CNST Group Seminars: 2006
THE CONNECTION BETWEEN 2D COMPUTATION THEORY AND CHEMICAL SELF-ASSEMBLY
University of Texas at Austin, Department of Chemical Engineering. Austin, TX
Tuesday, November 28, 2006, 10:30AM, Rm. H107, 217 Bldg.
Three decades of exponential gains in semiconductor performance have been made possible by the continuing development of top-down/ subtractive methodologies. However, fundamental limitations exist as top-down processes are extended into the sub-30nm regime. The cost of inputting information into tens of nanometer sized pixel regions becomes economically (and practically) limiting at this critical dimension. The question is- how else can we input information into resists with high fidelity in order to create arbitrary binary patterns? Innovative methods will need to be developed to deliver high contrast information to future resists.
Computational theory provides a logical connection between chemical self-assembly and the patterning of 2D tiles into arbitrarily complicated mathematical tilings. Work with DNA has shown that equilibrium systems containing pre-designed DNA "tiles" can be made to self assemble into pseudo-crystalline shapes with high fidelity (see Rothemund et. al. PLoS Biology p. 2041, 2004). We believe that this connection has implications to the way in which future types of resists are designed. Algorithmic self-assembly may provide an alternative method for delivering information to resists through bottom up design rules and directed self-assembly. We are therefore exploring the connection between information (in the language of computation) and chemical self-assembly. In this talk, model systems will be discussed in which DNA is used to "glue" colloidal components together into macroscopic 2D algorithmic crystals.For further information contact Alexander Liddle, 301-975-6050, firstname.lastname@example.org
Electron Physics Group Seminar
GIANT SPIN-SPLITTING IN THE LEAD AND BISMUTH SURFACE ALLOYS ON AG(111)
Christian R. Ast
Postdoctoral Associate, Max-Planck Institute of Solid State Research. Stuttgart, Germany
Monday, October 2, 2006, 10:30AM, Rm. H107, 216 Bldg.
Surface alloying is shown to produce electronic states with a very large spin-splitting. We have used angle-resolved photoemission spectroscopy (ARPES) to study the long range ordered bismuth/silver(111) and lead/silver(111) surface alloys where an energy bands separation of up to one eV is achieved. As ARPES is a real-space averaging technique and only sensitive to occupied states, a more complete picture with respect to local detection of the spin-splitting as well as sensitivity in the unoccupied states is desirable. We show that a singularity at the band edge in the density of states induced by the characteristic “momentum shift” in the spin-split band dispersion can be observed in the local density of states measured by STS. The singularity introduces a characteristic energy, which is in direct relation to the spin-splitting and can be extracted from the experimentally determined density of states. For further information contact Jason Crain, 301-975-3744, email@example.com
DEFFRACTED SECOND-HARMONIC GENERATION FROM SYMMETRIC NANOPARTICLE ARRAYS
Research Associate, Vanderbilt University. Nashville, TN Tuesday, August 22, 2006, 10:30AM, Rm. H107, 217 Bldg.
I will discuss experiments studying second-harmonic generation (SHG) from ordered arrays (2-D diffraction gratings) of lithographically-prepared gold nanoparticles. We have shown that it is possible to measure SHG from symmetric nanoparticles even when illuminated in a symmetric manner. Due to symmetry constraints, no light is emitted in the forward or backward directions from a particle having in-plane symmetry. At nonzero angles, however, a signal may be measured. By specifying the grating constant of the arrays, we control the radiated angles. By constructing and measuring several arrays with different grating constants, we can effectively map the angular second-harmonic radiation pattern of a single particle with far-field excitation and detection. More recent measurements dealing with resonantly-enhanced thermal effects in the nanoparticles, and the effects of electric-field hotspots, will also be discussed. For further information contact Alexander Liddle, 301-975-6050, firstname.lastname@example.org
EXCHANGE BIAS IN NANOSTRUCTURES
S. H. Chung
Postdoctoral Research Associate, Argonne National Laboratory. Argonne, IL
Monday, August 7, 2006, 10:30AM, Rm. H107, 217 Bldg.
The rapid advancement in lithography technique for fabricating nanostructures with controlled dimensions and geometry has triggered increased research in magnetic nanostructures. In this work, the interplay between the exchange bias and the geometrical confinement has been systematically investigated in (i) nanometer sized strips of NiFe/FeMn bilayers, and (ii) submicron disks of NiFe/IrMn bilayers. The magnetic behavior of these patterned structures was characterized using magneto-optic Kerr effect and magnetic force microscopy for different orientations of the applied magnetic field, and micromagnetic simulations/numerical calculations provide a quantitative prediction of the hysteretic behavior. The samples exhibit peculiar magnetic behavior when the exchange bias, the uniaxial anisotropy, and the applied magnetic field are not collinear. In case (i), when the applied magnetic field is perpendicular to the exchange bias, the magnitude and the orientation of the uniaxial anisotropy determines the magnitude and the sign of the loop shift. Case (ii) exhibits different magnetization reversal mechanisms depending on the direction of the magnetic applied field. These results show clearly that, in order to understand and control interfacial coupling, in addition to the loop shift, it is also necessary to characterize all other magnetic anisotropies. For further information contact Daniel T. Pierce, 301-975-3711, email@example.com
NEW GENERATION OF INSTRUMENTS FOR SPIN POLARIZED ELECTRON SPECTROSCOPY: ULTRAFAST COMPACT CLASSICAL MOTT POLARIMETER
Professor, St. Petersburg State Polytechnical University. St. Petersburg, Russia
Monday, July 10, 2006, 1:30PM, Room H107, 217 Building.
Ultrafast compact classical electron spin detector based on Mott scattering will be discussed. The bulk size of the polarimeters is 25×15cm. Polarimeter is most effective to date. The design of the polarimeter goes back to the classical 100 kV Mott detector. In this Mott polarimeter an electrostatic acceleration voltage up to 40 kV can be applied. The electron detectors are energy sensitive photomultipliers with scintillators. The detectors and amplifiers are floated on the top of the acceleration voltage and allow field-free travel of the electrons from the scattering gold foil to the detectors. Such features reduce the polarimeters sensitivity to slight motion or changes in the shape of the incoming beam. The maximal counting rate is 2 Mcounts/second. Experimental results of separate magnetic contributions of Fe and Ni in FeNi3 sample and Fe2+ and Fe3+ in Fe3O4 sample studied by Spin Polarized Auger Electron Spectroscopy will be discussed. For further information contact Daniel T. Pierce, 301-975-3711, firstname.lastname@example.org
ENHANCED AND DIRECTIONAL TRANSMISSION THROUGH SUBWAVELENGTH APERTURES IN METAL FILMS
Dr. Henri J. Lezec
Director of Research, CNRS, France and California Institute of Technology. Pasedena, CA
Monday, June 12, 2006, 10:30AM, Conference Room C103, Building 215.
Interesting optical transmission properties arise when subwavelength apertures are patterned in proximity with other nanoscale features on metallic surfaces. Using focused-ion-beam milling, we have fabricated a variety of such structures, leading to novel effects such as strongly enhanced transmission and beaming.
Recent interferometric experiments involving only two subwavelength apertures suggest a new model for enhanced transmission through hole arrays which is based not on the commonly proposed mechanism of grating coupling to a surface-plasmon-polariton, but rather on the efficient generation of a continuum of diffracted evanescent waves at each hole. In addition to the salient optical properties of passive aperture systems, I will report recent results in inducing active switching behavior via the integration of non-linear materials such as ferroelectrics or semiconductor nanoparticles. Novel metal-insulator-metal waveguides fed by subwavelength slits will also be described, including configurations specifically designed to investigate the possibility of left-handed behavior.For further information contact Jabez McClelland, 301-975-3721, email@example.com
TRANSPORT PROPERTIES OF FE/MGO/FE TUNNEL JUNCTIONS
Martin Luther University. Halle-Wittenberg, Germany
Monday, April 24, 2006, 10:30AM, Rm. H107, AML 217 Bldg.
Disk read/write heads based on the tunnelling magneto resistance (TMR) effect have partly replaced the heads based on the giant magneto resistance (GMR). Another challenging task is magnetic RAM (MRAM) which needs high TMR ratio to yield a high storage density. Therefore a microscopic understanding of the transport properties in particular the bias voltage dependence of the TMR ratio is necessary. Ab initio calculations based on density functional theory (DFT) using a Korringa-Kohn-Rostoker (KKR) method in terms of Green’s functions will be presented. Tunnel junctions are mainly characterized by the interface geometry, the barrier thickness, and the structure of the leads. Their influences on the transport properties will be discussed in detail. For further information contact Mark Stiles, 301-975-3745, firstname.lastname@example.org
ELECTRICAL TRANSPORT THROUGH DOMAIN WALLS IN NANOWIRES
Sylvia H. Florez
University of Maryland.
Wednesday, April 5, 2006, 10:30AM, Rm. B145, Physics Bldg.
The static and dynamic properties of nanoscale magnetic systems are of crucial interest in the field of spintronics. An important focus is the interaction between conduction electrons and the domain walls (DW) in a ferromagnet. The physics may be analogous to the giant magnetoresistance (GMR) given that in both cases there is spin-dependent transport through non-collinear magnetizations. We have studied two effects that arise from this interaction. These are domain wall resistance (DWR) and current induced domain wall motion (CIDWM). From a technological standpoint these effects are of great interest for device applications. DWR is an efficient transducer of magnetization state, while CIDWM is an effective means of altering the magnetic state. In this talk I will describe my experiments on permalloy nanowires. The work has two major parts. In the first, we measure the contribution of a single DW to the electrical resistivity by using constrictions as artificial traps for DW's. The measurements are correlated with the specific micromagnetic distribution induced by the constriction geometry. In the second part, we observe and characterize the effect of spin current induced magnetization reversal. This includes a measurement of the critical current/field phase space boundary between static and moving walls and an estimation of the intrinsic wall mobility. We also incorporate the effects of DW trapping and CIDWM into a nanometer scale spin-valve. The device exhibits significant MR and we show control of the resistance level through applied current. For further information contact Barbara Coalmon, 301-975-3707, email@example.com
EXPLORING ELECTRONIC STATES AND ELECTRONIC COUPLING METAL – ORGANIC MOLECULE INTERFACES
Mark S. Hybertsen
Thursday, February 16, 2006, 10:30AM, Rm. H107, AML 217 Bldg.
There is substantial interest in electronic transport in organic molecular systems ranging from crystals and thin films with well defined bulk transport to the limit of single molecule devices where the source and drain electrodes are integral to the transport processes. There are several common factors that are important for these different systems, including frontier electronic energy level alignment, electronic coupling and structural reorganization or vibrational coupling. In this talk, I will use results from first principles electronic structure calculations to discuss two topics: (1) the role of electron correlation in the alignment of frontier molecular energy levels at an organic-metal interface to the Fermi level of the metal; (2) new link chemistry for coupling organic molecules to a metal electrode. For further information contact Mark Stiles, 301-975-3745, firstname.lastname@example.org
CONTROLLING THE STRUCTURE OF DNA MONOLAYERS ON GOLD
Michael J. Tarlov
NIST, Process Measurements Division.
Friday, February 10, 2006, 10:30AM, Rm. C103-106, AML 215 Bldg.
The development of DNA-based sensors and micro-arrays for genetic assays has been an area of intense research over the last decade. DNA is also being studied as a material for the programmed self-assembly of nanostructures because of its unique base-pairing ability, variable base sequence, and ease of synthetic manipulation. All of these applications rely on the hybridization of complementary oligonucleotides with surface-immobilized, single-stranded DNA or RNA, so-called nucleic acid probes. Understanding and controlling the surface structure of bound nucleic acid probes is critical for optimizing hybridization reactions; however, the molecular scale structure of immobilized probes in most DNA sensors is typically poorly defined. The structure of DNA probes will depend on the how the probe interacts with itself, other surrounding probes, and the substrate surface. To elucidate the nature of these interactions, we have studied the adsorption of thiol-derivatized DNA molecules on gold surfaces as a model system. We have used various surface analytical methods to determine the coverage of DNA probes, their conformation, and the nature and extent of probe-surface and probe-probe interactions. This talk will review some of our recent studies including the finding that the four different nucleotides of DNA exhibit dramatically different affinities for gold, a property that can be exploited to form surface-confined DNA brushes of controlled graft density and length. For further information contact Barbara Coalmon, 301-975-3707, email@example.com
USING SURFACE CHEMISTRY TO CONTROL THE SUPRAMOLECULAR STRUCTURE OF ADSORBED COLLAGEN
NIST, Biochemical Science Division.
Friday, January 27, 2006, 10:30AM, Room C103-106, AML 215 Building.
Collagen type I is the most abundant extracellular matrix protein in the body. In vivo and in vitro, it polymerizes into nanometer size supramolecular fibrils to form an adhesion scaffold for cells in tissues. To develop a model system for understanding the effects of collagen on cellular behavior, we prepared thin film mimics of collagen gels by adsorbed native type I collagen to substrates covered in alkanethiol self-assembled monolayers. Ellipsometric and atomic force microscopy analysis indicated the films were on average 30 nm thick and composed of collagen fibrils that are microns long and as large as several hundred nanometers in diameter. Interestingly, we found that we could dramatically change the supramolecular structure of the adsorbed collagen by varying the surface energy of the adsorbing surface. This feature allows us to systematically control the structural nature of collagen when it is presented to cells as an adhesion matrix. The use of thin film analytical techniques to investigate the structural properties of adsorbed collagen has provided valuable insights into how real extracellular matrices influence cellular behavior. The stability and reproducibility of these protein films suggests they will be useful as reference biological materials for develop of new nanoscale analytical techniques applicable to biological systems. For further information contact Barbara Coalmon, 301-975-3707, firstname.lastname@example.org
FRACTIONAL VORTICES AND COMPOSITE DOMAIN WALLS IN MAGNETIC NANORINGS
John Hopkins University.
Monday, January 23, 2006, 10:30AM, Rm. H107, AML 217 Bldg.
Nanoparticles made out of a soft ferromagnetic material exhibit intricate magnetic patterns during the process of magnetization reversal. We show that the complex structures are superpositions of a few elementary topological defects: vortices in the bulk and halfvortices confined to the edge of the sample. Domain walls observed in experiments and simulations in magnetic strips and rings are composite objects containing two or three elementary defects. For further information contact Casey Uhlig, 301-975-8269, email@example.com