Thermal and Thermoelectric Properties of Molecular Junctions
Prof. Shintaro Fujii
Tokyo Institute of Technology, Japan
Abstract
The ability to measure transport and thermoelectric properties of molecular junctions at the single molecule scale is essential to achieving the ultimate goal of molecular electronics, which is to design and control molecular electronic devices with desired functionality. Here, we present two topics form our recent work [1−4] on thermal conductivity and thermoelectric voltage of molecule junctions on the molecular scale.
The first topic is on the thermal transport properties of self-assembled monolayers (SAMs) [1]. Thermal transport properties of patterned binary SAMs on Au(111) are examined using scanning thermal microscopy (SThM). Two separate SAM domains of thiolated molecules with different molecular backbones or anchoring groups are fabricated as two-dimensional patterns on the Au electrode. We compare thermal conductivity of the SAM domains and visualized the spatial distribution of thermal conductivity according to molecular framework and metal‑molecule interface structure [1]. To control the thermal conductivity, it is necessary to design the molecular skeleton and to control the interface structure at the molecular level.
The second topic is tuning of the thermopower of single-molecule junctions of fullerene (C60), 4,4’-bipyridine (BPY), and p-phenylenediamine (PPD) by scanning tunneling microscopy (STM)-based break junction technique [2]. Single-molecule junctions are prepared in a nanogap between a Au-STM tip and a Au(111) electrode. Upon applying a temperature difference across the junction, a thermoelectric voltage is generated across it. By mechanically controlling the tip–electrode separation distance, the thermoelectric voltage of the junction is tuned. The absolute value of the thermopower decreases with decreasing tip–electrode separation distance for BPY and PPD, while it increases for C60. Atomistic simulations of the junction illustrate how this arises from shifts in the conduction orbital energies induced by the mechanical compression of the junctions.
The results of our study on thermal and thermoelectric properties provide additional insight into the fundamental correlation between the geometrical and electronic structure of molecular junctions and their function.
References:
S. Fujii et al., J. Am. Chem. Soc. 143, 18777 (2021).
S. Fujii et al., Adv. Electron. Mater. 2200700 (2022).
Y. Isshiki et al., ACS Appl. Mater. Interfaces 14, 11919 (2022).
S. Fujii et al., Carbon Rev. 1, 79–88 (2022).