Stretching-induced conductance variations as fingerprints of contact configurations in single-molecule junctions

📝 Abstract

Molecule-electrode contact atomic structures are a critical factor that characterizes molecular devices, but their precise understanding and control still remain elusive. Based on combined first-principles calculations and single-molecule break junction experiments, we herein establish that the conductance of alkanedithiolate junctions can both increase and decrease with mechanical stretching and the specific trend is determined by the S-Au linkage coordination number (CN) or the molecule-electrode contact atomic structure. Specifically, we find that the mechanical pulling results in the conductance increase for the junctions based on S-Au CN two and CN three contacts, while the conductance is minimally affected by stretching for junctions with the CN one contact and decreases upon the formation of Au monoatomic chains. Detailed analysis unravels the mechanisms involving the competition between the stretching-induced upshift of the highest occupied molecular orbital-related states toward the Fermi level of electrodes and the deterioration of molecule-electrode electronic couplings in different contact CN cases. Moreover, we experimentally find a higher chance to observe the conductance enhancement mode under a faster elongation speed, which is explained by ab initio molecular dynamics simulations that reveal an important role of thermal fluctuations in aiding deformations of contacts into low-coordination configurations that include monoatomic Au chains. Pointing out the insufficiency in previous notions of associating peak values in conductance histograms with specific contact atomic structures, this work resolves the controversy on the origins of ubiquitous multiple conductance peaks in S-Au-based single-molecule junctions.

💡 Analysis

Molecule-electrode contact atomic structures are a critical factor that characterizes molecular devices, but their precise understanding and control still remain elusive. Based on combined first-principles calculations and single-molecule break junction experiments, we herein establish that the conductance of alkanedithiolate junctions can both increase and decrease with mechanical stretching and the specific trend is determined by the S-Au linkage coordination number (CN) or the molecule-electrode contact atomic structure. Specifically, we find that the mechanical pulling results in the conductance increase for the junctions based on S-Au CN two and CN three contacts, while the conductance is minimally affected by stretching for junctions with the CN one contact and decreases upon the formation of Au monoatomic chains. Detailed analysis unravels the mechanisms involving the competition between the stretching-induced upshift of the highest occupied molecular orbital-related states toward the Fermi level of electrodes and the deterioration of molecule-electrode electronic couplings in different contact CN cases. Moreover, we experimentally find a higher chance to observe the conductance enhancement mode under a faster elongation speed, which is explained by ab initio molecular dynamics simulations that reveal an important role of thermal fluctuations in aiding deformations of contacts into low-coordination configurations that include monoatomic Au chains. Pointing out the insufficiency in previous notions of associating peak values in conductance histograms with specific contact atomic structures, this work resolves the controversy on the origins of ubiquitous multiple conductance peaks in S-Au-based single-molecule junctions.

📄 Content

1 Stretching-induced conductance variations as fingerprints of contact configurations in single-molecule junctions Yong-Hoon Kim,1,* Hu Sung Kim,1 Juho Lee,1 Makusu Tsutsui,2,* and Tomoji Kawai2 1Graduate School of Energy, Environment, Water, and Sustainability, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea.
2 The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Molecule-electrode contact atomic structures are a critical factor that characterizes molecular devices, but their precise understanding and control still remain elusive. Based on combined first-principles calculations and single-molecule break junction experiments, we herein establish that the conductance of alkanedithiolate junctions can both increase and decrease with mechanical stretching and the specific trend is determined by the S-Au linkage coordination number (CN) or the molecule-electrode contact atomic structure. Specifically, we find that the mechanical pulling results in the conductance increase for the junctions based on S-Au CN two and CN three contacts, while the conductance is minimally affected by stretching for junctions with the CN one contact and decreases upon the formation of Au monoatomic chains. Detailed analysis unravels the mechanisms involving the competition between the stretching-induced upshift of the highest occupied molecular orbital-related states toward the Fermi level of electrodes and the deterioration of molecule-electrode electronic couplings in different contact CN cases. Moreover, we experimentally find a higher chance to observe the conductance enhancement mode under a faster elongation speed, which is explained by ab initio molecular dynamics simulations that reveal an important role of thermal fluctuations in aiding deformations of contacts into low-coordination configurations that include monoatomic Au chains. Pointing out the insufficiency in previous notions of associating peak values in conductance histograms with specific contact atomic structures, this work resolves the controversy on the origins of ubiquitous multiple conductance peaks in S-Au- based single-molecule junctions. Introduction
A major breakthrough in the molecular electronics research over the past decade was the introduction and refinement of scanning tunneling microscopy or mechanically controllable break junction methods to establish single-molecule junctions in a systematic and robust fashion.1-2 Repeated formation of single-molecule junctions accompanied with the statistical data analysis has not only significantly improved the quantitative agreement among different experimental data but also continues to elucidate scientifically important yet complex phenomena that can potentially lead to novel applications.2-3 The characteristic feature of these molecular junctions is that the molecule-electrode interfaces often play a role as much important as the molecules themselves.4-5 As a representative manifestation, multiple energetically favorable contact atomic structures were claimed to produce separate conductance peaks.6-19 While this feature could be ideally exploited as a potential route to realize a switching device,13 it demonstrates the inherent variability and controllability issues in the single-molecule junction platform.4-5 Particularly, it is still unfortunately the case that the correlation between contact atomic structures and charge transport characteristics is not fully established for the ubiquitous gold-surfur contacts,20-21 which represent the most well-established venue toward molecular self- assembly and nanofabrication for the electronic, energy, and bio applications.22-24
Applying a combined computational and experimental approach, herein we show that the conductance variations during the mechanical stretching represents a unique fingerprint of contact atomic structures in covalent thiolate-gold (RS-Au) bond-mediated single molecule junctions. We particularly note a recent experiment21 that has successfully observed the theoretically-predicted25-26 conductance increase in pulled benzendithiolate (BDT) junctions. The counterintuitive observation was explained by the upshift of the BDT highest-occupied molecular orbital (HOMO) and related states toward the Fermi level (EF) of Au electrodes upon mechanical stretching and the resulting enhancement of resonant transmission. However, questions still remain on why the conductance decrease rather than increase with junction elongation is more commonly observed irrespective of the HOMO upshift, and whether such behavior is general to all Au-S-based 2 single-molecule junctions. In fact, the energetic positions of benzene molecular core and thiol contacts are comparable and their interplay can complicate the charge transport process throughout the molecular junctions

This content is AI-processed based on ArXiv data.