Colloquium Fall 2010

 

 

August 30
Plumber's Wonderland Found on Graphene
Professor Ju Li
School of Engineering and Applied Science
University of Pennsylvania

Abstract

Curvy nanostructures such as carbon nanotubes and fullerenes have extraordinary properties but are difficult to pick up and assemble into devices after synthesis. We have performed experimental and modeling research into how to construct curvy nanostructures directly integrated on graphene, taking advantage of the fact that graphene bends easily after open edges have been cut on it, which can then fuse with other open edges, like a plumber connecting metal fittings. By applying electrical current heating to few-layer graphene inside an electron microscope, we observed the in situ creation of many interconnected, curved carbon nanostructures, such as graphene bilayer edges (BLEs), aka "fractional nanotubes"; BLE polygons equivalent to "squashed fullerenes" and "anti quantum-dots"; and nanotube-BLE junctions connecting multiple layers of graphene. The BLEs, quite atypical of elemental carbon, have large permanent electric dipoles of 0.87 and 1.14 debye for zigzag and armchair inclinations, respectively. An unusual, weak AA interlayer coupling leads to a twinned double-cone dispersion of the electronic states near the Dirac points. This entails a type of quantum Hall behavior markedly different from what has been observed in graphene-based materials, characterized by a magnetic field-dependent resonance in the Hall conductivity. Further simulations indicate that multiple-layer graphene offers unique opportunities for tailoring carbon-based structures and engineering novel nano-devices with complex topologies. (PNAS 106 (2009) 10103; Phys. Rev. B 80 (2009) 165407; Nano Research 3 (2010) 43; Carbon 48 (2010) 2354).

September 6
Labor Day, University Holiday

September 13
Non-linear Photointeractions under Strong X-ray Radiation: A new laser revolution?
Professor Peter Lambropoulos
Institute of Electronic Structure and Laser - FORTH
Physics Department, University of Crete
Greece

Abstract

Fifty years ago, a new optical radiation source, the LASER, ushered in a revolution in radiation-matter interactions, which to a large extent is still going on. Owing to its intensity, coherence and eventually short pulse duration (down to subfemtoseconds), it became possible to go beyond traditional single-photon transitions, revealing aspects of atomic and molecular structure and dynamics which, until then, could only be speculated about.

It is only rather recently that new, accelerator-based Free Electron Laser (FEL) sources, with similar properties, but much higher photon energies, reaching up to 7 keV, have become available. If the past can serve as a guide, they should revolutionize once again radiation matter interactions, involving this time inner shell electrons and much shorter time scales.

I present a very brief overview of key developments over the last fifty years, followed by an outlook, for at least the near future, including a few examples of processes studied so far at the FEL facilities FLASH (Hamburg) and LCLS (Stanford).

September 20
Defects at the Nanoscale: The Role of Quantum Confinement and Dimensionality
Professor Jim Chelikowsky
Departments of Physics, Chemical Engineering, and Chemistry and Biochemistry
Director, Center for Computational Materials, Institute for Computational Engineering and Sciences
University of Texas at Austin

Abstract

One of the most challenging issues in materials physics is to predict the properties of defects in matter. Such defects play an important role in functionalizing materials for use in electronic devices. As the length scale for such devices approaches the nano-regime, the interplay of dimensionality, quantum confinement and defects can be complex. In particular, the usual rules for describing defects in the bulk may be inoperative, i.e., a shallow defect level in the bulk may become a deep level at the nanoscale. The development of theoretical methods to describe the properties of nanoscale defects is a formidable challenge. Nanoscale systems may contain numerous electronic and nuclear degrees of freedom, and often possess little symmetry. My presentation will center on recent advances in this area based on new algorithms, which allow a solution of the eigenvalue problem without any explicit diagonalization. I will apply these algorithms to nanoscale systems, and present calculations for the structural and electronic properties of dopants (Li, Zn, B, P, Mn..) in semiconductor (Si, InP, ZnO, CdSe, ...) nanostructures. By considering nanocrystals, nanowires and nanofilms, I will vary the size and dimensionality of these nanoscale systems.

September 27
Phenomenology of Ungravity
Professor Jonas Mureika
Department of Physics
Loyola Marymount University

Abstract

Unparticle physics is a conformally-invariant theory whose interactions resemble those of a non-integer number of fundamental particles. While these curious effects could manifest themselves at the TeV scale as alterations to cross-sections and decay modes, the experimental consequences are not just limited to accelerator physics. One such circumstance is known as "ungravity," where the unparticle matter alters the effective gravitational field strength via a coupling to the stress energy tensor. This talk will provide a general overview of the physics of ungravity, from black hole formation in high energy collisions to potential cosmological influences, and will demonstrate how its unique phenomenology might be detectable in current and future experiments.

October 4
Micro- and Nanotechnology for Label-free Biophotonics
Professor Andrea Armani
Mork Family Department of Chemical Engineering and Materials Science
Ming Hsieh Department of Electrical Engineering-Electrophysics
University of Southern California

Abstract

Innovation in technology routinely leads the way for discovery in chemistry and biology. Most notably, x-ray diffraction data was instrumental in the elucidation of the structure of DNA. To explore the inherent complexity present in biological systems, existing technologies are being pushed to their limits. Once again, scientists are looking to engineers to create innovative solutions to enable their exploration and discovery. Many of the new methods currently being developed focus on increasing the sensitivity of the detection technique by inventing new devices as well as increasing the specificity of the device by engineering synthetic targeting moieties and improved attachment methods. This talk will present an overview of both aspects of technology development which are currently being investigated in the Armani Lab. Specifically, the experimental and theoretical optical properties of several new optical devices, which were designed for the express purpose of biological and chemical detection, will be discussed. Additionally, biomolecule attachment strategies which can improve both the stability and specificity of the sensor’s surface functionalization will be presented. Applications of these devices which are actively being pursued include measurements of substrate-enzyme affinity and single cell behavior.

October 11
Nanoscalesystems assembled with DNA: from principles to rational design

Dr. Oleg Gang
Center for Functional Nanomaterials
Brookhaven National Laboratory

Abstract

Incorporation of biomolecules with specific recognitions into nano-object design provides a unique opportunity to establish highly selective and reversible interactions between the components of nanosystems. The interaction encoding can tremendously expand the concept of self-assembly by providing programmable instructions for the formation of structures from the multiple types of components. Our work explores how DNA can be engaged in the encoded interactions between inorganic nano-components, and how the morphology of self-organized structures can be regulated. By tailoring interplay of specific recognition interactions and non-specific physical effects, we have demonstrated an ability to control assembly kinetics, clustering, and ordering of DNA-capped nanoparticles. I will discuss our recent progress in understanding the principles of programmable assembly and effects associated with polymeric nature of DNA and nano-object anisotropy. I will review approaches developed in our group for the rational fabrication of nanosystems with well-defined architectures, including 3D superlattices, finite size clusters of nanoparticles and switchable assemblies. Applications of these classes of systems for biosensing and for nano-optics will be also discussed.

October 18
Recognition Tunneling - a new approach to DNA sequencing
Professor Stuart Lindsay
Department of Chemistry and Biochemistry
Biodesign Institute
Arizona State University

Abstract

Electron tunneling is exponentially sensitive to the positions of atoms in a tunnel gap, giving it enormous potential for interfacing chemistry with electronics. However, it is just as sensitive to contamination and thermal fluctuations. To make it work in conditions compatible with biology, we functionalize electrodes with recognition reagents that capture their targets via hydrogen bonding, clamping the target in place and completing an electron-tunneling path. Single molecule targets can be identified with reasonable confidence in a single read, and high spatial resolution is achieved because the chemical contact area is smaller than the grain size of the metal electrodes. Thus, a single base can resolved from neighbors in a DNA oligomer. Furthermore, targets appear to remain trapped for surprisingly long times, but in an energy landscape compatible with control of the reading speed. By using metallic carbon nanotubes as both electrical contacts and also as nanpores for DNA translocation, this approach might enable a new type of rapid sequencing of DNA and other heteropolymers.

 

October 25
Modeling Highly Resolved Spectroscopies of Complex Materials: From Qualitative to Quantitative
Professor Arun Bansil
Physics Department
Northeastern University

Abstract

We stand at the threshold of a new 'golden age' of spectroscopic studies of materials for unraveling how charge, spin, orbital and lattice degrees of freedom interact to produce emergent phenomena and exotic states of matter. In this connection, the need for realistic modeling of various highly resolved spectroscopies is becoming of critical importance in providing discriminating tests of competing theoretical models and as a rational basis for future experimentation. In this talk, I will discuss how as we move to model spectroscopic data from a qualitative to a quantitative level, surprising new insights into the nature of electronic states and correlation effects are obtained in high-temperature cuprate superconductors and other complex materials.[1-6] Illustrative examples in cuprates include: (i) Asymmetry of the scanning tunneling (STM) spectrum between positive and negative bias voltages and the extent to which it comes about within the conventional picture; (ii) Origin of the 'high-energy kink' or the 'waterfall effect' in the photoemission spectrum (ARPES) and the interplay therein between effects of the matrix element and the presence of strong coupling of the quasiparticles to electronic excitations; and (iii) The nature of the dichroic signal in photoemission and its relationship to the time-reversal symmetry breaking in the cuprates. I will also comment on our recent work on the manganites and topological insulators.

 

November 1
Inorganic-Organic Co-Assembly as a Route to Functional Nanostructured Materials - Nanomagnetics, Nonoelectronics and Nanomaterials for Energy
Professor Sarah H. Tolbert
Department of Chemistry and Biochemistry and the California NanoSystems Institute
UCLA

Abstract

Block-copolymer templating of inorganic frameworks provides a powerful route to the production periodic nanoporous materials from either sol-gel or nanoparticle building blocks. The periodicity provides mechanical robustness that allows the frameworks to withstand a broad range of chemical transformations with minimal changes in porosity. In this talk, we examine a range of systems that exploit these templating strategies to produce periodic nanostructured materials. We first focus on the use of nanoporous inorganic frameworks to produce novel composite materials by filling the pores space with either optical or magnetic materials. We then consider the unique physical properties that can be observed in magnetic and ferroelectric frameworks. Finally, we examine application of porous materials for electrochemical charge storage (i.e. batteries and supercapacitors), focusing on optimization of both charge storage capacity and charge/discharge rates. In all cases, our goal is to create complex nanoscale architectures which can be used to tune the physical properties of composite materials.

November 8
Characterizing passive lipid membrane transport with artificial cells
Professor Noah Malmstadt
Mork Family Department of Chemical Engineering and Materials Science
University of Southern California

Abstract

Passive transport—the diffusion of small molecules across the cell membrane without the involvement of any cellular machinery—is a key mechanism by which drugs and environmental toxins enter cells. This mechanism is especially important to drug delivery since drugs that passively diffuse into cells tend to have high bioavailability.

 

Developing quantitative relationships between molecular structure and passive membrane permeability requires accurate methods for measuring permeability. Unfortunately, most state-of-the-art methods are artifact prone, resulting in wildly varying literature values for the membrane permeability of even simple molecules.

 

We have developed a novel method to measure the membrane permeability of small molecules. “Artificial cells”—spherical lipid membrane structures with a scale and geometry similar to that of eukaryotic cells—are imaged by high-speed spinning-disk confocal microscopy as molecules permeate the membrane. These images capture the full temporal development of the transmembrane concentration field, providing a richer data set than other current techniques which measure only steady-state concentrations. This image-based approach can track the transport of both fluorescently labeled molecules and unlabeled acidic/basic molecules when combined with a pH-sensitive fluorophore.

 

To facilitate rapid buffer exchange for fast-permeating species, an artificial cell is immobilized in a microfluidic channel. The concentration of the species of interest can be tracked both inside and outside of the vesicle as the buffer is exchanged. To obtain precise permeability values, these concentration measurements are fit to a finite difference model of membrane transport.

 

This technique is compatible with new, more sophisticated, synthetic models of the cell membrane, including a synthetic membrane we have developed that mimics the compositional asymmetry of eukaryotic membranes. Such systems will allow for a detailed mapping of the relationships between drug structure, membrane composition, and membrane permeability.

 

November 15
The Energy-Development-Environment-Climate Challenge
Dr. Rajan Gupta
Clear and Particle Physics, Astrophysics and Cosmology Group T2
Los Alamos National Laboratory

Abstract

Modern Energy systems are enormous, complex, dynamical and adaptive. Understanding them is crucial because access to inexpensive energy is key to development and is the basis of modern technological societies. At the same time, because of rising concern for associated environmental impacts and green house gas emissions contributing to climate change, transitioning to carbon-neutral and sustainable energy systems has become a global imperitive. This talk will analyze different regions of the world and assess the energy choices they have made and are making, and their implications for the environment and the climate.


November 22
Hydrodynamics and Flunctuations of Flat and Curved Membranes with Applications to the Microrheology of Red Blood Cells
Professor Alex J. Levine
Department of Chemistry and Biochemistry
UCLA

Abstract

The dynamics of viscous or viscoelastic membranes surrounded by a viscous fluid plays an important biophysical role in e.g. understanding the mobility of proteins in cell membranes.  These fluid membranes also provide an interesting physical system in which low-Reynolds-number hydrodynamics acquires an inherent length scale and where membrane curvature can dramatically change the mobility of membrane-bound particles.  In this talk, I first discuss the hydrodynamics of flat fluid membranes surrounded by bulk fluids, initially developed by Saffman and Delbrück (SD), and compute the hydrodynamic interactions between particles in such flat membranes.  I then extend these theories to study flows in curved membranes, showing how curvature modifies the SD picture.  Finally, I used these geometric ideas to study the effect of curvature on the fluctuation spectrum of (visco-) elastic membranes and apply that analysis to membrane microheology.  Using this theory, I analyze the fluctuation spectrum of human red blood cell membranes (measured by G. Popescu, UIUC) in order to extract their mechanical properties in various morphological states of the cell.


November 29
The Sustainable Energy Challenge

Dr. George W. Crabtree
Materials Science Division
Argonne National Laboratory

Abstract

The dependence on oil and other fossil fuels for over 80% of our energy and the continued emission of carbon dioxide threatening stable climate are captured in a single term: sustainability. Although we generally agree that sustainability is valuable, there is less agreement on how much sustainability is necessary or desirable.  In this talk, three criteria describing increasingly strict features of sustainability will be presented and applied to evaluate the alternatives to oil and carbon dioxide emission, such as tapping unused energy flows in sunlight and wind, producing electricity without carbon emissions from clean coal and high efficiency nuclear power plants, and replacing oil with biofuel or electricity.  Implementing these more sustainable alternatives requires new materials of increasing complexity and functionality that control the transformation of energy between light, electrons and chemical bonds at the nanoscale.  Challenges and opportunities for developing the complex materials and controlling the chemical changes that enable greater sustainability will be presented. 

  • Department of Physics and Astronomy
  • University of Southern California
  • 825 Bloom Walk
  • ACB 439
  • Los Angeles, CA 90089-0484