Transition Voltage Spectroscopy Research Articles

Coupling effects in bipyridines linked to carbon nanowires

In this work, we conducted an investigation into the electronic transport properties of four distinct devices formed by bipyridines (BPY) as the central region, linked to carbon nanowires (polyyne-type carbyne) electrodes. These four devices have two symmetrical (2,2′ and 4,4′ BPY) and two asymmetrical (2,4′ and 2,6′ BPY) configurations. We employed Density Functional Theory combined with Non-Equilibrium Green's Function (DFT-NEGF) to calculate the molecular geometry and the electronic transport properties. Our analysis, using Fowler-Nordheim (F-N) and Millikan-Lauritsen (M-L) plots, reveals that electronic transport primarily occurs through direct tunneling without any inflection points, indicating the absence of nonlinear effects such as resonance or negative differential resistance (NDR). This demonstrates the importance of the study including an analysis of Transition Voltage Spectroscopy (TVS) which shows that the presence of nitrogen atoms in the electrode-molecule contacts, specifically in the cases 2,4′ and 4,4′ BPY, significantly alters the structural and electronic characteristics of the devices. This alteration leads to a strong coupling of the junction, improved structural and electronic properties. These findings are supported by the Device Density of States (DDOS) and Frontier Molecular Orbitals (FMOs). Moreover, we observed that the electronic characteristics of all four devices remain consistent even under bias voltage (Ve).

Secondary Structure-Dependent Charge Transport in iLOV Protein Junctions.

The composition and structure of proteins are crucial for charge migration in the solid-state charge transport (CTp). Despite much progress, it is still challenging to explore the relationship between conformational change and CTp in the complex protein system. Herein, we design three improved light-oxygen-voltage (iLOV) domains, and efficiently regulate the CTp of the iLOV self-assembled monolayers (SAMs) by pH induced conformation variation. The current density can be controlled in the range of one order of magnitude. Interestingly, the CTp of iLOV displays negative linear relations with the β-sheet contents. Single-level Landauer fitting and transition voltage spectroscopy analysis suggest that β-sheet-dependent CTp would be related to the coupling between iLOV and electrodes. This work proposes a new strategy to explore the CTp in complex molecular system. Our findings deepen the understanding on protein structure-CTp relationship, and provide predictive mode of protein CTp responses for the design of functional bioelectronics.

High-Temperature Electronic Transport Properties of PEDOT:PSS Top-Contact Molecular Junctions with Oligophenylene Dithiols

In this study, we investigated the high-temperature electronic transport behavior of spin-coated PEDOT:PSS top-contact molecular ensemble junctions based on self-assembled monolayers (SAMs) of oligophenylene dithiols. We observed irreversible temperature-dependent charge transport at the high-temperature regime over 320 K. The effective contact resistance and normalized resistance decreased with increasing temperature (320 to 400 K), whereas the tunneling attenuation factor was nearly constant irrespective of temperature change. These findings demonstrate that the high-temperature transport properties are not dominated by the integrity of SAMs in molecular junctions, but rather the PEDOT:PSS/SAMs contact. Transition voltage spectroscopy measurements indicated that the contact barrier height of the PEDOT:PSS/SAMs is lowered at elevated temperatures, which gives rise to a decrease in the contact resistance and normalized resistance. The high-temperature charge transport through these junctions is also related to an increase in the grain area of PEDOT cores after thermal treatment. Moreover, it was found that there was no significant change in either the current density or normalized resistance of the annealed junctions after 60 days of storage in ambient conditions.

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.

Tuning transport properties for 1D and 2D ψ-Graphene

In this work, we built two equivalent molecular devices via DFT/NEGF replicating the 2D and 1D ψ-Graphene unit cell composed by C12 and C24H6. Our results exhibit that the hydrogenation at the edge for 1D ψ-Graphene makes energetically stable, more structurally stable and reduces driving on device signature between 0 V and 0.25 V (Switching) and 0.25 V-0.5 V (resonant tunnel diode - RTD) while the 2D device has RTD signature. The density of states and transmission eingenchannels confirm metallic behavior and semiconductor–metal transition for two devices. Thenegative differential resistance (NDR) is identified in the two devices by transition voltage spectroscopy. Our discoveries make the1D ψ-Graphenehighly promising in nanoelectronics applications for tuning transport properties.

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.

Tunneling rules for electronic transport in 1-D systems

Carbynes have gained highlight due to their unique properties and applications in molecular electronics exhibiting resonant tunnel effect. We studied the electronic transport through a molecular tunnel junction composed of two polyyne-type carbynes separated by distance (4 Å ≥ dn ≥ 2.1 Å) higher than bond length (dn > dC–C ) ranging from 0.1 Å based on Density Functional Theory/Non-Equilibrium Green’s Function (DFT/NEGF). The frontier molecular orbitals and bond lengths of the polyyne (dC–C ≈ 1.36 Å, dC ≡C ≈ 1.26 Å) at 0.0 V become equal to those in cumulene-type carbynes (dC= C ≈ 1.28 Å) at 0.9 V occurring a semiconductor–metal transition for dn < 4 Å. I–V (dI/dV–V) curve presents switching behaviour up to 0.6 V and after a diode behaviour favoured as dn decreases, that is, to 4.0 Å is 3.3 nA (5 nS) while for 2.1 Å is 3750 nA (1500 nS) showing metallic nature. The log(I)–V [log(dI/dV)–V] curve exhibits the behaviour for low bias voltage while the I–dn curve is exhibited for high bias voltage. The exponential expression for G(d) fits perfectly with the Gmax result obtained with DFT/NEGF calculations. Transition Voltage Spectroscopy (TVS) confirms rectangular barrier and direct tunneling for Vd–s < ϕB /e playing a tunneling rule for electronic transport of these 1-D systems.

Molecule–Electrode Electronic Coupling Modulates Optoelectronics of (Bio)Molecular Junctions

The charge transport across a molecular junction formed by sandwiching molecules between two electrodes in testbed architectures depends not only on the work function of the metal electrodes and energy gap of the molecules but also on the efficacy of the molecule–electrode electronic coupling. Insights into such molecule–electrode coupling would help to understand the relation between the coupling strength and electron transport processes. With this aim, the optoelectronic modulation across bacteriorhodopsin-based molecular junctions has been studied using experimental current–voltage traces obtained by conducting-probe atomic force microscopy under various illuminations. The energy barrier $$ \left( {\varepsilon_{0} } \right) $$ , molecule–electrode coupling (Γg), and other transport parameters were determined utilizing the Landauer model with a single-Lorentzian transmission function, transition voltage spectroscopy, and the law of corresponding states in the universal tunneling model approach. The findings reveal that the optoelectronic modulation of bacteriorhodopsin molecular junctions originate from alteration of the molecule–electrode coupling, which could originate from modulation of electronic states and the electrostatic environment of retinal chromophores made of the protein under dark and green or green–blue illumination conditions.

Transitions in electrical behavior of Molecular Devices based on 1-D and 2-D graphene-phagraphene-graphene hybrid heterojunctions

This work proposes an electronic transport investigation without spin polarization based on DFT/NEGF for six Graphene-like and PhaGraphene-like heterojunctions including (57575) topological defects as inducers of localized states. We can summarize our findings for two-dimensional (2-D) and uni-dimensional (1-D) heterojunctions as: (i) for zero bias exhibit non-degenerate bands with topological insulator, metal or semiconductor signature depending on the defect distribution; (ii) for devices under bias voltage, the I–V curves and transition voltage spectroscopy (TVS) exhibit: (1) negative differential resistance (NDR) and resonance behavior signing as resonant tunneling diode (RTD) and field effect transistor (FET) type for 2-D and 1-D heterojunctions, respectively; (2) electronic transitions as (2.1) topological insulator-semiconductor, topological insulator-metal, semiconductor-metal and metal-semiconductor transitions confirmed by density of states (DOS) analysis for voltages > 0. The analysis of rectification ratio for asymmetric heterojunctions presents better conductance for reverse bias. Our discoveries make the heterojunctions highly promising in nanoelectronics applications.

Nanoscale junctions for single molecule electronics fabricated using bilayer nanoimprint lithography combined with feedback controlled electromigration

Nanoimprint lithography (NIL) is a fast, simple and high throughput technique that allows fabrication of structures with nanometre precision features at low cost. We present an advanced bilayer nanoimprint lithography approach to fabricate four terminal nanojunction devices for use in single molecule electronic studies. In the first part of this work, we demonstrate a NIL lift-off process using a bilayer resist technique that negates problems associated with metal side-wall tearing during lift-off. In addition to precise nanoscale feature replication, we show that it is possible to imprint micron-sized features while still maintaining a bilayer structure enabling an undercut resist structure to be formed. This is accomplished by choosing suitable imprint parameters as well as residual layer etching depth and development time. We then use a feedback controlled electromigration procedure, to produce room-temperature stable nanogap electrodes with sizes below 2 nm. This approach facilitates the integration of molecules in stable, solid-state molecular electronic devices as demonstrated by incorporating benzenethiol as molecular bridges between the electrodes and characterizing its electronics properties through current–voltage measurements. The observation of molecular transport signatures, showing current suppression in the I–V behaviour at low voltage, which is then lifted at high voltage, signifying on- and off-resonant transport through molecular levels as a function of voltage, is confirmed in repeated I–V sweeps. The large conductance, symmetry of the I–V sweep and small value of the voltage minimum in transition voltage spectroscopy indicates the bridging of the two benzenethiol molecules is by π–stacking.

Evidence That Molecules in Molecular Junctions May Not Be Subject to the Entire External Perturbation Applied to Electrodes

Whether molecules forming molecular junctions are really subject to the entire external perturbation applied to electrodes is an important issue, but so far, it has not received adequate consideration in the literature. In this paper, we demonstrate that, out of the temperature difference ΔTelectr between electrodes applied in thermopower measurements, molecules only feel a significantly smaller temperature difference (ΔTmolec < ΔTelectr). Rephrasing, temperature drops at metal-molecule interfaces are substantial. Our theoretical analysis to address this problem of fundamental importance for surface science is based on experimental data collected via ultraviolet photoelectron spectroscopy, transition voltage spectroscopy, and Seebeck coefficient measurements. An important practical consequence of the presently reported finding is that the energetic alignment of the frontier molecular orbital (HOMO or LUMO) of the embedded molecules with respect to the metallic Fermi level position deduced from thermopower data-and this is frequently the case in current studies of molecular electronics-is substantially overestimated. Another important result presented here is that, unlike the exponential length dependence characterizing electric conduction (which is a fingerprint for quantum tunneling), thermal conduction through the molecules considered (oligophenylene thiols and alkane thiols) exhibits a length dependence compatible with classical physics.

Giant Conductance Enhancement of Intramolecular Circuits through Interchannel Gating

Summary For neutral intramolecular circuits with two constitutionally identical branches, a maximum 4-fold increase in total conductance can be obtained according to constructive quantum interference (CQI). For charged intramolecular circuits, however, the strong electrostatic interactions entangle the quantum states of these two parallel pathways, thus introducing complicated transport behavior that warrants experimental investigation of the intramolecular circuit rules. Here, we report that a tetracationic cyclophane with parallel channels exhibits a 50-fold conductance enhancement compared with that of a single-channel control, an observation that supplements intramolecular circuit law in systems with strong Coulombic interactions. Flicker noise measurements and theoretical calculations show that strong electrostatic interactions between charged parallel channels—serving as the chemical gate to promote the effective conductance of each channel—and CQI boosts the total conductance of the two-channel circuit. The molecular design presented herein constitutes a proof-of-principle approach to charged intramolecular circuits that are desirable for quantum circuits and devices.

Electronic transport in zigzag PhaGraphene NanoRibbon doped with boron and nitrogen

In this paper is investigated the transport and electronic properties of 1-D material, such as zigzag PhaGraphene NanoRibbons (zzPGNR) substitutionally B- and N-doped, i.e., B-doped zzPGNR and N-doped zzPGNR. A comparison is made with 1-D pure (zzPGNR and zzGNR) and doped (B- and N-doped zzGNR) materials already well known in the literature. The question is what changes and which applications occur in the properties of these materials when it has its crystalline lattice modified? The calculations were performed using DFT without spin for the electronic properties of the unit cell via SIESTA/INELASTICA packages and posteriorly combined with NEGF (DFT/NEGF) via TranSIESTA package to obtain the transport properties and transition voltage spectroscopy (TVS) of the molecular device. The results exhibit Field Effect Transistor (FET) behavior and semiconductor-metal transition for B- and N-doped zzGNR, while resonant tunnel diode (RTD) behavior due to emergence of negative differential resistances (NDRs) and metal-semiconductor transition for B- and N-doped zzPGNR. B- and N-doped zzGNR exhibit p- and n-semiconductor behavior with non-degenerate bands, while B- and N-doped zzPGNR exhibit metallic behavior with non-degenerate and degenerate bands, respectively.

Topological insulator-metal transition and molecular electronics device based on zigzag phagraphene nanoribbon

In this work, we investigate the electronic transport properties of a graphene allotrope composed of 5–6-7 carbon aromatic rings called phagraphene and compare with the results of the transition-voltage spectroscopy (TVS) and propose the behavior at low voltage characteristic of a topological insulator. Phagraphene properties were compared to those of graphene in a zigzag nanoribbon configuration, zigzag graphene vs zigzag phagraphene nanoribbon (zzGNR and zzPGNR). The molecular geometry and the electronic properties were calculated by density functional theory (DFT) without spin, and the electronic transport and TVS were obtained by means of DFT combined with non-equilibrium Green´s function when we couple the optimized geometry of zzGNR and zzPGNR to the leads (left and right), forming the molecular junction that will be subjected to the action of an external bias voltage (Ve) to generate the molecular device. The results exhibit (i) a metal-insulator transition when Ve is increased until Ve = 1.4 V which corresponds to the nonlinear region (resonance), showing the field effect transistor behaviour for zzGNR junctions; and (ii) two nonlinear regions (two negative differential resistances), showing a resonant tunnel diode behaviour with two operation windows (Ve = 0.5 V and Ve = 1.7 V) for the zzPGNR junction. In addition, the zzPGNR junction exhibits topological insulator characteristics upon introducing topological defects such as pentagons and heptagons in the hexagonal lattice of graphene, and when Ve = 1.7 V, there occurs a topological insulator-metal transition that can be seen in the behaviour of the density of states, transmittance, and frontier molecular orbitals with Ve.

Efficient Fabrication of Stable Graphene‐Molecule‐Graphene Single‐Molecule Junctions at Room Temperature

We present a robust approach to fabricate stable single-molecule junctions at room temperature using single-layer graphene as nanoelectrodes. Molecular scale nano-gaps in graphene were generated using an optimized fast-speed feedback-controlled electroburning process. This process shortened the time for creating a single nano-gap to be less than one minute while keeping a yield higher than 97 %. To precisely control the gap position and minimize the effects of edge defects and the quantum confinement, extra-narrow grooves were pre-patterned in the graphene structures with oxygen plasma etching. Molecular junctions were formed by bridging the nano-gaps with amino-functionalized hexaphenyl molecules by taking advantage of chemical reactions between the amino groups at the two ends of the molecules and the carboxyl groups at the edges of graphene electrodes. Electronic transport measurements and transition voltage spectroscopy analysis verified the formation of single-molecule devices. First-principles quantum transport calculations show that the highest occupied molecular orbital of hexaphenyl is closer to the Fermi level of the graphene electrodes and thus the devices exhibit a hole-type transport characteristics. Some of these molecular devices remained stable up to four weeks, highlighting the potential of graphene nano-electrodes in the fabrication of stable single-molecule devices at room temperature.

Work function and temperature dependence of electron tunneling through an N-type perylene diimide molecular junction with isocyanide surface linkers.

Conducting probe atomic force microscopy (CP-AFM) was employed to examine electron tunneling in self-assembled monolayer (SAM) junctions. A 2.3 nm long perylene tetracarboxylic acid diimide (PDI) acceptor molecule equipped with isocyanide linker groups was synthesized, adsorbed onto Ag, Au and Pt substrates, and the current-voltage (I-V) properties were measured by CP-AFM. The dependence of the low-bias resistance (R) on contact work function indicates that transport is LUMO-assisted ('n-type behavior'). A single-level tunneling model combined with transition voltage spectroscopy (TVS) was employed to analyze the experimental I-V curves and to extract the effective LUMO position εl = ELUMO - EF and the effective electronic coupling (Γ) between the PDI redox core and the contacts. This analysis revealed a strong Fermi level (EF) pinning effect in all the junctions, likely due to interface dipoles that significantly increased with increasing contact work function, as revealed by scanning Kelvin probe microscopy (SKPM). Furthermore, the temperature (T) dependence of R was found to be substantial. For Pt/Pt junctions, R varied more than two orders of magnitude in the range 248 K < T < 338 K. Importantly, the R(T) data are consistent with a single step electron tunneling mechanism and allow independent determination of εl, giving values compatible with estimates of εl based on analysis of the full I-V data. Theoretical analysis revealed a general criterion to unambiguously rule out a two-step transport mechanism: namely, if measured resistance data exhibit a pronounced Arrhenius-type temperature dependence, a two-step electron transfer scenario should be excluded in cases where the activation energy depends on contact metallurgy. Overall, our results indicate (1) the generality of the Fermi level pinning phenomenon in molecular junctions, (2) the utility of employing the single level tunneling model for determining essential electronic structure parameters (εl and Γ), and (3) the importance of changing the nature of the contacts to verify transport mechanisms.

Controlling destructive quantum interference in tunneling junctions comprising self-assembled monolayers via bond topology and functional groups.

Quantum interference effects (QI) are of interest in nano-scale devices based on molecular tunneling junctions because they can affect conductance exponentially through minor structural changes. However, their utilization requires the prediction and deterministic control over the position and magnitude of QI features, which remains a significant challenge. In this context, we designed and synthesized three benzodithiophenes based molecular wires; one linearly-conjugated, one cross-conjugated and one cross-conjugated quinone. Using eutectic Ga-In (EGaIn) and CP-AFM, we compared them to a well-known anthraquinone in molecular junctions comprising self-assembled monolayers (SAMs). By combining density functional theory and transition voltage spectroscopy, we show that the presence of an interference feature and its position can be controlled independently by manipulating bond topology and electronegativity. This is the first study to separate these two parameters experimentally, demonstrating that the conductance of a tunneling junction depends on the position and depth of a QI feature, both of which can be controlled synthetically.

Effect of surface passivation on breakdown voltages of 4H-SiC Schottky barrier diodes

In this study, we fabricated and characterized graphene/molecule/graphene (GMG) vertical junctions with aryl alkane monolayers. The constituent molecules were chemically self-assembled via electrophilic diazonium reactions into a monolayer on the graphene bottom electrode, while the other end physically contacted the graphene top electrode. A full understanding of the transport properties of molecular junctions is a key step in the realization of molecular-scale electronic devices and requires detailed microscopic characterization of the junction’s active region. Using a multiprobe approach combining a variety of transport techniques, we elucidated the transport mechanisms and electronic structure of the GMG junctions, including temperature- and length-variable transport measurements, and transition voltage spectroscopy. These results provide criteria to establish a valid molecular junction and to determine the most probable transport characteristics of the GMG junctions.

Single Nucleobase Identification Using Biophysical Signatures from Nanoelectronic Quantum Tunneling.

Nanoelectronic DNA sequencing can provide an important alternative to sequencing-by-synthesis by reducing sample preparation time, cost, and complexity as a high-throughput next-generation technique with accurate single-molecule identification. However, sample noise and signature overlap continue to prevent high-resolution and accurate sequencing results. Probing the molecular orbitals of chemically distinct DNA nucleobases offers a path for facile sequence identification, but molecular entropy (from nucleotide conformations) makes such identification difficult when relying only on the energies of lowest-unoccupied and highest-occupied molecular orbitals (LUMO and HOMO). Here, nine biophysical parameters are developed to better characterize molecular orbitals of individual nucleobases, intended for single-molecule DNA sequencing using quantum tunneling of charges. For this analysis, theoretical models for quantum tunneling are combined with transition voltage spectroscopy to obtain measurable parameters unique to the molecule within an electronic junction. Scanning tunneling spectroscopy is then used to measure these nine biophysical parameters for DNA nucleotides, and a modified machine learning algorithm identified nucleobases. The new parameters significantly improve base calling over merely using LUMO and HOMO frontier orbital energies. Furthermore, high accuracies for identifying DNA nucleobases were observed at different pH conditions. These results have significant implications for developing a robust and accurate high-throughput nanoelectronic DNA sequencing technique.

Tunneling mechanism and contact mechanics of colloidal nanoparticle assemblies

Nanoparticle assemblies with thiol-terminated alkyl chains are studied by conducting atomic force microscopy (c-AFM) regarding their use as strain gauges for touch-sensitive panels. Current–force spectroscopy is used as a characterization tool complementary to the macroscopic setup since it allows a bias to be applied to a limited number of junctions, overcoming the Coulomb blockade energy and focusing on the contact electromechanics and the transport mechanism across the ligand. First, transition voltage spectroscopy is applied with varying force to target the underlying tunneling mechanism by observing whether the transition between the ohmic and exponential current–voltage behavior is force-dependent. Secondly, current–force spectroscopy in the ohmic range below the transition voltage is performed. The current–force behavior of the AFM probe in contact with a nanoparticle multilayer is associated with the spread of force and current within the nanoparticle lattice and at the level of adjacent particles by detailed contact mechanics treatment. The result is twofold: concerning the architecture of sensors, this work is a sample case of contact electromechanics at scales ranging from the device scale down to the individual ligand molecule. Regarding transport across the molecule, the vacuum tunneling mechanism is favored over the conduction by coherent molecular states, which is a decision-making aid for the choice of ligand in applications.