MODIFICATION OF SILICON-NITRIDE TIPS WITH TRICHLOROSILANE SELF-ASSEMBLED MONOLAYERS (SAMS) FOR CHEMICAL FORCE MICROSCOPY

Chemical modification of Si3N4 atomic force microscopy tips was perfor med by a mild pretreatment using ozone and alkaline/acid solutions and subsequent formation of self-assembled monolayers (SAMs) with two kin ds of omega-alkenyltrichlorosilanes (CH2=CH(CH2)(n)SiCl3, n = 4, 9; HT S and UTS, respectively). Hydroboration of their vinyl groups gave hyd roxyl-terminated SAMs. The extent of SAM formation was assessed by det ermining the adhesion between Si3N4 tips on one hand and bare mica or mica substrates covered with UTS SAMs on the other hand. AU adhesion f orces were determined with the samples immersed in water to eliminate effects of capillary forces. Contact angles on analogously treated pla nar Si3N4 substrates were measured for comparison. Proper conditions f or SAM formation on Si3N4 tips were found to be similar to those for f ormation of SAMs on Si3N4 substrates, as shown by the good temporal co rrelation between the adhesion forces observed between UTS-modified ti ps and mica and the contact angles of water an planar Si3N4 substrates modified under identical conditions. Due to hydrophobic interactions, strong adhesion between tips modified with HTS or UTS SAMs on one han d and UTS SAMs on mica on the other hand was observed. The adhesion of tips modified with HTS to hydrophobic surfaces decreased only slightl y upon hydroboration of the vinyl groups. Furthermore, hydroxyl-termin ated tips derived from HTS adhered more strongly to vinyl-terminated m onolayers on mica than to hydroxyl-terminated monolayers. This suggest s substantial disorder in the HTS-type SAMs formed on Si3N4 tips, resu lting in strong hydrophobic interactions between exposed alkyl chains on the tips and the hydrophobic surfaces. This seems to conceal the sp ecific interactions between the terminal functional groups. Evidence f or such disorder is also given by FTIR spectra and by the larger frict ion forces obtained for HTS monolayers on Si3N4 than for UTS monolayer s an 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 d ecrease in the adhesion to hydrophobic UTS monolayers and a small incr ease in the adhesion to hydroxyl-terminated monolayers derived from UT S. This shows that formation of SAMs of trichlorosilanes with long alk yl chains on Si3N4 tips and subsequent functional group conversion is a promising method for the preparation of modified tips for chemical f orce microscopy.