ambient-method-for-production-an-ionically-gated-carbon-nanotube

Ambient Method for the Production of an Ionically Gated Carbon Nanotube Common Cathode in Tandem Organic Solar Cells

A method of fabricating organic photovoltaic (OPV) tandems that requires no vacuum processing is presented. These devices are comprised of two ...

The University of Texas at Dallas

Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.

Carbon nanotubes--the route toward applications.

Individual carbon nanotubes are like minute bits of string, and many trillions of these invisible strings must be assembled to make useful macroscopic articles. We demonstrated such assembly at rates above 7 meters per minute by cooperatively rotating carbon nanotubes in vertically oriented nanotube arrays (forests) and made 5-centimeter-wide, meter-long transparent sheets. These self-supporting nanotube sheets are initially formed as a highly anisotropic electronically conducting aerogel that can be densified into strong sheets that are as thin as 50 nanometers. The measured gravimetric strength of orthogonally oriented sheet arrays exceeds that of sheets of high-strength steel. These nanotube sheets have been used in laboratory demonstrations for the microwave bonding of plastics and for making transparent, highly elastomeric electrodes; planar sources of polarized broad-band radiation; conducting appliqués; and flexible organic light-emitting diodes.

Strong, transparent, multifunctional, carbon nanotube sheets.

Carbon nanotubes (CNTs) have unique chemical and physical properties anticipated to enable broad novel biomedical applications. Yet the question concerning their biocompatibility remains controversial. We recently reported a method for rapidly preparing strong, highly electrically conducting sheets and yarns from multi-walled CNTs. The present studies demonstrate that highly oriented 50-nm-thick semi-transparent CNT sheets and yarns, produced with a minimal residual content of catalytic transition materials, support the long-term growth of a variety of cell types ranging from skin fibroblasts and Schwann cells, to postnatal cortical and cerebellar neurons. We show that CNT sheets stimulate fibroblast cell migration compared to plastic and glass culture substrates; entice neuronal growth to the level of those achieved on polyornithine-coated glass and can be used for directed cellular growth. These findings have positive implications for the use of CNTs in applications such as tissue engineering, wound healing, neural interfaces and biosensors.

Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns.

Improved electrically powered artificial muscles are needed for generating force, moving objects, and accomplishing work. Carbon nanotube aerogel sheets are the sole component of new artificial muscles that provide giant elongations and elongation rates of 220% and (3.7 x 10(4))% per second, respectively, at operating temperatures from 80 to 1900 kelvin. These solid-state-fabricated sheets are enthalpic rubbers having gaslike density and specific strength in one direction higher than those of steel plate. Actuation decreases nanotube aerogel density and can be permanently frozen for such device applications as transparent electrodes. Poisson's ratios reach 15, a factor of 30 higher than for conventional rubbers. These giant Poisson's ratios explain the observed opposite sign of width and length actuation and result in rare properties: negative linear compressibility and stretch densification.

Giant-stroke, superelastic carbon nanotube aerogel muscles.

Low efficiencies and costly electrode materials have limited harvesting of thermal energy as electrical energy using thermo-electrochemical cells (or "thermocells"). We demonstrate thermocells, in practical configurations (from coin cells to cells that can be wrapped around exhaust pipes), that harvest low-grade thermal energy using relatively inexpensive carbon multiwalled nanotube (MWNT) electrodes. These electrodes provide high electrochemically accessible surface areas and fast redox-mediated electron transfer, which significantly enhances thermocell current generation capacity and overall efficiency. Thermocell efficiency is further improved by directly synthesizing MWNTs as vertical forests that reduce electrical and thermal resistance at electrode/substrate junctions. The efficiency of thermocells with MWNT electrodes is shown to be as high as 1.4% of Carnot efficiency, which is 3-fold higher than for previously demonstrated thermocells. With the cost of MWNTs decreasing, MWNT-based thermocells may become commercially viable for harvesting low-grade thermal energy.

Harvesting waste thermal energy using a carbon-nanotube-based thermo-electrochemical cell.

Three-dimensionally ordered arrays of submicron-sized mesoporous titania spheres with high surface area and high crystallinity have been prepared through triblock copolymer templating within the confinement of polymer inverse opals.

Template synthesis of ordered arrays of mesoporous titania spheres.

We demonstrate an aerosol CVD process to dry deposit large-area SWCNT networks with tunable conductivity and optical transmittance on a wide range of substrates including flexible polymers. These SWCNT networks can be chemically doped to reach a sheet resistance of as low as 110 Ω/⟨ at 90% optical transmittance. A wide application potential of these networks is demonstrated by fabricating SWCNT network-based devices such as a transparent capacitive touch sensors, thin-film transistors (TFTs), and bright organic light-emitting diodes (OLEDs).

Aerosol-synthesized SWCNT networks with tunable conductivity and transparency by a dry transfer technique.

Multifunctional applications of textiles have been limited by the inability to spin important materials into yarns. Generically applicable methods are demonstrated for producing weavable yarns comprising up to 95 weight percent of otherwise unspinnable particulate or nanofiber powders that remain highly functional. Scrolled 50-nanometer-thick carbon nanotube sheets confine these powders in the galleries of irregular scroll sacks whose observed complex structures are related to twist-dependent extension of Archimedean spirals, Fermat spirals, or spiral pairs into scrolls. The strength and electronic connectivity of a small weight fraction of scrolled carbon nanotube sheet enables yarn weaving, sewing, knotting, braiding, and charge collection. This technology is used to make yarns of superconductors, lithium-ion battery materials, graphene ribbons, catalytic nanofibers for fuel cells, and titanium dioxide for photocatalysis.

Biscrolling nanotube sheets and functional guests into yarns.

A structural model is developed for describing the solid-state transformation of a vertically oriented carbon multiwall nanotube (MWNT) forest to a horizontally oriented MWNT sheet or yarn. The key element of our model is a network of individual carbon nanotubes or small bundles interconnecting the array of main large-diameter MWNT bundles of the forest. The dry-draw self-assembly mechanism for MWNT sheet formation involves two principal processes that reconfigure the interconnection network: (1) unzipping by preferentially peeling off interconnections between the bundles in the forest and (2) self-strengthening of these interconnections by densification at the top and bottom of the forest during draw-induced reorientation of the bundles. It is shown that interconnection density is a key parameter that determines the ability of a MWNT forest to be dry-drawable into sheets and yarns. This model describes the principal mechanism of solid-state draw (confirmed by dynamic in situ scanning electron microscopy), the range of forest structural parameters that enable sheet draw, and observed dependencies of sheet properties on the parent MWNT forest structure.

Structural model for dry-drawing of sheets and yarns from carbon nanotube forests.

The effects of processing conditions and apparent nanotube length on properties are investigated for carbon nanotube yarns obtained by solid-state drawing of an aerogel from a forest of multi-walled carbon nanotubes. Investigation of twist, false twist, liquid densification and combination methods for converting the drawn aerogel into dense yarn show that permanent twist is not needed for obtaining useful mechanical properties when nanotube lengths are long compared with nanotube diameters. Average mechanical strengths of 800 MPa were obtained for polymer-free twist-spun multi-walled carbon nanotube (MWNT) yarns and average mechanical strengths of 1040 MPa were obtained for MWNT yarns infiltrated with 10 wt% polystyrene solution. Strategies for increasing the mechanical properties are suggested based on analysis of intra-wall, intra-bundle and inter-bundle stress transfer.

Structure and process-dependent properties of solid-state spun carbon nanotube yarns.

We demonstrated the replacement of the Pt catalyst normally used in the counter electrode of a dye-sensitized solar cell (DSSC) by a nanocomposite of dry spun carbon multi-walled nanotube (MWNT) sheets with graphene flakes (Gr-F). The effectiveness of this counter electrode on the reduction of the triiodide in the iodide/triiodide redox (I(-)/I(3)(-)) redox reaction was studied in parallel with the use of the dry spun carbon MWNT sheets alone and graphene flakes used independent of each other. This nanocomposite deposited onto fluorinated tin-oxide-coated glass showed improved catalytic behavior and power conversion efficiency (7.55%) beyond the use of the MWNTs alone (6.62%) or graphene alone (4.65%) for the triiodide reduction reaction in DSSC. We also compare the use of the carbon MWNT/Gr-F composite counter electrode with a DSSC using the standard Pt counter electrode (8.8%). The details of increased performance of graphene/MWNT composite electrodes as studied are discussed in terms of increased catalytic activity permitted by sharp atomic edges that arise from the structure of graphene flakes or the defect sites in the carbon MWNT and increased electrical conductivity between the carbon MWNT bundles by the graphene flakes.

Carbon nanotube/graphene nanocomposite as efficient counter electrodes in dye-sensitized solar cells.

Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. A vertically bicontinuous and interdigitized heterojunction between donor and acceptor has been regarded as one of the ideal structures to enable both efficient charge separation and transport. Synergistic control of polymer orientation in the nanostructured heterojunction is also critical to improve the performance of polymer solar cells. Nanoimprint lithography has emerged as a new approach to simultaneously control both the heterojunction morphology and polymer chains in organic photovoltaics. Currently, in the area of nanoimprinted polymer solar cells, much progress has been achieved in the fabrication of nanostructured morphology, control of molecular orientation/crystallinity, deposition of acceptor materials, patterned electrodes, understanding of structure-property correlations, and device performance. This review article summarizes the recent studies on nanoimprinted polymer solar cells and discusses the outstanding challenges and opportunities for future work.

Nanoimprinted polymer solar cell.

A novel symmetrical alkylsulfonyl-substituted poly(phenylenevinylene) derivative, poly [2,5-bis-(2'-ethylhexylsulfonyl)-1,4-phenylene)vinylene] (SO2EH-PPV), was synthesized via palladium-catalyzed Stille coupling, and its electronic and optical properties were investigated. The novel PPV derivative was characterized by NMR, UV-visible absorption, photoluminescence, gel permeation chromatography, infrared spectroscopy, and cyclic voltammetry (CV). The polymer with Mw of 27,800 and a polydispersity index of 2.6 is readily soluble in common organic solvents, such as THF, chloroform, and toluene. The fluorescence quantum yield of the polymer, determined against rhodamine 6G in dilute aqueous solutions, was 0.95. The HOMO and LUMO levels of SO2EH-PPV were calculated to be -6.0 and -3.61 eV, respectively. The results obtained by CV suggest that SO2EH-PPV is a strong electron acceptor polymer. Single layer stable polymer light-emitting diode devices with the configuration of (ITO/PEDOT:PSS/SO2EH-PPV polymer/Al) were fabricated exhibiting a green light emission.

Synthesis and characterization of a novel symmetrical sulfone-substituted polyphenylene vinylene (SO2EH-PPV) for applications in light emitting devices.

Single-walled carbon nanotubes (SWCNTs) exhibit excellent nonlinear optical (NLO) properties due to the delocalized π electron states present along their tube axis. Using the open aperture Z-scan method in tandem with X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, we demonstrate the simultaneous tailoring of both electronic and NLO properties of SWCNTs, from ultrafast (femtosecond) to relatively slow (nanosecond) timescales, by doping with a single substituent, viz., boron. SWCNTs were doped via a wet chemical method using B2O3, and the boron content and bonding configurations were identified using XPS. While in the ns excitation regime, the nonlinear absorption was found to increase with increasing boron concentration in the SWCNTs (due to the increasing disorder and enhanced metallicity of the SWCNTs), the saturation intensity in the fs excitation regime decreased. We attribute this counter-intuitive behavior to excited state absorption on ns timescales, and saturable absorption combined with weak two-photon transitions on fs timescales between van Hove singularities.

Nonlinear optical properties of boron doped single-walled carbon nanotubes.

We demonstrate the effects of nanostructure geometry on the nanoimprint induced poly(3-hexylthiophene-2,5-diyl) (P3HT) chain alignment and the performance of nanoimprinted photovoltaic devices. Out-of-plane and in-plane grazing incident X-ray diffraction techniques are employed to characterize the nanoimprint induced chain alignment in P3HT nanogratings with different widths, spacings and heights. We observe the dependence of the crystallite orientation on nanostructure geometry such that a larger width of P3HT nanogratings leads to more edge-on chain alignment while the increase in height gives more vertical alignment. Consequently, P3HT/[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) solar cells with the highest density and aspect ratio P3HT nanostructures show the highest power conversion efficiency among others, which is attributed to the efficient charge separation, transport and light absorption.

Effects of nanostructure geometry on nanoimprinted polymer photovoltaics.

In this work, we demonstrate the feasibility of using nanoimprint lithography (NIL) to make efficient low bandgap polymer solar cells with well-ordered heterojunction. High quality low bandgap conjugated polymer poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) nanogratings are fabricated using this technique for the first time. The geometry effect of PCPDTBT nanostructures on the solar cell performance is investigated by making PCPDTBT/C70 solar cells with different feature sizes of PCPDTBT nanogratings. It is found that the power conversion efficiency (PCE) increases with increasing nanograting height, PCPDTBT/C70 junction area, and decreasing nanograting width. We also find that NIL makes PCPDTBT chains interact more strongly and form an improved structural ordering. Solar cells made on the highest aspect ratio PCPDTBT nanostructures are among the best reported devices using the same material with a PCE of 5.5%.

Efficient Low Bandgap Polymer Solar Cell with Ordered Heterojunction Defined by Nanoimprint Lithography.

Magnesium diboride coated carbon nanotube arrays are synthesized by templating carbon nanotube aerogel sheets with boron and then converting the boron to MgB2 . The resultant MgB2 -CNT sheets are twisted into flexible, light-weight yarns that have a superconducting transition around 37.8 K and critical current and critical field comparable to existing MgB2 wires, but have about 20 times lower density than bulk MgB2 .

Article	Year
Carbon nanotubes--the route toward applications. Science (New York, N.Y.)\| PubMed ID: 12161643	2002
Strong, transparent, multifunctional, carbon nanotube sheets. Science (New York, N.Y.)\| PubMed ID: 16109875	2005
Robust cell migration and neuronal growth on pristine carbon nanotube sheets and yarns. Journal of biomaterials science. Polymer edition\| PubMed ID: 17939884	2007
Giant-stroke, superelastic carbon nanotube aerogel muscles. Science (New York, N.Y.)\| PubMed ID: 19299612	2009
Harvesting waste thermal energy using a carbon-nanotube-based thermo-electrochemical cell. Nano letters\| PubMed ID: 20170193	2010
Template synthesis of ordered arrays of mesoporous titania spheres. Chemical communications (Cambridge, England)\| PubMed ID: 20198237	2010
Aerosol-synthesized SWCNT networks with tunable conductivity and transparency by a dry transfer technique. Nano letters\| PubMed ID: 20863125	2010
Biscrolling nanotube sheets and functional guests into yarns. Science (New York, N.Y.)\| PubMed ID: 21212350	2011
Structural model for dry-drawing of sheets and yarns from carbon nanotube forests. ACS nano\| PubMed ID: 21294525	2011
Structure and process-dependent properties of solid-state spun carbon nanotube yarns. Journal of physics. Condensed matter : an Institute of Physics journal\| PubMed ID: 21386511	2010
Carbon nanotube/graphene nanocomposite as efficient counter electrodes in dye-sensitized solar cells. Nanotechnology\| PubMed ID: 22293392	2012
Nanoimprinted polymer solar cell. ACS nano\| PubMed ID: 22394246	2012
Synthesis and characterization of a novel symmetrical sulfone-substituted polyphenylene vinylene (SO2EH-PPV) for applications in light emitting devices. The journal of physical chemistry. B\| PubMed ID: 23167914	2013
Nonlinear optical properties of boron doped single-walled carbon nanotubes. Nanoscale\| PubMed ID: 23817830	2013
Effects of nanostructure geometry on nanoimprinted polymer photovoltaics. Nanoscale\| PubMed ID: 24890562	2014
Efficient Low Bandgap Polymer Solar Cell with Ordered Heterojunction Defined by Nanoimprint Lithography. ACS applied materials & interfaces\| PubMed ID: 25295533	2014
Flexible, Ultralight, Porous Superconducting Yarns Containing Shell-Core Magnesium Diboride-Carbon Nanotube Nanofibers. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 25319360	2014

Anvar Zakhidov

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