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

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.Comment: 11 pages, 4 figures; to be published in J. Am. Chem. So

Ionic Signal Amplification of DNA in a Nanopore

Ionic signal amplification is a key challenge for single-molecule analyses by solid-state nanopore sensing. Here, a permittivity gradient approach for amplifying ionic blockade characteristics of DNA in a nanofluidic channel is reported. The transmembrane ionic current response is found to change substantially through modifying the liquid permittivity at one side of a pore with an organic solvent. Imposing positive liquid permittivity gradients with respect to the direction of DNA electrophoresis, this study observes the resistive ionic signals to become larger due to the varying contributions of molecular counterions. On the contrary, negative gradients render adverse effects causing conductive ionic current pulses upon polynucleotide translocations. Most importantly, both the positive and negative gradients are demonstrated to be capable of amplifying the ionic signals by an order of magnitude with a 1.3-fold difference in the transmembrane liquid dielectric constants. This phenomenon allows a novel way to enhance the single-molecule sensitivity of nanopore sensing that may be useful in analyzing secondary structures and genome sequence of DNA by ionic current measurements.This is the pre-peer reviewed version of the following article: Tsutsui, M., Yokota, K., He, Y., Kawai, T., Ionic Signal Amplification of DNA in a Nanopore. Small Methods 2022, 6, 2200761, which has been published in final form at https://doi.org/10.1002/smtd.202200761. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving

Electrochemical response of biased nanoelectrodes in solution

Novel approaches to DNA sequencing and detection require the measurement of electrical currents between metal probes immersed in ionic solution. Here, we experimentally demonstrate that these systems maintain large background currents with a transient response that decays very slowly in time and noise that increases with ionic concentration. Using a non-equilibrium stochastic model, we obtain an analytical expression for the ionic current that shows these results are due to a fast electrochemical reaction at the electrode surface followed by the slow formation of a diffusion layer. During the latter, ions translocate in the weak electric field generated after the initial rapid screening of the strong fields near the electrode surfaces. Our theoretical results are in very good agreement with experimental findings

MCBJ ギホウ オ モチイタ ゲンシ ブンシ ドウデンタイ ノ デンシ デンドウ ニ カンスル ケンキュウ

京都大学0048新制・課程博士博士(工学)甲第12324号工博第2653号新制||工||1375(附属図書館)24160UT51-2006-J316京都大学大学院工学研究科材料工学専攻(主査)教授 酒井 明, 教授 杉村 博之, 教授 松重 和美学位規則第4条第1項該当Doctor of EngineeringKyoto UniversityDA

Electrical breakdown of short multiwalled carbon nanotubes

Electrical breakdown of short (∼50nm) multiwalled carbon nanotubes(MWNTs) is studied utilizing mechanically controllable break junction technique with goldelectrodes. Measurements of the conductance-biascharacteristics near the breakdown point revealed two different types of breakdown behavior for the short MWNTs. In one type designated as type I, the conductance increases nearly linearly with the bias and, over the breakdown point, decreases stepwise. On the other hand, in the type-II breakdown, the conductance shows a transient and irreversible increase right before the breakdown and subsequently jumps to zero. We consider that the type-II breakdown in vacuum is contact related, whereas the type-I breakdown occurs through the usual MWNT heatin