Argonne National Laboratory Center for Nanoscale Materials U.S. Department of Energy

Archive: Seminars 2014

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June 26, 2014

"Single-Particle Laser Spectroscopy of Semiconductor Nanocrystal Quantum Dots," by Young-Shin Park, Los Alamos National Laboratory and University of New Mexico, hosted by Gary Weiderrecht

Abstract: Size-controlled emission colors and high intrinsic photoluminescence quantum yields are among the reasons that make nanocrystal quantum dots (QDs) attractive materials for a range of applications, for examples, light-emitting diodes (LEDs), lasing, and nonclassical photon sources. One problem in all of these applications, however, is significant quenching of emission from charged- and multi-excitons due to nonradiative Auger recombination. In this process, the electron-hole recombination energy is not emitted as a photon, but rather transferred to a third carrier, finally dissipating in thermal form. Thus, the ability to harvest the emission from charged- and multi-exciton states could dramatically enhance the potential of QDs, especially for LEDs and lasing.

In this talk, I will present our recent progress on the realization of Auger suppression in two types of heterostructured QDs ("giant" and "alloyed" QDs) that are characterized by an interfacial alloy layer between the core and the shell. We found that the interfacial alloying, which makes the confinement potential "softened," can lead to significant reduction in Auger decay rates of these QDs. Auger suppression of individual QDs was quantitatively evaluated by a time-tagged, time-resolved single-photon counting technique, demonstrating high quantum yields and elongated lifetimes of biexciton states in these QDs with an interfacial alloy. I will also discuss the use of Auger-suppressed QDs for the applications of LEDs and lasers.

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.

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