James J. Watkins

Nanoparticle-Driven Assembly of Block Copolymers: A Simple Route to Ordered Hybrid Materials

The addition of nanoparticles that selectively hydrogen bond with one of the segments of a block copolymer is shown to induce order in otherwise disordered systems. This enables the fabrication of well-ordered hybrid materials with spherical, cylindrical, or lamellar domains at particle loadings of more than 40%, as evidenced by TEM and SAXS. The approach described is simple and applicable to a wide range of nanoparticles and block copolymers, and it lays the groundwork for the design of cooperatively assembled functional devices.

Additive-driven assembly of block copolymer-nanoparticle hybrid materials for solution processable floating gate memory.

Floating gate memory devices were fabricated using well-ordered gold nanoparticle/block copolymer hybrid films as the charge trapping layers, SiO(2) as the dielectric layer, and poly(3-hexylthiophene) as the semiconductor layer. The charge trapping layer was prepared via self-assembly. The addition of Au nanoparticles that selectively hydrogen bond with pyridine in a poly(styrene-b-2-vinyl pyridine) block copolymer yields well-ordered hybrid materials at Au nanoparticle loadings up to 40 wt %. The characteristics of the memory window were tuned by simple control of the Au nanoparticle concentration. This approach enables the fabrication of well-ordered charge storage layers by solution processing, which is extendable for the fabrications of large area and high density devices via roll-to-roll processing.

Relating chemical structure to device performance via morphology control in diketopyrrolopyrrole-based low band gap polymers.

We investigated the structure-morphology-performance relationship of diketopyrrolopyrrole (DPP)-based low band gap polymers with different donor cores in organic field effect transistors (OFETs) and organic photovoltaics (OPVs). The change in the chemical structure led to strong physical property differences, such as crystalline behavior, blend morphology, and device performance. In addition, the choice of solvents and additives enabled one to fine tune the properties of these materials in the condensed state. For instance, when thin films were processed from solvent mixtures, both in the pure polymer and in a blend, we observed an enhanced edge-on orientation and the formation of thinner and longer polymer fibrils. In the BHJ blends, processing from a solvent mixture reduced the size scale of the phase separation and promoted the formation of a fibrillar network morphology, having a polymer-PCBM mixture filling the interfibrillar regions. The characteristic length scale of the fibrillar network dictated the specific inner surface area, which directly correlated to the performance in the OPV devices. When the BHJ mixture was processed from a single solvent, a large-scale phase separated morphology was observed that was stratified, normal to the film surface. A strong scattering anisotropy was observed in the resonant soft X-ray scattering of the blends that provided insight into the packing of the polymer chains within the fibrils. The morphology and performance trend in OPVs paralleled the performance in an OFET, suggesting that similar processing conditions should be considered in OFET fabrication.

Supercritical fluids for the fabrication of semiconductor devices: emerging or missed opportunities?

Next generation semiconductor devices and evolving opportunities in nanoelectronics present new challenges for fabrication technology. These include patterning and structure generation to define the smallest features, the deposition of metals, metal oxides, and other functional layers within challenging topographies, and development of the associated integration steps necessary to create working devices. Implementation of emergent technical solutions, however, is also subject to economic realities that require high volume process tools, reliability, and low cost per device layer. Changes in semiconductor process technology have to date largely been driven by the continued downscaling of device features to increase transistor density. Leading edge microprocessors are currently in production at the 45 nm device node. Device dimensions approaching 22 nm will be realized withinin the next several years. As manufacturing moves to smaller and smaller features, it is necessary to re-evaluate process technology and decide if evolutions in current methods are sufficient or if technical and/or economic considerations will mandate change. Semiconductor fabrication relies on a combination of gas (or vapor) based techniques and liquid phase processing. Each has its advantages. Gas phase techniques allow for dry processing, complete wetting of surfaces, and the absence of surface tension, which in turn allows facile transport of reagents into confined geometries. Vapor phase transport of reagents and processing aids, however, are subject to species volatility constraints. Liquid phase processes have the advantage of species transport in solution, but the presence of the liquid phase can give rise to contamination issues, sluggish mass transport, and difficulties such as pattern collapse for the smallest device features. The latter arises from surface tension and associated capillary forces. These limitations are especially relevant for processing and fabrication at the nanoscale.

Deposition of Cu films from supercritical fluids using Cu(I) β-diketonate precursors

Cu films were deposited onto planar and etched silicon substrates by the hydrogen-assisted reduction of a series of copper(I)(1,1,1,5,5,5-hexafluoro-2,4-acetylacetonate)L compounds [Cu(I)(hfac)L] where L is (2- butyne), (1,5-cyclooctadiene), (vinyltrimethylsilane) or (2-methyl-1-hexene-3-yne) in supercritical solvents using a cold wall, high pressure reactor. At substrate temperatures of 225 °C and a pressure of 200 bar, H2 reduction of Cu(I)(hfac)(2-butyne) in CO2 yielded high purity films with exceptional step coverage in features as narrow as 100 nm with an aspect ratio of eight. Planar films deposited by H2 reduction of each of the precursors in supercritical CO2 and Cu(I)(hfac)(2-butyne) in supercritical C2F6 exhibited little carbon or oxygen contamination. Attempts to deposit high purity Cu films by the thermal disproportionation of the Cu(I)(hfac)L precursors in supercritical CO2 in the absence of H2 at 225 °C were unsuccessful and yielded highly oxidized Cu films in each case. Thermal disproportionation of Cu(I)(hfac)(2-butyne) in supercritical C2F6 also yielded poor quality films, although oxygen contamination was less severe than in CO2.

Large-area, continuous roll-to-roll nanoimprinting with PFPE composite molds.

Successful implementation of a high-speed roll-to-roll nanoimprinting technique for continuous manufacturing of electronic devices has been hindered due to lack of simple substrate preparation steps, as well as lack of durable and long lasting molds that can faithfully replicate nanofeatures with high fidelity over hundreds of imprinting cycles. In this work, we demonstrate large-area high-speed continuous roll-to-roll nanoimprinting of 1D and 2D micron to sub-100 nm features on flexible substrate using perfluoropolyether (PFPE) composite molds on a custom designed roll-to-roll nanoimprinter. The efficiency and reliability of the PFPE based mold for the dynamic roll-to-roll patterning process was investigated. The PFPE composite mold replicated nanofeatures with high fidelity and maintained superb mold performance in terms of dimensional integrity of the nanofeatures, nearly defect free pattern transfer and exceptional mold recovering capability throughout hundreds of imprinting cycles.

Triisopropylsilylethynyl-functionalized dibenzo[def,mno]chrysene: a solution-processed small molecule for bulk heterojunction solar cells

This communication reports the synthesis of a new polycyclic aromatic hydrocarbon and its unique packing motif. This molecule is shown to be an efficient electron donor in organic bulk heterojunction solar cells, exhibiting a power conversion efficiency of [similar]2.0%.

Controlled supramolecular self-assembly of large nanoparticles in amphiphilic brush block copolymers.

To date the self-assembly of ordered metal nanoparticle (NP)/block copolymer hybrid materials has been limited to NPs with core diameters (D(core)) of less than 10 nm, which represents only a very small fraction of NPs with attractive size-dependent physical properties. Here this limitation has been circumvented using amphiphilic brush block copolymers as templates for the self-assembly of ordered, periodic hybrid materials containing large NPs beyond 10 nm. Gold NPs (D(core) = 15.8 ± 1.3 nm) bearing poly(4-vinylphenol) ligands were selectively incorporated within the hydrophilic domains of a phase-separated (polynorbornene-g-polystyrene)-b-(polynorbornene-g-poly(ethylene oxide)) copolymer via hydrogen bonding between the phenol groups on gold and the PEO side chains of the brush block copolymer. Well-ordered NP arrays with an inverse cylindrical morphology were readily generated through an NP-driven order-order transition of the brush block copolymer.

Fractionation of high density polyethylene in propane by isothermal pressure profiling and isobaric temperature profiling

Abstract High density polyethylene was fractionated with respect to molecular weight and dissolution temperature in supercritical and near-critical propane. Isothermal pressure profiling in the liquid (polymer)-super-critical fluid regime resulted in 14 fractions with narrow polydispersity. The liquid-crystal phase separation technique of Pennings was extended from organic solvents to compressed propane for fractionation with respect to dissolution temperature and therefore crystallizability in the semicrystalline solid-supercritical fluid regime by isobaric temperature profiling above the second critical endpoint pressure. The influence of molecular weight, crystalline content, and mass transfer limitations is discussed. The authors believe this is the first time a crystal phase fractionation has been reported in supercritical fluids. The process is suggested as an alternative to temperature rising elution fractionation (TREF) developed to fractionate linear low density polyethylene on the basis of short chain branching.

Solution Processable High Dielectric Constant Nanocomposites Based on ZrO2 Nanoparticles for Flexible Organic Transistors

A solution-based strategy for fabrication of high dielectric constant (κ) nanocomposites for flexible organic field effect transistors (OFETs) has been developed. The nanocomposite was composed of a high-κ polymer, cyanoethyl pullulan (CYELP), and a high-κ nanoparticle, zirconium dioxide (ZrO2). Organic field effect transistors (OFETs) based on neat CYELP exhibited anomalous behavior during device operation, such as large hysteresis and variable threshold voltages, which yielded inconsistent devices and poor electrical characteristics. To improve the stability of the OFET, we introduced ZrO2 nanoparticles that bind with residual functional groups on the high-κ polymer, which reduces the number of charge trapping sites. The nanoparticles, which serve as physical cross-links, reduce the hysteresis without decreasing the dielectric constant. The dielectric constant of the nanocomposites was tuned over the range of 15.6-21 by varying the ratio of the two components in the composite dielectrics, resulting in a high areal capacitance between 51 and 74 nF cm(-2) at 100 kHz and good insulating properties of a low leakage current of 1.8 × 10(-6) A cm(-2) at an applied voltage of -3.5 V (0.25 MV cm(-1)). Bottom-gate, top-contact (BGTC) low operating voltage p-channel OFETs using these solution processable high-κ nanocomposites were fabricated by a contact film transfer (CFT) technique with poly(3-hexylthiophene) (P3HT) as the charge transport layer. Field effect mobilities as high as 0.08 cm(2) V(-1) s(-1) and on/off current ratio of 1.2 × 10(3) for P3HT were measured for devices using the high-κ dielectric ZrO2 nanocomposite. These materials are promising for generating solution coatable dielectrics for low cost, large area, low operating voltage flexible transistors.