Validating the Mott Formula with Self-Assembled Monolayer (SAM)-Based Large-Area Junctions: Effect of Length, Backbone, Spacer, Substituent, and Electrode on the Thermopower of SAMs

Abstract

Understanding how the Seebeck effect of organic thermoelectric devices is associated with the chemical structure of active molecules within the devices is a key goal in organic and molecular thermoelectrics. This paper describes a series of physical–organic studies that investigate structure–thermopower relationships in self-assembled monolayers (SAMs) through measurements of the Seebeck coefficient (S, μV/K) using the eutectic gallium–indium (EGaIn)-based junction technique. Several hypotheses were derived from a transmission function-based simple toy model, the Lorentzian transmission function-based Mott formula. These hypotheses were tested by comparing values of S for simple alkyl and aryl molecules with different structures in terms of backbone, length, spacer, anchor, and substituent, and for different electrodes (Au vs Ag), and by monitoring responses of S to the structural modifications. Experimentally obtained S values were further reconciled with values simulated by the Mott formula and with interfacial electronic structure and molecule-electrode coupling strength, independently measured by ultraviolet photoelectron spectroscopy and transition voltage spectroscopy.

Thermopower in Transition from Tunneling to Hopping

“…Molecular epitaxystepwise growth of (semi)­crystalline molecular overlayer on a substrateis useful for fabricating molecular thin films composed of vertically aligned, ordered oligomers from monomers. , Here, we show thermopower measurements over molecular junctions based on molecular epitaxy of an oligophenyleneimine (OPI n where n = 1–9) layer, formed via imine condensation between aryl aldehyde and amine derivatives on an ultrasmooth template-stripped gold (Au TS ) substrate (Figure ), , using eutectic Ga–In (EGaIn) for a top electrode (Figure ). ,− As the number of epitaxy cycles ( n in OPI n ) increased, the Seebeck coefficient ( S , μV/K) increased linearly. Notably, we observed a noticeable transition in the increasing rate at the point of n = 4 (length of molecule ( d ) = 3.4 nm), which was attributed to the transition in the charge transport regime from tunneling to hopping.…”

Exploring seebeck-coefficient fluctuations in endohedral-fullerene, single-molecule junctions

“…They are higher than those found for thiol-terminated alkyl chains, which were found to be 0.3 AE 1 and almost independent of length. 70 Studies [71][72][73][74] have also shown that increasing the applied temperature difference can lead to additional fluctuations in the thermovoltage. As a first step in the calculations leading to Table 1, energy barriers to rotation about the above axes were computed, to obtain the preferred angles of rotation, which minimise the total energy.…”

“…They are higher than those found for thiol-terminated alkyl chains, which were found to be 0.3 ± 1 and almost independent of length. 70 Studies 71–74 have also shown that increasing the applied temperature difference can lead to additional fluctuations in the thermovoltage.…”

Transformable Gallium‐Based Liquid Metal Nanoparticles for Tumor Radiotherapy Sensitization

“…toxicity, which enable their suitability not only for microfluidics, printed circuits, flexible electronics, and artificial intelligence, but also for biomedical applications. [10][11][12][13][14][15][16][17][18][19][20] Compared with "hard" NPs, "soft" NPs like Gallium and Gallium-Indium eutectic alloys (EGaIn, 75% Ga, 25% In, wt%) with moderate melting points (29.8 °C, 15.7 °C, respectively) possess good biocompatibility and low toxicity. More importantly, their mechanical parameters are close to biological tissue, which allows their in vivo administration, acrossing the biological barrier, and increased tissue retention.…”

“…In recent years, room temperature Gallium‐based liquid metal (LM) has attracted widespread interest due to its versatile physicochemical properties, [ 6–9 ] such as high electrical conductivity, flexibility, large surface tension, good biocompatibility, and low toxicity, which enable their suitability not only for microfluidics, printed circuits, flexible electronics, and artificial intelligence, but also for biomedical applications. [ 10–20 ] Compared with “hard” NPs, “soft” NPs like Gallium and Gallium‐Indium eutectic alloys (EGaIn, 75% Ga, 25% In, wt%) with moderate melting points (29.8 ℃, 15.7 ℃, respectively) possess good biocompatibility and low toxicity. More importantly, their mechanical parameters are close to biological tissue, which allows their in vivo administration, acrossing the biological barrier, and increased tissue retention.…”