Argonne National Laboratory

2008 Colloquium Archive

Date Title

December 10, 2008

"Screw Dislocation Driven Nanowire Growth: Nanowire Trees, Helice, and Beyond," Song Jin, University of Wisconsin - Madison, hosted by Matthias Bode

I will discuss a "new" nanowire formation mechanism that is completely different from the well-known metal catalyzed vapor-liquid-solid (VLS) mechanism. The screw component of an axial dislocation provides the self-perpetuating steps to enable 1-dimensional crystal growth, unlike previously understood mechanisms that require metal catalysts. This mechanism was found in hierarchical nanostructures of lead sulfide (PbS) nanowires resembling "pine trees" that were synthesized via chemical vapor deposition. Structural characterization reveals a screw-like dislocation in the nanowire trunks with helically rotating epitaxial branch nanowires. The rotating trunks and branches are the consequence of the Eshelby twist of screw dislocations. With the help of X-ray microdiffraction and transmission X-ray microscopy experiments carried out at APS, we have further used dislocation mechanism and elasticity theory to explain the spontaneous formations of nanotubes and helical nanostructures. We suggest that screw dislocation growth is overlooked and underappreciated in modern literature on one-dimensional nanomaterials. The proposed nanowire growth mechanism will be general to many materials and enable more complex nanostructures to be synthesized in the future to enable diverse applications.

November 12, 2008

"Probing microscopic conduction mechanisms in tunable superconducting thin films," Sambandamurthy Ganapathy, University of Buffalo - SUNY, hosted by Nathan Ramanathan

Abstract: I will present experimental results from our transport measurements on amorphous indium oxide thin films that can be controllably tuned between the insulating and superconducting phases. In particular, the transport behavior of the films at magnetic fields larger than the critical point of the phase transition will be presented. Our results show the emergence of an collective insulating phase that may still sustain superconducting correlations at submicron length scales.

November 12, 2008

"Diamonds Are Forever: Solid-State Quantum Optics Electron-Nuclear Spin Registers," M. V. Gurudev Dutt, University of Pittsburgh, hosted byeffrey Guest

Abstract: Building scalable quantum information systems is a central challenge facing modern science. One promising approach is based on quantum registers composed of several quantum bits that are coupled together via optical channels. I will discuss experiments that demonstrate addressing, preparation, and coherent control of individual nuclear spin qubits in the diamond lattice at room temperature. We have measured spin coherence times exceeding milliseconds, and observed coherent coupling to nearby electronic and nuclear spins. Robust initialization of a two-qubit register and transfer of arbitrary quantum states between electron and nuclear spin qubits has been achieved. Our results show that coherent operations are possible with individual solid-state qubits whose coherence properties approach those for isolated atoms and ions. The resulting electron-nuclear few-qubit registers can potentially serve as small processor nodes in a quantum network where the electron spins are coupled by optical photons to generate entanglement, and the nuclear spins serve as a resource for quantum memory and quantum logic operations. Future prospects along these directions as well as potential applications in nanoscale precision magnetometry and magnetic imaging will be discussed.

November 5, 2008

"Revealing Deterministic Mesoscopic Mechanisms of Polarization Switching: Switching Spectroscopy PFM of Defect-Engineered Structures," Sergei V. Kalinin, Oak Ridge National Laboratory, hosted by Stephen Streiffer

Abstract: Polarization switching in ferroelectric materials is controlled by structural defects that act both as the local nucleation centers and the pinning centers for moving domain walls. Progress in nanoscale ferroelectric device applications ranging from FeRAM to data storage to tunneling barriers necessitates understanding of polarization switching mechanism on a single structural or morphological defect level. In this talk, I will present recent results on local studies of fundamental polarization reversal mechanisms in ferroelectrics]. The direct imaging of a single nucleation center on sub-100-nm level is demonstrated. Using switching spectroscopy piezoresponse force microscopy of the systems with engineered defect structures and phase-field modeling, we demonstrate that deterministic mesoscopic polarization switching mechanism on a single known structural defect can be determined. In particular, the artificial bicrystal grain boundary in (100) BiFeO3 is found to impede ferroelectric switching, but facilitate ferroelastic switching for one of the constituent crystals. The coupling between ferroelastic domain walls and ferroelectric polarization switching is demonstrated and attributed to the kinetic effects. These studies open the pathway for probing kinetics and thermodynamics of local bias-induced phase transitions and dissipation on a single-defect level using field confinement by an SPM tip. The future potential for atomistic studies is discussed. Research was supported by the U.S. Department of Energy Office of Basic Energy Sciences Division of Scientific User Instruments and was performed at Oak Ridge National Laboratory, which is operated by UT-Battelle, LLC.

October 29, 2008

"Mycobacterial Mimics, Microparticle Manipulation, and More," Raghuveer Parthasarathy, University of Oregon, hosted by Xiao-Min Lin

Abstract: Lipid membranes, the underlying architecture of all cellular membranes, are remarkable materials: self-assembled, two-dimensional fluids. Membranes can be constructed on solid supports -- these "supported membranes" enable controlled investigations of a variety of membrane properties as well as new sorts of composite materials. In this talk I'll describe what my group is presently exploring: * Mycobacteria, which include the pathogens that cause tuberculosis and leprosy, have unusual membranes. Building a supported mimic of the mycobacterial envelope, we have discovered that an important mycobacterial lipid has the surprising ability to make membranes resistant to dehydration. * Can we harness interactions between membranes to organize non-biological materials? As "building blocks," we focus on lipid membrane-functionalized silica microparticles. To measure their interaction energies, we have invented a new type of optical trap that sculpts the trapping potential landscape into desired forms. With this approach, we have begun to quantify relationships between interaction energies and membrane properties, such as lipid composition and protein binding.

October 22, 2008

Special Colloquium: "Formation, melting and freezing of Bi nanoparticles embedded in a sodium borate glass" A. F. Craievich, Institute of Physics, University of Sao Paulo, hosted by Brian Stephenson

Abstract: The processes of nucleation and growth of liquid bismuth nanodroplets embedded in a soda-borate glass - submitted to isothermal annealing at different temperatures - were studied by in situ small-angle x-ray scattering (SAXS). Our experimental results indicate that the formation of bismuth droplets occurs in two successive stages. The first stage is characterized by the nucleation and growth of spherical droplets promoted by the diffusion of isolated bismuth atoms through the glass matrix and the second stage by droplet coarsening. The experimental functions describing the time variation of the droplet average radius and density number, at advanced stages of the growth process, agree with those predicted by the classical Lifshitz-Slyozov-Wagner theory for particle coarsening. A combined use of the SAXS and WAXS techniques allowed us to establish that the melting temperature of bismuth nanocrystals strongly decreases for decreasing radius R and is a linear function of 1/R. The freezing temperature of bismuth nanodroplets was determined to be lower than the melting temperature of nanocrystals with the same radius (overcooling effect). The freezing temperature also decreases linearly for increasing values of (1/R), but, in this case, the slope is lower than that determined for the melting temperature. Thus the magnitude of the overcooling progressively decreases for decreasing radius, and it vanishes for a critical radius Rc=1.9A. These mentioned dependences of the melting and freezing temperatures could be explained by a simple model that assumes the nanoparticles being composed of a crystalline core surrounded by a disordered shell. On the other hand, the reduction of the volume of bismuth nanocrystals across the melting transition was determined from the temperature dependence of the integral of the SAXS intensity. This volume reduction - derived from SAXS results - is smaller than that a priori expected for homogeneous bismuth nanocrystals. This finding corroborates the model that suggests the heterogeneous nature of the (crystalline-amorphous) bismuth nanoparticles with R>Rc. Our experimental results also indicate that bismuth nanoparticles with R<Rc are fully amorphous. Thus, for bismuth nanoparticles with subcritical radii, neither liquid-to-crystal nor crystal-to-liquid transitions are expected to occur.

October 15, 2008

"A DIM view of science," Paul Russo, Louisiana State University, hosted by Seth Darling and Nathan Ramanathan

Abstract: Disease-inspired materials science includes two avenues of approach: a) emulating the systems nature has evolved to topple other living systems and b) learning to regulate bio-inspired platforms, possibly for medicinal purposes but also just for the challenge of it. The talk will emphasize systems designed around a fibrillar motif.

October 1, 2008

"Pure Spin Currents," Axel Hoffman, Argonne National Laboratory, hosted by Seth Darling

Abstract: The new development of spintronics aims at utilizing the spin degree of freedom for electronic applications. To this date, in most investigated spintronics systems and devices, the spin and charge currents are generally in parallel and therefore directly coupled. However, using nonlocal geometries allows us to separate spin and charge currents, which enables the investigation of pure spin currents. Because spin states are not necessarily conserved due to spin-flip scattering, they behave differently than charge currents. In particular, this opens up the opportunity to transport spin information via exchange interactions instead of actual spin transport. Thus there is a possibility of significantly reduced dissipation for devices based on pure spin currents. In this talk, I will review our work on pure spin currents as well as alternative approaches to the generation of spin currents, such as spin Hall effects and spin pumping.

September 17, 2008

"Photophysics of nanostructured molecular crystals: from nanorod bending to exciton fission," Christopher Bardeen, University of California Riverside, hosted by Gary Wiederrecht

Abstract: Our research concerns the photophysics of organic molecular crystals, from understanding their basic spectroscopic properties to making new materials. In this talk, we will discuss two interesting properties of molecular crystal nanostructures that motivate our studies. Transforming light into mechanical energy can be done in a controlled way by taking advantage of photochemical reactions in organic crystalline nanorods. Both reversible and irreversible shape changes in these structures can be accomplished using one or two-photon excitation. The ability to make a nanorod bend or expand using light provides a novel approach to making nanoscale photoactuators.

On the other hand, transforming light into electrical energy relies on the production of excitons in a material which can then be ionized into electron-hole pairs. We will discuss our research on exciton fission in organic polyacene structures, where an initially created singlet exciton spontaneously splits into a pair of triplets. This phenomenon provides a possible mechanism for increasing the quantum yield of charge carrier generation above unity.

September 3, 2008

"Arrays of metallic and bimetallic nanoparticles on alumina ultrathin films," Claude Henry, Universite Marseille & CiNaM, hosted by Stefan Vajda

Abstract: Regular arrays of metallic nanoparticles are grown by UHV atomic deposition on in situ prepared ultrathin alumina films. The alumina films (0.5 nm thick) are prepared by oxidation of a Ni3Al (111) surface at high temperature. AFM and STM studies show that the surface of the film is nanostructured, and it presents two interrelated hexagonal lattices of defects with parameters of 2.4 and 4.1 nm. Through the deposition of palladium atoms, palladium clusters are nucleated exclusively on the 4.1-nm lattice of defects and form an ordered array, as seen by STM. By evaporating a second metal, such as gold, only the predeposited palladium clusters grow, forming a perfect lattice of bimetallic clusters. The size (a dozen atoms to about 2 nm) and the composition of the bimetallic clusters can be independently controlled by the amounts of the two deposited metals. If we deposit gold first and then palladium, the result is different because the defects of the films are not perfect sinks for the gold clusters. Both pure palladium and bimetallic PdAu clusters coexist in that case. Kinetic Monte Carlo calculations explain these results.

The growth of the clusters have also been studied by grazing incidence small-angle X-ray scattering. The results show clearly that until coalescence, the lattice of the cluster array is perfect and extends on the whole substrate (1 cm²). The first results on the reactivity of arrays of palladium clusters will be presented. The adsorption of CO has also been studied by pulsed molecular beam techniques.

August 20, 2008

"Modulating the Power Factor with the Morphology of in situ Grown Nanostructured Thermoelectric Films," Clemens Burda, Case Western Reserve University, hosted by Matthew Pelton

Abstract: Thermoelectrics, which convert heat to electricity or vice versa, provide a promising approach to help overcome the current energy challenge by making use of thermal energy, such as waste heat from combustion engines. Recent theoretical and experimental progress on low-dimensional thermoelectric materials demonstrated that the ZT figure of merit could be greatly enhanced through nanoengineering of materials. However, this nanoengineering also increased the cost of thermoelectrics. Therefore, techniques that could compromise both the cost and efficiency requirements are going to be the focus of future studies. This presentation reports the exploration of nanostructured p-type PbSe thin films and fine-tuning of their thermoelectric properties through a cost-efficient wet chemical approach.

August 15, 2008

"Self-Organized Nanoparticle Assemblies: A Panoply of Patterns," Philip John Moriarty, University of Nottingham, hosted by Xiao-Min Lin

Abstract: Nanoparticle-solvent films deposited on solid substrates are associated with a rich dynamic behavior which gives rise to a wide variety of striking self-organized patterns]. Although close-to-equilibrium self-assembly of nanoparticle arrays has been studied in some depth, there has been rather less work on solvent-nanoparticle systems driven far from equilibrium (via, for example, spin coating). In the far-from-equilibrium regime, a remarkably broad array of intricate, spatially correlated patterns form including "foam-like" cellular networks, labyrinthine structures similar to those formed in spinodal decomposition of binary fluids, and well-defined fractal morphologies. I shall focus on our recent results in two areas: (i) "coerced coarsening" of nanoparticle arrays where the system is mechanically driven towards equilibrium], and (ii) the use of scanning probe-defined silicon oxide patterns to direct solvent dewetting and thus control pattern formation in drying nanofluids.

August 6, 2008

"Power Dissipation in Nanoscale CMOS and Carbon Nanotubes," Eric Pop, University of Illinois at Urbana-Champaign, hosted by Matthew Pelton

Abstract: High power densities are considered a major roadblock in the evolution of nanoelectronics. At the device level, such challenges are compounded by reduced thermal conductance and the thermal resistance of material interfaces. This talk will focus on power dissipation in nanoscale CMOS and carbon nanotubes, incorporating interface effects, temperature transients, and hot phonon scattering. Simple experiments are used to gain new insight into the fundamental behavior of nanotube devices. This work suggests much room for the optimization of nanoscale devices and circuits through bottom-up power-aware geometry, interface, and materials design.

July 23, 2008

"Magnetic nanostructures fabricated using block copolymer self-assembly," Caroline Ross, Massachusetts Institute of Technology, hosted by Seth Darling

Abstract: Block copolymers, which microphase-separate into ordered periodic nanoscale structures, provide a path to accomplish large-area patterning of arrays of dots or lines with periodicity on a scale of about 10-100 nm. We will describe the block copolymer lithography process and the magnetic properties of nanostructures made using this method, including CoCrPt dot arrays, Co/Cu/NiFe multilayer antidot arrays, and Co ring structures. We will describe the selection of diblock and triblock copolymer chemistry and processing methods, including substrate functionalization and thermal or solvent annealing, and describe the advantages of silicon-containing block copolymers for lithography. Long-range order can be imposed on the self-assembled block copolymer microdomains using topographical features such as trenches or small pillars made using optical or electron-beam lithography, giving well-ordered arrangements of nanoscale features. For example, in circular pits, concentric ring patterns can form. Of particular interest is the use of "'sparse" templates to produce densely packed block copolymer microdomain arrays with periodicity significantly less than that of the template. Applications in patterned magnetic media and ring-shaped magnetoelectronic memory devices will be discussed.

July 16, 2008

"Graphene-based Functional Materials and Devices," Yong P. Chen, Purdue University, hosted by Derrick Mancini

Abstract: Graphene (two-dimensional carbon) has attracted interest as a novel material promising diverse applications ranging from nanoelectronics to energy conservation. Ongoing research projects on material properties and functional devices of exfoliated and epitaxial graphene will be described. We have characterized epitaxially deposited carbon on metals (then transferred to insulators) and on sapphire and demonstrated their excellent promise as large-area graphene for electronic applications. The fabrication of various nanostructures based on exfoliated graphene using e-beam lithography as well as AFM-tip induced local oxidation and a novel hysteretic field effect found in junctions between graphene with different thickness and its potential application for nonvolatile memory will be discussed. Finally, the prospect of using functional graphene nanostructures to store, convert and manage energy will be discussed.

July 9, 2008

"Three Short Stories about Gold Nanorods," Cathy Murphy, University of South Carolina, hosted by Xiao-Min Lin

Abstract: Our research group has developed synthetic methods to make gold nanorods that are monodisperse in size and shape. The structure-directing agent we use is cetyltrimethylammonium bromide (CTAB) which forms a bilayer on the gold nanorod surface.

  • In Short Story 1, "Outside and Inside," I will describe experiments in which we coat the outside of the CTAB bilayer to introduce chemical functionality to the nanorods on the outside, and also use the CTAB bilayer to take up hydrophobic molecules from aqueous solution to the inside of the bilayer, which has interesting environmental implications.
  • In Short Story 2, "Cells and Gels," I describe experiments in which we incubate living cells, either in supported on a film or in a collagen gel, with nanorods in order to determine effects on the cells (toxicity, changes in gene expression, etc.).
  • In Short Story 3, "The Estuary," I describe experiments in which we introduce gold nanorods into model coastal ecosystems and measure (by ICP-MS) how the gold is distributed in water, sediment, plants and animals.

June 25, 2008

"Advancing the Function of 3D Photonic Crystals through Materials Chemistry," Paul Braun, University of Illinois at Urbana-Champaign, hostd by Yugang Sun

Abstract: Three-dimensional photonic crystals have been of interest for some time. However, simple photonic crystals, such as those formed through colloidal crystallization of polymers or silica microspheres, have generally limited application. Most photonic applications, including solar energy harvesting, lasers, waveguides, and chemical sensors, require the structures to be formed of optically active materials, often materials with high refractive index or metallic materials, and may also require the incorporation of aperiodic defect structures within the periodic structure of the photonic crystal. We have made significant strides in both expanding the materials available for forming photonic crystal through various templating schemes and adding functional defect structures within photonic crystals. Through electrochemical routes, metal inverse opals were formed that may provide the functional element for a new class of solar cells. We found that simple electrochemical infilling of a colloidal crystal followed by removal of the colloidal template was not sufficient to form an optically interesting structure. It was only after the filling fraction of the metal was reduced by a subsequent electropolishing step that optical properties became three-dimensional in nature. High-refractive-index photonic band-gap crystals containing embedded three-dimensional waveguides were formed through a combination of multiphoton polymerization, atomic layer deposition, and chemical vapor deposition. The optical transmission through these waveguides was measured in the near-infrared. Both pH- and glucose-responsive photonic crystal-based sensors were generated through photopolymerization of a chemical responsive hydrogel within a colloidal crystal. In all thes systems, complete optically and structural characterization was performed. Alternative (noncolloidal) routes to photonic crystals, including ink-based direct writing and holographic routes will also be discussed. As well as being interesting means to form photonic crystals, these techniques are also proving powerful in synthesizing nonspherical colloids that may assemble into unique structures.

June 11, 2008

"Semiconductor spintronics (Just the facts)," Nitin Samarth, The Pennsylvania State University, hosted by Anand Bhattacharya

May 28, 2008

"The Spin on Electronics!," Stuart Parkin, IBM Almaden Research Center, hosted by Matthias Bode

Abstract: Today, nearly all microelectronic devices are based on storing or flowing the electron¹s charge. The electron also possesses a quantum mechanical property termed "spin" that gives rise to magnetism. Electrical current is comprised of "spin-up" and "spin-down" electrons, which behave as largely independent spin currents. The flow of these spin currents can be controlled in thin-film structures composed of atomically thin layers of conducting magnetic materials separated by nonmagnetic conducting or insulating layers. The resistance of such devices, so-called spin valves and magnetic tunneling junctions, respectively, can be varied by controlling the relative magnetic orientation of the magnetic layers, giving rise to magnetoresistance tailored for different applications. Recent advances in generating, manipulating, and detecting spin-polarized electrons and electrical current make possible new classes of spin-based sensor, memory, and logic devices, generally referred to as the field of spintronics. In particular, the spin valve is a key component of all magnetic hard-disk drives manufactured today and has enabled their nearly thousandfold increase in capacity over the past eight years. The magnetic tunnel junction allows for a novel, high-performance random-access solid-state memory that maintains its memory in the absence of electrical power. The respective strengths of these two major classes of digital data storage devices, namely the very low cost of disk drives and the high performance and reliability of solid-state memories, may be combined in the future into a single spintronic memory-storage technology, the magnetic Racetrack. The Racetrack is a novel three-dimensional technology that uses nanosecond long pulses of spin polarized current to move a series of magnetic domain walls along magnetic nanowires.

May 19, 2008

"Incorporating Abiological Function in de Novo Designed Proteins," H. Christopher Fry, University of Pennsylvania, hsoted by Tijana Rajh

Abstract: The de novo design of helical bundles capable of binding natural heme complexes is well established. Extending this knowledge to include abiological metalloporphyrin-based chromophores will enhance the utility as well as test the reliability of computationally derived proteins. Success has been found in the design and characterization of a heterotetrameric a-helical bundle (AHis:BThr) that binds the photo-activatable, abiological chromophore, zinc diphenylporphyrin (ZnDPP). In addition, the incorporation of an abiological, nonlinear optic chromophore ((porphinato)zinc-ethyne-(terpyridyl)ruthenium complex, RuPZn) into a computationally designed, asymmetric single-chain helix bundle (SC_RPZ) exploits the natural helical chirality potentially enhancing and macroscopically organizing the complex of interest.

May 14, 2008

"Probing molecular-level organizational structure and electronic properties of weakly surface bound metallic nanoparticles, chiral domains, and single biomolecules," Thomas Pearl, North Carolina State University, hosted by Seth Darling

Abstract: Mechanisms of adsorption and organization of organic molecules on metallic surfaces play a significant role in the growth of chemically and electronically tuned, monolayer thin films. Intercommunication between functional groups for individual adsorbates can serve as the primary driving force for monolayer crystallinity as well as electronic structure especially in the limit of weak interaction between the adsorbate and substrate. In this talk, I will present a series of examples involving weakly bound surface species probed with high-spatial-resolution scanning tunneling microscopy (STM) and spectroscopy. As a first example, data will be discussed regarding spectral diffusion features for ligand encapsulated Au11 nanoparticles supported and isolated on alkanethiolate monolayers. The bulk of the work presented will involve submonolayer ordering of a chiral molecule, tartaric acid (C4H6O6), weakly bound to an achiral metal surface, Ag(111), as studied with low-temperature STM and density functional theory. Molecularly resolved images of enantiomerically pure (R,R)- and (S,S)-tartaric acid domains on Ag(111) will be presented, and the role of intermolecular hydrogen bonding in stereospecific domain and superlattice formation will be addressed. Additionally, we will consider chiral domain formation and phase separation from a racemic mixture of tartaric acid enantiomers. Lastly, we will present differential conductance mapping of tartaric acid molecular domains that highlight an intrinsic decoupling of molecular film electronic states with respect to the metallic lattice. While the chiral expression that drives the formation of enantiomeric domains does not induce stereospecific conductance, we demonstrate electronic differentiation of submonolayer organic domains from the Ag(111) surface. Density functional theory calculations will be discussed as they relate to both the molecular organization as well as the deconvolution of electronic structure between the molecular film and the metallic substrate. Finally, I will also highlight recent work in our group involving the study of functionalized, single- and double-stranded DNA molecules anchored to both metallic and ferroelectric surfaces.

April 30, 2008

"Nanoscale assembly using conditions far from equilibrium," Heinrich Jaeger, University of Chicago, hosted by Seth Darling

Abstract: Far from equilibrium conditions open up a range of new possibilities for forming structures on the nanometer scale. This talk will explore two situations where such conditions can be utilized to self-assemble nanoparticles. The first deals with the selective metal decoration of diblock copolymer scaffolds, while the second concerns drying-mediated nanoparticle assembly. Both situations deal with structures in the range of 5 to 50 nm that is difficult to tackle with conventional approaches.

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