Issue 5, 2024

Resolving charge transfer mechanisms in molecular tunnel junctions using dynamic charge transfer and static current–voltage measurements

Abstract

Understanding charge transfer (CT) dynamics is important for controlling the tunneling mechanism in molecular junctions. Synchrotron-based core-hole clock (CHC) spectroscopy can quantify the femtosecond-scale CT time τCT across the metal–molecule interface, which affects the current density (J) produced with applied bias (V) in the junctions. However, directly determining the tunneling behavior from a comparison of the CHC τCT and the J(V) measurement of a junction requires prior knowledge of the molecular orbitals involved. To solve this problem, we examined CT dynamics across self-assembled monolayers (SAMs) based on oligophenylene ethynylene (OPE) wires with ferrocene (Fc) terminal groups with Au, Ag and Pt bottom electrodes. Density functional theory (DFT) helped identify the donor and acceptor levels, which are typically the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The measured J(V) response of the SAM junctions with gallium-indium (EGaIn) alloy as the top electrode demonstrates that the tunneling decay coefficient β provides an intensive parameter to assess CT efficiency. We find that more delocalized molecular wavefunctions (in this case, LUMO+2, with contributions from Fc and OPE) facilitate faster and more efficient CT than more localized acceptor levels (here, the more iron-centered LUMO+1). These orbital-specific effects explain why we measure comparable β values for CT via LUMO+1 and J via HOMO and LUMO at −1 V bias. Our study highlights the utility of τCT measured by CHC in experimentally confirming the orbitals participating in charge transport measurements and shows that higher-lying delocalized orbitals can in some instances dominate over frontier orbitals despite larger energy offset (or increase in tunneling barrier height).

Graphical abstract: Resolving charge transfer mechanisms in molecular tunnel junctions using dynamic charge transfer and static current–voltage measurements

Supplementary files

Article information

Article type
Paper
Submitted
14 Nov 2023
Accepted
21 Dec 2023
First published
23 Dec 2023
This article is Open Access
Creative Commons BY-NC license

J. Mater. Chem. C, 2024,12, 1701-1709

Resolving charge transfer mechanisms in molecular tunnel junctions using dynamic charge transfer and static current–voltage measurements

L. Cao, Z. Zhang, D. Thompson, D. Qi and C. A. Nijhui, J. Mater. Chem. C, 2024, 12, 1701 DOI: 10.1039/D3TC04184K

This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. You can use material from this article in other publications, without requesting further permission from the RSC, provided that the correct acknowledgement is given and it is not used for commercial purposes.

To request permission to reproduce material from this article in a commercial publication, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party commercial publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements