Energy Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy: Self-Consistency for the Case of Alkyl Thiols and Dithiols on Ag, Au, and Pt Electrodes

Breakdown of Ohm's Law in Molecular Junctions with Electrodes of Single-Layer Graphene

For sufficiently low biases, Ohm's law, the cornerstone of electricity, stating that current I and voltage V are proportional, is satisfied at low biases for all known systems ranging from macroscopic conductors to nanojunctions. In this study, we predict theoretically and demonstrate experimentally that in single-molecule junctions fabricated with single-layer graphene as electrodes the current at low V scales as the cube of V, thereby invalidating Ohm's law. The absence of the ohmic regime is a direct consequence of the unique band structure of the single-layer graphene, whose vanishing density of states at the Dirac points precludes electron transfer from and to the electrodes at low biases.

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.

CP-AFM Molecular Tunnel Junctions with Alkyl Backbones Anchored Using Alkynyl and Thiol Groups: Microscopically Different Despite Phenomenological Similarity

In this paper, we report results on the electronic structure and transport properties of molecular junctions fabricated via conducting probe atomic force microscopy (CP-AFM) using self-assembled monolayers (SAMs) of n-alkyl chains anchored with acetylene groups (CnA; n = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage (I-V) characteristics of CnA CP-AFM junctions can be very accurately reproduced by the same off-resonant single-level model (orSLM) successfully utilized previously for many other junctions. We demonstrate that important insight into the energy-level alignment can be gained from experimental data of transport (processed via the orSLM) and ultraviolet photoelectron spectroscopy combined with ab initio quantum chemical information based on the many-body outer valence Green's function method. Measured conductance GAg < GAu < GPt is found to follow the same ordering as the metal work function ΦAu < ΦAu < ΦPt, a fact that points toward a transport mediated by an occupied molecular orbital (MO). Still, careful data analysis surprisingly revealed that transport is not dominated by the ubiquitous HOMO but rather by the HOMO-1. This is an important difference from other molecular tunnel junctions with p-type HOMO-mediated conduction investigated in the past, including the alkyl thiols (CnT) to which we refer in view of some similarities. Furthermore, unlike in CnT and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in CnA an additional MO shift, whose independence of size (n) rules out significant image charge effects. Along with the prevalence of the HOMO-1 over the HOMO, the impact of the "second" (tip) electrode on the energy level alignment is another important finding that makes the CnA and CnT junctions different. What ultimately makes CnA unique at the microscopic level is a salient difference never reported previously, namely, that CnA's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present CnA from the CnT, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.

Reply to the 'Comment on "A single level tunneling model for molecular junctions: evaluating the simulation methods"' by I Baldea, Phys. Chem. Chem. Phys., 2024, , D2CP05110A (http://D2CP05110A)

We respond to the recent comment by Dr. Ion Bâldea concerning our work, "A single level tunneling model for molecular junctions: evaluating the simulation methods [E. M. Opodi et al.,Phys. Chem. Chem. Phys., 2022, 24, 11958]". Dr. Bâldea has raised concerns about the applicability map we developed based on the comparison study of the 3 analytical models used in the study of molecular electronics. In our response, we have dissected Dr. Bâldea's critique into four primary points. For each, we have provided comprehensive replies that address and rectify the misinterpretations of our initial study. We'd like to thank Dr. Ion Bâldea for his comments, which have resulted in further clarification of the applicability of the analytical models for the study of molecular electronics.

Comment on "A single level tunneling model for molecular junctions: evaluating the simulation methods" by E. M. Opodi, X. Song, X. Yu and W. Hu, Phys. Chem. Chem. Phys., 2022, , 11958"

The present Comment demonstrates important flaws of the paper Opodi et al. Phys. Chem. Chem. Phys., 2022, 24, 11958 Their crown result ("applicability map") aims at indicating parameter ranges wherein two approximate methods (called method 2 and 3) apply. My calculations reveal that the applicability map is a factual error. Deviations of I2 from the exact current I1 do not exceed 3% for model parameters where Opodi et al. claimed that method 2 is inapplicable. As for method 3, the parameter range of the applicability map is beyond its scope, as stated in papers cited by Opodi et al. themselves.

Can tunneling current in molecular junctions be so strongly temperature dependent to challenge a hopping mechanism? Analytical formulas answer this question and provide important insight into large area junctions

Analytical equations like Richardson-Dushman's or Shockley's provided a general, if simplified conceptual background, which was widely accepted in conventional electronics and made a fundamental contribution to advances in the field. In the attempt to develop a (highly desirable, but so far missing) counterpart for molecular electronics, in this work, we deduce a general analytical formula for the tunneling current through molecular junctions mediated by a single level that is valid for any bias voltage and temperature. Starting from this expression, which is exact and obviates cumbersome numerical integration, in the low and high temperature limits we also provide analytical formulas expressing the current in terms of elementary functions. They are accurate for broad model parameter ranges relevant for real molecular junctions. Within this theoretical framework we show that: (i) by varying the temperature, the tunneling current can vary by several orders of magnitude, thus debunking the myth that a strong temperature dependence of the current is evidence for a hopping mechanism, (ii) real molecular junctions can undergo a gradual (Sommerfeld-Arrhenius) transition from a weakly temperature dependent to a strongly ("exponential") temperature dependent current that can be tuned by the applied bias, and (iii) important insight into large area molecular junctions with eutectic gallium indium alloy (EGaIn) top electrodes can be gained. E.g., merely based on transport data, we estimate that the current carrying molecules represent only a fraction of f ≈ 4 × 10-4 out of the total number of molecules in a large area Au-S-(CH2)13-CH3/EGaIn junction.

Deciphering I-V characteristics in molecular electronics with the benefit of an analytical model

We share our perspective that a simple analytical model for electron tunneling in molecular junctions can greatly aid quantitative analysis of experimental data in molecular electronics. In particular, the single-level model (SLM), derived from first principles, provides a precise prediction for the current-voltage (I-V) characteristics in terms of key electronic structure parameters, which in turn depend on the molecular and contact architecture. SLM analysis thus facilitates understanding of structure-property relationships and provides metrics that can be compared across different types of tunnel junctions, as we illustrate with several examples.

Thickness-Dependent Charge Transport in Three Dimensional Ru(II)- Tris(phenanthroline)-Based Molecular Assemblies

We describe here the fabrication of large-area molecular junctions with a configuration of ITO/[Ru(Phen)3]/Al to understand temperature- and thickness-dependent charge transport phenomena. Thanks to the electrochemical technique, thin layers of electroactive ruthenium(II)-tris(phenanthroline) [Ru(Phen)3] with thicknesses of 4-16 nm are covalently grown on sputtering-deposited patterned ITO electrodes. The bias-induced molecular junctions exhibit symmetric current-voltage (j-V) curves, demonstrating highly efficient long-range charge transport and weak attenuation with increased molecular film thickness (β = 0.70 to 0.79 nm-1). Such a lower β value is attributed to the accessibility of Ru(Phen)3 molecular conduction channels to Fermi levels of both the electrodes and a strong electronic coupling at ITO-molecules interfaces. The thinner junctions (d = 3.9 nm) follow charge transport via resonant tunneling, while the thicker junctions (d = 10-16 nm) follow thermally activated (activation energy, Ea ∼ 43 meV) Poole-Frenkel charge conduction, showing a clear "molecular signature" in the nanometric junctions.

Large Seebeck Values in Metal-Molecule-Semimetal Junctions Attained by a Gateless Level-Alignment Method

Molecular junctions are potentially highly efficient devices for thermal energy harvesting since their transmission properties can be tailored to break electron-hole transport symmetry and consequently yield high Seebeck and Peltier coefficients. Full harnessing of this potential requires, however, a capability to precisely position their Fermi level within the transmission landscape. Currently, with the lack of such a "knob" for two-lead junctions, their thermoelectric performance is too low for applications. Here we report that the requested capability can be realized by using junctions with a semimetal lead and molecules with a tailored effect of their monolayers on the work function of the semimetal. The approach is demonstrated by junctions with monolayers of alkanethiols on bismuth (Bi). Fermi-level tuning enables in this case increasing the Seebeck coefficient by more than 2 orders of magnitude. The underlying mechanism of this capability is discussed, as well as its general applicability.

The Stark Effect: A Tool for the Design of High-Performance Molecular Rectifiers

Molecular electronic devices offer a path to the miniaturization of electronic circuits and could potentially facilitate novel functionalities that can be embedded into the molecular structure. Given their nanoscale dimensions, device properties are strongly influenced by quantum effects, yet many of these phenomena have been largely overlooked. We investigated the mechanism responsible for current rectification in molecular diodes and found that efficient rectification is achieved by enhancing the Stark effect strength and enabling a large number of molecules to participate in transport. These findings provided insights into the operation of molecular rectifiers and guided the development of high-performance devices via the design of molecules containing polarizable aromatic rings. Our results are consistent for different molecular structures and are expected to have broad applicability to all molecular devices by answering key questions related to charge transport mechanisms in such systems.

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.

Fine-Tuning the Molecular Design for High-Performance Molecular Diodes Based on Pyridyl Isomers

Better control of molecule-electrode coupling (Γ) to minimize leakage current is an effective method to optimize the functionality of molecular diodes. Herein we embedded 5 isomers of phenypyridyl derivatives, each with an N atom placed at a different position, in two electrodes to fine-tune Γ between self-assembled monolayers (SAMs) and the top electrode of EGaIn (eutectic Ga-In terminating in Ga2 O3 ). Combined with electrical tunnelling results, characterizations of electronic structures, single-level model fittings, and DFT calculations, we found that the values of Γ of SAMs formed by these isomers could be regulated by nearly 10 times, thereby contributing to the leakage current changing over about two orders of magnitude and switching the isomers from resistors to diodes with a rectification ratio (r+ =|J(+1.5 V)/J(-1.5 V)|) exceeding 200. We demonstrated that the N atom placement can be chemically engineered to tune the resistive and rectifying properties of the molecular junctions, making it possible to convert molecular resistors into rectifiers. Our study provides fundamental insights into the role of isomerism in molecular electronics and offers a new avenue for designing functional molecular devices.

Can room-temperature data for tunneling molecular junctions be analyzed within a theoretical framework assuming zero temperature?

Routinely, experiments on tunneling molecular junctions report values of conductances (G_RT) and currents (I_RT) measured at room temperature. On the other hand, theoretical approaches based on simplified models provide analytic formulas for the conductance (G_0K) and current (I_0K) valid at zero temperature. Therefore, interrogating the applicability of the theoretical results deduced in the zero-temperature limit to real experimental situations at room temperature (i.e., G_RT ≈ G_0K and I_RT ≈ I_0K) is a relevant aspect. Quantifying the pertaining temperature impact on the transport properties computed within the ubiquitous single-level model with Lorentzian transmission is the specific aim of the present work. Comprehensive results are presented for broad ranges of the relevant parameters (level's energy offset ε₀ and width Γ_a, and applied bias V) that safely cover values characterizing currently fabricated junctions. They demonstrate that the strongest thermal effects occur at biases below resonance (2|ε₀| - δε₀ - 0.3 ≲ |eV| - 0.3 ≲ 2|ε₀|). At fixed V, they affect an ε₀-range whose largest width δε₀ is about nine times larger than the thermal energy (δε₀ ≈ 3πk_BT) at Γ_a → 0. The numerous figures included aim to convey a quick overview on the applicability of the zero-temperature limit to a specific real junction. In quantitative terms, the conditions of applicability are expressed as mathematical inequalities involving elementary functions. They constitute the basis of a proposed interactive data-fitting procedure, which aims to guide experimentalists interested in data processing in a specific case.

Tunable Subnanometer Gaps in Self-Assembled Monolayer Gold Nanoparticle Superlattices Enabling Strong Plasmonic Field Confinement

Nanoparticle superlattices produced with controllable interparticle gap distances down to the subnanometer range are of superior significance for applications in electronic and plasmonic devices as well as in optical metasurfaces. In this work, a method to fabricate large-area (∼1 cm²) gold nanoparticle (GNP) superlattices with a typical size of single domains at several micrometers and high-density nanogaps of tunable distances (from 2.3 to 0.1 nm) as well as variable constituents (from organothiols to inorganic S^2-) is demonstrated. Our approach is based on the combination of interfacial nanoparticle self-assembly, subphase exchange, and free-floating ligand exchange. Electrical transport measurements on our GNP superlattices reveal variations in the nanogap conductance of more than 6 orders of magnitude. Meanwhile, nanoscopic modifications in the surface potential landscape of active GNP devices have been observed following engineered nanogaps. In situ optical reflectance measurements during free-floating ligand exchange show a gradual enhancement of plasmonic capacitive coupling with a diminishing average interparticle gap distance down to 0.1 nm, as continuously red-shifted localized surface plasmon resonances with increasing intensity have been observed. Optical metasurfaces consisting of such GNP superlattices exhibit tunable effective refractive index over a broad wavelength range. Maximal real part of the effective refractive index, n_max, reaching 5.4 is obtained as a result of the extreme field confinement in the high-density subnanometer plasmonic gaps.

Effects of Electrode Materials on Electron Transport for Single-Molecule Junctions

The contact at the molecule-electrode interface is a key component for a range of molecule-based devices involving electron transport. An electrode-molecule-electrode configuration is a prototypical testbed for quantitatively studying the underlying physical chemistry. Rather than the molecular side of the interface, this review focuses on examples of electrode materials in the literature. The basic concepts and relevant experimental techniques are introduced.

Increased Surface Density of States at the Fermi Level for Electron Transport Across Single-Molecule Junctions

Fermi's golden rule, a remarkable concept for the transition probability involving continuous states, is applicable to the interfacial electron-transporting efficiency via correlation with the surface density of states (SDOS). Yet, this concept has not been reported to tailor single-molecule junctions where gold is an overwhelmingly popular electrode material due to its superior amenability in regenerating molecular junctions. At the Fermi level, however, the SDOS of gold is small due to its fully filled d-shell. To increase the electron-transport efficiency, herein, gold electrodes are modified by a monolayer of platinum or palladium that bears partially filled d-shells and exhibits significant SDOS at the Fermi energy. An increase by 2-30 fold is found for single-molecule conductance of α,ω-hexanes bridged via common headgroups. The improved junction conductance is attributed to the electrode self-energy which involves a stronger coupling with the molecule and a larger SDOS participated by d-electrons at the electrode-molecule interfaces.

Chemical Conformation Induced Transport Carrier Switching in Molecular Junction based on Carboxylic-Terminated Thiol Molecules

The paper demonstrates the effect of the chemical conformation of the -COOH group on the transport characteristic including conductance, rectification, and length effect in molecular junctions (MJs) formed by self-assembled monolayers of carboxylic-terminated thiol molecules. For an alkyl chain shorter than C₁₁, the transport mechanism was attributed to a direct off-resonant tunneling of a hole carrier, located at the Au-S interface, whereas a hopping mechanism was assigned to the alkyl chain longer than the C₁₁ chain located at the -COOH group. The hopping mechanism may be operated by electron transport associated with the breaking of the -OH bonding likely driven by a voltage. Importantly, at the C₁₁ alkyl chain, we observed that the transport carrier operating in MJs could change from a hole carrier into an electron carrier. The result strongly proves that the chemical conformation should be considered in analyzing molecular electronics and provides a basis for the rational design of molecular electronic devices.

Estimating the Number of Molecules in Molecular Junctions Merely Based on the Low Bias Tunneling Conductance at Variable Temperature

Temperature (T) dependent conductance G=G(T) data measured in molecular junctions are routinely taken as evidence for a two-step hopping mechanism. The present paper emphasizes that this is not necessarily the case. A curve of lnG versus 1/T decreasing almost linearly (Arrhenius-like regime) and eventually switching to a nearly horizontal plateau (Sommerfeld regime), or possessing a slope gradually decreasing with increasing 1/T is fully compatible with a single-step tunneling mechanism. The results for the dependence of G on T presented include both analytical exact and accurate approximate formulas and numerical simulations. These theoretical results are general, also in the sense that they are not limited, e.g., to the (single molecule electromigrated (SET) or large area EGaIn) fabrication platforms, which are chosen for exemplification merely in view of the available experimental data needed for analysis. To be specific, we examine in detail transport measurements for molecular junctions based on ferrocene (Fc). As a particularly important finding, we show how the present analytic formulas for G=G(T) can be utilized to compute the ratio f=Aeff/An between the effective and nominal areas of large area Fc-based junctions with an EGaIn top electrode. Our estimate of f≈0.6×10-4 is comparable with previously reported values based on completely different methods for related large area molecular junctions.

Redox-controlled conductance of polyoxometalate molecular junctions

We demonstrate the reversible in situ photoreduction of molecular junctions of a phosphomolybdate [PMo₁₂O₄₀]^3- monolayer self-assembled on flat gold electrodes, connected by the tip of a conductive atomic force microscope. The conductance of the one electron reduced [PMo₁₂O₄₀]^4- molecular junction is increased by ∼10, and this open-shell state is stable in the junction in air at room temperature. The analysis of a large current-voltage dataset by unsupervised machine learning and clustering algorithms reveals that the electron transport in the pristine phosphomolybdate junctions leads to symmetric current-voltage curves, controlled by the lowest unoccupied molecular orbital (LUMO) at 0.6-0.7 eV above the Fermi energy with ∼25% of the junctions having a better electronic coupling to the electrodes than the main part of the dataset. This analysis also shows that a small fraction (∼18% of the dataset) of the molecules is already reduced. The UV light in situ photoreduced phosphomolybdate junctions systematically feature slightly asymmetric current-voltage behaviors, which is ascribed to the electron transport mediated by the single occupied molecular orbital (SOMO) nearly at resonance with the Fermi energy of the electrodes and by a closely located single unoccupied molecular orbital (SUMO) at ∼0.3 eV above the SOMO with a weak electronic coupling to the electrodes (∼50% of the dataset) or at ∼0.4 eV but with a better electrode coupling (∼50% of the dataset). These results shed light on the electronic properties of reversible switchable redox polyoxometalates, a key point for potential applications in nanoelectronic devices.

Thermopower in Transition from Tunneling to Hopping

The Seebeck effect of a molecular junction in a hopping regime or tunneling-to-hopping transition remains uncertain. This paper describes the Seebeck effect in molecular epitaxy films (OPI_n where n = 1-9) based on imine condensation between an aryl amine and aldehyde and investigates how the Seebeck coefficient (S, μV/K) varies at the crossover region. The S value of OPI_n linearly increased with increasing the molecular length (d, nm), ranging from 7.2 to 38.0 μV/K. The increasing rate changed from 0.99 to 0.38 μV·K^-1 Å^-1 at d = 3.4 nm (OPI₄). Combined experimental and theoretical studies indicated that such a change stems from a tunneling-to-hopping transition, and the small but detectable length-dependence of thermopower in the long molecules originates from the gradual reduction of the tunneling contribution to the broadening of molecular orbital energy level, rather than its relative position to the Fermi level. Our work helps to bridge the gap between bulk and nanoscale thermoelectric systems.

Self-Assembled Monolayers for Improved Charge Injection of Silver Back Electrodes in Inverted Organic Electronic Devices

Self-assembled monolayers (SAMs) formed from thiol compounds bound to Ag and Au electrodes have been used as an important strategy in improving the stability and efficiency of optoelectronic devices. Thiol compounds provide only one binding site with the metal electrode which limits their influence. Dithiolane/dithiol compounds can provide multiple binding sites and could be useful in enhancing the performance of the device. In this study, inverted organic semiconducting hole-only devices were fabricated by using Ag back electrodes in conjunction with SAMs formed from disulfide lipoic acid-based compounds and were compared to a long aliphatic chain thiol. The binding and the electronic properties as well as electrical characteristics of the SAMs on silver were studied to look at the influence of their structure on charge injection in the organic semiconductor devices. It was found that the SAMs formed with (±)-α-lipoic acid, isolipoic acid, and (±)-4-phenylbutyl 5-(1,2-dithiolan-3-yl) pentanoate significantly improved the charge injection by either changing the work function of the Ag or altering the physical interaction between the polymer and the metal surface. This study may lead to an understanding of how the nature of the functional groups of the SAM and the number of bonds formed between each SAM molecule and the metal electrode influence the contact resistance and the performance of organic semiconductor devices.

Molecular ensemble junctions with inter-molecular quantum interference

We report of a high yield method to form nanopore molecular ensembles junctions containing ~40,000 molecules, in which the semimetal bismuth (Bi) is a top contact. Conductance histograms of these junctions are double-peaked (bi-modal), a behavior that is typical for single molecule junctions but not expected for junctions with thousands of molecules. This unique observation is shown to result from a new form of quantum interference that is inter-molecular in nature, which occurs in these junctions since the very long coherence length of the electrons in Bi enables them to probe large ensembles of molecules while tunneling through the junctions. Under such conditions, each molecule within the ensembles becomes an interference path that modifies via its tunneling phase the electronic structure of the entire junction. This new form of quantum interference holds a great promise for robust novel conductance effects in practical molecular junctions.

Quantum interference effect in the conductance of single molecule junctions has been attracting intensive interest in recent years. Here, Li and Selzer show the presence of intermolecular quantum interference over 40,000 molecules in a molecular ensemble junction with bismuth as the top electrode.

Demonstration of Molecular Tunneling Junctions Based on Vertically Stacked Graphene Heterostructures

We demonstrate the fabrication and complete characterization of vertical molecular tunneling junctions based on graphene heterostructures, which incorporate a control series of arylalkane molecules acting as charge transport barriers. Raman spectroscopy and atomic force microscopy were employed to identify the formation of the molecular monolayer via an electrophilic diazonium reaction on a pre-patterned bottom graphene electrode. The top graphene electrode was transferred to the deposited molecular layer to form a stable electrical connection without filamentary damage. Then, we showed proof of intrinsic charge carrier transport through the arylalkane molecule in the vertical tunneling junctions by carrying out multiprobe approaches combining complementary transport characterization methods, which included length- and temperature-dependent charge transport measurements and transition voltage spectroscopy. Interpretation of all the electrical characterizations was conducted on the basis of intact statistical analysis using a total of 294 fabricated devices. Our results and analysis can provide an objective criterion to validate molecular electronic devices fabricated with graphene electrodes and establish statistically representative junction properties. Since many of the experimental test beds used to examine molecular junctions have generated large variation in the measured data, such a statistical approach is advantageous to identify the meaningful parameters with the data population and describe how the results can be used to characterize the graphene-based molecular junctions.

Empirical Parameter to Compare Molecule-Electrode Interfaces in Large-Area Molecular Junctions

This paper describes a simple model for comparing the degree of electronic coupling between molecules and electrodes across different large-area molecular junctions. The resulting coupling parameter can be obtained directly from current–voltage data or extracted from published data without fitting. We demonstrate the generalizability of this model by comparing over 40 different junctions comprising different molecules and measured by different laboratories. The results agree with existing models, reflect differences in mechanisms of charge transport and rectification, and are predictive in cases where experimental limitations preclude more sophisticated modeling. We also synthesized a series of conjugated molecular wires, in which embedded dipoles are varied systematically and at both molecule–electrode interfaces. The resulting current–voltage characteristics vary in nonintuitive ways that are not captured by existing models, but which produce trends using our simple model, providing insights that are otherwise difficult or impossible to explain. The utility of our model is its demonstrative generalizability, which is why simple observables like tunneling decay coefficients remain so widely used in molecular electronics despite the existence of much more sophisticated models. Our model is complementary, giving insights into molecule–electrode coupling across series of molecules that can guide synthetic chemists in the design of new molecular motifs, particularly in the context of devices comprising large-area molecular junctions.

A single level tunneling model for molecular junctions: evaluating the simulation methods

A single level tunneling model has been the most popular model system in both experimental and theoretical studies of molecular junctions. We performed a detailed simulation study on the performance of the single level tunneling model for analyzing the charge transport in molecular junctions. Three different modeling methods, including the numerical integration of the Landauer formula and two approximated analytical formulas that are extensively used for extracting key transport parameters, i.e. the energy offset and the coupling strength between molecules and electrodes from current-voltage (I-V) characteristics were compared and evaluated for their applicability. The simulation of I-V plots shows that the applicability of the two approximated analytical models is dependent on the energy offset and coupling strength. Model analysis based on the three methods performed on experimental data obtained from representative literature papers revealed that the two approximated analytical methods are neither suitable for small coupling strength nor suitable for low energy offset, and they also deviated from the exact results at high bias. These results imply that the transport parameters by the model analysis can be wrong if the models were not correctly applied under their intrinsic constraints, therefore providing wrong physical information about the system. We finally provided an applicability map as a guide for different modeling methods for charge transport studies in molecular devices.

HCnH- Anion Chains with n ≤ 8 Are Nonlinear and Their Permanent Dipole Makes Them Potential Candidates for Astronomical Observation

To be detectable in space via radio astronomy, molecules should have a permanent dipole moment. This is the plausible reason why HC_nH chains are underproportionally represented in the interstellar medium in comparison with the isoelectronically equivalent HC_nN chain family, which is the most numerous homologous series astronomically observed so far. In this communication, we present results of quantum chemical calculations for the HC_nH family at several levels of theory: density functional theory (DFT/B3LYP), coupled-cluster expansions (ROCCSD(T)), and G4 composite model. Contradicting previous studies, we report here that linear HC_nH⁻ anion chains with sizes of astrochemical interest are unstable (i.e., not all calculated frequencies are real). Nonlinear cis and trans HC_nH⁻ anion chains turn out to be stable both against molecular vibrations (i.e., all vibrational frequencies are real) and against electron detachment (i.e., positive electroaffinity). The fact that the cis anion conformers possess permanent dipole is the main encouraging message that this study is aiming at conveying to the astrochemical community, as this makes them observable by means of radio astronomy.

Charge Transport Characteristics of Molecular Electronic Junctions Studied by Transition Voltage Spectroscopy

The field of molecular electronics is prompted by tremendous opportunities for using a single-molecule and molecular monolayers as active components in integrated circuits. Until now, a wide range of molecular devices exhibiting characteristic functions, such as diodes, transistors, switches, and memory, have been demonstrated. However, a full understanding of the crucial factors that affect charge transport through molecular electronic junctions should yet be accomplished. Remarkably, recent advances in transition voltage spectroscopy (TVS) elucidate that it can provide key quantities for probing the transport characteristics of the junctions, including, for example, the position of the frontier molecular orbital energy relative to the electrode Fermi level and the strength of the molecule–electrode interactions. These parameters are known to be highly associated with charge transport behaviors in molecular systems and can then be used in the design of molecule-based devices with rationally tuned electronic properties. This article highlights the fundamental principle of TVS and then demonstrates its major applications to study the charge transport properties of molecular electronic junctions.

Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted Oligophenylene Ethynylene Dithiols

A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, ε_h, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, ε_h^trans and ε_h^UPS, respectively. We take the difference in these quantities to be the image potential energy eV_image. In the theoretical approach, we use density functional theory (DFT) to calculate directly eV_image between positive charge on an OPX molecule and the negative image charge in the Au. Both approaches yield eV_image ∼ -0.1 eV per metal contact, meaning that the total image potential energy is ∼-0.2 eV for an assembled junction with two Au contacts. Thus, we find that the total image potential energy is 25-30% of the total offset ε_h, which means that image charge effects are significant in OPX junctions. Our methods should be generally applicable to understanding image charge effects as a function of molecular size, for example, in a variety of SAM-based junctions.

Amine-Anchored Aromatic Self-Assembled Monolayer Junction: Structure and Electric Transport Properties

We studied the structure and transport properties of aromatic amine self-assembled monolayers (NH₂-SAMs) on an Au surface. The oligophenylene and oligoacene amines with variable lengths can form a densely packed and uniform monolayer under proper assembly conditions. Molecular junctions incorporating an eutectic Ga-In (EGaIn) top electrode were used to characterize the charge transport properties of the amine monolayer. The current density J of the junction decreases exponentially with the molecular length (d), as J = J₀ exp(-βd), which is a sign of tunneling transport, with indistinguishable values of J₀ and β for NH₂-SAMs of oligophenylene and oligoacene, indicating a similar molecule-electrode contact and tunneling barrier for two groups of molecules. Compared with the oligophenylene and oligoacene molecules with thiol (SH) as the anchor group, a similar β value (∼0.35 Å^-1) of the aromatic NH₂-SAM suggests a similar tunneling barrier, while a lower (by 2 orders of magnitude) injection current J₀ is attributed to lower electronic coupling Γ of the amine group with the electrode. These observations are further supported by single-level tunneling model fitting. Our study here demonstrates the NH₂-SAMs can work as an effective active layer for molecular junctions, and provide key physical parameters for the charge transport, paving the road for their applications in functional devices.

Quantitative analysis of weak current rectification in molecular tunnel junctions subject to mechanical deformation reveals two different rectification mechanisms for oligophenylene thiols versus alkane thiols

Metal-molecule-metal junctions based on alkane thiol (CnT) and oligophenylene thiol (OPTn) self-assembled monolayers (SAMs) and Au electrodes are expected to exhibit similar electrical asymmetry, as both junctions have one chemisorbed Au-S contact and one physisorbed, van der Waals contact. Asymmetry is quantified by the current rectification ratio RR apparent in the current-voltage (I-V) characteristics. Here we show that RR < 1 for CnT and RR > 1 for OPTn junctions, in contrast to expectation, and further, that RR behaves very differently for CnT and OPTn junctions under mechanical extension using the conducting probe atomic force microscopy (CP-AFM) testbed. The analysis presented in this paper, which leverages results from the previously validated single level model and ab initio quantum chemical calculations, allows us to explain the puzzling experimental findings for CnT and OPTn in terms of different current rectification mechanisms. Specifically, in CnT-based junctions the Stark effect creates the HOMO level shifting necessary for rectification, while for OPTn junctions the level shift arises from position-dependent coupling of the HOMO wavefunction with the junction electrostatic potential profile. On the basis of these mechanisms, our quantum chemical calculations allow quantitative description of the impact of mechanical deformation on the measured current rectification. Additionally, our analysis, matched to experiment, facilitates direct estimation of the impact of intramolecular electrostatic screening on the junction potential profile. Overall, our examination of current rectification in benchmark molecular tunnel junctions illuminates key physical mechanisms at play in single step tunneling through molecules, and demonstrates the quantitative agreement that can be obtained between experiment and theory in these systems.

Energy-Level Alignment and Orbital-Selective Femtosecond Charge Transfer Dynamics of Redox-Active Molecules on Au, Ag, and Pt Metal Surfaces

Charge transfer (CT) dynamics across metal-molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal-sulfur bond to the carbon-sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule-metal interface.

A single atom change turns insulating saturated wires into molecular conductors

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH₂)_(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å⁻¹. Impedance measurements show tripled dielectric constants (ε_r) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH₂)_nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in ε_r. Here, we demonstrate experimentally that \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\beta \propto 1/\sqrt{{\varepsilon }_{r}}$$\end{document}β∝1/εr, suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

In molecular junctions, where a molecule is placed between two electrodes, the current passed decays exponentially as a function of length. Here, Chen et al. show that this exponentially attenuation can be controlled by changing a single atom at the end of the molecular wire.

Tuning surface d bands with bimetallic electrodes to facilitate electron transport across molecular junctions

Understanding chemical bonding and conductivity at the electrode-molecule interface is key for the operation of single-molecule junctions. Here we apply the d-band theory that describes interfacial interactions between adsorbates and transition metal surfaces to study electron transport across these devices. We realized bimetallic Au electrodes modified with a monoatomic Ag adlayer to connect α,ω-alkanoic acids (HO₂C(CH₂)_nCO₂H). The force required to break the molecule-electrode binding and the contact conductance G_n=0 are 1.1 nN and 0.29 G₀ (the conductance quantum, 1 G₀ = 2e²/h ≈ 77.5 μS), which makes these junctions, respectively, 1.3-1.8 times stronger and 40-60-fold more conductive than junctions with bare Au or Ag electrodes. A similar performance was found for Au electrodes modified by Cu monolayers. By integrating the Newns-Anderson model with the Hammer-Nørskov d-band model, we explain how the surface d bands strengthen the adsorption and promote interfacial electron transport, which provides an alternative avenue for the optimization of molecular electronic devices.

Hopping Conductance in Molecular Wires Exhibits a Large Heavy-Atom Kinetic Isotope Effect

We report a large kinetic isotope effect (KIE) for intramolecular charge transport in π-conjugated oligophenyleneimine (OPI) molecules connected to Au electrodes. ¹³C and ¹⁵N substitution on the imine bonds produces a conductance KIE of ∼2.7 per labeled atom in long OPI wires >4 nm in length, far larger than typical heavy-atom KIEs for chemical reactions. In contrast, isotopic labeling in shorter OPI wires <4 nm does not produce a conductance KIE, consistent with a direct tunneling mechanism. Temperature-dependent measurements reveal that conductance for a long ¹⁵N-substituted OPI wire is activated, and we propose that the exceptionally large conductance KIEs imply a thermally assisted, through-barrier polaron tunneling mechanism. In general, observation of large conductance KIEs opens up considerable opportunities for understanding microscopic conduction mechanisms in π-conjugated molecules.

The energy level alignment of the ferrocene-EGaIn interface studied with photoelectron spectroscopy

The energy level alignment after the formation of a molecular tunnel junction is often poorly understood because spectroscopy inside junctions is not possible, which hampers the rational design of functional molecular junctions and complicates the interpretation of the data generated by molecular junctions. In molecular junction platforms where the top electrode-molecule interaction is weak; one may argue that the energy level alignment can be deduced from measurements with the molecules supported by the bottom electrode (sometimes referred to as "half junctions"). This approach, however, still relies on a series of assumptions, which are challenging to address experimentally due to difficulties in studying the molecule-top electrode interaction. Herein, we describe top electrode-molecule junctions with a liquid metal alloy top electrode of EGaIn (which stands for eutectic alloy of Ga and In) interacting with well-characterised ferrocene (Fc) moieties. We deposited a ferrocene derivative on films of EGaIn, coated with its native GaOx layer, and studied the energy level alignment with photoelectron spectroscopy. Our results reveal that the electronic interaction between the Fc and GaOx/EGaIn is very weak, resembling physisorption. Therefore, investigations of "half junctions" for this system can provide valuable information regarding the energy level alignment of complete EGaIn junctions. Our results help to improve our understanding of the energy landscape in weakly coupled molecular junctions and aid to the rational design of molecular electronic devices.

Interplay of Fermi Level Pinning, Marcus Inverted Transport, and Orbital Gating in Molecular Tunneling Junctions

This Letter examines the interplay of important tunneling mechanisms-Fermi level pinning, Marcus inverted transport, and orbital gating-in a molecular rectifier. The temperature dependence of the rectifying molecular junction containing 2,2'-bipyridyl terminated n-alkanethiolate was investigated. A bell-shaped trend of activation energy as a function of applied bias evidenced the dominant occurrence of unusual Marcus inverted transport, while retention of rectification at low temperatures implied that the rectification obeyed the resonant tunneling regime. The results allowed reconciling two separately developed transport models, Marcus-Landauer energetics and Fermi level pinning-based rectification. Our work shows that the internal orbital gating can be substituted with the pinning effect, which pushes the transport mechanism into the Marcus inverted regime.

The Potential of X-ray Photoelectron Spectroscopy for Determining Interface Dipoles of Self-Assembled Monolayers

In the current manuscript we assess to what extent X-ray photoelectron spectroscopy (XPS) is a suitable tool for probing the dipoles formed at interfaces between self-assembled monolayers and metal substrates. To that aim, we perform dispersion-corrected, slab-type band-structure calculations on a number of biphenyl-based systems bonded to an Au(111) surface via different docking groups. In addition to changing the docking chemistry (and the associated interface dipoles), the impacts of polar tail group substituents and varying dipole densities are also investigated. We find that for densely packed monolayers the shifts of the peak positions of the simulated XP spectra are a direct measure for the interface dipoles. In the absence of polar tail group substituents they also directly correlate with adsorption-induced work function changes. At reduced dipole densities this correlation deteriorates, as work function measurements probe the difference between the Fermi level of the substrate and the electrostatic energy far above the interface, while core level shifts are determined by the local electrostatic energy in the region of the atom from which the photoelectron is excited.

Revealing the electronic structure of organic emitting semiconductors at the single-molecule level

Single-molecule conductance measurements of OLED molecules show that the holes injected from metal electrode can be suppressed by adding electron-withdrawing arms, benefiting the electron–hole balance of OLED devices whose holes are excessive.