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

Molecular Engineering for Future Thermoelectric Materials: The Role of Electrode and Metal Components in Molecular Junctions

As global temperatures increase due to climate change, the accumulation of excess heat on Earth presents a valuable resource that can be harnessed for electricity generation using thermoelectric materials. However, the intricate structures of bulk thermoelectric materials pose significant challenges to their comprehensive understanding and limit performance. Additionally, their relatively high production costs present practical obstacles. A promising solution to these issues lies in molecular control and the use of molecular junctions. Molecules are predicted to surpass the performance of existing bulk materials in energy conversion because they can be chemically tuned to achieve high thermoelectric efficiencies. This review identifies the thermoelectric parameters that affect the performance of molecular junctions. It also explores various experimental platforms for measuring thermoelectric performance from single molecules to assemblies of hundreds of molecules. Finally, it highlights recent advancements in thermoelectric molecular junctions, focusing on the crucial roles of electrodes and metal components within the molecules, such as Ru complexes, metalloporphyrins, metallocenes, conjugated silane wires, and endohedral metallofullerenes. Ultimately, our review provides a comprehensive analysis of strategies to enhance the thermoelectric efficiency of molecular junctions.

Does Kirchhoff's Law Work in Molecular-Scale Structures?

This study aims to theoretically and comprehensively investigate the single-molecule electrical conductance of symmetric and asymmetric alkane cyclic (SAC and AAC) molecules and their corresponding linear chains with three different terminal end groups including thiol (-SH), direct carbon (-C), and amine (-NH 2). Here, we examine the validity of Kirchhoff's law concerning sigma nonconjugated molecules at the nanoscale level. Counterintuitively, the electrical conductance (G) of symmetric and asymmetric alkane cyclic molecules with two parallel conductance paths is lower than that of their corresponding single chains with only one conductance path. This completely contradicts classical rules for combining conductances in parallel, regardless of the anchor group type, in light of this study's use of symmetric and asymmetric cyclic molecules. A comparison of the DFT prediction trends with scanning tunneling microscopy measurements indicates that they are well-supported. The results of this investigation demonstrate an excellent correlation between our simulations and experimental measurements, for both SAC and AAC structures of different cavity size n,m = 3,3; 4,4; 5,5···10,10 and n,m = 3,5; 4,6; 5,7; 6,8; 7,9; 8,10; and 9,11 and for three different terminal end groups.

Orientational Effects and Molecular-Scale Thermoelectricity Control

The orientational effect concept in a molecular-scale junction is established for asymmetric junctions, which requires the fulfillment of two conditions: (1) design of an asymmetric molecule with strong distinct terminal end groups and (2) construction of a doubly asymmetric junction by placing an asymmetric molecule in an asymmetric junction to form a multicomponent system such as Au/Zn-TPP+M/Au. Here, we demonstrate that molecular-scale junctions that satisfy the conditions of these effects can manifest Seebeck coefficients whose sign fluctuates depending on the orientation of the molecule within the asymmetric junction in a complete theoretical investigation. Three anthracene-based compounds are investigated in three different scenarios, one of which displays a bithermoelectric behavior due to the presence of strong anchor groups, including pyridyl and thioacetate. This bithermoelectricity demonstration implies that if molecules with alternating orientations can be placed between an asymmetric source and drain, they can be potentially utilized for increasing the thermovoltage in molecular-scale thermoelectric energy generators (TEGs).

20-State Molecular Switch in a Li@C60 Complex

A substantial potential advantage of industrial electric and thermoelectric devices utilizing endohedral metallofullerenes (EMFs) is their ability to accommodate metallic moieties inside their empty cavities. Experimental and theoretical studies have elucidated the merit of this extraordinary feature with respect to developing electrical conductance and thermopower. Published research studies have demonstrated multiple state molecular switches initiated with 4, 6, and 14 distinguished switching states. Through comprehensive theoretical investigations involving electronic structure and electric transport, we report 20 molecular switching states that can be statistically recognized employing the endohedral fullerene Li@C₆₀ complex. We propose a switching technique that counts on the location of the alkali metal that encapsulates inside a fullerene cage. The 20 switching states correspond to the 20 hexagonal rings that the Li cation energetically prefers to reside close to. We demonstrate that the multiswitching feature of such molecular complexes can be controlled by taking advantage of the off-center displacement and charge transfer from the alkali metal to the C₆₀ cage. The most energetically favorable optimization suggests 1.2-1.4 Å off-center displacement, and Mulliken, Hirshfeld, and Voronoi simulations articulate that the charge migrates from the Li cation to C₆₀ fullerene; however, the amount of the charge transferred depends on the nature and location of the cation within the complex. We believe that the proposed work suggests a relevant step toward the practical application of molecular switches in organic materials.

Impact of the terminal end-group on the electrical conductance in alkane linear chains

This research presents comprehensive theoretical investigations of a series of alkane-based chains using four different terminal end groups including amine -NH₂, thiomethyl -SMe, thiol -SH and direct carbon contact -C. It is widely known that the electrical conductance of single molecules can be tuned and boosted by chemically varying their terminal groups to metal electrodes. Here, we demonstrate how different terminal groups affect alkane molecules' electrical conductance. In general, alkane chain conductance decreases exponentially with length, regardless of the anchor group types. In these simulations the molecular length varies from 3 to 8 -CH₂ units, with 4 different linker groups; these simulations suggest that the conductances follow the order G _C > G _SH > G _SMe > G _NH2 . The DFT prediction order of the 4 anchors is well supported by STM measurements. This work demonstrates an excellent correlation between our simulations and experimental measurements, namely: the percent difference ΔG, exponential decay slopes, A constants and β factors at different molecular alkane chain lengths.

Metallofullertube: From Tubular Endohedral Structures to Properties

Metallofullertubes are endohedral metallofullerenes with tubular fullerene cage possessing the segment of carbon nanotubes. Metallofullertubes have endohedral metal atom, fullerene cap and nanotube segment. Therefore, it is conceivable that this new kind of molecular materials would bring on many unexpected properties. In recent years, several pioneer metallofullertubes have been successfully reported, such as La₂ @D₅ (450)-C₁₀₀ , Ce₂ @D₅ (450)-C₁₀₀ , Sm₂ @D_3d (822)-C₁₀₄ . Apart from the great effort to synthesize molecules and determine their structures, the physical and chemical properties of metallofullertubes are still waiting to be explored. In this minireview, we revisit the structures of reported metallofullertubes, and then we highlight their electronic and supramolecular properties. Finally, some perspectives for the development of metallofullertubes are also discussed.

Orientational control of molecular scale thermoelectricity

Through a comprehensive theoretical study, we demonstrate that single-molecule junctions formed from asymmetric molecules with different terminal groups can exhibit Seebeck coefficients, whose sign depends on the orientation of the molecule within the junction. Three anthracene-based molecules are studied, one of which exhibits this bi-thermoelectric behaviour, due to the presence of a thioacetate terminal group at one end and a pyridyl terminal group at the other. A pre-requisite for obtaining this behaviour is the use of junction electrodes formed from different materials. In our case, we use gold as the bottom electrode and graphene-coated gold as the top electrode. This demonstration of bi-thermoelecricity means that if molecules with alternating orientations can be deposited on a substrate, then they form a basis for boosting the thermovoltage in molecular-scale thermoelectric energy generators (TEGs).

Advances in Thermoelectric Composites Consisting of Conductive Polymers and Fillers with Different Architectures

Stretchable wireless power is in increasingly high demand in fields such as smart devices, flexible robots, and electronic skins. Thermoelectric devices are able to convert heat into electricity due to the Seebeck effect, making them promising candidates for wearable electronics. Therefore, high-performance conductive polymer-based composites are urgently required for flexible wearable thermoelectric devices for the utilization of low-grade thermal energy. In this review, mechanisms and optimization strategies for polymer-based thermoelectric composites containing fillers of different architectures will be introduced, and recent advances in the development of such thermoelectric composites containing 0- to 3-dimensional filler components will be presented and outlooked.

Electrostatic Fermi level tuning in large-scale self-assembled monolayers of oligo(phenylene-ethynylene) derivatives

Understanding and controlling the orbital alignment of molecules placed between electrodes is essential in the design of practically-applicable molecular and nanoscale electronic devices. The orbital alignment is highly determined by the molecule-electrode interface. Dependence of orbital alignment on the molecular anchor group for single molecular junctions has been intensively studied; however, when scaling-up single molecules to large parallel molecular arrays (like self-assembled monolayers (SAMs)), two challenges need to be addressed: 1. Most desired anchor groups do not form high quality SAMs. 2. It is much harder to tune the frontier molecular orbitals via a gate voltage in SAM junctions than in single molecular junctions. In this work, we studied the effect of the molecule-electrode interface in SAMs with a micro-pore device, using a recently developed tetrapodal anchor to overcome challenge 1, and the combination of a single layered graphene top electrode with an ionic liquid gate to solve challenge 2. The zero-bias orbital alignment of different molecules was signalled by a shift in conductance minimum vs. gate voltage for molecules with different anchoring groups. Molecules with the same backbone, but a different molecule-electrode interface, were shown experimentally to have conductances that differ by a factor of 5 near zero bias. Theoretical calculations using density functional theory support the trends observed in the experimental data. This work sheds light on how to control electron transport within the HOMO-LUMO energy gap in molecular junctions and will be applicable in scaling up molecular electronic systems for future device applications.