research thrusts
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The
list of ISIS PIs encompasses research groups with a long-standing
commitment to research on nanometer-scale systems as well as new
faculty members who are in the process of initiating research programs
at UCI that will focus on exciting aspects of nanometer scale science
and technology including Ilya Krivorotov (magnetic nanostructures),
Regina Ragan (metal silicide nanowires), Zuzanna Siwy (transport in
nanopores), and Szu-wen Wang (biological nanomaterials).
fax:
(949) 824-8125Five research thrusts are outlined in Figure 1. Each of these thrusts is supported by faculty whose combined expertise will provide a fully vertically integrated coverage of this research topic.  Figure
1. Five “vertically integrated” ISIS research thrusts.
3.A. Nanocatalysis Surface scientists have understood for along time that “dispersion” – dividing the same amount of a catalyst into smaller pieces – increases catalytic activity for virtually any heterogeneous catalyst because the catalytic activity increases in direct proportion to the surface area. Recently, however, departures from this proportionality have been seen both in terms of the size of catalyst particles[1,2], and in terms of their shape[3]. These size and shape effects announce themselves when the particle diameter approaches the nanometer scale. ISIS scientists Wu, Hemminger, Ho, Shea, and Guan, are interested in understanding fundamental and practical aspects of “nanocatalysis”. Many heterogeneous catalysts consist of nanometer scale transition metal particles dispersed on a high surface area metal oxide support (e.g. Al2O3). Research is underway within ISIS to develop and use Density Functional methods to calculate the electronic properties of metal oxide surfaces and to understand how metal nanoparticles interact with such surfaces[4-6] (Wu). In addition, experiments using atomic resolution microscopy (STM [7-12]) are being combined with experimental methods to measure reaction rates (laser desorption mass spectrometry [13-16]) to develop a fundamental and quantitative understanding of how the size and shape of a metal nanoparticle impacts its catalytic activity (Ho and Hemminger). Other ISIS researchers are involved in the synthesis of novel high surface area materials[17-19] capable of functioning as catalysts, or as supports for heterogeneous catalysts (Shea, Guan). 3.B. Nanomechanics How do nanoscopic pores immersed in an electrolyte solution produce electrical rectification of the ionic current flowing through them? How does a droplet of superfluid helium decompose into two droplets when “pinched”? How does a material nucleate and grow on the corrugated surface of a protein-containing biological membrane? Understanding these and other physical processes is the goal of a large contingent of ISIS faculty (Fig. 1). At the risk of wearing out the prefix, three aspects of nanomechanics under investigation by ISIS faculty could be termed nanotransport, nanointerfaces, and biological nanoactuators. One nanointerface studied by ISIS faculty Peter Taborek and Jim Rutledge[20,21] is that which exists, transiently, as a single liquid droplet is divided into two. The dynamics of this process is determined by a competition between surface and inertial forces. They have demonstrated that highly quantitative data on the neck diameter versus time can be obtained for mercury droplets by measuring the ns time-resolved electrical resistance as a mercury droplet is divided[20]. These data have served as a baseline against which to compare the behavior of an even more exotic system: superfluid helium droplets on cesium surfaces[21]. Biological ion channels are capable of high selectivity and the ability to actively pump against concentration gradients. The function of these channels is modulated by an applied electric field, the binding of molecules to the membrane in which the channel resides, and/or the application of a mechanical stress to the membrane. The complexity of this problem in nanotransport requires a multi-pronged approach. The researches in this ISIS thrust combine experimental studies involving single polymer nanopores of tailored geometry and surface chemistry (Siwy)[22,23], with molecular dynamics modeling of the molecules in the channel (Tobias, Gerber, Wolfsburg)[24-28], as well as studies involving fundamental research in the area of theoretical and computational nonequilibrium thermodynamics (Martens, Gerber). Studies of biological nanoactuators by ISIS faculty are focusing on model protein-actin-lipid networks (Dennin) and studies intra-cellular transport systems (Gross)[29,30]. Within the cell, there is a complex network of filaments (actin and microtubules being the most common) along which motor proteins transport material, or within which motor proteins serves as an active element. The Gross lab is focused on understanding the details of bi-directional transport of cargo along this network (plus and minus motion) [29,30]. In terms of cellular motion itself, there are a number of questions involving both the actin/motor protein network within the cell and with regards to the response of the cell to external stimuli. As the cell moves, external stress and strains on the actin network impacts in a highly nonlinear fashion the response of motor proteins attached to the network. This in turn, generates stresses within the actin network that is attached to the cell membrane and can impact the cell’s motion. An example of this “active material” occurs in neuronal growth cones[31]. These issues are being studied in the Dennin lab using model actin network/protein systems attached to a Langmuir monolayer. 3.C. Nanomagnetism. The magnetic data bit keeps shrinking both in size and cost. Driving this technology are innovations in nanomagnetism, such as spin valves, which have spawned the new area of spintronics. ISIS scientists seek a fundamental understanding of magnetism at the nanoscale. Systems of interest to ISIS scientists range from single atoms, 1-D chains of atoms, and two-dimensional islands, ferromagnetic nanoparticles and 3-D clusters, and ferromagnetic nanowires and ultrathin films composed of one or several atomic layers. The combined effort of experimentalists and theorists is essential in order to obtain a detailed understanding of existing results and to formulate new problems. ISIS scientists engaged in this research have a track record for carrying out such collaborative effort between theorists and experimentalists, as indicated by the number of joint papers published. Research is focused on three main areas: 1. Spatially Resolved Hysteresis Loops in Nanostructures, 2. Spectroscopy of Magnetic Excitations in Restricted Dimensional Systems, and 3. Transport in Magnetically Modulated Nanowires. New instrumentation and procedures are being developed to perform these measurements. Theoretical calculations will be carried out to provide explanatory and predictive results. Research in these three areas to investigate the fundamentals of nanomagnetism will provide the scientific basis for advanced magnetic technologies of the future. The experimental probes include spatially resolved spin-dependent measurements with variable, low temperature scanning tunneling microscopes (Ho)[32,33], spin-polarized electron scattering and secondary electron emission (Hopster)[34], magneto optical Kerr effect (Hopster), and transport measurements through ballistic nanoconstrictions (Krivorotov)[35]. Nanoscale magnetic materials will be fabricated by thermal evaporation, electrochemical deposition, laser ablation, and atomic scale manipulation. The theoretical research in this thrust area involves ab initio density functional studies combined with empirical modeling to explore energetics and spin dynamics (Wu)[36-38]. 3.D. Nanooptics. ISIS has had a presence in nanooptics long before work in this direction was fashionable! The research group of Alex Maradudin has pioneered the study of multiple-scattering effects in the scattering of electromagnetic waves from surfaces with nanoscale random roughness[39-41]. Of particular interest are weak localization effects caused by the coherent interference of each multiply-scattered optical wave and its reciprocal partner. Another aspect of rough surface scattering that is being actively pursued is the design of one- and two-dimensional randomly rough surfaces that scatter or transmit light with a specified angular or spatial dependence of the intensity of the scattered or transmitted light, that produce scattered or transmitted light with specified coherence properties, or produce scattered or transmitted light with a specified independence of its intensity on the wavelength of the incident light for fixed angles of incidence and scattering. [42,43]. The latter type of surface can be used to synthesize the infrared absorption spectrum of a known compound. The Maradudin research group also predicted that surface electromagnetic waves they called channel plasmon polaritons can be guided by a groove of subwavelength width and depth cut into the otherwise planar surface of a metal and are tightly confined to the groove. This prediction has recently been confirmed in experiments that also show that such channels can be used in y-junctions and other ultracompact low-loss surface plasmonic components. Theoretical studies of these channel plasmon polaritons are continuing. Nanooptics now forms the basis for the NSF-funded Center for Chemical Innovation (http://www.chem.uci.edu/CCI/index.html) which is administered through ISIS and anchored by four ISIS faculty (Apkarian, Collins, Ho, and Mills). The global objective of this CCI is to develop the measurement and theoretical apparatus to access the inner workings of complex molecules – individual bonds, interactions among delocalized vibrations, electronically excited states and charge distributions. The field of femtochemistry is well established and has advanced to the limit where molecular processes can be followed at the light cycle limit in time (10-15 s in the visible, 10-16 s in the X-ray range). Also, time independent scanning tunneling microscopy (STM) measurements have been demonstrated with sub-atomic resolution. The execution of either of these feats separately requires demanding state-of-the-art skills. Combining the two is a formidable challenge that requires the manipulation of multiple ultrashort laser pulses in the tip area of a scanning probe that operates with atomic resolution. The scope of this effort requires the assembly of expertise in the areas of time-resolved nonlinear molecular spectroscopy (Apkarian)[44], theoretical quantum chemistry of open systems (Mills)[45,46], and scanned probe characterization of single molecules (Ho)[9] or single nanostructures (Collins)[47]. Other ISIS faculty are appoaching the domain of nanooptics via the top-down fabrication of optical devices. Rob Corn’s research group has pioneered the development of Surface Plasmon Resonance (SPR) imaging in conjunction with lithographically patterned silicon surfaces to create unique tools for probing biomolecular interactions, and for doing high-throughput label-free biosensing[48-50]. Chen Tsai directs the Integrated Optics Laboratory within the Department of Electrical Engineering at UCI. Integrated photonics involves the creation of optical analogues for all microelectronic functions, and the miniaturization and integration of these functions onto silicon surfaces. His current effort is focused on silicon-based current injection lasers[51] and nano optical interconnects[52]. 3.E. Nanowires Nanowires possess intriguing fundamental properties and tremendous technological promise. Eight ISIS faculty (Fig. 1) are actively pursuing projects involving nanowires. The breadth of these projects is stunning. At the extremes of the continuum shown in Fig. 1 is the highly fundamental work of Wilson Ho which involves investigations of the electronic structure gold atom chains prepared by atom-by-atom assembly[10], and, at the “applied” end of this continuum, are the microwave transistors prepared from carbon single walled nanotubes (SWNTs) by Peter Burke and his group[53]. In the laboratories of the six other ISIS faculty are investigations of transport in ferromagnetic nanowires (Krivorotov)[35], the self-assembly of rare earth silicide nanowires on silicon surfaces (Ragan)[54], investigations of single atomic defects in carbon SWNTs (Collins)[47], the synthesis and spectroscopy of ZnO nanowires (Lu)[55], growth and characterization of gold nanowires on stepped graphite (Hemminger), and the fabrication of nanowire-based sensors (Penner)[56]. Nanowires are mostly surface. In spite of the diversity of nanowire types under investigation by the ISIS faculty, a unifying theme is the strong coupling of nanowire surface chemistry and surface structure with wire properties. 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