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Archive: Seminars 2011

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Date Details
November 17, 2011

My STM Quest at CNMS: Phonons, Molecules and Oxides,” MInghu Pan, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, hosted by Jeff Guest

Abstract: Scanning tunneling microscopy (STM) has been well developed for decades and has proven to be a very useful technique to detect surface structure, nanostructures, surface defects, and related electronic properties. Here, I introduce two homemade STMs from the Center for Nanophase Materials Sciences. By choosing three examples, I will demonstrate the research we are performing now.

Inelastic electron tunneling spectroscopy (IETS) by STM becomes a powerful tool to detect vibronic excitation (phonons) on surface/molecular system. We will show the role surface state electrons play in vibronic excitations on Au surface. In second example, a small molecule, phenylacetylene, forms a hexamer - a six-member superamolecule - via attractive, cooperative, multicentral weak CH/pi bondings between ethyne groups. STM and theoretical calculations help us to investigate such molecular system. The third example is ruthenate. Sr3(Ru1-xMnx)2O7 is a strong correlated electron materials involving coupling of lattice, charge, orbital, and spin. The role of Mn dopants is especially surprising and interesting for understanding phase transitions in these materials. Here, we observe the √2 superstructure of surfaces, which reflects the octahedral rotation of the actual bulk unit cell for Sr3(Ru1-xMnx)2O7. By visualizing Mn dopants in different doping levels and different phases, we are able to elucidate the nature of doping-induced changes in the physical properties of doped ruthenate.

November 15, 2011

Joint CNM-ES Seminar

"Ultrasmall Nanocrystals: Colloidal Synthesis, Magnetic and Optical Properties, Growth Mechanism, and Electrophoretic Deposition," Weidong He, Vanderbilt University, hosted by Richard Brotzman and Xiao-Min Lin

Absract: As the size of a nanocrystal becomes small enough, size-dependent properties, increased astoichiometry, and defects can give rise to interesting property transitions, including a transition between diamagnetism and ferromagnetism. This talk focuses on colloidal synthesis, and size-dependent optical and magnetic properties of several nanocrystals, such as EuX (X=O, S, Se, Te), Eu2O2S, and tellurium nanoparticles and nanorods. Significant Néel temperature enhancement and diamagnetic-magnetism transition were observed on EuTe and Eu2O2S nanocrystals, respectively.

To better control the synthesis and improve the properties of these materials, the reaction kinetics and growth mechanism of the nanocrystals were evaluated by using our newly derived model for the van der Waals interaction between a nanorod and an attaching nanoparticle. Towards the application of these nanocrystals, the synthesized nanocrystals were deposited into uniform nanofilms by using an automatic electrophoretic deposition technique. A highly efficient nonchemistry method was also invented for the synthesis of gold nanocrystals, including gold nanocages and gold nanoprisms. These nanostructures may find applications in various optical fields, such as second-harmonic generation and surface-enhanced Raman spectroscopy.

November 10, 2011

"Spatially Resolved Ionic Diffusion and Electrochemical Reactions in Solids – A Biased View of Lithium-Ion Batteries," Nina Balke, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, hosted by Paul Fenter and Andreas Roelofs

Absract: The functionality of energy storage and generation systems such as lithium-ion batteries or fuel cells is not only based on but also limited by the flow of ions through the device. To understand device limitations and to draw a roadmap to optimize device properties, the ionic flow has to be studied on relevant length scales of grain sizes, structural defects, and local inhomogeneities (i.e., over tens of nanometers). Knowledge of the interplay among the ionic flow, material properties, and microstructure can be used to optimize the device properties. For example, to maximize energy density, increase charging/discharging rates, and improve cycling life for lithium-ion batteries for applications in electric vehicles and aerospace. Until recently, existing solid-state electrochemical methods were limited to a spatial resolution of ~10 mm or greater, well above the characteristic size of grains and subgranular defects. Our development of electrochemical strain microscopy (ESM) has reduced this resolution limit to length scales down to 100 - 10 nm, which allows studies of the local lithium-ion flow in electrode materials and across interfaces. ESM is a scanning probe microscopy (SPM) based characterization method that utilizes the intrinsic link between lithium-ion concentration and molar volume of electrode materials.

In this talk, we aim to provide an overview of current local SPM-based techniques applicable for the characterization of energy storage and energy conversion materials and discuss potential advantages and drawbacks of SPM-based approaches. We will explore in detail how ESM can be used to characterize lithium-ion transport in important battery materials The results can be used to conclude on the role of microstructure and interfaces on lithium-ion transport. Theoretical calculations are shown to support the experimental data and to give insight into the signal-generating mechanism.

October 27, 2011

"Hard X-ray microscopy activities at SPring-8 public beamlines," Yoshio Suzuki, Japan Synchrotron Radiation Research Institute, hosted by Gary Wiederrecht

Abstract: Before the SPring-8 synchrotron radiation facility was commissioned in 1997, the X-ray microimaging activities in Japan were at a very primitive stage. A few groups were struggling to develop microfocusing and imaging techniques using Photon Factory beamlines. Establishing imaging techniques and organizing user groups were some of the most important issues. The tentative goals were as follows:

  1. Routine experiments in the energy region of 5-37 keV with spatial resolution of 1-0.1 micron,
  2. Application of microprobe in various fields of X-ray science (e.g., scanning microscopy, micro-XAFS, fluorescent X-ray trace-element analysis, micro-diffraction)
  3. Three-dimensional imaging by computer tomography .

Now, microbeam applications are routinely capable of spatial resolution of about 100 nm. It is possible to carry out full-field imaging microscopy with a spatial resolution of better than 100 nm, and 200-nm resolution measurements are routinely performed in imaging tomographic microscopy. The best record of nanofocusing is now better than 30 nm for 8-keV X-rays using an electron-beam lithography zone plate fabricated at NTT Advanced Technology. Submicron resolution is also attained in the higher energy region up to 100 keV. Some phase-sensitive imaging techniques have also been developed and applied to user experiments. Experiments are now carried out at four public beamlines in SPring-8: 20B2, 20XU, 37XU, and 47XU. User groups cover a wide variety of disciplines, from medical applications to elementary particle physics.

In my talk, development of optics and results of microfocusing, full-field imaging microscopy, and microtomography will be shown, and some applications will be presented.

June 6, 2011

"Nanofabrication Laboratory at Chalmers University of Technology," Peter Modh, Chalmers University of Technology, hosted by Leonidas E. Ocola

Abstract: Our presentation is on the centralized cleanroom facility and Nanofabrication Laboratory at Chalmers University of Technology, with an overview of the organization, the cleanroom usage, and some equipment high lights. In addition, we will describe the Swedish cleanroom network Myfab, now recognized as a national research infrastructure where Chalmers is one of the three nodes.

May 23, 2011

"Single-Digit Nanofabrication using Directed Assembly," Deirdre Olynick, Lawrence Berkeley National Laboratory, hosted by Leonidas E. Ocola

Abstract: One of our themes at the Molecular Foundry is “Single-Digit Nanofabrication (SDN),” which describes our efforts to pattern materials with resolution, precision, and control at the sub-10-nm scale. To reach this scale, the nanofabrication facility has a focus on nanofabrication science in areas of resist development, electron-beam-induced deposition, atomic layer deposition, directed assembly, and plasma etching. In this talk, I will highlight general Molecular Foundry work and applications for SDN, and then focus on patterning with directed self-assembly and plasma etching.

As an alternative to directed assembly of block copolymers using substrate prepatterning, we guide the order of BCPs by using thermal imprint. Cylindrical PDMS phases in PS-b-PDMS thin films were imprinted and annealed to generate linear arrays of sub-10-nm half-pitch features on the substrates. I will discuss the critical roles of BCP-mold chemistry, polymer mobility, and polymer flow in the imprint guided alignment process.

With the ability to generate SDN with self-assembly techniques, the next critical challenge is how to reliably transfer these patterns. Plasma etching has been a workhorse in nanofabrication but at SDN we must ask, “What are the limits?” I will discuss nanoscale etching work from 30 to 3 nm to address this question for SDN nanopatterning

May 13, 2011

"Applications of Ultrasound to the Synthesis of Nanostructured Materials," Kenneth S. Suslick, University of Illinois at Urbana-Champaign, hosted by Elena Shevchenko

Abstract: Recent advances in nanostructured materials have been led by the development of new synthetic methods that provide control over size, morphology, and nano/microstructure. The use of high-intensity ultrasound offers a facile, versatile synthetic tool for nanostructured materials that are often unavailable by conventional methods.

The primary physical phenomena associated with ultrasound that are relevant to materials synthesis are cavitation and nebulization. Acoustic cavitation (the formation, growth, and implosive collapse of bubbles in a liquid) creates extreme conditions inside the collapsing bubble and serves as the origin of most sonochemical phenomena in liquids or liquid-solid slurries. Nebulization (the creation of mist from ultrasound passing through a liquid and impinging on a liquid-gas interface) is the basis for ultrasonic spray pyrolysis (USP) with subsequent reactions occurring in the heated droplets of the mist. In both cases, we have examples of phase-separated nano-reactors: for sonochemistry, it is a hot gas inside bubbles isolated from one another in a liquid, while for USP it is hot droplets isolated from one another in a gas.

Cavitation-induced sonochemistry provides a unique interaction between energy and matter, with hot spots inside the bubbles of ~5000 K, pressures of ~1000 bar, heating and cooling rates of >1010 K s-1. These extraordinary conditions permit access to a range of chemical reaction space normally not accessible, which allows for the synthesis of a wide variety of unusual nanostructured materials.

Complementary to cavitational chemistry, the microdroplet reactors created by USP facilitate the formation of a wide range of nanocomposites. In this lecture, we summarize the fundamental principles of both synthetic methods and recent development in the applications of ultrasound in nanostructured materials synthesis.

April 26, 2011

"Assembly, Chirality, and Polymorphism of Large Molecules on Metal Surfaces," Erin V. Iski, hosted by Nathan Guisinger

Abstract: The high-resolution capabilities of low-temperature UHV STM were exploited to examine the packing of a pharmaceutical compound, Carbamazepine (CBZ), on Au(111) and Cu(111) single crystals, which resulted in the formation of complex chiral molecular architectures that were previously unreported. The identity of the metal surface altered the way in which the molecules packed both in density and in chirality, indicating that different metallic surfaces could be used as templates to control the molecular packing density or polymorphism of pharmaceutical compounds. Furthermore, we were able to examine how the packing of CBZ changed as a result of an increase in surface coverage and how second layer growth began to replicate a predicted, but previously not observed, structure for the bulk crystal. The study of chiral surface chemistry was extended to include the spontaneous transmission of chirality through multiple length scales for a simple, robust polyaromatic hydrocarbon, Napht  ho[2,3-a]Pyrene (NP), on a Cu(111) surface. Additionally, the interaction of that technologically important molecule with a Au(111) surface was examined in an effort to understand the changes in the organic-metal interface as a function of various annealing treatments.

April 22, 2011

"Nematic Liquid Crystal Interfaces for Biological and Chemical Detection," by Bharat R. Acharya, Platypus Technologies, LLC, hosted by Amanda Petford-Long

Abstract: Nematic liquid crystals (NLCs), the electrically responsive fluid behind most of flat-panel displays, consist of anisotropic molecules that possess long-range orientational correlation but lack positional order. In recent years, there have been significant advances in unconventional applications of NLCs in photonics, sensors, and diagnostics.

In this talk, the application of NLCs for detection of biological entities and vapor-phase chemicals is presented. Interfaces functionalized with select biological and entities promote alignment of NLCs in predetermined orientations (perpendicular or parallel to that interface) that are primarily dictated by local interactions at the interface. When these interfaces are exposed to target analytes, the interactions at the interfaces are perturbed, and the NLC films undergo orientational transitions from perpendicular to parallel alignment, or vice versa. The orientational transition can easily be detected by viewing the film of NLCs between crossed polarizers. By engineering surfaces with different interfacial properties, sensors based on this principle have been demonstrated to selectively detect a wide variety of chemical and biological analytes that have relevance in industrial hygiene, environmental monitoring, homeland security, diagnostics, and biomedical applications.

April 18, 2011

"How nanotechnology can solve the End of the Silicon Roadmap problem," by Juan Alejandro Herbsommer, Texas Instruments, hosted by Amanda Petford-Long

Abstract: Moore’s Law is the empirical observation that component density and performance of integrated circuits doubles every year, which was then revised to doubling every two years. Guided by smart optimization, timely introduction of new processing techniques, device structures, and materials, Moore’s Law has continued unabated for 40 years. Driven by tremendous advances in lithography, the 65-nm logic technology node featuring ~30-nm transistors is currently in high-volume production. With such small feature sizes in high-volume production and under development, silicon CMOS technologies are now leading the field of nanotechnology for semiconductor applications.

However, this standard technology path of scaling down the size of the devices will not be able to be sustained in the near future. It is wrongly said that “the ultimate limit of the current silicon technology is the molecular dimension.” The “End of the Silicon Roadmap” is around the corner and will not be due to reaching the ultimate quantum dimension but to the parasitic at nanometer level dominating the physics of the devices.

In fact, transistor scaling limits arise from practical limits related to leakage current at small gate lengths. The problem at small gate lengths is that the drain voltage reduces the barrier height at the source, thereby causing a low source-to-channel barrier height even with the gate voltage off, which leads to undesirable large off-state leakage. This undesirable effect affects traditional MOSFET transistors, which constitute 99% of all the transistors manufactured today. The enormous leakage issue grows with every standard scaling-down generation of technology creating a thermal dissipation problem so big that we refer to it as the “thermal wall.”

I will show the results we obtained in the development of the most efficient power MOSFET of the market. The NexFet technology developed by my team at Ciclon Semiconductors reduces the leakage in a very innovative way to make the power loss of the device negligible.

I will also show how the End of the Silicon Roadmap could be addressed by using nanotechnology in a nonstandard way. I will describe the projects I am leading (in collaboration with the universities) to develop nanotechnologies linked to spintronics, new III-V nano-engineered devices and nano-composites to improve the materials used in the microelectronics industry.

April 7, 2010

"Imaging and manipulation of nanoscale materials with coaxial and triaxial AFM probes," by Keith Brown, Harvard University, hosted by Daniel Lopez

Abstract: The atomic force microscope (AFM) has had an enormous impact on the study of nanoscale materials as it allows the measurement of piconewton forces on nanometer length scales. In this talk, I will discuss our recent research developing custom AFM probes that enable new ways to image and manipulate nanoscale materials. We deposit shielding electrodes on conducting AFM probes and etch their tips to create nanoscale coaxial apertures. We begin by showing that the capacitive structure of these cantilevers couples the mechanical deformation of the cantilever to the electrostatic energy stored in the probe. This effect is used to create self-driving probes that can image topographical features without mechanical excitation. The electrical field created by a coaxial probe can be modeled as the field from an electric dipole, which makes coaxial probes useful for high-spatial resolution imaging of electrical properties. We show nearly an order of magnitude improvement in the step resol  ution of Kelvin probe force microscopy. We further demonstrate that coaxial probes can image materials with dielectrophoresis (DEP), which allows coaxial probes to effectively image with a smaller tip radius. Coaxial probes create strong DEP forces which we combine with the nanometer precision of the AFM to create a nanoscale pick and place tool. Finally, we construct triaxial probes with three coaxial electrodes that can create more complex electric fields. In particular, triaxial probes can create an electric field zero displaced from the tip which we use to trap nanoscale beads with negative dielectrophoresis. The Triaxial AFM Contact-free Tweezers have applications in novel force sensing and nanoscale pick and place.

April 1, 2010

"Nanocrystal Syntheses and Applications for Composite Magnet and Fuel Cell Catalysis," Yi Liu, Brown University, hosted by Xio-Min Lin

Abstract: I will present my graduate research studies on solution-phase chemistry syntheses of nanocrystals and their applications. I will first outline the controlled synthesis of Fe3BO5 nanorods to demonstrate how to use these nanorods to fabricate exchange-coupled Nd2Fe14B-Fe nanocomposite magnets via high-temperature reductive annealing. I will then focus on the syntheses of platinum alloy (PdPt and PtSn) nanocrystals as fuel cell catalysts for methanol oxidation reaction (MOR).

In the MOR condition, pure platinum catalysts suffer from CO poisoning and quickly lose their initial activity . By incorporating an appropriatesecond metal with platinum, the catalyst should lessen the CO poisoning problems and show higher MOR activity and stability. We synthesized PdPt alloy nanocrystals with Pd/Pt composition tunable from 5/1 to 1/2 and studied the composition-dependent MOR. We found thatPdPt catalyst at Pd/Pt = 2/1 showedMOR activity that was superior to any of the single-component palladium and platinum catalysts. When palladium was replaced with tin, the Pt3Sn nanocrystals showed even higher MOR activity. Our work proves that MOR catalysis can be optimized via a controlled nanoscale alloying process.

March 24, 2010

"Nanostructured Silicate-Based Materials: From Environmental Remediation to Energy Storage Applications," Eduardo Ruiz-Hitzky, Materials Science Institute of Madrid

Abstract: Nanostructured solids derived from synthetic and natural silicates can be considered eco-friendly materials for advanced applications in diverse fields ranging from energy storage to environmental remediation. Recent examples from our laboratory include:

  • Graphene-like based nanocomposites, or hybrid buckypapers comprised of carbon nanotubes with clay minerals or produced from precursors of natural origin (e.g., table sugar). These materials have unique characteristics such as simultaneous electrical conductivity and elevated specific surface areas. Examples of potential applications include electrodes for rechargeable lithium batteries, supercapacitors, electrochemical sensor devices, and electromagnetic shielding panels.
  • Bio-nanocomposites, prepared by the assembly of biopolymers and other entities of biological origin with inorganic solids. These materials are biodegradable, biocompatible and exhibit other properties such as fire retardancy or with cellular structures imparting ultralightweight characteristics. Applications lie in highly efficient thermal and acoustic insulating eco-materials, active phases of sensors, and materials for biomedical uses.
  • Clay-heterostructures based on the assembly of metal-oxide nanoparticles with silicates. Anatase and magnetite nanoparticles/clay systems, for example, are useful for environmental applications such as (a) improving photocatalytic behavior and (b) creating superparamagnetic materials for easy and safe recovery of pollutants and radionuclides in oil spill and nuclear accident remediation.
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February 15, 2011

"Nanoplasmonics: A foundation for new optical materials," Jonathan Fan, Harvard University, hosted by Gary Wiederrecht

Abstract: The self-assembly of colloids is an alternative to top-down processing that enables the fabrication of nanostructures. I will show that self-assembled clusters of metal-dielectric spheres are the basis for nanophotonic structures. By tailoring the number and position of spheres in close-packed clusters, plasmon modes exhibiting strong magnetic and Fano-like resonances emerge. The use of identical spheres simplifies cluster assembly and facilitates the fabrication of highly symmetric structures. Dielectric spacers are used to tailor the interparticle spacing in these clusters. These types of chemically synthesized nanoparticle clusters can be generalized to other two- and three-dimensional structures and can serve as building blocks for new metamaterials.

February 7, 2011

"Ultrafast Nanoscale Diffraction with Convergent Beam Electron Microscopy," Aycan Yurtsever, California Institute of Technology, hosted by Elena Shevchenko

Abstract: Diffraction with femtosecond electron pulses is a fast-emerging and promising technique for investigating ultrafast dynamics at the atomic scale. One common approach in the field is to use collimated electron beams with micrometer diameters. Although the spatial information obtained from such diffraction data is at the subatomic scale, any nanosized dynamical heterogeneity, such as propagating elastic waves, has been difficult to observe. This seminar will present recent experiments in which nanoscale diffraction with picosecond temporal resolution by using focused electron pulses inside an electron microscope has been achieved. In particular, I will discuss the intensity dynamics at the Bragg reflections and the induced ultrafast heating. In addition, I will talk about ultrafast Kikuchi nanodiffraction and its implication for propagating elastic waves.

February 1, 2011

"Characterization of Nanostructures and Defects in Functional Materials using Advanced Electron Microscopy," Jiaqing He, Northwestern University, hosted by Elena Shevchenko

Abstract: Understanding the structure-property relationship of thermoelectric materials and strongly correlated electron system,s including giant colossal magnetoresistive (GMR) manganite materials and superconductive materials, is a major challenge in condensed matter physics, material sciences, and nanosciences.

We present studies of PbTe-based thermoelectric materials by scanning transmission electron microscopy (STEM). We find and characterize nanoscale precipitates, dislocations, strains, and phase boundaries that have vital effects on phonon and electron scattering and the figure of merit (ZT) of materials. We also present an unusual observation of increase and then decrease in electrical conductivity as a function of temperature in a “dual-nanostructured” bulk PbTe thermoelectric system. By combining in situ TEM and theoretical calculations, we find a plausible new mechanism interdiffusion between the lead and antimony when they are present together.

We also report on measurements of the spin configuration across ferromagnetic domains in La0.325Pr0.3Ca0.375MnO3 films obtained by means of low-temperature Lorentz electron microscopy with in situ magnetizing capabilities. We also describe the MR as a function of applied magnetic field at constant temperature and show how local spin inhomogeneities contribute directly to macroscopic GMR properties in a strongly correlated electron system.

Finally, we detail our results in exploring mutilayered La1-xSrxCuO4 superconductor films on LaSrAlO4 substrates by STEM and EELS.

January 31, 2011

"Transmission Electron Microscopy of Nanomaterials," Yuzi Liu, Materials Science Division, Argonne National Laboratory, hosted by Elena Shevchenko

Transmission electron microscopy (TEM) is an advanced analytical tool used to investigate the critical role of microstructure on the unique properties exhibited by nanomaterials. Several examples will be presented of the application of advanced TEM techniques to the study of nanomaterials.

ZnO thin films exhibit a number of important properties (e.g., semiconductor, piezoelectric and bio-compatibility). To improve the quality of ZnO film electron diffraction, dark field imaging and high-resolution TEM were employed to study 30-deg rotation domains and inversion domains that are detrimental to ZnO properties. The distribution of dopants in the diluted magnetic film of ZnO:Co is critical in understanding the origin of the anomalous Hall effect in these materials. The distribution of cobalt was measured with TEM by a combination of atomic channeling enhanced microanalysis and electron energy loss spectroscopy.

A magnetic tunnel junction (MTJ) is another example of a thin-film nanostructure whose properties depend critically upon its microstructure at the nanoscale. The tunnel barrier of the MTJ (e.g., ~2 nm of MgO) is particularly important to its transport properties. Energy-filtered TEM and in situ off-axis electron holography were used to investigate the effects of annealing and electrical bias on the tunnel barrier shape. Electron tomography was also used to measure quantitatively the roughness of the top and bottom barrier interfaces before and after annealing. Annealing was observed to affect both the barrier interfacial roughness and transport properties of the MTJ. Electrical bias was also found to affect the symmetry of the barrier potential and the effective barrier width as a result of charge accumulation at the MgO-CoFe interface.

In situ microscopy experiments provide unique insights into how materials behave on the nanoscale. It is desirable, however, to investigate these materials in their native environment, which is typically incompatible with the high-vacuum requirements of TEM. Therefore, an environmental liquid cell is required to conduct these in situ TEM experiments in a realistic environment. Building up a liquid cell experiment platform and using it to understand the assembly of nanoparticles within biological materials in different liquid media and nanomaterials dispersion in different solvents is a very interesting future research direction.

   
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