Our research program examines fundamental physical and chemical interactions that drive self-assembled, functional organic surface structures. We are particularly interested in developing robust and highly-ordered architectures that will impact advances in catalysis, organic photovoltaics, and molecular electronics. Excellent ordering is important for each of these applications. In catalysis, having a well-defined single-site catalyst at a surface will lead to advances in heterogeneous catalysis. Highly-ordered organic layers at surfaces may offer significant improvement in charge transfer and device efficiency compared to less-ordered polymer films. We strive to achieve and understand ordering of functional organic materials at surfaces through a combination of atomic resolution scanning probe microscopies (structural characterization) and electron spectroscopies (chemical characterization).
Metal-organic Complexes for Catalysis
With support from the U. S. Department of Energy, we are developing surface-supported metal-organic complexes that combine advantages of high selectivity and excellent stability. We are exploring transition metal complexes at surfaces that have potential to advance our understanding of complex surface catalysts.
Robust 2D Organic Architectures through Ionic Bonding
We are studying methods to obtain thermally robust organic architectures that have atomically-precise ordering. One successful example of this is a reaction of carboxylic acid functional groups with NaCl on a copper surface (see JACS 2012). This system produces a crystalline organic architecture that is stable to temperatures as high as 180 C, as evidenced by molecular resolution microscopy at that elevated temperature (see JPCC 2013).
Multi-layer Stabilization by Interfacial Organic Layer
Recent results have allowed us to form crystalline multilayer structures (see PCCP 2012) using an interfacial organic layer (IOL). The IOL provides two key advantages to organic technologies: structural ordering and chemical stability of the film. Our studies provide a molecular resolution view of what is happening in the IOL system.
Dynamic Self-assembly at the Liquid-solid Interface
Experiments at the liquid-solid interface allow us to explore more complex organic systems and to observe key elements of dynamic self-assembly. In these studies the system is able to approach a dynamic equilibrium between solvated species and those adsorbed at the surface. Error correction and response to environmental triggers are achieved by active molecule exchange with the solution.
Vibration Spectroscopy at Surfaces
Experiments using high-resolution electron energy loss spectroscopy (HREELS) allow us to probe vibrational modes at the surface. These experiments provide information about bonding within the molecule, between molecules, and to the surface, as well as insight into the structure of monolayers and multilayers. HREELS has been applied to a model system of terephthalic acid on Cu(100) to elucidate the attractive driving force behind island formation in the sub-monolayer and the structural transition during bi-layer growth (see Langmuir 2010).
Advanced analytical tools are a key part of our work. We characterize structure with atomic resolution microscopy and use surface-sensitive spectroscopy methods to characterize chemical properties. Instrumentation in our group includes:
Scanning probe microscope and X-ray photoelectron spectroscopy vacuum system
Scanning tunneling microscope for studies of liquid solid interface
Pulsed supersonic molecular beam system
High-resolution electron energy loss spectroscopy system