The elastic, adhesive, and charge transport properties of a metal-molecule-metal junction are studied via conducting-probe atomic force microscopy (AFM) and correlated with molecular structure by near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The junctions consisted of Co-Cr-coated AFM tips in contact with methyl-terminated alkanethiols (CH3(CH2)n 1SH, denoted by Cn, where n is the number of carbons in the molecular chain) on Au substrates. AFM contact data were analyzed with the Derjaguin-Muller-Toporov (DMT) contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulator-metal junction where the thickness of the insulator is comparable to the Fermi wavelength of the conducting electrons. NEXAFS carbon K-edge spectra were used to compute the dichroic ratio RI for each film, which provided a quantitative measure of the molecular structure as a function of n. As n decreased from 18 to 5, there was a change in the molecular phase from crystalline to amorphous (RI 0) and loss of surface coverage (intensity of the NEXAFS C=C * resonance peak increased), and as a result, the work of adhesion w increased from 82.8 mJ m−2 to 168.3 mJ m−2, the Young s modulus of the film Efilm decreased from 1.0 GPa to 0.15 GPa, and the barrier height 0−EF decreased from 2.4 eV to 2.1 eV. For all n, the barrier thickness t decreased for small applied loads F and remained constant at ~ 2.2 nm for large F. One possible explanation for the change in behavior is the presence of two insulating layers: an oxide layer on the Co-Cr tip and the alkanethiol monolayer on the Au surface. XPS confirmed the presence of an oxide layer on the Co-Cr tip, and by performing high resolution region scans through the film, the thickness of the oxide layer toxide was found to be between 1.9 nm and 3.9 nm. Finally, it is shown that 0−EF is strain-dependent and the strain at which the film is completely displaced from under the tip is − 0.17 for all values of n.
Pub Type: Journals
self assembled monolayers, atomic force microscopy