Our research concerns chemical processes at surfaces, in particular the formation and patterning of molecular thin films (self-assembled monolayers, SAMs), and reactions of these patterned films with metals and biomolecules. Metal/SAM and biomolecule/SAM structures have applications in organic electronics, sensing and catalysis. We employ surface science techniques, in particular time-of-flight secondary ion mass spectrometry (TOF SIMS) and reflection absorption infrared spectroscopy (RAIRS) in this work. We also perform calculations of molecular structure (Density Functional Theory, DFT) to help interpret our experimental results.
Our current research includes:
New Methods for Constructing Metallized Organic Surfaces. We have introduced a simple method for making complex two-dimensional molecular structures using SAMs patterned with UV light (UV photopatterning) and selective surface reactions that deposit metals. This has significant advantages over previous methods: it affords precise nanoscale placement, is extensible to many types of materials, and is easily scaled up.
In the area of molecular electronics, individual devices (such as diodes and memory elements) are prepared by the deposition of metals (the “contact”) onto a SAM. Device-to-device variation and short device lifetimes due to small changes in the structure of these contacts are often observed. With this in mind, we are amassing a database of metal-molecule interactions to help guide the design of metallic contacts. We are also developing new metallization techniques for organic surfaces including chemical vapor deposition (CVD) and electroless deposition.
Mechanisms of UV Photopatterning of SAMs
UV photopatterning is a well-known method for the creation of patterned SAMs on metal substrates. Previous work demonstrated dependence on the length and chemistry of the SAM molecules. We have shown that UV photopatterning is also strongly dependent on the wavelengths of light that reach the sample, and is particularly sensitive to infrared light (which is also generated by UV lamps).
We have demonstrated for the first time that alkanethiolate SAMs adsorbed on gallium arsenide (a semiconductor) can also be UV photopatterned. In this case, both the SAM and the substrate photooxidize. We have determined in a separate study that the reaction pathways involved depend on the length of the SAM molecules, which is most likely due to surface reconstruction during UV exposure.
In collaboration with J. W. P. Hsu (Sandia National Laboratories) we are now using electron lithography in order to produce nanoscale patterns.
Analytical Methodology: Fundamental Studies of TOF SIMS
TOF SIMS is an imaging method that provides detailed information about the chemical composition of surfaces with ~200 nanometer lateral resolution. SIMS resolution is limited by low secondary ion yields. Using polyatomic primary ions, including Aun+ and Binx+ (n = 1-7, x = 1,2), can greatly enhance molecular ion yields in SIMS (nonlinear yield enhancement). We have shown that the mechanism of Bin+ sputtering is very similar to that of Aun+, and that the yield enhancement is due to efficient energy transfer between the polyatomic primary ion and the analyte molecules. We have also demonstrated that the mechanism of secondary ion generation is very different if doubly-charged primary ions such as Bi2+ and Bi32+ are employed.
Collaborative Work
We have active collaborations with two other groups at Washington University. With K.D. Moeller (Chemistry) we have developed a mass spectrometric cleavable linker that enables the use of TOF SIMS to analyze each microelectrode of a chip-based array. With K.L. Wooley (Chemistry), we have studied the interaction of a biomolecule mimic, a biotinylated shell cross-linked nanoparticle (SCK), with a strepatividin/biotin-functionalized patterned SAM surface. This can be used to integrate biofunctional surfaces with conventional or organic electronic circuitry.
New Research Directions
We are starting to study the preparation, chemical reactivity and catalytic activity of metallized polymers. Metallized polymer constructs are used in fuel cells, batteries, anti-corrosion barrier films, and catalysts. We plan to create a database of metal-polymer interactions and chemical reactivity to aid in the design robust metallized polymers. A second new project will concern the construction of 3D structures with molecular resolution. We will use chemical reactions with SAM terminal groups to grow 3D structures layer-by-layer. Each layer can in principle be independently patterned to add functionality, and at any point a metal (or a metal oxide) can be deposited. This approach has several advantages over available layer-by-layer techniques.