Modification of silicon nitride tips with trichlorosilane Self-Assembled Monolayers (SAMs) for chemical force microscopy

Chemical modification of Si₃N₄ atomic force microscopy tips was performed by a mild pretreatment using ozone and alkaline/acid solutions and subsequent formation of self-assembled monolayers (SAMs) with two kinds of ω-alkenyltrichlorosilanes (CH₂=CH(CH₂)_nSiCl₃, n = 4, 9; HTS and UTS, respectively). Hydroboration of their vinyl groups gave hydroxyl-terminated SAMs. The extent of SAM formation was assessed by determining the adhesion between Si₃N₄ tips on one hand and bare mica or mica substrates covered with UTS SAMs on the other hand. All adhesion forces were determined with the samples immersed in water to eliminate effects of capillary forces. Contact angles on analogously treated planar Si₃N₄ substrates were measured for comparison. Proper conditions for SAM formation on Si₃N₄ tips were found to be similar to those for formation of SAMs on Si₃N₄ substrates, as shown by the good temporal correlation between the adhesion forces observed between UTS-modified tips and mica and the contact angles of water on planar Si₃N₄ substrates modified under identical conditions. Due to hydrophobic interactions, strong adhesion between tips modified with HTS or UTS SAMs on one hand and UTS SAMs on mica on the other hand was observed. The adhesion of tips modified with HTS to hydrophobic surfaces decreased only slightly upon hydroboration of the vinyl groups. Furthermore, hydroxyl-terminated tips derived from HTS adhered more strongly to vinyl-terminated monolayers on mica than to hydroxyl-terminated monolayers. This suggests substantial disorder in the HTS-type SAMs formed on Si₃N₄ tips, resulting in strong hydrophobic interactions between exposed alkyl chains on the tips and the hydrophobic surfaces. This seems to conceal the specific interactions between the terminal functional groups. Evidence for such disorder is also given by FTIR spectra and by the larger friction forces obtained for HTS monolayers on Si₃N₄ than for UTS monolayers on Si₃N₄. On the other hand, conversion of the terminal vinyl groups of the UTS SAMs on the Si₃N₄ tips to hydroxyl groups led to a large decrease in the adhesion to hydrophobic UTS monolayers and a small increase in the adhesion to hydroxyl-terminated monolayers derived from UTS. This shows that formation of SAMs of trichlorosilanes with long alkyl chains on Si₃N₄ tips and subsequent functional group conversion is a promising method for the preparation of modified tips for chemical force microscopy.