Interplay of wall number and diameter on the electrical conductivity of carbon nanotube thin films

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Available at www.sciencedirect.com

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Interplay of wall number and diameter on the electrical conductivity of carbon nanotube thin films Guohai Chen a,b, Don N. Futaba Kenji Hata a,b,c,* a b c

a,b,* ,

Shunsuke Sakurai

a,b

, Motoo Yumura

a,b

,

Technology Research Association for Single Wall Carbon Nanotubes (TASC), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Japan Science and Technology Agency (JST), Honcho 4-1-8, Kawaguchi 332-0012, Japan

A R T I C L E I N F O

A B S T R A C T

Article history:

We report the interplay between the carbon nanotube (CNT) structure (wall number and

Received 26 April 2013

diameter) and assembly structure (packing density) on the electrical conductivity of CNT

Accepted 20 August 2013

thin films. By controlling the CNT average wall number from 1.0 to 5.5 (and inevitably

Available online 5 October 2013

changing of the diameter from 3.0 to 8.7 nm), the electrical conductivity of CNT films showed a unique and unexpected phenomenon, i.e. peaking for films made from an average wall number of 2.7 that was 3-times higher than that from single-walled CNTs and 1.6-times higher than that from 5.5-walled CNTs. By developing a first-order model, the individual contributions of individual CNT structure and assembly structure were estimated, and we found that the peak arose from offsetting factors: increase in the effective CNT electrical conductivity and decrease in the packing density with increased wall number. The synergetic effect between the CNT structure and the assembly structure would provide a scientific framework to deeply understand CNT assemblies. Ó 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

When carbon nanotubes (CNTs) are used in practical applications, they are in the form of assemblies as opposed to individual CNTs. Although great attention has been focused on CNTs based on their unique and excellent properties, the properties of CNT assemblies possess practical importance. Many CNT-based macroscopic assemblies with diverse configurations, compositions, and composed of different CNT structures have been developed as represented by CNT thin films, CNT fibers, CNT mats, CNT composites, and buckypapers to meet the specific demands of the target application [1–7]. There are two crucial factors which determine the properties of the assemblies: the CNT structure itself and the manner in which they are assembled, i.e. assembly structure. Variation

in the diameter, length, crystallinity, wall number of the CNTs greatly influences the properties of the assembly. For example, for CNT-rubber composites, the electrical conductivity and mechanical durability have been both improved by using long single-walled CNTs (SWCNTs) due to the long-ranged mesh-like network [8]. In addition, for transparent conductive thin films, double-walled CNTs were found to be most desirable because SWCNTs contained a large fraction of semiconducting tubes (lowering the electrical conductivity) and multiwalled CNTs (MWCNTs) absorb much more photons (reducing the transparency) [9]. Similarly, the assembly structure, e.g. the degree of alignment, packing density, anisotropy, degree of bundling, straightness of the bundles, etc. also influence the properties of the assembly. For instance, a CNT fiber with a considerably great toughness has been achieved by aligning

* Corresponding authors: Fax: +81 29 861 4851. E-mail addresses: [email protected] (D.N. Futaba), [email protected] (K. Hata). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.10.001

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MWCNTs and spinning them into a fiber form [10]. For MWCNT buckypapers, the electrical conductivity was greatly improved by increasing the packing density through both alignment and hydrostatic pressure [11]. Further, numerical simulation has shown that for a CNT composite, the CNT straightness plays an important role in the electrical conductivity [12]. Moreover, for MWCNT composites, the tensile strength and electrical conductivity could be significantly increased after enhancing bundle straightness through a simple mechanical-stretch process [13]. In reality, changes in the individual CNT structure also results in changes to the assembly structure, which increases the complexity of this matter. This interdependency might result in an unexpected and interesting dependence of the CNT assembly performance on the CNT structure, although this aspect, to the best of our knowledge, has not been greatly pursued in the literature. Instead, numerous researches on CNT assemblies in the literature have mainly focused on achieving an assembly showing high performance using a single CNT structure. One of the most important CNT assemblies is the CNT film because it possesses excellent properties, such as low density, highly porosity, flexibility, and high specific surface area, and is expected to be useful for various promising applications, such as supercapacitors, touch panels, solar cells, lighting, random-access memory, etc. [14–18]. For example, CNT thin films due to their highly porous and high specific area have been used as both electrodes and current collectors for high power supercapacitors [14]. Highly flexible and transparent CNT thin films have been used as components in flexible solar cells or touch panels when deposited on plastics [15,16]. Selfstanding CNT films have been used as planar incandescent light sources that emit polarized radiation [17]. Self-standing CNT films have also been assembled layer-by-layer and patterned by standard lithography into conventional electronics as demonstrated by low power resistive random-access memory [18]. These examples demonstrate that the applications of CNT films are highly diverse. Furthermore, these examples show that the electrical conductivity of the CNT film is the principle property that governs the performance of many devices. As such, an intense effort has been dedicated to fabricate highly conductive CNT thin films. For example, by increasing the CNT alignment within a SWCNT film using magnetic fields, the exceptionally low resistivity (as low as 0.095 mO cm) along the alignment axis has been demonstrated [19]. In addition to alignment, the density of CNTs is also known to play a significant role in the properties of CNT films. For the case of a MWCNT buckypaper, by increasing the bulk density from 0.77 to 1.39 g/cm3, the electrical conductivity was doubled from 320 to 640 S/cm [11]. Furthermore, through post-synthesis processes, such as conjugational cross-linking by chemical functionalization, the electrical conductivity of MWCNT buckypaper could reach as high as 6200 S/cm, which was among the highest reported results in recent literature [20]. Finally, the electrical conductivity of a transparent conducting SWCNT film fabricated by a vacuum filtration was shown to exceed 6700 S/cm [21]. As shown, although significant effort has been invested to

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develop highly conductive CNT films, the dependency of the electrical conductivity on the CNT structure and the assembly structure remains not well understood. In this work, we show the electrical conductivity of CNT thin films, in which the interplay between the CNT structure (wall number and diameter) and assembly structure (packing density) results in offsetting effects, which lead to an unexpected phenomenon. Specifically, when CNT thin films were fabricated with different wall number and diameter (average wall number: 1.0–5.5 and average diameter: 3.0–8.7 nm), we found, contrary to our expectations, that the electrical conductivity of CNT films exhibited a peak for films made from an average wall number of 2.7 that was 3-times higher than that from SWCNTs and 1.6-times higher than that from 5.5-walled CNTs. We constructed a simple model to explain this phenomenon and proposed that this phenomenon was a result of the interplay of two opposing effects: (1) the increase in metallicity (i.e. increase the electrical conductivity) with wall number and (2) the decrease in the packing number density (that would decrease the electrical conductivity) with wall number (diameter). We expect to find similar behavior in many other CNT assemblies, and propose that our simple model and analysis would provide a scientific framework to understand them.

2.

Experimental

We synthesized a series of six vertically aligned CNT forests with different average wall number and diameter spanning SWCNTs (mean diameter: 3.0 nm) to 5.5-walled CNTs (mean diameter: 8.7 nm) by controlling the Fe thin film thickness from 1.5 to 6 nm. All forests were grown by using water-assisted chemical vapor deposition (CVD) (super-growth CVD) [22]. Specifically, the detailed recipe is described in the super-growth CVD manual [23]. In short, CNT forests were synthesized with a diluted C2H2 (10% C2H2 in He) using a catalyst of Al2O3 (40 nm)/Fe (1.5–6.0 nm) film sputtered on Si substrates. The growth was carried out at 800 °C with optimized C2H2 of 30–60 sccm and water vapor concentration of 100–270 ppm for 10–15 min. The synthesized CNTs were characterized by transmission electron microscopy (TEM, TOPCON EM-002B) to estimate the wall number and diameter. The forest structures were characterized by scanning electron microscopy (SEM, Hitachi S-4800). The non-woven CNT film was made by dispersing the CNT forests into an organic solvent (Methyl isobutyl ketone) and standard vacuum filtration. An aligned CNT film was fabricated by first laying down the CNTs in the forest to retain the alignment using a custom-built roll-press, then densifying them with capillary force of ethanol (Fig. 1a). Both films were vacuum annealed (180 °C) to remove their respective organic solvents. We took great care to fabricate CNT films with similar mass density, as we found that the electrical conductivity greatly varied with density. The sheet resistance of these films was measured, and the electrical conductivity was calculated accordingly. Temperature dependent resistivity characterization for non-woven CNT films was performed from room temperature to 473 K.

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Fig. 1 – (a) Schematic of synthesizing SWCNT and MWCNT forests by controlling Fe thin film thickness. SEM images demonstrate the forest structure. The blue rectangle shows the fabrication of two types of free-standing CNT thin films. (b) Electrical conductivity (rFilm(E)) of non-woven CNT film vs. wall number. Inset: the schematic packing of CNTs with different wall numbers. (c) Electrical conductivity (rFilm(E)) of aligned CNT film vs. wall number. Inset: SEM image of the aligned CNT film. The double-headed arrows in (a) and (c) indicate the alignment direction. The dashed lines in (b) and (c) are to guide the eyes. (A colour version of this figure can be viewed online.)

3.

Results and discussion

A series of six vertically aligned CNT forests with different and controlled wall number and diameter was synthesized using the water-assisted CVD [22]. By this method, we could grow very long, high purity CNTs (above 99.98%) with high specific surface area that have been proven to be useful for various applications [22,24]. For all of CNT forests, the height was controlled to fall into the range of 500–600 lm to exclude the possible effects of CNT length (Fig. 1a). From this series of forests, two types of free-standing CNT thin films were fabricated: Non-woven (filtration) and aligned (liquid-induced collapse [24]). In this way, we could confirm that the results were not due to the dispersion process, and the agreement in the results would add to their generality. For both series of CNT films, we measured the electrical conductivities experimentally (rFilm(E)). We would like to note that an increase in the diameter is inevitably associated with an increase in the wall number within current synthetic technology, and thus it is not possible to separately control these

two structural parameters. When the electrical conductivity of the CNT films was plotted as a function of the CNT average wall number, we observed an interesting and unreported dependency (Fig. 1b and c). For the non-woven CNT film, the electrical conductivity was 90 S/cm for CNTs with average 1.4 walls and greatly increased with wall number. The interesting observation here was the presence of a peak where the electrical conductivity maximized for films made from an average wall number of 2.7 with electrical conductivity of 169 S/cm which is 2-times higher than that from 1.4walled CNTs. At larger CNT wall number, the electrical conductivity gradually dropped to 120 S/cm for films made from CNTs with average 5.5 walls (Fig. 1b). Interestingly, the identical behavior was observed for the aligned CNT film. For SWCNTs, electrical conductivity of the film was found to be 57 S/cm, which peaked at 178 S/cm for the film made from CNTs with 2.7 average wall numbers, and fell to 115 S/cm for average wall number of 5.5. It should be noted that a similar peaking behavior was observed both along and perpendicular to the alignment direction (Fig. 1c).

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The fact that we observed identical behavior for both films indicates that the phenomenon was independent of the fabrication process, particularly the dispersion of CNTs, but reflected the intrinsic properties of the CNT film. Second, we would like to note that a similar electrical conductivity was achieved for both the aligned and non-woven CNT films indicating that the processing-induced damage to the CNTs was very small. Third, we observed that the level of anisotropy in the electrical conductivity in both of the aligned and nonwoven films was very small to negligible. The small level of anisotropy for the aligned sample agreed with previous reports [19,24–26], and likely stemmed from the different level of alignment of CNTs and the thickness of the film. We would like to note that previous reports have shown that high electrical conductivity anisotropy for a CNT film requires both CNT alignment and very low thickness [19,26]. Many previous reports on CNT thin films that have used CNTs with higher crystallinity have shown much higher electrical conductivity. In this work, the observed electrical conductivity of the CNT films, both aligned and nonaligned, is lower than the highest electrical conducting CNT films in the literatures [19–21]. We believe that our lower observed electrical conductivity is likely a result of the lower crystallinity of our CNTs, as opposed to the assembly structure (e.g. contact resistance, etc.). In addition, most of the researches used only SWCNTs or MWCNTs, and the wall number and diameter were not optimized, and thus we believe the application of the results reported herein could be applied toward many of the previous reports to improve the electrical properties as well. As described in the introduction, the property of an assembly depends on both the CNTs themselves and the manner in which they are assembled. We chose the intrinsic electrical conductivity of the CNT and the CNT packing density as the defining factors in determining the electrical conductivity of a CNT film. To first order, we analytically expressed the electrical conductivity of the CNT films as a product of the effective electrical conductivity of the average CNTs in the film, the CNT number density, and the cross-sectional area of the film: rFilmðCÞ ¼ rCNT  n  A;

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rather is intended to elucidate the contributions of the two factors, i.e. CNT effective electrical conductivity and packing density on the electrical conductivity of CNT thin films.). Direct electrical conductivity measurement of an individual CNT within a forest is difficult because the forest itself is a self-organized aligned assembly of CNTs where CNTs are in contact with many neighboring CNTs and currently no practical method has been developed to distinguish the different effects. We propose a strategy to separately estimate the contributions of the intrinsic electrical conductivity of the CNTs and the packing on the wall number. To address the dependence of the intrinsic electrical conductivity of the CNTs on the wall number, we use the following relationship to estimate the relative effective electrical conductivity of the CNTs (rCNT), rCNT ¼ r0 exp½Eg =ð2kB  298Þ;

ð2Þ

ð1Þ

where rFilm(C) is the calculated electrical conductivity of CNT films (S/cm); rCNT is the ‘‘effective’’ electrical conductivity of individual CNTs (S/cm); n is the number density of CNTs (cm2); A is the cross-sectional area of CNT film (cm2), for comparison, it is assumed to be 1 cm2. The term ‘‘effective’’ was used as this value does not represent the true intrinsic electrical conductivity of the CNT, but includes environmental effects, such as purity, junctions, bundle straightness, crystallinity, etc. Both the effective electrical conductivity and number density depend on the wall number because of the different semiconducting contributions and synthetic dependence of wall number and diameter. As discussed in more detail in the following, the effective electrical conductivity of CNTs was estimated from the effective aggregate band gap of the CNT film as experimentally determined from the temperature dependent resistivity. The CNT number density was estimated from the mass density of the CNT film, the experimentally observed mean diameter, and mean wall number for each CNT forest. (Note, we recognize that our modeling cannot provide a full, accurate description, but

Fig. 2 – (a) Temperature dependent electrical conductivity of the series of non-woven CNT films (Normalized by the electrical conductivity at 298 K). Inset: temperature dependent resistivity with fitting lines to the standard model of temperature dependent resistivity of semiconductors. (b) Relative effective electrical conductivity of CNTs (rCNT) as a function of wall number. Inset: the effective band gap fit from the temperature dependent resistivity. (A colour version of this figure can be viewed online.)

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where Eg is the effective band gap of CNTs and r0 is the preexponential factor. After surveying the literature, we were unable to determine usable values for this pre-exponential factor. Although we do recognize that this factor may vary with the different kind of CNTs (SWCNT, MWCNT, etc.), we assumed that this factor was a constant to allow our model to be solvable. Thus, the relative effective electrical conductivity of the CNTs could be compared for CNTs of differing wall number. To estimate the effective band gap Eg, we treated the CNT films as a semiconducting material with different effective band gaps. The effective band gap represents the band gap of the CNT film as an aggregate which, in reality, is composed of a mixture of semiconducting and metallic CNTs, in which the density of states at the Fermi surface is expected to be modified [27,28]. Therefore, the effective bad gap (Eg) could be estimated by treating the CNT film as a semiconductor using the standard temperature-dependent resistivity (q, X cm) behavior for semiconductors [29,30], q / exp½Eg =ð2kB TÞ;

ð3Þ

we experimentally measured the temperature dependent resistivity q (X cm) of the series of non-woven CNT films from room temperature to 473 K. We would like to remind that CNTs can be either metallic or semiconducting, and the metallicity/semiconductivity increases/decreases with increased wall number [31,32]. By measuring the temperature dependence of the resistivity, we sought to measure the dependence of metallicity/semiconductivity on the wall number that could, in turn, be used to estimate the dependence of CNT effective electrical conductivity on the wall number.

For all six CNT films, the electrical conductivity increased (electrical resistivity decreased) with increased temperature (Fig. 2a). This increase in electrical conductivity means that all the CNT films possess some level of semiconductivity, because truly metallic character would show a fairly constant or a decreased electrical conductivity within this temperature range. The relative increase in the electrical conductivity was observed to be highest for 1.4-walled CNT films (45%) and decreased with increased wall number (less than 10% for the film made from CNTs with 5.5 average walls). This reflects an increase in metallicity of the CNTs with wall number as expected [32]. Using Eq. (3), the effective band gap can be estimated for each wall number (Inset in Fig. 2b). The lines in the inset of Fig. 2a indicate the temperature dependent resistivity was well fitted with Eq. (3). As expected, the effective band gap was highest for SWCNTs and rapidly decreased with increased wall number and plateaued for CNTs above 3walls, indicating the increased metallicity/conductivity with increased wall number. Applying the resultant effective band gap for each case to Eq. (2), the relative effective electrical conductivity of the CNTs (rCNT) could be estimated for each wall number (Fig. 2b). This plot showed that rCNT rose and plateaued above 3-walls with a 6.5-times increase in effective electrical conductivity compared with SWCNTs. Inevitably, within the current CVD technology, an increase in wall number results in an associated increase in diameter. This means that the number of CNTs within the CNT film decreases. Hence, despite increase to the electrical conductivity of the individual CNT, the number of available CNTs contributing to the electrical conductivity decreases. First, in order to

Fig. 3 – (a) Wall number distributions, (b) Diameter distributions, and (c) TEM images of CNTs grown from various catalyst Fe thin film thickness. (A colour version of this figure can be viewed online.)

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estimate the CNT number density for the series of forests, we experimentally determined the average wall number and diameter from TEM observation of each member to generate histograms of wall number and diameter distributions, which were fit to Gaussian functions (Fig. 3). The average wall number was plotted as a function of average diameter, which showed the linear relationship between the diameter and wall number (inset, Fig. 4a). For example, for 1.4 average wall number, the average diameter of CNTs was 3.9 nm, but for 5.5 average wall number, the average diameter of CNTs was 8.7 nm, which is an over 2-times increase in diameter. The method for calculating the CNT number density of an aligned ensemble was an extension of a previous method based on the average size and wall number within the ensemble [33,34]: n ¼ /=k;

ð4Þ 2

where n is the CNT number density (cm ), / is the bulk, macroscopic mass density (g/cm3) of the CNT film; and k is the average linear mass density (g/cm) for an individual CNT. The average linear mass density for the CNTs in the film was calculated from the average wall number and diameter as determined by TEM (inset, Fig. 4a). Previous reports have

Fig. 4 – (a) Linear mass density as a function of wall number. Inset: the diameter vs. wall number. (b) Calculated number density of CNTs in the non-woven CNT films. Inset: the mass density of the series of CNT films. (A colour version of this figure can be viewed online.)

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shown the calculation of the linear mass density, as derived from an atomic model, for SWCNTs [33,34]. Within the scope of SWCNTs, this calculation is solely dependent on diameter. We extended this methodology to multi-shelled CNTs by considering MWCNTs as a linear sum of SWCNTs with a difference in diameter of 0.34 nm. In this manner, we could estimate the linear mass density of any arbitrary CNT with any diameter and wall number (Fig. 4a). As mentioned above, we took great care to fabricate CNT films with similar mass density to avoid the effect from the density. The mass density of the CNT films was estimated directly from the macroscopic mass and volume (inset, Fig. 4b). Therefore, from Eq. (4), the number density for each wall number could be calculated and was plotted as a function of the average wall number (Fig. 4b). The significant decrease in CNT number density is clearly apparent with wall number due to the increased diameter. Following Eq. (1), we calculated the relative electrical conductivity (rFilm(C)) for each member of the series of the nonwoven CNT films (red line in Fig. 5, normalized by the highest value). This plot was overlaid onto the experimentally measured conductivities (rFilm(E) in Fig. 1b and normalized for the purpose of comparison, blue line in Fig. 5). Despite the simplicity of this model, the model agreed well with the experimentally observed electrical conductivity. In particular, a peak was observed in a similar region as experimental observation. In fact, according to our model, a peak should exist because the electrical conductivity of the CNT film is a product of two factors showing opposing behavior with increased wall number. Specifically, with an increase in wall number, CNT conductivity/metallicity increases while the number density decreases. Although our model encapsulates the overall experimentally observed behavior, we do observe some deviation particularly at the SWCNT region. We recognize that, in reality, the CNT film is much more complex assembly than we have presented where other factors such as purity, junctions, bundle straightness, crystallinity, etc. can all contribute. All of these factors were taken to be independent and constant within

Fig. 5 – Normalized experimental electrical conductivity (rFilm(E), Blue) and calculated electrical conductivity (rFilm(C), Red). The dashed lines indicate the peak position. (A colour version of this figure can be viewed online.)

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our model. For example, it is likely that thicker diameter and MWCNTs possess a higher degree of defects, and therefore lower crystallinity. It is noteworthy that, if we were to include crystallinity level into our model, i.e. the crystallinity drops with wall number, our model would show better agreement with the experimental data. However, for simplicity in this work, we have assumed that the crystallinity was constant for all CNT varieties. This assumption was decided on the point of practicality as the accurate and numerical estimation of crystallinity is virtually impossible within current technology, and even if crystallinity could be quantified (e.g. 0–100%), the analytical relationship with electrical conductivity is unknown, which is a highly important research topic. Therefore, to make our simple model solvable, we assume that the crystallinity levels are constant. In addition, it is known that contacts between CNTs are important. However, the importance of the contacts might be less important in this work as the highly dense packing assembled from long CNTs in which the loading of CNTs is quite high. We would like to note that the CNT films were assembled from CNTs with lengths of 500–600 lm which is orders longer than conventional commercial HiPco and other commercial CNTs so that the number of contacts could be minimized. Experimentally, the strong dependence of the electrical conductivity of the films on the wall number means that the electrical conductivity is not dominated by the contact resistance between the CNTs but the internal resistance of the CNTs due to the low crystallinity of MWCNTs. In this manuscript, we are examining the relative changes in the electrical conductivity and therefore the contact resistance throughout our samples is assumed as a constant. Furthermore, SWCNTs are empirically known to be more flexible due to the smaller flexural modulus compared to MWCNTs and thus would generally exhibit less straightness within the assembly. This effect would lead to a decrease/increase in the electrical conductivity for few wall/many wall compared to our model leading to better agreement with experimental results. Finally, a message which deserves great attention is the relationship between the CNT wall number and the diameter. The experimental and calculated results shown here are for a fixed relationship between the wall number and diameter. As noted earlier, these two CNT structural features are linked. Therefore, our results have shown that films comprised of CNTs with an average wall number of 2.7 and an average diameter of 5.4 nm exhibited the highest electrical conductivity. It is now clear through our simple model (Eq. (1)), this peak position can be shifted through a change in the effective CNT electrical conductivity (i.e. wall number) and/or packing density (i.e. diameter). This message underscores an important limitation on the synthesis of CNTs (i.e. the independent control of diameter and wall number).

4.

Conclusion

We have demonstrated the interplay between the CNT structure and the assembly structure on the electrical conductivity of CNT thin films. By examining the electrical conductivity of a series of CNT thin films made from CNT forests of different average wall number (1.0–5.5) and the associated change in

diameter (3.0–8.7 nm), an unexpected peak, higher than that from SWCNTs and MWCNTs, was observed for a film made from CNTs with an average wall number of 2.7. We developed a simple model to separately estimate the contributions of the intrinsic electrical conductivity of the CNTs and the CNT packing in the thin film with increased wall number. By this first-order model, the peak in the electrical conductivity was found to be the result of the offsetting effects of increasing wall number: increase in the CNT electrical conductivity versus decrease in the CNT packing density. The synergetic effect between the CNT structure and assembly structure would give deep insight into the understanding of the mechanism of CNT assemblies and provide valuable information for the practical application of CNTs.

Acknowledgments Support by Technology Research Association for Single Wall Carbon Nanotubes (TASC) is acknowledged. The authors are grateful for the valuable input from Jinping He, Lijuan Yu, and Rakesh Voggu.

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