Destructive quantum interference in heterocyclic alkanes: the search for ultra-short molecular insulators

Interpretation of molecular electron transport in ab initio many-electron framework incorporating zero-point nuclear motion effects

A computational methodology, founded on chemical concepts, is presented for interpreting the role of nuclear motion in the electron transport through single-molecule junctions (SMJ) using many-electron ab initio quantum chemical calculations. Within this approach the many-electron states of the system, computed at the SOS-ADC(2) level, are followed along the individual normal modes of the encapsulated molecules. The inspection of the changes in the partial charge distribution of the many-electron states allows the quantification of the electron transport and the estimation of transmission probabilities. This analysis improves the understanding of the relationship between internal motions and electron transport. Two SMJ model systems are studied for validation purposes, constructed from a conductor (BDA, benzene-1,4-diamine) and an insulator molecule (DABCO, 1,4-diazabicyclo[2.2.2]octane). The trends of the resulting transmission probabilities are in agreement with the experimental observations, demonstrating the capability of the approach to distinguish between conductor and insulator type systems, thereby offering a straightforward and cost-effective tool for such classifications via quantum chemical calculations.

Robust binding between secondary amines and Au electrodes

While primary amines are one of the most widely used linker groups for forming single-molecule junctions, it remains elusive whether and how the substitution of one hydrogen in a primary amine with a methyl group (secondary amine) can alter its functional properties as a linker group. Here we show that a robust binding between a secondary amine and an Au electrode is absent with the use of a non-coated Au tip and is achieved when in contact with a wax-coated Au tip, which we propose is catalyzed by the more frequent formation of Au adatoms in measurements with a wax-coated tip.

Intramolecular London Dispersion Interactions in Single-Molecule Junctions

This work shows the first example of using intramolecular London dispersion interactions to control molecular geometry and quantum transport in single-molecule junctions. Flexible σ-bonded molecular junctions typically occupy straight-chain geometries due to steric effects. Here, we synthesize a series of thiomethyl-terminated oligo(dimethylsilmethylene)s that bear [CH2-Si(CH3)2]n repeat units, where all backbone dihedral states are sterically equivalent. Scanning tunneling microscopy break-junction (STM-BJ) measurements and theoretical calculations indicate that in the absence of a strong steric bias concerted intramolecular London dispersion interactions staple the carbosilane backbone into coiled conformations that remain intact even as the junction is stretched to its breakpoint. As these kinked conformations are highly resistive to electronic transport, we observe record-high conductance decay values on an experimental junction length basis (β = 1.86 ± 0.12 Å-1). These studies reveal the potential of using intramolecular London dispersion interactions to design single-molecule electronics.

Radical Single-Molecule Junctions

Radicals are unique molecular systems for applications in electronic devices due to their open-shell electronic structures. Radicals can function as good electrical conductors and switches in molecular circuits while also holding great promise in the field of molecular spintronics. However, it is both challenging to create stable, persistent radicals and to understand their properties in molecular junctions. The goal of this Perspective is to address this dual challenge by providing design principles for the synthesis of stable radicals relevant to molecular junctions, as well as offering current insight into the electronic properties of radicals in single-molecule devices. By exploring both the chemical and physical properties of established radical systems, we will facilitate increased exploration and development of radical-based molecular systems.

Manipulating Quantum Interference between σ and π Orbitals in Single-Molecule Junctions via Chemical Substitution and Environmental Control

Understanding and manipulating quantum interference (QI) effects in single molecule junction conductance can enable the design of molecular-scale devices. Here we demonstrate QI between σ and π molecular orbitals in an ∼4 Å molecule, pyrazine, bridging source and drain electrodes. Using single molecule conductance measurements, first-principles analysis, and electronic transport calculations, we show that this phenomenon leads to distinct patterns of electron transport in nanoscale junctions, such as destructive interference through the para position of a six-membered ring. These QI effects can be tuned to allow conductance switching using environmental pH control. Our work lays out a conceptual framework for engineering QI features in short molecular systems through synthetic and external manipulation that tunes the energies and symmetries of the σ and π channels.

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.

Substituent Control of σ-Interference Effects in the Transmission of Saturated Molecules

The single-molecule conductance of saturated molecules can potentially be fully suppressed by destructive quantum interference in their σ-system. However, only few molecules with σ-interference have been identified, and the structure-property relationship remains to be elucidated. Here, we explore the role of substituents in modulating the electronic transmission of saturated molecules. In functionalized bicyclo[2.2.2]octanes, the transmission is suppressed by σ-interference when fluorine substituents are applied. For bicyclo[2.2.2]octasilane and -octagermanes, the transmission is suppressed when carbon-based substituents are used, and such molecules are likely to be highly insulating. For the carbon-based substituents, we find a strong correlation between the appropriate Hammett constants and the transmission. The substituent effect enables systematic optimization of the insulating properties of saturated molecular cores.

Site and length dependent quantum interference and resonance in the electron transport of armchair carbon nanotube molecular junctions

The destructive quantum interference (DQI) effect in molecular devices, as characterized by a sharp valley in the transmission function and conductance suppression with several orders of magnitude, is of great interest for both fundamental reasons and technical applications. Planar π conjugated systems, such as benzene, graphene molecules and graphene nanoribbons, are typical examples showing DQI and have been studied most frequently. Carbon nanotubes (CNTs) can be considered as extended planar π conjugated systems, but with a different topology from graphene. In this work, using the Hückel analytical theory, we investigated the transport properties of molecular junctions constructed with armchair CNTs which are weakly coupled to the leads with single site connections. It is found that the transport properties demonstrate obvious oscillation with a period of 3 in nanotube length as defined by the number (n) of atomic planes along the transport direction, which is not observed in graphene nanoribbons. Specifically, when the length is n = 3p or 3p + 1, DQI will be observed at the Fermi level when both leads are connected to the same sublattice, but not observed when they are connected to different sublattices. In contrast, when the length is 3p + 2, the DQI sharp valley will never be observed at the Fermi level. Instead, a resonant peak will appear at the Fermi level when the two leads are connected to the same sublattice. Nevertheless, this resonant peak will not appear for connections with different sublattices. All these results are well explained in terms of the energy spectrum of an armchair graphene nanoribbon model and spatial distribution of the frontier molecular orbitals. The findings demonstrate the great difference between the DQI patterns of graphene nanoribbons and carbon nanotubes due to topology differences.

Electronic Conduction through Monolayer Amorphous Carbon Nanojunctions

In molecular electronic conduction, exotic lattice morphologies often give rise to exotic behaviors. Among 2D systems, graphene is a notable example. Recently, a stable amorphous version of graphene called monolayer amorphous carbon (MAC) was synthesized. MAC poses a new set of questions regarding the effects of disorder on conduction. In this Letter, we perform an ensemble-level computational analysis of the coherent electronic transmission through MAC nanofragments in search of defining characteristics. Our analysis, relying on a semiempirical Hamiltonian (Pariser-Parr-Pople) and Landauer theory, showed that states near the Fermi energy (E_F) in MAC inherit partial characteristics of analogous surface states in graphene nanofragments. Away from E_F, current is carried by a set of delocalized states that transition into a subset of insulating interior states at the extreme portions of MAC's energy spectrum. Finally, we also found that quantum interference between frontier orbitals is a common feature among MAC nanofragments.