Molecular design and control of fullerene-based bi-thermoelectric materials

Resolving molecular frontier orbitals in molecular junctions with kHz resolution

Designing and building single-molecule circuits with tailored functionalities requires a detailed knowledge of the junction electronic structure. The energy of frontier molecular orbitals and their electronic coupling with the electrodes play a key role in determining the conductance of nanoscale molecular circuits. Here, we developed a method for measuring the current-voltage (I-V) characteristics of single-molecule junctions with a time resolution that is two orders of magnitude higher than previously achieved. These I-V measurements with high temporal resolution, together with atomistic simulations, enabled us to characterize in detail the frontier molecular states and their evolution in sub-angstrom stretching of the junction. For a series of molecules, changes in the electronic structure were resolved as well as their fluctuations prior to junction breakdown. This study describes a new methodology to determine the key frontier MO parameters at single-molecule junctions and demonstrates how these can be mechanically tuned at the single-molecule level.

Enhancing the thermopower of single-molecule junctions by edge substitution effects

Heteroatom substitution and anchoring groups have an important impact on the thermoelectric properties of single-molecule junctions. Herein, thermoelectric properties of several anthracene derivative based single-molecule junctions are studied by means of first-principles calculations. In particular, we pay great attention to the edge substitution effects and find that edge substitution with nitrogen can induce a transmission peak near the Fermi energy, leading to large transmission coefficients and electrical conductance at the Fermi energy. Additionally, the steep shape of the transmission function gives rise to a high Seebeck coefficient. Therefore, an enhanced power factor can be expected. The robustness of this edge substitution effect has been examined by altering the electrode distance and introducing heteroatoms at different positions. The enhancement of the power factor due to edge substitution makes the studied single-molecule junction a promising candidate for efficient thermoelectric devices.

Seebeck Effect in Molecular Wires Facilitating Long-Range Transport

The study of molecular wires facilitating long-range charge transport is of fundamental interest for the development of various technologies in (bio)organic and molecular electronics. Defining the nature of long-range charge transport is challenging as electrical characterization does not offer the ability to distinguish a tunneling mechanism from the other. Here, we show that investigation of the Seebeck effect provides the ability. We examine the length dependence of the Seebeck coefficient in electrografted bis-terpyridine Ru(II) complex films. The Seebeck coefficient ranges from 307 to 1027 μV/K, with an increasing rate of 95.7 μV/(K nm) as the film thickness increases to 10 nm. Quantum-chemical calculations unveil that the nearly overlapped molecular-orbital energy level of the Ru complex with the Fermi level accounts for the giant thermopower. Landauer-Büttiker probe simulations indicate that the significant length dependence evinces the Seebeck effect dominated by coherent near-resonant tunneling rather than thermal hopping. This study enhances our comprehension of long-range charge transport, paving the way for efficient electronic and thermoelectric materials.

Structure and single-molecule conductance of two endohedral metallofullerenes with large C88 cage

Endohedral metallofullerenes are capable of holding peculiar metal clusters inside the carbon cage. Additionally, these display many chemical and physical properties originating from the complexation between the metal clusters and carbon cages, which could be acquired for wide applications. In this study, two metallofullerenes (Ce2O@C88 and Ce3N@C88) with an identical large C88-D2(35) cage, and their molecular structures and single-molecule conductance properties were investigated comparatively. Characterizations of UV-vis-NIR absorption spectroscopy, Raman spectroscopy, and DFT calculations were employed to determine the geometries and electronic structures of Ce2O@C88 and Ce3N@C88. These molecules revealed varied energy gaps, structural parameters, vibrational modes, and molecular frontier orbitals. Although the two metallofullerenes have an identical cage isomer of C88-D2(35), their different endohedral clusters can influence their structures and physicochemical properties. Furthermore, the single-molecule conductance properties were measured using the scanning tunneling microscopy break junction technique (STM-BJ). The experimental results revealed that Ce2O@C88 has a higher conductance than Ce3N@C88 and C60. This revealed the cluster-dependent electron transportation as well as the significant research value of metallofullerenes with large carbon cages. These results provide guidance for fabricating single-molecule electronic devices.

Determination of electric and thermoelectric properties of molecular junctions by AFM in peak force tapping mode

Molecular thin films, such as self-assembled monolayers (SAMs), offer the possibility of translating the optimised thermophysical and electrical properties of high-Seebeck-coefficient single molecules to scalable device architectures. However, for many scanning probe-based approaches attempting to characterise such SAMs, there remains a significant challenge in recovering single-molecule equivalent values from large-area films due to the intrinsic uncertainty of the probe-sample contact area coupled with film damage caused by contact forces. Here we report a new reproducible non-destructive method for probing the electrical and thermoelectric (TE) properties of small assemblies (10-103) of thiol-terminated molecules arranged within a SAM on a gold surface, and demonstrate the successful and reproducible measurements of the equivalent single-molecule electrical conductivity and Seebeck values. We have used a modified thermal-electric force microscopy approach, which integrates the conductive-probe atomic force microscope, a sample positioned on a temperature-controlled heater, and a probe-sample peak-force feedback that interactively limits the normal force across the molecular junctions. The experimental results are interpreted by density functional theory calculations allowing quantification the electrical quantum transport properties of both single molecules and small clusters of molecules. Significantly, this approach effectively eliminates lateral forces between probe and sample, minimising disruption to the SAM while enabling simultaneous mapping of the SAMs nanomechanical properties, as well as electrical and/or TE response, thereby allowing correlation of the film properties.

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.

Molecular Thermoelectricity in EGaIn-Based Molecular Junctions

ConspectusUnderstanding the thermoelectric effects that convert energy between heat and electricity on a molecular scale is of great interest to the nanoscience community. As electronic devices continue to be miniaturized to nanometer scales, thermoregulation on such devices becomes increasingly critical. In addition, the study of molecular thermoelectricity provides information that cannot be accessed through conventional electrical conductance measurements. The field of molecular thermoelectrics aims to explore thermoelectric effects in electrode-molecule-electrode tunnel junctions and draw inferences on how the (supra)molecular structure of active molecules is associated with their thermopower. In this Account, we introduce a convenient and useful junction technique that enables thermovoltage measurements of one molecule thick films, self-assembled monolayers (SAMs), with reliability, and discuss the atomic-detailed structure-thermopower relations established by the technique. The technique relies on a microelectrode composed of non-Newtonian liquid metal, eutectic gallium-indium (EGaIn) covered with a native gallium oxide layer. The EGaIn electrode makes it possible to form thermoelectric contacts with the delicate structure of SAMs in a noninvasive fashion. A defined interface between SAM and the EGaIn electrode allows time-effective collection of large amounts of thermovoltage data, with great reproducibility, efficiency, and reliable interpretation and statistical analysis of the data. We also highlight recent efforts to utilize the EGaIn technique for probing molecular thermoelectricity and structure-thermopower relations. Using the technique, it was possible to unravel quantum-chemical mechanisms of thermoelectric functions, based on the Mott formula, in SAM-based large-area junctions, which in turn led us to set various hypotheses to boost the Seebeck coefficient. By validating the hypotheses again with the EGaIn technique, we revealed that the thermopower of junction increases through the reduction of the energy offset between accessible molecular orbital energy level and Fermi level or the tuning of broadening of the orbital energy level. Such alterations in the shape of energy topography of junction could be achieved through structural modifications in anchoring group and molecular backbone of SAM, and the bottom electrode. Molecular thermoelectrics offers a unique opportunity to build a well-defined nanoscale system and isolate an effect of interest from others, advancing fundamental understanding of charge transport across individual molecules and molecule-electrode interfaces. In the Account, we showed our recent work involving carefully designed molecular system that are relevant to answering the question of how thermopower differs between the tunneling and thermal-hopping regimes. The field of molecular thermoelectrics needs to address practical application-related issues, particularly molecular degradation in thermal environments. In this regard, we summarized the results highlighting the thermal instability of SAM-based junctions based on a traditional thiol anchor group and how to circumvent this problem. We also discussed the power factor (PF)─a practical parameter representing the efficiency for converting heat into electricity─of SAMs, evaluated using the EGaIn technique. In the Conclusion section of this Account, we present future challenges and perspectives.

Structural Regulation of Mechanical Gating in Molecular Junctions

In contrast to silicon-based transistors, single-molecule junctions can be gated by simple mechanical means. Specifically, charge can be transferred between the junction's electrodes and its molecular bridge when the interelectrode distance is modified, leading to variations in the electronic transport properties of the junction. While this effect has been studied extensively, the influence of the molecular orientation on mechanical gating has not been addressed, despite its potential influence on the gating effectiveness. Here, we show that the same molecular junction can experience either clear mechanical gating or none, depending on the molecular orientation in the junctions. The effect is found in silver-ferrocene-silver break junctions and analyzed in view of ab initio and transport calculations, where the influence of the molecular orbital geometry on charge transfer to or from the molecule is revealed. The molecular orientation is thus a new degree of freedom that can be used to optimize mechanically gated molecular junctions.

Enhanced Thermoelectricity in Metal-[60]Fullerene-Graphene Molecular Junctions

The thermoelectric properties of molecular junctions consisting of a metal Pt electrode contacting [60]fullerene derivatives covalently bound to a graphene electrode have been studied by using a conducting-probe atomic force microscope (c-AFM). The [60]fullerene derivatives are covalently linked to the graphene via two meta-connected phenyl rings, two para-connected phenyl rings, or a single phenyl ring. We find that the magnitude of the Seebeck coefficient is up to nine times larger than that of Au-C₆₀-Pt molecular junctions. Moreover, the sign of the thermopower can be either positive or negative depending on the details of the binding geometry and on the local value of the Fermi energy. Our results demonstrate the potential of using graphene electrodes for controlling and enhancing the thermoelectric properties of molecular junctions and confirm the outstanding performance of [60]fullerene derivatives.

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.

High Seebeck Coefficient Achieved by Multinuclear Organometallic Molecular Junctions

This paper describes the thermoelectric properties of molecular junctions incorporating multinuclear ruthenium alkynyl complexes that comprise Ru(dppe)₂ [dppe = 1,2-bis(diphenylphosphino)ethane] fragments and diethylnyl aromatic bridging ligands with thioether anchors. Using the liquid metal technique, the Seebeck coefficient was examined as a function of metal nuclearity, oxidation state, and substituent on the organic ligand backbone. High Seebeck coefficients up to 73 μV/K and appreciable thermal stability with thermovoltage up to ∼3.3 mV at a heating temperature of 423 K were observed. An unusually high proximity of the highest occupied molecular orbital (HOMO) energy level to the Fermi level was revealed to give the remarkable thermoelectric performance as suggested by combined experiments and calculations. This work offers important insights into the development of molecular-scale devices for efficient thermoregulation and heat-to-electricity conversion.

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.

Room-temperature logic-in-memory operations in single-metallofullerene devices

In-memory computing provides an opportunity to meet the growing demands of large data-driven applications such as machine learning, by colocating logic operations and data storage. Despite being regarded as the ultimate solution for high-density integration and low-power manipulation, the use of spin or electric dipole at the single-molecule level to realize in-memory logic functions has yet to be realized at room temperature, due to their random orientation. Here, we demonstrate logic-in-memory operations, based on single electric dipole flipping in a two-terminal single-metallofullerene (Sc₂C₂@C_s(hept)-C₈₈) device at room temperature. By applying a low voltage of ±0.8 V to the single-metallofullerene junction, we found that the digital information recorded among the different dipole states could be reversibly encoded in situ and stored. As a consequence, 14 types of Boolean logic operation were shown from a single-metallofullerene device. Density functional theory calculations reveal that the non-volatile memory behaviour comes from dipole reorientation of the [Sc₂C₂] group in the fullerene cage. This proof-of-concept represents a major step towards room-temperature electrically manipulated, low-power, two-terminal in-memory logic devices and a direction for in-memory computing using nanoelectronic devices.

Excellent Medium-Temperature Thermoelectric Performance of Monolayer BiOCl

Recently, a ternary-layered material BiOCl has elicited intense interest in photocatalysis, environmental remediation, and ultraviolet light detection because of its unique band gap of around 3.6 eV, low toxicity, and earth abundance. In particular, Gibson et al. reported a measurement of the in-plane thermal conductivity of BiOCl experimentally using a four-point-probe method [Science, 373, 1017-1022 (2021)], which is only 1.25 W/m K at 300 K. Motivated by the work, we studied the thermoelectric property of monolayer BiOCl using first-principles calculations combined with the Boltzmann transport equation. The calculated phonon thermal conductivity of monolayer BiOCl is 3 W/m K at 300 K, which is far below that of other promising 2D thermoelectric materials like graphyne and MoS₂. A comprehensive analysis of phonon modes is conducted to reveal the low thermal conductivity. Moreover, the maximal ZT value is as high as 1.8 at 300 K and 5.7 at 800 K for the p-type doping with the 2 × 10¹⁵ cm^-2 concentration. More importantly, we found that the thermoelectric efficiency of such 2D materials is significantly enhanced to 8 at 800 K by applying 1.5% tensile strain, which clearly outperforms that of the reported 2D thermoelectric material SnSe. The results shed light on the promising application in medium-temperature (600-900 K) thermoelectric devices.

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

For the purpose of creating single-molecule junctions, which can convert a temperature difference ΔT into a voltage ΔV via the Seebeck effect, it is of interest to screen molecules for their potential to deliver high values of the Seebeck coefficient S = -ΔV/ΔT. Here we demonstrate that insight into molecular-scale thermoelectricity can be obtained by examining the widths and extreme values of Seebeck histograms. Using a combination of experimental scanning-tunnelling-microscopy-based transport measurements and density-functional-theory-based transport calculations, we study the electrical conductance and Seebeck coefficient of three endohedral metallofullerenes (EMFs) Sc₃N@C₈₀, Sc₃C₂@C₈₀, and Er₃N@C₈₀, which based on their structures, are selected to exhibit different degrees of charge inhomogeneity and geometrical disorder within a junction. We demonstrate that standard deviations in the Seebeck coefficient σ_S of EMF-based junctions are correlated with the geometric standard deviation σ and the charge inhomogeneity σ_q. We benchmark these molecules against C₆₀ and demonstrate that both σ_q, σ_S are the largest for Sc₃C₂@C₈₀, both are the smallest for C₆₀ and for the other EMFs, they follow the order Sc₃C₂@C₈₀ > Sc₃N@C₈₀ > Er₃N@C₈₀ > C₆₀. A large value of σ_S is a sign that a molecule can exhibit a wide range of Seebeck coefficients, which means that if orientations corresponding to high values can be selected and controlled, then the molecule has the potential to exhibit high-performance thermoelectricity. For the EMFs studied here, large values of σ_S are associated with distributions of Seebeck coefficients containing both positive and negative signs, which reveals that all these EMFs are bi-thermoelectric materials.

Multi-component self-assembled molecular-electronic films: towards new high-performance thermoelectric systems

The thermoelectric properties of parallel arrays of organic molecules on a surface offer the potential for large-area, flexible, solution processed, energy harvesting thin-films, whose room-temperature transport properties are controlled by quantum interference (QI). Recently, it has been demonstrated that constructive QI (CQI) can be translated from single molecules to self-assembled monolayers (SAMs), boosting both electrical conductivities and Seebeck coefficients. However, these CQI-enhanced systems are limited by rigid coupling of the component molecules to metallic electrodes, preventing the introduction of additional layers which would be advantageous for their further development. These rigid couplings also limit our ability to suppress the transport of phonons through these systems, which could act to boost their thermoelectric output, without comprising on their impressive electronic features. Here, through a combined experimental and theoretical study, we show that cross-plane thermoelectricity in SAMs can be enhanced by incorporating extra molecular layers. We utilize a bottom-up approach to assemble multi-component thin-films that combine a rigid, highly conductive 'sticky'-linker, formed from alkynyl-functionalised anthracenes, and a 'slippery'-linker consisting of a functionalized metalloporphyrin. Starting from an anthracene-based SAM, we demonstrate that subsequent addition of either a porphyrin layer or a graphene layer increases the Seebeck coefficient, and addition of both porphyrin and graphene leads to a further boost in their Seebeck coefficients. This demonstration of Seebeck-enhanced multi-component SAMs is the first of its kind and presents a new strategy towards the design of thin-film thermoelectric materials.

A Metallofullertube of Ce2 @C100 with a Carbon Nanotube Segment: Synthesis, Single-Molecule Conductance and Supramolecular Assembly

Tubular fullerenes can be considered as end-capped carbon nanotubes with accurate structure, which are promising nanocarbon materials for advanced single-molecule electronic devices. Herein, we report the synthesis and characterization of a metallofullertube Ce₂ @D₅ (450)-C₁₀₀ , which has a tubular C₁₀₀ cage with a carbon nanotube segment and two fullerene end-caps. As there are structure correlations between tubular Ce₂ @D₅ (450)-C₁₀₀ and spherical Ce₂ @I_h -C₈₀ , their structure-property relationship has been compared by means of experimental and theoretical methods. Notably, single-molecule conductance measurement determined that the conductivity of Ce₂ @D₅ (450)-C₁₀₀ was up to eight times larger than that of Ce₂ @I_h -C₈₀ . Furthermore, supramolecular assembly of Ce₂ @D₅ (450)-C₁₀₀ and a [12]CPP nanohoop was investigated, and theoretical calculations revealed that metallofullertube Ce₂ @D₅ (450)-C₁₀₀ adopted a "standing" configuration in the cavity of [12]CPP. These results demonstrate the special nature of this kind of metallofullertube.

Nickel-Fullerene Nanocomposites as Thermoelectric Materials

Nickel films with nanovoids filled with fullerene molecules have been fabricated. The thermoelectric properties of the nanocomposites have been measured from room temperature down to about 30 K. The main idea is that the phonon scattering can be enhanced at the C₆₀/matrix heterointerface. The distribution of atoms within the Ni and Ni-C₆₀ layers has been characterized by Auger depth profiling. The morphology of the grown samples has been checked using cross-sectional scanning electron microscopy (SEM). The Seebeck coefficient and electrical conductivity have been addressed employing an automatic home-built measuring system. It has been found that nanostructuring using Ar⁺ ion treatment increases the thermopower magnitude over the entire temperature range. Incorporating C₆₀ into the resulting voids further increased the thermopower magnitude below ≈200 K. A maximum increase in the Seebeck coefficient has been measured up to four times in different fabricated samples. This effect is attributed to enhanced scattering of charge carriers and phonons at the Ni/C₆₀ boundary.

Soft Organic Thermoelectric Materials: Principles, Current State of the Art and Applications

The enormous demand for waste heat utilization and burgeoning eco-friendly wearable materials has triggered huge interest in the development of thermoelectric materials that can harvest low-cost energy resources by converting waste heat to electricity efficiently. In particular, due to their high flexibility, nontoxicity, cost-effectivity, and promising applicability in various fields, organic thermoelectric materials are drawing more attention compared with their toxic, expensive, heavy, and brittle inorganic counterparts. Organic thermoelectric materials are approaching the figure of merit of the inorganic ones via the construction and optimization of unique transport pathways and device geometries. This review presents the recent development of the interdependence and decoupling principles of the thermoelectric efficiency parameters as well as the new achievements of high performance organic thermoelectric materials. Moreover, this review also discusses the advances in the thermoelectric devices with emphasis on their energy-related applications. It is believed that organic thermoelectric materials are emerging as green energy alternatives rivaling their conventional inorganic counterparts in the efficient and pure electricity harvesting from waste heat and solar thermal energy.

Thermal and Thermoelectric Properties of SAM-Based Molecular Junctions

In molecular thermoelectrics, the thermopower of molecular junctions is closely interlinked with their thermal properties; however, the detailed relationship between them remains uncertain. This study systematically investigates the thermal properties of self-assembled monolayer (SAM)-based molecular junctions and relates them to the thermoelectric performance of the junctions. The electrode temperatures for the bare AuTS, AuTS/EGaIn, and AuTS/TPT SAM//Ga2O3/EGaIn samples placed on a hot chuck were measured under different conditions, such as air vs vacuum and the presence and absence of thermal grease, which generates a heat conduction channel from a hot chuck to gold. It was revealed that the SAM was the most efficient thermal resistor, which was responsible for the creation of a temperature differential (ΔT) across the junction; ΔT in an air atmosphere is overestimated to some extent, and air mainly contributes to large dispersions of thermovoltage (ΔV) data. While junction measurements in air were possible at low ΔT (up to 13 K), the new optimal condition, under a vacuum and with thermal grease, allowed us to examine a wide temperature range up to ΔT = 40 K and obtain a more reliable Seebeck coefficient (S, μV/K). The value of S under the new condition was ∼1.4 times higher than that measured in air without thermal grease. Our study shows the potential of liquid-metal-based junctions to reliably investigate heat conduction across nanometer-thick organic films and elaborates on how the thermal properties of molecular junctions affect their thermoelectric performance.

Surface-Enhanced Raman Scattering Stimulated by Strong Metal-Molecule Interactions in a C60 Single-Molecule Junction

Specifying the geometric and electronic structures of a metal-molecule interface at the single-molecule level is crucial for the improvement of organic electronics. A single-molecule junction (SMJ) can be used to investigate interfaces because it can be regarded as an elementary unit of the interface structure. Although considerable efforts have been made to this end, the detection of structural changes in SMJs associated with metal-molecule interactions remains challenging. In this study, we detected the surface-enhanced Raman scattering (SERS) signal originating from the metal-molecule interaction change induced by a local structural change in a C₆₀ SMJ. This junction has attracted wide attention owing to its unique electronic and vibronic properties. We fabricated a C₆₀ SMJ using a lithographically fabricated Au electrode and measured the SERS spectra along with the current-voltage (I-V) response. By continuous measurement of SERS for the C₆₀ SMJ, we obtained SERS spectra dependent on the local structural change. The analysis of the I-V response revealed that the vibration energy shift originates from the change in the local structure for different Au-C₆₀ interactions. Based on the discrimination of the states in accordance with the Au-C₆₀ interaction, we found that the probability of SERS for geometry with a large Au-C₆₀ interaction was enhanced.

Unconventional Thermoelectric Materials for Energy Harvesting and Sensing Applications

Heat is an abundant but often wasted source of energy. Thus, harvesting just a portion of this tremendous amount of energy holds significant promise for a more sustainable society. While traditional solid-state inorganic semiconductors have dominated the research stage on thermal-to-electrical energy conversion, carbon-based semiconductors have recently attracted a great deal of attention as potential thermoelectric materials for low-temperature energy harvesting, primarily driven by the high abundance of their atomic elements, ease of processing/manufacturing, and intrinsically low thermal conductivity. This quest for new materials has resulted in the discovery of several new kinds of thermoelectric materials and concepts capable of converting a heat flux into an electrical current by means of various types of particles transporting the electric charge: (i) electrons, (ii) ions, and (iii) redox molecules. This has contributed to expanding the applications envisaged for thermoelectric materials far beyond simple conversion of heat into electricity. This is the motivation behind this review. This work is divided in three sections. In the first section, we present the basic principle of the thermoelectric effects when the particles transporting the electric charge are electrons, ions, and redox molecules and describe the conceptual differences between the three thermodiffusion phenomena. In the second section, we review the efforts made on developing devices exploiting these three effects and give a thorough understanding of what limits their performance. In the third section, we review the state-of-the-art thermoelectric materials investigated so far and provide a comprehensive understanding of what limits charge and energy transport in each of these classes of materials.

Synthesis and Characterization of Two Isomers of Th@C82: Th@C2v(9)-C82 and Th@C2(5)-C82

Actinide endohedral fullerenes have demonstrated remarkably different physicochemical properties compared to their lanthanide analogues. In this work, two novel isomers of Th@C₈₂ were successfully synthesized, isolated, and fully characterized by mass spectrometry, X-ray single crystallography, UV-vis-NIR spectroscopy, Raman spectroscopy, and cyclic voltammetry. The molecular structures of the two isomers were determined unambiguously as Th@C_2v(9)-C₈₂ and Th@C₂(5)-C₈₂ by single-crystal X-ray diffraction analysis. Raman and UV-vis-NIR spectroscopies further confirm the assignment of the cage isomers. Electrochemical gaps suggest that both Th@C_2v(9)-C₈₂ and Th@C₂(5)-C₈₂ possess a stable closed-shell electronic structure. The computational results further confirm that Th@C_2v(9)-C₈₂ and Th@C₂(5)-C₈₂ exhibit a unique four-electron charge transfer from the metal to the carbon cage and are among the most abundant isomers of Th@C₈₂.

Optimised power harvesting by controlling the pressure applied to molecular junctions

A major potential advantage of creating thermoelectric devices using self-assembled molecular layers is their mechanical flexibility. Previous reports have discussed the advantage of this flexibility from the perspective of facile skin attachment and the ability to avoid mechanical deformation. In this work, we demonstrate that the thermoelectric properties of such molecular devices can be controlled by taking advantage of their mechanical flexibility. The thermoelectric properties of self-assembled monolayers (SAMs) fabricated from thiol terminated molecules were measured with a modified AFM system, and the conformation of the SAMs was controlled by regulating the loading force between the organic thin film and the probe, which changes the tilt angle at the metal-molecule interface. We tracked the thermopower shift vs. the tilt angle of the SAM and showed that changes in both the electrical conductivity and Seebeck coefficient combine to optimize the power factor at a specific angle. This optimization of thermoelectric performance via applied pressure is confirmed through the use of theoretical calculations and is expected to be a general method for optimising the power factor of SAMs.

Probing the activity of transition metal M and heteroatom N4 co-doped in vacancy fullerene (M-N4-C64, M = Fe, Co, and Ni) towards the oxygen reduction reaction by density functional theory

In this study, a novel type oxygen reduction reaction (ORR) electrocatalyst is explored using density functional theory (DFT); the catalyst consists of transition metal M and heteroatom N₄ co-doped in vacancy fullerene (M–N₄–C₆₄, M = Fe, Co, and Ni). Mulliken charge analysis shows that the metal center is the reaction site of ORR. PDOS analysis indicates that in M–N₄–C₆₄, the interaction between Fe–N₄–C₆₄ and the adsorbate is the strongest, followed by Co–N₄–C₆₄ and Ni–N₄–C₆₄. This is consistent with the calculated adsorption energies. By analyzing and comparing the adsorption energies of ORR intermediates and activation energies and reaction energies of all elemental reactions in M–N₄–C₆₄ (M = Fe, Co, and Ni), two favorable ORR electrocatalysts, Fe–N₄–C₆₄ and Co–N₄–C₆₄, are selected. Both exhibited conduction through the more efficient 4e⁻ reduction pathway. Moreover, PES diagrams indicate that the whole reaction energy variation in the favorable ORR pathways of Fe–N₄–C₆₄ and Co–N₄–C₆₄ is degressive, which is conducive to positive-going reactions. This study offers worthwhile information for the improvement of cathode materials for fuel cells.

In this study, a novel type oxygen reduction reaction (ORR) electrocatalyst is explored using density functional theory (DFT); the catalyst consists of transition metal M and heteroatom N₄ co-doped in vacancy fullerene (M–N₄–C₆₄, M = Fe, Co, and Ni).

The impact of Fe atom on the spin-filter and spin thermoelectric properties of Au-Fe@C20-Au monomer and dimer systems

Based on density functional theory and non-equilibrium Green's function formalism, we explore the effect of Fe atom in Au-Fe@C20-Au monomer and dimer systems in comparison with the C20 fullerene molecular junctions. We calculate the spin-dependent transmission coefficient, spin polarization and also their spin thermoelectric coefficients to investigate magnetic properties in the system. Our results indicate that the presence of Fe atoms enhances substantially the spin-filter and increases the spin figure of merit in the dimer system. We suggest that the Au-(Fe@C20)2-Au system is a suitable junction for designing spin-filtering and spin thermoelectric devices and eventually it is a good candidate for spintronic applications.

Selective Fabrication of Single-Molecule Junctions by Interface Engineering

Recent progress in addressing electrically driven single-molecule behaviors has opened up a path toward the controllable fabrication of molecular devices. Herein, the selective fabrication of single-molecule junctions is achieved by employing the external electric field. For molecular junctions with methylthio (-SMe), thioacetate (-SAc), amine (-NH₂ ), and pyridyl (-PY), the evolution of their formation probabilities along with the electric field is extracted from the plateau analysis of individual single-molecule break junction traces. With the increase of the electric field, the SMe-anchored molecules show a different trend in the formation probability compared to the other molecular junctions, which is consistent with the density functional theory calculations. Furthermore, switching from an SMe-anchored junction to an SAc-anchored junction is realized by altering the electric field in a mixed solution. The results in this work provide a new approach to the controllable fabrication and modulation of single-molecule junctions and other bottom-up nanodevices at molecular scales.

Molecular-scale thermoelectricity: as simple as 'ABC'

If the Seebeck coefficient of single molecules or self-assembled monolayers (SAMs) could be predicted from measurements of their conductance-voltage (G-V) characteristics alone, then the experimentally more difficult task of creating a set-up to measure their thermoelectric properties could be avoided. This article highlights a novel strategy for predicting an upper bound to the Seebeck coefficient of single molecules or SAMs, from measurements of their G-V characteristics. The theory begins by making a fit to measured G-V curves using three fitting parameters, denoted a, b, c. This 'ABC' theory then predicts a maximum value for the magnitude of the corresponding Seebeck coefficient. This is a useful material parameter, because if the predicted upper bound is large, then the material would warrant further investigation using a full Seebeck-measurement setup. On the other hand, if the upper bound is small, then the material would not be promising and this much more technically demanding set of measurements would be avoided. Histograms of predicted Seebeck coefficients are compared with histograms of measured Seebeck coefficients for six different SAMs, formed from anthracene-based molecules with different anchor groups and are shown to be in excellent agreement.

Tolerance to Stretching in Thiol-Terminated Single-Molecule Junctions Characterized by Surface-Enhanced Raman Scattering

We investigated the change in the metal-molecule interaction in a 1,4-benzenedithiol (BDT) single-molecule junction using a combination of surface-enhanced Raman scattering spectra and current-voltage curves. During the stretching process, the conductance of the junction systematically decreased, accompanied by an increase in the vibrational energy of the CC stretching mode. By analyzing the current-voltage curves and Raman spectra, we found that the interaction between the π orbital of BDT and the electronic states of Au was diminished by the orientation change of BDT during the stretching process. A comparison with a 4,4'-bipyridine single-molecule junction revealed that the reduction of coupling of the Au-S contacts was smaller than that of Au-pyridine contacts. Therefore, the electronic states originating from the contact geometry are responsible for the tolerance to the stretching of thiol-terminated molecular junctions.

Exploring the thermoelectric properties of oligo(phenylene-ethynylene) derivatives

Seebeck coefficient measurements provide unique insights into the electronic structure of single-molecule junctions, which underpins their charge and heat transport properties. Since the Seebeck coefficient depends on the slope of the transmission function at the Fermi energy (EF), the sign of the thermoelectric voltage will be determined by the location of the molecular orbital levels relative to EF. Here we investigate thermoelectricity in molecular junctions formed from a series of oligophenylene-ethynylene (OPE) derivatives with biphenylene, naphthalene and anthracene cores and pyridyl or methylthio end-groups. Single-molecule conductance and thermoelectric voltage data were obtained using a home-built scanning tunneling microscope break junction technique. The results show that all the OPE derivatives studied here are dominated by the lowest unoccupied molecular orbital level. The Seebeck coefficients for these molecules follow the same trend as the energy derivatives of their corresponding transmission spectra around the Fermi level. The molecule terminated with pyridyl units has the largest Seebeck coefficient corresponding to the highest slope of the transmission function at EF. Density-functional-theory-based quantum transport calculations support the experimental results.

Connectivity dependent thermopower of bridged biphenyl molecules in single-molecule junctions

We report measurements on gold|single-molecule|gold junctions, using a modified scanning tunneling microscope-break junction (STM-BJ) technique, of the Seebeck coefficient and electrical conductance of a series of bridged biphenyl molecules, with meta connectivities to pyridyl anchor groups. These data are compared with a previously reported study of para-connected analogues. In agreement with a tight binding model, the electrical conductance of the meta series is relatively low and is sensitive to the nature of the bridging groups, whereas in the para case the conductance is higher and relatively insensitive to the presence of the bridging groups. This difference in sensitivity arises from the presence of destructive quantum interference in the π system of the unbridged aromatic core, which is alleviated to different degrees by the presence of bridging groups. More precisely, the Seebeck coefficient of meta-connected molecules was found to vary between -6.1 μV K^-1 and -14.1 μV K^-1, whereas that of the para-connected molecules varied from -5.5 μV K^-1 and -9.0 μV K^-1.

Tuning the thermoelectrical properties of anthracene-based self-assembled monolayers

It is known that the electrical conductance of single molecules can be controlled in a deterministic manner by chemically varying their anchor groups to external electrodes. Here, by employing synthetic methodologies to vary the terminal anchor groups around aromatic anthracene cores, and by forming self-assembled monolayers (SAMs) of the resulting molecules, we demonstrate that this method of control can be translated into cross-plane SAM-on-gold molecular films. The cross-plane conductance of SAMs formed from anthracene-based molecules with four different combinations of anchors are measured to differ by a factor of approximately 3 in agreement with theoretical predictions. We also demonstrate that the Seebeck coefficient of such films can be boosted by more than an order of magnitude by an appropriate choice of anchor groups and that both positive and negative Seebeck coefficients can be realised. This demonstration that the thermoelectric properties of SAMs are controlled by their anchor groups represents a critical step towards functional ultra-thin-film devices for future molecular-scale electronics.

Thermal magnetoresistance and spin thermopower in C60 dimers

We theoretically investigate the spin-related thermoelectric properties in C₆₀ dimer bridged between zigzag graphene nanoribbon electrodes using the tight-binding model, equilibrium Green's function method, and Landauer-Büttiker transport formalism. By applying a thermal gradient, our proposed device could generate a notable spin thermopower. Moreover, by switching the magnetization of the electrodes, different spin currents, and giant thermal magnetoresistance (MR) can be achieved. Interestingly, various types of C₆₀ dimers also produce a thermal MR, which is sensitively modified by the gate voltages.

Molecular-scale thermoelectricity: a worst-case scenario

This article highlights a novel strategy for designing molecules with high thermoelectric performance, which are resilient to fluctuations. In laboratory measurements of thermoelectric properties of single-molecule junctions and self-assembled monolayers, fluctuations in frontier orbital energies relative to the Fermi energy E_F of electrodes are an important factor, which determine average values of transport coefficients, such as the average Seebeck coefficient 〈S〉. In a worst-case scenario, where the relative value of E_F fluctuates uniformly over the HOMO-LUMO gap, a "worst-case scenario theorem" tells us that the average Seebeck coefficient will vanish unless the transmission coefficient at the LUMO and HOMO resonances take different values. This implies that junction asymmetry is a necessary condition for obtaining non-zero values of 〈S〉 in the presence of large fluctuations. This conclusion that asymmetry can drive high thermoelectric performance is supported by detailed simulations on 17 molecules using density functional theory. Importantly, junction asymmetry does not imply that the molecules themselves should be asymmetric. We demonstrate that symmetric molecules possessing a localised frontier orbital can achieve even higher thermoelectric performance than asymmetric molecules, because under laboratory conditions of slight symmetry breaking, such orbitals are 'silent' and do not contribute to transport. Consequently, transport is biased towards the nearest "non-silent" frontier orbital and leads to a high ensemble averaged Seebeck coefficient. This effect is demonstrated for a spatially-symmetric 1,2,3-triazole-based molecule, a rotaxane-hexayne macrocycle and a phthalocyanine.

Solvent-molecule interaction induced gating of charge transport through single-molecule junctions

To explore solvent gating of single-molecule electrical conductance due to solvent-molecule interactions, charge transport through single-molecule junctions with different anchoring groups in various solvent environments was measured by using the mechanically controllable break junction technique. We found that the conductance of single-molecule junctions can be tuned by nearly an order of magnitude by varying the polarity of solvent. Furthermore, gating efficiency due to solvent-molecule interactions was found to be dependent on the choice of the anchor group. Theoretical calculations revealed that the polar solvent shifted the molecular-orbital energies, based on the coupling strength of the anchor groups. For weakly coupled molecular junctions, the polar solvent-molecule interaction was observed to reduce the energy gap between the molecular orbital and the Fermi level of the electrode and shifted the molecular orbitals. This resulted in a more significant gating effect than that of the strongly coupled molecules. This study suggested that solvent-molecule interaction can significantly affect the charge transport through single-molecule junctions.

Scale-Up of Room-Temperature Constructive Quantum Interference from Single Molecules to Self-Assembled Molecular-Electronic Films

The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up of QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on CQI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ∼50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step toward functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.

Constructive Quantum Interference in Single-Molecule Benzodichalcogenophene Junctions

Heteroatom substitution into the cores of alternant, aromatic hydrocarbons containing only even-membered rings is attracting increasing interest as a method of tuning their electrical conductance. Here, the effect of heteroatom substitution into molecular cores of non-alternant hydrocarbons, containing odd-membered rings, is examined. Benzodichalcogenophene (BDC) compounds are rigid, planar π-conjugated structures, with molecular cores containing five-membered rings fused to a six-membered aryl ring. To probe the sensitivity or resilience of constructive quantum interference (CQI) in these non-bipartite molecular cores, two C₂ -symmetric molecules (I and II) and one asymmetric molecule (III) were investigated. I (II) contains S (O) heteroatoms in each of the five-membered rings, while III contains an S in one five-membered ring and an O in the other. Differences in their conductances arise primarily from the longer S-C and shorter O-C bond lengths compared with the C-C bond and the associated changes in their resonance integrals. Although the conductance of III is significantly lower than the conductances of the others, CQI was found to be resilient and persist in all molecules.

Enhanced thermopower in covalent graphite-molecule contacts

The Seebeck effect is very attractive for technological applications as it leads to the direct conversion of heat into electricity. One of the key quantities determining the efficiency of such conversion is the thermopower S. In this paper we explore theoretically what electronic properties are responsible for the Seebeck effect in molecular junctions with graphite or graphene electrodes. We propose that S can be enhanced because of the combined effect of the dip in the density of states at the Fermi energy of these materials and the molecular resonance. Then to understand the impact of the covalent vs. non-covalent molecule-carbon bonding we calculate from first principles the electronic and transport properties of graphite/molecule/Au junctions, where both types of bonding have been reported experimentally. We ultimately predict that S is about 120 μV K^-1 at room temperature for a 3,5-dimethyl-4-aminobenzene (DMAB) molecule covalently attached to the graphite electrode. This value is one order of magnitude larger than the typical values measured to date for molecular junctions and it is a signature of the direct C-C molecule-graphite bond. Finally we also demonstrate how one can control not just the absolute magnitude of S, but also its sign by designing the graphite-molecule contact. Our results lead the way towards the use of junctions with molecules covalently attached to a C-based substrate as possible new improved platforms for molecular thermoelectric devices.

Taming quantum interference in single molecule junctions: induction and resonance are key

Chemical bond induction and mesomerism/resonance are theoretically demonstrated to control quantum interference in single molecule junctions.

Functional Metallofullerene Materials and Their Applications in Nanomedicine, Magnetics, and Electronics

Endohedral metallofullerenes exhibit combined properties from carbon cages as well as internal metal moieties and have great potential in a wide range of applications as molecule materials. Along with the breakthrough of mass production of metallofullerenes, their applied research has been greatly developed with more and more new functions and practical applications. For gadolinium metallofullerenes, their water-soluble derivatives have been demonstrated with antitumor activity and unprecedented tumor vascular-targeting therapy. Metallofullerene water-soluble derivatives also can be applied to treat reactive oxygen species (ROS)-induced diseases due to their high antioxidative activity. For magnetic metallofullerenes, the internal electron spin and metal species bring about spin sensitivity, molecular magnets, and spin quantum qubits, which have many promising applications. Metallofullerenes are significant candidates for fabricating useful electronic devices because of their various electronic structures. This Review provides a summary of the metallofullerene studies reported recently, in the fields of tumor inhibition, tumor vascular-targeting therapies, antioxidative activity, spin probes, single-molecule magnets, spin qubits, and electronic devices. This is not an exhaustive summary and there are many other important study results regarding metallofullerenes. All of this research has revealed the irreplaceable role of metallofullerene materials.

Single-molecule level control of host-guest interactions in metallocycle-C60 complexes

Host−guest interactions are of central importance in many biological and chemical processes. However, the investigation of the formation and decomplexation of host−guest systems at the single-molecule level has been a challenging task. Here we show that the single-molecule conductance of organoplatinum(II) metallocycle hosts can be enhanced by an order of magnitude by the incorporation of a C₆₀ guest molecule. Mechanically stretching the metallocycle-C₆₀ junction with a scanning tunneling microscopy break junction technique causes the release of the C₆₀ guest from the metallocycle, and consequently the conductance switches back to the free-host level. Metallocycle hosts with different shapes and cavity sizes show different degrees of flexibility to accommodate the C₆₀ guest in response to mechanical stretching. DFT calculations provide further insights into the electronic structures and charge transport properties of the molecular junctions based on metallocycles and the metallocycle-C₆₀ complexes.

Studying the single-molecule behavior of host-guest complexes can provide fundamental insights into their supramolecular interactions. Here, the authors use the scanning tunneling microscopy break junction technique to show that encapsulation of a C₆₀ molecule significantly enhances the conductance of an organoplatinum metallocycle; mechanical stretching of the junction releases the guest, returning the conductance to free-host level.

Symmetry based molecular design for triplet excitation and optical spin injection

Spintronics, as a relatively new scientific field, is developing rapidly together with our understanding of spin related phenomena and spin manipulation. One of the challenges in the field is spin injection, which has been achieved optically in inorganic crystalline semiconductors, but not yet in organic semiconductors. Here, we introduce an approach whereby we apply group theory and computational methods to design molecular materials in which spin can be injected optically via circularly polarized light (CPL). Our approach is based on the use of group theory and double group theory to identify families of molecules whose symmetry satisfies design rules for optical excitation of triplets of particular properties. Employing such screening prior to detailed calculation can accelerate design by first identifying any structures that fail some criterion on grounds of symmetry. Here, we show using group theory and computational methods that particular families of molecules possess a low lying triplet state that can be excited with circularly polarized light causing spin polarization of an excited electron. Such structures are of potential interest for organic or molecular spintronics. We present an efficient procedure to identify candidate point groups and determine the excited state symmetry using group theory, before full calculation of excited states using relativistic quantum chemistry.

Crystallographic characterization of Er3N@C2n (2n = 80, 82, 84, 88): the importance of a planar Er3N cluster

A series of Er-based nitride clusterfullerenes (NCFs), Er₃N@C_80-88, have been successfully synthesized and isolated. In particular, Er₃N@I_h(7)-C₈₀, Er₃N@D_5h(6)-C₈₀, Er₃N@C_2v(9)-C₈₂, Er₃N@C_s(51365)-C₈₄, and Er₃N@D₂(35)-C₈₈ have been characterized by single-crystal X-ray diffraction (XRD) for the first time. The planar configuration of the inserted Er₃N cluster is identified unambiguously and the Er-N distances increase in accordance with cage expansion to maintain strong metal-cage interactions. Additionally, the electrochemical properties of the Er₃N@C_80-88 series are studied by means of cyclic voltammetry. It is found that the first reduction potentials are roughly similar for all compounds under study, while the first oxidation potentials are cathodically shifted along with the increase of the cage size in the Er₃N@C_2n (2n = 80, 84, 86, 88) series, leading to a decrease in the corresponding electrochemical band gaps. Nevertheless, for Er₃N@C_2v(9)-C₈₂, a good electron donating ability is manifested by its relatively small first oxidation potential, which results from the relatively higher energy level of the highest occupied molecular orbital. The redox behaviors observed in such Er₃N-based NCFs may promise their great potential applications in donor-acceptor systems.

Atomically defined angstrom-scale all-carbon junctions

Full-carbon electronics at the scale of several angstroms is an expeimental challenge, which could be overcome by exploiting the versatility of carbon allotropes. Here, we investigate charge transport through graphene/single-fullerene/graphene hybrid junctions using a single-molecule manipulation technique. Such sub-nanoscale electronic junctions can be tuned by band gap engineering as exemplified by various pristine fullerenes such as C₆₀, C₇₀, C₇₆ and C₉₀. In addition, we demonstrate further control of charge transport by breaking the conjugation of their π systems which lowers their conductance, and via heteroatom doping of fullerene, which introduces transport resonances and increase their conductance. Supported by our combined density functional theory (DFT) calculations, a promising future of tunable full-carbon electronics based on numerous sub-nanoscale fullerenes in the large family of carbon allotropes is anticipated.

All-carbon electronics holds promise beyond the conventional silicon-based electronics, but it remains challenging to manufacture them with well-defined structures thus tunability. Tan et al. control charge transport in single-molecule junctions using different fullerenes between graphene electrodes.

Thermoelectric Properties of 2,7-Dipyridylfluorene Derivatives in Single-Molecule Junctions

A series of 2,7-dipyridylfluorene derivatives have been synthesized with different substituents (2H, 2Me, 2OMe, 2CF3, and O) at the C(9) position. Experimental measurements on gold|single-molecule|gold junctions, using a modified scanning tunneling microscope-break-junction technique, show that the C(9) substituent has little effect on the conductance, although there is a more significant influence on the thermopower, with the Seebeck coefficient varying by a factor of 1.65 within the series. The combined experimental and computational study, using density functional theory calculations, provides insights into the interplay of conductance and thermopower in single-molecule junctions and is a guide for new strategies for thermopower modulation in single-molecule junctions.