Yunnan Fang

Solvent and polymer matrix effects on TIPS-pentacene/polymer blend organic field-effect transistors

We report on a systematic study of solvent and polymer matrix effects on the phase segregation behavior of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) blends incorporated into two different amorphous polymer matrices, poly (α-methyl styrene) and poly (triarylamine), and using two solvents, chlorobenzene and tetralin. Optical microscopy, X-ray diffraction analyses, and optical absorption measurements are used to evaluate the film morphology, crystallinity, and optical density, respectively. These analyses are correlated with the extent of vertical segregation of TIPS-pentacene, as observed for the blended films by depth-profile XPS analyses. The microstructure and vertical phase segregation of TIPS-pentacene in blend films are found to be strongly influenced by the choice of solvent. Tetralin, a solvent with a high boiling temperature, was found to be more desirable for achieving distinct phase segregation/crystallization of TIPS-pentacene in blend films and best performance in OFETs with a dual-gate geometry. The electrical properties of top and bottom channels were consistent with the morphological characterization and OFETs processed from tetralin showed higher mobility values than those from chlorobenzene. Further modification of the annealing conditions in the TIPS-pentacene/PTAA/tetralin ternary system led to top-gate OFETs with mobility values up to 2.82 cm2/Vs.

Inkjet Catalyst Printing and Electroless Copper Deposition for Low-Cost Patterned Microwave Passive Devices on Paper

A scalable, low-cost process for fabricating copper-based microwave components on flexible, paper-based substrates is demonstrated. An inkjet printer is used to deposit a catalyst-bearing solution (tailored for such printing) in a desired pattern on commercially-available, recyclable, non-toxic (Teslin®) paper. The catalystbearing paper is then immersed in an aqueous copper-bearing solution to allow for electroless deposition of a compact and conformal layer of copper in the inkjet-derived pattern. Meander monopole antennas comprised of such electroless-deposited copper patterns on paper exhibited comparable performance as for antennas synthesized via inkjet printing of a commercially-available silver nanoparticle ink. However, the solution-based patterning and electroless copper deposition process avoids nozzle-clogging problems and costs associated with noble metal particle-based inks. This process yields compact conductive copper layers without appreciable oxidation and without the need for an elevated temperature, post-deposition thermal treatment commonly required for noble metal particle-based ink processes. This low-cost copper patterning process is readily scalable on virtually any substrate and may be used to generate a variety of copper-based microwave devices on flexible, paper-based substrates.

Identification of peptides capable of inducing the formation of titania but not silica via a subtractive bacteriophage display approach

A “subtractive” bacteriophage display biopanning approach has been used to identify 12-mer peptides that are capable of binding selectively to, and inducing the precipitation of, titania but not silica. Unlike the conventional bacteriophage display procedure, this approach consists of two selection steps: i) removal of phage particles containing silica-binding peptides from the phage library (the subtractive step), and then ii) isolation of phage particles bearing peptides that bind strongly to titania. While prior phage display biopanning with silica and titania targets has led to the isolation of polycationic peptides enriched in basic residues, the subtractive biopanning process yielded several acidic peptides enriched in hydroxyl-bearing residues. These peptides were found to induce the precipitation of titania, but not silica, from aqueous precursor solutions at pH 3–8. To our knowledge, this is the first demonstration that a subtractive bacteriophage biopanning process may be used to identify specific peptides possessing the ability to induce the formation of one oxide while lacking the ability to form a second, chemically similar oxide.

Structure-based optical filtering by the silica microshell of the centric marine diatom Coscinodiscus wailesii

Diatoms are a renewable (biologically reproducible) source of three-dimensional (3-D) nanostructured silica that could be attractive for a variety of photonic devices, owing to the wide range of quasi-periodic patterns of nano-to-microscale pores available on the silica microshells (frustules) of various diatom species. We have investigated the optical behavior of the silica frustule of a centric marine diatom, Coscinodiscus wailesii, using a coherent broadband (400-1700 nm) supercontinuum laser focused to a fine (20 µm diameter) spot. The C. wailesii frustule valve, which possessed a quasi-periodic hexagonal pore array, exhibited position-dependent optical diffraction. Changes in such diffraction behavior across the frustule were consistent with observed variations in the quasi-periodic pore pattern.

Syntheses of nanostructured Cu- and Ni-based micro-assemblies with selectable 3-D hierarchical biogenic morphologies

A combined layer-by-layer (LbL) surface amine amplification and electroless deposition process has been developed to convert biologically replicable three-dimensional (3-D) nanostructured micro-assemblies (such as siliceous diatom frustules) into freestanding Cu-bearing or Ni-bearing structures that retain the starting biogenic microscale 3-D shapes and nanoscale patterns. After reacting the hydroxyl-bearing surfaces of these biotemplates with an aminosilane, a LbL polyacrylate/polyamine deposition process was used to dendritically amplify the surface amine concentration. Subsequent binding of metal chloride catalysts to these amine-enriched surfaces enabled the rapid electroless deposition of thin, conformal, continuous, and nanocrystalline or amorphous metallic coatings on the 3-D biotemplates. Selective removal of the underlying templates then yielded freestanding Cu-bearing or Ni-bearing structures. The conformality and continuity of the thin coatings, and the fidelity with which the biogenic shape and fine features were preserved in the freestanding structures, were significantly enhanced by the amplification of surface amines (and the associated enrichment of catalytic sites) resulting from the LbL polyacrylate/polyamine treatment. Monolithic and multicomponent structures (e.g., Cu, multilayer Au/Cu, CuO, and Ni–P alloy) with bio-derived morphologies have been synthesized utilizing this approach. This readily-scalable process may be used to convert self-assembled rigid templates (of biological or synthetic origin) into nanostructured transition metal-based micro-assemblies with a wide variety of selectable 3-D hierarchical morphologies for use in numerous functional and structural applications.

Fully inkjet-printed microfluidics: a solution to low-cost rapid three-dimensional microfluidics fabrication with numerous electrical and sensing applications

As the needs for low-cost rapidly-produced microfluidics are growing with the trend of Lab-on-a-Chip and distributed healthcare, the fully inkjet-printing of microfluidics can be a solution to it with numerous potential electrical and sensing applications. Inkjet-printing is an additive manufacturing technique featuring no material waste and a low equipment cost. Moreover, similar to other additive manufacturing techniques, inkjet-printing is easy to learn and has a high fabrication speed, while it offers generally a great planar resolution down to below 20 µm and enables flexible designs due to its inherent thin film deposition capabilities. Due to the thin film feature, the printed objects also usually obtain a high vertical resolution (such as 4.6 µm). This paper introduces a low-cost rapid three-dimensional fabrication process of microfluidics, that relies entirely on an inkjet-printer based single platform and can be implemented directly on top of virtually any substrates.

Inverted polymer solar cells with amorphous indium zinc oxide as the electron-collecting electrode

We report on the fabrication and performance of polymer-based inverted solar cells utilizing amorphous indium zinc oxide (a-IZO) as the electron-collecting electrode. Amorphous IZO films of 200 nm thickness were deposited by room temperature sputtering in a high-purity argon atmosphere. The films possessed a high optical transmittance in the visible region (≥ 80%), a low resistivity (3.3 × 10⁻⁴ Ωcm), a low surface roughness (root mean square = 0.68 nm), and a low work function (4.46 ± 0.02 eV). Inverted solar cells with the structure a-IZO/P3HT: PCBM/PEDOT:PSS/Ag exhibited a power conversion efficiency of 3% estimated for AM 1.5G, 100 mW/cm² illumination.

Morphology-Preserving Conversion of a 3D Bioorganic Template into a Nanocrystalline Multicomponent Oxide Compound†

The development of a robust process for generating functional assemblies possessing complex (three-dimensional, 3D) morphologies with well-controlled patterns of fine (micro-tonanoscale) features along with complex (multicomponent) and tailorable inorganic compositions remains a considerable challenge. One strategy for such versatile fabrication is to convert a 3D micro/nanostructured solid template, generated through a biological or synthetic self-assembly process, into a new inorganic composition by a morphology-preserving chemical transformation process; that is, to decouple the 3D template formation and chemical tailoring processes. The purpose of this paper is to introduce a broadly applicable bottom-up approach for chemical conversion of intricate 3D nanostructured (bio)organic templates into positive replicas comprised of multicomponent oxide compounds. This general process consists of the following steps: 1) the generation of a thin conformal coating on a 3D (bio)organic template by a layer-by-layer (LbL) process, 2) pyrolysis of the template and collapse of the thin coating into the space previously occupied by the prior solid (bio)organic material, and 3) conversion of the resulting inorganic structure into a nanocrystalline multicomponent oxide compound by a low-temperature hydrothermal reaction. This work reveals, for the first time, how a 3D (bio)organic template may be converted into a positive replica comprised of a nanocrystalline multicomponent oxide compound by coupling the precise structural (sub-nanometer) and versatile chemical control of conformal coatings provided by an automated surface sol–gel (SSG) process with the lowtemperature compound formation provided by microwave hydrothermal (MWHT) processing. To our knowledge, no prior report exists of the use of hydrothermal processing to convert conformally coated intricate 3D organic structures into functional multicomponent oxide replicas. This shape-preserving process has been demonstrated by converting the 3D micro/nanostructured wing scales of a Morpho helenor butterfly into tetragonal barium titanate (BaTiO3) replicas. M. helenor wing scales are comprised of a natural polysaccharide, chitin, that contains an abundance of hydroxy groups for reaction with metal alkoxide precursors used in the SSG process. 4] Barium titanate was selected as a representative multicomponent oxide product owing to the extensive use of compositions based on this perovskite compound in a variety of applications (e.g., multilayer capacitors, thermistors, microwave dielectrics). Secondary electron (SE) images revealing the morphology of the scales present on the brown side of a female M. helenor butterfly are shown in Figure 1 a. As seen in the lower magnification inset image, the scales possess an overall rectangular shape with pointed tips. The higher-magnification image reveals parallel ridges that are aligned with the long dimension of the scale. Adjoining ridges are spaced several micrometers apart and are connected by perpendicular struts. The scales also contain parallel ribs spaced about 150 nm apart. Thin, conformal, and continuous titania-bearing coatings were applied to these nanostructured scales in a computer-controlled LbL SSG process. Each cycle of this process involved exposure to a titanium(IV) isopropoxide solution in anhydrous isopropyl alcohol (IPA), washing with IPA, exposure to a solution of 40 vol% IPA in water, washing again with IPA, and then drying with warm flowing nitrogen.

Graphene enhanced wireless sensors

In this paper we demonstrate the design and development of a family of low-cost, self-powered, wireless sensor solutions utilizing both analog and digital principles. The sensors will utilize Graphene-based thin films integrated directly into the structure. For an immediately deployable digital sensing solution compatible with current commercial technologies we will utilize the Intel WISP platform, which can be read with current COTS products. Our thin films are produced from water-based, inkjet printed graphene oxide (GO) on paper/Kapton, developed using both conventional thermal and laser reduction techniques. In addition to reporting the first ever integration of inkjet-printed water soluble GO inks into low cost, flexible RF electronics, we also bring gas sensing capabilities to RFID tags relying on purely wireless digital transmission. The introduction of low cost, mass producible, eco-friendly, reduced graphene oxide (RGO) films on paper substrates lays the foundation for the development of a wide range of new low-cost, high performance Graphene-based electronic devices.

A novel, facile, layer-by-layer substrate surface modification for the fabrication of all-inkjet-printed flexible electronic devices on Kapton

A facile, environmentally-friendly, low-cost, and scalable deposition process has been developed and automated to apply polyelectrolyte multilayers (PEMs) on flexible Kapton HN substrates. Two weak polyelectrolytes, poly(acrylic acid) and polyethylenimine, were deposited in an alternating, layer-by-layer fashion under controlled pH and ionic strength. Compared to strong polyelectrolytes, weak electrolytes can control the properties of the PEMs more systmatically and simply. To our knowledge, this work on surface modification of Kapton is not only the first to use only weak polyelectrolytes, but is also the first to take advantage of the surface properties of calcium-bearing additive particles present in Kapton HN. The resulting surface-modified Kapton HN substrate allowed for the inkjet printing of water-based graphene oxide (GO) inks and organic solvent-based inks with good adhesion and with desired printability. While the deposition of a single PEM layer on a Kapton substrate significantly reduced the water contact angle and allowed for the inkjet-printing of GO inks, the deposition of additional PEM layers was required to maintain the adhesion during post-printing chemical treatments. As a conceptual demonstration of the general applicability of this PEM surface modification approach, a flexible, robust, single-layered gas sensor prototype was fully inkjet printed with both water- and ethanol-based inks and tested for its sensitivity to diethyl ethylphosphonate (DEEP), a simulant for G-series nerve agents. The electrical conductivity and morphology of the sensor were found to be insensitive to repeated bending around a 1 cm radius.