Probing metal-molecule contact at the atomic scale via conductance jumps

Letter

Probing metal-molecule contact at the atomic scale via conductance jumps

Biswajit Pabi, Debayan Mondal, Priya Mahadevan, and Atindra Nath Pal

Phys. Rev. B 104, L121407 – Published 24 September 2021

Abstract

Understanding the formation of metal-molecule contact at the microscopic level is the key towards controlling and manipulating atomic-scale devices. Employing two isomers of bipyridine, $4, 4^{'}$ bipyridine and $2, 2^{'}$ bipyridine between gold electrodes, here, we investigate the formation of a metal-molecule bond by studying charge transport through single molecular junctions using a mechanically controlled break junction technique at room temperature. While both molecules form molecular junctions during the breaking process, closing traces show the formation of molecular junctions unambiguously for $4, 4^{'}$ bipyridine via a conductance jump from the tunneling regime, referred to as “jump to molecular contact,” being absent for $2, 2^{'}$ bipyridine. Through statistical analysis of the data, along with molecular dynamics and first-principles calculations, we establish that contact formation is strongly connected with the molecular structure of the electrodes as well as how the junction is broken during the breaking process, providing important insights for using a single molecule in an electronic device.

Received 19 June 2021
Revised 3 September 2021
Accepted 8 September 2021

DOI:https://doi.org/10.1103/PhysRevB.104.L121407

Physics Subject Headings (PhySH)

Atomic & molecular structure Chemical bonding

Molecular junctions

Density functional theory Molecular dynamics Single molecule techniques

Atomic, Molecular & OpticalInterdisciplinary PhysicsCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Biswajit Pabi , Debayan Mondal , Priya Mahadevan^*, and Atindra Nath Pal ^†

Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Sector III, Block JD, Salt Lake, Kolkata 700106, India

^*priya@bose.res.in
^†atin@bose.res.in

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Issue

Vol. 104, Iss. 12 — 15 September 2021

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Images

Figure 1
(a) Schematic of a mechanically controllable break junction (MCBJ) set up, along with the schematic representation of the molecules used in this work: $4, 4^{'}$ bipyridine ( $4, 4^{'}$ -BPY) and $2, 2^{'}$ bipyridine ( $2, 2^{'}$ -BPY). Conductance traces for (b) $4, 4^{'}$ -BPY and (d) $2, 2^{'}$ -BPY. Blue (red) represents trace during pull (push). 1D logarithmic conductance histogram for (c) $4, 4^{'}$ -BPY and (e) $2, 2^{'}$ -BPY, constructed from 5000 and 8000 traces, respectively, without any data selection. Blue and red represent the corresponding pull and push, respectively. White dashed line shows the Gaussian fit to the corresponding molecular peaks of the histogram.
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Figure 2
2D Conductance-displacement density plot for (a) $4, 4^{'}$ -BPY and (b) $2, 2^{'}$ -BPY, for push traces used in Figs. 1 and 1, with 100 bins per decade. Inset: Histogram of the conductance of the molecular contact formed after the jump, having 3750 traces (see Supplemental Material [40] for details).
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Figure 3
Conditional histogram of $4, 4^{'}$ -BPY for (a) pull and (d) push traces. The black dash-dotted line represents the histogram of the selected pull (push) traces for which plateaus are formed in the corresponding push (pull) traces, respectively. The blue and red area graph represents the histogram for all traces, as a reference for (a) and (d), respectively. (b) 2D correlation histogram, indicating the correlation between the pull (horizontal axis) and push (vertical axis). The histogram is constructed from 10 000 traces without any data selection using 50 bins per decade. (c) Bar diagram showing the percentage of traces forming a molecular junction in push via jump (black) and the percentage of traces having both molecules in the pull and push (gray) for eight different data sets.
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Figure 4
(a) MD snapshot of molecular junction with $4, 4^{'}$ -BPY for effective length of electrode (1) $11.10 Å$ , (2) $11.60 Å$ , (3) $12.20 Å$ , and (4) $12.45 Å$ . (b) MD snapshot of molecular junction with $2, 2^{'}$ -BPY for effective length of electrode (1) $6.65 Å$ , (2) 6.80 Å, and (3) $6.89 Å$ . (c) Change in total energy with respect to stretching of the bond of molecule-Au (black) and Au-Au atom (red).
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Figure 5
Complete optimized structure for vertically aligned and horizontally aligned orientations of the molecules shown in panels (a) and (b) for $4, 4^{'}$ -BPY and in panel (c) for $2, 2^{'}$ -BPY. Charge density of a state formed as a result of interactions between Au and the molecule (see Fig. S7 in the Supplemental Material [40]) is shown in the inset of panel (a).
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