Understanding the doping effects on the structural

Understanding the doping effects on the structural and electrical properties of ultrathin carbon nanotube networks Ying Zhou, Satoru Shimada, Takeshi Saito, and Reiko Azumi Citation: Journal of Applied Physics 118, 215305 (2015); doi: 10.1063/1.4937137 View online: http://dx.doi.org/10.1063/1.4937137 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/118/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structure, electronic properties, and aggregation behavior of hydroxylated carbon nanotubes J. Chem. Phys. 141, 174703 (2014); 10.1063/1.4900546 Electrical properties of air-stable, iodine-doped carbon-nanotube–polymer composites Appl. Phys. Lett. 91, 173103 (2007); 10.1063/1.2801353 The doping effect of multiwall carbon nanotube on Mg B 2 ∕ Fe superconductor wire J. Appl. Phys. 100, 013908 (2006); 10.1063/1.2209188 Terahertz absorption and dispersion of fluorine-doped single-walled carbon nanotube J. Appl. Phys. 98, 034316 (2005); 10.1063/1.2001751 Amphoteric doping of carbon nanotubes by encapsulation of organic molecules: Electronic properties and quantum conductance J. Chem. Phys. 123, 024705 (2005); 10.1063/1.1931547

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JOURNAL OF APPLIED PHYSICS 118, 215305 (2015)

Understanding the doping effects on the structural and electrical properties of ultrathin carbon nanotube networks Ying Zhou,1,a) Satoru Shimada,1 Takeshi Saito,2 and Reiko Azumi1 1

Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, 305-8565 Tsukuba, Japan 2 Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, 305-8565 Tsukuba, Japan

(Received 3 September 2015; accepted 21 November 2015; published online 7 December 2015) Similar to other semiconductor technology, doping of carbon nanotube (CNT) thin film is of great significance for performance improvement or modification. However, it still remains a challenge to seek a stable and effective dopant. In this paper, we unitize several spectroscopic techniques and electrical characterizations under various conditions to investigate the effects of typical dopants and related methods. Nitric acid (HNO3) solution, I2 vapor, and CuI nanoparticles are used to modify a series of ultrathin CNT networks. Although efficient charge transfer is achieved initially after doping, HNO3 is not applicable because it suffers from severe reliability problems in structural and electrical properties, and it also causes a number of undesired structural defects. I2 vapor doping at 150  C can form some stable C-I bonding structures, resulting in relatively more stable but less efficient electrical performances. CuI nanoparticles seem to be an ideal dopant. Photonic curing enables the manipulation of CuI, which not only results in the construction of novel CNT-CuI hybrid structures but also encourages the deepest level of charge transfer doping. The excellent reliability as well as processing feasibility identify the bright perspective of CNTC 2015 AIP Publishing LLC. CuI hybrid film for practical applications. V [http://dx.doi.org/10.1063/1.4937137]

I. INTRODUCTION

Thin film of carbon nanotube (CNT), being composed of a random or oriented network, is one of the most promising technologies for future electronics.1–3 For example, CNT films exhibiting high electrical conductivity and high optical transparency can be a transparent electrode instead of conventional indium tin oxide (ITO). Both of experiments and theory have shown that CNTs can have high electronic mobilities of 10 000–100 000 cm2/V s and high conductivities of up to 400 000 S/cm, while it still remains a challenge to translate the excellent electrical properties of individual CNTs to two-dimensional networks.4,5 For instance, without further chemical doping, even high-quality CNT films exhibit sheet resistances of 300–1000 X/sq at around 85% optical transmittance,6–10 which are significantly higher than ITO (10–20 X/square at 90% transmittance). The poor electrical transport is attributed to the complexity in CNT films: first, CNTs are unavoidably doped by oxygen or oxygen functional groups, which lead to damages in the graphitic structure, and correspondingly a serious deterioration in the electrical properties,11–14 because these bonds on the CNT sidewalls cause the localization of the delocalized p electrons occupying a one-dimensional density of states;15 second, the Schottky barrier between metallic and semiconducting CNTs greatly suppresses the electrical transport;16 finally, the spacing between CNTs due to the weak van der

a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]

0021-8979/2015/118(21)/215305/9/$30.00

Waals interaction may significantly increase the overall resistance by a factor as high as 10.17,18 Chemical modification and doping are usual strategies to improve the electrical conductivity of CNT films. Many dopants including halogen (Br2),19 acids (HNO3, and HCl),20,21 chlorides (FeCl3, and AuCl3),22,23 metal oxides (MoOx),24 and organic molecules25 have been investigated. Those dopants encourage the charge transfer between them and CNT and enable the Fermi level tuning of CNT for desired electronic properties. Moreover, doping treatments can reduce the CNT-CNT contact resistance and increase the carrier concentration of semiconducting CNT, which can provide more conductive films than metallic CNTs no matter they are doped or not.26,27 However, most of the dopants are not stable to air, temperature, or humidity, resulting in unstable electrical properties in CNT films. Therefore, the reliability of doping is becoming one of the biggest challenges for practical application. Recently, a technique to construct interconnecting nodes in CNT networks with copper halides including copper iodide (CuI), copper bromide (CuBr), and copper chloride (CuCl) has been reported.28 The CNT-halide hybrid films exhibited extremely durable sheet resistance of 55–65 X/sq at 85% transmittance in air for more than 1000 h. It has shown that besides its potential in organic photovoltaics,29,30 copper halides can be excellent dopants for CNT. Apparently, understanding these dopants and related treatment is critical to develop efficient and reliable methods for future nano-carbon based electronics.3,12,31–35 In this paper, we systematically investigate the doping effects on structural and electrical properties of the single

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walled CNT films. We develop a high-quality CNT dispersion to prepare CNT films with well-controlled thickness and CNT density. HNO3, iodine (I2), and CuI are utilized to modify CNT; these treatments cover typical processes based on liquid, gaseous, and solid phases, respectively. Several spectroscopic techniques including Raman scattering, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) are used to study the variations in the structural defects of CNT. The electrical behaviors under different environmental conditions (i.e., varying the temperature from 80 K to 450 K) are discussed in detail. The results indicate that HNO3 doping causes a large number of undesired structural defects (C-O bonds), which are very sensitive to vacuum or heating at 450 K. On the other hand, CuI is an efficient and reliable dopant, which not only significantly suppresses the formation of structural defects but also greatly improves the electrical conductivity of ultrathin CNT networks by more than 400 times. The outstanding electrical properties and stability identify that CuI nanoparticles act as interconnecting nodes in CNT network to achieve ideal CNT-CNT contacts for efficient and stable charge transport. II. EXPERIMENTAL

Single-walled CNT with a mean diameter of 1.5 nm was synthesized by enhanced direct injection pyrolytic synthesis (eDIPS) method.36 5 mg of CNTs was added in 20 ml purified water mixed with 0.1 g of hydroxypropyl cellulose (HPC, Nippon-Soda),37 and then, the solution was ultrasonicated for 30 min with a probe type homogenizer (Sonifier S-450 400 W). A homogeneous solution was obtained after centrifugation at 40 000 rpm for 1 h at 20  C. The solution was spin-coated on glass and Si substrate with spin speeds ranging from 1000 to 5000 rpm. Then, CNT films were treated differently for doping. For CuI doping, 0.5-nm-thick CuI film was vacuum evaporated on CNT-HPC film. The thickness and growth rate were monitored with a quartz crystal oscillator. A photonic curing process was unitized to remove HPC dispersant and form CNT-CuI hybrid films. For other samples, the films were also treated by the photonic curing process, followed by ultrasonic bath in purified water for 1 h to thoroughly remove HPC dispenser. For HNO3 doping, CNT film was immersed in 13 M HNO3 solution for 30 min, followed by rinsing with purified water. For I2 doping, CNT film was exposed to I2 vapor in a sealed box on a hotplate for overnight heating at 150  C. The atomic force microscopy (AFM) was carried out using a SPA300 (SII) system with a dynamic force mode. Raman spectroscopy was performed using a RamanStation 400 (PerkinElmer) spectrometer with an excitation laser at 785 nm. XPS was performed at a PHI 5000 VersaProbe (ULVAC) spectrometer with a monochromatic Al Ka source (1486.6 eV) at an incidence angle of 45 . Carbon 1 s line (284.8 eV) was used to calibrate the binding-energy scale. FTIR measurement was utilized to investigate the chemical structure with FT/IR 6600 (JASCO) in vacuum. To investigate the structural stability, samples were annealed at 200 for 2 h at 1  104 Pa.

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For electrical measurement, Cr (2 nm) /Au (40 nm) pattern was evaporated on CNT film with a mask as electrode contact. Four-probe electrical measurement was performed with Keithley 2000 in a vacuum system by following the procedure: (1) initially measure in air atmosphere at around 300 K (room temperature); (2) measure in vacuum of less than 104 Pa; (3) decrease the temperature to 80 K with liquid nitrogen, hold for 30 min; (4) increase the temperature to 450 K with a rate of 2 K/min, keep every 10 K for measuring; (5) hold at 450 K for 2 h; (6) decrease the temperature to 80 K, keep every 10 K for measuring; (7) wait for temperature returning to around 300 K, break the vacuum, finally measure in air atmosphere. III. RESULTS AND DISCUSSION A. Structural characterization

Figure 1(a) schematically illustrates the fabrication process of CNT-CuI hybrid. Here, HPC, a derivative of cellulose is utilized to prepare a stable aqueous CNT dispersion, which enables the control of the thickness and density of ultrathin CNT networks by spin coating. Figures 1(b)–1(f) show the surface morphology of the undoped CNT films, and Table I gives the detailed structural characteristics. With simply varying the spin speed from 5000 to 1000 rpm, the CNT density is increased by three times from 39 to 123 CNTs/lm2, resulting in a great increase in CNT-CNT contact from 60 to 412 counts/lm2. In addition, doping with HNO3 or I2 does not change the surface morphology of CNT bundles. On the other hand, CuI doping leads to a unique CNT-CuI hybrid film, as shown in Figures 1(g)–1(k). Interestingly, very few of CuI nanoparticles is observed on bare substrate (also see Figure S1),38 although the distribution of CuI is quite uniform before photonic curing. Increasing the CNT density leads to an increase in CuI density with a consequent decrease in size. The morphology of CNT networks plays an important role in the structural evolution of CuI nanoparticles. Apparently, CuI prefers to grow on CNT rather than on the substrate, possibly due to the strong interaction between CuI and CNT.35 Note that above 75% of CuI nanoparticles are located at CNT-CNT contact areas (Table I), especially, for the CNT film with a low density of 39 CNTs/lm2. The photonic curing can realize a rapid heating and cooling process to control the melting and solidifying process of CuI at a microsecond timescale. We have shown the detailed structural evolution of thick CNT-CuI films, suggesting that photonic curing enables the manipulation of CuI nanoparticles.28 Here, it is identified that the strong interaction between CuI and CNT39 also makes this technique feasible for CNT networks with any desired density (thickness), which can expand its applications for transistor and other optoelectronic devices. Doping is expected to modify the electronic and photonic properties of sp2-hybridized CNT, which is evaluated by Raman spectroscopy at k ¼ 785 nm excitation, as shown in Figure 2. Undoped CNT film exhibits typical defectinduced D-, graphitic G- and G0 -bands near 1310, 1593, and 2598 cm1, respectively. The G0 -band exhibits extreme sensitivity to the p electronic structure, and accordingly, it

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FIG. 1. (a) Schematic illustration showing the fabrication process of CNT-CuI hybrid film. AFM images of CNT films spin-coated with spin speeds of (b) 5000 rpm, (c) 4000 rpm, (d) 3000 rpm, (e) 2000 rpm, (f) 1000 rpm and CNT-CuI hybrid films spin-coated with spin speeds of (g) 5000 rpm, (h) 4000 rpm, (i) 3000 rpm, (j) 2000 rpm, and (k) 1000 rpm, respectively. The scan area is 1  1 lm2. The height contrast is used to evaluate the film thickness, as given in Table I.

TABLE I. Structural characteristics of CNT and CNT-CuI networks. Spin-coat (rpm)

5000

4000

3000

2000

1000

CNT

Thickness (nm) Density (CNTs/lm2) Node density (counts/lm2)

3.5 6 0.2 39 60

4.0 6 0.2 64 150

4.4 6 0.2 82 214

4.5 6 0.2 95 246

6.6 6 0.2 123 412

CNT-CuI

Thickness (nm) CuI density (counts/lm2) Node density (counts/lm2)

4.9 6 0.4 77 54

6.0 6 0.5 126 91

5.4 6 0.3 213 155

7.0 6 0.3 249 208

7.5 6 0.3 360 252

FIG. 2. Raman spectra of CNT films with different doping treatments before (a) and after (b) vacuum heat treatment at 200  C. The spectra were normalized for clarity. The blueshifts of peaks indicate p-type doping of CNT.

provides a very useful probe for characterizing the doping of CNT.40 Doping with HNO3, I2, and CuI shifts the G0 -band to 2604, 2604, and 2612 cm1, respectively (see Figure 2(a)). It indicates the electron transfer from CNT into the dopants, corresponding to a downshift of Fermi level toward the valence band of CNT.41 Thus, these dopants act as electron acceptors for typical p-type doping. The largest blueshift of 14 cm1 suggests that CuI can be an efficient dopant for nano-carbon materials. On the other hand, a double-fold increase is identified in the intensity of D-band for HNO3 doping, while only a slight increase is observed for doping with I2. Interestingly, CuI doping leads to a large decrease in D-band. It implies that doping treatments also modify the structural defects in CNT.42 Despite the fact that sp2-

hybridized structure of CNT is extremely stable, the dopant molecules are capable of intercalating the CNT bundles or reacting with the structural defects in CNT. CuI doping exhibits a feasibility to improve the quality of CNT by reducing the undesired structural defects. Heating at 200  C plays a negligible role in Raman spectra of undoped CNT film (see Figure 2(b)). The G0 -band shifts to 2599, 2606, and 2614 cm1 for doping with HNO3, I2, and CuI, respectively. Obviously, HNO3 doping is very unstable, and its effects on charge-transfer doping almost disappear after heating. Moreover, the intensity of D-band is further increased, indicating that structural defects are increased, accompanying with removal of the physically absorbed dopant molecules induced by HNO3. On the contrary, for I2 and CuI doping,

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no clear variation can be observed in Raman spectra, showing their stability. XPS offers further insight into the structural variations induced by doping treatment. Here, relatively thicker CNT films are prepared to avoid the influence of glass substrate. Figure 3 shows the XPS spectra for CNT with different doping treatments in C 1 s, O 1 s, and I 3 d regions, where C-C 1 s line (284.8 eV) is used to calibrate the binding energy scale and intensity. Here, we cannot find any clear signal in N 1 s region for all of the samples, indicating that HNO3 induced molecules are desorbed from CNT in the ultra-highvacuum system of XPS. Figure 3(a) reveals the bonding environment of carbon atom. Generally, CNTs are chemically modified with oxygen or oxygen functional groups, which result in the appearance of carbon 1 s spectra at higher binding energy ranging from 285 to 290 eV.43 HNO3 doping further increases the spectra for C-O bonding, and consequently, its O composition reaches 18% in Figure 3(b), which is much higher than those for CNT with other doping (around 8%–11%). CuI doping leads to the decrease in the spectra of 286–290 eV, suggesting the reduction of C-O components. Moreover, CuI doped film exhibits a clear peak shift from 533.2 to 532 eV. It is attributed to a replacement reaction of C-O by Cu-O bonds, which has been previously discussed in detail.28 Heating of the CNT films causes significant spectra variations in O 1 s rather than C 1 s. The O composition is further increased to 26% and 15% for HNO3 and I2 doping, respectively, while it is slightly increased from 8% to 9% for CuI doping. The results have a good agreement with the variations in D-band in Raman spectra (in Figure 2). The construction of CNT-CuI interactions cannot only

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reduce the oxygen content previously doped in CNT, but also prevent CNT from further oxidation. Figures 3(c) and 3(f) show that the I composition keeps at around 2% in I2 doped film after heating, indicating the formation of stable C-I bonding structures. FTIR spectroscopy is utilized to identify the chemical bonding in CNT films, as shown in Figure 4. The undoped CNT film exhibits an absorption peak centered at 1105 cm1 (see Figure 4(a)), corresponding to stretching C-O bonds. Such C-O structures have been widely reported as covalently bonded groups (structural defects) in CNT and graphene.44–46 Moreover, a few of weak absorption peaks ranging from 800 to 1000 cm1 possibly correspond to stretching C-H bonds. We cannot observe any absorption spectra ranging from 1200 to 4000 cm1, implying that the polymer dispersant has negligible effect on the spectra. HNO3 or I2 doping increases the absorption peak of C-O bonds, while these treatments do not change the absorption for C-H bonds. Interestingly, those C-O bonds cannot be identified for CuI doping, indicating a significant reduction of the functional groups in CNT. Heating treatment increases the intensity of the absorption for C-O bonds for all the films. Note that for CuI doping, only a slight absorption peak centered at around 1090 cm1 appears, showing a shift of 15 cm1 towards lower wavenumber side for C-O vibration. Since the frequency of vibration is inversely proportional to mass of vibrating molecule, it is believed that the existence of Cu atoms plays a role in the functional groups induced by oxidation. In spite of the fact that the formation of Cu-O bonds cannot be proven in FTIR, oxidation of CuI preferentially occurs, and it protects CNT from further oxidation.

FIG. 3. XPS spectra of CNT films with different doping treatments before (a)–(c) and after (d)–(f) vacuum heating treatment at 200  C. Carbon 1 s line (284.8 eV) was used to calibrate the binding-energy scale.

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FIG. 4. FTIR spectra of CNT films with different doping treatments (a) before and after (b) vacuum heating treatment at 200  C.

CNTs are chemically doped with oxygen and other functional groups (carboxyl, carbonyl, and/or hydroxyl groups),47,48 which are much more active comparing with the inert sp2 hybridized graphitic structures. The increased oxygen concentration of CNT in vacuum heating can be attributed to two reasons: (1) CNTs react with residual oxygen, (2) vacuum heating remove some weakly absorbed molecules, and oxygen in vacuum and air are absorbed on the defect sites to form relatively stronger C-O bonds. Raman spectroscopy shows that CNTs can be efficiently modulated by the charge transfer doping with HNO3, I2, or CuI. These electron acceptors can react with the functional groups in CNT or intercalate the CNT bundles. HNO3 induced molecules are weakly absorbed in CNT bundles, and would be easily desorbed in vacuum, because no clear spectra can be seen in XPS N 1 s spectra. However, the chemically inert surface of CNTs is converted into a reactive surface after doping, and consequently, many undesired structural defects are formed during drying or heating process. I2 doped film exhibits very similar structural stability to undoped film, while it remains a challenge to control the doping grade by introducing more stable C-I bonding structures in CNT. Furthermore, our experiments show that stable I2 doping requires a heating treatment at above 100  C, which unavoidably causes the formation of undesired oxygen-containing groups in CNT. On the other hand, CuI seems to be an ideal dopant for CNTs. Here, the photonic curing process encourages a deep level of CuI doping, which results in the most efficient charge transfer between CNT and dopants investigated here.

Surprisingly, CuI doping is capable of reducing the undesired defects, which may lead to a degradation in the electronic and transport properties of CNTs. B. Electrical characterization

It is well known that doping treatments have two main effects for improving the conductivity of CNT: (1) dopant molecules intercalate the CNT bundles or modify the CNT to reduce the CNT-CNT node resistance, and (2) dopant molecules increase the majority charge carrier concentration by donating electrons to the conduction band or accepting holes in the valence band. Figure 5(a) gives the initial currentvoltage characteristics of the films with a spin speed of 5000 rpm. Figures 5(b), 5(c), and Table II summarize the electrical conductivity measured at initial and final measurements, respectively. The undoped CNT film with a density of 39 CNTs/lm2 shows an initial sheet resistance of over 107 X/sq, corresponding to an electrical conductivity of 20 S/m. The initial conductivities are exponentially improved to 3.3 6 0.5  103, 6.5 6 1.0  102, and 8.4 6 1.0  103 S/m, for doping with HNO3, I2, and CuI, respectively. Besides the doping, film thickness (more accurately, CNT density), and measurement environments also strongly influence the electrical behaviors. For undoped film, 3-fold increase in CNT density from 39 to 123 CNTs/lm2 results in around 50-fold increase in electrical conductivity from 20 to 1.0 6 0.2  103 S/m. Generally, more CNTs improve the capacity for carrier carrying, and consequently, the conductivity has a proportional correlation with CNT density. Here,

FIG. 5. (a) Current–voltage characteristics of CNT films spin-coated with 5000 rpm. The electrical conductivity of CNT films at the (b) initial and (c) final electrical measurements.

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TABLE II. Electrical conductivity (S/m) of CNT films under different doping treatments. Initial values are measured at air atmosphere, while final values are measured after vacuuming and heating processes. Spin speed (rpm)

1000

2000

3000

4000

5000

No doping

Initial Vacuum Final

1.0 6 0.2  103 7.0 6 1.0  102 6.5 6 0.8  102

3.0 6 0.5  102 2.4 6 0.5  102 1.9 6 0.2  102

1.9 6 0.2  102 1.5 6 0.2  102 1.4 6 0.2  102

2.6 6 0.5  10 2.0 6 0.5  10 1.7 6 0.5  10

2.0 6 0.5  10 1.5 6 0.5  10 1.4 6 0.5  10

HNO3

Initial Vacuum Final

1.6 6 0.2  104 4.3 6 0.5  103 7.2 6 1.0  102

7.0 6 1.0  103 3.0 6 0.5  103 5.5 6 0.8  102

7.2 6 1.0  103 2.1 6 0.5  103 3.4 6 0.5  102

5.2 6 0.8  103 2.0 6 0.5  103 1.1 6 0.2  102

3.3 6 0.5  103 9.9 6 1.0  102 6.1 6 1.0  10

I2

Initial Vacuum Final

9.1 6 1.0  103 6.8 6 1.0  103 3.8 6 0.4  103

7.5 6 1.0  103 4.6 6 0.5  103 3.0 6 0.5  103

7.3 6 1.0  103 6.1 6 1.0  103 2.8 6 0.5  103

2.3 6 0.4  103 2.0 6 0.4  103 9.0 6 1.0  102

6.5 6 1.0  102 5.6 6 1.0  102 2.2 6 0.5  102

CuI

Initial Vacuum Final

3.8 6 0.4  104 3.9 6 0.4  104 3.5 6 0.4  104

2.5 6 0.4  104 2.4 6 0.4  104 2.1 6 0.4  104

1.4 6 0.2  104 1.4 6 0.2  104 1.0 6 0.2  104

1.0 6 0.2  104 1.1 6 0.2  104 8.2 6 1.0  103

8.4 6 1.0  103 8.8 6 1.0  103 5.8 6 0.8  103

the contact resistance is the principal characteristic for electrical transport in CNT random networks. Apparently, more CNTs also cause more CNT-CNT contacts. As overlap increases, the contact resistance decreases because more atoms from opposing CNT are able to exchange carriers through van der Waals interactions. It is estimated that 7-fold increase in the density of CNT-CNT nodes increase the conductivity by a factor of 15. Znidarsic et al. had systemically investigated the factors affecting the resistance between two individual CNTs and shown that the contact resistance is strongly dependent on the diameter of CNT, the contact angle, and doping.33 Here, the random networks use CNTs with randomly distributed diameter, length, and contact, while the small deviation in conductivity (Table II) indicates that these improvements in conductivity are mainly caused by the doping methods. On the other hand, doped films exhibit less thickness dependence on conductivity. Although the efficient charge transfer doping is achieved, I2 doping is not so effective on the conductivity for relatively thinner films. Exposure to I2 vapor causes a random absorption of iodine molecules in CNT, which will have larger opportunities to intercalate the CNT bundles in CNT films with larger density. Note that, for doping with HNO3 or CuI, CNT films show an approximately proportional increase in conductivity with CNT density (see Figure S2), and about 4fold increases in conductivity are observed for the films with 3 fold larger CNT density. It suggests that both of HNO3 and CuI doping efficiently reduce the contact resistance, being independent to the CNT density. HNO3 is regarded as one the most efficient doping method, it not only introduces NO3– ions, which can intercalate CNT bundles but also lowers the Schottky barrier height between semiconducting and metallic CNTs via modifying the energy level of CNTs.49 Interestingly, better electrical conductivity is achieved for CuI doping. The deeper level of CuI doping causes more charge transfer, and consequently, higher carrier concentration. It also suggests that CuI nanoparticles exactly connect CNTs to realize efficient electric transport. Most of gaseous or ionic dopants are not stable by vacuuming or heating, which is usually required for practical device processing. As shown in Figure 5(c), all the films

exhibit more or less decrease in the final conductivities. For HNO3 doped film with the largest CNT density, the conductivity degrades greatly, being close to that of undoped films. Figure 6 shows the detailed variation in resistance to distinguish the influences of processes. For undoped films, the resistances increase by a factor of 1.2–1.4 after vacuuming, and further increase by a factor of 1.1–1.2 after heating. Vacuuming and heating can remove the weakly absorbed dopant molecules. For example, it has been reported that O2 can intercalate the CNT bundles to improve the charge transport in CNT films.50 Desorption of the gaseous molecules could be the reason for the increased resistance in undoped films by vacuuming. Moreover, the contact resistance increases with spacing between CNTs. The van der Waals interactions allow the carrier transfer between the CNTs, and weaken with increasing spacing. Vacuuming and heating may cause CNT films more porous with larger distance. The further increased resistance at final measurement may be attributed to such morphological variation, despite it cannot be identified by AFM. For HNO3 doping, vacuuming processes leads to 3-fold increase in resistance, which is consistence with the well-known phenomenon that CNT films exhibit a rapid 3-fold increase in resistance after doping.51 Further, more than 5-fold increase induced by heating suggested that CNT films lose almost all of the HNO3 induced dopant molecules. Thus, the reliability can be a more serious issue for practical uses. For I2 doping, vacuuming and heating treatments lead to less than 3 fold increase in resistance due to the relatively stable iodine contents (Figure 3). On the other hand, CNT-CuI hybrid films exhibit extremely stable electrical performances, the resistance is only increased by factors in a range of 1.1–1.5, which is comparable to those for undoped films. Obviously, unlike the gaseous molecules, CuI nanoparticles are capable of surviving in various processes for practical applications. The temperature dependence of electrical conductivity is an important indicator to understand metallic or semiconducting characteristics as well as the conduction mechanisms. We performed a cycle measurement by alternating the temperature between 80 and 450 K, as shown in Figure 7. All of the films exhibit semiconductor-like behavior, showing that the

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FIG. 6. The resistance variations of CNT networks: (a) no doping, (b) nitric acid solution, (c) iodine vapor, and (d) copper iodide nanoparticles. The initial resistances measured in air atmosphere are set to 1. The variations indicate the stability of resistance for doped CNT films.

conductivity increases with temperature. Figures S3 and S4 give the normalized conductivity-versus-temperature curves to clearly distinguish the influences of film thickness and doping treatments.38 Thinner undoped films (lower CNT density) exhibit stronger temperature dependence on conductivity. Especially, the thinnest film shows an exponentially increased conductivity with temperature, which is very similar to the carrier concentration dependence on temperature in silicon. Despite the effects of temperature on the resistance of individual metallic or semiconducting CNT have been experimentally and theoretically studied, and some mathematic

models have been established,52–55 CNT network is a much more complicated system, where the contact resistance plays an important role in overall electrical conductivity. The contact resistances decrease with temperature, because carriers cross the potential barrier between CNTs more easily at high temperature. Figure 5 implies that the contact resistance rather than the resistance of individual CNT is more dominate in thinner film. Therefore, thinner undoped films seem to be more semiconducting. Moreover, a hysteresis loop is observed in the forward-reverse temperature cycle in Figure 7(a). It may be attributed to the variations in contact

FIG. 7. The temperature dependent conductivity of CNT films with different doping treatments: (a) no doping, (b) nitric acid solution, (c) iodine vapor, and (d) copper iodide nanoparticles.

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FIG. 8. The AFM of CNT/CuI films: (a) before heating; (b) after overnight heating at 350  C; and (c) at 450  C in vacuum (1  103 Pa). All of the scale bar are 200 nm.

resistance due to desorption of molecules intercalated in CNT bundles at high temperature. Apparently, the electrical behaviors of CNT are strongly influenced by doping. Doped films exhibit significantly smaller temperature dependence. Moreover, Figure S3 shows that film thickness has a negligible influence in the temperature dependence, especially for CuI doping. The results indicate that doping suppresses the potential barrier for carrier transfer between CNTs, and consequently, doped CNT films become relatively metallic. Interestingly, all of the doped films show very similar curves for the forward direction with slightly gradual increases in conductivity with temperature up to around 300 K (see Figure S4). However, the conductivity is very sensitive to temperature above 300 K. It is considered that higher temperature may activate the ionic dopants intercalating the CNT bundles, and simultaneously further remove the volatile dopants from CNTs. It is consistent with the results that HNO3 doped films show clear peak conductivities at around 400 K, but such variations are slight in I2 or CuI doped films. The curves for the reverse direction become simply with a gradually decreased conductivity with temperature. CuI doped films remain metallic performances, while HNO3 doped films become more semiconducting with increased slopes in Figure S3. The hysteresis in forward-reverse curves should be attributed to the structural stability of the doped films. Beside the removal of dopants, the significant structural defects induced by HNO3 are another possible reason. The thermal activated hopping of the localized electrons is strongly dependent on temperature. Overall, CuI doped films exhibit the smallest hysteresis, demonstrating their excellent stability in the electrical performances. Moreover, in order to understand CuI doping, we further investigated vacuum heating (1  103 Pa), as shown in Figure 8, and compared its effects on CNT-CuI with photonic curing method. In Figure 8(a), evaporation of a 5-nm-thick CuI film leads to a uniform distribution of CuI nanoparticles with an average grain size of around 40 nm. Vacuum heating at 350  C leads to an increase in the grain size of CuI to 80 nm in Figure 8(b). By further increasing the temperature to 450  C, the grain size reaches 120 nm (Figure 8(c)). Although vacuum heating results in similar variations in Raman spectra, (not shown here) comparing with the results by photonic curing (Figure 2), apparently, these CuI nanoparticles only cover the surface of CNT network to form continuous film, while the interconnecting nodes cannot be identified. On the other hand, the sheet resistances of CNT films (85% transmittance at k ¼ 550 nm) are 400 and 350 X/square for the film after

vacuum heating at 350 and 450  C, respectively. Compared with the undoped CNT films (around 1000 X/square with 85% transmittance), the decrease in sheet resistance implies that the CuI doping can enhance the electrical conductivity of CNT films by a factor of 2–3. However, the sheet resistance of CNT-CuI film with 85% transmittance can be reduced to 60–70 X/square after photonic curing.28 The discussion on structural characterization shows that photonic curing can provide a unique rapid heating and cooling treatment, which not only encourages CuI doping in deep level but also results in the construction of interconnecting nodes. Thus, it is believed that the novel hybrid structures with CuI interconnecting nodes in CNT networks (Figure 1) are more crucial to improve the electrical conductivity of CNT films. IV. CONCLUSION

We have systematically investigated the doping effects on the structural and electrical properties of single walled CNTs. HNO3 solution, I2 vapor, and CuI nanoparticles are used to modify a series of ultrathin CNT networks with well controlled CNT density. These treatments cause efficient p-type doping and significantly improve the electrical conductivity of CNT networks. (1) As a well-known doping method, HNO3 solution leads to a deep level of doping with efficient electron transfer from CNT to dopants. However, HNO3 induced molecules are very volatile and are much sensitive to vacuum and temperature. HNO3 doping also causes a large number of undesired structural defects. Despite such doping increases the electrical conductivity of the thinnest CNT network with a density of 39 CNTs/lm2 by a factor of 150 initially, which is finally decreased to be 3, after vacuuming and heating. Thus, HNO3 doping should be avoided due to the seriously reliable problems. (2) Exposure to I2 vapor also leads to similarly efficient charge transfer doping to HNO3 doping. It increases the conductivity of the thinnest CNT network by a factor of 30 initially, which is finally decreased to be 10. The doping effects on structural and electrical characteristics are much more stable. However, it remains challenges to control I2 vapor process to improve the intercalation of I2 induced molecules in CNT bundles. (3) Photonic curing enables the manipulation of CuI crystallites in ultrathin CNT networks, which not only results in the construction of novel CNT-CuI hybrid structures but also encourages the deepest level of charge transfer

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doping. CuI doping is able to reduce and suppress the oxidation induced structural defects of CNTs during the processes. Such CNT-CuI hybrid exhibits stable structural and electrical characteristics. The initial conductivity of the thinnest CNT network is improved from 20 to 8.4 6 1.0  103 S/m, which keeps to 5.8 6 0.8  103 S/m after vacuuming and heating. Moreover, the electrical behaviors by varying CNT density indicate that CuI nanoparticles interconnect CNTs to realize efficient carrier transport. In summary, the results indicate that CuI can be an ideal dopant, which can be used to form a stable hybrid composite with CNTs but does not degrade the electronic and transport property of CNTs. The processing feasibility and excellent reliability identify the bright perspective of CNT-CuI hybrid film for practical applications. ACKNOWLEDGMENTS

This work was supported by JSPS Grant-in-Aid for Research Activity Start-up. The authors acknowledge the help from Ms. Y. Yokota, Dr. Z. Wang, and Dr. M. Chikamatsu. 1

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