Energy level alignment in molecular tunnel junctions by transport and spectroscopy

• Verfasst von: Xie, Zuoti [VerfasserIn]
• Verfasst von: Bâldea, Ioan [VerfasserIn]
• Verfasst von: Frisbie, C. Daniel [VerfasserIn]
• Titel: Energy level alignment in molecular tunnel junctions by transport and spectroscopy
• Titelzusatz: self-consistency for the case of alkyl thiols and dithiols on Ag, Au, and Pt electrodes
• Verf.angabe: Zuoti Xie, Ioan Bâldea, and C. Daniel Frisbie
• E-Jahr: 2019
• Jahr: October 16, 2019
• Umfang: 11 S.
• Fussnoten: Gesehen am 07.01.2020
• Titel Quelle: Enthalten in: American Chemical SocietyJournal of the American Chemical Society
• Ort Quelle: Washington, DC : American Chemical Society, 1879
• Jahr Quelle: 2019
• Band/Heft Quelle: 141(2019), 45, Seite 18182-18192
• ISSN Quelle: 1520-5126
• DOI: doi:10.1021/jacs.9b08905
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1686520921

Abstract

We report here an extensive study of transport and electronic structure of molecular junctions based on alkyl thiols (CnT; n = 7, 8, 9, 10, 12) and dithiols (CnDT; n = 8, 9, 10) with various lengths contacted with different metal electrodes (Ag, Au, Pt). The dependence of the low-bias resistance (R) on contact work function indicates that transport is HOMO-assisted (p-type transport). Analysis of the current-voltage (I-V) characteristics for CnT and CnDT tunnel junctions with the analytical single-level model (SLM) provides both the HOMO-Fermi energy offset εhtrans and the average molecule-electrode coupling (Γ) as a function of molecular length (n), electrode work function (Φ), and the number of chemical contacts (one or two). The SLM analysis reveals a strong Fermi level (EF) pinning effect in all the junctions, i.e., εhtrans changes very little with n, Φ, and the number of chemical contacts, but Γ depends strongly on these variables. Significantly, independent measurements of the HOMO-Fermi level offset (εhUPS) by ultraviolet photoelectron spectroscopy (UPS) for CnT and CnDT SAMs agree remarkably well with the transport-estimated εhtrans. This result provides strong evidence for hole transport mediated by localized HOMO states at the Au-thiol interface, and not by the delocalized σ states in the C-C backbones, clarifying a long-standing issue in molecular electronics. Our results also substantiate the application of the single-level model for quantitative, unified understanding of transport in benchmark molecular junctions.