Session S18: Hybrid Organic-Inorganic Nanomaterials II: Assembly and Fabrication

Transparent Organic Field-Effect Transistors with Carbon Nanotube Electrodes

Carbon nanotube (CNT) films are promising as electrode material for organic field-effect transistors (OFETs). We have fabricated CNT films over large areas by airbrushing and patterned them either using shadow masks, or via photolithography and reactive ion etching. Pentacene thin film transistors bottom-contacted by CNT source/drain electrodes on Si/SiO$_{2}$ substrates show moderate mobility (up to 0.1 cm$^{2}$/Vs) and contact resistance as low as 20 k$\Omega $*cm. The contact resistance varies linearly with the gate voltage, eventually saturating at high negative gate voltage. OFETs were measured at temperatures from 228 to 353.5 K. After accounting for contact resistance, the intrinsic mobility of the transistors is found to be activated in temperature with activation energy between 0.137 and 0.151 eV in reasonable agreement with values in the literature. An all-carbon transparent OFET with CNT source, drain and gate electrodes and polymethylmethacrylate (PMMA) gate dielectric has been assembled on a polyethylene terephthalate substrate by transfer printing, with a field-effect mobility up to 0.06 cm$^{2}$/(Vs).   

Directed self assembly of macroscopic nanowires from single-wall carbon nanotubes suspended in aqueous bile-salt solutions

Length purified and chirality enriched single-wall carbon nanotubes (SWNTs) suspended in aqueous bile-salt solutions are found to spontaneously self assemble into macroscopically long straight nanowires, both in confined geometries and on patterned substrates. By patterning surfaces with ordered arrays of hydrophobic and hydrophilic regions, we tailor the self assembly of the nanowires for potential applications in the rapid and cheap fabrication of transparent films with strong directional conductivity. We report a phase diagram in the plane of SWNT concentration and bile-salt concentration that delineates regions of stable nanotube dispersion, bulk and surface phase separation, and nanowire self assembly. The predominant mechanisms for this phase behavior are identified as hydrogen-bonding interactions between contacted bile-salt micelles, which lead to the natural formation of macroscopic fibrils, and entropic depletion interactions mediated by free surfactant micelles.   

Time and Temperature Dependent Rheological Behavior of Single-Walled Carbon Nanotubes Dispersed in Thermoreversible Acrylic Copolymer {\&} Alcohol Solutions

SWCNT stabilized by A-B diblock and A-B-A triblock copolymers are excellent model systems for studying the relationship between nanotube dispersion and mechanical response. We have investigated the mechanical properties of these materials with low-amplitude oscillatory shear rheological measurements. The solvent used here, 2-ethyl-1-hexanol, is a poor solvent for PMMA (A) at low temperatures but a good solvent for PnBA (B) over the entire temperature range studied. The solubility of the PMMA blocks in 2-ethyl-1-hexanol drives the formation of an elastic gel in the ABA triblock copolymer at low temperatures. In these SWCNT/copolymer materials the storage and loss moduli have been observed to increase with time at fixed temperature. When triblock copolymer gels are used as the matrix, we find that the aging effect is erased by cycling the temperature through the gel transition. An increase in storage modulus is observed upon cooling before the gel formation. However, the moduli revert back to lower values when the gel dissolves on heating. We believe this is a result of semi-permanent nanotube junctions being pulled apart when the gel forms. This reversibility is not observed when the nanotubes are dispersed in solutions of diblock copolymer, which do not form gels.   

Forces between nanorods with end-adsorbed chains in polymer melts

Adsorbed or grafted polymers are often used to provide steric stabilization of colloidal particles. When the particle size approaches the nanoscale, the curvature of the particles becomes relevant. Here I use a classical density functional theory to study the polymer-mediated interactions between two nanorods. The rods are immersed in an athermal, melt polymer blend consisting of: 1) a small fraction of chains of length N=20 with ``sticky'' ends that are attracted to the rods with energy e/kT so that they form a polymer brush on the rods; and 2) a matrix of chains of length P which have no interactions with the rods. The structure of the brushes depends on the nanorod diameter, P, and e/kT. There is an attractive well in the force between the rods near contact, followed by a strong repulsion as the brushes are compressed. The depth of the well increases with increasing P. I will discuss the implications for experimental systems. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Shape and size selection of Au nanorods by reversible flocculation.

For gold nanorods (GNRs) synthesized by wet chemical method, spherical particles as a byproduct is inevitable. Efficient and rapid approaches to separate the NRs are critical to optimize the nanostructure-dependent optical properties and not bias conclusions due to existence of spherical impurities. Relatively monodisperse GNRs can be separated from smaller size spheres through repeated centrifugation steps. The number of centrifugation steps, though, must be minimized to avoid irreversible aggregation of rods due to the loss of their capping surfactant. As an alternative, we demonstrate that size selection of GNRs can be achieved by the formation of controlled flocculates of GNRs, driven by an attractive depletion interaction between the NRs induced by surfactant micelles above a critical concentration. The flocculates sediment after a few hours. Separation yields as high as 90 {\%} in number of particles were obtained without any damage to the surface of NRs. Flocculation is reversible upon varying the concentration of surfactant. The effect of the aspect ratio of rods and surfactant micelle structure on the flocculation is discussed with regards to concentration, type and mixing ratio of binary surfactants. The effect of electrostatic interaction is also considered through the impact of different types of electrolytes.   

Industrial viable process of making nanoparticles of various shapes and interior structures

Over the past 10 years, we attempted to develop industrial viable processes which were of significance in manufacturing the nanoparticles in good quality and large volume. Our effort relied on the self-assembly concepts of block macromolecules in solutions to prepare particles with a hard core made of crosslinked plastics and a soft shell made of low Tg elastomer. Depending on the type and microstructure of the copolymers, the solvent concentration and other process parameters chosen, a variety of shell-core nano-particles of different shapes (spheres, hollow spheres, ellipsoids, cylinders, linear and branched strings, disks and etc.) and sizes (5-100 nm diameter) were reproducibly synthesized. Scale-up studies led to an optimization of the manufacturing process and the production of nanoparticles in large quantities for various product application efforts. The unique performance of those nanoparticles as performance tuning additives and novel rubber reinforcing elements was explored in rubber compounds. This review describes the synthesis methods used to produce the polymer nanoparticles, the technology to modify the particles through functionalization, the means to optimize their performance for specific applications, and the methods to use those particles in rubber compounds. Collaborators: Victor J. Foltz, Kurasch Jessica, Chenchy J. Lin, Jeff Magestrelli, Sandra Warren, Alberto Scuratti, James E. Hall, Jim Krom, Mindaugas Rackaitis, Michael W. Hayes, Pat Sadhukhan, Georg G. A. Bohm   

Performance of ZnO nanowire-based hybrid solar cells decorated with CdTe quantum dots deposited by a pulsed electron beam technique

More efficient nanocomposite hybrid solar cells require facile charge transport to the collecting electrodes as well as photon-to-electron conversion over the broad solar spectrum. In this work, we employ ZnO nanowires (ZnO-NW) as direct conduction pathways for electrons to the collecting electrode by growing them directly on ITO-coated glass substrates. Photovoltaic nanocomposites are then formed by spin-coating of poly(3-hexylthiophene). By decorating the ZnO-NW with CdTe quantum dots deposited by a pulsed electron beam technique, we have extended the photon-to-electron conversion sensitivity beyond 600 nm.   

Organic and Carbon-based Thin-film Transistors on Flexible Substrates.

Fabrication of organic {\&} carbon-based thin-film transistors (TFT) was achieved on plastic substrates using transfer printing. Each device component (Au electrodes, polymer dielectric layer and semiconductor layer) was printed using only pressure and temperature, eliminating all chemical processing on the device substrate. Pentacene (Pn), poly(3-hexylthiophene) (P3HT), carbon nanotube mats (CNTM) and graphene TFTs were all fabricated on polyethylene terephthalate (PET) substrates, yielding mobilities of 0.237 cm$^{2}$/Vs for Pn and 0.04 cm$^{2}$/Vs for P3HT. Bottom-gate CNTM TFTs are p-type, with mobilities of 13.7 cm$^{2}$/Vs, on/off ratio of 10$^{3}$ and minimal hysteresis. Top-gate graphene TFTs have mobilities of 1.0x10$^{4}$ cm$^{2}$/Vs for holes and 4x10$^{3}$ cm$^{2}$/Vs for electrons. P3HT TFTs showed little variation in mobility, but strong variation in threshold voltage for different dielectric layers. These TFTs printed onto plastic substrates with a variety of polymer dielectric layers will be presented and discussed..   

Self-assembled contacts to nanoparticles using metallic Ga droplets

We demonstrate a pragmatic approach to forming electronic materials and devices, in which metal droplets serve as electrodes and their spacing is controlled spontaneously, \textit{via} self-assembly, to allow tunneling contact with nanoparticles. We have fashioned devices consisting of droplets of molten metal (Ga). Ga is suspended in acidic solution. Ligand-stabilized Au nanoparticles in solution assemble on the metal surface, as shown by electron microscopy. Coated droplets which are then placed on a substrate and the solvent removed. Electron-transport measurements reveal the Coulomb blockade, in which current is suppressed below a tunable threshold voltage by the energy of charging individual nanoparticles. The threshold voltage for two different sizes of nanoparticles agrees with theory. Our approach provides a straightforward approach to creating nanoscale-precision contacts to nanoparticles and might lead to formation of a large number of microscopic devices from suspension.   

ATRP of MMA on Asymmetrically Functionalized Gold Nanoparticles

Metal nanoparticles have attracted enormous interest due to their unique optical and electronic properties. After the pioneer work of Brust and Schiffrin, a lot of reports have been focused on the modification of the surface of metal nanoparticles with functional groups. However, it still remains a challenging task to synthesize Janus metal nanoparticles, which could potentially leads to directed assembly of the functionalized nanoparticles, an essential step towards using these nanoparticles in real world applications. Asymmetrical gold nanoparticles (AuNPs) modified with two different kinds of polymers on the opposite sides were synthesized using poly(ethylene oxide) single crystals with thiol end groups as the substrate. After the immobilization of AuNPs, room temperature ATRP was performed using the `grafting from' method to obtain asymmetrical AuNPs, which possess the `Janus' nature: i.e. two types of polymer chains were selectively pattered on the different locations of the AuNPs. The asymmetric nature of these AuNPs was demonstrated by NP decoration.   

DNA guided assembly of well-organized nano-architectures.

An incorporation of DNA in nano-object design provides a unique opportunity to establish reversible, chemically weak and highly selective interactions between the components of nanosystems. Assembly approaches based on this addressable interactions promise a possibility for creation of rationally designed multicomponent system. However, understanding interplay of interactions, cooperative phenomena leading to phase formation and experimental realizations of ordered phases has remained elusive. Using in-situ x-ray scattering methods, we have studied an assembly kinetics, structure development, and phase formation of DNA-capped nanoparticles on surfaces and in bulk for various DNA assembly schemes. The observed changes in the 2D DNA/nanoparticle array layer reveal an evolution of particle-surface separations and surface coverages. For 3D systems, formation of 3D assemblies with crystalline long-range order in two-component nanoparticle systems was observed. The DNA design, assembly schemes and thermodynamic pathway leading to this crystallization has been explored.   

Schottky nanodiodes based on electrospun polymer nanofibers: Effect of varying fiber diameter

We report on a simple method to fabricate, under ambient conditions and within seconds, Schottky nanodiodes using electrospun polyaniline nanofibers and an inorganic $n$-doped semiconductor. The objective of the present work is to investigate the role of surface states on the device operation by fabricating Schottky nanodiodes using fibers of varying diameter. The standard thermionic emission model of a Schottky junction was utilized in analyzing the data. As the fiber diameter gets smaller, the diode rectification ratio and the diode turn-on voltage shifts to lower values, while the diode barrier height and the ideality factor increase. The simple construction and high surface to volume ratio of the nanofiber also makes these devices attractive candidates in the potential fabrication of low power, supersensitive and rapid response reusable sensors.   

Effects of severe confinement on the structure and dynamics in polymer nanocomposites

The structure and dynamics of PEO/Na$^{+}$MMT nanocomposites is investigated by XRD, DSC, Raman spectroscopy, and quasi-elastic neutron scattering (QENS). For concentrations up to 20 wt\% PEO, the PEO chains within the galleries form either a single- or a double-layer structure of intercalated chains; at higher PEO content only double-layers of intercalated PEO chains are formed within the 0.9nm galleries. For PEO content below 70 wt\%, the absence of XRD peaks that can be assigned to crystalline PEO and of any DSC melting transition as well as the observation of broad Raman lines reveal that the PEO chains remain liquid-like. It is only for PEO content higher than 70 wt\% that diffraction peaks characteristic of bulk PEO are observed together with sharp Raman lines, proving crystallization of only the excess polymer outside the completely full galleries. QENS investigated the dynamics of PEO in bulk and in confinement. A jump of the bulk PEO dynamics at $T_{m}$ is observed whereas the dynamics of confined PEO shows only weak temperature dependence and goes smoothly through the bulk $T_{m}$. Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU.   

pH and Protein Sensing with Functionalized Semiconducting Oxide Nanobelt FETs

We report solution pH sensing and selective protein detection with high-performance channel-limited field-effect transistors (FETs) based on single semiconducting oxide (ZnO and SnO$_{2})$ nanobelts$^{1}$. The devices were integrated with PDMS microfluidic channels for analyte delivery and the source/drain contacts were passivated for in-solution sensing. pH sensing experiments were performed on FETs with functionalized and unmodified nanobelts. Functionalization of the nanobelts by APTES was found to greatly improve the pH sensitivity. The change in nanobelt conductance as functions of pH values at different gate voltages and ionic strengths showed high sensitivity and consistency. For the protein detection, we achieved highly selective biotinylation of the nanobelt channel with through APTES linkage. The specific binding of fluorescently-tagged streptavidin to the biotinylated nanobelt was verified by fluorescence microscopy; non-specific binding to the substrate was largely eliminated using PEG-silane passivation. The electrical responses of the biotinylated FETs to the streptavidin binding in PBS buffers of different pH values were systematically measured. The results will be presented and discussed. $^{1}$Y. Cheng et al., Appl. Phys. Lett. \textbf{89}, 093114 (2006). *Supported by NSF NIRT Grant ECS-0210332.   

On the Miscibility of Polymer / Layered Silicate Nanocomposites

In the present work we attempt to control the structure in polymer / layered silicate nanocomposites by understanding and / or altering the interactions between the chains and the surfaces. In this respect, hydrophilic and organophilic systems have been utilized and the final structure of the composites is characterized by X-Ray Diffraction and Transmission Electron Microscopy. The effect of the solvent quality on the final structure, in the case of solution mixing, has been examined and the results are compared with the respective obtained from melt intercalation whereas the role of the chemical structure or of the different glass transition temperature of the polymer has been evaluated. In the case of very immiscible systems like for example polyolefin/silicate composites the effect of a more polar additive has been examined. Phase separated up to exfoliated structures can be obtained in a controlled way by varying the compatibilizer to organoclay ratio. Sponsored by NATO's Scientific Affairs Division, by the Greek GSRT and by the EU.   

Magnetic fluorescent particles with polypeptide shell

Magnetic fluorescent particles with a hydrophobic polypeptide shell were synthesized and characterized. The first step was the preparation of an iron oxide magnetic core from ferric chloride and ferrous chloride in presence of ammonium hydroxide. A silica shell was grown on this central nougat by the St\"{o}ber method. In order to introduce fluorescence, a mixture of tetraethoxy silane, a complex of fluorescein isothiocyanate (FITC) with 3-aminopropyl triethoxy silane (APTES), ammonia, and ethanol were added. These particles were further functionalized to place amine groups on the surface. Polypeptide chains were grown from the amine initiators by ring opening polymerization of the N-carboxyanhydride of the glutamate. These particles were characterized by light scattering, transmission electron microscopy, x-ray photoelectron spectroscopy, infrared spectroscopy and fluorescence spectroscopy.