Tunneling mechanism implications from an STM study of H3C(CH2)(15)HC=C=CH(CH2)(15)CH3 on graphite and C14H29OH on MoS2

The observations reported herein confirm that the bright spots in high-reso lution STM images of adsorbed alkanes and alkanols are predominantly due to the electronic and topographic structure of the molecule, and not predomin antly due to the substrate. STM images of a monolayer of 17,18-pentatriacon tadiene, H3C(CH2)(15)-HC=C=CH(CH2)(15)CH3, adsorbed on graphite were obtain ed to evaluate whether changes in the orientation of the exposed methylene hydrogen atoms relative to the STM tip produced changes in the observed pat tern of bright spots in a STM image. STM images of this system showed a pat tern of bright spots within individual molecules that appears to change on either side of the allene -C=C=C- functionality. STM images were also obtai ned for tetradecanol overlayers on graphite and MoS2 surfaces. The angles a nd distances observed in the images of tetradecanol on MoS2 were nearly ide ntical to those measured previously in our laboratories for alkanol and alk ane monolayers on graphite despite that fact that the separation between br ight spots in an STM image of graphite is 2.46 Angstrom, while the separati on between bright spots in STM images of MoS2 is 3.16 Angstrom.