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:
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.
Recently we have extended this work to include the construction of semiconductor/SAM and biomolecule/SAM complex surfaces for applications including photovoltaics and sensing. We have also demonstrated that this approach can be employed to construct 3D organic structures with molecular resolution. We employ specific chemical reactions with different SAM terminal groups to grow these structures layer-by-layer.
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; now at U. T. Dallas) we have also invesigated the use of electron beam lithography to pattern SAMS adsorbed on GaAs and Au. Upon electron beam exposure, the monolayers dehydrogenate,leading to the formation of C=C bonds, cross-links, and polycyclic aromatic hydrocarbons (PAHs). The extent of damage at a given electron dose is strongly dependent on the substrate conductivity and the detailed monolayer structure.
Chemical Lithography of SAMsPrevious work has always assumed that complete degradation of the SAM was necessary for the formation of well-defined multifunctional patterned surfaces, requiring large electron doses or long UVirradiation times. We have recently demonstrated that well-defined multifunctional patterned surfaces can be produced on GaAs (001) with only partial degradation of the SAM, allowing greatly reduced electron beam doses and UV irradiation times to be used. Using electron beam lithography we observe that sharp well-defined patterns can form after an electron dose as low as 450 μC cm-2.We also show that only 50% of the monolayer must be photooxidized in UV photopatterning, reducing the exposure time needed by a factor of 3! In contrast, patterning of alkanethiolate SAMs adsorbed on Au requires much higher electron doses (≥1250 μC cm-2) and photooxidation times (2 h). The substantial differences in the chemical lithographic processes observed on these two substrates seem to arise from differences in the SAM structure on GaAs and Au. These results also suggest that alkanethiolate SAM resists may be a suitable technology for nanometer scale lithography of GaAs and possibly other semiconductors.
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.
We have recentlydeveloped ionic liquid matrix-enhanced SIMS. Room temperature ionic liquids (ILs) have many applications including as matrices in MALDI. We synthesized two ILs derived from α-cyano-4-hydroxycinnamic acid (CHCA). Using IL matrices, the molecular ion intensities of 1,2-dipalmitoylsn-glycero-3- phosphocholine (DPPC),1,2-dipalmitoyl-snglycero-3-phosphoethanolamine (DPPE), cholesterol, and bradykinin are greatly increased. Further, detection limits are also improved: for DPPC and DPPE, limits of detection were at least 2 orders of
magnitude better using IL matrices. The data also show that IL matrices are suitable for imaging MS. IL matrices do not cause
changes to the sample surface via matrix crystallization
or other processes; no "hot spots" are observed in the mass spectra. As a demonstration, we imaged an onion skin membrane. In the matrix-enhanced MS images, ions characteristic of proteins and other biomolecules were observed which could not otherwise be observed.We are currently investigating the detailed mechanism behind the matrix enhancement effect as well as optimizing the preparation method.
With K.D. Moeller (Washington University in St. Louis) 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 (Texas A&M), 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.