45256-G5
Molecular Control at the <1 nm Level Using Electron Standing Waves
The ACS-PRF grant has helped support two separate projects over the last year. They have a common theme of using low-temperature scanning tunneling microscopy (LT-STM) to study molecular adsorption and interaction with surfaces at the single-molecule level, but are distinct in their goals.
Dimethyl Sulfide Self-Assembly on Cu
Self-assembled monolayers (SAMs) have been thoroughly investigated due to their utility in many fields such as sensing, device assembly, molecular electronics, and microelectromechanical systems. Thiol (RSH) SAMs are by far the most extensively studied in the literature due to their robustness and the ability to control their assembly in the dimension perpendicular to the surface of the metal. There are, however, problems with thiol SAMs because they contain structural defects like etch pits and rotational domain boundaries, which make them more susceptible to oxidation and displacement.
Our current study revealed the effect of the shorter alkyl chain length of dimethyl sulfide on both the rate of diffusion and the packing structure of the molecule.1 At a medium surface coverage and at 78 K, it was found that dimethyl sulfide is mobile and forms large, ordered islands without the 120 K annealing that was required for dibutyl sulfide to arrange. Also, the molecular packing structure evolves from quadrupole-quadrupole interactions and results in a perpendicular arrangement of neighboring molecules instead of the parallel arrangement observed for dibutyl sulfide.2 We recorded high-resolution images of the dimethyl sulfide islands in which submolecular features are revealed. These high-resolution data allowed us to propose a structural model for the adsorption site of each dimethyl sulfide molecule within the ordered structures. These results demonstrate that the length of the alkyl side chain is an important factor in determining how thioethers self-assemble on metal surfaces.
Surface Chemistry of Palladium Alloys:
Palladium and its alloys play a central role in a wide variety of industrially important applications such as hydrogenation reactions, separations, storage devices, and fuel cell components. The exact mechanisms of many of these processes are yet to be discovered. Low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) was used to investigate the atomic-scale structure of Pd/Au and Pd/Cu alloys, created by depositing Pd on both Au(111) and Cu(111) at a variety of temperatures.3, 4 Control over the deposition temperature enables us to study Pd particles as a function of size; from individual isolated atoms to pure islands > 10 nm in diameter.
In catalytic hydrogenation applications the rates at which H2 molecules: adsorb, diffuse to the active Pd sites, dissociate, and repopulates the Cu surface with H atoms are crucial for determining the overall reaction rate and selectivity. By depositing H2 at catalytically relevant temperatures in the preparation chamber and subsequently cooling the sample to 7 Kelvin we are able to image the population of the various regions on the alloy surface as a function of deposition time, temperature and coverage. From these data we were able to determine the mechanism of H uptake and spillover on these alloys.5
1. Jensen, S. C.; Baber, A. E.; Tierney, H. L.; Sykes, E. C. H. Dimethyl Sulfide on Cu{111}: Molecular Self-Assembly and Submolecular Resolution Imaging. ACS Nano 2007, 1, 423–428.
2. Jensen, S. C.; Baber, A. E.; Tierney, H. L.; Sykes, E. C. H. Adsorption, Interaction and Manipulation of Dibutyl Sulfide on Cu{111}. ACS Nano 2007, 1, 22-29.
3. Baber, A. E.; Tierney, H. L.; Sykes, E. C. H. Structural, Electronic and Chemisorption Properties of Au/Pd Alloys. In preparation 2008.
4. Tierney, H. L.; Baber, A. E.; Sykes, E. C. H. Atomic-Scale Electronic Structure of Catalytic Sites on Pd/Cu Near Surface Alloys. Submitted to JPCC 2008.
5. Tierney, H. L.; Baber, A. E.; Sykes, E. C. H.; Kitchin, J. R. Novel energetic pathways to hydrogen spillover on Pd alloys. Submitted to Science 2008.
