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 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 (CH{sub 3}(CH{sub 2}){sub n-1}SH, 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 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 in which 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 (R{sub I} {yields} 0) and loss of surface coverage, and as a result, the work of adhesion w increased from 82.8 mJ m{sup -2} to 168.3 mJ m{sup -2}, the Young's modulus of the film E{sub film} decreased from 1.0 to 0.15 GPa, and the tunneling barrier height {psi}{sub 0} - E{sub F} 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 {approx}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 t{sub oxide} was found to be between 1.9 and 3.9 nm. Finally, it was shown that {psi}{sub 0} - E{sub F} 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.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- DOE - OFFICE OF SCIENCE
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 1019644
- Report Number(s):
- BNL-95490-2011-JA; LANGD5; TRN: US201115%%284
- Journal Information:
- Langmuir, Vol. 26, Issue 3; ISSN 0743-7463
- Country of Publication:
- United States
- Language:
- English
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