The focus of our work is to explore functional nanometer-scale architectures at surfaces formed by self-assembly of organic building blocks. The research in our lab combines the understanding of growth kinetics and materials characterization of physical and analytical chemistry with the rich building block library and supramolecular organization schemes of organic and inorganic chemistry. Efficient patterning of solid surfaces with organic materials is a challenging research problem that has the potential to open up new opportunities and new technologies in many fields, including molecular electronics, catalysis, molecular recognition/sensors and magnetism.
Our experiments are made in extremely clean and well-controlled vacuum chambers. Starting with atomically clean and flat surfaces, we sublimate molecular "building blocks" onto the surfaces from heated crucibles. We can control the deposition rate and final concentration of each component on the surface as well as the surface temperature during and after deposition. Thermal energy from the surface allows for diffusive motion of the molecular components so that they can assemble themselves into highly ordered two-dimensional structures. An exciting aspect to surface-supported studies of these model architectures is their accessibility for direct structural and electronic characterization with single molecule resolution using scanning tunneling microscopy (STM). Insight gained at the local level - a direct view into the molecular world - combined with integral techniques, such as photoelectron spectroscopy, expands our understanding of the fundamental interactions that determine the function of these systems.
In these highly controlled conditions we can directly observe interesting phenomena of the growth process, and characterize in a controlled way the functional aspects and chemical properties of the structures - and most importantly how we can tune those properties by selection of the organic components. In these model systems we can modify the system in several directions - changing metal centers, organic ligand substitutions, surface modification, growth conditions - and observe the effect of these parameters on properties such as bonding of adsorbates, reactivity, structure, charge transfer, electron transport, and magnetism.
Our current projects are focusing on the construction of such supramolecular networks on technologically relevant surfaces. We are also examining more closely the chemical functionality of the systems and how this depends on the ligand structure and supramolecular interactions. This interdisciplinary research will lead to a better understanding of the properties of organic-based nanostructures and nanotechnologies, especially with regard to electronic characterization, thermodynamics of assembly, surface interactions, and device development in controlled environments.
Our group uses a multi-technique surface analysis system to address structural and chemical aspects of self-assembled supramolecular structures. This instrument was recently (May 2009) delivered to our new lab in the Multidisciplinary Sciences Building (Simon Hall) at Indiana University. The experimental tools on this system include: Scanning Tunneling Microscopy to directly image the structure of supramolecular architectures on conducting surfaces. Imaging can be at temperatures in the range of 25 K to over 1000 K, which will allow for growth characterization, adsorption site identification for weakly adsorbed species, and system behavior at reaction conditions. Non-contact Atomic Force Microscopy allows for structural characterization on non-conducting surfaces and also measurements of the response of a system to locally applied forces or stresses. X-ray Photoelectron Spectroscopy measurements are sensitive to the elemental composition of the topmost atomic layers of a sample. This technique is critical for determining the composition as well as for identifying activity of the surface structures, as "chemical shifts" in the electronic energy levels. Temperature-Programmed Desorption measurements using a quadrupole mass spectrometer and temperature control loop will be used to measure the binding energy of adsorbates on the designed supramolecular surface structures.
Our group also has an active collaboration with Prof. Larry Kesmodel from the IU Physics department. In Professor Kesmodel's lab and in one of our labs in the Chemistry building, we are using High-resolution Electron Energy Loss Spectroscopy (HREELS) to study the effect of small ligand adsorption at surfaces on the interamolecular and molecule--surface bonds. HREELS can provide information about the orientation of the molecule to the surface and the nature and strength of the bonds. Those apparatus also use Auger Electron Spectroscopy for composition analysis of the samples and Low-energy Electron Diffraction for integral surface structure characterization and epitaxial overlayer growth characterization. We are also in the process of constructing a pulsed molecular beam on one of the HREELS systems for further adsorption and reactivity studies.
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