Preface
Recently, new physical properties reported in the nano scale structures. Nano-sciences
have great potential in various research areas such as madicine, environment, energy, and
etc. CNT has been great spotlight because of great potential of business and new physical
properties in nanostructures.
After Kroto and Smalley reported the Fullerene (C60) in 1985, Iijima, who worked in NEC,
reported CNT in nature in 1991. CNT is a hexagonal structures with sp2 hybridization and
several nanometer. Ebbesen and Ajayan reported CNT yield depending on Helium pressure
in 1992. Bethune (IBM) and Iijima (NEC) reported SWNT synthesis using the arc discharge
in 1993, and Smally reported a bundle SWNT (rope nanotube) using the laser ablation in
1996.
Ren reported high purity vertically aligned CNT synthesis using the plasma CVD on the
glass substrates in 1998. After their frontier research, many interesting CNT researches
have been reported recently both synthesis and devices.
Structures and physical properties of carbon nanotubes
Structures of carbon nanotubes
Having diameters of about one nanometer, carbon nanotubes are synthesized
when graphite sheets are rolled roundly and they show metallic characteristics or
semiconductor characteristics with the rolling angles of graphite sheets.
With wall numbers making up of carbon nanotubes, carbon nanotubes are classfied to
single-walled nanotubes, multi-walled nanotubes and rope nanotubes.
As it is shown in Figure 1, carbon nanotubes have two possible symmetric structures
known as zigzag and armchair types. Actually, most carbon nanotubes have chiral
structures where hexagons are aligned helically on tubular axis in stead of having those
structures mentioned above.
A simple way to classify each nanotube structure is a vector which connects tow points on
the graphene lattice and it is designated with Ch. Rolled to meet the two points of the
vector, the plane becomes a cylinder.
Figure 2(a) shows graphene layer designated by Dresselhaus’ way. In the figure, the
integer couple(n,m) expresses the possible structures of carbon nanotubes. Therefore, we
can designate vector Ch with n and m as follows.
Ch = na1 + ma2
a1 and a2 are the unit vector of graphene layer and n ³m. In zigzag tubes m=0 and in
armchair tubes, n=m values. The tubes different from the above conditions called “chiral”.
The electrical properties of carbon nanotubes are metallic or semiconductor periodically
as a function of diameter and chirality, and according to theoretical research a third of
SWNT is metallic and the other is semiconductor whose band gap is inversely proportional
to diameters of nanotubes. Generally when n-m=3q (where q is integer), (n,m) nanotubes
have metallic properties as it is shown in Figure 2(b). All armchair structure nanotubes and
about a third of zigzag structure nanotubes have metallic properties.
While general metals have plane DOSes (density of state), each peak of the nanotube DOS
has a lot of singularities in accordance with single quantum subband. These singularities
are important to analysis the experimental data of STS (scanning tunneling spectroscopy)
and resonant raman spectra.
Physics of carbon nanotubes
As we have dicussed, carbon nanotubes have the cylindrical structures which nanoscaled
graphite sheets are rolled roundly, and they are macromolecules having unique physical
properties with their sizes and shapes.
Even if many researching groups have preceeded a lot of experiments to exam the physical
properties of nanotubes on structures, we still have many problems to be resolved since
carbon nanotubes have various physical properties on their diameters, lengths and
chiralities.
In the next, we are going to describe electrical, thermal and mechanical properties, known
to public recently, of carbon nanotubes.
(1) The electrical properties
In 1998, Frank measured conductivity of carbon nanotubes with SPM (Scanning Probing
Microscopy) as dipping carbon nanotube into liquid mercury.
As a result, he reported that carbon nanotubes show quantum mechanical behaviors and
also have ballistic conductance. The conductivity of MWNT increased as much as 1 Go
whenever nanotubes were added into liquid mercury. The value of 1 Go is 1/12.9 kW-1 .
Sanvito et. al. measured conductivity of MWNt using scattering method and they
reconfirmed Frank’s results.
They also observed reduction of quantum mechanical conductive channel in the inside of
MWNT and rearrangement of electron flow of each carbon nanotube by interwall reactions.
Thess group found that the resistivity of rope shaped metallic SWNT is about 10-4 W-cm
measured with four-point probe at 300 K and it is considered to be a higher value than that
of high conductive carbon nano fiber. Frank et. al. and Avouris et. al. observed that the
stable current densities are 107 A/cm2 and 1013 A/cm2 respectively.
(2) Thermal properties
The carbon nanotube’s thermal conductivity is dependent on a temperature and mean free
path of phonons.
In 1999, Hone et. al. reported that the conductivity of a carbon nanotube has a linear
function of temperature : it has linear relationship at 7~25 K range, the gradient increases
at 25~40 K range
Where kz is thermal conductivity, C is the heat capacity, v is sound velocity. Hone et. al.
introduced at room temperature, the conductivity of a single-walled nanotube rope is
existing between 1,800 and 6,000 W/mK range.
In 1999, Goddard group numerically calculated that the conductivity of (10,10) nanotube
approaches to 2,980 W/mK as increasing applied current.
In 2000, Tomanek group researched the correlation between thermal conductivity and
temperature and it became a chance to reconfirm that at room temperature the thermal
conductivity is very high value of 6,600 W/mK, which Hone et. al. suggested, and they
theoretically verified that the value is caused by the very long mean free path of the
phonon.
However, Barber et. al. insisted that the correlation between thermal conductivity and
temperature should have different characteristics rather than linearity. Namely, they
reported it rises to the maximum value of 37,000 W/mK to 100K and it falls abruptly to
3,000 W/mK at 40 K.
(3) Mechanical properties
Recently, many research groups have studied the elastic behavior of SWNT in nanotube
field. Mostly, SWNT is from 10 to 100 times stiffer than steel and it also shows a
considerably strong endurance against physical shocks. When a force is given on a tip of a
nanotube, the nanotube bends without any damage and when the force is removed, the
nanotube goes back to its original state.
However, to quantityfy this phenomena is known as a very difficult problem. The research
teams of Princeton and Illinois in USA measured that the average Young’s modulus is 1.8
TPa in 1996. After they stood tubes randomly, the micro-photographs of tips were taken
and the Young’s Modulus was calculated from the blur quantity at various temperatures. In
1997 Goddard showed various Young’s Moduluses of nanotubes: (10,10) armchair
nanotube 640.30 GPa, (17,0) zizag nanotube 648.43 GPa and (12,6) chiral nanotube
673,93 GPa. These values were calculated from potentional’s two dimensional differential
coefficient and it is shown that they considerably differ from the 1.8 TPa calculated above.
In 1998, Treaty group reported the elastic modulus is 1.25 TPa and it is comparable to 1.28
TPa which was observed by Wong et. al. in MWNT in 1997.
They made a tip of a nanotube, which doesn’t adhere on a supporting structure, deviated
from the equilibrium state with an AFM and measured the modulus by recording the force
applied on the tip. In 1999, Rubio et. al. showed the Young’s modulus of SWNT depends on
its diameter and chirality through tight-binding calculation and also reported the
moduluses are 1.22 TPa for the (10,0) and (6,6) tubes and 1.26 TPa for the (20,0)
nanotube.
In above results, we can know that for SWNT, the elastic modulus is strongly dependent on
the diameter and structure. In the meanwhile, in 1999 Forro et. al. reported that for the
MWNTs, their diameters don’t influence to modulus considerably, but mostly the modulus
depends on their structural sides such as defects in nanotubes.
They also reported the SWNT bundles, whose diameters are 15~20 nm, have the modulus
of 100 GPa. They continue controversy about the Modulus and it is reported that this is
caused by the reason why the researchers have analyzed the thickness of nanotubes by
their own ways.
Generally, if carbon molecules have a cylinder shaped perfect solid state, modulus would
show smaller values than the values discussed in advance and we can imagine as
decreasing the wall thickness of tubular carbon molecules, the modulus would increase.
Synthesis of carbon nanotubes
The arc-discharge, laser vaporization, and pyrolysis methods have been reported
for the carbon nanotubes synthesis. After carbon nanotube synthesis, very complicate
purification process needs to get the high purity carbon nanotube, and also carbon
nanotube has been trouble with diameter and vertical growth control.
Recently, CVD method (chemical vapor deposition method) has been proposed to vertically
synthesize carbon nanotube, and also many different CVD methods have been reported
such as thermal CVD.
For various scientific functional application researches, many difficult technical problems
have to solve such as the low temperature synthesis, vertically aligned synthesis, large
scale synthesis, high purity synthesis methods. Recently, vapor phase (VP) growth method
has been reported for the large scale carbon nanotube synthesis. This method use the
reaction of both metal catalyst and carbon contained gas (C2H2, C2H4) in the reaction
chamber, but not use substrate. The VP method has a great technical benefit for the large
scale carbon nanotube synthesis. We introduce various carbon nanotube synthesis
methods in this section.
Arc-discharge
The figure 1 shows the cross-sectional view of a carbon arc generator that
can use to synthesize carbon nanotube. Two graphite electrodes have been used in this
system. Once the arc is in operation, a carbon deposit forms on the low temperature
negative electrode. This carbon contains carbon nanotube and carbon nanoparticle. In this
system, chamber is connected with both vacuum pump and ~several Torr He with ~several
ml/s flow rate for cooling purpose. High purity graphite usually use for the electrode.
Typically the positive electrode (anode) has 6 mm diameter, and negative electrode
(cathode) has 9 mm diameter.For the synthesis of high purity carbon nanotube, cathode
cooling requires. Because anode position does not fix, the distance between the anode and
cathode can fix constantly during the arc for carbon nanotube synthesis. Typical conditions
for operating a carbon arc for the synthesis of carbon nanotubes include the use of carbon
rod electrodes separated by ~1 mm with a voltage of 20-40 eV across the electrodes and a
dc electric current of 50-100 A flowing between the electrodes.
Laser vaporization
Smalley Group synthesized the carbon nanotube using the laser vaporization method.
The condensing vapor of the heated flow tube operated at 1200 ℃ in chamber, laser
pulse were used to evaporate a target containing carbon mixed with a small amount of
transition metal from target. Flowing argon gas sweeps the entrained nanotubes from the
high temperature zone to the water-cooled collector downstream, just outside the furnace.
Carrier gas used Ar and He with ~several hundred Torr. A carbon nanotube deposit forms
on the cooled collector mixing with both multi-wall carbon nanotube and carbon
nanoparticle. The target is mixed with catalyst Co/Ni/Fe instead of pure graphite, and the
uniform single wall carbon nanotube synthesized. The figure 5 showed synthesizing a
bundle of single-wall carbon nanotube.
Plasma Enhanced Chemical Vapor Deposition
The plasma CVD has a good advantage with the low temperature carbon nanotube
synthesis comparing with arc-discharge and laser vaporization synthesis methods. The
plasma method use a dc electric current or high frequency. Generally, plasma CVD use a dc
current, RF(13.56 MHz), and Microwave(2.47 GHz).
Thermal Chemical Vapor Deposition
Carbon nanotube synthesis on large area substrate has great advantage for the FED or
other various field emission display technology.
Recently, CNT synthesis on the large area substrate has been reported using the thermal
CVD. Thermal CVD has various advantages, which this method can use various sources,
high purity CNT can synthesize, and also can control CNT structures.
Vapor Phase Growth
CVD method uses the reaction of both metal catalyst film and reaction gas (C2H2, C2H4,
CH4, C2H6). VPG method does not use substrate, but makes reaction in the chamber
supporting both reaction gas and metal catalyst in chamber. This method proposes for
good advantage of large scale CNT synthesizing.In this system, when reaction gas
supports one side, the metal catalyst boat is in the other side. Chamber designed two
different temperature regions. Metal catalyst powder sets on the low temperature region,
and the CNT synthesized at high temperature region. Although low temperature region
does not decompose carbon gas, this region has good enough vaporization temperature of
metal catalyst. At low temperature, vaporized metal catalyst is atomic state, and than
metal catalyst becomes several nanometer particle after scattering with other metal
catalyst. CNT synthesized from the reaction between vaporized atomic state metal catalyst
reacts with carbon gas in high temperature region. Carbon nanotube field emission
characterization
There are two CNT FED research areas. Because CNT is vertically aligned on the substrate,
the first research area directly uses recent silicon tip or metal tip technology of the
semiconducting process. ETRI is using this method to get the high quality FED. The second
research area is a screen printing method mixing with CNT and conducting polymer.
Samsung Advanced Institute of Technology is using this method for the large area FED.
This method has good advantage of reducing a FED producing cost. Recently, Samsung
Advanced Institute of Technology developed the 15 inch full color FED. In this section, we
describe the emission characteristic of the vertically aligned CNT.
Applied technology of carbon nanotubes
The Applications of Emitters FEDs
The research on the applications of carbon nanotubes as the electron emitter and
FED(Field Emission Display) is one of the intensively researched areas in many countries. It
is expected that LCD (Liquid Crystal Display), LED (Light Emitting Diode), PDP (Plasma
Display Panel), FED (Field Emission Display), followed by the CRT (Cathode Ray Tube) that
can be used as a display device until now, will be used in the next high technological
electronic information periods. The FEDs have been highly attentions as the next
generation electronic information devices. The main technology of the FEDs is the based
on the polishing technology and stabilities of emitter tips.
The silicon tips and molybdenum tips have the large problems on the lifetimes and the
stabilities and the efficiencies on the electron emitions. Thus, there are serious attentions
on the use of carbon nanotubes as the emitter tips because of their high conductivities and
sharp tips.
The FED constructions using carbon nanotubes are initiated by De Heer after Smalley
confirmed the SWNT’s FED electron emitter capacities. Chang and et. al. constructed the
diode typed FEDs using screen printing with carbon nanotube conductive epoxy and
expressed the characters on the screen by the methods of controlling on-off with applying
pulse signals on each pixel. They also showed the possibilities of stable electron emission
in the low 106 torr. Saito and et. at. showed the possibilities of FEDs using MWNTs.
Samsung Advanced Institutes of Technology successfully developed the realization of
moving pictures on 9 inch FEDs using screen printing methods and showed the higher
brightness (1800 cd/cm2 at 3.7 V/μm) than conventional Spindt-type FED in the low
voltages.
The Applications of Carbon Nanotubes as the secondary batteries and Fuel
batteries
We expect many anticipated effects in the case of applications of carbon nanotubes as the
secondary batteries and fuel cell batteries. If we use the carbon nanotubes instead of
currently used hydrogen attached alloys, we can reduce the weight of the secondary
batteries severely and increase the effectiveness of charging batteries. Thus, carbon
nanotubes have the great possibilities of using secondary battery electrodes in automobile
batteries, charging batteries, notebook computers. The empty spaces of carbon nanotubes
are used for improving the capacity of hydrogen storage. Carbon nanotubes have
advantages in not only the light weight but also much space to store hydrogen so that
carbon nanotubes are outstanding for the charge storage per unite weight. It is expected
that carbon nanotubes fuel cell can be in the highlights as the replaceable energy source.
Rodrigueze and et. al. announced that the maximum hydrogen storage capacities of
herringbone structured carbon nanotubes are 67.55 wt % at the temperature of 298 K and
the pressure of 11.35 MPa. If this fact is confirmed, we expect enormous applications of
carbon nanotubes as the hydrogen storages. In the case of SWNTs, they stated that the
maximum hydrogen storage capacities of carbon nanotubes are 5-10 wt % in the
temperature of 133 K and the pressure of 0.040 MPa. It is considered that the low
temperature and low pressure conditions are the severe problems to overcome for the real
applications. For the references, the goal of Department of Energy (DOE) in U.S.A. is to
make electrical automobiles to consume 6.5 wt % and 65 kg H2/m3 per 500 km driving
distances.
The Applications of electron switching nano devices
The metallic and semiconductor properties of carbon nanotubes are controlled by the
diameters and the wound structures. It is expected that carbon nanotubes with several
tens nm diameters will replace the present silicon devices, developing Tera memory
devices. Dekker in university of Delft in Netherland constructed the single molecule
nano-devices at the room temperatures using SWNTs
In this device, the 1 nm carbon nanotubes with semiconductor properties is connected
between two metallic electrodes on the Si substrates sputtered with SiO2 with 400 nm
spaces. They confirmed that there are two kinds of carbon nanotubes showing different
current-voltage (I-V bias) characteristics according to the gate voltages. One is a metallic
carbon nanotube showing linear I-V bias characteristics without the relations of gate
voltages, and the other is a semiconducting carbon nanotube showing nonlinear I-V bias
characteristics with the highly affected relation of Gate voltages.
Figure 4 shows the I-V bias courves of CNT-based devices that indicate the semiconductor
characteristics and the switching efficiency is about 106 on/off ratio. Based on these
characteristics, it is considered that the applied Field-Effect Transistor (FET) technologies
using carbon nanotubes have advantages not only on device execution speed, but also on
the minute size. Recently it is announce that professor Im-Ji Sun and et.al. developed 10
nm size triod type carbon nanotube transistors through the cooperative research with
University of California at Berkeley in U.S.A. It is revaluated that this technologies reduce
the currently used 256 MDRAM semiconducting devices to 1 over 10 thousand sizes and we
expect the appearance of Terabit Dram in the future.
The Mechanics and the applications of high functional composites
Carbon nanotubes can be used in the tips of SPM, STM and AFM using the excellent
electronic transport and mechanical strength. Now, the current AFM tips consisted of Si or
SiN materials have disadvantages due to the limitation in the observation of minute defects
inside samples. In the case of the use of carbon nanotube tips, these disadvantages can be
solved. It is anticipated that the minute sized carbon nanotubes can be applied to the nano
connections of nano systems, the nano pipes, the nano liquid injection systems, the gas
sensors employed the gas adhesion properties of carbon nanotubes, and the applications
of the medical system devices used the affinity between carbon and biological tissues.
Especially, Liber and et. al. suggested the possibilities of special sensors detecting special
applicants on the surface of materials. These carbon nanotubes visualize the chemically
patterned surfaces with various molecules, oxidizing and deleting the tips of carbon
nanotubes, and attaching carboxyl functionals on the tips. Carbon nanotubes are also
applied in observing the correspondence reactions of liquid ?receptors in biological and
chemical ways.
On the other side, it is expected that the applications of high functional composites will
give a great affection on the whole industrial areas.
As the existing carbon black and carbon fibers are employed for the purpose of the
electron transport of high molecule composite, recent research is in progress on the
composite materials for the purpose of optoelectronic applications using high electron
transport of carbon nanotubes.
Until now, it is considered that the carbon nanotube composite research had a limitation
because of the high manufacturing expense. However, the carbon nanotube composite
research will be boosted due to the publications of the low cost of carbon nanotube
synthesis. Future Prospect
It has been much progress since Iijima discovered the carbon nanotubes in 1991.
There are large advancements in large synthesis methods, purification methods, the
synthesis methods of SWNTs and MWNTs, the vertical synthesis methods and the insertion
methods of different materials after deleting the end-caps. However, much research of
carbon nanotubes synthesis and applications is required because of the characteristics of
complexities, varieties and minuteness of nanosystems. Carbon nanotube synthesis has
many problems to be solved in not only the current technologies, but also the structures,
structure controls, large area synthesis, and low temperature synthesis technologies of
carbon nanotubes since the transitions from electric arc discharge methods and laser
depositions to CVD methods in 1998.
Many scholars confirmed theoretically and experimentally that carbon nanotubes have the
excellent mechanical and electrical properties. It is expected that much progress will be the
application research of carbon nanotubes such as the applications of emitters and displays,
secondary batteries and fuel cell batteries, the nano-devices and nano-systems, and high
technical complexities because of the excellent properties of carbon nanotubes.
Especially, it is anticipated that the activation of the synthesis and application research of
carbon nanotubes contributes the technological advancements of next generation plane
displays through the rapid developments of the high technological electronic
communication industries.
There are many undeveloped research areas of carbon nanotubes even though one and
more journal papers about carbon nanotubes are published and much research is in
progress in carbon nanotube research.
It is anticipated that we will have the international competitiveness in the carbon nanotube
synthesis and application research if we concentrate our research into the carbon
nanotube research areas because the research groups of foreign countries are in just early
stages in carbon nanotube research.