High thermopower of mechanically stretched single-molecule junctions

Makusu Tsutsui,Takanori Morikawa,Yuhui He,Akihide Arima,Masateru Taniguchi

doi:10.1038/srep11519

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.

Highlights

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 molecular-sized electrode gap can be formed by breaking Au nanobridge through mechanical deflection of the substrate
The sequential G - Δ Vc measurements were performed in the course of repeated formation and breaking of BDT molecular junctions. c-d, Partial G – t curve (c) and corresponding two-dimensional histogram (d) obtained at Vh = 3.0 V. e-f, The simultaneously recorded Δ V – t trace (e) and the twodimensional histogram (f)

Summary

Introduction

Electron transport in an organic molecule connected to metal leads has been extensively explored for applications in nanosensors and nanoelectronics[1,2,3,4] In this virtually zero-dimensional nanostructure, simultaneous enhancement of electrical conductance and thermopower can be achieved by exploiting the steep peak in the density of states at the frontier molecular orbital levels, thereby opening new perspectives for achieving high thermoelectric figure of merit[5,6] Owing to the advance in single-molecule technologies, it has become increasingly feasible to experimentally address the thermoelectric transport in individual molecular wires[7,8,9,10] Despite the progress, while it is well-established that the single-molecule conductance varies widely depending on the molecular conformations and the electrode-molecule contact structures, little is known about the thermopower sensitivity on the molecular junction configurations, which is of fundamental importance for the development of efficient molecular thermoelectric devices[11,12] geometry-sensitive nature of Seebeck coefficient was found in nanosystems similar to molecular junctions such as metal atom-sized contacts[13,14]. Au-BDT-Au bridge is known to possess a broad range of electric conductance due to the geometry-sensitive electronic coupling strength at the electrode-molecule links[15,16,17], and serves as a suitable model system for examining the structural dependence of thermoelectricity in a single-molecule wire

Methods

Results

Conclusion