Carbon Nanotubes: History Edit

Since their first presentation to the scientific world in 1991 by Iijima [13], at the Japanese NEC Corporation, carbon nanotubes (CNTs) have attracted the attention of specialists from different domains. Research on carbon nanotubes was greatly stimulated by this first scientific paper on the observation of nanoscale carbon tubes [13] and, subsequently, theoretical and simulation works have been conducted to understand this nanoscale material and related phenomena [14].

A series of publications followed with observations on the technological conditions required for the synthesis of large quantities of nanotubes [15,16]. The emergence of these early studies led to the intensification of investigation, the results highlighting that carbon nanotube belong to a family of fullerenes structures and can be seen as hollow cylinders, rolled-up graphitic layers into cylinders [17] except at the ends. Carbon nanotubes can be formed by one or more graphite layers, under the form of hexagonal networks of carbon atoms bonded in the sp2 hybridization state [18] except that, in some cases, tube diameters are small enough to present the effects of a one-dimensional (1D) periodicity [19,20].

By discovering the preparation methods of single-wall nanotubes [19,21,22] it is now possible to test and verify predictions and theoretical calculations previously performed. After rolling the graphene sheets, a rearrangement of the marginal carbon atoms to form the nanotube caps takes place. There are cases where the nanotube does not appear closed at the ends. In this case, edge effects are manifested by an increased chemical reactivity of atoms in the bond formation with different radicals.

Depending on the arrangement of the graphene cylinders, there are three types of nanotubes: single-walled nanotubes (SWCNTs), double-walled nanotubes (DWCNTs), and multi-walled nanotubes (MWCNTs). In the case of SWCNTs they have approximately 1 nm diameter and are typically 1–100 microns in length.

In theory, SWCNTs are obtained by twisting graphene sheets having honeycomb-distributed carbon atoms (Figure 1) [14]. Their geometrical structures which are uniquely specified by a pair of chiral indexes (n,m) are directly associated with their electronic properties. Depending on the orientation of the graphene lattice with respect to the tube axis they are twisted, three typical types can be obtained: armchair (n,n), zigzag (n,0), and chiral (n,m). If n-m is divisible by 3, the SWCNT present metallic behavior, otherwise they present semiconductor behavior [23].

Figure 1. The imaginative process of forming a SWCNT by rolling a graphene sheet in different directions.

Of particular importance to the properties of carbon nanotubes are the many possible geometries that can be made on a cylindrical surface without introducing stress factors into the carbon nanotube. For 1D system, on a cylindrical surface, symmetry with a screw shape axis can affect the electronic structure and associated properties. Nearly exotic electronic properties of 1D nanotubes are predominantly resultant of band structure of SWCNTs [24], intra-wall interactions between multiple layers within the same single nanotube (for MWCNTs) [25] rather than between two different nanotubes.

This interesting structure provides them with unique electrical, mechanical, physical, optical, and chemical properties coupled with a high aspect ratio. Those properties are summarized in Table 1 [4–6,14,18].

Table 1. CNTs properties

Electrical

- semiconducting; metallic

- high conductivity

- current carrying capacity: ≈ 1 TA/cm3 [5]

Mechanical

- tensile strength: 75 GPa (SWCNTs), 150 GPa (SWCNTs) [14]

- Young’s modulus: 1054 GPa (SWCNTs), 1200 GPa (SWCNTs) [14]

- diameter: 0.4 to >3 nm (SWCNTs); 1.4 to > 100 nm (MWCNTs) [5]

- density: 1.3 g/cm3 (SWCNTs); 2.6 g/cm3 (MWCNTs) [14]

- strength / weight ratio 500 times greater than aluminum

Thermal

- thermal conductivity: 0.2 kW/mK to 6 kW/mK [14]

- specific heat: 0.3 mJ/gK (SWCNTs) to 10 mJ/gK (MWCNTs bundle) [14]

- thermoelectric power (at room temperature): 280 µV/K (semiconducting SWCNTs) [14]

Chemical

- chemical and biological stability obtained by functionalization

- stability in solvent, acids, and bases

Optical

- light affects conductivity

- field emission tip generates X-ray

- IR detection/emission possible

Owing to these superior material properties, CNTs suggest that their use as very sensitive sensing elements in the sensors domain will allow for further designing and development of measurement devices, with superior characteristics to others of similar size. Their high mobility and ballistic transport characteristics, for example, make them serious candidate for the replacement of Si in future devices, especially when miniaturization—as one of the solutions for improved performance—is becoming more and more difficult. Field effect transistor (FET) compatibility and small intrinsic capacitance for possible operation at terahertz frequencies are advantages over Si technologies of similar design [26]. Challenges are related to the reproducibility of properties from one device to another and to their homogeneous growth, without defects, with the desired orientation and the necessary length.

Their mechanical properties—including high strength, high rigidity, and low density—make them highly attractive for various applications by controlling the band structure and thereby modifying the electronic transport properties. Most importantly, this can be achieved reversibly, opening the way to the vast possibility of designing electromechanical sensors, high current field effect transistors, and low resistance interconnects in electronic devices [27].

However, there is currently no transducer available on the market, that incorporates a sensing device capable of exploiting the superior properties of carbon nanotubes due to the fact that nanoscale manufacturing processes are still very costly and technologically advanced.

Although many companies have activities in place related to the integration of carbon nanotubes in their currently manufactured products, the products offered on the market are still in their young phase of the life cycle. To be at the cutting edge of the nanotechnologies, due to the globalization of the market, it is a recipe for future achievements and for preservation of market share. With the notable domains of coatings, composites, and energy, there are only few companies from microelectronics selling CNT-based products [6]. There is a strong market expectation from the domains of transparent conductors, thermal interfaces, wind turbine blades, and antiballistic equipment to be areas of excellence following the implementation of carbon nanotubes.

Historically, in the field of electronics components and electronics manufacturing, the technological advance caused by penetration into the micro-scale constituted a significant development step. It was obvious that smaller electronic devices would mean using less space, better portability, and most importantly, saving materials by using as little as possible. On the other hand, the materials used must be of particular purity, since even in small numbers the impurities can represent a significant percentage of the amount of the used material.

As technology has advanced, the possibility of building small elements has grown, and through the intensive use of carbon nanotubes, science enters the nanometer scale era.

In addition to the dimensional differences between the two size scales—micro and nano—there are other considerations that need to be considered for the choice of manufacturing processes. Physical handling is still possible in these dimensions even if it requires precision that is difficult to achieve even with the help of very expensive equipment, such as a powerful microscope. However, in particular, it is even more complicated to physically move, assemble, or modify objects at a nanoscale using standard microfabrication equipment.

For the design and development of nanometer-scale devices, it is essential to discuss and understand what steps are required to successfully manufacture and characterize a device at such a size. One of the most important differences between how ordinary objects (millimeter) and nano-scale objects behave is determined by the forces that control the state of matter. On a regular scale (meso-scale) the dominant force is gravity together with the friction force. In the micro-scale field, dominant forces are surface forces. These surface forces include static friction, friction, electrostatic forces, and van der Waals forces. At the nanometric scale, the main forces are intermolecular and atomic forces, the effects of which are often neglected in meso-scale analyzes. It is therefore important that further research be directed to understanding the hypotheses that can be used to accurately determine the behavior of ultra-small devices.

Conventional assembly (meso-scale) processes are very simple and involve either the use of the operator's hand or automated manufacturing equipment. By contrast, micro-scale manufacturing processes require the use of very precise equipment, equipment that cannot be used in the nano-scale field. At the nano-scale, the product manufacturing process requires more complicated steps, including self-assembly and processes that involve direct handling of materials for example.

At present, cost is an important issue in the development of nanotechnology. ‘Nanomanufacturing’ processes involving individual CNTs, as defined and used by U.S. National Nanotechnology Initiative (NNI) [28], still produce a very small number of products that are sent to the market due to both the lack of specific manufacturing methods, the precision measurement solutions of the physical dimensions and also due to the lack of necessary commercial infrastructure. For example, carbon nanotubes are produced/grown by using multiple technologies, but the final products where they are employed are not promoted on the market in a consistent manner. Most products using CNTs today incorporate CNT powders dispersed in polymer matrices [6]. The reasons are also given by the synthesis processes, which are costly and slow, a fact which currently represents a limitation in the development of nanotechnologies. Even if manufacturing prices are expected to become competitive, it is still too early to discuss the marketing of CNT-based nanomaterial technologies.