Due to the high cost, brittle nature, and suboptimal electronic and chemical properties of indium tin oxide (ITO), [ 1 − 4 ] alternative transparent conducting anode materials have played an increasingly important role in organic photovoltaic (OPV) device research. For example, thin fi lms of single-walled carbon nanotubes (SWNTs) have been identifi ed as a promising option due to their excellent electronic properties, solution processability, elemental abundance, environmental stability, and robust mechanical fl exibility. [ 5 − 7 ] Recent reports have demonstrated OPVs incorporating SWNT fi lms as the transparent anode, with the primary barrier to greater effi ciencies being the relatively high sheet resistance of the SWNT fi lm. [ 8 − 10 ] One of the common methods employed to overcome this obstacle has been chemical doping of the fi lms prior to device fabrication, either intentionally or as a byproduct of roughnessreducing acid treatments. In particular, the adsorption of electron-withdrawing species both lowers the SWNT fi lm sheet resistance and bleaches the primary peaks in the optical absorption spectrum, thereby increasing the fi lm transparency. [ 11 − 14 ] However, this chemical doping strategy introduces limited environmental stability, which compromises performance in many device applications. [ 5 , 15 ] Additionally, because previous studies of SWNT-based OPV anodes have employed thin fi lms formulated from electronically polydisperse SWNT mixtures, the role and relative importance of metallic versus semiconducting SWNTs has not been clarifi ed.Herein, we present the use of electronically monodisperse arc discharge SWNTs, sorted via density gradient ultracentrifugation (DGU), [ 7 , 16 ] as the transparent anode material in OPVs. By varying the ratio of semiconducting and metallic species in the SWNT thin fi lms, we fi nd that a composition of 70% or greater metallic SWNTs affords 50× higher OPV power conversion effi ciency (PCE) than monodisperse semiconducting SWNT thin fi lms. Analysis of the stability after chemical doping with nitric acid, which is used to lower the fi lm roughness, indicates that the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electron-blocking layer reverses the effects of the doping process and reduces the SWNT electrical conductivity. X-ray photoelectron spectroscopy (XPS) further reveals that nitric oxide (NO) is the primary adsorbed dopant species that is removed upon PEDOT:PSS deposition. Since the electronic and optical properties of metallic SWNTs are less affected by chemical doping, they remain effective transparent conductors following OPV fabrication, thus explaining the 50× difference in device PCE for metallic versus semiconducting SWNT-enriched anodes. Overall, this study establishes that SWNT chemical doping is incompatible with PEDOT:PSS, thus demonstrating the importance of metallic SWNT-enriched materials in OPV anodes.Previous studies recognized the importance of minimizing the roughness of SWNT thin fi lms in organic electron...

Noble metal nanoparticle clusters underlie a variety of plasmonic devices and measurements including surface-enhanced Raman spectroscopy (SERS). Because of the strong dependence of plasmonic properties on nanoparticle cluster aggregation state, the elimination of non-SERS-active structures and the refinement of the nanoparticle cluster population are critical to realizing uniform and reproducible structures for plasmonic nanoantenna applications such as SERSbased sensors. In this Letter, we report a centrifugal sorting technique for gold core/ silica shell nanoparticles that host SERS reporter molecules at the gold/silica interface. The relatively massive nanoparticle clusters are sorted by sedimentation coefficient via centrifugation in a high-viscosity density gradient medium, iodixanol, which yields solutions that contain a preponderance of one aggregation state and a diminished monomer population, as determined by transmission electron microscopy, extinction spectroscopy, and SERS. A quantitative analysis of the nanoparticle sedimentation coefficients is presented, thus allowing this approach to be predictably generalized to other nanoparticle systems. SECTION Nanoparticles and NanostructuresT he intense electromagnetic field arising at the surface of metallic nanostructures from the excitation of the localized surface plasmon resonance (LSPR) allows for the enhancement of the Raman intensity of adsorbed molecules by a factor up to 4 Â 10 8 or greater. 1 The structures supporting this plasmonic phenomenon, known as surfaceenhanced Raman scattering (SERS), are diverse. The most sensitive examples, with an enhancement factor large enough to observe spectra at the single molecule level, 2-5 are aggregated metallic nanoparticles. Correlative structure-activity studies have indeed shown that the presence of a nanometersized junction 6,7 or crevice 1,8 creates the electromagnetic "hot-spot" (i.e., "nanoantenna") required to observe singlemolecule SERS. Recent investigations of the hot-spots at the junction of silver cubes 9 and gold pyramidal shells 10 or at the interface between a gold nanostar and a gold surface 11 have highlighted how the control over the structure of this nanometer-scale region is crucial for achieving high enhancement factors. Whereas the early fundamental studies of single-molecule SERS have been performed on inhomogeneous samples, the integration of plasmonic nanoantennas into reliable technological applications, such as high sensitivity biological and chemical sensors, requires improved structural reproducibility.Homogeneous nanostructure populations can be realized via precisely controlled fabrication or postsynthetic sorting techniques. Although much effort has been devoted to the controlled synthesis of nanoparticles, structural polydispersity remains an issue. 12-14 Consequently, postfabrication separation methods have become important for characterizing or refining populations of nanoparticles based on their size, shape, and aggregation state. 15 For example, electrophoretic methods, 1...

Dual‐iteration density gradient ultracentrifugation isolates nearly single diameters of monodisperse metallic arc discharge SWCNTs. Subsequently fabricated conductive thin films possess distinct colors due to well‐defined transmittance windows flanked by sharp optical transitions. Measurements of uniform sheet resistances and work functions confirm the largely invariant electronic properties between metallic arc discharge SWCNT films of differing diameters.

We report the use of microcapsules containing suspensions of polymer-stabilized carbon nanotubes and/or graphene flakes for the autonomic restoration of conductivity in fractured gold lines. Multilayered samples were prepared in which microcapsules were embedded in layers of epoxy above and below a glass slide patterned with gold lines. Upon sample fracture, conductivity was lost as a crack formed in the gold line. Simultaneous release of carbon nanotubes and/or graphene suspensions from capsule cores restored conductivity in minutes. We suggest a healing mechanism in which the released carbon nanomaterials bridge gaps in the gold lines. V

Highly refined shape populations of gold nanoparticles (AuNPs) are important for emerging applications in catalysis, plasmonics, and nanomaterials growth. To date, research efforts have focused on achieving monodisperse shape by synthetic control or postsynthetic processing that relies on centrifugal sedimentation-based sorting schemes where differences in the particle mass and aspect ratios (e.g., rods and spheres) provide a driving force for separation. Here, we present a technique to reversibly modify the sedimentation coefficients of AuNPs possessing different shapes that would otherwise be virtually indistinguishable during centrifugal sedimentation due to their similar densities, masses, and aspect ratios by exploiting the preferential affinity of the surfactant cetyltrimethylammonium bromide (CTAB) for the Au(100) facet. The resulting tailored sedimentation coefficients enable AuNP shape sorting via density gradient centrifugation (DGC). DGC-refined populations of faceted AuNPs are shown to significantly enhance the growth rate of InAs nanowires when used as seed particles, emphasizing the importance of shape control for nanomaterials growth applications.

The image depicts a series of electronic‐type sorted single‐walled carbon nanotubes suspended in solution. The rays of light emanating from behind the samples is suggestive of the solar energy that these materials help capture in organic photovoltaic cells. Mark Hersam and co‐workers demonstrate that the green metallic solutions yield transparent electrodes with improved stability compared to their red semiconducting counterparts.