Contorted Octabenzocircumbiphenyl Sorts Semiconducting Single

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Contorted Octabenzocircumbiphenyl Sorts Semiconducting Single-Walled Carbon Nanotubes with Structural Specificity Jia Gao, Nikita Sengar, Ying Wu, Steffen Jockusch, Colin Nuckolls, Paulette Clancy, and Yueh-Lin Loo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04018 • Publication Date (Web): 01 Jan 2017 Downloaded from http://pubs.acs.org on January 8, 2017

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Chemistry of Materials

Contorted Octabenzocircumbiphenyl Sorts Semiconducting Single-Walled Carbon Nanotubes with Structural Specificity Jia Gao,† Nikita Sengar,‡ Ying Wu,|| Steffen Jockusch,|| Colin Nuckolls,|| Paulette Clancy‡ and Yueh-Lin Loo*†§ †

Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544, United States

‡

Robert F. Smith School of Chemical and Biomolecular Engineering and the Cornell Institute for Computational Science and Engineering, Cornell University, Ithaca, NY 14853 United States ||

Department of Chemistry, Columbia University, MC 3130, 3000 Broadway, New York, New York 10027, United States

§

Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544, United States

ABSTRACT: In this study, we use a non-planar aromatic molecule, contorted octabenzocircumbiphenyl (c-OBCB), to sort semiconducting single-walled carbon nanotubes (SWNTs) by their chiral angles. From absorption spectroscopy, photoluminescence excitation spectroscopy, and Raman spectroscopy studies, we find that c-OBCB preferentially binds and sorts for a number of semiconducting carbon nanotubes with chiral angles greater than 12o. Molecular Dynamics simulations reveal that the contorted aromatic core of c-OBCB binds strongly to only certain SWNTs, especially those with matching curvature, and that this discriminatory binding interaction is reinforced by preferences of the side chains on the c-OBCB to stick to SWNT surface rather than interact with the solvent. This opens the door to side chain/solvent engineering to bias the selection of certain (m,n) SWNT variants. We also investigate the temperature dependence of hole mobility in field-effect transistors comprising c-OBCB-sorted semiconducting carbon-nanotube networks and find hole transport in these networks to be thermally activated.

■ INTRODUCTION A major limitation of using single-walled carbon nanotubes (SWNTs) for applications in logic circuits is the coexistence of both semiconducting and metallic species in SWNTs prepared from conventional methods, such as chemical-vapor deposition, arc discharge and laser ablation.1-3 Numerous efforts have been targeted for controlling the growth of carbon nanotubes, as well as separating and purifying SWNTs to isolate the semiconducting species from their metallic counterparts.4,5 In the last few years, we have witnessed tremendous progress in the ability to sort SWNTs; high purity semiconducting SWNT (sSWNT) dispersions are commercially accessible today and are being explored as alternatives for electronic and optoelectronic devices in the post-silicon era.6-10 One of the most effective approaches for sorting SWNTs with specific structural or electrical properties entail the use of macromolecular dispersants, such as pi-conjugated polymers1118 and single-stranded DNA with specific sequences.19,20 These sorted s-SWNTs have shown broad applications in electronic devices, including field-effect transistors (FETs), light-emitting transistors, photodetectors, and photovoltaics.21-31 Interestingly, aromatic molecules (e.g., pyrenes, porphyrins, phthalocyanines, or perylenediimides) that exhibit strong π-π interactions with the surfaces of carbon

nanotubes,32-39 have not shown pronounced selectivity for s-SWNTs with specific chiralities. For example, while flavin and hexaaza-pentacene were reported to sort sSWNTs from metallic ones with 99% purity,40-42 they does not provide any specificity to the chirality of the sorted sSWNTs. Further, the mechanism by which these molecular dispersants isolate s-SWNTs from metallic ones is not understood.41,42 “Nanotweezers” is another class of molecular compounds reported to sort SWNTs.43-49 These derivatives are composed of two planar conjugated anchoring groups connected by a rigid linker. The anchoring groups each adsorb on the exterior surface of SWNTs.48 Even though such molecules interact with select SWNTs whose diameters are commensurate with the space defined by the anchoring groups and the rigid linker, the anchoring groups themselves are not able to conform effectively to the walls of SWNTs given their rigid and planar conformation. As a result, such molecules have had some limited success in separating s-SWNTs from metallic ones but essentially no success in sorting s-SWNTs with chiral specificity. Of these studies, nanotweezer compounds with tert-butylpyrene anchoring groups have been most effective at sorting s-SWNTs, with a reported purity of 93%44 but little/no selectivity in terms of chirality. Other molecules that fall in this category, including those based on perylene, porphyrin, tetrathiafulvalene, etc., do not

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show any selectivity towards s-SWNTs.47,49,50 In summary, molecular dispersants have shown limited ability in sorting s-SWNTs with high specificity. If we are to progress towards having predictive guidelines for sorting carbon nanotubes on demand, we will need a deep understanding of the structures of dispersants and molecular-scale mechanisms responsible for their resulting selectivity toward carbon nanotubes. This understanding is currently absent. In the study reported here, we explore how a nonplanar, contorted molecular semiconductor can form a complementary interface with s-SWNTs to both effectively disperse and sort them. For this study we employed a large, contorted, polycyclic aromatic core, c-OBCB,51 for sorting s-SWNTs. Absorption spectroscopy, photoluminescence excitation spectroscopy, and Raman spectroscopy, reveal that c-OBCB selectively sorts for a number of sSWNTs species, including (6,5), (7,6), (10,3), (10,5), (8,7), (11,7), (10,9) and (13,5) from Hipco carbon nanotube mixtures, and for (6,5), (7,6) and (10,3) species from Comocat SWNT mixtures. Molecular Dynamics (MD) simulations suggest that selectivity results from an interplay between the propensity of the alkyl side chains of c-OBCB to wrap the carbon nanotubes and the binding of the curved core of c-OBCB to the SWNTs. Temperature dependent electrical characterization of FETs based on c-OBCB-sorted carbon-nanotube networks reveal charge transport can be described with either the fluctuation-induced tunneling (FIT) model or the variable-range hopping (VRH) model over the range explored.

■ EXPERIMENTAL SECTION Materials and dispersion preparation: Hipco and Comocat carbon nanotubes were obtained from Unidym and Sigma Aldrich, respectively. A solution, comprising 10 mL of toluene with 2 mg of raw SWNTs and 10 mg of cOBCB, was kept in an ice-water bath and sonicated using a probe sonicator (model FB505, Fisher Scientific) at intensity 5 for 30 min. The solution was then centrifuged at 20,500 g for 1 h and the supernatant was extracted for optical characterization and device fabrication. Computation: Molecular-Dynamics simulations were performed using Sandia’s open-source LAMMPS software package.52 The “Optimized Potentials for Liquid Simulations-All Atoms” (OPLS-AA) force field was used to describe the inter- and intra-molecular forces between all the molecules in the system.53,54 Non-bonded interactions were calculated using Lennard-Jones and Coulombic potentials with a cut-off of 10 Å. Tail corrections and particle-particle, particle-mesh approximations55 were used to take into account long-range van der Waals and Coulombic interactions, respectively. We employed an isobaric, isothermal Nosé-Hoover NPT ensemble, in which the number of molecules N, pressure at P = 1 bar, and temperature at T = 298 K were kept constant. All the simulations used a time step of 1 fs. Each simulation was allowed to equilibrate for 2 ns (2 million time steps), with production data collected for the next 12 ns.

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Periodic boundary conditions (PBCs) were used in all three Cartesian directions. The size of the simulation box is (30 Å × 30 Å × 100 Å) in the x-, y-, and z- directions, respectively. A single SWNT was placed in the center of the simulation box with its long axis aligned along the zdirection. To emulate the scale of experimental studies, the SWNT was made infinitely long56,57 by connecting it to its images across the ends of the box (using PBCs). The zdimension of the simulation box was kept fixed to keep the SWNT from deforming; x- and y- dimensions were allowed to deform in response to the NPT barostat. This condition was chosen to avoid end effects and to allow cOBCB to move freely over the SWNT surface. After equilibration, the SWNT was maintained as a rigid component throughout the simulation. All the carbon atoms in the SWNT were treated as Lennard-Jones spheres.58 To determine the role of entropy in the binding of the aromatic core of c-OBCB to the SWNT, we used the wellknown Thermodynamic Integration (TI) simulation approach to calculate the potential of mean force (PMF).59 c-OBCB was pulled slowly from its equilibrium position with respect to SWNT at a rate of 0.125 Å per ns for the first 14 Å of separation, and then at a slightly higher rate of 0.25 Å per ns for the next 8 Å. This two-tier pulling rate was adopted to conserve computational time without affecting the results. The distance between c-OBCB and SWNT is measured as the distance between the center of masses of c-OBCB and SWNT. A separation of 22 Å, obtained by sampling the system at 74 small windows, was found to be sufficient to capture the salient aspects of the PMF. As a separate test of the PMF, we also sampled two windows in which c-OBCB was 0.5 Å closer to the SWNT than its equilibrium position (0.25 Å in each window) to assess its energy profile. The c-OBCB side chains were allowed to move freely as the intermolecular forces dictated, while the core of the c-OBCB and the SWNT were kept rigid. All the TI simulations were allowed to equilibrate for 1 ns and data were collected over the next 2 ns. Photophysical characterization: Absorbance measurements were performed on a Varian Cary 5000 spectrometer. Raman spectroscopy was performed using a Horiba ARAMIS Raman spectrometer with three excitation wavelengths of 532, 633 and 785 nm. Atomic Force Microscopic (AFM) images were collected using a Veeco Dimension NanoMan AFM. The average thickness of the carbon-nanotube networks was measured using a KLA surface profilometer. Photoluminescence spectra were recorded on a modified Fluorolog-3 spectrometer (HORIBA Jobin Yvon) with a NIR sensitive photomultiplier tube (H10330A-45, Hamamatsu). A 450W Xe-lamp was used for excitation. Emission spectra with 1 nm spectral resolution were scanned from 950 - 1400 nm with excitation at 600 - 800 nm in 10 nm intervals from which the 2D PLE contour plot, as shown in Figure 1c, was constructed. Field-effect transistor fabrication and electrical characterization: Bottom-gate, top-contact field-effect transistors comprising c-OBCB-sorted SWNT networks were fabricated on heavily doped Si substrate with a thermally grown 90-nm thick silicon dioxide as gate die-


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lectric. Contact pads having Ti (5 nm)/Au (50 nm) were defined by standard photolithography, electron-beam evaporation and lift-off. We prepared SWNT-based FETs by vacuum filtration. In this case, the SWNT dispersion was filtered through mixed cellulose ester membranes with a 220-nm pore size (Millipore). The filter cake comprising SWNTs was then transferred onto Si/SiO2 substrates with pre-defined contact pads by dissolving the membrane in acetone and methanol. E-beam lithography was performed to define palladium source/drain electrodes (70-nm thick) and to make electrical connections to the contact pads. The device channel width was kept constant at 80 µm with variable channel lengths ranging from 5 to 30 µm. A final photolithography step was performed to pattern the active-channel region. Oxygen plasma treatment removed the SWNTs outside the channel regions and electrically isolated neighboring devices. SWNT transistors were placed in a Lakeshore probe station (Lake Shore Cryotronics, Inc., Westerville, USA) and cooled with liquid nitrogen. The electrical characteristics were measured in vacuum (< 5 x 10-5 torr) using an Agilent 4155C semiconductor parameter analyzer as a function of temperature from 78 K to 295 K.

■ RESULTS AND DISCUSSION Figure 1 summarizes the spectroscopic characterization of c-OBCB-sorted Hipco SWNTs in toluene. Figure 1a shows the chirality map, obtained from optical measurements, of the SWNTs dispersed with c-OBCB. The angle (α) in Figure 1a is defined as that between the chiral vector and the zigzag direction of a graphene sheet; it is specific to the chirality of the SWNT. The molecular structure of c-OBCB is provided in Figure 1b; its synthesis is reported elsewhere.51 Figure 1c shows the absorption spectrum of c-OBCB-sorted Hipco SWNTs and those dispersed with a common surfactant, sodium dodecyl sulfate (SDS). The absorption spectrum of the c-OBCB-sorted SWNT dispersion (red) shows lower background intensity and better-resolved features compared to that of the SDSdispersed SWNTs. While SDS disperses SWNTs, it does so with no selectivity (black).60 The absorption spectrum from the c-OBCB-sorted SWNT is lower in background intensity but contains better-resolved features, implying that c-OBCB is more effective at isolating and dispersing individual tubes. More importantly, comparison of the spectroscopic features in Figure 1c confirms that c-OBCB only selects for a few types of s-SWNTs. The photoluminescence excitation (PLE) map in Figure 1d reveals that c-OBCB preferentially selects (6,5), (7,6), (10,3), (8,7) and (10,5) s-SWNTs.61,62

Figure 1. (a) The graphene sheet map of SWNTs that are sorted by c-OBCB in toluene. (b) Chemical structure of cOBCB. (c) Absorption spectra of Hipco SWNTs sorted with c-OBCB (red) and with SDS (black). (d) 2D photoluminescence excitation (PLE) contour plot of c-OBCB/Hipco SWNTs dispersion identifying strong selectivity for (10,3) and some selectivity for (7,6), (8,7), (10,5) and (6,5) SWNTs. (e) Radial breathing mode (RBM) regions of Raman spectra of Hipco SWNTs sorted with c-OBCB (red) and with SDS (black), excited at 633 nm. The red and blue shaded areas correspond to the metallic and semiconducting SWNTs regions, respectively. (f) Absorption spectra of Comocat SWNTs sorted with c-OBCB (red) and with SDS (black).

Of the sorted s-SWNTs, (10,3) tubes have the smallest chiral angle at 12.73o. The other sorted tubes have substantially larger chiral angles, with (6,5) and (7,6) tubes having chiral angles of 27.00o and 27.46o, respectively. In comparison, s-SWNTs with smaller chiral angles, such as (9,1), (11,1) or (12,1), are not present in the supernatant. These observations are strong indications that c-OBCB preferentially selects for s-SWNTs with chiral angles of, and often substantially greater than, 12o. We used a multi peak-fit (Lorentzian) function to analyze the absorption spectra of SDS-dispersed and cOBCB-sorted SWNT dispersions; the results of which are shown in Supporting Information (Figures S1a and S1b). By deconvolutiong the absorption spectra, we were able to assign individual absorptions to SWNTs with different chiralities based on previous studies.14, 16, 63 The purity was estimated by comparing the relative absorbances of sSWNTs having different chiralities. Among these four



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species, c-OBCB shows the strongest selectivity for (10,3) and (6,5) SWNTs. (10,3) and (6,5) SWNTs are 15% and 6%, respectively, of the mixture of SDS-dispersed Hipco SWNTs. In c-OBCB-sorted Hipco SWNTs, the content of (10,3) tubes is 38%, and of (6,5) tubes is 7%. More interestingly, we observe pronounced absorption between 1450 to 1600 nm, corresponding to the absorption of largediameter (1.2-1.3 nm) s-SWNTs, including (11,7), (10,9) and (13,5) SWNTs.63 In comparison, we do not observe any distinct features besides the broad absorption that is slightly above background in the spectrum of the SDSdispersed Hipco SWNTs in this wavelength range. By analyzing the absorption intensity of each carbon nanotube species extracted from the absorption spectrum of cOBCB-sorted Hipco SWNTs dispersion, we estimate these large-diameter tubes to comprise 49% of the dispersion. Because only SWNTs that are resonant with the excitation energy can be detected by Raman spectroscopy,64 we performed Raman characterization of c-OBCB-sorted carbon nanotubes by exciting at different energies to assess the composition of the mixture. Figure 1e shows the radial breathing mode (RBM) region of the Raman spectra of networks of c-OBCB-sorted (red) and SDS-dispersed (black) SWNTs at 633 nm. This excitation wavelength is resonant with both metallic SWNTs that show RBM peaks in the range from 180 to 230 cm-1 and with s-SWNTs having RBM peaks in the range from 230 to 320 cm-1. While the Raman spectrum of SDS-dispersed SWNTs reveals the presence of metallic SWNTs, we do not observe any peaks in the radial breathing mode region in the Raman spectrum of c-OBCB-sorted Hipco SWNTs that is attributable to the presence of metallic tubes. The single Raman peak at 251 cm-1 in the spectrum of the c-OBCB-sorted SWNTs can be assigned to (10,3) tubes;16 this observation is consistent with the PLE measurements described above. The Raman spectrum of c-OBCB-sorted Hipco SWNTs excited at 532 nm (shown in Supporting Information) further confirms that c-OBCB-sorted Hipco SWNTs are enriched with semiconducting tubes, as evidenced by the fact that the ratio of the integrated area of the RBM in the wavenumber range that corresponds to semiconducting SWNTs to that of the RBM in the range that corresponds to metallic SWNTs in c-OBCB-sorted Hipco SWNTs is larger than that in the pristine Hipco SWNTs We, however, observe RBM peaks in the range of 210 to 320 cm-1 that indicates the presence of residual metallic carbon nanotubes when an excitation energy of 532 nm is employed.64,65 The two peaks at 271 cm-1 and 229 cm-1 can be assigned to (9,3) and (9,6) tubes, respectively. These tubes are not in resonance with excitation at 633 nm and are thus not observed in the Raman spectrum in Figure 1e. In addition to Hipco SWNTs, we also investigated cOBCB’s selectivity of Comocat SWNTs. Unlike the Hipco SWNTs, Comocat SWNTs only comprise tubes with diameters less than 1.1 nm. The absorption spectrum of cOBCB-sorted Comocat SWNTs in Figure 1f provides evidence for the presence of only three, small-diameter sSWNTs, namely (6,5), (7,6) and (10,3), in the final dispersion. The observation further confirms that c-OBCB only

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interacts with a few s-SWNTs species and indicates cOBCB’s selectivity across SWNTs from different sources. We also tested other types of SWNTs, including the arcdischarge carbon nanotubes from Nanolab and the SWNTs from Hanwha Chemical that have average diameters of 1.4 nm and observed that these tubes do not disperse with c-OBCB under the experimental conditions. The Raman spectra of c-OBCB-sorted-Comocat SWNTs are shown in Supporting Information. Taking into account the absorption cross section for individually dispersed s-SWNTs is approximately 1.5 × 10-17 cm2/atom,66 we estimate the concentration of s-SWNTs sorted using cOBCB to be about 0.15 µg/mL in the case of Hipco SWNTs and to be about 0.03 µg/mL the case of Comocast SWNTs. Those values indicate an overall yield of sorting s-SWNTs with c-OBCB to be in the range of 0.03 to 0.1%; this yield is comparable to that of sorting s-SWNTs with poly[9,9dioctylfluorene-2,7-diyl] (PFO).21 While the overall purity of the sorted s-SWNTs with c-OBCB remains lower than that sorted with polymeric dispersants, such as PFO and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6’-{2,2’bipyridine})]), PFO-BPy,11, 15, 18 c-OBCB is the first molecular dispersant that sorts s-SWNTs with structural specificity. In contrast to non-dispersant processing technologies that sort s-SWNTs effectively, including gel chromatography,7 two-phase extraction,67 or pH-value gradient weak-field centrifuge,68 sorting with c-OBCB is straightforward, comprising two simple steps of sonication and centrifugation.

Figure 2. Potential of mean force as a function of the distance between the center-of-mass of c-OBCB and SWNT in toluene. Color key shown in the inset.

In order to account for the effective interactions between the contorted aromatic core of c-OBCB and SWNTs, we calculated the Potential of Mean Force (PMF) averaged over the conformations of all other components of the system. For comparison, and as negative controls, we also modeled the interactions between c-OBCB and (8,6) and (7,5) tubes; these tubes are not selected by cOBCB in toluene. The PMF between c-OBCB and s-


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SWNTs describes the free energy as a function of reaction coordinate of the binding of the aromatic core of c-OBCB to the SWNT surface. Here, the reaction coordinate refers to the distance between centers of masses. The results shown in Figure 2 are based on Thermodynamic Integration simulation data.59 The depth of the first minimum in the PMF can be seen in Figure 2; this minimum marks the extent to which the core of c-OBCB binds to the SWNT. For c-OBCB core interactions with (10,3) and (6,5) tubes, binding is considerably stronger (10-20 kcal/mol) than binding of the c-OBCB core with (7,5) tubes. Our simulations, however, reveal one anomaly. We observe that the core of c-OBCB also binds strongly with (8,6) tubes, a chirality for which c-OBCB does not select in experiments. We believe that the unusually strong binding of (8,6) observed in these simulations stems from a match of its curvature to c-OBCB. The curvature of c-OBCB, κ, was calculated using κ = 1/R, where R is the circumradius of cOBCB (details in Supporting Information). Here, we specifically refer to the curvature of the short axis of c-OBCB because our theoretical calculations show that c-OBCB adsorbs on SWNT with its long axis parallel to the longitudinal axis of the SWNT (details in Supporting Information). This departure from our experimental results implicates that, while the strength of c-OBCB core interactions with the nanotubes is an important factor, it is not the sole factor governing SWNT selectivity (details in Supporting Information).

Figure 3. Heat maps (spatial distribution maps) showing the relative probability of finding the carbon atoms of the cOBCB side chains in the (x, y) plane in toluene. These configurations were obtained by rotating c-OBCB such that its center of mass is at x = 0 and y ≥ 0. Dark blue color represents zero probability of finding the side chain at that (x, y) location, and red represents a probability of 1. Color bars show the relation to the probabilities. Red and yellow (high probability dots) are seen for the positive controls (10,3) and (6,5) SWNTs; yellow and orange are the highest probabilities for the negative controls. SWNTs are not shown.

To assess the role the side chains of c-OBCB play in dictating the interactions between c-OBCB with s-SWNTs with specific chiralities, we performed MD simulations to generate a “heat map” depicting the locations of carbon atoms in the alkyl side chains (-OC12H25) of c-OBCB in the (x, y) cross-sectional plane in toluene (Figure 3). In the MD simulations, c-OBCB is allowed to roll freely about the SWNT. In order to assist viewing the results, we rotate all the resulting configurations of c-OBCB to one particular configuration such that its center of mass is, at all times, located at x = 0.0 and above the SWNT (i.e., y > 0), with the SWNT centered at the origin. Blue in the “heat map” indicates that the side chains spend little time adsorbed to the SWNT (less than 20% probability), whereas yellow and red indicate higher probabilities (~70% for yellow and ~90% for red) of finding the side chains adsorbed on the exterior wall of the SWNT.

Figure 4: (a) Relative probability of finding the carbon atoms of side chains of c-OBCB molecule at a particular distance from the surface of the SWNTs (shown in nm) in toluene. There is an exclusion zone from 0 - 0.3 nm away from the SWNTs, due to the van der Waals radius of the SWNTs and side chains. There is strong peak at 0.4 - 0.5 nm for side chains stuck to the SWNT. The first minimum at around 0.6 nm occurs because, at that distance between the side chain atoms and the surface of the SWNT, there is not enough space for solvent molecules to enter between them. Hence, side chain atoms prefer to lie on the surface rather than stay at these intermediate distances and create room for a “vacuum” of sorts. As the solvent rushes in, the probability rises, as depicted by the small second maximum in the histograms occurring at around 0.8 nm. The probability of finding side chains stuck to the SWNT is markedly highest for the (10,3) variant with the other SWNTs’ first maxima being comparable. (b), (c) and (d) represent MD simulation snapshots of



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SWNT/c-OBCB configurations: (b) reflects the tight adsorption of side chains to the SWNT surface corresponding to the first maximum ~0.45 nm in plot (a); (c) shows the beginnings of solvent incursion between c-OBCB and SWNT (see grey dots between the side chains and SWNT; this is the situation that gives rise to the small minimum at ~0.6 nm where desorption of the side chains is beginning; (d) shows complete desorption of the side chain from the SWNT surface, reflecting the zero probability of adsorption of side chains at ~2.0 nm.

Comparison of the “heat maps” (Figure 3) and quantitative analyses of the probability of finding side chains a given distance away from the SWNT walls (Figure 4) indicate that c-OBCB’s side chains are more likely to be adsorbed to the walls of (10,3) and (6,5) tubes compared to those of the (7,5) and (8,6) SWNTs. In the “heat maps”, this observation is manifested by the appearance of red and yellow (i.e., high probability) regions in simulations of the positive controls, but only yellow and green (i.e., lower probability) regions in simulations of the negative controls. Examination of the distance of side chain atoms from the surface of the SWNT averaged over time also provides a consistent picture. Figure 4 shows the histograms that quantify the probability of finding a carbon chain atom at a particular distance from the SWNT surface. We observe that the relative probability of finding side chains adsorbed on the SWNT (at ~0.45 nm) is as high as 100% for (10,3) tubes. The likelihood of finding cOBCB side chains adsorbed on the surfaces of the other SWNTs is considerably lower at 55-70%. Combining these computational results from the PMF and side chain locations with experimental observations, we conclude that the selective interactions between cOBCB and s-SWNT stem from a synergy of the contributions from both the strength of the binding of the piconjugated core of c-OBCB to a particular chirality of SWNTs, as well as the tendency for c-OBCB side chains to preferentially adsorb on the sidewalls of the SWNTs. Solvents also play a role in dictating sorting; our investigation on the role of solvents is detailed in Supporting Information.

Figure 5. (a) Representative transfer characteristics (IDS-VG) of a FET having 16-nm thick c-OBCB-sorted Hipco SWNT network at VDS = -1 V (W = 80 µm and L = 20 µm). (b) The output characteristics of the same device shown in (a). (c) Average resistance as a function of channel length extracted from the transfer characteristics of FETs having different channel lengths at VDS= -1 V and VG= -30 V. Solid line represents a linear ﬁt to the data. Inset shows the AFM image of a 16-nm thick c-OBCB sorted Hipco SWNT network. (d) Device parameters, including hole mobility and on/off ratio, of FETs having different channel lengths (squares: L = 5 µm, circles: 10 µm, triangles: 20 µm, diamonds: 30 µm) and cOBCB-sorted Hipco SWNT network thickness (red: thickness = 7 nm, black: 11 nm, blue: 16 nm). (e) Hole mobility as a function of temperature of the FET whose device characteristics are shown in (a). Symbols are experimental data and the solid line represents a fit to the data using the fluctuationinduced tunneling (FIT) model. (f) The same experimental data (symbols) fitted with the Efros and Shklovskii variable range hopping (ES-VRH) model.

Using a vacuum filtration method,31 we formed c-OBCBsorted Hipco SWNTs networks with varying thicknesses for electrical characterization. Figure 5 summarizes the electrical characteristics of FETs whose active layers comprise networks of c-OBCB-sorted Hipco SWNTs. These FETs show p-type electrical behavior as the gate bias is swept from 10 to -30 V. Figures 5a and 5b show representative transfer (IDS vs VG) and output (IDS vs VDS) characteristics of FETs based on c-OBCB-sorted Hipco SWNTs networks. We observe a leading hysteresis that arises from defects at the carbon nanotube-dielectric interface.30 Specifically, charge carriers in SWNTs can charge and discharge the hydroxyl groups on the dielectric surface. In the case of a p-type SWNT-network FET, these hydroxyl


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groups trap electrons during the forward sweep (VG sweeps from 10 to -30 V) and the trapped electrons increase screening of the gate dielectric, which in turn facilitates hole conduction. During the reverse sweep (VG sweeps from -30 to 10 V), trapped electrons are released and they can combine with holes in the SWNTs in the conduction channel. This action, in turn, causes a negative shift in threshold voltage. That the output characteristics of the same FET (the inset in Figure 5a) are linear at low source-drain bias indicates that the contact resistance in this device is low. To quantify, we adopted the standard transmission-line method 69 to extract the contact resistance of FETs having a 16 nm-thick c-OBCB-sorted Hipco SWNT active layer, the data of which are shown in Figure 5c. The channel-width normalized contact resistance for hole transport is extracted to be 57 ohm-cm (the red line in Figure 5c).This value is within reported values for other s-SWNTs-based FETs.70,71 We do not observe n-type behavior in c-OBCB-sorted s-SWNT-based FETs, presumably due to the large electron-injection barrier at the SWNT-Pd interface and/or to the extensive exposure of s-SWNT network to ambient conditions during device preparation, which p-dopes carbon nanotubes.30 Figure 5d demonstrates the effects of network thickness, as well as channel dimensions, on the device performance of FETs based on c-OBCB-sorted Hipco SWNT networks. The circles group FETs having SWNT networks of the same thickness. The hole mobilities were calculated from the transfer curve in the linear regime using equation (1) below: ‫݃ = ݑ‬௠ ×

௅ ௐ

×

ଵ ஼೚ೣ

×

ଵ ௏ವೄ

(1)

where gm is the transconductance, L and W are the channel length and width, respectively, of the transistor. VDS is the source-drain bias, and Cox is the gate capacitance. Here, we estimated the capacitance by assuming a dielectric constant of 38.3 nF/cm2 for the 90nm thick SiO2 gate dielectric. The hole mobility in our cOBCB-sorted Hipco s-SWNTs network-based transistors is more than two orders of magnitude higher than those of c-OBCB transistors having the same channel dimensions (2 x 10-3 cm2/V-s),51 confirming that transport through the s-SWNTs network dominates charge transport in such FETs. In addition, we observe a trade-off between charge carrier mobility and current on/off ratio as the thickness of the s-SWNT networks in the channel increases. Specifically, the hole mobility increases from 0.2 cm2/V-s to 1.3 cm2/V-s as the thickness of s-SWNT network increases from 7 nm to 16 nm. This increase in mobility takes place at the expense of the on/off current ratio, which decreases from 105 to 103 over the same thickness range. A closer look at the source-drain current (IDS) confirms that both the on and off currents increase with increasing SWNT network thickness. However, the off current increases by approximately 3 orders of magnitude while the on current increases by an order of magnitude over the same thickness range, leading to a net decrease in the on/off ratio with increasing network thickness. This

observation is consistent with prior reports72,73 and points to the addition of conducting pathways as the thickness of the networks increases, likely due to the presence of a small fraction of residual metallic tubes. We also evaluated the effects of channel length, ranging from 5 to 30 µm, on the hole mobility and current on/off ratio of the FETs. In each circle, we find that FETs with 5-µm channel length show smaller current on/off ratio compared to those with larger channel lengths. This observation can also be explained by the presence of residual metallic tubes as increasing the channel length decreases the probability of charge transport through metallic nanotubes.74 Hole mobility of FETs comprising c-OBCB-sorted sSWNT networks in the range of 78 K to 295 K extracted from the linear regime indicates that charge transport is thermally activated. Thermally activated charge transport had previously been reported for carbon-nanotube systems75-79 and quantified by either the fluctuation-induced tunneling (FIT) model75,76 or the variable range hopping (VRH) model.77-79 The FIT model describes charge transport in composites comprising unsorted SWNTs embedded in polymer matrices;75,76,80,81 the VRH model has been used to quantify the temperature dependence of charge transport in carbon nanotube networks comprising semiconducting and metallic SWNTs in the absence of any dispersants.77-79 Considering that our SWNTs networks comprise adsorbed c-OBCB, we first applied the FIT model to quantify the temperature dependence of hole mobilities extracted from our FETs (Figure 5e). We extracted characteristic temperatures of T0 = 35 K and T1 = 255 K where T0 represents the characteristic temperature above which thermally activated conduction over the barrier begins to occur, and T1 is the temperature required for charge carriers to traverse the tunnel junction and is thus proportional to the activation energy.82 The activation energy can be evaluated using kT1, where k is the Boltzmann constant, yielding Ea = 22 meV. This value appears to be much smaller than the 120 meV extracted for charge transport in devices with PFO-BPy-sorted s-SWNTs networks.31 We also observe a large discrepancy in T0 for devices comprising c-OBCB-s-SWNT networks compared to those comprising PFO-BPy-sorted s-SWNTs networks. At 318K, T0 for PFO-BPy-sorted s-SWNTs networks is an order of magnitude larger than that of c-OBCB sorted s-SWNT networks. Both T0 and T1 are functions of the potential barrier height for charge transport.82 The literature values of T0, for instance, vary over a broad range from 13 K at the low end for SWNT/silica fiber composites83 to 520 K for single-stranded DNA-wrapped carbon nanotubes composites at the high end.80 The values of T1 for these SWNT networks are 53 K and 4330 K, respectively. While the electronic properties of the polymer, loading of SWNTs, ratio of semiconducting and metallic SWNTs in the composite and doping concentration, all impact the extracted values of T0 and T1,75,76,80,81 T0 and T1 for c-OBCBsorted Hipco SWNT networks are at the lower end of the ranges reported in the literature.



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Our data over this temperature range can also be described by the Efros and Shklovskii VRH (ES-VRH) model84 with comparable quality fit (Figure 5f). This ES-VRH model with a characteristic exponent d = 1 indicates the presence of Coulombic interactions between localized electrons in the s-SWNT network.84 Similar observations have been made in high-purity s-SWNT networks.78, 85,86 The characteristic temperature, Tes, in this model has been used to extract the localization length of charges in carbon nanotube networks.87,88 Nevertheless, the value of Tes is influenced by many extrinsic factors, such as the content of metallic carbon nanotubes, charge concentration, density of defects and the bundling of carbon nanotubes85-88 and a broad distribution of Tes that is in the range of 34 to 1000 K have been reported.78,85,89 For example, Shiraishi et al., reported Tes = 34 K for networks comprising unsorted SWNTs.89 Benoit et al., investigated the charge transport in SWNTs/polymer composites and demonstrated that Tes can be inversely correlated to the volume fraction of SWNTs in the composites. Tes is approximately 1000 K in such composites with 1 % SWNTs and decreases to 200 K with 8 % SWNTs.85 Another study by Yanagi et al., shows Tes of 650 K for 99% s-SWNT networks.78 We interpret the observation that our Tes is 673 K, comparable to that observed by Yanagi, to be an independent verification of the high purity of the s-SWNTs sorted with c-OBCB. Despite differences in the physics of the FIT and the ESVRH models, both have been used to describe the temperature dependence of charge transport in networks comprising high purity semiconducting carbon nanotubes (>90%),78,86,87,90 with little consensus on the exact charge transport mechanism. The recent study by Itkis et al., suggests that the midgap (Fermi level) electronic states in carbon nanotubes also contribute to charge hopping in individual SWNTs in addition to tunneling of charges at intertube junctions. Such midgap states originate from structural defects of SWNTs and/or residual impurities in the networks.86 This scenario complicates the elucidation of the charge transport mechanism in s-SWNT networks and can perhaps explain the lack of consensus among studies with different s-SWNTs.78,86 ■ CONCLUSIONS We have demonstrated the effectiveness of sorting semiconducting carbon nanotubes with high selectivity and chiral specificity using a contorted semiconducting molecule, c-OBCB, and uncovered the mechanism by which c-OBCB interacts with SWNTs with the aid of Molecular Dynamic simulations. Both the tendency of cOBCB’s side chains to wrap the carbon nanotubes and the binding between the non-planar core of the c-OBCB and the SWNTs play important roles in determining selectivity. Tuning this subtle balance of interactions between side-chain and solvent as well as shape complementarity could open the door to predictive route to chiral selectivity in the future. Variable-temperature electrical measurements of field-effect transistors comprising c-OBCBsorted SWNT networks reveal thermally activated charge transport. This work charts a path towards the use of con-

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torted molecules for sorting SWNTs with specific diameters and chiralities.

ASSOCIATED CONTENT Supporting Information Optical characterization of materials, computational calculations, temperature dependence of hole mobility in c-OBCBComocat SWNTs-based FETs. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.-L.L.).

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT J.G. acknowledges the Netherlands Organisation for Scientific Research (NWO) for a Rubicon Post-doctoral Fellowship (Research Grant 680-50-1202). We acknowledge funding from the NSF through grants CHE-1124754 and CMMI1537011, as well as from NRI (Gift # 2011-NE-2205GB) under its joint initiative “Nanoelectronics Beyond 2020” with the NSF. We also thank Prof. R.K. Prud'homme at Princeton University for access to his centrifuge facilities and Dr. J.D. Saathoff at Cornell University for many helpful discussions regarding the free-energy calculations. Partial support of this project was provided by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy (DOE), under award no. DE-FG0201ER15264.

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