Filament poisoning at typical carbon nanotube deposition
conditions by hot-filament CVD
C. J. Oliphant Æ C. J. Arendse Æ G. F. Malgas Æ
D. E. Motaung Æ T. F. G. Muller Æ S. Halindintwali Æ
B. A. Julies Æ D. Knoesen
Received: 3 December 2008 / Accepted: 11 February 2009 / Published online: 6 March 2009
Ó Springer Science+Business Media, LLC 2009
Abstract We report on the poisoning of tungsten fila-
ments during the hot-filament chemical vapour deposition
process at typical carbon nanotube (CNT) deposition con-
ditions and filament temperatures ranging from 1400 to
2000 °C. The morphological and structural changes of the
filaments were investigated using scanning electron
microscopy and X-ray diffraction, respectively. Our results
conclusively show that the W-filament is not stable during
the carburization process and that both mono- and
ditungsten-carbides form within the first 5 min. Cracks and
graphitic microspheres form on the carbide layer during the
first 15 min at the temperatures C1600 °C. The micro-
spheres subsequently coalesce to form a graphite layer,
encapsulating a fully carburized filament at the temperature
of 2000 °C after 60 min, which inhibits the catalytic
activity of the filament to produce atomic hydrogen. The
structural changes of the filament also induce variations in
its temperature, illustrating the instability of the filament
during the deposition of CNTs.
Introduction
The hot-filament chemical vapour deposition (HFCVD)
process has been extensively used for the deposition of
various materials, including diamond [1–4], polymers [5],
silicon thin films [6], boron–carbon–nitride layers [7],
tungsten oxide nanoparticles [8] and carbon nanotubes
(CNTs) [9]. The process relies on the catalytic decompo-
sition of precursor gases into reactive gaseous species by a
resistively heated filament. A variety of transition metals,
such as tungsten (W), tantalum (Ta) and rhenium (Re),
have been utilized as the filament during the HFCVD
process [10–12]. This is primarily due to their high melting
points and superior mechanical stabilities at temperatures
above 1500 °C. Furthermore, different filaments have dif-
ferent abilities to dissociate the precursors. For example, a
Ta filament dissociates molecular hydrogen (H
2
) into
atomic hydrogen (H
0
) twice as efficient as a W filament
during the deposition of microcrystalline silicon thin films
[10].
Reactions between the precursor gases and the heated
filament result in changes of the structural properties of the
filaments; a process referred to as filament ageing. The
most comprehensive study on the filament ageing process
in a methane atmosphere, hereafter referred to as filament
carburization, was executed for the synthesis of diamond
[1, 11, 13–18]. Filament carburization has several impli-
cations for HFCVD; of which the most notable is that the
catalytic ability of the filament changes during carburiza-
tion, since the structure and the temperature of the filament
vary during the deposition [10], which consequently results
in unstable deposition conditions. Furthermore, from a
manufacturing point of view, carbides are brittle materials
and therefore the formation thereof reduces the lifetime of
the filament.
C. J. Oliphant Á C. J. Arendse (&) Á D. E. Motaung Á
T. F. G. Muller Á S. Halindintwali Á B. A. Julies Á D. Knoesen
Department of Physics, University of the Western Cape,
Private Bag X17, Bellville 7535, South Africa
e-mail: cjarendse@uwc.ac.za
C. J. Oliphant Á C. J. Arendse Á G. F. Malgas (&) Á
D. E. Motaung
National Centre for Nano-Structured Materials, CSIR Materials
Science and Manufacturing, P.O. Box 395, Pretoria 0001,
South Africa
e-mail: gmalgas@csir.co.za
123
J Mater Sci (2009) 44:2610–2616
DOI 10.1007/s10853-009-3341-y