The synthesized CNTs have the diameter of about 30 nm and the length of 1.2mm, which are more uniform compared with those grown on Ni-coated Si substrates.. To verify the effects of SUS
Trang 1Growth of carbon nanotubes on stainless steel substrates by DC-PECVD
a
Division of Microelectronics and Display Technology, College of Natural Sciences, Wonkwang University, Iksan, 344-2 Shinyong Dong, Iksan, Jeonbuk 570-749, Republic of Korea
b
Regional Innovation Center for Next Generation Industrial Radiation Technology, 208 College of Natural Sciences, Wonkwang University, Iksan, 344-2 Shinyong Dong, Iksan, Jeonbuk 570-749, Republic of Korea
c Faculty of Physics, HaNoi University of Science, VietNam National University, 334 Nguyen Trai, Thanh Xuan, HaNoi, VietNam
1 Introduction
Since the discovery of carbon nanotubes (CNTs) in 1991[1],
they have attracted considerable interest because of their unique
physical properties and many potential applications With the
nanometer-size diameter and very large aspect ratio, CNTs exhibit
unique electronic properties such as excellent electron emission
efficiency and extraordinary mechanical properties They are the
potential building blocks for field emission displays, tips for
scanning probe microscopy, X-ray sources using field emission
cathode, hydrogen storage, chemical sensors, high-strength
mechanical composites, etc [2–5] It is reported that X-ray
sources with field emission cathodes have several intrinsic
advantages over thermionic X-ray tubes, including low
tempera-ture, instantaneous response, and the potential for
miniaturiza-tion Field emission X-ray tubes with CNT emitters [6] had
recently been demonstrated to have significantly improved
properties compared to those with metal [7]or diamond tips
[8] For field emission X-ray tubes with CNT emitters, the
synthesis of CNTs directly on metallic substrates will greatly
simplify the preparation of cold cathodes However, it has been
reported that the synthesis of CNTs on metallic or electrically
conducting substrates is rather difficult compared to that on
insulators such as glass or silicon wafers It has been attributed to the high mobility and lack of localization of carbon atoms on metallic surfaces, or to the difficulty of catalyst island formation due to the diffused reaction and interfacial bonding between the catalytic layer and the metallic surface at the growth temperature
[9] Although metal substrates, in recent years, have been more frequently researched [10–12], their effects on growth of CNTs have not been fully understood
In this research, we report the growth of carbon nanotubes on Ni-coated stainless steel (SUS) substrates by dc plasma enhanced chemical vapor deposition (DC-PECVD) The synthesized CNTs have the diameter of about 30 nm and the length of 1.2mm, which are more uniform compared with those grown on Ni-coated Si substrates It is attributed to the higher uniformity of Ni catalyst particles on the SUS substrates after the pretreatment with NH3 plasma, compared to those on the Si substrates Field emission properties of the CNT films were measured in the diode configuration The turn-on electric field of 3.87 V/mm and the field enhancement factorbof about 1737 were obtained from the synthesized CNTs at the gap of 500mm between the SUS substrate and the anode
2 Experimental CNTs had been grown on Ni-coated SUS substrates with a TiN buffer layer by dc plasma enhanced chemical vapor deposition (DC-PEVCD) The Ni layer and the TiN buffer layer with a thickness
A R T I C L E I N F O
Article history:
Available online 2 June 2009
PACS:
72.80
Keywords:
Carbon nanotube
DC-PECVD
Pretreatment
SUS
Field emission
A B S T R A C T
We report on the fabrication of carbon nanotubes (CNTs) on Ni-coated stainless steel (SUS) substrates by using dc plasma enhanced chemical vapor deposition The synthesized CNTs have the diameter of about
30 nm and the length of about 1.2mm To verify the effects of SUS substrates on the growth of CNTs, CNTs had also been grown on Ni-coated Si substrates CNTs grown on the SUS substrates were more uniform compared with those grown on the Si substrates Field emission properties of the CNT films were measured in the diode configuration, and the turn-on electric field of 3.87 V/mm and field enhancement factorbof about 1737 were obtained from the synthesized CNTs at the gap of 500mm between the SUS substrate and the anode These results have not only clarified the effects of the substrate on the growth of CNTs, but also shown the potential of CNTs in field emission applications, especially CNT-based cold-cathode X-ray tubes
ß2009 Elsevier B.V All rights reserved
* Corresponding author Tel.: +82 63 850 6784; fax: +82 63 850 7138.
E-mail address: chlee@wonkwang.ac.kr (C.H Lee).
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Applied Surface Science
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / a p s u s c
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Trang 2of 50 and 1000 A˚, respectively, were deposited by using a
radio-frequency magnetron sputtering system It has been reported that
the formation of Ni grains during the pretreatment process plays a
key role in growing CNTs; when Ni forms alloys such as NiFe, NiCr,
etc (with SUS substrates) or NiSi2 (with Si substrates), other
isomers of carbon such as carbon nanotips are formed instead of
CNT[13], and therefore TiN buffer layers with excellent electrical
conductivity (resistivity 25 mVcm) and high melting point
(3200 8C)[9]are usually added to prevent the reaction between
catalyst layers and substrates[14] To create uniform Ni particles,
NH3gas was introduced for 6 min During this process, the cathode
voltage, the temperature, and the flow rate were kept at 550 V,
600 8C, and 60 sccm, respectively The base pressure of the reactor
was maintained 3.4 10 6Torr Before the CNT growth, we have performed annealing procedures at the temperature of 600 8C in H2 environments After the pretreatment and annealing processes, CNTs were grown at 600 8C for 15 min using a mixture of acetylene and ammonia with the flow rates of 30 and 100 sccm, respectively
To examine the effects of SUS substrates on the growth of CNTs, CNTs were grown also on Ni-coated Si substrates at the same synthesizing conditions as above
The morphology, density, and quality of the CNTs were analyzed using field emission scanning electron microscope (FESEM, Hitachi S-4800), a high resolution transmission electron microscope (HRTEM, JEM 2200FS), and Raman spectroscopy (Ar+ laser 514 nm, 2.42 eV), respectively The morphology of Ni particles and Ni catalyst films were investigated using an atomic force microscope (AFM) The field emission properties of the CNT films were measured in the diode configuration in a vacuum chamber with pressure below 3.0 10 7Torr The anode was a Mo electrode, and the gap between the SUS substrate and the anode was 500mm
3 Results and discussion
Fig 1(a) and (b) shows the AFM images of the Ni catalyst layers
on SUS and Si substrates after the NH3 plasma pretreatment, respectively The catalyst layers on both substrates were observed
to aggregate and form Ni nanoparticles However, Ni nanoparticles
on SUS substrates (Fig 1(a)) seem to be more uniform in size than those on Si substrates: most nanoparticles on SUS substrates have diameter from 20 to 40 nm while those on Si substrates have from
Fig 1 Typical AFM images of Ni catalyst layers (a) on SUS substrates and (b) on Si
substrates after the NH 3 pretreatment process, and (c) on SUS substrates before the
NH pretreatment process.
Fig 2 Typical SEM images of CNTs grown (a) on SUS substrates and (b) on Si
Trang 310 to 140 nm, although the area uniformity of Ni particles on Si
substrates is better compared with that on SUS substrates We
suggest that because of floating voltage in DC-PECVD[15], etching
of the Ni layer on the metal substrate could be more effective due
to its low resistivity, and therefore the Ni nanoparticles were more
uniform The morphology of surfaces of Ni catalyst layers on the
SUS substrates (Fig 1(a)) seems to be inhomogeneous To interpret
the phenomena, the Ni catalyst layers before the NH3 plasma
pretreatment (Fig 1(c)) were also examined by AFM Before the
NH3 plasma pretreatment, the morphology of the surface of Ni
catalyst layers on SUS substrates was inhomogeneous (Fig 1(c))
and the inhomogeneity is attributed to the non-uniform surface of
SUS substrates before the sputtering process
The diameter of CNTs has been reported to be related to the
size of the catalyst particles and the thickness of the catalytic
layer[16], and a single CNT was grown on a single nanoparticle
below a critical size[17] This means that the diameters of the
CNTs can be controlled by changing the size of Ni nanoparticles
during the pretreatment process.Fig 2(a) and (b) shows SEM
images of the CNTs grown on SUS and Si substrates, respectively
The CNTs are well aligned perpendicular to the surface of both
substrates, and have lengths of about 1.2mm However, the
diameters of the CNTs grown on the SUS (Fig 2(a)) are more
uniform than those grown on the Si substrates (Fig 2(b))
Fig 3(a) and (d) shows TEM images of the CNTs grown on the SUS
and Si substrates, respectively The diameter of the CNTs grown
on the SUS substrates ranges from 10 to 50 nm, and most of them
have the diameter of about 30 nm (Table 1) The diameters of the
CNTs are much smaller than those previously reported [18]
Although most of the CNTs grown on the Si substrates also have
the diameter of about 30 nm, the diameters range from 10 to
140 nm (Table 1) It could be explained by the size uniformity of
the Ni catalyst particles on the SUS substrates.Fig 3(b) and (c)
shows TEM images of the CNTs grown on the SUS substrates The
CNTs are multiwalled with the number of walls around 31 The
clear walls of CNTs (Fig 3(c)) show that the crystallinity of CNTs
is as good as those previously reported[18]
Fig 4shows the Raman spectra of the CNTs grown on both Si
and SUS substrates The Raman spectra of the CNTs grown on both
substrates are observed to have two prominent peaks at
1346 cm 1 (noted as D-band) and 1588 cm 1 (noted as
G-band) with the intensity ratio ID/IGof 0.99, which is related to the
size of the sp2carbon clusters in the graphene sheet or the defect
density [19] We suggest that the ID/IGratio and therefore the
quality of CNTs grown by PECVD do not strongly depend on their
diameters as well as details of individual CNTs[20]
Fig 5(a) shows the field emission current density (J) as a
function of the applied electric field (E) from the CNTs film grown
on the SUS substrate The turn-on field, defined as the field for the
emission current of 1mA, was 3.87 V/mm with the gap of 500mm
between the SUS substrate and the Mo anode According to
previous works [6,21], the lowest turn-on electric field for
multiwall CNTs was observed to be less than 1 V/mm at the gap
of less than 300mm between substrate and anode.Fig 5(b) shows
the Fowler–Nordheim (F-N) plot of ln(I/V2) vs 1/V for the J–E
curves shown inFig 5(a) The plot shows a linear fit, indicating that the emission current of CNTs follows the F-N behavior Since the gap between the substrate and the anode was 500mm, assuming a work function of 5.0 eV for CNT, the field emission factor b is estimated to be about 1737 The CNTs grown on the SUS substrates served as cold cathodes in the X-ray generation in our previous reports[5]
Fig 3 Typical TEM images of CNTs grown (a, b, and c) on SUS substrates and (d) on
Si substrates.
Table 1
Distribution of the diameter of CNTs grown on SUS and Si substrates.
Trang 44 Conclusion
We had successfully grown carbon nanotubes on Ni-coated SUS
substrates by DC-PECVD The synthesized CNTs have the diameter
of about 30 nm and the length of about 1.2mm, which were more
uniform compared to those grown on Ni-coated Si substrates It is
attributed to Ni catalyst particles on the SUS substrates formed by
the pretreatment with NH3gas before the growth of CNTs, which were more uniform than those on the Si substrates Field emission properties of the CNT films were measured in the diode configuration, and turn-on electric fields of 3.87 V/mm and field enhancement factor b of about 1737 were obtained from the synthesized CNTs Those results have shown the potential of CNTs
in field emission applications, especially CNT-based cold-cathode X-ray tubes
Acknowledgements This research was financially supported by the Ministry of Education, Science, and Technology (MEST) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Regional Innovation, and Regional Innovation Center For Next Generation Industrial Radiation Technology of Wonkwang University
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Fig 5 (a) Emission current density vs applied electric field and (b) F-N plot for the
CNTs grown on SUS substrates.
Fig 4 Raman spectra for the CNTs grown on SUS substrates and on Si substrates.