Journal
ACS NANO
Volume 3, Issue 3, Pages 527-536Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nn800673d
Keywords
self-assembly; self-assembled monolayers; carboranethiol; contact angle; Kelvin probe; Fourier transform infrared spectroscopy; scanning tunneling microscopy; dipole moment
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Funding
- National Science Foundation funded Center for Nanoscale Science
- Penn State's National Nanotechnology Infrastructure Network
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Self-assembled monolayer (SAM) structures and properties are dominated by two interactions: those between the substrate and adsorbate and those between the adsorbates themselves. We have fabricated self-assembled monolayers of m-1-carboranethiol (M1) and m-9-carboranethiol (M9) on Au{111}. The two isomers are nearly identical geometrically, but calculated molecular dipole moments show a sizable difference at 1.06 and 4.08 D for M1 and M9 in the gas phase, respectively. These molecules provide an opportunity to investigate the effect of different dipole moments within SAMs without altering the geometry of the assembly. Pure and co-deposited SAMs of these molecules were studied by scanning tunneling microscopy (STM). The molecules are indistinguishable in STM images, and the hexagonally close-packed adlayer structures were found to have (root 19 x root 19)R23.4 degrees unit cells. Both SAMs display rotational domains without the protruding or depressed features in STM images associated with domain boundaries in other SAM systems. Differing orientations of molecular dipole moments influence SAM properties, including the stability of the SAM and the coverage of the carboranethiolate in competitive binding conditions. These properties were investigated by dynamic contact angle goniometry, Kelvin probe force microscopy, and grazing incidence Fourier transform infrared spectroscopy.
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