Carbon Nanotube
Final Report
Deanna Zhang
Chuan-Lan Lin
May 14, 2003
Introduction
Carbon nanotubes are formed from pure carbon bonds. Pure carbons only have two covalent
bonds: SP 2 and SP 3 , the former constitutes graphite and the latter constitutes diamond. Figure
1 shows the structure of graphite and diamond.
Graphite
Diamond
Figure 1- Structure of graphite and diamond [5]
SP 2 is a strong bond within a plane but weak between planes. SP 2 is composed of one S orbital
and two P orbitals. When more SP 2 bonds come together, they form six-fold structures, like
honeycomb pattern, which is the plane structure, the same structure as graphite. Graphite is
stacked layer by layer so it’s only stable for one single sheet. That’s why graphite is used in
pencils. Viewing these layers perpendicularly shows the honeycomb patterns of graphite.
Wrapping these patterns back on top of themselves, joining the edges, and close one end while
leave one end open, we form a tube of graphite, which is a Nanotube [5].
Two types of nanotubes exist in nature: single-wall nanotube and multi-wall nanotube.
Figure 2 shows the structure of single-wall nanotube and multi-wall nanotube.
Single Nanotube
Multiwall Nanotube
Figure 2- Structure of single-wall and multi-wall carbon nanotubes [5].
Single-wall nanotubes has only one single layer and with a diameter of 1 to 5 nm [5]. The
properties of SWNT are more stable than MWNT so it is more favorable. MWNT is the first
nanotube discovered in 1991. MWNT is a little bigger than SWNT because MWNT has about 50
layers. MWNT’s inner diameter is from 1.5 to 15 nm and the outer diameter is from 2.5 nm to 30
nm. SWNT have better defined shapes of cylinder than MWNT, thus MWNT has more
possibilities of structure defects and its nanostructure is less stable [5]. Most researchers focus
on SWNT and develop applications based on SWNT due the physical stability of SWNT.
Properties of Carbon Nanotubes
Electrical Properties
Carbon nanotubes have some distinct electrical properties. One of the important
properties of carbon nanotube is that it can exhibit the characteristics of a metal or a
semiconductor [6]. Specially, the energy gap is determined by the rolling direction of nanotube.
Figure3- Diagram showing rolling direction of nanotube [6].
In Figure 3, Ch is Mamada vector connecting two crystallographical equivalent sites, and a1 and
a 2 are the unit vectors of the unit cell. Ch= n1 a1 + n2 a 2 , where n1 and n2 are integers. The
nanotube is formed by connecting A and A’ point. There is a simple rule to determine if the
nanotube acts as a metal or a semiconductor: if ( n1 + n2 )/3 = integer, nanotube acts as a metal
otherwise it acts as a semiconductor [6] Due to the miniscule size of the nanotube, the electrical
conductivity is measured by the four point probe method to determine the sheet resistance. The
four point probe method measures the resistivity of any semiconductor material [1]. Figure 4
shows the setup of the four point probe method. At the same time, nanotubes have been studied
to make switches and transistors, which would be much smaller than the silicon chips currently
used. The wires made by nanotubes are capable of currents that are 100 times greater than metal
wires, making nanotubes useful in the production of flat panels [6].
Figure 4-Four Point Probe method setup. The sheet resistivity Rs= K
V
[1].
I
Additional properties of nanotubes:
Nanotubes are the strongest fibers that are currently known. A single-wall nanotube can
be up to 100 times stronger than that of steel with the same weight. The Young’s Modulus of
SWNT is up to 1TPa, which is 5 times greater than steel (230 GPa) while the density is only 1.3
g/cm^3 [6]. That means that materials made of nanotubes are lighter and more durable. There
may also be other applications due to these properties, such as car bumpers, and strong wires.
Nanotubes also have a very high aspect ratio. The lengths of nanotubes are usually around 1 µm,
while the diameter for SWNT is only 1 nm (50 nm for MWNT) [6]. This property makes
nanotubes useful for tips and nanowires. In addition, the thermal conductivity (2000W/m.K) is
five times greater than that of copper (400W/m.K) [6].
History of Carbon Nanotubes:
The discovery of the carbon nanotube is based off of the discovery of the Buckyball.
Buckyballs, also know as C60 , are made up of 60 carbon atoms, arranged in the shape of a
sphere. Figure 4a shows the Buckyball structure. The exact chemical name of C60 is
Buckminsterfullerene but is often referred to as fullerene or Buckyball [5]. The discovery of the
Buckyball was accidental. While researching Radioastronomy, scientists found unusual long
molecules that had not been synthesized in laboratory. The concentration of these long molecules
was much higher than anyone expected. Scientists attempted to synthesize these long carbon
chains in the laboratory. They used a powerful laser to evaporate small amounts of graphite into
a hot cloud of particles, then cooled the cloud with a stream of helium gas [5]. This allowed the
atoms to condense into clusters. They then used a mass-spectrometer to identify the molecular
mass. The result was a molecular mass of 720amu, which corresponds to 60 carbon atoms
(12amu x 60=720amu) [5]. Because the amount synthesized was too little to allow a structure
analysis, they hypothesized that 60 carbon atoms had formed themselves into a sphere.
Scientists around the world duplicated these results, creating Buckyballs in order to perform
structural analysis. From these experiments, the scientists finally proved the existence of the
spherical structure.
Figure 4a: Buckyball structure
of C60 [5]
Figure 4b: Nanotubes viewed under
electron microscope [5]
Using carbon arc-evaporation synthesis, more Buckyballs were created. When the carbon arc
power supply was changed to DC current instead of AC, nanotubes were created. In 1991, a
Japanese Scientist, Sumio Iijima, first observed and reported the existence of entirely carbonmade nanotubes. He found that the graphite structures contained nanoparticles and nanotubes,
which were later identified as multilayer nanotubes [5]. Figure 4b shows these nanotubes as
viewed under an electron microscope.
Timeline of the discovery of Nanotubes:
When
1940s
Who
German Chemist Otto Hahn
1970s
England Chemists Harry
Kroto and Dave Walton
1985
Kroto and his American
colleague, Rick Smalley
Late
1980s
Scientists around the world
1991
Japanese Scientist, Sumio
Iijima
S, Iijima and T, Ichihashi
1993
1995
1996
A.G. Rinzler
Professor Robert F. Curl, Jr.,
Rice University, Houston,
USA, Professor Sir Harold
W. Kroto, University of
Events
When trying to create heavier atoms by arc
carbon method of neutron, Hahn reported
the existence of carbon chains. Because he
was interested in only metal atoms, the
research of carbon chains was not
continuing.
They were synthesizing long carbon chains
to make something like gas cloud in the
galaxy.
They collaborated a project to simulate
conditions of red giant stars in the
laboratory.
Buckyball was synthesized and confirmed
as C60
Discovery of multi wall carbon nanotubes
Synthesis of single wall carbon nanotubes
Begin to use Laser ablation method
Research of nanotubes as field emitters
Awarded 1996 Nobel Prize in Chemistry
for the discovery of Buckyball
Sussex, Brighton, U.K., and
Professor Richard E.
Smalley, Rice University,
Houston, USA,
1998
1999
Samsung Company
2001
IBM research group
2001
M. Kociak
Development of HiPCO, CVD methods
Demonstrated Flat Panel display prototype
(4.5”, full-color) using nanotube as filedemission source.
The first computer circuit composed of only
one single carbon nanotube was announced
Intrinsic superconductivity of carbon
nanotubes
Fabrication of Carbon Nanotubes
The physical mechanisms and reactions that cause the formation of carbon nanotubes are
still unknown. However, four methods currently exist to fabricate these nanotubes. The four
methods are all gas-phase processes that start with a source of carbon that is evaporated from a
surface [3]. The four techniques used produce carbon nanotubes are the carbon arc-discharge
method, the laser vaporization technique, chemical vapor deposition, and high-pressure carbon
monoxide method.
Carbon Arc-Discharge
The arc-discharge method produces good quality multi-wall and single-wall nanotubes.
This process produces nanotubes at a greater rate than the pulsed laser vaporization technique.
This technique utilizes two graphite electrodes to generate an arc by a high dc current. After arc
discharging for a period of time, a carbon rod builds up at the cathode. Carbon nanotube bundles
and amorphous carbon both form at the cross section of the rod. Figure 8 shows the setup of the
arc-discharge method. This method does not produce clean results due to the existence of these
amorphous carbons. Helium gas is present to increase the speed of carbon deposition. Some
parameters that are critical in this process are the pressure of the helium, the temperature, and the
dc current. Efficient cooling is necessary to form homogenous deposition of carbon nanotubes
[9].
The group at University of Montpellier, France perfected the arc-discharge method by
using yttrium and nickel metal catalysts. The group produced carbon nanotubes from vapors of
carbon containing a small amount of nickel and yttrium catalysts. “An electric arc vaporizes an
anode containing the catalysts” [4]. An electrical current of 100 amps and 35 volts provide the
energy to generate the discharge. The flow system is controlled via gas pressure and is pumped
by a mechanical vacuum pump [4].
Figure 8- Carbon Arc-Discharge Method
Pulsed Laser Ablation or Vaporization
The pulsed laser vaporization (PLV) of graphite in the presence of an inert gas and
catalyst forms single-walled carbon nanotubes. Without the presence of these catalysts, the
vaporized graphite would form buckyballs instead. Buckyballs are formed in nature when both
ends of the graphite sheet closes. However, in the presence of a catalyst, the graphite layers
stays open on one end [4].
The PLV of carbon containing metal catalysts produces the purest SWNT. Some factors
that determine the amount of carbon nanotubes produced are the amount and type of catalysts,
laser power and wavelength, temperature, pressure, type of inert gas present, and the fluid
dynamics near the carbon target [4]. The PLV method reduces the amount of amorphous carbon
contaminates [3]. The PLV of carbon is an expensive method that avoids the high electric fields
involved in the arc-discharge method [9].
The NASA Johnson Space Center Carbon Nanotube Project uses two lasers to impinge
on a composite graphite and metal catalyst target. These two lasers are Neodymium: YttriumAluminum-Garnet pulsed. The frequency of pulse operation is 60 Hz, which is very fast. The
lasers operate at infrared and green wavelengths at 300 mJ. The laser is focused onto a carbon
targets containing one atomic percent each of the catalysts cobalt and nickel. The carbon target
is ablated in a 5 cm flow tube pressurized with argon at 500 Watt, in an oven at 1473 K [4].
Figure 9 shows the setup. The Argon gas carries the vapors from the high temperature chamber
into a water-cooled copper collector positioned downstream [9]. The nanotubes self-assemble
from the carbon vapors and condense on the walls of the flow tube. The nanotubes produced
have a very narrow distribution of diameters [9].
[4].
Figure 9- Pulsed Laser Vaporization Apparatus [4]
The yield is about 0.3=0.4 grams per hour
Chemical Vapor Deposition
The chemical vapor deposition (CVD) method deposits hydrocarbon molecules on top of
heated catalyst material. The catalyst is usually predeposited on a substrate. Metal catalysts
dissociate the hydrocarbon molecules. Figure 10 shows the apparatus of the CVD process. The
CVD method produces both single-wall and multi-wall nanotubes. The CVD process uses
hydrocarbons as the carbon source. Hydrocarbons flow through the quartz tube where it is
heated at a high temperature. The dissociation of the hydrocarbons breaks the hydrogen carbon
bond, producing pure carbon molecules. At high temperatures, the carbon form carbon
nanotubes. Growth of SWNTs also occurs at a higher temperature than MWNTs [7]. Some
advantages of the CVD are low power input, lower temperature range, relatively high purity and
possibility to scale up the process [9].
Figure 10-Apparatus of the CVD process [9]
The main focus in the CVD process is the prepattern of the substrate with a catalyst.
Selective growth is achieved through prepatterning the substrate with a metal catalyst. One of
the ways of prepatterning the substrate is to use standard lithography techniques. In this process,
a photoresist is deposited onto the substrate. The resist is exposed and developed, creating a
pattern of resist on the surface. Metal catalysts are deposited on the film using the resist pattern
as a mask. The catalyst is lift off with the unexposed substrate through wet etching. The
photoresist not covered by the catalyst is etched away. The CVD growth of the carbon
nanotubes exists on the catalysts. Figure 11 shows the prepatterning process [7].
Figure 11-Prepatterning Process [7]
High Pressure Carbon Monoxide Method
The high pressure carbon monoxide (HiPCO) method can produce large quantities of
carbon nanotubes. The high yield demonstrates the high potential of this method for bulk
production of SWNTs. “Catalysts for SWNT growth form in situ by thermal decomposition of
ion pentacarbonyl in a heated flow of carbon monoxide at pressures of 1-10atm and temperatures
of 800-1200 C” [2]. Previous methods for growing CNTs using hydrocarbons as source have
resulted large quantities of amorphous carbon and graphitic deposits due to the thermal
breakdown of hydrocarbons at high temperatures [2]. The amorphous carbon overcoating would
have to be removed in subsequent steps. The HiPCO method uses carbon monoxide as the
carbon feedstock and Fe(CO)5 as the iron-containing catalyst precursor. The SWNT yield and
the diameter of the nanotubes produced can vary over a wide range determined by the condition
and flow-cell geometry [2].
The HiPCO method produces SWNTs by flowing CO mixed with a small amount of
Fe(CO)5 through a heated reactor [2]. Figure 12 shows the setup of the HiPCO method. “The
products of the Fe(CO)5 thermal decomposition react to produce iron clusters in gas phase.
These clusters act as nuclei upon which SWNTs nucleate and grow” [2]. The solid carbon is
formed through CO disproportionation, also known as the Boudouard reaction:
CO+CO C(s)+CO2
This reaction occurs catalytically on the surface of the iron particles. The iron particles “promote
the formation of the tube’s characteristic graphitic carbon lattice” [2]. The flow tube has a thick
quartz wall and is contained within the furnace. The rate at which the reactant gases are heated
determines the amount and quality of the SWNTs produced. The CO and Fe(CO)5 gases are
maintained at a low temperature initially through a water-cooled injector. This low temperature
is maintained so that rapid heating can occur inside the furnace [2]. This apparatus of the HiPCO
method yielded high quantities of SWNTs.
Figure 12-HiPCO apparatus [2]
Applications of Carbon Nanotubes
Field Effect Transistors
Carbon nanotubes are a new form of carbon with unique electrical and mechanical
properties. Varying the diameter of the cylinder can control the band gap of semiconducting
nanotubes. The width of the diameter of the nanotube is inversely proportional to the size of the
band gap [16]. Semiconducting nanotubes can be used to build molecular field-effect transistors
(FETs) while metallic nanotubes can be used to build single-electron transistors [17].
Single wall carbon nanotube transistors are electronic devices based on a single rolled-up
sheet of carbon atoms. When the first SWNT transistors were formed, they could not operate at
room temperature. However, recent research has developed SWNT transistors that do operate at
room temperature. Early nanotube FETs used a non-local back-gate with the nanotube sidebonded to noble metal electrodes [16]. This setup gave large contact resistance and poor
characteristics [16]. The recent CNFETs are now built with top-gate geometry and resemble
more conventional silicon CMOS devices. Figure 13 shows the layout of the carbon nanotube
transistor along with its I-V characteristics. The SWNT transistors consist of a semiconducting
carbon nanotube about 1 nm in diameter bridging two closely separated metal electrodes a top a
silicon surface coated with SiO2 [16]. Applying an electric filed to the silicon via the gate
electrode turns on and off the flow of the current across the nanotube by controlling the
movement of charge carriers onto it.
Figure 13- Carbon nanotube transistor layout and its I-V characteristics [17]
The research group at IBM has studied methods of producing single- and multi-wall
carbon nanotubes to be used as field-effect transistors. The SWNTs were formed via laser
ablation of graphite doped with cobalt and nickel catalysts [17]. The SWNTs were ultrasonically
treated with H2SO4/H2O2 solution to clean the NTs. The MWNTs were formed by arc discharge
evaporation methods [17]. Figure 13 shows the schematic of the NT device. The field transistor
consist of either an individual SWNT or MWNT bridging two electrodes deposited on a 140 nm
thick gate oxide films on a doped SI wafer. The Au electrodes were generated through electron
beam lithography.
The behavior of the NT transistors is similar to that of a p-channel MOSFET. The I-V
characteristic of the NTs was measured at room temperature. Figure 13 shows the I-VSD of a
device consisting of a single SWNT with a diameter of 1.6 nm. The source-drain current
decreases strongly with increasing gate voltage indicates that the NT device operates as a filed
effect transistor and also that the transport through semiconducting SWNT is dominated by
positive carriers [17]. The source of these carriers is possibly from the carrier concentration
inherent to the NT or these carriers can be the majority carriers that were injected at the goldnanotube contacts.
The CNFETs have several important differences from the conventional semiconductor
transistors. One of the difference is that carbon nanotube is one-dimensional which greatly
reduces the scattering probability [16]. As a result, the devices may operate in the ballistic
regime. Another difference is that the nanotube conducts essentially on the surface where are the
chemical bonds are saturated and stable [16]. The stable bond indicates that there is no need for
careful passivation of the interface between the nanotube channel and the gate dielectric. The
MOSFET devices do not have such an interface between the silicon/silicon dioxide interfaces. A
third difference is that the Schottky barrier at the metal-nanotube contact is the active switching
element in an intrinsic nanotube device [16]. The switching behavior involves mostly the
contacts as oppose to the bulk of the nanotube.
Recent research at IBM produced the highest nanotubes transistors that can output
perform leading MOSFETs [15]. Through experiments with different device structures, the
researchers achieve the highest transconductance, the measure of current carrying capability, of
any carbon transistor to date [15]. This high transconductance implies that the transistors can
operate faster. The IBM group also has produced large arrays of carbon nanotubes bypassing the
need to meticulously separate metallic and semiconducting nanotubes.
Logic circuits have been realized through the production of both n-type and p-type
CNFETs. The n-type CNFETs were generated by doping the p-type FETs with potassium. The
modification at the contacts introduces an increase of the barrier height for hole injection,
leading to an n-type CNFET. There is an intermediate stage in which the Fermi level is around
mid-gap. This intermediate stage leads to ambipolar behavior of the CNFET in which
conduction of both holes and electrons are possible, thus making it both n-type and p-type [16].
Complementary CNFETs are placed in circuit to operate simple logic functions. This kind of
nanotube-based circuit is the analogue to the conventional CMOS based logic gates and has the
same advantages. A simple example of this complementary CNFET is the bonding of two
ambipolar CNFETs. An offset voltage between the isolated transistor back-gates allows
adjustment of the threshold of the p- and n- CNFET characteristic so that the inversion function
is optimal [16].
Field Emission Displays
The unusual properties of nanotubes make them likely candidates for next-generation
display devices. The idea of a Field Emission Device (FED) is to add a high electric field to a
wire in order to produce electrons. The efficiency of generating electrons is dependant on the
strength of the electric field, and the geometry of the wire [14]. The field amplification increases
with a higher electric field and sharper radius. Carbon nanotubes exhibit both of these properties
(100 times the electric field strength and a diameter of ~1nm). Samsung has been developing a
Field-Emission flat display, and has demonstrated a prototype of a 9-inch full color display (576
x 242 pixels) in 1999 [14]. The structure of a SWNT based flat panel display is shown in
Figure12. Each pixel in this device is primarily composed of two glasses: an anode glass, which
is coated with a layer of indium-tin-oxide (ITO) phosphor, and a cathode glass that is coated with
SWNT [14]. The role of the SWNT in this device is to generate an electron source. Assembling
these pixels into a matrix creates the flat panel display [14]. Using the plasma-enhanced or
conventional CVD on top of substrates prepatterned with transitional metal catalysts yielded
highly aligned arrays of carbon nanotubes growing vertically to the surface of the substrate [8].
This device is only 2.4 mm in depth, has a brightness of 1800 cd/m^2, and has a low power
consumption (3.7 V) [14]. Error!
Figure 14 - Schematic structure of nanotube flat panel display [14]
Figure 15 - The Samsung 4.5” full-color nanotube display [14]
Additional Applications:
Nanotube sensors:
The electrical resistance of a semiconductor SWNT changes dramatically when exposed
to gaseous molecules [13]. This unusual property makes it useful for chemical sensors.
Nanotubes act as the wire between two electrodes so that changes in conductance can be
measured [13].
Figure 16 – Schematic of Nanotube sensor [13]
Lighting Elements:
Nanotubes are excellent electron sources, thus they are useful in the creation of light
elements. The electrons produced by nanotubes are used to bombard a surface coated with
phosphor in order to produce light. The brightness of this light is usually 2 times brighter than
conventional lighting elements (due to the high electron efficiency) [12].
Hydrogen Storage:
Another property of carbon nanotubes is their ability to quickly adsorb high densities of
hydrogen at room temperature and atmospheric pressure. The research group at the National
Renewable Energy Laboratory already confirms that SWNTs are capable of storing hydrogen at
densities of more than 63kg/m^3 [11]. Researchers have found that the interaction of hydrogen
and SWNT is between the Van der Waals force of the SWMT and the chemical bonds of the
hydrogen molecule (as opposed to being due to hydrogen dissociation) [10].
Figure 17- Temperature-dependent behavior of desorption of hydrogen [10]
Memory device:
Because of its ability to store information as a single electronic charge, nanotubes have the
potential to be used in the design of memory devices. A single electron is discrete, and thus
needs less energy in order to change the state of the memory. Such a design would also take
advantage of the high mobility of SWMT, which is ten times greater than that of silicon [11].
Figure 18 - Nanotube single-electron memory cell. The dark blocks at the top and the bottom are
the source and drain and the vertical dark line is nanotube [11].
Conclusion
The unique properties of carbon nanotubes have promoted research in the fabrication of
devices composed of carbon nanotubes. No commercial products made of carbon nanotubes are
available yet due to the fabrication limits. Improvements on current fabrication of carbon
nanotubes are necessary to increase the production of carbon nanotubes. Even though much
research has been completed on fabricating nanotubes, there is still little knowledge about carbon
nanotube growth mechanism. Scientists have only been able to make observations about the
growth of carbon nanotubes with different environmental conditions.
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