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Inelastic electron tunneling process for alkanethiol self-assembled monolayers
Kanazawa University.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
Tohoku University.
2013 (English)In: Progress in Surface Science, ISSN 0079-6816, E-ISSN 1878-4240, Vol. 88, no 1, 1-38 p.Article, review/survey (Refereed) Published
Abstract [en]

Recent investigations of inelastic electron tunneling spectroscopy (IETS) for alkanethiol self-assembled monolayers (SAMs) are reviewed. Alkanethiol SAMs are usually prepared by immersing a gold substrate into a solution of alkanethiol molecules, and they are very stable, even under ambient conditions. Thus, alkanethiol SAMs have been used as typical molecules for research into molecular electronics. Infrared spectroscopy and electron energy loss spectroscopy (EELS) have frequently been employed to characterize SAMs on the macroscopic scale. For characterization of alkanethiol SAMs on the nanometer scale region, or for single alkanethiol molecules through which electrons actually tunnel, IETS has proven to be an effective method. However, IETS experiments for alkanethiol SAMs employing different methods have shown large differences, i.e., there is a lack of standard data for alkanethiol SAMs with which to understand the IET process or to satisfactorily compare with theoretical investigations. An effective means of acquiring standard data is the formation of a tunneling junction with scanning tunneling microscopy (STM). After explanation of the STM experimental techniques, standard IETS data are presented whereby a contact condition between the tip and SAM is tuned. We have found that many vibrational modes are detected by STM-IETS, as is also the case for EELS. These results are compared with LET spectra measured with different tunneling junctions. In order to precisely investigate which vibrational modes are active in IETS, isotope labeling of alkanethiols with specifically synthesized isotopically substituted molecule has been examined. This method provides unambiguous assignments of IET spectra peaks and site selectivity for alkanethiol SAMs such that all parts of the alkanethiol molecules almost equally contribute to the IET process. The LET process is also discussed based on density functional theory and nonequilibrium Green's function calculations. These results quantitatively reproduce many the experimentally observed features, whereas Fermi's golden rule for JETS qualitatively explains the propensity rule and site selectivity observed in the experiments. However, comparison between experiment and theory reveals a large difference in JETS intensity for the C H stretching mode that originates from the side chains of the alkanethiol molecules. In order to explain this difference, we discuss the importance of an intermolecular tunneling process in the SAM. Application of STM-IETS to identify a hydrogenated alkanethiol molecule inserted into a deuterated alkanethiol SAM matrix is also demonstrated. (C) 2012 Elsevier Ltd. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2013. Vol. 88, no 1, 1-38 p.
Keyword [en]
Inelastic electron tunneling spectroscopy, Scanning tunneling microscope, Density functional theory, Nonequilibrium Green's function, Alkanethiol self-assembled monolayer, Isotope labeling
National Category
Physical Sciences
Research subject
Physics, Condensed Matter Physics
Identifiers
URN: urn:nbn:se:lnu:diva-25530DOI: 10.1016/j.progsurf.2012.11.001ISI: 000317027600001OAI: oai:DiVA.org:lnu-25530DiVA: diva2:619724
Available from: 2013-05-06 Created: 2013-05-06 Last updated: 2013-06-14Bibliographically approved

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