Oscillating Seebeck coefficients in π-stacked molecular junctions

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).

Energy gap and aromatic molecular rings

The manuscript combines rational density functional theory simulations and experimental data to investigate the electrical properties of eight polycyclic aromatic hydrocarbons (PAHs). The optimized geometries reveal a preference for one-row, two-row and three-row ring distributions. Band structure plots demonstrate an inverse correlation between the number of aromatic rings and band gap size, with a specific order observed across the PAHs. Gas phase simulations support these findings, though differences in values are noted compared to the literature. Introducing a two-row ring distribution concept resolves discrepancies, particularly in azulene. The B3LYP function successfully bridges theoretical and experimental gaps, particularly in large PAHs. The manuscript highlights the potential for designing electronic devices based on different-sized PAHs, emphasizing a multi-ring distribution approach and opening new avenues for practical applications.

Investigation of thermoelectric properties of cadmium selenide CdnSen (n= 7, 11, 13) molecular junctions: a DFT study

CONTEXT

The thermoelectric properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated before and after adding hydrogen atoms. The effects of hydrogen passivation on the transmission and thermopower curves were analyzed. CdSe-diamantane (Cd7Se7) and CdSe-tetramantane (Cd11Se11) junctions exhibited the best thermoelectric performance due to their low surface reconstruction energy, which is attributed to the number of dangling and unsaturated bonds. This study guides the design of new molecular junctions with desired thermoelectric properties.

METHOD

The electrical and thermal properties of cadmium selenide (CdnSen) molecular junctions (n = 7, 11, 13) were investigated using a ballistic quantum transport method based on the non-equilibrium Green's function (NEGF) approach. Thermoelectric properties were calculated for the molecular junctions with different structures before and after hydrogen passivation. Density functional theory (DFT) calculations were performed at the B3LYP level with the 3-21G basis set for the Cd atoms and the 6-31G** basis set for the Se atoms. The SIESTA and GOLLUM codes were used to study the effect of changing the shape and size of each structure on its electrical and thermal characteristics.

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.

Characterization and Application of Supramolecular Junctions

The convergence of supramolecular chemistry and single-molecule electronics offers a new perspective on supramolecular electronics, and provides a new avenue toward understanding and application of intermolecular charge transport at the molecular level. In this review, we will provide an overview of the advances in the characterization technique for the investigation of intermolecular charge transport, and summarize the experimental investigation of several non-covalent interactions, including π-π stacking interactions, hydrogen bonding, host-guest interactions and σ-σ interactions at the single-molecule level. We will also provide a perspective on supramolecular electronics and discuss the potential applications and future challenges.

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.

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).

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.

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.

Through an experimental and theoretical study, cross-plane thermoelectricity in Self-Assembled Monolayers (SAMs) was enhanced by adding extra molecular layers, presenting a new strategy towards the design of high thermoelectric materials.

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.

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.

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.