The characterization of organic monolayers at gold surfaces using scanning tunneling microscopy and atomic force microscopy correlation with macrostructural properties
Monolayer films formed by self-assembly of organothiols at epitaxially grown Au(111) films at mica were examined in air using scanning tunneling (STM) and atomic force microscopies (AFM). n-Alkanethiolate monolayers exhibit a hexagonal packing arrangement with nearest-neighbor and next-nearest-neighbor spacings of 0.50 and 0.87 nm. This arrangement is consistent with the ({radical}3 {times} {radical}3)R30{degrees} adlayer structure at Au(111). STM reveals the structure of the Au-bound sulfur, while AFM details the structure at the monolayer/air interface, revealing that the order at the Au-S interface is retained up to the monolayer/air interface. The investigation of the self-assembled CF{sub 3}(CF{sub 2})7(CH{sub 2}){sub 2}SH monolayer at Au(lll) by AFM reveals a (2 {times} 2) adlayer structure, with nearest-neighbor and next-nearest-neighbor spacings of 0.58 {plus_minus} 0.02 nm and 1.0 {plus_minus} 0.02 nm, respectively. This is consistent with the larger van der Waals diameter of the fluorinated chain. Coverage of this fluorinated thiolate monolayer is (6.3{plus_minus}0.8) {times} 10{sup {minus}10} mol/cm{sup 2}, consistent with the expected 0.25 monolayer coverage of the (2 {times} 2) adlayer structure at Au(l11). Infrared reflection spectroscopy also confirm this. Upon prolonged exposure to air, the thiolate species is oxidized to elemental sulfur in the forms of cyclooctasulfur (cyclo-S{sub 8}) and other allotropes. STM reveals square structures on aged thiolate monolayers. Dimensions of these squares (0.40--0.50 rum per side) are close to those of cyclo-S{sub 8}. Electrochemical reductive desorption experiments also reveal a change in the surface species with time, with a second desorption wave.
- Research Organization:
- Ames Lab., IA (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- W-7405-ENG-82
- OSTI ID:
- 10185216
- Report Number(s):
- IS-T--1628; ON: DE93001218
- Country of Publication:
- United States
- Language:
- English
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