SCANNING-TUNNELING-MICROSCOPY IMAGE-CONTRAST AS A FUNCTION OF SCAN ANGLE IN HYDROGEN-BONDED SELF-ASSEMBLED MONOLAYERS

Scanning tunneling microscopy (STM) has been performed on monolayers o f the nucleic acid base, adenine, adsorbed to molybdenum disulfide (Mo S2) surfaces. Analysis of the real space images of the adsorbates, tog ether with molecular mechanics simulations suggest that the molecules form monolayers stablized by cyclic hydrogen bonds. We present data sh owing the effect on image contrast of varying scan angles of the STM t ip. These indicate an anisotropic response of components of the monola yer to the scan direction and support a structural model implicating c yclic hydrogen bonds in both the stability of the monolayer and in the STM image contrast mechanism.