Team led by Prof. Hyo Jae Yoon publishes paper on new molecular-scale thermoelectric measurement system in Nano Letters

World’s first large-area junction-based thermoelectric measurement system at the molecular scale

▲ Professor Hyo Jae Yoon (left) and first author Sohyun Park (right)

A team led by Professor Hyo Jae Yoon of the Department of Chemistry under the College of Science developed a new system of molecular scale thermoelectric measurement using liquid eutectic gallium-indium (EGaIn), enabling easier high yield measurement of the Seebeck coefficient and validating the thermoelectric performance of self-assembled monolayers (SAMs).

Supported by the National Research Foundation of Korea (nanomaterial technology development project, conductive material technology development program), the study was published in Nano Letters (impact factor: 12.712), a leading journal in the field of materials and nanoscience, on November 13.

※ Title of paper: New Approach for Large-Area Thermoelectric Junctions with a Liquid Eutectic Gallium-Indium Electrode

※ Authors: Sohyun Park (first author, Korea University), Hyo Jae Yoon (corresponding author, Korea University)

The significance of the study is that it pioneers a new research area in the field of molecular thermoelectrics by establishing, for the first time, a large-area (up to μm2) thermoelectric system on a molecular scale.

The team showed that the Seebeck coefficient of organic molecules on the surface of self-assembled monolayers (SAMs) can be easily measured with gallium-indium alloy, which can be fabricated at room temperature and does not damage delicate SAM surfaces. The study of thermoelectrics on single molecules or SAMs is highly challenging. In particular, it is technically difficult to fabricate a thermoelectric device and then analyze its properties without damaging its molecules. Existing technologies are based on single molecule junctions that rely on expensive Scanning Tunneling Microscope (STM) and Atomic Force Microscope (AFM) equipment, or require the use of a laser. This study presented the world’s first technique of large-area thermoelectric measurement on a molecular scale without such equipment. An advantage of this technique is that it minimizes the damage to SAMs and produces highly reliable data in a relatively short period. In addition to enabling large-area thermoelectric measurement of various molecular structures, the study is expected to serve as a valuable reference for future research into structural thermoelectrics at the atomic scale as well as the development of high-efficiency organic thermoelectric materials.

[Terminology]

1) Thermoelectric Effect

◯ The thermoelectric effect is a key mechanism that uses the difference in surrounding temperature to produce electrical energy. This recycling of heat makes it an interesting research topic in fields related to the environment as well as science. Thermoelectric devices have a variety of applications across industries, including semiconductor, automobile, space, aerospace, biology, optics, computer, power, and household appliances.

2) Eutectic Gallium-Indium (EGaIn)

◯ Comprised of 75% Ga and 25% In, EGaIn exists as a liquid at room temperature as it melts at approximately 15 ℃. When exposed to air, a 1 nm thin film of gallium oxide (Ga2O3) is formed on its surface. This is shaped like the conical tip shown in Fig. 1, and can create large-area junctions. The liquid metal causes minimal damage to SAMs, allows high yields of working junctions, and provides vast amounts of data.

3) Self-Assembled Monolayer (SAM)

◯ The self-assembled monolayer refers to thin films of single molecules, self-assembled as a layer on a metal surface.

4) Seebeck Coefficient

◯ The Seebeck coefficient (S) represents the efficiency of converting heat into electrical energy. It is calculated by S=-ΔV/ΔT, and the absolute value increases when a higher voltage is produced by the same temperature differential.

[Fig. 1] Schematic showing the molecular structure and detailed structure of the large area junction (AuTS/S(Ph)n//Ga2O3/EGaIn). Here, “//” represents van der Waals contact, and AuTS is an ultraflat gold substrate fabricated by template-stripping.)

[Fig. 2] (a) Examples of thermoelectric data covering trace-to-trace, junction-to-junction, electrode-to-electrode, and sample-to-sample variations (b) Histograms of ΔV for S(Ph)3 SAM (c) Plot of ΔVmean as a function of ΔT of S(Ph)n SAM (n=1,2,3) (d) Plot of SSAM as a function of molecule length ((n) in S(Ph)n)