Effect of high-pressure fluorination on electrical properties of multi

Eur. Phys. J. Appl. Phys. (2015) 72: 20403 DOI: 10.1051/epjap/2015150137

THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS

Regular Article

Effect of high-pressure fluorination on electrical properties of multi-walled carbon nanotubes sheet Alnura Omarbekova1 , Sriram Yagneswaran2 , Zharkynay Kuanyshbekova1,2 , Mikhail Kozlov2 , Nicholas Cornell2 , Ramachandran Ramakrishnan3,a , Anvar Zakhidov2 , and Dauletkhan Smagulov1 1 2 3

Machine Tool Industry, Materials Technology and Engineering Production Sub Department, K. Satbayev Kazakh National Technical University, Kazakhstan Nanotech Institute, University of Texas at Dallas, USA Institut de Recherche en Communications et Cybernétique de Nantes (IRCCYN), Ecole Centrale de Nantes, France Received: 6 March 2015 / Received in final form: 25 June 2015 / Accepted: 30 June 2015 c EDP Sciences 2015 Published online: 3 November 2015 – Abstract. High-pressure fluorination of multi-walled carbon nanotubes (MWNTs) sheet was performed at room temperature. This fluorine treatment increases the electrical conductivity of MWNTs sheet. In addition, low-temperature conductivity measurement data gives a good fit to variable-range hopping (VRH) model. Thus, VRH model was adopted to study and understand the increase in electrical conductivity. Estimation of the degree of disorder from the experimental data and VRH model indicate that degree of disorder of fluorinated multi-walled carbon nanotubes (FMWNTs) is higher than that of MWNTs. Higher degree of disorder of FMWNTs sheet substantiate the increase in electrical conductivity when compared to MWNTs. Scanning Electron Microscopy (SEM) and Raman analysis shows that change in structural disorder of FMWNTs network. Effect of fluorination on activation energy was also studied. The activation energy reduces with lower temperatures. The estimated values of temperature coefficient of resistance also show the increase in degree of disorder of FMWNTs. The change in electrical properties of FMWNTs is a much-needed requirement in the application of nano-electronics.

1 Introduction Carbon nanotubes are widely used in diverse applications such as sensors, conductive composites, electronic switches and energy conversion devices [1]. In order to comply with needs of such devices, electrical properties of pristine (as-prepared) material often have to be adjusted to a particular practical requirement. This involves fine-tuning the electrical conductivity of the material, carrier mobility, and density of states at Fermi level. In addition, bulk doping, and property tuning of carbon nanotubes in conventional semiconductors can be achieved using a material functionalization. It is known that diverse functional groups can be attached to a surface of the carbon nanoentities to provide substantial variations in their electrical behavior [2]. Tuning electrical properties of carbon nanotubes facilitates the application of this unique material in electronic devices of next generation [3]. The nanotubes sheet consists of macroscopic assemblies of carbon nanotubes formed by partially aligned multi-walled carbon nanotubes (MWNTs) bundles and it possess well-defined electrical properties [4]. If attached to polymer films, MWNTs sheet make their surface a

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electrically conductive. Therefore, this is important for applications in the area of flexible solar cells and display screens [5]. Because of very low areal density, substantial gravimetric strength, and uncommon structure, nanotubes sheet exhibit exceptional performance as high-stroke electrostatic actuators and wide band thermoacoustic sound transducers [6]. It has been reported in literature [7] that nanotubes fluorination results in binding of fluorine to carbon atoms that are believed to be a promising way of property modification. The evaluation of property tuning enabled by the fluorination of carbon nanotubes sheet is explored in this paper. To achieve the direct covalent functionalization, MWNTs sheet were treated with fluorine gas at high pressure similar to conditions in related study reported by Plank et al. [8]. Chai et al. [9] suggested that changes in electrical properties of the fluorinated multi-walled carbon nanotubes could be beneficial for their applications in diverse electronic devices. Model of DC conductivity (electrical property) of MWNT was developed by Ram et al. [10] to study the variations of DC conductivity against composition and temperature. In 2015, Jung et al. [11] performed thermal fluorinations on MWNTs at various temperatures and its surfaces were modified to obtain electric double-layer capacitors.

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The European Physical Journal Applied Physics

High temperature fluorination of nanotubes involves treating the nanotube with fluorine in rich reactive environment (at high temperature). After high temperature fluorination of nanotubes, resistance measurements along with vibrational spectroscopy in literature [12] by Mickelson et al. confirm the formation of new chemical bonds to the nanotube superstructure. The fluorination study [12] on single walled nanotubes also indicates that at 500 ◦ C, the single-wall tubular structure does not survive the fluorination process and then some MWNTs like structures are being formed. The studies [13,14] illustrates that excessive fluorination destroys the internal structure and reduces electrical performance of nanotubes samples. Continued accumulation of covalent C-F bonds eventually results in a significant drop of bulk electrical conductivity of the pristine material. It can be avoided upon optimization of treatment conditions, lowering temperature in particular. It has been reported in literatures [15,16] that fluorinated carbon nanotubes used for electrodes of capacitors and batteries, often enhance their performance. Advantage of high-pressure fluorination over the high temperature fluorination [12] is that multi-walled tubular structure of fluorinated MWNTs survives the fluorination and remains the same after the process. However, the present work focuses on fluorination of sheet of multiwalled carbon nanotubes in high pressure followed by their scanning electron microscopy (SEM), Raman and electrical characterization. From the literatures, it is evident that meager work has been carried out in investigating the uncommon electrical properties of fluorinated MWNTs sheet such as electrical conductivity and temperature coefficient of resistance. In this research paper, an attempt has been made to study the increase in electrical conductivity of MWNTs sheet by fluorination process. The following sections will elaborate the experiments carried out and results obtained.

2 Experimental part The dry-spun MWNTs forests were grown by the chemical vapor deposition (CVD) method on Si wafers using acetylene gas as a precursor [17]. The MWNTs sheet used in the experiment were prepared by pulling the sidewall of a MWNTs forest using a technique developed at the University of Texas at Dallas. A 70 × 30 mm rectangular sheet of prepared MWNTs was attached to a Teflon holder in free form as shown in Figure 1a. Schematic diagram of fluorination experimental setup was shown in Figure 1b. It consists of components such as N2 gas cylinder, F2 gas cylinder, stainless steel reactor, scrubber, bubbler, vacuum, wheel valves (V), pressure regulators (PR) and pressure gauge (PG). The MWNT sample was then placed in the fluorination reactor (a gas flow reactor). Initially, before the experiment, the wheel valves V1 , V2 and V6 are closed and the rest of valves are in open condition. At this stage, the vacuum is filled inside the stainless steel reactor and tubes. After this stage, V3 and V7 are closed and rest of the

(a)

(b)

Fig. 1. (a). MWNTs sheet attached to Teflon holder and (b) schematic of flow reactor used for sample fluorination.

valves is open. The reactor was pacified with F2 gas mixture (5% F2 gas diluted in N2 gas 95%, air products) for 30 min, and then thoroughly flushed with nitrogen. In the next step, V3 , V5 , V6 and V7 are closed and the rest of the valves are open. Meanwhile, PR1 and PR2 are opened to fill F2 and N2 gas mixture in to the stainless steel reactor. The fluorination treatment was performed (sample was exposed to fluorine gas, which is mixture of 5% F2 in N2 ) under high pressure in a stainless steel cylinder at room temperature (300 K). During this experiment (V4 , & V5 is closed), the gas pressure in the reactor was kept at 276 kPa for 168 h. Finally, wheel valves V1 , V2 and V7 are closed and rest of the valves are open. At this stage, the gases inside the reactor and pressure line are vented into the atmosphere through scrubber and bubbler. Pristine and treated samples further in the text will be labeled MWNTs and FMWNTs (fluorinated multi-walled carbon nanotubes), respectively. SEM images of sheet were acquired using a LEO 1530 field-emission scanning electron microscope. Then, Raman measurements were performed using Jobin Yvon LabRam HR800 Raman Microscope equipped with He:Ne laser (λ = 632.8 nm). For electrical conductivity characterization, MWNTs and FMWNTs sheet were transferred into a holder. Then the measurement of change in electrical resistance from the room temperature to 3 K was performed in Quantum Design PPMS system using a four-probe

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A. Omarbekova et al.: Effect of high-pressure fluorination on electrical properties of multi-walled carbon nanotubes sheet

(a)

(b)

Fig. 3. Raman spectra of MWNTs and FMWNTs sheet.

Fig. 2. SEM images of (a) MWNTs and (b) FMWNTs sheet.

technique. Change in electrical resistance of sheet over time was recorded with a Keithley 2402 Source meter at room temperature using a two-probe technique. Measurements of MWNTs and FMWNTs samples at the same conditions enabled direct comparison of their properties. The results obtained are analyzed and the discussions about the results are explained in the following sections.

electronic properties ranging from non-metallic to metallic [16]. On the other hand, the properties of fluorinated carbon nanotubes are controlled by the type of nanotubes, fluorine attachment pattern and material stoichiometry [18]. Moreover, when fluorine atoms are normally bound to sp3 carbon, π bonding in the sp2 nanotubes is affected by the partial fluorination resulting in the variation of electrical conductivity. In particular, substantial changes can be expected in the material surface morphology [16,18]. The theoretical analysis reported in [19] predicts the formation of helical poly-acetylene-like chains of π-bonded sp2 carbon atoms on the nanotubes surface. Because of high electrical conductivity, the chains can serve as nanoscale wires forming nano-solenoids and other components of molecular electronics. A quite rough surface pattern visible on SEM images of fluorinated sheet (Fig. 2b) seems to support this prediction. Certainly much higher magnification than that available from the present SEM is required in order to validate this assumption. In any case, a significant increase in room-temperature electrical conductivity of FMWNTs sheet is highly beneficial for applications in electronic devices.

3 Results and discussion 3.1 SEM analysis

3.2 Raman spectral analysis

Scanning electron microscope (SEM) images of pristine and fluorinated MWNTs sheet are shown in Figures 2a and 2b respectively. The nanotubes bundles in the pristine material have a uniform smooth surface as seen in Figure 2a. However, the appearance of bundles substantially changes after fluorination. The surface after fluorination reveals some roughness that can be associated with structural changes as seen in Figure 2b. These changes are caused by the fluorine attachment on the outer surfaces of MWNTs. It is known that treatment of carbon nanotubes with fluorine may lead to structures with quite different

The comparison of Raman spectra of pristine and fluorinated sheets are shown in Figure 3. Both specimens exhibit intense D and G bands with pristine specimens (MWNTs) banding at 1344 cm−1 and 1576 cm−1 respectively and these values are typical for MWNTs [20]. Similarly, Figure 3 shows the D and G bands values of FMWNTs banding at 1356 cm−1 and 1585 cm−1 respectively. Raman shift shows a slight shift in the intensity of MWNTs and FMWNTs specimens. Relative intensities of these peaks change more substantially so that D/G intensity ratio increases from about 0.38 for pristine to 0.51 for fluorinated sheets.

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This increase in D/G ratio can be ascribed to the structural disorder of nanotubes surface induced by the fluorination. This conclusion is in good agreement with acquired SEM images (Fig. 2) and literature data [21]. Corroboration of these findings comes from the data given in the literature [22] explaining how functionalized carbon nanotubes experience chemical modification on the surface. Surface modification results in the appearance of disorders, which include adatoms, vacancies and geometrical alterations of the covalent bonding that produces diverse pentagon and heptagon linkages. This defects cause problems such as alteration of Raman spectra, variation of intensity of D, and G bands similar to those presented in Figure 3. On the other hand, fluorine treatment causes the appearance of structural deficiencies in investigated nanotubes assemblies. The deficiencies enhance D band in Raman spectra of FMWNTs samples and substantially affects their low temperature electrical properties. Within the framework of the variable-range hopping (VRH) model, the disorder can be characterized by the parameter T0 that for treated material is almost three times higher than that for pristine MWNTs sheet. Structural imperfections can be related to the appearance of some sp3 sites on nanotubes surfaces caused by covalent fluorine bonding. This assumption agrees with theoretical calculations [18] that evidence significant increase of band gap for fluorine phase fabricated at low temperatures. Moreover, the high-pressure fluorine treatment deteriorates contact between nanotubes in investigated assemblies causing an increase of activation energy. Its impact should be especially strong in regions with numerous “dangling” bonds near nanotubes ends and vacancies that are sensitive to fluorine binding. 3.3 Electrical conductivity The comparison of time dependences of sheet resistance (R) for MWNTs and FMWNTs sheet are shown in Figure 4a. FMWNTs sheet was moved from oxygen-free environment (inside the reactor) to open atmosphere. Sheet resistance of MWNTs and FMWNTs were recorded right after fluorination. The initial sheet resistance (that was lowest) of MWNTs and FMWNTs were found to be 688 Ω/ (a common unit used for sheet resistance is “ohms per square”, which is dimensionally equal to an ohm, but is exclusively used for sheet resistance) and 317 Ω/ respectively. Material property stabilization of FMWNT is reached at 1020 min due to fluorine release. It was observed the sheet resistance of FMWNTs increases gradually from 317 Ω/ to 418 Ω/ until 1020 min, that is, 11% increase in resistance. However, MWNTs show a decrease in resistance from 688 Ω/ to 672 Ω/ until 1020 min, that is, 2% reduction in resistance. It was estimated that the resistance value of FMWNTs was 40% lower than that of MWNTs sheet after stabilization. It is evident from the above results that fluorination causes a decrease of sheet resistance that can be ascribed

(a)

688 418

Material property stabilization

MWNT FMWNT 672

377

1020

(b)

MWNT

FMWNT

Fig. 4. (a) Time dependence of sheet resistance of FMWNTs and MWNTs and (b) temperature dependences of normalized conductivity of MWNTs and FMWNTs.

to charge carriers created by fluorine doping. The conclusion is consistent with the data reported in literature [6] where carbon nanotubes are chemically functionalized. Since, it is obvious that the resistance is inversely proportional to electrical conductivity. From the sheet resistance response, it can be also concluded that there is an increase in electrical conductivity with fluorination. To study and understand the mechanism of increase in electrical conductivity, temperature dependences of electrical conductivity σ(T )/σ (300) (normalized by electrical conductivity at room temperature) versus T −1/4 for both MWNTs and FMWNTs sheet are plotted as shown in Figure 4b. At 80 K, the dependences are linear and it follows the same trend of curve obtained in variable-range hopping (VRH) model. According to the VRH model [23], the electrical conductivity of a material can be expressed with the following equation: 1/4 T0 σ (T ) = σ0 exp − , T

(1)

where, σ0 is the pre-exponential factor, T is the temperature and T0 is the parameter related to degree of disorder in the system.

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A. Omarbekova et al.: Effect of high-pressure fluorination on electrical properties of multi-walled carbon nanotubes sheet Table 1. Estimation of activation energy. T (K) 220 150 75 25 4

−5

KB (eV/K) (×10 8.62 8.62 8.62 8.62 8.62

)

σ0 (Ω

−1

cm−1 )

1 1 1 1 1

σ (Ω−1 cm−1 ) MWNT FMWNT 1.53 1.73 1.59 1.83 1.74 2.05 1.92 2.57 3.34 4.47

The phenomenon of low temperature transport in disordered systems was discussed by Mott [24]. He has explained about the variable-range hopping concept of localized electrons between the different sites in the disordered system. In this conduction process, the electron will hop between localized states with energy as low as possible. These states are kept in a hopping distance. As per the VRH model, the hopping distance increases with decrease in temperature [25,26]. Hence, the electrical conductivity equation of VRH model is adopted to analyze the results obtained in this study. Fitting experimental data points with equation (1), T0 values are calculated as 7 K and 20 K for MWNTs and FMWNTs sheet respectively. It is observed that degree of disorder value of FMWNTs is higher than that of MWNTs sheet. According to the equation (1), electrical conductivity increases with increase in degree of disorder. Hence, this numerical analysis from the experimental data conclude that fluorination process improve the electrical conductivity of FMWNT nanotubes. In addition, activation energy is also estimated to study the effect of fluorination on activation energy. In the literature [27], the relation between the electrical conductivity and the activation energy is given by: ΔE σ (T ) = σ0 exp − , (2) kB T where, ΔE is the activation energy and kB is Boltzmann constant. The activation energy with respect to different temperature was estimated with the help of the experimental data and then it is summarized in Table 1. It is observed that, the activation energy reduces with decrease in temperature. Moreover, substantial increase in T0 of FMWNTs can be associated with the disorder in transport systems with fluorinated nanotubes. This phenomenon is in good agreement with SEM and Raman data that clearly shows a substantial increase of structural deficiency of FMWNTs as compared with the pristine material.

3.4 Temperature coefficient of resistance To identify a right FMWNTs material for applications such as electronic switches, energy conversion devices and sensors, it is essential to estimate the temperature coefficient of resistance (TCR). In here, it is estimated using

T0 (K) MWNT FMWNT 7 20 7 20 7 20 7 20 7 20

ΔE (eV) (×10−3 ) MWNT FMWNT 8.01 10.40 6.01 7.81 3.57 4.65 1.41 2.04 0.42 0.52

experimental data acquired. The temperature coefficient of resistance (α) [28] is given as: dR(T ) 1 , (3) α= R(T ) dT where, R(T ) is sheet resistance as a function of temperature. From equation (3), near room temperature, TCR value of MWNTs and FMWNTs were estimated and it is found to be 2.3 × 10−4 K−1 and 5.1 × 10−4 K−1 respectively. Higher TCR values of FMWNTs emphasis the increase in degree of disorder in transport system induced by the fluorination [29,30].

4 Conclusions Present work focuses on fluorination of dry-spun sheet of MWNTs sheet followed by their SEM, Raman and electrical characterization. Unlike nanotubes powders, investigated material consists of macroscopic carbon nanotubes assemblies that greatly simplify fabrication of diverse electronic devices. The electrical sheet resistance of FMWNTs decreases upon exposure of sheet to air but still remains 40% lower than that of MWNTs (pristine). From the sheet resistance curve, it is also concluded that high-pressure fluorination of the nanotubes sheet increases the electrical conductivity. Variable-range hopping (VRH) model was adopted to study the increase in electrical conductivity. Analysis of low temperature electrical conductivity of FMWNTs sheet using the variable-range hopping model indicates an increase in degree of disorder in the transport system. The estimated degree of disorder values of MWNT and FMWNT are 7 K and 20 K respectively. FMWNTs sheet possess higher degree of disorder when compared to MWNTs sheet. From the Mott’s equation, it is believed that FMWNTs sheet have higher electrical conductivity than MWNTs due to increase in degree of disorder values. This is in a good agreement with SEM and Raman data’s and it clearly show the enhancement of structural deficiency in FMWNTs. The effect of fluorination on activation energy was studied. It was observed that activation energy of MWNT and FMWNT decreases with decrease in temperature. Finally, the temperature coefficient of resistance (TCR) is estimated from the experimental data to identify the exact FMWNTs material for the electronic devices application. The estimated TCR values of FMWNTs and MWNTs are 2.3×10−4 K−1

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and 5.1 × 10−4 K−1 respectively. Increase in degree of disorder in transport system induced by the fluorination is reflected in higher TCR values of FMWNTs when compared to TCR value of pristine material.

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