Effect of hydrogen plasma treatment on the growth and microstructures of multiwalled carbon nanotubes (original) (raw)
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The effects of hydrogen plasma pretreatment on the formation of vertically aligned carbon nanotubes
Applied Surface Science, 2007
The effects of H 2 plasma pretreatment on the growth of vertically aligned carbon nanotubes (CNTs) by varying the flow rate of the precursor gas mixture during microwave plasma chemical vapor deposition (MPCVD) have been investigated in this study. Gas mixture of H 2 and CH 4 with a ratio of 9:1 was used as the precursor for synthesizing CNTs on Ni-coated TiN/Si(1 0 0) substrates. The structure and composition of Ni catalyst nanoparticles were investigated by using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (XTEM). Results indicated that, by manipulating the morphology and density of the Ni catalyst nanoparticles via changing the flow rate of the precursor gas mixture, the vertically aligned CNTs could be effectively controlled. The Raman results also indicated that the intensity ratio of the G and D bands (I D /I G ) is decreased with increasing gas flow rate. TEM results suggest H 2 plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles and, thus, is playing a crucial role in modifying the obtained CNTs structures. #
2006
Well-aligned carbon nanotubes (CNTs) were grown on iron coated silicon substrates by microwave plasma enhanced chemical vapor deposition. Effect of plasma composition on the growth and microstructures of CNTs were investigated by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and optical emission spectroscopy. Morphology and microstructure of nanotubes were found to be strongly dependent on the plasma composition. Aligned bamboo-shaped nanotubes consisting of regular cone shaped compartments were observed for C 2 H 2 /NH 3 /N 2 and C 2 H 2 /NH 3 /H 2 gas mixtures. Randomly oriented or no nanotubes growth were observed in C 2 H 2 /H 2 and C 2 H 2 /N 2 gas mixtures respectively. CNTs grown in nitrogen rich plasma had more frequent short compartments while compartment length increased with decreasing nitrogen concentration in the plasma. Raman spectroscopy of CNTs samples revealed that CNTs prepared in nitrogen rich plasma had higher degree of disorder than those in low nitrogen or nitrogen free plasma. In-situ optical emission spectroscopy investigations showed that CN and H radicals play very important role in both the growth and microstructure of CNTs. Microstructure of CNTs has been correlated as a function of CN radical concentration in the plasma. It is suggested that presence of nitrogen in the plasma enhances the bulk diffusion of carbon through the iron catalyst particles which causes compartment formation. Based on our experimental observations, growth model of nanotubes under different plasma composition has been suggested using base growth mechanism.
Nanotechnology, 2010
Catalysts play a key role in the growth of carbon nanotubes. The microwave plasma-assisted chemical vapor deposition (MPACVD) method is now commonly used for directional and conformal growth of carbon nanotubes (CNTs) on substrates. In this work, we report on the effect of H 2 plasma pre-treatment on the diameter and density of iron catalyst nanoparticles for different iron layer thicknesses in order to grow isolated bundles of CNTs. Atomic force microscopy shows first that as plasma power density increases, iron nanoparticle diameters decrease, which is due to the increasing of gas dissociation giving more ion bombardment energy, and second that the diameter of nanoparticles decreases with the catalyst thickness. The growth of CNT was carried out under different CH 4 concentrations for different iron film thicknesses. Transmission electron microscopy and Raman spectroscopy show that the synthesized CNT were of good quality and had an outer diameter between 5 and 10 nm.
Journal of Physics and Chemistry of Solids, 2007
Deposition of carbon nanotubes in microwave plasma torch at atmospheric pressure is discussed in terms of factors influencing the deposition uniformity and amorphous carbon overlayer formation. The depositions were carried out on silicon substrates with a thin iron catalyst layer from the mixture of argon, methane and hydrogen. Samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Substrate temperature gradients and a size distribution of catalytic particles were main reasons for a deposit non-uniformity. The uniformity was improved by a substrate heating in Ar/H 2 discharge before the deposition. The catalyst poisoning with a subsequent amorphous carbon deposition was responsible for the formation of amorphous carbon overlayer containing catalytic particles. Shorter deposition time and optimization of carbon feedstock dilution in hydrogen were suggested for a minimization of this effect. r
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 2003
Influence of morphology of Fe-catalyst on the microstructure and electron field emission properties of carbon nanotubes were systematically investigated. H2-plasma treatment imposed a most significant effect on the morphology of Fe clusters. The 1200 W H2-plasma treated Fe catalysts are nanosized (40–80 nm), isolated, and uniformly distributed, which result in carbon nanotubes of highest purity and most density population, exhibiting the largest electron field emission capacity (Je=3600 μA/cm2 at 6 V/μm applied field). Too low (1000 W) or too high (1400 W) H2-plasma treated Fe catalysts are interconnected or coarsened particulates, which result in large proportion of carbon soots, in addition to the carbon nanotubes. The electron field emission capacity of the films is thus markedly decreased. The prebaking process before H2-plasma treatment also imposes a marked effect on those characteristics. These results imply that spin coating of Fe(OR)3 is a simple and inexpensive method for ...
Diamond and Related Materials, 2003
The growth behaviour of carbon nanotubes (CNTs), produced by radio-frequency plasma enhanced chemical vapour deposition, is studied here as a function of a CH yAr ratio and Ni catalyst layer thickness. The composition of the plasma mixture was 4 observed as being crucial for the morphology of the nanotubes, indicating a transition from a random to a more uniform orientation, when argon is added to the plasma atmosphere. Scanning tunnelling microscopy shows, as a result of the argon dilution, the formation of a defective structure (i.e. pentagons) at the tip of vertically aligned CNTs while hexagonal atomic arrangement was detected on the sidewall of randomly oriented CNTs deposited by pure methane. The electronic structure of CNTs was then investigated by C 1s' photoemission spectroscopy. The results show a shift of the overall spectral to the higherbinding-energy side, indicating the formation of metallic aligned tubes when argon is added to the plasma atmosphere. Experimental results are applied to develop a coherent picture of the relationship between the deposition parameters and the microstructural features, as well as to check the relationship of the electronic properties predicted for nanotubes with the plasma chemical composition. ᮊ
Nanotechnology, 2007
A two-step catalyst annealing process is developed in order to control the diameter of nickel catalyst particles for the growth of carbon nanotubes (CNTs) by microwave plasma-enhanced chemical vapour deposition (MW PECVD). Thermal annealing of a continuous nickel film in a hydrogen (H 2 ) environment in a first step is found to be insufficient for the formation of nanometre-size, high-density catalyst particles. In a second step, a H 2 MW plasma treatment decreases the catalyst diameter by a factor of two and increases the particle density by a factor of five. An x-ray photoelectron spectroscopy study of the catalyst after each step in the annealing process is presented. It is found that the catalyst particles interact with the substrate during thermal annealing, thereby forming a silicate, even if a buffer layer in between the catalyst and the substrate is intended to prevent silicate formation. The silicate formation and reduction is shown to be directly related to the CNT growth mechanism, determining whether the catalyst particles reside at the base or the tip of the growing CNTs. The catalyst particles are used for the growth of a high-density CNT coating by MW PECVD. CNTs are analysed with electron microscopy and Raman spectroscopy.
Plasma Chemistry and Plasma Processing, 2008
A mixture of acetylene, hydrogen and ammonia (C 2 H 2 /H 2 /NH 3 ) is used to produce carbon nanotubes (CNTs) by a plasma-enhanced catalytic chemical vapor deposition process either without (PE CCVD) or with hot filaments-assistance (PE HF CCVD). A mathematical model based on Chemkin computer package is used for analyzing specific conditions of nanotube synthesis. Simulations are compared with optical emission spectroscopy (OES) measurements. Morphological and structural investigations on the grown carbon nanostructures are also performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was shown that the significant change in the density and the morphology of the CNTs grown in the presence of NH 3 could be mainly explained by the gas phase formation of CN and HCN. Both species display a high etching activity, whereas the species C, CH, CH 2 , CH 2 (S), C 2 and C 2 H are expected to be the most probable carbon nanotube precursors.
The microwave plasma torch (2.45 GHz) was used for the synthesis of carbon nanotubes from the mixture of CH 4 /H 2 /Ar or C 2 H 2 /H 2 /Ar on different substrates with iron catalyst. Iron catalyst was prepared by vacuum evaporation of iron on Si, Si/SiO x or Si/AlxO y substrates or by deposition of iron oxide nanoparticles on Si/SiO x substrate by decomposion of Fe(CO) 5 in gas feed. Such prepared substrates were used for growth of carbon nanotubes. Recostruction of the iron catalyst layer into nanoparticles was also studied in dependence on substrate buffer layer, gas atmosphere and temperature. Samples were studied by scanning and transmission electron microscopy and Raman spectroscopy. Synthesis resulted in rapid growth of MWNTs on all samples but the density, purity and nanotube diameter distribution varied. Such prepared carbon nanotube layers were used for sensing applications.