Nanotechnologies / Materials
Electronics, Smart devices

Measurement and analysis of electrical conductivity ofsingle molecular devices
Division of Frontier Materials Science, Graduate School of Engineering Science
Assistant Professor Tatsuhiko Ohto

Researchmap https://researchmap.jp/2ru3_y8_2d

Assosicate Professor Ryo Yamada

Researchmap https://researchmap.jp/_ryoy

Professor Hirokazu Tada

Researchmap https://researchmap.jp/read0051974

Abstract

Nanotechnologies / Materials

Single-molecule devices attract attentions because of possible
bottom-up construction, potential for device miniaturization, andutilization of unique electrical responses of organic materials. Since
it is not possible to directly observe the bridging structure of themolecule between electrodes, the presence of the molecule should
be confirmed through the current-voltage characteristics. We havedeveloped a method to unveil the junction structures by combining the break junction technique, unsupervised clustering, andfirst-principles calculations and applied it for a tripodal anchoring
molecule.

▍ Background & Results

The research field “single molecule electronics” aims for realizing
the smallest electrical devices fabricated by a bottom-up method.
The break junction method, which is widely used to measure theelectrical conductivity of single-molecule devices, is a method
in which a narrow gold wire is slowly stretched and broken whilemeasuring electrical conductivity. If a molecule is bridged between
electrodes in a certain probability, the electrical conductivity of themolecule appears as a peak in the accumulated conductance histogram. This measurement is normally performed more than thousands of times, and the resulting histogram becomes broad. Sucha broad distribution originates from the multiple junction structures,
exhibiting different electrical conductivities depending on thebridging structures, while classification of the peaks is not feasible.
On the other hand, the measurement data of the break junction
method is considered to be suitable for application of data science methods, because the data possesses so-called 3V (volume,velocity, and variety) characteristics of big data. In this study, weanalyzed the break junction data of a tripodal anchoring moleculeusing unsupervised clustering. By employing the similarity between
the averaged current-voltage curve and the individual current-voltage curve as the feature value, all the data could be classified into
three electrical conductivity peaks. First-principles calculations
also suggested that the existence of three different bridging structures characterized by the conductivity and contact asymmetry.
This method will be useful for the analysis of single-molecule junctions with various bridging structures.

▍ Significance of the research and Future perspective
By using unsupervised clustering, which is one of the data science methods, a large amount of data could be classified intomeaningful clusters, which could be linked to the molecular bridging structures. In the future, we aim for more precise analysis byapplying state-of-the art data science techniques and accurate
theoretical predictions of structure-conductivity relationship using
quantum computers.

Schematic diagram for the mechanically controllable break junction (MCBJ)
measurement and the obtained conductance histogram. The arrows indicate
the peak conductances. Clusters obtained by vector-based feature values in thek-means clustering method (k = 4) is also shown. The right panel shows the conductance histograms calculated from the I–V curves for clusters 1–4 and the normalized I–V curves averaged within each cluster and. The reference I–V curves, R,
are represented by the black dotted lines.

Potential energy surfaces and conductance values calculated by density functional theory for the metastable molecular junction structures. We assumed twoseries of potential energy surfaces (PESs) and calculated the molecular binding
energy by changing the electrode gap length. PES1 has a deep minimum, wherethe molecule achieves the self-standing structure. On the other PES (PES2), oneof the tripodal anchors was laid on its side and there are several minima. Thesestructures would appear during the formation of molecular junctions after the contact of two electrodes.

P a t e n t
Ohto, Tatsuhiko; Yamada, Ryo; Tada, Hirokazu et al. Single-Molecule Conductance of a π-Hybridized Tripodal Anchor while Maintaining Electronic Communication. Small. 2021; 17: 2006709. doi: 10.1002/smll.202006709
UR
L http://molectronics.jp/K e y w o r d single molecular electronics, break junction, first-principles calculation, machine learning
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