Controlled Growth of Well-Defined Conjugated Polymers from the

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Controlled Growth of Well-Defined Conjugated Polymers from the Surfaces of Multiwalled Carbon Nanotubes: Photoresponse Enhancement via Charge Separation Wenpeng Hou, Ning-Jiu Zhao, Dongli Meng, Jing Tang, Yi Zeng, Yu Wu, Yangziwan Weng, Chungui Cheng, Xiulai Xu, Yi Li, Jian-Ping Zhang, Yong Huang, Christopher W. Bielawski, and Jianxin Geng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00673 • Publication Date (Web): 16 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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Controlled Growth of Well-Defined Conjugated Polymers from the Surfaces of Multiwalled Carbon Nanotubes: Photoresponse Enhancement via Charge Separation

Wenpeng Hou,† Ning-Jiu Zhao,‡ Dongli Meng,⊥ Jing Tang,§ Yi Zeng,† Yu Wu,† Yangziwan Weng,† Chungui Cheng,† Xiulai Xu,§ Yi Li,† Jian-Ping Zhang,‡ Yong Huang,† Christopher W. Bielawski,⊥,∏ Jianxin Geng†,*

†

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, China, Email: [email protected] ‡

§

Department of Chemistry, Renmin University of China, Beijing 100872, China

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China ⊥

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea

∏

Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of Korea

ABSTRACT: The installation of heterojunctions on the surfaces of carbon nanotubes (CNTs) is an effective method for promoting the charge separation processes needed for CNT-based electronics and optoelectronics applications. Conjugated polymers are proven state-of-the-art candidates for modifying the surfaces of CNTs. However, all previous attempts to incorporate conjugated polymers to CNTs resulted in unordered interfaces. Herein we show that well-defined chains of regioregular poly(3hexylthiophene) (P3HT) were successfully grown from the surfaces of multiwalled CNTs (MWNTs)

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using surface-initiated Kumada catalyst-transfer polycondensation. The polymerization was found to proceed in a controlled manner as chains of tunable lengths were prepared through variation of the initial monomer-to-initiator ratio. Moreover, it was determined that large-diameter MWNTs afforded highly-ordered P3HT aggregates, which exhibited a markedly bathochromically shifted optical absorption due to a high grafting density induced planarization of the polymer chains. Using ultrafast spectroscopy, the heterojunctions formed between the MWNTs and P3HT were shown to effectively overcome the binding energy of excitons, leading to photoinduced electron transfer from P3HT to MWNTs. Finally, when used as prototype devices, the individual MWNT-g-P3HT core-shell structures exhibited excellent photoresponses under a low illumination density.

KEYWORDS: Carbon nanotubes, poly(3-hexylthiophene), surface-initiated Kumada catalyst-transfer polycondensation, photoresponse, charge separation

Carbon nanotubes (CNTs) offer great potential for use in nanosized optoelectronic devices due to their one-dimensional structures and unique physical properties. In contrast to the metallic CNTs, including the metallic single-walled CNTs (SWNTs) and multiwalled CNTs (MWNTs), semiconducting SWNTs are particularly appealing because they feature a band gap that is associated with the first van Hove singularity transition in a one-dimensional electronic density of states.1 Unfortunately, the large-scale synthesis and separation of semiconducting SWNTs remains challenging,2−4 and recent studies have shown that the photoexcitation of SWNTs results mainly in the formation of excitons rather than free charge carriers,5,6 For example, the exciton binding energy was measured to be ca. 0.4 eV for semiconducting SWNTs with a diameter of 0.8 nm via two-photon excitation spectroscopy.7 Collectively, these limitations have made it difficult to measure the photoresponse of pristine CNTs.


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While photoconductivities have been reported for individual SWNTs or SWNT films, the results are obtained indirectly via bolometry or modeling excitons formed using high-powered lasers.8,9 An effective way to overcome the aforementioned challenges is to incorporate heterojunctions on the surfaces of CNTs, where the potential energy difference between the CNTs and an electron donor facilitates the separation of the excitons into free electrons and holes. Conjugated polymers are ideal candidates for such purposes because of their intrinsic advantages, including tunable band gaps, good solution processibility, and high optical absorption in visible light region.10 Indeed, conjugated polymers and CNTs constitute donor-acceptor systems where the former function as donors and the latter serve as acceptors.11−13 CNT/conjugated polymer composites are typically prepared by mixing the two components together, which results in the formation of stabilizing π-stacking interactions, and are usually used as photoactive materials in optoelectronic devices.14,15 Other methods for fabricating CNT/conjugated polymer composites have also been reported and include the covalent attachment of conjugated polymers to the surfaces of CNTs using “grafting-to” or “grafting-through” approaches16−18 as well as the electrochemical deposition of conjugated polymers onto CNT-based electrodes.19 Unfortunately, the aforementioned methods do not provide access to composites that feature welldefined heterojunctions or regioregular conjugated polymers, which are needed to accurately measure the optoelectronic properties displayed by individual CNT-based devices. Herein we report a controlled “grafting-from” approach for linking poly(3-hexylthiophene) (P3HT) to CNTs and demonstrate the photoresponses of individual P3HT-grafted MWNTs (MWNT-g-P3HT). The

polymerization

method

utilized

employs

a

surface-initiated

Kumada

catalyst-transfer

polycondensation (SI-KCTP) to grow P3HT brushes from the surfaces of functionalized MWNTs. The SI-KCTP technique is a powerful synthetic tool for grafting conjugated polymers with high regioregularities, controlled molecular weights, low polydispersities, and high grafting densities.20 As a


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result, these features were successfully used to enhance the dispersibility of MWNTs in organic solvents and, through grafting density induced planarization of the P3HT chains, bathochromically shift the optical absorption of the composite. Notably, the well-defined heterojunctions at the interfaces of MWNTs and P3HT brushes were found to facilitate the dissociation of photogenerated excitons, which resulted in enhanced photoresponses of the MWNTs through the formation of photogenerated charge carriers. Such photoresponse properties outperform those exhibited by previously reported CNT-based devices, which were measured using the Schottky barrier model or via bolometry with high-powered lasers.8,21−23

RESULTS AND DISCUSSION Fabrication of well-defined layers of P3HT on the surfaces of MWNTs. The successful development of a chain-growth polymerization through metal-mediated cross-coupling chemistry, as pioneered by Yokozawa24 and McCullough,25 has enabled the surface-initiated polymerization of a broad range of conjugated monomers. The process that we employed to grow P3HT from MWNTs using SI-KCTP is summarized in Figure 1a. A key step in the overall transformation is the immobilization of the Ni(II) species ligated to bidentate phosphine ligands, such as 1,2-bis(diphenylphosphino)propane (dppp), as these complexes effectively initiate the polymerization reaction.26 To gain access to the aforementioned complexes, 4-bromoaniline was converted to its respective diazonium salt with isoamyl nitrite in the presence of MWNTs.27 At elevated temperatures (60 °C), the diazonium salt decomposed to its respective bromophenyl radical and added to the surface of the aforementioned MWNTs. The resulting functionalized MWNTs (1) were then transmetallated with Et2Ni(bipy) (bipy = bipyridyl),28 affording 2, which underwent ligand exchange upon exposure to excess of dppp.26 The desired MWNT-PhNi(dppp)-Br initiator (3) was purified by a series of cycles that involved centrifugation followed by dispersion in anhydrous tetrahydrofuran (THF) and CH2Cl2 under an atmosphere of argon. 4 ACS Paragon Plus Environment

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Figure 1. Synthesis and characterization of MWNT-g-P3HT composites. (a) Schematic illustration of the synthesis of the MWNT-Ph-Ni(dppp)-Br initiator and subsequent growth of P3HT from the surfaces of the MWNTs. For clarity, only the outermost layer of the MWNTs is shown. (b) Schematic representation of a core-shell structure formed by a compact P3HT layer wrapping around a MWNT. The interface of the MWNT core and the P3HT shell constitutes a heterojunction. (c) Schematic illustration of P3HT brushes grafted to the surfaces of MWNTs. Note that low surface curvature (i.e., for MWNTs with large diameters) results in an extended conformation of P3HT backbones.

Subsequent addition of 2-bromo-5-chloromagnesio-3-hexylthiophene (4) to a suspension of the initiator 3 ([4]0/[3]0 = 100:1) at room temperature resulted in the formation of a uniform, nanometer scale layer of P3HT grafted to the MWNTs. As discussed below, the coverage of P3HT was found to be 5 ACS Paragon Plus Environment

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higher than those obtained using “grafting-to”16 and “grafting-through” techniques,17 and ultimately afforded a composite with a well-defined core-shell structure. Such core-shell structures effectively form cylindrical heterojunctions at the interfaces of the MWNTs and the P3HT layer (Figure 1b), which facilitate radial charge separation. Moreover, recent studies have indicated that core-shell structures improve carrier collection and overall efficiency due to the short collection distances for excited charge carriers.29 Notably, Figure 1c shows that the low surface curvature of a large-diameter MWNT effectively planarizes the conjugated backbones of the grafted P3HT chains.26 Such planarization was demonstrated to facilitate the formation of highly-ordered P3HT aggregates, which was reported to enable fast transportation of holes in the P3HT phase30 and was found to shift the absorption maxima to longer wavelengths (see below). Utilizing these design principles, a series of P3HT brushes were grown from the surfaces of MWNTs with relatively large diameters (ca. 40 nm in average, see Figure S1). Several techniques were employed to characterize the aforementioned composites and precursors. First, the number of the initiating sites on MWNTs was estimated from the weight loss of the bromophenyl component (ca. 6 wt%) via thermogravimetric analysis (TGA) (see Figure S2).27 Next, NMR spectroscopy was used to characterize the structure of the composites and evaluate the regioregularity of the P3HT chains grown from the surfaces of MWNTs (Figure 2). The 1H NMR spectrum recorded for a MWNT-g-P3HT composite displayed a weak signal at 7.55 ppm and a strong signal at 7.01 ppm, which were assigned to the hydrogens on the phenyl and the thiophene moieties, respectively. The coexistence of these two signals was consistent with the covalent attachment of the P3HT chains to the surfaces of MWNTs through a phenyl-based linkage. The resonances appearing in the high-field region of the 1H NMR spectrum were assigned to the hydrogens on the hexyl groups. In particular, the signal recorded at 2.82 ppm was attributed to the methylene moieties alpha to the 3hexylthiophene repeat units positioned in a head-to-tail (HT) manner. The absence of a 1H NMR signal


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at 2.58 ppm, which is typically observed for the P3HT chains that contain head-to-head (HH) linkages between the repeat units, indicated that the grafted P3HT featured high HT regioregularity. The high regioregularity of the grafted P3HT, which is an important parameter for P3HT in electronics and optoelectronics applications, was attributed to the mechanism of the Ni-mediated catalyst-transfer polycondensation and consistent with other reports.31

Figure 2. 1H NMR spectrum of the MWNT-g-P3HT prepared using a [4]0/[3]0 = 100:1, with the extended view of the indicated regions shown in the insets.

The kinetics of the SI-KCTP were evaluated by performing a polymerization reaction ([4]0/[3]0 = 50:1) for different periods of time. The intensity ratio of the G band associated with the MWNTs to the Raman band of P3HT (IG/IP3HT) was used to monitor the change in quantity of P3HT in the MWNT-gP3HT composites over time (Figure 3a). The Raman band recorded at 1445 cm−1 was assigned to the C– C skeletal stretching vibrations in the P3HT main chains. Figure 3b shows that the IG/IP3HT decreased markedly during the initial stage of polymerization and reached a plateau in ca. 8 h, consistent with other reports of KCTP reactions that were performed in solution.25 To evaluate the ability to control the lengths of the P3HT brushes on the surfaces of the MWNTs, the SI-KCTP was performed with various 7 ACS Paragon Plus Environment

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monomer to initiator ratios (i.e., [4]0/[3]0 = 25:1, 50:1, 75:1 and 100:1). In these experiments, the polymerization reactions were performed for 24 h to ensure a complete consumption of monomer. Figure 3c shows the TGA data recorded for MWNTs and the MWNT-g-P3HT composites obtained with the different [4]0/[3]0 ratios. While the MWNTs were found to be stable up to 600 °C, nearly all of the composites exhibited a mass loss at > 400 °C that corresponded to the thermal decomposition of the P3HT component. The MWNT-g-P3HT composite prepared using a [4]0/[3]0 ratio of 25:1 exhibited a mass loss at ca. 300 °C, which is lower than that of the P3HT obtained with other [4]0/[3]0 ratios and likely due to the relatively lower molecular weight of the P3HT. Regardless, the contents of P3HT were calculated to be ca. 10.6, 19.6, 30.2, and 36.8 wt% in the composites, respectively. Figure 3d shows that the content of grafted P3HT increased linearly with the [4]0/[3]0 ratio, consistent with a controlled polymerization process;32 this result underscores the advantage of the SI-KCTP in controlling the lengths of the P3HT brushes on MWNTs.


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Figure 3. Controlled growth of P3HT from the surfaces of MWNTs. (a) Raman spectra of MWNTs and the MWNT-g-P3HT composites obtained with different polymerization periods using a [4]0/[3]0 = 50:1. (b) The change in IG/IP3HT as function of polymerization period. (c) TGA curves of MWNTs and the MWNT-g-P3HT composites obtained with different [4]0/[3]0 ratios (i.e., 25:1, 50:1, 75:1, and 100:1). (d) The weight loss of P3HT component as function of the [4]0/[3]0 ratio.

The grafted P3HT brushes on the surfaces of the MWNTs were visualized using high-resolution transmission electron microscopy (HRTEM) (Figure 4). The thickness of the P3HT layers attached to the surfaces of the MWNTs increased (from ca. 2 to 6 nm) with the [4]0/[3]0 ratio. Unlike the rough


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surfaces often observed when polymer chains are linked to the surfaces of CNTs using “grafting-to” approaches,16 the P3HT layers prepared in the MWNT-g-P3HT composites were well-defined as a result of the high-density immobilization of the initiating sites and the controlled nature of KCTP chemistry.32 The controlled growth of P3HT from the surfaces of MWNTs was further supported by a series of FT-IR and Raman spectroscopy experiments (see Figure S3 and S4).

Figure 4. HRTEM images of the MWNT-g-P3HT composites obtained using [4]0/[3]0 = (a) 25:1, (b) 50:1, (c) 75:1, and (d) 100:1, with the P3HT layers marked by red dashed lines. The P3HT layers shown in the HRTEM images may be smaller than their actual values as electron beam irradiation can cause deformation.

Steady-state optical spectra of the MWNT-g-P3HT composites. Figure 5a displays the optical absorption spectra of P3HT and the MWNT-g-P3HT composites. Under dilute conditions (0.01 mg mL−1 in THF), P3HT showed a broad and featureless absorption maximum at 445 nm that corresponds to the π−π* transition of the conjugated backbone. By contrast, suspensions of the MWNT-g-P3HT composites exhibited markedly red-shifted and resolved absorption signals at 605, 554, and 514 nm, 10 ACS Paragon Plus Environment

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which were ascribed to the 0−0, 0−1, and 0−2 vibronic features of orderly packed P3HT chains.33 In addition, the content of the P3HT aggregates as well as the resolved absorption signals were found to increase with the length of the P3HT chains.34 Indeed, the spectroscopic features recorded for the MWNT-g-P3HT composites were similar to those of annealed P3HT films,35,36 but different from those reported for highly concentrated P3HT solutions.37 Based on these results, we believe that the P3HT chains in the MWNT-g-P3HT composites underwent planarization that effectively enhanced the interchain interactions. This conclusion was also supported by a control experiment where a mixture of P3HT and MWNTs did not exhibit a red-shifted optical absorption (see Figure S5).

Figure 5. Steady-state optical spectra of the MWNT-g-P3HT composites. (a) Optical absorption spectra of a solution of P3HT in THF and a suspension of MWNT-g-P3HT composites in THF. (b) PL spectra of 11 ACS Paragon Plus Environment

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the P3HT solution and the suspensions of MWNT-g-P3HT composites in THF, with the enlarged spectra shown in the inset. The red-shifted and resolved optical absorption and PL emission are consistent with the formation of highly-ordered P3HT aggregates in the MWNT-g-P3HT composites.

Photoluminescence (PL) spectroscopy also indicated that the P3HT chains in the MWNT-g-P3HT composites were highly ordered (Figure 5b). A solution of P3HT (0.01 mg mL−1 in THF) exhibited a PL emission at 571 nm (black line), which corresponded to the relaxation of excited electrons from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO). By contrast, suspensions of the MWNT-g-P3HT composites (0.027 mg mL−1 in THF, with the concentration of P3HT component = 0.01 mg mL−1 for the MWNT-g-P3HT prepared using a [4]0/[3]0 ratio of 100:1) revealed that the PL emission of P3HT was effectively quenched, consistent with photoinduced electron transfer from P3HT to the MWNTs. Moreover, inspection of the PL emission spectra of the composites revealed two new signals at longer wavelengths (639 and 694 nm), in agreement with the emissions that have been ascribed to the formation of highly-ordered P3HT aggregates.33 To elucidate the electronic structures of the aforementioned composites, a series of PL excitation (PLE) spectra were recorded at 573, 639, and 694 nm (see Figure S7). Comparison of the absorption and the PLE spectra supported the assignment of the PL emissions at 639 and 694 nm to 0−0 and 0−1 vibronic features, respectively; the PL emission recorded at 573 nm was thus ascribed to the disordered P3HT. The PL quantum yields of the suspensions of MWNT-g-P3HT composites in THF were measured by comparing to a reference sample (Rhodamine 6G). While a high PL quantum yield was measured for a solution of P3HT (ca. 33.7%, consistent with the literature38), the corresponding values recorded for the P3HT in the MWNTg-P3HT composites were relatively low and correlated with the content of P3HT (i.e., 1.0, 1.5, 2.9, and 4.3% for the composites prepared with [4]0/[3]0 = 25:1, 50:1, 75:1, and 100:1, respectively), consistent


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with the efficient quenching properties of the MWNTs. As a control experiment, a mixture of P3HT and MWNTs was prepared with the same ratio of the two components as that of the MWNT-g-P3HT prepared using a [4]0/[3]0 ratio of 100:1. The PL emission of the P3HT component in the mixture was not bathochromically shifted and was relatively less quenched than that recorded for the MWNT-gP3HT composite (see Figure S6). The lifetimes of MWNT-g-P3HT composites were measured to be 650, 640, 610 ps for the composites prepared with [4]0/[3]0 = 50:1, 75:1, and 100:1, respectively (see Figure S8). These values are similar to that measured for P3HT (ca. 630 ps), indicating that a static quenching process may be operative.13 Collectively, these results underscore the importance of forming welldefined interfaces between MWNTs and P3HT to facilitate photoinduced electron transfer in their corresponding composites. To further examine the impact of the surface curvature of MWNTs on the packing order of the polymer chains, P3HT was grown from MWNTs with relatively small diameters (ca. 7 nm on average, see Figure S1) and the composites were analyzed by FT-IR spectroscopy, Raman spectroscopy, and TGA (Figure S9−S11). Reflective of relatively unordered P3HT chains, red-shifted absorption signals were not detected in the optical absorption spectrum recorded for the material (Figure S12). Based on these results, we concluded that MWNTs with relatively large diameters were critical for inducing the highly-ordered packing of the grafted P3HT brushes. Such ordering not only results in red-shifted optical absorption characteristics, which enable the effective use of sunlight, but also improves the charge carrier mobility.30

Photoresponse of the MWNT-g-P3HT composites. Since the aforementioned MWNT-g-P3HT composites were found to exhibit excellent optical, structural, and interfacial properties, our subsequent efforts were directed toward investigating their photoresponse characteristics. Photoresponse devices


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were fabricated from individual MWNT-g-P3HT core-shell structures using a focused ion beam/scanning electron microscope (FIB/SEM) system (Figure 6a). The length between the two contacts was determined to be several microns (Figure 6b). As illustrated in Figure 6c, the band difference between the LUMO of P3HT and the Fermi level of the MWNTs facilitates exciton dissociation and results in a photoinduced electron transfer from the P3HT to the MWNTs. The photocurrents of individual MWNT-g-P3HT core-shell structures were collected upon periodic exposure to the incident light. In contrast to pristine MWNTs that do not generate a photoresponse due to their metallic nature,23,39 all of the MWNT-g-P3HT materials that featured core-shell structures were found to exhibit excellent photoresponse characteristics. As the illumination path was periodically interrupted, the current of the devices increased or decreased accordingly with the absolute values measured in the nanoampere range. It was previously demonstrated that the electrical conductivities displayed by MWNTs are to a large extent governed by the electrons confined in the outmost layer.40,41 Switching between these two states was found to be sensitive and repeatable. The photostimulated conductivity was measured under ambient conditions where P3HT may absorb trace amounts of water, oxygen or other traps of photogenerated electrons.42,43 Thus, we attribute the slow rise of the photocurrent to filling of the traps by photogenerated electrons and the slow decay to emptying of the traps.44 Because the time required for the electron traps to be filled or emptied by the photogenerated electrons is much longer than the photoinduced charge transfer process (on the picosecond time scale as confirmed latter), the photoinduced charge transfer is not reflected in the response speeds of the photocurrents.44 Photosensitivity was defined as the percentage of enhanced current upon illumination with respect to the current measured in the absence of light. As shown in Figure 7a, the photosensitivity of the MWNT-gP3HT core-shell structures gradually enhanced with the length of the grafted P3HT chains (e.g., 1.00, 1.20, 1.42, and 1.75% for the four different composites, respectively). These results were ascribed to the


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photoinduced charge separation at the interfaces of the MWNTs and the P3HT, which led to an enhanced density of free electrons in the MWNTs. Moreover, a thicker P3HT layer gave rise to an enhanced absorption of light, and thereafter led to improved electron transfer from P3HT to the MWNTs.

Figure 6. (a) Schematic illustration of the fabrication of photoresponsive devices using individual MWNT-g-P3HT core-shell structures. (b) An SEM image of a photoresponsive device based on individual MWNT-g-P3HT that was prepared using a [4]0/[3]0 = 100:1, with an enlarged image shown in the inset. (b) Energy-band diagrams of P3HT and MWNTs, showing that the band offset facilitates photoinduced charge separation at the interface of MWNTs and P3HT.


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Figure 7. Photoresponses of individual MWNT-g-P3HT core-shell structures. (a) Photosensitivities of individual MWNT-g-P3HT composites with core-shell structures obtained with different [4]0/[3]0 ratios (incident light density: 82 mW cm−2). (b) Changes of the photocurrent of an individual MWNT-g-P3HT core-shell structure prepared using a [4]0/[3]0 = 100:1 as the incident light (with intensities of 29, 33, 45, 58, 66, and 82 mW cm−2) is opened/closed periodically. The inset shows that the photosensitivity increases linearly as function of the intensity of the incident light.

Exciton dissociation is a key step in order for the MWNT-g-P3HT composites to exhibit a photoresponse. Upon photoexcitation in the P3HT layer, the excitons diffuse to the MWNT/P3HT interface and dissociate. The diffusion of excitons in the P3HT phase can be probed by changing the 16 ACS Paragon Plus Environment

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intensity of illumination. A MWNT-g-P3HT core-shell structure prepared using a [4]0/[3]0 = 100:1 was found to display photocurrents that were dependent on the intensity of the incident light (Figure 7b); in addition, the photosensitivity was found to linearly increase with the light intensity (Figure 7b, inset). Since the use of higher intensity light increased the generation of excitons in the P3HT phase, the correlation with the linearly increased photocurrent indicated fast diffusion of the excitons in the P3HT phase as well as efficient exciton dissociation at the MWNT/P3HT interface. This feature can be attributed to the highly-ordered P3HT aggregates and the formation of well-defined MWNT/P3HT heterojunctions. While the photoresponse characteristics of individual CNTs or CNT films have been previously reported, the corresponding results were obtained through the use of Schottky barrier models or via bolometry.21−23 By contrast, the photoresponses of the MWNT-g-P3HT composites described herein were achieved through charge separation. Furthermore, a low-powered white light (as low as 29 mW cm−2) generated by a solar simulator was used to produce a photoresponse in this research in lieu of the high-powered lasers (e.g. 0.7 and 5.6 kW cm−2) reported in the literature.21−23 As will be described in more detail below, the photoresponses of the MWNT-g-P3HT composites was exclusively attributed to exciton dissociation at the MWNT/P3HT interface.

NIR TA spectra of the MWNT-g-P3HT composites. To gain a deeper understanding of the aforementioned photoresponse properties, the photophysics displayed by the MWNT-g-P3HT composites were studied using near-infrared time-resolved absorption (NIR TA) spectroscopy. The excited state properties of P3HT in solutions and in solid films have been extensively investigated.45,46 To avoid singlet exciton annihilation, TA spectra were acquired under a low excitation-photon fluence of 2.3×1013 photons cm−2 pulse−1. Immediately after excitation at 445 nm (at a delay time ∆t = 0.00 ps), a dilute solution of P3HT in THF (0.01 mg mL−1) was found to display a broad photoinduced absorption


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(PIA) signal at ca. 950 nm (Figure 8a). As the delay time elapsed, the maximum of the PIA signal shifted to a longer wavelength with increasing intensity. At ∆t = 30 ps, the maximum of the PIA signal shifted to ca. 1080 nm and exhibited the greatest intensity over the course of the experiment. Moreover, signal decay was observed at longer delay times. The aforementioned PIA signal was assigned to singlet excitons generated in P3HT (1P3HT*) and the 130-nm dynamic Stokes shift was attributed to a downhill excitation energy transfer as well as conformational relaxation of the polymer backbones.47 At ∆t = 600 ps, a PIA signal attributed to the generation of triplet excitons in P3HT (3P3HT*) was recorded at ca. 830 nm38 and did not appear to decay until ∆t = 1800 ps. Over this time scale, the PIA signal assigned to 1

P3HT* was almost completely decayed.


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Figure 8. NIR TA spectroscopy of P3HT and MWNT-g-P3HT composites. (a, b) TA spectra at various delay times for (a) a dilute solution of P3HT in THF and (b) a suspension of MWNT-g-P3HT prepared using a [4]0/[3]0 = 100:1 in THF excited at 445 nm. (c) TA spectra for the MWNT-g-P3HT suspension excited at 605 nm. The blanked sections in (b) and (c) are due to laser interference. (d) Time evolution of 1P3HT* at 1300 nm for the P3HT solution and the MWNT-g-P3HT suspension excited at 445 and 605 nm. The solid lines were obtained by least-square curve fitting based on multi-exponential models (see text for additional details).

When the MWNT-g-P3HT in THF (0.03 mg mL−1) was excited at 445 nm (Figure 8b), the corresponding TA spectra exhibited two independent PIA signals at ca. 1050 and 1250 nm, which were recognized from the transients in the time domains of up to 30 ps. While the PIA signal recorded at 1050 nm decayed more slowly than the signal observed at 1250 nm, both signals were different from the single PIA signal observed with dynamic Stokes shift for P3HT (Figure 8a). The dual PIA characteristics of the MWNT-g-P3HT composites resembled those reported for annealed P3HT films, where the PIA bands at 1050 and 1250 nm have been assigned to localized polarons of P3HT (P3HT•+) and 1P3HT*.35,48 It is known that the high crystallinity of P3HT films facilitates the generation of P3HT•+ and inhibits the formation of 3P3HT*.49 Thus, the appearance of a sizable P3HT•+ signal and the disappearance of the 3P3HT* signature suggested to us that the P3HT brushes were highly ordered in the MWNT-g-P3HT composite. To selectively excite the highly-ordered P3HT aggregates, the TA spectra of the MWNT-g-P3HT composites were recorded after excitation at 605 nm, the shoulder of the ground-state absorption of the highly-ordered P3HT aggregates (Figure 8c). While the broad TA spectra were biased by the 1P3HT* absorption at ca. 1250 nm in the earlier time domain of up to 0.16 ps, the P3HT•+ feature at ca. 1050 nm


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became dominant at a later time stage (e.g., at ∆t = 700 and 1500 ps). Previously, we have shown that low photon fluence and band-edge photoexcitation does not yield detectable P3HT•+ in annealed P3HT films.35 Therefore, the observation of P3HT•+ in the MWNT-g-P3HT composite under excitation at 605 nm suggested that the MWNTs functioned as an electron acceptor and facilitated photoinduced electron transfer at the MWNT/P3HT interface. Figure 8d summarizes a comparison of the time evolution of 1P3HT* at 1300 nm for P3HT and the MWNT-g-P3HT composite (prepared using a [4]0/[3]0 = 100:1) upon excitation at different wavelengths. Excitation of P3HT or the MWNT-g-P3HT composite at 445 nm resulted in a rise phase prior to the decay phase, whereas excitation of the MWNT-g-P3HT composite at 605 nm resulted in only a decay phase. As summarized in Table 1, different kinetics can be fitted using bi- or tri-exponential functions, ∆OD = A1exp(−t/τ1) + A2exp(−t/τ2) + A3exp(−t/τ3), where Ai and τi represent the amplitudes and the decay or rise time constants. For the solution of P3HT, the rise and decay time constants of 1P3HT* were calculated to be 8.1±0.7 and 705±28 ps, respectively. The rise phase was due to the aforementioned dynamic Stokes shift of the 1P3HT* absorption (Figure 8a), while the decay time constant (∼705 ps) was consistent with the lifetime of 1P3HT*.38 By contrast, the rise and decay time constants were calculated to be 0.30±0.13 and 360±28 ps, respectively, for the MWNT-g-P3HT composite excited at 445 nm. The relatively rapid rise phase, compared to that of P3HT (∼8 ps), was attributed to the orderly-packed P3HT chains and reflected a strong interchain electronic coupling. The decay time constant (∼360 ps) was consistent with the lifetime of 1P3HT* reported for annealed P3HT films.35,38


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Table 1. Time constants of 1P3HT* (1300 nm) measured for P3HT and the MWNT-g-P3HT composite at different excitation wavelengths (λex). Sample

λex (nm)

τ1 (ps)

P3HT

445

8.10±0.70 (rise)

705±28

MWNT-g-P3HT(100:1)

445

0.30±0.13 (rise)

360±28

MWNT-g-P3HT(100:1)

605

0.80±0.13

10.00±1.10

τ2 (ps)

τ3 (ps)

176±15

When the MWNT-g-P3HT composite was excited at 605 nm, no rise phase was observed; rather, two fast decay phases (∼0.8 and ∼10 ps) and a slow decay phase (∼180 ps) were recorded. The fastest decay (∼0.8 ps) was attributed to an instantaneous electron transfer at the MWNT/P3HT interface, since the low photon fluence excluded the involvement of singlet exciton annihilation and the low photon energy minimized the deposition of excess excitation energy. Indeed, the phenylene units that link the P3HT chains to the surfaces of the MWNTs may facilitate the photoinduced electron transfer from P3HT to MWNTs. The decay phase with the intermediate rate (~10 ps) was assigned to the diffusion of 1

P3HT* to the interfaces of MWNTs and P3HT, where the charge separation takes place. The timescale

recorded for exciton diffusion was similar to that observed in films of phenyl-C61-butyric acid methyl ester (PCBM)/P3HT blends.35,48 Finally, the lifetime of 1P3HT* decay was found to shorten to ∼180 ps because the longer wavelength radiation (λex=605 nm) excited merely the highly-ordered P3HT aggregates which quench the excitons more efficiently. Collectively, these results indicated that photoinduced charge separation occurred at the interfaces of MWNTs and P3HT in the MWNT-g-P3HT composites.


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CONCLUSION Well-defined P3HT chains were successfully grown from the surfaces of MWNTs using SI-KCTP. The process resulted in the formation of well-defined heterojunctions between the MWNTs and the P3HT layers. The polymerization was found to proceed in a controlled manner as chains of tunable lengths were achieved through variation of the initial monomer-to-initiator ratio. It was determined that largediameter MWNTs facilitated the grafted P3HT chains to form highly-ordered aggregates, which were subsequently found to exhibit well resolved and red-shifted optical absorption. Moreover, NIR TA spectroscopy revealed photoinduced charge separation in the MWNT-g-P3HT composites, which resulted in electron transfer from P3HT to MWNTs. Finally, the individual MWNT-g-P3HT composites with core-shell structures were found to exhibit excellent photoresponses under low illumination intensity due to efficient photoinduced charge separation processes at the interfaces of the MWNTs and the P3HT. Since the construction of heterojunction is important in carbon-based electronics and optoelectronics applications, the methodology described herein is expected to provide a universal approach for the controlled modification of various materials, including CNTs, graphene, carbon quantum dots, and fullerenes.

METHODS Synthesis of MWNT-g-P3HT composites. MWNTs (purity > 95%) with two different diameters (ca. 40 and 7 nm in average, see Figure S1) were purchased from Chengdu Organic Chemicals Co. Ltd. Solvents were dried by distillation from Na/benzophenone. Complex 1, Et2Ni(bipy)2, and 4 were synthesized by following procedures reported in the literature (see Section 6 and 7 in the Supporting Information). All manipulations were performed using standard Schlenk technique or in an Ar-filled glovebox.


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Polymerization initiators were installed on the surfaces of MWNTs using a two-step procedure that involved the exposure of 1 to Et2Ni(bipy), followed by ligand exchange with dppp. In a typical reaction, 1 (70 mg, ca. 0.026 mmol of PhBr as determined by TGA) was first dispersed in anhydrous THF (2.5 mL), followed by the addition of a solution of Et2Ni(bipy) (0.0074 M, 3.5 mL in THF, 0.026 mmol) at room temperature in several portions over the course of 2 hours. The relatively slow addition procedure facilitated the efficient conversion of the PhBr groups to afford 2. Afterward, the ligand exchange reaction was accomplished through the addition of excess dppp (11 mg in 2 mL of THF, 0.027 mmol) to the aforementioned suspension. After 12 hours at room temperature, the resulting product 3 was isolated by repeated alternate centrifugation/dispersion in anhydrous THF and CH2Cl2 under an atmosphere of argon. The centrifugation/dispersion cycles were repeated until the supernatant was colorless. The SI-KCTP from MWNTs with a daverage = 40 nm was performed in an Ar-filled glovebox. In a typical reaction, 4 (0.397 g, 1.30 mmol) was added to a suspension of 3 (83 mg in 4 mL THF, ca. 0.026 mmol of initiating sites). After 24 hours at room temperature, the resultant composite was designated as MWNT-g-P3HT(50:1) and subjected to repeated alternate centrifugation/dispersion in THF. Finally, the composite was dried in a vacuum oven at 60 °C overnight. Various [4]0/[3]0 ratios, i.e., 25:1, 50:1, 75:1, and 100:1, were employed and the resultant composites were designated as MWNT-g-P3HT(25:1), MWNT-g-P3HT(50:1), MWNT-g-P3HT(75:1), and MWNT-g-P3HT(100:1), respectively. To test the effect of surface curvature of MWNTs on the optical absorption of grafted P3HT, MWNTs with daverage = 7 nm were used for the SI-KCTP with a [4]0/[3]0 = 50:1. The composite was designated as MWNT(7nm)-g-P3HT(50:1). Device fabrication. The MWNT-g-P3HT composites were dispersed in chloroform (0.1 mg mL−1) with the aid of sonication for 15 min and deposited on Si wafers covered with a 100 nm thick SiO2 layer by spin casting. The individual MWNT-g-P3HT composites were readily found using SEM due to the


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excellent dispersibility. Contacts were coated using a FIB/SEM system (FEI Helios NanoLab 600I). The polymer on the two ends of the MWNTs was first etched using a Ga+ ion beam, followed by deposition of Pt pads on the naked MWNT ends using electron beam. Instrumentation. See Section 8 in the Supporting Information for details. ASSOCIATED CONTENT Conflict of Interest: The authors declare no competing ﬁnancial interest.

Supporting Information: Characterization and surface functionalization of MWNTs, FT-IR and Raman spectra of MWNT-gP3HT composites (daverage of MWNTs = ca. 40 nm), spectroscopic characterization of the mixture of MWNTs and P3HT (daverage of MWNTs = ca. 40 nm), PL excitation spectra and PL decay curves of MWNT-g-P3HT composites (daverage of MWNTs = ca. 40 nm), grafting P3HT brushes to MWNTs with daverage = ca. 7 nm, synthesis of diethyldipyridylnickel, synthesis of 2-bromo-5-chloromagnesio-3hexylthiophene, instrumentation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *[email protected] Acknowledgements. This research was supported by the “Hundred Talents Program” of Chinese Academy of Sciences and the National Natural Science Foundation of China (21274158, 91333114). CWB is grateful to the IBS (IBS-R019-D1) and the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea.


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