Elastic, adhesive, and charge transport properties of a metal-molecule-metal junction: The role of molecular orientation, order, and coverage

The elastic, adhesive, and charge transport properties of a metal-molecule-metal junction were studied via conductingprobe 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 (CH 3(CH2)n-1,SH, denoted by C., where n is the number of carbons in the molecular chain) on Au substrates. AFM contact data were analyzed with the Derjaguin-Muller-Toporov contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulatormetal junction in which the thickness of the insulator is comparable to the Fermi wavelength of the conducting electrons. NEXAFS carbon A'-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, 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 to 0.15 GPa, and the tunneling barrier height φo - Ep decreased from 2.4 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. The change in behavior was explained by the presence of two insulating layers: an oxide layer on the Co-Cr tip, and the alkanethiol monolayer on the Au surface. X-ray photoelectron spectroscopy 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 and 3.9 nm. Finally, it was shown that φo ∼ 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.