Home Institution:
Major/Minor:
MRSEC Mentor: Allen Goldman
Effect of Ultraviolet Light on the Conduction of Tetracene in Field Effect Transistors
In this study we hope to discover the effect of shining ultraviolet light on a semiconductive crystal of tetracene. The material, as used in organic field effect transistors (OFETs), has many superior qualities such as being mechanically flexible and both cheaper and easier to produce than its silicon counterparts. What is holding us back from using OFETs in advanced electronics is the significantly lower charge mobility that causes reduced conduction through the crystal. It is believed that, along with its p-type conduction, tetracene could also have n-type charge carriers if the electrons are freed from the traps located at the substrate interface. Through this study, we will be able to test the resistance of the UV illuminated tetracene crystal and find results that may increase the use of OFETs and better the understanding of the charge carrier mechanism.
Home Institution:
Major/Minor:
MRSEC Mentor: Eray Aydil
Effect of Ultraviolet Light on the Conduction of Tetracene in Field Effect Transistors
Oriented nanoscale wire arrays show promise in applications such as microelectronics, sensors, and solar-to-electrical energy conversion. The dye sensitized solar cell (DSSC) is one application which utilizes semiconducting nanowire arrays to convert solar energy into electricity. Nanowire based DSSC's utilize a photosensitive dye adsorbed onto the nanowires which in turn are immersed in an electrolyte. DSSC's require a fine structured morphology because the interfacial area between the semiconducting material, the dye and the electrolyte must be very large in order to absorb the solar energy incident upon the solar cell.
One material that has often been used as the semiconductor in DSSC's is zinc oxide. In this work, oriented nanometer size ZnO "rods" will be grown on substrates seeded with 5-10 nm diameter ZnO particles, using a low temperature solution method that utilizes the reactants zinc nitrate and methenamine. Processing conditions such as reactant concentrations, pH, and temperature strongly influence the morphology of the nanoscale ZnO rods produced - as do the seed ZnO particles. This project will focus on synthesis of ZnO rods using a variety of processing conditions, including steady-state reactant concentrations and ZnO seed particles with different morphologies.
Synthesized nanoscale ZnO arrays will also be characterized for morphology and crystal structure using electron microscopy and XRD. The characterization will provide further insight into the processing-structure relationships for this oriented ZnO array method.
Home Institution:
Major/Minor:
MRSEC Mentor: Frank Bates
The Effects of Asymmetry on Mechanical Properties of Block Copolymers
We propose to study the effects of asymmetry on the morphology and mechanical properties of poly(styrene-b-isoprene-b-styrene-b-isoprene-b-styrene) (SISIS) pentablock copolymers. These pentablocks are synthesized with controlled anionic polymerization by sequentially alternating between styrene and isoprene addition. The sizes of the middle three blocks are kept constant while the lengths of the two terminal styrene blocks are varied. The mechanical properties and morphological domain spacing of pentablock copolymers with identical overall compositions depends on the relative lengths of these two terminal blocks. We will investigate the effect of varying the terminal block lengths on domain spacing using small-angle x-ray scattering (SAXS) and on tensile strength using tensile testing. The fundamental understanding elucidated from these studies will aid in the design of mechanically robust plastics for a variety of uses, including medical and electronics applications.
Home Institution:
Major/Minor:
MRSEC Mentor: Peter McMurry
Laser Ablation Time-of-flight Mass Spectrometry
Laser ablation time-of-flight spectrometry is a technique that has been devised recently to characterize the size and composition of small individual gas-borne particles. The main mechanism is to direct a high-intensity ultraviolet pulse laser at the particles suspended in the gas with the purpose of blasting them apart into its chemical constituents. These latter, now ionized as a result of the laser light impact, will be mass- analyzed by time-of-flight mass spectrometry to determine the composition of the particles. The focus of my research is on sulfates. Sulfates are known to be a major component of atmospheric particles; however, they are hard to detect using this technique and consequently pose an obstacle to an accurate compositional analysis of the particles. The objective of this project is thus to improve the efficiency with which sulfates can be detected. It is believed that sulfates become more readily detectable when coated with certain UV-absorbing organic compounds prior to the laser ablation process. My goal is to develop a technique for coating ammonium sulfate particles with a controlled amount of selected organic compounds. The instrument to be used in coating the particles will be a commercial condensation particle counter (CPC). It will be adjusted as needed to work appropriately with the target compounds that will have been chosen.
Home Institution:
Major/Minor:
MRSEC Mentor: Uwe Kortshagen
Nanowires and nanotubes have been a recent topic of interest to aid in many different applications. The focus of this project is to successfully grow silicon nanowires with the use of plasma enhanced chemical vapor deposition aided by a gold catalyst. This method allows the growth of nanowires at lower temperatures than other processes. To facilitate the nanowire production, we are designing and building a device that uses an applied voltage at a controlled temperature with silane gas to produce the plasma required by the specifications of the project. Other applications include uses in polymers (which is intended to be researched after the first phase of this project is accomplished,) integrated circuits, and many more.
Home Institution:
Major/Minor:
MRSEC Mentor: David Norris
Semiconductor nanocrystals, also called quantum dots, have unique optical properties that make them desirable for biological imaging applications. One advantage of using quantum dots over conventional dyes is that quantum dots do not photo‑bleach. They also have a broad absorption spectrum and emit a specific wavelength that can be adjusted by merely changing their size.The purpose of my research will be to conjugate quantum dots with antibodies. Ultimately, these dots will be used in a cancer study. To accomplish this conjugation, I will take previously prepared quantum dots in phospholipid micelles and attempt to conjugate them with anti-biotin using cross-linking molecules, particularly SMCC and EDC, described in Bioconjugate Techniques by Greg T. Hermanson. A dot blot test will be used to test the quantum dot bioconjugates. Different concentrations of biotin will be exposed to the bioconjugated dots. The biotin should fluoresce if the antibodies were successfully conjugated to the quantum dots. This test will not only determine whether or not the antibodies were successfully coupled to the quantum dots, but will also, assuming successful conjugation, verify the sensitivity of the antibodies to the corresponding antigens.
Home Institution:
Major/Minor:
MRSEC Mentor: Jim Kakalios
The focus of this research involves hydrogenated amorphous silicon thin films. A sample of amorphous silicon was heated and cooled in order to explore the conductivity and photoconductivity of the semiconductor in both light and dark settings. When the sample changes temperature in a dark setting, the sample's energy output is stronger as opposed to a light environment. Because of the Staebler-Wronski effect, each time the sample is exposed to light, the energy output is weakened. The main interest is to harness the full potential of amorphous silicon thin films by bypassing the Staebler-Wronski effect.
Home Institution:
Major/Minor:
MRSEC Mentor: Kent Mann
Synthesis and Characterization of Naphthalene-Capped Oligothiphenes for use as Semiconducting Materials in Thin Film Transistors
Organic semiconductors have the potential to be used in the manufacture of solution processable thin film transistors (TFTs), and may provide an inexpensive alternative to current silicon technology. One promising family of semiconducting organic molecules is oligothiophenes. The purpose of this research is to synthesize and characterize naphthalene-capped oligothiphenes with alkyl substituents for use as the semiconducting layer in a TFT. Unsubstituted molecules show p-type behavior and a herringbone packing motif. The addition of alkyl substituents will force the molecules into a π-stacking structure in the solid state and change the behavior of the semiconductor. The alkyl substituents will also provide increased solubility, which can provide ease in the characterization and possible spin coating of films. All the molecules synthesized in this study will be characterized by methods that include: nuclear magnetic resonance spectroscopy, UV-VIS spectroscopy, X-ray crystallography, mass spectrometry, and cyclic voltammetry.
Home Institution:
Major/Minor:
MRSEC Mentor: Xiaoyang Zhu
What allows for the conduction of electrons in certain materials? This has been researched for many years, and I intend to work with monolayers on top of Au 111. Previous research has shown that several compounds, when placed on Au 111 with an appropriate connector molecule, will form upright strands. In this formation the conduction band (the lowest unoccupied electron energy state of the atoms) of these molecules merge together to form one long conduction band that electrons can travel in, which allows for conduction along these paths. My research proposes to observe monolayers of this sort (such as benzenethiol) using a STM (scanning tunneling microscope) to take topographical images of these monolayers. By analyzing these images one can determine if the monolayers are the upright ones that we expect. The goal is to create several different monolayers, observe their structure, and verify that they form upright strands, which will assist in the development of new compounds for the conduction of electrons.
Home Institution:
Major/Minor:
MRSEC Mentor: Frank Bates
Effects of Block Sequencing on Mechanical Properties of Polyolefin-containing Block Copolymers
Block copolymers are fascinating materials that are unique in the way they self assemble into micro-phase separated structures. These materials have been used in a wide range of applications, mainly as thermoplastic elastomers. The ability to control block copolymer properties by manipulating the constituent blocks and resulting microstructures is leading to the use of these materials in much more diverse applications. We are studying the mechanical properties of lamellae-forming block copolymers composed of poly (cyclohexylethylene) "C", and poly (ethylene) "E". We have seen that a CEC triblock copolymer exhibits ductile behavior (strain-to-failure ~ 300%), while its ECE counterpart is an extremely brittle material (strain-to-failure ~ 1%). This dramatic difference likely stems from the presence of bridging E chains in the CEC material. In this work, we plan to gradually introduce E bridging by starting with neat ECE material and adding varying amounts of CEC material to prepare a series of blends. Investigation of the tensile properties of these blends will reveal the degree of E bridging necessary to obtain a ductile material.
Home Institution:
Major/Minor:
MRSEC Mentor: Chris Macosko
Effects of PS/PMMA Blend Compositions on Morphology
The current drive to fabricate improved, cheaper materials for advanced technologies is seen in every aspect of the global market, including polymers. To enhance a material, it is necessary to begin by looking at it from the microscopic level. Take, for example, the most common element found in polymers - carbon. Carbon is found in two forms the majority of us are familiar with: diamond and graphite. While diamond is clear and hard (Moh's hardness of 10), graphite is grey and soft (Moh's hardness of about two). The tremendous difference in properties seen in these two materials is due to crystalline structure. Changing carbon on the microscopic level is just one example of the profound influence that a material's structure has on its macroscopic properties. Now look at the macromolecule polystyrene; PS alone is a brittle material. However, the addition of rubber particles into the PS matrix (such as in the ABS copolymer) decreases the brittleness of the PS, thereby allowing the use of ABS in high-impact applications. With this idea in mind, investigation of polymer blend morphology is being conducted as a means to enhance current polymer capabilities.
Polymer blends are known to evolve into several known morphologies; of these morphologies, cocontinuous is of particular interest because of its potential in "property enhancement, static charge control, desiccant entrained polymers for applications with moisture sensitive products and barrier property improvement."[1] Cocontinuous polymers form what appears like a roadmap of many intertwining streets, where the streets represent one polymer and areas other than streets represent another polymer(s). There are two basic definitions available to describe cocontinuous polymers; the first definition explains the phenomenon as at least two continuous structures within the same volume that subsist with one another. The second definition describes the morphology as "one in which at least a part of each phase forms a coherent continuous structure that permeates the whole volume." [2]
Identification of cocontinuous morphologies is commonly done using solvent extraction and image analysis techniques. Galloway, et. al [3] apply algorithms to microtomed SEM images to calculate interfacial area concentrations. These algorithms are responsible for detecting phase edges and eliminating false edges within the SEM image. Self extraction methods require one phase of the polymer blend to be removed; if the remaining polymer and polymer that was removed are self-supporting, the blend is deemed cocontinuous. A linear relationship was found between the degree of continuity and the reciprocal of the sample thickness.[4]
Stability of cocontinuous polymers tends to be problematic. Currently, several studies have been completed that use block copolymers to aid in stabilization of these morphologies. Harrats, et. al [5] found that the use of diblock copolymers were more effective in stabilizing PE/PS blends. Similarly, Galloway, et. al [6] also found that the use of block copolymers had a stabilizing effect at the polymer interface. Currently, effects of temperature, mixing speed and annealing time are being investigated to understand how processing influences polymer morphology. Moriera, et. al [7] found that compression-molding temperatures can effect the range of cocontinuity, while altering the mixing and molding temperatures changes the morphology from cocontinuous to droplet-matrix. In addition, Chaundry, et. al [8] found a "strong influence" of mixing time and temperature on PC/ABS blend morphologies. These studies emphasize the importance of identifying how processing parameters influence morphology, thereby allowing optimization of polymer blend processing techniques.
The focus of this study is on PS/PMMA blends. Currently, PS/PMMA blends from 10% - 90% PS (in 10% intervals) have been created. One mm thick disks for rheological measurements will be fabricated, along with two mm thick disks for solvent extraction experiments. Blends from 10 - 90 % PS (in 5% intervals) will also be studied to classify PS/PMMA blends and effects of blend composition on morphology. Time permitting, effects of processing parameters will also be investigated.
Home Institution:
Major/Minor:
MRSEC Mentor: Paul Crowell
Home Institution:
Major/Minor:
MRSEC Mentor: Chris Leighton
Measurement of Magnetization as a Function of Temperature in Pr0.5xLa0.5(1-x)Sr0.5CoO3
The cobaltites Pr0.5Sr0.5CoO3 and La0.5Sr0.5CoO3 are similar in crystalline structure, and the atoms Pr and La have similar atomic radii. However, the magnetization curve as a function of temperature in La0.5Sr0.5CoO3 is in agreement with quantum theory, while the magnetization curve for Pr0.5Sr0.5CoO3 is anomalous. We will examine the transition from normal to anomalous magnetization curve by measuring the magnetization curves for the family of alloys PPr0.5xLa0.5(1-x)Sr0.5CoO3 where x varies from 0 to 1. It is expected that the specific nature of this transition will help to explain the cause of the anomalous Pr0.5Sr0.5CoO3 magnetization curve. The samples' crystalline structures will be characterized by x-ray diffraction, while their magnetic properties (such as the magnetization vs. temperature curve) will be characterized by neutron diffraction and magnetometry.
Home Institution:
Major/Minor:
MRSEC Mentor: Victor Barocas
A Lag After Pulsed Separation (LAPS) Flow Sensor and Pumping MIcroDevice for Crystallization Micro-Array (cryma)
The interactions between proteins have been of great interest in modern life sciences. In order to study proteins, their 3-D crystalline structure must be identified. To crystallize the proteins, the CRYMA system can be employed. To obtain large (50-500 μ) and defect-free crystals, small volumes of proteins must be controlled into the crystallization reactor. It has been difficult to control such small volumes; therefore, the creation of a micropump and a LAPS microflowmeter to control the transport of the protein solution into the reactor is proposed for the CRYMA. Techniques for making the components of the microdevice are presented. Photolithography will be used to create the microchannels and then, the electromagnetic pump will be adapted. The microchannels will be constructed to form the LAPS microflowmeter: a main microchannel, which will hold a pair of electrodes, will bifurcate into two smaller microchannels (these smaller microchannels will also hold a pair of electrodes) that will recombine again. Electrophoretic concentration of the particles in the protein solution in one of the small microchannels should occur after a DC pulse is applied. This change in concentration will be measured as a change in AC resistance. The flow-rate will be determined measuring the time elapsed between the application of the DC pulse and the measured change in AC resistance. For creating the pump, a microchannel will lead to a deformable chamber. Above the chamber an actuator will be placed and beneath it an electromagnet; a magnetic field will be applied and the actuator should deform the chamber pushing the solution out of the chamber. Variation of the width of the microchannels and variation of the thickness of the layers forming the microchannels will be performed. A controlled flow of small volume of protein solution through the microchannels is expected.
Home Institution:
Major/Minor:
MRSEC Mentor: Allen Goldman
Characterization of Indium Oxide Films at Insulator-Superconductor Transitions
The effects of ultraviolet radiation on the transition temperature and characteristics of Indium Oxide films will be investigated through measurement of electrical properties at low temperatures and high magnetic fields. Spectra from the ultraviolet source, a mercury-ion fluorescent bulb, will be further characterized. Alternate filters and light sources will be tested for controlling the peak frequencies of emitted light. Changes in oxygen content as a result of ultraviolet exposure and subsequent annealing will be examined as a possible mechanism for the varying resistivity of irradiated samples.
Home Institution:
Major/Minor:
MRSEC Mentor: Daniel Frisbie
Synthesis of Air Stable Polythiophenes with Application for Organic Photovoltaics
Recently organic solar cells have become of increased interest over traditional inorganic solar cells. Inorganic solar cells have a high cost of production to power output ratio and therefore are not currently an economical source of energy conversion. Organic solar cells, on the other hand, can be produced at a lower cost and have gained interest in preliminary studies. Organic solar cells in the past have been made from poly (3-hexylthiophe), P3HT, which has limited usefulness due to its instability in air. When P3HT is exposed to air the polymer begins to lose its electrical transport properties, which makes P3HT impractical for device fabrication. In my research I will synthesize Poly(3,3'''-didodecyl-quaterthiophene), PQT-12, a conjugated thiophene polymer. PQT-12 has similar electrical properties to P3HT, but has the added quality of being air stable which makes PQT-12 particularly interesting for solar cells fabrication. In the solar cell, the PQT-12 will act as the hole transporting material. After synthesizing PQT-12 I will characterize the polymer and then fabricate devices. During the course of my research I will use several analysis techniques: NMR, size exclusion chromatography (SEC), and atomic force microscopy (AFM). I will use NMR to identify the species synthesized and SEC to determine the molecular weight of the polymer. Finally, I will AFM to analyze the thin film morphology of PQT-12.
Home Institution:
Major/Minor:
MRSEC Mentor: Michael Tsapatsis
Synthesis and Characterization of Zeolite - Polymer Composite Membranes
Gas separation by membranes offers an alternative to often economically and/or environmentally taxing distillation, absorption, and adsorption processes. The focus of our research is synthesis of a composite membrane for industrial hydrogen separation scale-up featuring the high selectivity and permeability inherent to zeolites in molecular sieves combined with favorable mechanical and coating process properties of polymer films. Using mass transport theory we attribute the separation performance of the composite membrane to the aggregation and orientation of anisotropic zeolite platelets dispersed in the polymer network. Permeation and selectivity of synthesized composites will be determined by flux from measured partial pressure changes across the films at different temperatures. Resolved microstructure by X-ray diffraction and various electron microscopies coupled with associated separation properties will motivate subsequent formulation and synthesis.
Home Institution:
MRSEC Mentor: Dan Dahlberg
We're studying the magnetic reversal characteristics of a nano scale ferromagnetic and antiferromagnetic layered particle. The reversal is caused by both static and pulsed magnetic fields, and verified with MFM microscopy. Initially it was assumed that the magnetic reversal caused by the applied external magnetic field could be characterized with a standard deviation, but recently this idea has come under question. Therefore we've set aside some time to investigate a new idea that involves a probability of magnetic reversal before moving on. Once this problem is addressed, we will attempt to cause magnetic reversal solely with a pulsed external magnetic field. If this pulsed magnetic reversal of the particle proves successful and consistent this summer, we aim to measure the constant of the decaying gyration of the single domain particle.
Home Institution:
MRSEC Mentor: Daniel Frisbie
Characterization and Analysis of a Au Particle Arrayed G-wire Scaffold
The feature of programmable self-assembly makes nucleic acids an attractive material for the creation of nanoscale circuitry. The self-assembling scaffold material used in this work is a modified ten base pair oligonucleotide sequence, GGGGT*TGGGG (TM1-6) that bears a thiol functional group on one of the thymine (T*) bases. The oligonulceotide, TM1-6 is capable of self-assembling into long linear quadruple helical DNA filaments called G-wires with thiol functional groups enabling Au nanoparticle decoration. Initially, thiol modified G-wires are deposited on the desired substrate surface. A G-wire nanoparticle array is subsequently formed in situ by incubation of the coated substrate with a solution of Au nanoparticle in dichloromethane. The resulting structure manifests itself in the form of a G-wire scaffold with branched thiol groups each bearing a conductive Au moiety.
The Au nanoparticle/G-wire structures will be characterized by Transmission Electron Microscopy (TEM) to elucidate the orientation, spacing and density of Au particles arrayed on the G-wire scaffold. Tapping Mode Atomic Force Microscopy (AFM) will be employed to quantify topological variations before and after decoration of G-wire with the Au particles. Finally, Kelvin Probe Microscopy (KPM) will be utilized to detect the electrostatic potential of the Au modified G-wire verses its grounded substrate to examine its electrostatic potential and conductive properties.
Funded by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013
UMN MRSEC
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