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

Archive: Seminars 2008

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December 18, 2008

"Interactions of Single Molecules with Gold Surfaces: Kondo Effect and Molecular Rotors," Li Gao, University of California ­ Los Angeles, hosted by Nathan Guisinger and Jeffrey Guest

Abstract: Detecting and manipulating the properties of single molecules are of great importance for the development of single molecular devices. Scanning tunneling microscope (STM) has been used to study the electronic and mechanical properties of single molecules on the metal surfaces. Kondo resonances are a very precise measure of spin-polarized transport through magnetic impurities. We detected Kondo resonances for single iron phthalocyanine molecules on Au(111) surface. The Kondo resonances were found to be greatly dependent on the molecular adsorption site, which provides a method for manipulating molecular Kondo effect. Precisely controlling the motion of single molecules is crucial for fabricating smart molecular machinery. Molecular rotors with a fixed off-center rotation axis have been observed for single tetra-tert-butyl zinc phthalocyanine molecules on Au(111). Experiments and first-principles calculations reveal that the rotation axis is formed by a chemical bonding between a nitrogen atom in the molecule and a gold adatom on the surface. These single-molecule rotors self-assemble into large scale ordered arrays on cryogenic gold surfaces.

December 18, 2008

 

"Chemical Routes to the Nanocrystalline Thermoelectric Materials: AgPbmSbTem+2 and Pb1-xSnxTe Nanocrystals," Indika Arachchige, Northwestern University, hosted by Matthew Pelton

Abstract: Significant progress in the synthesis of narrow-gap quantum dots, such as PbTe nanocrystals, has triggered recognition of their potential in photovoltaic, thermovoltaic, and thermoelectric (TE) applications. Some recent advances in enhancing the TE figure of merit (ZT) are associated with these materials, due to the strong quantum-confinement effect as well as large phonon scattering. This presentation will highlight the recent efforts in the synthesis of thermoelectrically relevant nanocrystalline lead chalcogenide materials that have been the major focus of my research over the last two years.

Nanocrystals of the Quaternary Thermoelectric Materials AgPbmSbTem+2: Materials with compositions AgPbmSbTem+2 or LAST ( Lead-Antimony-Silver-Telluride) have been shown to have promising TE properties with large ZT values ranging from 1.2 to 1.7 at 700K. Although by many measures, bulk LAST materials behave as solid solutions, high-resolution TEM micrographs indicate phase segregation with the presence of coherent, endotaxially embedded nanodots that are rich in Ag and Sb. The nanocrystals are believed to affect the TE properties at least in part by causing enhanced phonon scattering leading to low thermal conductivity.

However, the nature of the Ag/Sb-rich nanodots found in bulk LAST materials and their full impact on TE performance is not well understood. It would therefore be interesting to develop a synthetic methodology to prepare nanocrystals of LAST materials to investigate the link to enhance TE performance. We have recently developed a new general methodology for the synthesis of nanocrystalline LAST materials with control over size, shape and atomic composition. Herein we describe the influence of synthetic parameters on the primary particle size, morphology, optoelectronic properties as well as the TE performance.

Anomalous Band Gap Evolution from Band Inversion in Pb1-xSnxTe Nanocrystals: Ternary Pb1-xSnxTe alloys are narrow gap semiconductors with absorption band energies that are nonlinearly proportional to the Sn concentration (x). At 300 K, the energy gap of PbTe is 0.29 eV and pure SnTe is a p-type semiconductor with energy gap of 0.18 eV, but with conduction and valence bands are inverted from those of PbTe. Hence, the energy gap of Pb1-xSnxTe materials initially decreases with increasing Sn concentration (x) and vanishes for an intermediate alloy composition. With further increase in Sn fraction (x), the energy gap starts to increase up to the SnTe value.

However, it is very difficult to precisely determine the band edge structure of the intermediate compositions near and beyond the band inversion region, since only high carrier concentration samples can be obtained due to deviation from stoichiometry. On the other hand, nanocrystals of Pb1-xSnxTe would not have high carrier concentrations and can serve as ideal samples to determine the band edge structure near and beyond the inversion region.

Recently we have developed a general methodology for the synthesis of nanocrystalline Pb1-xSnxTe materials which exhibit anomalous trend in optoelectronic properties. Herein we describe the effect of synthetic parameters on the primary particle size, morphology, atomic composition, and optoelectronic properties.

December 17, 2008

"Synthesis and Self-Assembly of One-Dimensional Nanostructures," Bishnu P. Khanal, Rice University, hosted by Matthew Pelton

Abstract: Gold nanorods exhibit strong optical scattering and absorption in the visible and near-infrared regions due to the localized surface-plasmon resonance. The longitudinal plasmon of gold nanorods is dependent on their aspect ratio. However, the nanorods synthesized by classical seed-mediated method have a very limited range of achievable aspect ratios (3-5). Numerous attempts to increase the aspect ratio to values >5 have met little success so far. This presentation will describe methods to tune the plasmon peak in a reversible fashion and obtain nanorods with any aspect ratio ranging from 1 to 300. One-dimensional growth and shortening of nanorods in aqueous solution in the presence of Au (I) and Au (III) ions, respectively, will be discussed. In addition, the self-assembly of one-dimensional nanostructures in the form of ring like superstructures and colloidal crystals will be discussed.

December 16, 2008

"Second-Order Nonlinear Optic Materials and Bridging Nanoscale Devices with Functional Molecular Wires," Yiliang Wang, Northwestern University, hosted by Matthew Pelton

Abstract: There are two different topics in the area of material chemistry in this talk. The first part is the research on second-order NLO materials, including EO chromophore synthesis, EO thin film fabrication, and the design of a transparent conductive oxide-based EO modulator. The second part is the study of the electrical properties of molecular wires. Functional molecular wires were used to to covalently connect nanoscale electrodes, which were cut single-walled carbon nanotubes. Reliable and reproducible results were obtained, and oligoaniline wires were fully studied for the pH-sensitive conductance.

December 15, 2008

 

"Patterning of Nanoparticles Using DNA Directed Self-Assembly," Jaswinder Kumar Sharma, Arizona State University, hosted by Matthew Pelton

Abstract: Recently research in the field of noble metal nanoparticles has received a great interest due to their unique optical properties. Uniqueness of these optical properties arises from the enhanced electromagnetic fields near the particle surface, which in turn are result of resonant oscillation of their conduction electrons caused by their interaction with the incident light. This collective oscillation of conduction electrons is called localized surface plasmon, which can be exploited to create a variety of photonic devices like plasmon waveguides and plasmonic rulers. But all these devices can only be created by organizing nanoparticles with constant interparticle spacings at nanoscale. DNA nanotechnology has emerged as an excellent approach to organize metallic nanoparticles at nanometer scale with predetermined position and interparticle distances. In today’s presentation, I will talk about the use of DNA scaffolds for patterning gold nanoparticles and Quantum Dots. I will also talk about some strategies developed to strengthen the bond between DNA oligonucleotides and gold nanoparticles, and their use in increasing the yield of nanoparticles patterned by DNA scaffolds.

November 17, 2008

 

"Micromagnetic investigations of current and field-induced vortex core reversal," Sebastian Gliga, Forschungszentrum Jülich, hosted by Matthias Bode

Abstract: Micromagnetic simulations are routinely employed for the modeling of magnetic structures in mesoscopic ferromagnets. In recent years, the simulations have achieved a very high degree of reliability and accuracy, yielding in most cases perfect agreement with experiments, thereby demonstrating their predictive power. Micromagnetic simulations are now therefore being applied to investigate phenomena on time and length scales beyond the present limits of experimental resolution, where new dynamic processes are expected to be found.

One such recently discovered process is the ultrafast reversal of vortex cores in magnetic thin-film elements, which may find application in non-volatile storage media. Magnetic vortices are naturally-forming fundamental magnetization structures possessing a nanometric core around which the magnetization direction circulates in the film plane. In the core, the magnetization aligns perpendicular to the plane, pointing 'up' or 'down'. Recent experiments have shown that the core magnetization can be switched by means of weak in-plane pulses, but the details of the switching mechanism could not be resolved. Our three-dimensional micromagnetic finite-element simulations elucidate the dynamics of this complex mechanism and describe the fastest field-induced switching process ever reported.

I will present recent results on the reversal of the vortex core induced by external fields and, more technologically relevant, by electric currents. I will show that in both cases, the switching mechanism is identical.

Moreover, two distinct routes exist through which a vortex core can be switched: a slow route, in which the reversal is achieved within nanoseconds and an ultrafast route unfolding on the picosecond time scale. In both cases, the switching occurs as soon as the internal energy exceeds well-defined threshold values, providing insight into the origin of this process.

Micromagnetic simulations have allowed furthering the understanding of arguably the most complicated micromagnetic switching mechanism known to date. They also make clear predictions for possible experimental studies.

November 11, 2008

"Growth, Characterization and Application of Nanocrystalline Diamond Films," James E. Butler, Naval Research Laboratory, hosted by Anirudha Sumant

Abstract: The nucleation and growth of nanocrystalline diamond films by chemical vapor deposition will be presented. Nanodiamond films grown by chemical vapor deposition exhibit a number of remarkable properties desirable for MEMS and NEMS. These include high Young's Modulus, thermal diffusivity, dielectric breakdown strength, mass density, secondary electron yields, fracture toughness, optical transparency, corrosion resistance, biological stability, and more. The nucleation, growth, and doping of these films on diverse substrate materials, including silicon, poly silicon, SiO2, and various metals, will be described along with various methods of processing into structures and devices.

November 3, 2008

"MEMS Micromirror Arrays for Spatial and Temporal Light Shaping and Its Application in Optical Communication and Infrared Image Detection," Roland Ryf, Alcatel-Lucent Bell Laboratories, hosted by Daniel Lopoez

Abstract MEMS micromirror arrays exhibit some unique properties, such as high optical throughput, pixel size as small as 3 um, and fast response in the order of 10 us. I will describe two types of MEMS array fabricated at Bell Labs. The first type has mirrors that have an electrostatic actuated piston motion for the individual pixel-mirrors and is in essence a spatially programmable phase modulator. The second type of array offers the additional ability to tilt the individual mirror around two axis and thus can act as a programmable Fresnel optical element. I will describe how these two arrays can be used for applications such as adaptive optics, optical tweezers, or optical lattices. Further, with the help of a diffractive element, the arrays can be used for time domain filtering, which is useful for pulse shaping and dispersion compensation. I will present the result for a programmable dispersion compensator with no impairment on the passbands. Finally I will report on a MEMS-based infrared detector array (8- to 14-um wavelength range) based on an optical readout scheme at visible wavelength.

October 23, 2008

"Accelerating development of membranes using materials modeling: metal organic frameworks and metal alloys," David Sholl, Georgia Institute of Technology, hosted by Jeffrey Greeley

Abstract: Membranes have potential to play an important role in many energy-related chemical separations, but experimental development of new membrane materials is challenging and time-consuming. Materials modeling can play an important role in this area by screening potential membrane materials in advance of experimental studies. I will describe work in my group on several classes of materials following this theme, including metal organic frameworks as potential nanoporous membranes for light gas separations and metal alloys as dense films for hydrogen purification.

October 23, 2008

"Revisiting optical manipulation with surface plasmons," Romain Quidant, ICFO ­ The Institute of Photonic Sciences, Barcelona, Spain, hosted by Gary Weiderricht

Abstract :Force fields originating from evanescent waves open new opportunities to integrate optical manipulation in a coplanar geometry and extend optical trapping to the subwavelength scale. Within this scope, it was recently suggested that surface plasmon (SP) fields bound to metal-dielectric interfaces be used. In addition to offering enhanced magnitude over conventional evanescent fields, SP fields are also expected to achieve further spatial confinement toward the nanoscale. We review recent advances achieved in the use of SP fields engineered at a patterned metal surface for on-a-chip optical manipulation of small objects in solution.

We first discuss trapping of microsized colloids with gold micropads illuminated under total internal reflection. It is shown how enhanced and confined SP fields at the pad surface can be engineered to provide stable wells able to trap colloids at predefined locations of the surface with laser intensity much weaker than is required for conventional three-dimensional optical tweezers. This method also enables parallel trapping with a single unfocused beam.

We then discuss the specificities of SP traps over conventional three-dimensional tweezers. In particular, we show how the dependency of the gradient and scattering force components on the illumination parameters enables tuning their trapping ability and achieving a trapping selectivity to the specimen polarizability.

Finally, we explore new trapping platforms, directly inspired from the latest advances in plasmon nano-optics to extend our method to the manipulation of subwavelength objects that otherwise could not be trapped with conventional tweezers.

October 23, 2008

"Magnetism on the Atomic Scale," Andreas J. Heinrich, IBM - Almaden Research Center, hosted by Matthias Bode

Abstract: Understanding and controlling the magnetic properties of nanoscale systems is crucial for the implementation of future data storage and computation paradigms. Here we show how the magnetic properties of individual atoms can be probed with a low-temperature, high-field scanning tunneling microscope when the atom is placed on a thin insulator. We find clear evidence of magnetic anisotropy in the spin excitation spectra of individual magnetic atoms embedded in a nonmagnetic surface. In extended one-dimensional spin chains, which we build one atom at a time, we find strong spin coupling into collective quantum spins, even for the longest chains of length 3.5 nm. The spectroscopic results can be understood with the model of spin excitations in a system with antiferromagnetic coupling, controlled on the atomic scale.

High-spin atoms can show an interesting form of the Kondo effect when the magnetic anisotropy places a degenerate, low-spin Kramers doublet in the ground state.

October 15, 2008

"Engineering Nanomaterial Platforms: Toward Highly Sensitive and Specific Biomedical Detection," Jong-in Hahm, Pennsylvania State University, hosted by Seth Darling

Abstract: This talk presents an overview of our ongoing nanomaterials research, aiming to provide more rapid, sensitive, and accurate detection of genetic and protein markers. Current research efforts pertaining to nanotubes, nanowires, and polymers will be briefly highlighted.

Following the introduction of various nanomaterials research in our group, this talk will focus on the remarkably enhanced optical detection of DNA and proteins which is enabled by the use of nanoscale zinc oxide platforms. Fluorescence detection is currently one of the most widely used methods in the areas of basic biological research, biotechnology, cellular imaging, medical testing, and drug discovery. Using model protein and nucleic acid systems, we demonstrate that engineered nanoscale zinc oxide nanostructures can significantly enhance the detection capability of biomolecular fluorescence. Without any chemical or biological amplification processes, nanoscale zinc oxide platforms enabled increased fluorescence detection of these biomolecules when compared to other conventional substrates. This ultrasensitive detection was due to the presence of ZnO nanomaterials which contributed greatly to the increased signal to noise ratio of biomolecular fluorescence.

We also demonstrate the easy integration potential of zinc oxide nanostructures into periodically patterned platforms which, in turn, will promote the assembly and fabrication of these materials into multiplexed, high-throughput, optical sensor arrays. These zinc oxide platforms will be extremely beneficial in accomplishing highly sensitive and specific detection of biological samples involving nucleic acids, proteins and cells, particularly under detection environments involving large scale population screening and early biomarker detection.

October 9, 2008

"Organizing atoms, clusters, and proteins on surfaces," Richard Palmer, University of Birmingham, hosted by Stefan Vajda

Abstract: We will address two complementary methods to organize many-atom systems on the sub-10-nm scale: room temperature STM manipulation of individual polyatomic molecules and deposition of size-selected atomic clusters. The common theme will be precision and uncertainty in the organization of atoms.

Bond-selective molecular manipulation is one of the frontiers of atomic manipulation with the STM. Traditionally such experiments are conducted in the stable, low-temperature regime; room-temperature manipulation is much more challenging. Here we demonstrate room-temperature, bond-selective manipulation ("molecular dissection") in a polyatomic molecule, chlorobenzene (C6H5Cl), anchored to the Si(111)-7x7 surface by chemisorption. Electron (or hole) injection from the STM tip into the p* LUMO (p HOMO) orbitals of the benzene ring leads to controlled molecular desorption - a one-electron process. We focus on C-Cl bond dissociation in the chemisorbed chlorobenzene molecule. Detailed STM images identify the azimuthal orientation of the individual chlorobenzene molecules and allow us to correlate the final location of the liberated chlorine "daughter" atoms with their parents. We identify Cl atoms up to 50Å from the parents. We find that dissociation is a two-electron process and propose a vibrationally mediated electron attachment mechanism.

The controlled deposition of size-selected clusters, assembled in the gas phase, is an alternative route to the fabrication of surface features of size 1-10 nm - also the size scale of biological molecules such as proteins.

Scaling relations that describe the implantation and pinning of the clusters enable the preparation of stable, three-dimensional surface features, which can act as protein binding sites. Specifically, we report the pinning of size-selected AuN clusters (N = 1­100) to the (hydrophobic) graphite surface to create films of arbitrary, submonolayer density. Gold presents an attractive binding site for sulphur and thus for cysteine residues in protein molecules. AFM measurements in buffer solution show that GroEL chaperonin molecules (15-nm rings), which contain free cysteines, bind to the clusters and are immobilized]. Peroxidase and oncostatin molecules behave similarly. By contrast, green fluorescent protein does not bind, consistent with detailed analysis of the protein surface; the cysteine residues lie in the interior of the folded protein. The results provide "ground rules" for residue-specific protein immobilization by clusters and have led to the development of a novel biochip for protein screening by a spin-off company.

October 2, 2008

"Label-free Mapping of Near-field Transport Properties Using Surface Plasmon Resonance Reflectance Imaging," Iltai Kim, University of Tennessee, hosted by David Tiede and Gary Wiederrecht

Abstract: A label-free visualization technique based on surface plasmon resonance (SPR) reflectance sensing is presented for real-time and full-field mapping of microscale concentration and temperature profiles. The key idea is that SPR reflectance sensitivity varies with the refractive index of the near-wall region of the test mixture fluid. The Fresnel equation, based on Kretschmann’s theory, correlates the SPR reflectance with the refractive index of the test medium, and the refractive index correlates with the mixture concentration or temperature. The basic operation principle is that when noble metal is illuminated by p-polarized light or electron at a specific angle above critical, an evanescent wave is generated to excite a surface plasmon wave in the interface between the thin noble metal film and the test medium. The surface plasmon wave is highly sensitive to variation of the refractive index of the test medium with an accuracy of 10-8 in RIU to provide a powerful tool for chemical/bio sensing. The existing SPR-related techniques, however, are effectively pointwise and mostly lack a systematic, quantitative approach.

In this study, SPR is applied and demonstrated to detect near-field microfluidic properties, such as concentrations and temperatures, in a label-free, real-time, and full-field manner using a laboratory-developed SPR reflectance imaging system. Three example applications are presented: (1) micromixing concentration field development of ethanol penetrating into water contained in a microchannel, (2) full-field detection of the near-wall salinity profiles for convective/diffusion of a saline droplet into water, and (3) SPR imaging thermometry of the cooling of a hot-water droplet in air and cold water medium as well as a parametric study of optical properties. Also presented are discussions on measurement sensitivity, uncertainties, and a parametric study of optical properties such as refractive indices of prisms and thin metal films, and detection limitations of the implemented SPR imaging sensor. Furthermore, SPR reflectance imaging can be effectively applied in biochemical sensing, such as interactions between cell and substrate, because of its high sensitivity in the near surface less than 1 μm, and SPR can be coupled with nanoparticles to present various applications, such as in nanophotonics and nanomedicine.

September 26, 2008

"Nano-Nanocomposites: An Emerging Class of Materials," Pushan Ayyub, Tata Institute of Fundamental Research Mumbai, India, hosted by Derrick C. Mancini

Abstract: The possibility of obtaining some degree of control over different physico-chemical properties by means of varying the size and shape of the crystallographic grains constituting bulk matter is now well known. In this talk, we will review our recent results on nano-nanocomposites (NNC), defined as a random, bi-phasic nanodispersion, in which the characteristic size of both phases present is in nanometers. This is a subset of the larger family of nanocomposites, which commonly consist of one phase nanodispersed in an extended matrix phase. NNCs afford the possibility of tuning physico-chemical properties via a large number of parameters, such as the nature of the components, the composition, and the size and morphology of both phases. In certain especially interesting situations, the properties of the NNC are not simple linear superpositions of the properties of the individual nanocrystalline species. I will discuss three such special cases:

  1. NNCs of II-VI semiconductors with different band gaps that show enhanced photoluminescence, photoconductivity, and nonlinear optical properties;
  2. NNCs of a superconductor and a normal metal, whose properties are predominantly controlled by the superconducting proximity effect; and
  3. Metal-metal NNCs that are actually nanoscale phase-separated alloys of "immiscible" metals.
September 11, 2008

"Multiple Excitons in PbSe Nanocrystals: From Generation using Single Photons to Auger Recombination," Richard D. Schaller, Los Alamos National Laboratory, hosted by Stephen K. Streiffer

Abstract: PbSe nanocrystals are efficient infrared (IR) emitters with potential applications ranging from telecommunications to optical tagging. These nanocrystals can be synthesized with narrow size dispersion and high photoluminescence quantum yields (up to ~80% at room temperature), providing size-controlled tunability of the energy gap from near- to mid-IR wavelengths. By means of novel transient absorption experiments, we were able to observe in 2004 that single photons of sufficient energy can produce multiple electron-hole pairs in PbSe nanocrystals with a low energetic onset in comparison to the bulk. This result has been verified by other research groups, and is of significant interest to the photovoltaics community because of the potential for this physical effect to increase power conversion efficiencies of single junction devices above the apparent Shockley-Queisser thermodynamic limit. We find experimentally that the generation of multiple excitons takes place exceptionally fast, which has invited novel mechanisms to be posited. Time permitting, I will present research into the mechanism of Auger recombination in nanocrystals via the unique approach of using hydrostatic pressure to tune nanocrystal energy gap via the material deformation potential. In these studies, we find that Auger recombination is energetically barrierless in distinction from bulk materials. This finding has several important ramifications for nanocrystal material applications.

September 8, 2008

"Self-Assembly for Nanostructured Photovoltaic Devices," Charles T. Black, Center for Functional Nanomaterials, Brookhaven National Laboratory, hosted by Stephen K. Streiffer

Abstract The Center for Functional Nanomaterials at Brookhaven National Laboratory is a science-based user facility devoted to nanotechnology research addressing challenges in energy security. The five internal research groups (Electronic Nanomaterials, Catalysis/Surface Science, Biology/Soft Materials, Electron Microscopy, and Theory/Computation) accompany a broad portfolio of scientific capabilities and an active external user program. Our research program in electronic materials incorporates nanostructured materials with precisely defined and tunable internal dimensions as experimental platforms for understanding and improving the three critical steps of photovoltaic energy conversion: solar light absorption by the active photovoltaic material; dissociation of photogenerated electron-hole pairs into free charge carriers; and charge collection.

Nanometer-scale material self-assembly has applications in both today¹s and future technology. I will present thoughts and preliminary experimental results showing the promise of self-assembling materials for improving photovoltaic device performance, while at the same time framing the discussion around our prior successful implementation of self-assembly processes in high-performance semiconductor devices.

August 22, 2008

"Heusler Compounds: Materials for Spinelectronic Devices," Andy Thomas, Bielefeld University, hosted by Tiffany Santos

Abstract: Spinelectronic devices try to utilize the electric charge as well as the electron spin to enable new devices such as magnetic sensors, memory, and logic. These devices could be based on magnetic tunnel junctions (MTJs). The signal change -- which is crucial for most applications -- of the MTJs depends on the spin polarization of the ferromagnetic electrodes. There are a few material classes that have a predicted spin polarization of 100%, one of which is the Heusler alloys. Here, we present our investigations of Heusler alloys integrated in magnetic tunnel junctions and compare them with band structure calculations.

July 14, 2008

"Exploring the complex magnetic phase space at surfaces," Stefan Heinze, University of Hamburg, hosted by Matthias Bode

July 11, 2008

"Synchrotron Microbeam X-Ray Radiation Damage in Crystalline Semiconductor Layers," Ismail C. Noyan, Columbia University, hosted by Jorg Maser

Abstract: Radiation-induced structural damage is observed in silicon-on-insulator and SiGe samples illuminated with monochromatic (11.2 keV) X-ray microbeams approximately 250 nm in diameter. The X-ray diffraction peaks from the irradiated layers degrade irreversibly with time, indicating permanent structural damage to the crystal lattice. The size of the damaged regions is almost an order of magnitude larger than the beam size, and the magnitude of damage drops as one moves away from the center of the illuminated volume. We discuss the threshold dosage required for damage initiation and possible mechanisms for the observed damage.

July 10, 2008

"Tunneling and spin transfer torques in Fe/MgO/Fe," Christian Heiliger, Center for Nanoscale Science and Technology, National Institute of Standards and Technology/ Maryland NanoCenter, University of Maryland, hosted by Nathan Guisinger

Abstract: Very high tunneling magnetoresistance (TMR) ratios in crystalline Fe/MgO/Fe tunnel junctions result from coherent transport. The MgO barrier material selects high symmetry states that are only present in the majority spin channel of Fe leading to a high spin polarization and to a high TMR. In this talk, I show that this half metallicity of Fe with respect to the high symmetry states leads to a strong localization of the spin transfer torque (STT) to the interface. As a result, the STT in realistic tunnel junctions is independent of the free layer thickness for more than three monolayers of Fe. For ideal samples, however, quantum size effects are visible. I also discuss the bias dependence of the STT and compare our results to recent measurements.

July 8, 2008

"Polyvalent DNA-Functionalized Gold Nanoparticles: Investigating DNA Loading and Aggregate Stability," Sarah Hurst, Northwestern University, hosted by Tijana Rajh

Abstract: Polyvalent DNA-functionalized gold (DNA-Au) nanoparticles (13-30 nm in diameter) have become widely used as nanoscale building blocks in assembly strategies, antisense agents in nanotherapeutics, and probes in biodiagnostic systems. Recently, we have developed protocols to finely tune the density of DNA on the surface of gold nanoparticles ranging from 15 to 250 nm in diameter. These protocols have afforded us the opportunity to investigate the relationship between nanoparticle size and the cooperative melting transition associated with aggregates of DNA-Au nanoparticles. Specifically, we have determined the minimum number of base pairings necessary to stabilize DNA-Au nanoparticle aggregates as a function of salt concentration for particles between 15 and 150 nm in size. Significantly, we have found that under certain conditions a single possible base pair interaction per DNA connection is capable of evoking hybridization of 150-nm DNA-Au nanoparticles. In addition, we have investigated the curvature-induced contributions of non-Watson-Crick base pairings to nanoparticle aggregate stability. Knowledge of the relative contributions of these parameters to the stability of DNA-Au nanoparticle aggregates will be important in the aforementioned applications where DNA-Au nanoparticles, especially those greater than 50 nm, are used.

June 30, 2008

Miniature Flow Cytometry Systems Employing Microfluidics, Dielectrophoresis, and Microacoustics for Medical Diagnosis and Biothreat Detection,” Surendra Kumar Ravula, Sandia National Laboratories, hosted by Derrick Mancini

Abstract: Flow cytometry is an indispensable tool in clinical diagnostics for screening of cancer, AIDS, infectious disease outbreaks, microbiology,and others. The cost and size of existing cytometers precludes their entry into field clinics, water monitoring, agriculture/veterinary diagnostics, and rapidly deployable biothreat detection. Much of the cost and footprint of conventional cytometers is dictated by the high speed achieved by cells or beads in a hydrodynamically focused stream. This constraint is removed by using dielectrophoretic and acoustic focusing in a parallel microfluidic architecture. In this presentation, I will describe our progress towards a microfabricated flow cytometer that uses bulk planar piezoelectric transducers in microfluidic channels. In addition to experimental data, initial modeling data to predict the performance of our systems are discussed.

June 26, 2008

"Single-Molecule Absorption Detected by Scanning Tunneling Microscopy," Erin Carmichael, University of Illinois at Urbana-Champaign, hosted by Nathan Guisinger

Abstract: Scanning tunneling microscopy (STM) consistently provides the highest spatial resolution among the scanning probe methods, allowing surfaces to be investigated on the atomic level. With the addition of optical excitation, STM promises to become a powerful technique for single molecule spectroscopy, enabling one to examine the response of single molecules on a surface. Atomic scale laser-assisted STM has thus far remained an elusive goal, with few recent experiments reaching subnanometer resolution. This is due to the many difficulties faced as a result of light perturbing the tunneling junction. We combine a novel rear-illumination geometry with frequency-modulated laser excitation to probe the optical absorption of single-walled carbon nanotubes on silicon surfaces. Modulations in the local electronic density of states can be detected with near-atomic resolution, and measurements of the molecular absorption coefficients can provide information about the chirality of a nanotube.

June 24, 2008

"Extreme Ultraviolet Interferometric and Holographic Lithography at the University of Wisconsin-Madison," Artak Isoyan, Center for Nano Technology, University of Wisconsin-Madison, hosted by Derrick Mancini

Abstract: Initial results from a 4X reduction interferometric lithography technique using extreme ultraviolet radiation from a new undulator on the Aladdin storage ring at the Synchrotron Radiation Center of the University of Wisconsin-Madison will be reported. Extended traditional interferometric lithography by using second diffraction orders instead of first orders will be discussed. This change considerably simplifies mask fabrication by reducing the requirements for mask resolution. Interferometric fringes reduced by 4X (from 70-nm half-period grating to 17.5 nm) have been recorded in a 50-nm-thick hydrogen silsesquioxane photoresist using 13.4 nm wavelength EUV radiation.

June 12, 2008

"Synthesis and applications of conducting polymer nanostructures," Jiaxing Huang, Northwestern University, hosted by Dave Gosztola and Yugang Sun

Abstract: There are exciting opportunities at the interface between the fields of nanostructured materials and conducting polymers. In this talk, conducting polymer polyaniline nanofibers will be used as an example to illustrate how nanoscale morphology impacts the material¹s processing and applications. A chemical method was developed to synthesize the nanofibers in high purity and large quantity without the need for templates. With this nanofiber morphology, the dispersibility and processibility of polyaniline are now much improved. The nanofibers show dramatically enhanced performance over conventional polyaniline applications such as in chemical sensors. They can also serve as a template to grow inorganic/polyaniline nanocomposites that lead to exciting properties for nonvolatile memory devices and catalysis. Additionally, a flash welding technique for the nanofibers has been developed that can be used to make asymmetric polymer membranes, form patterned nanofiber films, and create polymer-based nanocomposites based on an enhanced photothermal effect observed in these highly conjugated polymeric nanofibers. Polyaniline nanofibers are also a great model material for use in chemical / materials science education

May 29, 2008

"Lead Sulfide Nanocrystals Embedded in Organic Films for Nonlinear Optical Devices," Luke Hanley, University of Illinois at Chicago, hosted by Seth Darling

Abstract: Lead sulfide nanocrystal/oligomer composite films are examined here for nonlinear optical materials, but are also of interest for applications in photodetectors, light-emitting diodes, and photovoltaics. PbS-oligomer nanocomposite films are deposited with a modified commercial cluster beam deposition source and also prepared by traditional colloidal methods, then transferred to polymer films. These PbS nanocomposite films are examined by X-ray photoelectron spectroscopy, transmission electron microscopy, and nonlinear optical absorption measurements. It is well established that the linear optical properties of such composites can be tuned by varying the nanocrystal size and concentration within the organic matrix. Controlling the PbS nanocrystal surface properties at the nanocrystal-oligomer or polymer interface strongly affects their nonlinear optical absorption at 532 nm. This effect will be discussed in terms of differences in excited state absorption controlled by this interface. The relative merits of gaseous deposition and colloidal synthesis will also be discussed for the preparation of nanocomposite films.

May 2, 2008

"Nanomechanical switching at 1 ns and 1 V using an in-plane switch," David Czaplewski, Sandia National Laboratories, hosted by Derrick C. Mancini

Abstract: Nanomechanical switches have been proposed to replace CMOS switches to operate at temperatures greater than 200°C and with almost no static power dissipation. Ideally, the NEMS switches would be integrated with standard CMOS to realize power savings in computing applications. Ideally, to realize this goal, the switches would switch at similar voltages and with similar times to their CMOS counterparts. For integration purposes, the NEMS switch fabrication would have to be CMOS compatible.

This presentation will summarize the progress of the work on a program to create a nanomechanical switch that switches in 1 ns using 1-V drive actuation. A summary of the modeling effort will show the critical parameters of the design for fast, stable switching while avoiding mechanical failures of the switch. Then the fabrication performed to realize these devices will be shown. Finally, electrical testing results will be presented on the switches that have been fabricated and released.

May 1, 2008

"Synthesis and Tailoring the Surface Chemistry of Nanostructured Carbon Materials for Applications in Micro/Nano Systems," Anirudha V. Sumant, Argonne National Laboratory, hosted by Derrick C. Mancini

Abstract: The current progress in fabricating microelectromechanical and nanoelectromechanical systems (MEMS/NEMS) involving rotating, sliding, or impacting surfaces in contact based on silicon has achieved limited success primarily due to the poor mechanical, chemical, and tribological properties of silicon. At the nanoscale, due to increased surface-to-volume ratio, surface properties, such as adhesion/stiction dictates the performance of a device, and therefore development of new materials with superior mechanical, chemical, and tribological properties than those of silicon are necessary. Nanostructured carbon materials such as ultrananocrystalline diamond (UNCD) and tetrahedral amorphous carbon (ta-C) have demonstrated exceptional physical, chemical, and tribological properties at the macro- and microscale, and therefore are considered promising materials for MEMS and NEMS applications. However, little is known about their surface chemistry, morphology, and bonding configuration at the tribological interface and how it will affect tribological performance at the nanoscale. I will discuss methodologies to tune surface chemistry, morphology, and bonding configuration of UNCD surface via changes in the UNCD nucleation and growth process. Using these methodologies, we achieve extremely low adhesion energies (down to van der Waal's limit) and friction forces at the nanoscale.

Additionally, recent preliminary measurements of nanomechanical properties of UNCD carried out using in situTEM nano-indentation will also be presented. In case of ta-C, I will discuss how film-annealing processes alter both the bonding in the film and the nanotribological response.

In the next part of my talk, I will discuss current progress in synthesizing large area UNCD films (up to 8-in diameter) at low temperatures (400°C) at CNM and demonstration of its CMOS compatibility with a goal to develop monolithically integrated UNCD based MEMS/NEMS devices driven by CMOS. I will present work under progress in this direction on the fabrication of RF-MEMS switches based on UNCD by taking advantages of its unique dielectric properties as well as on the fabrication of UNCD based resonators by integrating with piezoelectric materials. At the end of my talk, I will present brief highlights of research that is being carried out under collaborative CNM user's proposal that I have developed with various outside collaborators.

April 22, 2008

"How Macromolecules Pass Through a Nanopore or Ultrafiltration of Macromolecules through Nanopores," Chi (Qi) Wu, The Chinese University of Hong Kong, hosted by Yugang Sun and Gary Wiederrecht

Abstract: Using a special double-layer membrane to avoid interaction among flow fields generated by different pores, we have, for the first time, observed the predicted discontinuous first-order transition in ultrafiltration of flexible linear polymer chains. That is, the chain could pass through a pore much smaller than its unperturbed radius only when the flow rate is higher than a certain value. When only one chain and one pore are considered, in theory, such a threshold is surprisingly independent of both the chain length and the pore size. Our results reveal that for a membrane with many pores and at a microscopic flow rate (q) lower than the threshold, the inevitable blocking of some pores by longer nonstretched coiled chains increases q in those nonblocked pores because the macroscopic flow rate (Q) is a constant.

Our results reveal that the force needed to stretch a polymer coil in an athermal solvent is only ~10 fN. Further, using this method, we are able to measure how "soft" a polymer chain is and how strong the interchain interaction is when they are collapsed and entangled with each other.

April 17, 2008

"MEMS-Based Spatial Light Modulators: Recent Developments and Future Directions," Daniel Lopez, Alcatel-Lucent, hosted by Eric Isaacs

Abstract: Micro electro mechanical dystems (MEMS) technology enables mass production of microscopic mechanical systems through batch fabrication techniques similar to the ones used in the electronics integrated circuit (IC) industry. Similar to ICs containing millions of individual transistors, mechanical systems with millions of independent moving parts (degrees of freedom) can now be fabricated on a single small silicon chip. While a variety of applications will benefit from this technology, nowhere is the potential as clear and compelling as in optical systems, at the intersection of electronic, mechanical and optical domains.

The unique advantages of MEMS are high-speed and excellent optical quality, wavelength and polarization independence, and low optical loss. Dense integration of millions of individually controlled micro-mirrors onto a single silicon chip leads to creation of a reconfigurable fast digital diffractive optical element that allows an unprecedented degree of control and manipulation of the optical field. This new kind of spatial light modulator (SLM) reveals potential to bring revolutionary new capabilities for projection, information processing, and telecommunications.

In this talk I will describe the fundamentals of this technology and our recent work in the development and fabrication of a MEMS-based phase-only SLM originally envisioned as a substitute for expensive optical photomasks. I will also describe the new and distinctive capabilities that these SLMs offer for miniature projection systems, optical nano-manipulation, holographic data storage, and free space communications.

April 14, 2008

"Device Implications of Spin Transfer Torques," Jordan Katine, Hitachi San Jose Research Center, hosted by Axel Hoffman

Abstract: This presentation looks at spin transfer torques from the perspective of three technological applications: hard disk drives, MRAM, and current-tunable high-frequency oscillators. In hard disk drives, spin transfer torques are a source of noise, and I will discuss the implications spin transfer noise will have on future sensor designs. For MRAM, I evaluate the feasibility of spin transfer driven switching. Finally, I consider the possibility of GHz communication applications enabled by nanoscale spin transfer oscillators, including Hitachi's recent results in MgO-based devices.

March 19, 2008

"Shape Changes induced by Chemical Transformation of Nanocrystals," Can K. Erdonmez, Massachusetts Institute of Technology, hosted by Yugang Sun and Gary Wiederrecht

Abstract: Colloidal nanoparticles (NPs) of a great number of materials can now be produced with well-controlled size, shape and surface properties. However, it is still often difficult to extend an existing synthetic recipe. For example, a particular size range and shape might be easy to produce for one composition, but not another, similar one. Analogous to cases in organic chemistry, inorganic NPs can participate in chemical reactions as preformed precursors and form product particles with modified size, composition, and properties. Recent research shows that this approach not only allows the realization of new compositions, but can also yield morphologies derived from, but also distinct from, the shapes of the starting materials.

I will present several examples of morphological evolution of NPs as a consequence of chemical transformation. The major focus is on the transformation of solid particles into hollow nanoshells due to directional diffusive mass transport (the Kirkendall effect) accompanying chemical transformation. This process is predicted to be easily achieved for a large number of materials and experimental results support the general nature of the effect. In one case, the transformation of cobalt NPs into Co3S4 nanoshells, existing bulk studies are complete enough and the nanoshell synthesis mature enough for making inferences about the shell formation process.

I will also highlight two more examples of simultaneous composition and shape modification and discuss their likely mechanisms: transformation of solid silver particles into octahedral cages upon galvanic exchange and formation of periodically ordered Ag2S inclusions in CdS nanorods upon partial exchange of Cd2+ with dissolved Ag+. The former process illustrates one possible end result of etching and redeposition occurring simultaneously in a nanoscale system; the latter involves a complex and still not fully understood interplay between elastic stress, nucleation and diffusion-limited segregation kinetics in a one-dimensional system.

February 21, 2008

"Quantum dots based energy transfer to photodynamic therapy agents," Smita Dayal, Case Western Reserve University, hosted by Matthew Pelton

February 12, 2008

"Nanoscale Imaging with the Coherent Diffraction Microscope," Changyong Song, University of California, Los Angeles, hosted by Jorg Maser and Ian McNulty

Abstract: The coherent diffraction microscope (CDM) is a versatile imaging probe with applications spanning a wide range of nano- and biosystems at nanometer resolution. The simple experimental schemes of the CDM have led to immediate adaptations over broad spectra of coherent light sources including hard X-ray, soft X-ray, and tabletop EUV lasers. Extensive research over the past several years has advanced the technique significantly – three-dimensional tomography, resonant imaging and tabletop diffraction microscopy – to address certain scientific questions as a practical microscope. The ultimate interest for the technique is to resolve structures within single macromolecule complexes at near atomic resolution, which will be feasible with the emergent X-ray free electron lasers (XFELs). The coherent diffraction microscope combined with the XFELs will shed a new light on nano- and biotechnology.

January 29, 2008

"Spectroscopic Studies of Novel Nanomagnetic Materials," Saritha Nellutla, National High Magnetic Field Laboratory, hosted by Tijana Rajh

Abstract: Nanomagnets are fascinating materials not only from a basic research point of view but also because of their relevance in memory storage, quantum computation, spintronics, and sensors. They can be thought of as "bridges" between the isolated single-ion spin systems and bulk magnetic materials.

The talk will focus on magnetic and electron paramagnetic resonance (EPR) studies of polynuclear transition metal ion clusters, Cu3, Cu20, and Mn6, while addressing such potential applications as catalysts and nanomagnets.

The two extreme diluted spin systems compared with correlated magnetic lattices will also be highlighted. Potassium niobate doped with Cr5+ ions, a dilute spin S = 1/2 system, has been characterized by using pulsed EPR and electron nuclear double resonance techniques and will be introduced as a new transition metal-ion-based electron spin qubit. As illustrative examples of correlated magnetic lattices, thermomagnetic data of spin S = 1/2 and S = 1 antiferromagnetic peroxychromates will be presented and nature and origin of the three-dimensional magnetic ordering in these compounds will be discussed.

January 15, 2008

"Nanocrystal based 'artificial solids': a modular approach to materials design," Dmitri Talapin, The University of Chicago, hosted by Tijana Rajh

Abstract: The development of applications ranging from displays and photovoltaic cells to thermoelectric, light-emitting devices and sensors could be accelerated by introducing lower cost alternatives to conventional silicon technology.

Chemically synthesized semiconductor nanocrystals are considered promising candidates that allow inexpensive solution-based device fabrication with precise engineering of electronic structure due to quantum size and shape effects. Self-assembly of chemically synthesized nanocrystals can yield complex long-range ordered and quasicrystalline structures that can be used as model systems for studying transport in low-dimensional materials. At the same time, employing nanocrystals in these and other electronic and optoelectronic applications require deep understanding of charge transport and collective phenomena in nanocrystal solids.

We propose the techniques for engineering nanocrystal surfaces to improve exchange coupling in self-assembled nanocrystal solids. The conductivity of nanocrystal solids can be switched between n- and p-type transports by surface transfer doping. Thus, hydrazine-capped PbSe nanocrystal solids show n-type conductivity ~1.4 S cm -1 with electron mobility of 2.5 cm2V-1s-1, successfully competing with organic electronic materials. Doping of PbSe and PbTe nanocrystal solids can also occur through the exchange coupling with other semiconductor (Ag 2Te) or metal (Au) nanocrystals intentionally introduced in the nanocrystal solids. By using various approaches to nanoparticle surface engineering, we demonstrated n- and p-channel field-effect transistors based on PbS, PbSe, PbTe, CdSe, and SnTe nanocrystals.

We develop a general approach to solution-processed semiconductor nanocomposite materials based on incorporation of nanocrystals into a matrix of another crystalline inorganic semiconductor. This approach opens up broad avenues for designing novel functional materials. We demonstrate memory devices using CdSe/ZnS core-shell nanocrystals as floating gates, solar cells employing CdS nanorods integrated into CuIn(1-x)Ga(x)Se 2 matrix and Sb2Te3-Bi2Te3 nanocomposites promising for thermoelectric applications.

January 9, 2008

"Development of novel photocatalytically active materials on the base of porous silica and titania," Galyna Krylova, Université de Rennes, hosted by Tijana Rajh and Elena Shevchenko

Abstract: Photocatalysis is a promising, energy-saving approach for purification of the environment from toxic pollutants. The most current studies are based on TiO2 photocatalyst as a rather inexpensive and chemically stable material. Surface modification of TiO2 photocatalyst allows efficient use of sunlight, leads to an increase in the active surface, and improves charge separation. We discovered that the addition of 3-aminopropyltrimethoxysilane (APTES) to the precursors of the mixed TiO2/ZnO sol-gel nanostructure films drastically affects the size of crystallites and their crystal lattices, leading to much smaller (6-10 times) domains of Zn2TiO4 spinel and TiO2 anatase, instead of relatively large ZnTiO3 and TiO2 rutile crystallites. As compared with widely used titania films prepared from P25 Degussa sol, our APTES-modified mixed-oxide nanostructures demonstrate two times higher photocatalytical efficiency in such model reactions as decomposition of methylene blue (model pollutant of waste water) and stearic acid (model reagent to study antifouling properties). The excellent hydrophobic properties of TiO2/ZnO sol-gel films allows their use as self-cleaning, antifogging glass and mirror coatings.

Highly adsorptive porous silica is another excellent platform for designing hybrid photocatalysts. Modification of porous silica with photoactive organic molecules promotes the reduction of metals cations noble and allows the formation of nanoparticles of noble metals. Surface modification of mesoporous powders of films of silica with benzophenone shifts the barrier of photosensitized reduction of Ag+, AuCl4–, Pd2+, Сu2+, Cr2O72–, and Hg2+ ions from far (253.7 nm) to near (365 nm) ultraviolet region. The mechanism of photosensitized reduction in these systems was proposed. Variation in the reaction conditions allows both synthesis of stable colloidal solutions of noble metals with controllable sizes and shapes and silica-noble metal nanostructures composites. As compared witih the chemical synthesis of such structures, our photo-induced approach has a number of advantages associated with low energy consumption and the possibility of removing side reaction products by absorption at silica surface.

January 4, 2008

"Enhancing the open-circuit voltage of dye-sensitized solar cells by coadsorbents and alternative redox couples," Zhipan Zhang, Swiss Federal Institute of Technology, hosted by Gary Wiederrecht

Abstract: The interface between a nanocrystalline semiconducting metal oxide film and a redox electrolyte was optimized in order to enhance the photovoltage and performance of dye-sensitized solar cells. It features the application of ω-guanidino acids as the coadsorbent with ruthenium amphiphilic sensitizers.

Meanwhile, fast one-electron-transfer couples were employed as alternative redox mediators to the normal iodide/iodine system to reduce the potential mismatch between the Nernst potential of the dye cation and that of the redox mediator. Although a much faster recombination was observed as compared with I- / I3- redox, the device with 2,2,6,6 -Tetramethylpiperidine-1-oxyl (TEMPO/TEMPO+) showed an overall solar to electric power conversion efficiency of 5.4 % under AM 1.5 illumination at 100 mW/cm2.

 

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