Variation of Composition and Structure over the Height in Arrays of Vertically Aligned CNT

Makunin A.V., Bukunov K.A., Vorobyeva E.A., Pankratov D.A., Petrov D.V., Chechenin N.G. Variation of Composition and Structure over the Height in Arrays of Vertically Aligned CNT. /XII International Conference on Nanostructured Materials (NANO 2014), July 13-18, 2014 Moscow, Russia (2014), pp01.116

Arrays of vertically aligned multiwall carbon nanotubes (MWNT) were grown by the method of pyrolitic gas-phase deposition were studied by a number of techniques. Vibrational modes variations over the length (height) of CNTs with abnormal large aspect ratio of the order of 10⁴ ÷ 10⁵ were studied using the Raman scattering spectra. The results made it possible to estimate structure degree of disorder variation over the length of MWNTs indirectly. Microstructure and composition data were obtained using scanning electron microscopy (SEM) and X-ray dispersion analysis. The method of Mössbauer spectrometry was used to investigate the phases of iron present in MWNTs. Based on the experimental results, various iron phases distribution influence on the MWNTs growth mechanism was discussed.

Materials and methods.

The process of carbon deposition on silicon substrate is based on continuous injection of cyclohexane with ferrocene addition as a catalyst into the IC-CVD reactor at a temperature of 800°C with the support gas (N₂) flow. Average process duration is 80 min. VANTs arrays with uniform heights (up to 1 mm) are produced systematically [1]. Raman spectroscopy was performed using NTEGRA Spectra system with 633 nm and 473 nm excitation lasers wavelengths (λ_ex). Spectra were collected from a surface of 1 – 2 µm in diameter. Mössbauer spectrometry was performed using МС1104ЕМ spectrometer. SEM measurements were performed using LYRA3 TESCAN system with X-ray dispersion analysis module X-Max Oxford 80 mm².

Results. Raman spectroscopy data.

In sp² bonded carbon materials peak intensity ratio (I_D/I_G) is the characteristic of the system degree of disorder, as well as I_2D/I_G parameter including second-order Raman band intensity. Degree of disorder is associated with defects and correlate with sp² crystallite size (L_a) [2]. Therefore I_D/I_G variation over the height (h) appears to be informative for establishing defect distribution in MWNTs arrays. D’ band is also defect dependent but comparatively weak, so it is important to specify reliable spectrum deconvolution method. Experimental spectrum is not approximated well enough using a sum of 3 authorized Lorentz peaks compared to approximation with 2 extra phenomenological ones.

Main peaks positions remain constant over the height variation. Due to the mechanism of stimulated Raman scattering and number of interacting excitatory photons conservation I_D/I_G and I_2D/I_G slope values are close to opposite. Assuming I_D/I_G is linear over the height and using general equation for the calculation of the L_a [2] the latter was estimated.

Mössbauer spectrometry data. X-ray dispersion analysis.

Deconvoluted bands of Mössbauer spectrum [3] correspond with various iron phases or coordination environment. Singlet line in the center of the spectrum corresponds with metastable (under 910°C) gamma iron. Its formation could be enabled only in the synthesis conditions with further carbon stabilization and formation of austenite structure (γ-Fe-C). Band with maximum relative area corresponds with Fe₃C. This phase represents the major contribution into the iron phases distribution in MWNTs arrays and probably takes significant part in MWNT formation. Fe-graf band could refer ether to some Fe-C compound or Fe₃C oxidation product.

Conclusions.

• Disorder parameter I_D/I_G variation over the height of MWNTs arrays was estimated as linear. Constant and proportional contributions shows that MWNTs contain defects of at least 2 different origins. One of them depends on the height and probably associated with iron phases impurities.
• Estimated quantitative dependency of sp² crystallite size over the array (total) height is in good correlation with previously established facts [2].
• Mössbauer spectrometry results give evidence of MWNTs (like SWNTs) Fe₃C phase growth mechanism.

References.

[1] A.V. Makunin, N.G. Chechenin, A.A. Serdyukov, K.E. Bachurin, and E.A. Vorobyeva, Technological characteristics of the processes of carbon nanostructure production by the methods of plasma_arc and gas_pyrolytic deposition, //Inorganic Materials: Applied Research, 2 (3) (2011), pp. 252–255.

[2] N. Soin, S.S. Roy, S.C. Ray and J.A. McLaughlin, Excitation energy dependence of Raman bands in multiwalled carbon nanotubes, //Journal of Raman Spectroscopy, 41 (2010), pp. 1227–1233.

[3] A. Fionov, G. Yurkov, V. Kolesov, D. Pankratov, E. Ovchenkov, and Y. Koksharov, Composite material based on iron-containing nanoparticles for applications in the problems of electromagnetic compatibility, //Journal of Communications Technology and Electronics, 57 (2012), pp. 543–552.

Makunin A.V., Bukunov K.A., Vorobyeva E.A., Pankratov D.A., Petrov D.V., Chechenin N.G. Variation of Composition and Structure over the Height in Arrays of Vertically Aligned CNT. /XII International Conference on Nanostructured Materials (NANO 2014), July 13-18, 2014 Moscow, Russia (2014), pp01.116

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