GISAXS study of the alignment of oriented carbon nanotubes grown on plain SiO₂/Si(100) substrates by a catalytically enhanced CVD process

Abstract

We report a quantitative Grazing Incidence Small Angle X-Ray Scattering (GISAXS) study of the alignment of mutually oriented carbon nanotubes (CNTs) grown by a catalytically-activated, a plasma direct current and hot filaments-assisted (DC HF CCVD) process. Metallic catalytic (Co) islands were dispersed on plain SiO 2 (5 nm thickness)/Si(100) substrates prior the growth of CNTs which can be considered as highly anisotropic 1D nanostructures. The GISAXS pattern analysis in the framework of the DistortedWave Born Approximation (DWBA) has been expanded to multilayered non-correlated surface science systems (non-correlated carbon nanotubes) and is based on the determination of carbon nanotubes density, characteristic lengths, atomic Co dispersion throughout the CNTs and roughnesses of the uncorrelated particles. Even dominated by envelope features of disordered objects, they provide noticeable information about CNTs films: both structural (orientation, size and length distribution) and correlation (density, mutual alignment) information. The results stand in rather good agreement with the Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) observations. Moreover, the GISAXS patterns could only be satisfactorily reproduced by adding a mixed C -Co contribution between the pure carbon and the metallic cobalt let on top of the CNT, inferring that cobalt continuously fills the nanotube in the course of the growth and that the CNTs experience a large tendency toward mutual alignment. The effect of variable X-ray incidence angles has been investigated. With a weak addition of ammonia (from 1% to 3% of the gas mixture) the density has been found to decrease by more than one order of magnitude.

Localized CVD growth of oriented and individual carbon nanotubes from nanoscaled dots prepared by lithographic sequences

“…The third part of table 1 summarizes the comparative analysis regarding the characteristics of MWNTs grown on micronsized dots as a function of the catalytic metal and the ammonia concentration during the CVD process, the other experimental conditions remaining unchanged. As we have seen for sample D, the addition of NH 3 in the gaseous mixture strongly increases the MWNT growth rate [28], but also modifies the micron-sized dot shape (at high NH 3 content), the density but also the quality of the nanotubes. Thus, accounting for all these parameters a good compromise would be to use 0.5% NH 3 [29].…”

Alignment of Vertically Grown Carbon Nanostructures Studied by X-Ray Absorption Spectroscopy

“…The flux rate at 973 K is estimated to 0.025 nm of equivalent layer per minute from an in situ XPS analysis of the Co2p/Si2p signal. In other cases the transition metal is deposited by sputtering within conditions that have been elsewhere described [39].…”

“…The nature of the carbon deposit was probed by Raman spectroscopy on a Renishaw apparatus with a He-Ne light source. More structural and spectroscopic data are reported in [39].…”

“…Under conditions where the catalyst is deposited by UHV atomic evaporation at moderate pressure (5 -15 mbar), it is possible to control the nature of the carbon nanostructures [39]. Carbon nanowalls (CNWs) are prepared at low pressure (5 mbar) (Figure 1(f)).…”

A GISAXS study of the angular dependence of carbon nanotubes grown on a plain substrate by the dc HF CCVD process

“…This process requires the presence of transition metal particles (Co, Fe, Ni) as a catalyst spread onto the surface. It has been shown in previous studies that the size and density of these metallic particles govern the size and distribution of the grown CNTs [5][6][7][8]. So, an in situ process by Ultra High Vacuum (UHV) metal evaporation at high temperature on an SiO 2 /Si(100) substrate has been used to get monodisperse and uniform spatial distributions of these particles [6,7].…”