Angstrom-Scale Ruler Using Single Molecule Conductance Signatures

Abstract

Experimental techniques that determine atomic arrangements in single metal–molecule–metal junctions will enable a mechanistic understanding and control of electronic properties on the nanoscale. Here, we develop a method to determine average gold and silver nanogap widths with Angstrom resolution using single molecule junction conductance and distance measurements of N,N′-diamino alkanes in a scanning tunneling microscope break junction setup. Our experiments are supported by density functional theory (DFT) calculations, which suggest that the alkane-conducting trans-configurations can be outcompeted by the nonconducting cis conformers when the nanogap distance is shorter than the length of the molecule. As a result, the distribution of binding geometries during the conductance plateau is peaked when the gap width is comparable to the molecule length. We apply this conductance ruler to determine the binding geometry of N,N′-oligophenyl amines which have been observed to have two distinct conductance signatures. Our measurements and DFT calculations show that for the high conductance geometry, oligophenyls preferentially bind away from the apex of the electrodes, so that the tip–tip nanogap distance is less than the full length of the molecule and the π-system can overlap with the electrodes. Significantly, our new conductance ruler method allows us to determine that the low conductance of oligophenyls occurs when the interelectrode distance is greater than the N–N length of the molecule, requiring two π–π stacked molecules to bridge the junction.

Hard–Soft Chemistry Design Principles for Predictive Assembly of Single Molecule-Metal Junctions

“…Such 2D histograms bin the log of the conductance versus electrode displacement relative to G 0 rupture and can identify the length and binding geometry of the molecule in the junction. 26,37,46 For example, the lengths of molecular signatures in 2D histograms have been shown to scale with the length of the molecular complex binding in the junction. We compare the molecular plateau signatures measured in the presence of pySMe (N−S distance ∼4.5 Å), a heterobimetallic compound containing one axial pySMe (5b, Pt-SMe distance ∼9 Å) and a homobimetallic complex containing two pySMe linkers (6, SMe-SMe distance ∼16 Å) in 2D histograms, plotted in parts B, C, and D of Figure 3, respectively (see Figure S14 for length information).…”

Grid-Based Correlation Analysis to Identify Rare Quantum Transport Behaviors

“…Single-molecule electronics have the potential to enable cheap and efficient circuit fabrication at the ultimate size limit and also provide an appealing test-bed for exploring intriguing physical phenomena at the nanoscale such as quantum interference, − spin filtering, , and interfacial coupling. − A significant and ongoing challenge in the investigation of transport through single-molecule systems, however, is extracting meaning from the large and stochastic data sets typically produced by experimental techniques such as the scanning tunneling microscope break junction (STM-BJ) − and mechanically controlled break junction (MCBJ). − Both of these methods involve forming and then breaking a thin metal constriction to create a single-molecule junction in the nanogap between two metal electrodes. The primary data collected is the conductance ( G = I / V ) through the junction during the breaking process as a function of how much the two sides have been pulled apart, known as a “breaking trace”.…”

“…The magnitude of this so-called “molecular yield” varies depending on the binding group strength, − molecular concentration, and other unknown or uncontrolled variables. , Figure uses simulated traces to illustrate that, for short molecules whose molecular plateaus mostly overlap with the tunneling background, low molecular yield can make the molecular signature functionally impossible to identify in both the 1D and 2D histograms. Partially for this reason, most break junction experiments focus on systems with molecular yields >10%, ,,,,, and often approaching 100%, ,, because this produces histograms with clear molecular features. However, high molecular yields increase the risk of measuring multi-molecule rather than the desired single-molecule features .…”

Decoding Forming Processes of Different Contact Configurations in Au- and Ag-Electrode Single-Molecule Junctions

“…The electron transport properties of molecular junctions are closely related to the structures of the functional molecules, molecule–electrode interface configurations, electrode–electrode distances, and so forth. − However, different electrode materials can also result in much different transport properties for the same functional molecule, − which are generally attributed to different mechanisms, such as different energy band structures of the electrodes, , different molecule–electrode coupling strengths, and different molecule–electrode interface configurations. ,, Due to the excellent conductivity and prominent extensibility, gold is the most commonly used electrode material in molecule junction investigations. − The conductivity of silver is higher than that of gold, so silver has also been extensively studied in molecular electronics. ,,− It is found that the conductance of ethynyl-terminated molecular junctions with silver electrodes is much higher than that of the molecular junctions with gold electrodes . However, this conductance difference is obviously not due to the different conductivities between gold and silver. − The preliminary calculations show that it may be attributed to the different contact configurations of the molecular junctions with different metal electrodes .…”