Atomistic structure of thiol-gold self-assembled monolayers

Motivation

Self-assembled monolayers (SAMs) of thiols on metal surfaces attract a lot of interest both from fundamental perspective and due to their potential applications.

Why structure? Understand what we have to understand it's formation process and be able to control it.

Problem of direct imaging (STM) while GIXRD, LEED and PED are only averaging and sacrifice feasibility of the model for the quality of the fit.

History

rt3 c(4x2) striped phase prediction of surface reconstruction [Hammer] experimental confirmation of reconstructions so what is the exact structure?

Main research groups working on alkanethiol SAMs on Au(111) structure

• Scoles
• Woodruff
• Salvarezza
• Ulstrup
• Reimers
• Hakkinen

Our publications

• O. Voznyy, J. J. Dubowski, J. T. Yates Jr., P. Maksymovych
  The Role of Gold Adatoms and Stereochemistry in Self-Assembly of Methylthiolate on Au(111),
  JACS, ASAP, 2009.

  On the basis of high resolution STM images and DFT modeling, we have resolved low- and high-coverage structures of methylthiolate (CH3S) self-assembled on the Au(111) surface. The key new finding is that the building block of all these structures has the same stoichiometry of two thiolate species joined by a gold adatom. The self-arrangement of the methylthiolate?adatom complexes on the surface depends critically on their stereochemical properties. Variations of the latter can produce local ordering of adatom complexes with either (3x4) or (3x4rt3) periodicity. A possible structural connection between the (3x4rt3) structure and commonly observed (rt3 x rt3)R30° phase in methylthiolate self-assembled monolayers is developed by taking into account the reduction in the long-range order and stereochemical isomerization at high coverage. We also suggest how the observed self-arrangements of methylthiolate may be related to the c(4x2) phase of its longer homologues.

• O. Voznyy, J. J. Dubowski,
  c(4x2) Structures of alkanethiol monolayers on Au (111) compatible with the constraint of dense packing,
  Langmuir, 25, 2009, p.7353.

  Using alkanethiol dense packing as a starting point, we have found six prototypical packing structures commensurate with the (3 x 2rt3) supercell of the Au (111) surface. Five of the six structures are not compatible with the flat surface conditions but can be fitted to a reconstructed surface. Combined with density functional theory calculations and simulations of grazing incidence X-ray diffraction maps and of scanning tunneling microscopy images, this allowed us to refine and assess the recently proposed models of the c(4x2) self-assembled monolayers involving thiolate-adatom and thiolate-adatom-thiolate species and to propose a new model with four gold adatoms per unit cell.

Other references

A regularly updated list of the most important papers related to structure determination of alkanethiol SAMs on Au(111) can be found in my Mendeley public 'thiol gold SAMs' collection