Growth Morphology and Spectroscopy of Multiwall Carbon Nanotubes Synthesized by Pyrolysis of Iron Phthalocyanine

Abstract

Carbon nanotubes (CNTs) were synthesized by Chemical Vapor Deposition (CVD) from the pyrolytic decomposition of Iron Phthalocyanine (FePc) molecules, on SiO2/Si(111) substrates in the presence of a hydrogen flow. FePc molecules contribute simultaneously both to the formation of the precursor Fe nanoparticles and also as a Carbon source. Different experimental conditions were examined. Samples were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and inverse photoemission. The resulting samples are highly oriented multiwall carbon nanotubes films, with heights in the range between: 4 and 20 microm. The tubes diameter is strongly dependent on growth temperature. Our experimental results show evidence of a transition in the growth mechanism, from a tip growth to a base growth mode, as the decomposition temperature is increased. Preliminary spectroscopic measurements performed on these MWCNTs, show the unoccupied density of states has several resonances close to Fermi level, related both to the graphene electronic structure and the formation of the tube.

Inverse photoemission spectroscopy of multiwall carbon nanotubes

“…Growth method I: (sample I) This procedure has been previously reported [5]. In a typical reaction, 0.05g of Fe-Pc were decomposed over SiO 2 /Si(111) wafers at 1000 o C. The reaction under these conditions takes about 20 min.…”

“…One of the films was obtained by pyrolysis of Iron (II) Phthalocyanine [5]. This compound has a double function, first supplying iron atoms for the formation of the catalytic nanoparticles and then as a carbon source.…”

Carbon nanotubes grown by catalytic CO2 laser-induced chemical vapor deposition on core-shell Fe/C composite nanoparticles

“…Two main mechanisms are given in the literature (see for instance [32][33][34][35][36]), for explaining carbon nanotube growth: ''a tip-growth mechanism'', where the CNTs push the nanoparticles to the top and ''a basegrowth mechanism'' where the nanoparticles seem to stay attached to the supporting substrate. In [36] it is demonstrated that, depending on the experimental parameters, both mechanisms could coexist.…”

“…Two main mechanisms are given in the literature (see for instance [32][33][34][35][36]), for explaining carbon nanotube growth: ''a tip-growth mechanism'', where the CNTs push the nanoparticles to the top and ''a basegrowth mechanism'' where the nanoparticles seem to stay attached to the supporting substrate. In [36] it is demonstrated that, depending on the experimental parameters, both mechanisms could coexist. Of these two models, we consider here for simplicity the possibility of a ''tip-growth mechanism'' where the following stages may occur: (i) at temperatures higher than about 600-700°C, the hydrocarbon molecules decompose at the surface of the Fe catalyst nanoparticles at locations where discontinuities in the thin graphenic sheet appear, accumulating the carbon fragments at the surface and superposing to the already existing carbon layers; (ii) carbon diffuses through the catalyst particle, rapidly leading to the formation of a supersaturated solution of C and Fe; (iii) C precipitation at the base of the catalyst particle will lift the Fe particle on the growing CNT tip.…”

“…It was often observed (see e.g. [37,36] that nanoparticles increase due to the more efficient agglomeration on the surface of substrates. At higher temperature the iron atoms have a greater tendency to cluster thus forming larger nuclei.…”