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Simultaneous growth of MWCNTs at different temperatures in a variable gradient furnace

Valentina Grossi ,  Alessandro Urbani ,  Andrea Giugni ,  Sandro Santucci ,  Maurizio Passacantando 

Università dell'Aquila, Dipartimento di Fisica, Via Vetoio, L'Aquila I-67010, Italy

Abstract

Multi-walled carbon nanotubes (MWCNTs) have been grown, simultaneously at different temperatures, by thermal chemical vapour deposition (CVD) of acetylene (C2H2) gas, in ammonia (NH3) atmosphere. The reactor for the CVD growth is a horizontal quartz tube mounted inside a furnace. An accurate calibration of the temperature, at various distances from the tube mean position has been performed by a thermocouple. Silicon oxide substrates, coated with 3 nm of nickel (Ni) film, have been placed at different positions in the reactor and the CNTs have been grown at different temperatures: 750 °C, 725 °C, 700 °C and 600 °C. The tube has been heated by radiation, in an argon (Ar) flux, up to 980 °C (temperature in the middle of the reactor). In order to obtain the Ni nanoclusters, only a NH3 gas has been introduced into the quartz tube for 20 min at 980 °C. The CNT growth has been performed at 980 °C for 1 hour introducing C2H2 and NH3 gasses with the flux ratio (C2H2:NH3) of 1:5. Scanning electron microscopy (SEM) images show a uniform and a highly dense distribution of CNTs. But, as the growth temperature of substrate increases, the CNT average diameter and density distribution decrease. There is a temperature limit (725 °C) for the CNT growth. At 750 °C we obtain only the Ni clusters with the amorphous carbon. The Raman spectra show that we produce the MWCNTs. The presence of the D-peak indicates that the CNTs have the structural defects to respect to the high crystalline graphitic layers, indicated by the G-peak. Where the temperature of the substrates is higher the D-band line-width is smaller but the ratio of intensities of D and G peaks is still high. The Raman spectra confirm that when the substrate temperature increases the CNT density distribution decreases. The CVD method of growth is totally selective. It is possible to obtain the isolated arrays of CNTs and then to realize the gas sensing (NO2) devices.

 

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Related papers

Presentation: Oral at E-MRS Fall Meeting 2008, Symposium C, by Valentina Grossi
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-05-12 13:11
Revised:   2009-06-07 00:48