DIFFERENTIATING FUNCTIONAL-GROUPS WITH THE SCANNING TUNNELING MICROSCOPE

Scanning tunneling microscopy (STM) images of 1-docosanol, 1-docosanet hiol, didocosyl disulfide, and 1-chlorooctadecane on graphite are comp ared. The images of the 1-docosanethiol and didocosyl disulfide show b right spots which are attributed to the positions of the S-H and S-S f unctional groups. The STM images of the 1-docosanol and 1-chlorooctade cane do not show such bright spots. The fact that both the S-H and S-S groups appear bright in the STM images indicates that the presence of an S atom on graphite results in a higher tunneling current when the tip scans over it compared to the tunneling current over a C, O, or Cl atom. The different behavior of the S atoms compared to the O, C, and Cl atoms is discussed in terms of the interactions between these atom s and the underlying graphite substrate. The persistent brightness of S atoms in the images of molecular adsorbates suggests that sulfur may serve as a useful ''chromophore'' for molecules imaged by STM.