On the Growth and Microstructure of Carbon Nanotubes Grown by Thermal Chemical Vapor Deposition
Carbon nanotubes (CNTs) were deposited on various substrates namely untreated silicon and quartz, Fe-deposited silicon and quartz, HF-treated silicon, silicon nitride-deposited silicon, copper foil, and stainless steel mesh using thermal chemical vapor deposition technique. The optimum parameters for the growth and the microstructure of the synthesized CNTs on these substrates are described. The results show that the growth of CNTs is strongly influenced by the substrate used. Vertically aligned multi-walled CNTs were found on quartz, Fe-deposited silicon and quartz, untreated silicon, and on silicon nitride-deposited silicon substrates. On the other hand, spaghetti-type growth was observed on stainless steel mesh, and no CNT growth was observed on HF-treated silicon and copper. Silicon nitride-deposited silicon substrate proved to be a promising substrate for long vertically aligned CNTs of length 110–130 μm. We present a possible growth mechanism for vertically aligned and spaghetti-type growth of CNTs based on these results.
KeywordsCarbon nanotubes Substrates Morphology Microstructure
Over the last several years, a large number of experiments have been carried out to study the growth and structure of carbon nanotubes (CNTs) and to correlate the results with theoretical predictions [1, 2, 3]. Owing to wide applications of CNTs in nano electronics, energy storage devices, optics, medical, and many others fields, an intense research is being carried out to find substrate and CNT combinations that can directly act as active/passive components of devices in specific applications [4, 5]. Along with silicon, many other substrates such as glass, nickel, sapphire, quartz, and alumina have been explored in this direction [3, 4, 5, 6, 7, 8, 9, 10]. Further, the role of various catalysts such as Fe, Co, and Ni has also been extensively explored in conjunction with these substrates. It has been demonstrated that the morphology and microstructure of CNTs depend on the substrate, precursors, and the catalyst used [3, 11]. It is also possible that a substrate suitable for optimized growth of CNTs may itself be a serious limitation for a particular application.
Dielectrics such as silicon oxide and silicon nitride, which have good compatibility with silicon, have shown promise as substrates for the growth of CNTs. Silicon nitride-deposited silicon substrate has an edge over silicon oxide-deposited silicon owing to its high dielectric constant that can overcome the limitation of device shrinkage imposed by Moore’s law.
In this paper, we present a comparative study on the growth and morphology of CNTs synthesized by thermal chemical vapor deposition on various substrates commonly used in various applications.
The following substrates were used for the growth of CNTs in the present study : (1) untreated silicon, quartz, silicon oxide, copper, and stainless steel, (2) n-type silicon and quartz with an iron film of ~20 nm thickness deposited on them by thermal evaporation (3) n-type silicon wafer with a ~20-nm film of amorphous hydrogenated silicon nitride (a-SiNx:H) deposited on it by photo-enhanced CVD, and (4) n-type silicon with HF treatment. Further, the (2) and (3) categories of substrates (Fe and silicon nitride-deposited silicon) were heat-treated in air at 900°C for 20 min in a CVD chamber prior to growing CNTs on them. The experimental setup used for the growth of CNTs by thermal CVD has been described elsewhere [12, 13]. Each substrate was placed in a quartz boat and then loaded into a quartz reaction tube (growth chamber) of a horizontal single-stage tubular furnace. The deposition temperature was kept at 900°C for all the substrates. Prior to spraying the solution of ferrocene in xylene (0.02 gm/ml) with a glass sprayer, the growth chamber was purged with Ar gas.
Size distribution of Fe and silicon nitride particles on the silicon wafer was analyzed using atomic force microscope in contact mode (model Nanoscope IIIa, Veeco Metrology Group). The surface morphology, cross section, and growth behavior of the CNT samples were analyzed with scanning electron microscope (SEM: EVO) operated at 30 kV. The microstructure of the samples was analyzed with transmission electron microscope (TEM: Phillips CM 12) and high-resolution TEM (HRTEM: Technai G2, EDAX company USA) operated at 100 and 200 kV, respectively. For TEM and HRTEM, samples were scratched from the substrates, refluxed, ultrasonicated in ethanol, and then transferred on to a carbon-coated copper grid.
Results and Discussion
CNTs growth characteristics on various substrate and their potential applications
Potential applications in
Well aligned, uniform
Well aligned, uniform, well adhered
Optical limiting devices
No CNT growth
Densed, uniform, long aligned growth
Field emission devices, nanosensor devices
Uniform, well-aligned growth
Nanoelectronics, nanosensor devices, CNTFET
Silicon nitride-deposited silicon
Uniform, long aligned growth, bundle form
Nanoelectronics, CNTFET, composites (mechanical strength), nanosensor
Stainless steel mesh
No CNT growth
Uniform, non-aligned, not well adhered
Field emission devices super capacitor
We believe that the addition of Fe layer plays a crucial role in alignment of CNTs due to the formation of uniform layer of Fe catalyst nano particles on the surface of the substrate . During the CNT growth process, the precursor solution of xylene and ferrocene readily decomposes on the substrates (silicon oxide or well-dispersed catalytic Fe on silicon) resulting in the growth of well-aligned CNT arrays. The alignment of the CNT arrays may be explained based on the “crowding effect” and on Van der Waal interactions, with each CNT being supported by a neighboring CNT. The vertically aligned CNT arrays grown on silicon oxide and Fe-deposited silicon had similar morphologies.
The above-mentioned mechanism is applicable to describe the growth of aligned CNTs on most of the substrates used here. However, HF-treated silicon and copper fail to allow the nucleation of CNTs. In the case of copper, we believe that the tendency of copper to alloy with iron (provided by ferrocene) hinders the availability of catalyst for the growth of CNTs. On the other hand on HF-treated silicon, iron silicide formation takes place i.e. Fe provided by the dissociation of precursor solution reacts with the silicon substrate yielding a catalytically inactive FeSi2 phase which prevents the growth of CNTs on this substrate . In addition, silicon nitride layer acts like a diffusion barrier and thus reduces the probability of iron silicide formation between the silicon substrate and the iron catalyst. When this silicon nitride layer is annealed in air, silicon nitride decomposes to crystalline silicon oxide in a matrix of amorphous silicon nitride . In our experiments, the alignment of the CNTs on silicon nitride-deposited silicon substrates could be due to nucleation process catalyzed by Fe on active sites or unsaturated bonds created on silicon nitride clusters (as shown in AFM, Fig. 1b). In the presence of Fe catalyst, silicon nitride clusters efficiently catalyze the continuous synthesis of CNTs. The length of the CNTs on this substrate is around 100–130 μm, whereas on silicon oxide it is around 50–60 μm . The CNTs grown on silicon nitride sample are seen to form bundles, which are well adhered to the substrate. The TEM and HRTEM images of the samples (Figs. 3f, 4c, 4d) and CNTs grown on silicon nitride are highly crystalline, showing all graphene planes are exactly parallel to each other with no defects, and the total number of graphene planes is ~8 to 10 with interplanar distance of ~ 0.338 nm. These results suggest that silicon nitride-coated silicon is very well suited as a substrate for the growth of CNTs.
Silicon oxide and silicon nitride are dielectric materials, widely used in semiconductor industry  and hence controlled growth of long vertically aligned CNTs on these substrates can open up new pathways for the futuristic microelectronic devices having hybrid structures of dielectrics and CNT [19, 20]. On the other hand bare and catalyst deposited quartz substrates used for aligned growth of CNTs can find applications in various optical and photonics devices . CNTs grown on the highly conducting copper and stainless steel mesh substrates may find application in field emission devices and super capacitors .
From the present study on growth and morphology of CNTs grown on various substrates, it is concluded that quartz, silicon oxide, silicon nitride, and Fe-deposited silicon favored the aligned growth of CNTs, whereas stainless steel resulted in random growth (‘spaghetti’ like). It is also evident that bare silicon and copper do not allow nucleation and growth of CNTs. The silicon nitride-deposited silicon seemed to be the best-suited substrate for long aligned CNTs, which may find application in various nano devices.
One of the authors, Sangeeta Handuja is grateful to the Council of Scientific and Industrial Research, Government of India for providing a research fellowship.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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