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ABSTRACT: Carbon nanofibres (CNFs) of controlled diameter and length were grown on different metal substrates using plasma-enhanced chemical vapour deposition (PECVD). The diameter control of catalyst dots (and hence CNF diameter) was obtained by using the shot modulation technique in electron beam lithography. Catalyst dots of different sizes within arrays of different pitch were prepared and the dependence of the growth of vertically aligned CNFs on these parameters was studied for different metal underlayers. Good quality vertically aligned CNFs with a narrow length distribution were grown on Mo and W substrates. The structures grown on Nb substrates were significantly shorter for identical growth conditions and showed a lower nucleation rate. We demonstrate that through the shot modulation technique it is possible to control the diameter variation of CNFs from a single design geometry for the catalyst deposition. Individual VACNFs can be grown down to a pitch within the range 100–500 nm.
Nanotechnology 01/2006; 17(3):790. · 3.98 Impact Factor
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ABSTRACT: Nickel-catalysed multiwall carbon nanotubes synthesized by plasma-enhanced chemical vapour deposition on a silicon substrate with acetylene and ammonia at 700 °C have been characterized by high-resolution and analytical transmission electron microscopy. The nucleation of the carbon nanotubes occurs as a consequence of the carburization and dusting of supported preformed nickel- and silicon-rich particles. This process yields disintegrated silicon-containing nickel particles dispersed in dome-shaped carbon islands adherent to the substrate. The particles act as catalysts for tube growth, resulting in aligned multiwall carbon nanotubes with a bamboo-like structure anchored to the dome-shaped carbon islands. The bottom part of the carbon islands contains bundles of graphene sheets orientated parallel to the substrate. The nanotubes are capped with fcc nickel particles containing dissolved silicon. Most of these particles have a conical shape orientated with a <110> direction along the tube growth axis, and with {110} and {111} planes as exposed faces.
Journal of Microscopy 09/2005; 219(2):69 - 75. · 1.63 Impact Factor
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ABSTRACT: One important requirement for future applications of carbon nanotube electronic devices is the ability to controllably grow carbon nanotubes on metal electrodes. Here we show that it is possible to grow small diameter (<10 nm) vertically aligned carbon nanotubes on different metal underlayers using plasma-enhanced chemical vapour deposition. A crucial component is the insertion of a thin silicon layer between the metal and the catalyst particle. The electrical integrity of the metal electrode layer after plasma treatment and the quality of the metals as interconnects are also investigated.
Nanotechnology 02/2005; 16(4):458. · 3.98 Impact Factor
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ABSTRACT: The interface between the silicon substrate and a carbon nanotube film grown by thermal CVD with acetylene (C2H2) and hydrogen at 750 or 900 C has been characterized by high resolution and analytical transmission electron microscopy, including electron spectroscopic imaging. Silicon (0 0 2) substrates coated with a thin (2.8 nm) iron film were heat treated in the CVD furnace at the deposition temperature in a mixture of flowing argon and hydrogen whereby nanosized particles of (Fe,Si)3O4 formed. These particles were reduced to catalytic iron silicides with the –(Fe, Si), 2–Fe2Si and 1–Fe2Si structures during CVD at 900 C, and multi-wall carbon nanotubes grew from supported particles via a base-growth mechanism. A limited number of intermediate iron carbides, hexagonal and orthorhombic Fe7C3, were also present on the substrate surface after CVD at 900 C. The reduction of the preformed (Fe, Si)3O4 particles during thermal CVD at 750 C was accompanied by disintegration leading to the formation of a number of smaller (
Journal of Materials Science Materials in Electronics 07/2004; 15(8):533-543. · 1.08 Impact Factor
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ABSTRACT: Carbon nanotubes can be obtained from a multitude of molecular precursors in chemical vapor deposition (CVD) processes. Here we demonstrate that the use of C60 as the carbon feedstock gas in an iron-catalyzed thermal CVD experiment leads to the formation of films of multi-walled carbon nanotubes. The critical role of the diameter of the catalyst particles in determining the efficiency of nanotube growth is clearly demonstrated. Electron microscopy and Raman spectroscopy were employed for the characterisation of the nanotube material. The structural properties of the individual nanotubes show distinctive differences to acetylene-grown multi-walled nanotubes.
Applied Physics A 01/2004; 78(3):253-261. · 1.63 Impact Factor
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ABSTRACT: ZnO nanowires have been grown on sapphire and Si substrates using catalytic growth. A strong near-band-gap ultraviolet emission is observed at room temperature. By carefully studying the temperature dependence of ZnO wire emission, we found that the room-temperature UV emission contains two different transitions; one is related to the ZnO free exciton and the other is related to the free-to-bound transition. The bound state has a binding energy of about 124 meV. The results from optical measurements show that a high quality of ZnO nanowires grown on sapphire and Si substrates has been achieved. © 2003 American Institute of Physics.
Applied Physics Letters 07/2003; 83(1):165-167. · 3.84 Impact Factor
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ABSTRACT: The catalytic particle size dependence of chemical vapor deposition growth of multiwall carbon nanotubes was systematically investigated using two different molecules, C2H2 and C60, as carbon feedstock gases. In the particle size range between 25 and 500 nm, the use of C2H2 leads exclusively to growth of carbon nanotubes. The nanotube diameters increase with increasing catalytic particle sizes but do not scale 1:1. In contrast, nanotube formation from C60 is observed only if the particle sizes are sufficiently small with an optimum between 20 and 30 nm. For catalyst samples with considerably larger diameters, C60 is transformed into a nontubular deposit. A growth model is given that explains the different behavior. © 2003 American Institute of Physics.
Journal of Applied Physics 03/2003; 93(7):4185-4190. · 2.17 Impact Factor
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ABSTRACT: Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison
between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs)
which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with
clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist
of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The
presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the
disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties
with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact
on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current
densities and larger critical fields.
Applied Physics A 09/2001; 73(4):409-418. · 1.63 Impact Factor
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ABSTRACT: The catalytic particle size (Fe) and temperature dependence of multi-walled nanotubes growth from C2H2 and C60 precursor molecules is studied. The structure and density of the carbon nanotubes produced is critically dependent on the particle size, the growth temperature and the carbon flux rate. Under certain conditions, bundles of single-walled nanotubes, where the bundles appear to consist of nanotubes with the same diameters, can be produced. The nanotubes are characterised by electron microscopy and Raman spectroscopy. Field emission properties of aligned films are studied and the electron emission is correlated with light emission measurements.
Physica B: Condensed Matter.
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ABSTRACT: Films of aligned multi-walled carbon nanotubes (MWNT) are produced by two different methods, thermal chemical vapour deposition (thermal CVD) and plasma chemical vapour deposition (plasma CVD), on silicon substrates. Electron field emission measurements on these films show that the thermal CVD produced films have excellent emission properties, while the plasma CVD films seem to give a lower electron emission with lower threshold and turn-on fields on the initial voltage scan. The electron emission for some of the films is accompanied by light emission from the carbon nanotubes at high emission current densities. The light emission is a result of strong ohmic heating and can be explained in terms of the one dimensional heat equation. This heating effect in the nanotube film is more important for the thermal CVD films than for the plasma films and can be qualitatively explained by considering the nanotube morphology in each case.
Carbon.
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ABSTRACT: DC plasma-enhanced chemical vapour deposition (PECVD) was used to grow films of aligned carbon nanotubes on a silicon wafer using Fe as catalyst and a C2H2/H2 gas mixture. The films were of high quality and showed an exceptionally high growth rate compared with other plasma growth techniques. For long growth times, the upper parts of the nanotubes developed additional outer graphite flakes. The onset of the ‘tube decoration’ correlates with a decrease in linear growth rate and can be related to the gradient of plasma parameters in the cathode sheath.
Chemical Physics Letters.
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F. Dumitrache,
I. Morjan,
R. Alexandrescu, R.E. Morjan,
I. Voicu,
I. Sandu,
I. Soare,
M. Ploscaru,
C. Fleaca,
V. Ciupina,
G. Prodan,
B. Rand,
R. Brydson,
A. Woodword
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ABSTRACT: We present the single step preparation of nano-iron cores embedded in carbon layers and their preliminary application to the growth of carbon nanotubes. CO2 laser pyrolysis of volatile iron and carbon precursors in a gas flow reactor was used in order to obtain the Fe–C nanocomposites. The structure and composition of the obtained nanopowders were analysed by transmission electron microscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy. The results indicate that the synthesized carbon embedded iron nanoparticles (∼3–7 nm mean diameter) present sharp particle distributions. The degree of agglomeration and the number of surrounding carbon layers depend on the relative concentration of reactive donors. Silicon substrates seeded with a dilution of Fe–C nanocomposites were further used to catalyze nanotubes/nanofibres growth by the laser-induced CVD method. A deeper insight in the inception and growth mechanism of these structures is needed in order to achieve different levels of encapsulation and subsequent nanotube growth.
Diamond and Related Materials.
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I. Morjan,
I. Soare,
R. Alexandrescu,
L. Gavrila-Florescu, R.-E. Morjan,
G. Prodan,
C. Fleaca,
I. Sandu,
I. Voicu,
F. Dumitrache,
E. Popovici
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ABSTRACT: The synthesis of carbon nanotubes (CNTs) by catalytic laser-induced chemical vapor deposition (C-LCVD) was investigated. C-LCVD uses both ex situ synthesized catalyst nanoparticles and the controlled decomposition of gas-phase hydrocarbon mixtures. As catalysts, Fe/C composites of the core-shell type were used. A continuous-wave CO2 laser was employed to irradiate the ethylene/acetylene hydrocarbon precursors and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown. The effects on carbon nanotube growth of both the iron-based nanocomposite particles and of the ethylene concentration were studied. The analysis suggests the feasibility of the C-LCVD process, in which the core-shell Fe/C catalysts comply with the prerequisite conditions of the CNT growth namely dispersion and supersaturation.
Infrared Physics & Technology.
