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

Archive: Seminars 2013

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December 10, 2013

"Hierarchical Semiconductor, Metal and Hybrid Nanostructures and the Study of their Light-Matter Interactions," by Anna Lee, University of Toronto, hosted by Jeff Guest

Abstract: The interdisciplinary work during my Ph.D. and postdoctoral studies (Dept. of Chemistry and Dept. of Electrical Engineering, University of Toronto) explore the optical properties of hierarchical structures composed of nanoscale building blocks ranging from metals to semiconductors and composites, organized through bottom-up design methods.

This talk is comprised of three main research projects for which the common thread is the rational design of nanoscale assembled structures and their interactions with light.

Recent advances in spectrally tunable solution-processed metal nanoparticles have provided unprecedented control over light at the nanoscale. The plasmonic properties of metal nanoparticles have been explored as optical signal enhancers for applications ranging from sensing to nanoelectronics. Specifically,

  1. By following the dynamic generation of hot spots in self-assembled chains of gold nanorods (NRs), we have established a direct correlation between ensemble-averaged surface- enhanced Raman scattering (SERS) and extinction properties of these nanoscale chains in a solution state. Experimental results were supported by comprehensive finite-difference time-domain simulations. Building from this,
  2. We studied an alternate geometry, namely side-by-side assembled NRs. There is a general misconception that aggregates of metal nanoparticles are more efficient SERS probes than individual nanoparticles, due to the enhancement of the electric field in the interparicle gaps. However, we have shown through theoretical and experimental analyses that this is not the case for side-by-side assembled gold NRs.
  3. Progress in colloidal quantum dot photovoltaics offers the potential for low-cost, large-area solar power; however, these devices suffer from poor quantum efficiency in the more weakly absorbed near infrared portion of the sun's spectrum. Here, I will talk about a plasmonic-excitonic solar cell that combines two jointly tuned solution processed infrared materials. We show through experiment and theory that a plasmonic-excitonic design using gold nanoshells with optimized single-particle scattering-to-absorption cross section ratios leads to a strong enhancement in near-field absorption and resultant photocurrent in the performance-limiting near infrared spectral region.

The present work offers guidance towards the establishment of "design rules" for the development of colloidal nanoparticle assembled systems for plasmonic sensing applications.

December 10, 2013

"Study of Molecules on Graphene by Low-Temperature Scanning Tunneling Microscopy," by Haigang Zhang, University of California, Irvine, hosted by Jeff Guest

Abstract: This talk has three parts First, I will introduce a step-by-step how-to for building a low-noise scanning tunneling microscopy (STM) system for optical experiments. Many details were considered to minimize the noise level of the system. Then I will talk about some experiments on epitaxial graphene grown on a Ru(0001) surface. A periodic Moiré pattern is formed on the graphene surface due to the lattice mismatch between graphene and substrate. The graphene Moiré pattern is periodically corrugated and electronically inhomogeneous as characterized by STM spectroscopy. Electrons and phonons are localized on the surface. The physical and dynamic properties of molecules on the graphene Moiré surface were investigated by using both experiments and calculations. To continue the low-temperature experiments, a lab-based liquid helium recovery facility was built. The homemade facility is performing better than commercial ones and saving tons of liquid helium every year.

December 9, 2013

"Scanning Tunneling Microscopy Studies of Self-Assembly and Intramolecular Charge Localization," by Rebecca Quardokus, University of Nore Dame, hosted by Jeff Guest

Abstract: This talk will focus on two projects. The first project involves large mixed-valence iron-based organometallic molecules that were studied on gold surfaces using low-temperature ultrahigh-vacuum scanning tunneling microscopy (STM). Oxidation of these molecules results in mixed-valence complexes with an asymmetric distribution of electron density. STM was used to image the electronic properties of individual molecules, and it was found that the details of a molecule's internal structure determined whether the charge distribution was symmetric or asymmetric when placed on a metal surface in the absence of bulk solvent. Further investigation of mixed-valence dinuclear organometallic molecules with asymmetric electron state density may open up the possibility for their use in molecularly based electronic devices.

The second part of the talk will focus on hydrogen-bonded networks of ferrocenecarboxylic acid that form both dimers and five-membered rings (pentamers) on a gold surface with longer-range quasicrystalline ordering. Five-membered rings are stabilized by C--HO hydrogen bonding and the combination of dimers and pentamers on the surface allow for two-dimensional self-assembled quasi-crystalline ordering with fivefold symmetry. This is the first example of two-dimensional quasi-crystalline ordering of a small self-assembling molecule.

October 16, 2013

"Molecular Design of Fullerene Acceptors for High-Performance Bulk-Heterojunction Solar Cells," by Leeyih Wang, National Taiwan University, hosted by Seth Darling

Abstract: Organic bulk-heterojunction solar cells have attracted considerable research interest from both academia and industry because they have the advantages of light weight, low material cost, good mechanical flexibility, and simplicity of manufacture. Especially, recent advances in low-bandgap conjugated polymers and novel fullerene derivatives have further increased the PCE to 6-9%. The active layer in this type of solar cell is usually a binary blend of two compounds, one with a higher lying LUMO acting as electron donor (D) and the other as electron acceptor (A). Because the excitons generated in organic materials typically have large binding energy and can be efficiently dissociated into free carriers only at the interface of D/A with the aid of the energetic driving force originating from the differences in the electronic levels of the materials. Since Heeger et al. discovered that C60 is an effective fluorescence quencher of conjugated polymer, fullerenes and their derivatives have been extensively used as acceptor materials in fabricating polymer solar cells. In this seminar, the design strategy of fullerene acceptors and the effect of their molecular structure on the photovoltaic performance will be systematically discussed.

October 9, 2013

"Nonlinear light-matter interaction in photonic crystals: Solitons, plasma, and single-photon sources," by Chad Husko, University of Sydney, hosted by Daniel Lopez

Abstract: Photonic crystals are periodic dielectric media that possess photonic band gaps, a frequency range in which certain light wavelengths, or colors, are forbidden. These ordered materials are the optical equivalent to electronic semiconductors, such as silicon, that pervade our daily lives.

Photonic crystals enable exciting opportunities for nonlinear optics at the chip scale because of their unique ability to localize light and to strongly enhance light-matter interactions via slow light, along with highly tunable dispersion. In addition to these more fundamental studies, these compact and energy-efficient devices are compelling candidates for ultrafast all-optical applications in integrated photonics.

In this talk, we report on solitons in semiconductor photonics crystals, nonlinear optical waves in the multiphoton plasma regime, and nonlinear loss limits to single-photon sources in integrated waveguides.

August 19, 2013

"X-Ray Speckle Visibility Spectroscopy and Approach to Protein Dynamics," by Luxi Lil, Brookhaven National Laboratory, hosted by Ian McNulty

Abstract: The dynamics of mesoscale soft matter material has become a popular topic during the past decade. The collective motion of colloids and polymers has been studied by X-ray photon correlation spectroscopy (XPCS). However, instrumental limit, such as the readout rate of area detectors, and the high risk of the sample radiation damage, are obstacles to using the XPCS method. In this work, I will introduce a novel variation of XPCS, namely X-ray speckle visibility spectroscopy (XSVS). This method is capable of studying sample motions one order of magnitude faster than XPCS does under the same experimental conditions. In addition, XSVS is considered as a single-shot experiment. Coherent X-ray scattering patterns taken with different acquisition times were analyzed by pixel photon count statistics. The acquisition-time-dependent speckle contrast was fit to extract the characteristic time of the sample motion.

A PMMA suspension in decalin was studied by XSVS as an example of this new method. Protein function is related to the stochastic motion of the protein molecules, particularly collective motion in the mesoscale. In macromolecular diffraction, diffuse scattering around the Bragg peaks results from protein collective motion, such as domain fluctuations. We performed an XSVS experiment on Concanavalin A (ConA) crystals. The diffuse component from the ConA diffraction pattern was analyzed. The preliminary results will be presented in this talk.

August 6, 2013

"Publishing at Nature Photonics," by David F.P. Pile, Nature Photonics, hosted by Matt Pelton

Bio: David received a PhD in physics (on diffraction theory) from Queensland University of Technology in 2003 (under Dr. Dimitri Gramotnev) and then was a JSPS postdoctoral research fellow at the University of Tokushima under Prof. Masuo Fukui (discoverer of long-range plasmons). From 2006-2008, David was a postdoctoral researcher in Prof. Xiang Zhang's group at UC Berkeley.

David's research experience includes theoretical analysis and experimental investigation of plasmon-polaritons; subwavelength waveguiding; nano-focusing and slow and fast light in metallic structures; guided and surface modes of photonic crystals; and metamaterials for applications in micro- and nano-optics, telecommunications, sensors, imaging, etc. His research articles have been cited more than 2200 times and his H-index is 21, despite ceasing primary research and publication in 2008.

In November 2008, David moved from research to publishing, as an editor at Nature Photonics, the highest ranked journal in the fields of optics. He is responsible for both front half (News, Correspondence, Industry and Technology articles, etc.) and back half (primary research manuscripts and reviews) content. Since joining, he has been the "owner" of the following topics:

  1. Plasmonics
  2. Photonic crystals
  3. Metamaterials
  4. Fundamental and theoretical optical physics
  5. X-Ray optics
  6. Quantum optics
  7. Solar cells
  8. Exciton-polaritons.
August 2, 2013

From pinwheels to quasicrystals: investigations of C60 and pentacene via scanning tunneling microscopy," by Joseph A. Smerdon, University of Central Lancashire, hosted by Jeffrey Guest

Abstract: C60 and pentacene are archetypal molecules for the study of adsorption at surfaces because of their simple structures. Because of their strong optical absorption and complementary organic semiconductor properties, they are also used in the active layers of bulk heterojunction solar cells. In this talk, I will describe how we can construct heterojunctions inside the scanning tunneling microscope tunnel junction from single layers of these molecules, and how they perform as diodes. When adsorbed on a Cu(111) surface in a mixed molecular layer, strong interactions between the molecules lead to the formation of supramolecular chiral pinwheel structures, which again show charge transfer between the molecules in a manner consistent with a picture of the pinwheel as a heterojunction. Finally, I will describe how these molecules can be induced to form extended two-dimensional quasi-crystalline networks, when adsorption is templated by using quasi-crystalline substrates.

June 21, 2013

Nanosecond dynamics of polarization domains and lattice structure in ferroelectric oxide thin films," by Pice Chen, University of Wisconsin, Madison hosted by Ian McNulty

Abstract: Novel ferroelectric materials, including ferroelectric and dielectric superlattices, provide additional means to modify electronic and structural properties. These functional properties are closely linked to the response of spontaneous polarization to external stimuli. Insight into the coupling of polarization to other degrees of freedom can be inferred from the dynamics of ferroelectrics under electric field and optical excitations.

We have studied the switching mechanism of polarization domains at a nanosecond timescale in ferroelectric/dielectric PbTiO3/SrTiO3 superlattices by using time-resolved X-ray microdiffraction. In a superlattice with weakly coupled component layers, the competition between the energy associated with the depolarization field and the energy of domain walls leads to the formation of striped domains. The dielectric layers are polarized with a weaker polarization than the ferroelectric layers. The striped domains and the nonequal distribution of the polarization have important consequences in the response of the superlattice to applied electric fields. We found that switching of the striped domains occurs heterogeneously over the areas under applied electric fields, with a nanosecond timescale. In each component layer, however, the responses are different to applied electric fields. The dielectric SrTiO3 layers are less stable and show larger distortion of domains than the ferroelectric PbTiO3 layers at the early-time regime of switching. A larger piezoelectric expansion in the SrTiO3 layers is found at the late-time regime, commensurate with polarization change due to the elimination of striped domains.

We have also studied the dynamics of structural modification in multiferroic BiFeO3 thin films under above-bandgap femtosecond optical excitation. Understanding the mechanism of ultrafast structural change is key to achieve an optical manipulation of both ferroelectric and ferromagnetic orders in complex oxides. A photo-induced strain on the order of 0.5% develops within 100 ps after a 400-nm laser pulse. This lattice distortion is consistent with the piezoelectric effect in response to the screening of the depolarization field in the presence of photo-induced carriers. Relaxation of the strain, which can be interpreted as a carrier recombination process, is on the order of 1 ns depending on the film thickness.

June 18, 2013

A Semiclassical, Time-Dependent Approach to Localized Surface Plasmons in Presence of Gain Elements," by Alessandro Veltri, University of Calabria, hosted by Gary Wiederrecht

The study of gain-assisted localized surface plasmons (LSPs) is of growing interest in different fields of nanotechnology. In fact, the embedding of optical gain is possibly the most promising strategy to compensate the losses they show in visible range. Moreover, metallic nanostructures with gain elements are nanoscale source of strong optical fields. This intriguing feature, culminating in the conception of the spaser, widened their applicability to nanoscale lithography, probing, microscopy, and more.

It is possible to demonstrate, through a purely classical and steady state approach, that a single metallic nanoparticle immersed in a gain medium may show new types of optical responses as the gain level is modified, producing amplification and distortions in the spectra. This classical, steady state approach naturally fails when instable, amplifying regimes are reached.

Here we present a time-dependent model, integrating a quantum formalism to describe the gain while the metal is treated classically. This new model does contain previous results as steady state solutions and allows to investigate the system in time domain. By means of this dynamical approach it is possible to describe transient regimes, to study instabilities and to account for the effects of a pulsed pump.

Furthermore, the geometrical solidity of the model allows to study the stability of the spasing mode; describing how, in many geometrical configurations, population saturation shifts the most of the energy into parasitic modes, thereby destroying the amplification effect. Being this phenomenon related to the geometrical configuration, our approach can be used to identify the best design possible to enhance the stability of the amplified mode.

June 18, 2013

"Fluid FM: combining AFM and microfluidics for single-cell and nanoparticle manipulation in liquid," by Tomaso Zambelli, ETH Zurich, hosted by Tijana Rajh

Abstract: Glass micropipettes are the typical instrument for intracellular injection, patch clamping or extracellular deposition of liquids into viable cells. The micropipette is thereby slowly approached to the cell by using micromanipulators and visual control through an optical microscope. During this process, however, the cell is often mechanically injured, which leads to cell death and failure of the experiment. To overcome these challenges and limitations of this conventional method we developed the FluidFM technology, an evolution of standard AFM microscopy combining nanofluidics via cantilevers with integrated microfluidic channel. The channel ends at a well-defined aperture at the apex of the AFM tip, while the other extremity is connected to a reservoir. The instrument can therefore be regarded as a multifunctional micropipette with force feedback working in liquid environment. We are focusing on three applications for single-cell biology:

  1. Cytosolic and intranuclear injection
  2. Cell adhesion
  3. Single virus deposition on cell surfaces

At the same time, we are using the FluidFM as a lithography tool in liquid.

June 17, 2013

"Magnetic nanohybrids for thermo-chemotherapy," by Dhirendra Bahadur, Indian Institute of Technology, hosted by Elena Rozhkova

Abstract: Magnetic nanoparticulates with different shapes, composites, hybrids, core shell structure and magnetic fluids have been developed by various soft chemical methods. Magnetic nanostructures with sufficient biocompatibility are the best candidates for several therapeutic and diagnostic applications, such as treatment for cancer through hyperthermia, targeted and sustained drug delivery, as contrast agents and, in other biosensing applications. We discuss here some of these aspects based on the work carried out in our laboratory.

In addition, we discuss development of multifunctional magnetic hybrid nanostructures, which may be used for a combined therapeutic and diagnostic approach. For efficient delivery of magnetic nanoparticulates with drug to the diseased site, magnetic-fluid-based release systems will be discussed with different possibilities of thermosensitive and pH-sensitive hydrogels, liposomes, and dendrimers as carrier. The encapsulation of chemotherapeutic drug and magnetic nanoparticulates with tagging targeting moiety (such as folic acid) on the surface have been investigated.

We will particularly emphasize some of our recent in vitro as well as in vivo results on lipid and hydrogel based nanohybrids with multifunctional capabilities. We have further investigated the synergistic effects of dual drugs and dual therapy. The deliberate design of nanoparticulates for biological applications has been enabled by new advances in synthetic procedures through different soft chemistry routes. Such nanostructures, when properly functionalized, can be used as effective vehicles for biological entities in vivo. In this respect, we will also discuss some of the other porous nanostructures we have used for specific drug delivery applications. In addition, the mechanism of cell death during controlled experimental conditions for hyperthermia treatment of cancer will be discussed.

June 7, 2013

"Molecular biophotonics for diagnostics and treatment," by Jonathan T.C. Liu, Stonybrook University, hosted by Il Woong Jung

Abstract: The molecular biophotonics lab, directed by Dr. Jonathan Liu, is developing optical strategies for biomedical diagnostics and therapy. These endeavors require multidisciplinary advances in optical devices, contrast agents, image processing, and preclinical/clinical studies.

For example, over the past few years, our lab has published on the simulation and development of a miniaturized advanced volumetric microscopy technology to enable real-time point-of-care pathology, as well as the development of a molecularly targeted fluorescent contrast agent to guide tumor resection in the brain. These complementary technologies have the potential to revolutionize patient care by providing surgeons with a real-time alternative to invasive biopsy and frozen-sectioning pathology for confirming the status of tissues at the final stages of surgery.

In addition, our lab is developing spectral imaging devices in conjunction with multiplexed Raman nanoparticles for endoscopic visualization of large panels of disease biomarkers. This has the potential to allow physicians to better visualize and understand the molecular mechanisms of disease progression for improved early detection and to monitor the molecular response to personalized therapies.

May 28, 2013

"Aberration-Corrected Scanning Transmission Electron Microscopy: Including Light Elements," by Patrick Phillips, University of Illinois at Chicago, hosted by Yuzi Liu

Abstract: Scanning transmission electron microscopy (STEM) offers a multitude of characterization techniques for a wide range of relevant materials, most commonly realized through chemical analysis combined with high-angle annular dark-field (HAADF) imaging. However, with the recent advent of annular bright-field (ABF) STEM, it is now possible to image light and heavy elements simultaneously. By coupling ABF imaging to HAADF and chemical analyses, a full-scale materials characterization can be performed.

The first part of this talk will focus on the STEM-based characterization of AlxGa1xN nanowires for UVLED applications. To assist subsequent growth processes while striving for optimum efficiency, both structural and chemical characterization methods are necessary, which can be provided at sufficiently high resolutions by advanced STEM instruments. Specifically, structural characterization will focus on determining layer thicknesses and wire polarity, as well as visualizing any short-range ordering and/or stacking faults that may be present. Chemically, both energy dispersive X-ray (EDX) and electron energy loss (EEL) spectroscopies will be discussed in various capacities, ranging fromquantum well composition (EDX) to N K-edge fine structure of both GaN and AlN (EELS).

The second portion of the talk will present numerous example materials that required structural, chemical, and/or electronic characterization via aberration-corrected STEM methods. Additionally, the technique of ABF STEM (coupled with HAADF) will be discussed in terms of imaging very thin samples. Notably, preliminary research has indicated that some of the phenomenological theory of ABF breaks down in the limit of thin specimens. Where necessary, supporting STEM multislice image simulations will be presented.

April 19, 2013

"Ferroelectricity in hybrid organic-inorganic compounds," by Alessandro Stroppa, CNR-SPIN (Italy), hosted by Saw Wai Hla

Abstract: Ferroelectric materials, whose spontaneous polarizations can be switched under an external electric field, have a wide range of applications in device electronics. Recent discoveries of ferroelectricity in organic solids have been limited to some well-known polymer ferroelectrics or a few low-molecular-mass compounds. Computational approaches based on density functional theory represent a valuable tool in predicting or suggesting new organic ferroelectrics with the large polarization values needed for device applications. In particular, the modern theory of polarization is used to estimate the polarization in insulating compounds and symmetry analysis gives an important help for gaining insights into the mechanisms responsible for the ferroelectric polarization.

In this contribution, we will focus on the description of the ferroelectric properties of complex organic-inorganic systems, such as metal-organic frameworks (MOFs). In particular, MOFs with a perovskite topology show promising new routes for the cohexistence of ferroelectricity and magnetism (i.e., multiferroicity).

April 10, 2013

"Scanning probe characterization of energy nanomaterials and devices," by Liwei Chen, Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, hosted by Saw Wai Hla

Abstract: Energy is the biggest challenge in the next 50 years. Energy nanotechnology, which combines nanomaterials and nanoscale effects of special properties for specific applications in energy, has become the focus of a mainstream campaign of research in the last decade. In this talk, I will present a few examples of scanning force microscopy investigations on materials and devices, especially on interfaces in devices. Firstly, a new scanning dielectric force microscopy technique will be introduced and its application in characterization of charge transport properties will be demonstrated. We will then move on to the interfacial dipole measurement in organic photovoltaic device and finally we show force spectroscopy study of solid-electrolyte interphase in lithium-ion batteries.

April 9, 2013

"Controlling Gold Nanoparticles with Atomic Precision," Rongchao Jin, Carnegie Mellon University, hosted by Yugang Sun and Gary Wiedderrecht

Abstract: Controlling nanoparticles with atomic precision has long been a major goal in nanoscience research. Gold nanoparticles are particularly attractive because of their chemical stability and elegant optical properties. The synthesis of atomically precise gold nanoparticles, however, remained a major challenge in the past, which hampered the pursuit of fundamental science of such nanoparticles (e.g., surface structure, quantum size effect).

This talk will present a size-focusing methodology successfully developed for synthesizing a series of atomically precise gold nanoparticles protected by thiolates [denoted as Aun(SR)m, with n ranging from a few dozens to several hundreds, also called nanoclusters)]. Such ultrasmall nanoparticles (ca. 1-3 nm) exhibit distinct quantum size effects and interesting electronic/optical properties, which are fundamentally different from those of larger counterparts, such as plasmonic nanoparticles. New types of atom-packing structures have been discovered in Aun(SR)m nanoclusters through X-ray crystallographic analysis.

A few representative size-specific Aun(SR)m nanoclusters will be discussed in detail. These well-defined nanoclusters also hold potential in catalysis as new model catalysts, and atomic-level correlation of the catalytic properties of Aun(SR)m with crystallographic structures will ultimately offer fundamental understanding on nanogold catalysis.

March 25, 2013

"Exerting Mechanical Force on Single Protein Molecules," Robert Szoszkiewicz, Kansas State University, hosted by Xiao-Min Lin and Tijana Rajh

Abstract: Using AFM force spectroscopy, one can measure physiologically relevant pN forces between an AFM tip and a biomolecule with a mean displacement resolution of about 0.1 nm. The last 15 years have witnessed an explosion of interest in single-molecule force spectroscopy fueled by

  • New possibilities to advance in protein folding,
  • Possibilities to elucidate molecular mechanisms of various cellular processes, and diseases, and
  • Efforts to understand the nanomechanical properties of proteins, polysaccharides and DNAs in order to design biomimetic and/or mechanically functional materials.

In this seminar, we will present several examples of our AFM force spectroscopy data. First, we will present the results of mechanical unfolding of on a recombinant protein comprising an NRR domain from mammalian Notch 1. Notch is a transmembrane cell signaling protein, and understanding its mechanical properties at the single-molecule level is expected to help elucidate Notch's role in processes relevant to embryonic development, tissue homeostasis, and some breast cancers. Second, we will concentrate on elucidating early folding events in a simple model protein from changes of molecular compliance and dissipation factors. Using such measurements, we hope to provide basic understanding of early-folding events. Time permitting, we will show how mechanical force can influence the rate and mechanisms of an enzymatic cleavage of a single disulfide bond embedded in a protein.

March 25, 2013

"Manipulative Scanning Tunneling Microscopy and Single-Molecule Spintronics," by Andrew DiLullo, Ohio University, hosted by Saw Wai Hla

Abstract: Scanning tunneling microscopy (STM), a real-space local probe of nanoscale topology and electromagnetic properties, is applied to further our understanding of surfaces and surface supported atomic and molecular systems. In addition, STM manipulation techniques are implemented for bond dissociations, lateral manipulations, and surface augmentations. Diverse applications of STM techniques will be presented, with the primary focus of characterizing surface supported spintronic molecular systems. The controlled creation of surface nano-cavities will be shown, along with a method for extracting local surface and probe work functions through judicious measurement and data analysis. Surface-catalyzed molecular chain formation will be demonstrated, resulting in linear, covalently coordinated networks of spin-centers (cobalt ions) that are antiferromagnetically linked and interact individually with the Au(111) substrate through Kondo interactions. The spin polarization of molecular orbitals is mapped by application of spin-polarized STM, and reversible probe-induced molecular conformation switching is demonstrated. Results are summarized as related to the primary goal of creating functional spintronic molecular systems, and a brief outlook for future measurements will be presented

March 14, 2013

"Atomic and Molecular Nanocontacts: Structure, Magnetism, and Kondo Anomalies from First Principles," Erio Tosatti, SISSA, Trieste, Italy, hosted by Daniel Lopez

The nature and properties of atomic and molecular metallic nanocontacts, break junctions, of tip-surface tunnel contacts are difficult to access geometrically. Yet, they are rich in phenomena connected with structure, electron transport, and magnetism. Structurally, the formation of magic nanowires in gold is a remarkable phenomenon, explained by minima of the string tension. Electronically, first-principles calculations account well for the ballistic conductance of metal nanocontacts, both in magnetic and nonmagnetic metals.

Magnetic impurities bridging nonmagnetic metal contacts yield zero bias anomalies in STS spectra and/or in ballistic conductance because of the Kondo effect — a remarkable example of magnetically controlled current. Conductance is in this case ruled by the specificities of the atomic and molecular states involved, accessible only through a first-principles electronic structure approach.

The unsolved difficulty in combining the Kondomany body physics with standard electronic structure poses a problem to the theorist. We address this problem by means of a recently devised joint density functional plus numerical renormalization group approach. I will illustrate applications to transition metal impurities on gold nanowires and on carbon nanotubes, to platinum nanocontacts, and to magnetic molecules on gold surfaces. The circumstances leading to an exotic ferromagnetic Kondo effect, as opposed to an ordinary antiferromagnetic Kondo effect, will also be outlined along with possible systems where that unusual situation could be realized.

Feb. 8, 2013

"Atomic Structure of Carbon and Nitrogen on the Pt(111) Surface," Michael Trenary, University of Illinois at Chicago, hosted by Tijana Rajh

Abstract: The structure and reactivity of elemental carbon and nitrogen on transition metal surfaces are important to a variety of problems in heterogeneous catalysis. Many of the surface chemical properties of both carbon and nitrogen can be deduced through studies that employ techniques that average over monolayers, while scanning tunneling microscopy (STM) can provide direct information on the structure of surface layers, often with atomic resolution. The techniques of reflection adsorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and low-energy electron diffraction (LEED) have been used to study the formation of carbon and nitrogen on Pt(111) through dehydrogenation reactions.

In the case of carbon, the dehydrogenation of acetylene and ethylene was found to first produce ethylidyne (CCH3), which then decomposes to form CxHy clusters of various sizes as observed with room-temperature STM. At higher temperatures, these clusters would undergo further dehydrogenation to form graphene islands. Under conditions that resulted in complete coverage of the Pt(111) surface with graphene, various rotational domains of graphene were observed. The boundaries between graphene domains provide nucleation sites for the growth of Pt nanoclusters when Pt is deposited onto the graphene covered Pt(111) surface. In the case of nitrogen, it was found that reaction between ammonia and molecularly adsorbed O2 would result in the formation of H2O, which desorbs below 200K to leave behind a well-ordered p(2×2)-N layer on Pt(111). This N layer readily reacts with H2 to form NH molecules on the surface, as observed with RAIRS. Through collaborative research with a group in Japan, a low-temperature STM operated at 5K was used to obtain atomically resolved images of the p-(2×2)-N layer and of its hydrogenation to NH.

January 31, 2013

"3D Composition Profiling at the Nanoscale: Doping Limits in Semiconducting Nanowires," by Justin Connell, Northwestern University, Vanderbilt University, hosted by Amanda Petford-Long

Abstract: The vapor-liquid-solid (VLS) mechanism of semiconductor nanowire growth provides a means to fabricate one-dimensional structures with control over doping and aspect ratio provided in situ during growth. Developing deep understanding and precise control of the structure and chemical composition of VLS-grown nanowires is crucial, as any unintended gradients in dopant concentration can severely degrade the ultimate device performance. This is particularly important for optoelectronic applications such as solar cells and LEDs, where broadened axial and/or radial doping junctions lower efficiencies.

Contrary to the traditional model of VLS growth, where dopant species are assumed to incorporate uniformly across a planar liquid-solid interface, we demonstrate that VLS-mediated doping is highly radially anisotropic, with dopant concentration variations across the nanowire diameter of as much as two orders of magnitude. Finite-element modeling of the doping process, coupled with recent in situ TEM observations reported in the literature, suggest that this radially inhomogeneous dopant distribution is a direct consequence of growth from a faceted liquid-solid interface, rather than the commonly assumed planar interface.

These observations suggest that this doping inhomogeneity is general to all nanowire systems, motivating the search for novel catalysts for nanowire growth that can alleviate or eliminate this side faceting behavior. Using both aqueous solution and e-beam lithographic techniques, we are able to fabricate composition-controlled Au-Cu alloy catalysts for nanowire growth, providing a platform on which to study the limits to which varying catalyst phase and chemistry can be used to control doping in VLS nanowire growth.

January 9, 2013

"Hybrid Plasmonic Phase-Changing Nanostructures: Active Reconfigurable Devices to Ultrafast Dynamics," by Kannatassen Appavoo, Vanderbilt University, hosted by Matt Pelton

Abstract: Ultrafast photo-induced phase transitions in quantum materials could revolutionize data storage and telecommunications technologies by modulating transport in integrated nanocircuits at terahertz speeds. In phase-changing (PC) materials, microscopic charge, orbital and lattice degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known when such PC materials are nanostructured and implemented in nanoscale switching configurations.

This talk presents a generalizable concept of incorporating a quantum material — vanadium dioxide (VO2) — to create functionality in plasmonics, a new device technology that interfaces electronic and photonic components in a single chip. By designing, simulating, and fabricating hybrid plasmonic/PC nanostructures, we demonstrate at the single nanostructure level how signal modulation can be achieved when the VO2 component undergoes its characteristic insulator-to-metal transition. Furthermore, a subwavelength hybrid nanomodulator is demonstrated that is both thermodynamically and wavelength tunable. Reconfigurability is enabled by spatially confining electromagnetic fields to nanoscale volumes by using a metallic nanostructure while simultaneously tailoring its near-field environment with a PC nanostructure.

By providing the first ultrafast optical studies of a hybrid nanomaterial, we also report a novel all-optical technique to trigger VO2 PT on a timescale faster than its single-phonon cycle, accompanied by a decrease in switching threshold. The mechanism is based on ballistic hot electrons created by ultrafast optical excitation of gold nanoparticles, which are injected through the gold/VO2-nanostructure interface. Density functional calculations show that the injected electrons cause the catastrophic collapse of the 6-THz optical phonon mode, associated with the structural phase transition of VO2.

Most importantly, the hybrid nanostructures discussed here combine generic plasmonic (gold) and PC (VO2) components. Therefore, this work aims to be generalizable, serving as a platform for designing other hybrid nanostructures operating at nanometer length scale and on femtosecond timescale for next-generation all-optical nanophotonic devices. Key scientific issues regarding the viability of such hybrid nanomodulators are also addressed, such as interfacial effects, intrinsic size-dependent switching of VO2 and the potential for coherent control of the structural dynamics in VO2.

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