Molecular Thermoelectricity in EGaIn-Based Molecular Junctions

Engineering Charge Transport by Tunneling in Supramolecular Assemblies through Precise Control of Metal-Ligand Interactions

Coordination-driven supramolecular assemblies are promising for nanometer-sized electronic devices due to the potential to manipulate metal-ligand interactions and thereby control charge transport via tunneling through these assemblies. Cross-plane charge tunneling is investigated in assemblies of metalloporphyrins and pillar molecules, specifically palladium(II) and zinc(II) octaethylporphyrin (PdOEP and ZnOEP) monolayers and bilayers with bidentate (DABCO) and monodentate (ABCO) pillar ligands on highly oriented pyrolytic graphite (HOPG). Junction measurements and quantum-chemical calculations reveal that metal-ligand interactions significantly influence charge transport via tunneling and thermoelectric effects. Weak interactions in PdOEP assemblies create isolated molecular orbitals on interior pillar ligands, compressing the HOMO-LUMO gap and enhancing tunneling currents with unusual, inverted attenuation behavior and high thermopower. Conversely, strong interactions in ZnOEP assemblies induce localized orbitals on the porphyrin, leading to conventional tunneling decay behavior and low thermopower. The study highlights the potential of metal-ligand interactions as a strategy to engineer molecular orbital distribution, enhancing quantum transport efficiency in molecular-scale devices.

Recent advances for core-shell gallium-based liquid metal particles: properties, fabrication, modification, and applications

Gallium-based liquid metal micro-nanoparticles (Ga-LMPs) have attracted extensive attention in recent years due to their unique physicochemical properties, such as biocompatibility, fluidity and large specific surface area. However, the surface of gallium-based liquid metal is prone to oxidation, forming a solid insulating gallium oxide shell that limits its functionality and applications. Therefore, it has become a hot research topic to endow Ga-LMPs with new functionalities by surface modification. This review summarizes the surface properties, preparation methods, and surface modification mechanisms of Ga-LMPs, with a focus on the diverse functionalities gained through surface modification, such as enhanced particle stability, electrical conductivity, drug delivery, stimulus responsiveness, thermoelectric property and catalytic activity. The potential applications of these properties in fields such as sensing, energy storage, and catalysis are also discussed.

Formation, Structure, and Thermal Annealing Effects of Ordered Self-Assembled Monolayers of 4-Fluorobenzeneselenol on Au(111)

The formation, surface structure, and thermal annealing effects of self-assembled monolayers (SAMs) via vapor deposition of 4-fluorobenzeneselenol (4-FBSeH) on Au(111) at room temperature were investigated using scanning tunneling microscopy (STM). The most prominent structural feature is that 4-fluorobenzeneselenolate (4-FBSe) SAMs on Au(111) are composed of numerous SAM-covered Au adatom islands, regardless of the deposition time. High-resolution STM observations revealed that the ordered phase of 4-FBSe SAMs was formed after very short deposition times of 30 s and 3 min, whereas the disordered phase was formed after long deposition times of 1 h and 24 h. The ordered phase can be described as a (4 × 2√3) structure, and the average areal molecular density of the SAMs was calculated to be 29.0 Å2/molecule, suggesting the formation of densely packed monolayers with a standing-up adsorption structure. Interestingly, after thermal annealing at 373 K for 30 min, the (4 × 2√3) ordered phase of the SAMs was transformed to randomly distributed, short, single-molecular rows ranging from several nanometers to approximately ten nanometers in length, which has not been observed previously in organic thiolate SAMs. The high-resolution STM results of this study can provide very meaningful information for understanding the formation, surface structure, and thermal annealing effects of 4-FBSe SAMs on Au(111).

Advances in single-molecule electrical transport studies of peptides

The charge transport between peptide molecules is one of the crucial factors in sustaining various biochemical processes within biological organisms. Understanding the charge transport processes between peptide molecules is of great significance for further investigating life reaction processes. Through single-molecule electronic characterization techniques, we have reviewed the effects of peptide molecular stuctures and external experimental factors on charge transport. Additionally, we have summarized the latest research on supramolecular interactions between peptide chains, particularly focusing on the even-odd effect. This not only enhances our understanding of the charge transport mechanisms between peptide molecules but also lays a solid theoretical foundation for the widespread application of peptide molecules in fields such as biological devices.

Thermoelectric and thermal properties of molecular junctions: mechanisms, characterization methods and applications

The rapid development of artificial intelligence requires tremendous energy consumption. Due to the limitations of cooling and energy recovery systems, effectively lowering power dissipation and utilizing the waste heat of electronic devices remain challenges. Molecular electronics, with its potential for low energy consumption and high-efficiency thermoelectric conversion, offers a feasible solution for future computational devices. Over the past two decades, researchers have made significant progress in the study of thermal and thermoelectric properties of molecular junctions. In this feature article, we first introduced four mechanisms of thermal and thermoelectric transport in molecular junctions guided by quantum theory. We then reviewed the evolution of characterization techniques for assessing the local temperature, thermopower, and thermal conductance of molecular junctions. Subsequently, we introduced the practical applications that have been implemented so far. This review concludes by addressing the principal challenges currently faced in the field and identifying crucial directions for future research.

Unusually High Thermopower in Molecular Junctions from Molecularly Induced Quantized States in Their Semimetal Leads

The efficiency of a thermoelectric (TE) device depends on the extent to which its electron/hole transport symmetry at the Fermi level is broken. This requirement makes molecular junctions promising for TE applications as their transmission characteristics are highly nonlinear. Yet, in the absence of an efficient method to tune the position of the Fermi level within their transmission landscape, the typical Seebeck values of metal-molecules-metal junctions are |S| ≤ 100 μV/K, while considering their electrical and thermal conductance, it should be |S| ≥ 1 mV/K to be relevant for applications. Here, we report metal-molecules-semimetal junctions with |S| in the required mV/K range. This is achieved by molecularly induced quantized two-dimensional (2D) interfacial states within the semimetal that result in nonlinear features in their transmission properties. The importance of the presented approach goes beyond TE applications as it demonstrates a novel strategy to form and tune 2D interfacial layers within bulk materials by molecular monolayers.

Thermoelectricity in Molecular Tunnel Junctions

The growing interest in thermoelectric energy conversion technologies has recently extended to the molecular scale, with molecular tunnel junctions emerging as promising platforms for energy harvesting from heat in a quantum-tunneling regime. This Review explores the advances in thermoelectricity within molecular junctions, highlighting the unique ability of these junctions to exploit charge tunneling and controlled molecular structure to enhance thermoelectric performance. Molecular thermoelectrics, which bridge nanoscale material design and thermoelectric applications, utilize tunneling mechanisms, such as coherent tunneling and hopping processes, including coherent and incoherent pathways, to facilitate energy conversion. Complementing these mechanisms is an array of high-precision fabrication techniques for molecular junctions, from single-molecule break junctions to large-area liquid metal-based systems, each tailored to optimize heat and charge transfer properties. With novel design strategies such as the incorporation of electron-dense ligands, customizable anchor groups, and advanced junction architectures, molecular tunnel junctions hold promise for addressing challenging targets in thermoelectricity. This Review focuses on theoretical models, experimental methodologies, and design principles aimed at understanding the thermoelectric function in molecular junctions and enhancing the performance.

Liquid metal composite with carbon nanotubes for reliable interconnection between Pt electrodes

We report a CNT/eGaIn composite that suppresses dissolutive wetting on platinum, maintaining interconnect stability for up to 30 days. Minimizing CNT aggregation prevents gallium penetration, enhancing the reliability of liquid metal components in electronics.

Formation and Surface Structures of Long-Range Ordered Self-Assembled Monolayers of 2-Mercaptopyrazine on Au(111)

The effect of solution pH on the formation and surface structure of 2-pyrazinethiolate (2-PyzS) self-assembled monolayers (SAMs) formed by the adsorption of 2-mercaptopyrazine (2-PyzSH) on Au(111) was investigated using scanning tunneling microscopy (STM) and X-ray photoelectron microscopy (XPS). Molecular-scale STM observations clearly revealed that 2-PyzS SAMs at pH 2 had a short-range ordered phase of (2√3 × √21)R30° structure with a standing-up adsorption structure. However, 2-PyzS SAMs at pH 8 had a very unique long-range ordered phase, showing a "ladder-like molecular arrangement" with bright repeating rows. This ordered phase was assigned to the (3 × √37)R43° structure, consisting of two different adsorption structures: standing-up and tilted adsorption structures. The average arial density of 2-PyzS SAMs on Au(111) at pH 8 was calculated to be 49.47 Å2/molecule, which is 1.52 times more loosely packed compared to the SAMs at pH 2 with 32.55 Å2/molecule. XPS measurements showed that 2-PyzS SAMs at pH 2 and pH 8 were mainly formed through chemical interactions between the sulfur anchoring group and the Au(111) substrates. The proposed structural models of packing structures for 2-PyzS SAMs on Au(111) at different pHs are well supported by the XPS results. The results of this study will provide new insights into the formation, surface structure, and molecular orientation of SAMs by N-heteroaromatic thiols with pyrazine molecular backbone on Au(111) at the molecular level.

Radioisotope thermoelectric generators (RTGs): a review of current challenges and future applications

Nuclear power sources can be effectively employed to generate renewable energy as a counter to global reliance on fossil fuels and increasing energy demands. Nuclear radiation can be utilized in numerous ways to produce energy. Along with their use as fuel for nuclear power plants, the decay process of radioisotopes can also be used to create electrical energy. A radioisotope thermoelectric generator (RTG) is one such example, as it is tasked to convert the decay energy of radioisotopes into heat energy, which is later converted into electrical power using the Seebeck effect. Radioisotopes have high energy densities and certain isotopes can generate considerable heat energy for prolonged timescales. As such, RTGs have found applications in powering interplanetary exploration and interstellar space missions due to their long half-life. They are also commonly used in remote regions of the earth where frequent replacement, charging and maintenance of power sources is difficult. In this review, we will discuss the working principle and architecture of RTGs, challenges faced in further development and potential applications of this technology.

Long-Range Charge Transport in Molecular Wires

Long-range charge transport (LRCT) in molecular wires is crucial for the advancement of molecular electronics but remains insufficiently understood due to complex transport mechanisms and their dependencies on molecular structure. While short-range charge transport is typically dominated by off-resonant tunneling, which decays exponentially with molecular length, recent studies have highlighted certain molecular structures that facilitate LRCT with minimal attenuation over several nanometers. This Perspective reviews the latest progress in understanding LRCT, focusing on chemical designs and mechanisms that enable this phenomenon. Key strategies include π-conjugation, redox-active centers, and stabilization of radical intermediates, which support LRCT through mechanisms such as coherent resonant tunneling or incoherent hopping. We discuss how the effects of molecular structure, length, and temperature influence charge transport, and highlight emerging techniques like the Seebeck effect for distinguishing between transport mechanisms. By clarifying the principles behind LRCT and outlining future challenges, this work aims to guide the design of molecular systems capable of sustaining efficient long-distance charge transport, thereby paving the way for practical applications in molecular electronics and beyond.

Self-Assembled Molecular Layers as Interfacial Engineering Nanomaterials in Rechargeable Battery Applications

Rechargeable batteries have transformed human lives and modern industry, ushering in new technological advancements such as mobile consumer electronics and electric vehicles. However, to fulfill escalating demands, it is crucial to address several critical issues including energy density, production cost, cycle life and durability, temperature sensitivity, and safety concerns is imperative. Recent research has shed light on the intricate relationship between these challenges and the chemical processes occurring at the electrode-electrolyte interface. Consequently, a novel approach has emerged, utilizing self-assembled molecular layers (SAMLs) of meticulously designed molecules as nanomaterials for interface engineering. This research provides a comprehensive overview of recent studies underscoring the significant roles played by SAML in rechargeable battery applications. It discusses the mechanisms and advantageous features arising from the incorporation of SAML. Moreover, it delineates the remaining challenges in SAML-based rechargeable battery research and technology, while also outlining future perspectives.

Electrically Driven Deterministic Plasmon Light Sources Based on Arrays of Molecular Tunnel Junctions

Surface plasmons excited via inelastic tunnelling have led to plasmon light sources with small footprints and ultrafast response speeds, which are favored by integrated optical circuits. Self-assembled monolayers of organic molecules function as highly tunable tunnel barriers with novel functions. However, limited by the low effective contact between the liquid metal electrode and the self-assembled monolayers, it is quite challenging to obtain molecular plasmon light sources with high density and uniform emission. Here, by combining lithographic patterning with a solvent treatment method, we have demonstrated electrically driven deterministic plasmon emission from arrays of molecular tunnel junctions. The solvent treatment could largely improve the effective contact from 9.6% to 48% and simultaneously allow the liquid metal to fill into lithographically patterned micropore structures toward deterministic plasmon emission with desired patterns. Our findings open up new possibilities for tunnel junction-based plasmon light sources, laying the foundation for electrically driven light-emitting metasurfaces.

Interface of gallium-based liquid metals: oxide skin, wetting, and applications

Gallium-based liquid metals (GaLMs) are promising for a variety of applications-especially as a component material for soft devices-due to their fluidic nature, low toxicity and reactivity, and high electrical and thermal conductivity comparable to solid counterparts. Understanding the interfacial properties and behaviors of GaLMs in different environments is crucial for most applications. When exposed to air or water, GaLMs form a gallium oxide layer with nanoscale thickness. This "oxide nano-skin" passivates the metal surface and allows for the formation of stable microstructures and films despite the high-surface tension of liquid metal. The oxide skin easily adheres to most smooth surfaces. While it enables effective printing and patterning of the GaLMs, it can also make the metals challenging to handle because it adheres to most surfaces. The oxide also affects the interfacial electrical resistance of the metals. Its formation, thickness, and composition can be chemically or electrochemically controlled, altering the physical, chemical, and electrical properties of the metal interface. Without the oxide, GaLMs wet metallic surfaces but do not wet non-metallic substrates such as polymers. The topography of the underlying surface further influences the wetting characteristics of the metals. This review outlines the interfacial attributes of GaLMs in air, water, and other environments and discusses relevant applications based on interfacial engineering. The effect of surface topography on the wetting behaviors of the GaLMs is also discussed. Finally, we suggest important research topics for a better understanding of the GaLMs interface.

Two-dimensional Mo1-xB2 with ordered metal vacancies obtained for advanced thermoelectric applications based on first-principles calculations

The study and development of high thermoelectric properties is crucial for the next generation of microelectronic and wearable electronics. Derived from the recent experimental realization of layers of transition metal molybdenum and boride, we report the theoretical realization of advanced thermoelectric properties in two-dimensional (2D) transition metal boride Mo1-xB2 (x = 0, 0.05, 0.10, 0.125, 0.15)-based defect sheets. The introduction of metal vacancies results in stronger d-p exchange interactions and hybridization between the Mo-d and B-p atoms. Meanwhile, the ordered metal vacancies enabled transition metal borides (n-type Mo0.9B2) to widen the d-bandwidth and raise the d-band center, leading to a relatively high carrier mobility of 3262 cm2 V-1 s-1 and conductivity twice that of a bug-free n-type MoB2 layer, which indicates that it presents good electronic and thermal transport properties. Furthermore, investigations of the thermoelectric performance exhibit a maximum ZT of up to 3.29, which is superior to those of currently reported 2D materials. Modulation by defect engineering suggests that 2D transition metal boride sheets with ordered metal vacancies have promising applications in microelectronics, wearable electronics and thermoelectric devices.

Gaining insight into molecular tunnel junctions with a pocket calculator without I-V data fitting. Five-thirds protocol

The protocol put forward in the present paper is an attempt to meet the experimentalists' legitimate desire of reliably and easily extracting microscopic parameters from current-voltage measurements on molecular junctions. It applies to junctions wherein charge transport dominated by a single level (molecular orbital, MO) occurs via off-resonant tunneling. The recipe is simple. The measured current-voltage curve I = I(V) should be recast as a curve of V5/3/I versus V. This curve exhibits two maxima: one at positive bias (V = Vp+), another at negative bias (V = Vp-). The values Vp+ > 0 and Vp- < 0 at the two peaks of the curve for V5/3/I at positive and negative bias and the corresponding values Ip+ = I(Vp+) > 0 and Ip- = I(Vp-) < 0 of the current is all information needed as input. The arithmetic average of Vp+ and |Vp-| in volt provides the value in electronvolt of the MO energy offset ε0 = EMO - EF relative to the electrode Fermi level (|ε0| = e(Vp+ + |Vp-|)/2). The value of the (Stark) strength of the bias-driven MO shift is obtained as γ = (4/5)(Vp+ - |Vp-|)/(Vp+ + |Vp-|) sign (ε0). Even the low-bias conductance estimate, G = (3/8)(Ip+/Vp+ + Ip-/Vp-), can be a preferable alternative to that deduced from fitting the I-V slope in situations of noisy curves at low bias. To demonstrate the reliability and the generality of this "five-thirds" protocol, I illustrate its wide applicability for molecular tunnel junctions fabricated using metallic and nonmetallic electrodes, molecular species possessing localized σ and delocalized π electrons, and various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).

Dichotomy between Level Broadening and Level Coupling to Electrodes in Large Area EGaIn Molecular Junctions

Choosing self-assembled monolayers (SAMs) of fluorine-terminated oligophenylenes adsorbed on gold as an illustration, we show that a single-level [molecular orbital (MO)] model can excellently reproduce full I-V curves measured for large area junctions fabricated with a top EGaIn contact. In addition, this model unravels a surprising dichotomy between MO coupling to electrodes and MO broadening. Importantly for the coherence of the microscopic description, the latter is found to correlate with the SAM coverage and molecular and π* orbital tilt angles.

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.

Thermopower in Underpotential Deposition-Based Molecular Junctions

Underpotential deposition (UPD) is an intriguing means for tailoring the interfacial electronic structure of an adsorbate at a substrate. Here we investigate the impact of UPD on thermoelectricity occurring in molecular tunnel junctions based on alkyl self-assembled monolayers (SAMs). We observed noticeable enhancements in the Seebeck coefficient of alkanoic acid and alkanethiol monolayers, by up to 2- and 4-fold, respectively, upon replacement of a conventional Au electrode with an analogous bimetallic electrode, Cu UPD on Au. Quantum transport calculations indicated that the increased Seebeck coefficients are due to the UPD-induced changes in the shape or position of transmission resonances corresponding to gateway orbitals, which depend on the choice of the anchor group. Our work unveils UPD as a potent means for altering the shape of the tunneling energy barrier at the molecule-electrode contact of alkyl SAM-based junctions and hence enhancing thermoelectric performance.

Do quantum interference effects manifest in acyclic aliphatic molecules with anchoring groups?

The ability to control single molecule electronic conductance is imperative for achieving functional molecular electronics applications such as insulation, switching, and energy conversion. Quantum interference (QI) effects are generally used to control electronic transmission through single molecular junctions by tuning the molecular structure or the position of the anchoring group(s) in the molecule. While previous studies focussed on the QI between σ and/or π channels of the molecular backbone, here, we show that single molecule electronic devices can be designed based on QI effects originating from the interactions of anchoring groups. Furthermore, while previous studies have concentrated on the QI mostly in conjugated/cyclic systems, our study showcases that QI effects can be harnessed even in the simplest acyclic aliphatic systems-alkanedithiols, alkanediamines, and alkanediselenols. We identify band gap state resonances in the transmission spectrum of these molecules whose positions and intensities depend on the chain length, and anchoring group sensitive QI between the nearly degenerate molecular orbitals localized on the anchoring groups. We predict that these QI features can be harnessed through an external mechanical stimulus to tune the charge transport properties of single molecules in the break-junction experiments.