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Research Our research focuses on molecular structure and dynamics of surfaces and condensed phase interfaces. Two complementary aspects of our research program are (1) Development of new and more powerful surface spectroscopy techniques with better sensitivity and detection limits, ultrafast time resolution, and improved molecular-level information content; (2) Application of these techniques for studies of molecular structure, organization, and dynamics in these interesting complex environments. Techniques To achieve surface selectivity, we utilize even-order nonlinear optical processes, such as Sum Frequency Generation (SFG) and Second Harmonic Generation (SHG), which are symmetry-forbidden in isotropic bulk media in the electric dipole approximation. One particularly interesting spectral region for us is the mid-infrared: vibrational spectroscopy provides a wealth of information on the molecular structure and organization at surfaces and interfaces using the intrinsic vibrational chromophores present in every molecule. In addition to the standard frequency-domain spectroscopic measurements that characterize ensemble averaged structures, we are working on an array of ultrafast (femtosecond) time-domain techniques, such as SFG-FID (Free Induction Decay) for complementary studies of the molecular dynamics in real time and Spectrally- and Time-Resolved SFG (STiR-SFG), a mixed time-frequency domain technique capable of measuring spectral evolution of vibrational coherences at surfaces. Another recent development is the heterodyne-detected SFG (HD-SFG) capable of ultrasensitive detection on a few percent of a monolayer level while simultaneously providing phase information on the molecular vibrations. Systems We are interested in systems important in life sciences, nano- and bio-technology, and material science. Current projects fall into three categories: Ø Dynamics and relaxation at liquid interfaces. The most important class of these are aqueous interfaces, which are ubiquitous in biology, environmental/atmospheric chemistry, and many areas of technology (e.g., electrochemistry). The hydrogen bonding network underlies most of the important and unique properties of water. Our research aims to understand how the interface influences the structure and dynamics of the of the aqueous H-bonds. We utilize both frequency- and time-domain vibrational spectroscopy of small probe molecules and/or the OH-stretch of water itself to study rotational, vibrational, and H-bond switching dynamics, and systematically investigate how they are affected by the chemical composition and electrostatics of interfaces. Ø Monolayer and surface materials. Molecularly ordered monolayers and thin films are emerging as the base materials for nano- and bio-technologies and molecular electronics. The ‘tailored’ surface materials under investigation include: Self-Assembled Monolayers (SAMs), Langmuir-Blodgett monolayers, polymer surfaces, and functionalized surfaces used, e.g., for photoswitching and cell adhesion. We study details of molecular orientation, conformation, packing, dynamics and relaxation, and surface functionalization chemistry at these surfaces. We also apply nonlinear spectroscopy to study surfaces and interfaces of nanostructures. A common motif in many emerging nanotechnology applications involves nanostructures covered with chemi- or physisorbed organic molecules which perform the desired physical, chemical, or biological function. Interesting new effects arise when the characteristic size of the nanostructure approaches the molecular scale. Ø Surfaces and interfaces of organic and hybrid photovoltaics. Interfaces play important role in determining the efficiency of photovoltaic devices, in particular the donor-acceptor (D/A) interface where the charge carrier separation occurs, and the electrical contacts. The 'buried' D/A interface is difficult to access, and thus our knowledge of the molecular structure and organization at this junction is very limited. All-optical surface-selective probe such as SFG can therefore be very useful in filling this knowledge gap. In collaboration with other OPV groups at USC, we study orientation and conformation of the donor and acceptor molecules at interfaces.
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Current Projects
"Seedling" funding under the USC CEN (Center for Energy Nanoscience) Thanks to Sean's and Purnim's preliminary results on SFG spectroscopy of OPV (organic photovoltaics) films, our group has been selected for funding under the USC Center for Energy Nanoscience, an EFRC (Energy Frontiers Research Center) funded by DOE.
More studies under way...!
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