Electron Transport in the Assemblies of Multiwall Carbon Nanotubes

The assemblies (films) of carbon nanotubes (CNTs) possess very stable, repro­ ducible, and extraordinary electronic properties. These films have been considered as attractive materials for various nanosensors and as electrodes of electrochemical energy storage devices, like supercapacitors, with low equivalent series resistance and highly developed internal surface. In order to develop CNT devices operating at the room temperature, it was necessary to determine the assembled films’ properties, such as the mechanism of conductivity, carrier concentration, and mobility. In this study, we are focused on the assemblies (monolayers, arrays, and films) of multiwall carbon nanotubes (MWCNT). We applied a wide temperature range resistance and magnetoresistance as a tool to determine the transport characteristics of MWCNT films. The measurements of the electrical transport (temperature dependence of the resistance) in the assemblies of nanotubes were tested in the temperature range T = 1.5–300 K, and the magnetoresistance measurements were carried out in pulsed magnetic fields up to 35 tesla in the temperature range 1.5–300 K. The mechanisms responsible for the transport in these systems, including weak localization, antilo­ calization, Luttinger liquid, Shubnikov–de Haas oscillations, and intertube coupling, were observed.

The electrical transport properties are the most important for a number of applications of carbon nanotubes. Usually, singletube samples are used for electri cal transport studies. The kinetic inductance of individual nanotubes contributes to Electron Transport in the Assemblies of Multiwall Carbon Nanotubes DOI: http://dx.doi.org /10.5772/intechopen.89937 We describe the methods of modification and assembly of CNTs and the specificity of the electrical transport in the layers in respect to the technological approach utilized.

Pristine MWCNT (no modification/functionalization)
The purified MWCNTs were dissolved in chloroform/chlorobenzene mixture with the concentration up to 10 −2 mg/ml of visually nonscattered solution after sonication. A droplet of this solution was spread on the water surface, and after evaporation of the solvent, an array of MWCNT was collected on the substrate with inplane macroscopic fingershaped electrodes. The resistance was measured as a function of temperature and magnetic field in the Laboratoire National des Champs Magnetiques Intenses de Toulouse in the range 1.8-300 K and up to 35 T. We review here the most typical features of our results obtained on these nonfunctionalized selfassembled arrays of MWCNT.
As a power law can fit the temperature dependence of the resistance, the forma tion of a Luttinger liquid in which the transport is managed by coherent backscat tering effects could be suggested [35]. Similar power law temperature dependence with the same exponent was found earlier in SWCNT samples [25,26].
With the magnetic field B perpendicular to the plane of our sample, a negative magnetoresistance (NMR) is observed in the whole range of magnetic fields and in the temperatures between 1.8 and 80 K, with a tendency to a saturation at high magnetic fields.
Also, the magnetoresistance shows some oscillations in the magnetic field up to 30 T, at 1.8 K, as it can be seen in Figure 2. At higher temperatures these oscillations tend to vanish. The oscillations can be attributed to the Shubnikov-de Haas (SdH) oscillations. They are clearly seen without background subtraction.
A maximum occurs in the resistance at magnetic field B n when the Fermi energy E F crosses a quantized energy level E n in the field. In graphene [57],  [58] and B n = B 0 /n, where B 0 = E F 2 /(2ev 0 ħ 2 ) = ħk F 2 /2e. Each maximum is labeled with n and plotted as a function of 1/B n in a Landau plot (inset of Figure 2). The wellidentified peaks (n = 1-6) define a straight line. From the slope we determine k F and n s = 4k F 2 /4π ≈ 1.9 × 10 12 cm −2 , which is consistent with the experiment [59].
At this stage of our investigation in these samples, we intend to suggest some explanation of the behavior of the NMR. The transverse NMR has been observed at low magnetic field with a superposition of some aperiodic oscillations consistent with the universal conductance fluctuations (UCF) [3,30]. From a classical point of view, the interference term leading to UCF originates from adding the prob ability amplitudes of all paths that connect the source and drain. The NMR at low field is caused by interference contributions due to closed electron trajectories, which add up constructively at zero magnetic field; that could be considered as the definition of WL [60]. A correct treatment would need the use of the digamma functions [61,62].
While WL is expected to decay slower with the temperature than UCF, a study of the NMR at different temperatures will allow us to discriminate from these two processes. If not, it will signify that the fluctuations could be due to the band struc ture of the ensemble of nanotubes and might be caused by magnetic depopulation of a onedimensional subband, a phenomenon which, in extended thin films, gives rise to SdH oscillations [60].
It must be noticed that the MWCNT in our samples were not arranged as a film but rather form a "carpet" which, under some conditions, should be considered as a 2DEG. In the MWCNT films, the NMR was observed also at low magnetic fields [48], followed by a positive magnetoresistance (PMR), till 12 T. It was argued that the NMR may come from (i) quantum interference [63,64], (ii) thermal fluctua tioninduced tunneling [65], or (iii) Landau levels in disordered graphite [66]. In our case, as the NMR was observed till 25 T and at low temperature, the hypotheses (i) and (ii) can be rejected. As far as the Landau levels, or SdH oscillations [3], could be considered, we notice that the oscillations observed in our experiments appear to be mainly periodic in 1/B (in contradiction with which was observed in [3,30]) and might be related to the SdH oscillations.
As a result, in this section, we report the experiments on temperature and magnetic field dependence of the resistance of selfassembled assemblies of pristine

Chemically functionalized multiwall carbon nanotubes
The electrical transport properties are the most important for a number of applications of carbon nanotubes. Usually, singletube samples are used for electri cal transport studies. Quantum transport properties have been obtained both in singlewall (SWCNT) [5,9,[24][25][26][27][28] and multiwall nanotubes (MWCNT) [3,[30][31][32][33][34][35]. For the nanotube array samples, only mean values of the characteristic parameters were expected [38]. But, in some cases, the transport in the arrays of nanotubes was found to show single nanotube properties at low temperatures due to the mostly conductive nanotubes responsible for the transport [25]. On the other hand, for the specific applications, like chemical [15,16] and bio [17][18][19][20][21] sensors, the synergetic properties of the arrays of nanotube samples are important. They are based on their large surface area per volume and intertube coupling in electrical transport in the arrays of nanotubes [40].
We have shown experimentally that chemically functionalized multiwall carbon nanotubes could be assembled into 2D layers (dense arrays) covering large surfaces with inplane electrodes for electrical and magnetotransport testing.
In contrast to the standard morphologies of the samples of arrays of nanotubes involving definitions of bundles (ropes), mats, networks, etc., based on hardly con trolled deposition from an organic solvent dispersion of pristine nanotubes, we use the LB technique for chemically functionalized multiwall carbon nanotubes. The method we propose offers a radical departure from the existing methodology due to the possibility of covering large surfaces with dense and defectfree, molecularly thin films of carbon nanotubes.
The electrical and magnetotransport properties in the assembled monolayers of carbon nanotubes have been tested.
The organic functionalization of carbon nanotubes had been realized [54][55][56][67][68][69]. But, to our knowledge, up to now, the experimental data on electrical and magnetotransport properties' characterization of dense monolayers manufactured using LB assembling of functionalized nanotubes is very limited. This method is expected to be used for obtaining the layers of very dense arrays of nanotubes for utilizing them for new applications in chemical and biosensors, controlled by electrical transport.
Organic functionalization of MWCNTs was based on the scheme described in [69]. Heterogeneous mixture was heated at 130°C for 3 days. The scheme of the reaction is described in Figure 3. After the reaction was stopped, the organic phase was separated from unreacted material by centrifugation and washing five times with chloroform (CHCl 3 ) and vacuum drying. The obtained dark solid phase was easily soluble in CHCl 3 up to a few mg/ml without sonication. The functional ized nanotubes were tested by HRTEM (Figure 4) and FTIR ( Figure 5) methods. Due to the distinct layer on the surface of the nanotubes, observed by HRTEM (Figure 4), which is responsible for the absorption peaks in 1400-1500 cm −1 and 1800-1900 cm −1 on FTIR spectra (Figures 5 and 6), we conclude that the    functionalization procedure described in [69] for singlewall carbon nanotubes works for MWCNTs. An important feature of FTIR spectrum of MWCNT is the absence of COOH group peak near 3350 cm −1 . This fact has proven covalent bonding of 3methylhippuric acid and panisaldehyde with MWCNTs, and not just physical adsorption of the surfactant.
The deposition of the layers (arrays) of nanotubes on the surface of the devices with the electrodes was done by using the cell, imitating the LB trough. Once a droplet of the solution of functionalized nanotubes in chloroform was spread on the water surface, a droplet of diblock copolymer PSPMMA solution was added in order to create surface pressure of ~9 mN/m. Functionalized nanotubes were self assembled in dense arrays (monolayers), covering without empty space the whole surfaces of the fingershaped electrode devices, when the monolayer was picked up from the water surface (Figure 7). The same layer of MWCNTs on the substrate at higher magnification with SEM is shown in Figure 8.
Utilizing LB method for functionalized nanotubes has shown reliable and repro ducible pA isotherms. Based on this, we expect to use LB method for covering large surfaces with dense and defectfree, molecularly ordered ultrathin films of carbon nanotubes with controlled thickness and orientation.  Perspective of Carbon Nanotubes 8 The electrical and magnetotransport properties of the layers (arrays) of multi wall carbon nanotubes have been tested in the temperature range 1.8-300 K and in magnetic fields up to 35 T.
The nanotube samples on the electrodes with "fingershape" geometry ( Figure 7) have shown low resistance (<1 kOhm at room temperature) and a "weak" temperature dependence of the resistance in the shape of power law in the temperature range T = 4.2-300 K.
The temperature dependences of the resistance in linear and log-log scales for arrays of the functionalized nanotubes are presented in Figure 9. It is interesting to point out that the temperature dependences of the resistance are represented by the power law with the exponent −0.22. While a power law can fit the temperature dependence of the resistance in MWCNT, a behavior suggestive of the formation of a Luttinger liquid, in which the transport is managed by coherent backscattering effects, can be considered [35]. Similar power law temperature dependence with the same value of the exponent was found earlier in SWCNT "bundle" samples depos ited on the top of the contacting electrodes [25,26].
The saturation at low temperatures is noticeable on R(T) dependence of function alized nanotubes. This could be explained in the framework of Coulomb blockade and tunneling between tubes through thin organic layers, covering MWCNT surfaces.
The magnetoresistance measurements were carried out in pulsed magnetic fields up to 35 T in the temperature range 1.8-80 K (Figure 10). The magnetic field  Electron Transport in the Assemblies of Multiwall Carbon Nanotubes DOI: http://dx.doi.org /10.5772/intechopen.89937 orientation in relation to the current direction was considered "normal" to the current direction.
With the magnetic field B perpendicular to the plane of the sample, NMR was observed in the whole range of available magnetic fields between 1.8 and 80 K in the arrays of functionalized nanotubes (Figure 10). As already reported in section 2, the NMR was believed to be observed due to the interference contributions of the closed electron trajectories, which add up constructively at zero magnetic field; that could be considered as the definition of WL [60]. A correct treatment should be based on the use of the digamma function [61,62].
In addition to the NMR at high fields for functionalized nanotubes, we can see a positive magnetoresistance at low fields (inset in Figure 10). The positive magne toresistance increases quadratically and saturates at fields above B = 2 T. The weak antilocalization effect [70][71][72] in multiwall carbon nanotubes [73] appears to be responsible for the positive magnetoresistance. It is attributed to the spindephasing process, arising from the local interfacial fields as a genuine property of the curved multiwall tubes [73].
We have reported the experimental observation of the electrical transport properties of assembled (layers) arrays of nonfunctionalized and functionalized MWCNTs. The negative magnetoresistance as a characteristic of weak localiza tion state was observed. In addition to the negative magnetoresistance at high fields for functionalized nanotubes, we observed positive magnetoresistance at low fields.

Oxidized carbon nanotubes
This technology introduces the formation of oxidized multiwalled carbon nanotubes. Oxidized MWCNTs become hydrophilic and soluble in water. In order to assemble uniform monolayers of MWCNTs on large surfaces we have developed, the socalled "inverted" Langmuir-Blodgett technique, the essence of which is clear from Figure 11. The method consists of the following major technology steps. MWCNT oxidation can be carried out by means of oxidizing liquids such as sulfuric and nitric acids.
MWCNT is dispersed in acid solution (3:1 = H 2 SO 4 :HNO 3 ) and ultrasonicated for a few hours in a water bath at the elevated temperatures. The solution was stirred very well simultaneously. The mixture is filtered by using 0.1micron polyvinyli dene fluoride (PVDF) filter. The filtered oxidized MWCNT is extensively washed with distilled water until pH became neutral. The powder was dried in the oven.
This method generates oxygenated functional groups (OH, C=O, and COOH). Hydroxyl (OH) groups are not highly reactive, but they readily form hydrogen bonds and contribute to making molecules soluble in water.
Carbonyl (C=O) groups have one oxygen atom doublebonded to a carbon atom (symbolized as C=O). Like hydroxyl groups, carbonyl groups contribute to making CNTs watersoluble.
The carboxyl group (symbolized as COOH) has both a carbonyl and a hydroxyl group attached to the same carbon atom, resulting in new properties-it can be ionized-releasing the H from the hydroxyl group as a free proton (H + ), with the remaining O carrying a negative charge.
MWCNT monolayers were prepared by dispersion in deionized water at the concentration of 0.04-0.05 mg/ml. The surfactant hexadecylamine is dissolved in chloroform to prepare the spreading solution at the concentration of 1 mg/ml and spread onto the surface of the oxidized MWCNT solution in the LB trough. The floating monolayer of highly polarized surfactant molecule, having positively charged hydrophilic ends, makes monolayers with negatively charged oxidized car bon nanotubes through electrostatic interaction. The hybrid monolayer compressed in the LB trough at a rate of a few mm/min. Surface pressurearea (pA) isotherm  The electrical transport properties of the monolayers (arrays) of the oxidized multiwall carbon nanotubes have been tested in the temperature range 4.2-300 K and represented in Figure 12.
The oxidized MWCNTs can be assembled in the freestanding layers of arbitrary thickness, utilizing vacuum filtering technique. The resulting MWCNT membranes are very stable and can be further utilized for sensor and energy storage applica tions. Figure 13 shows the SEM image of a few micrometerthick membrane.

Nano-catalysis
The common catalysisoriented goals are to understand and predict the proper ties of nanosized materials and control how they facilitate chemical reactivity. Another critically important issue deals with the manufacture of nanoscale com ponents from the bottom up and finally to integrate nanoscale components into macroscopic scale objects and catalysts for realworld uses.
We have shown a very strong catalytic activity of an outer surface of MWCNT, modified by acid oxidation. Nanocrystals of NaF were formed on the surface of MWCNT (Figure 14). We have also observed metal nanocrystal "decoration" on the surface of MWCNTs.
For example, electrolytic water splitting represents the most environmentally friendly alternative to generate hydrogen gas. However, the kinetics of the oxygen evolution reaction (OER) are slow and require a catalyst. Most catalysts to date have been limited to transition metal oxides or noble metals-both of which are expensive and unsustainable. The OER activity rationalized by the oxygen containing functional groups on the surface of oxidized MWCNTs alters the electronic distribution of the surrounding carbon atoms at the MWCNT surfaces, thereby facilitating the adsorption of water oxidation intermediates. This opens the door to new applications of surfaceoxidized MWCNTs for catalyzing a class of important anodic reactions in water splitting and fuel cells. Further improve ments of the activity of the surfaceoxidized carbon nanomaterials may enable the finetuning of the structure and compositions of hybrid carbon materials for specific applications.

Carbon nanotube assembling and physical cross-linking
We have developed a novel physical method of stable linkage between neigh boring carbon nanotubes in 2D layers (dense arrays), by intertube bridging using Ar + ion beam (Figure 15).
The carbon nanotube layer has modified using ion beam irradiation. It intro duces stable link formation between neighboring carbon nanotubes, in other words, bridging or physical crosslinking.
The intertube bridging (crosslinking) of MWCNTs and SWCNTs in the arrays was observed under the Ar + ion irradiation. This method can be utilized for the improvement of the electrical and thermal conductivity properties of carbon nanotube layers for electron transport heat transfer applications.

Nanosensor applications of carbon nanotube films
In this chapter we would like to point out an example of application of films of MWCNT as a new icing condition resistive sensor that we have developed. These sensors are based on the adsorption of a molecular thin layer of water on the surface of carbon nanotubes and on the detection of the firstorder phase transi tion of water molecules into ice. This transition is very well detected as a result of nonmonotonous dependence of the resistance of the sensor vs. temperature in the vicinity of the freezing point due to a virtual "field effect transistor." Electronic transport in carbon nanotube films, assembled into the resistive films, was found to be extremely sensitive to the adsorption of polar H 2 O molecules.
Modern sensors of icing conditions (optical or piezo devices) are based on the detection of the actual (significantly thick) layers of ice formed on the surfaces. The accumulation of the ice layer is a fast process, and detection of the massive ice formation is too late for the safety of the aircrafts.
We have developed a method of assembling MWCNT films of arbitrary thickness for sensor applications. This method involves oxidation of carbon nanotubes and does have several major advantages over the conventional methods of carbon nano tube assembling via their functionalization. The assembled carbon nanotube films are dense, homogeneous, and strong on the macro level, but internally they consist of disordered structure of selfassembled carbon nanotubes, forming conductive medium, as it is seen in Figure 16. Besides they are hydrophilic and adsorb water mol ecules strongly. Based on our experience of multiwall CNT characterization [22,74], we were able to study them at fixed values of humidity and temperature variation.
Standard sensors of relative humidity based on multiwall carbon nanotubes were developed earlier [75,76].
We have found that the adsorption of the water vapor at the temperatures close to freezing conditions generates a specific nonmonotonous dependence of the resistance of the sensor vs. temperature.
The intensive precipitation of the water vapor, when the temperature is decreas ing, results in the increase of the resistance of the nanosensor, due to the "field effect" created by the adsorbed polar water molecules on the surface of slightly charged CNT tubes. A further decrease of the temperature passing the freezing point results in the sudden drop of the resistance ("lambdapoint"type curve at the phase transitions of the first order) (see the insert in Figure 17) due to the water transition to nonpolar ice crystal. As a result, the "field effect" disappears, and the resistance of the carbon nanotubes decreases again. In order to verify, if the positions of the peaks correspond to the humidity (dew point or frost point), we did temperature scans at different controllable values of the humidity. The dependences of the resistance of the CNT sensor measured at fast temperature scan at different humidity levels and temperature variations from approx +50°C down to −50°C are plotted in Figure 18. The temperature depen dences of the resistance of the CNT sensors measured at the slow temperature scan (≤0.01°C/s in the cryocell of our own design) are shown in Figure 19. As can be seen in Figures 18 and 19, if the temperature drops down, a significant resistance increase takes place, followed by the maximum point of the resistance and sudden resistance drop due to the ice formation.
There is a significant difference in the observation of the dew points and the frost points using CNT sensors. If we observe T dew , which is higher than the freez ing point, we observe significant resistance increase due to condensation. Then the saturation occurs and, at the freezing point, is characterized by a sudden resistance decrease. In the meantime, if we observe frost points instead, while the temperature decreases, the condensation corresponds to the frost point.

Coating carbon nanotubes with diamond-like carbon films
A commercial plasma CVD setup was used to deposit a diamondlike carbon (DLC) film over the CNT matrix (Figure 20).
We found out that thin (~50 nm thick) DLC films have significantly improved mechanical properties.
DLC coating on the CNT layers has a high degree of wettability. We also deter mined that it is adding significant reinforcement to the MWCNT matrix.

Conclusions
Several methods of assembling MWCNTs into monolayers and freestanding films of arbitrary thickness have been developed.
We have studied assemblies of pristine (nonmodified) CNTs, CNTs modified by organic chemical functionalization, and the oxidized CNTs.
We have experimentally tested the electrical transport properties of assembled layers (films) of nonfunctionalized, functionalized, and oxidized MWCNTs.
The temperature dependence of the resistance and magnetoresistance of the selfassembled arrays of MWCNT, tested in the wide temperature range, were con sidered as a tool to determine the transport characteristics of the films, important for further applications.  The negative magnetoresistance as a characteristic of weak localization state was observed. In addition to the negative magnetoresistance at high fields for function alized nanotubes, we observed positive magnetoresistance at low fields.
The layers of MWCNTs have been considered as attractive materials for various nanosensors and as the electrodes of electrochemical energy storage devices. We have shown an example of application of films of MWCNT as an icing condition resistive sensor. A very strong catalytic activity of an outer surface of MWCNT, modified by oxidation, has been pointed out as well.
We have developed a novel method of stable physical linkage between the neighbor ing carbon nanotubes in 2D layers (dense arrays) by intertube bridging using Ar + ion beam and the method of MWCNT coating with diamondlike carbon films as well.