Marks focused on the design, synthesis, and utilization of new precursors for efficient MOCVD growth of TCOs, including ligands which encapsulate metal ions of interest in protective, volatility-enhancing, non-polar ligation–especially challenging for ions having large radius:charge ratios. Marks demonstrated successful epitaxial growth of CdO, and MxCd1-xO (M = Sc+3, Y+3, Ga+3, Sn+4, Mo+6). A combined microstructural, charge transport, and theoretical study with Dravid, Kannewurf, and Freeman led to new guidelines for the design of high-performance TCOs. With Hersam (IRG#3), Marks applied conductive scanning probe techniques for characterization of TCO surfaces as a function of growth and cleaning procedure. This work shows now evidence for “hot spots” and “dead spots” on the surface, and that scanning probe techniques can be used to “write” nanoscale patterns on TCO surfaces. Marks also developed new families of volatile copper precursors for MOCVD growth of Cu2S thin films for photovoltaic applications, which are the most conductive to date.

Mason showed (in collaboration with Klein- Darmstadt) that indium oxide and indium-tin oxide (ITO) exhibit a propensity for chemical depletion at their surfaces, leading to Fermi levels inside the band gap at the surface. This is attributable to the bixbyite crystal structure (with readily available oxygen interstitial sites) and resulting bulk point defect structure (including donor-interstitial associates in ITO), leading to chemical depletion by surface oxidation. These findings have major ramifications for TCO applications (e.g., in OLEDs and in organic solar cells), and may explain why ITO tends to be so variable and unstable in such devices. In contrast, conventionally-doped TCOs without available oxygen interstitial sites (e.g., Al-doped ZnO and Sb-doped SnO2) seem to be relatively unaffected by such chemical depletion effects. Mason model accounts for the difference in band gaps measured by diffuse reflectance in bulk specimens vs. those measured by transmission in thin films. Stimulated by Mason results, Freeman developed and applied thin film FLAPW method and code to calculate work functions for ZnO films having both nonpolar and polar surfaces.

Freeman performed first principles FLAPW calculations on CdO and MxCd1-xO. A combined theoretical/experimental study with Dravid, Kannewurf and Marks led to new guidelines for the design of high temperature TCO’s. A unique feature of this work is the use of the self-consistent screened-exchange – local density approximation (sX-LDA), which provides a considerably improved description of the optical properties obtained with the LDA. Freeman predicted Cu doped ZnO to be a strong half-metallic ferromagnet and estimated its Curie temperature, by mean field theory, to be around 380 K, and confirmed by Chang and Ketterson.

Barnett focused on measurements of the nanometer-scale layers in the ZnO/ZnMgO structures, utilizing a combination of x-ray diffraction and computer simulations. Fitting to the measured x-ray scans using the simulation allowed detailed information about the nano-layers, such as the exact layer thicknesses, the roughness of the interfaces between layers, as well as the extent of intermixing between layers. Such structural information will be important for interpreting results on electrical properties, i.e. how fast can electrons move along these interfaces. In regards to improvement in the quality of films on sapphire substrates, Barnett has grown ZnO buffer layers and annealed in air at elevated temperatures. Results indicate that annealing temperatures of 800-900oC yield substantial improvements in crystal quality. These annealed ZnO surface can then be used for nano-layer growth, and should yield substantially improved crystalline perfection.

Dravid developed a facile route to pattern solid-state inorganic materials, termed Soft Electron Beam Lithography (Soft-eBL). Soft-eBL synergistically combines the advantages of high resolution e-beam lithography with versatile wet chemistry (sol-gel method, in particular). It expands the scope of nanopatterning capabilities to fabricate multifunctional and multidimensional nanostructures on virtually any substrate. Dravid successfully employed the soft-eBL to fabricate diverse functional oxide nanopatterns (e.g., ZnO, BiFeO3, PZT and CoFe2O4), and 3D hierarchical multifunctional nanostructures, such as radially-stacked heterostructures with ferroelectric PZT nano-shell and magnetic CoFe2O4 core. Wessels is assisting in FE measurements of these structures and MOKE studies.
Ellis has developed algorithms to simulate growth and curing (cross-linking) of semi-disordered films, grown on crystalline substrates. Key parameters of deposition density, defect density, and cross-linking frequency are controlled to produce interface structures suitable for both classical and quantum modeling. Density Functional theory has been applied in both periodic supercell and embedded cluster. Ellis has begun the extension of DFT and transport theory to model effects of multilayer and interfacial structures important for spintronic applications. These approaches complement first-principles studies by Freeman.

Torkelson used solid-state shear pulverization (SSSP) to form nanoscale dispersions of two glassy polymers, polystyrene (PS) and poly(methyl methacrylate) (PMMA). Simulataneous pulverization of this PS/PMMA blend with 5 wt% styrene-methyl methacrylate gradient copolymer was shown to yield a well-compatibilized, nanoscale blend. A compatibilized, nanoscale blend was also produced by melt mixing of PS and polycaprolactone (PCL) in the presence of a styrene-hydroxystyrene diblock copolymer. Because the PCL phase was dispersed at the nanoscale, there was a confined crystallization effect, meaning that the PCL dispersed phase is rubbery at room temperature rather than semi-crystalline as it normally found at room temperature in the bulk state. Thus, the production of nanoscale blends with a crystallizable dispersed phase can yield different properties from a conventional blend of the same polymers.

In ongoing research on highly comptibilized immiscible blends that form equilibrium bicontinuous microemulsions, Burghardt initiated a new kind of experiment, using x-ray photon correlation spectroscopy to study the equilibrium dynamics of concentration fluctuations in a polystyrene-polyisoprene microemulsion sample. These results complement existing rheological and rheo-x-ray studies, and should allow for detailed testing of phenomenological Landau-Ginzburg model predictions of microemulsion rheology. Related theoretical work in the Shull group has focused on the application of self-consistent field theories to the formation of emulsified droplet phases formed from homopolymer/copolymer blends where hydrogen bonding interactions play an important role.

Gradient Copolymers

Torkelson and Nguyen produced gradient copolymers of styrene (S) and acrylic acid (AA) by controlled radical polymerization. When added at low content to selective solvents, these gradient copolymers yield higher critical aggregation or micelle concentrations than block copolymers. An important discovery was also made regarding the glass transition temperature (Tg) behavior of gradient copolymers in comparison with random and block copolymers. Using derivative heat curves from differential scanning calorimetry, it was shown that S-AA, S-hydroxystyrene, and S-tertbutyl acrylate gradient copolymers exhibit an unusually broad glass transition, in some cases as much as 80 K in breadth, which can be rationalized by the sinusoidal composition distribution that was predicted by Lefebvre, Shull, and Olvera to be present in the ordered state of gradient copolymers. In contrast, ordered block copolymers made from these comonomers yield two distinct Tgs while random copolymers yield one Tg of moderate breadth. Shull and Nguyen also compared the interfacial activity of model gradient copolymers to self-consistent mean-field predictions.

Broadbelt has developed kinetic models describing nitroxide-mediated controlled radical polymerization of 4 acetoxystyrene and its copolymerization with styrene. Previous modeling studies involved the use of continuum models based on the method of moments to successfully describe the kinetics of these systems. The recently developed stochastic models, based on kinetic Monte Carlo methods, allows for a more explicit description of the key characteristics of copolymerization systems. These characteristics include species evolution, molecular weight distribution, and chain sequencing. In developing this framework, approaches have been incorporated to handle the inherent stiffness of the system due to large differences in reaction timescales.

Nanocomposites

Nanocomposite studies performed by the Torkelson group have demonstrated that incorporation of low levels of well-dispersed silica nanofiller that can hydrogen bond to amorphous polymer can provide for a new application for nanocomposites – the production of nonequilibrium-state glassy materials that exhibit little or no physical aging (relaxation toward the equilibrium state). As physical aging is often accompanied by deleterious effects to material properties, including embrittlement, reduced barrier properties, etc., the ability to prepare glassy materials that exhibit little or no aging may have major technological implications. Burghardt and Torkelson have collaborated on X-ray scattering experiments to study flow-structure relations in polymer clay nanocomposites. In addition, the Shull, Brinson and Burghardt groups have collaborated to study the rheological and structural features of model nanocomposite systems that incorporate carbon nanotubes into the triblock copolymer gels that are also used for the thermoreversible gelcasting work described below.

Brinson has focused on the nature of the interphase formed in the vicinity of nanoparticles within polymer nanocomposites. Model nanocomposites were formed from graphite particles dispersed in Poly (methyl methacrylate)(PMMA) or Polycarbonate. The results show that a significant increase in glass transition temperature (Tg) for low concentrations of thermally expanded graphite oxide (TEGO) in PMMA. In contrast to this, TEGO/PC nanocomposite systems showed a slight decrease in Tg. A possible explanation for the different behavior of TEGO in different matrix systems is the presence of hydroxyl groups on TEGO surfaces that can have a secondary bonding with PMMA, thus increasing their Tg. This chemical interaction is lacking for PC. Brinson has also developed a multiscale modeling method that explicitly includes the interphase and the in situ nanoparticle morphology to ensure accurate prediction and design of nanocomposites. This method, based on a coupling of finite element and micromechanics methods, operates at relatively low computational cost and is highly accessible. Preliminary results, when benchmarked with experimental results, have indicated the capability of this multi-scale modeling method in capturing the strong impact of nanoinclusion morphology and interphase on the over-all performance, particularly the viscoelastic behavior of the nanocomposites.

Glass Transition Dynamics

Broadbelt and Torkelson have used fluorescence to characterize physical aging and quantitatively compare the extents of aging toward equilibrium in bulk and ultrathin polymer films. Evidence was obtained for nearly isochoric (constant volume) glass formation in rapidly quenched, ultrathin films. The temperature dependence of the fluorescence of a bulk PMMA film was examined, revealing that a decrease in temperature results in an increase in intensity and a red shift in the spectrum. The effect of temperature in shifting the spectrum was quantified using a ratio of intensities at wavelengths below and above the spectral maximum.

Thermoreversible Gelcasting

Shull and Faber have continued work on a thermoreversible gelcasting technique that can be used to form ceramic objects with complex shapes in a very convenient manner. In the last funding cycle, the relaxation behavior of three different acrylic triblock gels, provided by Kuraray Co. Ltd., was characterized. This allowed a determination of the optimal endblock length for ceramic processing. If the block length is too short, the relaxation time of the gel is too low, and details in ceramic pieces are not retained. Alternately, too long an endblock raises the processing temperatures and increases the viscosity of the ceramic slurry.

In an effort to produce highly porous ceramics for polymer infiltration, Faber initiated work to determine the maximum fugitive filler volume fraction in PMMA-PtBA-PMMA/Al2O3 slurries. Corn starch was chosen as the first fugitive filler to be tested and has been successfully loaded at up to 60 vol% of solids. Correlation between the volume fraction of fugitive filler and the volume fraction of observed porosity after gelcasting and sintering has also been confirmed. Preliminary studies of polymer infiltration of ceramics are currently being carried out using poly-butyl-acrylate and porous Al2O3.

The thermoreversible gelcasting technique has also been extended to TiO2. Slurries with solid loadings as high as 45 vol.% can be obtained, resulting in cast ceramic pieces with densities of at least 95% . These materials are prepared for infiltration of DOPA-polymers which are known to form strong bonds with oxides.

1. developing new, anisotropic nanomaterials,
2. creating passive and active plasmonic devices,
3. developing coherent control strategies to manipulate plasmons within nanoparticle arrays,
4. understanding the coupling mechanism between molecular chromophores and surface plasmons, and
5. understanding the coupling between plasmons and other nano- and micro-scale resonantors.

1. Synthesis: organic high-k dielectrics and inorganic semiconductor nanowires.
2. Processing: single-walled carbon nanotube sorting and printable electronics.
3. Characterization: scanning probe microscopy and synchrotron x-ray techniques.
4. Theory: non-equilibrium transport and quantum electronic structure.

AIM #1: Synthesize elastomeric poly(diol citrate) nanocomposite biphasic scaffolds that have a high burst pressures (tensile strength of 0.80 MPa, similar to early bypass grafts) and degradation rates (total degradation time of 10-12 weeks). Specifically, we will synthesize poly(diol citrate) nanocomposites where the nanophase consists of poly(lactide-co-glycolide) (PLGA) nanoparticles or poly(L-lactic acid) (PLLA) nanofiber meshes. The chemical, mechanical, and degradation properties will be characterized.

AIM #2: Investigate the effect of long-term in vitro culture (12 weeks) on burst pressure, compliance, and suturability of the biphasic scaffold. Assess the effect of cyclic radial strain on the formation of a small-diameter vessel in vitro. Specifically, we will seed tubular bi-phasic scaffolds with vascular cells and culture the cell/scaffold construct under 2% (similar to low compliance vessels such as ePTFE) and 12% (similar to human arteries) pulsatile radial strain. The quality of the resulting tissue will be assessed via histology, biochemical assays, and mechanical tests and correlated with the amount of cyclic radial strain.

1. 2001 Heart and stroke statistical update. 2001, American Heart Association: Dallas, TX.
2. Haruguchi, H. and S. Teraoka, Intimal hyperplasia and hemodynamic factors in arterial bypass and arteriovenous grafts: a review. Journal of Artificial Organs, 2003. 6(4): p. 227-335.
3. Thompson, W., Reflection of the pathogenesis of abdominal aortic aneurysms. Cardiovascular Surgery, 2002. 10: p. 389-394.
4. Niklason, L.E., et al., Functional arteries grown in vitro. Science, 1999. 284(5413): p. 489-93.
5. Yang, J., A. Webb, and G. Ameer, Novel citric acid-based biodegradable elastomers for tissue engineering. Advanced Materials, 2004. 16(6): p. 511-516.

The bacterium Escherichia coli, for which there exists a staggering array of genetic engineering methods and genomic data, is well suited for such tasks. Efforts at bioengineering of E. coli depend on understanding unresolved basic questions of the mechanisms underlying control of gene expression (i.e., modulation of which genes are expressed at any given time, largely through DNA-protein interactions on the chromosome) and metabolic processes (i.e., the biochemical reaction pathways through which the basic molecules of a cell are processed).

The overall objective of this joint project between the Motter and Marko labs is to biologically engineer bacterial cells, and design them to perform specific tasks and to produce specific materials. Two complementary projects focus on understanding mechanistic aspects of gene expression and metabolic processes in E. coli. Key strengths of the project are the complementary abilities of groups in the areas of theoretical study of metabolic and genetic networks of E. coli (Motter), study of information in genetic sequences (Motter), theoretical and experimental study of protein-DNA interactions at the single-molecule level (Marko), experimental study of chromosome structure and function (Marko), and theoretical and experimental study of soft materials including supramolecular assemblies of biomolecules (Marko).

Co-investigator:
Shaoyan Chu

This initiative has a strong focus on basic research, where the aim is to discover and understand the exotic phases that arise in strongly interacting electron systems. The materials of interest involve quantum spins on low-dimensional lattices for which strong fluctuations and/or geometric frustration tend to suppress the classically expected ground states. As a result, states with unusual properties (such as superconductivity, anamolously high thermopower, or infinite ground-state degeneracy) can emerge. Single crystals will be synthesized and studied with the specialized techniques of neutron and synchrotron x-ray scattering and scanning tunneling microscopy (with atomic resolution).