SELF-ASSEMBLY OF N-ALKANETHIOLATE MONOLAYERS ON SILVER NANOSTRUCTURES- DETERMINATION OF THE APPARENT THICKNESS OF THE MONOLAYER BY SCANNING-TUNNELING-MICROSCOPY

The scanning tunneling microscope (STM) is employed to monitor the eff ect of adsorption of an n-alkanethiolate monolayer, from an aqueous so lution of the thiol, on the apparent topography of nanoscopic silver a dsorption sites. Silver nanodisk structures, 1000-1500 Angstrom in dia meter and 50- 150 A in height, were electrochemically deposited on gra phite surfaces by using the STM. The topography of these adsorption si tes was then determined in situ by STM imaging prior to the exposure o f the nanostructure to an aqueous solution of an n-alkanethiol. Immedi ately following exposure to the thiol, STM images revealed an increase in the height of the nanostructure resulting from the self-assembly o f an n-alkanethiolate monolayer on the silver surface. The apparent th ickness of the self-assembled monolayer (SAM), estimated from the diff erence in the nanostructure height measured before and after adsorptio n of the monolayer, increased linearly with the alkyl chain length for five n-alkanethiols (i.e., CnH(2n+1)SH) having even-numbered chain le ngths from n = 10 to 18. For SAMs of tetradecanethiol (CH3(CH2)(13)SH) , hexadecanethiol (CH3(CH2)(15)SH), and octadecanethiol (CH3(CH2)(17)S H), the average height increments (20, 24, and 28 Angstrom, respective ly) equaled the expected all-trans chain length of these molecules wit hin the experimental precision of our measurement, indicating that lit tle penetration of the STM tip into the surface of the monolayer occur s during the STM imaging experiment.