 300 < lCNTs < 500 nonuniform lCNTs < 150 uniform 180 < lCNTs < 210 uniform 200 < lCNTs < 250
15 15 20 15 15
40 40 30 30 20
The first column indicates the catalyst, the gas mixture, and the substrate temperature.
TABLE 2: N/C and H/C Ratio Performed by ERDA and XPS Measurement samples
N/C (%) ERDA
N/C (%) XPS
H/C (%) ERDA
N-doped CNTs (Ni 700 °C) N-doped CNTs (Ni 600 °C) N-doped CNTs (Pd 700 °C) CNTs (Ni 700 °C)
6.2 ( 0.5 5.3 ( 0.5 4.9 ( 0.4 0.6 ( 0.1
5(1 6(1 7 ( 1.5 -
6.2 ( 0.2 6.1 ( 0.2 4.4 ( 0.1 8.1 (0.2
edge, revealed that nitrogen atoms occupy preferentially conjugated positions related to carbon atoms in the graphene sheet (e.g., N-C-N or N-C-C-N).13,18 The nitrogen content measured in the CNTs films by XPS is estimated ∼5 atom %. It appears quite low compared with others CNx “bamboo-like” nanostructures reported by other groups (∼20 atom %).27 Note that the CNTs presented in this work are MWNT rather than nanofibres (Figure 3a). They do not present regular and large compartments, in the vicinity of which “bamboo-like” structures are shown to concentrate their nitrogen.28,29 The structural features of the synthesized CNTs (length, diameter) are summarized in Table 1. The above results clearly show the existence of a critical synthesis temperature between 550 and 600 °C. Below this critical value, the carbon deposit order worsens and we commonly call it nanofibres (CNFs). Nanofibres walls are misaligned (with graphite fringes out of the axis) and their growth rate is drastically reduced (estimated ∼1 nm.s-1 rather than 2.5 nm.s-1). This radical modification of the growth kinetics is certainly the expression of an important change of the growth regime. As recently suggested, nanotubes structuring may be predominantly controlled at low temperature by the carbon diffusion on the catalyst surface.30 The dominant
Figure 6. Depth profiles calculated from ERDA measurements on MWNT synthesized at 600 °C with nickel (cf. Figure 1c). Note that the legend of the y-coordinate cannot be labeled “atomic density” but atomic ratio because of the porosity of the MWNT films. For the same reason, the x-coordinate only provides a virtual depth. The apparent thickness of the MWNT film, obtained from the derivative of the profile is denoted lERDA.
mechanism that governs nanotube growth may switch from the carbon surface diffusion to the bulk diffusion by increasing the synthesis temperature (T > 550 °C). b. Effect of the Catalyst and the Hydrogen Carrying Diluent. Palladium has been used as target for in situ PVD catalyst preparation. It has succeeded the growth of carbon nanotubes with the same synthesis conditions as described in the previous section (700 °C, C2H2/NH3 1:2, Vgrid ) 100 V). Using palladium (2 nm), CNTs observed length appears uniform and close to 250 nm (Figure 4b). TEM mainly shows twisted structures with small diameters (
