Argonne National Laboratory

2014 Seminars Archive

Date Title
December 10, 2014
4:00 p.m.
Bldg. 440, A105-106
"Graphene: A new emerging lubricant," Diana Berman, Nanoscience & Technology Division
November 26, 2014
11:00 a.m.
Bldg. 440, A105-106

"Tunable Properties of Coupled Quantum Dots," Eric Stinaff, Ohio University, hosted by Saw Wai Hla

Abstract: Quantum dot molecules (QDMs) provide the ability to control the effects of quantum mechanical coupling for multiple confined carriers with applied electric field. This work is focused on the understanding and control of the effects of molecular carrier states on the luminescence lifetimes, quantum confined Stark effect (QCSE), and spin polarization in these coupled systems.

In quantum dots the discrete nature of the energy levels restricts the processes through which relaxation can occur and for exciton energy separations of only a few meV, the dominant scattering mechanism involves acoustic phonons. In QDMs the carrier wavefunction is spatially delocalized between the dots, both reducing the electron-hole overlap and modifying the carrier-phonon interaction and theoretical investigations have suggested that a reduction of acoustic phonon scattering rates can be achieved in QDMs. We observe evidence of such a modulation and find that the measured lifetime of the indirect exciton can be understood as a combination of a changing wavefunction, carrier tunneling, and phonon relaxation.

We have also observed a continuous and controllable change in the QCSE of excitons in InAs QDMs as a function of the GaAs barrier which may be useful for tunable single photon emission or optical switches. The changes can be qualitatively understood by a simple model of the QDM as two coupled 1D potential wells in a field. Solutions to Schrödinger's equation result in molecular symmetric (bonding) and antisymmetric (antibonding) wavefunctions for the electron and hole. As the dots are brought together the electron and hole symmetric molecular states shift toward the midway point between the dots. Due to the difference in effective masses this will occur to differing amounts for the electron and hole resulting in a tunable QCSE.

Lastly, the molecular nature of the carriers is also found to have an effect on spin interactions. With applied electric field we observe an increase in polarization memory in the indirect exciton. This indicates a possible reduction of the anisotropic exchange interaction which may provide a possible mechanism to regain degenerate polarization states to produce entangled photon pairs. However, we find a sudden unexpected dip in the circular polarization memory of the spatially indirect exciton state. This effect, along with a discussion of the polarization of various charge states, will be presented.

November 19, 2014
11:00 a.m.
Bldg. 440, A105-106

"Designed Chemical Synthesis and Assembly of Uniform-sized Nanoparticles for Medical and Energy Applications," Taeghwan Hyeon, Center for Basic Science, hosted by Elena Shevchensko

Abstract: Over the last 10 years, our laboratory has focused on designed chemical synthesis, assembly, and applications of uniform-sized nanocrystals. In particular, we developed a novel generalized procedure called the "heat-up process" for the direct synthesis of uniform-sized nanocrystals of many metals, oxides, and chalcogenides.

Recently our group has focused on medical applications of various uniform-sized nanoparticles. Using 3-nm iron oxide nanoparticles, a new nontoxic contrast agent was realized for high-resolution magnetic resonance imaging (MRI) of blood vessels down to 0.2 mm. We demonstrated that ceria nanoparticles protect against ischemic stroke in an in vivo animal model. We reported the first successful demonstration of high-resolution in vivo three-photon imaging using biocompatible and bright Mn2+-doped ZnS nanocrystals. We fabricated tumor pH-sensitive magnetic nanogrenades composed of self-assembled iron oxide nanoparticles and pH-responsive ligands for theranostic application, enabling the visualization of small tumors of < 3 mm via pH-responsive T1 MRI and fluorescence imaging and superior photodynamic therapeutic efficacy in highly drug-resistant heterogeneous tumors. We synthesized tumor pH-sensitive nanoformulated triptolide coated with folate targeting ligand to treat hepatocellular carcinoma (HCC), which has one of the worst prognoses for survival as it is poorly responsive to both conventional chemotherapy and mechanism directed therapy.

We reported the large-scale synthesis of magnetite nanocrystals embedded in a carbon matrix and hollow iron oxide nanoparticles. We demonstrated galvanic replacement reactions in metal oxide nanocrystals. When Mn3O4 nanocrystals were reacted with iron(II) perchlorate, hollow box-shaped nanocrystals of Mn3O4/γ-Fe2O3 ("nanoboxes") were produced. These iron oxide-based nanomaterials exhibited very high specific capacity and good cyclability for lithium ion battery anodes.

November 12, 2014
10:00 a.m.
Bldg. 440, A105-106

NOTE TIME CHANGE
 

"Integrating Graphene with Ferroelectrics," Xia Hong, University of Nebraska-Lincoln, hosted by Anand Bhattacharya and Jason Hoffman

Abstract: The interface between graphene and various dielectric materials presents a rich playground for exploring nanoscale control of low dimensional electronic behavior and rational design of new functionalities. For example, integrating graphene with ferroelectrics allows one to explore graphene-based nanoelectronics such as high frequency analog devices and nonvolatile memories.  One can also use graphene as a sensitive tool to probe the properties of ferroelectric thin films and nanostructures, where conventional electrical characterization becomes challenging.

In this talk, I will discuss our recent studies of the prototype graphene-ferroelectric hybrid devices. Using graphene as a sensor, we have examined the dielectric, pyroelectric, and ferroelectric properties of epitaxial ferroelectric thin films of Pb(Zr,Ti)3 and (Ba,Sr)TiO3. It is observed that the competition between the ferroelectric polarization and interfacial screening charge dynamics has a profound effect on the ferroelectric field effect control of graphene. I will also discuss how the presence of the ferroelectric neighbor layer affects the mobility of graphene. Our work reveals the important roles of the dielectric properties and interface chemistry of the ferroelectric gate on the electronic behavior of graphene.

November 6, 2014
2:00 p.m.
Bldg. 440, A105-106

"Scanning Tunneling Microscopy Study of Artificial Molecular Rotors andDipolar Molecule Self-Assembly Mechanisms," Yuan Zhang, Ohio University, hosted by Saw Wai Hla

Abstract: Compared with biomolecular rotors in nature that depend sensitively on the environment to function, artificial molecular rotors are more practical for molecular nanodevice engineering. Here we show the results of two low-temperature scanning tunneling microscopy studies with respect to artificial molecular rotor.

In the first artificial molecular rotor study, we show that a single stand-alone molecular motor adsorbed on a gold surface can be made to rotate in a clockwise or counterclockwise direction by selective inelastic electron tunneling through different sub-units of the rotor. The directional rotation originates from sawtooth-like rotational potentials, which are solely determined by the internal molecular structure and are independent of the surface adsorption site. In the second molecular rotor study, we add molecular communications in a ferroelectric nanomachine network. Here we show that by exploiting dipolar interactions between the rotor arms of self-assembled double-decker class molecular rotors in a hexagonal network, synchronized and coordinated rotations of the rotors can be performed by using an electric field of a scanning tunneling microscope tip as an energy source. Synchronized rotation occurs due to the degeneracy of the ground state rotational energy in the hexagonal dipole network, and it can be coherently transferred to multiple destinations within nanometer range. This work is a step forward for the development of solid state compatible and responsive multicomponent molecular machines.

We further investigate the influence of molecular dipole interaction on molecular self-assembly processes. We introduce a slight modification on 6P by replacing two hydrogen (H) atoms from an end π-ring with fluorine atoms to have a dipole moment in the molecule. Molecules with two different fluorinated end π-rings have been selected to further distinguish the effects of dipolar interactions in molecular self-assembly processes. Analysis of cluster dipolar energy and atom specific interaction between molecules reveals that molecular self-assembly process is under more influence of H-F interaction rather than dipolar interaction at close intermolecular distance.

November 4, 2014
11:00 a.m.
Bldg. 440, A105-106

"Nanoclusters as a new family of high Tc superconductors," Vladimir Kresin, Lawrence Berkeley National Laboratory, hosted by Saw Wai Hla

Abstract: Metallic nanoclusters display a remarkable feature: their electronic states form energy shells similar to those in atoms and nuclei. Under special but perfectly realistic conditions, superconducting pairing in some nanoclusters can become very strong. Such clusters form a new family of high-temperature superconductors. In principle, it is possible to raise Tc up to room temperature. The experimental search for the phenomenon has been carried out in several laboratories. Recent observation of high Tc state in aluminum clusters will be described. Delocalized electrons in aromatic molecules and biologically active systems also represent an example of superconducting nanosystems. The phenomenon is promising for building the nano-based tunneling networks capable to transfer a macroscopic supercurrent at high temperatures.

October 23, 2014
1:00 p.m.
Bldg. 440, A105-106

"Modeling film structure and electron transport processes in stable free-radical organic polymers," by Ross Larsen, National Renewable Energy Laboratory, Hosted by Kenley Pelzer

Abstract:: Organic radical batteries (ORBs) comprise a novel technology that uses cathodes based on stable organic radical-based polymers. Polymeric organic nitroxide radical materials, such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), have received great interest for various energy storage applications. These materials are readily synthesized from environmentally benign precursors, and TEMPO in particular has been shown to have very fast charge transfer kinetics. These types of materials show great promise as cathode materials because the neutral, radical species are remarkably stable, and the one-electron oxidation is fully reversible.  In order to guide the development of new organic radical electrode materials and to aid in the design of improved electrode structures, a detailed understanding of the fundamental mechanisms involved in electronic charge transfer and anion interfacial mobility in the polymer matrix is required. The work reported here has focused on combining large-scale molecular dynamics simulations with ab initio electronic structure calculations to predict charge transport in organic radical films. The simulations predict that electron transport occurs over a range of different distances so charge diffusion and mobility is predicted using a novel formalism that accounts for inhomogeneity in electron hopping rates and distances. Some implications of these calculations for how electrons move through these materials will be discussed in context of a variety of experiments such as AC impedance that have been performed in our group.

October 13,  2014
11:00 a.m.
Bldg. 440, A105-106

"Semiconductor Nanocrystals for Photocatalytic Hydrogen Production," by Fen Qiu, University of Rochester, hosted by Yugang Sun

Abstract:: Homogeneous systems for light-driven reduction of protons to hydrogen (H2) typically suffer from short lifetimes because of decomposition of the photosensitizers (PSs). I will present a robust and highly active system for solar H2 generation in water that uses CdSe quantum dots (QDs) capped with dihydrolipoic acid (DHLA) as the PS and a soluble Ni2+-DHLA catalyst for proton reduction with ascorbic acid as an electron donor, which gives >600,000 turnovers. Nanocrystals (NCs) whose proton reduction activity were compared include spherical CdSe QDs, spherical CdSe/CdS core/shell QDs, CdSe quantum rods (QRs), and CdSe/CdS dot-in-rods (DIRs) in this system. In order of decreasing H2 generation activity, the NCs order as: CdSe QDs > CdSe QRs > CdSe/CdS core/shell QDs > and CdSe/CdS DIRs, which is surprisingly related inversely to the electron-hole separation efficiency. Electron and hole transfer rates of CdSe/CdS NCs measured by fluorescence quenching were positively correlated to the H2 production rate and suggest neither step is strongly rate-limiting. Calculations of photoexcited surface charge densities are also positively correlated with the H2 production rate and suggest the size of the NC plays a critical role in determining the relative efficiency of H2 production in the current system.

October 10, 2014
11:00 a.m.
Bldg. 440, A105-106

Study of Advanced Batteries Using in situ/operando Techniqiues,” by Qi Liu, Purdue University, hosted by Yugang Sun

Abstract: For lithium-ion batteries (LIBs), the cathode materials are typically metal oxides or phosphates, which serve as the host for Li+ ion intercalation during discharge. Many different metal oxides or phosphates have been explored as cathode materials. Among the commonly used cathode materials, LiFePO4 and V2O5 have been considered to be very promising. This presentation will center on the study of LiFePO4 and V2O5 as the cathode materials for LIBs. Typically, the in-depth investigation of process-structure-property relationship in LiFePO4-based and V2O5-based LIBs has been studied using advanced in situ synchrotron techniques and existing tools.

The first topic is about the investigation of lithium-ion insertion/deinsertion behavior of LiFePO4 cathodes in commercial 18650 LiFePO4 cells. In the study, we have performed operando synchrotron high-energy X-ray diffraction (HEXRD) to obtain nonintrusive, real-time monitoring of the dynamic chemical and structural changes in commercial 18650 LiFePO4/C cells under realistic cycling conditions. The results indicate a nonequilibrium lithium insertion and extraction in the LiFePO4 cathode, with neither the LiFePO4 phase nor the FePO4 phase maintaining a static composition during lithium insertion/extraction. Based on our observations, we propose that the LiFePO4 cathode simultaneously experiences both a two-phase reaction mechanism and a dual-phase solid-solution reaction mechanism over the entire range of the flat voltage plateau, with this dual-phase solid-solution behavior being strongly dependent on charge/discharge rates. The proposed dual-phase solid-solution mechanism may explain the remarkable rate capability of LiFePO4 in commercial cells.

The second topic is the investigation of structure evolution of vanadium oxide (V2O5) nanocrystals during the Li+ ion intercalation and deinsertion processes using in operando HEXRD and in operando x-ray adsorption near edge spectroscopy (XANES). The HEXRD results clearly show that V2O5 undergoes phase transformations during the first Li+ ion intercalation (i.e., discharge) process. Analysis of the XANES data suggests that the average oxidation state of vanadium in fully charged V2O5 nanocrystals decreases to less than +5 after the first four cycles.  The combined results of the unchanged crystal structure (HEXRD) and the decreased oxidation state (XANES) lead to the conclusion that some of the Li+ ions are trapped within the V2O5 framework and the V2O5 exists as Li0.18V2O5 instead of pure V2O5 after the first four cycles, while the trapped Li+ ion may increase the stability of V2O5 framework. 

The following part is about the development of a novel method to sandwich graphene sheets into V2Onanoribbons via the sol-gel process. Graphene-modified nanostructured V2O5 hybrids have been developed with extraordinary electrochemical performance, 438 mAh/g, almost achieving the theoretical specific capacity (443 mAh/g), with only ~2% graphene in the composite. An in operando HEXRD study revealed that such performance is the result of the enhanced thermal stability of the V2O5 hybrids. The graphene sheets help to preserve the V2O5 xerogel structure and keep the xerogel from collapsing by maintaining 0.3 water molecules per V2O5 (water molecules serve as a pillar between the V2O5 layers) during the annealing process. The AC impedance indicates that the electric conduction, vanadium redox reaction, and Li+ diffusion in the graphene modified nanostructured V2O5 hybrid have been greatly improved, resulting in a significant improvement on rate performance and cycle life. This method provides a new avenue to create nanostructured metal oxide/graphene xerogel with improved properties, as long as they can be synthesized via the sol-gel process or by a reaction in solutions.

October 3, 2014
11:00 a.m.
Bldg. 440, A105-106

Photo-induced doping in heterostructures of graphene and boron nitride,” by Jairo Velasco, Jr., University of California-Berkeley, hosted by Adina Luican-Mayer

Abstract: The design of stacks of layered materials in which adjacent layers interact by van der Waals forces has enabled the combination of various two-dimensional crystals with different electrical, optical and mechanical properties, and the emergence of novel physical phenomena and device functionality. I will discuss our studies on photo-induced doping in van der Waals hetero-structures(VDHs) consisting of graphene and boron nitride layers. We find this photo-induced doping enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that it maintains the high carrier mobility of the graphene–boron nitride heterostructure, thus resembling the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially varying doping profiles such as p–n junctions. We show that this photo-induced doping arises from microscopically coupled optical and electrical responses of graphene–boron nitride heterostructures, including optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene. Our work contributes towards understanding light matter interactions in VDHs, and innovates a simple technique for creating inhomogeneous doping in high mobility graphene devices. This opens the door for new scientific studies and applications.

September 18, 2014
4:00 p.m.
Bldg. 440, A105-106

"Stenotrophomonas maltophilia: From Nocosomal Pathogen to Nanoparticle," by Bryan W. Berger, Lehigh University, hosted by Gary Christopher

Abstract: Stenotrophomonas maltophilia is a ubiquitous, gram-negative bacteria that has become an increasing important, multidrug-resistant (MDR) nocosomial pathogen in immunocompromised patients. One unique mechanism that S. maltophilia has evolved to become MDR is its production of highly branched, anionic exopolysaccharides (EPS) to form biofilms. Recent studies indicate the additional negative charge of S. maltophilia EPS due to the branched HexA-Lac group enables biofilm binding to metallic or plastic surfaces, as well as creating a heat-, acid-, and detergent-resistant environment that allows S. maltophilia to colonize water purification systems, ventilators and stents. As a result, S. maltophilia is the second leading cause of ventilator-associated pneumonia, with over 70% of all S. maltophilia-specific, ventilator-associated MDR infections caused by biofilm formingstrains.

First, I will discuss our recent work identifying and characterizing a series of novel polysaccharide lyases (PLs) unique to clinical isolates of S. maltophilia. PLs are a broad class of enzymes that depolymerize polysaccharides via a b-elimination mechanism. We find that in contrast to pathogens such as P. aeruginosa that produce PLs specific to a particular EPS, S. maltophilia PL smlt1473 exhibits broad substrate specificity, but with strict pH regulation of activity against a given substrate. Furthermore, mutations to conserved regions flanking thepredicted active site provide further diversification of both enzymatic activity as well as possible substrates accepted by the PL. The basis for this versatility in both substrate specificity as well as pH-dependent suggests this class of enzymes has evolved considerably flexibility in order to process increasingly diverse biofilm EPS chemical structures as well as function as a possible virulence factor active against mammalian polysaccharide substrates such as hyaluronic acid.

Second, I will discuss mechanisms of heavy-metal resistance in S. maltophilia clinical isolates. We have identified a series of extracellular proteins that are responsible for conferring resistance to silver, lead, cadmium, and other toxic heavy metals at concentrations 10-100 fold greater than that of E. coli and other gram-negative bacterium. We have exploited these proteins to engineer strains with enhanced tolerance to a wide range of heavy metals that produce nanostructured metal sulfides with well-defined chemical and optical properties. Applications in scalable biosynthesis of nanostructured metal sulfides will be discussed.

June 25, 2014

"Understanding Non-Equilibrium Charge Transport and Rectification at Nanoscale Interfaces," by Pierre T. Darancet, Columbia Univrsity, hosted by Stephen Gray

Abstract: Understanding and controlling nonequilibrium charge transport across nanoscale interfaces and in supramolecular assemblies is central to developing an intuitive picture of fundamental processes in nanoelectronics, photovoltaics, and other energy conversion applications. In this talk, I will discuss our theoretical studies of finite-bias transport at prototypical organic/metal interfaces, single-molecule junctions, small organic molecules trapped between gold electrodes. I will show how many-body effects influence energy level alignment in these systems, and that a simple model of nonlocal correlations on the top of density functional theory leads to quantitative agreement with experiments. Finally, I will discuss the implications of this theory in the context of transport in molecular diodes; in particular, how to systematically optimize rectification by tuning the competing energy scales in single-molecule junctions via molecular conformation.

June 24, 2014

"Ultrafast optical manipulation of magnetoelectric coupling at a multiferroic interface," Yu-Min Sheu, Los Alamos National Laboratory, hosted by Gary Weiderrecht

Abstract: A new paradigm for all-optical detection and control of interfacial magnetoelectric coupling on ultrafast timescales is achieved by using femtosecond time-resolved second-harmonic generation (SHG) to study a ferroelectric/ferromagnet oxide heterostructure. I use femtosecond optical pulses to photoinduce interfacial coupling in a Ba0.1Sr0.9TiO3(BSTO)/La0.7Ca0.3MnO3 (LCMO) heterostructure and selectively probe the ferroelectric response using SHG. In this heterostructure, the pump pulses photoexcite nonequilibrium quasiparticles in LCMO, which rapidly interact with phonons before undergoing spin-lattice relaxation on a timescale of tens of picoseconds. This relaxes the spin-spin interactions in LCMO, applying stress on BSTO through magnetostriction. This in turn leads to a transverse magnetoelectric effect that occurs much faster than laser-induced heat diffusion from LCMO to BSTO.

During this seminar, I will demonstrate how an ultrafast interfacial magnetoelectric effect can be mediated through elastic coupling, which could lead to future high-speed optically controlled magnetoelectric devices.

June 11, 2014

"Direct imaging of nanostructure surfaces and interfaces to the atomic scale using both scanning probe and synchrotron light based microscopy," Anders Mikkelsen, Lund Univesity, hosted by Volker Rose

Abstract: We work toward combining novel atomic scale microscopy/spectroscopy on complex nanostructures, advanced light sources, and material science for tailoring low-dimensional structures. This allow development and use of a new generation of imaging techniques with orders of magnitude better resolution in both time and space for direct studies of functional nanoscale materials and devices - even during operating. Two main themes will be covered.

First, we have developed and used scanning probe microscopy and high-brightness synchrotron-based microscopy to determine structure, chemistry, and physical properties of III-V semiconductor nanostructures. We have, for example, developed novel methods to directly image surfaces inside, outside, and topside of nanowires down to the single-atom level, revealing geometric structure as well as both electrical and mechanical properties. Using our rather diverse toolbox, we can obtain a real understanding of the connections among structure, growt, and function of these nanowires — some with potential applications in informatoin technology, energy, and life science.

Second, we are working on NanoMAX, which will be the first X-ray imaging beamline at the new Swedish synchrotron radiation source, MAX IV. It is a hard X-ray undulator beamline for micro- and nano-beams and will enable imaging applications exploring diffraction, scattering, fluorescence, and coherent diffractive imaging methods. The beamline will feature two experimental stations. One has beam sizes down to the 100-nm range, well-suited for diffractive and coherence experiments in flexible sample environments. The second experimental station will use zone plates to eventually reach 10-nm spot sizes for diffraction and fluorescence experiments. NanoMAX was funded late in 2011, and the beamline is planned to open for users in late 2016.

June 10, 2014

"p x n Transverse Thermoelectrics: A Novel Paradigm for Thermoelectric Materials," Matthew Grayson, Northwestern University, hosted by Nathan Guisinger

Abstract: A new class of electronic materials has been identified with promising thermoelectric applications due to its scalability, geometric figure-of-merit enhancement, and ability to operate at cryogenic temperatures. The so-called p x n-type transverse thermoelectric ("p-cross-n") with p-type Seebeck in one direction and n-type orthogonal is a narrow gap semiconductor with both electrons and holes carrying comparable magnitudes of orthogonally directed heat currents. Off-diagonal terms in the Seebeck tensor can drive the net heat flow transverse to the net electrical current. Whereas thermoelectric performance is normally limited by the figure of merit ZT, these p x n-type materials can be geometrically shaped for enhanced performance equivalent to an increased effective figure of merit. The single-leg nature of these thermoelectric devices allows for integrated device applications, leading to simpler, more compact, and efficient thermal elements for Peltier cooling or waste heat generators. Anisotropic p x n-type materials that show promise include type II InAs/GaSb superlattices and noncubic bulk crystals of narrow-gap semiconductors such as CeBi(4)Te(6) and ReSi(1.75). Unconventional geometries for possible detector pixel cooling, and solar cell waste heat recovery are described.

June 10, 2014

"Magnetic Domain Formation in La1-xSrxMnO3 Nanowires using Resonant Soft X-Ray Scattering," Xiaoqian Chen, University of Illinois at Urbana-Champaign, hosted by Ian McNulty

Abstract: Spatial confinement effects can be a useful tool to disentangle the complexity of strongly correlated systems. In the case of manganites, factors such as super exchange, double exchange, Hund's rule coupling, or electron kinetic energy can compete for determination of a ground state. When the spatial dimension of a material is reduced, its ground state can be altered by the difference in correlation lengths of these underlying competing orders. This raises the question of how boundary effects influence the phase of such a system, and whether spatial confinement can influence the properties of a nanoscale object.

To answer these questions, we fabricated arrays of nanowires from the CMR material La1-xSrxMnO3 (LSMO) using e-beam lithography. In bulk or thin film, LSMO undergoes a para- to ferromagnetic phase transition at the Curie temperature (Tc). Our magnetization measurements performed on these wires suggest the existence of an additional magnetic ordering at a temperature much lower than Tc. Around this temperature, domain switching was also observed in transport measurements.

To understand these observations, we performed resonant soft X-ray scattering studies at the Mn L edge. We observed a series of grating reflections and superlattice reflections whose magnetic signals are temperature dependent. These observations indicate the emergence of a nontrivial magnetic ordering inside the wires at different length scales as the temperature is lowered. To determine the exact magnetic structure, we are in the process of modeling the scattering using a numerical method combining least-squares fitting and algorithmic phase retrieval. This analysis will reveal the real-space magnetization density distribution inside the nanowires with nanometer resolution.

June 6, 2014

"Geometrical frustration and energy landscape in patterned magnetic nanostructures," Sheng Zhang, Argonne National Laboratory, hosted by Ian McNulty

Abstract: Frustration, arising from competing interactions, is a ubiquitous condition in many physical systems and leads to degeneracy and disorder. Artificial spin ice, consisting of lithographically fabricated single-domain ferromagnetic nanostructures, allows us to control the degree of frustration and manipulate competing energy terms by tuning their geometries and lattice parameters. Of prominent interest in frustrated systems is the experimental achievement of equilibrium ground states and the novel phases that evolve during the approach to such states. Recent research on successful routes to equilibrium involved a protocol of thermal annealing that achieved unprecedented long-range ground-state ordering in spin ice arrays and realized incipient magnetic charge crystallization in kagome arrays. Frustrated arrays comprised of nanostructures with magnetic moments perpendicular to the substrate plane were also investigated, which realized isotropic Ising model. These perpendicular arrays exhibit striking similarity to their in-plane counterparts, indicating a universality in frustrated systems. Our research sheds light on the nature of magnetism in patterned arrays and provides a new avenue to study the physics of frustration.

Control of magnetic domain behavior in patterned nanostructures can be achieved by modifying their energy landscape. Critical to that control is the ability to obtain quantitative measures of the contributing energy terms, including pinning sites that arise as a result of competing energy contributions. Patterned discs consisting of ferromagnetic, nonmagnetic, and antiferromagnetic multilayer heterostructures show novel magnetization reversal mechanism as a combination of magnetization rotation in the pinned layer and localized vortex nucleation in the free layer. The vortex nucleation was observed to be strongly influenced by the magnetization in the pinned layer along with an unexpected jump in its trajectory as a function of in situ application of magnetic field. The jump of the vortex was identified as a result of the competition of several energy terms (e.g., the exchange bias energy, magnetostatic interaction energy and Zeeman energy), supported by integrated phase shift of the coupled discs and micromagnetic simulations. This work provides new opportunities for macroscopic control of the energy landscape of magnetic heterostructures for functionalapplications.

June 5, 2014

"Antiferromagnetic Domain Wall Manipulation and Measurement," Jonathan Logan, The University of Chicago, hosted by Ian McNulty

Abstract: Antiferromagnets are described by their local magnetization as well as how this magnetization evolves with position. This contrasts with ferromagnets, which can be described by their magnetization vector alone. This feature adds an extra layer of complexity and richness to antiferromagnetic domain walls. Learning how these domain walls form, move, and affect electron transport is at the heart of understanding many intrinsic properties of antiferromagnetic materials.

The absence of a net magnetic moment renders antiferromagnetic ordering invisible to imaging techniques commonly used for ferromagnets. As a result, local probes such as X-ray microdiffraction and X-ray photon correlation spectroscopy are used to obtain information on the structure and dynamics of antiferromagnetic domains. In this talk, I discuss the domain structure and dynamics of antiferromagnetic Cr as measured in a bulk sample, and discuss domain wall manipulation in Cr films.

By using surface pinning and magnetic frustration effects, we developed a method to lithographically pattern individual antiferromagnetic spin-density wave (SDW) domain walls in epitaxial Cr(001)/Fe/Au films on MgO(001). The creation of SDW domain walls was verified with X-ray microdiffraction and their location was confirmed with X-ray microfluorescence to coincide with the lithographic pattern. These engineered domain walls have a precisely defined shape and are pinned to a known nucleation site, allowing precise measurements of electrical transport.

Several devices, each containing a single lithographically patterned antiferromagnetic domain wall, were used to collect electrical transport data. From this data, it is possible to isolate the resistance of an individual antiferromagnetic domain wall across the entire temperature range of the SDW order parameter.  The domain wall resistivity shows an unexpected temperature dependence that elucidates the possible mechanisms responsible for electron scattering in individual SDW domain walls.

At the end of the talk, I will discuss a future study of perovskite oxide interfaces in antiferromagnet/ferromagnet bilayer systems such as La0.7Sr0.3FeO3/ La0.7Sr0.3MnO3. I propose using hard X-ray dichroic coherent diffractive imaging to investigate interfacial coupling of domain structures in these bilayer films and to obtain detailed information on their spin, charge and lattice states. This study will be possible with the unique capabilities at APS 34-ID and the Hard-Xray Nanoprobe facility, and will be performed in collaboration with Yayoi Takamura of UC Davis.

June 2, 2014

"Thermal Transport in Isotope Substituted Carbon Nanomaterials: From Fundamentals to Design," Ganesh Balasubramanian, Iowa State University, hosted by Subramanian Sankaranarayanan

Abstract: Thermal conductivity in carbon nanomaterials such as nanotubes (CNTs) and graphene nanoribbons (GNRs) is governed by lattice vibrations (also called phonons) and the various energy scattering phenomena associated with them. Impurities such as atomic vacancies, dopants and isotopes enhance the scattering effects, further reducing the energy transfer ability of these materials. We present results from quantum mechanical and classical molecular simulations on the effects of isotopes on the thermal conductivity of CNTs and GNRs. Strong shifts in the characteristic vibrational frequencies of the phonon modes are observed in the mass disordered structures that decrease the energy carrying capacity of the nanomaterials.

Our investigations reveal that contrary to intuitive understanding the out-of-plane modes in a graphene sheet contribute significantly to thermal transport through them. An ordered arrangement of these isotope impurities can facilitate engineering of material systems for targeted thermal transport behavior. Results from our recent efforts at employing informatics and optimization tools show the importance of high-frequency modes in the vibrational spectra toward designing mass disordered structures for desired thermal conductivities.

May 30, 2014

"Applications of Femtosecond Spectroscopy: From two-dimensional materials to mimicking neuro-functioning," by Keshav M. Dani, Okinawa Institute of Science and Technology, hosted by Richard Schaller

Abstract: The Femtosecond Spectroscopy Unit at the newly established Okinawa Institute of Science and Technology studies applications of ultrafast and nonlinear spectroscopy in a variety of phenomena ranging from opto-electronic properties of two-dimensional materials to mimicking neurotransmitter dynamics of the brain. In this talk, I will present the experimental facilities developed over the past two years. I will present a broad overview of our recent studies in heterostructures of two-dimensional materials; and mimicking neurotransmitter dynamics of the brain using femtosecond pulses.

May 1, 2014

"Atomic-Scale Assessment of Graphene-Substrate Interactions, Grain Boundaries, and Materials for Heterostructures," by Justin Koepke, University of Illinois at Urbana-Champaign, hosted by Nathan Guisinger

Abstract: Graphene is an atomically thin honeycomb lattice of sp2-bonded carbon atoms with a linear, low-energy band structure. Despite its exceptional electronic properties, the primary challenges to development of graphene for device applications are wafer-scale synthesis methods and graphene-substrate interactions. Chemical vapor deposition (CVD) growth of graphene on copper foil provides one path to wafer-scale graphene.

Typical graphene CVD on copper yields rotationally misoriented graphene domains that form grain boundaries (GBs) when these domains merge. These graphene GBs strongly perturb the local graphene electronic structure. These GBs lead to localized states and decrease the local work function, leading to p-n-p and p-p'-p (p' < p) potential barriers at the GBs that act as scatter charge carriers. The effects of the GBs decay over a length ~1 nm.

Graphene-substrate interactions are critical in determining key properties such as carrier mobility. Graphene deposited in UHV on GaAs(110), InAs(110), and Si(111) – 7×7 surfaces exhibits an electronic semitransparency effect in which the substrate electronic structure is observable "through" the graphene by scanning tunneling microscopy (STM). The mechanical force of the STM tip leads to a reduction of the graphene-substrate spacing, which induces the observed semitransparency. Transport experiments and STM studies of graphene on hexagonal boron nitride (h-BN) show that it is an ideal substrate for graphene.

However, a full understanding of the growth mechanisms for CVD growth of h-BN on copper foil is lacking. The chamber pressure during the growth step has a dramatic effect on the morphology, chemical structure, and growth rate of the resulting h-BN. Experiments varying the chamber pressure for h-BN synthesis clearly shows that h-BN growth by low-pressure CVD yields more planar, uniform h-BN than that obtained by atmospheric pressure CVD

. Understanding the perturbative effects of GBs on the electronic properties of graphene and the interactions between graphene and its substrate are critical to device development. Furthermore, understanding the role of pressure in the CVD growth of h-BN will further the development of flexible graphene and transition metal dichalcogenide-based electronics and enable the growth of their heterostructured combinations.

March 20, 2014

"Template Direct Assembly of Bio-based Materials for Advanced Applications," by Handan Acar, Iowa State University, hosted by Tijana Rajh

Abstract: Engineering at the nanoscale has been an active area of science and technology over the last decades. Inspired by nature, synthesis of functional inorganic materials using synthetic organic templates constitutes will be the theme of the first part of this talk.

Developing an organic template-directed synthesis approach for inorganic nanomaterial synthesis was our goal. For this purpose, an amyloid-like peptide sequence capable of self-assembling into nanofibers under convenient conditions was designed and decorated with functional groups showing a relatively high affinity to special inorganic ions, which are present at the periphery of the one-dimensional peptide nanofibers. These chemical groups facilitated the deposition of targeted inorganic monomers onto the nanofibers, yielding one-dimensional organic-inorganic core-shell nanostructures. The physical and chemical properties of the synthesized peptide nanofibers and inorganic nanostructures were characterized by both qualitative and quantitative methods. The results obtained in these studies encourage use of a new bottom-up synthesis approach.

In the second part of the talk, a new concept of transient materials for bioelectronics and biomedical applications will be presented. The precise control over transiency of polymer composites based on biocompatible and biodegradable polymers is demonstrated. These transient materials can be used in the fabrication of bioelectronic devices that are capable of dissolving in their surrounding environment with no traceable remains and maintain full functionality until triggered for degradation. Further, precise control over the degradation of these biodegradable polymers serve as a matrix for encapsulation of susceptible bioactive materials, such as proteins and growth factors. These nontoxic degradable polymers are suitable platforms for slow delivery of bioactive materials with tunable mechanical properties to match that of the host living tissue.

March 12, 2014

"Supra-molecular Architectures at Surfaces for Probing Structure, Electron and Spin States," by Thomas A. Jung, Paul Scherrer Institute, hosted by Saw Wai Hla

Abstract: Well-defined electronic and spintronic interfaces can be architected by combining self-assembly and surface science. The atomically clean metal surface in ultrahigh vacuum provides a very specific environment affecting the behavior of the ad-molecules as well as the adsorbent-adsorbate interaction. Depending on the bonding at the interface, complex electronic and magnetic interaction can occur that can be explored by using spectromicroscopy correlation, in this case, photoemission and photoabsorption spectroscopy and scanning tunnelling microscopy.

One example is provided by the emergence of quantum dot states from the interaction of a porous network with the two-dimensional (Shockley) surface state of Cu(111), which exhibit sufficient residual coupling to show the emergence of a band-like structure in angle-resolved photoemission experiments. In another example, specifically chosen surface supported molecules have been shown to exhibit ferromagnetic or antiferromagnetic exchange interaction, and their spin system has shown change induced by physical parameters and/or chemical stimuli. By combining supramolecular chemistry with on-surface coordination chemistry, the reversible spin switching of self-assembled bimolecular arrays has recently been demonstrated.

These examples all have in common that the molecular interfaces are well defined by their production from atomically clean substrates and molecular building blocks. The physics and chemistry of these unprecedented systems, which are addressable by scanning probes, provide insight into novel materials in their assembly and their electronic and spintronic properties, which emerge from the interaction of their components down to the scale of single atoms, molecules, and bonds.

February 19, 2014

"When Structured Light Meets Structured Matter at the Nanoscale," by Xiaobo Yin, University of Colorado, Boulder, hosted by Jun Rho

Abstract: The rapid development of nanoscale science and technology not only permits exploration of advanced scientific ideas and observation of unprecedented phenomena, but also offers practical solutions to the world's most serious issues, such as energy and pollution crises, health and food safety concerns, and military and homeland security needs. Exploiting and enhancing originally weak light-matter interactions, we will be able to devise better imaging and manufacturing tools, catalyze more efficient photochemical reactions, and sense and diagnose contaminants at the single-molecule level.

This talk will focus on how judiciously designed nanostructures and materials can tailor and eventually control light-matter interactions at the deep-subwavelength scale. I will illustrate these design principles by using specific examples. In particular, I will elaborate strategies to harvest substantial amounts of energy-efficient emissions from a sub-wavelength laser cavity and to achieve close-to-unit utilization of light, providing coherent sources at the nanoscale. These nanolasers can perform much brighter and faster when quantum engineering is employed and show great potential in ultra-trace chemical sensing. More interestingly, introducing uniquely structured quantum materials, such as monoatomic layer transition metal dichalcogenides, the light-matter interaction at the nanoscale senses the atomic structural and topological symmetries that are embedded in the system, revealing the fascinating physics and redefining applications based on these unique physical processes. I will discuss some of the preliminary assessments of the observed valley physics and illustrate the structure and function relationship in these impactful materials that have been widely utilized in both mechanical systems and energy sciences.

February 10, 2014

"Optical and electronic microscopic characterization of plasmonic modes on a self-assembled metallic grating," by Clotilde Lethiec, Universite Pierre et Marie Curie, hosted by Gary Wiederrecht

Abstract: Efficient coupling of single-photon nanoemitters to photonic or plasmonic structures requires spatial and spectral matching of the emitting dipoles to the nanostructure. It is especially crucial to match the orientation between the electrical field of the photonic or plasmonic mode and the fluorescent dipole. Therefore, it is necessary to determine the distribution of the electrical field associated to the excited mode as well as the dipole orientation.

This seminar will be divided into two parts. First, I will present the polarimetric method I have developed and applied to high-quality CdSe/CdS dot-in-rods and spherical nanocrystals in order to retrieve the three-dimensional orientation of the associated dipole. I could correlate optical dipolar properties to the shape of the emitter.

In the main part of my talk, I will focus on the coupling between light and surface plasmons polaritons (SPPs). SPPs are known to enhance light matter interaction with applications in fields such as bio-imaging, light-emitting diodes, photovoltaics, and single-photon sources. Metallic surface gratings offer the opportunity to absorb light with almost 100% of efficiency and to enhance the fluorescence of nanoemitters close to their surface. In order to take advantage of SPP modes, which are not coupled to far-field radiative modes in the case of a planar metallic surface, a periodically patterned metallic surface can be used. We used self-assembly to produce centimeter-sized plasmonic crystals with 400-nm periodic structure, by evaporating a thick layer of gold on artificial silica opals used as a periodic template. We performed optical specular reflection spectra and evidenced a dip of almost complete absorption. This dip is explained by theoretical calculations and can be attributed to a coupling to SPPs. We demonstrated, at a given incidence angle, a broad continuum of coupled wavelengths over the visible spectrum.

Complementary photo-emission electron microscopy (PEEM) measurements give a high-resolution (25-nm) map of the electric field of the photo-excited plasmonic modes. This technique enables us to distinguish between the coupling of incident light modes to SPP and localized plasmons by the observation of interference fringes and hot spots. The arrangement of the hot spots is discussed as a function of the crystallographic quality of the opal. These results stress the important role of disorder at different scales and open new possibilities for the study of optical disordered media.