Long-Lived Gold Single-Atom Junctions Formed by a Flexible Probe for Scanning Tunneling Microscopy Applications

Abstract

Flexible probes have shown promise in improving resolution and sensitivity in various scanning probe microscopy based imaging and spectroscopy techniques. However, it has not been utilized in scanning tunneling microscopy (STM) based break-junction (BJ) and fixed-junction (FJ) techniques. In this report, flexible STM tips are facilely fabricated by using goldcoated quartz nanopipettes. The feasibility of using conductive nanopipettes as STM tips is demonstrated by using the STM BJ technique to repeatedly form stable gold quantum point contacts and reliably measure their conductance at an ambient condition. These nanopipette tips are surprisingly durable in the measurements. The junction formation rate and the lifetime of the formed gold atomic junctions are found to be greatly improved using flexible tips. In addition, the most probable lifetime of single-atom junctions formed by the STM FJ method reaches the natural bond lifetime. Thermally activated gold atom relaxations near the banks of the long-lived single-atom contacts are clearly revealed, permitting the insightful study of the mechanical and electrical properties of single gold atoms or atomic-chain structures. In contrast to the conventional solid metal probes, the flexible probes show significant advantages in creating and stabilizing the single-atom junctions, providing exciting opportunities for STM based techniques, molecular electronics, and biosensors.

Developing Longer‐Lived Single Molecule Junctions with a Functional Flexible Electrode

“…A Au‐coated nanopipette electrode (NPE) is used as the tip of a home‐built STM on a gold substrate as described elsewhere. [ 14–17 ] The NPE is fabricated by sequentially depositing chromium and gold on a laser‐pulled quartz nanopipette (see Section S1.1, Supporting Information), and the produced electrode possesses a conducting exterior surface, an open channel and a long shank length (6.1 ± 0.1 mm) with a high flexibility [ 14 ] (k NP ≈ 0.13 N m −1 , see atomic force microscopy (AFM) measurement of the NPE spring constant in Section S2.1, Supporting Information). The optical image of a typical NPE and its SEM images of the tip apex are displayed in the inset of Figure 1a.…”

“…This trend is similar to the measurements of gold single‐atom junctions using both types of probes. [ 14 ] Figure 1c shows the molecular conductance distribution generated by statistically compiling thousands of conductance decay traces without data selection. For both measurements, a conductance peak appears at ≈10 −4.3 G 0 , which is consistent with the reported single‐molecule conductance value of octanedithiol molecule.…”

“…In a recent publication, we have demonstrated that a flexible gold tip fabricated from gold coated quartz nanopipette can effectively stabilize gold single‐atom junctions formed by STM BJ and fixed‐junction (FJ) methods. [ 14 ] A lifetime of several minutes of gold single‐atom junction could be realized at room temperature in ambient environment, in contrast to the duration of several tenths of ms established using a conventional solid gold probe. This has inspired us to use the flexible gold tip to investigate the single‐molecule junctions.…”

Preparation and Application of Microelectrodes at the Single‐Molecule Scale

“…A typical example is the application of different lengths of tips in STM‐BJ measurement. Molecular junctions formed by STM methods are usually unstable at room temperature [33] . In addition to reported chemical methods, some physical methods, such as flexible electrodes, can be regarded as a buffer, which can significantly improve the formation rate of the single‐molecule junction and reduce noise.…”

pH-Mediated Single Molecule Conductance of Cucurbit[7]uril

“…Molecular electronics aims at investigating single molecules at electrical junctions for fabricating electronic devices, which has been extensively investigated (Cui, 2001;Choi and Mody, 2009;Herrer et al, 2018). A variety of methods have been developed to measure the electrical properties of single molecules by constructing metal-moleculemetal junctions, including scanning probes techniques such as scanning tunneling microscopy (STM) techniques (Xu and Tao, 2003;Haiss et al, 2006;Venkataraman et al, 2006;Chen et al, 2020;Yu et al, 2020), conducting atomic force microscopy (AFM) (Cui, 2001), mechanically controlled break junctions (MCBJ) (Reed et al, 1997;Smit et al, 2002), and nanoparticle dimers (Dadosh et al, 2005;Fernandez et al, 2014).…”