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

Archive: Seminars 2007

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December 14, 2007

"Tailoring the Plasmonic Properties of Silver and Gold Nanostructures through Shape-Controlled Synthesis," Younan Xia, Washington University, hosted by Matt Pelton and Yugang Sun

Abstract: By controlling the presence of twin defects in silver nanocrystallite seeds, we can selectively grow them into pentagonal nanowires, nanocubes with controllable corner truncation, right bipyramids, or triangular plates. The key to removal of twinned particles is oxidative etching. Without oxygen in the reaction, fivefold twinned decahedrons preferentially form because of their lower surface energy, and these seeds grow anisotropically into pentagonal nanowires. With the addition of chloride and oxygen, twinned particles are etched away, leaving only single crystal seeds that grow to form nanocubes. If bromide is added in place of chloride, an intermediate level of etching results in formation of seeds with a single twin that grow to form right bipyramids. The symmetry of plasmon resonance within each of these nanostructures is distinct, and thus each has a unique spectral signature that may find application in multiple-analyte surface plasmon resonance sensors. We can further tune the spectral properties of silver nanostructures through a galvanic replacement reaction that converts them into hollow gold nanostructures. Gold nanocages obtained from silver nanocubes have extremely high extinction coefficients in the near-infrared (800-1000 nm, transparent window for soft tissues). For this reason, we functionalized gold nanocages with tumor antibodies to enable contrast-enhanced imaging of cancer tissue and photothermal destruction of tumor cells.

December 12, 2007

 

A tightly regulated, information-dependent molecular motor based upon T7 RNA polymerase,” William T. McAllister, University of Medicine and Dentistry of New Jersey, hosted by Elena Rozhkova

Abstract: Controlled movement of materials or molecules within the nanometer range is essential in many applications of nanotechnology. Here we report the capture, movement, and release of cargo molecules along DNA by a modified form of T7 RNA polymerase (RNAP) in a manner that is controlled by the sequence of the DNA. Using single-molecule methods, we visualize the assembly and manipulation of nanodevices and the ability to harness rotary and linear forces of the RNAP motor.

December 7, 2007

"Fundamental Insights and Improvements in Solar Cells from Theoretical Calculations," Jeffrey Grossman, Berkeley Nanosciences and Nanoengineering Institute, hosted by Larry Curtiss

Abstract: Classical and quantum mechanical calculations are employed to understand important microscopic mechanisms in photovoltaic materials and interfaces.

Our goal is to predict key properties that govern the conversion efficiency in these materials, including structural and electronic effects, interfacial charge separation, electron and hole traps, excited state phenomena, and band level alignment. An overview of our work will be presented, with focus given to two different types of solar cells, based on polymer blends and amorphous silicon, that illustrate how these computational approaches can improve our understanding and lead to more efficient devices.

December 5, 2007

"Interface Effect on the Activity of Metal/Oxide and Catalyst Searching for Multistep Reactions using DFT Simulation," Tao Song, University of California San Diego, hosted by Jeffrey Greeley

Abstract: NO dissociation at the interfaces of metal/Al2O3 (metal = gold, silver, and copper) and on the corresponding pure metal surfaces is studied to obtain the physical origin of the metal/oxide interface effect using density functional theory calculations. It is found that: (i) the barriers are significantly reduced at the interfaces of metal/Al2O3, (ii) there is a linear relationship between the reaction barrier at the interface and the total chemisorption energy, and (iii) the significant decrease in reaction barrier at the interface is mainly due to an increase in oxygen chemisorption energy at the interface. The anomaly of Ag/Al2O3 and the general implications of the results on metal/oxide system are discussed.

To find good catalysts for yield efficient processes is a major task in physical science. Up to now, new catalysts have been searched by utilising try-and-error methods experimentally, despite a huge effort spent to discover an approach using which a good catalyst for a particular reaction can be identified. A fundamental discovery in heterogeneous catalysis regarding the catalyst search and its understanding is the volcano curve, and it is traditionally explained by the Sabatier¹s principle. However, the principle remains largely empirical. Here we investigate all the relevant elementary steps for a key catalytic reaction, ammonia synthesis, from hydrogen and nitrogen on the entire 4d transition metal series. Our first principles volcano curves, which are obtained without any key assumptions agree with experiment. A simple equation containing only a few thermodynamic properties, using which the volcano curves can be reproduced, is derived.

This method may lead to better catalyst search strategies.

December 3, 2007

“Electronic Transport through Single-Molecule-Based and -Controlled Devices using Metal-Molecule-Metal Structures,” Ajit K. Mahapatro, Purdue University, hosted by Seth Darling

Abstract: In this work, we studied electronic transport through various organic/DNA molecules in a metal-molecule-metal (MMM) device structure using newly developed techniques for fabrication of molecular devices at micro- and nanometer dimensions and demonstrated potential nanoelectronics and sensor applications.

An enabling technology has been developed for realizing atomically flat gold surfaces by e-beam evaporation of gold over oxidized silicon (SiO2) substrates using an organic adhesion. High-resolution scanning tunneling microscope (STM) topography reveals atomically flat surface and gold atomic arrangements in face -centered-cubic (111) and hexagonal closed packed (111) domains. These gold layers have been further used in fabricating single- or few-molecule contained nanogap molecular junctions (NMJs) and molecular monolayer based large area (≈ 100 µm2) molecular devices (LAMDs).

Electronic transport properties of various π-bonded and diruhenium contained redox-active organic molecules were studied using STM and NMJ techniques. The energy states for the molecular orbitals were estimated from the observed conductance peaks. Devices with evidence of molecular states close to the contact Fermi level would allow resonant tunneling. Devices employing redox-active molecules, which allow resonant tunneling, along with observation of the molecules in specific charge states, could provide suitable structures for memory or chemical-sensing applications. Sequence specific electronic conduction through short (15 base pairs) double-stranded DNA molecules in NMJ structure infers an exponential decrease in single DNA conductance values with the length of A:T pairs replacing G:C pairs at the center of a G:C rich strand, consistent with a barrier at the A:T sites. Junction conductance of LAMDs containing n-alkanedithiol SAMs scales linearly with the device area and decreases exponentially with increasing number of carbons in the chain.

This study demonstrates realization of molecular electronics at micro/nano-meter scale device dimension. Self-organization and electronic properties of various organic/bio molecules could be characterized and tested using currently developed test beds (atomically flat gold layer, NMJ and LAMD) for specific utilization in nanoelectronic circuitry and biosensor applications.

November 30, 2007

"Atomic Structure and Domain Specific Chemical Reactivity on Ferroelectric Oxide Surfaces," Dongbo Li, University of Pennsylvania, hosted by Matthias Bode

Abstract: Although ferroelectric oxides have been extensively studied over the last five decades, surface structures and properties have received relatively little attention as a consequence of a variety of experimental challenges. The fundamental aspects of reactions on ferroelectric surfaces are critical to a range of device applications. This talk will present our recent studies on the atomic structure of a model ferroelectric surface, BaTiO3 (001), and polarization-dependent surface reactivities on BaTiO3 and lead zirconate titanate (PZT). For a surface structure study on BaTiO3 (001), we have used STM, nc-AFM, LEED, and AES to show that this surface adopts a family of reconstructions depending on thermochemical history. Most of these structures have not been observed previously.

Using a combination of density functional theory and ab initio thermodynamics, we compute the surface phase diagrams with oxygen potential as a dependent variable, since this is a critical variable in the experiment. The stabilities of reconstructions with Ba-adatoms, O-vacancies and Ti-O clusters are compared for different thermochemical conditions. Comparisons of the results from the calculations with the STM and nc-AFM results are used to construct atomic models for the reconstructed surfaces.

For domain-specific chemical reactivity studies, we have used an in situ method to control domain orientation and quantify the effect of polarization on CO2 molecular adsorption on BaTiO3 and PZT surfaces. Using a conductive AFM tip, we pole the ferroelectric substrates such that submicron-sized out-of-plane domains terminate the surfaces. Surface potentials of these positive and negative domains are then measured by a frequency modulation scanning surface potential microscopy. The influence of domain polarization on molecular adsorption is examined by comparing surface potential variation as a function of CO2 exposure. Positive and negative domains have exhibited quantitatively different variations in surface potential. The differences are discussed in terms of possible adsorption mechanisms. The effect of defects is examined by comparing results on BaTiO3 single crystals before and after UHV annealing to produce oxygen vacancies.

October 19, 2007

"Enhancing and Tailoring the Optical Responses of Metallic Nanostructures," Stephen Gray, Argonne National Laboratory, hosted by Peter Zapol

Abstract: Optical interactions with metallic nanostructures are of interest in part because it may be possible to excite and manipulate surface plasmons in them. Surface plasmons are electronic excitations confined near metallic surfaces that may be useful in a variety of applications, including chemical and biological sensing, and optoelectronics. Since plasmons can be intense and localized, correctly describing their behavior in complex systems requires numerically rigorous modeling techniques. I discuss recent results of electrodynamics modeling aimed at learning how to enhance or tailor the optical responses of metallic nanostructures. Two recent studies are highlighted. In one study, it is shown how the propagation lengths of surface plasmon polaritons (SPPs) on thin metal films can be significantly increased by coupling SPPs into waveguiding structures. In the other study, it is shown how to design arrays of subwavelength holes in thin metal films such that their transmission spectra are especially sensitive to substrates with specific refractive index values. A special resonance involving both a Wood's anomaly diffraction effect and an SPP is responsible for this enhanced sensing capability. Finally, I indicate future research directions.

October 19, 2007

“Simple Models for Molecular Transport Junctions,” Michael Galperin, Los Alamos National Laboratory, hosted by Eric Isaacs

Abstract: We review our recent research on role of interactions in molecular transport junctions. We consider simple models within a nonequilibrium Green function approach in the steady-state regime. An important feature of molecular junctions is the role played by nuclear motions in the conduction process.

We start by reviewing approaches used to describe limits of weak and strong electron-phonon interactions. We treat the weak interaction case in a self-consistent Born approximation. The scheme is used to describe features observed in the inelastic electron tunneling spectroscopy. We treat the strong electron-phonon coupling within nonequilibrium equation-of-motion (EOM) method coupled with linked cluster expansion. The approach is useful in describing resonant inelastic tunneling.

Electron-electron interaction is responsible for effects such as Coulomb blockade in molecular junction transport. We consider electron-electron and electron-phonon interactions within Hubbard-Holstein Hamiltonian. considerations similar to the equilibrium EOM approach by Meir et al. are used on the Keldysh contour to account for the nonequilibrium nature of the junction, and dressing by appropriate Franck-Condon factors is used to account for vibrational features. Results of the equilibrium EOM scheme by Meir et al. are reproduced in the appropriate limit. Strong electron-vibration interaction may lead to formation of polaron on the bridge. We propose a polarization model as a possible mechanism for the negative differential resistance and hysteresis observed in molecular junctions. The consideration is based on the static limit of the Born approximation. We discuss theimportance of taking into account the open character of the system. Electron-vibration interaction is the cause of junction heating. We obtain a unified description of heating in current-carrying molecular junctions as well as electron and phonon contributions to the thermal flux, including their mutual influence. Ways to calculate these contributions, their relative importance and ambiguities in their definitions are discussed. A general expression for the phonon thermal flux is derived and used in a new “measuring technique,” to define and quantify ‘local temperature’ in nonequilibrium systems. Finally, in our ongoing project, we use a two-level (HOMO-LUMO) model to study Raman scattering of current-carrying molecular junctions.

October 12, 2007

“Controlling Magnetic Anisotropy and Probing Magnetic Structure in Magnetic Nanoparticles and Ferromagnetic/Antiferromagnetic Bilayers,” Minn-Tsong Lin, National Taiwan University, hosted by Matthias Bode

Abstract: Controlling magnetic orientation and imaging magnetic structure are two crucial issues in both fundamental science and application for magnetic nanomaterials. In particular, tuning perpendicular magnetic anisotropy by a more concise and efficient process draws much attention because of the possible application of a perpendicular medium with high-storage density. In this work, an enhanced perpendicular magnetic anisotropy of ferromagnetic thin films is demonstrated by introducing an antiferromagnetic (AF) underlayer. A new kind of spin-reorientation transition is observed with varying thickness of the AF layer. This finding is shown to be related to the strength of the AF coupling of the AF layer.

Controlling the magnetic anisotropy can be also important in magnetic domain imaging with in-plane sensitivity by spin-polarized scanning tunneling microscopy (SP-STM). A simple method using a ring-shaped magnetically coated wire as the tip of SP-STM is shown to be able to show spin contrast easily in the in-plane direction of the film. A well-defined magnetization orientation of magnetic tip is achieved with controlled anisotropy caused by geometrical asymmetry.

Finally, magnetic coupling and magnetic structure in magnetic self-aligned iron particles grown on a single-crystalline oxide layer Al2O3/NiAl(100) will be also discussed. By using scanning electron microscopy with polarization analysis (SEMPA), the magnetic domain is imaged, revealing a vortex structure, which may be attributable to a dipole-dipole interaction. Furthermore, capping the magnetic particles with a nonmagnetic metallic layer (copper) can enhance the magnetic coupling, and in turn the Curie temperature of the system. This finding can also be confirmed in the enhanced spin contrast observed by SEMPA for magnetic particles with capping layer. The magnetic coupling under magnetic particles is shown to be able to propagate through the copper layer.

October 10, 2007

"Bottom-Up Fabrication as a Route towards Unique and Multifunctional Nanostructured Materials: From Biomimetics to Ultrastrong Nanocomposites," Paul Podsiadlo, University of Michigan, hosted by Tijana Rajh and Elena Shevchenko

Abstract: Layer-by-layer (LBL) assembly technique, based on sequential adsorption of oppositely charged compounds, is one of the most popular and well-established methods for the preparation of multifunctional nanostructured thin films. Since its inception nearly two decades ago, a wide variety of species, ranging from polymers and biomolecules, to nanoparticles and viruses, have been successfully used as assembly components. This remarkable versatility has led to a number of novel designs and applications, including: superhydrophobic surfaces, chemical sensors and semi-permeable membranes, drug and biomolecules delivery systems, optically active and responsive films, cell and protein adhesion resistant coatings, fuel cells and photovoltaic materials, biomimetic and bio-responsive coatings, semiconductors, catalysts, and magnetic devices.

Recently, our group has also shown preparation of architecturally and mechanically unique LBL nanocomposites incorporating carbon nanotubes and clay nanosheets. Assembly of negatively charged clay platelets of Na+-montmorillonite (MTM) with a polycation, Na+poly(diallyldimethylammonium chloride) (PDDA), resulted in a nanocomposite with very high loading of uniformly distributed MTM nanosheets (~70 wt.% of clay) organized into a well-defined layered architecture. Further, the structure, deformation mechanism, and mechanical properties (ultimate tensile strength, σUTS ~100 MPa and Young’s modulus, E ~11 GPa) of the material were found to be similar to those of two exceptional natural composites: seashell nacre and lamellar bones.

In this talk I will present my results from the exploration of mechanical properties of the LBL assembled nanocomposites from MTM nanosheets and another natural nanomaterial: cellulose nanocrystals, with an ultimate goal of developing high strength, high stiffness, and tough nanocomposites. I will show that understanding of the nanoscale mechanics and the interfacial interactions is a key to preparation of high-performance nanocomposites. To this end we have been able to generate a transparent clay nanocomposite with record-high strength and stiffness: σUTS ~400 MPa and E’ ~110 GPa, which on the per-weight basis can be compared to steel and its alloys. I will further discuss my ongoing work with different designs of the LBL nanocomposites and their applications.

October 3, 2007

"Chemical- and Bio-Inspired Route to Functional Nanostructures: Synthesis, Preparations, and Some Applications," Yongdong Jin, University of California, Los Angeles, hosted by Yugang Sun and Gary Wiederrecht

Abstract: Functional metallic (or organic) nanoparticles , nanostructures, hybrids, and composites are being exploited for a variety of applications, including sensing, catalysis, and electronics. This presentation focuses on the wet chemical- and bio-inspired route to functional nanostructures, and some interesting applications will be discussed. Examples include

  1. A wet-chemical and colloidal approach for preparing surface plasmon resonance active gold substrates;
  2. One-pot synthesis of shell-type Ag-Au bimetallic nanoparticles (and their fractal monolayer films) with nano-spikes, based on colloid seed-engaged replacement reaction and colloid-mediated deposition reactions;
  3. An electrochemical strategy to nanoparticle-based catalyst design using the underpotential deposition redox replacement technique,
  4. Vesicle or protein cage-templated synthesis of functional core and shell nanostructures;
  5. Proton pumping protein, Bacteriorhodopsin-based metal-protein-metal planar junctions using biomimetic monolayer preparation; and
  6. Plasmonic gold nanoparticles based monolayer junctions and enhancement in current transport through nanometer-scale insulating layers.

September 14, 2007

"Studying a single Kondo atom in a precisely known anisotropic environment," Sander Otte, Leiden University, hosted by Matthias Bode

Abstract: Using a 3He STM, we employ a new technique called spin excitation spectroscopy to study the magnetic properties of single d-metal atoms( manganese, iron, and cobalt) on a thin insulating copper nitride layer. This surface provides a well-defined anisotropic environment that partly breaks the degeneracy of the spin states even in the absence of an external magnetic field. For cobalt, this results in an effective S = 1Ž2 system that exhibits Kondo behavior. We show that the splitting of the Kondo peak, though in itself a multibody effect, is dictated by the quantum mechanics of the single spin. By performing atom manipulation we can let a Kondo spin interact with other spins and tune their coupling strength, opening the way to building Kondo chains and lattice.

August 8, 2007

“Carbon Nanostructures – New Opportunities in Material Science,” Dirk Guldi, Friedrich-Alexander-Universität Erlangen-Nürnberg, hosted by Tijana Rajh

July 11, 2007

“Near 100% Spin Filtering in Europium Oxide,” Tiffany S. Santos, Massachusetts Institute of Technology, hosted by Anand Bhattacharya

Abstract: Essential to the emergence of spin-based electronics is a source of highly polarized electron spins. Conventional ferromagnets have at best a spin polarization P~50%. Europium monoxide (EuO) is a novel material capable of generating a highly spin-polarized current when used as a tunnel barrier. EuO is both a Heisenberg ferromagnet (Tc=69 K) and a semiconductor. Exchange splitting of the conduction band creates different tunnel barrier heights for spin-up and spin-down electrons, thus filtering the spins during tunneling. High-quality EuO films at the monolayer level are necessary for efficient spin-filtering. Because nonferromagnetic, insulating Eu2O3 forms more readily, growth of an ultrathin, high-quality film is quite challenging. For this reason, EuO on such a small thickness scale had never been studied previously.

Films of EuO were grown by reactive thermal evaporation, and various thin film characterization techniques were employed to determine the structural, optical, and magnetic properties, even on the thickness scale needed for tunneling (<3 nm). A reduction in Curie temperature for ultrathin EuO was experimentally demonstrated, in agreement with theoretical calculation. Controlling the smoothness and chemical nature of the interfaces between EuO and metallic electrodes was found to be of critical importance, as proven by careful interfacial chemical and magnetic analysis at the monolayer level, using X-ray absorption spectroscopy, magnetic circular dichroism, diffuse X-ray resonance scattering, and polarized neutron reflectivity techniques, through collaborative efforts.

EuO was successfully prepared as the barrier in Al/EuO/Y tunnel junctions. By fitting the current-voltage characteristics of these junctions to tunneling theory, exchange splitting in an ultrathin layer of EuO was quantitatively determined for the first time, indicating near-complete spin filtering, P=100%. In an alternative approach, P was directly measured using the superconducting aluminum electrode as a spin detector. Spin filtering in EuO barriers was also observed in magnetic tunnel junctions (MTJs), using a ferromagnetic electrode as the spin detector. In Cu/EuO/Gd MTJs, a tunnel magnetoresistance (TMR) of 280% was measured by changing the relative alignment of magnetization of EuO and gadolinium, which is the largest TMR measured by using a spin-filter barrier. Co/Al2O3/EuO/Y junctions, in which the Al2O3 barrier magnetically decoupled cobalt and EuO, also showed substantial TMR. Its matching band gap (1.1 eV) and compatibility with silicon open up the novel possibility of using EuO to inject highly polarized spins into silicon-based semiconductors.

June 29, 2007

“Intermittent fluorescence from CdSe core and core-shell nanorods,” Catherine Crouch, Swarthmore College, hosted by Matthew Pelton

Abstract: One intriguing feature of the fluorescence observed from a wide variety of single fluorophores is intermittency, colloquially called “blinking.” Under steady excitation, single fluorophores do not emit light steadily, but switch between bright and dark states, remaining bright (“on”) or dark (“off”) for milliseconds to minutes at a time. The mechanism of this intermittency is still poorly understood in semiconductor nanocrystals. In particular, the durations of “off” periods observed from NCs follow a power-law distribution rather than the exponential distribution that would be expected from transitions between a single “on” state and a single “off” state. This talk will present our measurements of fluorescence intermittency measured from single CdSe nanorods of seven different sizes with aspect ratio ranging from 3 to 11 and compare our findings with measurements on spherical CdSe core and CdSe/ZnS core/shell nanocrystals, to explore the effect of sample geometry on intermittency. Our findings are consistent with the increasingly one-dimensional nature of excitonic states in nanorods as the aspect ratio increases.

June 27, 2007

"Shaping the Future of Desalination and Water Treatment: the Union of Nanotechnology and Membranes," Eric M.V. Hoek, University of California, Los Angeles, hosted by Ron Faibish

Abstract: The basic reverse osmosis (RO) membrane technology that has revolutionized desalination is now more than 30 years old. Optimal separation performance, energy efficiency, and fouling resistance of conventional polymer membranes are nearly fully realized, but RO processes remain relatively nonselective, energy-intensive, and fouling-prone. These constraints remain in the face of rising worldwide demand for clean water and the sustainability imperatives to control energy use. However, the "age of nanotechnology" has brought forth entirely new classes of functional materials that can be explored for use in desalination as well as other water treatment applications.

I will review the history of RO membrane development and relevant aspects of nanotechnology and discuss examples of how the future of desalination may be shaped from the union of nanotechnology and membranes. Specifically, I will present preliminary results from our efforts to create of a new class of RO membranes through use of nanotechnology. Low-energy and fouling-resistant membranes already have been created and tested in the laboratory at UCLA. Eventually, nanotechnology may produce membranes with advanced functionality like on-demand tuning of rejection and catalytic reactivity as well as sensing, antifouling, and regenerative interfaces.

June 21, 2007

"Field-Resolved Femtosecond Vibrational Spectroscopies of Solutions and Nonequilibrium Studies of Anisotropy in Protein Unfolding,” Rene Nome, The James Franck Institute, The University of Chicago, hosted by Norbert Scherer

Abstract: In Part I, solute and solvent femtosecond dynamics in the visible and infrared spectral regions are studied with full-electric field-resolved techniques. Resonant linear propagation of ultrashort mid-infrared pulses through optically dense samples of HDO in liquid D2O is studied experimentally and computationally. This combined approach was used to test the accuracy of various correlation functions in describing the fast vibrational dynamics of water. Using visible laser pulses, we have performed electric field-resolved transient grating measurements of a pyridinium iodide charge transfer complex. These experiments showed how the pump-probe delay may be used to directly control the magnitudes of resonant and nonresonant third-order polarizations. In Part II, we present a comprehensive study that integrates experimental and theoretical nonequilibrium techniques to map energy landscapes along well-defined pull-axis specific coordinates to obtain thermodynamic, kinetic, and structural information about protein unfolding. Single-molecule force extension experiments along two different axes of photoactive yellow protein (PYP) combined with nonequilibrium statistical mechanical analysis and atomistic simulation reveal energetic and mechanistic anisotropy in protein unfolding.

June 15, 2007

“High-Frequency EPR and ENDOR Spectroscopy on Semiconductor Nanocrystals,” Jan Schmidt, Leiden University, hosted by Tijana Rajh

Abstract: There is great interest in semiconductor nanocrystals because the electronic and optical properties of these structures are strongly affected by quantum confinement owing to the reduced dimensions of these systems. We have recently shown that ZnO nanocrystals can be doped with shallow donors by the introduction of interstitial lithium and sodium atoms. This finding opens up the possibility of studying the effect of quantum confinement on the electronic structure of these donors. I will show that high-frequency EPR and ENDOR spectroscopy at 95 GHz and 275 GHz is the method of choice to identify the atomic structure of these donors and to probe for the first time the effect of confinement on their electronic wave function.

June 14, 2007

“Magnetic reversal, magnetodynamics and imaging of magnetic nanostructures,” Justin M. Shaw, National Institute of Standards and Technology, hosted by Kristen Buchanan

Abstract: Magnetic nanostructures are currently receiving much attention due to their potential use in “beyond CMOS” technology, sensors, and media storage. In the current hard drive technology, information is stored in magnetic domains written to a continuous magnetic thin film. In order to increase storage densities beyond 1 Tbit/in2, the size of these domains must be reduced below 12 nm. In thin-film media, lower exchange-coupled systems must be used to prevent neighboring bits or domains from interacting. However, these lower exchange-coupled materials also have lower thermal stability, and at a critical grain size, they become superparamagnetic at room temperature and can no longer store information. A proposed solution to this problem is patterned bit media, whereby information is stored in nanopatterned single-domain structures or islands that are laterally isolated. Because the structures are physically isolated, highly exchange-coupled materials can be used that have significantly more thermal stability. However, when these magnetic layers undergo nanopatterning, a switching field distribution (SFD) results whereby each nanostructure magnetically reverses at a different value of applied magnetic field. The fundamental source of the SFD and understanding the reversal mechanisms in these nanostructures have been and continue to be a large effort. We use several magnetic imaging techniques, such as magnetic force microscopy, Lorentz microscopy, and electron holography, to probe the “nanomagnetic” structure as well as high-resolution transimission electron microscopy. The combination of magnetic imaging with materials characterization allowed us to isolate the source of switching field distributions in Co/Pd nanodots. Although still under investigation, this source is found to reside in the intrinsic material properties of the Co/Pd and is not due to lithographic variations or other extrinsic sources as previously thought. In addition, temperature-dependent studies indicate that the reversal mechanism is not that of coherent rotation as predicted.

The increased dimensional confinement of magnetic nanostructures also has a profound effect on the high-frequency (ferromagnetic resonance) behavior of the structures. Direct measurement of such properties will require new methods and techniques as many traditional techniques do not have the signal-to-noise and/or spatial resolution needed. We have recently developed a new frequency resolved magneto-optic Kerr effect system to study ferromagnetic resonance in our magnetic nanostructures. We have demonstrated excellent signal-to-noise in nanodot arrays in 3-nm-thick magnetic layers down to 50 nm in diameter. With this technique we can directly measure the ferromagnetic resonance frequency, damping, dynamic inhomogeneity, edge effects, and (more indirectly) interdot interactions.

In this talk, I will present our recent results on magnetic reversal, ferromagnetic resonance and magnetic (and non-magnetic) imaging of both in-plane and perpendicularly magnetized nanopatterned structures. Emphasis will be on understanding sub-100-nm features which are required in patterned bit media and spin-momentum transfer devices. I will also discuss the new techniques we developed to study the magnetic properties of such structures.

June 13, 2007

“Spin-torque induced dynamics and switching behavior in spin-valve devices,” Michael Schneider, National Institute of Standards and Technology, hosted by Kristen Buchanan

Abstract: The state of a magnetic device is commonly manipulated using an applied magnetic field. However, in 1996 it was predicted that a magnetic device could also be manipulated with a spin-polarized dc current. In particular, a spin-polarized current propagating into a ferromagnetic layer will exert a torque on that layer. This effect is often referred to as spin torque. It has been shown that the spin torque effect is an efficient way to change the orientation of the free layer in a spin valve nanopillar when using a high current density perpendicular to the film plane. This effect has recently been demonstrated as a viable alternative to the cross-point writing scheme of conventional magnetic random access memory (MRAM). Spin torque switching of MRAM has the advantage that as the device size is reduced, the current needed to switch the free layer orientation is decreased.

We investigated the influence of thermal effects on the critical switching current in spin torque nano-pillars. We compared zero temperature critical current values extrapolated from room temperature pulsed current switching measurements to those of quasi static current sweeps at 5 K. In addition, we compared both of these experimental results to zero temperature Slonczewski theory. There is substantial variation in the hysteretic region from device to device at room temperature for devices of the same nominal size and resistance. While this is not expected, it has been attributed to thermal effects having a strong influence on the response of the freelayer to applied field as well as the coercivity. We find that by reducing the temperature, and thus any thermal fluctuations, the device to device variations are drastically reduced. Furthermore, the values extrapolated from the low temperature measurements were robust with respect to device size and in quantitative agreement with theoretical predictions from Slonczewski theory.

In addition to complete switching of the orientation of the free layer in a spin valve device, the spin torque can be used to counteract the damping torque and thus set up sustained microwave oscillations of the magnetic moment in the free layer. There are two distinct regions of microwave oscillations that can be accessed with this method. The first, and best characterized, is consistent with uniform magnetization precession similar to ferromagnetic resonance oscillations. These oscillations can occur at frequency from several GHz to more than 40 GHz. In addition to uniform mode oscillations, we also observe lower frequency oscillations with small in-plane applied fields. Unlike previous measurements, the frequency of oscillation is far below the uniform-mode ferromagnetic resonance frequency and is only a weak function of applied field. These oscillations can be hysteretic with applied dc bias current; once they are “turned on” they remain active at currents below the initial current necessary to instigate the oscillations. These observations are consistent with dynamics of a vortex-like state in the vicinity of the contact, one nucleated by the Oersted fields generated by the dc current and with dynamics driven by the spin transfer torque.

June 12, 2007

“Using Ab Initio Methods to Help Design Materials and Applications at the Atomic Scale: Importance of Resolving Energetic Differences Associated with Minor Changes in Geometry or Configuration,” Rees B. Rankin, University of Pittsburgh, hosted by Larry Curtiss and Jeffrey Greeley

Abstract : As the need for novel complex materials rises appreciably every year, the ability to eventually develop fully predictive multiscale models often relies firstly and most importantly on the development of a comprehensive understanding of atomic-scale processes and their associated energies. This presentation will highlight the application of ab initio DFT methods in two distinct research problems: the first involving design of multicontaminant sorbent material for remediation of IGCC syngas streams, the second involving control of chiral adlayer assembly in amino acids with reconstruction of copper surfaces. In both cases, it will be shown that the use of high-level calculations at this scale allows for the resolution of extant questions that were previously generated from experimental observations regarding the energetics of nonequivalent structures. In both cases, the critical fundamental result will be that even seemingly minor changes in geometry and/or configuration of the materials in question can lead to interesting effects and exploitable differences in energy for potential applications.

May 30, 2007

“Quantum mechanics, classical mechanics and molecular dynamics on bio- and nanosystems,” Kurt Mikkelsen, University of Copenhagen, hosted by Larry Curtiss

May 22, 2007

“Study of Nanostructures and Surface Plasmons by Ultrafast THz-TDS,” Jiaguang Han, Oklahoma State University, hosted by Norbert Scherer

Abstract: I will intrroduce some of my present work in the study of nanostructures and surface plasmons by ultrafast terahertz lasers, such as terahertz dielectric properties and low-frequency phonon resonances of ZnO nanostructures; phonon confinement in ZnS nanoparticles; transition from photonic effect to surface plasmon resonance based on pump-probe measurement on subwavelength hole array; and surface plasmon sensors.

May 17, 2007

“Light-Induced Charge Separation across Bio-Inorganic Interface,” Nada Dimitrijevic, Chemistry Division, hosted by Tijana Rajh

Abstract: Rational design of hybrid biomolecule – nanoparticulate semiconductor conjugates enables coupling of functionality of biomolecules with the capability of semiconductors for solar energy capture, which can have potential application in energy conversion, sensing, and catalysis. The particular challenge is to obtain efficient charge separation analogous to the natural photosynthesis process. The synthesis of axially anisotropic TiO2 nano-objects, such as tubes, rods, and bricks, as well as spherical and faceted nanoparticles has been developed in our laboratory. Depending on their size and shape, these nanostructures exhibit different domains of crystallinity, surface areas, and aspect ratios. Moreover, in order to accommodate for high curvature in the nanoscale regime, the surfaces of TiO2 nano-objects reconstruct, resulting in changes in the coordination of surface titanium atoms from octahedral (D2d) to square pyramidal structures (C4v). The formation of these coordinatively unsaturated titanium atoms thus depends strongly on the size and shape of nanocrystallites, and they affect trapping and reactivity of photogenerated charges. We have exploited these coordinatively unsaturated titanium atoms to couple electron-donating (dopamine) and electron-accepting (pyrroloquinoline quinone) conductive linkers that allowing wiring of biomolecules and proteins, resulting in enhanced charge separation, which increases the yield of ensuing chemical transformations.

May 17, 2007

"Nanostructure Matters," Olle Heinonen, Seagate Technology, hosted by Larry Curtiss

Abstract: Bulk systems generally have well-defined physical properties, examples of which are conductivity, charge density, or magnetization density. These are macroscopic properties obtained by averaging over many microscopic volumes or configurations. When the characteristic dimensions of a system, such as system size or layer thickness, become equal to or smaller than characteristic length scales, such as mean free path, magnetic exchange length, or phase-breaking length, the physical properties may become distinctly different from those of a bulk system. In particular, the observable properties depend very critically on the material micro- or nanostructure.

In this talk, I will give some examples of the interplay between dimension, nanostructure, and physical properties for a variety of systems, as well as of the tools that can be used to study them theoretically. These latter include unusual flavors of density functional theory, semiclassical transport theory coupled with first-principle electronic structure calculation, and finite-temperature micromagnetics. The first example is given by quantum Hall dots. By changing the shape of the potential confining the electrons in these dots, the spin-charge texture of the edge can be modified. This is important because the behavior of these dots is controlled by the ground-state electronic structure of their edges. A second example is given by giant magnetoresistive spin-valve sensors. One can design quantum-mechanical properties that directly affect the magnetoconductance of these devices by carefully controlling the layer structures and interfaces. Another example is given by magnetic tunnel junctions. In these structures, there is a very delicate interplay between the nanostructure near the ultrathin insulating tunneling barrier and the conductance and thermal magnetic noise. The final example is given by bilayer Permalloy/IrMn disks. By setting the exchange bias of the IrMn in a vortex configuration, one can tune the dynamics of both the gyrotropic motion as well as the spin-wave eigenmodes by controlling the exchange bias strength.

April 19, 2007

“Magnetic Nanostructures Probed on the Atomic Scale,” Cyrus F. Hirjibehedin, IBM Almaden Research Center, Matthias Bode

Abstract: Magnetic nanostructures are increasing data storage capacities and are promising candidates for implementations of novel spin-based computation techniques. The relative simplicity and reduced dimensionality of nanoscale magnetic structures also make them attractive model systems for studying fundamental interactions between quantum spins. We used a scanning tunneling microscope to probe the interactions between spins in individual atomic-scale magnetic structures. Linear chains of 1 to 10 Mn atoms were assembled one atom at a time on a thin decoupling layer of copper nitride on bare copper. The spin excitation spectra of these structures were measured with inelastic electron tunneling spectroscopy. We observed excitations of the coupled atomic spins that can change both the total spin and its orientation. Comparison with a model spin interaction Hamiltonian yielded the collective spin configuration and the strength of the exchange coupling between the atomic spins. Anisotropy effects were directly manifested in the excitation spectra as finite energy excitations in the absence of a magnetic field. The effects of anisotropy were found to be relatively weak for Mn atoms but were substantially larger in atoms with strong spin-orbit coupling, such as iron.

 

April 13, 2007

“Multiscale Coarse-Grained Modeling of Condensed-Phase and Nanoscale Systems,” Sergey Izvekov, University of Utah, hosted by Peter Zapol

Abstract: The multiscale coarse-graining (MS-CG) method for condensed phase and nanoscale systems is presented. The MS-CG approach systematically maps atomistic interactions into effective pairwise interactions between coarse-grained structural units. The MS-CG method is based on a rigorous force-matching procedure to determine the generalized forces associated with the coarse-grained degrees of freedom. The resulting simulation of the coarse-grained representation of the system is much faster than its all-atom counterpart and while accurately reproducing structural and thermodynamic properties. The MS-CG method has been successfully applied to many complex condensed phase systems, including water, ionic liquids, carbonaceous nanoparticles, lipid bilayers, peptides, and various solvent-free models. The MS-CG method has also been recently used to study hydrophobic hydration and the hydrophobic effect. Future applications include studies of the mesoscopic structures formed by systems of biomolecules and nanoparticles.

April 12, 2007

“The Characterization, Processing, and Application of Hybrid Organic/Inorganic Nanomaterials,” Nathan Guisinger, Center for Nanoscale Science and Technology, NIST, hosted by Matthias Bode

Abstract: The study of organically functionalized semiconductor surfaces has become an active pursuit, both for its fundamental significance and technical relevance. This talk covers three topics relevant to the application of hybrid organic/inorganic nanomaterials: silicon-based molecular electronics, molecular charge transfer visualized at the atomic scale, and epitaxial graphene grown on SiC.

April 12, 2007

"Synthesis of Nanoparticles and Their Self-Assembly into Periodic Arrays," Elena Shevchenko, Lawrence Berkeley National Laboratory, hosted by Tijana Rajh

Abstract: Nanoparticles of different size and functionality (e.g., noble metals, semiconductors, oxides, magnetic alloys) can be synthesized by using a solution phase approach. Nanoparticle building blocks can self-assemble into ordered binary superlattices (also known as opals or colloidal crystals) that retain the size tunable properties of their constituents. Control over particle size and their monodispersity are key issues in the self-assembly process. Depending on nanoparticle type, different approaches to control their sizes, shapes, and monodispersity can be applied.

A variety of multicomponent superlattices from monodisperse nanoparticles nanocrystals can be obtained by mixing and matching these nanoscale building blocks to yield multifunctional nanocomposites. Superlattices with AB - AB13 stoichiometry with cubic, hexagonal, tetragonal, and orthorhombic symmetries have been identified. Assemblies with the same stoichiometry can be produced in several polymorphous forms by tailoring the particle size and deposition conditions. Electrical charges on sterically stabilized nanoparticles, in addition to such parameters as particle size ratio and their concentrations, can provide the formation of much broader pallet of binary nanoparticle superlattices as compared with the limited number of possible superlattices formed by hard noninteracting spheres. Design of more complex nanoparticle structures, such as dumbbells and hollow nanoparticles, allows extending the number of challenging multicomponent periodic systems. Multifunctional superlattices are presented as a new class of materials with a potentially unlimited library of constituents over a wide range of tunable structures.

April 6, 2007

“Pressure-induced Hydration,” Thomas Vogt, University of South Carolina, hosted by Eric Isaacs

Abstract: We have recently established that in some zeolites (i.e., zeolite Y, zeolite A, natrolite, mesolite, scolecite) a reversible and selective pressure-induced hydration to a "superhydrated phase" occurs at pressures near 1 GPa. Pressure-induced hydration involves a concomitant volume and pore expansion. In some cases, the framework topology permits an auxetic-like behavior responsible for this effect. This talk will present structural models of various systems exhibiting pressure-induced hydration and discuss some of the heuristic rules and potential applications of this effect.

March 28, 2007

“Bioinspired Biomimetic Systems for Advanced Materials,” Elena A. Rozhkova, The University of Chicago, hosted by Tijana Rajh

Abstract: Biologically occurring organometallic supramolecular architectures give priceless lessons for the design and synthesis of sophisticated bio-inspired materials and engineered systems. Oxidoreductases are enzymes found throughout nature that catalyze versatile biologically important reactions, such as oxidation of hydrocarbons, biosynthesis of secondary messengers and hormones, and reversible oxidation of molecular hydrogen (H2). In the first part of my talk, I will discuss some iron-containing systems such as hydroxylases P450, nitric oxide synthase, non-heme AlkB, and multidisciplinary methods of their studying — recombinant DNA/protein technique, synthetic modeling, and chemical substrates probing. Application of modern method — synthetic biology for engineering of bioinspired hydrogen-producing systems on the base of iron-only hydrogenase will be also discussed.

In the second part of this talk, I will focus on bioinorganic nanohybrid materials for biomedical applications. Such nanocomposites combine cells receptors recognition bio-functionality and certain physical property (e.g., magnetic, photosensitivity) for controlled cytotoxic effects. Resulting nano-bio-conjugates are designed to possess synergistically enhanced activity with high potential for combined photo, thermal, and immune cancer therapy.

March 2, 2007

"Magnetic and Structural Properties of Mass-Filtered Three-Dimensional Transition Metal Nanoparticles, " Armin Kleibert, Rostock University

Abstract

February 26, 2007

““Minima Hopping: An Efficient Way to Optimize Geometry on the Potential Energy Surface and on the Free Energy Surface,” Abdel Kenoufi, University of Basel, hosted by Jeffrey Greeley

Abstract: It is well known that mechanical properties of materials are strongly dependent on the way their geometries have been optimized on the potential energy surface. The study of shear band localization in bulk metallic glasses is one example of this phenomenon. The shear band width depends clearly on the cooling rate that has been defined to decrease the control temperature during the simulated annealing procedure. Therefore, before studying mechanical properties, one has to find a well-cooled geometrical configuration. The global optimization is achieved by minimizing the potential energy in configuration space.

I'll first present the concept and the flowchart of an adapted version of the minima hopping method, which is an efficient way to avoid inconvenient features of simulated annealing methods. The second part will be devoted to numerical comparisons of metallic glasses of minima hopping and simulated annealing methods, performed within the EMT framework. I will show that minima hopping is an efficient way to optimize complex systems such as BMG. In the third part, I'll introduce a version of minima hopping that involves calculations of forces with DFT methods, the dual minima hopping method.

February 21, 2007

"Superconductivity on the Localization Threshold and Superconductor-Insulator Quantum Phase Transition," Tatiana Baturina, Institute of Semiconductor Physics, Novosibirsk, Russia, hosted by Eric Isaacs and Valerii Vinokour

Abstract: The interplay between superconductivity and localization is a phenomenon of fundamental interest. Effects of disorder become critical in two dimensions, and the nature of two-dimensional disordered superconductors is currently the focus of theoretical and experimental attention. The main questions are the possible ground states of two-dimensional electronic systems with Cooper pairing and the structure of the disorder-magnetic-field phase diagram.

I review recent experimental progress and discuss how new experimental findings relate to theories modeling the influence of disorder and/or magnetic field on the superconductor to insulator quantum phase transition. I demonstrate that in the systems near the localization threshold on its superconducting side, applying a perpendicular magnetic field drives the films from a superconducting to an insulating state, with very high values of resistance. Further increase of the magnetic field leads to an exponential decay of the resistance towards the finite value. This value depends on temperature and, in the limit of low temperatures, extrapolates to the universal quantum resistance h/e2. Comparison of studies on different materials indicates that the quantum metallic phase that appears at high magnetic field is a generic property of two-dimensional superconducting systems close to the disorder-driven superconductor-insulator transition.

February 16, 2007

“Nucleic acid sequence is a universal information substrate of all life. Advancement of DNA sequencing technology is central to enabling its utilization,” Viktor Stolc, NASA Ames Research Center, hosted by Kevin White and Eric Isaacs

Abstract: NASA Ames Genome Research Facility is a state-of-the-art research laboratory designed to provide service in large-scale genome research to government, public, and private organizations and to advance the development of single-molecule sensitive biosensors through collaborative projects with academic and industry partners. In addition to the development of faster and less expensive DNA sequencing technologies, advances in the ability to assay functionality of genes and proteins on a genome-wide scale have revolutionized biological sciences.

In partnership with several universities, NASA Ames Genome Research Facility has developed novel applications of high-density DNA oligonucleotide microarrays for probing RNA expression of both protein- and non-protein-coding sequences in the completed genomes of several model organisms and the human genome. Computational methods for the design and analysis of the whole-genome tiling microarrays were developed by using the NASA Ames supercomputer. This work resulted in the confirmation of nearly all previous gene expression results generated with other techniques, and identified thousands of genes and RNA transcripts that were previously undescribed by all other methods.

In partnership with commercial partners, NASA Ames Genome Research Facility also has been developing a solid-state nanopore device for direct reading of DNA sequence from single molecules faster than is currently possible with any other method. Because individual molecules are counted, the output is intrinsically quantitative. The nanopore device will be able to directly sequence single molecules of nucleic acid, DNA, or RNA, without labeling or biochemical manipulation, at a rate of a million bases per second by electrophoresis of the charged polymers through a channel of molecular dimensions. The channel is a solid-state nanopore that can analyze electronic properties of DNA or RNA to obtain a linear composition of each individual genetic polymer molecule. The nanopore-based analysis of nucleic acid polymers is revolutionary because no other technique can determine information content in single molecules of genetic material at the speed of 1 subunit per microsecond. Significant advancement in the realization of this device has been made; currently it is able to measure the length of individual polynucleotides heteropolymers and distinguish the identity of individual subunits (A,C,G,T) in monomer form with single-molecule resolution using nanospectroscopy. Current advances in the nanofabrication process and utility of the solid-state nanopore suggest that in the next few years, ultra-rapid, direct, and single-molecule-sensitive DNA sequencing will radically change life sciences and medicine because no other technique can determine the complete sequence of the genetic information of life.

February 15, 2007

“Electronic Conducting States in Nano- and Mesoscale Molecular Devices,” Nikolai Zhitenev, Lucent Technologies, hosted by Eric Isaacs

Abstract: Organic materials can offer new electronic functionality not available in inorganic devices. However, the integration of organics within nanoscale electronic circuitry poses new challenges for material physics, chemistry, and nanofabrication. I will discuss two very different approaches to designing useful electronic properties in small molecular devices. In the first case, the electronic functionality is to be provided by backbones of short molecules. We have developed a set of fabrication techniques that allow us to build devices with self-assembled monolayers from nearly single-molecule size up to ~300 nm on a side. For the first time, systematic experimenting with the topography, chemical bonding at the metal-molecule interface, and defect generation is performed. Surprisingly, the results consistently demonstrate that the tunneling conductance of common short molecules is 4-6 orders of magnitude smaller than is commonly believed. In the second approach, we build devices with monolayers of macromolecules. The electronic properties are engineered by the composition and by the chemical conversions of the side groups. Voltage-induced reversible switching between low- and high-conductive states is observed in devices fabricated with polyelectrolyte monolayers. In this case, multiple chemical modifications can be performed within completed devices significantly affecting the electrical behavior. We suggest that the switching is caused by ionization of the polymer creating a conducting channel of electronic levels aligned with the contact Fermi level.

February 12, 2007

“Optical Forces and Slow Light in Nanophotonics,” Michelle L. Povinelli, Stanford University, hosted by Larry Curtiss and Gary Wiederrecht

Abstract: Advances in nanofabrication techniques now allow us to pattern materials on a scale smaller than the wavelength of light. I will show how computational simulations can be used to explore novel optical phenomena in nanofabricated devices such as photonic crystals and guide device design. First, I explore the use of optical (or radiation-pressure) forces for reconfiguration and positioning of integrated optical devices. Calculations on waveguide, microsphere, and photonic-crystal systems show that light forces should lead to significant displacements, opening a new route for all-optical control. Second, I describe how dynamically-tuned photonic crystals can be used to slow down the speed of light on-chip, mimicking atomic systems. Thirdly, I describe mechanisms for reducing the propagation losses due to disorder and light leakage in miniaturized nanophotonic waveguides.

February 9, 2007

“Modeling the Metastability and Configurations of 0-D and 2-D Defects in Nanocrystals,” Amanda Barnard, University of Oxford, hosted by Larry Curtiss

Abstract: Although thermodynamically metastable, defects are often observed in polyhedral nanocrystals, nanorods and nanowires, even following annealing. These may include zero-dimensional point defects such as substitutional impurities, or two-dimensional planar defects such as stacking faults and twin planes. For example, many bulk metals have the face-centered cubic structure, but small nanocrystals and nanorods of the same material may exhibit structural modifications such as single or multiple (symmetric) contact twinning, as well as fivefold cyclic twinning resulting in pentagonal nanostructures. Unfortunately such defects are often neglected by most models and computational studies, which consequentially limits theoretical descriptions of nanostructure materials to more idealized (less realistic) approximations. Presented in this talk are examples of how computational and theoretical methods may be used to model the metastability of different configurations of zero-dimensional and two-dimensional defects in faceted nanocrystals, over a range of sizes.

January 30, 2007

“Dynamic Microscopic Patterns at Air/Liquid and Liquid/Liquid Interfaces,” Nathan Ramanathan, Florida State University, hosted by Seth Darling

Abstract: From the striped coats of zebras to the ripples in windblown sand, the natural world abounds with patterns. Such patterns have been of great interest throughout history, and, in the last 20 years, scientists in a wide variety of fields have been studying the patterns formed in well-controlled experiments that yield enormous quantities of high-precision data. The major part of my talk will be focused on the temperature-gradient-driven dynamic flow patterns at air/liquid interfaces, highlighting applications in biomembranes and microfluidics. During the last decade of the 20th Century, physicists and chemists have shown that a neutrally buoyant droplet in a fluid possessing a temperature gradient migrates under the action of thermocapillarity. The drop pole in the high-temperature region has a reduced surface tension. The surface pulls away from this low-tension region, establishing a Marangoni stress that propels the droplet into the warmer fluid. Recently, we have shown a way to generate thermocapillary flow in two-dimensional Langmuir monolayers. Locally irradiating a Langmuir monolayer with an infrared laser opens up a cavitation bubble. The bubble exerts a thermal stress on the adjacent liquid that generates a flow around the bubble that ultimately results in the formation of collapse patterns and vortical flow patterns. The generated thermocapillary flow depends on the magnitude of the applied laser power, surface pressure, temperature-dependent surface tension, and the duration of local heating. At the end of my talk I will discuss some of the magnetic patterns that are formed at the liquid/liquid interface and measurement of femto-Newton range forces.

January 26, 2007

"Mechanical Response of Nanocrystalline Ceramics: Massively Parallel Molecular Dynamics Simulations," Izabela Szlufarska, University of Wisconsin, hosted by Derrick Mancini

Abstract: Silicon carbide is an outstanding material for applications in harsh environments because of its high oxidation resistance and the stability of its properties in high-temperature, high-radiation, and high-frequency conditions. It also has excellent mechanical properties and has recently been explored as potential material for MEMS and NEMS applications. Its mechanical properties can be further improved by decreasing the grains size to the nanometer regime. We employ massively parallel molecular dynamics simulations to study atomistic events at the onset of plasticity in nanocrystalline ceramic thin films. The increased volume fraction of highly disordered intergranular films as compared with nanometals manifests itself in new deformation mechanisms. Our nanoindentation studies of nanocrystalline silicon carbide provide a scenario for the interplay between grain rotation, corporate grain motion, sliding at grain boundaries, and integranular deformation to produce a rich and unique load-displacement response. We predict a crossover from continuous corporate grain response to discrete intergrain plasticity at a critical depth that is a fraction of the grain size. We will also present results of tensile and shear testing in nc-SiC and describe changes in the mechanical response resulting from the reduction of grain size. Understanding the fundamental phenomena that govern elastic and plastic deformations is crucial for design and fabrication of nanocrystalline materials with enhanced mechanical properties. Preliminary results on simulations of diamond nanostructures and implementation of the REBOII potential with long-range interactions into our massively parallel code will also be discussed.

January 19, 2007

 

“Nanoimprint Lithography, Nanophotonic Devices and Applications,” Shufeng Bai, Princeton University, hosted by Derrick Mancini

Abstract: Nanoimprint lithography (NIL) is a high-throughput, low-cost, nonconventional lithography method with proven sub-10-nm resolution. It has been listed in theInternational Technology Roadmap for Semiconductors for 32-nm node and is quoted as an engine to nanomanufacturing.

The first part of the talk focuses on nanofabrication technologies. A process was developed to fabricate highly ordered array of nano-rings (as small as 40-nm radius) over large areas. The nanoring array has excellent periodicity, good circularity, and uniformity over wafer scale areas. Concentric double rings were also fabricated, and plasmon resonance in gold nano-rings was observed at infrared wavelength. The potential applications of nanoring arrays include plasmonic devices and magneto-resistive memory, which may replace both computer memory and hard drives in the future. Fabrication and applications of low-cost nanofluidic channels using nanoring lithography will also be discussed. The channels measure 90 nm in cross section and several centimeters in length. Stretching DNA molecules by using the nanochannels was demonstrated.

In the second part of the talk, a novel photonic device called subwavelength resonant grating (SRG) will be introduced. A SRG consists of a single layer of subwavelength surface relief gratings, and it acts as a band stop filter. SRGs are narrow band, ultrathin (< 1 um), highly efficient (~100%), tunable, and suitable for integration. They can be fabricated by using NIL.

By incorporating a SRG with a commercial multimode diode laser, a tunable single mode laser was demonstrated. It operates at 1.5-um wavelength with a side mode repression ratio of 36 dB and wavelength tuning range of 7 nm. Unlike DBR and DFB lasers, strict temperature control is not needed to keep the wavelength stable. A SRG was also used to narrow the spectra and stabilize the wavelengths of high power broad area lasers (BAL). The full-width at half-maximum (FWHM) of a BAL was reduced from several nanometers to less than 0.06 nm, and the wavelength drift against temperature was reduced by a factor of 40. The setup is much simpler than the approaches using conventional diffraction gratings.

Other subwavelength optical elements, such as subwavelength quarter wave plate, may also be discussed if time permits.

 

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