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Abstract
Conducting probe atomic force microscopy (CP-AFM) was employed to examine electron tunneling in self-assembled monolayer (SAM) junctions. A 2.3 nm long perylene tetracarboxylic acid diimide (PDI) acceptor molecule equipped with isocyanide linker groups was synthesized, adsorbed onto Ag, Au and Pt substrates, and the current-voltage (I-V) properties were measured by CP-AFM. The dependence of the low-bias resistance (R) on contact work function indicates that transport is LUMO-assisted ('n-type behavior'). A single-level tunneling model combined with transition voltage spectroscopy (TVS) was employed to analyze the experimental I-V curves and to extract the effective LUMO position ϵl = ELUMO - EF and the effective electronic coupling (Γ) between the PDI redox core and the contacts. This analysis revealed a strong Fermi level (EF) pinning effect in all the junctions, likely due to interface dipoles that significantly increased with increasing contact work function, as revealed by scanning Kelvin probe microscopy (SKPM). Furthermore, the temperature (T) dependence of R was found to be substantial. For Pt/Pt junctions, R varied more than two orders of magnitude in the range 248 K < T < 338 K. Importantly, the R(T) data are consistent with a single step electron tunneling mechanism and allow independent determination of ϵl, giving values compatible with estimates of ϵl based on analysis of the full I-V data. Theoretical analysis revealed a general criterion to unambiguously rule out a two-step transport mechanism: namely, if measured resistance data exhibit a pronounced Arrhenius-type temperature dependence, a two-step electron transfer scenario should be excluded in cases where the activation energy depends on contact metallurgy. Overall, our results indicate (1) the generality of the Fermi level pinning phenomenon in molecular junctions, (2) the utility of employing the single level tunneling model for determining essential electronic structure parameters (ϵl and Γ), and (3) the importance of changing the nature of the contacts to verify transport mechanisms.
Original language | English (US) |
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Pages (from-to) | 964-975 |
Number of pages | 12 |
Journal | Nanoscale |
Volume | 10 |
Issue number | 3 |
DOIs | |
State | Published - Jan 21 2018 |
Bibliographical note
Funding Information:We thank NSF (CHE-1708173) for financial support. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. Mass spectra were obtained at the University of Massachusetts Mass Spectrometry Center. Z. X. thanks Liwei Wu of the National Synchrotron Radiation Laboratory, University of Science and Technology of China, for advice concerning Angle-Resolved X-Ray Photoelectron Spectroscopy. I. B. acknowledges financial support from the Deutsche Forschungsgemeinschaft (grant BA 1799/3-1) and computational support by the State of Baden-Wurttemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/467-1 FUGG.
Funding Information:
We thank NSF (CHE-1708173) for financial support. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. Mass spectra were obtained at the University of Massachusetts Mass Spectrometry Center. Z. X. thanks Liwei Wu of the National Synchrotron Radiation Laboratory, University of Science and Technology of China, for advice concerning Angle-Resolved X-Ray Photoelectron Spectroscopy. I. B. acknowledges financial support from the Deutsche Forschungsgemeinschaft (grant BA 1799/3-1) and computational support by the State of Baden-Württemberg through bwHPC and the German Research Foundation (DFG) through grant no INST 40/467-1 FUGG.
Publisher Copyright:
© 2018 The Royal Society of Chemistry.
How much support was provided by MRSEC?
- Shared
Reporting period for MRSEC
- Period 4
PubMed: MeSH publication types
- Journal Article
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MRSEC IRG-1: Electrostatic Control of Materials
Leighton, C., Birol, T., Fernandes, R. M., Frisbie, D., Goldman, A. M., Greven, M., Jalan, B., Koester, S. J., He, T., Jeong, J. S., Koirala, S., Paul, A., Thoutam, L. R. & Yu, G.
9/1/98 → …
Project: Research project
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MRSEC Program
THE NATIONAL SCIENCE FOUNDATION, UNIVERSITY OF TEXAS RIO GRANDE VALLEY
8/1/98 → 10/31/20
Project: Research project