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IRG3 Research Highlight

Polyprotic Buffers Affect Polyelectrolyte Micelles

Principal Investigators: Tim Lodge and Theresa Reineke (IRG-3)

Charged nanoparticles, such as polyelectrolyte micelles, are of increasing interest in diverse applications, including gene therapy. The dimensions of these objects are critical determinants of their performance, yet their size is affected by the surroundings. In particular, we have shown that it is not just the pH that matters, but also whether the pH is established by a monoprotic or polyprotic buffer. This can be explained by a selective partitioning of polyanions (e.g., phosphate, sulfate) into the outer region of the micelle. This effect has not been documented before, but is of direct relevance to physiological conditions, where polyanions are abundant.

IRG1 Research Highlight

Approaching a Two-Dimensional (2D) Metallic State on the Surface of the Organic Semiconductor Rubrene

Principal Investigators: Dan Frisbie and Chris Leighton (IRG-1)

Whether metallic behavior can exist in 2D materials is a question that has troubled condensed matter physics for decades. Although originally thought impossible, evidence f or such in ultra-clean doped inorganic semiconductors like Si and GaAs eventually changed the prevailing view. Research performed in IRG-1 using an approach to doping known as electrolyte gating has now shown that highly conductive (close to metallic) behavior can also be seen in 2D in an organic semiconductor, rubrene. This was enabled by techniques that increase the density of holes on the surface by a thousand times over prior work. The mobility of the holes in rubrene remains far lower than inorganic semiconductors, however, raising perplexing questions about the origin of the conductive state.

IRG1 Research Highlight

Facile Oxygen Vacancy Doping of the High Room Temperature Mobility Oxide Semiconductor BaSnO3

Principal Investigators: Bharat Jalan and Chris Leighton (IRG-1)

Complex oxide materials are extraordinarily functional, and are promising for next generation “oxide electronic” devices. A weakness of these materials, however, is that they support high electron mobility at cryogenic temperatures, but this is difficult to translate to room temperature. BaSnO3, an emerging material with record room temperature mobility is thus of high current interest. In work performed in IRG-1, researchers have now demonstrated an effective and simple approach to doping this material, simply by annealing it in vacuum to form oxygen vacancies. This offers a number of potential advantages over other methods, which require the introduction of impurities. The work could have impact in oxide electronics, as well as transparent conductors for devices.

BSO Highlight Figure

MRSEC Education-Related Accomplishment

MRSEC Research Experiences for Teachers (RET) Student Expo

On May 20, 2015, over 250 middle and high school students participated in the inaugural MRSEC Research Experiences for Teachers (RET) Student Expo. The Expo extends the impact of the MRSEC RET program beyond participating teachers to their students via direct interaction with UMN researchers. During the school year, a secure website was set up to allow students to ask questions of the same researchers who mentored their teachers during the summer at UMN. After successful completion of the classroom research experience, the students were invited to the UMN campus to present their work in person via the Student Expo Poster Session. A full day of activities was planned leading up to the poster session, which included an admissions presentation, scientific demonstration show, and tours of the Minnesota Nano Center, Valspar Materials Lab, and seven faculty laboratories.

MRSEC Research Experiences Expo

MRSEC Highlight

COACh Workshop sponsored by UMN MRSEC

Christy Hayes (IRG-2) and Theresa Reineke (IRG-3) organized two COACh ( Workshops at the University of Minnesota, one for female faculty, and one for female postdocs and grad students in the MRSEC affiliated departments - Chemistry, Chemical Engineering and Materials Science, Mechanical Engineering, Physics, and Electrical and Computer Engineering.  The 2 half-day workshops entitled, "Strategic Performance" and "Academic Leadership" were held on Thursday, November 5th from 8am to 5pm. All participants were invited to a noon lunch. The workshop was facilitated by Nancy Houfek and Jane Tucker.

IRG4- Research Highlight

Aydil group research featured on the cover of the Journal Energy and Environmental Science

Bin Liu's research in the Aydil group was published in and featured on the cover of the Journal Energy and Environmental Science. Converting sunlight to fuels is a sustainable approach to reduce our dependence on oil, coal and natural gas. The key barrier for solar-to-fuel conversion is the development of stable and efficient photocatalysts that absorb visible light. Liu et al. report a simple solvothermal method for synthesis of carbonate doped mesoporous titanium dioxide microspheres with high surface area. The key advance is the introduction of carbonate as a dopant to extend the light absorption of titanium dioxide from the the ultraviolet to the visible region, which significantly increases the photoactivity of this material. For more information see!divAbstract

IRG4- Research Highlight

Rapid facile synthesis of Cu2ZnSnS4 nanocrystals

Boris Chernomordik's recent article on facile synthesis of the earth abundant photovoltaic material copper zinc tin sulfide (CZTS) is featured on the cover of Journal of Materials Chemistry A. Boris collaborated with graduate students Nancy Trejo and Aloysius Gunawan and undergraduate students Amelie Béland and Donna Deng to synthesize CZTS nanocrystals via thermolysis of copper, zinc and tin diethyldithiocarbamates. Boris, Nancy, Amelie and Donna are advised by Prof. Eray Aydil while Aloysius is advised by Prof. Andre Mkhoyan. The average nanocrystal size could be easily tuned between 2 nm and 40 nm, by varying the synthesis temperature. The synthesis is rapid and is completed in only a few minutes. The MRSEC team showed that films cast from dispersions of 20-30 nm nanocrystals could be annealed to form crack-free polycrystalline thin films with microstructure suitable for solar cells. The article and supporting information is available at :

B. D. Chernomordik, A. E. Béland, N. Trejo, A. Gunawan, D. D. Deng, K. A. Mkhoyan and E. S. Aydil “Rapid Facile Synthesis of Cu2ZnSnS4 Nanocrystals,” J. Mater. Chem. A 2, 10389−10395 (2014).

IRG- Research Highlight

Kondo Physics in Non-Local Metallic Spin Transport Devices

In collaboration with the group of Dr. Valeria Lauter at Oak Ridge National Lab, MRSEC post-doc Dr. Liam O’Brien, MRSEC students Dima Spivak and Dr. Mike Erickson, and MRSEC faculty Paul Crowell and Chris Leighton have recently reported a solution to a long-standing puzzle in metallic spintronics. Their work addresses the perplexing non-monotonicity in the temperature dependence of the spin accumulation signal in metallic non-local spin-valves, where the spin signal unexpectedly decreases at low temperatures. Such devices generate pure spin currents, and thus great insight into the fundamentals of spin injection, transport and relaxation, as well as being candidates for highly-scalable read head technologies for hard disk drives. By significantly expanding the range of ferromagnetic and non-magnetic metals studied the team discovered that the effect in question is not a property of the magnetic injector or non-magnetic channel alone, but rather a property of the interface between the ferromagnet/non-magnet pair. The puzzling downturn in spin accumulation at low temperatures was shown to be due to interdiffusion of magnetic atoms into the non-magnetic channel at part-per-million levels, at which point a novel manifestation of the Kondo effect suppresses the spin injection efficiency. Significantly, designing interfaces with thin interlayers of non-magnetic metals that cannot support d-electron local moments was shown to almost completely eliminate the effect, restoring the spin injection efficiency.

The work, “Kondo physics in non-local metallic spin transport devices”, was recently published in Nature Communications; L. O’Brien, D. Spivak, M. Erickson, H. Ambaye, R.J. Goyette, V. Lauter, P.A. Crowell and C. Leighton, Nat. Comm. 5, 3927 (2013).


Figure: False-color Scanning electron microscopy image of an electron beam lithographed non-local metallic spin-valve used in the work.

Seed- Research Highlight

Persistent optically-induced magnetism in SrTiO3-d

In collaboration with the group of Scott Crooker at Los Alamos National Lab and Greg Haugstad of the CSE Characterization Facility, graduate student Palak Ambwani and faculty member Chris Leighton have recently reported a remarkable finding in the area of complex oxides. The team discovered that illuminating the archetypal oxide semiconductor SrTiO3 with circularly polarized light can induce and control magnetism in this nominally non-magnetic material. Most surprisingly, at cryogenic temperatures the induced magnetism persists for hours after ceasing the illumination, creating the ability to optically write, store, and read information (see image). The effect occurs only in samples deliberately prepared to have significant densities of oxygen vacancies, and the detailed results in fact implicate a localized defect complex as the fundamental origin of the effect. Work is underway to understand the nature of this defect, which could potentially hold the key to room temperature operation. The work (“Persistent optically induced magnetism in oxygen-deficient strontium titanate”) was recently published in Nature Materials. This work received partial support from a MSREC Seed award.

Seed- Research Highlight

Atomic Level and Nanoscale Design of Molecular Sieve Catalysts (SEED)

Developing more efficient technologies to capture hydrogen sulfide (H2S) has been under intense investigation to alleviate the negative environmental impact of processing and utilization of fossil-based resources. We hypothesized that spatially well-distributed active metal oxides or mixed metal oxides on mesoporous hosts can be highly stable during H2S adsorption/regeneration cycles. We demonstrated such stability coupled with high H2S adsorption capacity for Cu-ZnO clusters supported on the surfactant-templated mesoporous silicate, SBA-15. HAADF-STEM images of the sorbent were obtained using microtomed samples and a FEI Titan aberration-corrected transmission electron microscope operated at 300kV. The images show a uniform distribution of nanoparticles with diameters smaller than ~3 nm in SBA-15. This arrangement is preserved during sulfidation and regeneration of the sorbents accounting for their remarkable stability.

(B. Elyassi et al., Microporous and Mesoporous Materials, submitted)

Seed- Research Highlight

Understanding Doping in The Oxide Semiconductor SrTiO3

Using analytical scanning transmission electron microscopy, the local structural and electronic properties of STO:Nb epilayers have been studied. Even for films deposited under conditions that yield nominally ideal stoichiometry for undoped STO, our results yield expanded out-of-plane lattice parameter, and insulating electronic behavior, in stark contrast to bulk Nb-doped single crystals. The Nb incorporation was found to be highly inhomogeneous on nanoscopic length-scales, the spatial variations being closely correlated to LAADF intensity in STEM imaging, which was demonstrated to be due to large quantities of interstitial Nb. Secondary phase formation was ruled out, even at the nanoscopic level. EEL spectra reveal changes in the density of state (DOS) in STO:Nb films compared to undoped STO, but without the clear shift in the Fermi edge seen in bulk single crystal STO:Nb. Using simple simulations, it is argued that the strain field seen in experiments likely arises from interstitial Nb in the Nb0 state. The results thus point to the presence of electrically-inactive Nb interstitials in large quantities, explaining the poor bulk conductivity, an essential step towards the heterostructured oxide electronic/spintronic devices.

Electron microscopy image of a Nb-doped SrTiO3 film grown on a SrTiO3 substrate and corresponding maps of Nb (b), Ti(c), and O (d). The horizontal dotted lines indicate the interface between the film (top) and the substrate (bottom). Panels (e-h) show vertically-averaged signal variations obtained from the region of the film indicated by the rectangle in (a).

“Observation of electrically-inactive interstitials in Nb-doped SrTiO3”, J.S. Jeong, P. Ambwani, B. Jalan, C. Leighton and K.A. Mkhoyan, ACS Nano 7 4487 (2013).

IRG4 - Research Highlight

Ligand-free Colloids and Surface Doping of Silicon Nanocrystals

Films of inorganic nanocrystals are widely considered to hold great potential for printed electronic devices from solar cells to low-cost flexible displays. However, one significant hurdle to using nanocrystal colloids or inks in printed electronics is the need for organic surfactants, molecules which are required to stabilize nanocrystals in the ink solutions, but which present a severe obstacle to the conduction of electrical currents. Ting Chen, a graduate student working with Professor Kortshagen, was involved in a study that discovered a new mechanism to stabilize silicon nanocrystals in inks without the use of any ligands. The researchers found that chlorine (green atoms in figure) coverage of the silicon (brown atoms) nanocrystal surface enables weak “hypervalent” attractions to common solvent molecules that enable ligand-less dispersion of the silicon crystals (inset). Moreover, the interaction with the solvent molecules also leads to surface doping of the silicon nanocrystals, which enhances the conductivity of silicon nanocrystal films (bottom image) by a factor of 1,000 compared to films prepared without utilizing this mechanism.

(Wheeler et al., Nat. Commun., 4, 2197, 2013)

IRG4 - Research Highlight

Plasmonic Interactions through Surface Ligands on PbSe Nanocrystals

Atomic-resolution HAADF-STEM images and low-loss EELS of PbSe nanocrystals at different stages of oxidation: (a) as-synthesized, (b) partially oxidized, and (c) completely oxidized PbSe nanocrystals. Changes in ligands and reduction of nanocrystal size are visible as oxidation progresses. In the completely oxidized samples, the STEM probe beam first passes through the oxide shells on the outer surface resulting in additional spreading of the probe and hence the observed blurriness. (d) HAADF-STEM images of PbSe nanocrystals after hydrazine treatment. In hydrazine treated samples, the oleic acid ligands were removed and nanocrystals are structurally modified. The residuals of the hydrazine treatment can be seen on a:Si substrate.

(Gunawan et al., ACS Nano, submitted)

IRG3 - Research Highlight

Configurational Anisotropy and Noise in Nanoparticles

The orientation of magnetic nanoparticles (meaning the direction of their magnetization) is determined by the competition between various contributions to their energy. Their small size (less than 500 nm) and non-ideal shape makes the orientation difficult to determine. Unlike in an ideal compass needle, the spins making up the nanoparticle are not exactly aligned. Rather their orientation inside the particle will vary depending on the local magnetic field. Furthermore, the nanoparticle is constantly being buffeted by thermal fluctuations, which cause its orientation to fluctuate randomly. IRG investigators Dahlberg and Victora, working with graduate students Dan Endean and Chad Weigelt, have implemented an elegant experimental technique as well as micromagnetic simulations that allow them to determine the actual configuration of the magnetization inside the particle as well as its noise spectrum. The approach is based on measuring the anisotropic magnetoresistance (AMR) of an individual nanoparticle. By comparing the magnetic field dependence of the AMR with simulations, the local orientation of the magnetization within the nanoparticle can be determined. This departure from a uniform state is also observed in noise experiments. In fact, the noise measurements and their modeling are the first successful identification of the specific non-uniform magnetization configurations leading to random telegraph noise in a magnetic nanoparticle.

  Part of this work is published as D. Endean, C. T. Weigelt, R. H. Victora, and E. D. Dahlberg, Appl. Phys. Lett. 103, 042409 (2013)

IRG2 - Research Highlight

Intramolecular Exciton Transport in Conjugated Polymers

Emission quenching by fullerenes covalently attached to both ends of a series of size selected regio-regular poly(3-hexylthiophene) samples was quantified and used to determine the intrachain exciton diffusion length. The diffusion length was found to be LD = 7.0 ± 0.8 nm. When the distance dependence of the quenching mechanism is considered, this is the same value that has been reported for emissive excitons in thin films. This result indicates that intra-chain exciton transport is more facile for excitons localized to single chains than for excitons that are delocalized between chains. In the context of solar cells, the result indicates additional complexity and the potential for competing interests when considering morphological design of the film to enhance both exciton and charge transport.

IRG1- Research Highlight

Cavitation in Block Copolymer Modified Epoxy Revealed by In Situ Small-Angle X-Ray Scattering

Carmelo Declet-Perez (Bates in collaboration with Francis) has made significant advances in the characterization and understanding of block polymer modified epoxy thermosets that are widely used in commercial coatings. Carmelo developed an in-situ synchrotron small-angle x-ray scattering (SAXS) technique that was applied to the investigation of the mechanism of toughening in block copolymer modified epoxy. Experiments were conducted with a three-part epoxy system characterized by an adjustable molecular weight between crosslinks. Poly(ethylene-alt-propylene)-block-poly(ethylene oxide) (PEP-PEO) and poly(styrene)-block-poly(ethylene oxide) (PS-PEO) diblocks were blended with the liquid resin at a concentration of 5 weight % and cured leading to spherical micelles with rubbery (PEP-PEO) and glassy (PS-PEO) cores. SAXS measurements were performed at the Advanced Photon Source (APS, Argonne National Laboratory) while straining the cured nanocomposites in tension. Upon yielding the 2-dimensional SAXS patterns associated with the PEP-PEO modified epoxy displayed dramatic changes in the scattering intensity and profile while the PS-PEO based material showed no discontinuous scattering behavior. These results were modeled using well-established procedures resulting in the quantitative determination of the micelle shape, size and volume during deformation. The rubbery (PEP) micelle cores cavitate coincident with yielding and continue to dilate until specimen failure, resulting in a five-fold expansion in the core volume as illustrated in the figure. The glassy micelle cores (PS) deform in an affine fashion, with no evidence of cavitation or core volume expansion. These results are quantitatively accounted for by recent theory that relates the local triaxial strain generated at the front of a crack tip to fracture toughness in rubber-modified plastics.

IRG3- Research Highlight

Magnetic Charge Crystallization in Artificial Spin Ice

"Artificial spin ice" is a term used to describe arrays of nanoscale magnetic islands placed on lattices that geometrically frustrate inter-island magnetic interactions. Such systems are easily tunable and provide a new platform for the study of frustration, a physical concept of broad importance in nature. In recent work, post-doc Liam O'Brien and IRG3 faculty Chris Leighton, working in collaboration with the group of Prof. Peter Schiffer of the University of Illinois and other groups at Penn State and Los Alamos, have demonstrated a means to anneal artificial spin ice into a thermalized state. This provides the first glimpse of the true ground state of these arrays, leading to the discovery of small crystallites exhibiting magnetic charge ordering, a theoretically predicted phenomenon that could not previously be accessed.

Figure: Three dimensional depiction of magnetic force microscopy data from an artificial spin ice sample (an array of lithographically-patterned nanoscopic magnetic islands) based on a frustrated lattice known as a Kagome lattice. Image credit: Ian Gilbert.

S. Zhang, I. Gilbert, C. Nisoli, G.-W. Chern, M.J. Erickson, L. O’Brien, C. Leighton, P.E. Lammert, V.H. Crespi and P. Schiffer, “Crystallites of magnetic charges in artificial spin ice”, Nature 500, 553 (2013).

Seed- Research Highlight

The Impact of Irregularities and Disorder on Vibrations and Macroscopic Properties of Elliptic Systems (Seed)

In 2012, Seed Faculty, Svitlana Mayboroda and Marcel Filoche announced a discovery of a universal mathematical mechanism governing the localization in vibration systems. It is the first known method to determine and control the exact shape and location of the regions confining localized waves. The mechanism applies to any vibrating system - mechanical, acoustical, optical, or quantum. In particular, the figure on the left demonstrates the way in which this theory predicts the regions of quantum states of electrons in application to the famous Anderson localization.

The first five eigenmodes of the Schrodinger operator with random potential (Anderson localization). In red, the network that partitions the domain into localization subregions, as determined by the theory of Mayboroda and collaborators. One can observe how the network accurately defines the subregions that enclose the modes.

Seed- Research Highlight

Responsive Magnetoplasmonic Imaging Agents

Using iron oxide and gold nanoparticles, our team has been able to synthesize organic-inorganic nanocomposites which display high saturation magnetization and high plasmonic signal. Functionalization of the nanoparticles with reacting polymers enable multimodal imaging of medically relevant metals and markers, such as copper, simultaneously with ultra high resolution by dark-field microscopy, and in three dimension by Magnetic Resonance Imaging. This first example of a responsive multimodal nanoparticle imaging agent will impact diagnosis, biosensing and biomedical research.

Responsive Magnetoplasmonics. A team at the University of Minnesota has recently developed a nanocomposite that enables detection of biomarkers or medically relevant targets simultaneously by dark field microscopy with nm resolution (a) and in three dimensions by Magnetic Resonance Imaging (b). I. water, II. iron oxide nanoparticle only, III. gold nanoparticles only, IV. magnetoplasmonic nanocomposite, V. nanocomposites with increasing concentration of copper, VI. gold nanoparticles only.

IRG4,2- Research Highlight

First All-Gas Phase Manufacture of a Nanocrystal Based Electronic Device

Semiconductor nanocrystals hold great potential for the low-cost manufacture of electronic devices. To date, nanocrystal-based devices have exclusively been produced from colloidal solutions, which requires solvents and added processing steps to add and remove them. IRG-4 and IRG-2 researchers demonstrated the first all-gas-phase manufacture of a silicon nanocrystal light-emitting device. This study may establish a new paradigm for the solvent-free, "green" manufacture of nanocrystal-based electronics.

(Anthony et al., Nano Letters 2012, 12, 2822-2825)

International Collaboration

Visit of Chinese Academy of Sciences Researchers

To bolster the established collaborative relationship between the UMN MRSEC and the Institute of Chemistry Chinese Academy of Sciences, State Key Laboratory of Polymer Physics and Chemistry, the UMN MRSEC hosted Assistant Professor Zhibo Li (an ex-UMN MRSEC-funded graduate student) for three weeks in April 2012. Prof. Li was joined by two of his students, Wenxin Fu and Yu Liu. The students worked in the laboratory with Can Zhou (IRG-1) and other researchers in the groups of Prof. Lodge and Prof. Hillmyer during their three-week stay. In addition, Prof. Li gave a lecture to IRG-1 members and interacted with other MRSEC faculty. These collective interactions led to the initiation of a collaborative project focused on ABC triblocks containing polypeptide blocks for stimulus-responsive hydrogels. This project combines expertise from both the UMN MRSEC and Prof. Li's laboratory and has synergistic benefit to both institutions.

Prof. Li (center) and his students, Wenxin Fu and Yu Lin

IRG3- Research Highlight

Low Resistivity Magnetic Nano-Sensors

As electronic devices shrink deep into the nano-scale, low-resistivities become essential. Simply put, electrons scatter off surfaces, and surfaces are closer together in small devices. Mazin Maqableh of IRG-3 has worked with Professors Stadler and Victora to develop ultra-small (10nm) magnetic sensors and associated electrodes with low resistivities enabled by very smooth surfaces. These resistivities are 10-1000x lower than both predictions and those found in nanowires made by other means. These magnetic sensors had high signals,, 25 W total resistance, and high switching currents that met or exceeded all parameters of competing sensors, which are approximately 10 x larger.

(a) Schematic of 10-nm diameter Co/Cu/Co nanowire in a read sensor application. Despite being 4x smaller than the bulk scattering length for electrons, these nanowires exhibit almost bulk resistivities due to the very smooth surfaces of the aluminum oxide growth template. (b) Micrograph showing templates with smooth sidewalls and long range ordering (over in2).

IRG3- Research Highlight

Global Picture of Lateral Spin Valves

A large class of sensor and memory technology is based on devices made from "sandwiches" of ferromagnetic and normal metals. In spite of this fact, most information about interfaces between these different classes of materials has been derived from experiments on only a few different combinations of metals. IRG Postdoc Liam O'Brien and graduate students Michael Erickson and Dima Spivak, working with Leighton and Crowell, have developed an experimental approach that definitively separates purely interfacial effects from other factors that limit spin transport, such as relaxation at surfaces, for many different pairs of magnetic and normal metals.

Top: Micrograph of a lateral spin valve fabricated using the IRG's approach, which is capable of achieving transparent interfaces between numerous combinations of materials. Bottom: Non-local spin resistance of several devices of identical geometry but prepared with different metals.

IRG2,3- Research Highlight

First Observation of Hall Effect in Polymer Transistors

Printed transistors employing both the bench-mark polymer semiconductor poly(3-hexyl-thiophene) and ultra-high capacitance ion gel gate insulators exhibit unusually large hole mobilities near 1 cm2/Vs at high charge densities (0.2 holes/ring). The large mobility suggests delocalized carriers and the possibility of observing the Hall effect and insulator-metal transition. Postdoc Shun Wang has measured the Hall effect, the first time that the Hall effect has been observed in polymer transistors. The Hall voltage has the expected sign and scaling with magnetic field strength and carrier type. This work appeared in Nature Communications. Future work aims to observe the Hall effect in other polymers and to better understand transport in the high carrier density regime near the insulator-metal transition.

IRG1- Research Highlight

Multiblock Polymers: Panacea or Pandora's Box

Advances in polymer synthesis have enabled access to a vast array of multiblock polymer architectures, with rich opportunities for designing multiple functionalities into a single self-assembled material. Examples using three monomers (colors) are shown in the upper panel. However, even for three ingredients the number of possible combinations is so large that conventional strategies for predicting block polymer structure are inadequate. This review article examines how careful selection of block number and sequence can yield new structures in a systematic way (see lower panel for an example), and identifies new theoretical approaches for exploring the most promising candidates. Such materials could have impact across a plethora of technologies, ranging from portable energy storage to biomedicine.

Multiblock Polymers: Panacea or Pandora's Box

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.

Development of a Universal Template for Producing Nanoporous Materials by Nanocasting

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.

Efficient, Single-Layer Organic Light-Emitting Devices

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.

High Efficiency Silicon Nanocrystal Light Emitting Devices

(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.

The Microscopics of Pinning

) 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.

Giant Magnetoresistance Effect at Microwave Frequencies

(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.

Spin Hall Effect Image

(a) Micrograph of a spin Hall effect device; (b) the spin Hall conductivity as a function of temperature for InxGa1-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 kBT at room temperature (kB, 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 (CT1s) is more than one order of magnitude larger than kBT at room temperature. The CT1s 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 CT1s, 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 CT1s 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 Co1-xFexS2, 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 Co1-xFexS2, 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.

Printed Organic FETs

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.

Two Photon Photoemission

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.]

Spatial map of the distribution of magnetization directions in a 6.4 x 6.4 µm NiMn / NiFe sample according to a micromagnetic simulation. The sample is at the coercive field after demagnetizing from saturation. Comparison to similar images on the opposite side of the hysteresis loop reveals a magnetization reversal asymmetry also observed in experiment.

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.

Biorenewable Thermoplastic Elastomers

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, 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 ( 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]