Simultaneous determination of conductance and thermopower of single molecule junctions

Possible Routes for Efficient Thermo-Electric Energy Conversion in a Molecular Junction

In the context of designing an efficient thermoelectric energy-conversion device at nanoscale level, we suggest several important tuning parameters to enhance the performance of thermoelectric converters. We consider a simple molecular junction, which is always helpful to understand the basic mechanisms in a deeper way, where a benzene molecule is coupled to two external baths having unequal temperatures. The key component responsible for achieving better performance is associated with the asymmetric nature of transmission function, and in the present work, we show that it can be implemented in different ways by regulating the physical parameters involving the system. Employing a tight-binding framework we calculate electrical and thermal conductances, thermopower, and figure of merit (FOM) by using Landauer integrals, and thoroughly examine the critical roles played by molecule-to-lead (ML) interface geometry, magnetic field, chemical substituent group, ML coupling, and the direct coupling between the two leads. Our results show that a reasonably large FOM (≫1) can be obtained and lead to a possibility of regulating the efficiency by selectively tuning the physical parameters. We believe that the present analysis will enhance the understanding of designing efficient thermoelectric devices, and can be verified in a laboratory.

Discriminating Seebeck sensing of molecules

One of the fundamental challenges in molecular-scale sensors is the junction to junction variability leading to variations in their electrical conductance by up to a few orders of magnitude. In contrast, thermal voltage measurements of single and many molecule junctions show that this variation in the Seebeck coefficient is smaller. Particularly, the sign of the Seebeck coefficient is often resilient against conformational changes. In this paper, we demonstrate that this robust molecular feature can be utilised in an entirely new direction of discriminating molecular sensing of gas and bio-molecules. We show that the positive sign of the Seebeck coefficient in the presence of cytosine nucleobases changes to a negative one when cancerous cytosine nucleobases were absorbed on the molecular wire formed by metalloporphyrins. Furthermore, the sign of the Seebeck coefficient changes when chlorine gas interacts with the Mn-porphyrin molecular wire. The change in the sign of Seebeck coefficient is due to the formation of spin driven bound states with energies close to the Fermi energy of electrodes. Seebeck sensing is a generic concept and opens new avenues for molecular sensing with huge potential applications in the years ahead.

Direct Probe of the Seebeck Coefficient in a Kondo-Correlated Single-Quantum-Dot Transistor

We report on the first measurement of the Seebeck coefficient in a tunnel-contacted and gate-tunable individual single-quantum dot junction in the Kondo regime, fabricated using the electromigration technique. This fundamental thermoelectric parameter is obtained by directly monitoring the magnitude of the voltage induced in response to a temperature difference across the junction, while keeping a zero net tunneling current through the device. In contrast to bulk materials and single molecules probed in a scanning tunneling microscopy (STM) configuration, investigating the thermopower in nanoscale electronic transistors benefits from the electric tunability to showcase prominent quantum effects. Here, striking sign changes of the Seebeck coefficient are induced by varying the temperature, depending on the spin configuration in the quantum dot. The comparison with numerical renormalization group (NRG) calculations demonstrates that the tunneling density of states is generically asymmetric around the Fermi level in the leads, both in the cotunneling and Kondo regimes.

New Approach for Large-Area Thermoelectric Junctions with a Liquid Eutectic Gallium-Indium Electrode

A challenge in organic thermoelectrics is to relate thermoelectric performance of devices to the chemical and electronic structures of organic components inside them on a molecular scale. To this end, a reliable and reproducible platform relevant to molecular-level thermoelectric measurements is essentially needed. This paper shows a new, efficient approach for thermoelectric characterization of a large area of molecular monolayers using liquid eutectic gallium-indium (EGaIn). A cone-shaped EGaIn microelectrode permits access to noninvasive, reversible top-contact formation onto organic surfaces in ambient conditions, high yields of working devices (up to 97%), and thus statistically sufficient thermoelectric data sets (∼6000 data per sample in a few hours). We here estimated thermopowers of EGaIn (3.4 ± 0.1 μV/K) and the Ga₂O₃ layer (3.4 ± 0.2 μV/K) on the EGaIn conical tip and successfully validated our platform with widely studied molecules, oligophenylenethiolates. Our approach will open the door to thermoelectric large-area molecular junctions.

Quo vadis, unimolecular electronics?

This paper reviews the present status of unimolecular electronics (UME). The field started in the 1970s with a hope that some day organic molecules (∼2 nm in size), when used as electronic components, would challenge Si-based inorganic electronics in ultimate-high-density integrated circuits. The technological push to ever smaller inorganic device sizes (Moore's "law") was driven by a profit motive and by vast investments. UME, the underfunded pauper, may have lost that "race to the bottom", but some excellent science is left to be done.

MoS2 nano flakes with self-adaptive contacts for efficient thermoelectric energy harvesting

We examine the potential of the low-dimensional material MoS2 for the efficient conversion of waste heat to electricity via the Seebeck effect. Recently monolayer MoS2 nano flakes with self-adaptive Mo6S6 contacts were formed, which take advantage of mechanical stability and chemical covalent bonding to the MoS2. Here, we study the thermoelectric properties of these junctions by calculating their conductance, thermopower and thermal conductance due to both electrons and phonons. We show that thermoelectric figures of merit ZT as high as ∼2.8 are accessible in these junctions, independent of the flake size and shape, provided the Fermi energy is close to a band edge. We show that Nb dopants as substituents for Mo atoms can be used to tune the Fermi energy, and despite the associated inhomogeneous broadening, room temperature values as high as ZT ∼ 0.6 are accessible, increasing to 0.8 at 500 K.

Floppy molecules as candidates for achieving optoelectronic molecular devices without skeletal rearrangement or bond breaking

Molecular species investigated as possible candidates for molecular photoswitches often toggle between two (low and high conductance) conformations implying skeletal rearrangement, bond breaking, and substantial changes of molecular length. All these represent shortcomings that impede the switching speed and straightforward incorporation in nanodevices. In the present paper we propose a mechanism wherein the photoinduced switching is from a nonplanar conformation to a planar conformation, and involves neither skeletal rearrangement nor bond breaking or significant molecular length changes. Specifically, by choosing typical floppy molecules consisting of two benzene or benzene-like rings that can easily rotate relative to each other, we present results of both ab initio and DFT quantum chemical calculations demonstrating that the lowest electronic excitation corresponds to a planar molecular conformation (φ = 0), in contrast to the nonplanar ground state characterized by φ ≠ 0. Because the low bias conductance scales as G ∝ cos² φ, the planar conformation has a higher conductance than the non-planar conformation, acting therefore as ON and OFF states of the molecular switch, respectively. We analyze recent experimental data on illuminated single-molecule junctions (E.-D. Fung et al., Nano Lett., 2017, 17, 1255) and show that the measured photoinduced conductance enhancement is consistent with the presently proposed mechanism. Furthermore, based on recent results demonstrating the substantial impact of the SAM coverage on the twisting angle (I. Bâldea, Faraday Discuss., 2017, 204, 35) we show that a photoinduced conductance enhancement can be much stronger than the rather modest enhancement obtained in the aforementioned experiment.

High cross-plane thermoelectric performance of metallo-porphyrin molecular junctions

We investigated the thermoelectric properties of flat-stacked 5,15-diphenylporphyrins containing divalent metal ions Ni, Co, Cu or Zn, which are strongly coordinated with the nitrogens of pyridyl coated gold electrodes. Changing metal atom has little effect on the thermal conductance due to the phonons. The room-temperature Seebeck coefficients of these junctions are rather high, ranging from 90 μV K^-1 for Cu, Ni and Zn-porphyrins to -16 μV K^-1 for Co-porphyrin. These values could be further increased by lowering molecular energy levels relative to the DFT-predicted Fermi energy. In contrast, the phonon contribution to the thermal conductance of these junctions is rather insensitive to the choice of metal atom. The thermopower, thermal conductance and electrical conductance combined to yield the room-temperature values for the thermoelectric figure of merit ZT ranging from 1.6 for Cu porphyrin to ∼0.02 for Ni-porphyrin.

Peltier cooling in molecular junctions

The study of thermoelectricity in molecular junctions is of fundamental interest for the development of various technologies including cooling (refrigeration) and heat-to-electricity conversion ^1-4 . Recent experimental progress in probing the thermopower (Seebeck effect) of molecular junctions ^5-9 has enabled studies of the relationship between thermoelectricity and molecular structure ^10,11 . However, observations of Peltier cooling in molecular junctions-a critical step for establishing molecular-based refrigeration-have remained inaccessible. Here, we report direct experimental observations of Peltier cooling in molecular junctions. By integrating conducting-probe atomic force microscopy ^12,13 with custom-fabricated picowatt-resolution calorimetric microdevices, we created an experimental platform that enables the unified characterization of electrical, thermoelectric and energy dissipation characteristics of molecular junctions. Using this platform, we studied gold junctions with prototypical molecules (Au-biphenyl-4,4'-dithiol-Au, Au-terphenyl-4,4''-dithiol-Au and Au-4,4'-bipyridine-Au) and revealed the relationship between heating or cooling and charge transmission characteristics. Our experimental conclusions are supported by self-energy-corrected density functional theory calculations. We expect these advances to stimulate studies of both thermal and thermoelectric transport in molecular junctions where the possibility of extraordinarily efficient energy conversion has been theoretically predicted ^2-4,14 .

Pure spin current and phonon thermoelectric transport in a triangulene-based molecular junction

A triangulene-based molecular junction: a favorable spintronic device with pure spin current and efficient phonon thermoelectric transport.

Inelastic electron tunneling spectroscopy of difurylethene-based photochromic single-molecule junctions

Diarylethene-derived molecules alter their electronic structure upon transformation between the open and closed forms of the diarylethene core, when exposed to ultraviolet (UV) or visible light. This transformation results in a significant variation of electrical conductance and vibrational properties of corresponding molecular junctions. We report here a combined experimental and theoretical analysis of charge transport through diarylethene-derived single-molecule devices, which are created using the mechanically controlled break-junction technique. Inelastic electron tunneling (IET) spectroscopy measurements performed at 4.2 K are compared with first-principles calculations in the two distinct forms of diarylethenes connected to gold electrodes. The combined approach clearly demonstrates that the IET spectra of single-molecule junctions show specific vibrational features that can be used to identify different isomeric molecular states by transport experiments.

Field-Effect Control of Graphene-Fullerene Thermoelectric Nanodevices

Although it was demonstrated that discrete molecular levels determine the sign and magnitude of the thermoelectric effect in single-molecule junctions, full electrostatic control of these levels has not been achieved to date. Here, we show that graphene nanogaps combined with gold microheaters serve as a testbed for studying single-molecule thermoelectricity. Reduced screening of the gate electric field compared to conventional metal electrodes allows control of the position of the dominant transport orbital by hundreds of meV. We find that the power factor of graphene-fullerene junctions can be tuned over several orders of magnitude to a value close to the theoretical limit of an isolated Breit-Wigner resonance. Furthermore, our data suggest that the power factor of an isolated level is only given by the tunnel coupling to the leads and temperature. These results open up new avenues for exploring thermoelectricity and charge transport in individual molecules and highlight the importance of level alignment and coupling to the electrodes for optimum energy conversion in organic thermoelectric materials.

Photothermoelectric Effects and Large Photovoltages in Plasmonic Au Nanowires with Nanogaps

Nanostructured metals subject to local optical interrogation can generate open-circuit photovoltages potentially useful for energy conversion and photodetection. We report a study of the photovoltage as a function of illumination position in single-metal Au nanowires and nanowires with nanogaps formed by electromigration. We use a laser scanning microscope to locally heat the metal nanostructures via excitation of a local plasmon resonance and direct absorption. In nanowires without nanogaps, where charge transport is diffusive, we observe voltage distributions consistent with thermoelectricity, with the local Seebeck coefficient depending on the width of the nanowire. In the nanowires with nanogaps, where charge transport is by tunneling, we observe large photovoltages up to tens of mV, with magnitude, polarization dependence, and spatial localization that follow the plasmon resonance in the nanogap. This is consistent with a model of photocurrent across the nanogap carried by the nonequilibrium, "hot" carriers generated upon plasmon excitation.

High-performance thermoelectricity in edge-over-edge zinc-porphyrin molecular wires

If high efficiency organic thermoelectric materials could be identified, then these would open the way to a range of energy harvesting technologies and Peltier coolers using flexible and transparent thin-film materials. We have compared the thermoelectric properties of three zinc porphyrin (ZnP) dimers and a ZnP monomer and found that the "edge-over-edge" dimer formed from stacked ZnP rings possesses a high electrical conductance, negligible phonon thermal conductance and a high Seebeck coefficient of the order of 300 μV K^-1. These combine to yield a predicted room-temperature figure of merit of ZT ≈ 4, which is the highest room-temperature ZT ever reported for a single organic molecule. This high value of ZT is a consequence of the low phonon thermal conductance arising from the stacked nature of the porphyrin rings, which hinders phonon transport through the edge-over-edge molecule and enhances the Seebeck coefficient.

Too Hot for Photon-Assisted Transport: Hot-Electrons Dominate Conductance Enhancement in Illuminated Single-Molecule Junctions

We investigate light-induced conductance enhancement in single-molecule junctions via photon-assisted transport and hot-electron transport. Using 4,4'-bipyridine bound to Au electrodes as a prototypical single-molecule junction, we report a 20-40% enhancement in conductance under illumination with 980 nm wavelength radiation. We probe the effects of subtle changes in the transmission function on light-enhanced current and show that discrete variations in the binding geometry result in a 10% change in enhancement. Importantly, we prove theoretically that the steady-state behavior of photon-assisted transport and hot-electron transport is identical but that hot-electron transport is the dominant mechanism for optically induced conductance enhancement in single-molecule junctions when the wavelength used is absorbed by the electrodes and the hot-electron relaxation time is long. We confirm this experimentally by performing polarization-dependent conductance measurements of illuminated 4,4'-bipyridine junctions. Finally, we perform lock-in type measurements of optical current and conclude that currents due to laser-induced thermal expansion mask optical currents. This work provides a robust experimental framework for studying mechanisms of light-enhanced transport in single-molecule junctions and offers tools for tuning the performance of organic optoelectronic devices by analyzing detailed transport properties of the molecules involved.

Conductance in a bis-terpyridine based single molecular breadboard circuit

Controlling charge flow in single molecule circuits with multiple electrical contacts and conductance pathways is a much sought after goal in molecular electronics. In this joint experimental and theoretical study, we advance the possibility of creating single molecule breadboard circuits through an analysis of the conductance of a bis-terpyridine based molecule (TP1). The TP1 molecule can adopt multiple conformations through relative rotations of 7 aromatic rings and can attach to electrodes in 61 possible single and multi-terminal configurations through 6 pyridyl groups. Despite this complexity, we show that it is possible to extract well defined conductance features for the TP1 breadboard and assign them rigorously to the underlying constituent circuits. Mechanically controllable break-junction (MCBJ) experiments on the TP1 molecular breadboard show an unprecedented 4 conductance states spanning a range 10 -2G0 to 10 -7G0. Quantitative theoretical examination of the conductance of TP1 reveals that combinations of 5 types of single terminal 2-5 ring subcircuits are accessed as a function of electrode separation to produce the distinct conductance steps observed in the MCBJ experiments. We estimate the absolute conductance for each single terminal subcircuit and its percentage contribution to the 4 experimentally observed conductance states. We also provide a detailed analysis of the role of quantum interference and thermal fluctuations in modulating conductance within the subcircuits of the TP1 molecular breadboard. Finally, we discuss the possible development of molecular circuit theory and experimental advances necessary for mapping conductance through complex single molecular breadboard circuits in terms of their constituent subcircuits.

Guidelines and diagnostics for charge carrier tuning in thiophene-based wires

Reported experimental trends in charge carrier tuning in single molecule junctions of oligothiophene-based wires are rationalized by means of frontier molecular orbital theory.

Communication: Length-dependent thermopower of single-molecule junctions

In the present work, we theoretically study the length dependence of thermopower of a single-molecule junction with a chain-like molecular bridge of an arbitrary length using a tight-binding model. We analyze conditions bringing a nonlinear growth of the thermopower accompanying the extension of the bridge length. Also, we show that the thermopower may decrease with increasing molecular length provided that the molecular bridge is sufficiently long.

Estimating the Landauer-Büttiker transmission function from single molecule break junction experiments

When investigating the electronic response properties of molecules, experiments often measure conductance whereas computation predicts the transmission probability. Although Landauer-Büttiker theory usually relates the two, comparison between experiment and computation remains difficult because experimental data (specifically those from break junctions) are statistical and computational results are deterministic. In this work we develop tools to quantitatively estimate-with error bars-the shape of the Landauer-Büttiker transmission function directly from experimental statistics on conductance and thermopower (if the latter is also available). We subsequently apply these tools to existing data, demonstrating a rigorous statistical comparison between experimental and computational results on molecular electron transport.

Thermoelectric efficiency of molecular junctions

Focus of the review is on experimental set-ups and theoretical proposals aimed to enhance thermoelectric performances of molecular junctions. In addition to charge conductance, the thermoelectric parameter commonly measured in these systems is the thermopower, which is typically rather low. We review recent experimental outcomes relative to several junction configurations used to optimize the thermopower. On the other hand, theoretical calculations provide estimations of all the thermoelectric parameters in the linear and non-linear regime, in particular of the thermoelectric figure of merit and efficiency, completing our knowledge of molecular thermoelectricity. For this reason, the review will mainly focus on theoretical studies analyzing the role of not only electronic, but also of the vibrational degrees of freedom. Theoretical results about thermoelectric phenomena in the coherent regime are reviewed focusing on interference effects which play a significant role in enhancing the figure of merit. Moreover, we review theoretical studies including the effects of molecular many-body interactions, such as electron-vibration couplings, which typically tend to reduce the efficiency. Since a fine tuning of many parameters and coupling strengths is required to optimize the thermoelectric conversion in molecular junctions, new theoretically proposed set-ups are discussed in the conclusions.

Exploring quantum interference in heteroatom-substituted graphene-like molecules

If design principles for controlling quantum interference in single molecules could be elucidated and verified, then this will lay the foundations for exploiting such effects in nanoscale devices and thin-film materials. When the core of a graphene-like polyaromatic hydrocarbon (PAH) is weakly coupled to external electrodes by atoms i and j, the single-molecule electrical conductance σij depends on the choice of connecting atoms i,j. Furthermore, provided the Fermi energy is located between the HOMO and LUMO, conductance ratios σij/σlm corresponding to different connectivities i,j and l,m are determined by quantum interference within the PAH core. In this paper, we examine how such conductance ratios change when one of the carbon atoms within the 'parent' PAH core is replaced by a heteroatom to yield a 'daughter' molecule. For bipartite parental cores, in which odd-numbered sites are connected to even-numbered sites only, the effect of heteroatom substitution onto an odd-numbered site is summarized by the following qualitative rules: (a) when i and j are odd, both parent and daughter have low conductances, (b) when i is odd and j is even, or vice versa both parent and daughter have high conductances and (c) when i,j are both even, the parent has a low conductance and the daughter a high conductance. These rules are verified by comparison with density-functional calculations on naphthalene, anthracene, pyrene and anthanthrene cores connected via two different anchor groups to gold electrodes.

Identification of a positive-Seebeck-coefficient exohedral fullerene

If fullerene-based thermoelectricity is to become a viable technology, then fullerenes exhibiting both positive and negative Seebeck coefficients are needed. C60 is known to have a negative Seebeck coefficient and therefore in this paper we address the challenge of identifying a positive-Seebeck-coefficient fullerene. We investigated the thermoelectric properties of single-molecule junctions of the exohedral fullerene C50Cl10 connected to gold electrodes and found that it indeed possesses a positive Seebeck coefficient. Furthermore, in common with C60, the Seebeck coefficient can be increased by placing more than one C50Cl10 in series. For a single C50Cl10, we find S = +8 μV K(-1) and for two C50Cl10's in series we find S = +30 μV K(-1). We also find that the C50Cl10 monomer and dimer have power factors of 0.5 × 10(-5) W m(-1) K(-2) and 6.0 × 10(-5) W m(-1) K(-2) respectively. These results demonstrate that exohedral fullerenes provide a new class of thermoelectric materials with desirable properties, which complement those of all-carbon fullerenes, thereby enabling the boosting of the thermovoltage in all-fullerene tandem structures.

Mapping the Transmission Functions of Single-Molecule Junctions

Charge transport phenomena in single-molecule junctions are often dominated by tunneling, with a transmission function dictating the probability that electrons or holes tunnel through the junction. Here, we present a new and simple technique for measuring the transmission functions of molecular junctions in the coherent tunneling limit, over an energy range of 1.5 eV around the Fermi energy. We create molecular junctions in an ionic environment with electrodes having different exposed areas, which results in the formation of electric double layers of dissimilar density on the two electrodes. This allows us to electrostatically shift the molecular resonance relative to the junction Fermi levels in a manner that depends on the sign of the applied bias, enabling us to map out the junction's transmission function and determine the dominant orbital for charge transport in the molecular junction. We demonstrate this technique using two groups of molecules: one group having molecular resonance energies relatively far from EF and one group having molecular resonance energies within the accessible bias window. Our results compare well with previous electrochemical gating data and with transmission functions computed from first principles. Furthermore, with the second group of molecules, we are able to examine the behavior of a molecular junction as a resonance shifts into the bias window. This work provides a new, experimentally simple route for exploring the fundamentals of charge transport at the nanoscale.

Thermo-voltage measurements of atomic contacts at low temperature

We report the development of a novel method to determine the thermopower of atomic-sized gold contacts at low temperature. For these measurements a mechanically controllable break junction (MCBJ) system is used and a laser source generates a temperature difference of a few kelvins across the junction to create a thermo-voltage. Since the temperature difference enters directly into the Seebeck coefficient S = -ΔV/ΔT, the determination of the temperature plays an important role. We present a method for the determination of the temperature difference using a combination of a finite element simulation, which reveals the temperature distribution of the sample, and the measurement of the resistance change due to laser heating of sensor leads on both sides next to the junction. Our results for the measured thermopower are in agreement with recent reports in the literature.

A New Approach to Materials Discovery for Electronic and Thermoelectric Properties of Single-Molecule Junctions

We have investigated a large set of symmetric and asymmetric molecules to demonstrate a general rule for molecular-scale quantum transport, which provides a new route to materials design and discovery. The rule states "the conductance GXBY of an asymmetric molecule is the geometric mean of the conductance of the two symmetric molecules derived from it and the thermopower SXBY of the asymmetric molecule is the algebraic mean of their thermopowers". The studied molecules have a structure X-B-Y, where B is the backbone of the molecule, while X and Y are anchor groups, which bind the molecule to metallic electrodes. When applied to experimentally measured histograms of conductance and thermopower, the rules apply to the statistically most probable values. We investigated molecules with anchors chosen from the following family: cyano, pyridl, dihydrobenzothiol, amine and thiol. For the backbones B, we tested 14 different structures. We found that the formulas (GXBY)(2) = GXBX*GYBY and SXBY = (SXBX + SYBY)/2 were satisfied in the large majority of the cases, provided the Fermi energy is located within the HOMO-LUMO gap of the molecules. The circuit rules imply that if measurements are performed on molecules with nA different anchors and nB different backbones, then properties of nA(nA + 1)nB/2 molecules can be predicted. So for example, in the case of 20 backbones and 10 anchors, 30 measurements (or reliable calculations) can provide a near quantitative estimate for 1070 measurements of other molecules, at no extra cost.

Tuning the thermoelectric properties of metallo-porphyrins

We investigated the thermoelectric properties of metalloporphyrins connected by thiol anchor groups to gold electrodes. By varying the transition metal-centre over the family Mn, Co, Ni, Cu, Fe, and Zn we are able to tune the molecular energy levels relative to the Fermi energy of the electrodes. The resulting single-molecule room-temperature thermopowers range from almost zero for Co and Cu centres, to +80 μV K(-1) and +230 μV K(-1) for Ni and Zn respectively. In contrast, the thermopowers with Mn(II) or Fe(II) metal centres are negative and lie in the range -280 to -260 μV K(-1). Complexing these with a counter anion to form Fe(III) and Mn(III) changes both the sign and magnitude of their thermopowers to +218 and +95 respectively. The room-temperature power factors of Mn(II), Mn(III), Fe(III), Zn and Fe(II) porphyrins are predicted to be 5.9 × 10(-5) W m(-1) K(-2), 5.4 × 10(-4) W m(-1) K(-2), 9.5 × 10(-4) W m(-1) K(-2), 1.6 × 10(-4) W m(-1) K(-2) and 2.3 × 10(-4) W m(-1) K(-2) respectively, which makes these attractive materials for molecular-scale thermoelectric devices.

Organometallic molecular wires as versatile modules for energy-level alignment of the metal–molecule–metal junction

Organometallic Ru molecular wires show significantly higher conductance compared to their organic counterpart due to high-lying HOMOs.

Thermopower measurements in molecular junctions

The measurement of thermopower in molecular junctions offers complementary information to conductance measurements and is becoming essential for the understanding of transport processes at the nanoscale.

Tuning the conformation of floppy molecules by charge transfer

Recent advances in electrochemistry and nano- and molecular electronics made it possible to prepare and study molecular species having fractional charges (q ≠ 0, ±1, …) that can be continuously tuned by biases.

Thermoelectric properties of fullerene-based junctions: a first-principles study

This study is built on density functional calculations in combination with the non-equilibrium Green's function, and we probe the thermoelectric transport mechanisms through C₆₀molecules anchored to Al nano-electrodes in three different ways, such as, the planar, pyramidal, and asymmetric surfaces.

Oligoyne Molecular Junctions for Efficient Room Temperature Thermoelectric Power Generation

Understanding phonon transport at a molecular scale is fundamental to the development of high-performance thermoelectric materials for the conversion of waste heat into electricity. We have studied phonon and electron transport in alkane and oligoyne chains of various lengths and find that, due to the more rigid nature of the latter, the phonon thermal conductances of oligoynes are counterintuitively lower than that of the corresponding alkanes. The thermal conductance of oligoynes decreases monotonically with increasing length, whereas the thermal conductance of alkanes initially increases with length and then decreases. This difference in behavior arises from phonon filtering by the gold electrodes and disappears when higher-Debye-frequency electrodes are used. Consequently a molecule that better transmits higher-frequency phonon modes, combined with a low-Debye-frequency electrode that filters high-energy phonons is a viable strategy for suppressing phonon transmission through the molecular junctions. The low thermal conductance of oligoynes, combined with their higher thermopower and higher electrical conductance lead to a maximum thermoelectric figure of merit of ZT = 1.4, which is several orders of magnitude higher than that of alkanes.

Thiophene-based Tripodal Anchor Units for Hole Transport in Single-Molecule Junctions with Gold Electrodes

Molecule-metal junctions are inevitable for the realization of single-molecule electronics. In this study, we developed new tripodal anchors with electron-rich aromatic rings to achieve robust contact with gold electrodes, an effective hybridization of the π orbital with gold electrodes (π channel), and hole transport through π-channel hybridization. Cyclic voltammetry and X-ray photoelectron spectroscopy measurements of the monolayers indicated that the thiophene-based tripodal molecule exhibits anchoring characteristics as expected. The electrical conductance of thiophene-anchored bistripodal molecules using the scanning tunneling microscope (STM)-based break junction technique confirmed the formation of molecular junctions. The Seebeck coefficient of this compound estimated from thermoelectric voltage measurements using a STM was determined to be a positive value, which indicates that the charge carriers are holes. On the contrary, the corresponding pyridine-anchored molecules showed electron transport. These results reveal the versatility of π-channel tripodal anchors for the control of charge-carrier type in single-molecule electronics.

Adsorption-Induced Solvent-Based Electrostatic Gating of Charge Transport through Molecular Junctions

Recent experiments have shown that transport properties of molecular-scale devices can be reversibly altered by the surrounding solvent. Here, we use a combination of first-principles calculations and experiment to explain this change in transport properties through a shift in the local electrostatic potential at the junction caused by nearby conducting and solvent molecules chemically bound to the electrodes. This effect is found to alter the conductance of 4,4'-bipyridine-gold junctions by more than 50%. Moreover, we develop a general electrostatic model that quantitatively relates the conductance and dipoles associated with the bound solvent and conducting molecules. Our work shows that solvent-induced effects can be used to control charge and energy transport at molecular-scale interfaces.

High thermopower of mechanically stretched single-molecule junctions

Metal-molecule-metal junction is a promising candidate for thermoelectric applications that utilizes quantum confinement effects in the chemically defined zero-dimensional atomic structure to achieve enhanced dimensionless figure of merit ZT. A key issue in this new class of thermoelectric nanomaterials is to clarify the sensitivity of thermoelectricity on the molecular junction configurations. Here we report simultaneous measurements of the thermoelectric voltage and conductance on Au-1,4-benzenedithiol (BDT)-Au junctions mechanically-stretched in-situ at sub-nanoscale. We obtained the average single-molecule conductance and thermopower of 0.01 G0 and 15 μV/K, respectively, suggesting charge transport through the highest occupied molecular orbital. Meanwhile, we found the single-molecule thermoelectric transport properties extremely-sensitive to the BDT bridge configurations, whereby manifesting the importance to design the electrode-molecule contact motifs for optimizing the thermoelectric performance of molecular junctions.

Alternative types of molecule-decorated atomic chains in Au-CO-Au single-molecule junctions

We investigate the formation and evolution of Au-CO single-molecule break junctions. The conductance histogram exhibits two distinct molecular configurations, which are further investigated by a combined statistical analysis. According to conditional histogram and correlation analysis these molecular configurations show strong anticorrelations with each other and with pure Au monoatomic junctions and atomic chains. We identify molecular precursor configurations with somewhat higher conductance, which are formed prior to single-molecule junctions. According to detailed length analysis two distinct types of molecule-affected chain-formation processes are observed, and we compare these results to former theoretical calculations considering bridge- and atop-type molecular configurations where the latter has reduced conductance due to destructive Fano interference.

Single-Molecule Electronics: Chemical and Analytical Perspectives

It is now possible to measure the electrical properties of single molecules using a variety of techniques including scanning probe microcopies and mechanically controlled break junctions. Such measurements can be made across a wide range of environments including ambient conditions, organic liquids, ionic liquids, aqueous solutions, electrolytes, and ultra high vacuum. This has given new insights into charge transport across molecule electrical junctions, and these experimental methods have been complemented with increasingly sophisticated theory. This article reviews progress in single-molecule electronics from a chemical perspective and discusses topics such as the molecule-surface coupling in electrical junctions, chemical control, and supramolecular interactions in junctions and gating charge transport. The article concludes with an outlook regarding chemical analysis based on single-molecule conductance.