Study of thermoelectric effects with molecular junctions is an effective strategy in understanding the mechanisms of transition between heat and electricity at atomic/molecular level and is a promising system for future development of electrical energy generator or cooling elements at nanoscale. The thermoelectric measurements in our group are conducted by the EGaIn technique. Fig. 1a and b show schematic illustrations of a test platform for molecular thermoelectric junctions using EGaIn as the top-electrode, and a polyimide (PI) film embedded with multiple heating resistors is used to heat the bottom-electrode generating temperature difference across junctions. Molecules were assembled on gold (Au) bottom-electrodes and the conical shaped tips of EGaIn were used to conformally contact the top-surface of SAMs eventually forming molecular thermoelectric junctions. When Au electrode was heated, Seebeck coefficient of SAMs, according to Seebeck effect, can be determined by measuring the thermoelectric voltage across the junction and deducing the relationship between thermoelectric voltage and Seebeck coefficient.

Fig. 1 The demonstrations of thermoelectric measurements conducted by the EGaIn technique. (a) The schematic diagram of the thermoelectric testbed developed from EGaIn technique. (b) The schematic illustration of a SAM-based thermoelectric junction. (c) Thermoelectric voltage of Au^TS-SAMs//Ga₂O₃/EGaIn junctions at temperature difference = 2.0 K (red), 3.5 K (blue) and = 5.0 K (green).

Typical thermoelectric-voltage traces of the Au^TS-SAMs//Ga₂O₃/EGaIn junctions at different temperature difference are shown in Fig. 1c. Wherein,  V₁ refers to the initial voltage across a junction without temperature difference (the magnitude of V₁ originated from chemical potential difference at the SAM/electrode interface), the average of V₁ (<V₁>) and its standard deviation could be obtained over ~100s test duration (rate of data acquisition: 10 data points of voltage per second) with a Gaussian fit. Once we heated the bottom-electrode, temperature difference occurred and the measured voltage was positively shifted, indicating charge transport was dominated by the HOMO. When the junction reached thermal equilibrium, the average of V₂ (<V₂>) and its standard deviation were calculated over 200 s test time with a Gaussian fit. Then, the thermoelectric voltage was obtained by using <V₂> subtracting <V₁> at a certain temperature difference for each junction. Fig. 1c shows that the measured values of thermoelectric voltage of Au^TS-SAMs//Ga₂O₃/EGaIn junction were enhanced as temperature difference increased, verifiably demonstrating that our method is suitable for testing the thermoelectric effects of molecular junctions with high stability.