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Free keywords:
Disulfides; Gold; Monolayers; Alkyls; Annealing (metallurgy)
Abstract:
The investigation of the thermal stability of self-assembled monolayers of symmetrical and mixed alkyl fluoroalkyl disulfides on gold is reported. The monolayers were annealed in air at temperatures ranging from 60 to 110 °C for several hours and afterward characterized by contact angle measurements, polarized grazing incidence Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). In mixed alkyl perfluoroalkyl systems evidence was sought for heat-induced phase separation of the alkyl- and perfluoroalkyl-terminated molecules. The ester and amide linkages of the fluorocarbon chains are shown to be less stable than those of the hydrocarbon analogues. Fluoro substituents in α-position to the ester linkage make the ester bond susceptible to hydrolytic cleavage at temperatures between 60 and 70 °C, whereas the corresponding amide and the γ-fluoro-substituted esters remain stable up to 80−100 °C. After annealing for 10 h or longer at these temperatures contact angle measurements, XPS, and FTIR reveal partial loss of fluorocarbon chains in monolayers of symmetrical fluoroalkyl disulfides and mixed alkyl fluoroalkyl disulfides. In contrast, it is still possible to observe ordered domains with AFM in monolayers of mixed alkyl fluoroalkyl disulfides annealed at 100 °C for 17 h. AFM scans in molecular resolution confirm that the lattice constant of the hexagonal lattice remains unchanged (6.2 Å). Since the monolayers of both corresponding symmetrical disulfides show significantly smaller lattice constants, heat-induced phase separation can be excluded. The results indicate nonexisting to very low mobility of the molecules within the layer at higher temperatures. Alternatively, the existence of disulfides rather than thiolates bound to the gold can explain the observations.
