High thermopower of mechanically stretched single-molecule junctions

Abstract

Metal-molecule-metal junction is a promising candidate for thermoelectric applications that utilizes quantum confinement effects in the chemically defined zero-dimensional atomic structure to achieve enhanced dimensionless figure of merit ZT. A key issue in this new class of thermoelectric nanomaterials is to clarify the sensitivity of thermoelectricity on the molecular junction configurations. Here we report simultaneous measurements of the thermoelectric voltage and conductance on Au-1,4-benzenedithiol (BDT)-Au junctions mechanically-stretched in-situ at sub-nanoscale. We obtained the average single-molecule conductance and thermopower of 0.01 G0 and 15 μV/K, respectively, suggesting charge transport through the highest occupied molecular orbital. Meanwhile, we found the single-molecule thermoelectric transport properties extremely-sensitive to the BDT bridge configurations, whereby manifesting the importance to design the electrode-molecule contact motifs for optimizing the thermoelectric performance of molecular junctions.

Shaping the Atomic‐Scale Geometries of Electrodes to Control Optical and Electrical Performance of Molecular Devices

“…The tips on the electrodes surface become blunt after several break–reconnect processes (Figure d). An annealing process was further performed at 300 °C for 2 h. By this annealing process, the surface of the two quasi plane‐type electrodes was further smoothed due to the migration of metal atoms driven by the thermal excitation energy at the high temperature . In this way, a pair of atomic‐planar electrodes was obtained as shown in Figure e.…”

“…To elucidate the role of the atomic‐scale geometry of electrodes in determining the conductance of molecular devices and to find the optimized atomic‐scale geometries to improve the conductance of the molecular devices, we fabricated both atomic‐scale sharp (cone‐shaped) electrodes and atomic‐scale planar (plane‐shaped) electrodes employing mechanically controllable break junction (MCBJ) technique . We observed that the signal‐to‐noise ratio can be greatly improved with cone‐shaped electrodes, and we further observed that the conductance of the molecular junction with cone‐shaped electrodes can be 50% higher than the one with plane‐shaped electrodes.…”

Single-Molecule Analysis Methods Using Nanogap Electrodes and Their Application to DNA Sequencing Technologies

“…14,20 Incorporating a microheater structure into a nano-MCBJ facilitates the measurement of the Seebeck coefficient of SMJs, and it can also be used to examine whether the carriers flowing through an SMJ are holes or electrons. 23,24 Finally, the phases of molecular orbitals can be used to predict whether or not an electric current is flowing through an SMJ. 29 3.1 Single-Molecule Conductance Mechanism.…”

“…23 When the heater voltage was 3 V, the conductance and Seebeck coefficient of the single gold atom contacts were 1 G 0 (i.e., quantized conductance) and ΔV = 50¯V, respectively. The conductance and Seebeck coefficient of the SMJs were, conversely, approximately 1.0 © 10 ¹2 G 0 and about ΔV = ¹200 V, respectively.…”

“…While maintaining a state with single-molecule conductance, localized heating was performed using a microheater in order to raise the temperature of one electrode. 23,24 Figure 15a shows an electron microscope image of a nanostructure in which this heater was incorporated into a nano-MCBJ. 22 Alumina is an insulator that is often used as a substrate with good heat conductance; using such a substrate, the heat energy generated by heaters can be effectively transferred to the SMJs.…”