Applications of Carbon Nanotubes

Carbon nanotube after its discovery has attracted overwhelming number of applications and newer applications are being sought due to the unusual properties it displays in the field of engineering materials. As a result of the newness of carbon nanotube technology, most of the potential applications discussed in this section are closely related to its obvious physical properties. Few of such selected potential applications include electrical transport, hydrogen adsorption, catalysis, artificial muscles, mechanical reinforcement, fuel cells, field emission and computers. It is expedient to mention however that this list does not in any capacity exhaust the uniqueness of carbon nanotubes.

10.6.1 Electrical Transport of Carbon Nanotubes for FET

Transport characteristics of carbon nanotubes have shown a tremendous impact in the microelectronic technology, especially in nanoscale electronic devices. The transport measurements of nanotubes follow different patterns depending on the nanotubes package as a sample such as a SWNT, a single rope or bundle of SWNTs, a single bundle of MWNTs or a single MWNT. The extremely thin nanotube is characterised as almost perfect one-dimensional conductor that behaves as single electron charging, resonant tunnelling at specific energy level, as well as proximity induced superconductivity at low temperatures. While the one-dimensional Luttinger liquid phenomenon is attained at relatively high temperatures, whereby tunnelling conductance into these tubes obey the power law suppression as a function of temperature and bias voltage. In an electrical resistance experiment involving a single MWNT at temperature as low as 20mK and zero magnetic field, a logarithmic conductance decrease was observed with decreasing temperature and saturation less than 0.3 K (Langer et al 1996). But two-dimensional weak localisation was observed when a positive magnetoresistance was obtained in the presence of a magnetic field that is perpendicular to the axis of the nanotube.

In the comprehensive review on carbon nanotube by Popov a number of electrical transports of carbon nanotubes results were presented (Fig. 10.10). Such as the one with bundles of SWNTs with bridging contacts separated by 200-500 nm where the I/V curve exhibits strong suppression of conductance near zero voltage for temperature less than 10K (Fig. 10.10(a)). The linear response conductance, G of the bundle as a function of the gate Vg showed peaks separated by regions (Fig. 10.9(b)) of very low conductance (Bockrath et al 1997). Conductance of a single SWNTs on a two-probe measurement showed metallic with resistance of tens of kiloohms, while a tunnel barrier of high resistance about 1 MO was observed (Tans et al. 1997). Here, the I/V curves showed a clear gap of about zero bias voltage and at higher voltages, the current increases stepwise as presented in Fig. 10.9(c). The gap change with Vg around zero bias voltage signifies Coulomb charging of the nanotube, which is a peculiar metallic characteristic of carbon nanotubes.

Fig. 10.10. Electrical transport characteristics (a) I/V curves of a 12 nm diameter bundle of 60 SWNTs (Bockrath et al 1997), (b) conductance, G against gate voltage Vg at temperature of 1.3 K with peak spacing of about 1.5 V, (c) I/V curves of a single SWNTs at varying gate voltage Vg (Tans et al. 1997).

Fig. 10.10. Electrical transport characteristics (a) I/V curves of a 12 nm diameter bundle of 60 SWNTs (Bockrath et al 1997), (b) conductance, G against gate voltage Vg at temperature of 1.3 K with peak spacing of about 1.5 V, (c) I/V curves of a single SWNTs at varying gate voltage Vg (Tans et al. 1997).

In the carbon nanotube transport measurements by Tans et al (1998), a single semiconducting nanotube was contacted by two Pt electrodes on a SiO2 layer over a Si substrate (Fig. 10.11(a)). It exhibited field - effect transistor characteristics, when voltage was applied to the gate electrode, and the nanotube was switched from a conducting to non-conducting acting as an insulator, which was also tested at room temperature. This observation implies great application potentials in the microelectronics of transistors. Fig. 10.11(b) (Tans et al. 1998) presents typical I/V curves of the nanotube FET, where varying the gate voltage from positive to negative, the curves changed from large gap of insulating non-linear to strongly metallic characteristics. The phenomenon has been explained with the current conventional semi-classical band-bending models (Popov 2004).

Fig. 10.11. Carbon nanotube base fields effect transistor (FET) (a) schematic of the device (b) I/V curves of the FET for different values of the gate voltage Vg (Tans et al. 1998)

10.6.2 Computers

An obvious objective in modern computer development is the increase of number of switches while reducing size and increasing storage capacity. Thus the extremely thin, light weight nanowire of carbon nanotube, which has been displayed to exhibit metallic, semiconducting and insulating characteristics, becomes excellent materials for smaller switches and smarter computer chips. Its superiority over the conventional metal wire such as copper is reflected in its low heat resistance even at diameter of 2 nm, whereby large currents can be carried at different interconnections without heating or melting. Carbon nanotubes possess very high thermal conductivity, which make them good heat sinks, making it possible to transfer heat rapidly away from the chips. Similar to Tans et al (1998) work discussed above, it has been observed in a carbon nanotube based gold electrodes gate FET device (Poole and Owens 2003) that a small voltage applied to the gate changed the conductivity of the nanotube up to a factor greater than 106. This magnitude is comparable to the conventional silicon FETs, and the switching time of the device was estimated to be very fast at the clock speeds of a tetrahertz, almost 104 times faster than the present processors. One of the several models proposed for the physical description of current transport in carbon nanotube (CNT) based FET (CNTFET) is the vertical transistors concept that allows higher packing densities because the source and drain areas are arranged on top of one another. Thus a three-dimensional structure where the active devices are not bound to the surface of the usual mono-crystalline Si wafer was simulated from a 1 nm diameter, 10 nm long SWNT with a coaxial gate and a gate dielectric with 1 nm oxide are presented in Fig. 10.12 (Hoenlein et al. 2003). In their report, recently published works of two p-type CNTFETs were compared with three silicon MOS-field-effect transistors (Si-MOSFET) as shown in Table 10.3. The parameters of the CNTFETs were much better than the best ones for the MOSFET devices. This is thus a clear indication of the wonderful performance of the future CNTs based computers and other microelectronics.

1 -d scaling 2-d scaling 3-d scaling

Fig. 10.12. Three-dimensional modelling of vertical CNTFETs (Hoenlein et al. 2003) for better performed chips and switches

1 -d scaling 2-d scaling 3-d scaling

Fig. 10.12. Three-dimensional modelling of vertical CNTFETs (Hoenlein et al. 2003) for better performed chips and switches

Table 10.3. Performance comparison of selected CNTFETs with Si-MOSFETs

Parameters

P-

P"

MOSFET

MOSFET

MOSFET

CNTFET

CNTFET

1x10"4 nm

10 nm

14 nm

1.4x10"

3x10"3nm

(1.5 V)

(1.2 V)

(0.9 V)

3nm (1V)

(1.2 V)

Drive current Ids

2.99

3.5

1.04 nFET

0.450 nFET

0.215

(mA/^m)

0.46 PFET

0.360 PFET

pFET

Transconductance

6666

6000

1000 nFET

500 nFET

360

(^S/Am)

460 PFET

450 PFET

pFET

S (mV/dec)

80

70

90

125 101

71

On-resistance

360

342

1442 nFET

2653 nFET

4186

(fi/Am)

3260 PFET

3333 PFET

pFET

Gate-length (nm)

1400

2000

130

10

14

Normalised gate-

80/1 = 80

25/8 =

4/2 = 2

4/1.7 = 2.35

4/1.2 =

oxide (1/nm)

3.12

3.33

Mobility(cm2/(Vs))

1500

3000

-

-

-

Ioff (nA/^m)

-

1

3

10

100

10.6.3 CNT Nanodevices for Biomedical Application

The numerous properties of carbon nanotubes (CNTs) as metals, semiconductors, electron field emitters and electromechanical actuators (often known as artificial muscles) have made them extremely valuable as well as provide a great future for biomedical applications such as microsurgical and diagnostic devices, artificial limbs, implants like artificial ocular muscles, hearts, etc. This section therefore discusses briefly future applications of CNTs in artificial muscles and X-ray generation by CNT-based field emission in the biomedical laboratory.

10.6.4 X-Ray Equipment

X-ray machines generate exceedingly high frequency, short wavelength, high-energy electromagnetic waves that penetrate the body during medical diagnostic and therapeutic practices. X-ray equipment has been in use for several years to obtain tissue photographs of tumour, skeletal fractures, deformations, etc. Although they have been part of quality and modern medicine, however, they could be extremely dangerous whether in a short or long term exposures thereby causing nausea, vomiting, dizziness, sterility, burns, genetic mutations, cancers, and death if used incorrectly or in excess (Carr and Brown 1998). The conventional thermionic cathodes x-ray tube has a metal filament that is resistively heated to temperature over 10000C to emit electrons, which is in turn are targeted and bombarded on a metal anode to emit x-rays. The high temperature requirement in thermionic cathode is a limitation that is not experienced with the field emission mechanism because electrons are emitted at room temperature and controllable voltage. Nevertheless, field emission faces another inherent bottlenecks since most diagnostic applications require tube current in the order of 10-100 mA and 30-150 KV operating voltage, which are almost impossible to attain for the field emission x-ray device. In an effort to circumvent these problems, Cheng, Zhou and co-workers (Yue et al. 2002) developed a CNT-based x-ray tube that could emit sufficient x-ray flux for diagnostic imaging and photography (Fig. 10.13). The device, presented in Fig. 10.13(a) consisted of a field emission metal cathode coated with SWNTs, and the gate electrode was a tungsten mesh that was 50-200|im distance from the cathode.

Fig. 10.13. X-ray device by SMNTs-based field emission experiments at University of North Carolina (a) schematic of the CNT-based field emission x-ray emitter (Yue et al. 2002), (b) X-ray image of humanoid fingers (Cheng Zhou 2003)

Applying a relatively low voltage between the gate and the cathode produced electrons from the cathode, which were accelerated and bombarded on a copper target to produce x-ray beam through the Be window in a high voltage applied between the gate and the anode. They observed that 30mA emission current from a relatively small carbon nanotube cathode, and release x-ray wave forms that could be programmable pulse and repetitive in rate. Interestingly, the x-ray flux produced was sufficient to image humanoid fingers shown in Fig. 10.13(b) (Yue et al. 2002). Some obvious advantages of the CNT-based x-ray device proposed over the usual thermionic x-ray tube could include:

• Prolonged life span of the x-ray tube,

• Significant size reduction of the x-ray device for industrial or medical applications,

• Focus electron beam with programmable pulse width and repetition rate,

• Low temperature cathode tube.

10.6.5 CNTs for Nanomechanic Actuator and Artificial Muscles

Single walled carbon nanotubes have been known to deform when electrically charged. This implies that CNTs possess the characteristics of an actuator, which convert electrical energy to mechanical energy, or vice versa. In 1999, Baughman et al (1999) used "bucky" paper made as sheets of bundles of SWNTs, otherwise referred to as "artificial" muscles to demonstrate CNTs characteristics as actuators (Vohrer et al. 2004). The explanation given for the actuation properties was a double layer charge injection that produces dimensional changes in covalently bonded direction for conjugated CNTs materials (Sun et al. 2002). The nanostructure of the artificial muscles observed with scanning electron microscope (SEM) is shown in Fig. 10.14(a), where the CNTs are tightly packed into bundles with most of their axes in horizontal planes, and consisting 3 by 20 mm strips of 25-50 |im in thickness. In demonstrating the actuation properties of the artificial muscles, two of the CNTs strips were bonded together with a double-stick Scotch tape in to sheets (Fig. 10.14(b)).

Fig. 10.14. Actuation properties of artificial muscles (bucky paper) (a) SEM image showing the nanostructure, and (b) actuation characteristics of the artificial muscles displaying positive or negative deflection when voltage was applied (Baughman et al. 1999)

Fig. 10.14. Actuation properties of artificial muscles (bucky paper) (a) SEM image showing the nanostructure, and (b) actuation characteristics of the artificial muscles displaying positive or negative deflection when voltage was applied (Baughman et al. 1999)

The sheets were placed in a 1M sodium-chloride electrolyte solution. Upon applying few volts as shown in Fig. 10.13(b), about 1 cm deflection was obtained and changing the polarity of the voltage could reverse the phenomenon. Because the response of the device is dependent on the expansion of opposite electrodes and oscillation of cantilever could be produced when an AC current is applied, making it a bimorph cantilever actuator (Poole and Owens 2003). Moreover, since individual bundle of CNTs would behave in the same pattern, then employing only three filaments would create a truly miniaturised or nanosized actuators that could revolutionise the microelectromechanical systems (MEMSs) and nanoelectromechanical systems (NEMSs) technologies, especially in biomedical applications where nanotechnology would promise a great future. A number of CNT-based nanodevices have been proposed in the literature, with actuation effects such as nanotube tweezers as CNT scanning tunnelling microscope (STM) tip, attached to the cantilever arm of an atomic force microscope (AFM), simulated nanogears, etc.

Anode

Cathode

Anode

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