An experimental set up to probe the quantum transport through single atomic/molecular junction at room temperature

Understanding the transport characteristics at the atomic limit is the prerequisite for futuristic nano-electronic applications. Among various experimental procedures, mechanically controllable break junction (MCBJ) is one of the well adopted experimental technique to study and control the atomic or molecular scale devices. Here, we present the details of the development of a piezo controlled table top MCBJ set up, working at ambient condition, along with necessary data acquisition technique and analysis of the data. We performed conductance experiment on a macroscopic gold wire, which exhibits quantized conductance plateau upon pulling the wire with the piezo. Conductance peak up to $\sim 20 G_0$ ($G_0 = 2e^2/h$, $e$ being the electronic charge and $h$ being the plank's constant) could be resolved at room temperature. A well-known test bed molecule,$4, 4^\prime$ bipyridine, was introduced between the gold electrodes and conductance histogram exhibits two distinctive conductance peaks, confirming the formation of single molecular junction, in line with the previous reports. This demonstrate that our custom-designed MCBJ set up is capable of measuring quantum transport of a single molecular junction at ambient condition.Comment: 15 pages, 8 figure

Resonant transport in a highly conducting single molecular junction via metal-metal covalent bond

Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although, resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (> 1V), the conductance was limited to < 0.1 G$_0$, G$_0$ being the quantum of conductance. Here, we report electronic transport measurements by directly connecting a Ferrocene molecule between Au electrodes at the ambient condition in a mechanically controllable break junction setup (MCBJ), revealing a conductance peak at ~ 0.2 G$_0$ in the conductance histogram. A similar experiment was repeated for Ferrocene terminated with amine (-NH2) and cyano (-CN) anchors, where conductance histograms exhibit an extended low conductance feature including the sharp high conductance peak, similar to pristine ferrocene. Statistical analysis of the data along with density functional theory-based transport calculation suggests the possible molecular conformation with a strong hybridization between the Au electrodes and Fe atom of Ferrocene molecule is responsible for a near-perfect transmission in the vicinity of the Fermi energy, leading to the resonant transport at a small applied bias (< 0.5V). Moreover, calculations including Van der Waals/dispersion corrections reveal a covalent like organometallic bonding between Au and the central Fe atom of Ferrocene, having bond energies of ~ 660 meV. Overall, our study not only demonstrates the realization of an air-stable highly transmitting molecular junction, but also provides an important insight about the nature of chemical bonding at the metal/organo-metallic interface.Comment: 23 pages, 6 figures, supplementary include

Structural Regulation of Mechanical Gating in Molecular Junctions

In contrast to silicon-based transistors, single molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the molecular orientation on mechanical gating has not been addressed, despite its potential influence on the gating effectiveness. Here, we show that the same molecular junction can experience either clear mechanical gating or none, depending on the molecular orientation in the junctions. The effect is found in silver-ferrocene-silver break junctions and analyzed in view of ab initio and transport calculations, where the influence of the molecular orbital geometry on charge transfer to or from the molecule is revealed. The molecular orientation is thus a new degree of freedom that can be used to optimize mechanically gated molecular junctions