**1. Introduction**

In the modern world, transparent conductive films (TCF) are extremely common and criti‐ cally important in electrical devices. In our homes or offices, they are found in flat panel dis‐ plays such as in TVs, laptops and in touch panels, of phones, tablet computers, E-readers and digital cameras [1]. Besides, they are also used as the electrodes for photovotaic devices such as solar cells [2] and organic light-emitting diodes (OLEDs) [3]. Liquid crystal display (LCD) is by far the largest user of transparent conductive films but many devices are show‐ ing rapid growth in popularity such as touch panels (362 million units in 2010 with annual growth of 20% through 2013), E-paper (30 fold growth expected from 2008 to 2014), and thin film solar cells (expected sales of over \$13 billion by 2017) [4].

The dominant transparent conductive material used today is tin doped indium oxide (ITO) with a demand growing at 20% per annum [5]. ITO has been studied and refined for over 70 years, and as a result, the material offers many beneficial properties. However, ITO has cer‐ tain drawbacks, mainly reflected on the depleted supply of raw material and their brittle‐ ness. The supply of indium is constrained by both mining and geo-political issues, which leads to dramatic price fluctuations over the last decades, from \$ 100-\$ 900, as shown in Figure 1. The high price of indium determined the high cost of ITO, since they compose nearly 75wt % of a typical ITO film [6]. In addition to the raw materials, the expense of set‐ ting up and maintaining a sputtering deposition line, as well as the low deposition yield (3-30%) [7] also increases the cost of ITO. Though current devices are typically based on rig‐ id substrates, there is a continued trend toward flexible devices. As ITO tend to fracture at strains of 2%, they are completely unsuitable for using in flexible electronics. Therefore, new transparent electrode materials have rapidly emerged in recent years, including carbon nanotubes (CNTs), graphene and metal nanowires. The intrinsically high conductivity cou‐

© 2013 Sun and Wang; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Sun and Wang; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

pled with high aspect ratio yields films with high transmittance, adequately low sheet resist‐ ance, and superior mechanical flexibility. These material properties, combined with inexpensive material and deposition costs make these emerging nanomaterials very attrac‐ tive for as transparent electrodes. Of the three dominant nanoscale materials, CNTs are per‐ haps the most promising and mature intensively investigated.

Finally, the latest progress on CNT transparent conductive films and their applications on

The two most important features for a transparent conductingmaterial are its sheet resist‐ ance ( Rs ) and optical transparency. The sheet resistance is defined as Rs = R(W/L), where R is DC resistance, W and L are width and length of the film. Grüner et al. [8] developed a suitable merit, the DC conductivity/optical conductivity (σdc/σop), to compare the perform‐ ance of various transparent conductors based on the standard percolation theory, in which each bundle of nanotubes was counted as one conducting stick. They assumed the conduc‐ tivity ratio σdc/σop remains constant for nanotube networks with different densities in the measured optical frequency range. By plotting RsvsT and fitting the data to equation 1, one can estimate the value ofσdc/σop. This value is often used as a Figure of Merit for transparent

conductors since high values of σdc/σop leads to films with high T and low Rs.

*T* =*t* ∙(1 +

*<sup>T</sup>* <sup>=</sup> <sup>1</sup> <sup>1</sup> <sup>+</sup> <sup>2</sup>*πσOP*

*cRsσdc*

Geng et al. [9] found that this equation can be fitted well to the curve of single-walled car‐ bon nanotube TCFs, nevertheless can not be fitted well with carbon nanotubes of other

> 188 (Ω) *Rs*

The parameter t may represent the optical property of CNT films. A high t value gives a high transmittance for the CNT films. The t value of SWCNT films is 0.999, while that of

Recently, Coleman et al. [5] modified this model to evaluate thinner (more transparent) films. They found that the data tend to deviate severely from the fits for thinner films, as seen from Figure 2. This deviation has been observed before [10-12] and tends to occur for films with T between 50% and 92%.Thus,σdc/σac fails to describe the relationship between T and Rsin the relevant regime. The deviation from bulk-like behavior as described in Equa‐ tion1, can be explained by percolation effects [13]. Such effects become important for very sparse networks of nano conductors. When the number of nanoconductors per unit isvery low, a continuous conducting path from one side of the sample to the other will generally

*σop σdc* )


Carbon Nanotube Transparent Electrode http://dx.doi.org/10.5772/51783 315

(1)

electrical devices will be summarized.

**2. Optoelectronic properties**

types. They modified the equation as follows:

MWCNT is much lower, around 0.884.

not exist.

**Figure 1.** (a) Global demand for resistive style touch panels by area; (b) Average price of Indium over the last sever‐ al decades; Reprinted with permission from reference [4] copyright 2011 Wiley.

This review will focus on transparent electrode made of CNTs, and six main parts will be covered.


Finally, the latest progress on CNT transparent conductive films and their applications on electrical devices will be summarized.

#### **2. Optoelectronic properties**

pled with high aspect ratio yields films with high transmittance, adequately low sheet resist‐ ance, and superior mechanical flexibility. These material properties, combined with inexpensive material and deposition costs make these emerging nanomaterials very attrac‐ tive for as transparent electrodes. Of the three dominant nanoscale materials, CNTs are per‐

**Figure 1.** (a) Global demand for resistive style touch panels by area; (b) Average price of Indium over the last sever‐

This review will focus on transparent electrode made of CNTs, and six main parts will

**1.** At first, some basic theories and parameters for characterizing transparent conductive materials will be presented so that the following parts of the review can be profound‐

**2.** CNTs prepared from different methods or modified under various conditions have di‐ verse physical and chemical properties, which will yield films with distinct perform‐ ance. Therefore, in the second part, CNTs of different types will be investigated, and the

**3.** One of the major advantages in using CNTs is their ability to be applied to substrates from solution, which opens up many alternative deposition techniques. Therefore, one of the primary research areas for making transparent conductive films is to process the CNT material into printable inks.The third part will outline major approaches to dis‐ perse CNTs and focus on the most important details with regards to making transpar‐

**4.** In the fourth part, a variety of techniques for making transparent conductive CNT films

**5.** During the solubilization step, non-conducting dispersants are induced, which sacrifice the conductance of the films a lot. Therefore, post-treatment needs to be done to remove them for enhancing the performance of the films. In the fifth part, various methods used to improve the performance of the transparent conductive films after their preparation

haps the most promising and mature intensively investigated.

314 Syntheses and Applications of Carbon Nanotubes and Their Composites

al decades; Reprinted with permission from reference [4] copyright 2011 Wiley.

performance of the as prepared thin films will be compared.

be covered.

ly understood.

ent conductive films.

will be discussed.

will be presented and evaluated.

The two most important features for a transparent conductingmaterial are its sheet resist‐ ance ( Rs ) and optical transparency. The sheet resistance is defined as Rs = R(W/L), where R is DC resistance, W and L are width and length of the film. Grüner et al. [8] developed a suitable merit, the DC conductivity/optical conductivity (σdc/σop), to compare the perform‐ ance of various transparent conductors based on the standard percolation theory, in which each bundle of nanotubes was counted as one conducting stick. They assumed the conduc‐ tivity ratio σdc/σop remains constant for nanotube networks with different densities in the measured optical frequency range. By plotting RsvsT and fitting the data to equation 1, one can estimate the value ofσdc/σop. This value is often used as a Figure of Merit for transparent conductors since high values of σdc/σop leads to films with high T and low Rs.

$$T = \frac{1}{1 + \ ^{\frac{2\pi\sigma\_{\text{CP}}}{\text{C}^{\text{R}}\text{-}\text{C}^{\text{R}}\text{-}\text{C}^{\text{R}}\text{-}\text{C}^{\text{R}}\text{-}\text{-}\text{C}^{\text{R}}\text{-}\text{-}1}}}\tag{1}$$

Geng et al. [9] found that this equation can be fitted well to the curve of single-walled car‐ bon nanotube TCFs, nevertheless can not be fitted well with carbon nanotubes of other types. They modified the equation as follows:

$$T = t \bullet \left(1 + \frac{188 \text{ ( $\Omega$ )} }{R\_s} \frac{\sigma\_{sp}}{\sigma\_{dc}}\right) \text{-} 2 \tag{2}$$

The parameter t may represent the optical property of CNT films. A high t value gives a high transmittance for the CNT films. The t value of SWCNT films is 0.999, while that of MWCNT is much lower, around 0.884.

Recently, Coleman et al. [5] modified this model to evaluate thinner (more transparent) films. They found that the data tend to deviate severely from the fits for thinner films, as seen from Figure 2. This deviation has been observed before [10-12] and tends to occur for films with T between 50% and 92%.Thus,σdc/σac fails to describe the relationship between T and Rsin the relevant regime. The deviation from bulk-like behavior as described in Equa‐ tion1, can be explained by percolation effects [13]. Such effects become important for very sparse networks of nano conductors. When the number of nanoconductors per unit isvery low, a continuous conducting path from one side of the sample to the other will generally not exist.

**Figure 2.** Typical graph of transmittance (generally measured at 550 nm) plotted versus sheet resistance for thin films of nanostructured materials. Reprinted with permission from reference [5] copyright MRS.

As more nanoconductors are added, at some point (the percolation threshold) the first conducting path will be formed. As more material is added, more conductive paths are formed, and the conductivity of the network increases rapidly. Eventually it reached a "bulklike" value above which it remains constant. Percolation theory describes how the dc con‐ ductivity of sparse networks depends on network thickness and predicts a non-linear, power law dependence:

$$
\sigma\_{dc} \propto (t - t\_c)^n \tag{3}
$$

of data in Figure 1 was fitted using Equation 4, and good fits allow the calculation of both nand Π. Analysis of these equations shows that large values of Π but low values of nare de‐ sirable to achieve low Rs coupled with high T, which are used to evaluate the performance

In addition to their sheet resistance and optical transparency, the stability and mechanical durability are also critical criteria to evaluate the performance of transparent conductors. Undoped CNT films exhibit excellent stability upon exposure to atmospheric conditions, as seen in Figure 3 [14]. Doping with nitric acid or SOCl2 could decrease the sheet resistance significantly, however at the expense of sacrificing their stability [15-17]. The sheet resist‐ ance of undoped SWCNT films decreases slightly with increasing temperature, which is consistent with the electrical behavior of semiconductors. Thermal stability of doped CNTs is dependent on dopants since elevated temperatures may increase chemical reactions or en‐ hance the desorption of dopants out of the films. CNT-PET thin films are significantly more

nificant change in resistance, [18] and the conductivity of the films can be retained after 500

**Figure 3.** Absolute sheet resistance versus time in air of four SWNT films. Reprinted with permission from reference

Carbon nanotubes synthesized from different methods or processes have diverse material qualities, such as the degree of purity, the defects, their length and diameters, and the chiral‐

without a sig‐

Carbon Nanotube Transparent Electrode http://dx.doi.org/10.5772/51783 317

flexible than commercial ITO/PET films. They can be bent all the way to 180o

of CNT films with high performance.

bending cycles [19].

[14] copyright Wiley.

**3. The choice of Carbon Nanotubes**

where tis the estimated thickness of the network, tc is the thickness associated with the per‐ colation thres hold, and nis the percolation exponent. This leads to a new relationship be‐ tween T and Rs, which applies to thin, transparent networks:

$$T = \left[1 + \frac{1}{\Pi} \left(\frac{Z\_0}{R\_s}\right)^{\text{l}}\right]^2 \tag{4}$$

where Π is the percolative FoM:

$$\mathbf{T} = \mathbf{2} \begin{bmatrix} \frac{\sigma\_{\rm dc} / \sigma\_{op}}{(Z\_{\rm d} t\_{\rm min} \sigma\_{op})^n} \end{bmatrix}^{1/(n+1)} \tag{5}$$

Here, tmin is the thickness below which the dc conductivity becomes thickness dependent. It scales closely with the nanostructures' smallest demision, tmin ≈ 2.33 D. The high Tportion of data in Figure 1 was fitted using Equation 4, and good fits allow the calculation of both nand Π. Analysis of these equations shows that large values of Π but low values of nare de‐ sirable to achieve low Rs coupled with high T, which are used to evaluate the performance of CNT films with high performance.

In addition to their sheet resistance and optical transparency, the stability and mechanical durability are also critical criteria to evaluate the performance of transparent conductors. Undoped CNT films exhibit excellent stability upon exposure to atmospheric conditions, as seen in Figure 3 [14]. Doping with nitric acid or SOCl2 could decrease the sheet resistance significantly, however at the expense of sacrificing their stability [15-17]. The sheet resist‐ ance of undoped SWCNT films decreases slightly with increasing temperature, which is consistent with the electrical behavior of semiconductors. Thermal stability of doped CNTs is dependent on dopants since elevated temperatures may increase chemical reactions or en‐ hance the desorption of dopants out of the films. CNT-PET thin films are significantly more flexible than commercial ITO/PET films. They can be bent all the way to 180o without a sig‐ nificant change in resistance, [18] and the conductivity of the films can be retained after 500 bending cycles [19].

**Figure 3.** Absolute sheet resistance versus time in air of four SWNT films. Reprinted with permission from reference [14] copyright Wiley.

### **3. The choice of Carbon Nanotubes**

**Figure 2.** Typical graph of transmittance (generally measured at 550 nm) plotted versus sheet resistance for thin films

As more nanoconductors are added, at some point (the percolation threshold) the first conducting path will be formed. As more material is added, more conductive paths are formed, and the conductivity of the network increases rapidly. Eventually it reached a "bulklike" value above which it remains constant. Percolation theory describes how the dc con‐ ductivity of sparse networks depends on network thickness and predicts a non-linear, power

where tis the estimated thickness of the network, tc is the thickness associated with the per‐ colation thres hold, and nis the percolation exponent. This leads to a new relationship be‐

1/(*n*+1)

Here, tmin is the thickness below which the dc conductivity becomes thickness dependent. It scales closely with the nanostructures' smallest demision, tmin ≈ 2.33 D. The high Tportion

*σdc* ∝ (*t* - *tc*)*<sup>n</sup>* (3)

(4)

(5)

of nanostructured materials. Reprinted with permission from reference [5] copyright MRS.

316 Syntheses and Applications of Carbon Nanotubes and Their Composites

tween T and Rs, which applies to thin, transparent networks:

*T* = 1 +

<sup>П</sup>=2 *<sup>σ</sup>dc* / *<sup>σ</sup>op* (*Z* <sup>0</sup>*tminσop*)*<sup>n</sup>*

1 <sup>П</sup> ( *<sup>Z</sup>*<sup>0</sup> *Rs* ) 1 (*n*+1) -2

law dependence:

where Π is the percolative FoM:

Carbon nanotubes synthesized from different methods or processes have diverse material qualities, such as the degree of purity, the defects, their length and diameters, and the chiral‐ ities, which are presumably important factors in determining the film performance.There‐ fore, the choice of CNTs as well as their further treatment is markedly important. Young Hee Lee group [9,20] did systematical analysis to investigate the CNT quality dependence. In their work, single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), thin multiwalled carbon nanotubes (t-MWCNT) and multiwalled carbon nano‐ tubes (MWCNT) powders were separately dispersed in deionized water with sodium do‐ decyl sulfate (SDS) and dichloroethane (DCE) by sonication and sprayed onto poly (ethylene terephthalate) (PET) substrates to fabricate thin films. The sheet resistance and transmittance of each film was measured and compared. As seen in Figure 4, the film's performance changes dramatically for different types of CNTs dispersed in deionized water with SDS, as well as in DCE. The TCFs fabricated with SWCNTs show the best film performance among all the selected CNTs. The trends of film performances are similar for the TCFs fabricated by using the CNT solution dispersed in deionized water and in DCE, which is SWCNTTCF>DWCNTTCF> t-MWCNTTCF>MWCNTTCF. Furthermore, they analyzed the defects and metallicity by Raman spectra, and found that CNTs with fewer defects and high content of metallic tubes leads to TCFs with higher conductivity. Nevertheless, in Li's re‐ port, [21]. MWCNTTCFs exhibit better performance than SWCNTTCFs. They indicated that MWCNT have more conductive π channels than SWCNTs does, therefore MWCNTs have better electronic transportability. In the case of a MWCNT where conduction occurs through the outer most shell, the large diameter of the outernanotube causes the gap to approach 0 eV and the nanotubeto become basically metallic. On the contrary, 2/3 of SWCNTs are semi‐ conducting. The other reason they mentioned is that the MWCNTs they used are longer than SWCNTs, which could decrease the contacts numbers. Another point needs to be ad‐ dressed is that dimethylformamide (DMF) which was chosen as the solvent in their work is actually not efficient to exfoliate SWCNTs. Therefore, SWCNTs bundled together which would open up an energy gap or pseudo gap owing to intertube interactions. We believe this is a critical reason for the worse performance of SWCNTTCFs in their work.

SWCNTs synthesized by different methods such as arc discharge (Arc), catalytic chemical vapor deposition (CVD), high pressure carbon monoxide (Hipco), and laser ablation (Laser) were also analyzed systematically [20]. After the SWCNT powder was characterized, each of them was dispersed in deionized water with sodiumdodecyl sulfate (SDS) by sonication fol‐ lowed by aspray process to fabricate the SWCNT film onto PET substrates.By analyzing the SWCNT film performance varying with the SWCNT parameters, they found that the metal‐ licity of the SWCNTsextracted from G'-band intensity of Raman spectros copy and the de‐ gree of dispersion in the solutionare the most decisive factors in determining the film performance. Figure 5 shows that the film performance changes dramatically with different types of SWCNTs. The TCFs fabricated with Arc SWCNTs result in the best film perform‐ ance, consistent with previous report [22]. The sheet resistance of the Arc TCF is ~160Ω/sq at a transmittance of 80%, which can be used in a wide range of applications from touch panels

Carbon Nanotube Transparent Electrode http://dx.doi.org/10.5772/51783 319

**Figure 5.** Characteristic curves of sheet resistance-transmittance of TCFs fabricated by various SWCNTs. Reprinted with

In order to investigate the underlying reason, CNTs were characterized with SEM, TEM, TGA and Rama spectra. TEM analysis showed that the diameter of individual nanotube syn‐ thesized with CVD and Hipco process were about 1nm, smaller than those (~1.4 nm) of La‐ ser and Arc SWCNTs. The CVDSWCNTs had the smallest average bundle size, as estimated from the SEM images, where as the Laser sample exhibited the largest average bundle size among samples. Carbonaceous particleson the SWCNT bundles are present in the CVDSWCNTs. The Arc SWCNTs have relatively well-defined crystallinity without amor‐ phous carbonson the tube walls, although the bundle size of the Arc sample is smaller than that of the Laser sample. Figure 6 disclosed that the influence of the purity of the SWCNT is

to electrodes for future flexible displays.

permission from Ref. [20].

**Figure 4.** Characteristic sheet resistance-transmittance curves for various CNT-films. Each curve contains several data points from films with different numbers of sprays by a CNT solution dispersed in (a) deionized water with SDS and (b) DCE without dispersant. Reprinted with permission from reference [9].

SWCNTs synthesized by different methods such as arc discharge (Arc), catalytic chemical vapor deposition (CVD), high pressure carbon monoxide (Hipco), and laser ablation (Laser) were also analyzed systematically [20]. After the SWCNT powder was characterized, each of them was dispersed in deionized water with sodiumdodecyl sulfate (SDS) by sonication fol‐ lowed by aspray process to fabricate the SWCNT film onto PET substrates.By analyzing the SWCNT film performance varying with the SWCNT parameters, they found that the metal‐ licity of the SWCNTsextracted from G'-band intensity of Raman spectros copy and the de‐ gree of dispersion in the solutionare the most decisive factors in determining the film performance. Figure 5 shows that the film performance changes dramatically with different types of SWCNTs. The TCFs fabricated with Arc SWCNTs result in the best film perform‐ ance, consistent with previous report [22]. The sheet resistance of the Arc TCF is ~160Ω/sq at a transmittance of 80%, which can be used in a wide range of applications from touch panels to electrodes for future flexible displays.

ities, which are presumably important factors in determining the film performance.There‐ fore, the choice of CNTs as well as their further treatment is markedly important. Young Hee Lee group [9,20] did systematical analysis to investigate the CNT quality dependence. In their work, single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), thin multiwalled carbon nanotubes (t-MWCNT) and multiwalled carbon nano‐ tubes (MWCNT) powders were separately dispersed in deionized water with sodium do‐ decyl sulfate (SDS) and dichloroethane (DCE) by sonication and sprayed onto poly (ethylene terephthalate) (PET) substrates to fabricate thin films. The sheet resistance and transmittance of each film was measured and compared. As seen in Figure 4, the film's performance changes dramatically for different types of CNTs dispersed in deionized water with SDS, as well as in DCE. The TCFs fabricated with SWCNTs show the best film performance among all the selected CNTs. The trends of film performances are similar for the TCFs fabricated by using the CNT solution dispersed in deionized water and in DCE, which is SWCNTTCF>DWCNTTCF> t-MWCNTTCF>MWCNTTCF. Furthermore, they analyzed the defects and metallicity by Raman spectra, and found that CNTs with fewer defects and high content of metallic tubes leads to TCFs with higher conductivity. Nevertheless, in Li's re‐ port, [21]. MWCNTTCFs exhibit better performance than SWCNTTCFs. They indicated that MWCNT have more conductive π channels than SWCNTs does, therefore MWCNTs have better electronic transportability. In the case of a MWCNT where conduction occurs through the outer most shell, the large diameter of the outernanotube causes the gap to approach 0 eV and the nanotubeto become basically metallic. On the contrary, 2/3 of SWCNTs are semi‐ conducting. The other reason they mentioned is that the MWCNTs they used are longer than SWCNTs, which could decrease the contacts numbers. Another point needs to be ad‐ dressed is that dimethylformamide (DMF) which was chosen as the solvent in their work is actually not efficient to exfoliate SWCNTs. Therefore, SWCNTs bundled together which would open up an energy gap or pseudo gap owing to intertube interactions. We believe

318 Syntheses and Applications of Carbon Nanotubes and Their Composites

this is a critical reason for the worse performance of SWCNTTCFs in their work.

**Figure 4.** Characteristic sheet resistance-transmittance curves for various CNT-films. Each curve contains several data points from films with different numbers of sprays by a CNT solution dispersed in (a) deionized water with SDS and (b)

DCE without dispersant. Reprinted with permission from reference [9].

**Figure 5.** Characteristic curves of sheet resistance-transmittance of TCFs fabricated by various SWCNTs. Reprinted with permission from Ref. [20].

In order to investigate the underlying reason, CNTs were characterized with SEM, TEM, TGA and Rama spectra. TEM analysis showed that the diameter of individual nanotube syn‐ thesized with CVD and Hipco process were about 1nm, smaller than those (~1.4 nm) of La‐ ser and Arc SWCNTs. The CVDSWCNTs had the smallest average bundle size, as estimated from the SEM images, where as the Laser sample exhibited the largest average bundle size among samples. Carbonaceous particleson the SWCNT bundles are present in the CVDSWCNTs. The Arc SWCNTs have relatively well-defined crystallinity without amor‐ phous carbonson the tube walls, although the bundle size of the Arc sample is smaller than that of the Laser sample. Figure 6 disclosed that the influence of the purity of the SWCNT is less deterministic, particularlyin CVD and HiPCOSWCNTs, where as the diameter has a strong correlation to the sheet conductance of SWCNT film. The sheet conductance of the film increases consistently with increasing diameters of nanotubes, as shown in Figure 6. This can be attributed to the decreasing band gap with increasing diameters of semi-con‐ ducting SWCNTs. Although individual metallic tubes are independent of the diameters, there are usually a pseudogap induced by tube-tube interactions, which is also inversely proportional to thetube diameter. Thus, the conductivityof the metallic nanotubes reveals the similar diameter dependence to semiconducting ones.

( ) /2 /2 ( )/ *E kT pq B E kT q B E E kT ifB Q Pe I e e I m M <sup>S</sup>*

SWCNTs, P is the purity of the sample, Ei

the system. Here IS (IM) is defined as

where Eg = 0.82/D (eV), Epg = 0.105/D (eV), D is the average diameter of individual

for the extrinsic semiconductors, kB is the Boltzmann constant and T is the temperature of

where AS(AM) is the areal intensity of semiconducting (metallic) peaks of RBMs from Raman shift. After calculation, it was observed that the sheet conductance reveals a linear relation‐ ship with the material quality factor. Although this empirical formula is not rigorous, it can provide atleast a means to estimate material quality that governs the conductivity of the SWCNTTCFs. Forinstance, large diameter, higher purity, less defects (lower intensity of Dband), and more metallic nanotubes (higher intensity of G'-band) will give better conductiv‐ ity of the SWCNTTC. From this point of view, the Arc TCF is the best sample providing the

In addition to the material parameters discussed above, the length of SWCNTs is also crucial to the TCF performance. According to the percolation theory, a conducting path could be formed at a lower density for longer nanotubes, which means at the same sheet resistance, TCFs prepared with longer nanotubes should exhibit higher optical transparency. This con‐ jecture has been confirmed by experiments [27,28]. In order to optimize the CNTs quality, such as their purity, their dispersibility and the content of metallic tubes, some pretreat‐ ments need to be done. Several attempts have been tried to purify the CNT powders.Gener‐ ally, Gas phase reaction or thermal annealing in air or oxygen atmosphere is used to remove amorphous carbon [29,30]. The key idea with these approaches is a selective oxidative etch‐ ing processes, based on the fact that the etching rate of amorphous carbons is faster than that of CNTs. Since the edge of the CNTs can be etched away as well as carbonaceous parti‐ cles during the annealing, itis crucial to have a keen control of annealing temperatures and annealing times to obtain high yield. Liquid-phasereactions in various acids are always con‐ ducted to remove the transitionmetal catalysts [31-33]. Hydrochloric acid, nitric acid and sulfuric acid are the most commonly used acid, and the purification effect is dependent on the concentration, the reaction temperature and the reaction time. In addition to their reac‐ tion with metal catalysts, nitric acid and sulfuric acid could induce some carboxyl or hy‐ droxyl groups onto the walls of nanotubes, which will improve their dispersibility in water [34,35]. However, some damages were introduced during this process. Therefore, subse‐ quent annealing or ammonium treatment was sometimes carried out to repair the wall structures of the nanotubes to fulfill some special requests.[36]. In order to enhance the con‐

( ) ( ) '/

*S M GD*

highest conductivity in comparison toTCFs made by other types of SWCNTs.

*II I*


*S M*

*A A*

*M S*

=´ ´ + ´ ´ å å (6)

is the intrinsic Fermi Level, Ef is the Fermi Level

Carbon Nanotube Transparent Electrode http://dx.doi.org/10.5772/51783 321

*A A* = ´ <sup>+</sup> (7)

The radical breathing modes (RBM) of Raman spectra were used to characterize the metal‐ licity of SWCNTs [20]. At 514 nm, the Laserand Arc SWCNTs reveal the semiconducting be‐ havior exclusively, on the other hand, CVD and HiPCOSWCNTs containboth metallic and semiconducting nanotubes. At 633 nm, the Laser and Arc SWCNTs pick up mostly metallic SWCNTs, where as the CVDSWCNTs retain mostly semiconducting properties (less promi‐ nent Fano line) and the HiPCOSWCNTscontain both the metallic and the semiconducting behaviors. Other than RBM mode, the G'-band intensity is strongly correlated with the met‐ allicity of SWCNTs. Despite the abundance of metallicity, the presence of defects on then anotube walls that may act as scattering centersdegrades the conductivity of the SWCNT network [23]. The intensity of the D-band indicates the amount of defects on the nanotube walls. Therefore, anappropriate parameter to express conductivity of nanotubes for SWCNTs is the intensity ratio, G'-band/D-band. High content of metallicity and few defects on the nanotube walls will be desired for high conductivity of the SWCNT films.

**Figure 6.** The sheet conductance of TCFs at transmittance of 70% and 80% versus (a) purity and (b) diameter of SWCNT powders. Reprinted with permission from Ref. [20].

The purity affects the conductivity.The diameter contributes to the conductivity via bandg‐ ap described in the previous paragraph. More defects reduce the *mean free path* of carriers and decrease the mobility of carriers in nanotubes.The conductivity is proportional to the metallicity of nanotubes and inversely proportional to the number of scattering centers or defects [24-26]. Considering all these factors, a material quality factor Qm was defined to govern the conductivity of SWCNTs:

$$\underline{Q}\_{u} = P \times \left( e^{E\_{\mu\_{l}}/2k\_{d}T} \times \overline{\sum I\_{M}} \right) + e^{-E\_{q}/2k\_{d}T} \times e^{\left(E\_{l} - E\_{f}\right)/k\_{d}T} \times \overline{\sum I\_{S}} \tag{6}$$

where Eg = 0.82/D (eV), Epg = 0.105/D (eV), D is the average diameter of individual SWCNTs, P is the purity of the sample, Ei is the intrinsic Fermi Level, Ef is the Fermi Level for the extrinsic semiconductors, kB is the Boltzmann constant and T is the temperature of the system. Here IS (IM) is defined as

less deterministic, particularlyin CVD and HiPCOSWCNTs, where as the diameter has a strong correlation to the sheet conductance of SWCNT film. The sheet conductance of the film increases consistently with increasing diameters of nanotubes, as shown in Figure 6. This can be attributed to the decreasing band gap with increasing diameters of semi-con‐ ducting SWCNTs. Although individual metallic tubes are independent of the diameters, there are usually a pseudogap induced by tube-tube interactions, which is also inversely proportional to thetube diameter. Thus, the conductivityof the metallic nanotubes reveals

The radical breathing modes (RBM) of Raman spectra were used to characterize the metal‐ licity of SWCNTs [20]. At 514 nm, the Laserand Arc SWCNTs reveal the semiconducting be‐ havior exclusively, on the other hand, CVD and HiPCOSWCNTs containboth metallic and semiconducting nanotubes. At 633 nm, the Laser and Arc SWCNTs pick up mostly metallic SWCNTs, where as the CVDSWCNTs retain mostly semiconducting properties (less promi‐ nent Fano line) and the HiPCOSWCNTscontain both the metallic and the semiconducting behaviors. Other than RBM mode, the G'-band intensity is strongly correlated with the met‐ allicity of SWCNTs. Despite the abundance of metallicity, the presence of defects on then anotube walls that may act as scattering centersdegrades the conductivity of the SWCNT network [23]. The intensity of the D-band indicates the amount of defects on the nanotube walls. Therefore, anappropriate parameter to express conductivity of nanotubes for SWCNTs is the intensity ratio, G'-band/D-band. High content of metallicity and few defects

on the nanotube walls will be desired for high conductivity of the SWCNT films.

**Figure 6.** The sheet conductance of TCFs at transmittance of 70% and 80% versus (a) purity and (b) diameter of

The purity affects the conductivity.The diameter contributes to the conductivity via bandg‐ ap described in the previous paragraph. More defects reduce the *mean free path* of carriers and decrease the mobility of carriers in nanotubes.The conductivity is proportional to the metallicity of nanotubes and inversely proportional to the number of scattering centers or defects [24-26]. Considering all these factors, a material quality factor Qm was defined to

the similar diameter dependence to semiconducting ones.

320 Syntheses and Applications of Carbon Nanotubes and Their Composites

SWCNT powders. Reprinted with permission from Ref. [20].

govern the conductivity of SWCNTs:

$$I\_S\left(I\_M\right) = I\_{G^\dagger D} \times \frac{A\_S\left(A\_M\right)}{A\_M + A\_S} \tag{7}$$

where AS(AM) is the areal intensity of semiconducting (metallic) peaks of RBMs from Raman shift. After calculation, it was observed that the sheet conductance reveals a linear relation‐ ship with the material quality factor. Although this empirical formula is not rigorous, it can provide atleast a means to estimate material quality that governs the conductivity of the SWCNTTCFs. Forinstance, large diameter, higher purity, less defects (lower intensity of Dband), and more metallic nanotubes (higher intensity of G'-band) will give better conductiv‐ ity of the SWCNTTC. From this point of view, the Arc TCF is the best sample providing the highest conductivity in comparison toTCFs made by other types of SWCNTs.

In addition to the material parameters discussed above, the length of SWCNTs is also crucial to the TCF performance. According to the percolation theory, a conducting path could be formed at a lower density for longer nanotubes, which means at the same sheet resistance, TCFs prepared with longer nanotubes should exhibit higher optical transparency. This con‐ jecture has been confirmed by experiments [27,28]. In order to optimize the CNTs quality, such as their purity, their dispersibility and the content of metallic tubes, some pretreat‐ ments need to be done. Several attempts have been tried to purify the CNT powders.Gener‐ ally, Gas phase reaction or thermal annealing in air or oxygen atmosphere is used to remove amorphous carbon [29,30]. The key idea with these approaches is a selective oxidative etch‐ ing processes, based on the fact that the etching rate of amorphous carbons is faster than that of CNTs. Since the edge of the CNTs can be etched away as well as carbonaceous parti‐ cles during the annealing, itis crucial to have a keen control of annealing temperatures and annealing times to obtain high yield. Liquid-phasereactions in various acids are always con‐ ducted to remove the transitionmetal catalysts [31-33]. Hydrochloric acid, nitric acid and sulfuric acid are the most commonly used acid, and the purification effect is dependent on the concentration, the reaction temperature and the reaction time. In addition to their reac‐ tion with metal catalysts, nitric acid and sulfuric acid could induce some carboxyl or hy‐ droxyl groups onto the walls of nanotubes, which will improve their dispersibility in water [34,35]. However, some damages were introduced during this process. Therefore, subse‐ quent annealing or ammonium treatment was sometimes carried out to repair the wall structures of the nanotubes to fulfill some special requests.[36]. In order to enhance the con‐ tent of metallic tubes, discriminated adsorption and separation or ion change chromatogra‐ phy was generally used.

non-hydrogen Lewis base theory, [43] polar π system and optimal geometry theory [44] and Hansen parameter [42]. According to non-hydrogen Lewis base theory, all of the solvents can be divided into three groups on the basis of their properties. Class 1consists of the best solvents, *N*-methylpyrrolidone (NMP),*N*,*N*-dimethylformamide (DMF), hexamethylphos‐ phoramide(HMPA), cyclopentanone, tetramethylenesulfoxide and*ε*-caprolactone (listed in decreasing order of optical densityof the dispersions), which readily disperse SWNTs, for‐ minglight-grey, slightly scattering liquid phases. All ofthese solvents are characterized by high values for electron-pair donicity*β*[45], negligible values for H-bond donation parameter *α*,[46] and high values for solvochromic parameter*π*∗. Thus, *Lewis basicity* (availability of a free electronpair) without H-donors is key to good solvation of SWNTs.Class 2 contains the good solvents, toluene, 1,2-dimethylbenzene (DMB),CS2, 1-methylnaphthalene, iodoben‐ zene,CHCl3, bromobenzene and 1,2-DCB. They show *α* ≈ *β* ≈ 0 and high valueof *π*∗. Class 3 entails the badsolvents, *n*-hexane, ethylisothiocyanate, acrylonitrile, dimethyl sulfoxide (DMSO),water and 4-chloroanisole. Badsolvents would have *α* = *β* = *π*∗ ≈ 0. However, the high electron-pair donicity alone has proven tobe insufficient, as dimethyl sulfoxide (DMSO) is not an effectivesolvent for SWNTs even though it contains three lone pairs [47]. A systematic study of the efficiency of a series of amide solvents to disperse as-produced and purified laser-generated SWNTssuggested that the favorable interaction between SWNTs andalkyl amide solvents is attributable to the highly polar *π* systemand optimal ge‐ ometries (appropriate bond lengths and bondangles) of the solvent structures [48]. Howev‐ er, this conclusion is some what undermined by the poor solubility of SWNTs intoluene [47]. Recently, Coleman et al found that the dispersibility of SWCNTs was intimately related with the Hansen parameters of the solvents and it is more sensitive to the dispersive Hansen pa‐ rameter than thepolar or H-bonding Hansen parameter. The dispersion, polar, and hydro‐ gen bonding Hansenparameter for the nanotubes is estimated to be<δD> = 17.8 MPa1/2,<δP> = 7.5 MPa1/2, and<δH> = 7.6 MPa1/2. Success ful solvents exist in only a small volume of Hansen space, which is 17 <δD< 19 MPa1/2, 5 <δP< 14 MPa1/2, 3 <δH< 11 MPa1/2. Hansen parameters have been used successfully to aid solvent discovery. Unfortunately they are not perfect. A number of non-solvents exist in the region of Hansen parameter space close to the solubility

Carbon Nanotube Transparent Electrode http://dx.doi.org/10.5772/51783 323

Compared with organic solvent, it is more efficient to exfoliate SWCNTs into thin bundles or even individual tubes with the assistant of dispersants. The most common dispersants used in TCFs are anionic surfactants including sodium dodecyl sulphate (SDS) and sodium dodecylbenzenesulphonate (SDBS). They are preferable dispersants because nanotubes can be highly exfoliated by them at rather high concentrations [49]. Besides, they nearly have no absorption over the visible spectrum region. However, they are not without disadvantage. Large amount of them is needed to exfoliate nanotubes into thin bundles; usually the CMC (critical micelle concentration) value should be reached [50]. Their residue will increase the sheet resistance of nanotube films significantly since they are nonconductive. In recent years, a lot of research has been done on the dispersion of CNTs with biomolecules such as DNA and RNA [51-54]. There are a number of advantages using them as dispersants.First, they can coat, separate, and solubilize CNTs more effectively with their phosphate back‐ bones interacting with water and many bases binding to CNTs [55]. DNA wrapped around

parameters of nanotubes.
