NANOTUBES *
CHAPTER 7 CHAPTER 7
7.5 APPLICATIONS OF CNTs
7.5.1 EXISTING APPLICATIONS .1 CNT composites
Outstanding mechanical properties of CNT have sparked great interest in their use as structural materials.
CNTs can be used as reinforcing fibers due to their strength and ultrahigh aspect ratios. Emphasis has been put on polymer composites with a number of commercial CNT reinforced resins already available on the market. Achieving proper dispersion and suitable CNTmatrix bonding are the two major ongoing challenges in this area of research. Dispersion is much more complex than simply mixing a powder of NTs into the liquid matrix material because CNTs tend to adhere together after purification. Functionalization of the tubes and the use of surfactants in combination with sonication are some of the methods developed
Table 7.3 Summary of the Major Production Methods of NTs
Method Arc Discharge Method Chemical Vapor Deposition Laser Ablation Pros Can easily produce SWNT
and MWNT. SWNTs have few structural defects;
MWNTs without catalyst, not too expensive
Easiest to scale up to industrial production; long length, simple process, SWNT diameter controllable, quite pure
Primarily SWNTs, with good diameter control and few defects. The reaction product is quite pure
Cons Tubes tend to be short with random sizes and directions; often needs a lot of purification
NTs are usually MWNTs and often riddled with defects
Costly technique, because it requires expensive lasers and high power requirement, but it is improving
From Daenen, M., et al. (2003). The wondrous world of carbon nanotubes. Eindhoven University of Technology.
(Continued)
155 7.5 APPLIcATIONS OF cNTs
to control dispersion. Typical CNT polymer composites experience fiber pullout under low loads and do not achieve high strengths and so research is ongoing in this area (Baugham et al., 2002).
Advances have also been made in development of CNTmetal and CNTceramic composites.
Complications arise due to the higher temperatures needed to sinter the matrix materials. However, CNTaluminum composites do not seem to experience the carbide formation seen in carbon fiber com
posites. A strengthened alumina ceramic is made by adding alumina/ethanol slurry to CNT dispersed in ethanol. The resulting powder is then sieved and ball milled to produce evenly distributed CNT fol
lowed by spark sintering. The initial stages produce evenly distributed CNT, while the sintering method ensures a fully dense material while maintaining nanometric grain sizes in the alumina and avoiding damaging the CNT (Kuzumaki et al., 2002; Ma et al., 1998).
7.5.1.2 Field emission displays
Research on electronic devices has focused primarily on using SWNTs and MWNTs as field emission electron sources for flat panel displays, lamps, gas discharge tubes providing surge protection, and Xray and microwave generators. As a result of the small radius of the nanofiber tip and the length of the nanofiber, a potential applied between a CNTcoated surface and an anode produces high local fields that cause electrons to tunnel from the NT tip into the vacuum. Electric fields direct the fieldemitted electrons toward the anode, where a phosphor produces light for the flat panel display application as shown in Fig. 7.9. NT tip electron emission arises from discrete energy states, rather than continuous electronic bands emitted from ordinary bulk materials. In addition, the emission behavior depends
FIGURE 7.9
(A) Schematic illustration of a flat panel display based on cNTs. ITO, indium tin oxide. (B) Scanning electron microscopy (SEM) image of an electron emitter for a display, showing well-separated SWNT bundles protruding from the supporting metal base. (c) Photograph of a 5-in. (13-cm) NT field emission display made by Samsung.
From Baugham et al., 2002. Carbon nanotubes—the route toward applications. doi:10.1126/science.1060928.
156 CHAPTER 7 NANOTUBES
critically on the NT tip structure: enhanced emission results from opening NT tips. Compared to tung
sten and molybdenum tip arrays, NT fieldemitting surfaces are easier to manufacture and require less vacuum for operation. NTs provide stable emission, long lifetimes, and low emission threshold poten
tials (Baugham et al., 2002).
Flat panel displays are one of the most promising applications being developed by industry.
However, their commercialization is currently hindered by technical complexity of the device requir
ing concurrent advances in electronic addressing circuitry, the development of lowvoltage phosphors, methods for maintaining the required vacuum, spacers withstanding the high electric fields, and the elimination of faulty pixels. The advantages of NTs over liquid crystal displays are low power con
sumption, high brightness, a wide viewing angle, a fast response rate, and a wide operating temperature range. Samsung has produced several prototypes including a 9in. (23cm) redbluegreen color display that can reproduce moving images (Baugham et al., 2002).
NTbased gas discharge tubes may soon find commercial use for protecting telecommunication networks against power surges. NTcontaining cathodes separated from an anode by a millimeterwide argonfilled gap provided a 4 to 20fold improvement in breakdown reliability and a 30% decrease in breakdown voltage, as compared to commercial devices. When the phosphorescent screen at the anode in a field emission device is replaced by a metal target and the accelerating voltage is increased, Xrays are emitted instead of light. The resulting Xray source has provided improved quality images of biological samples. The compact geometry of NTbased Xray tubes suggests their possible use in Xray source arrays for medical imaging, possibly even for Xray endoscopes for medical exploration and microwave generation (Baugham et al., 2002).
7.5.1.3 Nanoelectronic devices
Radically different device materials, architectures, and assembly processes are developed in order for electronic circuits to continue to shrink by orders of magnitude and provide corresponding increases in computational power. Dramatic recent advances have fueled speculation that NTs will be useful for downsizing circuit dimensions. Currentinduced electromigration causes conventional metal wire interconnects to fail when the wire diameter becomes too small. The covalently bonded structure of CNTs militates against similar breakdown of NT wires, and because of ballistic transport, the intrinsic resistance of the NT essentially vanishes (Baugham et al., 2002).
In nanotube fieldeffect transistors (NTFETs), gating is achieved by applying a voltage to a sub
merged gate beneath an SWNT (as shown in Fig. 7.10A and B) which is contacted at opposite NT ends by metal source and drain leads. The transistors are fabricated by lithographically applying electrodes to NTs that are either randomly distributed on a silicon substrate or positioned on the sub
strate with an atomic force microscope. It is possible to selectively peel outer layers from an MWNT until an NT cylinder with the desired electronic properties is obtained as shown in Fig. 7.10C (Baugham et al., 2002).
Great advances have been made in research toward nanoscopic NTFETs, which aims to replace the sourcedrain channel structure with an NT. The approach is to construct entire electronic circuits from interconnected NTs. A diode has to be produced by grafting a metallic NT to a semiconducting NT as shown in Fig. 7.11. The electronic properties depend on helicity, which can be tailored on both sides to produce a kinked structure. A revolutionary advance for nanoelectronics would be the development of rational synthesis routes to multiply branched and interconnected lowdefect NTs with targeted helicity (Baugham et al., 2002).
FIGURE 7.10
Nanoelectronic devices: (A) Schematic diagram for a carbon NT-FET. The semiconducting NT, which is on top of an insulating aluminum oxide layer, is connected at both ends to gold electrodes. The NT is switched by applying a potential to the aluminum gate under the NT and aluminum oxide. Vsd, source-drain voltage; Vg, gate voltage. (B) Scanning tunneling microscope (STM) picture of an SWNT field-effect transistor made using the design of (A). The aluminum strip is overcoated with aluminum oxide. (c) Image and overlaying schematic representation for the effect of electrical pulses in removing successive layers of an MWNT, so that layers having desired transport properties for devices can be revealed.
From Baugham et al., 2002. Carbon nanotubes—the route toward applications. doi:10.1126/science.1060928.
FIGURE 7.11
Nanoelectric device: STM image of an NT having regions of different helicity on opposite sides of a kink, which functions as a diode; one side of the kink is metallic and the opposite side is semiconducting.
From Baugham et al., 2002. Carbon nanotubes—the route toward applications. doi:10.1126/science.1060928.
158 CHAPTER 7 NANOTUBES
7.5.1.4 Hydrogen storage
NTs have been in a lengthy development process as potentially useful material for hydrogen storage for fuel cells. However, there has been a lot of controversy regarding claims of high hydrogen stor
age levels that have been shown to be incorrect which await confirmation. More recent research has suggested that CNTs are unlikely to be effective storage devices (Baugham et al., 2002; Makar and Beaudoin, 2003).
7.5.1.5 Sensors and probes
Because NT electronic transport and thermopower (voltages between junctions caused by interjunc
tion temperature differences) are very sensitive to substances that affect the amount of injected charge, chemical sensor of nonmetallic NTs hold great potential for stateofthe art sensor applications. The minute size of the NT sensing element and the correspondingly small amount of material required for a response are the main advantages of CNTs. However, major challenges remain in making devices that differentiate between absorbed species in complex mixtures and provide rapid forward and reverse responses. CNT scanning probe tips for atomic probe microscopes are now commercially avail
able from Seiko Instruments. The mechanical robustness of the NTs and the low buckling force dramat
ically increase probe life and minimize sample damage during repeated hard crashes into substrates.
The cylindrical shape and small tube diameter enable imaging in narrow, deep crevices and improve resolution in comparison to conventional nanoprobes, especially for high sample feature heights.
Mapping of chemical and biological functions is enabled by covalently modifying the NT tips, such as by adding biologically responsive ligands. Nanoscopic tweezers that may be used as nanoprobes for assembly are driven by the electrostatic interaction between two NTs on a probe tip (Baugham et al., 2002).