Compositional mapping of micropatterned, mixed self-assembled monolayers by lateral force microscopy
Lateral force microscopy (LFM) was used to image patterned organic surfaces and showed consistent changes in image contrast with surface composition. Self-assembled monolayers (SAMs) of ω-substituted alkanethiolates were patterned on surfaces of gold films coated on silicon wafers. The patterns consisted of areas of gold modified by SAMs formed from dodecanethiol (HS(CH2)11CH3) and areas derivatized by mixed SAMs formed by exposure to solutions of mixtures of HS(CH2)11CH3 and hexaethylene glycol-terminated alkanethiol [HS(CH2)11(OCH2CH2)6OH]. Several types of patterned SAMs were investigated, each of which differed in the relative mole fractions of methyl- and hexaethylene glycol-terminated alkanethiolates in the mixed SAMs. Analysis of the relative friction measured for the different mixed SAMs suggests that LFM can be used as a semiquantitative technique for compositional mapping of mixed SAMs. Correlation between relative friction and the composition (as estimated by X-ray photoelectron spectroscopy) and the wettability of mixed monolayers are presented for the types of SAMs investigated. The friction is directly proportional to the fraction of hexaethylene glycol-terminated alkanethiolates present in the mixed SAMs. The correlations can be applied in the semiquantitative estimation of the compositional and wetting properties of microscopic patterns in the mixed SAMs. As an example, we have used the compositional correlation together with LFM imaging to estimate the degree of displacement of an hexaethylene glycol-terminated SAM by a microdroplet of HS(CH2)11CH3.