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2004 World Technology Awards Winners & Finalists

Hongkun Park

Please describe the work that you are doing that you consider to be the most innovative and of the greatest likely long-term significance.

Nanometer-sized materials represent a natural size limit of the miniaturization trend of current technology, and they exhibit physical and chemical properties significantly different from their bulk counterparts. My research interest lies in developing detailed physical and chemical understanding of chemically derived nanostructures through novel experimental methods and applying this knowledge to possible technological applications. My current research efforts toward these general goals are centered on two areas: single-molecule electronics and transition-metal-oxide nanostructures. Single-Molecule Electronics: Over the past 40 years, we have witnessed a remarkable miniaturization trend in semiconductor-based electronic circuits, which, were the trend to continue, would soon reach the size scale of molecules. As a consequence, electron transport through molecules has received considerable attention as a model system for nanoscale electronic devices and also as a possible paradigm for future electronic architectures. Despite rapid progress in this area, however, a detailed understanding of the electron transport mode through individual molecules is yet to emerge. The research in my group is focused on addressing this question by combining advanced lithographic and self-assembly methods with transport and scanned probe measurements to fabricate and characterize single-molecule devices. Central to our experimental investigation is the fabrication of single-molecule transistors, which are subsequently characterized by transport and scanned probe measurements. Transport characterization of single-molecule transistors provides detailed insight into how the electrons move through individual molecules while the number of electrons on a molecule is systematically varied, as demonstrated by our studies on nanomechanical oscillations of C60 between two electrodes, the excitation of ring torsion in metallocene molecules induced by single-electron hopping, the appearance of Kondo resonance in a divanadium molecule caused by correlated spin screening, the competition between the Kondo correlation and magnetic field, and magnetoresistance of individual Mn12O12 clusters caused by quantum tunneling of magnetization. Moreover, the same devices can be readily coupled to various scanned probe techniques that can locally monitor and influence electronic motion. Coupled transport and scanned probe investigations of single-molecule-transistors have the potential to revolutionize our understanding of electronic motion through a molecule, the knowledge of which is crucial for assessing the viability of future molecular electronic applications. Transition-Metal-Oxide Nanostructures: Transition metal oxides generally undergo complex phase transitions that are closely intertwined with their electronic and magnetic properties. Furthermore, as exemplified by colossal magnetoresistive materials and high-Tc superconductors, electronic and magnetic modulations with nanometer-scale wavelengths often play a critical role in determining the properties of these materials. The connection between these complex phase behaviors and material properties is currently not well understood, however. Research in my group aims to address these questions by developing general synthetic methods for transition-metal-oxides with nanocrystal and nanowire morphologies and by probing their phase transitions using variable-temperature scanned probe and transport measurements. We have developed experimental methods to synthesize various transition metal oxides, such as BaTiO3, SrTiO3, and La1-xBaxMnO3, into a nanostructured form with well-defined crystallinity and shape. We have also developed an experimental probe based on variable-temperature scanned probe microscopy that allows the study of thermodynamics and kinetics of phase transitions without the complication of ensemble averaging. Specifically, we applied this technique to investigate ferroelectric properties of individual BaTiO3 nanowires and showed that the ferroelectric phase transition temperature of individual BaTiO3 nanowires is significantly depressed as the diameter of the wire becomes smaller, changing from Tc = 130 C to room temperature as the nanowire diameter approaches 3 nm from bulk. This measurement provides the first unequivocal evidence for the depression of phase transition temperature in a single isolated nanowire, and provides crucial insight into the minimum size of ferroelectric non-volatile memory devices.

Brief Biography

The general research interest of Hongkun Park is to develop detailed understanding of physics and chemistry of nanostructured materials and to apply this knowledge to possible technological applications. Current research efforts in his group focus on two specific aspects of this general goal: (1) To study electrical properties of individual molecules, clusters, carbon nanotubes, and their arrays using transport measurements and scanning probe microscopy and to test the viability of such structures for future molecular electronic applications, and (2) to develop synthetic schemes for transition-metal-oxide nanostructures with novel electronic and magnetic properties and to study the role of phase transitions in determining their properties at the individual-nanostructure level using variable-temperature scanning probe microscopy.

Hongkun Park received his B.S. degree in Chemistry from the College of Natural Sciences at Seoul National University, Korea, where he graduated summa cum laude and Valedictorian in 1990. Following a year of mandatory military service in the Republic of Korea Army, he proceeded to Stanford University, where he obtained his Ph.D. in Chemistry in 1996 under the direction of Richard N. Zare, with a thesis on photoionization dynamics of nitric oxide probed by angle- and energy-resolved photoelectron spectroscopy. He joined the faculty of the Department of Chemistry and Chemical Biology at Harvard University in 1999 after a three-year postdoctoral fellowship with Paul Alivisatos and Paul McEuen at Lawrence Berkeley National Laboratory, where his research dealt with electron transport through individual nanocrystals and nanocrystal arrays. He received the Camille and Henry Dreyfus New Faculty Award and Research Corporation Research Innovation Award in 1999, the David and Lucile Packard Foundation Fellowship for Science and Engineering in 2001, the NSF-CAREER Award and the Alfred P. Sloan Research Fellowship in 2002, the Visiting Miller Research Professorship from the University of California at Berkeley, the Ho-Am Foundation Prize in Science and the Camille Dreyfus Teacher-Scholar Award in 2003.