Mechanical Deformation Distinguishes Tunneling Pathways in Molecular Junctions

Zuoti Xie; Ioan Bâldea; Greg Haugstad; C. Daniel Frisbie

doi:10.1021/jacs.8b11248

Mechanical Deformation Distinguishes Tunneling Pathways in Molecular Junctions

Zuoti Xie, Ioan Bâldea, Greg Haugstad, C. Daniel Frisbie^*

^*Corresponding author for this work

Research output: Contribution to journal › Article › peer-review

19 Scopus citations

Abstract

Developing a clearer understanding of electron tunneling through molecules is a central challenge in molecular electronics. Here we demonstrate the use of mechanical stretching to distinguish orbital pathways that facilitate tunneling in molecular junctions. Our experiments employ junctions based on self-assembled monolayers (SAMs) of homologous alkanethiols (CnT) and oligophenylene thiols (OPTn), which serve as prototypical examples of σ-bonded and π-bonded backbones, respectively. Surprisingly, molecular conductances (G _molecule ) for stretched CnT SAMs have exactly the same length dependence as unstretched CnT SAMs in which molecular length is tuned by the number of CH ₂ repeat units, n. In contrast, OPTn SAMs exhibit a 10-fold-greater decrease in G _molecule with molecular length for stretched versus unstretched cases. Experiment and theory show that these divergent results are explained by the dependence of the molecule-electrode electronic coupling δ on strain and the spatial extent of the principal orbital facilitating tunneling. In particular, differences in the strain sensitivity of δ versus the repeat-length (n) sensitivity can be used to distinguish tunneling via delocalized orbitals versus localized orbitals. Angstrom-level tuning of interelectrode separation thus provides a strategy for examining the relationship between orbital localization or delocalization and electronic coupling in molecular junctions and therefore for distinguishing tunneling pathways.

Original language	English
Pages (from-to)	497-504
Number of pages	8
Journal	Journal of the American Chemical Society
Volume	141
Issue number	1
DOIs	https://doi.org/10.1021/jacs.8b11248
State	Published - 9 Jan 2019
Externally published	Yes

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title = "Mechanical Deformation Distinguishes Tunneling Pathways in Molecular Junctions",

abstract = " Developing a clearer understanding of electron tunneling through molecules is a central challenge in molecular electronics. Here we demonstrate the use of mechanical stretching to distinguish orbital pathways that facilitate tunneling in molecular junctions. Our experiments employ junctions based on self-assembled monolayers (SAMs) of homologous alkanethiols (CnT) and oligophenylene thiols (OPTn), which serve as prototypical examples of σ-bonded and π-bonded backbones, respectively. Surprisingly, molecular conductances (G molecule ) for stretched CnT SAMs have exactly the same length dependence as unstretched CnT SAMs in which molecular length is tuned by the number of CH 2 repeat units, n. In contrast, OPTn SAMs exhibit a 10-fold-greater decrease in G molecule with molecular length for stretched versus unstretched cases. Experiment and theory show that these divergent results are explained by the dependence of the molecule-electrode electronic coupling δ on strain and the spatial extent of the principal orbital facilitating tunneling. In particular, differences in the strain sensitivity of δ versus the repeat-length (n) sensitivity can be used to distinguish tunneling via delocalized orbitals versus localized orbitals. Angstrom-level tuning of interelectrode separation thus provides a strategy for examining the relationship between orbital localization or delocalization and electronic coupling in molecular junctions and therefore for distinguishing tunneling pathways.",

author = "Zuoti Xie and Ioan B{\^a}ldea and Greg Haugstad and {Daniel Frisbie}, C.",

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N2 - Developing a clearer understanding of electron tunneling through molecules is a central challenge in molecular electronics. Here we demonstrate the use of mechanical stretching to distinguish orbital pathways that facilitate tunneling in molecular junctions. Our experiments employ junctions based on self-assembled monolayers (SAMs) of homologous alkanethiols (CnT) and oligophenylene thiols (OPTn), which serve as prototypical examples of σ-bonded and π-bonded backbones, respectively. Surprisingly, molecular conductances (G molecule ) for stretched CnT SAMs have exactly the same length dependence as unstretched CnT SAMs in which molecular length is tuned by the number of CH 2 repeat units, n. In contrast, OPTn SAMs exhibit a 10-fold-greater decrease in G molecule with molecular length for stretched versus unstretched cases. Experiment and theory show that these divergent results are explained by the dependence of the molecule-electrode electronic coupling δ on strain and the spatial extent of the principal orbital facilitating tunneling. In particular, differences in the strain sensitivity of δ versus the repeat-length (n) sensitivity can be used to distinguish tunneling via delocalized orbitals versus localized orbitals. Angstrom-level tuning of interelectrode separation thus provides a strategy for examining the relationship between orbital localization or delocalization and electronic coupling in molecular junctions and therefore for distinguishing tunneling pathways.

AB - Developing a clearer understanding of electron tunneling through molecules is a central challenge in molecular electronics. Here we demonstrate the use of mechanical stretching to distinguish orbital pathways that facilitate tunneling in molecular junctions. Our experiments employ junctions based on self-assembled monolayers (SAMs) of homologous alkanethiols (CnT) and oligophenylene thiols (OPTn), which serve as prototypical examples of σ-bonded and π-bonded backbones, respectively. Surprisingly, molecular conductances (G molecule ) for stretched CnT SAMs have exactly the same length dependence as unstretched CnT SAMs in which molecular length is tuned by the number of CH 2 repeat units, n. In contrast, OPTn SAMs exhibit a 10-fold-greater decrease in G molecule with molecular length for stretched versus unstretched cases. Experiment and theory show that these divergent results are explained by the dependence of the molecule-electrode electronic coupling δ on strain and the spatial extent of the principal orbital facilitating tunneling. In particular, differences in the strain sensitivity of δ versus the repeat-length (n) sensitivity can be used to distinguish tunneling via delocalized orbitals versus localized orbitals. Angstrom-level tuning of interelectrode separation thus provides a strategy for examining the relationship between orbital localization or delocalization and electronic coupling in molecular junctions and therefore for distinguishing tunneling pathways.

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Mechanical Deformation Distinguishes Tunneling Pathways in Molecular Junctions

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