Literature Review:
Introduction to Research
MCEN 5028
Synthesis of carbon nanotubes by chemical vapor deposition
Graduate student: Amit Khosla
Graduate Advisor: Professor Roop. L. Mahajan
2/12/2016
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Table of Contents:
Abstract: .......................................................................................................... 3
Introduction:.................................................................................................... 3
Synthesis of Carbon nanotubes: ..................................................................... 5
Arc-Discharge and Laser Ablation: ................................................................................ 5
Synthesis by CVD Mechanism ....................................................................................... 6
Single-Walled Nanotube Growth and Optimization ...................................... 8
Directed Growth of Single-Walled Nanotubes............................................... 9
Conclusions ..................................................................................................... 9
References:...................................................................................................... 9
Figure References : ....................................................................................... 10
Table of Figures:
Figure 1: Schematic of Arc Discharge method
Figure 2:
Schematic of Laser Ablation .................................................................... 6
Figure 3: CNT synthesis by laser ablation. ..................................................... 6
Figure 4: Schematic of CVD process ............................................................. 7
Figure 5: Growth process of CNT from nucleating spot ................................ 7
Figure 6: Graph of wetting of Fe by carbon atom .......................................... 7
Figure 7 ........................................................................................................... 8
Figure 8: Image of CNT generated by CVD process ..................................... 8
Figure 9: Structured pattern of SiO2 on Si wafer
Figure 10:
Directed growth of CNT on Pegs ............................................................ 9
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Abstract:
Carbon nanotubes with their extraordinary mechanical, thermal and electric properties
hold a strong potential for future applications. This review discusses the various growth
methods of carbon nanotubes with emphasis on chemical vapor deposition (CVD). CVD
methods hold the advantage of producing high quality nanotubes in large scales. Their
advantages over other forms of synthesizing CNT’s are discussed. In the lights of
experimental and theoretical approaches, various mechanisms, for single and multi-wall
nanotubes nucleation and growth are reviewed. A fundamental understanding of
nanotube growth at the atomic level is probably one of the main issues to develop a
nanotube mass production process. Techniques for building micro- and nano –scale
architectures with carbon nanotubes using chemical vapor deposition have been studied.
This controlled growth of CNT point to the strong possibility of the building nano scale
devices.
Introduction:
Till1985 primary allotropes of carbon were diamond and graphite. In 1985 a new
allotrope was discovered by a group (Smalley) at Rice University, called Bucky Ball or
C60. It comprises of 60 carbon atoms combined to form a hollow closed sphere and
resembles to that of soccer ball. It was in 1991, while working on the C60 structure,
another allotrope of carbon, now known as carbon nanotube, was found accidentally by
Sumiya IIjama. He first found carbon nanotube while examining soot from the arc
discharge chamber used commonly to form C60. Since then these nanotubes have opened
a new world of possibilities because of their amazing mechanical [1, 2, 3], thermal [4]
and electric properties [5, 6]. Structurally carbon nanotubes consist of graphene sheet
rolled up into a seamless, cylindrical tube. The side walls of the tubes are made of
hexagons while the end caps are formed of heptagons or pentagons. The diameters of the
nanotubes range from few nanotubes for single walled nanotubes (SWNTs) to several
tens of nanometers for multi walled nanotubes (MWNTs).
The mechanical property of carbon nanotube is attributed to the nature of their C-C bond
strength and their seamless structure [1]. Carbon nanotubes are predicted to have far
superior mechanical properties in comparison to carbon fibers. Young’s modulus of
SWNT has been theorized to be ~1TPa [1]. Mechanical behavior of MWNTs is more
complex than SWNTs due to the interaction between the multiple cylindrical layers. It is
assumed that the individual layers in the MWNT should only interact by van der Walls
forces [3]. This weak interaction may lead to the sliding of walls within one other thereby
lowering the young modulus. CVD grown tubes had moduli ranging from 30-50 GPa.
This could be because of many defects associated with CVD growth method. Another
advantage of carbon nanotubes besides their high strength is their ability to undergo
elastic deformation without breaking. The combination of high strength and flexibility
makes carbon nanotubes an ideal choice for use in structural reinforcements.
Beyond their mechanical properties, carbon nanotubes also possess unique electrical
properties. Depending on the nanotubes helicity, how the carbon atoms wrap around the
nanotube, nanotubes can be metallic or semi conducting [7,8]. Due to their nanometer
size, carbon nanotubes are one dimensional (I-D) conductors, meaning that the electrons
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only flow in one direction, along the axis of the tube. The electrical properties of carbon
nanotubes can be modified by doping of the tube’s lattice with other elements. The
addition of dopants alters the local structure of the nanotube [9]. Adding defects not only
allow for the combination of two or more different types of nanotubes but they can also
facilitate the attachment of carbon nanotubes with biological structures like DNA , which
assist in self assembly of carbon nanotubes. This may lead to the future possibilities of
integrated structures on the order of a few nanometers with the nanotube acting as the
transistor and/or metal interconnect .
The thermal properties of nanotubes like the mechanical and electrical are also
unique. The thermal conductivity of carbon nanotubes is higher than most known
materials at moderate temperatures. This is primarily due to the phonon contribution at all
temperature ranges [4]. Thermal conductivity values of aligned ropes of SWNTs at room
temperature have been measured to be greater than 200W/mK while mats of nanotubes
possess a thermal conductivity of ~35 W/mK [4]. This property of carbon nanotubes can
be exploited for the development of thermal interface material.
The main hindrance to employing carbon nanotubes in real world devices is the
inability to control the growth of the nanotube (direction and location) and to synthesize
them in bulk quantities. There are primarily three main methods to grow carbon
nanotubes: arc discharge [5], laser ablation [5] and chemical vapor deposition (CVD) [10,
11]. The first two methods are high temperature processes, (2500oC) that yield high
quality but poor density of carbon nanotubes. The CVD technique has the ability to grow
bulk amounts of carbon nanotubes, but these nanotubes contain defects along the length
of tubes due to the relatively low synthesis temperatures of 600-1200oC. There are also
problems with the ability to selectively grow carbon nanotubes at specific sites. Some
progress however have been done in that area by forming micro- nano patterns of
nanotubes on structured substrates using conventional lithography and E-beam writing.
However, much research is still required for the selective growth of SWNTs. No one, as
of yet has been able to grow aligned SWNTs on a substrate or consistently control the
direction of the SWNT growth.
My research shall discuss detailed studies towards understanding the factors that
contribute to the growth of SWNTs and MWNTs during the thermal decomposition of
acetylene gas in the presence of liquid precursor, ferrocene. A fundamental knowledge of
the process would help in finding applications of MWNTs and SWNTs in real world
devices. This work would try to determine what operating properties promote the growth
of SWNTs. A first approach would be done by utilizing extensive literature in this area.
The established ideal growth parameters i.e. the gas type, metal catalyst and flow rate
would be kept constant. After these variables have been determined a close examinations
of the effect of temperature, growth time, ferrocene concentration and growth pressure
shall be carried out to optimize the process.
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Synthesis of Carbon nanotubes:
Now that the brief overview of the mechanical, electric and thermal properties of carbon
nanotubes has been provided, the synthesis of CNTs and the previous work done shall be
discussed. There are three main methods to fabricate carbon nanotues: arc discharge,
laser ablation, chemical vapor deposition.
Arc-Discharge and Laser Ablation:
Arc-discharge and laser ablation methods for the growth of nanotubes have been actively
pursued in the past ten years. Both methods involve the condensation of carbon atoms
generated from evaporation of solid carbon sources. The temperatures involved in these
methods are close to the melting temperature of graphite, 3000–4000oC. In arc-discharge,
carbon atoms are evaporated by plasma of helium gas ignited by high currents passed
through opposing carbon anode and cathode [fig 1]. Arc-discharge has been developed
into an excellent method for producing both high quality multi-walled nanotubes and
single-walled nanotubes. MWNTs can be obtained by controlling the growth conditions
such as the pressure of inert gas in the discharge chamber and the arcing current .In 1992,
a breakthrough in MWNT growth by arc-discharge was first made by Ebbesen and
Ajayan who achieved growth and purification of high quality MWNTs at the gram level
[5]. The synthesized MWNTs have lengths on the order of ten microns and diameters in
the range of 5-30nm. The nanotubes are typically bound together by strong van der Waals
interactions and form tight bundles. MWNTs produced by arc-discharge are very straight,
indicative of their high crystallinity. For as grown materials, there are few defects such as
pentagons or heptagons existing on the sidewalls of the nanotubes. The by-product of the
arc-discharge growth process is multi-layered graphitic particles in polyhedron shapes.
Purification of MWNTs can be achieved by heating the as grown material in an oxygen
environment to oxidize away the graphitic particles [11]. The polyhedron graphitic
particles exhibit higher oxidation rate than MWNTs; nevertheless, the oxidation
purification process also removes an appreciable amount of nanotubes. For the growth of
single-walled tubes, a metal catalyst is needed in the arc-discharge system. The first
success in producing substantial amounts of SWNTs by arc-discharge was achieved by
Bethune and coworkers in 1993 [12].They used a carbon anode containing a small
percentage of cobalt catalyst in the discharge experiment, and found abundant SWNTs
generated in the soot material. The growth of high quality SWNTs at the 1–10 g scale
was achieved by Smalley and coworkers using a laser ablation (laser oven) method [Fig
2.] [5]. The method utilized intense laser pulses to ablate a carbon target containing .05
atomic percent of nickel and cobalt. The target was placed in a tube-furnace heated to
1200oC. During laser ablation, a flow of inert gas was passed through the growth
chamber to carry the grown nanotubes downstream to be collected on a cold finger. The
produced SWNTs are mostly in the form of ropes consisting of tens of individual
nanotubes close-packed into hexagonal crystals via van der Waals interactions[Fig3 ].
The optimization of SWNT growth by arc-discharge was achieved by Journet and
coworkers using a carbon anode containing 10 atomic percentage of yttrium and 42 at %
of nickel as catalyst [5]. In SWNT growth by arc-discharge and laser ablation, typical byproducts include fullerenes, graphitic polyhedrons with enclosed metal particles, and
amorphous carbon in the form of particles or over coating on the sidewalls of nanotubes.
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A purification process for SWNT materials has been developed by Smalley and
coworkers [5] and is now widely used by many researchers. The method involves
refluxing the as-grown SWNTs in a nitric acid solution for an extended period of time,
oxidizing away amorphous carbon species and removing some of the metal catalyst.
Figure 1: Schematic of Arc Discharge method [1]
Figure 2: Schematic of Laser Ablation[2]
Figure 3: CNT synthesis by laser ablation [3]
Synthesis by CVD Mechanism
Chemical vapor deposition (CVD) methods have been successful in making carbon fiber,
filament and nanotube materials since more than 10–20 years ago [16, 17, 18, 19]. A
schematic experimental setup for CVD growth is depicted in [Fig 4]. The growth process
involves heating a catalyst material to high temperatures in a tube furnace and thereafter
flow a hydrocarbon gas through the tube reactor for a period of time. Materials grown
over the catalyst are collected upon cooling the system to room temperature. The key
parameters in nanotube CVD growth are the hydrocarbons, catalysts and growth
temperature. The active catalytic species are typically transition-metal nanoparticles
formed on a support material such as silicon. The general nanotube growth mechanism
[Fig 5] in a CVD process involves the dissociation of hydrocarbon molecules catalyzed
by the transition metal, and dissolution and saturation of carbon atoms in the metal
nanoparticle. The precipitation of carbon from the saturated metal particle leads to the
formation of tubular carbon solids in sp2 structure. Tubule formation is favored over
other forms of carbon such as graphitic sheets with open edges. This is because a tube
contains no dangling bonds and therefore is in a low energy form. For MWNT growth,
most of the CVD methods employ ethylene or acetylene as the carbon feedstock and the
growth temperature is typically in the range of 550–750C. Iron, nickel or cobalt
nanoparticles are often used as catalyst. The rationale for choosing these metals as
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catalyst for CVD growth of nanotubes lies in the phase diagrams[Fig 6] for the metals
and carbon. At high temperatures, carbon has finite solubility in these metals, which leads
to the formation of metal-carbon solutions and therefore mentioned growth mechanism.
Noticeably, iron, cobalt and nickel are also the favored catalytic metals used in laser
ablation and arc-
Figure 4: Schematic of CVD process [4]
discharge. A major pitfall for CVD grown MWNTs has been the high defect densities in
their structures. The defective nature of CVD grown MWNTs remains to be thoroughly
understood, but is most likely be due to the relatively low growth temperature, which
does not provide sufficient thermal energy to anneal nanotubes into perfectly crystalline
structures. Growing perfect MWNTs by CVD remains a challenge to this day.
Figure 5: Growth process of CNT from nucleating spot [5]
Figure 6: Graph of wetting of Fe by carbon atom [6]
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Single-Walled Nanotube Growth and Optimization
For a long time, arc-discharge and laser-ablation have been the principal methods for
obtaining nearly perfect single-walled nanotube materials. There are several issues
concerning these approaches. First, both methods rely on evaporating carbon atoms from
solid carbon sources at ≥ 3000C, which is not efficient and limits the scale-up of SWNTs.
Secondly, the nanotubes synthesized by the evaporation methods are in tangled forms
that are difficult to purify, manipulate and assemble for building addressable nanotube
structures. Recently, growth of single-walled carbon nanotubes with structural perfection
was enabled by CVD methods. For an example, it has been found that by using methane
as carbon feedstock, reaction temperatures in the range of 850–1000C, suitable catalyst
materials and flow conditions one can grow high quality SWNT materials by a simple
CVD process [20, 21, 22]. High temperature is necessary to form SWNTs that have small
diameters and thus high strain energies, and allow for nearly-defect free crystalline
nanotube structures. Among all hydrocarbon molecules; methane is the most stable at
high temperatures against self-decomposition. Therefore, catalytic decomposition of
methane by the transition-metal catalyst particles can be the dominant process in SWNT
growth. The choice of carbon feedstock is thus one of the key elements to the growth of
high quality SWNTs containing no defects and amorphous carbon over-coating. Another
CVD approach to SWNTs was reported by Smalley and coworkers who used ethylene as
carbon feedstock and growth temperature around 800oC [24]. In this case, low partialpressure ethylene was employed in order to reduce amorphous carbon formation due to
self-pyrolysis/dissociation of ethylene at the high growth temperature. Gaining an
understanding of the chemistry involved in the catalyst and nanotube growth process is
critical to enable materials scale-up by CVD [22] .The choice of many of the parameters
in CVD requires to be rationalized in order to optimize the materials growth. Within the
methane CVD approach for SWNT growth, we have found that the chemical and textural
properties of the catalyst materials dictate the yield and quality of SWNTs. This
understanding has allowed optimization of the catalyst material and thus the synthesis of
bulk quantities of high yield and quality SWNTs [22] . Evident from the TEM image[Fig
7,8] is that the nanotubes are free of amorphous carbon coating throughout their lengths
Figure 7 [7]
Figure 8: Image of CNT generated by CVD process [8]
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Directed Growth of Single-Walled Nanotubes
Ordered, single-walled nanotube structures can be directly grown by methane CVD on
catalytically patterned substrates. A method has been devised to grow suspended SWNT
networks with directionality on substrates containing lithographically patterned silicon
pillars [25]. Contact printing is used to transfer catalyst materials onto the tops of pillars
selectively [Fig 9]. CVD of methane using the substrates leads to suspended SWNTs
forming nearly ordered networks with the nanotube orientations directed by the pattern of
the pillars [Fig 10]
Figure 9: Structured pattern of SiO2 on Si wafer[9]
Figure 10: Directed growth of CNT on Pegs[10]
Conclusions
I have presented an overview of various growth methods for multi-walled and singlewalled carbon nanotubes. It is shown that chemical vapor deposition approaches are
highly promising for producing large quantities of high quality nanotube materials at
large scales. It can be envisioned that in a foreseeable future, controlled growth will yield
nanotube architectures used as key components in next generations of electronic,
chemical, mechanical and electromechanical devices.
It is fair to say that progress in nanotube research has been built upon the successes in
materials synthesis. This trend shall continue. It is perhaps an ultimate goal for growth to
gain control over the nanotube chirality and diameter, and be able to direct the growth of
a semiconducting or metallic nanowire from and to any desired sites. Such control will
require significant future effort, and once successful, is likely to bring about
revolutionary opportunities in nanoscale science and technology.
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Figure References :
All figures have been obtained from the book: Carbon nanotubes synthesis. Structure ,properties and
applications. M.S. Dresselhaus, G.Dresselhaus and Ph. Avorius .eds. Springer. New York.2001
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