Principal Investigators: Rafael Fernades, (IRG-1), Chris Leighton (IRG-1), Boris Shklovskii (IRG-2)
A new concept being developed in MRSEC IRG-1 at UMN allows for external electrical control over the electronic and magnetic properties of materials. This is based on the use of electrolytes in transistor devices, using electrical voltage to inject electrons into material surfaces, thereby controlling their properties. In this work, researchers in IRGs 1 & 2 have studied this problem theoretically, making predictions about the surface electron density required to induce transitions from insulating to metallic, and from non-magnetic to magnetic. The latter proceeds by connecting together (“percolating”) isolated magnetic clusters, forming a full long-range magnet. This work predicts that even though the electrons are added only at the surface, the percolated magnetism can extend much deeper into the material than expected, an exciting prediction that is now being tested in experiments.
Principal Investigators: Dan Frisbie (IRG-1), Chris Leighton (IRG-1)
Transistors, the building blocks of all computer technologies, are currently based on semi-conductors such as silicon, manufactured using energy-intensive processes. Materials that can be processed into electronic devices using cheaper and less energy-intensive methods are of high interest for a number of applications. In work recently performed in IRG-1, UMN MRSEC researchers have demonstrated landmark performance in transistors based on the widely studied transparent semiconductor indium oxide, fabricated via solution processing. Solution processing is a low temperature, low cost approach (in this case essentially a form of inkjet printing), but was shown here to be capable, in conjunction with cutting-edge electrolyte dielectrics, of voltage-induced metallic behavior at interfaces. This metallic conductivity is important, as it maximizes current output, improving device performance and applicability.
Principal Investigators: K. Andre Mkhoyan (IRG-2), Renata Wentzcovitch (IRG-1), Bharat Jalan (IRG-1)
We report a discovery of a new line defect in crystalline materials, observed in the single crystal perovskite oxide NdTiO3. Unlike defects of other dimensionality in crystals, only two types of line defect have previously been observed: dislocations and disclinations. The more prominent of the two, dislocations, are widely observed in crystalline materials and critically influence their properties. Perovskite oxides contain at least three elements and allow many degrees of freedom for alterations of the crystal structure (composition, crystal symmetry, strain etc.), and therefore can host a wider variety of defects. NdTiO3 is one such perovskite, in which we discovered a new line defect. Thorough analysis reveals uniquely different local electronic properties of this new line defect, giving hope that in different materials this defect can have varied exotic properties.
Principal Investigator: Boris Shklovskii (IRG-2)
Thin films comprising nanocrystals pressed against each other find applications in televisions and solar cells. It is important to understand the creation, annihilation and movement of electrical charges through these films. When a solar cell made of these nanocrystal films absorbs light, a positive (hole) and a negative charge (electron) are created. These charges move together in the film as a single entity called an exciton. Because positive and negative charges attract each other, the exciton does not live long and must hop towards an electrode to create current before it is annihilated. The research team found a new fast mechanism of exciton hopping: first the electron moves from one nanocrystal to another, and then the hole follows the electron. The team showed that in compact nanocrystal films this new hopping mechanism is faster than all the other mechanisms that were identified previously. The study was reported in ACS Nano (2016).
Principal Investigator: David Flannigan (seed)
Heat impacts an enormous range of technology and infrastructure, and major efforts have been devoted to understanding and controlling its behavior. Direct imaging of thermal-energy motion would greatly increase our understanding, but doing so has been extremely challenging owing to the small lengths and very fast speeds at which it operates. This has now been accomplished with a state-of-the-art electron microscope capable of capturing snapshots of processes happening on billionths of a meter (nanometer) and in a millionth of a billionth of a second. Nanoscale energy waves moving at many miles per second in semiconducting materials were directly imaged, thus providing the first glimpses of precisely how heat behaves at these scales.
Principal Investigators: Bharat Jalan (IRG-1, Seed), Vlad Pribiag (Seed)
An important recent advance in the materials science of metal oxides is the discovery that interfaces between intrinsically non-conductive complex oxide materials can exhibit conductive behavior. This, and other advances, have led to the concept that “oxide electronics” could be developed, with functionality not possible in current devices. In this work, IRG-1 researchers, along with a SEED researcher and collaborators at the Pacific Northwest National Laboratory, have identified, for the first time, both the source of the electrons that conduct, and the means to control their number. In essence the interfacial electrons are controlled remotely (“by their tail”) by tuning the composition away from the interface. This is a significant advance in thin-film engineering of oxides, in that properties are controlled at the level of the individual atoms that make up the materials. Some of the materials used are only a single atomic layer thick, yet their properties can still be controlled. This discovery has several intriguing implications, including the possibility of new electronic and photonic devices
Principal Investigators: Uwe Kortshagen (IRG-2), Eray Aydil (IRG-2), and Andre Mkhoyan (IRG-2)
Doping is essential for controlling the optical and electronic properties of compound semiconductor nanocrystals. In solution-based synthesis, often doping efficiencies are low and dopants are excluded from the nanocrystals’ central cores. The research team developed a fundamentally different plasma-based process for synthesizing aluminum-doped zinc oxide nanocrystals. Due to the high chemical reactivities of the doping species in the plasma, dopants were incorporated irreversibly throughout nanocrystal growth, resulting in high doping efficiency and uniform dopant distribution.
Principal Investigators: James Johns (Seed)
Electronic devices require interfaces between semiconductors. Creating these interfaces in 2D materials can be challenging because of the weak interactions between these van der Waals solids. Atomically abrupt, covalent interfaces between MoS2 and WS2 were fabricated via chemical vapor deposition. The atomically sharp termination of the MoS2 inner crystal was achieved by introducing hydrogen gas during the synthesis. These perfect crystals of MoS2 were then used as seed crystals to nucleate subsequent WS2 growth. The figure displays a chemical model of the ideally abrupt interface, an SEM image of the monolayer flakes, and a TEM image confirming the atomically sharp interface. The abrupt, lateral chemical junction creates an abrupt electrical, p-n junction which could be used for optoelectronics.
Principal Investigators: Marc Hillmyer (IRG-3)
Block polymers are capable to produce high density nanostructured arrays by the attractive “bottom-up” strategy of self-assembly. Strongly segregated block polymers with low degree of polymerization are needed to prepare ultrahigh density features for emerging applications in microelectronics and high density magnetic data storage. A series of novel poly(cyclohexylethylene)-block-poly(lactide) (PCHE-PLA) and of poly(cyclohexylethylene)-block-poly- (ethylene oxide) (PCHE-PEO) block polymers have been synthesized to achieve ultra-small nanostructured arrays with sub-10 nm domain sizes. Ordered block polymers thin films with ultra-small hexagonally packed cylinders oriented perpendicularly were prepared by spin-coating and subsequent solvent vapor annealing for use in three distinct templating strategies. Selective hydrolytic degradation of the PLA domains generated nanoporous PCHE templates with an average pore diameter of 5 ± 1 nm. Alternatively, an Al2O3 nanoarray from the PCHE-PLA template was produced on diverse substrates including silicon and gold with feature diameters less than 10 nm. In a third approach, selective inclusion of inorganic precursor within the PEO domain enabled the formation of inorganic oxide nanodots with exceptionally small feature sizes of 6 ± 1 nm.
Principal Investigators: Uwe Kortshagen (IRG-2) and Boris Shklovskii (IRG-2)
Understanding the transport of electrons in films of touching nanocrystals is of central importance for their future use in printed electronic devices such as light emitting diodes, solar cells, or transistors. The research team developed a new theory that describes the transition of the electron conduction in doped nanocrystal films from a semiconducting to a metallic behavior. The theory predicts that the transition occurs under strikingly different conditions from those previously known for bulk semiconductors. Associated experimental studies of the electron conduction in phosphorous-doped silicon nanocrystal films largely support the new theory. This study was reported in Nature Materials.
Principal Investigators: Frank Bates (IRG-3) and Lorraine Francis (IRG-2)
Glassy thermosets, such as epoxy, are brittle and lack the mechanical toughness needed for many applications. This work demonstrates that dispersing small amounts of nanoscale micelles of poly(ethylene-propylene)-b-poly(ethylene-oxide) (OP) diblock copolymer (5 wt%) and amine modified graphene (GA) (0.04 wt%) to an epoxy results in an unprecedented 20-fold increase in the strain energy release rate (GIc), a measure of toughness. Remarkably, the improvement is multiplicative: graphene addition boosts the GIc of block copolymer modified epoxy by 1.8 times, the same increase noted for its addition to the neat epoxy material. Future work will focus on the underlying toughening mechanisms and the properties of composite coatings.
Principal Investigators: Allen Goldman (IRG-1) and Javier Garcia-Barriocanal
High temperature superconductivity remains one of the biggest challenges in condensed matter physics. One of the major materials issues is the difficulty of chemically doping materials such as copper oxides (cuprates) over a wide range. Recently developed methods using ionic liquids provide an elegant potential solution as they enable doping not chemically, but rather by applying an external electrical voltage. In this recent work in IRG-1, investigators have shown that this ionic liquid method can be used to establish a special scaling relation between the penetration depth, transition temperature and conductivity in cuprates. This approach is highly efficient compared to prior methods, the scaling being demonstrated in a single sample, tuned via an external parameter.
Principal Investigators: Bharat Jalan (IRG-1) and Andre Mkhoyan (IRG-2)
Complex oxide materials are extraordinarily functional, and are promising for next generation “oxide electronic” devices. One particularly attractive direction with such materials is the formation of 2D conductive layers at the interfaces between insulators. In this work an exciting development with these interfaces has been made by IRG-1 researchers, working with IRG-2 and the Pacific Northwestern National Lab. The interface between the Mott insulator NdTiO3 and SrTiO3 was shown to have an unusual energy band alignment that enables additional transfer of electrons from NdTiO3 to SrTiO3, thus creating an electron gas with almost ten times the electron density of standard interfaces. The discovery has several intriguing implications, particularly for new photonic device concepts.
In 2015, the University of Minnesota MRSEC expanded its American Indian Outreach activities with the inaugural American Indian Visit Day. On November 7, 2015, 270 American Indian middle and high school students were invited to UMN for a day of activities to introduce opportunities in STEM available at UMN. The students participated in hands on activities presented by each of the three MRSEC IRGs exposing them to research in multiple science and engineering fields. Additionally, the students toured campus cultural offices and heard from American Indian student and alumni speakers. Students also participated in a college application workshop and were able submit applications to UMN on site at no cost.
Principal Investigators: Tim Lodge and Theresa Reineke
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.
Principal Investigators: Dan Frisbie and Chris Leighton
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.
Principal Investigators: Bharat Jalan and Chris Leighton
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.
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.
Christy Hayes and Theresa Reineke 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.
Principal Investigator: Eray Aydil
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. Read more
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.
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.
The work, "Persistent optically induced magnetism in oxygen-deficient strontium titanate”, was recently published in Nature Materials.
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. Read more
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.
The work, "Kondo physics in non-local metallic spin transport devices”, was recently published in Nature Communications.
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.
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.
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.
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.
"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.
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.
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.
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.
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.
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.
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 (a previous 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.
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.
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.
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.
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.
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