IRG1- Research Highlight

Development of a Universal Template for Producing Nanoporous Materials by Nanocasting

Solid materials with 100 nanometer pores are highly desirable for water ultrafiltration membranes, catalyst supports, conducting electrodes, and photovoltaics. However, this size scale is hard to achieve by standard chemical or processing routes. By using an equilibrium bicontinuous molten polymer blend as a precursor, a porous template with 45% void space is prepared by cooling. One polymer (polyethylene, PE) crystallizes, and the other (polyethylenepropylene) is rinsed out. Then, the precursor to any desired solid can be infiltrated into the pores and solidified by chemical or thermal means. The remaining PE can also be washed away at high temperature. This process can be used to generate a wide variety of nanoporous materials, such as the conducting polymer PEDOT.

IRG2- Research Highlight

Efficient, Single-Layer Organic Light-Emitting Devices

Efficient OLEDs often require the use of an intricate device architecture. Graduate student Nicholas Erickson has instead taken an alternative approach, focusing on the use of a doped, graded emissive layer (G-EML) architecture that permits high efficiency in devices comprising only a single layer. Device composition varies continuously from nearly 100% hole-transporting material (HTM) at the anode to nearly 100% electron-transporting material (ETM) at the cathode, with an emitter uniformly doped throughout the structure. Erickson has demonstrated efficient, single-layer OLEDs emitting in the blue, green, and red. The tunable gradient allows for the optimization of electron-hole charge balance and low-voltage operation while preserving charge and exciton confinement.

IRG4 & IRG2- Research Highlight

High Efficiency Silicon Nanocrystal Light Emitting Devices

Hybrid light-emitting devices based on organic semiconductors and inorganic semiconductor nanocrystals are of great interest for applications in optical displays and solid-state light sources. Silicon, a poor light emitter in bulk form, can exhibit strong luminescence in nanocrystal form; however, efficient electrical excitation had not been demonstrated. In this study, Cheng et al. showed a silicon nanocrystal device with an external quantum efficiency of 8.6%, the highest reported efficiency for any nanocrystal light emitting device.

(Cheng et al., Nano Letters 2011, 11, 1952-1956)

IRG2 & IRG3- Research Highlight

Why Most Plastics Can't be Metals

Conductive polymers, i.e. plastics, that conduct electricity, are important in science and technology as they offer the potential for cheap, flexible electronic devices. This work examines the mechanisms by which electrons are transported in such materials, a process that remains far from understood. One of the main results of the work is that the behavior of such materials, at very high densities of charge carriers, is radically different to simple expectations. In particular, it is found that the typical methods used to “dope” the materials, i.e. to increase their conductivity, can create large amounts of disorder in the polymer material, preventing the expected transition to metallic behavior. The result has important consequences for design of future polymer electronic devices.

(a) Influence of temperature on the resistivity of P3HT (see structure in (c) at multiple levels of electrostatic doping. (b) shows a schematic of the transistor.

IRG3- Research Highlight

The Microscopics of Pinning

Although pinning is a generic phenomenon in magnetic materials, surprisingly little information is available about its origins. IRG-3 investigators have addressed this problem by applying a nanoscale probe of pinning dynamics developed by graduate student, Te-Yu Chen, in Paul Crowell’s group to a set of samples prepared by graduate student, Michael Erickson, in Chris Leighton’s laboratory. The IRG’s approach allowed the group to measure the pinning energies and ranges in a family of Ni0.81Fe0.19 films and to establish definitively that surface roughness is the dominant source of pinning in soft ferromagnetic films.

) Map of pinning dynamics in a one micron diameter Ni0.81Fe0.19 disk as a function of the applied magnetic field. (b) Pinning energy as a function of roughness measured at a length scale (~ 20 nm) corresponding to a magnetic vortex core diameter.

IRG3- Industrial Collaboration Highlight

Giant Magnetoresistance Effect at Microwave Frequencies

The resistance of a tri-layer of a normal metal such as copper sandwiched by two ferromagnets can depend strongly on the relative orientation of the ferromagnets. This effect, known as giant magnetoresistance (GMR), is employed widely in a variety of sensor technologies but has not been explored extensively at high frequencies. In collaboration with Stefan Maat at Hitachi Global Storage Technologies, IRG student Dan Endean, working with E. Dan Dahlberg, has shown that the microwave properties of a tri-layer can be predicted from the low-frequency GMR. The IRG investigators showed that the magnetization dependence of the microwave transmission is larger in magnitude than the ordinary GMR, and they developed a quantitative model for this behavior as well as the weaker effects observed in reflection.

(a) Measurements of Δρ/ρ (ordinary transport GMR) and the magnetization-dependent microwave transmission (ΔT/T) and reflection (ΔR/R ) coefficents. (b) Transmission and reflection coefficients as a function of the ordinary GMR.

IRG4- Research Highlight

Imaging ‘Invisible’ Dopant Atoms in Semiconductor Nanocrystals

The deliberate introduction of impurity atoms into semiconductors, also known as doping, is widely used to tailor the properties of bulk semiconductors. However, the doping of semiconductor nanocrystals has been found to be surprisingly different from their bulk counter parts. The physics of doped nanocrystals may strongly depend on the exact position of the dopants inside the crystal. However, many impurities of interest cannot be observed with currently available imaging techniques and new methods are needed to determine their location. Aloysius Gunawan, a graduate student working with Professor Andre Mkhoyan combined electron energy loss spectroscopy (EELS) with annular dark-field scanning transmission electron microscopy (ADF-STEM) to image individual manganese (Mn) impurities inside zinc selenide (ZnSe) nanocrystals. In ADF-STEM mode, a highly focused electron beam is scanned across a nanocrystal while scattered electrons are collected with an annular dark field detector. This allows obtaining high-resolution images according to the material’s atomic-number (Z) contrast. However, a dopant must have a large Z-contrast with the surrounding semiconductor atoms to ensure its visibility. This is not the case for many dopants commonly used in semiconductors. Gunawan combined ADF-STEM with EELS to overcome this problem. Atomic columns with a Mn atom appear brighter when an EELS signal characteristic for Mn is detected. To our knowledge, this is the first demonstration of this technique that enables locating individual doping atoms in semiconductor nanocrystals. This technique may significantly enhance the understanding of the physics of doped semiconductor nanocrystals.

IRG4- Industrial Collaboration Highlight

Semicrystalline Tin Dioxide Films for Improving the Damp Heat Stability of Copper Indium Gallium Diselenide Solar Cells

In a collaboration that materialized from a juncture between an industrially-funded project and MRSEC goals, graduate students Selin Tosun (funded by Dow Solar) and Aloysius Gunawan (funded by MRSEC), co-advised by Aydil, Campbell and Mkhoyan, worked together on determining the structure of tin dioxide (SnO2) films, which improve the damp heat stability of copper indium gallium diselenide (CIGS) solar cells. While CIGS thin film solar cells with laboratory efficiencies exceeding 20% have been reported, these high efficiencies degrade with time as the devices are exposed to humid environments. Grain boundary diffusion of water is implicated in this degradation and must be reduced or stopped to increase the solar cell lifetime. Tosun showed that thin SnO2 layers, deposited on top of the completed CIGS solar cells can significantly increase the device lifetime by forming a barrier against water diffusion. She collaborated with Gunawan to show that these films are semicrystalline with SnO2 nanocrystals embedded in an amorphous matrix. This difference is attributed to the lack of grain boundaries and hence grain boundary diffusion of water in semicrystalline SnO2 films. This approach is a significant technological achievement in increasing long-term reliability of CIGS solar cells and has been patented by Dow Solar.

Seed- Research Highlight

Interfacial Events in Functional OFETs

Professor Aaron Massari and colleagues at the University of Minnesota have married an interface specific spectroscopic technique, called vibrational sum frequency generation, with electrical measurements on organic field-effect transistors. In these devices, the semiconducting material is an organic polymer or small molecule film. During operation in an electrical circuit, a mere 1-2 nanometers of the material at a buried interface dictate the device performance. The location and dimensions of this active region make it difficult to characterize, and typically the behaviors of molecules are inferred from indirect electrical measurements. By characterizing the organic interfaces in OFETs with a non-invasive spectroscopy, the Massari group is able to obtain information about these interfaces that is not available through other methods. In a recent study, the approach was applied to a polymeric OFET to demonstrate that the chemistry of the surface onto which the polymer was deposited directly influenced the orientations of molecules at that interface, leading to better performance. This work settled a decade-long debate about the mechanism of device improvement from surface chemistry changes (Figure A). These results also uncovered spectral features (shown in Figure B) that can be used to track defects and damage from within an operating device.

Interfacial Structure and Spectroscopy. A) Schematic of the influence of surface chemistry on molecular orientation at the buried interface of polymeric OFETs. B) Vibrational spectra from a polymeric OFET with inset electrical performance. The strong vibrational bands at 1260 cm-1 indicate structural defects (black and blue curves) when the device is activated, confirming ambipolar accumulation despite unipolar electrical performance (inset).

Seed- Research Highlight

ZnO Nanowires for DNA Electrophoresis

Professor Kevin Dorfman, Professor Eray Aydil and colleagues at the University of Minnesota have developed a method to integrate ZnO nanowires into a microfluidic device for DNA electrophoresis. The classical method for separating long DNA, known as pulsed-field electrophoresis, requires hours to days to achieve adequate resolution. Recent work demonstrated that the same separations can be performed in arrays of nano-sized posts in the manner of minutes. Unfortunately, these so-called "nanopost arrays" require expensive, time consuming clean room fabrication methods. In the method developed at the University of Minnesota, the nanoposts are grown from solution into a specified region of a glass microchannel. As a result, the cost-limiting nanopatterning step is eliminated. Moreover, the density of the nanowires is easily controlled by the surface preparation prior to nanowire growth. The Minnesota team used a sparse array of nanowires to perform a fundamental study of the collision of a large DNA molecule with an isolated nanowire, whose physics lie at the heart of the separation process. In addition to confirming a number of theoretical predictions, they discovered a new dynamic criterion governing the collision process. The fabrication method developed here could be used in large-scale manufacture of inexpensive DNA separation microdevices.

Method to fabricate ZnO nanowire matrices in a microfluidic channel. By controlling the seeding step, nanowire matrices of different densities are fabricated without using any nanopatterning steps

International Collaboration Highlight

Student exchange with the Chinese Academy of Sciences

From October 25 through December 1, 2010 two Ph.D. students from the Institute of Chemistry Chinese Academy of Sciences, State Key Laboratory of Polymer Physics and Chemistry participated in research projects focused on the thermodynamics of polyolefin blends and the synthesis of new low band gap polymers for organic photovoltaics. They both worked closely with students and postdocs in Prof. Hillmyer's research group, participated in the group meetings of Prof. Hillmyer and Prof. Lodge, and engaged in broader IRG-1 activities. This exchange complements the visit by UMN MRSEC students to the Chinese Academy of Sciences in the summer of 2009 and paves the way for future collaborative activities.

Left: Ye and Jin with two Minnesota Ph.D. students Brad Jones and Erica Redline. Right: Ms. Ye Huang and Ms. Jing Jin upon their arrival to Minnesota

IRG1 Research Highlight

Discovery of a Frank-Kasper σ Phase in Sphere-Forming Block Copolymer Melts

Sphere-forming block copolymers are known to self-assemble into body-centered cubic crystals near the order-disorder transition temperature. Small-angle x-ray scattering and transmission electron microscopy experiments on diblock and tetrablock copolymer melts have revealed an equilibrium phase characterized by a large tetragonal unit cell containing 30 microphase-separated spheres. This structure, referred to as the sigma (σ) phase by Frank and Kasper more than 50 years ago, nucleates and grows from the body-centered cubic phase similar to its occurrence in metal alloys and is a crystal approximant to dodecagonal quasicrystals. Formation of the σ phase in undiluted linear block copolymers (and certain branched dendrimers) appears to be mediated by macromolecular packing frustration, an entropic contribution to the interparticle interactions that control the sphere-packing geometry. The image is a projection of the sigma-phase crystal structure illustrating the square (tetragonal) lattice and columns of dodecagonally arranged spheres positioned with five nearest neighbors. Published in Science 2010, v.330, p. 349-353.

IRG2 Research Highlight

Printed, Flexible Carbon Nanotube Digital Circuits

Graduate student Mingjing Ha working with Optomec, Inc. and Northwestern University collaborators (Mark Hersam) has demonstrated low voltage, fast carbon nanotube (CNT) circuits printed on flexible plastic substrates. The circuits are fabricated by aerosol jet printing from a liquid dielectric ink (ion gel) and a purified semiconducting CNT ink (Northwestern). The printed semiconducting CNTs form the channels in thin film transistors and printed circuits. A five-stage ring oscillator operating at 2 kHz with a 3 V supply voltage was demonstrated, corresponding to a signal delay time of 50 ms. This is the shortest delay time reported for a printed circuit on plastic operating below 3 V, as shown in the lower figure.

IRG2 Research Highlight

Graded Heterojunction Organic Photovoltaic Cells

Graduate student Richa Pandey has demonstrated a new architecture for efficient solar cells through the use of engineered composition gradients of the organic semiconductor active materials. Cells based on graded heterojunctions outperform conventional bilayer and uniformly mixed structures, with the graded cell showing a high power conversion efficiency of 4.2% under 1 sun illumination. This work highlights a new approach to engineering film composition while also providing insight into charge transport in mixtures of different organic semiconductors.

IRG3- Research Highlight

Electrical Detection of the Direct Spin Hall Effect

One of the important goals of semiconductor spintronics is the completely electrical control of spins in the solid state. One classic proposal, dating from the early 1970's, is to use the spin-orbit interaction (an effective magnetic field in the rest frame of an electron) to deflect charge carriers either "left" or "right" depending on their spin. IRG researchers have carried out the first transport measurement of this phenomenon, which is known as the spin Hall effect. This experiment builds on several years of IRG efforts in the integration of ferromagnetic metals into semiconductor heterostructures.

(a) Micrograph of a spin Hall effect device; (b) thespin Hall conductivity as a function of temperature for In_xGa_1-xAs. Solid curves are a theoretical model.

IRG4 Research Highlight

Hot Electron Transfer from Semiconductor Nanocrystals

Will Tisdale and Brooke Timp, two graduate students coadvised by Aydil, Norris and Zhu and partially supported by MRSEC showed for the first time that electron transfer from the higher excited states of a colloidal semiconductor nanocrystal (PbSe) to a common electron acceptor (TiO₂) occurs on an ultrafast time scale (<50 fs). In typical p-n junction solar cells, photons with energies above the semiconductor's bandgap generate "hot" electrons and holes that quickly "cool" to the conduction and valence band edges, respectively. The excess energy is converted to phonons and lost as heat, a process that limits device efficiency. Although semiconductor nanocrystals offer a solution due to their ability to slow this cooling, until Tisdale's work, the extraction of hot carriers had not yet been demonstrated. Tisdale et al. used time-resolved optical second harmonic generation to observe hot electron transfer from colloidal PbSe nanocrystals to ₂, a common wide bandgap semiconductor. Tisdale showed that with ethane dithiol treatment of the nanocrystal surface, this transfer occurs much faster than expected. Hot electron extraction is the necessary first step towards making hot electron solar cells. This study was reported in Science (Tisdale et al., Science 2010, 328, 1543). The next goal is to demonstrate electrical current arising from this hot electron transfer and solar cells that exhibit photovoltaic effect due to hot electron transfer.

Seed Research Highlight

Interfacial Events in Functional OFETs

Professor Aaron Massari and colleagues at the University of Minnesota have married an interface specific spectroscopic technique, called vibrational sum frequency generation, with electrical measurements on organic field-effect transistors. In these devices, the semiconducting material is an organic polymer or small molecule, and during operation in an electrical circuit, only 1-2 nanometers of the material at a buried interface generate the entire device performance. The location and dimension of this active region make it exceedingly challenging to characterize, and typically the behaviors of molecules are inferred from indirect electrical measurements. When these assemblies deteriorate and fail, this indirect approach is no longer viable, leaving many questions unanswered as to the underlying causes of device failure. By characterizing the organic interfaces in OFETs with a non-invasive spectroscopy, the Massari group is able to obtain information about these interfaces that is not available through other methods. In a recent study, the approach was applied to a polymeric OFET that has been reported for over a decade to conduct electrical holes (positive charges) but not electrons (negative charges). Surprisingly, the spectroscopic measurements demonstrated that both positive and negative charges accumulate at this buried interface, though only the positive charges were mobile. This is an important finding that suggests that many materials could be altered to carry both types of charges (ambipolar devices) if the appropriate interfacial chemistry could be developed.

Probing buried interfaces. A team at the University of Minnesota has combined interfacial spectroscopy with electrical testing of OFETs. In this manner, molecular structural changes at buried interfaces can be probed in these devices when electrical measurements cannot be performed. (top) the beam and OFET device geometries, and (bottom) an expanded view of the buried interfaces during device operation.

Industrial Collaboration Highlight

High-Temperature Annealing of Co/Pd-Based MTJ's

The use of MgO as a tunnel barrier material has revolutionized magnetic tunnel junctions, enabling tunneling magnetoresistances of several hundred percent to be achieved. This technology, however, requires recrystallization of the tunnel barrier so that it is registered with the crystalline structure of the surrounding ferromagnetic layers. Unfortunately, this annealing process is not compatible with requirements for the growth and processing of high anisotropy magnetic multilayers. Ultimately, nanoscale tunnel junctions will require both high tunneling magnetoresistance (for signal to noise) and high anisotropy (for stability). Working with researchers at IBM Yorktown Heights have developed an annealing recipe that preserves both the high anisotropy of a Co/Pd multilayer while obtaining the required texture for a FeCoB/MgO/FeCoB tunnel barrier incorporated into the same structure. Ongoing research is dedicated to increasing the tunneling magnetoresistance of these devices.

Left: XTEM micrograph of a CoFeB/MgO tunnel junction with schematic of the registry of FeCo (rotated unit cell) to MgO. Right: Magnetization of a Co/Pd multilayer MgO-based MTJ showing independent switching of the hard and soft layers

IRG1 Highlight

Mechanism of Molecular Exchange in Block Polymer Micelles

Mr. Soohyung Choi, a graduate student co-advised by Professor Bates and Professor Lodge, investigated the molecular exchange dynamics in diblock copolymer micelles. Two pairs of structurally matched poly(styrene-b-ethylene-alt-propylene) (PS-PEP) were synthesized and dispersed in squalane, which is highly selective for the PEP block. Each pair includes polymers with fully deuterated (dPS-PEP) and normal (hPS-PEP) PS blocks. Since the neutron scattering intensity is proportional to the fraction of hPS in the micelles, the molecular exchange rate is measurable by the contrast matching technique in time-resolved small-angle neutron scattering measurements. Not only does temperature impact the exchange rate in a way consistent with polymer melt dynamics, increasing the core PS block length by 60% produces a remarkable 10,000-fold decrease in exchange rate due to the unfavorable interactions associated with ejecting a core block into the solvent. This work provides fundamental understanding of block copolymer micelle dynamics and the attainment of equilibrium structures, which are both relevant to a host of potential applications including drug delivery. Phys. Rev. Lett. 2010, in press. Supported by UMN MRSEC Award DMR# 0819885.

IRG1 Highlight

Multicompartment Micelle Morphology Evolution in Degradable Miktoarm Star Terpolymers

Multicompartment Micelle Morphology Evolution in Degradable Miktoarm Star Terpolymers (IRG-1). Dr. Naohiko Saito, a postdoctoral research associate, and graduate student Chun Liu in the research groups of Professor Hillmyer and Lodge investigated the structural evolution of multicompartment micelles formed in dilute aqueous dispersions of the miktoarm star terpolymer µ-EOC [E: poly(ethylethylene); O: poly(ethylene oxide); C: poly(γ-methyl- ε-caprolactone)] upon the hydrolytic degradation of the C block (pH=12 buffer solution at 50 °C). In neutral water, µ-EOC self-assembles into multicompartment wormlike micelles with the cores consisting of alternating E and C segmented subdomains surrounded by hydrophilic O corona. After two weeks under degradation conditions, most of the starting µ-EOC terpolymers were degraded into EO diblock copolymer and C homopolymer as indicated by size exclusion chromatography (SEC). Correspondingly, the initial segmented wormlike micelles evolved into raspberry-like vesicular (or disklike) micelles with the spherical C subdomains embedded in the E matrix. This dramatic change in the morphology of the multicompartment micelles is due to rearrangement of µ-EOC/EO/C composite micelles to a structure that minimizes unfavorable interfacial interactions between the three mutually immiscible polymers. This type of micelle-to-micelle morphological evolution induced by block degradation in a miktoarm star terpolymer system holds promise for the development of "smart" delivery capabilities including advanced drug delivery. ACS Nano 2010, submitted. Supported by UMN MRSEC Awards DMR# 0212302 and 0819885.

IRG2 Highlight

Charge Transfer Excitons at Organic Semiconductor Interfaces

When a material of low dielectric constant is electronically excited by a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic-hydrogen-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor (D/A) interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. From typical dielectric constants of organic semiconductors and sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a D/A interface is one order of magnitude greater than k_BT at room temperature (k_B, the Boltzmann constant; T, temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? Dr. Matthias Muntiwler, a postdoc in the Zhu lab addressed the details of this important question. He uses a thin film of an organic semiconductor, pentacene, as a model system and excites an electron above the surface to probe the CT excitons. When there is only the excess electron, the electron is bound to the surface by the positive polarization cloud in the pentacene molecules; this attractive potential leads to a bound electronic state called an image band. When a positive hole is present on a pentacene molecule, the electron is attracted to the surface by both the positive hole and the polarization cloud; the result of this combined attractive potential is approximately the CT exciton referenced to the image band. We observe in time-resolved two-photon photoemission spectroscopy a series of CT excitons with binding energies ≤0.5 eV below the image band minimum. These CT excitons can be accurately modeled as solutions to the atomic-hydrogen-like Schrödinger equation with cylindrical symmetry, characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character (CT_1s) is more than one order of magnitude larger than k_BT at room temperature. The CT_1s exciton—often referred to as the so-called exciplex—has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells for three reasons. (1) Compared to CT_1s, hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated. (2) Density-of-states of these hot excitons increase with energy in the Coulomb potential. (3) Electronic coupling from a donor exciton to a hot CT exciton across the D/A interface can be higher than that to CT_1s as expected from energy resonance arguments. Accordingly, we suggest a design principle in organic heterojunction solar cells: there must be strong electronic coupling between molecular excitons in the donor and hot CT excitons across the D/A interface.

M. Muntwiler, Q. Yang, W. A. Tisdale, X.-Y. Zhu, "Coulomb barrier for charge separation at an organic semiconductor interface," Phys. Rev. Lett. 101, 196403 (2008).
X.-Y. Zhu, Q. Yang, M. Muntwiler, "Charge transfer excitons at organic semiconductor surfaces and interfaces," Acct. Chem. Res. 42 (2009) 1779-1781.
M. Muntwiler, Q. Yang, X.-Y. Zhu, "Exciton dynamics at interfaces of organic semiconductors," J. Elec. Spec. Relat. Phenom. 174 (2009) 116-124 .
Q. Yang, M. Muntwiler, X.-Y. Zhu, "Charge transfer excitons and image potential states on organic semiconductor surfaces," Phys. Rev. B 80 (2009) 115214 .

IRG3 Highlight

Enhancement of Electrical Spin Detection Sensitivity

In collaboration with Los Alamos National Laboratory, MRSEC students Madhukar Reddy and Qi Hu, working with Professors Palmstrøm and Crowell, showed how the spin detection sensitivity of an Fe/GaAs epitaxial Schottky tunnel barrier could be enhanced with electrical bias [Phys. Rev. B 80, 041305R (2009)]. In the IRG's previous work, spin-polarized electrons were pumped into a semiconductor channel and then detected by an ordinary voltage measurement. The recently published work showed how this signal can be enhanced when a second bias current is added to the device (see figure). The additional electric field due to a charge current flowing in the detector also enhances the spin current. In the most recent devices, the observed spin signals can be enhanced by a factor of 50 – 100 over the values obtained without a biased detector. This provides a definitive enhancement of spin accumulation by electric fields, an approach that is possible only because the spin transport medium is a semiconductor. This work also showed how the effects of interfacial electronic structure can be separated from the enhancement of the signal by electric field effects. Supported by UMN MRSEC Awards DMR#0212302 and 0819885.

IRG2 Highlight

Enhanced Exciton Diffusion using Phosphor Sensitization

Graduate student Wade Luhman working with Professor Holmes is examining approaches to overcome the exciton diffusion bottleneck in organic solar cells. In contrast to the morphology-based approaches often employed, Luhman is instead using energy transfer via a phosphorescent sensitizer to populate the long-lived triplet exciton state of a fluorescent donor material, permitting an increase in LD and active layer thickness. Here, the donor layer consists of an N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPD) host doped with the phosphorescent guest fac-tris(2-phenylpyridine) iridium (Ir(ppy)3), with a thin acceptor layer of C60. The enhancement in donor LD relies on a multi-step process beginning with the absorption of light and formation of singlet excitons on the NPD host. Photogenerated excitons are transferred to the singlet state of the Ir(ppy)3 guest by Förster transfer, followed by rapid intersystem crossing to the triplet state. A final transfer occurs from the triplet level of Ir(ppy)3 to that of NPD. With the photogenerated excited state occupying the NPD triplet, a larger LD is possible due to the long lifetime of the triplet relative to the singlet. An increase in the NPD LD from (6.5±0.3) nm to (11.8±0.6) nm is extracted from measurements of the external quantum efficiency for donor layers containing 5 wt.% Ir(ppy)3. This enhancement leads to a ~80% improvement in the power conversion efficiency relative to devices containing an undoped donor layer. This approach is of interest since it allows for a decoupling of the functions of optical absorption and exciton diffusion, potentially broadening the scope of materials suitable for use in OPVs.

IRG4 Highlight

Auger Recombination in Quantum Dot Materials

Auger recombination is a mechanism in which the excitation energy stored in an electron-hole pair is transferred to another charge carrier. It is an important mechanism that can limit the performance of semiconductor lasers and solar cells. Ryan Gresback, a graduate student in Professor Kortshagen's group, was involved in a study with the group of Dr. Victor Klimov at Los Alamos National Lab, comparing Auger recombination rates of direct and indirect band gap semiconductor nanometer-sized crystals, so called quantum dots. In bulk semiconductors, Auger recombination rates differ by 4-5 orders of magnitude for direct and indirect band gap materials. In this study, the researchers reported the first experimental observation of a striking convergence of Auger recombination rates in quantum dots of both direct (InAs, PbSe, CdSe) and indirect band gap (Ge) semiconductors, which is in contrast to the dramatic difference in the Auger decay rates in respective bulk solids. This study supports that at the nanometer-size scale the difference in the photophysics of direct and indirect band gap materials ceases to exist. The figure shows the Auger recombination rate coefficients measured in this study. Supported by UMN MRSEC Award DMR# 0819885.

IRG3 Highlight

Probing the Spin Polarization of Sulfide Spintronic Materials

This work was performed by graduate student Mike Manno and undergraduate student Rachel Frakie in Chris Leighton's group in IRG3. Collaborators include B. Bolon (Hamline University, funded by the MRFN), C. Utfeld, S. Giblin, J. Taylor, J. Duffy, C. Shenton-Taylor, J. Laverock and S. Dugdale (University of Bristol, ISIS, and the University of Warwick) and M. Itou and Y. Sakurai (Spring-8, Japan). The research published in the Phys. Rev. Lett. paper involved the development of a new method to determine bulk spin polarization based on high resolution Compton scattering analyzed using first principles electronic structure calculations. The method was applied to the unique Co_1-xFe_xS₂, using spin-dependent intergranular tunneling to estimate spin polarizations as material that our IRG has recently established as a tunable highly spin-polarized system, ideal for fundamental studies of heterostructured spintronic devices. In addition to establishing reliable new means to determine bulk polarization, this work also provided critical support for the concept of a sign reversal in the spin polarization of this material with increasing doping. The Appl. Phys. Lett. paper built on our recent development of reliable means to deposit high quality thin films of Co_1-xFe_xS₂, using spin-dependent intergranular tunneling to estimate spin polarizations as high as 90 % at x = 0.15. The figure shows a typical intergranular MR (magnetoresistance) vs. H (magnetic field) curve, along with a schematic of the spin orientations of individual grains at the coercive point and in saturation. The grain boundaries between grains act as weak tunnel barriers. Supported by UMN MRSEC Awards DMR#0212302 and 0819885

M. Manno, R. Frakie*, B. Bolon, C. Leighton, Appl. Phys. Lett. 95 182510 (2009)

C. Utfeld, S.R. Giblin, J.W. Taylor, J.A. Duffy, C. Shenton-Taylor, J. Laverock, S.B. Dugdale, M. Manno, C. Leighton, M. Itou, Y. Sakurai, Phys. Rev. Lett. 103 226403 (2009)

Seed Highlight

Ultrasmooth Patterned Metals for Plasmonics

Professor David Norris, Professor Sang-Hyun Oh, and colleagues at the University of Minnesota have developed a simple method for fabricating patterned metal films with ultrasmooth surfaces. When light strikes such films, it can excite surface plasmons, which are electron density fluctuations that travel along the metal interface. Because these waves allow the concentration of light in nanometer-scale volumes, they have implications not only for fundamental studies but also for applications such as imaging, sensing, and solar cells. Thus, over the last decade the field of plasmonics has arisen to study and harness surface plasmons. Unfortunately, this field has been hindered by difficulties in fabricating patterned metal films that are smooth. Even nanometer-scale roughness at the interface can cause significant scattering of propagating plasmons, which greatly diminishes their ability to concentrate light. Avoiding these problems has been a major challenge with no reported solutions. By patterning a silicon wafer and depositing a metal film on it, the Minnesota team showed that the metal could be peeled off to expose an ultrasmooth metal replica of the patterned wafer. This process works due to the poor adhesion of noble metals on silicon. Using their approach, the Minnesota team demonstrated many high-quality plasmonic structures for the first time (see figure). This includes not only simple patterns in silver, gold, and copper, but also ultrasharp tips that are ideal for extreme localization of light. Therefore, this method solves a major problem that had previously impeded the field of plasmonics. Supported by UMN MRSEC Awards DMR#0212302 and 0819885

IRG-1 Research Highlight

ABC Triblock Thin Films for Large-Area Nanolithography

A general method for nanopatterning of large-area inorganic anti-dot arrays based on the use of cylinder forming triblock terpolymers and a simple pattern transfer process was discovered by Toshi Kubo and Dr. Ruifang Wang working with Professors Leighton (IRG 3) and Hillmyer (IRG 1). The use of a low surface energy B block in ABC triblock terpolymers drives spontaneous perpendicular alignment of the cylindrical domains, eliminating the need for lengthy anneal steps, external field alignment procedures, or specialized surface preparations that could restrict potential technological applications. Scanning probe microscopy combined with pattern transfer characterization of underlying magnetic thin films demonstrates that the results are comparable to those obtained with conventional thermal annealing. The figure depicts (a) a schematic of the initial structure consisting of a 50 nm thick triblock film on a NiFe/Au/Si/SiO2 substrate; (b) an AFM image after spin casting of the polymer; (c) an AFM image after removal of the cylinders [purple in panel (a)] by chemical etching; and (d) an AFM image of the nanostructured NiFe film after Ar ion beam milling. Utilization of these and related triblock templates for the formation of nanodot arrays is currently under investigation. These new thin film templates hold great promise for the ready generation of large-area magnetic nanodot arrays for high-density storage media applications.

IRG-1 Research Highlight

Robust Hydrophilic Nanoporous Membranes

Dr. Monique Roerdink, a postdoctoral research associate in Professor Hillmyer's group has expanded the applicability of ABC triblock terpolymers that contain a robust A block (polystyrene), a hydrophilic B block [poly(dimethylacrylamide)], and an etchable C block (polylactide) developed in the IRG (structure depicted below). She has been able to incorporate a novel metathesis reactive monomer into the polystyrene blocks and has used the multifunctional triblocks to prepare new robust bicontinuous membranes containing hydrophilic channels. The membranes were prepared by first polymerizing dicyclopentadiene (DCPD) using a ruthenium metathesis catalyst in the presence of the ABC triblock and a solvent. Optically clear membranes were cast from this solution and then were treated with dilute aqueous base to remove the polylactide component and generate the nanoporosity. An SEM image of the new nanoporous membranes is shown below. These membranes should be quite useful in water-purification applications given their hydrophilic nature, excellent hydraulic permeability and narrow pore size distribution.

IRG-2 Research Highlight

Printed Organic FETs on Plastic.

Tim Lodge (IRG 1) and Dan Frisbie (IRG 2). Industrial Collaborator: Mike Renn, Optomec, Inc. Lodge and Frisbie have developed a novel printable, high capacitance gate insulator material that facilitates low voltage operation of OFETs on plastic substrates. The insulator is a so-called ion gel composed of an ionic liquid and a gelating triblock copolymer, polystyrene-polyethyleneoxide-polystyrene. This material may be dissolved in solvents and printed. In a collaboration with an industrial manufacturer of aerosol jet printers (Optomec, Inc), Lodge, Frisbie, and their students have demonstrated successful low voltage operation of an array of ion-gel gated OFETs printed on flexible polyimide substrates. All components of the OFETs were printed—the metal electrodes (gold colloidal ink), the semiconductor (poly(3-hexylthiophene), and the gate insulator (the new ion gel material). The devices work at biases less than 3V and at speeds up to 10 kHz. Efforts to optimize the ion gel material and the transistor geometry for even better performance are underway. A patent on the new ion gel material has been filed by Frisbie and Lodge.

IRG-2 Research Highlight

Two Photon Photoemission (2PPE) Spectroscopy of Pentacene

Xiaoyang Zhu (IRG 2). Understanding electronic energy levels in organic semiconductors is critical to their application in devices. Zhu has used 2PPE to measure the energy to excite an electron out of a layer of the organic semiconductor pentacene. This measurement is relevant to the use of pentacene in organic (plastic) solar cells. In the experiment, a pump photon (blue) was used to excite an electron from a pentacene molecule to a charge transfer (CT) state, in which the excited electron occupies a level just outside the pentacene layer, i.e., in the adjacent vacuum environment. This state is called a CT exciton state. A second photon (red) kicked the electron out of the CT exciton state up to the vacuum level. By measuring the electron kinetic energy, Zhu determined the binding energy of the CT exciton to be 0.45 eV. This is the energy that is required to completely separate the electron from the positively charged hole on pentacene. Knowing this number is important to understanding how organic solar cells function. One conclusion is that charge separation at a pentacene/acceptor interface in an organic solar cell must involve hot CT excitons (higher energy CT states) so that the 0.45 eV Coulomb penalty is avoided.

IRG-3 Research Highlight

Understanding Magnetic "Exchange Pinning"

Magnetic storage of digital data is now possible at densities approaching 1 Terabit per square inch at a cost of only about a tenth of a cent per Megabit. To a large extent, the breathtaking progress in this area of technology is sustained by discovery of improved devices for detecting the magnetic field from increasingly miniaturized magnetic bits. The invention of "GMR" sensors based on stacks of ultra-thin films of magnetic metals (for which the Nobel Prize in Physics was awarded in 2007) is a perfect example. In these devices the ability to "pin" the magnetic orientation of one film in the stack while allowing another to respond freely to a magnetic field is a key principle. This "magnetic pinning", or "exchange pinning", is achieved by layering a simple magnet with a more complex magnetic material called an antiferromagnet. This pinning effect was discovered over 50 years ago, has been used in devices for over a decade, but is poorly understood. One of the major reasons for this is that the pinning effect is due to atomic defects in the antiferromagnet. Experiments are hindered by the fact that these defects are difficult to probe, while theoretical research is hampered by the fact that it is difficult to incorporate such defects in calculations. Recently, IRG 3 students Jyo Saha, Mike Lund, and Mun Chan, working with postdoc Jeff Parker and IRG 3 faculty Chris Leighton, Randy Victora, and Paul Crowell, have performed one of the most detailed studies of this exchange pinning in materials very similar to those used in hard disks. Most importantly, the experimental results were directly compared to realistic micromagnetic simulations. These supercomputer simulations break the sample up into hundreds of thousands of tiny magnetic elements (see figure), and are sufficiently powerful that the all-important defects can be properly accounted for. This approach has enabled the researchers to understand important factors such as the influence of layer thickness and grain size, complex magnetic switching, and even a mysterious effect where the exchange pinning deteriorates over time, a major problem for applications. [Saha, J.; Victora, R.H. Spontaneous Exchange Bias: Uniaxial Anisotropy in an Otherwise Isotropic System. Phys. Rev. B, 2007, 76, 100405(R). Saha, J.; Bolon, B.; Abin-Fuentes, A.; Parker, J.S.; Leighton, C.; Victora, R. Comparison Between Micromagnetic Simulation and Experiment for the Co/γ-Fe₅₀Mn₅₀ Exchange-biased System. J. Appl. Phys. 2007, 102, 073901; Chan, M.; Parker, J.S.; Crowell P.A.; Leighton, C. Identification and Separation of Two Distinct Contributions to the Training Effect in Polycrystalline Exchange Biased Co/FeMn Bilayers. Phys. Rev. B. 2008, 77, 014420; Lund, M.; Leighton C. Interplay Between Reversal Asymmetry, Training, and Induced Anisotropy in Epitaxial Exchange-Biased Bilayers. Phys. Rev. B 2007, 76, 104433.]

Proto-IRG (IRG4) Research Highlight

Plasmas for Ink-Jet-Printable Silicon

Silicon is the most widely used material in electronic devices and solar cells. At present, silicon is mainly used in the form of wafers cut from crystalline material, or amorphous material deposited using vacuum processes. Unfortunately, for applications such as solar cells the cost of these forms of silicon is still too high. Currently, efforts are underway in many industrial and academic laboratories to develop inks of silicon nanocrystals that may be used to ink-jet print silicon films. This manufacturing approach has the potential to significantly lower the cost in certain areas of silicon technology. Lorenzo Mangolini, a graduate student, and Xiaodong Pi, a post-doctoral researcher working with mechanical engineering professor Uwe Kortshagen, have developed a simple approach to produce inks of electrically doped silicon nanocrystals. Silicon nanocrystals are produced in a low-pressure plasma approach, in which silicon crystals between 2-6 nm in size are formed within a few milliseconds. By co-injecting dopant precursors the nanocrystals can be doped with donor and acceptor atoms. These doped nanocrystals are then immediately injected into a second plasma, in which organic molecules are grafted onto the silicon nanocrystal surfaces. The so functionalized nanocrystals are soluble in organic solvents and can be processed into nanocrystal inks. [Mangolini, L.; Kortshagen, U. Plasma-Assisted Synthesis of Silicon Nanocrystal Inks. Adv. Mater. 2007, 19, 2513.]

Seed Highlight

Biorenewable Thermoplastic Elastomers

(William Tolman, Thomas Hoye, Marc Hillmyer)

Given the rising price of oil and finite nature petrochemical reserves, there has been a recent drive to use biorenewable materials as feedstocks for the synthesis of new polymers with properties competitive with more traditional petroleum-based materials. Carolyn Wanamaker, a graduate student working with Prof. Marc Hillmyer and Prof. William Tolman, has developed a new approach for the preparation of all-biorenewable thermoplastic elastomers from ABA triblock copolymers with polylactide hard (A) segments and polymenthide (B) soft segments. Polymenthide is derived from the widely available terpene menthol and can be readily prepared using simple catalysts. Wanamaker successfully prepared a range of ABA triblocks using a difunctional initiator approach by first preparing dihydroxy polymenthide of controlled molecular weight. From the terminal hydroxy end groups of this material she was able to grow polylactide segments of specified length. Small-angle x-ray scattering and mechanical properties analyses were performed on the triblocks; they formed various ordered state morphologies and showed mechanical performance comparable to traditional polystyrene-polyisoprene-polystyrene triblock thermoplastic elastomers (see the figure below). Recently Wanamaker has demonstrated that using isotactic polylactide end blocks can significantly improve the mechanical strength and modulus of these novel triblocks, thus rendering them potentially useful for a range of applications. In addition, she has shown that these materials can degrade in aqueous media, making them attractive for uses in the biomedical device arena.
[Wanamaker, C.L.; O'Leary, L.E.; Lynd, N.A.; Hillmyer, M.A.; Tolman, W.B. Renewable Resource Thermoplastic Elastomers based on Polylactide and Polymenthide. Biomacromolecules 2007, 8, 3634.] Supported by UMN MRSEC Award DMR#0212302.

Seed Highlight

Zeolite Growth in Confined Spaces

(Lee Penn, Alon McCormick, Andreas Stein, Michael Tsapatsis)

Zeolites are aluminosilicate materials with pore size of molecular dimensions. The purpose of the current seed project was to investigate if zeolite particles with tightly controlled sizes and shapes can be made by hydrothermal growth under confinement in precisely defined pore spaces. Such zeolite particles are desirable for applications in adsorption, catalysis and membrane separations. Fabrication of ultra-thin molecular sieve membranes by seeded growth (1) is the application of interest for the group of co-PIs. In addition to the technological potential, synthesis of zeolites in confinement is of fundamental interest in understanding nucleation and growth phenomena (2,3). Under appropriate conditions, it was possible to produce zeolite spheres conforming precisely to the pore space of 3-DOM carbon templates (4). The SEM micrograph in the bottom shows zeolite spheres formed inside the 3DOM carbon pores. The TEM micrograph in the top left shows one zeolite sphere and the one in the top right shows a high magnification image, with the FFT in the inset, confirming the polycrystalline nature of an all-silica zeolite. Preliminary results from this seed grant led to a recently funded NIRT project (NIRT: Precise Building Blocks for Hierarchical Nanomanufacturing of Membranes with Molecular Resolution, CMMI 0707610). Work is in progress on extending the technique in order to isolate the smallest possible zeolite crystals in high yield and investigate their use in the fabrication of molecular sieve films thinner than 100nm. Supported by UMN MRSEC Award DMR#0212302.

Education Highlight

UMN MRSEC Hosts Davidson Institute Young Scholars.

On June 23, 2007, 100 students, ages 6-15, each with a parent, attended a day-long program Materials for Energy and Nanotechnology, put on by MRSEC faculty and students. The students were attendees at the Davidson Institute for Talent Development Young Scholars Annual Meeting, held in St Paul. Events included a plenary lecture on Chemical Energy by Frank Bates and Marc Hillmyer, visits to two or three faculty labs and characterization facilities, a barbeque on the Northrop Mall, and a show by the Physics Force.

Education Highlight

Polymer Day - You Make It, You Break It

Four faculty (Bates, Hillmyer, Lodge, Macosko) and seven graduate students (Erin Arndt, Mike Bluemle, Will Gramlich, Carlos Lopez, Louis Pitet, Zach Thompson, Ling Zhang) from IRG-1, Microstructured Polymers, led a day-long class for 23 high school students. The students were participants in a weeklong summer camp, Exploring Careers in Science & Engineering, put on by the Institute of Technology's Center for Educational Programs (ITCEP, http://www.itcep.umn.edu/index.php). During the morning the students synthesized three different polymer materials in the Chemistry Department, including a biorenewable polymer, polylactide. In the afternoon the students went over to Chemical Engineering & Materials Science, and subjected their polymers to various physical tests.

Industrial Partnership Highlight (IRG-1)

Block Copolymer Toughened Epoxy

Fundamental research conducted by IRG-1 on the self-assembly of amphiphilic block copolymers in water has led to the recent commercialization of copolymer additives as a toughness enhancer in advanced epoxy formulations. A crucial insight was the realization that the same physical principles that govern the choice of micellar morphology in water might also apply in low molar mass epoxy precursors. Then, the exciting discovery was made that the micellar morphology and dispersion could be preserved during epoxy curing, leading in some cases to a remarkably tougher thermoset material. The transmission electron microscopy image of poly(butyleneoxide)-poly(ethylene oxide) wormlike micelles formed in a cured bisphenol A epichlorohydrin / phenol novalac epoxy resin shown below is representative of the types of composites that give dramatically improved toughness over the pristine epoxy.

The following text is extracted from a press release of November 21, 2007 (http://news.dow.com/dow_news/prodbus/2007/20071121a.htm): Dow Epoxy, a global business unit of The Dow Chemical Company and its affiliates ("Dow"), today introduced FORTEGRA™ Epoxy Tougheners, low viscosity materials for use in amine, DICY, anhydride and phenolic cured epoxy systems.... The FORTEGRA line is based on a specially designed self-assembling block copolymer that creates the particles needed for toughening the cured epoxy and, at the same time, does not result in big changes to other properties such as viscosity, cure speed or chemical resistance. "With FORTEGRA Epoxy Tougheners, formulators no longer have to make big compromises on key properties to improve the toughness of epoxy systems as they are forced to do today with alternative materials," said Ohnishi Hideyuki, global business director for Dow Epoxy's Performance Resins group. And because these toughening products are effective at levels as low as 3 to 10 percent by volume, not as much is needed compared to similarly priced alternative products – making FORTEGRA Epoxy Tougheners a more reasonable choice."

IRG-3 Research Highlight

Electrical Detection of Spin Transport in Semiconductors

In semiconductor spintronics, the spin of the electron carries information for both storage and data processing. To some extent, the electron spin can be viewed as a miniature bar magnet that interacts with a magnetic field inside the semiconductor. The orientation of the bar magnet acts as a "bit" of information. Many laboratory demonstrations of spintronics have relied on sophisticated optical techniques for reading out the spin state of electrons. Future applications, however, will require a simple readout scheme using ordinary ferromagnetic materials (such as iron) and conventional electronics. IRG 3 graduate students Xiaohua Lou and Madhukar Reddy, IRG 3 postdoc Christoph Adelmann, and Professors Paul Crowell and Chris Palmstrøm have demonstrated a simple functional spintronic device in which spins are injected into a semiconductor channel from a ferromagnetic source and detected at a separate electrode. The measurement is completely electronic in character and is shown to be sensitive to the precession of the electron spin in the semiconductor. Working with collaborator Scott Crooker (Los Alamos National Laboratory), the IRG researchers have shown that their electronic device produces an output that is essentially identical to that obtained using more established optical techniques. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302. [Lou, X.; Adelmann, C.; Crooker, S.A.; Garlid, E.S.; Zhang, J.; Reddy, K.S.M.; Flexner, S.D.; Palmstrøm, C.J.; Crowell, P.A. Electrical Detection of Spin Transport in Lateral Ferromagnet-Semiconductor Devices. Nature Physics 2007, 3, 197]

Schematic of the spin transport device. The large arrows show the configuration of the magnetic electrodes. The small arrows show the electron spins in the semiconductor. The electron spins can be read out using an ordinary voltmeter (V), or with an optical Kerr microscope (shown focusing on the device).

Proto-IRG Research Highlight

Skimming the Surface

Many of the properties of a nanoparticle are determined, at least in part, by the structure and composition of the surface. This explains why researchers across the globe are developing ways of studying nanoparticle surface chemistry and methods for rationally functionalizing the surfaces of synthetic nanoparticles. Consider, for example, silicon nanocrystals. Their dispersibility in a given solvent, their stability with respect to oxidation, their photophysical behavior: all of these, and other, properties are strongly influenced by bonding of the very outer layer of silicon atoms. Jason Holm, a Mechanical Engineering graduate student working with Chemistry professor Jeff Roberts, has just described a new way of studying the surfaces of pristine silicon nanocrystals in the aerosol state. The method relies on the measurement of extraordinarily small changes in particle size, smaller than have ever been reported for this type of aerosol instrumentation. It was used to monitor desorption of hydrogen from the surfaces of 6 nm-diameter silicon nanoparticles. Hydrogen thermally desorbs over the same temperature range at which it desorbs from a silicon wafer, through a complicated process involving the gradual conversion of high hydride into low hydride species. These findings may be useful in developing processes for depositing organic monolayers onto the particle surfaces. This work was supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302. [Holm, J.; Roberts, J.T. Surface Chemistry of Aerosolized Silicon Nanoparticles: Evolution and Desorption of Hydrogen from 6-nm Diameter Particles. J. Am. Chem. Soc. 2007, 129, 2496]

IRG-1/IRG-2 Research Highlight

Ion Gel-Gated Polymer Thin Film Transistors

A major goal of organic electronics is the development of new kinds of solution processable organic dielectric materials that can serve as gate insulators in organic thin film transistors (OTFTs). An important figure of merit for prospective gate dielectrics is the specific capacitance, which determines how much charge can be induced in the semiconductor channel of an OTFT for a given applied gate voltage; higher capacitance translates into higher induced charge densities and therefore both higher ON currents and lower switching voltages. In an inter-IRG collaboration, postdoctoral fellows Jiyoul Lee (IRG 2) and Yiyong He (IRG 1) and graduate student Matt Panzer (IRG 2) demonstrated that a gel electrolyte (a so-called "ion gel") based on a mixture of an ionic liquid and a block-copolymer can provide both large specific capacitance (>10 mF/cm2) and greatly improved polarization response times (~1 ms) when used as the gate dielectric in a polymer TFT (Figure 1). The improved properties allow transistor operation at frequencies greater than 500 Hz, significantly faster than what has been demonstrated previously with conventional polymer electrolytes and opening the door to a broader range of applications. Furthermore, the ion gel material is solution processable, making it potentially compatible with high throughput patterning methods (e.g., ink jet printing). A provisional patent was filed on 2/9/07. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302. [Lee, J.; Panzer, M.J.; He, Y.; Lodge, T.P.; Frisbie, C.D. Ion Gel Gated Polymer Thin Film Transistors. J. Am. Chem. Soc. 2007, in press]

IRG-1 Research Highlight

Block Copolymer Toughening of Epoxy Resins

Frank Bates, Marc Hillmyer and their co-advised student Zach Thompson in the Department of Chemical Engineering and Materials Science at the University of Minnesota have been exploring the self-assembly of block copolymers in thermoset resins in order to uncover the fundamental mechanisms responsible for extraordinary fracture toughness observed at very low levels of block copolymer incorporation (1-5%). This work follows the Ph.D. thesis completed by June Wu in December of 2005. The transmission electron microscopy image of poly(butyleneoxide)-poly(ethylene oxide) wormlike micelles formed in a cured bisphenol A epichlorohydrin / phenol novalac epoxy resin shown below is representative of the types of composites that give dramatically improved toughness over the pristine epoxy. This new thrust represents a collaboration with the Dow Chemical Company and scientists at Texas A&M. This was supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302.

IRG-1 Research Highlight

Mesoporous Membranes Templated by a Polymeric Bicontinuous Microemulsion

Tim Lodge, Frank Bates, and their co-advised student Ning Zhou in the Department of Chemical Engineering and Materials Science at the University of Minnesota have demonstrated a new and versatile route to porous materials, with typical channel dimensions between 50 and 200 nanometers. This work has just appeared in Nano Letters. Ning showed that a bicontinuous microemulsion could be prepared from a mixture of polystyrene, polyisoprene, and a polystyrene-b-polyisoprene diblock copolymer surfactant. At room temperature the glassy polystyrene maintained the structure, and gaseous sulfur chloride was used to cross-link the polyisoprene domains. The polystyrene phase was then removed simply by solvent extraction leaving the crosslinked polyisoprene as a self-supported membrane. The porosity was characterized by small-angle x-ray scattering and by nitrogen adsorption analysis. After backfilling the pores with an ionic liquid, the resulting ionic conductivity proved the continuity of the pore structure through a 1 mm thick sample. A scanning electron micrograph of the material is shown in the figure. These materials may prove very useful as selective separation membranes, catalytic membrane reactors, and template for the formation of other nanostructured materials. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302. [Zhou, N.; Bates, F.S.; Lodge, T.P. Mesoporous Membranes Templated by a Polymeric Bicontinuous Microemulsion. Nano Lett. 2006, 6, 2354]