Chain length dependence of the structure of alkane thiol films on Au(111)

We report the results of a detailed LEED (low energy electron diffraction) study of the chemisorption of alkane thiols [CH3(CH2)(n-1)]SH (n = 4-10,12,18) on single crystalline Au(lll) surfaces. Upon vapor deposition the organic films form initially low coverage structures which can be characterized by disulfides, the carbon axes of which are oriented parallel to the surface plane. The length of the unit cells increases linearly with increasing length of the disulfides for n = 4-12. A further increase in coverage results in intermediate chain length dependent structures (n = 9-12) and eventually in a close-packed c(4 root 3 x 2 root 3)R30 degrees structure (n = 10-12). This structure, which is characterized by molecules with their carbon axes oriented upright with respect to the surface plane, is independent of chain length. It can also be obtained via organic film growth in an ethanolic solution. Low coverage and high coverage structures can be mutually transformed via the deposition or desorption of molecules. (C) 1997 Elsevier Science B.V.