Molecular ordering of carboxyl-terminated alkanethiols in self-assembled monolayers on Au (111)

Abstract

Surface properties of metal or semiconductor substrates can be easily controlled by introducing self-assembled monolayers (SAMs) with various terminal functional groups. Among them, carboxyl-terminated SAMs have been utilized as molecular templates for selective adsorption of biomolecules such as proteins and DNAs. In this study, we examined the molecular ordering of carboxyl-teminated alkanethiol SAMs with different alkyl chain lengths (HS(CH2)nCOOH, n=2, 5, 7, 10) adsorbed on Au(111) surface by scanning tunneling microscopy (STM). SAMs were fabricated by immersing the Au (111) substarte in the 1mM ethanol solution for 1 day. After the sample was taken out from the solution, it was rinsed with pure ethanol solvent and dried in a stream of nitrogen. From the peak intensity in the C1s X-ray photoemission spectra, we found that the absorption amounts of these SAMs are similar to those of the methyl-terminated alkanethiol SAMs. In the STM images of the SAMs with longer alkyl chain (n=5, 7, 10), well-ordered structures were obserbed. Especially, the SAM with the longest alkyl chain (n=10) has (√3×√3)R 30° packing structure shown in Fig. 1 with the lattice constant of 0.5 nm. This ordering is the same as methyl-terminated alkanethiol SAM. On the other hand, the molecules with the shortest alkyl chain (n=2) are almost disordered in the SAM. These results indicate that the longer alkyl chain leads to the higher ordering. The intermolecular interaction of alkyl chains plays an important role for molecular alkanethiol SAMs with long ordering of the carboxyl-terminated SAMs.