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Carbon nanotube and semiconductor nanorods hybrids: Preparation, characterization and evaluation of photocurrent generation Jugun Prakash Chinta, Nir Waiskopf, Gur Lubin, David Rand, Yael Hanein, Uri Banin, and Shlomo Yitzchaik Langmuir, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017

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Carbon nanotube and semiconductor nanorods hybrids: Preparation, characterization and evaluation of photocurrent generation Jugun Prakash Chinta1,2,†,‡, Nir Waiskopf 1,2,†, Gur Lubin3,4, David Rand3,4,Yael Hanein3,4, Uri Banin1,2, Shlomo Yitzchaik1,2* 1

Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 2

Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 3

4

School of Electrical Engineering, Tel Aviv University, Tel Aviv 69978, Israel

Tel Aviv University Center for Nanoscience and Nanotechnology, Tel Aviv University, Tel Aviv 69978, Israel

†

These authors contributed equally to this work

‡

Present address: Analytical Division and Centralized Instrument Facility, CSIR

Central Salt & Marine Chemicals Research Institute, Bhavnagar 364002, India. ABSTRACT Carbon nanotubes (CNTs) and semiconductor nanocrystals (SCNCs) are known to be interesting donor-acceptor partners due to their unique optical and electronic properties. These exciting features have led to the development of novel composites based on these two nanomaterials and to their characterization for use in various applications, such as components in sensors, transistors, solar cells and biomedical devices. Two approaches based on covalent and non-covalent methods have been suggested for coupling the SCNCs to CNTs. Most covalent conjugation methods used so far were found to disrupt the electronic structure of the CNTs or interfere with charge transfer in the CNT-SCNC interface. Moreover, it offers random and poorly organized nanoparticle coatings. Therefore non-covalent methods are considered to be ideal for better electronic coupling. However, a key common drawback of noncovalent methods is the lack of stability which hampers their applicability. 1 ACS Paragon Plus Environment

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In this article, a method has been developed to couple semiconductor seeded nanorods onto CNTs through π - π interactions. The CNTs and pyrene conjugated SCNC hybrid materials were characterized by both microscopic and spectroscopic techniques. Fluorescence and photocurrent measurements suggest the proposed pi-stacking approach results in a strong electronic coupling between the CNTs and the SCNCs leading to better photocurrent efficiency than that of a covalent conjugation method reported using similar SCNC material. Overall, the CNT-SCNC films reported in the present study open the scope for the fabrication of optoelectronic devices for various applications. 1. INTRODUCTION

Carbon nanotubes (CNTs) have been extensively studied in the last two decades revealing their unique mechanical, thermal, optical, and electronic properties.1-2 This comprehensive range of properties makes CNTs a potential tool for applications in various fields such as hydrogen storage, nanoelectronics and biomedical devices.3-8 A wide variety of organic donors and acceptors has been successfully introduced on CNTs and their charge transfer and transport properties were studied.9-16 CNT based donor-acceptor nanohybrids were found to show fast charge separation and slow charge recombination upon illumination with visible light, and therefore can be used for optoelectronic devices. Semiconductor nanocrystals (SCNCs) are known to be attractive donor partners due to their high photochemical stability and large absorption cross-section. Moreover, their properties can be tuned and optimized for a specific application by changing their composition, shape, dimensions and surface coating.17-20 Therefore, the combination of SCNCs with CNTs may lead to novel

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composite materials with interesting applications as sensitive and selective sensors, transistors and effective solar cells. Despite their many possible applications, only few methods have been demonstrated to form CNT-SCNC hybrid materials.21-25 For example, amine functionalized cadmium selenide (CdSe) quantum dots were attached to acid chloride modified CNTs via amide bond formation.26 In another example, cadmium telluride nanocrystals were grown in situ at oxygen-terminated sites on acid oxidized CNT surfaces.27,28 Such type of covalent attachments lead to disruption in the electronic structure of the CNT at the attachment site and in some cases would result in the destruction of the nanotubes.29-31 Covalent attachment strategies are also characterized by limited control and result with random and often poorly organized nanoparticle coverages. Thus noncovalent attachment was suggested as an alternative strategy to address these limitations. For example, CdTe nanocrystals were linked to CNTs through electrostatic interactions and created photoactive nanohybrid structures.32 An optoelectronic device realized through layer-by-layer deposition method of these nanohybrids exhibited appreciable photocurrent performance. SCNCs conjugation to CNTs was alternatively achieved through π - π interactions which were suggested to provide better electrical coupling between SCNCs and CNTs.33-36 However, although these methods were found to be simple and effective, hybrids prepared by π-π stacking and electrostatic interactions are not generally used due to their limited stability, and sensitivity to changes in pH and ionic strength. The combination of both covalent and non-covalent conjugation methods have also been used to develop CNT based hybrid materials with unique optoelectronic properties.37-41


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CNT-SCNC hybrids are becoming very attractive also in the realm of bio applications and specifically for photo stimulation of neuronal tissues. We have recently reported on the ability to photo-stimulate the retina with CdSe/ CdS seeded nanorods (NRs) coated CNT films.42 SCNCs coated with the tripeptide glutathione (GSH) were covalently conjugated to CNTs after plasma polymerization with acrylic acid (ppAAc), achieving highly dense mono-layer of the SCNCs on the CNTs. Light excitation of the SCNCs resulted in charge transfer at the CNT-SCNC interface eliciting electric field and stimulating in vitro isolated blind retinas. This former work, along with additional possible uses of the CNT-SCNCs hybrids in other applications, prompted us to develop and study additional routes to form such hybrid materials. Specifically, herein we examine π - π conjugation of SCNC coated by ligands with pyrene moiety and CNTs, demonstrating the ability to achieve by this conjugation route efficient photocurrent generation.

2. RESULTS AND DISCUSSION

2.1. Design and synthesis of thiol linked pyrene conjugates Two molecules were designed and synthesized to allow π-stacking interaction with CNT from one hand and a strong binding to SCNC surface on the other hand. Cysteamine-pyrene (CYS-PA), with a short spacer between a thiol group which has high affinity to SCNC surface and a CNT binding moiety, a pyrene group, were synthesized by condensation of pyrene aldehyde with cysteamine and the reduction of imine to amine by sodium borohydride (Figure 1a).43 Glutathione conjugated pyrene imine (GSH-PI) with a longer and biocompatible spacer were also synthesized by condensation of glutathione with pyrene aldehyde (Figure 1b). Both derivatives were 4 ACS Paragon Plus Environment

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characterized by 1H NMR,

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C NMR and HRMS (Figure S1-S6). The absorption

spectrum of the CYS-PA in water exhibited a broadened spectrum due to formation of aggregates with absorption maximum at 350 nm, corresponding to the pyrene moiety absorbance (Figure S7a). Excitation in this wavelength resulted in a broad emission spectrum with emission peaks at 380, 398 and 420 nm corresponding to the vibrational structure of pyrene and an additional emission peak at 475 nm corresponding to intermolecular π–π stacking of pyrene moieties (exciplex formation) (Figure S7b,c).

Figure 1. Synthesis scheme of cysteamine and glutathione conjugated pyrene derivatives.

2.2. Synthesis and characterization of CYS-PA or GSH-PI conjugated SCNC CdSe/CdS seeded NRs were synthesized by modification of a previously described procedure.44,45 Figure 2a shows the absorption and emission spectra and a transmission electron microscopy (TEM) image of these NRs. The absorption spectrum presents a weak first exciton peak at 600 nm associated to the CdSe seed, and a sharp increase in the absorption below 470 nm due to the additional absorption of the CdS rod. The emission peak of these NRs is at 610 nm and their fluorescence


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quantum yield is 45% (in toluene for excitation at 405 nm). TEM images of the CdSe/CdS NRs revealed a homogeneous size distribution, 40±5 nm in length and 5.0±0.8 nm in diameter (Figure 2b).

(a)

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Figure 2. (a) Absorption and emission spectra of CdSe/CdS seeded nanorods, black and red respectively. (b) TEM image showing NRs, 40±5 nm in length and 5.0±0.8 nm in diameter. The coating of the NRs by the synthesized CYS-PA or GSH-PI molecules was performed in two steps to prevent NRs aggregation in water due to over coating with these ligands. First the NR’s organic ligands from the synthesis stage were replaced through ligand exchange with GSH and the NRs were purified from excess of ligands using centrifugal filtration. During this process the fluorescence quantum yield of the SCNC dropped to 17%, compatible with previously reported quenching effect of thiolated ligands.46 Next, the addition of the synthesized CYS-PA or GSH-PI molecules to the SCNC solution resulted in their exchange with the glutathione ligands. These ligand exchanges were followed by changes in the spectroscopic properties of SCNC-ligands 6 ACS Paragon Plus Environment

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solutions. Figure 3a,b show the absorption and photoluminescence spectra respectively, before and after addition of CYS-PA to the SCNC solution. The absorption spectrum of CYS-PA conjugated NRs showed signatures of its two constituents, i.e., SCNC and organic ligands. Most importantly, the π-π* transition band in the region between 300-400 nm remains the same in conjugated NRs as in free ligand solutions. Addition of increasing CYS-PA concentration resulted in nonlinear quenching of the NRs fluorescence both for excitation at 341nm and at 450nm, where the ligands absorption is negligible (Figures 3c,d and S8). In addition, quantum yield and lifetime measurements showed reduction in QY and shortening in the fluorescence lifetime following the addition of the CYS-PA ligands (Figure S13). These observed results can indicate on different quenching mechanisms, such as combination of both static and dynamic quenching that is manifested by quadratic behavior, distance dependent quenching that is expressed in exponential behavior and by others.47-49 Our data showed satisfactory fitting to both quadratic equation and linear equation in logarithmic scale (Figures 3d and S9). Hence, further investigation will be required to uncover the exact quenching mechanism for our system. Similar characteristic changes have also been observed for the interaction of SCNCs with GSH-PI (Figures S10-S13). These results are compatible with an electron transfer process from the SCNC to the pyrene moiety that was suggested previously for similar systems.39,40 The data also suggest that higher ligands/NP ratio will result in a more efficient charge transfer as long as ligand coverage is not too high to result in aggregation of the NPs in the solution. Hence, we decided to use 1000/1 ligands/SCNC (i.e, 60 µM of CYS-PA or GSH-PI) ratio as an optimal concentration which provided stable SCNCs solution and a quenching to >45% of the nanoparticles initial fluorescence. 7 ACS Paragon Plus Environment

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Figure 3. Absorption (a) and emission (b) spectra of SCNC (black), CYS-PA (red) and SCNC+CYS-PA (green). (c) Fluorescence quenching of SCNC with CYS-PA; 0.5 mM solution of CYS-PA in DMSO was added in 50 µL increments to a solution of SCNC in water (Excitation wavelength (λex) = 341 nm). (d) Quadratic fitting (red) to the quenching of SCNCs by the CYS-PA.

2.3. Characterization of the conjugated SCNC interactions with CNT SCNC-Pyrene-CNT nanocomposites were prepared by vigorous sonication of 0.01% w/w CNTs followed by 1hr probe sonication of a mixture of SCNC-CYS-PA or SCNC-GSH-PI and CNTs in water. This process facilitated SCNC adhesion by breaking CNT bundles, which increased the exposed CNTs’ surface area to the SCNC. The hybrid materials obtained was characterized by standard techniques such as TEM, UV-vis-NIR, and fluorescence and Raman spectroscopy.50-52 TEM together 8 ACS Paragon Plus Environment

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with dark field images revealed a nanometric elongated crystalline structures on the CNTs (Figures 4a and S14) while energy dispersive X-ray spectrum confirmed the presence of cadmium and selenium on the CNTs (Figure 4b). The appearance of the additional elements, nitrogen, oxygen, phosphate, and sulfur may result from the organic surface coating of the NPs, such as GSH, CYS-PA and TOPO. Finally the elements Cu and Si are detected because of their presence in the microscope equipment and grid used. ICP-MS was used to quantify the number of NRs on each CNT film, confirming the NRs density on CNTs significantly increased following the functionalization of SCNCs with CYS-PA or GSH-PI. These experiments also showed there isn’t significant change in the number of SCNC conjugated to the CNT films following their incubation in buffered medium at least for 14 days (Figure S16). (a)

(b)

Figure 4. (a) Transmission electron microscopy (TEM) image of pyrene conjugated nanorods (SCNC-CYS-PA) adhered to CNTs through π-π stacking interactions. Inset: dark field image showing bright rod shape structures of NRs aligned on CNT. (b) EDS confirms the presence of NRs composing atoms (e.g. cadmium and selenium) on CNT bundles. CNT-SCNC-CYS-PA interaction was studied by absorption and emission spectroscopic techniques. The absorption spectrum of SCNC-CYS-PA and CNT hybrid solution showed no change in the absorption signatures of SCNC-CYS-PA. 9 ACS Paragon Plus Environment

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However, a new scattering feature and characteristic bands for S22 transition of semiconducting CNTs in the region of 600-800 were observed (Figure 5a). The addition of CNT, also resulted in further decrease in the SCNC fluorescence with no significant change in the fluorescent lifetime, suggesting a static quenching mode of action. (Figures 5b and S13). The hybrid materials were also characterized by Raman spectroscopic measurements. Raman spectra were recorded using the 514 nm and 785 nm excitations which are in resonance with the metallic and semiconducting transitions respectively. The spectra recorded using 514 nm exhibited dominant features from SCNCs which impedes the analysis of the CNT characteristics (Figure S17). The Raman spectra obtained after excitation with 785 nm showed no significant changes in tangential band at ∼ 1590 cm -1 whereas changes in the intensity of peaks in the radial breathing mode region were observed (Figure 5c,d). The peak intensity of ∼ 230 cm-1 in SCNC-CYS-PACNT and SCNC-GSH-PI-CNT increased relative to peak at 267 cm−1 indicating the presence

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interactions.51,52 Overall, the enhanced adhesion of pyrene containing SCNCs to CNTs is expected to facilitate better charge transfer between SCNCs (Donor) and CNTs (acceptor).53


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Figure 5. Absorption spectra (a) and time resolved decay profiles (b) of SCNC+CYSPA (green) and SCNC+CYS-PA+CNT (blue). Inset: Expanded absorption spectrum in the range of 500-800. (λex = 341 nm, Concentration of SCNC = 60 nM, CNTs = 0.01 mg/mL). RBM (c) and Tangential mode (d) regions of Raman spectra recorded at 785 nm excitation. 2.4. Preparation and Characterization of pyrene conjugated NR CNT films SCNC coated with CYS-PA were deposited on CNT films using up to 5 drop casting cycles and incubation at 50ºC overnight, each step followed by a thorough cleaning procedure. Loading and CNT-SCNC morphology were examined after each cycle using ICP-MS and SEM imaging, respectively (Figures 6a,b and S18). SCNC are deposited in bundles on the CNT film after the first cycle and the number of NPs and bundles increases with the number of cycles until a blanket of NPs can be seen on the 11 ACS Paragon Plus Environment

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CNT (Figure S18). In parallel, photocurrent responses of the CNT-SCNC-CYS-PA samples were measured as previously described using 405 nm LED for excitation.38 After two-three cycles of deposition, photocurrent values were as high as 8 µA/cm2. These data suggest that two-three deposition cycles give good conditions for improved loading and photocurrent without affecting CNT porosity. Importantly, the photocurrent stayed similar for numerous on-off light cycles.


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conjugated NRs on CNT films after two cycles (blue-SCNC-CYS-PA, green-SCNCGSH-PI). (d) The photocurrent efficiency, photocurrent per number of conjugated SCNC, for the π-stacking systems is higher than for covalent conjugation through ppAAc, indicating on better electronic coupling between SCNC and CNT using the πstacking approach. 2.5 Comparison of covalent versus non-covalent (π) conjugation of SCNCs on CNT films To examine the efficiency of the π-stacking of SCNCs to CNTs over covalent conjugation, GSH-PI conjugated SCNC were used as a model system. The SCNCGSH-PI deposition on CNT films showed similar behavior to that of SCNC-CYS-PA system with similar NPs loading and photocurrent after deposition cycles (Figure 6c). In addition, compared to previously reported covalent conjugation of CNT-SCNCGSH,27 higher photocurrent efficiency, photocurrent per number of conjugated nanoparticles, has been observed for non-covalent conjugation with SCNC-GSH-PI (Figure 6d), confirming that the π-stacking conjugation procedure can improve the electric coupling between SCNCs and CNTs.

3. CONCLUSIONS Two anchor molecules, which contain free -SH group at one end for quantum rod surface modification and a pyrene moiety for CNT binding at the other end, were designed and synthesized. Pyrene conjugated SCNC were prepared by ligand exchange of GSH with either CYS-PA or GSH-PI. Supported by steady state fluorescence and lifetime measurements, the decrease in quantum yield of these pyrene conjugated NRs is due to electron transfer from the SCNC to the pyrene moiety. The interaction of pyrene conjugated SCNC and CNT was studied by ICP-


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MS and fluorescent measurements and showed a strong and stable binding with a good electronic coupling between the two nanomaterials. This resulted in high photocurrent efficiency compared with covalent conjugation through amide bond with carboxylic groups, which were formed on the CNT by plasma polymerized acrylic acid. Both ligand systems i.e., CYS-PA and GSH-PI showed similar photocurrent generation ability indicating the generality of the current method. Overall, the present article describes a method for the preparation of stable and better electronic coupling between CNTs and SCNCs. CNT and SCNC films demonstrated in this investigation may help in the fabrication of photoactive devices for applications in various fields ranging from nanoelectronics to biomedical devices.

4. EXPERIMENTAL SECTION

4.1. Materials and methods Cysteamine HCl, L-glutathione reduced, 1-Pyrenecarboxaldehyde and Sodium borohydride were procured from Sigma Aldrich Chemical Co., USA and all other chemicals were procured from local sources. All the solvents used were procured from local sources and were dried and distilled by usual procedures immediately before use. Distilled and deionized water was used in the studies. Purified single walled carbon nanotubes (SWCNTs) produced by high pressure carbon monoxide (HiPco) process were procured from Unidym, USA. Trioctylphosphine-oxide (TOPO) capped CdSe/CdS nanorods were synthesized following previously reported procedure.44,45 CNT films grown on titanium nitride (TiN) coated silicon/silicon dioxide (Si/SiO2) substrates using chemical vapor deposition (CVD) were used for the preparation of CNT-nanorod composite films. Spectra were measured: 1H and

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NMR by Bruker NMR spectrometer working at 400 MHz, mass spectra on Q-TOF 15 ACS Paragon Plus Environment

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micromass (YA-105) or Varian Inc, USA, using electrospray ionization method, steady state fluorescence by Perkin Elmer LS55, and absorption spectra by Shimadzu UV2101 PC. Transmission electron microscopy (TEM) images were recorded with a high resolution transmission electron microscope Tecnai F20 G2 on carbon coated copper grids. Raman spectra of were measured by a micro Raman Spectrometer (λexc = 514 and 785 nm). The spectra obtained have been normalized to the G-band.

4.2. Synthesis of cysteamine conjugate of pyrene (CYS-PA) 1-Pyerenaldehyde (500 mg, 2.171 mmol) was dissolved in 30 mL of ethanol with slight heating. Cysteamine HCl (493 mg, 4.343 mmol) was added to another flask containing ethanol (10 mL). Triethylamine was added (446 mg, 4.408 mmol) to this suspension and stirred for 10 min at room temperature. These two solutions were mixed and refluxed for 6 h. The resulting solution was cooled, filtered to separate the imine. Sodium borohydride (165 mg, 4.342 mmol) was added in portions over 30 minutes and the solution stirred for another 2 h at room temperature. Solvent was removed by rotary evaporation and residue was dissolved in 50 mL of 1:1 water/dichloromethane. The organic layer was collected and dried over sodium sulfate. The solution was filtered and solvent removed by rotary evaporation. The residue was dissolved in 50 mL of 1:1 ethanol/diethyl ether under nitrogen and 100 mg of sodium borohydride to reduce possible disulfides. About 1 mL of hydrogen chloride was added dropwise, and after standing for 1 h, the yellow precipitate was collected, washed with ethanol and ether and then dried in vacuum to give CYS-PA in good yield. 1H NMR (400 MHz, DMSO-d6) δ (ppm): 10.12 (br, 2H), 8.09-8.64 (m, 9H), 4.99 (m, 2H), 3.51 (m, 2H), 3,30 (t, 2H).

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(ppm): 32.4, 46.4, 47.0, 123.5, 123.6, 123.9, 124.8, 125.6, 125.7, 126.5, 127.3, 128.2, 16 ACS Paragon Plus Environment

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129.2, 129.3, 130.2, 130.7, 131.3. HRMS (ESI+) calcd. for C19H18NS [M+H]+ (m/z): 292.1154, found: 292.1155.

4.3. Synthesis of glutathione conjugate of pyrene (GSH-PI) 1-Pyerenaldehyde (500 mg, 2.171 mmol) was dissolved in 30 mL of ethanol with slight heating. To this aqueous solution of glutathione (667 mg, 2.171 mmol) was added. The reaction mixture was refluxed for about 2 h, and yellow brown precipitate was obtained after standing overnight. It was purified by repeated washing with aqueous-ethanol (1:2) and dried in vacuum. 1H NMR (400 MHz, DMSO-d6) δ (ppm): 8.06-8.51 (m, 12H, pyrene, NH-Gly, NH-Cys), 6.69 (m, 1H, α–Glu), 4.49 (brs, 1H, α– Cys), 3.43-4.00(m, 8H).

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171.5, 171.1, 170.4, 134.4, 132.9, 131.3, 131.0, 130.7, 128.5, 128.2, 127.8, 126.8, 126.0, 125.8, 125.5, 124.4, 123.9, 123.5, 123.0, 68.8, 68.2, 66.5, 41.3, 37.9, 31.1. HRMS (ESI+) calcd. for C27H23N3O5S [M-H2O]+ (m/z): 501.1353, found: 501.1333.

4.4. Preparation of pyrene conjugated nanorods Trioctylphosphine-oxide (TOPO) capped CdSe/CdS nanorods were sonicated in hexane for five minutes and centrifuged at 5500 rpm for five minutes to remove undissolved residues. The supernatant solution was separated and the rods precipitated by adding ethanol (1:1 v/v hexane/ethanol) to remove the excess of TOPO ligands. The solid was separated by centrifugation at 5500 rpm for five minutes and redissolved it in 1mL of chloroform. To this solution 200 µL of Lglutathione (0.5M in MeOH) was added and extracted into water by adding basic water. The glutathione capped nanorods in water were purified from excess of glutathione ligand by repeated centrifugation with 100 kDa membrane filters. To a 60


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nM solution of glutathione capped nanorods, 60 µM solution of pyrene ligands in DMSO were added and stirred overnight and excess of pyrene ligands were removed by 12 kDa dialysis using cellulose membrane. Conjugation was confirmed by measuring the absorption and emission spectrum of the nanorods.

4.5. Preparation of pyrene conjugated nanorods and CNT composite material A suspension of nanorods-CNT hybrid material was obtained by adding SWCNTs (0.01 mg/ml) to 60 nM solution of pyrene conjugated nanorods. The solution was stirred vigorously for 24 hr and stored in dark. The solution was sonication for one min before doing any spectroscopic and microscopic measurements.

4.6. Fluorescence titrations Fluorescence titrations of SCNC with pyrene derivatives (CYS-PA or GSH-PI) were carried out by adding 0.5 mM solution of CYS-PA or GSH-PI in 50 µL increments to a solution of SCNC (60 nM) in water. During titration, the total volume of the sample was maintained constant and the contribution of the ligands to the total absorption of the solution was considered in the quenching calculations. Fluorescence spectra were measured by excitation of the samples at 341 nm and 452 nm.

4.7. Photocurrent measurements The photocurrent measurement setup consists of a 405 nm (Thorlabs) light emitting diode (LED), mounted on an upright metallurgical microscope (Meiji). The illumination was carried out using a 4× or a 40× water immersion objective which results in intensities within the range of 0.6-70 mW/cm2. The measurement unit equipped with a current amplifier (model 1212; DL Instruments). The photogenerated


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current was measured between the illuminated electrode and a reference electrode (platinum mesh) in PBS. ASSOCIATED CONTENT Supporting Information. Characterization of CYS-PA, Characterization of GSH-PI, Absorption and emission spectra

of

CYS-PA

and

GSH-PI,

Lifetime

measurements,

Spectroscopic

characterization of SCNC-GSH-PI, Electron microscopy characterization of SCNCGSH-PI-CNT, Spectroscopic characterization of SCNC-CYS-PA mixture with CNT, Spectroscopic characterization of SCNC-GSH-PI mixture with CNT, Spectroscopic characterization of SCNC mixture with CNT, Characterization of the pyrene conjugated nanorod CNT films. “This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] ACKNOWLEDGEMENTS This work was supported by the Israel Ministry of Science and Technology (SH, YH, UB), the European Research Council funding under the European Community's Seventh Framework Program (FP7/2007–2013)/ERC grant agreement FUNMANIA306707 (YH), and the Israel Science Foundation (UB, grant No. 811/13). We thank Ori Cheshnovsky for helpful discussions.

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