Chapter 2 Literature Review
Through the achievements of the pioneers and other procedures, many research records about the CNTs applied in field emission can be found in a readable literature. In fact, there are many review paragraphs appeared in papers, reports, theses and CNTs handbooks.
However, the aim of this chapter is to provide a basic understanding about the field emission related subject, and the material aspect of CNTs.
2.1 Introduction to Carbon Materials
Carbon is the sixth element of the periodic table and is listed at the top of column IV.
Carbon materials can be found in various forms as diamond, graphite, carbon fibers, fullerences, and CNTs. Crystal structures of these carbon materials are shown schematically in Fig. 2.1. The reason why carbon assumes many structural forms is that a carbon atom can form several distinct types of valence bonds (or hybridization of orbitals).
Fig. 2.1. Various forms of carbon: diamond, fullerene, graphite, and CNT [34].
Each carbon atom has six electrons 1s2, 2s2, and 2p2 theoretically. In carbon, there are three possible hybridized orbitals sp, sp2, and sp3. The spn hybridization is important for
determining the dimensionality of carbon-based molecules and carbon-based solids. Carbon shows a variety of stable forms ranging from 0D fullerenes [32] to 1D conducting or semiconducting CNTs to 2D semi-metallic graphite to 3D semiconducting diamond, as shown in Table 2.1 [33]. In spn hybridization, (n+1) σ bonds per carbon atom are formed. These σ bonds make a frame for the local structure of the n-dimensional structure.
Table 2.1 Isomers of carbon [33]
Dimension 0-D 1-D 2-D 3-D
In sp3 hybridization, 4 σ bonds defining a regular tetrahedron are sufficient to form a 3D diamond structure. In sp2 hybridization, it forms a planar structure in 2D graphite, and also forms a planar local structure in the closed polyhedron (0D) of fullerence and in the cylinders (1D) of CNT. Carbon fiber is macroscopic 1D material, because of the high length to diameter ratio. However, it consists of many graphitic planes and microscopically exhibits electronic properties that are mainly 2D. In sp hybridization, two σ bonds make only a 1D chain structure, which is known as a carbyne. Besides, amorphous carbon is a disorder, 3D material in which both of sp2 and sp3 hybridization are present randomly. Amorphous graphite is the
graphite with random stacking of graphitic layer segments, which consists mainly of sp2 hybridization. Amorphous graphite can behave like a 2D material.
Next, fullerences and carbon fibers, which are closely related to CNTs, are introduced. The main research object of this thesis, CNTs, is introduced apart in Section 2.2.
2.1.1 Fullerenes
Fullerence is an abbreviation of buckminsterfullerenes that honored the famous architect Buckminster Fuller, who designed and invented geodesic dome that was similar to the structure of C60. In 1985, Kroto, Smalley, Curl and coworkers [35] began a famous series of experiments on the vaporization of graphite. In the distribution of gas-phase carbon clusters, C60 was the dominant species. This dominance became even more marked under conditions which maximized the amount of time the clusters were ‘annealed’ in the helium. Figure 2.2 shows C60, which is a closed cluster containing precisely 60 carbon atoms would have a structure of unique stability and symmetry. The discovery of C60 marked the beginning of a new area in carbon science [36-39].
Fig. 2.2. C60 buckminsterfullerence [37].
2.1.2 Carbon Nanofibers
Carbon fibers represent an important class of graphite-related materials which are closely connected to CNTs, because of the high length to diameter ratio. Carbon fibers synthesized by traditional methods of extrusion from polymer slurries and from catalytic CVD vary in morphology and structure from fiber to fiber and area to area on each fiber. Despite the many precursors that can be used to synthesize carbon fibers, each having a different cross-sectional morphology, as shown in Fig. 2.3. Figure 2.3(a) shows carbon fibers as-deposited at 1100℃, and Fig. 2.3(b) shows carbon fibers which after treatment to 3000℃. The morphologies for commercial mesophase pitch fibers are shown in Fig. 2.3(c) for a “PAN-man” cross section with a radical arrangement of straight graphene ribbons and a missing wedge and Fig. 2.3(d) for a PAN-AM cross-sectional arrangement of graphene plane. A PAN fiber is shown in Fig.
2.3(e) with a circumferential arrangement of ribbons in the sheath region and a random structure in the core. The preferred orientation of the graphene planes is parallel to the fiber axis for all carbon fibers, thereby accounting for the high mechanical strength [40].
Fig. 2.3. Sketch illustrating the morphology of vapor grown carbon fibers [32].
2.1.3 Structure and Properties of Carbon Nanotubes
The main research object of this thesis, CNTs, is introduced here. Following paragraphs provide an overview of the structural and properties of CNTs, and there are many review articles [41-44] and books [32,45,46] surround this subject.
2.1.3.1 Structure
The detail structure of CNTs is discussed here. In the ideal case, a CNT consists of either one cylindrical graphene sheet (Single-walled nanotube, SWCNT) or of several nested cylinders with an inter-layer spacing of 0.34 - 0.36 nm that is close to the typical spacing of turbostratic graphite, i.e. MWCNT. There are many possibilities to form a cylinder with a graphene sheet [4] and a few configurations are shown in Fig. 2.4. Figure 2.4(a)-(c) are SWCNTs of (a) zig-zag, (b) armchair and (c) chiral type. Figure 2.4(d) represents a MWCNT formed by four tubes of increasing diameter with a layer spacing of 0.34 nm. One can roll up the sheet along one of the symmetry axis: this gives either a zig-zag tube, or an armchair tube.
It is also possible to roll up the sheet in a direction that differs from a symmetry axis: one obtains a chiral CNT. Besides the chiral angle, the circumference of the cylinder can also be varied.
Fig. 2.4. Models of different CNT structures [2].
Figure 2.5 shows the cutting graphite sheet along the dotted lines which connects two crystalline graphite equivalent sites on a 2-D [47]. Each carbon atom has three nearest neighbors, rolling sheet of graphite into cylinder form CNTs. The circumference of CNTs can be expressed in term of the chiral vector, Ch, and chiral angle, θ . The chiral vector is given by Eq. (1): lC-C is the length of C-C bond. The chiral angel determines the amount of twist in the tube.
The chiral angles exist two limiting cases that are at 0° and 30°. The chiral angle is defined in Eq. (2) as
Fig. 2.5. Schematic diagram showing how a hexagonal sheet of graphite is rolled to form a CNT [47].
The zig-zag CNT corresponds to the case of m = 0, and the armchair CNT corresponds to the case of n = m. The chiral CNT corresponds to the other (n, m) chiral vectors. The zig-zag CNT (n, 0) is generated from hexagon with θ= 0°, and armchair CNT (n, n) is formed from hexagon with θ= 30°. The chiral CNT is formed from hexagon with 0°＜θ＜30°.
The inter-atomic spacing of carbon atom is known so that the rolled up vector of CNT can define the CNT diameter. The properties of carbon CNTs depend on the atomic arrangement, diameter, length, and the morphology [48].
This diversity of possible configurations is indeed found in practice, and no particular type is preferentially formed. In most cases, the layers of MWCNTs are chiral [1,49] and of different helicities [50]. The lengths of SWCNTs and MWCNTs are usually well over 1 µm and diameters range from ~1 nm (for SWCNTs) to ~50 nm (for MWCNTs). Pristine SWCNTs are usually closed at both ends by fullerene-like halfspheres that contain both pentagons and hexagons [4]. A SWCNT with a well-defined spherical tip is shown in Fig. 2.6.
A MWCNT where the shape of the cap is more polyhedral than spherical is represented in Fig.
2.6(b). An open MWCNT where the ends of the graphene layers and the internal cavity of the tube are exposed is shown in Fig. 2.6(c). Defects in the hexagonal lattice are usually present in the form of pentagons and heptagons. Pentagons produce a positive curvature of the graphene layer and are mostly found at the cap as shown in Fig. 2.6(b) where each knick in the graphene layers points to the presence of pentagons in the carbon network. Heptagons give rise to a negative curvature of the tube wall [51]. Defects consisting of several pentagons and/or heptagons have also been observed. A simple model indicates that the diameter and/or chirality of the tube is changed from one side of the defect to the other [52]. Such an arrangement forms therefore a link between two different tubes and is accordingly called a junction.
Fig. 2.6. TEM pictures of the ends of (a) a SWCNT, (b) a closed MWCNT, and (c) an open MWCNT. Each black line corresponds to one graphene sheet viewed edge-on [2].
2.1.3.2 Properties
This section is an overview of the mechanic and electronic properties of CNTs. The electronic properties of SWCNTs have been studied in a large number of theoretical works [3,4,53-55]. These models show that the electronic properties vary in a calculable way from metallic to semiconducting, depending on the tube chirality (n, m) given by [32]
Metallic properties: n-m = 0 or (n-m)/3 = integer Semiconducting properties: (n-m)/3 ≠ integer
The study shows that about 1/3 of SWCNTs are metallic, while the other 2/3 of SWCNT are semiconducting with a band gap inversely proportional to the tube diameter. This is due to the very unusual band structure of graphene and is absent in systems that can be described with usual free electron theory. Graphene is a zero-gap semiconductor with the energy bands of the p-electrons crossing the Fermi level at the edges of the Brillouin zone, leading to a Fermi surface made of six points [56]. Graphene should show a metallic behavior at room temperature since electrons can easily cross from the valence to the conduction band.
However, it behaves as a semi-metal because the electronic density at the Fermi level is quite low [32,56]. Rolling up the graphene sheet into a cylinder imposes periodic boundary conditions along the circumference and only a limited number of wave vectors are allowed in
the direction perpendicular to the tube axis. When such wave vectors cross the edge of the Brillouin zone, and thus the Fermi surface, the CNT is metallic. This is the case for all armchair tubes and for one out of three zig-zag and chiral tubes. Otherwise, the band structure of the CNT shows a gap leading to semiconducting behavior, with a band gap that scales approximately with the inverse of the tube radius. Band gaps of 0.4-1 eV can be expected for SWCNTs (corresponding to diameters of 1.6-0.6 nm) [3,4,54]. This simple model does not take into account the curvature of the tube which induces hybridization effects for very small tubes [53] and generates a small band gap for most metallic tubes [55]. The exceptions are armchair tubes that remain metallic due to their high symmetry.
These theoretical predictions made in 1992 were confirmed in 1998 by scanning tunneling spectroscopy [5,6]. The scanning tunneling microscope has since then been used to image the atomic structure of SWCNTs [57,58], the electron wave function [59] and to characterize the band structure [58,60]. Numerous conductivity experiments on SWCNTs and MWCNTs yielded additional information [61-72]. At low temperatures, SWCNTs behave as coherent quantum wires where the conduction occurs through discrete electron states over large distances. Transport measurements revealed that metallic SWCNTs show extremely long coherence lengths [65,72,73]. MWCNTs show also these effects despite their larger diameter and multiple shells [74,75].
The fact that both MWCNT and SWCNT have indeed extraordinary mechanical properties has been indicated by a growing body of experimental evidence. Yakobson et al [76,77]
inspected the instability of CNTs beyond linear response. Their simulation results show that CNTs are remarkably resilient, sustaining extreme strain with no signs of brittleness or plasticity. Besides, some experimental measures of the Young’s modulus of CNTs have been reported. Treacy et al. [78] obtained a relation between amplitude of the tip oscillations and the Young’s modulus. Through TEM observations of some CNTs, they defined the amplitude of those oscillations and obtained an average value of 1.8 TPa for the Young’s modulus.
Another way to probe the mechanical properties of CNTs is to use the tip of AFM to bend anchored CNT. Young’s modulus can be extracted while simultaneously recording the force exerted by the tube as a function of the displacement from its equilibrium position. By this way, Wong et al. [9] reported a mean value of 1.28±0.59 TPa with no dependence on tube diameter for MWCNT. Walters et al. [79] investigated the elastic strain of CNTs bundles with the AFM. An experimental strain measurement and an elastic modulus of 1.25 TPa was assumed. Then yield strength of 45±7 GPa was calculated.
Yu et al. [12,80] reported the tensile of SWCNTs and MWCNTs ropes. For MWCNTs ropes, the tensile strengths of the outermost layer ranged from 11 to 63 GPa and the elastic modulus ranged from 27 to 950 GPa. For SWCNT ropes, the tensile strengths in the range from 13 to 52 GPa and the average elastic modulus of 320 to 1470 GPa were obtained.
In term of mechanical properties, CNTs are among the strongest and most elastic materials known to exist in nature [81]. Table 2.2 shows the mechanical properties of CNTs with other materials. It indicates that MWCNTs are of the most superior mechanical characteristic. The hollow structure and close topology of CNTs form a distinct mechanical response in CNT compared to other graphitic structure.
Table 2.2 Mechanical properties of CNTs compared with other Materials [81]
Materials Young’s modulus (GPa) Tensile strength (GPa) Density (g/cm3)
SWCNT 1054 ~150
2.2 Carbon Nanotube Synthesis
There are three major methods to synthesize CNTs: arc discharge, laser ablation and catalytically chemical vapor deposition.
2.2.1 Arc Discharge
The arc discharge was the first available method for the production of both MWCNTs [1,82]
and SWCNTs [83,84]. This is the classic method of preparing MWCNTs, and produces the best quality samples. The method has been in use for a long time for the production of carbon fibers. Therefore, it is very possible that CNTs were observed but not recognized before Iijima observed CNTs synthesized from this method in 1991 [85,86].
Figure 2.7 shows the schematic of arc discharge system and TEM micrograph of the grown CNT [2]. The arc discharge apparatus involves the use of two graphite rods as the anode and cathode. The rods are brought together under a gas atmosphere (usually He, but H2 [87] and Ar have also been used) and a voltage is applied until a stable arc is achieved. As the anode is consumed, a gap (~ 1mm) between cathode and anode is maintained by adjusting the position of anode. Carbon materials are deposited on the cathode to form CNTs and other carbon particles. MWCNTs produced by arc discharge are long and straight tubes closed at both ends with graphitic walls running parallel to the tube axis, as shown in Fig. 2.7.
Several factors have been shown to be important in producing a good yield of high quality of CNTs. Harris [88] repotted that perhaps the most important is the pressure of the helium in the chamber, as demonstrated by Ebbesen and Ajayan [82]. The Current in the arc discharge method is another important factor [89,90]. Efficient cooling of the electrodes and the chambers has also been shown to be essential in producing good quality CNT samples [89-91].
To synthesize SWCNTs, Iijima et al. [83] and Bethune et al. [84] reported in 1993 that an arc discharge with a cathode containing metal catalysts (such as cobalt, iron or nickel) mixed
to graphite powder results in a deposit containing SWCNTs. The yield has been significantly increased by optimizing the catalyst mixture [92] and the deposition conditions [93].
Fig. 2.7. Schematic illustration of the arc discharge system and TEM micrograph of the grown CNT [2].
2.2.2 Laser Ablation
Laser ablation was first used for synthesis of C60 in 1985 by Kroto et al. [35], and was demonstrated to grow SWCNTs and MWCNTs in 1995 by Samlley’s group at Rice University [94]. Subsequent refinements to this method led to the production of SWCNTs with unusually uniform diameter [95]. However, the length of MWCNTs grown by this method is much shorter than that by arc discharge method [94].
Thess et al. [95] showed that the synthesis could be carried out in a horizontal flow tube under a flow of inert gas at controlled pressure. In this setup the flow tube is heated to
~1200°C by a tube furnace as displayed in Fig. 2.8. Laser pulses enter the tube and strike a target consisting of a mixture of graphite and a metal catalyst such as Co or Ni. SWCNTs condense from the laser vaporization plume and are deposited on a collector outside the furnace zone [96]. The size of carbon sources limited the volume of sample. Besides, purification steps are necessary to separate the tube from undesirable by-product.
Nevertheless, this method has still become an important technique for synthesizing SWCNTs due to the high yield of CNTs.
Fig. 2.8. Schematic illustration of the laser ablation apparatus [43].
2.2.3 Catalytically Chemical Vapor Deposition (CVD)
The catalytic growth of CNTs is an alternative to the arc discharge and laser ablation methods. It is based on the decomposition of the hydrocarbon gas over transition metals to grow CNTs by using chemical vapor deposition (CVD). Since the 1960s [97], carbon filaments and fibers have been produced by thermal decomposition of hydrocarbons. Usually, a catalyst is necessary to promote the growth [98]. A similar approach was used to grow MWCNTs from the decomposition of acetylene over iron particles in 1993 [99]. A tube
produced by catalytic growth is shown in Fig. 2.9. In general, the diameter of CNTs grown by catalytic growth is larger than that of arc discharge, but along with an imperfectly graphitized crystalline structure. To grow MWCNTs, acetylene is usually used as carbon source at temperatures typically between 600-800°C. To grow SWCNTs, the temperature has to be significantly higher (900-1200°C) due to the fact that they have a higher energy of formation.
In this case carbon monoxide or methane must be used because of their increased stability at higher temperatures as compared to acetylene.
Fig. 2.9. Schematic illustration of the catalytic deposition and TEM micrograph of the grown CNT [2].
Up to now, the catalytic CVD has undergone many improvements. Co catalysts supported on silica particles produced straight as well as coiled MWCNTs [100], and the yield of CNTs was significantly increased by using zeolites as catalyst supports [101,102]. It was also reported that the continuous production of SWCNTs, where both the carbon and the catalyst are supplied in the gas phase. Besides, the yield and average diameter of SWCNTs could be varied by controlling the process parameters [103]. In addition, the type of catalyst support was found to control the formation of individual or bundled SWCNTs [104]. Transition metal
(e.g. Fe, Co, Ni) particles are known to be catalysts for vapor grown CNTs synthesis, in which hydrocarbons are used as carbon source. Metal catalysts are generally necessary to activate CNTs growth. A variety of other catalysts, hydrocarbons and catalyst supports have been used successfully by various groups the world wide to synthesize CNTs [105, 106]. Various metal compounds used as catalysts are listed in Table 2.3 [107]
The catalytic CVD is also an ideal method to grow CNTs on planar substrate (e.g. silicon or glass). Dense MWCNT arrays were thus deposited on mesoporous silica that was prepared by a sol-gel process [108] and long aligned CNTs were obtained over several square millimeters by using large-area mesoporous silica substrates [109]. Aligned MWCNTs were generated by pyrolysis of a triazine compound at 950°C with nearly no by-products [110]. There are some reasons behind the development of catalytic techniques to grow CNTs on planar substrates.
First, in many cases, purification steps are unnecessary because there is no or very few by-product. Second, substrates can be directly patterned with catalysts using lithographic techniques followed by catalytic growth.
CNTs were also deposited by plasma-assisted CVD (PACVD) of methane and hydrogen at 950°C [111], and the synthesis temperature could be decreased below 660°C by using plasma-enhanced hot filament CVD [112]. Since then, several papers describing the synthesis of films of CNTs on silicon substrates have been published [113-116]. Moreover, microwave
CNTs were also deposited by plasma-assisted CVD (PACVD) of methane and hydrogen at 950°C [111], and the synthesis temperature could be decreased below 660°C by using plasma-enhanced hot filament CVD [112]. Since then, several papers describing the synthesis of films of CNTs on silicon substrates have been published [113-116]. Moreover, microwave