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Functional Nanostructured Materials (including low-D carbon)

Enhanced Thermoelectric Properties of Boronsubstituted Single-walled Carbon Nanotube Films Wei-Hung Chiang, Yu Iihara, Wei-Ting Li, Cheng-Yu Hsieh, Shen-Chuan Lo, Chigusa Goto, Atsushi Tani, Tsuyoshi Kawai, and Yoshiyuki Nonoguchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14616 • Publication Date (Web): 17 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Materials & Interfaces

Enhanced Thermoelectric Properties of Boronsubstituted Single-walled Carbon Nanotube Films Wei-Hung Chiang,† Yu Iihara,‡ Wei-Ting Li,† Cheng-Yu Hsieh,§ Shen-Chuan Lo,§ Chigusa Goto,‡ Atsushi Tani,‡ Tsuyoshi Kawai,‡ and Yoshiyuki Nonoguchi‡∥*

†Department

of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei 10607, Taiwan. ‡Division

of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192,

Japan. §Material

and Chemical Research Laboratories, Industrial Technology Research Institute,

Hsinchu, 30013, Taiwan. ∥JST,

PRESTO, Kawaguchi 332-0012, Japan

Keywords doping, carbon nanotubes, thermoelectrics, waste heat, energy harvesting

ACS Paragon Plus Environment

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Abstract

Atomic doping is the most fundamental approach to modulating the transport properties of carbon nanotubes. In this paper, we demonstrate the enhanced thermoelectric properties of boronsubstituted single-walled carbon nanotube (B-SWCNT) films. The developed two-step synthesis of large quantities of B-SWCNTs readily enables the measurements of thermoelectricity of bulk B-SWCNT films. Complementary structural characterization implies the unique configuration of boron atoms at the doping sites of SWCNTs, successfully enabling carrier doping to SWCNTs. The developed boron substitution in combination with chemical doping is found to substantially improve the thermoelectric properties.


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Introduction Single-walled carbon nanotube (SWCNT) films have recently been investigated for application as inflexible thermoelectric materials.1 Thermoelectrics enables power generation from temperature differences. Most waste heat from industry is below 200 oC, and is dissipated into the environment. Such low quality heat cannot be reused by conventional turbine systems, highlighting the usefulness of thermoelectric power generators. Additionally, the efficient collection of heat energy can be achieved by closely fitting generators onto shaped heat resources. In this context, SWCNTbased materials having mechanical flexibility are adaptive to this requirement for emergent thermoelectric materials and power generators. Consequently, the development of SWCNT-based composites with superior thermoelectric properties will contribute to important advances in both scientific studies and innovative applications. Several approaches to increasing thermoelectric efficiency have been proposed, including the use of semiconducting SWCNTs,4 the tuning of the Fermi level,5 and selective carrier transport at the SWCNT-SWCNT junctions.6 Recently chemical doping has been demonstrated to improve the thermoelectric properties of SWCNTs by switching between p- and n-type polarities.7-16 The above approaches resulted in the successful modulation of thermoelectric properties, whereas the practical requirements (e.g. stability) and substantial improvement in thermoelectric properties from an atomic level have yet to be considered. The atomic doping of heteroatoms is an effective method to modulate the transport properties of SWCNTs.17 Different from chemical doping, this technique not only enables carrier doping, but also achieves high structural stability due to covalent bonding. It is well recognized that the substitution of a few carbons to electron acceptors [e.g. boron (B)] and donors ([e.g. nitrogen (N)] results in a dramatic change in electronic structures and the Fermi level.18-21 In this way, the low temperature transport properties of SWCNTs have been


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explored, where the atomic doping led to metallic conduction due to the deep Fermi level shift. In contrast, thermal transport was likely to be suppressed via atomic doping.22 Additionally, it was reported that B-doping resulted in the production of stable p-type SWCNTs while non-substituted SWCNTs showed complex polarity dependent on the number of oxidative adsorbates (e.g. oxygen and water).18 Regardless of such promising characteristics, however, there are few consistent studies concerning the effect of substitutional dopant ratios on the thermoelectric properties of atomically doped SWCNTs. Additionally, discrepancy between ideal and real dopant configurations has yet to be considered for further understanding a mechanistic insight into electronic structures and carrier doping. Here we examine the thermoelectric properties of B-SWCNT films. This work reveals the B-SWCNT films show improved conductivity and Seebeck coefficient, while their thermal conductivity is almost preserved before and after boron substitution. Toward this goal, we have developed a controlled bulk preparation (> milligrams) of atomically substituted B-SWCNTs using a simple two-step reaction route under atmospheric pressure. The present viable B-doping to SWCNTs achieves an approximately seven-fold enhanced power factor, relative to pristine SWCNTs, at raised temperatures above 100 oC. Chemical doping is found to further improve the thermoelectric properties. A structural origin for the enhanced thermoelectric transport is deduced by linking the spectroscopic characteristic to the thermoelectric properties.

Results and discussion


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(a) B

(b)

C

Bundled SWCNTs

SWCNT dispersion with boron precursor

B-SWCNT

Figure 1. Synthesis and preparation. (a) Schematic illustration of two-step reaction route to produce B-SWCNTs. The stepwise reaction route is composed with a wet-chemistry-assisted pretreatment and an atmospheric-pressure carbothermic reaction. (b) Flexible film fabricated by the as-produced B-SWCNTs.

Preparation and structural analysis It is empirically recognized that atomically doped SWCNTs can be prepared using various methods including the direct growth by chemical vapor deposition (CVD),23 arc discharge,24 laser ablation,25,26 and the post-growth substitution reaction.27 This problem has long made the thermoelectric characterization of bulk SWCNT films unattainable. To overcome this problem, we have developed a controlled bulk preparation (> milligrams) of atomically substituted B-SWCNTs using a simple two-step reaction route under atmospheric pressure (Figure 1a).27,28 The details of process is explained in the Experimental section and supporting information. The developed atmospheric-pressure synthesis method processes the advantage for industrial-scale production because of the simple sample preparation, easy process operation, scalability, and no vacuum system required. Because of the scalability of the developed process, film-based thermoelectric and thermal transport measurements can be performed by using the films fabricated by the asproduced B-SWCNTs (Figure 1b, Experimental section).


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Detailed transmission electron microcopy electron energy-loss spectroscopy (TEM-EELS) and X-ray photoelectron spectroscopy (XPS) characterizations indicate the B-SWCNTs with varying B content from 1.23 to 3.15 atomic percentage (atom %) were synthesized. Representative TEM images of starting SWCNTs and as-produced B-SWCNTs indicate that the tubular structure was remained after the high temperature reaction (Figure 2a). Sample information and reaction conditions are listed in Table 1. The as-produced B-SWCNTs are straight tube and smooth, as shown in the high-resolution (HR) TEM image (inset of Figure 2a), suggesting the absence of amorphous carbon or B4C particles. EELS were performed to study the elemental compositions of the as-produced B-SWCNTs. The TEM beam was carefully focused on the clean bundles of asproduced B-SWCNTs. Figure 2b and c show the representative EELS spectra of the B3 sample (Table 1) and raw SWCNT, respectively. The EELS spectrum of raw SWCNT shows only C Kedge around 284 eV (1s →* transition). In contrast, B3 sample shows C K-edge around 284 eV (1s →* transition) and B K-edge around 191 eV (1s →* transition). In fact, this B K-edge suggests the presence of B atoms doped into the sp2 carbon lattice of SWCNT.29 However, the ratio of the 1s →* transition to the 1s →* transition at 205 eV was significantly smaller than those of highly-conjugated boron compounds including boron nitrides.30,31 In addition, no peaks related to byproducts including B clusters, B4C, and B2O3 were observed during the EELS measurement. Therefore, we hypothesized that the boron substitution occurs partly at conjugated structures, and mainly at single-bond structures. It should be noted that the quantification of the EELS results gives an averaged dopant concentration of 3 atom % for boron in B3 sample.


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(a)

(b)

B-SWCNT

(c)

SWCNT

5 nm Counts (A.U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 nm 150

250

350

450

550

Binding energy (eV)

650

Figure 2. TEM and EELS. (a) TEM image of the as-produced B-SWCNTs (sample B3). Inset: HRTEM image of individual B-SWCNT. EELS spectra of (b) as-produced B-SWCNTs (sample B3) and (c) raw SWCNTs.


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Raw data Fitting Background BC3 189.9 BCO2 191.8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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196

194

192

190

188

186

Binding energy (eV)

Figure 3. Repesentative B1s XPS spectrum of as-produced B-SWCNTs (sample B3).

Table 1. XPS analysis of B, C, and O contents in the pristine SWCNT and as-produced B-SWCNT. Reaction time is 4 hours for all samples.

Tempearture

B

C

O

(oC)

(atom %)

(atom %)

(atom %)

SWCNT

--

0

96.32

3.68

B1

1000

1.23

96.67

2.10

B2

1100

2.27

93.71

4.02

B3

1200

3.15

91.47

5.38

A

1200

0.06

99.94

0

Sample


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XPS characterization was further performed to study the boron doping configuration. Detailed discussion can be found in the supporting information (Section S3). Representative XPS spectra of raw SWCNT and B-SWCNTs prepared with different temperatures are shown in Fig. S1. Spectra were normalized by the C1s (284.6 eV) intensity and averaged from 10 random positions on each sample. The peaks around 190.8, 284.6, and 534.6 eV respectively assigned to B1s, C1s, and O1s were observed for all as-produced B-SWCNTs, supporting that B atoms were existed in the structure of as-produced BCNTs and agreed with the EELS results (Figure 2b and c). Oxygen peaks could be due to the oxidation of as-produced BCNTs. We carefully estimated the boron concentrations in each sample using XPS elemental quantification. It is found that the boron concentrations in the as-produced samples can be controlled from 1.23, 2.27, and 3.15 atom % by adjusting the reaction temperature (Table 1). To further understand the boron doping configuration in the as-produced B-SWCNT, we performed HRXPS. Repesentative HRXPS spectrum of as-produced B-SWCNTs (sample B3) in a range of 184~196 eV reveals the existence of different B species (Fig. 3). The Gaussian deconvolution analysis indicated that the spectrum can be divided into two peaks with various intensities, which are centered at 189.9 and 191.8 eV, corresponding to BC3 and BCO2 chemical configurations, respectively.32-34 The peaks of byproducts including boron cluster and B4C were not found in the XPS speatra, which is in consistent with the TEM-EELS characterization. The above result suggests that the B-SWCNTs with a tunable B content were successfully prepared, providing a suitable platform for exploring the atomic substitution effect of SWCNTs on their thermoelectric properties. To emphasize the advantage of our pretreatment method, a reference experiment where raw SWCNT and B2O3 powder were mixed by homogenizer (Ultra Turrax T25, IKA) at 5,000 rpm for 10 minutes. The B dopant concentrations in the as-produced B-SWCNTs (sample A) prepared without the developed


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pretreatment were 0.05 at% by XPS analysis, showing the pretreatment processes the ability to overcome the SWCNT aggregation problem and to produce BCNTs with higher B dopant concentrations.

Electronic structure

Figure 4. (a) UV-Vis-NIR and (b) mid-infrared absorption spectra of SWCNT films before and after boron substitution at 1.23, and 3.15 atom%.

Solid-state absorption spectroscopy is helpful for elucidating the electronic structure that would reflect thermoelectric properties. The absorption spectrum of B-SWCNTs was quite similar to the corresponding pristine SWCNTs (Figure 4). A slight and relative decrease in S11 interband transition after boron substitution at around 0.6 eV can be attributed to weak hole doping. S11 reflects the first optical transition in semiconducting SWCNTs. If many boron atoms are incorporated into the sp2 framework of carbon nanotubes, the electronic structure, and the


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interband optical transition of B-SWCNTs would totally change from non-doped SWCNTs.35 Therefore, we assume that this atom substitution doesn’t mainly occur at conjugated systems but reflects sp3 nature. Due to the observation of complex fine structures, unfortunately, it was difficult to further analyze the second transition of semiconducting SWCNTs and the first transition of metallic ones in the higher photon energy range (> 0.8 eV). Once the XPS profile is reassessed, the BC3 peak (189.9 eV) slightly shifted to higher energy, compared to ideal sp2 BC3 (189.4 eV) (Figure 3). This oxidative shift suggests the contribution of B-C single bonds. We therefore deduce that boron substitution occurs preferentially at sp3, and partly at sp2 hybridization regions.

Figure 5. Possible boron configurations. (a) Borabenzene, (b) borole, (c) substituted boron, (d) CBH-C, and (d) C-BH2 forms.


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It is true that an ideal SWCNT should have fully conjugated structure with no defect, while the practically available SWCNTs have moderate amounts of structural defects. Additionally, the structural modification including boron substitution reactions would start with bond cleavage and then be followed by various reactions such as substitution, bond rearrangement, and structural defect formation. We assumed that such reactions would form conjugated (Figure 5 (a)) and less conjugated (Figure 5 (b,c)) structures.36 Addition reactions on C=C bonds and substitution reactions at structural defects such as a methyl group are also possible, which generates –BH- and –BH2 (Figure 5 (d,e)). Oxidation then proceeds after air exposure. In fact, boron-substituted SWCNTs made by various methods including our samples have exhibited boron oxide species probably due to the existence of unstable boron species.37 Thus, the formation of oxidative defects is possible through the boron addition/substitution reactions. This developed doping is attributed to a viable mass-processable method for selectively tuning the carrier concentration. This is then practically useful for tuning the thermoelectric properties of SWCNTs unless sp3 boron atoms significantly perturb lattice thermal conductivity. It should be noted that the products are air-stable; we compared the absorption spectra of BSWCNT films with identical stored feedstocks before and after 18 months storage on the shelf, where we observed no substantial difference in them (Figure S2). This result is advantageous over the chemically-doped SWCNTs that would gradually degrade.38

Thermoelectric properties


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Figure 6. Thermoelectric properties. (a) Electrical conductivity, (b) the Seebeck coefficient, and (c) power factor of SWCNTs before and after atomic boron doping at 1-3 atom%. (d) Maximum power factor values dependent on boron concentration in the temperature range from 37 to 200 oC.

We then evaluate the thermoelectric properties of B-SWCNT films at different atomic doping levels. For evaluation, we use the power factor described as 2 or a figure of merit (ZT) as

T/, where  is the Seebeck coefficient,  is electrical conductivity,  is thermal conductivity and T is the absolute temperature. First, electrical conductivity is the most basic component that reflects transport properties. Pristine SWCNT films showed moderate electrical conductivity ranging from 45 to 51 S cm-1 between 37 and 200 oC (Figure 6a). Upon atomic doping by boron,


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the conductivity increased to 124 S cm-1 for 1% and approximately 205 S cm-1 for 2.27 and 3.15% boron concentration. The electrical conductivity of B-SWCNT films decreased from room to high temperatures, which is an opposite trend against the pristine SWCNTs. A systematic increase in conductivity upon atomic doping and a change in temperature dependence support the successful carrier doping. The Seebeck coefficient is a generated voltage per temperature difference. Boron substitution would change the band structures or shift the Fermi level. Since the Seebeck coefficient reflects the slope of DOS around the Fermi level,3.5 we expect dramatic changes in the temperature dependence on the thermoelectric properties upon boron substitution. Surprisingly, the Seebeck coefficient of B-SWCNT films was larger than that of pristine SWCNTs and increased from 37 to 120 oC (Figure 6b). A difference in the temperature dependence between pristine and B-SWCNTs suggests electronic doping, or a change in electronic structures. Due to the negatively temperature-dependent conductivity, we assume that the observed thermoelectric properties were primarily dominated by B-doped species derived from semiconducting SWCNTs. At the present stage, we found only a small change in the band structures and doping level of semiconducting SWCNTs deduced from absorption spectra, which could not account for simultaneous increases in the Seebeck coefficient and electrical conductivity. Since the thermoelectric properties were examined using non-sorted SWCNT mixtures, such enhanced Seebeck coefficients might originate from partial changes in the electronic structures of metallic SWCNTs by B-substitution. The Bdoping shifts the Fermi level below the metallic first van Hove singularity points, where the Seebeck coefficient of metallic SWCNTs would increase upon doping.3


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We found a similar temperature dependence of the Seebeck coefficient regardless of boron concentration while conductivity systematically increased with an increase in boron concentration. As a result, we achieved a simultaneous increase in the Seebeck coefficient and electrical conductivity. The power factors of B-SWCNT films were maximized above 100 oC, while pristine SWCNT films showed a monotonic decrease (Figure 6c). We found an approximately seven-fold enhancement in the power factor () from 9 to 61 W m-1 K-2 (Figure 6d). Actually, the thermoelectric properties are likely to depend on the B substitution ratio, and similar between 2% and 3% B-SWCNTs. Due to a systematic increase in oxygen contents affording -BOx in XPS, the ratio of effective doping sites such as electron-withdrawing C-B-C bonding might not be changed significantly between 2 and 3% samples.

Table 2. In-plane thermal conductivity data. Sample B3 indicates B-SWCNT with 3.15 atom % of B in Table 1 Thermal

Thermal

diffusivity

conductivity

(mm2 sec-1)

(W m-1 K-1)

1.2

21.9

15

1.1

22.0

14

Density

Heat capacity

(g cm-3)

(J g-1 K-1)

SWCNT

0.57

B3

0.60

Sample

We evaluated the thermal conductivity of B-SWCNT films. It is empirically known that, due to significant thermal dissipation, it is somewhat difficult to evaluate the thermal conductivity of thin SWCNT materials. Considering this situation, we used a Xenon flash method with ~50 m-


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thick, 17mm-diameter B-SWCNT films. We estimated almost the same thermal conductivity before and after boron substitution (Table 2). Theoretical calculation based on sp2-doped BSWCNTs has predicted the suppression of electronic thermal conductivity upon boron doping.22 On the other hand, our result implies that the negligible suppression of thermal conductivity stems from extrinsic sp3 boron doping. Within the simple regime of the Wiedemann-Franz law, electrical thermal conductivity was estimated to be less than 0.6 W m-1 K-1. This contribution is small enough to primarily consider only the lattice thermal conductivity (~15 W m-1 K-1). Importantly, atomic doping in this paper leads to an increase in the power factor and the preservation of thermal conductivity. The both results are desirable for improving the thermoelectric figure of merit, ZT.

Chemical doping


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Figure 7. Chemical doping effects on thermoelectric properties. (a) IR absorption spectra of BSWCNT thin films dependent on chemical p-type doping with AgTFSI. “S“ and “1DPR“ indicate an interband transition and one-dimensional plasmon resonance. Colored lines indicates the films before (black), and after chemical doping with 0.5 (green), 1.0 (blue), and 2.0 mg mL-1 (red) AgTFSI. (b) Energy shift in the Fermi level of B-SWCNTs estimated by (a). (c) Conductivity and the Seebeck coefficient of chemically doped B-SWCNTs (sample B3) as a function of the Fermi level shift at room temperature. Diamonds and circles indicate conductivity and the Seebeck coefficient, respectively. (d) Power factor corresponding to (c). (e) Schematic valence band structures before and after hole doping.


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Further tunability in the electronic structure of B-SWCNT films upon chemical doping is examined by absorption spectroscopy. We used silver bis(trifluoromethanesulfonyl)imide (AgTFSI), a representative one-electron oxidizing agent through a Ag+/Ag0 redox couple.39 The absorption spectra of 3.15%B-SWCNTs upon AgTFSI addition clearly indicates p-type doping that modulates two characteristic absorption bands (Figure 7a). An interband transition at around 0.6 eV decreased and shifted to higher energy, corresponding to the Fermi level shift. Furthermore, a far infrared (FIR) band associated with the one-dimensional plasmon resonance (1DPR) band increased upon AgTFSI treatment. This structural evolution indicates an increase in free carriers.40 These results support the successful p-doping of B-SWCNTs. The quantification of the Fermi level is helpful for understanding the thermoelectric properties.5 The Fermi level shift was therefore calculated by estimating the onset energy shift in the S11 absorption band. While AgTFSI doping below a 0.1 mg mL-1 concentration changed almost no absorption spectra, higher concentration doping moderately shifted the Fermi level by 0.010.16 eV at 0.1-2.0 mg mL-1 concentrations (Figure 7b). Using the identical thin film samples for absorption measurements, thermoelectric properties were investigated on the basis of the Fermi level shift. The electrical conductivity increased and the Seebeck coefficient decreased as the Fermi level got deeper (Figure 7c). It should be noted that this tuning further resulted in the 2.5-fold enhancement of the power factor up to 102 W m-1 K-2 (Figure 7d). Considering both the B-substitution effect and the chemical hole doping, we potentially achieved a more than 15-fold increase in the thermoelectric power factor compared to that of pristine SWCNT films.


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Finally, the relationship of the electronic structures with thermoelectric properties is discussed. Conductivity seems proportional to the Fermi level shift in the range of 0.01-0.16 eV. Additionally, the Seebeck coefficient was roughly proportional to log ). At the Fermi shift below 0.01 eV, a gradual increase in electrical conductivity and a rapid decrease in the Seebeck coefficient were observed. We here assume that the present doping is attributed to the ejection of electrons from a relatively small gradient region in the total density-of-state (DOS). Considering an analogy to the thermoelectric properties of unsubstituted SWCNTs, the Fermi level is considered to originally locate between the van Hove singularity points in a valence band. When the Fermi level is adjusted at these edges, the Seebeck coefficient should become keenly maximized. In contrast, the present B-SWCNT films exhibited a simple inverse  relationship. This result suggests that the AgTFSI doping undergoes the Fermi tuning between the van Hove singularity points (Figure 7e).

Conclusion In conclusion, we have demonstrated a potentially >15-fold enhancement of thermoelectric power factor and ZT upon the atomic boron substitution of SWCNTs. Complementary structural and thermoelectric evaluation has suggested this atomic doping developed here is associated with concerted sp2 and sp3 B-C bond formation, achieving an increase in the thermoelectric power factor, and the preservation of thermal conductivity. This boron doping is expected to be advantageous in stability over simple chemical doping,41 and the precise thermoelectric property tuning can be realized by the combination of atomic and chemical doping. From a scientific point of view, the separation of doping sites and -conjugated carrier transport space can be categorized into the


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modulation doping regime.42-44 In terms of methodology, the procedure in this work is readily amenable to large-scale processing. These advantages further motivate us to investigate various characteristics of atomic doping enabling the p-type and n-type doping of SWCNTs in the future.

Experimental section Synthesis of B-SWCNTs To prepare raw SWCNTs, the Ni nanopcatalysts were first synthesized in an atmospheric pressure microplasma reactor45 and then deposited on substrate for SWCNT growth by water-assisted chemical vapor deposition (CVD).46,47 Details are listed in Section S1 of supporting information. B-SWCNTs were then prepared by a simple post growth substitution reaction route composed of a wet-chemistry-assisted pretreatment and atmospheric-pressure carbothermal reaction24 (details in Section S1). Characterization of B-SWCNTs Ex situ characterization of samples were performed by TEM, EELS, and XPS. Details are listed in Section S2 of supporting information. XPS (VG ESCALAB 250, Thermo Fisher Scientific, UK) was performed a monochromatic Al Kα X-ray radiation (10kV, 10 mA) to quantify the amount of boron doped into the SWCNTs. The TEM characterization was performed with a cold-field emission Cs-corrected TEM (JEOL ARM-200F, Japan) with 200 kV accelerating voltage. EELS was performed with STEM mode. The spatial resolution in STEM mode is 78 pm and the energy resolution is about 0.5 eV. Optical, thermoelectric and thermal transport measurements


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B-SWCNT films were prepared by filtering the dispersion and drying in vacuum at 80 oC. For optical and thermal measurements, the film thickness was controlled below 500 nm and above 30 m, respectively. Thin SWCNT films were prepared using the homogeneous solution with 1 % Pluronic F127 (BASF) and transferred to PET substrates by rubbing (Figure 1(b)).15,16 Bulk films were fabricated following a previous report.11 Thermoelectric properties were measured using a four-probe method (Advance Riko, ZEM-3M10). The standard sample shape was roughly a 16 mm length × 4 mm width rectangle. In-plane thermal diffusivity was evaluated by the Xenon flash analyzer (NETZSCH, LFA 467 HyperFlash). Heat capacity was calculated by comparing differential scanning calorimetry (Hitachi DSC6200) profile with that of an alumina standard.

ASSOCIATED CONTENT Supporting Information. The additional structural analyses of B-SWCNTs. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author: Yoshiyuki Nonoguchi ([email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgement


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We thank financial supports from JSPS KAKENHI grant number JP 26790014, JST-PRESTO grant number JPMJPR16R6, and Ministry of Science and Technology of Taiwan (MOST Grant no. MOST 105-2221-E-011-141 and MOST 104-2923-E-011-001-MY3). This work made use of NAIST common facilities supported by MEXT Nanotechnology Platform program. The TEM analysis was supported by Industrial Technology Research Institute (ITRI).

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