Archive: Colloquia

April 16, 2008

"Ag Nanocrystal Plasmons: Single-Molecule Raman Scattering and Photovoltage," Louis Brus, Columbia University, hosted by Tijana Rajh

Abstract: 30-nm Ag particles act as almost ideal "nano-antennas" for visible light. Two touching 30-nm Ag nanocrystals exhibit a junction "hot spot" in the local electromagnetic field enhancement. If a molecule is chemisorbed in the junction and also electronically resonant with the laser, this enhancement is sufficient to enable single-molecule Raman spectroscopy. The Ag metal "hot hole" coherent polarization also photo-oxidizes adsorbed stabilizing citrate anions, thus charging the nanocrystals. This creates a cathodic photovoltage, which can be observed as open-circuit photovoltage in an electrochemical cell containing nanocrystals adsorbed on a transparent electrode. Photovoltage creates enhanced reduction of Ag+ in solution, and thus, the nanocrystal grows in size. Photovoltage-driven Ostwalt ripening causes growth of aqueous colloidal 70-nm single-nanocrystal prisms from 8-nm Ag seeds in the presence of air. The visible irradiation wavelength controls the lateral size of the prisms.

April 2, 2008

"Towards a Single Quantum Dot Microdisk Laser," Glenn Solomon, National Institute of Standards and Technology (NIST) and University of Maryland, hosted by Matthew Pelton

Abstract: Ultra-low-threshold lasers have applications in low-power communications and emerging areas such as on-chip communications. At these very low thresholds, our common understanding of lasing breakdowns because the lasing occurs via only a few emitters and the gain medium is therefore highly nonuniform. An excellent example is the one studied here, where the gain medium is a very dilute array of semiconductor quantum dots (QDs). In such a case, the turn-on of lasing is not expected to be abrupt; the typically knee in the laser L-I curve, often used to define a threshold, will be transformed into a soft transition.

I will discuss an ultralow, sub-µW threshold microcavity laser. The gain medium is a randomly distributed ensemble of InAs QDs. The cavity is formed in a microdisk of GaAs, with a quality factor of approximately 17000. On average less than one QD spectrally and spatially aligned with a cavity mode.

March 19, 2008

"Nanoscale Spectroscopy with Optical Antennas," Lukas Novotny, University of Rochester, hosted by Gary Wiederrecht

Abstract: Antennas are devices that efficiently convert localized energy to free propagating radiation and vice versa. They are a key enabling technology in the microwave and radiowave regime but their optical counterpart is greatly unexplored.

In order to understand antenna-coupled light emission and absorption, we use a single molecule as an elementary light-emitting device. With an optical antenna in the form of a simple gold particle, we are able to increase the emission efficiency by more than a factor of 10. However, for very short distances between particle and molecule, the fluorescence yield drops drastically because of nonradiative energy transfer. A simple gold particle is not an efficient optical antenna, and it can be expected that favorably designed nanoplasmonic structures will yield much higher enhancement.

Optical antennas can be employed as light sources for high-resolution optical microscopy and spectroscopy. We demonstrate vibrational (Raman scattering) and nonlinear imaging with spatial resolutions down to 10 nm.

March 5, 2008

"Tools for the Synthesis and Characterization of NanoBio Interfaces," Milan Mrksich, University of Chicago

Abstract: This seminar will describe an approach that employs self-assembled monolayers for modifying man-made materials with biological functionality. These surface chemistries permit wide flexibility in patterning the attachment of proteins and cells with nanoscale control. The resulting interfaces are important for studies of cell adhesion, for high throughput assays of biochemical activities, for fundamental studies of enzyme reactions at interfaces and for integrating biological activities with electrical processes.

February 20, 2008

"Novel Carbon Cluster Materials and their Reactivity towards Hydrogen," Manfred Kappes, Institute of Physical Chemistry, University of Karlsruhe and, Institute of Nanotechnology, Forschungszentrum Karlsruhe, hosted by Stefan Vajda

Abstract: SIon beam soft-landing of mass selected fullerene ions has been used to generate multilayer films. We have studied their thermal and electronic properties as well as their reactivity towards atomic hydrogen and alkali atoms.

February 6, 2008

"Mach-Zehnder Interferometry and Microwave- Induced Cooling in Persistent Current Qubits," William D. Oliver, MIT Lincoln Laboratory, hosted by Matthew Pelton

Abstract: Superconducting persistent-current qubits are quantum-coherent artificial atoms with multiple energy levels. In the presence of large-amplitude harmonic excitation, the qubit state can be driven through one or more of the energy-level avoided crossings. The resulting Landau-Zener transitions mediate a rich array of quantum-coherent phenomena as a function of the driving amplitude and frequency.

In this talk, we present three such demonstrations of quantum coherence in a strongly driven niobium persistent-current qubit.

1. The first is Mach-Zehnder-type interferometry, for which we observe quantum interference fringes for 1-50 photon transitions.
2. The second is a new operating regime exhibiting coherent quasi-classical dynamics, for which the MZ quantum interference persists even for driving frequencies smaller than the resonance linewidth.
3. The third is microwave-induced cooling], for which we achieve effective qubit temperatures <3 mK, a factor 10-100x lower than the dilution refrigerator ambient temperature.

These experiments exhibit a remarkable agreement with theory, and are extensible to other solid-state qubit modalities. In addition to our interest in these techniques for fundamental studies of quantum coherence in strongly-driven solid-state systems, we anticipate they will find application to nonadiabatic qubit control and state-preparation methods for quantum information science and technology.

January 23, 2008

"Structure-Property Relationships in Functional Quantum Dots: From Biological Imaging to Solid-State Lighting," Sandra Rosenthal, Vanderbilt University, hosted by Stefan Vajda

"Polymer Self-Assembly in Semiconductor Microelectronics," C. T. Black, Center for Functional Nanomaterials, Brookhaven National Laboratory

Abstract: The challenge of defining semiconductor integrated circuit elements at sub-100-nm dimensions has created opportunities for alternative patterning approaches. One attractive nontraditional approach use the phenomenon of self-assembly. Under suitable conditions, certain materials self-organize into patterns offering promise for enabling further advances in semiconductor microelectronics. Diblock copolymers are particularly attractive for this application because — like photoresist materials used for lithography — they can act as sacrificial templates for patterning integrated circuit elements.

Self-assembly has applications in both current and future microelectronics, and materials integration is the critical first step for adoption into high-performance semiconductor technology. I will discuss my prior research program at the IBM T.J. Watson Research Center, which involved demonstrating key applications of self-assembly in high-performance semiconductor device fabrication, including its use in on-chip decoupling capacitors, nanocrystal FLASH memories, and advanced field-effect transistors. The discussion will highlight both the promise and versatility of this technique, as well as some limitations and challenges still to be addressed.

December 10, 2007

“Processes of Ordered Structure Formation iIn Polypeptide Thin-Film Solutions,” Ioan Botiz, Institut de Chimie des Surfaces et Interfaces, CNRS-UHA, hosted by Seth Darling

Abstract: Transforming thin solid films of polystyrene-poly(γ-benzyl-L-glutamate) into solutions, via exposure to solvent vapor, allowed us to nucleate and grow in such solutions ordered ellipsoidal three-dimensional structures containing millions of molecules. These structures were randomly oriented but homogeneously distributed on the film surface and possessed an anisotropic shape that we attributed to asymmetric growth in lateral directions (specific directional interactions act along the various axes of the molecules).

The experiments presented in this work proved that the size of ellipsoidal structures (limited by decrease of supersaturation) was found to be increasing with the decrease of polymer concentration in film solution. Consequently, the ordering of polypeptides in a film solution at two consecutive polymer concentrations (e.g., by performing a two-step experiment) led to a bimodal distribution of ellipsoidal structures on the film surface.

Performing experiments under different environmental conditions allowed us to conclude that polymer solubility could be influenced via surrounding gas-phase humidity variation. Increasing the humidity of the surrounding gas phase led to a decrease of the value of polymer solubility and an increase of interfacial tension between the ordered structures and the solution. Consequently, ordered three-dimensional ellipsoidal structures could be obtained even at very low polymer concentrations.

We propose that complexation of poly(γ-benzyl-L-glutamate) by water, via hydrogen bonding interactions, led to a “new” polymer which causes the formation of solid ordered structures also at low polymer concentrations. We tested this hypothesis by using besides water also other protic non-solvents like methanol or trifluoroacetic acid.

Furthermore, we asked the question if a reversible change of polymer solubility is possible by controlling the amount of protic non-solvent in the surrounding gas-phase. Adequate experiments, in which small amounts of water or methanol molecules were added and then removed from the surrounding gas-phase, proved that the process of structure formation and dissolution was reversible.

“Photo-induced Electron Transfer Processes at Nanostructured Interfaces: Advancing Excitonic Solar Cells,” Garry Rumbles, National Renewable Energy Laboratory, hosted by Tijana Rajh

Abstract: Excitonic solar cells are emerging as viable alternatives to conventional photovoltaic devices. At present, power-conversion efficiencies have only reached 5% and at least a threefold improvement is needed in order to be competitive with the existing technologies.

This presentation will focus on the nanostructured interface that is formed between two chemical systems: one a donor, the other an acceptor. The function of this interface is to dissociate excitons and create free carriers that do not immediately recombine. The technique used to follow this process is time-resolved microwave conductivity (TRMC). This electrode-less technique provides a means of studying the efficiency and kinetics of free carrier production (and loss) and therefore allows us to focus on the interface at the heart of the excitonic solar cell and therefore understand the fundamental processes that are occurring; processes that are key to advancing solar cell conversion efficiencies. Specific donor-acceptor systems that will be discussed includes the conjugated polymer poly(3-hexylthiophene) combined with C60, deposited on zinc oxide and solublized with single-wall carbon nanotubes. Each system will be used to highlight topics such as exciton dissociation, carrier mobility, and exciton annihilation.

November 14, 2007

“Plasmonic Route to Nanophotonics, ” Anatoly Zayats, The Queen’s University of Belfast, hosted by Gary Wiederrecht

Abstract: Recent advances in nanofabrication and subwavelength optical characterisation have led to the development of a new area of nanophotonics concerned with routing and manipulation of optical signals in scalable and integratable devices. In this context, plasmonics that is dealing with surface electromagnetic excitations in metallic structures, may provide a great deal of flexibility in photonic integration in all-optical circuits since with surface plasmons the problem of light manipulation can be reduced from three to two dimensions. Surface plasmon polaritons (SPPs), the electromagnetic excitations coupled to collective motion of conduction electrons near a metal surface, are emerging as a new optical information carrier that enables signal manipulation and processing on the subwavelength scale.

SPPs play a crucial role in determining optical properties of randomly rough and artificially structured metal surfaces and films, such as reflection, transmission, scattering, second-harmonic generation, surface-enhanced Raman scattering, etc. Various elements of two-dimensional optics-based on surface plasmon polaritonic excitations, such as mirrors, lenses, resonators, planar waveguides have been demonstrated. Band-gap effects, enhanced optical transmission through plasmonic crystals, surface plasmon polariton waveguiding along straight and bent line defects in such crystals have also been studied. Hybridization of plasmonic nanostructures with molecular species exhibiting nonlinear optical response allows the development of photonic components with the enhanced nonlinear response due to the electromagnetic field confinement related to surface plasmons.

In this talk the applications of plasmonic nanostructures to light guiding and manipulation in subwavelength photonic elements, the enhanced nonlinear functionalities and dispersion management using metallic nanostructures will be discussed. Surface-plasmon optics provides a basis for the implementation of novel photonic functionalities and development of a new class of active photonic devices for optical signal processing and all-optical integrated circuits. Numerous possible applications of surface plasmon polaritons can be envisaged in nanophotonics, classical and quantum optical information processing and optical communications as well as optical and magneto-optical data storage.

October 24, 2007

"Magnetization Reversals in Cobalt Nanorings and Nanoparticle Rings," Alex Wei, Purdue University

Abstract: Rings of weakly ferromagnetic cobalt nanoparticles (dispersed in toluene by resorcinarene-based surfactants), formed by dipole-directed self-assembly, have been determined to support bistable flux closure (FC) states at room temperature using off-axis electron holography. In many cases the FC polarization can be switched by exposure to a coaxial magnetic pulse, then switched again by applying M(z) in the opposite direction. This physical behavior has no known analogy at the macroscopic level, but the effect can be reproduced by magnetodynamic simulations, which provide some insights into the complex mechanism of field-induced FC reversal.

October 17, 2007

"Nanofluidics: Coulombic Dragging and Mechanical Propelling of Molecules," Petr Kral, University of Illinois at Chicago, hosted by Elena Shevchenko

Abstract: We show by molecular dynamics simulations that polar molecules and ions adsorbed on carbon nanotubes can be dragged by Coulombic scattering with water, individual ions or ionic solutions flowing inside the tubes. The phenomenon is presented on the manipulation of water droplets and inverse micelles in air, and biological molecules in natural solutions.

We also introduce molecular propellers that can pump liquids in the bulk and at the liquid surfaces, in dependence on the chemistry of the liquid-blade interface. The propellers can be attached to biological molecules and cellular components that could be manipulated in vitro and in vivo.

Finally, we model biomolecular docking on predesigned nests formed on doped and ligand-modified material surfaces. We show that the GFP protein can be selectively attached to modified graphene layers. This methodology could be used in the preparation of hybrid bionanostructures.

References

B. Wang and P. Kral, "Dragging of Molecules on Material Surfaces Induced by Flowing Liquids," J. Am. Chem. Soc., 128, 15984 (2006)

B. Wang and P. Kral, "Chemically tunable nanoscale propellers of liquids," Phys. Rev. Lett., 98, 266102 (2007)

B. Wang and P. Kral," Optimal Atomistic Modifications of Material Surfaces: Design of Selective Nesting Sites for Biomolecules," Small, 3, 153110 (2007)

October 3, 2007

"Metal Nanoparticle Plasmonics: Towards Quantum Nanoantennas," Garnett Bryant, National Institute of Standards and Technology, hosted by Matthew Pelton

Abstract: Understanding the nanooptics of metallic nanoparticles is critical for applications in nanometrology, nanosensors, nanoantennas, and nano-optical communication, where large optical response is needed. This understanding becomes even more essential for hybrid structures, which combine semiconductor quantum dots with metallic nanoparticles, where quantum coherent nanooptics is desired.

The response of metallic nanoparticles is provided by the surface plasmons excited in these confined structures. We first discuss the plasmonic excitations of single and coupled metallic nanoparticles based on classical calculations of their electromagnetic response. For isolated particles, the response is dipole-like, similar to a classical radio antenna. We map out the dependence of the plasmon resonance on particle size and shape to highlight critical differences with classical antennas.

In coupled systems, interaction across gaps distorts intraparticle dipolar response, localizes charge at the gaps, significantly red shifts the response and dramatically increases near fields, effects that are important for applications. These effects become singular for nearly touching nanoparticles, dramatically changing the interparticle coupling when particles touch. We discuss this transition in detail to provide a clear picture of coupling effects in these systems.

For nearly touching particles and for small nanoparticles, quantum effects, such as interparticle tunneling, surface scattering and state filling, may play a more important role in determining plasmon response. Density functional theory is being used to study strongly coupled systems.

Preliminary results from these DFT calculations will be discussed. Quantum effects will also play a key role in strongly coupled hybrid structures of metallic nanoparticles and quantum dots. Hybrid structures have been studied with the quantum dots treated quantum mechanically, via a density matrix approach, coupled by dipole interaction to classical metal nanoparticles.

The exciton response in the quantum dot is broadened and shifted by incoherent and coherent interactions with the metal nanoparticles.

Excitation transfer between semiconductor dots in hybrid structures is determined. These results will be discussed to assess prospects for quantum coherent communication and metrology.

September 5, 2007

"Characterization and manipulation of single molecule chemistry on silicon surfaces," Mark Hersam, Northwestern University, hosted by Seth Darling

Abstract: Organic molecules mounted on silicon surfaces present unique opportunities for electronics, photonics, and sensing at the nanometer scale. To help elucidate the potential of organosilicon nanostructures, this talk outlines recent efforts to characterize and manipulate organic chemistry on silicon surfaces down to the single molecule level using ultra-high-vacuum (UHV) scanning tunneling microscopy (STM). Specific topics include templated assembly of heteromolecular organosilicon nanostructures using room-temperature multistep feedback-controlled lithography and molecular resolution characterization of cryogenic UHV STM-driven organosilicon chemical reactions. In an effort to identify the underlying physical mechanisms that control single-molecule chemistry on silicon surfaces, the aforementioned experimental results will be quantitatively compared with density functional theory calculations.

August 22, 2007

"Studies of the Dynamics of Metal Nanoparticles fFrom Electronic Dephasing to Heat Dissipation," Gregory Hartland, University of Notre Dame, hosted by Matthew Pelton

Abstract: I will discuss recent work in my group, where optical spectroscopy has been used to probe the properties of metal particles of different sizes and shapes. Two types of experiments have been performed: ultrafast time-resolved spectroscopy studies and single-particle Rayleigh scattering measurements. The single particle experiments generate information about dephasing of the localized surface plasmon resonance due to electron-surface scattering and radiation damping. Understanding these effects is important for optimizing the size and shape of metal nanostructures for sensing applications. The materials studied include gold nanorods and hollow cubic structured particles made from silver and gold. The results show that both electron-surface scattering and radiation damping are significant effects. In the time-resolved experiments, the pump laser deposits energy into the electrons. Monitoring the response of the sample provides information about electron-phonon coupling and energy exchange with the environment. The laser-induced heating also coherently excites vibrational modes of the particle that correlate with the expansion coordinate. Comparing the measured periods to the results of continuum mechanics calculations gives unique information about the elastic properties of the particles. The vibrational beat measurements also provide a way of measuring the temperature of the particles, and can be used to determine the thresholds for explosive boiling in the solvent and photothermal reshaping of the particles.

August 8, 2007

"Amphiphiles at Bio-Interfaces ­ From Structure Control to Properties," Ka Yee Lee, University of Chicago, hosted by Seth Darling

Abstract: The cell membrane acts as a barrier, controlling the transport of molecules into and out of the cell. When the structural integrity of the membrane is compromised, so does its barrier function. We examine the way the membrane barrier function can be compromised by antimicrobial peptides and how leakage of intracellular materials from a structurally damaged cell membrane can be arrested by triblock copolymers.

Antimicrobial peptides are a class of peptide innate to various organisms that functions as a defense agent against harmful microorganisms by means of membrane disordering. Despite their enormous biomedical potential, progress towards developing these peptides into therapeutic agents has been hampered by a lack of understanding of their mechanism of action. A class triblock copolymer of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(PEO-PPO-PEO) has been shown to help seal electroporated cell membranes, arresting the leakage of intracellular materials of the damaged cell. However, the interaction mechanism between the cell membrane and poloxamer is still unclear. Using a variety of model systems and biophysical techniques, we have examined the targeting selectivity as well as the membrane disruption mechanism of antimicrobial peptide protegrin-1, and have explored the sealing capability of poloxamer 188.

July 25, 2007

“Mean-Field and Quantum Models for Magnetism in Gold Nanoparticles,” Vladimiro Mujica, Northwestern University, hosted by Tijana Rajh

Abstract: The onset of magnetism in gold nanoparticles is an intriguing phenomenon with a very rich physics. Macroscopic gold is diamagnetic, but bare gold nanoparticles may exhibit ferromagnetism in a certain size range. Chemical coating of nanoparticles may strongly modify their magnetic properties, either quenching or enhancing them.

I will present some recent theoretical work aiming to provide a model for the observed size and chemisorption dependence of magnetism in gold nanoparticles. Spin-polarized density functional calculations support the idea that bare spherical gold nanoparticles exhibiting ferromagnetism can be considered as a core-shell magnetic structure where the core is essentially diamagnetic and the non-zero contribution to the magnetic moment is concentrated on the surface atoms. I will also discuss quantum calculations on gold clusters aiming to assess the influence of different chemical adsorbates particularly in view of some recent experiments that indicate a striking difference between nitrogen and sulfur linkers used to stabilize the nanoparticles,

In addition to the quantum calculations, I will present a mean-field model that exploits the idea of a magnetic gold nanoparticle as a core-shell structure. Our results, regarding the behavior of the size-dependent magnetic moment, agree qualitatively with the experimental observation that the transition from quasi-atomic to bulk-behavior of the magnetic moment exhibits a maximum that is essentially determined by the relative numerical ratio of surface to bulk atoms, which in turn arises from the competition between the contributions to the magnetic moment due to the diamagnetic core and the ferromagnetic surface.