Research

Our group explores the behavior of surfaces and molecular interfaces. We use surface science techniques, in particular time-of-flight secondary ion mass spectrometry (TOF SIMS). TOF SIMS is a unique, widely applicable technique that provides detailed information about the chemical composition of surfaces with sub-micron lateral resolution, and is used in areas from biological systems to materials chemistry.

In TOF SIMS, an energetic ion beam (in our case Bi_n⁺ ions) strikes the sample surface. This leads to the desorption of both positively and negatively charged secondary ions which are characteristic of the species present on the surface. These ions are extracted into a time-of-flight mass analyzer and detected. The ion beam can be focused to a diameter of less than 100 nm. By rastering the beam over an area, a spatially-resolved mass spectrum image of the surface can be obtained (See Figure 1).

The principal focus of our research is molecular electronics - the use of molecules as active electronic devices. This is a promising new technology that could potentially supplant conventional silicon-based microelectronics by providing an inexpensive way to make and pattern smaller devices on microchips to improve their performance. A variety of molecular analogs of silicon-based electronic devices have been demonstrated, including switches, rectifying diodes, transistors and memory elements. Our goal is to develop reliable methods for the construction of complex molecular circuitry. One major obstacle to the broad application of molecular electronics is device-to-device variation due to the structure of the molecule-metal contact. To understand the underlying processes that govern the formation of metallic contacts, we study model systems of metal atoms deposited on self-assembled monolayers (SAMs). The metal atoms (e.g. Au, Ti, Cu, Al) are deposited using physical vapor deposition, chemical vapor deposition and wet chemical techniques. By "tuning" the metal-molecule interactions we are designing and developing new and better contacts.

We are also developing new ways to construct complex molecular circuitry using self-assembly methods. Here, we pattern surfaces with well-defined areas of SAMs and exploit the interaction of different SAM termini with deposited metal atoms to form circuits.

A second focus of our research is in the area of biomaterials and imaging of biological systems. We are using TOF SIMS and other techniques to examine and understand the interactions of proteins with polymeric surfaces to aid in the design of new biocompatible materials for medical implants such as stents. We are also developing methodologies to increase the ion yields observed from biological surfaces, e.g. cell structures, which will lead to an improvement in mass spectrum imaging of these type of systems.

Selected Publications

• A.V. Walker and N. Winograd, "Prospects for imaging with TOF-SIMS using gold liquid metal ion sources," SIMS XIII, Appl. Surf. Sci., 203-204, 198, (2003).
• A.V. Walker, T.B. Tighe, M.D. Reinard, B.C. Haynie, D.L. Allara, and N. Winograd., "Solvation of zero-valent metals in organic thin films," Chem. Phys. Letts., 369, 615, (2003).
• A.V. Walker, G.L. Fisher, A.E. Hooper, T. Tighe, R.L. Opila, N. Winograd, and D.L. Allara, "Nucleation and growth of vapor-deposited metal films on self-assembled monolayers studied by multiple characterization probes," in Metallization of Polymers 2, ed. E. Sacher, Kluwer Academic/Plenum, NY, NY, p. 117, (2002).