Principal Investigators: Ilja Siepmann (IRG-3), Timothy Lodge, (IRG-3)
The versatility of polymeric materials is inherent in their rich and varied molecular building blocks and architectures. The ability to quantitatively model polymer systems on the molecular level provides a route to engineer polymeric materials with desired properties. In this work, we utilize Monte Carlo simulations and transferable force fields to study the thermodynamic and structural properties of some olefin oligomers. It is shown that extrapolation of data for short oligomers can reproduce important thermodynamic properties of the polymers. In addition, the Flory−Huggins χ parameters, a measure of the unlike bead-bead interactions, are calculated directly from simulations of binary mixtures. The binary oligomer blend of propylene isomers is found to exhibit stabilized irregular mixing behavior, in agreement with experiments for polymers. Our results identify molecular simulations as a promising approach to predict and understand the polymerpolymer phase behavior.
Principal Investigators: Timothy Lodge, (IRG-3), Kevin Dorfman (IRG-3), David Morse (IRG-3), Theresa Reineke (IRG-3)
A significant challenge facing the development of polyelectrolyte complexes for biomedical and materials applications is kinetic trapping of the complexes far from their equilibrium states. Recent work from IRG-3 demonstrates that controlling the charge density of the polyelectrolyte chains can help promote rapid equilibration of these complexes. Through experiments on complexes of polyelectrolyte micelles, researchers in IRG-3 showed that incorporation of neutral, uncharged monomers facilitates rapid rearrangement from insoluble multi-micelle aggregates to soluble single-micelle complexes. Molecular dynamics simulations on model polymer brushes demonstrated that this results from the energy required to break ion-ion contacts in polymers with alternating charged and neutral units being lower than in polymers with many adjacent charges. These results provide critical new information for applications requiring rapid equilibration followed by long-term stability.
In 2016, the MRSEC broadened the reach of its popular Materials Week program by presenting hands-on demonstrations and lab activities for 30 Middle School girls participating in the Eureka! program. Eureka! is a partnership between UMN and the MPLS YWCA to encourage participation in STEM by providing opportunities to girls beginning in 7th grade and supporting them continuously through high school graduation. The MRSEC Materials Week program began as a weeklong summer camp for high school students but has since grown to include students as young as 7th grade. The library of activities developed for the program are now used is several MRSEC Education and Outreach programs and presented to nearly 300 middle and high school students each year.
Principal Investigators: Uwe Kortshagen, (IRG-2), Andre Mkhoyan (IRG-2)
Nanoparticles, consisting of only a few hundred to a few thousand atoms, are of interest as building blocks for bottom-up processing or next generation electronics. Because they are so small, a significant fraction of the atoms that make up these materials are found at the surface. Consequently, these surface atoms are extremely important in determining the properties and stability of the nanomaterial. In this work, the researchers demonstrated the ability to grow nanoparticles with a shell of a different material, all in the gas-phase using a plasma. This process allowed them to make core/shell nanocrystals that required high temperatures for synthesis that would be difficult or impossible to achieve in solution. They demonstrated the process by synthesizing germanium nanocrystals (<10 nm) with a silicon shell produced using a novel nonthermal plasma reactor design (K. Hunter, U. Kortshagen; Department of Mechanical Engineering). In collaboration with colleagues in the Department of Chemical Engineering and Material Science (J. Held, A. Mkhoyan), the core/shell structure of these nanocrystals was confirmed by scanning tunneling electron microscopy (STEM). Through careful analysis of these STEM images, the researchers were able to correlate the change in the properties of these particles with increasing shell thickness. The ability to use gas-phase processes to create nanoparticles with a core/shell structure opens up the possibility to produce nanomaterials that have so far been inaccessible by other approached. This study was reported in the journal ACS Applied Materials and Interfaces.
Principal Investigators: Rafael Fernandes, (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)
Defects, essentially locations in a crystal where the perfect arrangement of atoms is disturbed, are inherent in materials, and play a key role in their function. As they have been studied for so long, the discovery of new types of defects is rare. In recent work in IRGs 1 and 2 a completely new type of defect has been found in a class of complex oxide materials called perovskites, specifically the compound NdTiO3. This new defect is a “line” defect formed by a long local region of the crystal that rotates from the typical ordering pattern, as shown in the figure. This discovery was enabled by combining state-of-the-art synthesis methods with atomic resolution electron microscopy and complex computations, requiring collaboration between three MRSEC research groups with complementary expertise. One potential application lies in engineering such line defects to create atomic-scale “tunnels” for the flow of electrons or atoms.
Principal Investigator: Boris Shklovskii (IRG-2)
When a nanocrystal film absorbs a light quantum, its energy is stored in a nanocrystal exciton, which consists an electron and hole bound to each other by electrical attraction. The exciton hops between nanocrystals and, to release its electrical energy, should get converted into a free electron and hole before losing its energy. This is why it is important to increase the exciton hopping rate. 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 research team showed that for touching semiconductor nanocrystals such hopping mechanism can be more effective than any other known mechanisms. This study was reported in ACS Nano.
Principal Investigator: David Flannigan (seed)
Energy in the form of heat impacts all technologies and plays a major role in the design and engineering of infrastructure. It is also the largest form of waste energy in critical applications, including power transmission and transportation. Scientists and engineers have spent decades researching how to control thermal energy at the atomic level in order to use it to do useful work and ultimately increase efficiencies and reduce the use of fossil fuels. However, no one has yet been able to directly image what thermal energy looks like or how it moves through materials in real time. This is because the basic length scales are billionths of a meter (nanometers) and the speeds can be many miles per second. Here, using a cutting-edge electron microscope, we were able to directly image the emergence and motion of exceedingly fast energy waves moving through semiconducting materials. Even more exciting was our observation that these energy waves interact with particular features in materials in ways that would have been impossible to determine with such certainty by any other means. These observations represent a breakthrough in our ability to study and understand how energy moves through materials and could potentially change the way we approach thermal-energy management.
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 – one of the central challenges of nanocrystal engineering – is essential for controlling the optical and electronic properties of compound semiconductor nanocrystals. Conventionally, these materials are synthesized and doped by solution-based methods, but a significant obstacle is often encountered: dopants are excluded during the early stages of nanocrystal formation and growth, resulting in undoped central cores and low doping efficiencies. To address this problem, a team led by Professors Kortshagen, Aydil, and Mkhoyan developed a fundamentally different plasmabased process for synthesizing aluminum-doped zinc oxide nanocrystals, a prototypical material used in light-emitting diodes and solar cells. The key advantage of this approach was that the dopant Al atoms in the plasma were much more chemically reactive than their counterparts in solution-based synthesis. The team demonstrated that this high reactivity enabled irreversible dopant incorporation in all stages of nanocrystal growth, resulting in efficient and uniform doping throughout the nanocrystal cores.
Principal Investigators: James Johns (Seed)
The ability to control the flow of electrons in semiconductor electronics depends on the presence of junctions between materials with different electronic energy levels. Traditional techniques for creating interfaces in semiconductor materials do not work well for monolayer, or two dimensional, materials. Instead, junctions in 2D materials are typically formed by stacking one material on top of another, forming a weakly bound, vertical stack through which electrons can flow. Researchers sponsored by the SEED program have recently developed a new method for forming atomically abrupt junctions in 2D materials. In their work, they used chemical vapor deposition to grow monolayers of semiconductor MoS2. By introducing molecular hydrogen into the reaction, they were able to control the shape and cleanliness of their MoS2 flakes. They then performed a second reaction to deposit another monolayer semiconductor, WS2. They were able to chemically control whether the second material grew on top of MoS2, as a vertical stack, or whether it grew around the MoS2 forming a covalent junction similar to junctions found in traditional semiconductors. The figure below 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 such as solar cells and LEDs.
Principal Investigators: Marc Hillmyer (IRG-3)
Block polymers can 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)-blockpoly( lactide) (PCHE-PLA) and poly(cyclohexylethylene)-block-poly-(ethylene oxide) (PCHEPEO) 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)
The transition of a solid from an insulator to a metal as the number of free electrons in the material increases has been one of the central questions in semiconductor physics. In bulk semiconductors, this “metal-to-insulator transition” is described by the well-known Mott criterion. Understanding this transition in films of nanocrystals is of crucial importance to their future use in electronic devices such as light emitting diodes, solar cells, or transistors. An IRG-2 research team developed a new theory that relates the metal-to-insulator transition in films of doped nanocrystals to the “transparency” of the interface between nanocrystals for electrons. The theory predicts that the transition occurs under strikingly different conditions from those previously known for bulk semiconductors. In associated experimental studies of the electron conduction in phosphorousdoped silicon nanocrystal films, the team discovered a behavior that largely supports the predictions of the new theory. This study was reported in the journal 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-alt-propylene)-b-poly(ethyleneoxide) (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 with high temperature superconductors is the difficulty of chemically doping materials such as complex copper oxides (cuprates) over a wide range of charge carrier densities. Recently developed methods using ionic liquids in devices called electric double layer transistors provide an elegant potential solution to this problem 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 approach can be used to establish a special scaling relation between the superconducting penetration depth and transition temperature in cuprates, known as Homes scaling. The ionic liquid gating 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 oxides are extraordinarily functional materials, and are promising for next generation “oxide electronic” devices. One particularly attractive direction with such materials is the formation of two-dimensional (2D) conductive layers at the interfaces between insulating complex oxides. In work recently performed in IRG-1 an exciting development with these interfaces has been uncovered, arrived at by working in collaboration with researchers in IRG-2 and at the Pacific Northwestern National Lab. Specifically, the interface between the Mott insulator NdTiO3 and the band insulator 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. This phenomenon occurs at a critical thickness of the NdTiO3 layers, and is potentially externally controllable. 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 for such in ultra-clean high-purity doped inorganic semiconductor heterostructures based on materials such as 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, based on the use of ionic liquids, 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 fundamental origin of the conductive state.
Principal Investigators: Bharat Jalan and Chris Leighton
Complex oxides such as perovskites are extraordinarily functional materials, 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 operation. 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. High electron densities are obtained, at mobilities competitive with, in some cases even higher than, other methods. This offers a number of potential advantages over other doping approaches, which require the deliberate introduction of impurities. The work could have impact in oxide electronics in general, as well as in transparent conductors for a variety of devices.
On May 20, 2015, over 250 middle and high school students participated in the inaugural MRSEC Research Experiences for Teachers 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: Frank Bates, Lorraine Francis
Addition of rubber particles to epoxy thermosets has been successful for toughening these brittle materials. However, a complete description of the toughening mechanisms is still elusive. Of particular interest is understanding the enhanced toughness achieved upon adding small amounts of block copolymers to commercial epoxy resins. Simultaneous small-angle X-ray scattering (SAXS) and tensile experiments were performed on epoxies modified with 30 nm diameter rubbery and glassy core block copolymer micelles. The SAXS data revealed efficient cavitation of the spherical rubbery cores and absence of cavitation in the glassy nanodomains. These results are quantitatively anticipated by theory that accounts for cavitation in rubber-toughened plastics as a function of particle size. Read more
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
The interpretation of transport in ferromagnet/normal metal (F/N) bilayers is based on an idealized model of the interface between the two classes of materials. In general, the ferromagnet is treated as a source of spin-polarized electrons, while the polarization is zero in the normal metal. In part because of the absence of appropriate characterization tools, the interface between the two materials is usually considered as abrupt. The canonical model of spin-polarized transport is based on this picture. There are, however, strange puzzles that have emerged in simple “textbook” devices. One of these is the apparent temperature dependence of the non-local resistance in spin valves in which two ferromagnetic electrodes are separated by a non-magnetic channel. For certain combinations of F and N, the signal decreases as the temperature decreases below 100 K, as shown for Fe/Cu in the figure. IRG-3 investigators, with collaborators from Oak Ridge National Laboratory have resolved this mystery. The important conclusion is that the F/N interfaces are diffuse, and for certain combinations of F and N, the ferromagnetic atom forms a local moment at the interface. In the interfacial region, the Kondo effect (a screening cloud around the local moment) results in a reduction of the polarization in the current flowing into N. The IRG investigators find that the effect is absent in systems in which local moments do not form (e.g., any ferromagnet with Al), and is enhanced when the diffusion is promoted (e.g., by annealing). The interfacial character of the effect is definitely proved by showing that it disappears when a very thin layer of Al is inserted between Fe and Cu. This provides a complete explanation of the origins of the non-local resistance anomaly.
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. Using FEI Titan aberration-corrected TEM operated at 300kV and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, Prashant Kumar, a graduate student working with Professors Andre Mkhoyan and Michael Tsapatsis showed how a uniform distribution of nanoparticles with diameters smaller than ~3 nm is in SBA-15. The image on the left shows the SBA-15 mesopores, and the magnified view on the right shows that within the pores (pores are highlighted by circles for clarity) there are numerous nanoparticles evident by their bright contrast. This arrangement is preserved during sulfidation and regeneration of the sorbents accounting for their remarkable stability.
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.
When functional films are cast from colloidal dispersions of semiconductor nanocrystals, the length and structure of the ligands capping their surfaces determine the electronic coupling between the nanocrystals. Long chain oleic acid ligands on the surface of IV-VI semiconductor nanocrystals, such as PbSe, are typically considered to be insulating. Consequently, these ligands are either removed or replaced with short ones to bring the nanocrystals closer to each other for increased electronic coupling. Herein, using high-angle annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging combined with electron energy-loss spectroscopy (EELS), Aloysius Gunawan, a graduate student working with Professor Andre Mkhoyan showed that partial oxidation of PbSe nanocrystals forms conjugated double bonds within the oleic acid ligands, which then facilitates enhanced plasmonic interaction amongst the nanocrystals. The changes in the geometric configurations of the ligands are imaged directly and correlated with the changes in the surface plasmon intensities as they oxidize and undergo structural modifications. The work was performed in collaboration with Professor Eray Aydil’s group that provided PbSe nanocrystals.
The family of materials known as complex oxides display a range of electronic, magnetic, and other functional phenomena, leading to the idea that they could be used in an entirely new field of electronics known as oxide electronics. Oxide semiconductors such as SrTiO3 (STO) could play a key role in this field, but it is currently well understood to be difficult to dope such semiconductors in thin film form, particularly while maintaining high mobility of the electrons. The reasons for this are not well understood. In collaboration with IRG-4 member Mkhoyan, Seed investigators Bharat Jalan and Chris Leighton have recently studied the local structural and electronic properties of thin films of the prototypical oxide semiconductor, SrTiO3, attempting to dope it with Nb. The work reveals that, as a significant surprise, under the growth conditions used, Nb has a strong tendency to incorporate in interstitial sites. Detailed spectroscopy accompanying electron microscopy was then used to show that this Nb is electrically inactive, demonstrating that in this case the lack of conductivity is due to ineffective doping with electrically-inactive interstitial Nb. This is one of the first indications as to the fundamental problems with doping of thin films SrTiO3. It remains a challenge to understand how growth conditions can be tuned to mitigate interstitial Nb in favor of electrically-active substitutional Nb, thus gaining full control over thin film electronic properties.
This work is published in ACS Nano as: J. S. Jeong, P. Ambwani, B. Jalan, C. Leighton, and K. A. Mkhoyan, Observation of Electrically-inactive Interstitials in Nb-doped SrTiO3, ACS Nano, 2013 7, 4487.
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 (also called ligands), 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 coverage of the silicon nanocrystal surface enables week “hypervalent” attractions to common solvent molecules that enable ligand-less dispersion of the silicon crystals. Moreover, the interaction with the solvent molecules also leads to surface doping of the silicon nanocrystals, which enhances the conductivity of silicon nanocrystal films by a factor of 1,000 compared to films prepared without utilizing this mechanism.
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-3 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.
“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, postdoc Liam O’Brien and IRG-3 faculty member 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. Previous work in this field encountered difficulty in accessing this thermalized state as annealing above the Curie temperature of the ferromagnetic material used (Ni-Fe alloy) typically leads to lateral diffusion (thereby destroying the nanostructured islands) and vertical diffusion (thereby reacting the metal with the typically employed Si substrates). By using a Si-N layer as a diffusion barrier, this work opened up a small temperature window above the Curie/blocking point, into which the system could be heated then subsequently cooled in very low magnetic fields. This provided 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.
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. Research published in Science, 2012, 336, 434-440.
Professor Valerie Pierre and colleagues at the University of Minnesota have developed multimetallic, mixed organic/inorganic nanoparticles for dual imaging by dark field microscopy and magnetic resonance imaging. The incorporation of a thin organic layer between the iron oxide core and the gold layer preserves the magnetization of the nanoparticles which ensures that the resulting composites are powerful contrast agents for MRI while maintaining the small size of the assembly necessary for practical biological and medical imaging. This approach solves the problems with current multimodal agents, which are either agglomerates with size in the micrometer range which prevents cellular uptake and biomedical applications, or pure bimetallic nanoparticles which are characterized with low MRI and plasmonic sensitivity. 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 (Figure 1a) and in three dimension by Magnetic Resonance Imaging (Figure 1b). This first example of a responsive multimodal nanoparticle imaging agent will impact diagnosis, biosensing and biomedical research.
IRG-4 is interested in improving the luminescent performance of silicon nanocrystals and in developing light emitting devices (LEDs) by integrating the nanocrystals into novel device architectures. Rebecca Anthony, a postdoctoral researcher working with Professor Kortshagen, collaborated with Kai-Yuan Cheng, a graduate student advised by Professor Holmes (IRG-2), on incorporating silicon nanocrystals into hybrid nanocrystal/organic light emitting devices. Initial work with solution processed silicon nanocrystals was promising, resulting in record high external quantum efficiencies of 8.6% for electro-luminescent silicon nanocrystals, which was covered in two Nano Letters papers in 2010 and 2011. Now, work focused on a new, all-gas-phase approach to the manufacture of nanocrystal-based light emitting devices. Rebecca Anthony developed a gas-phase synthesis reactor that fulfilled three different functions: 1) The synthesis of high-quality silicon nanocrystals in a nonthermal plasma, 2) the in-flight surface functionalization of the silicon nanocrystals with organic monolayers in the downstream afterglow region of the reactor, and 3) the acceleration of the functionalized silicon nanocrystals via a rectangular nozzle and their inertial impaction onto a moving substrate holder. This novel reactor design enabled Rebecca to deposit ~100 nm thick, dense silicon nanocrystal films on ITO covered glass substrates. The device fabrication was completed by gas phase deposition of lithium fluoride/aluminum contacts. While the overall external quantum efficiency of these demonstration devices was only 0.02% and lacked behind solution processed devices, this study was the first demonstration of a completely gas phase based fabrication of a nanocrystal-based electronic device. Owing to the novelty of this approach, Rebecca’s study was published in Nano Letters.
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.
Models of resistivity predict degraded performance for 10-40 nm devices due to surface and grain boundary scattering (bulk electron scattering lengths are ~ 40nm). Recently, IRG-3 investigators have found almost bulk-like resistivities in 10 nm-diameter metallic wires prepared in densely-packed, vertical arrays inside anodized alumina nanochannels. These arrays are excellent candidates for devices such as read sensors [M. Maqableh et al., IEEE Trans. Mag. 48, 1744 (2012)], magnetic recording media, dense magnetic random-access memory, and self-assembled electronics. These low resistance devices will mitigate growing concerns that the ‘size-effect’ is threatening to halt progress along the International Technology Roadmap for Semiconductors (ITRS). Furthermore, to reach 1 -10 TBit/inch2, magnetic recording media will require < 20 nm diameter read sensors. Current (larger) read sensors have increasing resistivity and decreasing signal as they scale smaller due to sidewall damage during fabrication. This will lead to low signal-to-noise ratios (SNR), high resistance-capacitance (RC) time constants (low speeds), and resistive noise from heat which is difficult to dissipate. As a prototype device, 10-nm read sensors were first demonstrated by the IRG-3 team. The IRG work, published in Nano Letters [M. Maqableh et al., Nano Letters 12, 4102 (2012)], demonstrated sensors with 19% magnetoresistance,, 25 Ω total resistance, and high switching currents. These characteristics meet or exceed the performance of the closest competing sensors, which are approximately 10 times larger.
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 separates purely interfacial effects from other factors that limit spin transport, such as relaxation at surfaces. The IRG investigators developed a technique for preparing “lateral spin valves’ in which two ferromagnetic electrodes (Fe, Co, Ni, or NiFe) are connected to nanowires of aluminum or copper. The approach employs a double-angle shadow evaporation technique that does not require breaking vacuum at any point in the process. As a result, the interfaces between the materials are completely transparent. This type of systematic study addressing different binary combinations of materials has not been carried out previously. In parallel, the IRG developed a 3D Monte Carlo simulation that implements the Valet-Fert model of spin transport for the actual geometry of the devices. A quantitative understanding of many key aspects, such as spin relaxation at surfaces, has emerged from this program. Most importantly, the IRG has shown that the temperature-dependence of the non-local spin resistance (a measure of the spin transport efficiency) cannot be interpreted in terms of the material properties of the nanowire channel itself.
Printed transistors employing both the benchmark 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.
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.
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.
435 Amundson Hall, 421 Washington Ave. SE, Minneapolis, MN, 55455
P: 612-626-0713 | F: 612-626-7805