|December 17, 2012||
"Optically Induced Crystals of Submicron Particles," by Rachel Sapiro, University of Michigan, hosted by Jeff Guest
Abstract: While optical lattices have long been used to create fully controllable crystals in ultracold atom experiments, we have only recently applied them to trapping larger particles. In this talk, I will present the first optical lattice for large (submicron) particles. Four 1064-nm optical-tweezing beams with a common focus interfere to form a three-dimensional periodic trapping potential. Thousands of polystyrene particles of ~200-nm diamter are trapped, forming a three-dimensional, defect-free crystal. The trapping geometry shows strong polarization dependence; we see unexpected three-dimensional ordering even in polarization cases that should provide only two-dimensional potentials. The lattice is observed through both direct imaging and Bragg scattering with a 532-nm probe beam. The Bragg scattering patterns are consistent with the calculated trapping geometries. The decay and rise of the Bragg scattering intensity upon switching the lattice off and on reveals the Brownian motion dynamics of the particles in the periodic optical trapping potential. Experimental results agree well with results from trajectory simulations based on the Langevin equation.
|December 4, 2012||
"Domain wall dynamics in epitaxial ferroelectric thin films in investigated by piezoresponse force microscopy," by Sang Mo Yang, Seoul National University, hosted by Seungbum Hong
Abstract: Ferroelectric (FE) materials are fascinating and practical because of their spontaneous polarization, which is strongly coupled to long-range electric and stress fields, leading to outstanding properties (e.g., switchablity of electrical polarization and piezoelectricity). Based on such captivating properties, numerous efforts have been made to develop advanced FE-based electronic devices. The performance of such devices is mainly determined by the domain dynamics (i.e., the nucleation and subsequent growth of domains). Therefore, it is imperative to understand FE domain dynamics, especially at the nanoscale level.
Here, I present our recent studies on nanoscale visualization of FE domain wall dynamics in epitaxial Pb(Zr,Ti)O3 thin films using modified-piezoresponse force microscopy. First, I demonstrate that the domain wall motion in epitaxial FE thin-film capacitors can be well explained in terms of the nonlinear dynamics of elastic objects in disordered media. Second, I show that a time-dependent domain wall pinning process is the primary origin of polarization fatigue.
|November 8, 2012||
"Computational Materials Science in Lead-free Solders and Nanoparticles," Hyuck Mo Lee, KAIST, hosted by Seungbum Hong
Abstract: With the aid of phase diagrams and phase equilibria obtained from computational thermodynamics, lead-free tin-based solders for low-, mid- and high-temperature applications have been designed because of increasing environmental and health concerns over the use of lead. This approach also proved useful in studying interfacial reactions at solder joints. Some examples are presented.
Nanotechnology applied to lead-free solder alloys may result in synthesis of tin-based nanoparticles. Various sizes of bimetallic or unary nanoparticles were synthesized, and they were eventually used in inkjet printing. A eutectic composition shift was observed in nanosized particles as compared with bulk alloys. By controlling size and eutectic composition, a significant decrease in melting temperature was achieved, which proved useful in low-temperature printed electronics.
Nanophase diagrams of the corresponding material systems were obtained by using molecular dynamics simulations. It was revealed that in addition to the melting temperature decrease, the structure of the solid-state nanoparticles changed in relation to composition and size. Additionally, density functional theory calculations were performed to explore the CO oxidation of the nanoparticles. Among various determinant factors for catalytic performance, CO and O2 co-adsorption were calculated in critical compositions. In this way, their alloying effect on the catalytic properties was investigated.
November 5, 2012
Joint CNM/APS Seminar
"Unrippling and Imaging of Extra-Large Free-Standing Graphene with Atomic Precision," Woei Wu Larry Pai, National Taiwan University, hosted by Volker Rose
Abstract: Nanoscale ripple is believed to be a common feature in free-standing graphene; it is expected to play an important role in altering the coupling of electronic and geometric structures in graphene. Direct characterization of free-standing graphene ripple by atom-resolved transmission electron microscopy (TEM) is challenging because of its limited depth resolution. Recent scanning tunneling microscopy (STM) of free-standing graphene uses small suspended area (1 or 5 microns) samples and can introduce uncontrolled tension that alters the intrinsic graphene structure.
Here we report an STM study of suspended extra-large (~4000 µm2) CVD graphene prepared with a resist-free transfer and characterize its electromechanical response in detail. Controlled "Z-V" spectroscopy was conducted, in which the tip displacement vs. sample bias in closed-loop condition was recorded. This method hints at the nature of interaction forces and the mechanical response of graphene.
In contrast to a solid surface, the graphene membrane is very pliant, and the Z-V curves are characterized by a fast-rise regime and a plateau regime. Graphene deformation up to 100 nm was observed with a small ~1V bias ramp. We discovered that graphene is analogous to a curved rubber band that maintains a quasi-static shape until it is pulled or pushed to tensile stress regimes. The graphene can be manipulated by the STM tip through electrostatic and van der Waals forces, with the latter being significant when it is repulsive. In its transit to a tensile-stressed state, the graphene exhibits a series of sudden speed increases, which we interpret as unrippling of graphene ripples and render support with molecular dynamic simulation. Atom-resolved graphene images provide direct evidence of nanoscale structure ripples in its intrinsic state and the smoothing out of such ripples in the tensile regimes. Surprisingly, on rippled monolayer graphene, the coexistence of triangular and hexagonal graphene lattices without tip condition change was also observed. We also observed similar Z-V behaviors on partially suspended graphene (e.g., graphene/copper, graphene/BN).
Finally, the mechanical resonance of suspended graphene can easily be detected by interaction with a vibrating AFM cantilever. Our study provides a foundation to understand and control the electromechanical response of graphene (or other flexural atomic crystals) in its pristine two-dimensional form when subjected to a local proximal probe and therefore paves the way for further investigating structure-property correlation with atomic precision.
|November 1, 2012||
"Lead lanthanum zirconate titanate relaxor ferroelectric thin films capacitors for energy storage," Sheng Tong, Energy Systems, Argonne National Laboratory, hosted by Andreas Roelofs
Abstract: For stationary energy storage and vehicle propulsion, the demand for high-efficiency electric energy storage technology is on the rise. Capacitors, having orders of magnitude higher power density and life cycles compared with batteries, are able to smooth out momentary fluctuations and supply stable energy from renewable sources, to prolong the lifetime of batteries and improve the reliability of electric systems in hybrid vehicles. They also used to meet peak power needs in hybrid electric vehicles (HEVs) and keep internal combustion engines operating at optimized energy efficiency. Although capacitors have already been adopted widely in HEVs, more attention is now being directed toward improving the overall performance while reducing the size, weight, and cost of power electronics. Some of the desired improvements include high energy density, low electrical and thermal losses, better packaging, and improved reliability and lifetime. Thin-film ferroelectric relaxor capacitors demonstrate many attractive properties, such as high capacitance, low dielectric loss, high breakdown strength, and are capable of operating at high temperature under the hood. These properties makes relaxor ferroelectric capacitors a prime candidate to replace currently used polymer capacitors because it reduces size of the capacitor and does not require separate coolants. This presentation will discuss the performance of lead lanthanum zirconate titanate (PLZT) relaxor ferroelectric thin films on different combinations of bottom electrode platinum, LaNiO3) and substrate (nickel, silicon) for a more cost-effective solution to develop volumetrically efficient capacitors with high capacitance density, energy density, and energy storage efficiency.
|October 25, 2012||
"Quantum band engineering with new materials for infrared light emission," Oana Malis, Purdue University, hosted by Saw Wai Hla
Abstract: Quantum confinement in the conduction band of semiconductor heterostructures brings about fascinating optical properties in the infrared range of the spectrum. Research on intersubband transitions inrecent years has resulted in fundamental discoveries that eventually triggered practical device applications. Our goal is to creatively exploit the unique properties of nanostructured III-nitride materials for novel light emitters and detectors in the currently underdeveloped near- and far-infrared ranges. Because of the large electron effective mass, the nitride intersubband materials require the ability to fine-tune the atomic structure at an unprecedented subnanometer level. I will describe our efforts to understand, model, and control the effects of the nanostructure on optical properties and vertical transport in nitride heterostructures to realize the theoretical potential of this material system. Special attention will be given to the relationship between growth, structure, and optical properties in lattice-matched AlInN/GaN heterostructures. We also report the first observation of exactly reproducible low-temperature negative differential resistance in c-plane resonant tunneling diodes on low-defect quasi-bulk GaN substrates.
|August 31, 2012||
"Insights into lead-free ferroelectric materials from in situ X-ray diffraction," Goknur Tutuncu, University of Florida, hosted by Andreas Roelofs
Abstract: Most common technological devices containing ferroelectric materials utilize lead (Pb)-based piezoceramics owing to their superior properties. However, environmental and biocompatibility concerns have led to a desire for lead‐free materials, but such alternatives often fall short of the piezoelectric properties of the ubiquitous lead-containing materials. One alternative route for the development of feasible lead-free materials is designing methods for texturing materials. Here, advanced in situ X-ray diffraction techniques are used to gain new insights into the utility of this route to develop promising lead-free ferroelectric ceramics. The mechanism by which templated grain growth (TGG) assists in the formation of high texture in the promising piezoelectric ceramic (K,Na)NbO3 (KNN) was studied. To do this, high-energy X-ray diffraction was used to directly observe the interface between the template and matrix during the TGG sintering process, which allowed for a detailed understanding of how the template particles guide the phase and texture evolution of the bulk KNN. In another project, a series of tetragonal alloys of two constituent ceramics, (1-x)Ba(Zr0.2Ti0.8)O3- x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) under electric field were investigated. It was found that the composition's lattice aspect ratios play an important role in determining their piezoelectric properties.
|August 17, 2012||
"Block Copolymer Lithography in the Magnetic Storage Industry," Ricardo Ruiz, HGST, hosted by Leo Ocola
Abstract: Block copolymer directed self-assembly continues to make advances that place this technology as a potential candidate for sub-20-nm lithography. The naturally periodic features found in block copolymer films display superior size uniformity at ultrahigh densities, making them ideal lithographic masks to define the highly periodic data bits in the data sectors of hard disk drives for bit patterned media (BPM) technology at densities beyond 1Tbit/in2.
Nanofabrication challenges towards bit-patterned media, however, reach far beyond pattern formation at small length scales. We explore two potential architectures amenable to directed self-assembly: arrays of hexagonal close packed (hcp) circular dots and arrays of rectangular bits with a high aspect ratio. On the one hand, hcp patterns maximize feature density for a given lithographic dimension; on the other hand, rectangular patterns support wider write head poles in order to achieve the high write fields needed to write high-coercivity media. In both cases, a combination of e-beam lithography with block copolymer self-assembly ensures the small dimensions required for high-density media together with the flexibility to achieve accurate translational placement over circular tracks with constant angular pitch. Looking forward, extendibility toward higher areal densities remains very dependent on the ability of block copolymer lithography to deliver high image quality and high etch-contrast features for single-digit lithography at full-wafer scale. I will review the current challenges in pattern transfer at these small dimensions and evaluate some potential solutions that may enable the fabrication of patterned media templates beyond 1Tb/in2.
August 16, 2012
"Colloidal Self-Assembly: Membranes and Ribbons," Edward Barry, Center for Nanoscale Materials, Argonne National Laboratory, hosted by Xiao-Min Lin
Abstract: Understanding the self-assembly of nanostructured materials has been the cornerstone of my doctoral and postdoctoral studies, and it forms the basis for my future studies at Argonne. In this talk, I will outline results from my Ph.D. research for the self-assembly of liquid crystals, membranes, and ribbons from colloidal building blocks. Using a model experimental system based on nanometer-sized rod-like viruses (filamentous bacteriophages), I will describe a relatively simple and robust self-assembly pathway for these materials and demonstrate how macroscopic properties of the final assemblages respond to systematic variations in particle properties (e.g., rod aspect ratio, persistence length, and chirality). Of interest to the biological, chemistry, and materials science divisions at Argonne is the ability to couple these self-assembly methods with genetic and chemical modifications of the viral building blocks enabling their incorporation into devices such as lithium-ion batteries as well as light-harvesting systems. Lastly, I will give a brief overview of my most recent postdoctoral work using self-assembled monolayer membranes composed of nanoparticles with applications in filtration devices.
|August 14, 2012||
"Engineering Photonic-Plasmonic Devices for Spectroscopy and Sensing Applications," Alyssa Pasquale, Boston University, hosted by Daniel Lopez
Abstract: The control of light on the nanoscale has driven the development of novel optical devices such as biosensors, antennas, and guiding elements. These applications benefit from the distinctive resonant properties of thin films and nanoparticles consisting of noble metals.
Many optimization parameters exist to engineer nanoparticle properties for spectroscopy and sensing applications: for example, the choice of metal, the particle morphology, and the array geometry. By using various designs from simple monomer gratings to more complex engineered arrays, we model, fabricate, and experimentally characterize plasmonic arrays for sensing applications.
In this work, I have focused on the novel paradigm of photonic-plasmonic coupling to design, fabricate, and characterize optimized nanosensors. In particular, nanoplasmonic necklaces, which consist of circular loops of closely spaced gold nanoparticles, are designed using three-dimensional FDTD simulations, fabricated with electron-beam lithography, and characterized by using dark-field scattering and surface-enhanced Raman spectroscopy of pMA monolayers. I show that such necklaces are able to support hybridized dipolar scattering resonances and polarization-controlled electromagnetic hot spots. In addition, necklaces exhibit strong intensity enhancement when the necklace diameter leads to coupling between the broadband plasmonic resonance and the circular resonator structure of the necklace. These necklaces lead to stronger field intensity enhancement than nanoparticle monomers and dimers, which are also carefully studied.
|August 13, 2012||
"Topology optimization for the design of 'structure'," James Guest, Johns Hopkins University, hosted by Daniel Lopez
Abstract: Topology optimization has long been touted as a powerful tool capable of discovering innovative design solutions. It is currently being used to design "structures" defined at a range of length scales, from tens of microns to decameters, for performance properties governed by a range of mechanics, most notably solid and fluid mechanics. While its potential power is evident, it is well known that topology optimized solutions may be suboptimal when considering real-world manufacturing and operating conditions. This talk will briefly review topology optimization methodologies and discuss recent advancements in design for manufacturability, robustness, and multiple performance properties. These advancements will be presented in the context of porous material architectures and component design problems defined at the continuum scale.
|July 17, 2012||
"JSPS and its Programs to Support International Research Collaboration," Fumiyo Kaneko, Japan Society for the Promotion of Science, hosted by Elena Rozhkova
Abstract: The Japan Society for the Promotion of Science (JSPS) is a Japanese research-funding agency. JSPS supports all research fields: physics, mathematics, chemistry, engineering, geology, computer sciences, biomedical sciences, humanities, and social sciences. Its main missions are to (1) distribute research grants; (2) provide fellowships to Japanese young researchers; (3) carry out international programs; and (4) support university reform. Through its third mission, in addition to the international cooperative programs that JSPS runs with nearly 90 overseas counterpart agencies such as NSF and NIH, JSPS also offers various invitation fellowship programs to Japan for overseas researchers and graduate students in accordance with each research career. Through these programs, every year JSPS supports thousands of researcher/graduate student exchanges. I will introduce JSPS and its international programs to support international research collaboration.
|July 3, 2012||
“Coherent X-ray Diffraction Imaging and Bragg Geometry Ptychography on Highly Strained Silicon-on-Insulator Nanostructures,” Xiaowen Shi, University College London, hosted by Ian McNulty
Abstract: Silicon-on-Insulator (SOI) technology has been widely recognized as a major industrial breakthrough during the past decade, offering significant improvements in metal oxide semiconductor field-effect transistor (MOSFET) device performance. An SOI wire of dimensions XYZ was fabricated by electron-beam lithography and reactive ion etching and measured by coherent X-ray diffractive imaging (CDI). This technique is able to investigate strains, seen as atomic displacements from the ideal atomic position in a crystal lattice, of blocks of material with dimensions of 10 nm to 1 µm. We found that the structure of the SOI nanowires changed systematically as a function of exposure time in the beam. We explain this behavior as a radiation-induced bending effect caused by damage of the underlying oxide. Finite-element analysis (FEA) calculations were performed (by using the COMSOL Multiphysics© package) to simulate the diffraction patterns, taking into account the X-ray beam profile.
In the future, we plan to use the COMSOL-based FEA simulations to improve the algorithms for CDI. The present CDI method uses a hybrid input-output algorithm in alternation with a conventional error-reduction algorithm to phase the oversampled diffraction patterns measured by CXD. However, this approach generally fails to reconstruct the diffraction patterns of highly strained objects. We plan to use simulated models as starting points for reconstruction of experimental data, solving for deviations from an already close model. We believe that if the phase errors are less than π, the stagnation of the current algorithms will be avoided. Our initial results give us some optimism that a more detailed understanding of the bending and distortion of silicon nanostructures can be obtained in the near future. We hope this method and future Bragg geometry ptychography techniques will encourage further studies on SOI-based devices, which could be vital for pinpointing the underlying mechanisms that are responsible for observed improvements on carriers’ mobility. This may open a new path for direct observation and in situ characterization of nanostructure semiconductor devices ranging from sensors and optoelectronic devices.
|June 25, 2012||
“Elemental/Chemical TXM Imaging at SSRL,” Yijin Liu, SLAC National Accelerator Laboratory , hosted by Robert Winarski
Abstract: With the use of the third-generation synchrotron as X-ray source, full-field X-ray imaging and X-ray absorption spectroscopy (XAS) can be combined. In situ and ex situ elemental/chemical sensitive microscopy has shown great potential for application in various research areas. The development of the methodology will be described and discussed.
In addition, we will present results of the in situ and ex situ investigation of lithium-ion battery electrode materials using transmission X-ray microscopy. Morphological change of the active material is observed in real time as the battery is charged and discharged, revealing the correlation among structure, chemistry, and functionality. The chemical/elemental sensitive diagnostic of working catalyst solids will also be presented.
|June 13, 2012||
"Quantum phase transitions in SrCu2(BO3)2 and NiS2 revealed by X-rays and transport," Arnab Banerjee, The University of Chicago, hosted by Ian McNulty
Abstract: Quantum phase transitions (QPTs) are fundamentally different from their classical counterparts, mixing statics and dynamics. I will describe our experimental approaches to two such QPTs. In the first part, I will show that precise measurement of lattice distortions can reveal information about magnetic phase transitions in a material with strong spin-lattice coupling. Combining synchrotron X-ray and diamond anvil cell techniques, we consider a set of interacting dimers on a square lattice to uncover the magnetic phase diagram of the Shastry-Sutherland model in SrCu2(BO3)2. For the second part, I will consider quantum criticality at a Mott-Hubbard metal-insulator transition in NiS2. We have used X-ray diffraction to track magnetism and lattice symmetry to show that neither plays a driving role at the phase transition and that the transition is solely driven by electronic correlations. Finally, we explore the critical region of the phase transition in search for scaling laws by using high-pressure transport measurements.
|April 27, 2012||
"Record-Low Resistivity Zinc Oxide as a Transparent Electrode in Terawatt Photovoltaics," Meng Tao, Arizona State University, hosted by Dave Czaplewski and Liliana Stan
Abstract: This presentation will be divided into two parts. Part 1 will discuss natural resource limitations to terawatt-scale deployment of first- and second-generation solar cells, from which several strategic research directions are identified for solar electricity to become a significant source of energy in our society. Part 2 will showcase one example of our approach to post-silicon terawatt-scale photovoltaics (i.e., solution synthesis of an Earth-abundant, low-cost and high-performance transparent conducting oxide). Our recent demonstration of a record-low resistivity in electrochemically deposited zinc oxide will be highlighted. The major barriers for industrial adoption of this new TCO will be discussed.
|March 14 2012||
"Heterogeneous Catalysis for Energy and Sensor Applications," Subasri M. Ayyadurai, University of Cincinnati, hosted by Elena Rozhkova
Abstract: One of the most promising candidates for clean energy is hydrogen-based energy systems such as fuel cells. Platinum and platinum alloys are the most commonly used catalysts for proton-exchange membrane fuel cells (PEMFCs). Much research has been carried out to minimize the catalyst loading. This presentation demonstrates a promising alternative for preparing platinum catalysts through pulse electrodeposition directly over the gas diffusion layer (GDL). Specifically, surface activation of the GDL was found to affect the platinum loading. The performance of the catalyst layer was optimized by using a wetting agent, immersion time in the wetting agent, and various pulse parameters. Electrodes prepared by using pulse deposition under optimized conditions performed better than commercial electrodes, resulting in 1.92 ìm thickness, uniformly distributed platinum nanoparticles of 3-4 nm with loading of 0.33 mg/cm2 exhibited 746 mA/cm2 at 0.7 V.
I will also discuss how perfluorosulfonic acid (PSA) membranes such as Nafion can be used as heterogeneous catalysts in portable sensor applications. Continuous onsite monitoring of personal exposure to occupational chemical hazards in ambient air is a long-standing analytical challenge. Trimellitic anhydride (TMA) is used as starting material for various organic products and is recognized to be an extremely toxic agent by the National Institute for Occupational Safety and Health (NIOSH). The resorcinol molecule is shown to become immobilized in PSA membranes and diffusionally constrain an orange brown product that results from an acid-catalyzed reaction with TMA. FTIR, UV/VIS, reaction selectivity to TMA versus trimellitic acid (TMLA), and homogeneous synthesis are used to infer 5,7- dihydroxyanthraquinone- 2- carboxylic acid as the acylation product of the reaction. The color response has sensitivity to at least 3 ppb TMA exposure and, in addition to TMLA, excludes maleic anhydride and phthalic anhydride. This simple system based on catalysis on PSA membranes is extended to the identification of biomarkers such as acetone vapors for glucose level monitoring.
|March 6, 2012||
"Understanding moleculre-plasmon coupling, " Lasse Jensen, The Pennsylvania State University, hosted by Stephen Gray
Abstract: Controlling the optical behavior of molecules in the vicinity of noble metal nanoparticles continues to be an active research area in nanoscience. A molecular-level understanding of the optical properties of such metal-molecule complexes is important for many applications, such as energy harvesting, nanoscale optical circuits, and ultrasensitive chemical and biological sensors.
In this talk, we will discuss recent theoretical studies aimed at understanding the coupling between molecules and plasmons. We will show how electrodynamics simulations can be used to describe the optical properties of mixed exciton-plasmon states arising when strongly absorbing dyes interacts with plasmons. Electronic structure methods will be used to explore the chemical coupling in surface-enhanced Raman scattering (SERS), and resonance effects in SERS and surface-enhanced hyper-Raman scattering.
|February 20, 2012||
"Semiconductor Nanocrystals: From Quantum Dots to Quantum Disks," Zheng Li, University of Arkansas, hosted by Gary Wiederrecht and Yugang Sun
Abstract: Colloidal semiconductor nanocrystals are nanometer-sized fragments of corresponding bulk crystals synthesized in solution. When the size of semiconductor nanocrystals is smaller than the exciton Bohr radius, the exciton will experience quantum confinement, which results in size- and shape-dependent optical and electronic properties. While synthesis of semiconductor nanocrystals has been in rapid development, related molecular mechanism studies are rare, which leaves the synthetic chemistry of colloidal nanocrystals at an empirical level. This talk will illustrate that systematic and quantitative study of molecular mechanism for the formation of colloidal nanocrystals is possible, and the results will not only help us to understand formation mechanisms of colloidal nanocrystals but also advance their synthetic methods.
At present, synthetic methods for colloidal semiconductor nanocrystals with three-dimensional quantum confinement (quantum dots) and two-dimensional confinement (quantum rods) have been reasonably developed. This talk will discuss formation of colloidal-stable disk-shaped II-VI semiconductor nanocrystals as one-dimensional quantum confinement systems (quantum disks). Some unique properties of these new quantum objects, such as size-dependent lattice dilation, extremely sharp band-edge photoluminescence, and two-orders-of-magnitude faster photoluminescence decay compared with quantum dots, will be discussed.
|February 17, 2012||
"Graphene: Revisiting old questions in a new material," Shaffique Adam, Center for Nanoscale Science and Technology, National Institute of Standards and Technology , hosted byStephen Gray
Abstract: From the hard drives that harness giant magnetoresistance to the transistors that drive modern processors, solid-state physics is at the very heart of the technological revolution. Implied in this effort is a thorough understanding of electronic systems in nanoscale geometries. In this context, the complex interplay among disorder, electron-electron interactions, and quantum interference is an interesting backdrop to many of the unsolved mysteries in condensed matter physics.
About five years ago, a new electronic material appeared that was notable not only for its ease of preparation and theoretical simplicity, but also for its promise for future electronic devices. Single monatomic sheets of carbon, known as graphene, have an electronic dispersion that is reminiscent of light in that they can be described as a massless Dirac particle. In many ways, graphene is a textbook system to test physical models; for instance, similar to field-effect transistors, the electron density in graphene sheets can be modulated by a backgate. However, unlike conventional semiconductors, the carrier density can be continuously tuned from electron-like carriers for large positive gate bias to hole-like carriers for negative bias, with the Dirac point defined as the singularity that marks the transition from electrons to holes.
When graphene is close to charge neutrality, its energy landscape becomes highly inhomogeneous, forming a sea of electron-like and hole-like puddles, which determine the properties of graphene at low carrier density. In this talk, I will discuss how the electronic properties of the Dirac point provide an intriguing example of how the competing effects of disorder, electron-electron interactions, and quantum interference conspire together to give a surprisingly robust state whose properties can be described using semiclassical methods. Armed with this success, I will discuss how future graphene experiments could shed light onsome long-standing open questions in condensed matter physics.
|February 10, 2012||
"Functional Nanocomposites, Block Copolymers, and Nanocrystals: From Synthesis, Self-Assembly to Application," Lei Zhao, Georgia Institute of Technology, hosted by Xiao-Min Lin
Abstract: Conjugated polymer-nanocrystal nanohybrids, capitalizing on the advantages peculiar to solution-processable conjugated polymers (CPs) in conjunction with the high electron mobility and tunable optical properties of inorganic nanocrystals (NCs), have attracted considerable attention as candidates for achieving high-efficiency organic photovoltaics at low cost. The most elegant approach to obtaining CP-NC nanohybrids is to chemically tether CPs on the NC surface.
In our study, semiconductor organic-inorganic nanocomposites were synthesized by directly graftinga CP, poly(3-hexylthiophene), onto cadmium selenide nanorod surface (i.e., preparing P3HT-CdSe NR nanocomposites). The direct grafting was accomplished by two coupling reactions: Heck coupling of vinyl-terminated P3HT with bromobenzylphosphonic acid functionalized CdSe NRs (i.e., BBPA-CdSe), and a newly developed catalyst-free click reaction of ethynyl-terminated P3HT with azide functionalized CdSe NRs. Such rationally designed nanocomposites possess a well-defined interface between P3HT and CdSe NRs, thereby promoting the effective dispersion of CdSe NRs within the nanocomposites and facilitating their electronic interaction. Moreover, a star-like block copolymer templating method was pioneered to craft various extremely stable CP-NC nanocomposites(e.g., P3HT-CdSe and P3HT-TiO2) in which CPs were covalently bonded onto the NC surface, thereby eliminating the problem of the association and dissociation of surface ligands as in copious past work.
Novel strategies of nanoparticle fabrication are of fundamental significance in the advancement of nanoscience and nanotechnology. We developed a simple, convenient, and unified strategy for the fabrication of a large variety of nanoparticles (homo, core/shell and hollow) via a novel amphiphilic star-like block copolymer as unimolecular templates with different chemistries and properties, controllable diameters, and low dispersity. These nanoparticles include metal, ferroelectric, superparamagnetic ion oxide nanoparticles, semiconductor nanoparticles, and so on. Our strategy provides a simple and convenient route to a variety of building blocks for assembling nanomaterials with novel structure and properties in nanoscience and nanotechnology
As a typical film preparation method, the Langumuir-Blodgett (LB) technology has been widely use to produce copolymer films with mono- or multi-molecule layers for potential applications in microlithography, devices, and biomimetic thin films. In our study, self-assembly of a series of newly synthesized functional block copolymers (e.g., conjugated, biodegradable, responsive) with various novel structures (linear, brush, comb, and star-like) were systematically explored by using the LB technique. The influence of the chemical composition and molecular architectures on the self-assembly process was carefully investigated. Various models were proposed to elucidate the complex dynamic self-assembly process. This study not only complements the well-known models of self-assembly of amphiphilic block copolymers at the air/water interface, but also provides a general means of fabricating LB monolayers into controllable structures and integrating the intriguing functionalities in a desirable manner. In addition to the air/water interface, the oil/water interface was also used to create robust photoluminescent polymer emulsions that can be potentially used in drug delivery and solar cells.
|February 1, 2012||
"Electronic Transport and Multiferroic Nanostructures," Wei Ren, University of Arkansas, hosted by Stephen Gray
Abstract: Electronic transport of charge and spin can be controlled by external electric and magnetic fields. One of the most remarkable transport phenomena at mesoscale and nanoscale is universal conductance fluctuation. Because of quantum interference, the amplitude of such fluctuation stays constant independently of the average conductance and sample details. Based on a variety of nanomaterials, we will show results from perspectives of the electron's intrinsic charge and spin properties. Then we discuss multiferroic materials that possess both ferroelectric and magnetic orderings, such that the electric field can control magnetic properties and vice versa. Particularly, we focus on the formation and characteristisc of vortex in some perovskite oxide nanostructures. The computational and theoretical methods employed in these studies will be introduced.
|January 27, 2012||
"First-Principles Study of Redox Reactions: Oxidation States Electron Transfer and Reactions of a Novel H2-Evolving Electrocatalyst," Hoi Land Sit, Princeton University, hosted by Stephen Gray
Abstract: Quantum-mechanical simulations have been used extensively to provide valuable insights and realistic predictions in the study of problems of scientific and technological importance because of the ever-increasing computer power and the development of more accurate first-principles methodologies. In this talk, I will discuss the study of electron transfer reactions via first-principles molecular dynamics simulations. Accurate computational studies of electron transfer reactions require proper choice of the reaction coordinate, correct definition of the oxidation states, and careful consideration of the solvent effects on the electron transfer process. I have developed a novel approach to determine oxidation states from first-principles calculations. Accompanying the method is a simple and unambiguous definition of oxidation states that contributes to the resolution of the long-standing problem of defining oxidation states in quantum-mechanical calculations. It also helps to provide accurate computational descriptions of redox reactions. Using the ferrous-ferric self-exchange reaction as a prototypical example, I have introduced a method to calculate the diabatic free-energy surfaces of electron transfer full reactions by using first-principles molecular dynamics. Coupled with an approach to evaluate the electronic coupling parameter, the quantum-mechanical estimate of the electron transfer rate is significantly improved.
In the second part of the talk, I will report the recent study of a bio-inspired H2-producing electrocatalyst. This novel electrocatalyst consists of the active center of di-iron hydrogenase attached directly to a pyrite electrode. A detailed mechanistic understanding of the H2-evolution reaction is obtained through first-principles molecular dynamics simulations and the maximally localized Wannier function analysis technique. The O2 sensitivity of this eletrocatalyst is also investigated by using extensive density-functional calculations and first-principles molecular dynamics simulations.
|January 26, 2012||
“Nano-Optics and Chemistry,” Prashant K. Jain, University of Illinois, Urbana-Champaign, hosted by Gary Wiederrecht
Abstract: I will describe how simple solid-state chemistry can be used to manipulate, in completely new ways, the behavior of electrons and photons at the nanoscale. For instance, we can now generate localized surface plasmon resonances (LSPRs) in semiconductor quantum dots by introducing vacancies in the nanocrystal lattice. While reminiscent of their counterparts in metal nanoparticles, plasmons in quantum dots represent a paradigm shift, especially because these resonances, unlike those in metals, are actively tunable via electrochemistry, temperature, and crystallographic phase changes. As another example, in nanocrystals of semiconductors, all the cations can be simply replaced by another cation, without affecting the size, shape, and interfaces within the nanostructure. By means of ion exchange, one can essentially use nanocrystals as templates to synthesize materials that are otherwise inaccessible and engineer band-gaps in ways otherwise not possible. Finally, I will tell you about impurities in nanocrystals that can have a strong detrimental effect on optoelectronic performance, even at the level of few atoms/nanocrystal. We have, however, found that it is relatively easy to purify nanocrystals of such impurity atoms, resulting in two orders-of-magnitude enhancement in optical quality.
|January 10, 2012||
“Active Optical Metamaterials and Broadband Plasmonic Absorbers,” Koray Aydin, Northwestern University, hosted by Gary Wiederrecht
Abstract: Nanophotonics, the emerging field of photon-material interactions at the nanoscale, poses many challenges and opportunities for researchers engineering devices with subwavelength features. Plasmonic nanostructures and metamaterials exhibit optical properties not seen in conventional photonic materials and enable focusing, guiding, bending, and absorbing photons at the nanoscale. They are poised to revolutionize a broad range of applications including energy, communications, defense, and sensing.
In this seminar, I will describe the design, nanofabrication, and optical characterization of engineered nanophotonic materials that enable controlled and enhanced photonic functionalities. First, I will introduce frequency-tunable, hybrid infrared metamaterials, in which a dynamic optical response is achieved via a thermally induced phase transition in vanadium dioxide (VO2) nanostructures.
I will also present how the mechanical actuation of flexible polymers can be used to control the nanoscale distances between coupled metallic resonators, in turn enabling frequency-tunable, compliant optical metamaterials. Such reconfigurable nanophotonic materials significantly enhance the infrared reflection signal from a C-H vibrational mode, and could find use in biochemical sensing and environmental screening applications.
Finally, ultrathin, polarization-insensitive, broadband plasmonic super absorbers capable of absorbing light over the entire visible spectrum will be demonstrated. These uniquely shaped plasmonic nanostructures could be utilized in solar energy conversion applications for efficient light-trapping and photon management in photovoltaic and thermophotovoltaic cells.
|January 5, 2012||
“Structure, properties, and optimization: First-principles and atomistic modeling of renewable energy materials, ” Maria Chan, Center for Nanoscale Materials and Center for Electrical Energy Storage, Argonne National Laboratory, hosted by Stephen Gray
Abstract: Atomistic and first-principles computational modeling has become indispensable in the understanding and design of materials. At the heart of such modeling are three main questions: What are the physical atomic configurations? What are the relevant materials properties as predicted from these configurations? And can we invert the structure -> property maps to optimize the properties?
In this talk, I will discuss examples in which each of these is addressed by the use of novel algorithms extracted from the pertinent physics, in applications towards thermoelectrics, photovoltaics, and energy storage. Specifically, I will discuss configurational determination in lithium battery materials, accurate and efficient prediction of semiconductor band gaps and band edges for the screening of photovoltaic and photocatalytic materials, and optimization ofthermal conductivity in SiGe nanowires.