Destructive Quantum Interference Effects Research Articles

Bias switching in single-molecule junctions through destructive quantum interference

Highly nonlinear current-voltage (I-V) characteristics in single-molecule junctions based on destructive quantum interference (DQI) effects are key to realizing bias-controlled molecular switches. Using first-principles quantum transport calculations, we investigated the charge transport properties of 1,4-diphenyl-2,3-dioxa-7-tellurabicyclo(DPDT) coupled to gold electrodes via cyanide (-CN), isocyanide (-NC) and pyridyl (-PY) anchoring groups, respectively. We demonstrated that there are DQI effects between LUMO and LUMO+1 of the opposite phase in the three junctions. It was revealed that the p orbital of the tellurium atom can localize HOMO orbitals on central units of molecular systems, and weaken the amplitude of LUMO relative to LUMO+1. This eliminates the constructive quantum interference (CQI) between HOMO and LUMO, but enhances the DQI between LUMO and LUMO+1 near the Fermi energy level of gold electrodes (EF). As a result, the three molecular junctions dominated by two LUMOs exhibit extremely high nonlinear I-V characteristics, contributing to a current at a high bias (±0.8 V) >170 times larger than that at a low bias (±0.2 V). Our findings provide a potentially stable and low-loss bias switching in single-molecule junctions.

Destructive quantum interference in meta-oligo(phenyleneethynylene) molecular wires with gold-graphene heterojunctions.

Quantum interference (QI) is well recognised as a significant contributing factor to the magnitude of molecular conductance values in both single-molecule and large area junctions. Numerous structure-property relationship studies have shown that para-connected oligo(phenyleneethynylene) (OPE) based molecular wires exemplify the impact of constructive quantum interference (CQI), whilst destructive quantum interference (DQI) effects are responsible for the orders of magnitude lower conductance of analogous meta-contacted OPE derivatives, despite the somewhat shorter effective tunnelling distance. Since molecular conductance is related to the value of the transmission function, evaluated at the electrode Fermi energy, T(EF), which in turn is influenced by the presence and relative energy of (anti)resonances, it follows that the relative single-molecule conductance of para- and meta-contacted OPE-type molecules is tuned both by the anchor group and the nature of the electrode materials used in the construction of molecular junctions (gold|molecule|gold vs. gold|molecule|graphene). It is shown here that whilst amine-contacted junctions show little influence of the electrode material on molecular conductance due to the similar electrode-molecule coupling through this anchor group to both types of electrodes, the weaker coupling between thiomethyl and ethynyl anchors and the graphene substrate electrode results in a relative enhancement of the DQI effect. This work highlights an additional parameter space to explore QI effects and establishes a new working model based on the electrode materials and anchor groups in modulating QI effects beyond the chemical structure of the molecular backbone.

Photon blockade with a trapped Λ-type three-level atom in asymmetrical cavity.

We propose a scheme to manipulate strong and nonreciprocal photon blockades in asymmetrical Fabry-Perot cavity with a Λ-type three-level atom. Utilizing the mechanisms of both conventional and unconventional blockade, the strong photon blockade is achieved by the anharmonic eigenenergy spectrum brought by Λ-type atom and the destructive quantum interference effect induced by a microwave field. By optimizing the system parameters, the manipulation of strong photon blockade over a wide range of cavity detuning can be realized. Using spatial symmetry breaking introduced by the asymmetry of cavity, the direction-dependent nonreciprocal photon blockade can be achieved, and the nonreciprocity can reach the maximum at optimal cavity detuning. In particular, manipulating the occurring position of nonreciprocal photon blockade can be implemented by simply adjusting the cavity detuning. Our scheme provides feasible access for generating high-quality nonreciprocal single-photon sources.

Visualizing and comparing quantum interference in the π-system and σ-system of organic molecules.

Quantum interference effects in conjugated molecules have been well-explored, with benzene frequently invoked as a pedagogical example. These interference effects have been understood through a quantum interference map in which the electronic transmission is separated into interfering and non-interfering terms, with a focus on the π-orbitals for conjugated molecules. Recently, saturated molecules have also been reported to exhibit destructive quantum interference effects; however, the very different σ-orbital character in these molecules means that it is not clear how orbital contributions manifest. Herein, we demonstrate that the quantum interference effects in conjugated molecules are quite different from those observed in saturated molecules, as demonstrated by the quantum interference map. While destructive interference at the Fermi energy in the π-system of benzene arises from interference terms between paired occupied and virtual orbitals, this is not the case at the Fermi energy in saturated systems. Instead, destructive interference is evident when contributions from a larger number of non-paired orbitals cancel, leading to more subtle and varied manifestations of destructive interference in saturated systems.

Scaling of quantum interference from single molecules to molecular cages and their monolayers

The discovery of quantum interference (QI) is widely considered as an important advance in molecular electronics since it provides unique opportunities for achieving single-molecule devices with unprecedented performance. Although some pioneering studies suggested the presence of spin qubit coherence and QI in collective systems such as thin films, it remains unclear whether the QI can be transferred step-by-step from single molecules to different length scales, which hinders the application of QI in fabricating active molecular devices. Here, we found that QI can be transferred from a single molecule to their assemblies. We synthesized and investigated the charge transport through the molecular cages using 1,3-dipyridylbenzene (DPB) as a ligand block with a destructive quantum interference (DQI) effect and 2,5-dipyridylfuran (DPF) as a control building block with a constructive quantum interference (CQI) effect using both single-molecule break junction and large area junction techniques. Combined experiments and calculations revealed that both DQI and CQI had been transferred from the ligand blocks to the molecular cages and the monolayer thin film of the cages. Our work introduced QI effects from a ligand to the molecular cage comprising 732 atoms and even their monolayers, suggesting that the quantum interference could be scaled up within the phase-coherent distance.

Thermoelectric Performance Enhanced by Destructive Quantum Interference in Nanoporous Carbon Nanotube Based Junctions

Aided by density functional theory with nonequilibrium Green's functions simulations, the thermoelectric (TE) properties of nanoporous carbon nanotube junctions formed by covalently binding carbon nanotubes via benzene bridges with either meta‐ or para‐connections are investigated. The results show that the TE performances of meta‐connected nanoporous carbon nanotube junctions are significantly improved due to the influence of destructive quantum interference (DQI) effect. Moreover, the TE properties of meta‐connected carbon nanotube junctions can be further improved by substitution of nitrogen atom and gate voltage. The reason is that the quantum interference patterns can be transformed from DQI to constructive quantum interference (or Fano resonance). The theoretical analysis shows that the TE figure of merit (ZT) for meta‐connected carbon nanotube junctions at room temperature is close to 0.5 near the Fermi level, ≈20 times higher than that of the perfect carbon nanotube junction. These results reveal that quantum interference effect can indeed be used to enhance the TE performance of molecular junctions.

Modulation of destructive quantum interference by bridge groups in truxene-based single-molecule junctions.

Electron transport properties of polycyclic truxene derivatives have been investigated by the single molecule conductance measurement technique and theoretical study. Molecules with nitrogen and carbonyl substituents at the bridge sites exhibit higher single-molecule conductances by almost one order of magnitude compared with non-substituted analogues. It can be ascribed that the anti-resonance feature produced by destructive quantum interference (DQI) is alleviated and pushed away from the Fermi energy. These findings provide an effective chemical strategy for manipulating the DQI behavior in single molecular devices.

Mechanically Tunable Quantum Interference in Ferrocene-Based Single-Molecule Junctions.

Ferrocenes are ubiquitous organometallic building blocks that comprise a Fe atom sandwiched between two cyclopentadienyl (Cp) rings that rotate freely at room temperature. Of widespread interest in fundamental studies and real-world applications, they have also attracted some interest as functional elements of molecular-scale devices. Here we investigate the impact of the configurational degrees of freedom of a ferrocene derivative on its single-molecule junction conductance. Measurements indicate that the conductance of the ferrocene derivative, which is suppressed by 2 orders of magnitude as compared to a fully conjugated analogue, can be modulated by altering the junction configuration. Ab initio transport calculations show that the low conductance is a consequence of destructive quantum interference effects of the Fano type that arise from the hybridization of localized metal-based d-orbitals and the delocalized ligand-based π-system. By rotation of the Cp rings, the hybridization, and thus the quantum interference, can be mechanically controlled, resulting in a conductance modulation that is seen experimentally.

Wenjing Hong.

"The most significant scientific advance of the last 100 years has been quantum mechanics … What I look for first in a publication is something unexpected and counterintuitive …" Find out more about Wenjing Hong in his Author Profile.

Quantum Transport Through a ``Charge" Kondo Circuit: Effects of Weak Repulsive Interaction in Luttinger Liquid

We investigate theoretically quantum transport through the ``charge" Kondo circuit consisting of the quantum dot (QD) coupled weakly to an electrode at temperature \(T+\Delta T\) and connected strongly to another electrode at the reference temperature \(T\) by a single-mode quantum point contact (QPC). To account for the effects of Coulomb interaction in the QD-QPC setup operating in the integer quantum Hall regime we describe the edge current in the quantum circuit by Luttinger model characterized by the Luttinger parameter \(g\). It is shown that the temperature dependence of both electric conductance \(G\propto T^{2/g}\) and thermoelectric coefficient \(G_T\propto T^{1+2/g}\) detours from the Fermi-liquid (FL) theory predictions. The behaviour of the thermoelectric power \(S=G_T/G\propto T\) in a regime of a single-channel Kondo effect is, by contrast, consistent with the FL paradigm. We demonstrate that the interplay between the mesoscopic Coulomb blockade in QD and weak repulsive interaction in the Luttinger Liquid \(g=1-\alpha\) \((\alpha \ll 1)\) results in the enhancement of the thermopower. This enhancement is attributed to suppression of the Kondo correlations in the ``charge" circuit by the destructive quantum interference effects.

Design and optimization of a heat engine based on a porphyrin single-molecule junction with graphene electrodes

A molecular heat engine is proposed based on a porphyrin single-molecule junction. The important thermoelectric transport characteristics, such as electrical conductance, thermopower, output power, and efficiency have been calculated by means of a Landauer-B\uttiker approach combined with density functional theory. Different ways of engineering the heat engine performance have been investigated. (i) The efficiency of the heat engine can be considerably enhanced in special contact geometry between the molecule and electrodes that manifests a destructive quantum interference effect. (ii) Contrary to the usual length dependence of a single molecular junction, an increase of electrical conductance and thermopower with a number of porphyrin units have been found and used to optimize the molecular heat engine. (iii) The engine performance depends sensitively on the twisting angle between the molecule and graphene electrodes. Thus, a single molecular junction could be a promising candidate for a nanoscale heat engine utilizing quantum effect.

Tunable photon statistics in parametrically amplified photonic molecules

The proposal for single-photon generation utilizing quantum interference of two coupled cavity modes with weak Kerr nonlinearity under resonant coherent driving has been successfully put forward in previous work [T. C. H. Liew and V. Savona, Phys. Rev. Lett. 104, 183601 (2010)]. Here we suggest an alternative scheme to explore the statistical properties of the photons and their control in a parametrically amplified photonic molecule that is composed of a pair of coupled cavities, both containing the degenerate optical parametric amplifier (DOPA), by means of extending the model of Liew and Savona. Due to the destructive quantum interference effect between different paths for two-photon excitation, the photons in both cavity modes can exhibit a strong antibunching effect when one of the two cavities is coherently driven by an external laser field. In order to intuitively understand the strong photon antibunching phenomenon occurring in our DOPA photonic molecule, the approximate analytical solutions of the system are provided via the Schr\odinger equation and further are compared with the full numerical solutions via the master equation. The in-depth results reveal that the two solutions are in good agreement. For the two cavity modes, we achieve the optimal antibunching conditions in terms of the nonlinear gain and the strength ratio between the two pump laser fields based on analytical calculations, gaining deep insight into the rich physics of photon statistics. We also show that the statistical properties of the photons can be well controlled by regulating the relative phase between the two pump laser fields and the tunneling rate between the two cavity modes. Furthermore, we address the second-order cross-correlation function between the two cavity modes and find that a strong intermodal antibunching can be efficiently generated. Our approach via the DOPA photonic molecule architecture can be useful to generate tunable single-photon sources or controlled photonic quantum gates.

Interplay of Direct and Indirect Charge-Transfer Pathways in Donor-Bridge-Acceptor Systems.

Charge transfer in donor-bridge-acceptor (DBA) structures typically takes place through the combination of donor-bridge and bridge-acceptor overlap integrals forming an effective, indirect electronic coupling between the donor (D) and acceptor (A) moieties. Here we examine the effects of an additional direct DA electronic coupling of charge-transfer processes in DBA systems with local interaction with thermal baths. First, using the exact Nakajima-Zwanzig master equation (NZME) for the reduced density matrix, we rigorously define probability currents as the coherent part of the NZME, thereby allowing us to quantify the contribution of the different electronic pathways (direct and indirect) to the charge-transfer dynamics. Focusing on two minimal DBA systems of three sites (V and Λ models) and adopting well-developed methods, we find that the interplay between different transfer pathways can be assessed by the McConnell formula in the weak system-bath coupling regime. We then demonstrate that the combination of indirect and direct donor-acceptor coupling either enhances or leads to a destructive quantum interference effect on charge-transport processes, depending on the energy landscape of the DBA system.

Effect of electrophilic substitution and destructive quantum interference on the thermoelectric performance in molecular devices

Using density function theory combined with the non-equilibrium Green’s function method, the thermoelectric properties of para-Xylene-based molecular devices are investigated. It is found that destructive quantum interference can be triggered in n-type of para-connected para-Xylene-based molecular device and can obviously enhance the thermoelectric performance of the devices. Moreover, bridge atom electrophilic substitution can significantly improve the thermoelectric properties of p-type monolayer molecular device. The ZT value of p-type monolayer molecular device with doped electrodes can be optimized to 2.2 at 300 K and 2.8 at 500 K, and n-type bilayer molecular device can achieve the value of 1.2 at 300 K and 2.0 at 500 K. These results offer the information to design the complete molecular thermoelectric device with p-type and n-type of components and to promote the thermoelectric properties of bilayer molecular junctions by employing destructive quantum interference effects.

Systematic experimental study of quantum interference effects in anthraquinoid molecular wires.

In order to translate molecular properties in molecular-electronic devices, it is necessary to create design principles that can be used to achieve better structure-function control oriented toward device fabrication. In molecular tunneling junctions, cross-conjugation tends to give rise to destructive quantum interference effects that can be tuned by changing the electronic properties of the molecules. We performed a systematic study of the tunneling charge-transport properties of a series of compounds characterized by an identical cross-conjugated anthraquinoid molecular skeleton but bearing different substituents at the 9 and 10 positions that affect the energies and localization of their frontier orbitals. We compared the experimental results across three different experimental platforms in both single-molecule and large-area junctions and found a general agreement. Combined with theoretical models, these results separate the intrinsic properties of the molecules from platform-specific effects. This work is a step towards explicit synthetic control over tunneling charge transport targeted at specific functionality in (proto-)devices.

Quantum Interference Effects in Charge Transport through Single-Molecule Junctions: Detection, Manipulation, and Application.

Quantum interference effects (QIEs), which offer unique opportunities for the fine-tuning of charge transport through molecular building blocks by constructive or destructive quantum interference, have become an emerging area in single-molecule electronics. Benefiting from the QIEs, charge transport through molecular systems can be controlled through minor structural and environmental variations, which cause various charge transport states to be significantly changed from conductive to insulative states and offer promising applications in future functional single-molecule devices. Although QIEs were predicted by theoreticians more than two decades ago, only since 2011 have the challenges in ultralow conductance detection originating from destructive quantum interference been overcome experimentally. Currently, a series of single-molecule conductance investigations have been carried out experimentally to detect constructive and destructive QIEs in charge transport through various types of molecular junctions by altering molecular patterns and connectivities. Furthermore, the use of QIEs to tune the properties of charge transport through single-molecule junctions using external gating shows vital potential in future molecular electronic devices. The experimental and theoretical investigations of QIEs offer new fundamental understanding of the structural-electronic relationships in molecular devices and materials at the nanoscale. In this Account, we discuss our progress toward the experimental detection, manipulation, and further application of QIEs in charge transport through single-molecule junctions. These experiments were carried out continuously in our previous group at the University of Bern and in our lab at Xiamen University. As a result of the development of mechanically controllable break junction (MCBJ) and scanning tunneling microscope break junction (STM-BJ) techniques, we could detect ultralow charge transport through the cross-conjugated anthraquinone center, which was one of the earliest experimental studies of QIEs. In close cooperation with organic chemists and theoretical physicists, we systematically investigated charge transport through single-molecule junctions originating from QIEs in conjugated centers ranging from simple single benzene to polycyclic aromatic hydrocarbons (PAHs), heteroaromatics, and even complicated metalla-aromatics at room temperature. Then we further investigated the quantitative correlation between molecular structure and quantum interference by altering different molecular patterns and connectivities in homologous series of PAHs and heteroatom systems. Additionally, external chemical and electrochemical gating of single-molecule devices can be used for direct QIE manipulation via not only tuning molecular conjugation but also shifting the electrode Fermi level. Our study further suggested that distinguishable differences in conductance resulting from QIEs offer opportunities to detect photothermal reaction kinetics and to recognize isomers at the single-molecule scale. These investigations demonstrate the universality of QIEs in charge transport through various molecular building blocks. Moreover, effective manipulation of QIEs leads to various novel phenomena and promising applications in molecular electronic devices.

Controlling and Observing Sharp-Valleyed Quantum Interference Effect in Single Molecular Junctions.

The ability to control over the quantum interference (QI) effect in single molecular junctions is attractive in the application of molecular electronics. Herein we report that the QI effect of meta-benzene based molecule with dihydrobenzo[ b]thiophene as the anchoring group ( meta-BT) can be controlled by manipulating the electrode potential of the junctions in electrolyte while the redox state of the molecule does not change. More than 2 orders of magnitude conductance change is observed for meta-BT ranging from <10-6.0 to 10-3.3 G0 with varying the electrode potential, while the upper value is even larger than the conductance of para-BT ( para-benzene based molecule with anchoring group of dihydrobenzo[ b]thiophene). This phenomenon is attributed to the shifting of energy level alignment between the molecule and electrodes under electrode potential control. Calculation is carried out to predict the transmission function of single molecular junction and the work function of Au surface in the presence of the molecule, and good agreement is found between theory and experiments, both showing sharp-valley featured destructive QI effect for the meta-BT. The present work demonstrates that the QI effect can be tuned through electrochemical gating without change of molecular redox states, which provides a feasible way toward realization of effective molecular switches.

Quantum interference mediated vertical molecular tunneling transistors.

Molecular transistors operating in the quantum tunneling regime represent potential electronic building blocks for future integrated circuits. However, due to their complex fabrication processes and poor stability, traditional molecular transistors can only operate stably at cryogenic temperatures. Here, through a combined experimental and theoretical investigation, we demonstrate a new design of vertical molecular tunneling transistors, with stable switching operations up to room temperature, formed from cross-plane graphene/self-assembled monolayer (SAM)/gold heterostructures. We show that vertical molecular junctions formed from pseudo-p-bis((4-(acetylthio)phenyl)ethynyl)-p-[2,2]cyclophane (PCP) SAMs exhibit destructive quantum interference (QI) effects, which are absent in 1,4-bis(((4-acetylthio)phenyl)ethynyl)benzene (OPE3) SAMs. Consequently, the zero-bias differential conductance of the former is only about 2% of the latter, resulting in an enhanced on-off current ratio for (PCP) SAMs. Field-effect control is achieved using an ionic liquid gate, whose strong vertical electric field penetrates through the graphene layer and tunes the energy levels of the SAMs. The resulting on-off current ratio achieved in PCP SAMs can reach up to ~330, about one order of magnitude higher than that of OPE3 SAMs. The demonstration of molecular junctions with combined QI effect and gate tunability represents a critical step toward functional devices in future molecular-scale electronics.

Quantum interference effect in the charge transport through single-molecule benzene dithiol junction at room temperature: An experimental investigation

The quantum interference effect in the charge transport through single-phenyl molecules received intensive interests from theory but remained as an experimental challenge. In this paper, we investigated the charge transport through single-molecule benzene dithiol (BDT) junction with different connectivities using mechanically controllable break junction (MCBJ) technique. By further improving the mechanical stability and the electronic measuring component of the MCBJ set-up, we obtained the conductance histograms of BDT molecules (BDTs) from the statistical analysis of conductance-distance traces without data selection. By tuning the connectivity, the conductance of BDTs is determined to be 10−1.2G0, 10−2.2G0 and 10−1.0G0 for para, meta, and ortho connectivity, following the trend that ortho-BDT>para-BDT>meta-BDT. Furthermore, the displacements of the junctions followed the trend that para>meta>ortho, suggesting the charge transport through the molecules via the gold-thiol bond. The different trends between conductance and displacement for different connectivities suggests the presence of destructive quantum interference effect on meta-BDT, which provides the experimental evidence for the quantum interference effect through single-phenyl molecular junctions.

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

Destructive quantum interference effect is implemented in large area molecular junctions to improve signatures of electron-phonon interaction. Vertical molecular junctions based on a cross-conjugated anthraquinone layer were fabricated and low-noise transport measurements were performed by acquiring simultaneously the current-voltage characteristics, its second derivative, and the differential conductance. Signatures of vibrational modes excited by inelastic events are present in the whole measured voltage range and superpose to the conductance suppression induced by destructive quantum interference. As a consequence vibrational modes have improved visibility in the low energy window $(&lt;80\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$. Inelastic electron transport spectroscopy data are compared to infrared attenuated total reflection spectroscopy on Au/anthraquinone thin films. Common vibrational modes can be clearly identified, but inelastic electron tunneling spectroscopy reveals the existence of vibrational modes in a wider energy range $(0--400\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$ where infrared spectroscopy is lacking.