Fall 2009 Physics Colloquia

August 21, 2009

Department of Civil and Environmental Engineering, University of Virginia.

For the past decade, several non-profit organizations such as Potters for Peace have promoted the use of locally produced ceramic water filters impregnated with silver nanoparticles for point-of-use (household) water treatment. Despite their increasing production and use in developing-world communities, there has been relatively limited assessment of the technological performance, design, and social acceptance of these filters. In this presentation, I will we present data and analyses that critically evaluate these parameters in both field and laboratory settings. In the laboratory, bacteria (E. coli) and virus (MS2 coliphage) transport experiments have evaluated the technological performance of the filters, the design elements that maximize pathogen removal, and the effects of silver nanoparticles on filter performance. In the field, we have evaluated technological performance with respect to turbidity, coliform bacteria, and E. coli removal in the Guatemalan highland community of San Mateo Ixtatan.

August 28, 2009

VMEC Associate Professor
Applied Science Department,
Physics Department
College of William and Mary

Magnetic materials are interest for their fundamental properties and also for their use in a variety of applications. Our research group is interested in magnetic thin films and nanoparticles and recently we have been working on systems that exhibit proximity effects, on highly ordered magnetic alloys as well as "magnetoplasmonic" materials. In my talk I will present an overview of these various research thrusts with emphasis in the latter.

Light can be localized and manipulated in appropriately designed metallic and metallo-dielectric nanoparticle arrays and/or thin film structures. In particular, interesting phenomena occur near the plasmon frequency where optical extinction is resonantly enhanced and at the plasma frequency where the real part of the dielectric function changes sign. This phenomenon is very sensitive to slight changes in the dielectric constant at the surface and therefore Surface Plasmon resonance has been successfully applied to bio-sensing. [1] One interesting property of surface Plasmon excitation is that the associated electric field is strongly enhanced near the dielectric-metal interface. [2] Therefore we expect that Plasmon excitation can also enhance magneto-optical activity in magnetic layers and/or nanomagnets. The problem is that transition metals such as Fe, Ni and Co exhibit magneto-optical effects accessible at relatively low field, but their absorption coefficients are higher than those of Au and Ag, and therefore their SP's are very damped. One possibility to solve this is to use a composite noble-metal/ferromagnetic-metal system to achieve Plasmon enhancement of the magneto-optical activity. This opens up the interesting possibility of using magnetic fields to influence surface plasmon polaritons (SPP) and therefore mixing magnetic and plasmonic materials (i.e. "magnetoplasmonic" systems) seems a promising approach for obtaining externally controlled systems.

Our research group is currently investigating the optical and magneto-optical response of Au-Co layered structures and has recently initiated collaboration with Professor E. Carpenter to study core-shell AgFe nanoparticles. We study the main physical mechanisms playing a role in the magneto-optical activity these materials. We are currently investigating nano-patterning the film surface as an alternative mechanism to promote the coupling of the light and the surface Plasmon polaritons for magneto-plasmonic systems.

REFERENCES

1. Jung, L.S. ,Nelson, K.E., Campbell, C.T., Stayton, P.S., Yee, S.S., Perez-Luna, V., Lopez, G.P., Sensors and Actuators B 54, 137-144 (1999); Lu, H.B., Homola, J., Campbell, C.T., Nenninger, G.G., Yee, S.S., Ratner, B.D. , Sensors and Actuators B-Chemical 74(1-3), 91-99 (2001); Jung, L. S., Campbell, C. T., Chinowsky, T. M., Mar, M. N., Yee, S. S., Langmuir 14, 5636-5648 (1998).
2. C. Hermann, Phys. Rev. B 63, 235422 (2001)

September 4, 2009

Department of Chemistry
Virginia Commonwealth University.

The construction of highly porous organic and inorganic polymers with precise control over the dimensions and shapes of nanocavities have opened new avenues for applications in gas storage and separation. The design, synthesis, and characterization of selected examples will be presented and their performance in gas storage and separation is discussed. In particular, the ability to remove carbon dioxide from methane or hydrogen is highly valued since the processes provide higher energy densities. The interest in developing porous organic polymers stems from the fact that such materials would be of low density and are strongly linked by covalent bonds. An advantageous feature of the presented polymers is their functionalizable channels which can be created through post-synthesis modification. This feature is highly desired in hydrogen host materials because it increases surface area and allows for H2 molecule polarization and hence improved storage capability. Furthermore, the chemical and electronic nature of these polymers makes them also attractive candidates for use in photovoltaic applications. Theoretical calculations predict the 2D-polymers to have metallic or semiconductive properties while the 3D-polymers are insulators. Therefore, we believe such polymers will be of interest to researchers in engineering, materials, and physics.

September 11, 2009

Naval Research Laboratory

Recent price fluctuations at the gas pump have brought our attention to the phenomenal increase of global energy consumption in recent years. It is now evident that we have almost reached a peak in global oil production. Several projections indicate that total world consumption of oil will rise by nearly 60% between 1999 and 2020. In 1999 consumption was equivalent to 86 million barrels of oil per day, which has reached a peak of production extracted from most known oil reserves. These projections, if accurate, will present an unprecedented crisis to the global economy and industry. As an example, in the US, nearly 40 % of energy usage is provided by petroleum, of which nearly a third is used in transportation. The US Department of Defense (DOD) is the single largest buyer of fuel, amounting to, on the average, 13 million gallons per day. Additionally, these fuels have to meet different requirements that prevent use of ethanol additives and biodiesel. An aggressive search for alternate energy sources, both renewable and nonrenewable, is vital. The presentation will review national and DOD perspectives on the exploration of alternate energy with a focus on energy derivable from the ocean.

September 18, 2009

RB Murray Professor and Chair
Physics and Astronomy
University of Delaware

In this presentation, I will review our research projects for the past few years on the fabrication and characterization of magnetic nanoparticles for the following three novel applications; (i) high density recording media (ii) biomedical applications and (iii) next generation permanent magnets. For the high recording density media our efforts are focused on chemically synthesized FePt nanoparticles with controlled size, shape and tailored magnetic properties. For the biomedical applications our research is focused on metallic Fe-based nanoparticles with high magnetization, which are first made biocompatible and then functionalized for drug delivery. Our research on advanced permanent magnets is focused on the magnetically hard Sm-Co and Nd-Fe-B nanoparticles which can then be mixed with magnetically soft Fe(Co)-based nanoparticles in [3D] arrays to form nanocomposite magnets with giant energy products.

September 25, 2009

Naval Research Laboratory, Code 6390
Washington DC 20375-5345

A very brief introduction to density functional theory is provided within the context of a recent calculation on electron transport across a spin-unpolarized molecular system. For this system, it is shown that DFT and relatively standard transport theory may be used to qualitively explain how the current-voltage characteristics change when a gas-phase molecule docks onto an oligiophenylenevinulene wire-like molecule.[1] After doing so, a discussion is provided on recent attempts to develop a DFT-based many-electron approach that has sufficient complexity to simulate transport across molecular magnets or nanostructures.

In contrast to ordinary molecules, a molecular magnet is composed of a collection of six-fold coordinated spin-polarized transition-metal ions that are held together by ligands. When exchange interactions between neighboring metal centers are strong enough, the molecule can be viewed as being in a nearly classical configuration consisting of a single spin-ordered ground-state with a high degeneracy. Inclusion of the spin-orbit interaction breaks this degeneracy at second order. From a technological standpoint, an interesting case occurs when the molecule has uniaxial symmetry as this leads to a case where the energies of the lowest spin manifold can be characteristic of a nearly classical bar magnet. However these nanoscale molecules still behave as a quantum systems with 2S+1 magnetic states that are split into a collection of doublets and one singlet. Resonant tunneling of magnetization between these states has been observed and is now well established. Density-Functional Theory has proved to be a relatively accurate way to study molecular magnets.[2-3]

At the nanoscale, the DFT-based modeling of electron transport across a magnetic system may be significantly more complicated than for the case of spin-unpolarized molecules. Interactions between a molecular magnet and it's surroundings effect the collective magnetic behavior of the molecule. Possible effects that have been experimentally observed and/or computationally investigated include: (1) dramatic changes in spin ordering or magnetic anisotropy barriers due to the addition of one or two electrons, (2)Enhanced tunneling rates or the appearance of tunnel-splitting oscillations (Berry's Phase) due to weak interactions between the molecule and space-filling spectators (such as H₂O), (3) Changes in the magnetic anisotropy Hamiltonian due to effects such as pressure or physisorption of the molecular magnet on a surface/electrode.

With the Mn₁₂-Acetate molecule as a concrete example, I will discuss recent work aimed at developing a compact many-electron and many-spin representation of molecular magnets as a function of charge/excitation and applied electric and magnetic fields. A recent application of this theoretical and computational method will be presented as well (L. Michalak, C. Canali, M.R. Pederson, M. Paulson, and V. Benza, http://arxiv.org/PS_cache/arxiv/pdf/0812/0812.1058v1.pdf).

MRP thanks NRL and NRL CCMS for providing an environment for performing the recent parts of this work. Computational support has been provided by the DOD-HPCMO.

1. Nanoparticle networks as chemoselective sensing devices, N.A. Zimbovskaya, M.R. Pederson, A.S. Blum and B.R. Ratna, J. Chem. Phys 130 094702 (2009).
2. Molecular Magnets: Phenomenology and Theory, M.R. Pederson and T. Baruah, pp 1-22 in Handbook of Magnetism and Magnetic Materials, Ed. By S. Parkin (J. Wiley and Sons, London 2007).
3. Magnetic anisotropy barrier for spin tunneling in Mn₁₂O₁₂ molecules, M.R. Pederson and S.N. Khanna, Phys. Rev. B 60 9566-9572 (1999).

October 2, 2009

Department of Applied Science
The College of William and Mary

Graphene-based polymer nanocomposites are very promising candidates for new multi-functional, high-performance materials that offer improved mechanical, barrier, thermal and electrical properties. In our laboratory, we synthesize different kinds of single-layer, functionalized graphene sheets in gram quantities. The surface functionality of these sheets can be adjusted easily, which allows us to make stable dispersions in a large range of solvents. Employing the good solvent compatibility of these sheets, we make nanocomposites with different polymers via solution processing. Due to the outstanding characteristics of graphene, some of these nanocomposites exhibit significantly enhanced properties at small nanofiller concentrations, including increased stiffness, strength, electrical conductivity, as well as improved thermal and barrier properties. We believe that the mechanical performance of these materials can be even further improved once the underlying mechanisms are better understood. Therefore, we use scanning probe techniques to investigate and manipulate these materials at the level of individual filler nanoparticles. We are able to probe the mechanical properties of individual graphene sheets, and based on a new technique we can directly measure their interactions with the different polymer host materials. Moreover, we are developing new methods that will allow us to “look inside” nanocomposites. Our ultimate goal is a systematic design of new nanomaterials with tailored properties.

October 9, 2009

Jefferson Lab, Suite 1, 12000 Jefferson Ave., Newport News VA 23606 USA

Subtle deviations from our expectations have always served as a sign to possible new discoveries in science. In subatomic physics there has been considerable progress recently in the accuracy to which the predictions of QCD, a fundamental piece of the Standard Model, can be rigorously tested. The analysis of parity violating electron scattering has also led to new constraints on the mass scale associated with potential new physics. We shall review some highlights in these areas.

October 16, 2009

No Colloquium

October 23, 2009

Department of Chemistry
Virginia Commonwealth University

Abstract

October 30, 2009

Department of Electrical and Computer Engineering
University of Cincinnati

Disordered networks of covalent solids acquire unusual functionalities in a narrow but well defined range of connectivity, also known as Intermediate Phases¹ (IPs). Chalcogenide glasses1 are illustrative examples of covalent systems that display IPs. Experiments show IP glasses do not age much, form space filing networks, possess liquid-like entropies, exhibit dynamic reversibility, form rigid but stress-free networks, and display optimal glass formation, i.e., they are ideal glasses. We shall give an overview of IPs observed in these systems, and show that these data also provide clues on aspects of local structures that determine IPs, information that will assist numerical simulations of these phases.

1P. Boolchand, M.Micoulaut and P. Chen, "Nature of Glasses" in Phase Change Materials, Ed. S. Raoux and M. Wuttig, Springer Science + Business Media, LLC 2009, Chapter 3, p.39-59

November 6, 2009

University of Maryland

Understanding why there is only matter and no anti-matter in the universe has been a major puzzle in physics and astronomy for a long time. With the recent discovery of non-zero masses for a tiny particle called neutrinos, some clues to solving this problem by using new microphysical laws of nature seem to have emerged. Two apparently unconnected topics seem to be influencing each other. Much remains to be understood and verified. I will give a pedagogical review of our current understanding this exciting development and how to test the new ideas behind this using currently available facilities including the Large Hadron Collider in Europe.

November 13, 2009

James Madison University

Since the invention of Scanning Tunneling Microscopy by Binnig and Rohrer in 1981, a new class of microscopes, dubbed scanned probe microscopes (SPM) has been used to look into a wide variety of materials properties with unprecedented resolution. These microscopes are capable of measuring and/or manipulating over a dozen physical properties on length scales as small as Angstroms. Despite resolution that rivals the most sophisticated electron microscopes, the principles of operation can be readily understood from a few simple principles. In addition, a small SPM can fit in briefcase. This creates a unique opportunity, in which state of the art research tools can readily be brought into high school and college classrooms. In this talk I will present an overview of the operation of several types of SPM, including a brief survey of some interesting results from the literature. I will then focus on our efforts at James Madison University to incorporate scanned probe microscopy into the high school and undergraduate curriculum. The capability of a virtual reality interface for remote operation of an SPM will be demonstrated.