Thermal Stability of SAMs

Structural changes as function of annealing time and temperature in monolayers of dodecanethiol self-assembled on Au(111) were studied by X-ray photoelectron spectroscopy, contact angle microscopy and high gap-impedance scanning tunneling microscopy. Annealing in an oven caused the fusion of small domains into larger ones and the disappearance of gold defects. Small deficiencies (lack of molecules) due to evaporation caused the omission of lines. A slightly lower packing density of the organic chains perpendicular to the missing lines accommodated the mismatch and generated and almost defect-free surface. The shape of sections perpendicular to the missing lines allowed the direction of tilt angles in the domains to be deduced. Higher annealing temperatures eventually lead to the oxidation of thiols and to partial desorption fo the organic phase. On the way to full desorption and reduced lateral pressure, several metastable crystalline structures of film packing and, in later stages, of amorphous or liquid-like phases could be found.

Figure 1: STM image of 140 x 140 nm areas on dodecanethiol chemisorbed on gold(111) recorded at 7 pA and 1 V. All samples were pretreated in the adsorption solution at 50 degrees Centigrade for the same time (48 h). Image a represents the usual topography obtained after such preparation: gold terraces are decorated by depressions and domain boundaries (arrows). The five following images correspond to samples annealed at 100 degrees centigrade in air in an oven for 2 h (b), 5 h (c), 10 h (d), 18 h (e) and 40 h (f). Most noticeable changes are the formation of large depressions (b), their disappearance (c), and the formation of parallel stripes (b,c,d) with decreasing periodicity (e,f).

Bruno Michel <bmi@zurich.ibm.com>

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Last modified: Wed, 15 May 1996 11:34