Abstract:
Recently, Xuefeng Guo, Chuancheng Jia's research team and collaborators utilized the scanning tunneling microscope-based break junction technique (STM-BJ) to study the electrical and mechanical characteristics of van der Waals (vdW) interactions between alkyl chains and gold electrodes at the single-molecule level at the solid-liquid interface. At the sub-methane level, the charge transfer mechanism of alkyl chain/gold vdW contact and the detachment process of alkyl chains on the gold surface under force-induced conditions were revealed. The study found that there are two clear charge transfer channels in the alkyl chain/gold vdW contact, and under external force, alkyl chains detach from the gold surface with two sub-methane units as structural units. In addition, a biased electric field can adjust the conductivity and binding strength of the alkyl chain/gold vdW contact, with a larger electric field resulting in smaller interface contact resistance and higher binding strength.
Background:
Van der Waals (vdW) interactions, as the most commonly occurring weak interactions between molecules (substances), are widely used in chemical catalysis, interface science, organic electronics, and other fields. Accurate characterization of organic molecule/metal vdW contacts contributes to better understanding and utilization of the intrinsic properties of their microscale interfaces, which is of great significance in scientific, technological, and engineering applications. However, multidimensional characterization of weak interactions at the solid-liquid interface at the single-molecule level still remains challenging.
Highlights of the article:
Firstly, the authors utilized the STM-BJ technique to connect 1-octanethiol molecules with a single-end strong anchoring group to gold tips via Au-S bonds, where the alkane chain part exhibited four distinct conductive states when induced to detach from the gold surface under external force.
Theoretical calculations indicate that during the rupture process of 1-octanethiol molecules, detachment from the gold electrode surface occurs in units of two methylene groups, leading to a gradual decrease in molecular junction conductivity. Moreover, in the 1-octanethiol/gold vdW connection, there exist two distinct charge transfer channels: one along the shortest C-H/Au anchoring path and the other passing through the entire adsorbed alkyl chain between the electrodes. When both transport paths enter the bias window, the C-H/Au path contributes predominantly to the molecular junction conductivity.
To distinguish between vdW contacts and covalent contacts, the authors constructed single-molecule junctions with symmetric covalent Au-S contacts using 1,8-octanedithiol molecules and conducted experimental and computational studies. The molecular rupture junction exhibited a single conductive state. Furthermore, noise spectroscopy tests further indicated that the charge transfer between 1,8-octanedithiol and gold electrodes primarily occurs via bond coupling, unlike the spatial transfer found in the 1-octanethiol molecular junctions.
Furthermore, to investigate the effect of bias on alkane/gold vdW contacts, the conductive state characteristics were tested under biases of 0.1 V and 0.4 V, respectively. Experimental results showed that alkane/gold vdW contacts exhibit higher single-molecule junction efficiency and conductivity at high biases. Combined with theoretical calculations, it was shown that bias modulates the conductivity and binding strength of vdW contacts through the polarization effect of the molecular under the bias electric field.
Summary and Outlook:
In summary, this study utilized the STM-BJ technique to investigate the van der Waals (vdW) interactions between single alkane molecules and gold electrodes. At the sub-methane level, the detachment process of alkyl chains on the gold surface under external force and the charge transfer mechanism of alkane/gold vdW contacts were revealed. Furthermore, this study validated the mechanism by which bias electric fields regulate vdW interactions. This research advances our fundamental understanding of vdW contacts and holds significant importance for exploring other weak interactions and advancing molecular science.
Article details:
Single-Molecule Characterization of van der Waals Contact Between Alkane and Gold
Hongyu Ju†, Jinying Wang†, Wangping Liu†, Jie Hao†, Mengmeng Li, Yanxia Xu, Boyu Wang, Suhang He, Kunrong Mei*, Andrew C.-H. Sue*, Keqiu Chen*, Chuancheng Jia* and Xuefeng Guo*
Cite this by DOI: 10.31635/ccschem.024.202403861
Link: https://doi.org/10.31635/ccschem.024.202403861