2009-05-01
2009 MS&E Senior Thesis Research SymposiumTuesday, May 5, 2009 / 4:30 – 6:00 pm1st Floor Bard Hall
Session A: Materials for Energy and the Environment
Session B: Bio-mimetic and Bio-inspired Materials
Session C: Nanostructure Synthesis and Formation
Session D: Fundamental Materials Science
Session A: Materials for Energy and the EnvironmentA1.1: Advanced Thin Films for Solid Oxide Fuel Cells. Leo J. Small, John M. Gregoire, R. Bruce van Dover, Department of Materials Science and Engineering, Cornell University, 214 Bard Hall, Ithaca, NY 14853. The integration of solid oxide fuel cells (SOFCs) into everyday technologies remains limited by the elevated operating temperatures required for ionic conductivity in cell electrolytes. In an effort to ease this limiting factor, this study utilizes a combinatorial approach to identify thin film electrolyte materials with superior oxygen anion conductivity. Specifically, the doping of zirconia with yttria and scandia was investigated through composition spreads of dense, amorphous thin films created using off-axis rf sputtering. The conductivity of said systems were evaluated from 200 °C to 500 °C through the use of impedance spectroscopy. Appropriate equivalent circuits were developed to describe the impedance response of the system and identify the compositions of greatest conductivity. Although doped zirconia systems have been heavily studied, this work is unique in its examination of the effect of composition on the conductivity of these electrolytes in the form of thin films less than 100 nm thick.
A1.2: Tungsten Doped Titanium Oxide for use as a Catalyst Support Material in the Anode of Polymer Electrolyte Membrane Fuel Cells Andrew R. Dean, Dept. of Materials Science, Cornell University, Ithaca NY 14853. Oxide materials to be used as a catalyst support in the anode of fuel cells are of great interest to the alternative energy community. In this study, tungsten doped titanium oxide with different doping percentages are synthesized using a combined assembly by soft and hard chemistries (CASH method) with block co-polymers as structure directing agents. Both the anatase and rutile titanium oxide crystal structures are created by annealing to different temperatures and an assortment of polymers are used in an effort to optimize the oxides. To evaluate the potential of the oxides for use in fuel cells, compositional, structural, and electrochemical analyses are done. Compared to undoped titanium oxides synthesized by the same method, the doped oxides show significantly higher surface area, more micropore area, and better conductivity, although more difficulty has been encountered in controlling the mesostructure. The use of doped oxides in the anode of fuel cells is a novel idea which, if improved, could make fuel cells a realistic alternative source of energy.
A1.3: Air-stable and efficiently light-harvesting organic solar cells. Sungsoo Lee, Yee-Fun Lim, George G. Malliaras, Department of Materials Science & Engineering, Cornell University, Ithaca, New York, 14850. Despite the advantage of low-cost, lightweight, and flexibility, organic solar cells (OSCs) require further progresses in their performance to become commercially viable. In this work, two perylene tetracarboxylic derivatives (PTCMe and BPE-PTCDI) are exploited as p-type along with electron-withrawing, air-stable hexadecafluorophthalocyaninatocopper (F16CuPc) as n-type. It is proposed that the two materials groups not only align with respective energy levels to perform as a pn-junction, but their light absorption peaks also compliment each other to show a broader light absorption spectrum throughout the visible and infrared. Further, inserting naphthalene tetracarboxylic anhydride (NTCDA) as an exciton blocking layer at the organic/cathode contact show additional improvement in air-stability and shelf lifetime.
Session B: Bio-mimetic and Bio-inspired Materials
B1.1: Mechanical Properties of Calcite/Agarose Single Crystal Composites: a Model System for Biogenic Minerals. John M. Peloquin, Department of Materials Science, Cornell University, Ithaca NY 14853. In nature, mineral crystals with incorporated organic macromolecules demonstrate significantly improved mechanical properties compared to the raw mineral. In this study, calcite grown in agarose hydrogel was used as a model for biogenic minerals. The resulting calcite is single-crystalline despite the presence of incorporated agarose. Nanoindentation was used to measure the reduced modulus and hardness of the calcite/agarose composite with varying agarose concentration. Fracture toughness was estimated from crack length at high indentation loads. Geologic calcite and calcite prisms from the mollusc Atrina rigida were used as controls. The results of these mechanical tests will inform future synthesis experiments using biomimetic composites.
B1.2: Interactions Between Functionalized Surfaces and Organic Hydrogels A Biomemetic Approach for In Vitro Modeling. Vijay Ravichandran , Lara Estroff, Ellen Keene, Dept. of Materials Science, Bard Hall, Cornell University, Ithaca, NY 14853. Most traditional approaches to synthesis of materials require harsh conditions such as high temperature, pressure, extreme pH, and often produce toxic byproducts. Biomaterials, complex composites of inorganic crystals and organic compounds, however are assembled at ambient conditions and evolutionarily optimized for their properties. The mollusk shell is a model example of a biocomposite material and is the subject of much interest in materials science. Mollusk shells consist of 95-99% calcium carbonate by weight, while an organic component makes up the remaining 1-5%. The application of biomimetic principles derived from mollusk shell structure and assembly may help in fabricating new composite materials with enhanced properties. This focus of this research is to characterize the interaction between organic hydrogels and functionalized surfaces using a biomemetic approach to model mollusk shell in vitro developed by Levi-Kalisman et al. Functionalized surfaces of self-assembled monolayers of either n-alkanethiols on gold films or silanes on glass slides, and surfaces of various hydrophobicity were incubated with organic hydrogels and films in an effort to understand interactions at the interface the assembly process. Characterization was performed using FTIR, SAXS, and SEM techniques. Results identified the hydrophobicity of the surface as well as the hydrogel as a critical component of the system.
B1.3: Nanostructured Calcium Phosphate-Organic Composite Materials: Toward Next Generation Dental Composites. Christopher Sarra, Hiroaki Sai, Debra Lin, Lara Estroff, Ulrich Wiesner, Department of Materials Science, Cornell University, Ithaca, NY 14853. The shortcomings of current dental composite materials and implants have motivated a number of studies toward alternative routes for treatment and prevention of tooth decay. In this work, the self-assembling properties of block copolymers and colloidal suspensions of polystyrene microspheres are exploited to exert structure control over hydroxyapatite nucleation and growth at the nano- and meso- scales. The porous hydroxyapatite scaffolds produced by these methods were backfilled with methacrylate monomers, and were subsequently photo-polymerized to form a bicontinuous organic-inorganic composite for use in treating dental caries. It is proposed that the continuous inorganic scaffold is responsible for both the enhancement of the composite’s mechanical properties and the reduction in observed composite volume shrinkage upon photo-polymerization.
Session C: Nanostructure Synthesis and Formation
C1.1: Multi-walled Carbon Nanotube Composites in a Nafion Matrix for Use in Ionic Polymer Actuators. Joshua Gomberg, Luis Estevez, Huiqin Lian. Weizhong Quian, Emmanuel P. Giannelis, Department of Materials Science, Cornell University, Ithaca NY 14853. Ionic polymer metal composites (IPMCs) are smart biomimetic actuators that function by exploiting the effect of ion transport on the swelling of hydrated Nafion. When a voltage is applied across an electrode-coated Nafion layer, the mobile hydrated cations drift toward the cathode, and the resulting swelling at the cathode and contraction at the anode induces curvature. Traditionally, IPMCs have been electroded through the use of noble metal plating, which involves a procedure that is both difficult and time consuming. This project explores the possible use of multi-walled carbon nanotube (MWNT) / Nafion nanocomposites as a possible substitute for noble metal electrodes. These nanocomposite - based actuators can be synthesized through solution casting at elevated temperatures and pressures. Various methods of solvating MWNTs in Nafion solutions were explored. Samples with electrodes containing MWNT weight percentages ranging from 0-10% were synthesized and tested for blocking force. It was found that MWNTs most readily solvate in Nafion solutions with ethanol as solvent, though these solutions require special care during the casting procedure in order to produce bubble-free samples.
C1.2: The Effects of Different Capping Ligands on The Electrogenerated Chemiluminescence of PbS Quantum Dots. Armand A. Galan, Liangfeng Sun, George Malliaras, Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14850. PbS quantum dots combine the attractive optical and charging properties typical of bulk semi-conductors, with the additional benefit of bandgap tunability. The infrared photoluminescence (PL) of PbS QDs make them attractive for applications such as in-vivo biological tagging and imaging. However, the surfaces of these QDs contain many defects, vacancies and dangling bonds. During photoluminescence, these surface defects act as electron/hole traps, producing PL intensities much lower than that of the core. Because of the high surface area-to-volume ratio of QDs, photoluminescence is significantly affected by the surface. We explored the need to increase the PL intensity of QDs through surface passivation. We demonstrated the passivation of surface states by the addition of ligands containing thiol, phosphine, and carboxylic acid moieties. Electrogenerated chemiluminescence (ECL) tests were used to determine the particular degree of surface passivation due to these capping ligands. During ECL, QDs in solution within an electrochemical cell undergo charge injection and subsequent charge transfer, resulting in light emission. We observed different ECL intensities from QDs capped with different ligands, indicating the ability of ECL tests to serve as a metric of surface passivation. For example, PbS QDs capped with trioctylphosphine (TOP) exhibited ECL intensities three orders of magnitude higher than those capped with oleic acid, demonstrating that TOP passivates the surface well. Spectral analyses of the QDs have also shown a smaller peak, due to surface states, red-shifted from the PL peak. The TOP-capped QDs resulted in the removal of this red-shift, and a corresponding increase in the PL peak. ECL tests are thus essential for the examination of the surface passivation necessary for higher PL in PbS QDs, making them useful for future applications such as bioimaging and infrared solar cells.
C1.3: Synthesis of Fluorescent Silica Coated Non-Spherical Polystyrene Particles for the Real Time Study of Colloidal Self-Assembly Esther Fung , Stephanie Lee, Chekesha Liddell, Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. Colloidal self-assembly is an important area of research both for furthering out understanding of fundamental concepts such as crystallization and in developing new technologies such as light controlling materials or biomimetic composites. Computational studies have shown that non-spherical colloids have the potential to exhibit a diverse range of phases and thus properties that may prove of interest to researchers. Herein, we propose a method for studying such systems. The main challenge of in the study of these systems is the synthesis of particles with a suitable visibility and stability when used in conjunction with a real time imaging technique. Here, asymmetrical polystyrene dimer particles were pretreated with 3–aminopropyltriethoxysilane, coated with a fluorescent silica shell using a modified Stober process and stabilized with polyvinylpyrrolidone. These particles were then inserted into a confinement cell that was observed using confocal microscopy for two-dimensional characterization. Particles with viable visibility and stability were synthesized after careful optimization of experimental parameters. Three phases, oblique, rotator and out-of-plane hexagonal, were identified in this quasi-two-dimensional system of colloids. Future research will study the phases formed when using alternate shapes and attempt to remove the polystyrene cores to allow for three-dimensional characterization of these self-assembled structures.
C1.4 Synthesis of Colloidal CdTe Quantum Dots Yielding Sphere and Tetrapod Geometries. Neil Strom, Materials Science and Engineering, Cornell University, Ithaca NY 14853; Adam Bartnik, Byung-Ryool Hyun, Liangfeng Sun, and Frank Wise, Applied and Engineering Physics, Cornell University, Ithaca NY 14853. Cadmium chalcogenides have been synthesized into nanocrystals (quantum dots, QDs) small enough so that quantum-size effects cause variation in the electronic bandgaps and resulting photon absorption and fluorescence energies. With one end result in photovoltaics, specifically in matching the solar energy spectrum to increase photon absorption and charge transfer, CdSe QDs have been investigated as sensitizers that reliably trap light and transfer charge without degradation, but CdTe QDs are relatively less explored for this purpose. In this work, colloidal CdTe QDs are synthesized using a previously developed, more environmentally-friendly chemistry. Results indicate that altered geometries such as tetrapods can form, perhaps due to ligand binding during synthesis and preferential growth in specific directions in the wurtzite crystal structure, although spherical QDs may be more desirable for many applications. Growth is a delicate balance of parameters such as radii of nuclei, ligand and precursor concentrations, and growth temperature and duration. The QDs can be characterized to determine the absolute energy levels and quantum emission yields. Eventually for solar applications, attachment to other wide-bandgap materials that transfer charge to electrodes using an engineered energy scheme can be investigated. Other potential applications include fluorophores for biological labeling, light emitting diodes, and lasers.
C1.5: Effect of Large Molecular Weight Block Copolymer Hybrids on Structure Formation. Samantha Smith, Hiroaki Sai, Ulrich Wiesner, Cornell University, Department of Material Science and Engineering, Bard Hall, Ithaca, NY 14853. The use of block copolymer hybrids in industry is somewhat limited due to the fact that these materials are poorly understood when it comes to their characteristic lengths and structure formation. The structure of amorphous block copolymer hybrid poly(isoprene–block–dimethylaminoethyl methacrylate) (PI–b–PDMAEMA) with poly(ureamethylvinyl)silazane (Ceraset) ceramic precursor was studied using increasingly large molecular weight polymers on the order of 100,000 g/mol. Systems at each molecular weight were annealed for various times, creating a visible structure formation timeline when observed using Transmission Electron Microscopy (TEM). It was found that higher molecular weight polymers are subject to kinetic control as opposed to thermodynamic control earlier on in the structure forming process. Chain entanglements occurred over a large range of PI-b-PDMAEMA to ceraset weight ratios. The results of this experiment provide an important step toward finding a characteristic length scaling factor for block copolymer hybrids, which will drastically increase the use of these materials in the future.
Session D: Fundamental Materials Science
D1.1: Numerical Analysis of Spinel Formation by Reaction Between (CoxFe1-x)1-∆O and Fe2O3-ε at 1200 °C. Brenden Eng, Dept. of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. The formation of (CoxFe1-x)3-δO4 spinel has been experimentally observed to form in the solid state reaction between (CoxFe1-x)1-∆O and Fe2O3-ε at 1200 °C. Despite measurements, the evolution of local oxygen activities, local diffusivities and composition changes within the reaction assembly with time remains unresolved. To obtain more insight into the details of the solid state reaction, a computer simulation incorporating all relevant experimental data on the Co-Fe-O system was created.The Co-Fe-O phase diagram of log aO2 versus cationic composition (x) with constant temperature and pressure was used to determine the local phase near the interfaces between the rocksalt, spinel and sesquioxide phases. The deviation from nonstoichiometry (∆,δ) in the (CoxFe1-x)1-∆O and (CoxFe1-x)3-δO4 phases and cationic composition (x) were used to determine local oxygen activities. Finally, experimentally determined values of tracer diffusion coefficients of Co and Fe in (CoxFe1-x)1-∆O and (CoxFe1-x)3-δO4 spinel were parameterized as functions of cationic composition (x) and local oxygen activity.A finite difference method was used to compute local composition changes in the reaction mesh. Results from the computer simulation have provided quantitative data on the motion of boundaries between the phases occurring in the reaction assembly. The results of the simulation were compared to observations made in actual solid state reactions taking place in atmospheres of log aO2 = - 0.66 and aO2 = 0.0.
D1.2: Resistance of thin films of nanoporous gold on flexible substrates. Brian F. Donovan, Totka Ouzounova, Christopher Umbach, Material Science and Engineering, Cornell University, Ithaca NY 14850. Nanoporous gold thin films can be used for a variety of biomedical and sensor applications. Their high surface area and the biocompatibility of gold establish favorable properties for a thin film sensor; these properties are even more applicable when applied on a flexible substrate. A solid understanding the electrical properties of these materials is integral to their application. This study determined the resistance of thin films of nanoporous gold when on various flexible substrate systems. The nanoporous gold is very brittle and was placed on top of a layer of Kapton and sealed by a layer of Parylene, this enables flexibility while ensuring that the nanoporous gold will not flake off. The substrates were measured under various amounts of bending to determine the relationship between the resistance of the nanoporous gold and the amount of flexing of the system. These measurement methods and resistance values will enable further use of nanoporous gold in a flexible dynamic system.
D1.3: Crystallization of Amorphous Silicon Thin Films via Microsecond Annealing by CO2 and Diode Laser Spike Annealing. Byungki Jung, Michael O. Thompson, Dept. of Materials Science, Cornell University, Ithaca, NY 14853. Photovoltaic (PV) cells based on polycrystalline silicon (poly-Si) have been receiving increased attention due to the availability of high-rate deposition technologies for amorphous silicon (a-Si) films. Various methods of annealing a-Si have been extensively studied in order to obtain the largest grains and the fewest grain boundaries, which are generally achieved with the highest temperatures and the fastest growth rates. Thermal annealing in microsecond time frames at a temperature just below the melting point of a-Si may promote the growth of larger grains with smaller densities. Laser spike annealing via CO2 and diode laser was used for crystallizing a-Si with anneal times in sub-millisecond time frames. A scanned “line source laser” heats the surface to a controlled temperature (up to ~1300°C), followed by a thermal quench into the bulk substrate as the line source passes. Several poly-Si films with ~300 nm thicknesses were studied under 500 μs anneal times and laser irradiation at peak temperatures ranging from 600°C to 1300°C. Films annealed via CO2 laser spike show grains in the order of hundreds of nanometers to a micron, which is on par with conventional rapid thermal annealing methods. Spike annealing at higher temperatures and shorter times with CO2 or diode lasers may lead to larger grains and smaller grain density, which will improve PV and microelectronic technologies.
D1.4: Surface Diffusion of Na in (CaO•Al2O2)x(2SiO2)1x Glass. Jun Tong. Dept. of Materials Science, Bard Hall, Cornell University, Ithaca, NY 14853. The increasing use of glass in modern technology has brought about a new need for the optimization of this material. Both bulk and surface diffusions have major impacts on the many properties of glass, but as of now, the study of the diffusion of glass has been limited to that of the lattice. However, measurements of surface diffusion are extremely limited due to the complex nature of traditional grinding methods used to measure radiotracer motion from point or line sources. A new method is presented in which radioactive tracer is injected onto the glass and beta radiation that is emitted is then counted over a radial area around the injection spot. This method was tested on (CaO•Al2O2)x(2SiO2)1x glass with Na-22 tracer. Since our tracer emits both gamma and beta radiation, a chamber was built that allows for the focusing of a beta detector on specific points on the sample and for shielding to be placed between the detector and the sample to block unwanted particles. To ensure that only beta radiation from the desired area is counted, two shields are used, requiring counting to be done twice. A shield with no hole is used to count for the secondary beta radiation caused by the gamma particles and any background radiation that might be present while blocking off the beta particles from the sample. A second plate with a small hole is also used to collect all the counts that the first plate collected plus the beta particles from the sample. The difference of the two, which would be the beta particles from the specific point on the sample, would then be recorded and used to calculate the diffusion coefficient. These measurements produced diffusion coefficients near the expected values, confirming utility of this novel method.
D1.5: Optimization of the Vapor Phase Polymerization of PEDOT:Tosylate, Colin Eichinger, Materials Science and Engineering, Cornell University, Ithaca, NY 14850. Poly(3,4-ethylenedioxythiophene) (PEDOT) is an organic semiconductor routinely used in applications such as organic electrochemical transistors (OCETs), thin film transistors (TFTs) and biosensors. These devices suffer from numerous performance limitations such as low conductivities and delamination in analytic solutions. PEDOT doped with p-toluenesulfonate (PEDOT:Tosylate) has been shown to overcome many of these drawbacks, however the optimal deposition parameters have yet to be found. It is thought that by using a novel cluster tool developed by ReynoldsTech, each step in the deposition of PDOT:Tosylate thin films can be carefully controlled to produce consistent and reliable results. Each parameter in the deposition process is examined and co-optimized to produce highly conducing PEDOT:Tosylate films. Results show that conductivities of films produced with this cluster tool rival, and in some cases exceed, conductivities achieved by previously demonstrated methods.