Separated Metallic and Semiconducting Single-Walled Carbon

INVITED FEATURE ARTICLE pubs.acs.org/Langmuir

Separated Metallic and Semiconducting Single-Walled Carbon Nanotubes: Opportunities in Transparent Electrodes and Beyond Fushen Lu,† Mohammed J. Meziani,†,‡ Li Cao,† and Ya-Ping Sun*,† †

Department of Chemistry and Laboratory for Emerging Materials and Technology, Clemson University, Clemson, South Carolina 29634-0973, United States ‡ Department of Chemistry/Physics, Northwest Missouri State University, Maryville, Missouri 64468-6001, United States ABSTRACT: Ever since the discovery of single-walled carbon nanotubes (SWNTs), there have been many reports and predictions on their superior properties for use in a wide variety of potential applications. However, an SWNT is either metallic or semiconducting; these properties are distinctively different in electrical conductivity and many other aspects. The available bulk-production methods generally yield mixtures of metallic and semiconducting SWNTs, despite continuing efforts in metallicity-selective nanotube growth. Presented here are significant advances and major achievements in the development of postproduction separation methods, which are now capable of harvesting separated metallic and semiconducting SWNTs from different production sources with sufficiently high enrichment and quantities for satisfying at least the needs in research and technological explorations. Opportunities and some available examples for the use of metallic SWNTs in transparent electrodes and semiconducting SWNTs in various device nanotechnologies are highlighted and discussed.

’ INTRODUCTION Single-walled carbon nanotubes (SWNTs), while formed mostly spontaneously in available production schemes, may be considered conceptually as being rolled from a graphene sheet into a cylindrical structure. The operation requires a chiral vector Ch, consisting of two primitive vectors (Ch = na1 þ ma2) to match the graphene carbon atoms from edge to edge (Figure 1). The chiral vector, also commonly referred to as the chiral index (n, m) (or chirality, helicity), uniquely defines the diameter (d) and chiral angle (θ) of an SWNT pffiffiffi 3aC - C pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d ¼ ð1Þ n2 þ nm þ m2 π " pffiffiffi # 3m -1 ð2Þ θ ¼ tan 2n þ m where aC-C (∼0.142 nm) is the nearest-neighbor C-C distance. Depending on its chiral vector, an SWNT is either semiconducting or metallic (including semi- or quasi-metallic), which is often referred to as “metallicity”. When n - m 6¼ 3q (q is an integer), the electronic density of states (DOS) in the SWNT exhibits a significant band gap near the Fermi level and the nanotube is thus semiconducting; when n - m = 3q, the conductance and valence bands in the SWNT overlap and the nanotube is thus metallic (or semimetallic when n 6¼ m). Statistically, there are twice as many ways to roll a graphene sheet into a semiconducting SWNT as there are ways to roll the r 2010 American Chemical Society

same sheet into a metallic SWNT. Therefore, the semiconducting-to-metallic nanotube ratio of 2:1 should generally be expected in an as-grown mixture of SWNTs. Ever since the discovery of carbon nanotubes, there have been many reports and predictions of their superior properties for use in a wide variety of potential applications.1 For example, SWNTs have been used in high-mobility or even ballistic transistors and in integrated logic circuits such as inverters and ring oscillators. Individual SWNT field-effect transistors (FETs) have been demonstrated for their excellent performance over current silicon-based complementary metal oxide semiconductor (CMOS) devices. Among other widely investigated uses of SWNTs have been the development of electrodes for signal transmission and the development of detectors for sensing chemical and biological materials, including a variety of electrochemical and FET sensors in different configurations and mechanisms. In the ongoing pursuit of new and/or renewable energy sources, SWNTs are considered to be potentially excellent building blocks for a variety of energy conversion and storage technologies, such as in optoelectronic devices (photovoltaics, light-emitting diodes, etc.), batteries and supercapacitors, and various fuel cells. Obviously these and many others are all potentially exciting applications, for which much progress has been made in the development effort. However, something missing in many studies has been an acknowledgment of the fact that as-produced nanotube samples are mixtures of metallic Received: August 6, 2010 Revised: September 13, 2010 Published: October 13, 2010 4339

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Figure 1. Conceptual SWNT formation by rolling up a graphene sheet. As an example, the dashed lines represent the two edges that will merge in the rolling up of a (7, 3) semiconducting SWNT.5

and semiconducting SWNTs, which are distinctively different in electrical conductivity and many other aspects. Indeed, among the widely prescribed and/or predicted potential SWNT applications, some exploit the properties of metallic SWNTs whereas others require the properties of their semiconducting counterparts. However, the use of mixtures also has negative or even prohibitively negative consequences. For example, the presence of metallic SWNTs was found to be responsible for the poor, uncontrollable characteristics of FET devices, which was based exclusively on the properties of semiconducting SWNTs, resulting in an on/off ratio that was too low for most practical applications.2 Beyond electrical conductivity, metallic and semiconducting SWNTs also differ in many other physical and chemical properties, such as static polarizibility, doping effect, chemical reactivity, and those related to electronic structures. For example, it is known that semiconducting SWNTs are extremely sensitive to electrical gating and are capable of conductance changes by orders of magnitude under various electrostatic gate voltages.3 Conversely, metallic SWNTs are less sensitive to molecular adsorption and chemical gating because charge transfer does not significantly affect the charge density at the Fermi level.3 In the widely pursued use of SWNTs in transparent conductive coatings to compete with the currently predominant indium tin oxide (ITO) technology,4 metallic SWNTs are obviously required. Because semiconducting SWNTs are more absorptive than their metallic counterparts, their presence in the coatings is negative with respect to performance in terms of both lower electrical conductivity and reduced optical transmittance. Thus, for these specific examples and for general purpose use, the availability of SWNTs that are either metallic or semiconducting is much desired by the research community, with significant effort and progress already made in the development of methodologies for harvesting metallic and semiconducting SWNTs.5,6

’ SEPARATION The available bulk-production methods generally yield mixtures of metallic and semiconducting SWNTs (also mixed with carbonaceous impurities and catalysts), whereas the effort to develop the selective growth of one type only (either metallic or semiconducting) continues.7-9 Therefore, postproduction

INVITED FEATURE ARTICLE

separation has and will continue to represent a viable option for obtaining significant quantities of highly enriched metallic or semiconducting SWNTs. Metallic and semiconducting SWNTs differ in several aspects, in addition to their obvious differences in electrical conductivity, including static polarizability and surface characteristics, chemical reactivity, and so forth. They are also associated with SWNTs of different diameters, and this difference is particularly meaningful in nanotube samples with broad size distributions such as those from high-pressure carbon monoxide (HiPCO) and similar production methods. These differences can be and indeed most have been exploited for postproduction separation on various quantity scales, from the use of DNAs and surfactants (coupled with ultracentrifugation or electrophoresis) to separation agents with more specifically targeted selective interactions and thus for separation in relatively larger quantities, with major progress already made in terms of the availability of separated or enriched metallic and semiconducting SWNTs for the exploration of potential applications. Selectivity with Wrapping by DNA or Surfactant. There has been more recent emphasis on using the density gradient ultracentrifugation (DGU) method for postproduction separation.6,10-12 The antecedent of this DGU method, still generally used for separation on smaller scales, lies in the induced selectivity associated with the wrapping of SWNTs by singlestranded DNAs (ssDNAs). According to Zheng et al., ssDNAs readily adsorb onto the nanotube surface and efficiently disperse SWNTs upon ultrasonication.13 Because ssDNAs are negatively charged species, they express linear negative charges on the wrapped SWNTs.13 However, metallic and semiconducting SWNTs have different polarizabilities and also different size profiles, which result in different linear charge densities in the ssDNA-wrapped SWNTs. Zheng et al. exploited such differences for the separation of metallic and semiconducting SWNTs in an anionic exchange column.13 Hersam and co-workers first used centrifugation to separate ssDNA-wrapped SWNTs in solutions with a density gradient medium.10 Upon centrifugation, the nanotubes formed discrete colored bands in the centrifuge tubes, from which SWNTs of different diameter distributions were harvested, and in the resulting optical absorption spectra, semiconducting SWNTs with different chiralities were identified. Subsequently, the same group substituted DNAs with surfactants in the DGU method. For example, SWNTs were dispersed with a mixture of sodium cholate and sodium dodecyl sulfate (SDS), followed by ultracentrifugation in density gradient media. The discrete colored bands thus formed were correlated with different SWNTs in terms of optical absorption results (Figure 2).11 The development of nonlinear density gradients has apparently improved the separation for SWNTs enriched with a number of different chiralities (n, m).14 There has been continued use of ssDNA-dispersed SWNTs for separation purposes. For example, ion-exchange chromatography for the dispersion of SWNTs produced from the HiPCO process allowed the harvesting of 12 major single-chirality semiconducting species (Figure 3).15 However, the method was reported to be ineffective in the separation of metallic SWNTs. In another method used to exploit the dispersion of SWNTs by DNAs or conceptually similar surfactant species, agarose gels (originally developed for DNA separation) were used to separate metallic and semiconducting SWNTs.16,17 For the separation, 4340

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Figure 2. Sorting of SWNTs by density gradient ultracentrifugation (DGU). (A) Schematic of surfactant encapsulation and sorting, where F is density. (B, C) Photograph and optical absorbance (1 cm path length) spectra after the DGU separation of laser-ablation-grown SWNTs (1116 Å). Reproduced with permission from ref 11a (Copyright 2006, Nature Publishing Group).

a nanotube sample from HiPCO or laser-ablation production was dispersed in a surfactant (SDS) solution, followed by centrifugation to remove impurities and bundled SWNTs.16 The resulting dispersion was mixed with liquid agarose gel for gelation. The gel containing SDS-dispersed SWNTs was frozen, thawed, and squeezed to yield a solution of enriched (70%) metallic SWNTs, whereas the semiconducting SWNTs (95%) were left in the gel.16 The same separation was later demonstrated on columnbased gel chromatography.17 It seems that this method is more amenable to scaling up than the density gradient ultracentrifugation (DGU) or ion-exchange chromatography discussed above, though the separation efficiency may still be limited by how effectively SWNTs are dispersed to the individual nanotube level by SDS or a similar surfactant. Selective Interactions for Bulk Separation. Metallic and semiconducting SWNTs are apparently wrapped differently by DNAs or more generally, via the same concept, by surfactant molecules, as discussed above for various approaches to exploiting

Figure 3. (Upper left) DNA barrel on an (8, 4) nanotube formed by rolling up a 2D DNA sheet composed of two hydrogen-bonded antiparallel ATTTATTTATTT strands and (right) the same structure viewed along the tube axis. (Lower) Optical absorption spectra and atomic structures of 12 purified semiconducting SWNTs and the starting HiPCO mixture. The structure of each purified SWNT species (viewed along the tube axis) and its (n, m) notation are given on the right side of the corresponding spectrum. Reproduced with permission from ref 15 (Copyright 2009, Macmillan Publishers Limited).

such differences for postproduction separation purposes. More significant and useful for the same purposes is the fact that these nanotubes undergo different interactions with selected functionalization or solubilization agents. The selective interactions, sometimes considered to be noncovalent functionalizations, have been found to allow relatively facile postproduction separation for significant quantities.18-21 4341


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Figure 4. Pictorial representation of possible octadecylamine (ODA) interactions with oxidative acid-purified SWNTs through (A) zwitterion formation and (B) physisorption-assisted organization of ODA on SWNT sidewalls. Reproduced with permission from ref 18 (Copyright 2003, American Chemical Society).

Haddon and co-workers initiated the functionalization and solubilization of carbon nanotubes with octadecylamine (ODA),22 and similar alkylamines have since been used extensively in this field of research. In the thermal reaction of alkylamines with purified SWNTs (by oxidative acid treatment and thus the creation of carboxylic acid moieties on the nanotube surface),22,23 the functionalization was thought to be primarily composed of the formation of ammonium-carboxylate zwitterionic bonds. In addition to the chemical bonds, the chemical adsorption-like strong interactions of a great quantity of amino molecules on the nanotube surface was also considered to play a significant role in the functionalization and solubilization of the nanotubes.24 By recognizing this combination of chemical bonds and strong interactions in the functionalization of purified SWNTs with ODA (Figure 4), Papadimitrakopoulos and coworkers first reported that the solubilization was preferential toward the semiconducting SWNTs, thus their separation from the metallic counterparts.18,21 It was proposed that the observed selectivity toward the solubilization of semiconducting SWNTs could be attributed to the charge redistribution on the surface of semiconducting-enriched small bundles together with the formation of an ordered 2D arrangement of the NO3-/surfactant amine/water layer.21 According to the authors, this method was more effective for smaller-diameter SWNTs from the HiPCO process (0.8-1.3 nm) than for larger-diameter SWNTs derived from laser ablation production (1.15-1.55 nm).18 The selectivity with alkylamines was apparently subject to the manipulation of experimental conditions. Interestingly, though there were also reports on the use of simple amines (structurally


similar to ODA) for the same separation of as-produced HiPCO SWNTs via chemical functionalization, the solubilization was found to be selective toward metallic SWNTs instead of their semiconducting counterparts.25 Noncovalent functionalization with planar aromatic molecules, such as the derivatized porphyrin or pyrene (Figure 5), provided better selectivity between metallic and semiconducting SWNTs, as first reported by Sun and co-workers.19 The postproduction separation method19,20 simply “splits” the starting mixture by selectively solubilizing semiconducting SWNTs and leaving their metallic counterparts behind and consequently is capable of handling significant sample quantities. Experimentally, as-produced samples of SWNTs from arc discharge or laser ablation production were purified in the widely used procedure involving nitric acid treatment.20 The purified sample was dispersed in an organic solvent such as tetrahydrofuran (THF) with the selected planar aromatic molecule, derivatized porphyrin or pyrene with long alkyl chains, aided by homogenization and sonication. These conditions were similar to those commonly used in the noncovalent functionalization of carbon nanotubes. The resulting dispersion was separated via simple centrifugation to yield the soluble fraction containing noncovalently functionalized semiconducting SWNTs and the insoluble residue enriched with metallic SWNTs. The same separation could be improved by repeating the same procedure for a second time, though it was hardly necessary to repeat it a third time. The selective interaction between separation agents and semiconducting SWNTs was reflected by the diminishing of both the S11 and S22 absorption bands of the nanotubes (with doping effects due to the adsorption of aromatic moieties onto the nanotube surface).26 Separated metallic and semiconducting SWNTs could readily be recovered from the soluble fraction and residue, respectively, by removing the separation agents in repeated solvent washing and/or dialysis. The experimental conditions for the recovery were sufficiently mild so as not to change the dispersion characteristics of the separated metallic and semiconducting SWNTs. The separation agents could also be recovered nearly quantitatively, as verified by NMR results, and reused for the same separation purpose.27 Electron microscopy analyses of the separated fractions revealed no significant differences in their images from those of the preseparated mixtures.20,28 Major differences were observed in their resonance Raman and near-IR absorption spectra, consistent with the metallic and semiconducting fractions.20,28 In Raman spectra (Figure 6), the G band of the metallic fraction, known as the Breit-Wigner-Fano (BWF) feature, was much broader and more asymmetric than those for the preseparation nanotube sample and more so for the semiconducting fraction. The near-IR absorption spectrum of an as-prepared or purified sample of SWNTs is typically characterized by S11 and S22 bands (centered at ∼1800 and ∼1000 nm, respectively, in a sample from arc discharge production), which are due to only semiconducting SWNTs (electronic transitions associated with van Hove singularity pairs). As expected, these bands were more prominent in the spectrum of the separated semiconducting fraction and nearly absent in that of the separated metallic fraction (Figure 6). More quantitatively on the basis of the absorption results, the content of metallic SWNTs was estimated to be close to 85% in the separated metallic fraction (by assuming the original content of 33% in the preseparation sample), and the content of semiconducting SWNTs in the other fraction was much higher, though a truly quantitative determination of 4342


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Figure 5. Postproduction separation scheme by Sun and co-workers with the use of derivatized pyrene and porphyrin compounds.19,20 Two other compounds used in a similar separation33,34 are also shown.

Figure 6. Raman spectra of SWNTs in the separated metallic (—) and semiconducting (- - -) fractions. (Inset) Optical absorption spectra of SWNTs in neat films from the separated metallic (—) and semiconducting (- - -) fractions.28

metallic and semiconducting purities in their respective fractions still remains a significant technical challenge.29 One significant difficulty in quantifying purities in terms of metallicity in the separated metallic and semiconducting fractions was due to the fact that the purity, in terms of SWNTs (vs the contents of residual metal catalysts and other carbonaceous impurities) in the preseparation purified sample was only up to 85%.30 Obviously, there was no justification to assume that the impurities would be distributed evenly or in some predictable

proportion between the separated metallic and semiconducting fractions. It was also unclear as to what extent the presence of impurities interfered with both the separation process and results. Therefore, a much desired improvement to the postproduction separation method is to use a starting nanotube sample largely free of other impurities. Lu et al. have recently applied the same noncovalent functionalization approach to the purification of SWNTs. 30 A water-soluble pyrene derivative, 1-pyreneacetic acid, was used to solubilize the purified (typical nitric acid treatment) SWNTs in aqueous solution, allowing the nearly complete removal of residual metal catalysts and carbonaceous impurities. According to thermogravimetric analysis (TGA) results, the purified sample of SWNTs contained few other carbonaceous impurities and no more than 3% residual catalysts by weight (obviously much less by volume because of the much higher density of the metals than of carbon).30 The noncovalent functionalization of SWNTs with 1-pyreneacetic acid in an aqueous solution was nonselective toward either metallic or semiconducting SWNTs, with hardly any changes in their ratio during purification according to quantitative Raman results. Another important feature in the purification was such that the highly pure SWNTs remained solvent-dispersible,30 which is obviously valuable for the targeted use of the purified sample in the separation into metallic and semiconducting fractions. The Raman and near-IR absorption results of the metallic and semiconducting fractions were all consistent with the intended separation, as discussed above. In addition, a more direct 4343


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Figure 7. I-V curves for the P3HT/SWNT composite films (10 wt % nanotubes for both) with preseparation-purified SWNTs (0) and separated metallic SWNTs (O). Dashed lines represent the best fits from linear regression. Reproduced with permission from ref 20 (Copyright 2008, American Chemical Society).

verification in terms of electrical conductivity was pursued and demonstrated unambiguously. Conductive polymers poly(3-hexylthiophene) (P3HT) and other polythiophenes were used as matrices for the homogeneous dispersion of the separated metallic or semiconducting SWNTs, and the resulting nanocomposite thin films were evaluated for electrical conductive properties.20 It is known in the literature that polythiophenes are excellent agents in dispersing SWNTs to allow performance comparisons in films on a morphologically equivalent basis.31 The P3HT films dispersed with the separated metallic SWNTs were obviously more conductive than those with the preseparation nanotube mixture and even more so than those with the separated semiconducting SWNTs at the same nanotube loading (Figure 7). There have been computational studies on the selectivity in the noncovalent functionalization of SWNTs with planar aromatic molecules.32 The results, suggesting that the adsorption strength in the most stable configuration of porphyrin and pyrene on a semiconducting (10, 0) versus metallic (6, 6) SWNT was different (with the former being larger),32 were thus consistent with the experimental observation that these aromatic species were selective toward functionalizing semiconducting SWNTs over their metallic counterparts. The selectivity in the noncovalent functionalization of metallic versus semiconducting SWNTs is apparently dependent on the specific planar aromatic molecule, which is positive for one pyrene derivative (1-docosyloxymethyl-pyrene or DomP, Figure 5),26,27 and negative for another (1-pyreneacetic acid).30 Such a dependence actually strengthens the argument for the selectivity. Anthracene derivatives were also found to be generally poor or even incapable of selectively solubilizing semiconducting SWNTs for the intended separation.5,33 Nevertheless, a number of other aromatic compounds, including coronene tetracarboxylic acid,34 derivatized pentacene,33 and crown ether-terminated pyrene,35 have been used for the similar separation of metallic and semiconducting SWNTs, which has been the use of selective chargetransfer interactions.34,36 An interesting twist was that the crown ether-terminated pyrene was reported to be selective toward solubilizing metallic HiPCO SWNTs in chloroform.35 More studies for an improved mechanistic understanding of the selectivity of the preferential noncovalent functionalization of either


semiconducting or metallic SWNTs are needed. Regardless of the mechanism, the selectivity generally seems low, which may ultimately limit the achievable metallic or semiconducting purity in the separated fractions. Multiple separation cycles with alternating uses of different separation agents may be employed to compensate for the low selectivity. However, such a strategy is likely labor-intensive and risky in terms of the potential for success. Additional challenges include the need to develop batch separation into continuous processing. A continuous separation process is not only more desirable for scaling up but also enables or helps to implement the cycling strategy discussed here and the exploration of other approaches for very pure metallic and semiconducting fractions. Selectivity in Covalent Functionalization. The reactivity in various covalent addition reactions has long been thought to differ between metallic and semiconducting SWNTs, and the difference has been exploited for postproduction separation.37-42 For example, the addition of diazonium salts to the nanotube sidewall was found to favor metallic SWNTs,38 which upon solubilization allowed the enrichment of semiconducting SWNTs in the residue.39 Similarly, the higher reactivity of metallic HiPCO SWNTs in a [2 þ 2] cycloaddition reaction with fluorinated polyolefins was used to suppress the metallic conductivity of as-grown nanotubes.40 In another example, an azomethine ylide containing polycyclic aromatic moieties (pyrene or anthracene) was first anchored onto semiconducting SWNTs, taking advantage of the selective π-π stacking interactions, followed by the cycloaddition of the azomethine ylide to the nanotube sidewall.41 The selectivity in the functionalization (in terms of a noncovalent-covalent combination) toward semiconducting SWNTs and the resulting solubilization were responsible for the intended separation.41 As a generalized postproduction separation approach, covalent functionalization is obviously more invasive, introducing disorder on the nanotube sidewall that may damage the intrinsic properties of SWNTs. For the selective addition reaction toward metallic SWNTs, as an example, the functionalized metallic SWNTs could be defunctionalized to recover the nanotubes. However, the defunctionalization process might cause other problems, such as irreversibly changing the sample morphology to result in either poor or no dispersibility. Regarding selectivity, it is known that the reactivity of SWNTs is also dependent, sensitively in some reactions, on the nanotube diameter, with a smaller diameter generally corresponding to higher reactivity.42 It may compete with the selectivity between metallic and semiconducting SWNTs or add more complexity to the separation process.

’ OPPORTUNITIES WITH SEPARATED SWNTS Separated metallic and semiconducting SWNTs offer many unique opportunities for a variety of technological applications. Regarding metallic SWNTs, their extremely high electrical conductivity is well-established (estimated theoretically as high as 106 S/cm), and the propagation of electrons in metallic nanotubes is known to be ballistic, largely free from scattering over a distance of thousands of atoms. With their resistance approaching the theoretical lower limits, metallic nanotubes may, in principle, carry an electrical current density of 4 109 A/cm2, which is more than 1000 times greater than that in metals such as copper. Indeed, since the first fabrication and investigation of electrical devices based on metallic SWNTs in 1997,43 subsequently 4344


Langmuir pursued potential applications have included nanocircuitry, conductive polymeric nanocomposites, and, most extensively, transparent conductive coatings/films. In nearly all devices encompassing some type of conversion between electrons and photons, a component that is electrically conductive yet optically transparent is necessary.44 Though indium tin oxide (ITO) coating technology is the predominant method used to create transparent electrodes,4 significant deficiencies have been identified, including their incompatibility with flexible substrates and demanding processing conditions. Thus, the use of carbon nanotubes as potential replacements of ITO in transparent conductive coatings, especially those requiring high flexibility, has been a research endeavor of great interest ever since the discovery of SWNTs. Coatings with Nonseparated SWNTs. Nanotube films are quasi-2D interconnected networks in which the electrical conductivity is controlled by both the intrinsic conductivity of individual carbon nanotubes and tube-tube junctions.45 Many approaches have been applied to the fabrication of transparent conducting coatings from SWNTs, including spraying, filtration, rod/wire coating, layer-by-layer deposition, spin coating, and dip coating. Spray coating, a simple method for directly fabricating nanotube films of any size on various substrates, has been employed in many investigations. For example, Lee and co-workers used air spraying to fabricate films of arc-discharge-produced SWNTs on a flexible substrate.46 In their experiment, a suspension of surfactant (SDS)-dispersed SWNTs was sprayed onto a preheated (100 °C) polyethylene terephthalate (PET) substrate, followed by repeated rinsing in water to remove SDS. Upon doping in concentrated HNO3, the sheet resistance of the nanotube coating could reach ∼40 or 70 Ω/square for an optical transmittance of 70 or 80% at 550 nm, respectively.46 More recently, Blackburn and co-workers modified the spraying process by replacing the airbrush pistol with an ultrasonic spray head, allowing the controlled, uniform, reproducible coating of a specific number of SWNTs onto a large substrate.47 Generally speaking, although spray coating is simple and flexible, the resulting films have an inherent sparse density that may decrease the electrical conductivity, which has a negative impact on performance.46 Vacuum filtration is another popular method in the fabrication of transparent conductive films, which are morphologically like “bucky papers”. For example, Rinzler and co-workers used filtration to fabricate coatings from laser-ablation-produced SWNTs.48 Experimentally, the nanotube sample was first dispersed in a solution with surfactants and then filtered through a porous membrane to form a thin film on the membrane. Upon the removal of surfactants via careful washing, the film on the filter membrane could reach electrical sheet resistance down to 30 Ω/square, with an estimated optical transmittance of approximately 70% in the visible spectral region.48 A number of advantages have been discussed regarding the filtration method, including a more homogeneous distribution and improved packing of SWNTs in the coated films. The homogeneity is attributed to the compensation effect naturally associated with the filtration process such that the already deposited nanotubes would reduce the flow of the nanotube suspension, thus depositing additional nanotubes into other less-dense areas of the film, whereas the packing for improved contact between nanotubes is due to vacuum pressing in the filtration. However, films thus fabricated are limited by the filter size, a drawback for applications


requiring larger films. Because most membrane filters are largely opaque or incompatible with electronic devices, an additional process to transfer films from the filter membrane to the desired substrate is required for characterization and/or device construction. There are primarily two methods for such transfer: one involves dissolving the filter wet chemically to release the film,48 and the other involves using an adhesive stamp to peel the film off of the filter membrane.49 Rod/wire coating is another widely used fabrication method for transparent conductive films of SWNTs, which is compatible with various substrates and often used in the coating industry for the continuous and scalable production of liquid thin films.50 A critical aspect in the rod coating process is the preparation of coating fluids with specific rheological behavior and wetting properties. For example, a mixture of sodium dodecylbenzenesulfonate (SDBS) and Triton X-100 surfactants was used to disperse SWNTs for enhanced viscosity of the coating fluid.50 The nanotube films from the coating, upon acid treatment, exhibited sheet resistances of 80 and 140 Ω/square for 70 and 80% optical transmittance at 550 nm, respectively.50 Layer-bylayer deposition is another frequently used method of fabricating SWNT films51 in which thin films are created by alternately exposing a substrate to a polymer solution and an aqueous suspension of SWNTs (with a negatively charged surfactant such as poly(sodium 4-styrene sulfonate) or SDS as a dispersion agent).51 For example, a film thus fabricated was treated in the doping process with fuming sulfuric acid to reach a sheet resistance of 86 Ω/square for 80% optical transmittance at 550 nm, though additional measures were necessary to preserve the doping effect for performance stability over time.51 To align SWNTs partially in the transparent conductive films for improved performance, spin coating and dip coating have been found to be more effective.52 All of these wet-processing methods for nanotube films seem to share some common features, as determined by the properties of SWNTs and their networks in the coated films. It is known in the literature that the resistance at the intertube junction is higher when the junction is between nanotube bundles. Therefore, the homogeneous dispersion and individualization of SWNTs are prerequisites to the formation of more conductive nanotube films in terms of wet-processing methods. Indeed, dispersion strategies in these fabrication methods range from the use of surfactants or polymers as dispersion agents to aggressive sonication and their various combinations. A potentially negative outcome for overly aggressive sonication is the shortening of SWNTs, which according to percolation theory and related experiments is generally less favorable for producing the desired conductive films. A significant downside involving the use of surfactants or other dispersion agents is that their complete removal after fabrication is very challenging, to say the least, especially regarding the necessity to remove dispersion agents without affecting the morphology and other desired performance characteristics of the coated films.46 The extensive effort in the development of various coating methods has pushed transparent conductive films of SWNTs toward performance levels close to those of ITO coatings. In general, however, the potential for dramatic improvements in nanotube films through further modification and optimization of the different fabrication methods appears limited.4 Because nanotube samples from all available production, including those used in the studies discussed above, contain metallic SWNTs as only a minor fraction, the use of highly enriched or even pure metallic SWNTs for the fabrication of transparent conductive 4345


Langmuir films will more likely bring about the desired performance enhancement to surpass that of the presently predominant ITO coatings. Coatings with Enriched Metallic SWNTs. Experimental evidence suggests that the conductivity in nanotube films is dominated by resistance at the tube-tube junctions.45 Indeed, the contact resistance at metallic-semiconducting junctions is 3 orders of magnitude higher than that at the metallic-metallic junctions. It has been shown that nanotube films after fabrication may be doped via treatment with a strong acid (HNO3 or SOCl2) to “metallize” semiconducting nanotubes and to decrease the intertube resistance at the junctions (mitigation of the Schottky barrier), thus significantly enhancing the electrical conductivity in the films.53,54 However, these doped films are generally less stable thermally and chemically, often degrading in performance over time.51,53 Nevertheless, the demonstrated effect of “metallization” does point to the great potential of transparent conductive films from separated metallic SWNTs (no need for metallization and thus no associated problems either55). According to a direct comparison by Miyata et al., the film (90 nm in thickness) from enriched metallic SWNTs was ∼1 kΩ/square in sheet resistance whereas the thicker reference film (130 nm in thickness) by the same fabrication from nonseparated SWNTs exhibited a much higher sheet resistance of ∼20 kΩ/square.55 Sun and co-workers separated significant quantities of metallic SWNTs from arc-discharge-produced nanotube samples by using the method based on the selective noncovalent functionalization and solubilization of semiconducting SWNTs with planar aromatic molecules (Figure 5).19,20 In a study comparing the separated metallic SWNTs with preseparation as-purified SWNTs, transparent conductive films were fabricated by the vacuum filtration technique.28 Experimentally, all samples were dispersed in water with the assistance of surfactant SDS, and a porous alumina membrane was used as a filter in the vacuum filtration of the suspended SWNTs. After filtration, the film on the filter membrane was repeatedly and thoroughly washed to remove SDS. In order not to introduce any other effects associated with transferring films from the alumina membrane to another substrate, the as-fabricated films were compared directly such that the fabrication procedures remained identical and the number of separated metallic SWNTs or preseparation as-purified SWNTs in each film was determined quantitatively. In plots on variations in the observed surface resistivity against nanotube contents in the films (Figure 8), clearly the electrical conductive performance in the films from the separated metallic SWNTs was consistently much better than that in the films from as-purified SWNTs. The optical transmittance of selected films on the alumina filter was estimated by transferring the films to a transparent substrate. The results thus obtained suggested that at ∼80% transmittance (550 nm) the surface resistivity of the film from metallic SWNTs was less than 100 Ω/square,28 a performance level already competitive with that of ITO coatings for some applications. The same separated metallic SWNTs were included in a systematic evaluation by Yang and co-workers on transparent conductive films of SWNTs from different nanotube sources and various fabrication conditions.56 It is worth noting that the films were on a flexible substrate (PET), for which dip coating was used. Again, those films from the separated metallic SWNTs exhibited a sheet resistance of down to ∼130 Ω/square for 80% optical transmittance at 550 nm (Table 1). The comparison of nanotube films with ITO coatings on the flexible substrate was


Figure 8. (Upper) Films of SWNTs on alumina filters from the aspurified sample (left) and the separated metallic fraction (right). (Lower) Direct comparison of surface resistivity values in films of SWNTs (fabricated via vacuum filtration) from the as-purified sample (0) and the separated metallic fraction (O). Reproduced with permission from ref 28 (copyright 2010 Elsevier B.V.).

Table 1. Effect of Carbon Nanotube Chirality on the Conductivity (from Reference 56)a one coating

double coatings T%

Rs (Ω/sq)

T%

Rs (Ω/sq)

72

61 10

69

14 103

semiconducting 82 175 10

79

95 10

75

1493

metallic

82

262

80

130 ( 5

sample mixture

T% Rs (Ω/sq) 78 167 10

3 3

a

triple coatings

85 403

3 3

Substrate used: PET(control); T% = 85.

particularly striking, with the latter apparently breaking down at a bending angle of greater than about 30° (Figure 9).56 Enriched metallic SWNTs from other sources and separation methods have also been used for transparent conductive films. For example, Hersam and co-workers employed the density gradient ultracentrifugation (DGU) method to harvest metallic SWNTs of different diameter ranges, with the starting nanotube samples from different sources (HiPCO, laser ablation, and arc discharge),11 and used the enriched metallic SWNTs for transparent conductive films. The films were fabricated through vacuum filtration, followed by transfer to transparent hard (glass and quartz) and flexible (PET) substrates. For enriched metallic HiPCO SWNTs, the resulting film exhibited a sheet resistance of ∼231 Ω/square for 75% optical transmittance at 550 nm, in comparison with ∼1340 Ω/square in the reference film of the same optical transmittance from nonseparated HiPCO SWNTs. The films of enriched metallic SWNTs from laser-ablation- and arc-discharge-produced nanotube samples generally exhibited better performance, with less than 140 Ω/ square sheet resistance at optical transmittances of over 70% in the visible and near-IR spectral regions.11 An interesting twist was that the films of enriched metallic SWNTs from different sources exhibited distinctive colors because of the different diameter ranges for the nanotubes, which, as suggested by the 4346


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Figure 9. Flexibility study of SWNTs on PET vs ITO on PET with two probe resistances. Reproduced with permission from ref 56 (copyright 2009 Elsevier B.V.).

authors, might be exploited for applications in conductive optical filters.11 In another study,57 Maeda et al. fabricated transparent conductive films by air spraying enriched metallic SWNTs (from the amine-assisted postproduction separation18,25) onto both quartz and PET substrates. The sheet resistances in the films on PET were 690 Ω/square at an optical transmittance of 81% (550 nm) and 9000 Ω/square at 97% (550 nm), which represented reductions by a factor of 20 in comparison with the performance in films of nonseparated SWNTs.57 The separated metallic SWNTs can also enhance the transparent conductive performance in composite films with conductive polymers, particularly the poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) blend because it is optically transparent in the visible spectral region.20,58 In such composites, the conductive polymer blend serves as a dispersion agent, so no surfactants are necessary in the film fabrication. In the work by Wang et al.,20 suspensions of nanotubes (enriched metallic or nonseparated SWNTs) in DMSO were mixed with aqueous PEDOT/PSS in various compositions, and the resulting mixtures were sprayed onto an optically transparent substrate. The sheet resistance results demonstrated that the composite films with enriched metallic SWNTs were consistently and substantially better in performance than those with nonseparated SWNTs (and both better than films with neat PEDOT/PSS).20 Aqueous PEDOT/PSS is not as effective as commonly used surfactants in the dispersion of SWNTs, which is negative with respect to the performance of the resulting nanocomposite films. Appropriate structural modifications to PEDOT/PSS for derivatives of improved dispersion characteristics may be pursued to optimize the composite films. It should be pointed out that the use of conductive polymers in transparent electrodes may yield other benefits not reflected in the performance of low surface resistivity. For example, to adjust and improve the interfacial work function in electronic devices such as organic light-emitting diodes (OLEDs), PEDOT/PSS is often coated as an additional thin layer on top of a nanotube film or ITO coating in transparent electrodes. In fact, improved interfacial properties may be another benefit for the use of nanotube films to replace ITO coatings in certain energy-conversion devices. In dye-sensitized solar cells (DSSCs), for example, the photoanode is typically composed of semiconductor nanoparticles deposited on a transparent electrode (generally ITO-coated glass) and sensitized with a self-assembled monolayer of dye molecules. Several studies on the incorporation of SWNTs into the photoanode have found significant performance


improvements in the resulting DSSCs.59 The use of a transparent electrode made from metallic SWNTs in the photoanode may further enhance the performance of existing solar cells. There is now sufficient evidence to validate, in principle, the long-held expectation that metallic SWNTs may ultimately be used in transparent electrodes or at least as alternatives to ITO technology. In practice, many technical issues from materials (separated metallic SWNTs) to fabrication have yet to be addressed. Beyond transparent electrodes, metallic SWNTs may find other applications in which extremely high electrical conductivity and excellent optical properties are both required or even some in which optical transparency is not necessary. Again, for dye-sensitized solar cells as an example, great benefits to using metallic SWNTs to replace platinum metal in the cathode may be expected on the basis of available experimental results.59 There have been many studies on the potential of using semiconducting SWNTs in various applications, probably more so than those on their metallic counterparts, because of their better availability and because multiple-walled carbon nanotubes (MWNTs) are useless in terms of semiconducting properties. Among widely pursued applications of semiconducting SWNTs are those in transistors (especially FETs or field-emission transistors), chem-bio sensors, and various energy-conversion devices. FETs. The use of semiconducting SWNTs takes advantage of their ultrahigh on-off current ratio (∼105) and electron/hole mobility (∼100 000 cm2/(V s)), their ability to carry a high current density (109 A/cm2), and allowing operation at high frequencies (into the terahertz regime). These devices generally require high semiconducting purity in the nanotubes because even traces of metallic nanotubes and/or impurities (catalytic and amorphous particles) may short circuit the FET channels.60 A commonly used approach to retaining only semiconducting SWNTs in the fabricated FET devices has been the selective elimination of metallic SWNTs via current-induced electrical breakdown. For example, the IBM group used such an approach to fabricate FET devices with the desired characteristics, with their switching ratios limited primarily by contact resistance.61 A conceptually somewhat analogous approach has been the selective chemical modification of metallic SWNTs, such as the addition of diazonium salts to the nanotubes to reduce their electrical conductivity, to achieve high on/off ratios in the resulting FETs.62 Zhang et al. used the methane plasma hydrocarbonation reaction to etch and gasify metallic nanotubes selectively. The FET devices with the remaining semiconducting SWNTs exhibited on/off ratios of 104-105 at Vds = 1 V and high on currents (Ids ≈ 140 mA at Vds = 2 V).63 The direct selective growth of semiconducting SWNTs on the substrate has also been a popular approach in the development of FETs and other molecular electronic devices.7,8 More recently, high-density arrays of horizontally well-aligned semiconducting SWNTs were directly grown with high uniformity over a large area and were used to fabricate FET devices with high on/off ratios.8 The semiconducting SWNTs from postproduction separation methods discussed earlier have already been used and will likely see more use in FETs.52,64,65 For example, the semiconducting SWNTs recovered from the DGU (density gradient ultracentrifugation)-separated fractions were used to fabricate FET devices with enhanced performance.11,64 Similarly, the semiconducting SWNTs from agarose gel-based separation were used in thin-film FETs, with performance better than that in devices 4347


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Figure 10. Schematic of the SWNT TFT fabrication and structure. The dielectric (300 nm SiO2 on a heavily doped Si gate) is functionalized by either an amine-terminated (A) or a phenyl-terminated (B) silane. The SWNT solution is subsequently dispensed onto the spinning self-assembled monolayermodified substrate and dried, followed by source (S) and drain (D) gold electrode deposition. Upon spin coating, AFM tapping-mode topography images (10 μm 10 μm, z scale = 10 nm) of the nanotubes applied under identical conditions on amine (top) and phenyl (bottom) surfaces reveal that the density and alignment, represented by a histogram (Θ is angle, in degrees, of variation from an arbitrary direction) below the corresponding AFM images, are a direct function of the surface chemistry. Reproduced with permission from ref 52 (copyright 2008 the American Association for the Advancement of Science).

fabricated with nonseparated SWNTs.65 The postproduction separated semiconducting SWNTs may offer some advantageous flexibilities, such as their dispersion into a carbon inklike suspension to be compatible with various processing techniques, including inkjet printing. Interestingly, the selectivity found in the postproduction separation was exploited successfully during device fabrication with in-situ-enriched semiconducting SWNTs. In the work by Bao and co-workers, the SiO2/Si wafer surface was modified with amine- or phenyl-terminated silane, followed by spin coating a thin layer of SWNTs (Figure 10).52 It was found on the basis of Raman results that the amine-modified surface was enriched with semiconducting SWNTs, with the corresponding transistors exhibiting a higher on/off ratio and a lower off current than those of the transistors built upon the phenyl-modified wafer surface. Chem-Bio Sensors. There are primarily two classes of chembio sensors that use semiconducting SWNTs. One is based on FETs or, more generally, thin-film transistors (TFTs), including miniaturized sensors with high sensitivity and fast response times.3,66-68 TFTs with semiconducting SWNTs respond to a wide range of analyte interactions such that the conduction of current through a semiconducting SWNT in the device is sensitive to the environment and/or the adsorption of different types of molecules. For example, Kong et al. reported a conductance change of up to 3 orders of magnitude from that of individual semiconducting SWNTs within several seconds of exposure to NH3 and NO2 gases.3 In a more recent study,

Roberts et al. found that the TFT sensors with aligned, enriched semiconducting SWNT networks were much more sensitive than those fabricated with random, not semiconductingenriched, nanotube networks, with the detection limit lowered to 2 ppb for dimethyl methylphosphonate (DMMP) and trinitrotoluene (TNT) in aqueous solution.66 For biological species such as proteins, TFT sensors were demonstrated to achieve detection limits down to 100 pM,67 also with uses in studies of protein-protein interactions.68 The other class of sensors or sensing applications is based on the near-IR fluorescence properties of semiconducting SWNTs.69 Because the near-IR spectral region is more transparent with low absorption by blood and tissues, it is more useful in biosensing. The nanotube fluorescence, while resistant to photobleaching, was found to be sensitive to molecular adsorption on the nanotube surface,70 thus setting the stage for sensing. For example, Strano and co-workers demonstrated the selective modulation of the fluorescence from SWNTs in response to the adsorption of ss-D-glucose.70 The same group later applied this finding to the fabrication of DNA hybridization sensors with the detection limit down to 6 nM DNA.71 OLED and PV. As a unique semiconductor, semiconducting SWNTs have been widely pursued for various energy-conversion applications. One approach is again based on TFTs, which are considered for potential uses in displays or other flexible electronic devices. A representative recent work was by Avouris and co-workers,64 who were able to self-assemble and align separated semiconducting SWNTs into TFTs of superior performance, 4348


Langmuir including the generation of strong photocurrents and being both photoluminescent and electroluminescent. For organic light-emitting diodes (OLEDs), the control circuit with semiconducting SWNTs was demonstrated to have the modulation in the output light intensity exceeding 104.72 Because semiconducting SWNTs are absorptive in the near-IR, they can generate excitons in devices and also expand the range of light that can be absorbed simultaneously. For the use of semiconducting SWNTs in photon harvesting and generation, Avouris and co-workers demonstrated the feasibility of near-IR photoconductive and electroluminescent devices based upon the use of individual nanotubes in an ambipolar FET configuration.73 Kazaoui et al. isolated semiconducting SWNTs to couple with the established polymer film technology for evaluating photoconductive and photovoltaic properties over a broad spectral range (300-1600 nm).74 Their devices were based upon nanotube composites with poly(2-methoxy-5-(20 -ethylhexoxy)-1,4phenylenevinylene) or poly(3-ocylthiophene-2,5-diyl) and aluminum and ITO electrodes in a sandwich configuration. Haddon and co-workers recently demonstrated that the amplitude of electromodulation in nanotube thin film devices could be enhanced by an order of magnitude with the use of SWNTs having a higher semiconducting-to-metallic ratio.75 These devices are characterized by their operation in the infrared spectral region, high transparency, fast response time, and low operating voltage, which are valuable to the development of 3D-integrated arrays with an optoelectronic mode of interlayer communication.

’ SUMMARY AND PERSPECTIVE Metallic and semiconducting SWNTs, which are distinctively different in electrical conductivity and many other aspects, obviously do not belong in mixtures (compromising or even diminishing their respective superior properties). Although much effort has been expended to produce either metallic or semiconducting SWNTs directly, with seemingly somewhat more success for the latter, postproduction separation methods (including those for destroying one of the two) have seen significant advances and major achievements. The available separation methods are now capable of harvesting separated metallic and semiconducting SWNTs from different production sources, with sufficiently high enrichment and up to gram quantities to satisfy at least the needs in research and technological exploration. Further advances in the separation methods are anticipated, especially including the goals on higher purities (in terms of both nanotube sample and metallicity) and maturation for scaling up, though the latter might have to be driven by the implementation of one or more technological applications. Among the widely pursued applications, the separated metallic SWNTs are the most promising for transparent electrodes on both hard and flexible substrates, with the latter already competitive with the ITO coating technology. The separated semiconducting SWNTs will likely continue to find use in a variety of devices highlighted in this article, though higher semiconducting purity seems necessary for more high-end applications. Finally, the recent emergence of graphene nanosheets and related materials may offer great opportunities for the development of carbon tube-sheet hybrid nanotechnologies. ’ AUTHOR INFORMATION *E-mail: [email protected].


’ ACKNOWLEDGMENT Financial support from the Air Force Office of Scientific Research (AFOSR) through the program of Dr. Charles Lee is gratefully acknowledged. L.C. was supported by a Susan G. Komen for the Cure postdoctoral fellowship. We also acknowledge the efforts of Mr. Godfrey Kimball of Clemson University for editing this article. ’ REFERENCES (1) (a) Ajayan, P. M. Chem. Rev. 1999, 99, 1787. (b) Connell, M. J., Ed. Carbon Nanotubes: Properties and Applications; CRC/Taylor & Francis: Boca Raton, FL, 2006. (2) Unalan, H. E.; Fanchini, G.; Kanwal, A.; Du Pasquier, A.; Chhowalla, M. Nano Lett. 2006, 6, 677. (3) Kong, J.; Franklin, N.; Zhou, C.; Chapline, M.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (4) (a) Gordon, R. G. MRS Bull. 2000, 25, 52. (b) Kumar, A.; Zhou, C. ACS Nano 2010, 4, 11. (5) Lin, Y.; Fernando, K. A. S. Wang, W.; Sun, Y.-P. Separation of Metallic and Semiconducting Single-Walled Carbon Nanotubes. In Carbon Nanotechnology: Recent Developments in Chemistry, Physics, Materials Science and Device Applications; Dai, L., Ed.; Elsevier: Amsterdam, 2006; p 255. (6) Liu, J.; Hersam, M. C. MRS Bull. 2010, 35, 315. (7) Qu, L.; Du, F.; Dai, L. Nano Lett. 2008, 8, 2682. (8) Ding, L.; Tselev, A.; Wang, J.; Yuan, D.; Chu, H.; McNicholas, T. P.; Li, Y.; Liu, J. Nano Lett. 2009, 9, 800. (9) Harutyunyan, A. R.; Chen, G. G.; Paronyan, T. M.; Pigos, E. M.; Kuznetsov, O. A.; Hewaparakrama, K.; Kim, S. M.; Zakharov, D.; Stach, E. A.; Sumanasekera, G. U. Science 2009, 326, 116. (10) (a) Hersam, M. C. Nat. Nanotechnol. 2008, 3, 387. (b) Arnold, M. S.; Stupp, S. I.; Hersam, M. C. Nano Lett. 2005, 5, 713. (11) (a) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60. (b) Green, A. A.; Hersam, M. C. Nano Lett. 2008, 8, 1417. (12) Antaris, A. L.; Seo, J.-W. T.; Green, A. A.; Hersam, M. C. ACS Nano 2010, 4, 4725. (13) Zheng, M.; Jagota, A.; Strano, M. S.; Santos, A. P.; Barone, P.; Chou, S. G.; Diner, B. A.; Dresselhaus, M. S.; McLean, R. S.; Onoa, G. B.; Samsonidze, G. G.; Semke, E. D.; Usrey, M.; Walls, D. J. Science 2003, 302, 1545. (14) Ghosh, S.; Bachilo, S. M.; Weisman, R. B. Nat. Nanotechnol. 2010, 5, 443. (15) Tu, X.; Manohar, S.; Jagota, A.; Zheng, M. Nature 2009, 460, 250. (16) Tanaka, T.; Jin, H.; Miyata, Y.; Fujii, S.; Suga, H.; Naitoh, Y.; Minari, T.; Miyadera, T.; Tsukagoshi, K.; Kataura, H. Nano Lett. 2009, 9, 1497. (17) Liu, H.; Feng, Y.; Tanaka, T.; Urabe, Y.; Kataura, H. J. Phys. Chem. C 2010, 114, 9270. (18) Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2003, 125, 3370. (19) (a) Li, H.; Zhou, B.; Gu, L.; Wang, W.; Fernando, K. A. S.; Kumer, S.; Allard, L. F.; Sun, Y.-P. J. Am. Chem. Soc. 2004, 126, 1014. (b) Sun, Y.-P. U.S. Patent 7,374,685, May 20, 2008. (20) Wang, W.; Fernando, K. A. S.; Lin, Y.; Meziani, M. J.; Veca, L. M.; Cao, L.; Zhang, P.; Kimani, M. M.; Sun, Y.-P. J. Am. Chem. Soc. 2008, 130, 1415. (21) Ju, S.-Y.; Utz, M.; Papadimitrakopoulos, F. J. Am. Chem. Soc. 2009, 131, 6775. (22) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (23) (a) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105. (b) Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096. 4349


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