| AVS 62nd International Symposium & Exhibition | |
| Scanning Probe Microscopy Focus Topic | Thursday Sessions |
| Session SP+BI+NS+SS+TF-ThA |
| Session: | Probing Material Growth on the Surface |
| Presenter: | Holly Walen, Iowa State University |
| Authors: | H. Walen, Iowa State University D.-J. Liu, Ames Laboratory J. Oh, RIKEN, Japan H. Lim, RIKEN, Japan J.W. Evans, Iowa State University C.M. Aikens, Kansas State University Y. Kim, RIKEN, Japan P.A. Thiel, Iowa State University |
| Correspondent: | Click to Email |
The interaction of sulfur with copper and gold surfaces plays a fundamental role in important phenomena that include coarsening of surface nanostructures, and self-assembly of alkanethiols. Here, we identify and analyze unique sulfur-induced structural motifs observed on the (111) surfaces of these two metals. We choose very specific conditions: very low temperature (5 K), and very low sulfur coverage (≤ 0.05 monolayers, ML). In this region of temperature-coverage space, which has not been examined previously for these adsorbate-metal systems, the effects of individual interactions between metals and sulfur are most apparent and can be assessed extensively with the aid of theory and modeling. Furthermore, at this temperature diffusion is minimal and relatively-mobile species can be isolated. The primary technique is scanning tunneling microscopy (STM).
On Cu(111), at 0.004 ML S, we find unexpected heart-shaped Cu2S3 complexes on the terraces, made up of intersecting linear S-Cu-S units. With supporting density functional theory (DFT) and reaction-diffusion equation analysis, we propose that these hearts are a viable candidate for S-enhanced mass transport of Cu on Cu(111) at higher temperature. As S coverage increases (up to 0.05 ML), a diverse group of Cu-S structures develops which includes concatenated hearts, and eventually the known (√43 x √43)R±7.5◦ reconstruction[1-2]. Analysis of the step edges of Cu(111) indicates that S decorates the step edges preferentially (relative to the terraces) and that the complexes observed on terraces originate at the step edges.
In contrast, no metal-sulfur complexes are observed on Au(111) under similar conditions (0.03 ML). Instead, we observe striking √3R30◦ rows made up of S adatoms. Using DFT and ab-initio Monte Carlo analysis, we construct and test a lattice gas model. This analysis shows that these short rows of S adatoms form because of a complex set of through-metal interactions: a linear three-body attraction, as well as long-range pairwise interactions (up to 5a) between S atoms.
These experimental observations for Cu(111) and Au(111) surfaces—made under essentially-identical conditions—together with extensive DFT analyses, allow comparisons and insights into factors that favor the existence of metal-sulfur complexes, vs. chemisorbed atomic sulfur, on metal terraces.
[1] E. Wahlström, I. Ekvall, H. Olin, S. A. Lindgren, and L. Wallden, Phys. Rev. B 60 10699 (1999).
[2] E. Wahlström, I. Ekvall, T. Kihlgren, H. Olin, S. A. Lindgren, and L. Wallden, Phys. Rev. B 64 155406 (2001).