Tunneling Mechanism Implications from an STM Study of H₃C(CH₂)₁₅HC=C=CH(CH₂)₁₅CH₃ on Graphite and C₁₄H₂₉OH on MoS₂

The observations reported herein confirm that the bright spots in high-resolution STM images of adsorbed alkanes and alkanols are predominantly due to the electronic and topographic structure of the molecule, and not predominantly due to the substrate. STM images of a monolayer of 17,18-pentatriacontadiene, H₃C(CH₂)₁₅HC=C=CH(CH₂)₁₅CH₃, adsorbed on graphite were obtained to evaluate whether changes in the orientation of the exposed methylene hydrogen atoms relative to the STM tip produced changes in the observed pattern of bright spots in a STM image. STM images of this system showed a pattern of bright spots within individual molecules that appears to change on either side of the allene −C=C=C− functionality. STM images were also obtained for tetradecanol overlayers on graphite and MoS₂ surfaces. The angles and distances observed in the images of tetradecanol on MoS₂ were nearly identical to those measured previously in our laboratories for alkanol and alkane monolayers on graphite despite that fact that the separation between bright spots in an STM image of graphite is 2.46 Å, while the separation between bright spots in STM images of MoS₂ is 3.16 Å.