AVS 45th International Symposium
    Applied Surface Science Division Wednesday Sessions
       Session AS+BI+SS-WeM

Paper AS+BI+SS-WeM10
Bias-dependent Contrast in STM Images of Phenyloctadecylethers

Wednesday, November 4, 1998, 11:20 am, Room 307

Session: Organized Molecular Monolayers
Presenter: I.H. Musselman, University of Texas, Dallas
Authors: I.H. Musselman, University of Texas, Dallas
H.S. Lee, University of Texas, Dallas
S. Iyengar, University of Texas, Dallas
Correspondent: Click to Email
A homologous series of para-substituted phenyloctadecylethers (X-POEs, X = H, Cl, Br and I) was prepared and characterized using @super 1@H NMR and GC/MS. Scanning tunneling microscopy (STM) images were acquired from monolayers of the ethers physisorbed onto highly oriented pyrolytic graphite. The contrast exhibited by the X-POE molecules in these images varied as a function of tip-sample bias. For example, STM images acquired at biases of approximately -0.5 to -0.6 V (sample negative) exhibited a dim alkyl tail and a series of bright spots corresponding to functionalities in the head group (e.g. halogen, phenyl group, oxygen). However, at biases more negative than -1.2 V, the largest contribution to contrast was observed for the alkyl tail (e.g. octadecyl group). A comparison of STM images of the adsorbed X-POE molecules with electron density contours calculated using HyperChem suggested a bias-dependent participation of individual bonding molecular orbitals to tunneling. A resonance tunneling mechanism between the tip and molecular orbitals adjacent to and including the highest occupied molecular orbital (HOMO) is proposed.@footnote 1@ @FootnoteText@ @footnote 1@ The support of this research by a grant from the Robert A. Welch Foundation is gratefully acknowledged.