Substrate dependence of the surface structure and chain packing of docosyl mercaptan self-assembled on the (111), (110), and (100) faces of single crystal gold

Abstract

Low‐energy helium diffraction has been used to study the surface structure, chain packing, and thermal vibrations of docosyl mercaptan [CH₃(CH₂)₂₁SH] self‐assembled on single crystal Au(111), (110), and (100). The docosyl mercaptan molecules form monolayers with different periodicity on the different surfaces of gold. On Au(111) at low temperatures (≤100 K), the terminal methyl groups of the docosyl mercaptan molecules form domains of a hexagonal lattice with a unit mesh constant of 5.01±0.02 Å. The sulfur head groups are arranged in a commensurate (√3×√3)R30° structure and are believed to adsorb on the triple hollow sites of the Au(111) lattice. The unit mesh parameters for CH₃(CH₂)₂₁S/Au(110) are a=b=4.99±0.08 Å and α=109.5°, suggesting that the chemisorbed sulfur atoms remove the ‘‘missing row’’ reconstruction of the Au(110) surface and form a commensurate c(2×2) lattice. The adsorption of docosyl mercaptan molecules on a Au(100) surface results in a complicated diffraction pattern. Analysis of the data reveals an oblique unit mesh with a=b=5.97±0.09 Å and α=95°±5° with four kinds of equivalent domains present because of the fourfold symmetry of Au(100). The above results confirm that the sulfur–substrate interaction plays an important role in determining the periodicity and the packing density of the molecules within the monolayers. The estimated average domain size of the terminal methyl groups is 22.8, 38.6, and 23.4 Å for CH₃(CH₂)₂₁SH self‐assembled on Au(111), (110), and (100) faces, respectively. The chain packing and orientation within the unit cell are also discussed in this paper in conjunction with the latest results obtained via other techniques such as reflection IR spectroscopy and low‐energy electron diffraction.