A new perspective on structural and morphological properties of carbon nanotubes synthesized by Plasma Enhanced Chemical Vapor Deposition technique 1 3 4 5 6 7 8 9 10 11 1 3 14 15 16 17 18 19 20 21 22[.]
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7
8 A Salar Elahia,⇑, K Mikaili Agahb, M Ghorannevissa
Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
10 b
Department of Mathematics and Physics, East Tehran Branch, Islamic Azad University, Tehran, Iran
11
1 3 a r t i c l e i n f o
14 Article history:
15 Received 1 January 2017
16 Received in revised form 30 January 2017
17 Accepted 31 January 2017
18 Available online xxxx
19 Keywords:
20 Carbon nanotubes
21 Cobalt nanocatalyst
22 PECVD
23
2 4
a b s t r a c t
25 CNTs were produced on a silicon wafer by Plasma Enhanced Chemical Vapor Deposition (PECVD) using
26 acetylene as a carbon source, cobalt as a catalyst and ammonia as a reactive gas The DC-sputtering
sys-27 tem was used to prepare cobalt thin films on Si substrates A series of experiments was carried out to
28 investigate the effects of reaction temperature and deposition time on the synthesis of the nanotubes
29 The deposition time was selected as 15 and 25 min for all growth temperatures Energy Dispersive
X-30 ray (EDX) measurements were used to investigate the elemental composition of the Co nanocatalyst
31 deposited on Si substrates Atomic Force Microscopy (AFM) was used to characterize the surface
topog-32 raphy of the Co nanocatalyst deposited on Si substrates The as-grown CNTs were characterized under
33 Field Emission Scanning Electron Microscopy (FESEM) to study the morphological properties of CNTs
34 Also, the grown CNTs have been investigated by High Resolution Transmission Electron Microscopy
35 (HRTEM) and Raman spectroscopy The results demonstrated that increasing the temperature leads to
36 increasing the diameter of CNTs
37
Ó 2017 The Author Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license
38 (http://creativecommons.org/licenses/by-nc-nd/4.0/)
39 40
41 Introduction
42 All different types of materials like plastic, glass, metal, textiles,
43 etc can be coated with plasma based methods Also various
meth-44 ods using plasma are applied for a thin film formation with the
45 desired characteristics for the different applications[1–3] One of
46 these methods is plasma-enhanced Chemical Vapor Deposition
47 (PECVD) PECVD is a process used to deposit thin films from a
48 gas state to a solid state on a substrate Chemical reactions are
49 involved in the process, which occur after creation of aplasma of
50 the reacting gases The major advantage compare to simple
Chem-51 ical Vapor Deposition is that PECVD can operate at much lower
52 temperature[4,5] Carbon nanotubes (CNTs) are characterized as
53 a graphene sheet rolled-up to form a tube, for example a
single-54 walled tube (SWNT) When two or more concentric tubes are
55 placed one into another, multi-walled carbon nanotube (MWNT)
56 is formed Initially, the arc discharge was employed to produce
car-57 bon nanotubes This method was known enough and utilized for
58 the synthesis of carbon filaments and fibres Later on other
tech-59 niques such as laser ablation or Chemical Vapor Deposition
60 (CVD) were examined in the production of carbon nanotubes In
61 fact, these are the three main production methods Some efforts
62 were also made to look for other possibilities to grow nanotubes
63 but they had less success The cause may be the expensive reaction
64 apparatus, the state or the price of the catalyst material, the
65 strange reaction conditions, e.g., high pressure, temperatures of
66 liquid nitrogen So, ‘‘the old technologies” were improved, adapted
67
to new conditions more than to discover new technologies Today,
68 the arc discharge and Chemical Vapor Deposition methods are
69 widely applied for the formation of carbon nanotubes Many
stud-70 ies were made to improve either the quality or the quantity of the
71 produced material by optimizing the synthesis process As a result
72 some types of CVD method were discovered such as
plasma-73 enhanced, microwave-enhanced, radio-frequency-enhanced CVD
74 Nanotechnology based on CNTs is developing very fast leading to
75 decrease in the dimensions of electronic devices used in today’s
76 technological applications, such as field effect transistors[1], field
77 emitters[2], flat panel displays[3,4], sensors [5], etc Due to its
78 very small diameter, which is on the order of few nanometers with
79 the length up to centimeters [6], perfect electrical and thermal
80 conductance properties[7], CNTs are expected to find applications
81
in all industrial areas, also provide rich research subjects CNTs
82 have been grown by various methods, such as laser ablation,
ther-http://dx.doi.org/10.1016/j.rinp.2017.01.043
2211-3797/Ó 2017 The Author Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).
⇑ Corresponding author.
E-mail address: Salari_phy@yahoo.com (A.S Elahi).
Contents lists available atScienceDirect Results in Physics
j o u r n a l h o m e p a g e : w w w j o u r n a l s e l s e v i e r c o m / r e s u l t s - i n - p h y s i c s
Please cite this article in press as: Elahi AS et al A new perspective on structural and morphological properties of carbon nanotubes synthesized by Plasma
Trang 283 mal decomposition of hydrocarbons, and Chemical Vapor
84 Deposition (CVD) These three methods have been used the most
85 for producing CNTs Among them, CVD has been shown to be the
86 best method in producing vertically aligned CNTs uniformly in
87 large quantities due to the ease in controlling the catalysts and
88 temperature[8,9] Transition metals such as Fe, Ni, Co and their
89 compounds or alloys have been widely used as the catalysts[10]
90 CVD with these transition metals as a catalyst has several
advan-91 tages over other deposition methods [11] CVD with a catalyst
92 can be used to grow single-walled, double-walled, or
multi-93 walled CNTs by controlling the particle size and chemical nature
94 of the catalyst The adhesive force between the catalyst and the
95 substrate has been often attributed as an important factor in
deter-96 mining the growth mechanisms of CNTs While weak contact
97 between the catalyst and substrate favors a tip-growth
mecha-98 nism, a strong interaction promotes base-growth [12,13] The
99 growth of CNTs can be divided into four steps: (1) supply of carbon
100 source on the catalyst surface by adsorption and the subsequent
101 catalytic decomposition of the adsorbed carbon by carbon atoms;
102 (2) desorption of the carbon atoms into a gas phase; (3) diffusion
103 of the carbon atoms away from the catalyst surface through bulk
104 or surface diffusion; and (4) precipitation and formation of a
105 graphite structure[14–16] Formerly, some synthesis methods of
106 carbon nanotubes, related materials and metal atoms agglomerate
107 are analyzed[17–25] In this paper, we have systematically
inves-108 tigated the effects of growth temperatures and deposition time on
109 carbon nanotubes grown by Plasma Enhanced Chemical Vapor
110 Deposition using cobalt nanocatalyst
111 Experimental setup
112 P-type Si (400) wafers with the size of 1 cm 1 cm were used
113
as substrates The wafers were cleaned by ultrasonic method in
114 acetone and ethanol solutions to remove potential residual
con-115 taminants prior to deposition The samples were introduced into
116 the planar DC-sputtering system and then pumped down to a base
117 pressure of 4 101Pa A cobalt plate was used as a cathode and
118 was placed in parallel with the oven which was grounded The
dis-119 tance between the cathode and anode was about 1 cm Argon was
120 introduced into the chamber with a flow of 200 Standard
Centime-121 ter Cubic per Minutes (sccm) The cobalt nanocatalysts were
sput-122 tered on Si substrates when the substrate temperature gradually
123 increased up to 100°C Deposition time for cobalt sputtering was
124
30 min The Plasma Enhanced Chemical Vapor Deposition (PECVD)
125 system in the experiment (Fig 1) was an electric furnace composed
126
of a horizontal quartz glass tube with an internal diameter of
127 7.5 cm and a length of 80 cm which was operated at atmospheric
128 pressure Argon gas with a flow rate of 200 sccm was supplied into
129 the CVD reactor to prevent the oxidation of catalytic metal while
130 raising the temperature to 750°C The samples were placed in
131 the chamber and the temperature increased to 850°C After that,
132
Ar flow was switched off For CNT growth, we used C2H2/NH3at
133 35/60 sccm for 15 min The growth was terminated by turning
134 off C2H2/NH3flow and the samples were allowed to cool down to
135 room temperature under Ar gas flow Same experiments were
136 repeated at growth temperatures of 850°C, 900 °C, 950 °C and
137
1000°C during the deposition time of 15 min and again at the
138 deposition time of 25 min while keeping other growth parameters
139 constant
140 Results and discussions
141 All growth conditions are constant in order to study the
temper-142 ature effects Prior to carbon nanotube growth, Energy Dispersive
143 X-ray (EDX) measurements were done to investigate the elemental
144 composition of the cobalt catalyst deposited on Si substrates
145 (Fig 2) Atomic Force Microscopy (AFM) in contact mode was used
146 for analyzing the surface morphology of Co film deposited on Si Fig 1 Schematic diagram of PECVD system of nanotube synthesis.
Fig 2 Energy Dispersive X-ray (EDX) measurements show the elemental composition of the cobaltnanocatalyst deposited on Si substrates.
2 A.S Elahi et al / Results in Physics xxx (2017) xxx–xxx
9 February 2017
Trang 3147 substrates (Fig 3(a,b)) AFM images have been obtained in a
scan-148 ning area of 3mm 3 mm As it is clear, the formation of catalyst
149 particles with a relatively smooth surface can be observed For
150 the analysis of the uniformity of catalyst distribution along the
151 substrate surface, it is helpful to calculate the roughness value
152 The average roughnesses is 1.91 nm Root-Mean-Square (RMS)
153 roughnesses was measured over the whole area and it was
154 2.44 nm The RMS roughness of a surface is similar to the
155 roughness average, with the only difference being the mean
156 squared absolute values of surface roughness profile The effect
157 of deposition temperature on CNTs as a function of growth time
158 was investigated FESEM images of CNTs grown on the cobalt
159 catalyst at growth temperatures of 850°C, 900 °C, 950 °C and
160
1000°C during the deposition time of 15 min have been shown
161
inFig 4(a–d) For comparison purposes, the FESEM images of CNTs
162 grown at growth temperatures of 850°C, 900 °C, 950 °C and
163
1000°C during the deposition time of 25 min have been shown
164
in Fig 5(a–d) As can be seen in Fig 4(a), the CNTs grown at
165
850°C have smaller diameters and production yield is very high
166 When the temperature enhanced and reached to 900°C, the CNTs
167 diameters have been increased and the efficiency was very low
168 (Fig 4(b)) At 950°C, the yield slightly increased (Fig 4(c)) and at
Fig 3 (a) 2D and (b) 3D AFM Images of Co film deposited on Si substrates.
Fig 4 FESEM images of CNTs grown on the cobalt catalyst at growth temperatures of (a) 850 °C, (b) 900 °C, (c) 950 °C and (d) 1000 °C during the deposition time of 15 min.
Please cite this article in press as: Elahi AS et al A new perspective on structural and morphological properties of carbon nanotubes synthesized by Plasma
Trang 4169 1000°C, the efficiency enhanced again but CNTs have large
170 diameters From theFig 5(a–d), it is found that the deposition time
171 of 25 min gave less CNTs population particularly for 900°C, 950 °C
172 and 1000°C where large catalyst particles that remained
173 un-reacted amidst the carbon nanotubes were seen At the
temper-174 atures of 900°C and 1000 °C, nucleation was performed but the
175 growth has not taken place On the other hand, the grown CNTs
176 at the temperature of 850°C among all of the samples during
depo-177 sition time of 25 min have a minimum diameter and maximum
178 efficiency (Fig 5) It is supposed that at high temperature, the
179 metal atoms agglomerate into bigger clusters leading to thick
180 carbon nanotubes Simultaneously, high temperature promotes
181 acetylene decomposition leading to more carbon generation and
182 hence more wall formation Since agglomeration of catalyst
parti-183 cles produces greater particles with lower activities, the number of
184 active sites decreases and the density of grown CNTs is reduced
185
[25] Thus, The CVD temperature plays the central role in CNT
186 growth.Fig 6 shows the HRTEM image of the grown CNT at a
187 growth temperature of 850°C during the deposition time of
188
15 min using Co as catalyst, which confirms that the morphology
189 seen in the FESEM image (Fig 4(a)) have tubular structure, i.e they
190 are multi-walled carbon nanotubes (MWCNTs) The grown CNTs
Fig 6 the HRTEM image of the grown CNT at a growth temperature of 850 °C
during the deposition time of 15 min using Co as catalyst.
Fig 7 The Raman spectrum of the produced CNTs at different growth temperatures during deposition time of 15 min.
Fig 5 FESEM images of CNTs grown on the cobalt catalyst at growth temperatures of (a) 850 °C, (b) 900 °C, (c) 950 °C and (d) 1000 °C during the deposition time of 25 min.
4 A.S Elahi et al / Results in Physics xxx (2017) xxx–xxx
9 February 2017
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spec-192 trum of the produced CNTs at different growth temperatures
dur-193 ing deposition time of 15 min is shown inFig 7 The well separated
194 two Raman peaks at 1500–1605 cm1for G peaks (graphite band)
195 and at 1250–1450 cm1for D peaks (disorder induced band) was
196 observed for all samples Also, there is a signal peak in the region
197 of the radial breathing mode (RBM), i.e bellow 300 cm1 of the
198 spectrum The RBM Raman features correspond to the atomic
199 vibration of the C atoms in the radial direction The G0-band
fre-200 quency is close to twice that of the D band and is found from
201 2500 to 2900 cm1 The G0band is an intrinsic property of the
nan-202 otubes and graphite and present even in defect–free nanotubes for
203 which the D-band is completely absent Also, the Raman spectrum
204 of the produced CNTs at different growth temperatures during
205 deposition time of 15 min is shown inFig 8 The IG/IDratios were
206 calculated to estimate the variation of CNT crystallinity at different
207 growth temperatures during the deposition time of 15 and 25 min
208 using Co catalyst (Table 1) This reveals that the trend of CNT
209 crystallinity varies with synthesis temperature and deposition
210 time Here, the ratio of IG/IDis greater than others for the grown
211 CNTs at growth temperature of 950°C and deposition time of
212
25 min which indicates that these CNTs have good crystalline
gra-213 phite structure, while from FESEM results found that the diameters
214
of CNTs are raised and density is decreased From the FESEM
215 results, we observed that our CNTs grew with the tip growth
mech-216 anism as the catalyst nanoparticles are seen from FESEM images
217 that they are at the tip of the CNTs with brighter color than the
218 nanotubes[15]
219 Conclusions
220
In this work, we demonstrated how growth temperature and
221 deposition time affected the growth of carbon nanotubes using
222 the Co nanocatalyst by the PECVD technique In both of deposition
223 time (15 and 25 min), we changed the growth temperature while
224 keeping other parameters strictly constant It was found that by
225 raising the growth temperature, the degree of crystallinity of
226 grown CNTs increases however agglomeration of nanocatalysts
227 reduces their catalytic activities, which enhance graphite sheet
228 defects
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Fig 8 The Raman spectrum of the produced CNTs at different growth temperatures
during deposition time of 25 min.
Table 1
The ratios of the intensities of G and D peaks,I G /I D for produced CNTs at different
growth temperatures during deposition time of 15 and 25 min.
Deposition
Time
Growth temperature
G band (cm1)
D band (cm1)
I G /I D
Please cite this article in press as: Elahi AS et al A new perspective on structural and morphological properties of carbon nanotubes synthesized by Plasma