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

Archive: 2012 Colloquium Series

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Date Title
Special Colloquium
December 13, 2012

"Pathways to Complex Matter Far-Away-From Equilibrium: Developing Spatiotemporal Tools," by Gopal Shenoy, Argonne National Laboratory, hostged by Daniel Lopez

Abstract: From the Big Bang to the coming of humankind, every manifestation of nature has exhibited processes far-away-from equilibrium leading to increasingly complex structural orders from geological to atomic length and time scales. Examples include the evolution of galaxies, hurricanes, stars, and planets; prebiotic reactions; cyclical reactions; photosynthesis; and life itself. The organizational spatiotemporal evolution in soft, hard, and biological matter also follows the same path. It begins from a far-from-equilibrium state and develops over time into organizations with length scales between atoms and small molecules on the one hand and mesoscopic matter on the other.

Over the past century, almost all scientific effort has sought to understand equilibrium structural order, primarily in the crystalline regime. However, there is a host of nonequilibrium physical, chemical, and biological orders with unique emergent properties. Examples include glassy metal alloys with unusual mechanical strength, nanomachines, emergent hidden correlations such as colossal magnetoresistance behavior, and mesoscale self-assembly of crystalline hybrid nanoparticles. I

t is argued that progress in understanding higher orders of complexity can be made only from a complete understanding of the spatiotemporal evolution of nonequilibrium processes. How does one measure the evolution of complex order? Can we control the evolution? During the past decade, many new spatiotemporal experimental tools have become available, many of which are "table-top," that can probe electronic and chemical structure and properties over subatomic to micron length scales and over fs to ms temporal scales. This experimental approach along with powerful computational modeling tools will usher in a new era of realizing, understanding, and controlling emergent properties in complex matter far-away-from equilibrium.

In this light-hearted, cross-disciplinary talk, we will also provide a popular "good reads" list for the holiday season.

December 12, 2012

"Antenna-load interactions at optical frequencies: impedance matching to quantum systems," by Markus Raschke, University of COlorado at Boulder, hostoed by Matt Pelton

Abstract: The goal of antenna design at optical frequencies is to deliver optical electromagnetic energy to loads in the form of atoms, molecules, or nanostructures, to enhance the radiative emission from such structures, or both. A true optical antenna would, on a qualitatively new level, control the light-matter interaction on the nanoscale for controlled optical signal transduction, radiative decay engineering, quantum coherent control, and super-resolution microscopy, and provide unprecedented sensitivity in spectroscopy. However, in contrast to the radio frequency (RF), where exact design rules for antennas, waveguides, and antenna-load matching in terms of their impedances are well established, substantial physical differences limit the simple extension of RF concepts of antenna design to the optical regime.

I will discuss the generalization of the ideal antenna-load interaction at optical frequencies, characterized by far-field transformation from a propagating mode into an antenna resonance, the subsequent transformation of that mode into a nanoscale localization, and the free space transformation via an enhanced local density of states to a quantum load. These steps define the goal of efficient transformation of incident radiation into a quantum excitation in an impedance matched fashion. I will review the physical basis of the light-matter interaction at the transition from the RF to optical regime, discuss extension of antenna theory as needed for the design of impedance-matched optical antenna-load coupled systems, and provide several examples of the state of the art in design strategies and suggest future extensions. I will discuss new measurable performance metrics based on electric vector field, measurement, field enhancement, and capture cross section to aid the comparison between different antenna designs.

December 5, 2012

"Toward microscopy with direct electronic, chemical, and magnetic contrast at the atomic level," by Volker Rose, Argonne National Laboratory, hosted by Matt Pelton

Abstract: In this talk, we will discuss the development of a novel high-resolution microscopy technique for imaging of nanoscale materials with chemical, electronic, and magnetic contrast. It will combine the subnanometer spatial resolution of scanning tunneling microscopy (STM) with the chemical, electronic, and magnetic sensitivity of synchrotron radiation. Drawing upon experience from a prototype that has been developed to demonstrate general feasibility, current work has the goal to drastically increase the spatial resolution of existing state-of-the-art X-ray microscopy from only tens of nanometers down to atomic resolution.

The key enabler for high resolution is the development of insulator-coated "smart tips" with a small conducting apex. After entirely coating a sharp PtIr tip with an insulating SiO2 film by electron-beam physical vapor deposition, the insulating film has been removed from the apex by means of high-resolution focused-ion-beam milling using a shadow masking technique. Such tips drastically reduce the background of photo-ejected electrons that would otherwise cause an undesired signal at the sidewall of the tip. The novel microcopy technique will enable fundamentally new methods of characterization, which will be applied to the study of energy materials and nanoscale magnetic systems. A better understanding of these phenomena at the nanoscale has great potential to improve the conversion efficiency of quantum energy devices and lead to advances in future data storage applications.

November 7, 2012

"Semiconductor Nanostructured Materials for Next Generation Photovoltaics," by Zhiqun Lin, Georgia Institute of Technology, hosted by Xiao-Min Lin

Abstract: Functional polymers and nanocrystals are promising building blocks for advanced materials and devices. In this talk, I will present our efforts on nanostructured functional materials from synthesis and self-assembly to solar energy applications. Two studies will be discussed: (1) semiconductor conjugated polymer-quantum dot and conjugated polymer-quantum rod nanocomposites via directly grafting conjugated polymer onto quantum dot and quantum rod surface by Heck coupling and click reaction, and their potential applications in nanohybrid solar cells; and (2) low-cost, high-efficiency dye-sensitized solar cells through the use of nanostructured TiO2 (e.g., nanotubes and nanoflowers) as photoanode, and earth-abundant, environmentally friendly quaternary semiconductor copper zinc tin sulfide as counterelectrode.

October 10, 2012

"Molecular Graphene: Home-Baked Dirac Electrons," by Hari C. Manoharan, Stanford University, hosted by Esmeralda Yitamban

Abstract: The observation of massless Dirac fermions in monolayer graphene has propelled a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials. Using low-temperature scanning tunneling microscopy and spectroscopy, we show the emergence of Dirac fermions in a fully tunable condensed-matter system — molecular graphene — assembled via atomic manipulation of a conventional two-dimensional electron system. Into these electrons we embed, map, and tune the symmetries underlying the two-dimensional Dirac equation. With altered symmetry and texturing, these Dirac particles can be given a tunable mass, or even dressed with fictitious electric or magnetic fields (so-called gauge fields) such that the carriers believe they are in real fields and condense into the corresponding ground state. This talk will describe how molecular graphene seeds a versatile path, via tailored nanostructures, to synthesize unique devices and exotic topological phases in quantum materials.

September 26, 2012

"Optically Active Hybrid Nanostructures: Exciton-Plasmon Interaction, Fano Effect, and Plasmonic Chirality," by Alexander Govorov, Ohio University, hosted by Gary Wiederrecht

Abstract: Coulomb and electromagnetic interactions between excitons and plasmons in hybrid nanostructures lead to several interesting effects: energy transfer between nanoparticles, plasmon enhancement, exciton energy shifts, Fano interference, and new mechanisms of optical chirality. An interaction between a discrete state of exciton and a continuum of plasmonic states gives rise to interference effects (Fano-like asymmetric resonances and antiresonances). These interference effects can strongly enhance the visibility of relatively weak exciton signals and can be used for spectroscopy of single nanoparticles and molecules.

If a system includes chiral elements (chiral molecules or nanocrystals), the exciton-plasmon interaction is able to alter and enhance the circular dichroism (CD) of chiral components. In particular, the exciton-plasmon interaction may create new chiral plasmoniclines in CD spectra of a biomolecule-nanocrystal complex. Strong CD signals may also appear in purely plasmonic systems with a chiral geometry and a strong particle-particle interaction. Recent experiments on the proteinnanocrystal and multinanocrystal complexes showed the appearance of strong plasmonic signals in CD spectra. Potential applications of dynamic hybrid nanostructures include sensors and new optical and plasmonic materials.

September 19, 2012

"High-Resolution Fractionation of Biological Objects at High Speeds: The Bump Array," Robert Austin, Princeton University, hosted by Daniel Lopez

Abstract: Deterministic lateral displacement is a ratcheting particle-sorting method with excellent size selectivity, adaptability to sorting multiple particle sizes, and dynamic control of critical particle sizes. It has been demonstrated under a broad range of operating conditions, sorting particles from 100 nm to 30 microns. Using such an array, large particles can be concentrated from an input stream and harvested at the end of the array by collecting the output fluid stream separate from the rest of the fluid leaving the array. But real world-development of this technology has been slow because of a lack of understanding of the physics of the strange ratcheting motion and a lack of understanding of the biological problems that can be addressed. I'll try to explain how this technique works and the potential pitfalls, and point toward where I think real biological problems can be addressed.

September 12, 2012

"Using Nonlinearity to enhance Micro/NanoSensor Performance," by Kimberly Turner, University of California - Santa Barbara, hosted by Daniel Lopez

Abstract: Resonant microelectromechanical systems are key building blocks for many microsensor applications, including mass detection, inertial detection, and radio-frequency filters and timing oscillators. Especially in systems with low damping, amplitudes are such that nonlinearities are present. In many applications, these nonlinearities can be significant, and need to be accounted for. In this talk, I will give an overview of a few applications where understanding and cleverly utilizing nonlinearity actually results in improved sensor performance. Examples including mass sensors and oscillators will be utilized in this demonstration of the benefit in marrying nonlinear equations and micro/nanoscale devices.

August 29, 2012

"Electrochemistry in fabrication of nanodimensional materials and structures: history, accomplishments and potential," Peter Mardilovich, Hewlett-Packard, hosted by Andreas Roelofs

Abstract: Numerous challenges still limit the advancements in nanofabrication and fundamental understanding of nanoscale systems. For example, fab-friendly fabrication processes for nanoscale metamaterials and structures with wafer-level uniformity and reproducibility are needed. Also, new ways of fabricating nanodimensional devices are always of interest.

Electrochemical oxidation, or anodization, is one of the most powerful techniques in fabrication of materials, structures, and final devices with dimensions down to the nanometer scale. This presentation will cover the following topics:

  • A brief historical overview of unsteady progress and activities in the area of electrochemical oxidation of metals;
  • Porous and dense oxides and their dimensional variations;
  • Self-organized nanoporous anodic alumina and its unique morphology. Use of anodic alumina for highly anisotropic micromachining, fabrication of ceramic membranes for nanofiltration, as templates for deposition of nanotubes and nanowires;
  • Wafer-level fabrication of uniform sub-10-nm multilayer systems with unprecedented level of precision and reproducibility; planarization capabilities at subnanometer level; and
  • Fabrication of nanopillars with widely ranging shapes and sizes, as components of nanodevices.

Finally, prospects of the electrochemical nanofabrication method will be discussed.

August 15, 2012

"Femtosecond Transient Absorption Microscopy of Carrier Dynamics in Single Nanostructures," by Libai Huang, University of Notre Dame, hosted by Gary Wiederrecht

Abstract: I will present our recent work on transient absorption microscopy (TAM) as a novel tool to image carrier and phonon dynamics in single nanostructures with simultaneously high spatial (~ 200 nm) and temporal resolution (~ 200 fs). Until now, the majority of dynamical measurements on single nanostructures have been based on photoluminescence (PL). Transient absorption imaging approach offers two key advantages over PL-based methods:

  1. A time resolution of ~200 fs. This fast time resolution is important because many critical events such as electron-phonon coupling occur on such sub-picosecond time scales. 
  2. The measured signal is based on absorption, which means we can also study samples with low or even zero fluorescence quantum yield.

I will discuss two examples of such transientabsorption microscopic studies. Femtosecond transient absorption microscopy was employed to study the excited-state dynamics of individual semiconducting single wall carbon nanotubes (SWNTs). This unique experimental approach removes sample heterogeneity in ultrafast measurements of these complex materials. Transient absorption spectra of the individual SWNTs were obtained by recording transient absorption images at different probe wavelengths. These measurements provide new information about the origin of the photoinduced absorption features of SWNTs. Transient absorption dynamics traces were also collected for individual SWNTs. The dynamics show a fast ~1 ps decay for all the semiconducting nanotubes studied. We attributed this fast relaxation to coupling between the excitons created by the pump laser pulse and the substrate.

Recent success in fabricating graphene has inspired researchers to search for semiconducting analogues of graphene in hopes to retain two-dimensional crystallinity while providing a bandgap. In particular, monolayer MoS2 has recently emerged as a promising candidate. The second study I will present is the investigation of exciton dynamics in atomically thin and semiconducting MoS2 crystals. By controlling the dielectric environment around monolayers of MoS2 crystals, our measurements provide a comprehensive understanding on intrinsic exciton dynamics, quantum confinement effect, exciton-phonon coupling, as well as how the dielectric environment alters optical properties and energy relaxation processes in these novel two-dimensional crystals.

August 1, 2012

"Living Matter," by Margaret Gardel, University of Chicago, Hosted by Elena Rozhkova

Abstract: In this talk, I will discuss our efforts to understand how ensembles of proteins found in living cells self-organize into active, living matter that facilitates cell-shape change and force generation. We use a combination of experiment, theory, and simulation to understand how force is transmitted from the molecular to macroscopic length scales in networks and bundles containing molecular motors, semiflexible polymers, and accessory proteins.

July 25, 2012

Carbon Nanoswitches: Could This Possibly Work?” by Stephen Campbell, hosted by Suzanne Miller

Abstract: The process of shrinking semiconductor devices employed for 50 years has essentially reached its end. Fundamental limitations of the field effect device prevent much additional reduction in the supply voltage. This leads to unavoidable off-state leakage and very high operating power, an intolerable situation for mobile applications. Nonplanar (i.e., trigate or FINFET) devices will extend things for only a few generations. This talk discusses a rather surprising possibility: the use of carbon-based materials such as carbon nanotubes and graphene to make nanomechanical switches with performance somewhat faster than state-of-the-art CMOS with at least an order-of-magnitude lower power. The unusual properties of these materials make possible high-performance, very low-power digital logic. The reasons for this performance and potential implications of such a technology will be discussed.

July 18, 2012

"Synthetic Design Tools for Complex Inorganic Solids and Nanostructures," by Raymond Schaak, The Pennsylvania State University, hosted by Yugang Sun

Abstract: Synthesis is the gateway to new materials, and design strategies that provide a rational synthetic framework are critical for accessing increasingly complex solids and nanostructures. We have been developing a library of chemical design tools that conceptually parallel some of the guiding principles that underpin molecular organic synthesis. We use these tools and concepts to design new solids with crystal structures that are inaccessible through established methods, as well as new nanoscale heterostructures that contain multiple materials components with predefined spatial organization. This talk will highlight recent results involving the use of nanoparticles as templates to synthesize new compounds that are not generally considered to be stable, including magnetic intermetallic compounds and chalcogenide semiconductors. This talk will also describe a "total synthesis" paradigm for the multistep synthesis of complex colloidal nanoparticle heterostructures with direct solid-state interfaces. For example, a library of colloidal nanoparticle heterodimers, heterotrimers, and heterotetramers can be routinely synthesized with predictable and chemoselective control over phase nucleation using solid-state analogues of orthogonal reactivity, protection/deprotection, site-specific reactivity, and substituent effects.

July 11, 2012

"Plasmonics and Metamaterials to Control and Manipulate Light at the Nanoscale," Andrea Alu, University of Texas, hosted by Matt Pelton

Abstract: In this talk, I will discuss recent progress and research in the field of metamaterials, nanoantennas, and plasmonics, covering a wide range of topics, from theoretical approaches to model anomalous wave propagation and interaction with metamaterials, to various applications at optical frequencies, including enhanced nonlinearities, sensing, imaging, and energy harvesting devices. I will discuss our most recent experimental results in metamaterial research, including the concept of broadband "plasmonic Brewster funneling." I will also show our near- and far-field experimental verification of three-dimensional radio-frequency cloaking, which represents the first experimental realization of a metamaterial cloak in three dimensions for a free-standing object. Finally, I will discuss the concept and recent experimental realization of ultrathin, broadband, planarized circular polarizers based on the concept of twisted metamaterials, realized by using lithographically printed optical metasurfaces. Physical insights into these exotic phenomena will be discussed.

June 27, 2012

"Recent Advances in Diamond Nanoparticles," Olga Shenderova, International Technology Center, hosted by Anirudha Sumant

Abstract: Within the last few years, worldwide interest in applications of nanodiamond (ND) particles has grown rapidly. Nanodiamond particles with the smallest crystal size of just a few nanometers are produced by detonation of carbon-containing explosives (so-called detonation NDs) or by grinding microdiamond powders manufactured by static high-pressure, high-temperature synthesis. Diamond particles smaller than 10 nm have remarkable optical and mechanical properties in combination with biocompatibility, high specific surface area, and tunable surface structure. They are the least toxic of all carbon nanoparticles, and their properties make them a favorable platform for drug delivery and cellular labeling or imaging. Numerous applications of NDs are under development, including high-precision polishing, wear-resistant additives to metal coatings, antifriction additives to lubricants and oils, polymer nanocomposites and coatings with enhanced strength and scratch resistance, ultraviolet-protection coatings, seeding slurries for growth of CVD diamond films, and many other applications. Recently, several important new application areas have been implied, based on the potential of incorporating foreign atoms in the lattice of ND particles. Major thrust areas where controlled doping of NDs and on-demand production of N-V centers can revolutionize the field include quantum information processing, quantum computing, magnetometry, and photoluminescent probes. Synthesis, structure, surface chemistry, phase transformations, and applications of NDs will be surveyed and areas of future scientific research highlighted.

June 20, 2012

"Correlated Imaging of Nanoscale Structure and Properties," Lincoln Lauhon, Northwestern University, hosted by Andreas Roelofs

Abstract: Microscopy has played a central role in the advancement of nanoscience and nanotechnology by enabling the direct visualization of nanoscale structure and, by extension, predictive models of novel physical behaviors. Correlated imaging of nanoscale structure and properties is an important frontier that can provide a rational basis for engineering new materials and devices. I will describe our approach to correlated imaging with a focus on semiconductor nanowires. Nanocrystal growth modes such as the vapor-liquid-solid process provide the ability to tailor nanoscale structure and composition in three dimensions, creating new opportunities in a range of applications includinglight harvesting and solid state lighting. In this context, we have explored a number of important processing-structure-property relationships using atom probe tomography, scanning transmission electron microscopy, Raman microspectroscopy, and scanning photocurrent microscopy. In addition, electromagnetic fields are visualized by using finite-difference time-domain simulations. From these studies, we develop a more comprehensive understanding of the influence of geometry, size, defects, dopants, and interfaces on carrier generation, recombination, and transport in nanostructured materials. This quantitative approach to characterization of model systems aims to identify applications that can truly benefit from the adoption of unconventional nanostructured materials.

June 13, 2012

"Multivalent Ionic Interactions in Multicomponent Polyelectrolyte Mixtures: From New Physics to New Materials," Matthew Tirrell, Argonne National Laboratory, hosted by Daniel Lopez

Abstract: Highly charged polymer chains in monovalent salt media exhibit a fairly simple range of behaviors — swelling in low salt, shrinking in high salt — based on the screening of repulsive electrostatic interactions among the segments. In the presence of other multivalent constituents, attractive forces arise. These attractions produce strong collapse of polyelectrolyte chains, adhesion between polyelectrolyte bearing surfaces, precipitation, and in the case of mixtures of oppositely charged polyelectrolytes, formation of fluid complex coacervate phases. We have measured the attractive forces between layers of polyelectrolyte brushes immersed in multivalent ionic media as a function of ionic strength. We have characterized coacervate formation in mixtures of model polyelectrolytes. We demonstrate how coacervate formation can be used to create new self-assembled materials, such as physically cross-linked hydrogels.

June 6, 2012

Materials and Mechanics for Bio-Integrated Electronics,” John Rogers, University of Illinois, Urbana-Champaign, hosted by Yugang Sun

Abstract: Biology is curved, soft, and elastic; silicon wafers are not. Semiconductor technologies that can bridge this gap in form and mechanics will create new opportunities in devices that adopt biologically inspired designs or require intimate integration with the human body. This talk describes the development of ideas for electronics that offer the performance of state-of-the-art, wafer-based systems but with the mechanical properties of a rubber band. We explain the underlying materials science and mechanics of these approaches, and illustrate their use in bio-integrated, "tissue-like" electronics with unique capabilities for mapping cardiac electrophysiology, in both endocardial and epicardial modes, and for performing electrocorticgraphy. Demonstrations in live animal models  illustrate the functionality offered by these technologies, and suggest several clinically relevant applications.

May 23, 2012

High Spectral Resolution at the Atomic Scale: Resolving the Unresolved with the STM,” Wilson Ho, University of California at Irvine, hosted by Jeffrey Guest

Absract: One of the joys of instrumentation development is associated with making measurements that arepreviously not possible. While the scanning tunneling microscope (STM) is known for its inherent capability of sub-Angstrom spatial resolution, the extension to other limits of measurement continues to be an interesting challenge. The spectral resolution can be increased by lowering the temperature of the sample and the microscope to below 1 K. Combined with ultrahigh vacuum and high magnetic field, high spectral resolution has enabled the observation of states that would otherwise remain hidden. These hidden states reveal new interactions that are best observed in single molecules. The STM results provide stringent testing grounds for density functional theory and challenge our scientific understanding.

May 3, 2012

ExtremeNano: From Quantum Control to Nanotechnology by the Ton," Peter Littlewood, Argonne National Laboratory, hosted by Daniel Lopez

April 25, 2012

"The Power of Scanning Probe Microscopy: Proving Nanoscale Electrochemical Phenomena," Nina Balke, Oak Ridge National Laboratory, hosted by Maxim Nikoforov

Abstract: The functionality of energy storage and generation systems, such as lithium-ion batteries, supercapacitors, and fuel cells, is not only based on but also limited by the flow of ions through the device. To understand these limitations and to draw a roadmap for optimizing device properties, the ionic flow must be studied on relevant length scales of grain sizes, structural defects, and local inhomogeneities (i.e., over tens of nanometers). Knowledge of the interplay between the ionic flow, material properties, and microstructure can be used to optimize device properties; for example, to maximize energy density, increase charging/discharging rates, and improve cycling life for lithium-ion batteries for applications in electric vehicles and aerospace. Until recently, existing solid-state electrochemical methods were limited to a spatial resolution of ~10 microns or greater, well above the characteristic size of grains and subgranular defects.

Scanning probe microscopy (SPM) is a characterization tool with the capability to study local phenomena on the nanoscale and is an underutilized technique in the field of electrochemistry. In this talk, we aim to provide an overview of current local SPM-based techniques applicable to the characterization of energy storage and energy conversion materials and to discuss potential advantages and drawbacks of SPM-based approaches. We will explore in detail how SPM can be used to study reversible and irreversible electrochemical processes in battery materials, supercapacitors, and fuel cells.

April 18, 2012

"Opportunities for MesoscaleScience," George Crabtree, Argonne National Laboratory and University of Ilinois at Chicago, hosted by Daniel Lopez

Abstract: Mesoscale science embraces a wide variety of phenomena emerging at length scales larger than atomic and nano but smaller than macro. Mesoscale phenomena are often intermediate between quantum and classical, between isolated and interacting, and between simple and complex. They typically involve the interaction of many degrees of freedom, including mechanical, electrodynamic, electronic, ionic, and chemical. Biology is an inspiring example of meso phenomena, capable of striking functionality such as temperature regulation, muscle contraction, and self-healing of tissue. In contrast, inorganic systems typically show much lower levels of complexity and functionality. One vision of mesoscale science is achieving the complexity and functionality of biology with inorganic materials.

Examples of mesoscale materials and chemistry will be given, to stimulate audience input on the nature of mesoscale science and promising research directions. This input will be used in the coming Basic Energy Sciences Advisory Committee report on opportunities for mesoscale science.

April 4, 2012

Structure Measurements for Polymer-Fullerene Solar Cells,” Dean DeLongchamp, National Institute of Standards and Technology, hosted by Seth Darling

Abstract: Organic photovoltaic (OPV) technology has the potential to lower the cost of solar power by enabling solar cell fabrication by using high-throughput printing techniques. In bulk heterojunction (BHJ) OPV devices, the power conversion efficiency is widely thought to depend on the morphology of the polymer absorber and the fullerene electron acceptor, but robust correlations have been elusive. The first step toward correlation is the collection of quantitative structural information, which can be done using a variety of measurement methods. This talk will include several structure measurement themes including interface composition, order and orientation, solubility and miscibility, and nanoscale morphology.

A particular focus will be our recent work to establish a link between bimolecular recombination and microstructure/morphology. Very few polymer-fullerene BHJ activelayers exhibit slower-than-Langevin charge carrier recombination, which is a requirement for fabricating thick active layers (>100 nm) while maintaining high fill factors. In collaboration with the Mozer lab at Wollongong, we evaluate a variety of hypotheses related to nanoscale film structure in an attempt to determine why some special systems exhibit this unusual and desirable feature.

March 21, 2012

"Ferroelectric Thin Films and Nanostructures," Nazanin Bassiri-Gharb, Georgia Institute of Technology, hosted by Andreas Roelofs

Abstract: Ferroelectric (FE) thin films and nanostructures find a wide range of applications in capacitive elements, nonvolatile memories, micro- and nano-electromechanical system (MEMS/NEMS) sensors, actuators, transducers, actively tunable photonic and phononic crystals, and energy-harvesting nano- and micro-generators. With the drive towards miniaturization, multiple intrinsic and extrinsic factors need to be leveraged to increase and/or maintain the high response of the FEs and reduce size effects.

This presentation addresses work currently performed in our group to improve the dielectric and piezoelectric response of ferroelectric ultrathin films (≤ 300 nm in thickness), as well as introduce novel processing techniques for creation of FE nanostructures, enabling new device creation. Specifically, I will discuss optimization of chemical solution deposition of FE thin films to obtain dense and highly oriented columnar growth on nonepitaxial substrates, and the effects of chemical heterogeneities on the dielectric and piezoelectric response of FE thin films. I will also introduce two new techniques for creation of high- and low-aspect-ratio nanostructures, based on chemical solution deposition: soft-template infiltration and thermochemical nanolithography. Additionally, I will discuss size effects and impact of lateral and substrate-induced constraints on the extrinsic contribution to the piezoelectric response in polycrystalline, ferroelectric nanostructures.

March 14, 2012

"Novel Device Architectures and Deposition Methods for Organic-Based Energy Conversion Devices," Max Shtein, University of Michigan, hosted by Seth Darling

Abstract: The Van Der Waals nature of bonding in small molecular organic semiconductor films presents opportunities for developing novel device architectures and deposition methods, which in turn enable new capabilities. This talk will discuss examples of novel device architectures, including:

  1. Highly scalable, fiber-based organic solar cells and organic LEDs;
  2. Active sensors with nanoscale spatial resolution;
  3. Atmospheric pressure, guard flow-enhanced organic vapor jet printing – an approach that allows the deposition of device quality films in the ambient.

The discussion will include consideration of multiscale optics, energy transfer mechanisms, charge transport, and fluid dynamics relevant to the discovery and engineering of promising energy conversion devices.

February 29, 2012

"Density Functional Predictions of Materials Properties for Catalysis and Energy Storage," Jeff Greeley, Center for Nanoscale Materials, hosted by Matt Pelton

Abstract: Recent advances in density functional theory treatments of interfacial processes, together with corresponding improvements in computer hardware, have begun to open up important new avenues for both the fundamental understanding of materials properties and the prediction of improved materials from first principles. In this talk, I will suggest that there is often a natural progression of computational materials analysis that begins with the development of fundamental understanding through detailed investigations of specific systems and ends with combinatorial screening efforts that, in some cases, result in the identification of materials with enhanced properties. Drawing upon several examples from our work over the past several years in the fields of electrocatalysis, heterogeneous catalysis, and energy storage, I will illustrate how some systems are sufficiently well understood to enable materials screening and improved materials identification, while many more require further study and model development. In the latter cases, I will suggest strategies by which sufficient fundamental knowledge might be obtained to permit computational materials screening, and I will describe a few strategies that could accelerate the development of this level of understanding for general catalytic systems.

February 24, 2012

Scanning Probe Microscopy and Spectroscopy of Molecules on Thin Insulating Films,” Jascha Repp, University of Regensburg, hosted by Saw Wai Hla

Abstract: Ultrathin insulating films on metal substrates facilitate the use of the scanning tunneling microscope (STM) to study the electronic properties of single atoms and molecules, which are electronically decoupled from the metallic substrate. The ionic relaxations in a polar insulator lead to a charge bi-stability in some adsorbed atoms and molecules. It is shown that control over the charge-state of individual molecules in such systems can be obtained by choosing a substrate system with an appropriate work function. The distribution of the additional charge is studied using difference images. These images show marked intramolecular contrast.

Molecules on NaCl films allow studies on the surface motion of molecules in the limit of weak molecule-substrate binding. In this context, we investigated the influence of the molecular symmetry on the surface potential landscape of molecules. We studied the induced lateral motion of Cu(II)-tetra-azaphthalocyanine molecules, for which four symmetry distinct isomers exist. This nonthermal diffusion, induced by inelastic excitations, is found to be qualitatively different for all four symmetry distinct isomers, demonstrating that symmetry governs the surface potential landscape. We discuss how atomic force microscopy (AFM), in a combined STM/AFM based on the qPlus-sensor, reveals additional information that is truly complementary to the STM data set. In the case of a nonplanar molecule that shows two different adsorption geometries, only the AFM channel provides reliable information on the conformation of the molecule. In another example of an artificially formed molecule metal-molecule complex, the AFM channel provides information on the bonding that is important to understand the STM results.

February 15, 2012

Nanoscale Probing with Single Quantum Dots,” Edo Waks, University of Maryland, hosted by Matt Pelton

Abstract: The ability to position nanoscopic objects at precise locations on a surface isessential for a broad range of applications. One such application is the positioning of quantum dots (QDs) in nanophotonic structures such as cavities or waveguides for single-photon generation, quantum dot lasers, and nonlinear optical devices. Another example is the nanoscale positioning of metallic and dielectric particles on prepared metamaterial surfaces to engineer nanoscale electronic circuits. In addition, manipulation of QDs serving as biological tags could enable in situ characterization of biological molecules and controlled investigation of biological processes. The majority of these applications exploit optically resonant interactions that require the nanoscopic particles to have the correct spectral properties. These applications require particle manipulation techniques that can preselect nanoparticles with the correct spectral properties and placethem at the correct locations on a surface.

In this talk, I will describe a method for manipulating particles with nanometer accuracy by controlling the flow of the surrounding liquid. This technique uses electro-osmotic flow to achieve particle actuation, while imaging and feedback are used to continuously correct the particles position and move it towards the intended target. In contrast to optical tweezers, whose accuracy scales inversely particle volume, the accuracy of this approach scales inversely with particle diameter, making it advantageous for manipulation of nanoscopic objects. I will present our recent experimental work on the capture, quantum optical characterization, and manipulation of pre-selected single QDs with up to 45-nm precision using this flow control technique. I will also describe our recently demonstrated extensions of this method for positioning and immobilization of QDs on a two-dimensional surface, allowing us to create arrays and complex patterns of QDs with different spectral properties. Finally, I will describe our recent working on probing surface plasmon polariton modes of silver nanowires with single quantum dots using flow control. This technique enables us to image the electromagnetic mode of a silver nanowire with an accuracy as fine as 10 nm.

February 1, 2012

Structural Complexity at Nanoscale: From Single Nanoparticle to Membrane,” Xiao-Min Lin, Center for Nanoscale Materials, Argonne National Laboratory, hosted by Seth Darling

Abstract: Structural complexity on the nanometer scale, either within a single nanoparticle or in a nanoparticle assembly, can lead to intricate macroscopic physical and chemical properties. In this talk, I will illustrate this phenomenon through two examples.

At the single nanoparticle level, I will focus on our recent magnetic studies of chemically synthesized Fe@Fe3O4 core-shell and iron oxide hollow-shell nanoparticles. The ability to obtain highly monodisperse magnetic nanoparticles through chemical synthesis has created new opportunities to investigate exchange bias (EB) phenomena in these systems. By tuning structure and chemical composition at the nanometer scale, I will show that the EB effect is directly related to the amount of frozen spins in these systems. However, gradual oxidation in air can lead to a structural gap at the interface between core and shell and ultimately to magnetic decoupling between core and shell.

Self-assembly of colloidal nanoparticles can yield a new type of complex material: free-standing nanoparticle membranes. I will describe our recent experiments to study the mechanical properties of these membranes (Young's Modulus and Poisson's ratio) as well as our efforts to use these membranes as nanofiltration membrane. I will show that nano-size structural defects dominate the molecular transport through these membranes, leading to both size and charge selectivity.

January 25, 2012

Probing Dirac Fermions in Graphene by Scanning Tunneling Microscopy and Spectroscopy,” Adina Luican Mayer, Rutgers University, hosted by Nathan Guisinger

Abstract: In the two-dimensional lattice of graphene, consisting of carbon atoms arranged in a honeycomb lattice, the charge carriers are described by a Dirac-Weyl Hamiltonian. Seeking to understand their unique nature, we have performed scanning tunneling microscopy (STM) and spectroscopy (STS) experiments at low temperatures and in magnetic field. These techniques give access, down to atomic scales, to structural information as well as to the density of states.

In this talk, I will present our experimental results on graphene systems with different degree of disorder. The main findings include the observation of quantized Landau levels in the presence of magnetic field, their dependence on carrier density, effects of electron-electron interactions and disorder on the Landau level spectrum. Twisting graphene layers away from the equilibrium Bernal stacking leads to the formation of Moire patterns. I will discuss the effects of such rotations on the electronic properties as a function of twist angle. Lastly, I will briefly describe current projects probing electron correlation effects in heterostructures of two-dimensional materials such as graphene/boron nitride stacks through STM/STS as well as high-magnetic-field electrical transport.

January 18, 2012

Influence of Acceptor Structure on Charge Separation Dynamics in Organic Photovoltaic Materials,”John B. Asbury, Pennsylvania State University, hosted by Chunxing She

Abstract: The dynamics of charge separation in photovoltaic polymer blends following photo-induced electron transfer from the conjugated polymer, regioregular-P3HT, to electron acceptors are observed with ultrafast vibrational spectroscopy. The investigators take advantage of a solvatochromic shift of the vibrational frequency of the carbonyl (C=O) stretch of the acceptors to directly measure the rate of charge transfer state dissociation to form charge separated states. Two acceptor classes are examined: functionalized fullerenes (PCBM) and perylene diimides (PDIs). Charge separation in rr-P3HT:PCBM blends occurs through activationless pathways, whereas rr-P3HT:PDI blends exhibit activated charge separation. The variation in charge separation mechanism arises from differences in the degree of electron delocalization that the electron acceptors can support. The three-dimensional structure of fullerenes enables large electron delocalization, giving rise to low barriers to charge separation. The pseudo-two-dimensional structure of PDIs causes localization of electrons in the acceptor phase and larger barriers to charge separation.

   
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