Symposium on "Physics of Complex Materials"
Acknowledgments:This Symposium was made possible by the support received from the Liberal Studies Project of the College of Arts and Sciences. The Liberal Studies Project expands and enriches the intellectual life of the University of Louisville and the wider community through interdisciplinary initiatives such as lectures, workshops, exhibitions, symposia, and the Distinguished Visiting Scholars program.
| 09:00 - 09:30 am | C.S. Jayanthi, Department of Physics and Astronomy |
| 09:30 - 10:00 am | Prof. P. L. Taylor, Department of Physics, Case Western Reserve University |
| Band Gaps and Little Piggies -- Some thoughts from the 1960's regarding the density of states in disordered systems | |
| One of the principal difficulties in calculating the properties of disordered systems is that disorder is an ill-defined concept. An ordered lattice can be completely described, but an amorphous solid is just one exemplar of an infinite number of possible realizations. In this talk I will give an account of how Shi-Yu and I approached this problem in one dimension, and then outline a proof of the existence of band gaps in some three-dimensional disordered solids. | |
| 10:00 - 10:30 am | Abdelkader Kara, Department of Physics, University of Central Florida |
| A Real Space Approach à la Wu | |
| Volker Heine, in his seminal work (Solid State Physics Vol. 35, 1980), wrote about “throwing out k-space,” and Shi-Yu Wu responded with an equally provocative paper entitled, “Think globally and act locally.” Wu has developed an efficient approach to deal with systems with no long-range order. He formulated an iterative scheme that solves exactly any eigenvalue problem where the interaction between atoms is “short-sighted”. I had the privilege to work with Shi-Yu during the period 1992-94 and learned about his Real Space Green’s Function (RSGF). After that, I have applied this method to calculate the vibrational density of states and the corresponding thermodynamics functions for a variety of systems including vicinal surfaces, adatoms, 2D islands and nano-systems. I have also used it to re-determine the pre-exponential facto associated with surface diffusion. In my talk, I will present an overview of the research projects I have performed using the RSGF à la Wu. | |
| 10:30 - 10:45 am | Coffee Break |
| 10:45 - 11:15 am | Sidney Yip, Department of Nuclear Science and Engineering, MIT |
| Multiscale Materials Modeling: Then and Now | |
| In 1997 Shi-Yu Wu took a sabbatical leave at the Institute for Theoretical Physics at the University of California at Santa Barbara, where he was a core participant in a Long Program on Methods for Materials Research. An emerging concept at the time was multiscale modeling and simulation. He made notable contributions to the discussions on and further developments of the concept of Real Space Green’s Functions, including an application to probe the onset of crack nucleation in a crystal lattice under stress [1]. Seventeen years later, computational materials research has grown to be a major thrust in the landscape of science and technology. On the occasion of his retirement we take an outlook on current frontiers at the mesoscale, with a few specific examples of ‘dirty physics’ materials problems [2], in the hope that he might find in them irresistible challenges and opportunities (and therefore decide to stay in the game). [1] C. S. Jayanthi, M. Tang, S. Y. Wu, J. A. Cocks, S. Yip, Phys. Rev. Lett. 79, 4601 (1997). [2] S. Yip and M. P. Short, Nature Materials 12, 774 (2013). |
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| 11:15 - 11:45 am | Gamini Sumanasekera, Department of Physics and Astronomy, U. of Louisville |
| In-situ transport study of Graphene-Gas interaction | |
In the first part of the talk, the influence of environmental effects on transport properties of graphene will be discussed. Vacuum-annealed graphene is n-type, but becomes p-type when exposed to ambient air. Here we present evidence that this effect arises from pinning of the Fermi energy by the four-electron oxygen redox couple in an adsorbed water film, a type of surface transfer doping. The pronounced electrical sensitivity to O2 results from electron exchange between the redox couple and the graphene. This effect (performance volatility) must be considered when using graphene based devices for any application in humid air. In the second part of the talk, in-situ electrical properties of graphene subjected to controlled and sequential hydrogenation/fluorination using RF plasma will be discussed and correlated with ex-situ Raman scattering and X-ray photoemission (XPS) characteristics. For weak-hydrogenation, the transport is seen to be governed by electron diffusion and low temperature transport properties show metallic behavior (conductance G remains non-zero as T →0). For strong-hydrogenation, the transport is found to be describable by variable range hopping (VRH) and the low T conductance shows insulating behavior (G → 0 as T → 0). Weak localization (WL) behavior is seen with a negative MR for weakly-hydrogenated graphene and this WL effects are seen to diminish as the hydrogenation progresses. A clear transition to strong localization (SL) is evident with the emergence of pronounced negative MR for strongly-hydrogenated graphene. Transport study on fluorination of graphene shows clear evidence of band gap opening in graphene. |
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| 11:45 - 01:15 pm | Lunch Break |
| 01:30 - 02:00 pm | E. Kaxiras, Department of Physics and School of Eng. and Applied Sciences, Harvard |
| Ab-initio studies of optical excitations in solids and molecules. | |
| Time-dependent density functional theory has emerged as a reliable tool for studying optical excitations that provide insight to complex processes in molecules and on solid surfaces. In this presentation we will give a brief review of the underlying theoretical concepts and discuss several applications related to renewable energy sources, such as organic or hybrid photovoltaic devices and hydrogen fuel production through photocatalysis. | |
| 02:00 - 02:30 pm | Yufeng Zhao, National Renewable Energy Laboratory |
| Towards Rational Design of Functional Nanostructures: Predictive Theory Based on “Mechanism-Structure-Property” Paradigm | |
| The rapid progress in supercomputer technology and first-principles methods provides much more opportunities than before for bottom-up design of new materials, based on mechanistic modeling. Traditionally, the “structure-property” paradigm is widely used to guide research in materials science. The “property” part, representing the function of materials, is the interface between material sciences and engineering. The “structure” part, representing the materials to be designed, is considered as the core of material sciences. We show here that a “mechanism” part must be added to the basic paradigm, in order to take the advantage of atomistic modeling. The term “mechanism” refers to basic atomistic processes involved in material formation and their driving forces including localized chemical bonds, long-range electrostatic attraction or repulsion, delocalized electronic effect, etc. Therefore, the framework of predictive theory for material design should be constructed on the basis of a “mechanism-structure-property” paradigm with the mechanistic understanding emphasized. We will show a perspective of the relationship between these three core concepts, out of which general principles can be derived to guide material design. As case studies, we will discuss the formation of nanoscale structures such as junctions of carbon nanotubes, silicon nanowires and nanoparticles, organometallic nanostructures, and layered structures with versatile valence, and their potential application in microelectronics, hydrogen storage, catalysis, and metal-ion batteries. | |
| 02:30 - 02:45 pm | Coffee Break |
| 02:45 - 03:15 pm | C. Z. Wang, Ames Laboratory and Department of Physics, Iowa State University, Ames |
| Developing Theoretical Methods for Studying Complex Materials | |
| In this talk, I will present several theoretical and computational methods developed in our group at Ames Laboratory for studying complex materials. These methods include tight-binding molecular dynamics for large scale atomistic simulations, adaptive genetic algorithm for crystal structure prediction, and Gutzwiller density functional theory for studying strongly-correlated electron systems. | |
| 03:15 - 03:45 pm | Ming Yu**, Department of Physics and Astronomy, University of Louisville, KY |
| Materials Modeling via a Robust Semi-empirical Hamiltonian: SCED-LCAO | |
| In an effort to improve the predictive power and transferability of semi-empirical Hamiltonians as well as to extend the size-scales involved in quantum-mechanics based simulations, Professor Wu and his colleagues have developed a robust parameterized Hamiltonian (SCED-LCAO) that can predict the structure and electronic properties of complex materials of 10-15 nm size and beyond. A successful model for a semi-empirical Hamiltonian must capture charge redistributions and electron screening in a many-body environment so that bond-breaking and bond-forming processes associated with complex structural reconstructions can be described appropriately. The SCED-LCAO Hamiltonian, based on a linear combination of atomic orbitals (LCAO) framework, has been designed to capture these important processes through environment-dependent (ED) multi-center terms and through charge self-consistency (SC) in the calculation of energy and forces. Additionally, the framework of the SCED-LCAO Hamiltonian allows for charge self-consistency (SC) and environment-dependency (ED) to be treated on the same footing. The SCED-LCAO Hamiltonian has been developed for silicon, germanium, carbon, boron, phosphorous, and nitrogen using an extensive database of properties obtained from first-principles. Boron-based systems are notorious because of their chemical complexity and the success of the SCED-LCAO Hamiltonian in these systems is a testament to the robustness of this Hamiltonian to different atomic environments. In this talk, the success of the SCED-LCAO Hamiltonian will be elucidated through the following applications: (i) the phase transformations of carbon bucky-diamond cluster upon annealing, (ii) the discovery of bucky-diamond SiC clusters, (iii) the morphology and energetics of SiC nanowires (NWs), and (iv) the initial stage of growth of single-wall carbon nanotubes (SWCNTs). **This work was done in collaboration with several colleagues that include: Prof. Shi-Yu Wu, Prof. C.S. Jayanthi, Dr. C. Leahy, Dr. A. Tchernatinsky, Dr. P. Tandy, Dr. I. Chaudhuri, Mr. H. Simrall and Mr. S. Shen. | |
| 03:45 - 04:00 pm | Closing Remarks |
The symposium will be immediately followed by a Farewell Reception & Celebration
5:30pm-7:00pm, University Club Ballroom
Honoring Professor Shi-Yu Wu’s extraordinary academic and scientific contributions