Utilizing Atomic and Molecular Interactions to Guide Assembly of Functional Metal Nanosystems

Featuring Regina Ragan, Ph.D.
Assistant Professor, ChemicalEngineering and Materials Science
The Henry Samueli School of Engineering, UC Irvine

Location:  Donald Bren Hall,  Room 1300
Free and open to the public

Abstract:
The generation of new technology often stems from discovery of physical phenomena, bold ideas for utilization, and focused work on implementation.   In the case of metal systems, it has been discovered that unique optical, chemical and electrical properties are exhibited in structures when feature size is scaled down to molecular length scales. For example, metal nanoparticle based architectures have shown the capacity for transmission of electromagnetic energy and production of highly localized electromagnetic fields for use as optical interconnects and sensors.  Metal nanocatalysts exhibit chemical selectivity and higher activity than their bulk counterparts and one-dimensional, ballistic transport in self-assembled, metallic systems has been observed in carbon nanotubes and yttrium disilicide wires for use as low resistance interconnects. In my group, we are interested in both fundamental understanding of physical properties as feature size decreases and thermodynamic processes governing rational assembly of nanostructures on molecular length scales.

For implementation in devices as catalysts, low resistance inter-connects, and non-linear optical components, metal structures must be fabricated on length scales where properties are enhanced by quantum effects. Self-organization of atomic and molecular ensembles on surfaces allows for implementation of metal nanosystems having feature size and atomically controlled interfaces that are typically unattainable using lithographic techniques.  Ragan will present a few novel self-organization synthesis routes for the fabrication of metallic and bimetallic nanostructures in regular arrays immobilized on substrates followed by characterization of electronic and optical properties.  First, experimental results combined with molecular dynamics simulations reveal thermodynamic driving forces that lead to stable atomic structures of one-dimensional disilicide nanowires.  Rare earth disilicide nanowires exhibit one-dimensional transport and are used as a template for noble metal assembly to produce bimetallic systems.   Calculated adsorption energies of noble metal atoms on nanowires as well as theoretically predicted electronic properties will be compared with those measured using scanning tunneling microscopy and Kelvin probe force microscopy. Analyses in charge redistribution and density of states shed some light for the chemical properties of these core shell nanostructures, which are important for applications such as catalysis.  In the second case, self-organized diblock copolymers are used as a template for assembling metal nanoparticles from colloidal solution. Chemical domains of 20 nm and periodicity of 45 nm were achieved. Covalent substrate-particle interactions were utilized to produce nanometer scale inter-particle spacing and surfaces resistant to metal nanoparticle aggregation that typically results from Brownian motion on these length scales. The combination of diblock copolymer templates and metal colloids has the flexibility in design for producing ordered assemblies of noble metal polyhedron for fundamental understanding of how size, shape, material and architecture affects resonantly enhanced excitations of surface plasmons for sensor and non-linear optical applications.

About the Speaker:
Regina Ragan, Ph.D., assistant professor of chemical engineering and materials science, received her B.S. summa cum laude in material science and engineering in 1996 from the University of California, Los Angeles, and Ph.D. in applied physics in 2002 from the California Institute of Technology. From 2002-2004, she was a postdoctoral research scholar in the Information & Quantum Systems Laboratory at Hewlett Packard.  Since 2004, she has been an assistant professor of chemical engineering and materials science at the University of California, Irvine. Her research interests include self-assembly or directed self-assembly of molecular based or metallic nanosystems and the use of state of the art analytical techniques for (a) fundamental understanding of assembly processes and to (b) characterize the effect of structure, interfaces and materials on device performance. She is a recent recipient of the National Science Foundation Faculty Early Career (CAREER) Development Award.